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Increasing concerns over the environmental impact and sustainability of conventional polymer materials have motivated academia and industry to devote considerable efforts to the development of polymers from renewable resources. Among a few commercially available biobased or partially biobased thermoplastic polymers, poly(lactic acid) (PLA) has undergone the most investigation. PLA is synthesized either through polycondensation of lactic acid (2-hydroxy propionic acid) or ring-opening polymerization of lactide (LA) (the dimer of lactic acids), as illustrated in Figure 1. The monomer, lactic acids, can be produced via bacterial fermentation using enzyme-thinned corn starch or sugar directly as carbon sources. Lactic acid is one of the simplest chiral molecules and exists as the two stereo isomers: L- and D-lactic acid.
Advances in the polymerization technology have significantly reduced the production cost and have contributed to make PLA economically competitive with petroleum-based polymers. PLA has attracted increasing interest in various markets, such as packaging, textile, and automotive industries, as a very promising eco-friendly alternative to traditional petroleum-based commodity polymers. Despite its numerous advantages such as high strength and high modulus, the inherent brittleness significantly impedes its wide applications in many fields. Compared with the general purpose polystyrene (PS), a mainstream thermoplastic widely used in many industrial and home products, PLA not only has comparable tensile strength and modulus but also exhibits very similar inherent brittleness (as shown in Table 1). Just as the limitation of brittleness of PS led to the development of rubber-modified high impact PS and its copolymers [e.g., acrylonitrile–butadiene–styrene copolymer, (ABS)] for advanced engineering applications, in recent years PLA toughening has become the focus of numerous investigations. Many strategies have been developed in the literature to improve the toughness of PLA, including plasticization, copolymerization, addition of rigid fillers, and blending with a variety of flexible polymers or rubbers.
Table 1. Comparison of Typical PLA Properties with Several Petroleum-Based Commodity Thermoplastic Resins
It has been demonstrated that the variation in stereochemistry, molecular weight, and crystallinity of pristine PLA can improve its ductility and impact resistance to some extent. Nevertheless, such influences are usually marginal and the resulting increase of toughness properties is usually insufficient to satisfy the requirement of most practical applications. The detailed discussion regarding these factors can be found in a previous review entitled “Toughening Polylactide” by Anderson et al.1 and in other related literature.2–5 Therefore, this review will mainly focus on toughening modification of PLA by plasticization, copolymerization, and more industrially practical melt-blending technologies.
Plasticizers are used not only to improve the processability of polymers but also to enhance flexibility and ductility of glassy polymers. A preferred plasticizer for PLA should be one, which significantly lowers the glass transition temperature (Tg) of PLA, is biodegradable, nonvolatile, and nontoxic, and exhibits minimal leaching or migration during aging. The plasticizing efficiency of a plasticizer, which is usually evaluated in terms of the depression in Tg and enhancement in tensile toughness, depends on its miscibility with host polymers, molecular weight, and loading level. The closeness of solubility parameters (δ) and magnitude of interaction parameters (χT) between plasticizers and PLA as a host polymer is usually used to evaluate the miscibility between them, and thus provide a reference for the selection of effective plasticizers.8–16 Generally, small molecule plasticizers are more efficient than larger ones, especially in lowering Tg of the host polymer. The miscibility of a polymer with plasticizers from the same chemical family decreases with increase in the molecular weight of the plasticizers, because mixing with low-molecular weight plasticizers has high entropy of mixing. To date, various monomers and oligomers have been investigated as potential plasticizers for PLA. Among them, polyethylene glycol (PEG) and citrate esters are perhaps the most common investigated plasticizers.
With 19.2 wt % of LA in PLA, Sinclair17 demonstrated that the elongation of the plasticized PLA increased to 536%, and the corresponding elastic modulus and stress at break dropped to 0.66 GPa and 29.2 MPa, respectively. Tg was located between 32 and 40 °C with LA concentration varying from 15 to 20 wt %. Unfortunately, LA was reported to readily volatilize during melt processing because of its low boiling point. This study also reported plasticization of PLA using oligomeric lactic acid (OLA) but relatively lower efficiency in lowering Tg was achieved relative to using LA monomer.
Several citrate esters are commercial plasticizers for food contact films, including triethyl citrate (TEC), tributyl citrate (TBC), acetyltriethyl citrate (ATEC), and acetyltributyl citrate (ATBC). Labrecque et al.18 studied the plasticization of PLA using these citrate esters in extruded PLA films. All of the plasticized PLA compositions (up to 30 wt %) exhibited a single Tg which was lower than that of neat PLA. The elongation of PLA was improved on plasticization but the plasticizing efficiency was higher for ATBC. The citrate plasticizers appeared more effective in enhancing the elongation when its presence was in excess of 10 wt %. At a plasticizer content of 20 wt %, the plasticized PLA showed a minimum of 76% drop in yield strength compared to that (51.7 MPa) of neat PLA. Yield strength further decreased below 10 MPa when 20–30 wt % plasticizer was added. The loss of those low-molecular weight citrate plasticizers during processing was also observed because of their relatively lower boiling points. In another study, Ljungberg and Wesslen12 demonstrated that both glycerin triacetate (GTA; also known as “triacetin”) and TBC were more effective plasticizers for PLA than the other three citrates (TEC, TBC, and ATEC) on the basis of the extent of the Tg depression. Phase separation occurred when the content of both plasticizers were in excess of ∼25 wt %. Phase separation was also noted during heat treatment of the plasticized PLA. An increase in the crystallinity of PLA as a result of cold crystallization was considered to be responsible for the phase separation. With about 15.6 wt % TBC in PLA, phase separation after the storage for almost 30 days was also noted by Sierra et al.19
Murariu et al.8 studied the plasticization of PLA using three low-molecular weight ester-type plasticizers, bis(2-ethyldhexyl) adipate (DOA), GTA, and ATBC. Size exclusion chromatography results revealed that molecular weight and distribution of PLA were less affected by the amount and nature of the plasticizers used during melt blending. The thermal stability of the plasticized PLA correlated with the amount and volatility of the plasticizer used. Differential scanning calorimetry (DSC) analysis demonstrated that the addition of 20 wt % GTA which had the lowest molecular weight and the lowest interaction parameter with PLA among these three plasticizers resulted in the lowest Tg (∼29 °C). PLA plasticized with 20 wt % DOA exhibited phase separation and a smaller decrease in Tg (∼45 °C) but enhanced crystallization rate of PLA. Addition of up to 20 wt % plasticizer led to a gradual decrease in Young's modulus and increased ductility in the following order of efficiency: GTA > ATBC > DOA. The best notched impact performance was seen in PLA plasticized with 20 wt % GTA, in which specimens could not be broken in notched impact testing. By comparison, addition of TBAC led to the least improvement in the impact strength among the three plasticizers, only inducing a 77% increase on an addition of 20 wt %. Table 2 summarizes the molecular weight and solubility parameters (δ) of some monomeric plasticizers and their interaction parameter (χT) with PLA, as well as their plasticization effects on PLA.
Table 2. Molecular Weight and Solubility Parameters (δ) of Some Monomeric Plasticizers and their Plasticization Effects on PLA
Oligomeric and Polymeric Plasticizers
Small molecule plasticizers usually evaporate during melt processing12, 18, 21, 23 and also have a strong tendency to migrate toward the surfaces during storage.11, 13–16, 23 The driving force of the migration is attributed to the depletion in the amorphous PLA phase due to enhanced crystallinity of PLA in plasticized samples, and consequently, the ability of PLA to accommodate the plasticizer diminished.9, 11, 12, 15, 16, 23 Migration not only contaminates the food or beverage in contact with plasticized PLA but also causes plasticized PLA to regain part of the brittleness of neat PLA. The common way to reduce migration and evaporation of plasticizers is to increase the molecular mass of the plasticizer to an upper limit where migration will be minimized while the miscibility with the matrix is still retained.13 In recent years, increasing attention has been paid to the utilization of oligomeric or polymeric plasticizing agents for PLA.
Martino et al.21 compared the plasticizing effects of three commercial adipates as potential plasticizers for PLA. At 10 wt % of the plasticizer content, DOA resulted in much higher elongation (259%) of the plasticized PLA than the two polymeric adipates (5% and 7%, respectively). At 20 wt % of the plasticizer content, however, both polyadipates resulted in much higher elongation (>480%) of the plasticized PLA with respect to DOA (295%). Also, at 20 wt % DOA, lack of homogeneity and significant release of plasticizer during processing were noted, while good compatibility with PLA and higher plasticizing efficiency were observed for the other two polymeric adipates (especially polyadipate with the lower molecular mass) at 20 wt % content. In another work by Martino et al.,11 the plasticization of amorphous PLA using four commercially available adipates was also explored. Each plasticizer was miscible with PLA until a critical concentration was reached, which depended on the molecular mass of the individual adipate. A remarkable increase in elongation was achieved when the concentration of plasticizer reached 10 wt %, whereas the decreases in elastic modulus and tensile stress were noted for all the plasticizers investigated. It was shown that DOA and the polyadipate with the highest molecular mass (Glyplast® G206/7) were less efficient plasticizers. The former showed some migration at the concentrations higher than 10 wt %, while the latter easily caused phase separation to occur because of the lower compatibility with PLA matrix. It was evidenced that the other two polymeric adipates, Glyplast® 206/3 and Glyplast® 206/5, were miscible with PLA at least for the compositions ranging from 5 to 20 wt %. The best plasticizing effects were achieved with the polyadipate having lower molecular mass (Glyplast® 206/3), as it showed that Tg decreased from 55.1 °C for the neat PLA to 28.3 °C for the PLA with 20 wt % of the plasticizer. The elongation at break increased up to 250% and tear resistance increased by ∼135%. Meanwhile, the ultimate stress and elastic modulus decreased by ∼44% and ∼62%, respectively.
Ljungberg and Wesslen13, 15 prepared two oligomeric plasticizers of different molecular weights (Mw = 4,550 g/mol vs. 63,600 g/mol) in terms of transesterification of TBC with diethylene glycol, and investigated the effects of these TBC-based oligomers on thermal–mechanical and aging properties of the extruded PLA films. Both of investigated TBC oligomers did not lower the Tg of PLA as greatly as monomeric TBC. But among the two oligomeric plasticizers, a relatively larger reduction in Tg was achieved with the oligomer having the lower molecular weight (Mw = 4,550 g/mol). Partial phase separation occurred after the plasticized PLA with 10–20 wt % of the TBC-oligomer was aged at ambient temperature for several weeks. The higher the molecular weight of the plasticizer, the lower the critical saturation concentration, at which phase separation began to occur. Compared to the TBC monomer, the morphological stability of the PLA blends with the oligomer having lower Mw was enhanced when the concentration of the oligomers was relatively low (i.e., 10–15 wt %). By reacting diethyl bishydroxymethyl malonate (DBM) with acid dichlorides and/or diamines, a series of DBM-oligoesters and DBM-oligoesteramides were synthesized with different molecular weights, respectively.14–16 The oligomeric plasticizers resulted in a slightly smaller Tg depression of PLA than the monomeric DBM. The compatibility between PLA and the plasticizer and the enhancement in elongation were influenced by the molecular weight of the oligomer and the presence of polar amide groups that were able to positively interact with the PLA chains. With 15 wt % of either DBM-oligomester or DBM-oligoesteramide based on triethylene glycol diamine, the elongation increased to above 200%, whereas the oligoesteramide based on polypropylene glycol diamine only showed an elongation of around 20%. It was found that annealing of the plasticized PLA at 100 °C for 4 h promoted cold crystallization and phase separation, causing the plasticized PLA to regain the brittleness. On the contrary, physical aging at ambient temperature revealed that the enhanced flexibility and morphological stability of the film plasticized with the oligomers could be maintained.
Lapol®, LLC recently introduced a commercial bioplasticizer/impact modifier, Lapol™, which was specifically designed for PLA. Lapol is a viscous liquid modifier and is claimed to be a lactic acid-derived polymer.24 This liquid plasticizer comprises both polyester plasticizing units and compatibilizing units. It is therefore thought to be compatible and miscible with PLA and other biopolymers up to 20% and does not require any additional compatibilizers. Compared to traditional small molecule plasticizers, it is claimed that Lapol can improve “flexibility” of PLA without considerably sacrificing the modulus at relative low concentrations (5–10%).
PEG, conventionally referred to poly(ethylene oxide) of low molecular weight (<20,000 g/mol), is a class of nontoxic, water-soluble, and crystalline polymer commercially available over a broad range of molecular weights from 200 to 2 × 104 g/mol. The miscibility of PEG and PLA depends on molecular weight and content of PEGs.25–29 Lower molecular weight PEGs exhibit better miscibility with PLA and result in more efficient reduction of Tg, which can lead to drastic improvement in ductility and/or impact resistance of PLA at low concentrations. Baiardo et al.20 investigated the thermal and mechanical properties of PLA plasticized with PEGs of different molecular weights from 400 to 10,000. It was shown that Tg invariably dropped to a certain plateau value with the addition of PEG, and this limit concentration ranged from 15 to 30 wt %, depending on molecular weight of PEGs. The concentration at which maximum elongation was achieved also varied with the molecular weight of PEG. When PEG10000 was used, 20 wt % was needed to achieve an elongation of 130%, while the similar increase was obtained by 10 wt % in the case of the lower molecular weight PEG400. In another report by Martin and Avérous,30 PEG400 and OLA were found to be the most efficient plasticizers of amorphous PLA among various biocompatible monomeric and oligomeric plasticizer, while glycerol was the least efficient plasticizer.
Jacobsen and Fritz31 used PEG with a molecular weight of 1500 g/mol (PEG1500), glucose monoester (Dehydat® VPA1726), and partial fatty esters (Loxiol® GMS95) to plasticize PLA and examined the influences of these plasticizers on tensile and unnotched Charpy impact resistance of injection-molded PLA specimens. The significant improvement in both elongation (180%) and impact resistance (nonbreak under unnotched Charpy impact test condition) was reported when 10 wt % PEG1500 was added, whereas in the case of glucose monoester and partial fatty acid ester, elongation of PLA was improved but impact strength was slightly decreased at all concentrations examined (i.e., 2.5–10 wt %). No crazing was observed in the deformed tensile specimens plasticized with 10 wt % of PEG1500, which was different from the ones with 10 wt % of glucose monoester or partial fatty acid ester.
Pillin et al.9 investigated the thermal and mechanical properties of PLA blends plasticized with PEGs (Mw = 200, 400, 1000 g/mol) or several other plasticizers that can be used in food packaging, such as poly(1,3-butanediol) (PBOH), dibutyl sebacate (DBS), and acetyl glycerol monolaurate (AGM). The experimental results were further compared to the theoretically predicted results. Among these plasticizers, PEGs were the most efficient in reducing the Tg of PLA. For more than 20 wt % plasticizers, all plasticized PLA blends exhibited a limit of miscibility and a plateau of Tg reached. Also, thermal and mechanical results were found to contradict with the prediction of miscibility through empirical interaction parameters and Fox equations. For PEGs which should have optimum miscibility with the PLA matrix according to the theoretical predictions, macroscopic phase separation occurred at a certain PEGs concentration (20 wt % for PEG200 and 30 wt % for PEG400). Nevertheless, the improvement of miscibility was observed for the other three plasticizers that were expected to be less miscible with PLA. The authors attributed this discrepancy to a more remarkably enhanced crystallization of PLA in the presence of PEGs. Results of tensile tests showed a strong decrease in modulus and stress at break for plasticizer content higher than 20 wt %. At higher plasticizer contents (≥20 wt %), PEGs led to a lower elongation of blends in comparison to the other plasticizers. Thus, the authors stated that PBOH, AGM, and DBS at a loading level of 20–30 wt % were the more efficient according to the mechanical requirements. Kulinski and Piorkowska32 studied the effects of different end groups (hydroxyl vs. methyl) of PEG on the plasticization of both amorphous and semicrystalline PLA with plasticizer concentrations up to 10 wt %. No marked effects induced by different end groups of the plasticizer were found and thermal and mechanical properties were predominantly governed by the plasticizer content. All plasticizers used enabled Tg depression and improved the ability of the PLA to undergo cold crystallization. At the same plasticizer content, the amorphous plasticized PLA blends exhibited much higher elongation at break than the corresponding semicrystalline plasticized PLA blends. This difference was attributed to the reduced ability of PLA to plastic deformation due to the crystallization nature of the latter. It was found that the plastic deformation of both neat and plasticized PLA was associated with crazing.
Therefore, it was indicated that with lowering molecular weight and increasing concentration of PEGs, the crystallization temperature of PLA shifted to lower temperatures in parallel with the depression in Tg. At a certain PEG concentration depending on its molecular weight, the blends with PEGs would undergo a phase separation because of the slow crystallization of PEGs during aging, thereby resulting in gradual embrittlement of the materials.25–29, 33 Furthermore, because of the hydrophilic nature of PEG, leaching of PEG from the host polymer during contact with an aqueous environment was another drawback of the PEG plasticizers.26
To combat these aforesaid deficiencies, PLA-b-PEG block copolymers were synthesized and investigated as PLA plasticizers.34 The plasticization behaviors of these compounds were complicated by the dependence on the PEG block length. Some samples showed the microphase separation and crystallization of the PEG blocks, resulting in incomplete plasticization of the host polymer. In a separate strategy, the direct copolymerization of L-LA with ethylene oxide was reported to yield copolymers having a multiblock structure.35 Solvent-casting films from blends of these copolymers and PLA exhibited improved modulus and yield stress as well as comparable elongation with respect to the PLLA/PEG blend with an identical L-LA/EO (ethlene oxide) composition. The authors expected that leaching of these copolymer plasticizing agents was also greatly reduced when compared to PLLA/PEO blend. Although individual block sizes could be controlled to a certain degree by manipulating the reaction conditions, all of the reported block copolymers exhibited two melting transitions, suggesting that the blocks had sufficient length to undergo crystallization-induced microphase separation. In light of the above considerations, Bechtold et al.36 synthesized alternating copolymers of lactic acid and ethylene oxide poly(3-methyl-1,4-dioxan-2-one) (PMDO) as a potential macromolecular plasticizing agent for PLA. The miscibility of PLA and PMDO was evidenced by a single Tg that was well described by the Fox relationship of miscible blends.
Poly(propylene glycol) (PPG) also has been attempted in plasticization of PLA. Unlike the semicrystalline PEGs, PPG is amorphous. McCarthy and Song33 compared the plasticization of PLA using PPG and epoxy-capped PPG (referred to as PPG-E) of similar molecular weights (720 g/mol vs. 640 g/mol). DSC results showed that both PPG and PPG-E were miscible with PLA. The Tg of PLA decreased linearly with increasing concentration of either plasticizer, with PPG-E displaying a higher depression effect than PPG. Both plasticizers were very effective in improving tensile toughness. When the plasticizer content was above 15 wt%, the elongation increased to more than 250% for all the blends. For PPG-E at 15 wt % and for PPG at 15–20 wt %, the ductility of the blends were improved without sacrificing strength and stiffness. However, when the concentration of PPG-E was higher than 20 wt %, the modulus of the blends decreased to the range typical of elastomers. After aging for 1 month, the mechanical properties of the plasticized PLA did not change remarkably. This result indicated that PPG and PPG-E could prevent the physical aging embrittlement of PLA.
Subsequently, Kulinski and coworkers37, 38 studied the plasticization of PLA using PPG with a nominal molecular weight of 425 g/mol (PPG425) and 1000 g/mol (PPG1000), together with PEG600 (nominal Mw = 600 g/mol) as a comparison. The plasticized samples with both PPGs showed the decrease in Tg and the enhanced ability of PLA to crystallize but this effect induced by PPGs were relatively smaller when compared with that of the PLA plasticized by PEG600. As evidenced by the results, minor phase separation occurred in the blend containing 12.5 wt % of PPG1000, suggesting that the miscibility of PPGs with PLA for PPG decreased with the increase of PPG molecular weight. Unlike PEG600, however, phase separation of PPG from amorphous PLA did not deteriorate the drawability of the PLA materials. As one expected, increasing PPG content led to an increase in elongation and a decrease in yield stress. At the plasticizer content of 12.5 wt %, the use of PPG425 resulted in the maximum elongation (702%), which was significantly higher than that of neat PLA (64%). In addition, at higher contents of PPGs (≥7.5 wt %), the PLA samples exhibited strain-induced crystallization during deformation, whereas the evidences of crazing were noted in the deformed PLA samples containing the lower PPG concentrations. For semicrystalline PLA plasticized with the same PPGs, it was found that the crystallization in the blends was accompanied by phase separation.38 Increasing the plasticizer concentration in the amorphous phase and annealing the blends at crystallization temperatures contributed to the phase separation. With an increase of PPG content, yield stress decreased while the elongation increased. PLA/PPG blends universally exhibited higher elongations than the corresponding PLA/PEG600 ones. At 12.5 wt % of PPG content, the elongation values of the PLA/PPG425 and PLA/PPG1000 blends reached 105% and 65%, respectively, while in PLA/PEG blends, it decreased to only 15% at PEG content above 10 wt %. Neat PLA yielded an average elongation of ∼8%. The PLA/PPG1000 blends showed most intense phase separation, and the formation of tiny PPG droplets. Based on morphological analysis, the authors argued that tiny liquid pools of PPG facilitated local plasticization of PLA during plastic flow and had a positive effect on drawability, while solid inclusions of crystallizable plasticizers like PEG were undesirable as they deteriorated the blend drawability. Table 3 summarizes the molecular weight and solubility parameters (δ) of some oligomeric or polymeric plasticizers and their interaction parameter (χT) with PLA, as well as their plasticization effects on PLA.
Table 3. Molecular Weight and Solubility Parameters (δ) of Some Oligomeric or Polymeric Plasticizers and Their Plasticization Effects on PLA
While increasing the molecular weight of the plasticizer can slow down migration rate and thus improve morphological stability of PLA materials during storage, it also decreases its solubility and plasticizing efficiency. Additionally, high-molecular weight plasticizers are more prone to phase separation because of low saturation concentrations of plasticizers. To use the complementary advantages, the combination of small molecule plasticizers with polymeric or oligomeric ones was also attempted in the literature. Ren et al.39 used a mixture (1/1, w/w) of GTA and oligomeric poly(1,3-butylene glycol adipate) to plasticize PLA. Tg decreased from 59.7 °C for pure PLA to 37.4 °C for PLA containing 29 wt % mixed plasticizers. Tensile strength progressively decreased with an increase of the total content of mixed plasticizer, while a significant increase in elongation occurred at the content of about 5–9 wt %. The blends were brittle with less than 5% plasticizers and were ductile with great than 9 wt % plasticizers. Lemmouchi et al.22 recently reported the plasticization of PLA using a mixture of TBC and a more thermally stable low-molecular weight poly(D,L-LA)-b-poly(ethylene glycol) copolymer (PLA-b-PEG) with different molecular architecture (Table 4). The use of TBC alone was the most effective in lowering Tg and enhancing elongation of PLA, while the use of PLA-b-PEG copolymers alone well maintained tensile strength and modulus. Diblock copolymer (COPO1 or COPO2) seemed to be slightly more efficient in decreasing Tg than triblcok (COPO3) or star copolymers (COPO4). However, the combination of TBC and PLA-b-PEG copolymer (1/1, w/w) mixtures led to a medium level of depression in Tg and more balanced mechanical properties, compared to the use of an individual plasticizer. It was claimed that varying the structure of copolymers allowed tailoring of the end-use performance required for different targeted applications. Table 4 summarizes the molecular weight of some mixed plasticizers and their plasticization effects on PLA.
Table 4. Molecular Weight of Some Mixed Plasticizers and their Plasticization Effects on PLA22
Recently, two phosphonium type ionic liquids (ILs) with different anions, as shown in Figure 2, were evaluated as potential plasticizers and/or lubricants for amorphous PLA.40 Both ILs were found to lower the Tg of PLA and modify rheological characteristics as manifested by reduced viscosities, apparent phase separation, and lubrication effect. These effects were much more pronounced for the IL-1 containing a hydrophobic decanoate anion, presumably as a result of higher overall compatibility with the matrix with respect to the one containing a hydrophilic BF4 anion. Nevertheless, thermogravimetric analysis data showed that the presence of ILs had a catalytic effect on PLA degradation. Mechanical properties of the IL-plasticized PLA materials were not reported.
Epoxidized soybean oil (ESO) has long been used as a plasticizer for PVC. Ali et al.41 studied the plasticizing effects of ESO on PLA and found that the plasticization efficiency was relatively low. For instance, PLA containing 20 phr ESO only displayed an elongation of 38%, meanwhile, yield stress of the neat PLA decreased from 60 to 26 MPa.
In general, plasticization has been demonstrated to be very successful in improving the flexibility and ductility of PLA in the literature. However, there are still some problems associated with this method. Typically, relatively high percentage of plasticizers (15–20 wt %) are required to provide a remarkable reduction in Tg, adequate ductility or tensile toughness of the PLA matrix. The significant improvement in elongation is usually accompanied by substantial reductions in strength and modulus (even up to three orders of magnitude). Moreover, an excessive incorporation of plasticizer tends to result in the phase separation because of the saturation of plasticizer in the amorphous phase of PLA. In addition, there seems to exist a competition between plasticization efficiency and the kinetics of aging or unfavorable cold crystallization in the plasticized PLA. The more the material is plasticized, the larger the increase in chain mobility and the faster the cold crystallization process. It is therefore imperative to find an optimal balance at which PLA is sufficiently flexible for the desired application without the occurrence of overly fast cold crystallization.
Copolymerization has been extensively investigated as a powerful means to obtain polymer materials with properties unattainable by homopolymers. Properties including tensile and impact performances of a copolymer can be tailored in a versatile way by manipulating the architecture of the molecule, sequence of monomers, and composition. Copolymerization of PLA can be conducted either through polycondensation of lactic acid with other monomers (or polymer segments) or ring-opening copolymerization (ROC) of LA with other cyclic monomers (or polymer segments). Because the latter synthesis route gives a more precise control of chemistry and higher molecular weight of copolymers, it is more widely used to improve the toughness or flexibility of PLA in the literature. The polymerization can be ionic, co-ordination, or free radical depending on the type of catalyst system involved.42, 43 Figure 3 shows the chemical structures of some of the reported comonomers and block segments used for PLA toughening in the literature. According to the difference in molecular architecture, the resulting copolymers can be mainly classified into the following categories.
Linear Random Copolymers
Homopolymers of ε-caprolactone (CL) and trimethylene carbonate (TMC), that is, poly(CL) (PCL) and poly(TMC) (PTMC), are two biodegradable polyesters and are highly ductile. The Tgs of PCL and PTMC are approximately −60 and −20 °C, respectively. The excellent flexibility of the PCL and PTMC homopolymers prompted CL and TMC to be the most used comonomers to copolymerize with LA in achieving tough copolymers.
Effects of the comonomer ratio on the thermal and mechanical properties of the poly(CL-co-L-LA) and poly(CL-co-D,L-LA) copolymers were examined by Hiljanen-Vainio and coworkers.44 The monomer ratio was varied from 80/20 to 40/60 (w/w). The physical characteristics of resulting copolymers ranged from weak elastomers to tough thermoplastics as a function of CL/LA ratio and type of LA monomer in the copolymerization. Compared with the PLLA or PDLA homopolymers, the copolymers exhibited larger elongation (>100% for most copolymers) but lower tensile modulus and strength. The copolymers containing L-LA had greater tensile strength than those containing D,L-LA due to the crystalline nature of the former. Grijpma et al.45 also synthesized high-molecular weight copolymers of L-LA and CL by ROC. It was found that the copolymers (L-LA/CL = 1/1, mole ratio) exhibited a tensile strength of 34 MPa and an elongation as high as 500%. In addition, it was shown that the ROC temperature (110 °C vs. 80 °C) influenced mechanical properties of the resulting copolymers.46 The higher copolymerization temperature resulted in lower yield stress and tensile modulus but higher elongation, which was attributed to the less blocky copolymer formed at the higher polymerization temperature.
Grijpma et al.3, 47 further compared the influences of comonomer content and the mode of sample preparation (i.e., as-polymerized vs. compression-mold) on mechanical properties of L-LA/CL or LA/TMC copolymers. At low CL content when the Tg was still well above room temperature, the unnotched Dynstat impact strength of the copolymers differed slightly but yield stress, crystallinity, melting temperature, and Tg decreased with increasing CL content. It was not until the Tg of the materials approached room temperature (≥10 mol % CL) that the Dynsta impact strength began to increase continuously with comonomer content and high impact toughness and ductility (>100%) were obtained. Under these compositions, however, the copolymers showed low modulus and yield stress. For a given poly(L-LA-co-CL) composition, the as-polymerized samples had higher impact strength than the compression-molded ones. However, the situation was different when L-LA was copolymerized with TMC. In addition to the high impact strength achieved at high TMC content (∼30 mol %) at which the Tg approached room temperature, a very sharp maximum impact strength (34 kJ/m2) at 1.0 mol % concentration was also noted. At such low TMC content (∼1 mol %), the tensile properties of the L-LA-co-TMC copolymer were hardly affected and remained as high as those of the as-polymerized homo-PLLA. Similar but less drastic enhancement in toughness by copolymerization with TMC was reported by Ruckenstein and Yuan.48 The copolymer of L-LA and TMC (15 wt % TMC) showed the elongation of ∼15% (vs. ∼6% for pure PLLA) and tensile toughness of 7 MJ/m3 (vs. 2.5 MJ/m3 for pure PLLA). However, when the TMC content increased to 32 wt %, the copolymer displayed a rubbery behavior, and the elongation and tensile toughness increased significantly to 375% and 105 MJ/m3, respectively.
In addition to the CL and TMC comonomers, β-methyl-δ-valerolactone (MV) is also used to ring-copolymerize with LA.49 The Tg of the copolymers gradually decreased with increasing MV content. When the content of MV was higher than 20 mol %, the copolymer became amorphous. At 8 mol % MV, the elongation reached 680% and tensile strength was 37.8 MPa. With MV content ranging from 10 to 21 mol %, the elongation varied between 530% and 900%. Tensile strength did not change considerably within the range 8–15 mol % L-LA unit content. Table 5 summarizes the reported mechanical properties for these above linear random PLA copolymers.
Table 5. Summary of Reported Mechanical Properties for Some Linear Random PLA Copolymers
Star and Linear Block Copolymers
Grijpma et al.47 prepared a block copolymer using L-LA with a rubbery L-LA/CL (50/50 mol/mol) copolymer segment. With 34 wt % rubber block, the copolymer displayed an elongation of 1500% and did not fracture during Charpy impact test. However, tensile strength decreased to 30.8 MPa.
Using multifunctional alcohol as an initiator for ring-opening polymerization, star-shaped AB block50 and ABA block copolymers4, 50 were also synthesized by Grijpma and coworkers, respectively. In the above formulations, A is LA block; B is a TMC, a TMC/CL (50/50 in mole ratio) or a CL/d-valerolactone (VL) (60/40 in mole ratio) rubber block. It was found that all three rubber blocks adequately toughen PLA at concentrations higher than 15 wt %.50 While TMC/CL- or CL/VL-toughened star-shaped block copolymers exhibited significantly higher Dynstat unotched impact strengths than TMC-toughened star-block copolymers, higher tensile strength was achieved for the latter. This was attributed to the relatively high Tg of the TMC rubber block. The star-shaped block copolymer with 17 wt % TMC rubber merely exhibited an unnotched impact strength of 13.4 kJ/m2, while the copolymers with 15 wt % TMC/CL or CL/VL rubber was even nonbreakable in the Dynstat impact test. Overall, compared to neat PLA, the star-shaped block copolymers with TMC/CL rubber block exhibited much higher values in both ductility and impact strength with relatively small reductions in modulus and acceptable tensile strength. In addition, it seemed that the preparation route (bulk polymerization in the melt vs. polymerization in the solvents) as well as the molecular weight of star-shaped copolymers was crucial for obtaining good ultimate mechanical properties.
Grijpma et al.4 showed that the TMC rubber block (Mn = 65.1×103 g/mol) in the tri-block copolymers was also effective in toughening PLA. With varying weight content of TMC rubber from 10.9 to 21.4 wt %, the elongation increased from 135% to 210%. Similar to the corresponding PLA/PTMC blends, tensile strength of the tri-block copolymers decreased while impact strength increased with rubber content. However, elongation was much higher for triblock copolymers. The tri-block of PLA–PTMC–PLA containing 21 wt % TMC had a comparable notched Izod impact strength (66.7 J/m vs. 52–63 J/m) and tensile strength (36.9 MPa vs. 39.2 MPa) to the corresponding blend. But with molecular weight of 20.1 × 103 g/mol, the tri-block copolymer containing 21 wt % TMC block led to lower yield strength (24.4 MPa) but higher elongation (280%). This result was attributed to the plasticizing effect of low-molecular weight TMC blocks. The tri-block copolymer with 20 wt % TMC/CL rubber block (Mn = 46.5 × 103 g/mol) was much tougher during the impact test. No fracture during the Dynstat unotched impact test and a notched Izod impact strength of 446 J/m were observed. Tensile tests showed an yield strength of 41.4 MPa and an elongation of 120%. Also, its tensile and impact properties were superior to those of the corresponding PLA/P(TMC/CL) blend. In contrast, a triblock copolymer containing 20 wt % TMC/CL with relatively lower molecular weight (Mn = 41.5 × 103 g/mol) in the rubber block had a slightly inferior yield strength (35.1 MPa) but a clearly lower value in elongation (50%) and unotched Dynstat impact strength (only 6.6 kJ/m2). The much inferior impact strength in this case was attributed to the too small rubber domain to optimally toughen PLA matrix at the high strain rate.
Haynes et al.51 copolymerized L-LA with commercial perfluoropolyether oligomers (PFPE). The fluoropolyether segments improved ductility, optical clarity, and melt processability while reduced surface energy and water wettability. In contrast to the corresponding physical blend of PLLA and PFPE, the copolymers did not show macrophase separation but exhibited higher optical clarity and elongation. With 5 wt % PFPE block, the novel ABA tri-block copolymer film exhibited a dramatic increase in elongation (>300% vs. 10–15% for neat PLLA). Tensile strength and modulus of the copolymers were slightly lower than that of the PLLA homopolymer. Table 6 summarizes reported mechanical properties for the above star or block PLA copolymers.
Table 6. Summary of Reported Mechanical Properties for Some Star or Block PLA Copolymers
Graft copolymerization is a convenient method to impart a polymer with unique properties and is generally performed in a separate reaction step. Toughening modification of PLA has been also attempted by graft copolymerization. Jing and Hillmyer52 described the synthesis of a novel bifunctional monomer consisting of a LA substituted with a norbornene moiety. Ring opening of matathesis polymerization (ROMP) of this bifunctional monomer and 1,5-cyclooctadiene (COD) in a molar ratio of 3/97 yielded a rubbery statistical copolymer with pendant LA rings (PCOD/2). Subsequent ring-opening transesterification polymerization (ROTEP) of D,L-LA monomer in the presence of the rubberyPCOD/2 yielded a mixture composed of PLA graft copolymer and PLA homopolymer. Unlike the opaque physical blend of PLA and poly(COD), this in situ-synthesized PLA blend containing 20 wt % rubbery PCOD/2 was translucent and exhibited a unique nanophase separation. This PLA blend displayed higher elongation (65% vs. 5%) and tensile toughness (16 MJ/m3 vs 2 MJ/m3) than the PLA homopolymer but lower strength (24 MPa vs. 44 MPa).
Recently, Theryo et al.53 further adopted a “grafting-from” (polymer with functional groups which initiate the polymerization of monomer) approach to synthesize another graft copolymer of LA and COD. ROMP of COD with 5-norbornene-2-methanol was first conducted to obtain the pendant primary hydroxyl groups statistically distributed along a rubbery backbone (resulting block copolymer was referred as “PCN”), followed by ROTEP of LA initiated at those hydroxyl sites. The graft copolymer containing only 5 wt % rubbery backbone was transparent and exhibited about 18-fold increase in elongation (238% vs. 13%) and about 13-fold increase in tensile toughness (95 MJ/m3 vs. 7 MJ/m3) with respect to the neat PLA, respectively. Meanwhile, the tensile modulus (1.86 GPa vs. 2.03 GPa) and yield strength (64.8 MPa vs. 67.9 MPa) were only slightly lower than that of the neat PLA. Unfortunately, impact performance was absent in both of the above studies. Table 7 summarizes the reported mechanical properties for both grafted PLA copolymers.
Table 7. Summary of Reported Mechanical Properties for Some Grafted PLA Copolymers
Introduction of an appropriate level of crosslinking to PLA could also impart the simultaneous enhancements in tensile and impact strengths. Crosslinked PLA materials have been synthesized either by (1) copolymerization of LA with a multifunctional monomer or by (2) introducing a crosslinkable moiety into the polymer backbone and then performing postpolymerization crosslinking modifications.
By bulk copolymerization of LA with small amounts of tetra-functional spiro-bis-dimethylene-carbonate (spiro-bis-DMC), the chemically crosslinked PLA samples were obtained.3, 54 By copolymerizing L-LA copolymer with 0.2–0.3 mol % spiro-bis-DMC, even the occurrence of nonfracture in the unnotched Dynstat impact test was observed and tensile strength essentially increased to a limiting value (ca., 70 MPa) compared to 59.5 MPa of the neat PLLA. The authors argued that the increased interconnectivity of PLA molecular chains accounted for the simultaneous enhancement in tensile and impact strengths. The reinforcing effect in tensile strength was also observed for the crosslinking of L,D-LA copolymers with spiro-bis-DMC.54 But with incorporation of similar contents of the crosslinker, the unnotched impact strength of L,D-LA copolymers was less improved or even reduced, depending on the D-LA content in the PLA copolymers.3 The higher impact strength of crosslinked PLLA was attributed to the higher network strength of the networks due to the presence of not only chemical crosslinks but also physical crosslinks. By using the same copolymerization approach, another tetra-functional monomer, 5,5′-bis(oxepane-2-one) (5,5′-BO) as a crosslinker was also used to copolymerize with L-LA.55 It was found that the optimal mechanical properties of the crosslinked PLLA were obtained at relatively low polymerization temperatures and short reaction times with the crosslinker concentration close to 1.0 mol %. The Dynastat impact strength of crosslinked bulk-polymerized PLLA containing 1.00 mol % of 5,5′-BO was 24 kJ/m2 compared to 14 kJ/m2 of the linear PLLA. Meanwhile, the corresponding tensile strength was increased from 55 to 61 MPa.
By functionalizing telechelic star-shaped poly(CL/D,L-LA) oligomers with methacrylate anhydride followed by chemical crosslinking of the double bonds using dibenzoyl peroxide (DBPO) as a crosslinking agent, Helminen et al.56 obtained crosslinked PLA copolymers with a wide range of elastic properties. At a fixed DBPO content (0.5 wt %), tensile properties of the crosslinked copolymers were found to remarkably change with the monomer compositions and coinitiator (pentaerythritol) content. At the CL/D,L-LA molar ratio of 30/70 (mol/mol), the elongation reached 190%, while tensile strength (9.72 MPa) and modulus (5.2 MPa) were very low. Table 8 summarizes reported mechanical properties for some crosslinked PLA copolymers.
Table 8. Summary of Reported Mechanical Properties for Some Crosslinked PLA Copolymers
In summary, it should be pointed out that the majority of publications in this area either did not report impact properties at all or only reported unnotched impact strength. Because the energy to initiate a crack is emphasized in the unnotched test, the results of unnotched impact strength may not be valid for the comparison between materials and samples. In addition, the reproducibility of unnotched impact strength is usually not high. Although a broad spectrum of mechanical properties of PLA materials seemed achievable by manipulating the copolymerization, unfortunately, none of these copolymerization processes is currently economically viable.
MELT BLENDING WITH FLEXIBLE POLYMERS
Melt blending of polymers is a much more economic and convenient methodology than synthesizing new polymers to achieve the properties unattainable with existing polymers. Toughening is usually an integral part of blend design, especially for those blends involving rigid polymers. PLA has been blended with various polymers for different purposes. In this review, the discussion of blending of PLA is only limited to the literatures with the specific purpose of PLA toughening. A variety of biodegradable and nonbiodegradable flexible polymers have been used as toughness modifiers for PLA.
Biodegradable Polymer Modifiers
Aliphatic Polyesters and Their Copolyesters
PCL is a biodegradable polyester and possesses excellent flexibility and ductility. Its chemical structure is shown in Figure 4. Blending of PCL and PLA has been extensively investigated in the past years. However, the simple melt blending of PLA and PCL usually leads to a marginal improvement in toughness because of their immiscibility.57, 58
The use of small molecule reactive additives during compounding has been demonstrated to be an effective way to improve the compatibility between PLA and PCL. Wang et al.59 used triphenyl phosphate (TPP) as a catalyst or coupling agent in the preparation of PLA and PCL blends. The addition of 2 phr TPP to PLA/PCL (80/20, w/w; PCL used with Mn = 80,000 g/mol) blend during melt blending resulted in higher elongation (127% vs. 28%) and tensile modulus (1.0 GPa vs. 0.6 GPa) but lower tensile strength at break (33 MPa vs. 44 MPa). The balance between degradation of molecular weight and the formation of copolymer was thought to govern the final mechanical properties of the blends. Reaction time and molecular weight of PCL used were found to have remarkable effects on mechanical properties of the blends. Higher molecular weight PCL (Mn = 80,000 g/mol) and medium reaction time (15 min) promoted the largest improvement in elongation.
Semba et al.60 used dicumyl peroxide (DCP) during blending to promote reactive compatibilization of the PLA/PCL blends under different melt-compounding conditions (internal mixer vs. twin-screw extruder). The compression-molded film of the uncompatibilized PLA/PCL (70/30, w/w) blend displayed an elongation of only 15% compared to 3.6% of the neat PLA. When 0.1–0.2 phr DCP was added during blending of the PLA/PCL blend, the elongation of the resulting blend film was dramatically increased to the maximum 130% with yielding and necking observed during deformation. Further addition of DCP beyond the optimum amount had an opposite effect on elongation. For the compression-molded film samples, tensile modulus and tensile stress at break were independent of the DCP concentration but linearly decreased with increasing PCL content. Atomic force microscopy observation revealed that the diameter of the dispersed PCL domains decreased with increasing DCP content. Injection-molded specimens exhibited a similar trend of tensile properties as the compressed films. As for the impact strength (notched Izod test), addition of 0.3 phr DCP during blending resulted in the PLA/PCL (70/30, w/w) blend with an impact strength of 2.5 times more than that of neat PLA. In contrast, addition of DCP to PLA alone did not alter mechanical properties. It was considered that the crosslinking between PLA and PCL in the presence of DCP accounted for the improved interfacial adhesion. It was also found that tensile properties were not dependent on feeding procedure, but addition of DCP via the splitting feeding method resulted in a higher reverse Izod impact strength than feeding at once through the main hopper.61
Based on the high reactivity of isocyanate groups reacting with end hydroxyl or carboxylic groups, Takaya et al.62, 63 improved the compatibility of PLA and PCL using lysine triisocyanate (LTI) as a compatibilizer. Compatibility of PLA and PCL was also improved, resulting in the reduction of size of PCL spherulites. Impact fracture toughness was markedly improved by increasing LTI content, which was attributed to the strengthening structure of the blend as a consequence of crosslinking reactions. In another study, Harada et al.64 systematically compared the compatibilizing effects of LTI with four other reactive processing agents (Fig. 5) on the PLA/PCL (80/20, w/w) blends. Addition of 0.5 phr of each reactive agent resulted in an increase in the unnotched Charpy impact strength in the order of LTI > LDI (lysine diisocyanate) > Duranate TPA-100 [1,3.5-tris(6-isocyanatohexyl)-1,3,5-triazinane-2,4,6-trione] > Duranate 24A-100 [1,3,5-tris(6-isocyanatohexyl)biuret] > Epiclon 725 (trimethylolpropane triglycidyl ether). Among four isocyanates used, LTI induced the superior unnotched Charpy impact strength of the PLA/PCL blend (nonbreakable). The presence LDI or TPA-100 moderately increased the impact strength of the PLA/PCL blend (64 kJ/m2 and 58 kJ/m2). However, the addition of Epiclon 725 did not improve the impact strength (17 kJ/m2) of the binary blend. With 0–10 wt % PCL in the blends, the unnotched impact strength increased gradually with LTI concentration. However, with 20 wt % PCL in the blend, the addition of only 0.15 phr LTI led to the nonbreak during the unnotched Charpy impact test. With 0.5 phr of LTI, the notched Charpy impact strength and ultimate strain reached 17.3 kJ/m2 and 268%, respectively, while tensile strength was well maintained with respect to the binary PLA/PCL blend (47.3 MPa vs. 55.4 MPa). It was assumed that the reaction of isocyanates group with both terminal hydroxyl and carboxylic groups of polyesters accounted for improved compatibility at the PLA/PCL interfaces and thus the increases in the physical properties.
Poly(butylene succinate) and Their Copolyesters
Poly (butylene succinate) (PBS, Fig. 6) and copolyesters are commercialized under the trade name Bionolle® (Grade 1000 series). PBS and copolymers have low Tgs and are highly flexible. In addition to PBS, other PBS-based copolyesters, such as poly(butylene succinate-co-adipate) (PBSA; e.g., Bionolle® 3000 series) and poly(butylene succinate-co-L-LA) (PBSL; e.g., GS PLA® grade), have been used to toughen PLA.58, 65–68 Blends of PLA with these polymer modifiers are immiscible. Except notable increases in flexibility and elongation, significant improvement of impact toughness was seldom observed or only achieved at very high concentrations of the modifiers. In some studies, a third component as a compatibilizer was incorporated to improve compatibility.
Harada et al.69 studied the melt blending of PLA and PBS and their reactive compatibilization using LDI and LTI. Without compatibilization, the PLA/PBS binary blend (90/10, w/w) displayed a slightly higher elongation and almost the same unnotched Charpy impact strength (18 kJ/m2) compared with neat PLA. Even with PBS increased to 20 wt %, the impact strength still showed little change. However, on addition of 0.5 wt % LDI or 0.15 wt % LTI, elongation of the PLA/PBS (90/10, w/w) blend was increased to more than 150%. It was found that the magnitude of impact strength of the blends was independent of the molecular weight of PBS but was affected by concentrations of LTI and PBS. For the blends with 10–15 wt % PBS content, the impact strength was sharply increased with addition of LTI and saturated at 50–70 kJ/m2. Addition of LTI as low as 0.15 wt % significantly increased the impact strength of the PLA/PBS (80/20, w/w) blend, and the unnotched samples were not broken during the impact test. In contrast, even with addition of LDI to 0.5 wt %, the impact strength of PLA/PBS (80/20, w/w) blend only increased to 31 kJ/m2. The results implied that LTI was the more effective reactive processing agent to increase the toughness of the PLA/PBS blends. Also, on addition of 0.15 phr LTI into the PLA/PBS (90/10, w/w) blend, the size of dispersed PBS particles was significantly reduced and further increasing the content of LTI or PBS did not alter the size of PBS markedly.
Vannaladsaysy et al.70 investigated the effects of LTI on fracture behavior of the PLLA/PBSL blend. Similar to the PLA/PBS blend, the incorporation of LTI into the PLLA/PBSL blend effectively improved the compatibility between PLLA and PBSL, resulting in the suppression of spherulite formation of PBSL and the formation of a firm structure consisting of entanglements of PLLA and PBSL molecules and therefore higher energy dissipation during the initiation and propagation of crack growth.
As DCP was successfully used to compatibilize the PLA/PCL blends,60, 61 it was also incorporated to induce in situ compatibilization of the PLLA/PBS (80/20, w/w) blend by Wang et al.71 The uncompatibilized blend showed much higher elongation than PLLA (250% vs. 4%) but only slightly higher notched Izod impact strength (2.5 kJ/m2 vs. 3.7 kJ/m2). Addition of 0.1 phr DCP greatly increased the impact strength of the blend to 30 kJ/m2. Both strengths and moduli invariably decreased with increasing DCP content. It was found that the addition of DCP led to a reduction in the size of the PBS domains and improved interfacial adhesion between the PLLA and PBS phases. The toughening effect of the blends was considered to be related to the debonding-initiated shear yielding.
Polyhydroxyalkanoates and Their Copolyesters
Depending on the pendent alkyl chain length, bacterial polyesters, polyhydroxyalkanoates (PHAs), possess a wide array of mechanical properties ranging from stiff thermoplastics to elastomers (Fig. 7). According to the carbon atom numbers of the alkyl chains, PHAs are roughly divided into three classes, that is, short-chain-length PHA (scl-PHAs) with carbon atom numbers of monomers ranging from C3 to C5, medium-chain-length PHA (mcl-PHAs) with carbon atom numbers of monomers ranging from C6 to C14, and long-chain-length PHA (lcl-PHAs) with carbon atom numbers of monomers of more than C14.72 mcl-PHAs are less crystalline and elastomer-like, depending on their side-chain compositions. Thus, they have been used as modifiers to toughen PLA.
Lee and McCarthy73 used poly(3-hydroxy octanoate) (PHO) modified with hexamethylene diisocyanate (HDI) to melt-blend with PLA in a torque rheometer. DSC results suggested that the PLA/modified PHO blends were immiscible over the entire composition range. Elongation of PLA was only slightly increased at the modified PHO content less than 80 wt %. On the contrary, tensile strength and modulus were significantly reduced with the incorporation of modified PHO.
Nodax™ developed by Procter and Gamble Co., is a family of PHA copolymers of 3-hydroxybutyrate and a small amount of mcl 3-hydroxyalkanoate comonomers (Fig. 8).74, 75 Noda and coworkers75 melt-blended PLLA with a poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer (i.e., NodaxH6, containing 5 mol % 3-hydroxyhexanoate (3-HH) unit). In the PLA/NodaxH6 (90/10, w/w) blend, tensile toughness was 10 times more than that of neat PLA and elongation was >100%. When NodaxH6 content was less than 20 wt % in the blends, its crystallization in the blends was largely restricted and thus NodaxH6 was dispersed as rubbery amorphous droplets in PLA. Furthermore, it was interesting that the inclusion of these small amounts of PHA did not compromise the optical clarity of PLA itself.
Schreck and Hillmyer76 investigated the impact toughness of blends of PLLA with a NodaxH6 containing 7 mol % 3-HH. The PLLA/NodaxH6 (85/15, w/w) blend demonstrated a twofold increase in notched Izod impact strength (44 J/m) compared with that of PLLA (22 J/m). In an attempt to promote interfacial adhesion and hence increase impact performance, 5 wt % PLLA-b-NodaxH6 block copolymer was added to the binary blend. However, no positive effect was noted.
Poly(propylene carbonate) (PPC) (Fig. 9) is a biodegradable amorphous polymer produced from propylene oxide/carbon dioxide copolymerization. Ma et al.77 prepared the PLA/PPC blends and investigated their tensile properties. Elongation of the blends monotonically increased with PPC content and exceeded 200% at PPC content of more than 30 wt %. Meanwhile, tensile strength and modulus decreased linearly with increasing PPC content. From the analysis of mechanical properties, the authors concluded that there was good compatibility between PLA and PPC.
Poly(butylene adipate-co-terephthalate) (PBAT) is a fully biodegradable aliphatic–aromatic copolyester and its chemical structure is illustrated in Figure 10. PBAT is commercially available under the tradename of Ecoflex® (BASF Co.). PBAT polymer is said to be able to biodegrade in a few weeks in the presence of naturally occurring enzymes. PBAT is a thermoplastic with properties similar to those of low-density PE but has high mechanical properties. In view of its high flexibility and ductility (elongation > 700%) and excellent biodegradability, PBAT is thus considered a good choice for toughening of PLA without compromising the biodegradability of final materials. Currently, PBAT/PLA blends are being commercially produced by BASF Co. under the trademark Ecovio® for film and extruded foam applications.
Jiang et al.78 first reported the PLA/PBAT blends in the literature and detailed the morphology, tensile properties, and toughening mechanism. The PLA/ PBAT blend was immiscible and blending was conducted using a corotating twin-screw extruder. Even without use of compatibilizers, PBAT was evenly dispersed in PLA with an average particle size at the level of 0.3–0.4 μm. The elongation of neat PLA was merely 3.7%; however, with only 5 wt % PBAT, the blend exhibited an elongation of ∼115%. When PBAT content was increased to 20 wt %, elongation of the blend increased to more than 200%. On the other hand, tensile strength and modulus decreased monotonously with PBAT content. With 20 wt % PBAT, tensile strength decreased by 25% from 63 (neat PLA) to 47 MPa, while modulus decreased by 24% from 3.4 (neat PLA) to 2.6 GPa. It was revealed that the debonding-induced shear yield was responsible for the remarkable high extensibility of the blends. Because of weak interfacial adhesion in the blends, impact toughness was only slightly improved. For example, the impact strength of the blend with 20 wt % PBAT was only 4.4 kJ/m2 compared with that of 2.6 kJ/m2 for neat PLA.
To improve the compatibility between PBAT and PLA, Zhang et al.79 used a random terpolymer of ethylene, acrylate ester, and glycidyl methacrylate (referred as “T-GMA”) as a reactive compatibilizer in melt compounding. With the addition of only 2 wt % T-GMA, both ultimate strain and notched Charpy impact strength of PLA/PBAT blends were increased without decreasing tensile strength compared to the uncompatibilized binary blends. For the PLA/PBAT (70/30, w/w) blend with 1–3 wt % T-GMA, the notched impact strength reached 30–40 kJ/m2, approximately two times that of the uncompatibilized binary blend.
Elastomers and Rubbers
Thermoplastic poly(ether)urethane (PU) is a biodegradable elastomer and possesses good low-temperature properties. Li and Shimizu80 blended PLA with PU elastomer. It was demonstrated that PU markedly improved the impact toughness of PLA materials when its content was above 10 wt % in the blends. Elongation and impact strength continuously increased with PU content. Compared with the neat PLA, the PLA blend with 30 wt % PU had a lower tensile strength (31.5 MPa vs. 65 MPa), a greater elongation (363% vs. 4%) and higher unnotched impact strength (315 J/m vs. 64 J/m). PLA/PU blend was found to be a partially miscible system, and PU was dispersed in PLA with domain sizes at the submicrometer scale. Based on scanning electron microscopy (SEM) analysis of tensile and impact-fractured surfaces, matrix shear yielding initiated by debonding at the matrix/particle interface was considered to be responsible for the improved toughness.
Recently, NatureWorks81 reported the toughening of PLA using a PCL-based PU elastomer produced by Dow Chemical Company, Pellethane™ 2102-75A. With 30% of this elastomer, the notched Izod impact strength and elongation of the resulting PLA blend were increased to 769 J/m and 410% (27 J/m and 10% for neat PLA), respectively. Meanwhile, tensile yield strength of the blend was reduced by 32% with respect to neat PLA.
Biodegradable Polyamide Elastomer
Zhang et al. used a biodegradable thermoplastic polyamide elastomer (PAE) to toughen PLA.82 This PAE was a block copolymer consisting a polyamide-12 (22 wt %) block as the hard segment and a polytetramethylene oxide block (78 wt %) as the soft segment. SEM revealed that PAE indeed showed good interfacial compatibility with PLA. PAE was dispersed in the PLA matrix uniformly and the size of PAE domains was at the submicroscale. The incorporation of 5 wt % PAE into PLA resulted in a significant increase in elongation (161.5% vs. 5.1%), with little change in tensile modulus (1.5 GPa vs. 1.8 GPa) and strength (48.1 MPa vs. 46.8 MPa) compared with neat PLA. With further addition of PAE up to 20 wt %, the elongation at break was further increased to 184.6%, whereas the tensile strength of the blend was markedly reduced by ∼49% compared to neat PLA. Interestingly, the PLA/PAE blends exhibited a shape memory behavior after high deformation. For the blend containing 10 wt % PAE, the deformed specimens after stretching to 100% were able to restore to the original shape within 3–8 s after heating at 80–90 °C and retained 92% of the original mechanical properties.
HBPs possess a globular molecular architecture, cavernous interiors, and a large number of peripheral end groups. HBP has low hydrodynamic volume and viscosity and may have a level of good solubility or miscibility with other polymers. Therefore, HBPs have high potential for the use as modifiers in a variety of industrial applications.83
HBPs have been recently used by several groups to modify properties of PLA. Zhang and Sun84 investigated mechanical properties and crystallization behavior of the hydroxyl-terminated HBP-modified PLA. Neat PLA exhibited a tensile strength of 57.6 MPa and elongation of 3.33%. PLA containing 2 wt % HBP displayed a tensile strength of 70.8 MPa and an elongation of 5.16%. However, tensile strength decreased with increasing BHP but elongation remained about the same until 8 wt % BHP. Bhardwaj and Mohanty85 developed HBP-modified PLA blends through reactive blending of PLA, HBP, and polyanhydride (PA). During melt processing, the hydroxyls of HBP would react with the anhydride groups to crosslink in the PLA matrix. Compared with neat PLA, the PLA/HBP/PA (92/5.4/2.6, w/w) blend exhibited improved elongation (48.3% vs. 5.1%) and tensile toughness (17.4 MJ/m3 vs. 2.6 MJ/m3). However, tensile modulus and strength of the blend decreased from 3.6 GPa to 2.8 GPa and 76.6 MPa to 63.9 MPa, respectively. Lin et al.86 used a biodegradable aliphatic hyperbranched poly(ester amide) as a modifier for PLA. PLA blends showed gradual increase in elongation with HBP content without a severe loss in tensile strength. The elongation of the blend with 20 wt % HBP reached 50%, more than 10-fold over that of neat PLA (ca., 3.7%). Within 10 wt % content of HBP, the blend exhibited a somewhat increase in yield strength, as compared to neat PLA. Impact-fractured surfaces also demonstrated the change of fracture mode from brittle to ductile failure with the addition of HBP. Similarly, Zhang et al.87 reported the use of a biodegradable hyperbranched poly(ester amide) with aromatic rings to modify the brittleness of PLA. By increasing HBP content from 2.5 to 10 wt %, the blend exhibited a slight increase in tensile strength but a remarkable increase in elongation.
Soybean Oil Derivatives
Recently, Robertson et al.88 studied toughening of PLA using a polymerized soybean oil derivative, polySOY, which was prepared by crosslinking the double bonds of soybean oil molecules using a free radical crosslinking agent or oxygen under heating. A block copolymer, poly(isopropene-b-L-LA), was added as a compatibilizer. The elongation and tensile toughness of the PLA/polySOY blends were four and six times greater than those of unmodified PLLA, respectively.
Gramlich et al.89 used a conjugated soybean oil (CS) derivative to toughen PLA through reactive blending. First, a terminal-functionalized PLA was prepared by ring-opening polymerization of L-LA using N-2-hydroxyethylmaleimide (HEMI) as an initator and Sn(Oct)2 as a catalyst. The maleimide-terminated PLA (HEMI-PLLA) was then melt-blended with CS. It was demonstrated that the Diels-Alder reaction between the maleimide of HEMI-PLLA and the conjugated double bond of CS resulted in interfacial compatibilization between the two immiscible components. Blends of reactive HEMI-PLLA and 5 wt % CS resulted in an elongation of more than 17-fold at break that of neat HEMI-PLLA, as well as a more than 133% increase in elongation compared to the similar nonreactive PLA blend with 5 wt % CS. As the Tg and crystallinity of the PLA component was not significantly different from that of PLA homopolymer, the authors argued that the toughening of the blends did not originate from plasticization.
Nonbiodegradable Polymer Modifiers
While it is desirable for researchers to continue pursuing viable eco-friendly solutions to address the brittleness problem of PLA materials, blending PLA with nonbiodegradable but readily available petroleum-based thermoplastic polymers to modify the properties of PLA materials has gained momentum in recent years. NatureWorks81, 90 reported PLA blends with various commercial nonbiodegradable polymers in its Technology Focus Reports, such as ABS, acrylic impact modifiers, thermoplastic polyester elastomers, styrenic block copolymers, and polycarbonate (PC). Some of such blends are also commercially available.91, 92 Although it may not be a long-term solution, it provides an economic and viable means to meet the need of consumers. Usually, the majority of modifiers tend to be thermodynamically immiscible with PLA due to the lack of favorable interactions. To improve the compatibility between the modifier and matrix, a third component is added as a compatibilizer in most cases. The compatibilizer can be either premade or in situ formed during melt blending. For the latter, the rationale of reactive compatibilization is principally based on the reactions between end functional groups (i.e., OH or COOH) of PLA and other complementary functional groups (mainly epoxide groups) of the compatibilizers. As a result, improved interfacial adhesion and hence fine dispersion are achieved. Until now, various types of rubbery modifiers have been used to toughen PLA. A few super-toughned PLA blends (notched Izod impact strength > ∼530 J/m)93 have been successfully prepared in terms of melt blending.
Poly(ethylene-co-octene) (POE) is a thermoplastic polyolefin elastomer (TPO) and has been attempted in PLA toughening. POE and PLA are immiscible and have no strong interactions at the interface. Ho et al.94 prepared a series of POE-g-PLA copolymers as premade compatibilizers. The graft copolymers were synthesized by reacting terminal hydroxyl groups of PLA with maleic anhydride-functionlized POE (POE-MAH) using 4-dimethylaminopyridine as a catalyst. It was demonstrated that the copolymers significantly improved the compatibility of the PLA/TPO (80/20, w/w) blend. The size of the dispersed POE particles was substantially reduced with the addition of the compatibilizers until the equilibrium particle size was achieved at a certain critical concentration. As the concentration of POE-g-PLA copolymer increased, elongation and tensile toughness initially increased but then began to decline when the compatibilizer concentration was above 2.5 wt %. However, the presence of POE-g-PLA copolymer did not affect tensile strength or modulus markedly. It was found that the POE-g-PLA copolymers with long PLA segments resulted in higher elongation and tensile toughness. This work also showed that a POE-g-PLA copolymer was more efficient than POE-MAH to compatiblize the PLA/POE (80/20, w/w) blend.
In another study, Su et al.95 used GMA-grafted POE (mPOE) as a toughener of PLA. Both elongation and notched Charpy impact strength invariably increased with mPOE content. The uncompatibilized PLA/POE (85/15) blend exhibited an impact strength of only 19.4 kJ/m2. In contrast, when unreactive POE was replaced by mPOE in the blend, the impact strength reached 29.8 kJ/m2, more than seven times that of neat PLA (4.0 kJ/m2). With further addition of mPOE to 45 wt %, the impact strength increased to 54.7 kJ/m2. At the same time, both strengths and modulus suffered from a great loss because of the addition of an excessive amount of rubbery POE.
NatureWorks81 recently reported commercial toughening agents for PLA in a Technology Focus Report available in its website. They identified Blendex™ 338, an ABS resin containing 70% butadiene rubber, as an effective toughener among various impact modifiers. With 20% Blendex™ 338, the blend achieved a notched Izod impact strength of 518 J/m and an elongation of 281%. In contrast, neat PLA exhibited impact strength of 26.7 J/m and an elongation of 10%. As generally expected, tensile yield strength of the blend was decreased from 62 MPa for neat PLA to 43 MPa.
To enhance the compatibility between PLA and ABS, Li and Shimizu96 introduced styrene/acrylonitrile/GMA copolymer (SAN-GMA) as a reactive compatibilizer and ethyltriphenyl phhosphonium bromide (ETPB) as a catalyst during melt blending. Fourier transform infrared (FTIR) analysis revealed that the epoxy group of SNA-GMA reacted with PLA end groups under the mixing conditions and that addition of ETPB accelerated the reactions. It was also found that reactive compatibilization led to a remarkable decrease in the size of dispersed ABS domains. The compatibilized PLLA/ABS blends exhibited improved impact strength and elongation but slight reductions in modulus and tensile strength. For instance, adding 5 phr SAN-GMA to the PLLA/ABS (70/30, w/w) blend increased elongation from 3.1% to 20.5% and impact strength from 63.8 to 81.1 kJ/m2. By further incorporating 0.02 phr ETPB, the elongation and impact strength of the blend increased to 23.8% and 123.9 kJ/m2, respectively.
Oyama97 studied toughening of PLA using poly(ethylene-co-glycidyl methacrylate) (EGMA). It was shown that when the lower molecular weight PLA (L-PLA) and screw speed of 200 rpm during melt blending were used, the blend with 20 wt % EGMA had a much higher elongation (above 200%) relative to neat L-PLA (5%). But the notched Charpy impact strength of the blend was slightly increased, merely two times that of neat L-PLA (Fig. 11). Interestingly, much higher impact strength can be achieved after the injection-molded specimens of the L-PLA/EGMA (80/20, w/w) blend were annealed at 90 °C for 2.5 h. After annealing, the impact strength was significantly increased to 72 kJ/m2, about 50 times that of neat L-PLA. Also, the measurable improvement in strength and modulus of the blend was accompanied by a significant decrease in elongation at break. With the higher molecular weight PLA (H-PLA) as a matrix, such positive effect of annealing on impact strength appeared relatively less prominent. Based on DSC and wide-angle X-ray diffraction data, the author argued that the crystallization of the PLA matrix played a key role in such significant improvement, although the effects of annealing on phase morphologies and interfacial adhesion were not elucidated.
With PLLA-b-PE diblock copolymers as a compatibilizer, Anderson et al.98, 99 melt-blended PLA with linear low density PE (LLDPE) at a fixed PLA/LLDPE ratio (80/20, w/w). Addition of PLLA-b-PE block copolymers into the binary blend resulted in improved interfacial adhesion and finer dispersion of LLDPE in PLA matrix, as evidenced by SEM. The tacticty of the PLA matrix (amorphous vs. semicrystalline), molecular weight of the PLLA block (5 kg/mol vs. 30 kg/mol) in the PLLA-b-PE block copolymers, and its content (0–5 phr) were found to have a remarkable effect in determining the magnitude of ultimate notched Izod impact strength.
For the binary blend of amorphous PLA (a-PLA) and LLDPE, only a minor increment in the impact strength was observed with respect to neat a-PLA (34 J/m vs. 12 J/m). By adding 5 wt % of the block copolymer with the molecular weight of the PLLA block below its critical entanglement molecular weight (Mc = 9 kg/mol), that is, PLLA-b-PE (5-30), the compatibilized blend exhibited almost comparable impact strength to the uncompatibilized binary blend (36 J/m vs. 34 J/m). With the addition of 5 wt % of the block copolymer having the molecular weight of PLLA block above its Mc, that is, PLLA-b-PE (30-30), however, the impact strength was drastically increased to 460 J/m. This difference was attributed to the superior ability of the block copolymer with the long PLLA block to suppress the coalescence of dispersed phase. The situation was somewhat different in the case of the semicrystalline PLA (PLLA) matrix. Even without the PLLA-b-PE block copolymers, the PLLA/LLDPE blends exhibited significantly higher impact strength than that of the PLLA homopolymer (350 J/m vs. 20 J/m) despite a large standard deviation in impact strength values. The adhesion test gave an indication of the superior adhesion for the PLLA/LLDPE interface to the PLA/LLDPE interface. With the addition of the PLLA-b-PE block copolymers, the impact strength was further increased to 510 J/m for use of 5 wt % PLLA-b-PE (5-30) and 660 J/m for use of 5 wt % PLLA-b-PE (30-30), respectively. The authors proposed that the tacticty effects on either the entanglement molecular weight of PLA or miscibility degree of PLA matrix with LLDPE phase accounted for the difference between the two binary blend.
The dependence of impact toughness as well as LLDPE particle size on the amount of block copolymer was also examined. It was found that only 0.5 wt % of the block copolymer was sufficient to achieve the optimum impact toughness. With increasing amounts of PLLA-b-PE (30-30) block copolymer in the PLLA/LLDPE (80/20, w/w) blends, the dispersed LLDPE particle size was gradually reduced. At the block copolymer amount of 3 wt %, the size of the dispersed LLDPE particles began to level off at around 1.0 μm. As one of the important parameters determining ultimate final impact toughness,100, 101 the matrix ligament thickness (MLT) was further calculated with relation to impact resistance of the blends (Fig. 12). It was found that as the MLT decreased, the impact toughness increased and the critical MLT for PLA toughening was found to be approximately 1 μm.
At a fixed composition of compaibilized PLA/PE/PLLA-b-PE (80/20/5, w/w), notched Izod impact properties of the blends was also found to be highly dependent on the dispersed PE phase properties.99 The flexible LLDPE tended to result in blends with the high levels of toughness. On the contrary, the stiff high-density polyethylene (HDPE), even in the case of the ternary blends with a MLT of less than 1 μm, the maximum impact strength obtained was noticeably lower (64 J/m). Also, the level of interfacial adhesion needed to achieve maximum toughening varied with the PE used. Use of LLDPE, which relieve impact stresses by cavitation, required higher interfacial adhesion than use of HDPE, which was likely to dissipate energy by the debonding at the particle–matrix interface.
Recently, Hashima et al.102 toughened PLA using hydrogenated styrene-b-butadiene-b-styrene copolymer (SEBS) and a reactive EGMA. The PLA/SEBS/EGMA (70/20/10, w/w) blend achieved a notched Izod impact strength of 92 kJ/m2 and an elongation of 185%. After annealing the samples at 80 °C for 48 h, impact strength and elongation decreased dramatically to 32 kJ/m2 and 100%, respectively. The negative effect of annealing on the impact strength was also observed in the binary and quaternary blends. However, no detailed explanation for the reduction of impact toughness was given in this study.
By incorporating 40 wt % PC in the ternary blends, the heat deflection temperature and aging resistance were improved without severe deteriorations in impact toughness and ductility. Transmission electron microscopy (TEM) observation revealed that PC and SEBS were separately dispersed in the PLA matrix. For the PLA/PC/SEBS/EGMA (40/40/15/5, w/w) blend, the maximum notched impact strength attained was about 60 kJ/m2. The authors attributed the outstanding toughness and aging resistance of the quaternary alloy to the negative pressure of SEBS that dilated the bicontinuous PLA/PC matrix to enhance the local segment motions. The chart of the above development in notched Izod impact strength in the above PLA blends is briefly outlined in Figure 13.
A Novel Reactive Blend Systems Involving Dual Reactions
The majority of the above modifiers, when being used alone or in combination with a compatibilizer, proved to be fairly effective in enhancing tensile toughness and ductility of PLA. However, as for impact strength, especially in the notched state, these modifiers either had little effects or only introduced modest improvement. Even though a few supertough PLA blends have been reported in the literature,97–99, 102 a comprehensive understanding of the relationship between toughness and morphology is still lacking.
Liu et al.103 introduced a novel PLA ternary blend system consisting of an ethylene/n-butyl acrylate/GMA terpolymer elastomer (EBA-GMA) and a zinc ionomer of ethylene/methacrylic acid copolymer (EMAA-Zn). In the reactive ternary blend system, simultaneous vulcanization (crosslinking) of EBA-GMA and interfacial reactive compatibilization between PLA and EBA-GMA took place. Figure 14 shows the influence of extrusion temperature and EBA-GMA/EMAA-Zn (i.e., rubber/ionomer) weight ratio (total 20 wt % in the blends) on notched Izod impact strength and elongation of the blends. A remarkable dependence of impact strength on extrusion temperature was found.103 The ternary blends prepared at 185 °C only exhibited similar impact strength to that of binary blends, less than threefold that of the neat PLA control. In contrast, a tremendous toughening effect was observed in the ternary blends prepared at 240 °C. Furthermore, such improvement was more pronounced when the weight ratio of EBA-GMA/EMAA-Zn ≥1. Especially the ternary blend with 15 wt % EBA-GMA showed impact strength of 860 J/m, approximately 35 times that of neat PLA. It is noteworthy that the remarkable enhancement in impact strength was accompanied by high elongation (>200%). Evidently, this result was different from the results by Oyama97 in the study of PLA/EGMA (80/20, w/w) binary blend, which exhibited the similar toughness after annealing of the molded specimens but substantially lower strain at break (≤35%).
TEM observations revealed that the dispersed domains in the ternary blends displayed a unique “salami”-like phase structure. When the rubber content was higher than the ionomer content, this substructure was the occluded ionomer inside the rubber droplets which were dispersed in the PLA matrix (Fig. 15). In this case, the interface at the rubber droplet/PLA matrix exhibited good wetting and the blends exhibited high impact strength.104 When there was more ionomer than rubber in the blends, however, phase inversion occurred in the substructure during blend compounding.104 Consequently, the substructure turned out to be the rubber occluded inside the ionomer droplets. It was found that in the latter case the wetting at the ionomer droplet/PLA matrix interface became poor and the particle size of the dispersed phase was relatively larger. As a result, the impact strength of the ternary blends decreased rapidly.
Unlike other reactive rubber toughening which only involved reactive compatibilization at the rubber/matrix interface,95–97, 102 both reactive interfacial compatibilization and rubber crosslinking reactions simultaneously took place in this ternary blend system. Torque and dynamic mechanical analysis (DMA) data demonstrated that increasing EMAA-Zn content led to a faster vulcanization and progressively higher crosslink level of the EBA-GMA phase.104 FTIR spectra suggested that the variation in the EBA-GMA/EMAA-Zn ratio did not remarkably change the extent of compatibilization between PLA and EBA-GMA. Figure 16 proposes the possible reaction scheme that accounts for the remarkable dependence of impact strength on blending temperature.103 At 185 °C, moderate curing reactions took place between the carboxyl groups of EMAA-Zn and epoxy groups of EBA-GMA under the catalysis of Zn2+ ions, but the compatiblization reactions between the epoxy groups of EBA-GMA and hydroxyl groups of PLA were not significant. Hence, like many other soft polymer toughened PLA blends, the resulting ternary blend displayed high ductility but only limited improvement in impact strength. At 240 °C, not only the degree of curing of the EBA-GMA rubber was greatly increased but also the compatibilization reactions between the rubber and PLA phases were significantly enhanced. Therefore, the resulting interface was able to stabilize premature crack propagation at the early stage of impact test having a high-strain rate before massive matrix shear yielding took place.
By correlating dispersed particle size with notched Izod impact toughness, an optimum particle size range (ca. 0.7–0.9 μm) for PLA toughening was identified in the PLA/EBA-GMA/EMA-Zn (80/20-x/x, w/w) blend system, as shown in Figure 17.104 Likewise, the optimum particle size has also been widely reported for other thermoplastic matrices containing a variety of rubbers, such as semicrystalline nylon-6 (PA6: 0.2–0.5 μm),105–107 amorphous nylon (a-PA: 0.2–0.5 μm),108 PMMA (0.2–0.3 μm),109–111 PVC (0.2 μm),112 poly(styrene-co-acrylonitrile) (SAN: 0.75 μm),113 and PS (0.8–2.5 μm).93, 113. Wu correlated rubber particle diameter with chain structure parameter of the matrix and claimed that the optimum particle size for toughening decreased as the matrix becomes less brittle.92 Because PLA exhibited relatively higher intrinsic brittleness (characteristic chain ratio as a measure of chain flexibility, C∞ = 9.5–11.8114–116 depending on the L/D LA ratio) than other matrices (e.g., C∞ = 6.2 for PA6, C∞ = 5.4 for a-PA, C∞ = 7.6 for PVC, C∞ = 8.2 for PMMA, C∞ = 10.6 for SAN, and C∞ = 10.8 for PS),93, 117 this optimum particle size for the toughened ternary PLA system seemed reasonable. By correlating tensile toughness with dispersed particle diameter in PLLA/CS binary blends, Gramlich et al.89 also reported the similar range of optimum particle diameter (i.e., 0.5–0.9 μm) for toughening PLA.
In addition, the deformation mechanism of these blends was analyzed in terms of electron microscopic observation of the impact-fractured surfaces.104 SEM fractographic observation revealed that perceptible matrix plastic deformation only occurred in the ternary blends where there was more rubber than ionomer, for example, the PLA/EBA-GMA/EMAA-Zn (80/15/5 w/w) blend. TEM micrographs of the subfracture surface were further performed to identify the micromechanical deformation process (Fig. 18). For the binary PLA/EBA-GMA (80/20, w/w) blend, it was found that only tiny debonding around relatively larger particles was observed without internal cavitation [Fig. 18(a)]. Also, at the higher magnification, the existence of minute fibrillated crazes passing through other neighboring particles was also noted [Fig. 18(b)]. Therefore, the debonding in the PLA/EBA-GMA binary blend was unable to trigger the massive matrix plastic deformation required for high impact toughness. This situation was similar for the ternary PLA/EBA-GMA/EMAA-Zn (80/5/15, w/w) blend, in which the debonding at the ionomer/PLA interfaces prevailed. On the contrary, cavitation inside the grey EBA-GMA phase was noted in the PLA/EBA-GMA/EMAA-Zn (80/15/5, w/w) blend. Therefore, the evidence suggested that the high impact toughness observed for some of the ternary blends was attributed to the low cavitation resistance of dispersed particles in conjunction with suitable interfacial adhesion.
Commercial Impact Modifiers for PLA
In recent years, several series of commercial impact modifiers for biopolymers (especially PLA) have been launched. These modifiers are either linear elastomers of low Tg or crosslinked core–shell polymers. The core–shell modifiers typically consist of a low Tg rubbery core encapsulated by a glassy shell that has a good interfacial adhesion with the matrix polymer. When well-dispersed, these modifiers act as nanoscale or microscale rubbery domains that dissipate mechanical energy and retard or arrest initiation and propagation of microcracks through the polymer. These modifiers were reported to bring varying magnitudes of toughening effects to PLA. For the toughened PLA to retain good clarity, small particles with refractive indices similar to that of PLA are desired. To better match the refractive index of PLA matrix, low-Tg acrylates such as ethyl acrylate or butyl acrylate, are used to replace butadiene of the rubber core in the cores–shell impact modifier.
Sukano® PLA im Series
To overcome the inherent brittleness of PLA, Sukano Co. has developed a patented impact modifier (Sukano® PLA im S550) based on elastomer, and it was targeted for transparent applications (e.g., packaging).118 The special feature of this unique impact modifier, which has been optimized for use with FDA approved, biodegradable PLA, is that it does not impair the transparency or heat stability of PLA. At a concentration of just 4%, impact resistance of PLA was improved by a factor of 10, so preventing cracks and splinters in the PLA sheet and film during cutting or stamping. Furthermore, in addition to its compostability and excellent transparency, Sukano® PLA im S550 was claimed to be highly cost-effective in comparison to similar products on the markets. Recently, another new transparent impact modifier, Sukano® PLA im S555, was also launched by Sukano Co.119
OnCap™ BIO Impact Series
In 2010, PolyOne introduced a new transparent impact modifier (OnCap™ BIO Impact T) for PLA.120, 121 It is a masterbatch containing a specific elastomer. This modifier was said to improved impact properties in PLA while maintaining the desired transparency at the same time. Tear resistance of PLA was also improved with addition of OnCap BIO Impact T.
Recently, Scaffaro et al.122 compared toughening effects of OnCap™ BIO Impact T and Sukano® PLA im S550, on PLA. Both modifiers were immiscible with PLA but Sukano® PLA im S550 displayed a more homogeneous dispersion in the PLA matrix. It was found that neither impact modifiers brought obvious increase in elongation to PLA. The maximum Izod impact strength was achieved by using 8 wt % Sukano® PLA im S550 (141 J/m), while the impact strength only increased to 124 J/m even with addition of OnCap™ BIO Impact T.
Biomax® Strong Series
Biomax® Strong 100 and 120 are two commercial modifiers for PLA from DuPont Company. Both modifiers are said to be ethylene–acrylate copolymers and are designed to improve the toughness of PLA in packaging and industrial applications with minimal impact on transparency.123, 124 Biomax® Strong 100 is designed for nonfood applications and Biomax® Strong 120 for food packaging applications. It was claimed that addition of only 2 wt % Biomax® Strong to PLA, either amorphous or semicrystalline PLA, resulted in a significant increase in impact strength. With 5 wt % Biomax Strong or less, the blends maintained contact clarity similar to that of the clarified PP. Further increasing the loading level of Biomax® Strong in the range of 5–15% resulted in blends with different degrees of translucence, similar to that of the unclarified PP. Also, both modifiers improved the cutting and trimming performance of PLA.
Murariu et al.125 studied toughening effects of Biomax Strong® 100 on PLA and high-filled PLA/β-calcium sulfate anhydrite (AII) composites. Notched Izod impact strength of PLA with 5 and 10 wt % Biomax® Strong 100 increased from 2.6 kJ/m2 of the neat PLA to 4.6 and 12.4 kJ/m2, respectively. Elongation was above 25% for the blend with 10 wt % of the impact modifier, while tensile strength and modulus of PLA gradually decreased with addition of the impact modifier. Addition of 5 and 10 wt % of the impact modifier to the PLA/AII (70/30, w/w) composite also increased their impact strength to 4.5 and 5.7 kJ/m2, respectively. Impact strength slightly decreased with further increase of the filler loading to 40 wt % but remained higher than that of both the unmodified composites and the neat PLA. On the other hand, for the PLA composites with 40% filler, tensile strength and elongation markedly decreased with inclusion of the impact modifier.
Zhu et al.126 studied the films of the PLA blends containing either Biomax® Strong 100 or Sukano® PLA im S550 as a toughener. It was shown that the modulus decreased with increasing concentration of the former modifier but was relatively independent of the concentration of the latter toughener. The maximum elongation was 255% for the former at a 12 wt % loading and 240% for the latter at a 8 wt % loading, while elongation of neat PLA was about 90%. For a given composition, the latter modifier gave a clearer film than Biomax® Strong 100, but the clarity of films decreased with concentration for both tougheners.
Afrifah and Matuana127 compared the toughening effects of Biomax® Strong 100 on semicrystalline and amorphous PLA. Biomax® Strong 100 achieved superior toughening on semicrystalline PLA over amorphous PLA. With 40 wt % of the toughener, the notched Izod impact strength of the semicrystalline PLA increased from 16.9 J/m of pure PLA to 248.4 J/m. In addition, the presence of 15 wt % Biomax® Strong 100 lowered the brittle-to-ductile transition temperature of PLA, as revealed by the notched Izod impact test data of the freezed specimens under the designated temperatures.
Paraloid™ BPM Series
The former Rohm & Hass Co. introduced the Paraloid™ BPM-500 acrylic-based impact modifier for PLA resin. This modifier is a free-flowing white powder and is specially designed to improve impact properties without sacrificing the transparency of the product.128 It was claimed that improved impact properties were obtained with the addition levels as low as 3 wt %. At a 5 wt % loading, the dart drop impact strength of extruded sheets was increased by threefold with respect to neat PLA. In addition, PLA modified with BPM-500 also showed a marked improvement in cutting, slitting, and flexibility. Because of the combination of the nanoscale particle size and the excellent dispersability in PLA, this modifier has a minimal effect on the clarity of the PLA films. With addition up to 5 wt %, the haze measured on a 15 mil extruded sheet was increased to ∼6% compared with 3–4% for neat PLA. In 2009, Dow Chemical Company launched a new acrylic impact modifier in this series to impart toughness and maintain clarity of PLA.129 It was claimed that Paraloid™ BPM-515 offered the same benefits as Paraloid™ BPM-500 but with higher efficiency with a loading level as low as 1 wt %.129, 130 The haze measured on a 15-mil extruded sheet was less than 6% with addition up to 3 wt % Paraloid™ BPM-515.
Three grades of core–shell impact modifiers for PLA, Biostrength™ 130, 150, and 200, were launched by Arkema. These modifiers are white powders and are suggested to be added at 2–6 wt % in PLA. Both Biostrength™ 130131 and Biostrength™ 200132 are acrylic core–shell impact modifiers which are designed to increases toughness of PLA and retain adequate transparency.
Biostrength™ 150 is a methyl methacrylate–butadiene–styrene-type core–shell impact modifier for opaque applications and is said to be especially effective in durable injection molding applications requiring high ambient and low temperature durability.133, 134 Cygan et al.135 compared the effects of Biostrength™ 130 and Biostrength™ 150 on impact strength and clarity of the resulting PLA blends. It was demonstrated that compared to Biostrength™ 130, Biostrength™ 150 provided somewhat higher impact Gardner impact properties but much higher haze. In 2010, Arkema introduced another new clear acrylic core–shell impact modifier (Biostrength™ 280) and recommended its use in PLA applications that require toughness and high transparency.136 A small amount of Biostrength™ 280 impact modifier incorporated in PLA during extrusion turned the resulting sheet from brittle to ductile, allowing easier manufacturing and more durable end use properties of the thermoformed package.
Other Rubbery Modifiers
Ishida et al.137 studied the toughening of PLA using four rubbers: ethylene–propylene copolymer, ethylene–acrylic rubber, acrylonitril–butadiene rubber (NBR), and isoprene rubber (IR). Izod impact testing revealed that toughening was only achieved with the use of NBR which exhibited smaller particle size (3–4 μm) than the other three in the blends. In accordance with the morphological analysis, the interfacial tension between PLA and NBR phases was the lowest. The more polar structure of NBR was considered to be responsible for the better toughening effect. Tensile properties showed that NBR and IR without internal crosslinks possessed a high ability to induce plastic deformation before break as well as high elongation properties.
Ito et al.138 investigated fracture mechanism of neat PLA and PLA blends toughened with an acrylic core–shell modifier. The acrylic modifier was composed of a crosslinked alkyl acrylate rubber core and PMMA shell, and the particle size was in the range of 100–300 nm. Plane strain compression testing of PLA clearly showed strong softening after yielding. Because the stress for craze nucleation was close to that of yield stress, brittle fracture occurred for neat PLA. Addition of the acrylic modifier significantly lowered the yield stress and formed many microvoids. Release of strain constraint by microvoiding and decrease of yield stress led to the relaxation of stress concentration, and the toughness was improved moderately. Table 9 summarizes reported mechanical properties of some of highly toughened PLA blends prepared via melt-blending.
Table 9. Summary of Reported Mechanical Properties of Some Highly Toughened PLA Blends Prepared via Melt-Blending
Addition of Rigid Fillers
Usually, most of the fillers increase the stiffness of PLA materials with little benefit to toughness.140, 141 By in situ polymerization of L-LA in the presence of 36 wt % carbon fiber, Grijpma et al.47 found that the unnotched strength (Dynstat) was increased from 12.1 kJ/m2 of as-polymerized PLA to 62.8 kJ/m2. PLA with low viscosity-average molecular weight gave low impact strength.
To overcome the negative effect of rubber toughening on the stiffness of PLA blends, Xia et al.142 recently introduced a specially engineered mineral-EMforce™ Bio calcium carbonate (CaCO3) with a aspect ratio of 5.42 for PLA reinforcement. Interestingly, besides linearly increasing flexural modulus with the loading level, this mineral filler also provided a moderate improvement in room-temperature crack initiation energy (∼20 J) and notched Izod toughness (>120 J/m) at the 30 wt% loading. On the contrary, PLA composites filled with other mineral fillers such as mica, talc, and ground CaCO3, did not exhibit superior toughness to that of the neat PLA (∼3 J and ∼40 J/m). The moderate toughening effect of EMforce™ Bio CaCO3 on the PLA matrix was also identified in the NatureWorks' Technology Focus Report.140 With 30 wt % of the filler, the dart impact strength, unnotched and notched Izod impact strength were increased to 27 J, 294 J/m, and 123 J/m, respectively, with respect to that of the unmodified PLA being 4 J, 235 J/m, and 37 J/m.
Combination of Flexible Polymer and Mineral Filler
Toughening of PLA by incorporation of a flexible polymer is usually accompanied by sacrifices in strength and modulus. On the contrary, addition of mineral fillers generally leads to increased modulus but also reductions in elongation and impact strength in most cases. Therefore, in an attempt to achieve balanced overall properties, PLA ternary composites containing both flexible polymer and rigid inorganic fillers were recently studied.
Chen et al.143 studied the inclusion of organically modified clay in PLLA/PBS blends. The compatibility of clay and polymer was found to be critical for the property enhancement of resulting composites. Addition of 10 wt % Closite® 25A to the PLLA/PBS (75/25, w/w) blend increased tensile modulus from 1.08 GPa to 1.94 GPa but decreased elongation from 71.8% to 3.6% which was even lower than that of neat PLLA (6.9%). In contrast, the use of an epoxy-functionalized organoclay (TFC) at the same amounts not only retained high tensile modulus but also increased elongation to 118%. Chen et al.144 also noted the similar compatibilizing effect of TFC on the PLA/PBSA (75/25 w/w) blends, but the increase in elongation (46% vs. 5%) in this case was not as much as that in the PLA/PBS blends above.
Li et al.145 recently reported PLA/clay/core–shell rubber ternary composites. The core–shell rubber impact modifier was Paraloid™ EXL 2330, a polybutylacrylate (core)–polymethylmethacrylate (shell)-based material from the former Rohm and Haas Co. With 20% EXL 2330 and 5 wt % Cloisite 30B clay, the notched Izod impact strength increased from 2.2 kJ/m2 of neat PLA to 5.2 kJ/m2, whereas the tensile modulus only decreased from 1.81 MPa of neat PLA to 1.79 MPa. Elongation showed little change, being 7% compared to 6.6% of neat PLA. However, the tensile strength suffered a significant drop from 61.0 MPa of neat PLA to 43.8 MPa.
Hasook et al.146 investigated effects of PCL molecular weight on mechanical properties of ternary PLA/PCL/organically modified clay (OMMT) composites. Optimal mechanical properties were achieved with a PLA/PCL/OMMT (90/5/5, w/w) ternary composite in which the weight-average molecular weight of PCL was 40,000 g/mol. This ternary composite displayed increases in strength, modulus, and elongation of 19, 9, and 53%, respectively, with respect to neat PLLA.
Jiang et al.147 compared effects of OMMT and nanosized precipitated calcium carbonate (NPCC) on mechanical properties of PLA/PBAT/nanofiller ternary composites. Mechanical testing demonstrated that the composites containing OMMT exhibited higher tensile strength and modulus but lower elongation, compared with the ones containing NPCC. When 25 wt % of the PLA was replaced by maleic anhydride-grafted PLA (PLA-g-MA), the elongation of the ternary composites was substantially increased, possibly as a result of improved dispersion of the nanoparticles and enhanced interfacial adhesion. Among these composites, PLA/10 wt %PBAT/2.5 wt %OMMT with 25 wt % of PLA being PLA-g-MA demonstrated the best overall properties with 87% retention of tensile strength of pure PLA, slightly higher modulus and significantly improved elongation (16.5 times that of neat PLA). Teamsinsungvon148 also reinforced PLA/PBAT blends using microsized precipitated CaCO3 and achieved similar toughening effects on PLA/PBAT (80/20, w/w) blends.
Formation of Semi-Interpenetrating Network
Semi-interpenetrating network (SIPN) is defined as a blend in which one or more polymers are crosslinked and one or more polymers are linear or branched.149 Yuan and Ruckenstein150 toughened PLA by forming a network of thermosetting PU in the matrix. The PU was prepared using PCL diols and triols and toluene-2,4-diisocyanate and its degree of crosslinking was controlled by altering the diol/triol ratio. The toughening effect was influenced by crosslinking degree of PU. No crosslinking or excessive crosslinking both tended to lower the toughening effect. With 5 wt % of properly crosslinked PU, an optimum tensile toughness of 18 MJ/m3 could be achieved compared with 1.6 MJ/m3 of pure PLA. Elongation of the toughened PLA increased to ∼60%, meanwhile, yield strength, tensile strength, and Young's modulus decreased by ∼26, ∼30, and ∼22%, respectively. Optimum toughness was thought to result from the balance between the compatibility of the semi-interpenetrating PU network with PLA and the stiffness of this network.
If molecular orientation could be introduced conveniently and economically through processing techniques, it may provide a facile route to toughen PLA without compromising its tensile properties.
Bigg151 demonstrated that biaxial orientation of PLA induced a 5–10 fold increase in elongation with enhanced tensile strength. The considerable increase in tensile toughness for the initially amorphous PLA was attributed to strain-induced crystallization during orientation. Grijpma et al.152 showed that drawing of an injection-molded amorphous PLA sample at temperatures below the Tg of the polymer increased tensile strength from 47 MPa of the unoriented PLA to 73.3 MPa. At the same time, notched Izod impact strength was increased from 1.6 to 52.0 kJ/m2. Molecular orientation could be also manipulated via shear-controlled orientation during a nonconventional injection-molding (SCORIM), in which a macroscopic oscillating shear force was applied to orientate the solidifying polymer melt. A Charpy impact strength of 21.3 kJ/m2 was obtained, as compared with 15.1 kJ/m2 for the sample molded by conventional injection molding technique (CIM). It was noted that the SORIM process decreased the molecular weight of PLA slightly more than the CIM process. The degree of molecular orientation was not uniform throughout the specimen cross-section with the highest degree of orientation in the shell layers.
Recently, Ghosh et al.153 investigated the effects of operative parameters of SCORIM and compared the results of the molded PLLA samples prepared by SCORIM and CIM. PLLA molded by SCORIM demonstrated tensile toughness and strength which were 641 and 134% that of PLLA molded by CIM, respectively, without sacrificing modulus. The high ductility achieved by SCORIM was attributed to the preferential molecular orientation in the core sections. The orientation in the core was more pronounced at low mold temperatures and increased with increasing shearing time.
Due to the inherent rigidity of PLA chains, crazing deformation was favored over shear yielding in the case of neat PLA.93 The brittle fracture behavior of PLA in tensile and impact testing has been associated with the crazing deformation mechanism through which the polymer fails.3, 138 Various methods have been used to improve the toughness of PLA. Blending with polymeric tougheners has proved to be an economic and effective means to toughen PLA. The toughening effects of PLA blends are complicated by many variables, including size, volume fraction, substructure and inherent property of the dispersed phase, and interfacial adhesion.
It has been demonstrated that reactive blending is more effective in improving the toughness of PLA blends, particularly impact strength. In some cases, supertough PLA blends have been successfully achieved.97–99, 102, 103 In most of these blends, however, the achievement of superior impact toughness relies on the addition of a large amount of nonbiodegradable petroleum-based polymers (≥20 wt %), which compromise the integral biodegradability and compostability of the PLA materials. In addition, the significant improvement in impact toughness was usually accompanied by a great loss (30–50%) in strength and stiffness. Thus, how to greatly enhance impact toughness while minimizing the reductions in strength and stiffness of the PLA materials still remains a challenge. Furthermore, the roles of tacticty and crystallization (e.g., degree of crystallinity and crystalline structure) of PLA matrix in PLA toughening are not well understood yet until now and are thus worth more attentions.
Hongzhi Liu received his Ph.D. degree in polymer chemistry and physics at the Institute of Chemistry, Chinese Academy of Sciences, Beijing, China, in 2005. From 2006 to 2007, he was a postdoctoral fellow in Seoul National University. From 2007 to 2008, he was a postdoctoral researcher in Louisiana State University. Since 2008, he is now working as a post-doctoral research associate at the Composite Materials and Engineering Center of Washington State University. His current research interests focus on development and characterization of biobased polymeric materials derived from renewable natural resources.
Jinwen Zhang received his Ph.D. in polymer science from University of Massachusetts Lowell in 1997. Dr. Zhang is currently a tenured associate professor at the Composite Materials and Engineering Center of Washington State University. For the past 14 years, Dr. Zhang's research has been focused on biobased polymer materials ranging from synthesis of new renewable polymers, new processing techniques and application development.