Miscibility and Morphology
The dynamic viscoelastic curves for pristine polymers and blends are shown in Figure 1. As shown in Figure 1(a), each pristine polymer showed one peak (glass transition). All of the blends exhibited two distinct glass transitions, revealing a typical two-phase system, one for PBC at about −32°C and one for PLA at about 62°C. It was noticed that the glass transition temperatures are almost independent of PBC concentrations, indicating the lack of significant molecular interactions between PLA and PBC. As shown in Figure 1(b), the storage modulus (E′) at room temperature for PLA/PBC blends gradually decreased with increasing content of PBC. The E′ of pure PLA dropped sharply at about 50°C due to the glass transition and then increased at around 100°C due to the cold crystallization. Moreover, the temperature at which the E′ started to increase, due to the cold crystallization of PLA, shifted to a lower temperature with the addition of PBC. This result suggested that the incorporation of PBC enhanced the cold-crystallization ability PLA and therefore decreased the cold-crystallization temperature of PLA in the blend.
Figure 1. Dynamic viscoelastic curves for PLA/PBC blends: (a) loss modulus versus temperature; (b) storage modulus versus temperature.
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Figure 2 shows the SEM micrographs of the PLA/PBC blends. All the blends appeared a clear, phase-separated morphology with PBC dispersed in the PLA matrix. As shown in the graphs, PBC phase domains dispersed as spheres in the PLA matrix with distinct interface. With increasing content of PBC, there was a corresponding increase in the PBC particles size due to the coalescence phenomena. This phase-separated structure of the blends is in agreement with the result obtained from DMA measurements.
Figure 2. Phase morphologies of the PLA/PBC blends with various weight ratio: (a) 95/5, (b) 90/10, (c) 80/20, and (d) 70/30 (20-μm scale bar).
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It is well known that the solid-state morphology and crystallinity have great effect on the physical and mechanical properties of PLA.29–31 Consequently, it is important to study the influence of existence of the PBC on the crystallization of PLA. Figure 3 shows the DSC heating curves of neat PLA and the PLA/PBC blends after melt-quenched, with a heating rate of 5°C/min. Neat PLA exhibited the glass transition at about 61.7°C and displayed a cold crystallization exotherm at 117.5°C. The melt of these crystallized domains occurred at 165.6°C. The Tg of PLA in the blends almost unchanged with increasing PBC content, suggesting that the blends were totally immiscible. It was consistent with the results from DMA and SEM analysis. However, the cold-crystallization exothermic peak of the blend was significantly different from that of neat PLA. Compared to neat PLA, the incorporation of PBC decreased cold crystallization temperature by approximate 7°C and narrowed the peak width, indicating an enhanced crystalline ability of PLA. Thus, the cold crystallization of PLA can be promoted by the addition of PBC, as it occurs at a lower temperature with respect to pure PLA. However, a further increase in the PBC content had little effect on the cold crystallization of PLA in the blend.
As observed from DSC thermogram, neat PLA shows a melting peak at 165.6°C. The addition of PBC clearly separated the melting peaks of neat PLA into two individual peaks. The peaks at low temperature were around 5°C lower than the peak of neat PLA, whereas the peaks at high temperature almost unchanged. The very similar ΔHc and ΔHm values for each sample (Table I) indicate that the crystals cannot form during cooling from melt. The exothermal peaks during heating of the samples due to the cold crystallization indicate the formation of crystals. Thus, the double melting behavior can be well explained by the melting, recrystallization, and remelting mechanism.32–34 Therefore, it could be concluded that compared with neat PLA, the addition of PBC increased the crystallization rate of PLA but did not affect the final crystallinity of the PLA in the blends if given enough time. This finding may be related to the immiscibility of the blend. Considering the immiscibility of the PLA/PBC blends, the interface between the phase-separation domains may play a favorable nucleation sites for cold crystallization of PLA in the blend. Accordingly, the addition of PBC greatly increased the crystallization rate of PLA and this likely occurred through the increase in the nucleation rate. However, the nucleation effect of the interface did not simply increase with increasing PBC content due to coalescence phenomena. Thus, a further increase in the PBC content had little effect on the cold crystallization of PLA in the blend. The similar phenomena were also reported in other immiscible PLA blends such as PLA/PCL and PLA/PBAT.35, 36
Table I. Thermal Properties of PLA/PBC Blends
|Samples||Tg (°C)||Tcc (°C)||TmPLA (°C)||ΔHcc (J/g)||ΔHm (J/g)|
|Tm1 (°C)||Tm2 (°C)|
| || || || || || || |
The addition of PBC significantly changed the tensile behavior, from brittle fracture of neat PLA to ductile fracture of the blends. Figure 4 shows the stress–strain curves of neat PLA and PLA/PBC blends. Neat PLA is very rigid and brittle with tensile strength around 66.7 MPa, and the elongation at break only about 4.9%. Neat PLA showed a distinct yield point with subsequent failure by neck instability. In contrast, all the blends showed clear yielding and stable neck growth through cold drawing. The samples were finally broken at a drastically increased elongation and the elongation continuously increased with increasing PBC content. Surprisingly, it was interesting to notice that even at 10% of PBC, high elongation at break of 139% was obtained, whereas the tensile strength remained as high as 50.7 MPa (Table II). On the other hand, the tensile strength and modulus of the PLA/PBC blends decreased with increasing PBC content. The tensile strength decreased from 66.7 MPa (neat PLA) to 33.9 MPa (20% PBC), whereas modulus decreased from 2091 MPa (neat PLA) to 988 MPa (20% PBC). This consequence can ascribed to the difference of tensile and modulus between PLA and PBC.
Table II. Mechanical Properties PLA/PBC Blends
|Samples||Storage modulus (MPa)||Tensile strength (MPa)||Elongation at break (%)||Impact strength (kJ/m2)|
According to the literatures, the yield behavior of polymer blends is affect by the interfacial adhesion. When the interfacial adhesion is strong enough for stress transfer to occur between two phase, the yield stress obeys the law of mixtures:
where b is the blend, σ is the yield stress and subscripts 1 and 2 refer to component 1 (PLA) and component 2 (PBC), respectively. While in the case of a lack of interfacial adhesion, the yield stress calculated with eq. (2):
where superscript 0 denotes zero interfacial adhesion, subscript m is the matrix or continuous phase, and d is the dispersed phase. Figure 5 shows the comparison of the experimental date with the predication for extreme interfacial adhesion. The PLA/PBC blends have a significant positive deviation with respect to the predications by eq. (2). The Pukanszky model gave credit to modest interfacial adhesion between PLA and PBC although PLA/PBC is an immscible blend. According to literatures, the interfacial adhesion has a great influence in the micromechanical deformation processes.14, 37, 38 The cavitations form within the cores of rubber particles when there is a strong interfacial bonding between the components and relatively weak strength of rubber phase itself. Although when there is not sufficient interfacial adhesion, interfacial debonding will take place.
To further investigate the toughening mechanism of PLA/PBC blends, the morphology of different necking regions of the tensile specimen was cryofractured longitudinally to verify the interfacial adhesion effect on the micromechanical deformation processes. Neat PLA had almost the same smooth fracture surface for different regions without visible plastic deformation in the stress direction. However, the PLA/PBC blends showed different behaviors under tensile testing and the different deformation stages of the blend (20% PBC) during stretching are presented in Figure 6(a). The PBC particles act as stress concentrators because they have an elastic property that differed from the PLA matrix. The consequent stress concentration leads to the development of a triaxial stress in the PBC particles. Because of the lack of phase adhesion, debonding can easily take place at the particle matrix interface in the perpendicular external stress direction. Thus, the cavities arise and are clearly observed in the initial stage of the stretching, which is shown in Figure 6(b). Once the voids are formed, the hydrostatic stress state caused by stress concentration is released with the stress state in the ligaments of PLA between the voids being converted from a triaxial to more biaxial or uniaxial tensile stress state. With the continuous growth of voids, weak shear bands form in the matrix between the PBC particles. At this stage, these cavities are enlarged along the stress direction, as shown in Figure 6(c). With the continuous plastic growth of voids, PLA matrix between the PBC particles deforms more easily and therefore shear yielding is achieved. The oriented cavities in the stress direction along with the deformation of the matrix are shown in Figure 6(d). The plastic deformation, occurring via debonding process, is the important energy-dissipation process and leads to a toughened, biodegradable polymer blend. The conclusion is that the compatibility between the dispersed PBC phase and PLA matrix in the blending process is not neccessary for toughness, but for obtaining a fine dispersion of the dispersed phase. The important point is that the toughening mechanism requires only modest level of adhesion between particles and the polymer. The molecular mobility is a crucial factor for yield stress and plastic flow.
Figure 6. (a) Schematic diagram of the measurement locations B, C, and D of the SEM micrographs of the PLA/PBC blend (80/20) during the tensile testing; (b) morphology in region B; (c) morphology in region C; and (d) morphology in region D. The arrow indicates the stretching direction (20-μm scale bar).
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The impact strength of the PLA/PBC blends was also significantly increased, from 5.6 kJ/m2 for neat PLA to 25.1 kJ/m2 for the blend containing 30% PBC, as shown in Figure 7. The SEM micrographs of the impact-fracture surface, shown in Figure 8, can also confirm the toughening mechanism that the plastic deformation occurs via a single cavitations process inside the rubber particles. The neat PLA shows typical brittle fracture and the surface is very smooth. With increasing PBC content, the impact-fracture surfaces show more evidence of ductile fractures as more and longer fibrils can be observed. The important energy dissipation processes, involved in the impact fracture of toughened polymer, include crazing, cavitation, shear banding, crack bridging, and shear yielding. For the composites with 20 and 30% PBC contents, the impact caused not only fibers but also cavitations and a clear matrix deformation. Moreover, the extensive plastic deformation implied that the shear yielding of the PLA matrix has taken place.
Figure 8. SEM micrographs of the fracture surfaces of the PLA/PBC blends: (a) 100/0, (b) 90/10, (c) 80/20, and (d) 70/30 (20-μm scale bar).
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