3.1 Preliminary Considerations, Morphology, and Crystallinity
By considering the low density of hydroxyl groups on their surface compared to other silicates, consequently to lower interfacial forces of adhesion between nanotubes (e.g., by hydrogen bonds) the HNTs are considered easy dispersible in different polymers even using traditional methods, such as themost widely used melt-compounding technique. On the other hand, due to the hydrophilic nature of HNTs, in the literature different surface treatments of the nanofiller with hydrophobic organo-modifiers are reported, such as the functionalization with organosilanes.[20-22, 32]
In order to further increase the dispersion ability of nanotubes into PLA and to improve the affinity between the dispersed HNTs and the polyester chains, the additional treatment of HNT with selected silanes (e.g., TMSPM) has been performed. Furthermore, it is assumed that additional Si-O-Si layers can be formed by reaction of silanol molecules with each other to give a multimolecular structure on the surface of filler. The formation of additional alkyl layers, which will behave as interfacial zone playing an effectively compatibilizing role, will lead to hydrophobicity and improved nanofiller dispersion, factors that can finally affect the barrier and transport properties to water molecules. In relation to the morphology of nanocomposite films, it is important to precise that the systems contain a continuous phase represented by PLA matrix and as dispersed phase a “2D” nanofiller characterized by high aspect ratio. As it is illustrated in Figure 1a, HNTs assess a nanotubular morphology, with outer tube diameter of nanometric dimension, whereas the length of nanotubes was found ranging typically from hundreds of nanometres to about 1.5 µm. In nanocomposites, at loading of 6 and 12%, both nanofillers (additionally treated or not) are quite well distributed and dispersed through PLA matrix (Figure 1b–e), since the presence of large aggregates is difficult to be observed in the TEM images. However, using HNT(s), the improvement in dispersion is seen especially in the highly filled nanocomposites (e.g., at 12% loading, Figure 1e compared to Figure 1d). Lastly, the limited intertubular contact area and rod-like geometry, the lower number of hydroxyl groups on the surface of nanotubes, the additional surface treatment which decreases the surface free energy and nanotube/nanotube interactions, can be considered as key-factors that explain the good dispersion of HNT and HNT(s) into PLA.[20, 23, 29] As stated in introduction, the PLA nanocomposite films with multifunctional characteristics (mechanical, anti-UV, antibacterial, electrical, gas barrier properties, etc.) are potentially of high interest as packaging biomaterials.[15, 18, 19, 34, 35] In relation to the transport properties of PLA films, it is important to have information about their level of crystallinity. Following the results reported in the literature,[36, 37] it is generally expected that the changes and/or differences of crystallinity, will significantly affect the water vapor transmission rate (WVTR) properties. Furthermore, in semi-crystalline polymers, the crystalline regions are considered to be impermeable to the vapor molecules, and in this context it is of interest to have information about the morphology and structures of initial samples, as they are evaluated by the different techniques.
Figure 1. TEM images at different magnifications to illustrate the morphology of HNT nanotubes (a) and those of nanocomposites containing 6% (b and c) and 12% (d and e) nanofillers. The black circles in Figure 1d evidence the presence of some HNT aggregates in the highly filled nanocomposites (12HNT sample).
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The quantification of PLA crystallinity of films was carried out mainly by DSC, whereas XRD was used as additional tool of investigation. Figure 2a and b shows the DSC curves of all nanocomposite films compared to those of pristine PLA, whereas the calorimetric parameters are summarized in Table 2. From the calorimetric data it comes out that all samples are characterized by evident crystallization exotherms (ΔHc), which is a confirmation of the amorphous or low crystalline starting structures. Then, the glass transition temperature (Tg) and the endothermic enthalpy of relaxation (ΔHrel) show only minor modifications with nanofiller loading. The enthalpic relaxation, which is typical for PLA in the glassy state undergoing some physical aging, is exclusively related to the amorphous phase of the polymer. Furthermore, it was reported that the processing parameters such as mold temperature, packing pressure, cooling rate, etc., can also significantly influence PLA aging behavior as well. The multiple Tm peaks on DSC curves can be ascribed to the melting of crystalline regions of various size and perfection formed during cooling and crystallization processes, without excluding the influence of additional factors that can affect the melting of PLA as mentioned elsewhere. It is of interest to remark that in nanocomposites the increase of nanofiller loading is correlated with the decrease of Tc values, and accordingly, with some changes in the kinetics of PLA crystallization. However, by considering the low crystallization ability of PLA,[38-40] the high cooling rate during compression molding and the low thickness of samples as films, it is not surprising that in nanocomposites the χc is limited to relatively low values (χc < 10%), only slightly affected by the increase in nanofiller loading. Following the DSC analysis and by considering also the low value of χc of the neat PLA (5.2%), it is evident that the analyzed films were mainly characterized by an amorphous structure (low crystallinity), conclusion also supported by the results of XRD investigations.
Figure 2. DSC traces obtained during first heating scan (10 °C · min−1) of films: (a) PLA and PLA/HNT nanocomposites; (b) PLA and PLA/HNT(s) nanocomposites.
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Table 2. Comparative DSC data (first heating, 10 °C · min−1) of PLA and PLA nanocomposite films (samples after the compression molding process; all enthalpy values were normalized considering only the polymer mass in the sample)
|Entry||Sample code||Tg [°C]||ΔHrel [J · g−1]||Tc [°C]||ΔHc [J · g−1]||Tm [°C]||ΔHm [J · g−1]||χc [%]|
Figure 3a and b shows the XRD patterns of neat PLA films and those of all studied nanocomposites. All samples show a broad intensity with maximum appearing at approximately 2θ = 17° indicating mainly an amorphous structure. Furthermore, according to the XRD measurements and consistent with the DSC results, the samples filled with both nanofillers [HNT and HNT(s)] can be considered mainly amorphous or only low crystalline because the XRD spectra do not evidence the presence of significant traces of crystallinity. Moreover, the diffraction peaks evidenced at 2θ = 12.3, 20.1, and 24.6° are characteristic for halloysite nanofiller and are ascribed to (001), (020), and (002) reflection planes, respectively,[41, 42] whereas their intensity is clearly increasing with the nanofiller loading. For more clarity, it is essential to remind that the presence of interlayer water in HNTs is one of the most important features distinguishing HNTs from kaolinite. Allowing the state of hydration, HNTs are generally classified into two groups: hydrated HNTs with a crystalline structure of 10 Å (d001) spacing and dehydrated ones with 7 Å (d001) spacing. Thus, with the dehydration process, the d001 spacing of HNTs changes from 10 to 7 Å. Accordingly, the diffraction peaks recorded at approximately 2θ = 12.3°, which is related to (001) plane (basal distance around 7.2 Å), is indicating that the HNTs used in this study are rather dehydrated. Following these results, because the investigated PLA films were mainly amorphous, in the discussion of experimental data it was considered that the crystallinity is not an additional parameter that can significantly determine the transport properties and permeability to water vapor, but any reduction in permeability can more likely be attributed to the presence of the nanofiller itself.
3.2 Barrier Properties to Water Vapor
The nanofillers of different morphology (platelets, nanotubes, fibrous, etc.) can provoke a “tortuosity” (the term was used primarily in the case of OMLS) and introduce supplementary constraints, which result in longer path length in the diffusion of water vapor molecules, pure or mixed gas, finally leading to changes of PLA inherent permeation properties.[15, 19, 34, 35, 37, 43, 44] Obviously, a sheet-like morphology is particularly efficient in maximizing the path length due to high aspect ratio when compared to other nanofiller shapes such as spheres, cubes, etc., but additionally other factors can affect the permeability proprieties: loading of nanofiller, orientation, state of aggregation, surface treatments, etc. However, the adequate chemical modification of the nanofiller can represent a key factor, and the following example reported in literature is considered as relevant for the application of PLA based nanocomposites in packaging, by choosing the proper type of nanoclay and its optimum concentration. Accordingly, PLA nanocomposites have been prepared using two OMLS based on the same montmorillonite (MMT) commercially available (trade mark Cloisite). After the dispersion into PLA and realization of films, Cloisite 20A was found more effective in the reducing of water vapor permeability than Cloisite 30B, since the former was more hydrophobic than the latter. In relation to the barrier properties conferred by HNTs, in general this information is missing due to the relative novelty of the nanofiller. Noteworthy, it was recently reported that the presence of 5% HNTs in the epoxy resins is leading to the decrease of the water absorption by 20.1% when the nanocomposites are compared to the neat epoxy.
Sorption (S), diffusion (D) and permeability (P) to water vapor were analyzed on all film samples at the temperature of 30 °C, varying the water vapor pressure.
3.2.1 Sorption (S)
Figure 4a and b reports the equilibrium concentration of water vapor (Ceq) as function of water pressure (p) for the neat PLA and PLA nanocomposite films. All samples show an ideal behavior at low pressure (up to about 0.03 atm) that allowed evaluating the sorption coefficient (S) from Henry's law,
Figure 4. Sorption isotherms of PLA and PLA/HNT nanocomposites (a) and those of PLA and PLA/HNT(s) nanocomposites (b).
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For increased water vapor pressure (p > 0.03 atm), the isotherms deviate from linearity following a Flory-Huggins model of sorption. According to this behavior there is a preference for the formation of penetrant-penetrant pairs, so that the solubility coefficient continuously increases with penetrant pressure. The first sorbed molecules tend locally to “loose” the polymer structure, making easier the access of subsequent water molecules to enter in the proximity of the first one than to go elsewhere. These particular isotherms are observed when the penetrant effectively plasticizes the polymer, being as a strong solvent or swelling agent, like are the water vapor molecules for PLA matrix. Table 3 reports the S values (wt% · atm−1) determined according to Equation (1) for all samples. It comes out that is noticeable a decreasing of sorption in all nanocomposites, despite the presence of more or less hydrophilic nanofiller [i.e., HNT or HNT(s), respectively], although there is no a decreasing with halloysite loading.
Table 3. Sorption parameters, S evaluated from the isotherms of Figure 4 using Equation (1)
|Entry||Sample code||Sorption [wt% · atm−1]|
Figure 5a and b report the diffusion coefficients, D (cm2 · s−1), as function of the equilibrium sorbed water, Ceq (wt%), for both sets of nanocomposites. It is well known that for polymer-solvent systems the diffusion is correlated to equilibrium solvent content, Ceq, by the empirical law given by
where Do is the zero concentration diffusion coefficient (related to the fractional free volume and to the microstructure of the polymer) whereas γ is a coefficient which depends on the fractional free volume and on the effectiveness of the penetrant to plasticize the matrix.
Figure 5. Diffusion coefficients versus Ceq (wt%) of water for PLA and PLA/HNT nanocomposites (a) and those of PLA and PLA/HNT(s) nanocomposites (b).
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Both sets of nanocomposites follow Equation (2) up to Ceq of sorbed water of ≈0.35%, whereas after this value the diffusion is independent of concentration. As observed from the sorption isotherm curves, a solvent plasticizing effect was evident at vapor pressure ≥0.02 atm. The strong interaction of the PLA matrix with the penetrating molecules, leading to a high mobility of polymer chains, can induce structural transformations as clustering of solvent molecules, crazing, or partial dissolution. These systems lose their compactness and the diffusion becomes less dependent or even independent of the amount of water vapor absorbed. Following Equation (2), it was possible to extrapolate the thermodynamic diffusion coefficient, Do, for all samples in the concentration range 0–0.4%. The Do values are reported in Figure 6 as function of HNT and HNT(s) loadings (curves a and b, respectively). The nanocomposites containing HNT show a decrease of Do evidenced even by addition of low loading ofnanofiller (at 3%), than a less sharp decrement in the range 3–12%. On the other hand, the nanocomposites containing silane treated nanofiller show for 3% HNT(s) quite similar Do with respect to the unfilled PLA, and a sharp decrease of Do in the range 6–12%. The treatment with silane could lead to a better and homogeneous dispersion of HNT(s) into the polymer matrix in the range 6–12%, assumptions to some extent supported by the results of TEM investigations. Such experimental data allow demonstrating that the chemical modification of HNT with silanes has a significant role in determining the filler dispersion and in modification of interfacial regions PLA - dispersed phase. Following the literature on this subject, i.e., WVTR in nanocomposites, these factors can effectively affect the permeability properties.[49, 50] This is evidenced by a decrease of permeability, in particular of Do coefficients.
Figure 6. The evolution of D0 thermodynamic diffusion coefficient for nanocomposite films as function of the loading of HNT and HNT(s).
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3.2.3 Permeability (P)
The permeability of the samples to the water vapor was calculated according to Equation (3), as the product of sorption and diffusion:
It represents the thermodynamic P parameter, considering that we used the zero-concentration diffusion coefficient and the derivative of the equilibrium concentration versus pressure (at low vapor pressure). Although it is not a technological parameter, that can be only determined in the use conditions of the material, it is of fundamental importance with the purpose of correlating the physical properties to the structure of the samples. Figure 7a and b shows the evolution of permeability values, P [(wt% · atm−1) × (cm2 · s−1)] for both sets of nanocomposites. The nanocomposites containing HNT show a decreasing of P already for a loading of 3%, which is followed by a kind of plateau up to 12% (Figure 7a). The nanocomposites filled with the silane-treated nanofiller show a quite linear decreasing of P in the whole investigated composition range (Figure 7b). Accordingly, for nanocomposites containing 6 and 12% HNT(s), the effect of silane surface-treatment is demonstrated to be a promising strategy to make the PLA polymer matrix less permeable to water vapor. Moreover, as mentioned before, the improvements in barrier properties obtained with these nanofillers can be also ascribed to the reduction of matrix volume fraction for water diffusion, differences in the diffusivity and solubility of the dispersed and continuous phases, modification of the interface regions, etc.[49-51]
Figure 7. Ideal permeability (P), as product of thermodynamic diffusion coefficient (D0) and sorption evaluated following ideal Henry's behavior, to water as function of (a) HNT and (b) HNT(s) loading (wt%).
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3.3 Thermal Stability and Specific End-Use Characteristics
To highlight the effect of HNT and HNT(s) addition on the thermal stability of films, TG measurements under air of nanocomposites were compared to those of pristine PLA. For simplification and easier interpretation, Table 4 summarizes the temperatures for 5% weight loss (T5%), often considered as the initial decomposition temperature, and those of the max. rate of degradation (TD). It comes out that once more, the nanocomposites show comparable thermal stability (T5% as parameter) with respect to the unfilled PLA (T5% = 344 °C), since the addition of HNT or HNT(s) leads to differences of maximum 2 °C (decreasing or increasing in PLA thermal stability, respectively). Furthermore, TD results only slightly improved with silanization treatment, as evidenced for samples loaded with 6 and 12% HNT(s).
Table 4. TGA data of PLA, PLA-HNT and PLA/HNT(s) nanocomposites (under air flow, 20 °C · min−1)
|Entry||Sample||Temperature of 5% weight loss [°C]||Temperature of the max. rate of degradation (from d-TG) [°C]|
Also, the hydroxyl groups contained by the inorganic filler can play an important role in the hydrolytic degradation of PLA, especially at higher temperature. Thus, the slight improvement in PLA thermal stability using HNT(s) is attributed to the changes in hydrophobicity, the nanofiller being less sensitive to moisture, as well as to the reduction of hydroxyl groups. This also demonstrated that no degradation in PLA occurred after compounding at high temperature.
However, as suggested in previous studies, since these nanotubes display lower aspect ratio and specific surface area than most of the OMLS, it is reasonable to assume that the thermal barrier effect of HNTs may somewhat be inferior to those of layered silicates.
Regarding the opacity, this is a significant parameter that must be considered of interest in packaging applications since it affects the appearance of the products and it is seen as possibility to better visually mask the content of packaged products, or as opportunity for thinner packaging materials. The opacity is the direct result of the light absorption and scattering. Light scattering is accomplished by refraction and diffraction as the light passes through or near nanofillers. However, PLA and HNTs show differences between the refractive indices, respectively, values of 1.35–1.45 and 1.54.
As shown in Figure 8a and b that report the modification of the degree of opacity (O) of films by increasing the nanofiller loading [i.e., HNT or HNT(s), respectively], O shows a linear increase that is directly correlated to nanofiller percentage. Furthermore, O values were nearly 3 times higher for the sample 12HNT(s) with respect to the neat PLA. The more is nanofiller loading, the more light is scattered outward, yielding opaque films. This effect, usually obtained by addition of pigments such as TiO2 in polymers, must be taken into account since it could be a key parameter for some packaging applications. Additionally, the mechanical parameters of films (Table 5) were evaluated following the tensile tests performed under normal room conditions. Briefly, the addition of HNT or HNT(s) into PLA leads to the slight increase of the rigidity that is typically in correlation with the nanofiller loading (Young's modulus in the range 1550–1770 MPa, compared to 1500 MPa for the neat PLA), whereas the tensile strength at yield (σy) revealed interesting values for appliscations such as packaging (i.e., σy in the range 40–50 MPa) as they were compared to the neat PLA matrix (σy of about 50 MPa). The differences ascribed to the modification of nanofiller/polyester interface by silanization treatment (i.e., utilization of TMSPM) have concerned mostly the percent elongation at break (ϵb), the PLA/HNT(s) nanocomposites showing slightly higher ϵb than those of PLA-HNT (ϵb in the range 11.0–12.4% with respect to 7.5–9.9%, respectively), values that were found to be less influenced by the increase of nanofiller loading. Accordingly, it is assumed that the treatment of nanotubes with TMSPM helps to retain better values of ϵb, ascribed mainly to the changes of interfacial properties and somewhat to better dispersion of the nanofiller, i.e., HNT(s). However, PLA-HNT(s) films have shown slightly lower ϵb than those of the neat PLA films (ϵb of about 16%). Knowing that for utilization of PLA in packaging applications it is usually required the utilization of selected additives to improve this parameter of interest, i.e., the elongation at break, in this main goal the addition of plasticizers into PLA/HNT nanocomposites can be supplementary considered. Furthermore, a forthcoming paper will analyze the effect of HNT(s) addition on the barrier properties of PLA nanocomposite films to some common gases, i.e., oxygen and carbon dioxide.
Table 5. Comparative mechanical properties of PLA, PLA–HNT and PLA–HNT(s) films (at crosshead speed of 10 mm · min−1)
|Sample||Young's modulus [MPa]||Tensile strength at yield (σy) [MPa]||Tensile strength at break (σb) [MPa]||Elongation at yield [%]||Elongation at break [%]|
|PLA||1500 (±100)||50 (±4)||48 (±2)||4.0 (±0.5)||16.0 (±9.5)|
|3HNT||1600 (±160)||40 (±6)||37 (±5)||2.8 (±0.4)||9.9 (±2.7)|
|6HNT||1770 (±130)||42 (±7)||40 (±7)||2.8 (±0.3)||8.0 (±1.3)|
|12HNT||1730 (±110)||48 (±10)||47 (±8)||3.0 (±0.9)||7.5 (±2.4)|
|3HNT(s)||1550 (±100)||48 (±7)||47 (±5)||4.0 (±0.8)||12.0 (±3.0)|
|6HNT(s)||1650 (±90)||42 (±9)||40 (±6)||2.9 (±1.0)||12.4 (±1.8)|
|12HNT(s)||1670 (±110)||40 (±5)||40 (±4)||3.0 (±0.4)||11.0 (±4.0)|