Development of poly(lactic acid) and starch nanocrystals based nanocomposites

The materials used as polymeric packaging worldwide are derived from fossil-based resources. The issue with the use of such materials is that during manufacturing, they release noxious gases, and these materials are not biodegradable when discarded after their use.  Therefore, bio-based materials are required, which can replace or reduce the dependency on conventional polymer-based materials. Mostly the bio-based polymers such as poly(lactic acid), polycaprolactone, and polyhydroxyalkanoates are not suitable for commercial applications on their own. Nanocomposites produced by embedding nano polysaccharides into biodegradable polymers such as poly(lactic acid) can be beneficial to cope with global issues of environmental pollution. The nanofillers are added with the intention to improve the overall performance of the polymer in which they are incorporated. These nanocomposites have attracted significant attention from researchers due to their outstanding thermal, mechanical, barrier, and rheological properties when compared to conventional polymers. However, poor miscibility of the nanofiller in the polymer matrix and limited studies focusing on the effects of processing conditions of such nanocomposites, have limited its commercial application. This thesis reports on research to develop PLA based nanocomposites with improved properties by incorporating starch nanocrystals (SNC) and acetylated starch nanocrystals (Ac-SNC). Initially, the effect of processing conditions on neat PLA biopolymer has been studied. PLA films were fabricated using two different processing methods, which include solvent casting (SC) and melt processing (MP). Following that, the effect of adding SNC on the overall performance of PLA was studied by preparing the PLA(SC)-SNC and PLA(MP)-SNC nanocomposites. The optimum concentration of SNC, which can improve the properties of PLA, was determined. Subsequently, the SNCs were surface modified, and these acetylated SNC (Ac-SNC) were added to PLA. The prepared nanocomposites were characterized to determine the effect of nanofiller on the properties of PLA, using a range of characterization techniques. The morphology and crystallinity study for neat PLA indicated that PLA(SC) had a rough surface when fractured with crystalline regions, while the PLA(MP) had a smooth surface and a broad amorphous XRD peak. Polarised light microscopy revealed the presence of spherulites within the PLA(SC), which was absent in the PLA(MP). The rheology study additionally showed that the loss modulus for PLA(SC) was higher than PLA(MP) in the lower frequency range; however, this trend appeared to be reversed in the high-frequency range. Synchrotron-FTIR (S-FTIR) microspectroscopy revealed a distinct absorption band at 921 cm-1 and a shift of carbonyl peak to a higher wavenumber in the PLA(SC) associated with the crystallization of the spherulites that led to differences in molecular chain orientation and thereby the molecular interaction. The results obtained from various characterization techniques provided strong evidence of the effects of the solvent casting and melt processing techniques on a number of important properties of the two PLA films, as a result of differences in molecular orientation and interactions. SNC were synthesized using the most widely used acid hydrolysis method. PLA-based nanocomposites with SNC as nanofiller were prepared by solvent casting and melt processing technique. The SNCs were incorporated in PLA at three different loadings (1, 3, and 5 wt%). The shape and crystallinity of raw starch granules and SNC were characterised using SEM, TEM, and XRD. The results confirmed that SNC particles were in the nanoscale (50-100 nm) range and that the SNC had squared platelet morphology, which is the characteristic of the A-type crystalline starch. The morphological study through SEM and TEM of PLA(SC)-SNC nanocomposites showed that SNC dispersed well at lower loadings (1-3 wt%), above which agglomeration would occur. The XRD data depicted that the crystalline regions in PLA remained unaltered due to the small concentrations of SNC (1-5 wt%). TGA revealed that all the PLA(SC)-SNC nanocomposites maintained their thermal stability in the range of 347-355 °C. Enhanced thermal behaviour was noted in MDSC thermograms. Detailed investigation revealed that the percentage crystallinity was higher for PLA(SC)-SNC-3 wt% than the rest of the composites. Rheological properties of the PLA(SC)-SNC nanocomposites were significantly affected by the increase of SNC loading up to 3 wt% SNC, particularly in the low-frequency region. A further increase of SNC concentration decreased the storage modulus of the PLA(SC)-SNC nanocomposites slightly in the low-frequency region. All results obtained from morphology, thermal, and rheological confirmed that the 3 wt% SNC loading was the optimum concentration, which could be used to produce well-dispersed PLA(SC)-SNC nanocomposites. In summary, this research focused on developing and examining PLA based nanocomposite films with SNC and Ac-SNC as nanofillers, for their suitability for packaging applications. The incorporation of SNC and Ac-SNC in PLA has a prominent effect on the mechanical and barrier properties of the nanocomposite, even at lower loadings of 1 and 3 wt%. With the intention to further enhance the dispersion of SNC in PLA, a surface modification to obtain Ac-SNC was performed. The two nanofiller, SNC, and Ac-SNC, were incorporated in neat PLA(SC) at two different concentrations (1 and 3 wt%) through the solvent casting technique. The morphological examinations revealed that the PLA(SC)-SNC nanocomposites are rougher as compared to the PLA(SC)-Ac-SNC nanocomposites, due to better well-dispersed Ac-SNC in PLA. The surface modification created hydrophobic surfaces on Ac-SNC and favoured the dispersion of this nanofiller in the PLA matrix. The oxygen permeability and mechanical testing results indicated that both the nanofillers, SNC, and Ac-SNC have the potential to improve the oxygen barrier and tensile properties of PLA(SC) nanocomposite. As the SNC has a squared platelet shape, which creates tortuous diffusive paths for penetrant molecules, they are considered as suitable nanofillers for food packaging applications. The rheological investigations underlined the positive effect of surface modification of SNC as the rheological properties were improved for PLA(SC)-Ac-SNC nanocomposites compared to PLA(SC)-SNC nanocomposites. The current research highlighted the potential of SNC and Ac-SNC as nanofillers to develop sustainable, low-cost and high-performance nanocomposites.