Nanoamphiphilic Chitosan Dispersed Poly(lactic acid) Bionanocomposite Films with Improved Thermal, Mechanical, and Gas Barrier Properties

This article demonstrates the synthesis of lactic acid oligomer-grafted-chitosan (OLLA-g-CH), a nanoamphiphilic molecule, by in situ condensation polymerization and its effective use as a nanofiller for improvement in multiple properties of poly(lactic acid) (PLA) films, essential for stringent food packaging applications. Fourier transform infrared spectroscopy (FTIR) analysis shows the presence of amide-ester bond at 1539 cm(-1), which confirms the structural grafting of OLLA chains with chitosan molecules. This nanoamphiphilic OLLA-g-CH molecule act as surfactant containing hydrophilic chitosan head and hydrophobic OLLA tails with average size in the range of ∼2-4 nm. Prepared PLA/OLLA-g-CH bionanocomposite films appear with uniform dispersion of nanoamphiphilic OLLA-g-CH molecules with self-assembled micelles having size as low as ∼20 nm and as high as ∼150 nm with core-shell morphology in PLA matrix. This nanofiller is found very effective toward significant reduction in oxygen permeability (OP) by ∼10-fold due to the reduction in solubility of oxygen molecules and improvement in crystal nucleation density due to availability of nanonucleating sites. Ultimate tensile strength (UTS) of PLA/OLLA-g-CH bionanocomposite films are relatively comparable to that of PLA, however, elongation at break is improved significantly. The onset of thermal degradation of PLA/(OLLA-g-CH) films is also found comparable to that of PLA film. The glass transition temperature (Tg) of bionanocomposites is decreased by more than 18 °C with increase in OLLA-g-CH loading, which indicates the improved plasticization characteristics of PLA matrix. The crystallization kinetics suggest nonthree dimensional truncated spherical structures, which is controlled by the combination of thermal and athermal instantaneous nucleations. POM analysis suggested that the spherulite growth of PLA is improved significantly with the addition of OLLA-g-CH. The reduction in Tg of PLA with improvement in elongation at break and multifold reduction in oxygen permeability offers this bionanocomposite films, a promising candidate for stringent food packaging applications.