Cellulose Functionalized High Molecular Weight Stereocomplex Polylactic Acid Biocomposite Films with Improved Gas Barrier, Thermomechanical Properties

This work presents a facile, solvent-free approach for the fabrication of PLA biocomposites, followed by melt extrusion process to prepare stereocomplex PLA films with excellent thermomechanical and gas barrier properties. The presence of stereocomplex crystallites improves the thermal properties of polylactic acid (PLA); however, the formation of stereocomplex crystallites is predominantly lesser compared to homocrystallites in case of high molecular weight poly(l-lactic acid) and poly(d-lactic acid) blend. Grafting of biofillers with polymer matrix chains may help in homogeneous dispersion and formation of stereocomplex crystallites. Henceforth, stereocomplex PLA was fabricated with cellulose microcrystals (CMC) as filler, after chemical modification by in situ ring opening polymerization of d-lactide. The stereocomplexation in the blend system was found to be enhanced by the extended molecular surface area provided by grafted CMC. As confirmed by morphological analysis, the modification of CMC drives the homogeneous dispersion into the matrix and reduction in the size of CMC in the range of ∼200 nm diameter. Increased melting temperature (∼209 °C) with no evidence of homocrystallites confirm the role of grafted CMC in the formation of stereocomplex crystallites by suppressing the development of homocrystals. The fraction of stereocomplex crystallites was found to be 100% when analyzed using X-ray analysis. The enhanced stereocomplexation in the composites resulted ∼96% improvement in the tensile strength in comparison to pristine PLLA/PDLA blend. Interestingly, the oxygen permeability and water vapor permeability were reduced by ∼25% and ∼35%. The improved thermomechanical properties of the biocomposites through enhanced stereocomplexation may comply with the requirement for high temperature engineering and packaging applications.