Stiff and thermal stable PLA/nanocellulose biocomposites prepared by Pickering emulsion method and cold crystallization

Development of biodegradable polymer composites with superior properties is a promising way to mitigate plastic pollution. In the present study, poly(L-lactic acid) (PLLA, as the bio-based matrix) and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofiber (TOCNF, as the bio-based filler) were combined by Pickering emulsion method to prepare PLLA/TOCNF composite microspheres. The yarn-ball-like microspheres had diameters of approximately 0.5-8.0 mu m. The composite microspheres were hot-pressed to prepare PLLA/TOCNF composite films with layered structures. The composite films with 7.5% and 10% TOCNF addition amounts (PT7.5 and PT10 films) further underwent cold crystallization to increase the crystallinity of the PLLA matrices to 28-30%. The neat PLLA film exhibited Young's modulus (YM) of 2.8 GPa, contact angle (CA) of 69.7 degrees, and coefficient of thermal expansion (CTE) of 12923 ppm K-1. The cold-crystallized PT7.5 (C-PT7.5) film exhibited superior properties such as YM of 4.0 GPa, CA of 85.0 degrees, and CTE of 239 ppm K-1. The parameters were 142.9%, 122.0%, and 1.8% of those of the PLLA film, respectively. Hence, the properties of the PLLA film can be simultaneously significantly improved by adding the TOCNFs and cold crystallization, enhancing the potential of the PLA-based materials for packaging applications.