High Barrier and Flexible Biodegradable PLA/CNC Based Multilayer Films via Large-Scale Forced Assembly Layer Multiplication Coextrusion: Role of Crystalline Polymer Layers Under Confinement

In this work, a novel and pioneering route was developed for the large-scale fabrication of PLA-based multilayer films with high barrier properties by forced assembly layer multiplication coextrusion. The process involved coextruding cellulose nanocrystal (CNC)-filled polylactide (PLA) biocomposite, obtained by a liquid feeding method, with highly crystalline poly(3-hydroxybutyrate-<i>co</i>-3-hydroxyvalerate) (PHBV) into alternating multilayer structures with a number of layers up to 513. The degree of CNC dispersion in the PLA/CNC-based composite and the multilayer architecture were first evaluated by TEM observations. Subsequently, the role of PHBV layers under confinement on the crystallization behavior and gas permeability was thoroughly investigated. It was found that the obtained films basically maintained their multilayer structure and architectures, with CNC particles overall well-dispersed at a mean length of 318 nm. Nevertheless, layer instability and breakup began to occur at 129 layers due to the formation of microscale CNC aggregates. The geometric confinement effect resulted in a gradual restriction of crystallization behavior of both PLA and PHBV phases/layers as the number of layers increased. Notably, an increase in the oriented edge-on lamellar/crystal structure in the PHBV layer along the normal direction was detected. Consequently, a remarkable reduction in oxygen transmission rate (OTR) was realized when increasing the number of layers and confinement. Additionally, multimicro/nanolayers with the large number of layers exhibited higher flexibility, while maintaining a considerable tensile strength. In conclusion, this study provides the innovative and novel solution for the continuous melt-processing of biodegradable flexible films with enhanced barrier properties, making them highly suitable for food packaging applications.