Modulation Of Structure And Properties Of Poly(Lactic Acid)-Based Blends By Aluminosilicate Nanosheets

Polylactic acid(PLA)is the most commonly utilized biodegradable polymer,which possesses excellent processing properties,mechanical properties and transparency.However PLA has some major disadvantages such as poor toughness,inferior crystallization ability,low heat resistance and limited water vapor barrier performance.Polymer blending and nanocomposite are the most effective and efficient strategy to fabricate desired PLA-based materials with simultaneous enhanced overall performance.Nanosheets with anisotropic features can modulate the phase morphology(e.g.,domain size and distributions,interfacial curvature)of immiscible blends while providing some intriguing functionalities.The structure and properties of polymer composites are closely correlated with the dispersion of nanosheets.However,the underlying correlation remains elusive.In this thesis,multicomponent polymer composites of PLA and aluminum silicate nanosheets(Als-NS)complemented with a second biodegradable polymer component,namely poly(butylene adipate/terephthalate)(PBAT),polymethyl ethyl carbonate(PPC),were prepared by melt blending.The effects of Als-NS on the morphology and properties of the biodegradable polymer nanocomposites were systematically investigated.This work aims to explore the critical factors of the dispersion of nanofillers and establish the correlation of nanofiller loading/dispersion with the microstructure and properties of polymer composites.The results of this work will help design and prepare high-performance PLA-based nanocomposites with improved mechanical properties,thermal resistance,water vapor barrier property,etc.Following are the main results of this work:(1)Dispersion of Als-NS and its effect on the properties of the polymer matrix:Als-NS was introduced into PLA,PBAT and PPC by melt blending and the dispersion of Als-NS in different polymers was evaluated.The effects of Als-NS on the crystallization behaviors and thermal stability of the polymers were investigated.AlsNS uniformly dispersed in the polymers and effectively improved the mechanical strength of the polymers.Meanwhile,Als-NS acted as the nucleating agents to enhance the crystallization ability of PLA.(2)Structure and performance of PLA/PBAT/Als-NS composites: Als-NS were introduced into the PLA/PBAT blends by melt blending to prepare PLA/PBAT/Als-NS nanocomposites.The effects of the Als-NS content,the component composition and the processing conditions on the distribution and dispersion of Als-NS were systematically investigated.Theoretical calculation of the surface energy and wetting coefficient reveals that Als-NS are thermodynamically stable at the PLA/PBAT two-phase interface.The shear flow during processing promoted the migration of AlsNS and the localization at the interface.The localization of Als-NS was dominated by the domain size(for the typical “sea-island” phase structure)when it is small.For small domains,the interface failed to accommodate Als-NS,resulting in the location of AlsNS in the “sea” phase.The selective localization of Als-NS enhanced the mechanical strength,crystallization ability and water vapor barrier properties of the polymer composites.(3)Structure and properties of PLA/PPC/Als-NS composites: Als-NS were introduced into the PLA/PPC blends by melt blending,and the effects of Als-NS dispersion on the mechanical properties,crystallization behaviors,thermal stability and water vapor barrier properties of the composites were systematically investigated.The incorporation of Als-NS induced the transformation of a “sea-island” phase structure of a PLA/PPC(50/50)blend into a co-continuous phase structure.The Als-NS enriched at the interface between the two phases of the co-continuous phase structure produced a network to impede water vapor and enhanced the water vapor barrier performance of the polymer composites.