Modification of Poly(lactic acid) by Melt Blending

Abstract

Poly(lactic acid) (PLA), a renewable, biodegradable, and biocompatible polyester, is one of the most successful solutions to revolutionize renewable plastic production. Nevertheless, application of pristine PLA is largely limited by its brittleness in solid state and low melt strength in melt state. Various strategies have been developed to improve the performance of PLA, among which melt processing is the most viable and economical for industrial use. This thesis covers several independent aspects of PLA modification and presents some new possibilities in melt processing of PLA and PLA-based blends. Chapter 2 and Chapter 3 focus on branching of PLA with multifunctional aziridine to improve melt strength. Multifunctional aziridine as a branching agent has its advantage in fast reaction kinetics that leads to stable final product properties. Extensional rheology was extensively used to clarify the correlation between melt strength and chain structure. Chapter 4 seeks to toughen PLA by blending with poly(ethylene oxide)-poly(propylene oxide )-poly (ethylene oxide) (PEO-PPO-PEO) triblock copolymers, which are commercially known as Pluronic® (by BASF). Pluronic copolymers with large PPO block size and low PEO content can increase the elongation of break from 5% to more than 100% at a low loading of 5 wt.%, along with the additional benefits of reduced blend viscosity and easy mold-release. Chapter 5 and chapter 6 are less property-oriented, focusing on cocontinuous immiscible polymer blends. In Chapter 5, carboxylic acid/oxazoline reaction is used to compatibilize cocontinuous PLA/polystyrene (PS) blend. Reactively formed interfacial graft copolymer reduced the phase domain size to submicron scale, which is hard to achieve in melt-processed cocontinuous blends. Hierarchically porous PLA, with primary pore size 5 – 20 µm and secondary pore size 0.5 – 2 µm, was further made from compatibilized PLA/PS/linear low density polyethylene (LLDPE) blend after selective extraction of PS and LLDPE. By studying the wetting behavior of ternary PLA/PS/PE blend, we confirmed that PLA/PE interfacial tension is much higher than PLA/PS interfacial tension. Chapter 6 uses branched PE and PLA to study the effect of extensional viscosity on cocontinuity formation in immiscible polymer blends. Blending with two branched polymers broadened the range of cocontinuity. This was attributed to the ability of a strain hardening matrix to promote elongation, and hence percolation, of the minor phase.