Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) quality and property control by engineering immiscible copolymer blends

Abstract

Plastic waste motivates the development of products and services from renewable, biodegradable polymers, like polyhydroxyalkanoates (PHAs). Approaches for quality control and engineering of PHA property specifications (i.e. crystallinity, crystallization rate, mechanical properties, processability, etc.) are going to be needed for industrial scale production. Methods of PHBV, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), extraction from biomass with non-halogenated solvents were applied to formulate immiscible PHBV copolymer blends. The goal was to test, in principle, if material properties could be controlled as part of the step of PHBV extraction since solution blending is anyway inherent to the biomass extraction process. Homogenous solution blends with average 3-hydroxyvalerate (3HV) content from 0 to 38 wt% were formulated in dimethyl carbonate with proportions of the more crystalline polyhydroxybutyrate (PHB) mixed with a less crystalliine, miscible, pre-eutectic PHBV copolymer blend. Respective PHBV grade properties and microstructures were characterized using Pyrolysis GC/MS, solution rheology, DSC, DMTA, AFM, and melt rheology. Blends exhibited immiscibility (two distinct glass transition temperatures) and phase-separated microstructure morphologies (dispersed or layered-network) of interpenetrating harder and softer phases, as evidenced using Peak Force QNM. Blending systematically modulated elongation at break (from 3% to > 100%) and stiffness (from 1500 to 250 MPa). The more crystalline PHB component progressively effected melt stiffening temperature and rate, which are important to melt processing. Blending reproduced properties of an independently recovered PHBV grade with 37 wt% 3HV that was similarly independently extracted from a dried mixed microbial culture. Thus, solution blending of especially immiscible PHBV grades during PHA recovery is proposed as a novel and practical scalable route for effective property quality control and application-specific PHBV bioplastic masterbatch production.