SusChEM: Studies of Molecular Orientation, Degradation and Thermoreversible Gelation in Environmentally Sustainable Polymers: Poly(hydroxybutyrates) and Their Copolymers

University Of Delaware, Newark DE

Investigators

Abstract

NON-TECHNICAL ABSTRACT: Plastic packaging is usually disposed of in landfills or incinerated, adding to the air pollution which is endemic in many regions of the world. Many third world countries, too poor to recycle, have few sanitary landfills and few facilities for incineration. Most of the current polymers used for packaging are not biodegradable and persist in landfills for many decades. These problems could be addressed if packaging materials were based on biodegradable polymers from renewable sources. However, polymers that are biodegradable often suffer from reduced mechanical properties in terms of strength and flexibility. This research group is trying to address these issues using polymers that are biodegradable and biosynthetically produced. This interdisciplinary research on the study of chemical and biological degradation of a class of sustainable polymers, poly(hydroxybutyrates) (PHB) and their random copolymers, combines approaches involving chemical synthesis, novel processing using an electrospinning technique, as well as characterization of the resulting materials using new combinations of instrumentation. The group will assess the effects of randomly incorporating a second component into the PHB polymer molecules in order to improve their mechanical properties while maintaining the rate of degradation and the nature of the degradation products. The ultimate goal is a sustainable polymeric material that is inexpensive, processable, and degrades rapidly (30-45 days) in landfills, while providing improved properties of strength and flexibility for packaging. If successful, this approach to the development of optimized, sustainable, and biodegradable polymers may be extended beyond the PHB systems and have significant impact on development and production of new sustainable materials. TECHNICAL ABSTRACT: Poly(hydroxybutyrate) (PHB) is a biodegradable, aliphatic polyester that can be produced by chemical processes or bacterial fermentation. The facts that it can be produced biologically with the properties of a thermoplastic and is naturally biodegradable have made it a subject of research in many industrial and academic laboratories worldwide. However, bacterially produced PHB results in a polymer that is brittle and lacks flexibility. In order to modify these properties 3-hydroxyhexanoate (3HHx) has been added as a co-monomer and these new materials, referred to as PHB-HHx, exhibit a significantly reduced crystalline content, resulting in improved mechanical properties and processability. While this approach definitely leads to important improvements, there is always the question of concomitant changes in other important properties like biodegradability. If the degradability of these sustainable, thermoplastic polymers can be optimized so as to enzymatically degrade rapidly (30-45 days) in landfills, this would be an extremely important contribution to the environment and society with impacts on land use, soil contamination, and the economics of material disposal. It is this hypothesis that researchers will explore in this project using a wide range of copolymers with varying amounts of 3HHX comonomer (0 mol% (pure PHB), 3.9 mol%, 5.8 mol%, 6.2 mol%, 7.6 mol%, 9.4 mol%, 11.9 mol%, and 13 mol%). In addition, the researchers will explore the correlation between structure, processing, and chain orientation/crystallinity, and test the hypothesis that improved chain orientation in electrospun nanofibers can increase modulus and tenacity even while crystallinity is disrupted by appropriate comonomer compositions. They will also explore alternate processing approaches to produce a fibrous structure similar to an electrospun membrane without significantly changing desired properties. Thermoreversible gelation, which has been discovered for PHB and PHB-HHx, offers a route to a non-woven fiber-like structure when gelation is followed by lyophilization. The researchers hypothesize that this processing method may significantly improve throughput while maintaining structures with the same range of properties as electrospun membranes.