Multi-Scale Investigation of Metastable Phases in Sustainable Polymers

University Of Delaware, Newark DE

Investigators

Abstract

NON-TECHNICAL SUMMARY: Plastic materials have played a major role in everyone's life for the last half century. However, the widespread use of plastics has led to intense problems with respect to disposal as many millions of tons of used plastics that occupy landfills, roadsides, as well as open and confined waters. In order to keep the beneficial aspects of plastic materials it is therefore critical to explore materials that are renewable, i.e. both sourced from nature and biodegradable. Polyhydroxybutyrates (PHBs) fill this need since they are synthesized by microbes and also biodegrade to form carbon dioxide and water on relevant timescales. However, the switch from petroleum-based plastics to materials like PHBs requires that the new biodegradable plastics possess similar mechanical and electrical properties compared to the petroleum-based plastics. Properties such as strength, stretch, and stiffness must be at least equivalent to current commodity plastics. It has been discovered in previous work that the ultimate mechanical properties of PHB can be changed by how the PHB is processed. Mechanical strength can be increased by stretching or compressing the material rapidly during the formation process, resulting in a plastic that is still biodegradable but has the desired properties. If the processing is fast enough, then new properties can be generated in PHB plastics, such as improved mechanical properties, as well as piezoelectricity (i.e., an electrical response to a mechanical stimulus, which is a desirable electronic property). This work focuses on the rapid processing of PHB plastic materials into fibers and films with higher strength, improved toughness, and a piezoelectric response. These new properties arise from a different arrangement of the PHB molecules in a crystalline lattice, obtained when spinning fibers or rapidly stretching films. The resultant material can be "locked" into this new arrangement, which yields improved properties. This project is directed towards finding the appropriate conditions that maximize this new polymer structure, and could thus enable the production of sustainable plastics with new possibilities for packaging, environmental sensors, and electronic applications. TECHNICAL SUMMARY: The processing of semi-crystalline polymers to produce films, fibers and molded articles involves manipulating the secondary and tertiary structure to produce the required macroscopic properties for the intended application. Since the bulk of the processing methods are slow relative to the chain and segmental motion, generally the thermodynamically stable crystalline structure is obtained. It has been shown that when poly(hydroxybutyrate) (PHB) is copolymerized with hydroxyhexanoate (Hx) comonomers, using microbes, a random copolymer, PHBHx, is formed that exhibits very different properties compared to PHB homopolymer. When these PHBHx copolymers are processed (e.g., electrospun from a polymer solution into nanofibers), a metastable beta crystalline form is produced in which the polymer backbones adopt a planar zig-zag conformation. This crystalline modification gives rise to a piezoelectric response and higher mechanical strength. The focus of this research will address the following question: Under what conditions (solvent and solvent evaporation kinetics, high pressure and temperature, tensile deformation) can metastable crystalline form(s) be produced and stabilized and how do these phases affect the mechanical properties and the piezoelectric response of PHBHx as a function of Hx concentration? This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.