Understanding crystallization mechanisms of a biodegradable copolymer poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBHx) and the inhibition of its crystallization using aluminum oxide and pseudoboehmite

Abstract

Bacterially derived, biodegradable poly[(R)-3-hydroxybutyrate] (PHB) and its random copolymer poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBHx) have gained substantial interest as environmentally friendly polymers. PHB and PHBHx serve as a carbon and energy storage material in vivo in micro-organisms. However, neat PHB suffers from an excessively high crystallinity, leading to high brittleness, a high melting point, and subsequent poor processing ability. Incorporating an unexpectedly high amount of 3HHx is required to decrease the crystallinity. These drawbacks limit the application areas for PHB and PHBHx. Why does the homopolymer PHB exhibit high crystallinity, and why does PHBHx require an abnormally high commoner concentration to decrease the crystallinity? The goal of the current dissertation is to understand the crystallization mechanism for PHB and PHBHx. In addition, novel ways of tuning the crystallization are explored. ☐ The first part of this thesis focuses on understanding the crystallization mechanism of PHBHx by isothermally growing single crystals. Single crystals of PHBHx, with a relatively high (R)-3-hydroxyhexanoate (3HHx) content of 3.9 mol% were grown from dilute solutions over a wide temperature range from -20 °C to 75 °C. PHBHx single crystals were found to adopt the α crystalline form of the homopolymer for all temperatures. The comonomer, 3HHx, was found to be excluded from the crystal lattice as a non-crystallizable molecular defect. A unique needle-shaped anisotropic growth pattern was identified. This anisotropic growth pattern was significantly enhanced at Tc=20 °C, wherein the unit cell packing velocity along the a direction is approximately 55 times faster than that along the b direction. ☐ The second part of this thesis focuses on investigating a crystallization retardation phenomenon of PHB and PHBHx ultrathin films on an aluminum oxide (AO) surface. Infrared reflection absorption spectroscopy (IRRAS) was used to study crystallization kinetics of polymer films. Avrami analysis showed that the crystallization rate constant k (min−1) for all of the polymers on AO is approximately 3 to 4 orders of magnitude less than that found for the same polymers on gold. The retardation mechanism was explained as being a sum of the dipole−dipole interactions of −C=O of PHB or PHBHx and the −O−Al−O− groups of AO coupled with the rigid disordered amorphous nature of the AO surface. In the most recent development, a poorly crystalline aluminum oxide hydroxide, or, pseudoboehmite (PB) was found to be capable of forming intermolecular H-bonding with PHB ultrathin films. Grazing incident wide-angle X-ray diffraction and polarized optical microscopy results indicate the crystallization of PHB films is inhibited for a longer term. These findings are believed to have practical potential to allow tuning of PHB and PHBHx crystallinity.