Reorientation of the poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) crystal in thin film induced by polyethylene glycol

Abstract

Understanding the crystal structure and thermal behavior of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHx) and its blends thin film under spatial confinement is crucial for its application in molecular scale devices and advanced nanotechnology. In this study, we investigate the influence of polyethylene glycol (PEG) on the morphology and molecular orientation of PHBHx thin films using scanning probe microscope and grazing-incidence wide-angle X-ray scattering. Unexpectedly, we found that with addition of PEG no hydrogen bonds were detectable in the thin film of PHBHx/PEG blend. PEG destroyed the edge-on lamellar orientation of PHBHx leading to the chain axis to pack preferentially in a direction between the out-of-plane and in-plane along with a flat surface. More importantly, analysis of the q space of (020) diffraction indicated that PHBHx crystal tends to form a densely packed lamellae after the addition of PEG. The thermal behaviors of such densely packed lamellae crystals were examined by temperature-dependent infrared-reflection absorbance spectroscopy and two-dimensional correlation analysis. It was revealed that the crystal structure of PHBHx/PEG blends were stable during the melting process despite the decrease of the crystallinity. We speculate that the reorientation and thermal stability of the crystalline PHBHx/PEG blends were due to PEG acting as an effective solvent that not only lowered the surface energy of chain folding surface but also changed the kinetics of PHBHx crystallization. This interesting observation will provide new insights into understanding of the crystallization kinetics of PHBHx thin film under spatial confinement.