Intermolecular Hydrogen Bonding between Poly[(R)-3-hydroxybutyrate] (PHB) and Pseudoboehmite and Its Effect on Crystallization of PHB

In the present study, we focused on the intermolecular H-bonding interactions of poly[(R)-3-hydroxybutyrate] (PHB) with an inorganic material, pseudoboehmite (PB), and their effect on PHB crystallization. Noncrystallizable atactic PHB and crystallizable isotactic PHB (a-PHB and i-PHB) ultrathin films were spin-coated on a PB substrate, as well as an aluminum oxide (AO) and a gold substrate for comparison. Infrared reflection-absorption spectroscopy (IRRAS) data show an absorption peak in the carbonyl region located at 1724 cm(-1) for a 2.8 nm a-PHB film deposited on PB. A peak at this frequency, often observed for thick bulk crystalline i-PHB films, was not observed for a 1.4 nm a-PHB film deposited on a gold or AO substrate, indicating that the 1724 cm(-1) peak observed for a-PHB on PB is not due to a geometric confinement effect or crystallization but due to the existence of intermolecular H-bonding (H-bond(inter)) between -C=O of a-PHB and -OH from PB. Supercooled, amorphous i-PHB was also found to exhibit the same H-bond(inter) with PB. It was found that a PB surface significantly modified the crystal orientation and morphologies of the films. Grazing incident wide-angle X-ray diffraction (GIWAXD) data show that the crystallites in i-PHB on PB are randomly oriented, whereas those on AO are predominantly edge-on oriented. Polarized optical microscopy (POM) images show spherulites for i-PHB on AO, whereas no spherulites were observed for i-PHB on PB. This study demonstrates a novel method of using PB to modulate the crystallization behavior of PHB thin films.