Crystal and Lamella Structure and C−H···OC Hydrogen Bonding of Poly(3-hydroxyalkanoate) Studied by X-ray Diffraction and Infrared Spectroscopy

Abstract

Temperature-dependent X-ray diffraction and infrared (IR) spectra were measured for poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) P(HB-co-HHx) (HHx = 2.5, 3.4, 10.5, and 12 mol %) to explore their crystal and lamella structure and the C−H···OC hydrogen bonding in them. The X-ray diffraction and IR measurements of PHB and P(HB-co-HHx) revealed that the smaller the a lattice parameter, the higher the frequency (∼3008 cm-1) of the C−H stretching band of the C−H···OC hydrogen bonding along the a axis between the CH3 group of one helix and the CO group of another helix. Therefore, it seems that the C−H···OC hydrogen bonding becomes strong with the decrease in the a lattice parameter. To investigate the relation between the C−H···OC hydrogen bonding and the lamella structure, we estimated the number of C−H···OC hydrogen bonding along the c axis (the direction of the lamella thickness) based on the reported lamella thickness. It is about 8 or 9 for PHB and about 3 for P(HB-co-HHx) (HHx = 10.5 and 12 mol %). It is very likely that the C−H···OC hydrogen bondings break much more easily in P(HB-co-HHx) than in PHB because of the bulkiness of large amounts of amorphous parts. However, the polymer chains still keep the lamella structure even in the copolymers with the HHx content of more than several percent. This is the reason why the P(HB-co-HHx) copolymers show high crystallinity and essentially have the same lattice spacing as the PHB homopolymer even if the HHx content is more than 10%. We have concluded that the C−H···OC hydrogen bonding stabilizes the chain folding in the lamella structure of PHB and P(HB-co-HHx) and the high crystallinity of PHB and P(HB-co-HHx) partly comes from the C−H···OC hydrogen bonding.