X-ray diffraction and infrared spectroscopy studies on crystal and lamellar structure and cho hydrogen bonding of biodegradable poly(hydroxyalkanoate)

Temperature-dependent, wide-angle, x-ray diffraction (WAXD) patterns and infrared (IR) spectra were measured for biodegradable poly(3-hydroxybutyrate) (PHB) and its copolymers, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) P(HB−co−HHx) (HHx=2.5, 3.4, 10.5, and 12 mol), in order to explore their crystal and lamellar structure and their pattern of C−H−O=C hydrogen bonding. The WAXD patterns showed that the P(HB-co-HHx) copolymers have the same orthorhombic system as PHB. It was found from the temperature-dependent WAXD measurements of PHB and P(HB-co-HHx) that thea lattice parameter is more enlarged than theb lattice parameter during heating and that only thea lattice parameter shows reversibility during both heating and cooling processes. These observations suggest that an interaction occurs along thea axis in PHB and P(HB−co−HHx). This interaction seems to be due to an intermolecular C−H−O=C hydrogen bonding between the C=O group in one helical structure and the CH3 group in the other helical structure. The x-ray crystallographic data of PHB showed that the distance between the O atom of the C=O group in one helical structure and the H atom of one of the three C−H bonds of the CH3 group in the other helix structure is 2.63 Å, which is significantly shorter than the sum of the van der Waals separation (2.72 Å). This result and the appearance of the CH3 asymmetric stretching band at 3009 cm−1 suggest that there is a C−H−O=C hydrogen bond between the C=O group and the CH3 group in PHB and P(HB-co-HHx). The temperature-dependent WAXD and IR measurements revealed that the crystallinity of P(HB-co-HHx) (HHx=10.5 and 12 mol) decreases gradually from a fairly low temperature, while that of PHB and P(HB-co-HHx) (HHx=2.5 and 3.5 mol) remains almost unchanged until just below their melting temperatures. It was also shown from our studies that the weakening of the C−H−O=C interaction starts from just above room temperature and proceeds gradually increasing temperature. It seems that the C−H−O=C hydrogen bonding stabilizes the chain holding in the lamellar structure and affects the thermal behaviour of PHB and its copolymers.