Infrared Spectroscopy and X-ray Diffraction Studies of Thermal Behavior and Lamella Structures of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(HB-co-HV)) with PHB-Type Crystal Structure and PHV-Type Crystal Structure

Thermal behavior and lamella structures of poly(3-hydroxybutyrate-co-3-hydroxyvalerate), hereafter P(HB-co-HV) (HV = 9, 15, 21, 28.8, 58.4, 73.9, and 88.6 mol %), were investigated using infrared (IR) spectroscopy, wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC). Temperature-dependent IR and WAXD measurements revealed that P(HB-co-HV) with low HV content (HV = 9, 15, and 21 mol %) has a PHB-type crystal structure with CH3···OC hydrogen bonds, whereas P(HB-co-HV) with high HV content (HV = 73.9 and 88.6 mol %) has a PHV-type crystal structure with CH2···OC hydrogen bonds. The results of IR measurements showed that the strength of the CH3···OC hydrogen bonds in the P(HB-co-HV) (HV = 9, 15, and 21 mol %) copolymers is almost the same as that in PHB, whereas in the P(HB-co-HV) (HV = 28.8 mol %) copolymer, the hydrogen bonds become weaker. In the case of P(HB-co-HV) with higher HV content (HV = 73.9 and 88.6 mol %), the effect of lamella packing on CH2···OC hydrogen bonding in the PHV crystal is not considerable because the side chain of the HB unit is shorter than that of the HV unit. Additionally, it seems that in the crystal structure of P(HB-co-HV) (HV = 58.4 mol %) there are only very weak intermolecular interactions between a CO group and either a CH2 group or a CH3 group because the region of transition of the crystal structure of P(HB-co-HV) from the PHB type to the PHV type occurs at around 50 mol % HV content. Therefore, it is very much possible that with increasing temperature the crystal structure of P(HB-co-HV) (HV = 58.4 mol %) is more likely to collapse than other P(HB-co-HV) copolymers.