Thermally Induced Exchanges of Hydrogen Bonding Interactions and Their Effects on Phase Structures of Poly(3-hydroxybutyrate) and Poly(4-vinylphenol) Blends

Effects of various hydrogen bondings (HBs) and thermally induced exchanges of them on cold crystallization and melting poly(3-hydroxybutyrate) (PHB) and poly(4-vinylphenol) (PVPh) blends were explored as a function of the PVPh weight fraction (W-PVPh) by using Fourier transform infrared spectroscopy. The HBs investigated in this work include intramolecular HBs within PHB crystals (intro PHB) as observed in the C=O and CH stretching band regions, intramolecular HBs within PVPh (intro PVPh) as observed in the OH stretching band region, and intermolecular HBs (inter) between the CO groups of PHB in amorphous phase and the OH groups of PVPh. The results elucidated the following pieces of evidence. (i) Inter and intro PVPh suppress the rate of the crystallization and hence the crystallinity of PHB crystals in the blend via formation of physical cross-links in PHB and PVPh chains in the amorphous phase or melt. The crystallization and melting occur in two steps: (ii) the crystallization involves the dissociation of inter and intra PVPh, followed by the association of infra PHB and the reassociation of intro PVPh; (iii) the melting involves the dissociation of intro PHB, followed by the association of inter. (iv) Among C=O groups of PHB in the melt blends existing in either free C=O or inter, the fraction of inter linearly increases with W-PVPh, which in turn suppresses the PHB crystallinity under given crystallization conditions to a greater extent due to an increased number of physical cross-links and hence due to the suppressed crystallization rate.