Effects of intermolecular hydrogen bondings on isothermal crystallization behavior of polymer blends of cellulose acetate butyrate and poly(3-hydroxybutyrate)

摘要

Effects of hydrogen bondings (HBs) on the isothermal crystallization kinetics and mechanisms of poly(3-hydroxybutyrate) (PHB) in neat PHB and its blends with cellulose acetate butyrate (CAB), amorphous melts at the crystallization temperature, were investigated by using time-resolved Fourier transform infrared spectroscopy (FTIR). The FTIR investigation focused on the characteristic bands of PHB in the crystalline phase and in the amorphous phase. The results revealed the following pieces of evidence. (1) The crystallization of PHB in both neat PHB and PHB/CAB blends commonly involves a multistep process: (i) transformation from amorphous melts to an intermediated structure between amorphous and crystals (process (b1) shown above), (ii) from the intermediate structure to crystals without intramolecular HBs (intra) (b2), and (iii) finally to crystals with intra (b3). (2) These transformations sequentially occur with increasing time. More specifically, the transformations (ii) and (iii) dominantly occur respectively in the primary and secondary crystallization process in both neat PHB and the blends. The Avrami analysis revealed that (3) the crystallization in both neat PHB and the blends involves essentially the same nucleation and growth mechanism, as evidenced by almost the same value of the Avrami exponent n; however (4) the crystallization rate constant kA, also determined from the Avrami analysis, is strikingly reduced by blending CAB (see the kA value for the PHB/HCAB blend in the text), which is attributed to physical cross-links formed by intermolecular HBs between PHB and CAB in addition to molecular entanglements.