Thermally induced phase transformation of nodax copolymer monitored by two-dimensional infrared spectroscopy

Abstract

The crystalline/amorphous phase transformation behaviors monitored at the bulk and interface regions of a newly developed biopolymer, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(HB-co-HHX)), and those of its basic unit, poly(3-hydroxybutyrate) (PHB), were investigated by using transmission and attenuated total reflection (ATR) infrared (IR) spectroscopy coupled with the generalized two-dimensional (2D) correlation analysis. Wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) were employed to study the thermal properties of the polymers. The asynchronous 2D correlation spectra generated from the dynamic IR spectra in the C=O stretching region for the two polymers reveal the coexistence of two crystalline components assignable to the highly ordered and less-ordered crystalline parts of the polymers. The phase transformation process of P(HB-co-HHx) and that of PHB take place through an intermediate state. The distributions of the crystalline phase detected by ART and transmission techniques suggest that the population of polymer crystals at the surface is higher than that in the bulk for both polymers. The time-dependent IR spectral variations indicate that P(HB-CO-HHx) crystallizes much slower than PHB. The HHx unite incorporated in P(HB-co-HHx) markedly reduce not only the degree of crystallinity but also the crystallization rate of PHB. The highly ordered and less-ordered crystalline parts of P(HB-co-HHx) exhibit different crystallization behaviors, and the orders of crystal growth steps are inverse between the gradual cooling crystallization of the melt and the isothermal crystallization of the supercooled liquid.