Two-Dimensional Raman Correlation Spectroscopy Study of Poly[(R)-3-hydroxybutyrate- co -(R)-3-hydroxyhexanoate] Copolymers

Abstract

Two-dimensional correlation analysis was applied to the time-dependent evolution of Raman spectra during the isothermal crystallization of bioplastic, poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] or PHBHx copolymer. Simultaneous Raman measurement of both carbonyl stretching and low-frequency crystalline lattice mode regions made it possible to carry out the highly informative hetero-mode correlation analysis. The crystallization process of PHBHx involves: (1) the early nucleation stage; (2) the primary growth of well-ordered crystals of PHBHx; and (3) the secondary crystal growth phase. The latter stage probably occurs in the inter-lamellar region, with an accompanying reduction of the amorphous component, which occurs most dominantly during the primary crystal growth. The development of a fully formed lamellar structure comprising the 2₁ helices occurs after the primary growth of crystals. In the later stage, secondary inter lamellar space crystallization occurs after the full formation of packed helices comprising the lamellae.