Further Spectroscopic Study of Solution Crystallization of a Biodegradable Polyester

The growing interest in high-purity bioplastics for emerging specialized applications motivated us to investigate the crystallization of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHx) from a chloroform solution using advanced spectroscopic techniques. Previous study of the carbonyl stretching region of time-dependent attenuated total reflection infrared (ATR-IR) spectra had revealed that solution crystallization process of PHBHx involving distinct intermediate species was markedly different from the more traditional melt crystallization. IR study of PHBHx solution crystallization is now extended to more complex C-H stretching and fingerprint regions. Characteristic IR bands of the system showing the least correlated and most independent behaviors with each other were identified using a new technique based on a two-dimensional (2D) discrimination spectrum. Correlation filters based on the characteristic bands were used to selectively attenuate interfering spectral contributions in conjunction with the hetero-mode two-dimensional correlation spectroscopy (2D-COS) analysis. Traditional classification of decreasing and increasing IR bands, respectively, during the crystallization process of PHBHx copolymers simply to the amorphous and crystalline components probably needs to be reconsidered. Dynamics of bands in the C-H stretching and fingerprint regions are not fully synchronized with the behavior of amorphous or crystalline species characterized by the carbonyl stretching region. The result strongly suggests the presence of some intermediate species appearing after the consumption of amorphous components and prior to the formation of crystals.