C-H••O=C hydrogen bonding and isothermal crystallization kinetics of poly(3-hydroxybutyrate) investigated by near-infrared spectroscopy

We have investigated the C-H center dot center dot center dot O=C hydrogen bonding and isothermal crystallization kinetics of poly(3-hydroxybutyrate) (PHB) at 125 degrees C by using near-infrared (NIR) spectroscopy, which allows us to investigate thick films. The NIR spectrum of a PHB film changes remarkably with the evolution of isothermal crystallization process. Accordingly, NIR bands due to the amorphous and crystalline states have been assigned. Particular attention has been paid to a NIR band at 5973 cm(-1) due to the first overtone of the C-H stretching mode and an IR band at 3435 cm(-1) due to the first overtone of the C=O stretching mode of the C-H center dot center dot center dot O=C hydrogen bonding combining two parallel helical structures of PHB. The anharmonicity of C-H (5973 cm(-1)) and C=O (3435 cm(-1)) bands arising from the crystalline parts and that of the amorphous C-H (5954 cm(-1)) and C=O (3457 cm(-1)) bands was investigated by using both IR and NIR data. The anharmonicity of crystalline C- H and C=O bands is significantly different from that of the amorphous C- H and C=O bands, supporting the idea that both CH and C=O bonds in the crystalline state are involved in the C-H center dot center dot center dot O=C hydrogen bonding. Principal component analysis (PCA) was used to analyze the real-time NIR difference spectra measured during the isothermal crystallization process. The result suggests that the crystallization process may be not a simple transition of binary mixture of the amorphous and ordered crystalline states. Moreover, the crystallization kinetics parameters of PHB have been obtained by combining PCA and Avrami equation based on the spectral regions instead of the intensity changes of certain bands.