Infrared spectroscopy studies of CH•••O hydrogen bondings and thermal behavior of biodegradable poly(hydroxyalkanoate)

Infrared (IR) spectra of new types of bacterial copolyester, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), P(HB-co-HHx) (HHx = 2.5, 3.4, and 12 mol %), and poly(3-hydroxybutyrate) (PHB) were measured over a temperature range of 20 degreesC to higher temperatures (PHB, 185 degreesC; HHx = 2.5 mol %, 160 degreesC; HHx = 3.4 mol %, 160 degreesC; HHx = 12 mol %, 140 degreesC) to explore their structure and thermal behavior. The temperature-dependent IR spectral variations were analyzed for the CH stretching, C=O stretching, CH3 deformation, and C-O-C stretching vibration regions, and bands characteristic of crystalline and amorphous parts were identified in each region. It has been found from the anomalous frequencies of the CH3 asymmetric stretching bands of the four polymers and the X-ray crystallographic structure of PHB that there is an inter- or intramolecular interaction (C-(HO)-O-... hydrogen bond) between the C=O group in one helical structure and the CH3 group in the other helical structure in PHB and P(HB-co-HHx). The bonding energy of the C-(HO)-O-... hydrogen bond seems to be smaller than 4 kJ/mol, but considering the heat of fusion (12.5 kJ/mol) of PHB, it is likely that a chain of C-(HO)-O-... hydrogen bond pairs link two parallel helical structures in the crystalline parts. The temperature-dependent IR spectral variations have shown that the crystallinity of P(HB-co-HHx) (HHx = 12 mol %) decreases gradually from a fairly low temperature (about 60 degreesC), while the crystallinity of PHB and P(HB-co-HHx) (HHx = 2.5 and 3.4 mol %) remains almost unchanged until just below their melting temperatures. It has also been revealed from the present study that the weakening of the C-(HO)-O-... interaction starts from just above room temperature and proceeds gradually with increase in temperature, but the collapse of helical structure occurs at a much higher temperature for all the polymers investigated.