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 °C to higher temperatures (PHB, 185 °C; HHx = 2.5 mol %, 160 °C; HHx = 3.4 mol %, 160 °C; HHx = 12 mol %, 140 °C) to explore their structure and thermal behavior. The temperature-dependent IR spectral variations were analyzed for the CH stretching, C O stretching, CH 3 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 CH 3 asymmetric stretching bands of the four polymers and the X-ray crystallographic structure of PHB that there is an inter- or intramolecular interaction (C−H···O hydrogen bond) between the C O group in one helical structure and the CH 3 group in the other helical structure in PHB and P(HB- co -HHx). The bonding energy of the C−H···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−H···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 °C), 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−H···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.