Melting behavior of poly(3-hydroxybutyrate) investigated by two-dimensional infrared correlation spectroscopy

The melting behavior of a bacterially synthesized biodegradable polymer, poly(3-hydroxybutyrate) (PHB), was investigated by using generalized two-dimensional infrared (2D IR) correlation spectroscopy. Temperature-dependent spectral variations in the regions of the CH stretching (3100-2850 cm−1), CO stretching (1800-1680 cm−1), and COC stretching (1320-1120 cm−1) bands were monitored during the melting process. The asynchronous 2D correlation spectrum for the CO stretching band region resolved two crystalline bands at 1731 and 1723 cm−1. The intense band at 1723 cm−1 may be due to the highly ordered crystalline part of PHB, and the weak band at 1731 cm−1 possibly arises from the crystalline part with a less ordered structure. These crystalline bands at 1731 and 1723 cm−1 share asynchronous cross peaks with a band at around 1740 cm−1 assignable to the CO band due to the amorphous component. This observation indicates that the decreases in the crystalline components do not proceed simultaneously with the increase in the amorphous component. In the 3020-2915 cm−1 region where bands due to the asymmetric CH3 stretching and antisymmetric CH2 stretching modes are expected to appear, eight bands are identified at 3007, 2995, 2985, 2975, 2967, 2938, 2934, and 2929 cm−1. The bands at 2985 and 2938 cm−1 are ascribed to the amorphous part while the rest come from crystal field splitting, which is a characteristic of polymers with a helical structure.