Thermally induced phase transition of poly(3-hydroxybutyrate- co-3-hydroxyhexanoate) investigated by two-dimensional infrared correlation spectroscopy

Abstract

The thermally induced crystalline/amorphous phase transition process of a newly developed biodegradable polymer, poly(3-hydroxybutyrate- co-3-hydroxyhexanoate) (P(HB- co-HHx)) (HHx=12 mol%), was investigated by using generalized two-dimensional infrared (2D IR) correlation spectroscopy. Three spectral regions, the CH stretching (3100–2800 cm −1), CO stretching (1780–1680 cm −1), and COC stretching (1330–1200 cm −1) band regions were analyzed in order to explore the phase transition behavior of the copolymer. The asynchronous 2D spectrum generated from the temperature-induced variations in IR spectra in the CO stretching band region clearly revealed the coexistence of two crystalline bands at 1731 and 1723 cm −1. The dominant band at 1723 cm −1 may arise from the highly ordered crystalline component of the copolymer, and the weaker band at 1731 cm −1 is possibly due to the minor crystalline component with a less ordered structure. The major crystalline band at 1723 cm −1 shares asynchronous cross peaks with the amorphous band at 1740 cm −1. This observation suggests that the melting of the crystalline structure does not simultaneously result in the formation of the completely amorphous structure. The phase transition process of P(HB- co-HHx) (12 mol% HHx) takes place through an intermediate state. It is noted that the vibrational frequencies of the CO, COC, and CH stretching bands due to the crystalline components of P(HB- co-HHx) (12 mol% HHx) are almost identical to those of PHB. These observations indicate that the helical structure of P(HB- co-HHx) (12 mol% HHx) is very similar to that of PHB. The inclusion of the HHx comonomer locally disrupts the highly ordered and helical structure of PHB from place to place, thereby reducing the crystallinity of the copolymer. It was also found that the helical structure of P(HB- co-HHx) (12 mol% HHx) is deformed gradually from much lower temperature than that of PHB.