Formation and stability of β-structure in biodegradable ultra-high-molecular-weight poly(3-hydroxybutyrate) by infrared, Raman, and quantum chemical calculation studies

Infrared (IR) and Raman spectra were measured for four kinds of ultra-high-molecular-weight poly[(R)-3-hydroxybutyrate] (UHMW-PHB) films: a solvent-cast film, a cold-drawn film, a two-step-drawn film, and a hot-drawn film. Quantum chemical calculations were made for octamer models of UHMW-PHB with a helix conformation (α-form) and a planar zigzag conformation (β-form). Comparison of the results between the Raman spectra of four kinds of films and the quantum chemical calculations of octamer models revealed that only two-step-drawn film yields additional bands at 1735, 966, 935, 908, and 858 cm−1 assignable to the β-structure, suggesting that it contains the β-form as well as the α-form. Detailed comparison of the frequencies and intensities of Raman bands between the observed and calculated values for the β-form indicates that the amount of β-form is relatively small and that the β-structure has a less ordered structure. The infrared and Raman spectra of two-step-drawn film also indicate that it has more amorphous parts than other films. When the two-step-drawn film was further heated up to 130 °C and then cooled down to room temperature, the above additional bands due to the β-form disappeared in the Raman spectra, suggesting that the β-form is less stable than α-form.