High-resolution solid-state 13C NMR studies of poly[(R)-3-hydroxybutyric] acid

Solid poly[(R)-3-hydroxybutyric] acid was examined by high-resolution ¹³C MAS NMR, differential scanning calorimetry and infrared spectroscopy. The ¹³C methyl group resonance consists of three components: the rigid amorphous phase, the crystalline phase and the mobile amorphous phase. Spectral deconvolution, using the Lorentz function, reveals the relative amounts as 62% crystalline and 38% amorphous at 333 K, and 42% crystalline, 40% pure amorphous and 18% β-orthorhombic at 413 K. NMR indicates a large difference in molecular mobility between the crystalline and amorphous regions of the sample. Infrared spectroscopy shows that the stretching at 1725 cm⁻¹ (characteristic of the -form) comes from the crystalline region, and the bands at 1744 cm⁻¹ (characteristic of the β-form) and 1800 cm⁻¹ come from the amorphous region.