Temperature dependent poly(L-lactide) crystallization investigated by Fourier transform terahertz spectroscopy

Poly(L-lactide) (PLLA) was investigated by Fourier transform terahertz (THz) spectroscopy over the frequency range of 1.0–8.5 THz. THz absorption spectra were acquired for PLLA samples isothermally crystallized at the temperature of 80–140 °C after melt-quenching. Characteristic PLLA absorption peaks were identified at 1.8, 4.0, 4.7, and 7.1 THz for the 80 °C sample; higher crystallization temperature resulted in a blue shift of the absorption peak frequency. The higher two peaks (4.7 and 7.1 THz), which are present not only in α-form but also in δ-form, were attributed to the intermolecular vibration of the PLLA chain, depending on the lattice spacing and crystallinity. In contrast, the 4.0 THz peak was intrinsically in α-form, thus enabling its use as an indicator in the optimization of PLLA polymorphism. Broadband THz spectroscopy is a nondestructive and noninvasive evaluation technique for biodegradable polymers.