Crystallization behavior and crystalline structural changes of poly(glycolic acid) investigated via temperature-variable WAXD and FTIR analysis

Poly(glycolic acid) (PGA) is a unique biodegradable, aliphatic polyester having a higher melting point (T-m) and better mechanical properties and chemical resistance than its analogs. The unusual physical properties of PGA can be attributed to its crystalline structure, chain packing and interactions. Herein, we systematically investigated the crystalline structure, spherulitic morphology, and structural evolution of PGA in the crystallization and melting processes by time-resolved wide-angle X-ray diffraction (WAXD) and FTIR spectrometry. PGA exhibits fast crystallization and it completely crystallizes in the fast cooling process. The T-m, degree of crystallinity, and long period of PGA increase as the crystallization temperature increases. PGA forms unique hedrites but not normal spherulites during crystallization. Because of the thermal expansion of the crystal lattice, the a and b axes of PGA crystal cells increase linearly with temperature in the heating process. The band splitting in the FTIR spectrum of PGA is observed in the nu(C=O) and delta(CH2) regions and the spectral splitting intensifies with cooling, indicating the existence of intermolecular dipolar interactions in PGA crystals. It is speculated that the C-H center dot center dot center dot O=C H-bonds are formed in the crystalline phase of PGA, as indicated by the significant red shifts of the nu(C=O) band during crystallization.