Study of Co-crystallization and Intermolecular Hydrogen Bondings of Poly(glycolide-co-L-lactide) Copolymers by Terahertz and Low-Frequency Raman Spectroscopy

The co-crystallization and intermolecular hydrogen bonding of poly(glycolide-co-L-lactide) [P(GA-co-LLA)] copolymers have been studied using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering, and infrared (IR), terahertz, and low-frequency Raman spectroscopy. The DSC and WAXD results confirm that the melting point of the P(GA-co-LLA) copolymers decreases linearly and the spacing of the (110) and (020) planes expands as the concentration of polyglycolic acid (PGA) in the copolymers decreases. Furthermore, the lamellar thickness of the P(GA-co-LLA) copolymers increases with an increase in the concentration of polylactic acid (PLA), which is the same as that of poly(3-hydroxybutyrate-co-3-hydroxyvalerate), a co-crystallized copolymer. These results indicate that P(GA-co-LLA) copolymers are co-crystallized. The copolymers show a terahertz band at approximately 75 cm(-1) and Raman bands at approximately 130 and 80 cm(-1). The corresponding terahertz band at 75 cm(-1) and Raman bands at 130 and 80 cm(-1) of the PGA homopolymer show a shift to a relatively low frequency during the heating process because of thermal expansion. However, the terahertz band of the P(GA-co-LLA) copolymers at approximately 75 cm(-1) shifts to a relatively high wavenumber, and their Raman bands at approximately 130 and 80 cm(-1) do not show any shift despite the expansion of the crystal lattice of the P(GA-co-LLA) copolymers with the increasing concentration of PLA. The different behaviors of the shifts between the PGA homopolymer and P(GA-co-LLA) copolymers are due to the presence of PLA, and the co-crystallization can induce the formation of new intermolecular interactions between PGA and PLA. The temperature dependence of the IR bands of the P(GA-co-LLA) copolymers in the C=O, CH2, and C-O-C stretching regions suggests that the new intermolecular interaction of C=O center dot center dot center dot H3C is formed because of the co-crystallization of the P(GA-co-LLA) copolymers and the C-H center dot center dot center dot O(ether) hydrogen bonding in the PGA homopolymer is maintained in the P(GA-co-LLA) copolymers. Therefore, P(GA-co-LLA) copolymers that form co-crystallization have two types of hydrogen bondings: C=O center dot center dot center dot H3C and C-H center dot center dot center dot O(ether) hydrogen bondings.