Insights into the Structural Dynamics of PLGA at Terahertz Frequencies

The mechanical properties of an amorphous copolymer are directly related to the dynamic processes occurring at the molecular level. Poly lactic-co-glycolic acid (PLGA) is a biodegradable co-polymer, and in this work we investigate the dynamics of PLGA and its glass transition behaviour by performing variable temperature terahertz time-domain spectroscopy (THz-TDS) experiments. We correlate PLGA dynamics, as measured at terahertz frequencies, their temperature dependence, molecular weight (MW), lactide to glycolide ratio, and free volume. The THz-TDS data can be used to detect two distinct glass transition processes, T_g,α and T_g,β. To complement our analysis, we use dynamic mechanical analysis (DMA) to probe the β- and α-relaxation processes in PLGA, and compare the results obtained from the DMA experiments with those obtained using THz-TDS. We attribute T_g,β to the change in dipole moments associated with the β-relaxation process, originating from the local rotation of C-O macromolecular chain segments, and T_g,α to the change in dipole moments due to large segmental motion of the copolymer backbone associated with the α-relaxation process. We connect our experimental results to the free volume theory proposed by Fox and Flory, and demonstrate our results are consistent with the relationship between the experimentally determined T_g,β and T_g,α and free volume and PLGA dynamics.