Insights into the Structural Dynamics of PLGA at Terahertz Frequencies

2018-06-07T19:46:08Z (GMT) by Talia A. Shmool J. Axel Zeitler
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<sub>g,α</sub> and T<sub>g,β</sub>. 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<sub>g,β</sub> 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<sub>g,α</sub> 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<sub>g,β</sub> and T<sub>g,α</sub> and free volume and PLGA dynamics.