We first attempt to explain the origin of the transition temperatures, corresponding to T g,β and T g,α , by proposing a physical picture of the change in PLGA dynamics with temperature. Second, we link this physical picture to the concept of free volume in order to offer an understanding of temperature-dependent PLGA dynamics and their contribution to the observed α-and β-relaxation processes. It is worth noting at the outset that the values of T g,α , as determined from the THz-TDS experiments, are in good agreement with our own calorimetric measurements, T g,DSC , and our own DMA measurements, T g,DMA , as well as the values reported in the literature for these materials.