The THz-TDS data shown in Figure demonstrates that there is a significant dependence of T g,β and T g,α on the MW of the PLGA copolymers. As shown by the schematic in Figure , for PLGA 50:50 an increase in the MW of the sample results in a decrease of the free volume. (In the THz-TDS data for PLGA 50:50, we observe three regions with a distinct T g,β separating the lower temperature regions for each sample (Figures and). The T g,β value of the low MW material (T g,β = 136 K), is significantly lower than that of the medium MW samples (T g,β = 233 K) and of the high MW samples (T g,β = 243 K)). For PLGA 50:50, the long chains of the medium and high MW material entangle with each other far more than the shorter chains of low MW material. Therefore, the higher MW materials have more chain interactions to overcome before the chains can begin moving; and, thus require greater energy and sufficient free volume for the rotational motions to proceed despite chain entanglement occurring, which increases the entropy of these systems and the values of T g,β as well as T g,α . As temperature is increased, sufficient thermal energy is supplied to the system to overcome local and segmental chain interactions that limit molecular mobility at lower temperatures, thereby increasing the free volume (and mobility). Additionally, the greater chain entanglement associated with the higher MW samples could contribute to the higher barriers of the PES which would explain the raised values of T g,β and T g,α with increasing MW.