Free Volume In order to better understand the contribution of the methyl side group of PLA and the effect of free volume on the glass transition behaviour of PLGA, we examined PDLLA. Specifically, in the case of poly(lactic acid) (PLA), previous work has shown that the structure of the polymer chain can have a significant influence on T g . For example, it was found that the value of T g (as determined by DSC) is reduced in hyper-branched PLA due to an increase in free volume. Figure shows that the T g,β value of PDLLA in our measurement is 212 K (T g,β = 0.70 T g,α ), which is lower than the T g,β value observed for the PLGA 50:50 material of similar MW. This finding suggests that the presence of the methyl group results in steric hindrance to rotations about the free C-O bonds in PDLLA. Notably, the primary difference between the PDLLA and PLGA 75:25 is that the co-polymer PLGA backbone includes more flexible C-O bonds, which would increase the free volume between the polymer chains. Finally, to further investigate the T g,β and the β-transition, we used DMA to measure the temperature dependence of tan δ for samples of low, medium, and high MW PLGA 75:25, and medium and high MW PLGA 50:50, shown in Figures . Both the β-and α-relaxation processes are clearly detected using DMA. At temperatures below 300 K, the βrelaxation process is observed as a broad shallow peak, followed by a sharp peak associated with the α-relaxation. The values of T g,DMA , the onset of the α-relaxation measured by DMA, are in good agreement with the T g,α values obtained from the THz-TDS data (Table ). It is difficult to compare the β-process measured by DMA compared to that measured by THz-TDS, since we define T g,β for the THz-TDS measurements as an onset temperature, but the DMA instrument cannot access sufficiently low temperatures to resolve the onset temperature of the β-process. We therefore report the temperature for the peak of the shallow β-process together with the range of temperatures for the β-process (Table ). In the context of this discussion it is important to highlight that both the terahertz dielectric measurement as well as the DMA method are measuring the adiabatic process, i.e. the frequency of the measurement is much higher compared to the relaxation time of the process probed in the sample. Notably, we can see similarities between the gradient of the absorption coefficient versus temperature in the THz-TDS data to the tan δ of the DMA data. In the DMA data we can see that for low MW PLGA 75:25, T g,β is significantly higher compared to that of the medium and high MW PLGA 75:25 materials (shown in Figure ). This is in good qualitative agreement with the THz-TDS data. The measurements of the β-process for PLGA 50:50 using DMA also agree very well with the THz-TDS experiments (shown in Figure ): The β-process is observed to shift to higher temperatures with increasing MW. It is worth noting that the strength of the tan δ peak in the DMA data has previously been linked with an increase in the freedom of movement of the polymer. We note that the intensity of the tan δ peak is increasing with increasing MW, in line with lower molecular mobility in the low MW material compared with the high MW PLGA. In contrast, for high MW PLGA 50:50, the tan δ associated with the β-process is not only pronounced but also much broader than that of medium MW PLGA 50:50 (shown in Figure ). It has been suggested that the width of the tan δ peak correlates with the heterogeneity of the environments of the co-polymer and the distribution of mobilities of the polymer chains. For the higher MW PLGA 50:50 material, the high chain entanglement leads to more heterogeneous regions in this material compared to the medium MW PLGA 50:50 material, which forms a more homogenous polymer network. This preliminary observation supports the idea that the width of the tan δ peak may correlate with the number of heterogeneous environments in the polymer network and in turn the distribution of mobilities of the PLGA chains.