The calorimetric T g,DSC was determined for each sample and the resulting values are listed in Table . As expected from Fox-Flory theory, we observed an increase in T g with increasing average MW of the respective polymer samples. Table : Transition temperatures determined by different characterisation techniques. For THz-TDS T g,β and T g,α are reported as onset temperatures. For DMA, T g,β is reported as the temperature range which the tan δ spans below 300 K, and T g,α is reported as an onset temperature. The terahertz spectra of all PLGA materials showed an increase in absorption with frequency and temperature over the entire investigated range, which is in line with previous measurements of disordered molecular solids in general and amorphous polymers in particular (Figure ). As expected, no discrete spectral features were present in the frequency range covered by our instrument and the spectra were dominated by the monotonous increase with frequency that is characteristic for the rising flank of the broad peak due to the vibrational density of states (VDOS). In contrast, the refractive index decreases slightly with increasing frequency (see ESI). To further investigate the relationship between the increase of absorption coefficient and temperature we examined the temperature dependent changes in absorption losses at a frequency 1 THz in more detail. Given the lack of distinct spectral features we chose the frequency of 1 THz as the signal-to-noise ratio of the measurement at this frequency is highest for our spectrometer and the absorption is clearly dominated by the VDOS in this spectral region. It is well established that the α-relaxation and β-relaxation processes are associated with different molecular motions in the bulk of the polymer material. Overall, the glassy co-polymer chains are more densely packed at low temperatures, compared to at higher temperatures. The onset of T g,β corresponds to the lowest temperature at which the copolymer system is excited with sufficient energy to allow for localised reorientational motion of smaller segments within the co-polymer macromolecule to occur. Thus, at low temperatures, motions which require high activation energies and or large amounts of free volume are restricted. In contrast, for the α-relaxation this involves the movement of large segments of the copolymer backbone. At temperatures above the glass transition the polymer chains are more loosely packed and sufficient thermal energy is available for the copolymer to cross higher energy barriers, giving rise to the α-relaxation. The motions of the co-polymer chains can therefore be divided into two categories: (1) motions associated with the β-relaxation process that require relatively low activation energy and a small amount of free volume, and