Here i represents a point of intersection where i In order to analyse the DMA data we applied the following methodology: (1) The data was binned into temperature intervals of 1 K and averaged; (2) the peak position of the local maximum in the tan δ-curve was determined at low temperatures (below 300 K); (3) the onset value of the peak due to α-relaxation (which in all cases was well separated from the β-process above 300 K) was determined. Here, tan δ is defined as the loss tangent, i.e. the ratio of the loss modulus to storage modulus. When determining T g,α in the DMA data we used the onset value of the tan δ-peak at temperatures above the β-relaxation. This is consistent with the methodology we implement when detecting T g,β and T g,α from the THz-TDS data. However, it is not possible to define T g,β at low temperatures in terms of its onset value in the DMA data, as the DMA instrument cannot acquire data at sufficiently low temperatures to reliably determine the onset of T g,β . Therefore, we instead report the range of the temperatures which the tan δ peak spans at low temperatures corresponding to the β-relaxation process.