Detecting the α-and β-relaxation processes can be achieved using various techniques, such as Brillouin light scattering, neutron scattering, dynamic mechanical analysis (DMA), Raman spectroscopy, dielectric spectroscopy, and THz-TDS. Terahertz time-domain spectroscopy (THz-TDS) is a relatively recent technique that can be used to investigate the molecular dynamics of relaxation processes at high frequencies. It is a non-contact technique which can be used to measure polymer mobility over a broad temperature range at frequencies of 0.1 -3 THz and can be applied to investigate the microscopic mechanisms of amorphous polymer dynamics. Using THz-TDS we previously showed that a sub-T g transition can be found in organic molecular glasses. The behaviour of materials at the temperature region below T g,β has been explored using various techniques, including THz-TDS and calorimetric techniques. To date, the majority of THz-TDS studies presented in the literature have investigated small organic molecules or hydrogen bonded systems. This present work systematically examines a number of polymer systems by investigating the behaviour and trends these exhibit as a function of and the values of T g,β and T g,α obtained. The purpose of this study is to better understand the relationship between the relaxation dynamics and the molecular structure of PLGA systems that vary in molecular weight (MW) as well as in lactide to glycolide ratio using THz-TDS and DMA. Finally, we propose an explanation of the mechanism of the α-and β-relaxation processes in amorphous PLGA. For each sample, a sandwich structure consisting of a layer of polymer embedded between two z-cut quartz windows of 13 mm diameter and 2 mm thickness each (Crystran Ltd., Dorset, UK), was formed by vacuum compression moulding (VCM), using the VCM Essentials tool (MeltPrep, Graz, Austria) operating with a circular die of 13 mm inner diameter die. For THz-TDS experiments, the resulting sandwich structure consists of two z-cut quartz windows enclosing the sample, which was separated by a glass fibre reinforced Teflon spacer. The sample thicknesses was adjusted with the spacer and ranged between 300 -600 µm. Using the VCM Essentials tool, initially, the samples were subjected to vacuum compaction and pre-drying, followed by a vacuum melting step and subsequent cooling and solidification. Suitable temperatures for sample preparation were identified in the range of 408 -413 K. Vacuum compaction and pre-drying at room temperature were performed for 3 minutes at 20 mbar. The vacuum melting step to form the quartz/polymer/quartz sandwich moulding structure was completed within an additional 10 minutes under continued vacuum at 20 mbar, allowing for sufficient time for the polymer sample to melt. Then, the VCM tool was manually transferred to the cooling unit of the VCM Essentials setup and the sample was allowed to cool to room temperature for 5 minutes without active cooling.