In order to optimise the formulation of microspheres and achieve release delivery of the active drug, the microsphere product ought to be stable. When determining the chemical stability of a material, molecular mobility is a key factor and it has been shown that an increase in molecular mobility is directly linked to an increase in the chemical degradation of a material and aggregation, and therefore its storage stability. Thus, it is critical to understand the molecular mobility behaviour of a material and its dependance on temperature, and specifically the molecular dynamics of the material leading up to the glass transition temperature (T g ). It has been shown that a great number of polymers and amorphous pharmaceuticals, ranging from small molecules to high molecular weight peptides and proteins exhibit at least two dielectric relaxation processes: the primary, or α-relaxation process and and a secondary, or β-relaxation process. The former can be observed at temperatures above T g and can be designated as T g,α , and the latter occurs at temperatures below T g and is associated with a secondary glass transition process, which can be indicated as T g,β . In previous work we investigated these processes in neat PLGA 50:50 and 75:25 of low, medium, and high molecular weights: For all the copolymer PLGA film samples we observed three regions of relaxation behaviour with distinct T g,β and T g,α transition temperatures. We showed that with an increase in temperature, the copolymer chains can transition through different conformational environments, each constrained by its characteristic potential energy landscape, and that the movement of each PLGA chain is restricted by adjacent entangled chains. At T g,β , the system has sufficient thermal energy and free volume to overcome the energy barrier arising from chain entanglement, allowing for local mobility to occur. Furthermore, we show that there is a relationship between free volume and the value of T g,β for the different film samples.