Upon increasing the temperature the density of the copolymer will decrease further due to thermal expansion, which means that in turn the free volume increases. As a result, cooperative motions can gradually take place alongside the more localised motions that are responsible for the β-relaxation process. It is commonly accepted that the T g,α coincides with the glass transition and that this process corresponds to the structural relaxation of the polymer. With an increase in temperature, segmental co-polymer movement becomes possible, and at the glass transition the dominating motions are likely to be the large-scale chain motions. We believe that these motions give rise to T g,α of the PLGA materials.