As the MW is increased in PLGA 75:25, the values of T g,β decrease as the free volume effects on the molecular dynamics start to dominate. For the medium and high MW PLGA 75:25, the increase of methyl side groups, which can freely rotate, disrupt the dense packing of the co-polymer chains, resulting in an increase in available free volume and the molecular mobility of the copolymer backbone (Figure ) and (Figure , rotation 2). Consequently, this increase in free volume between chains loosens the steric hinderance on the C-O bond rotational motion of the co-polymer backbone and reduce T g,β , which is exactly what we observe in medium and high MW PLGA 75:25 (Figure ). As the polymer segments become more mobile, small, isolated regions of free volume between the individual chains merge into larger regions of free volume, thereby reducing the energy barrier for segmental motions and a lower onset temperature of T g,α .