The structural units of the polymer chain act as a collection of kinetic units such that each is capable of moving independently alongside the cooperative motion of adjacent macromolec- is expected to be low as the conformation is defined by relatively shallow potential energy minima that govern its dihedral angles. This motion could be coupled or accompanied, and indeed facilitated, by the rotational motion of small polymer segments which include the methyl side group 5 (rotation 2 albeit the methyl rotation itself is not infrared active). It is highly conceivable that such motions can readily occur at low temperatures, as the rotational barrier for methyl groups is well known to be rather low, of the order of a few kJ mol -1 . We believe that it is these types of molecular motion that give rise to T g,β of the PLGA materials.