Thermal dilatation of polymers

The glass-transition temperature, Tg, is the narrow temperature interval (usually approximated by a point) above which long-range mobility of macromolecular segments exists and below which it ceases. For such mobility to be realized two prerequisites are required: (1) a free volume sufficiently large to accomodate the moving segment must exist, or be formed, adjacently to it, and (2) the segment itself must possess sufficiently high thermal energy to overcome the energy barrier confining it to its equilibrium position. The high segmental mobility reflects a relatively low packing density, D, and is reflected in a relatively high coefficient of thermal expansion, α.