Glass Transition, Crystallinity and Entanglement in Polymers

Polymers can be fully amorphous or partly crystalline. At the glass transition, Tg, motion of amorphous chain segments becomes fast enough on the observation timescale, and those regions soften. Crystals do not melt at Tg. This kinetic change has a heat-capacity change, not the latent heat of equilibrium melting. Measured Tg depends on timescale and thermal history. A broad temperature range alone does not distinguish Tg from polymer melting.

Rigid backbones and interactions that restrict segment motion often raise Tg. Bulky side groups can restrict motion, but also change packing and free volume. Polarity alone cannot fix a Tg ranking. Compare measured values: amorphous regions below Tg are glassy; above Tg they are more mobile on the relevant timescale. Crystallinity and crosslinks also affect whole-sample stiffness.

Entanglements are temporary constraints between long chains that slow relaxation, not new bonds. Me describes molar mass between entanglements; the viscosity crossover Mc is related but need not equal Me. Compare similar linear-polymer melts at the same temperature using zero-shear viscosity, the low-rate Newtonian plateau where viscosity is independent of shear rate. Above Mc it rises more steeply with molar mass, without a jump. At higher shear rates, entangled melts can shear-thin.

Differential scanning calorimetry, DSC, measures heat flow during a temperature scan. Tg usually appears as a baseline step because heat capacity changes. Crystallisation releases heat; melting absorbs heat. Check whether the plot draws heat absorption upward or downward before assigning peak direction. A heating scan need not show all three events. A sample that remains fully amorphous has no crystalline melting peak.