Physical aging and enthalpy relaxation in polypropylene

Abstract

The physical aging of semi-crystalline polypropylene at 20°C has been studied by low strain torsional creep, and compared with the enthalpy and density relaxation behaviours. The physical aging rate of 0.72 is much smaller than the value of unity usually found for amorphous polymers. Differential scanning calorimetry studies reveal the appearance of an endothermic peak at or above approximately 35°C, which increases in magnitude and shifts to higher temperatures on aging. Simultaneously, the glass transition temperature, T g, increases and the relaxation strength at T g decreases. The kinetics of enthalpy and density relaxation at 20°C in polypropylene are shown to be inconsistent with the usual behaviour of amorphous polymers below T g. It is suggested that these observations could be rationalised if the structural changes in polyprophylene on aging at 20°C involve an ‘ordering’ of an interfacial region between the crystalline lamellae and the bulk amorphous material.