Volumetric rheology of polymers: The influence of shear flow, cooling rate and pressure on the specific volume of iPP and P/E random copolymers

A novel dilatometer has been used to measure the evolution of specific volume at different cooling rates (0.1-1.5 °C/s) and at elevated pressures (100-600 bar) under quiescent conditions and under shear (shear rate: 67 s-1, total shear: 135, shear temperatures: T=164 °C and 142 °C) for a series of commercial iPP homopolymers and polypropylene-ethylene random copolymers. Higher cooling rates shift the transition temperature Ttr to lower values, while an increase in nucleation density is observed, higher pressures increase Ttr. Shearing significantly increases Ttr and its influence is directly related to molecular weight and polydispersity and of course the temperature at which shearing is applied. In the copolymers, the composition defined by the ethylene content (0 – 8%) determines the position of the transition temperature Ttr and the pronounced influence of shearing is now counteracted by the negative influence of chain irregularities on the polymer’s ability to crystallize. Morphologies show a change from standard spherulites towards irregularly shaped objects.