Rheological Properties of a Partially Molten Polypropylene Random Copolymer during Annealing

Abstract

Semicrystalline polymers exhibit a broad melting transition. This effect is due to the lamellar thickness distribution in the materials. The melting temperature T m (1) of a given lamella increases with its thickness 1. When heating to a temperature T a in the melting range, the crystalline lamellae with T m (1)< T a melt. This process is accompanied by the thickening of the nonmolten lamellae which suck in molten polymer chains. We used a polypropylene random copolymer as a convenient material to investigate rheological and structural aspects of this reorganizational process. The temperature and time dependence of the dynamic moduli G' and G″ in oscillatory shear flow and for the first time of the transient uniaxial elongational viscosity were measured during the reorganization of the partially molten material. Data from differential scanning calorimetry (DSO runs on the annealed samples are correlated with the respective data from the rheological experiments. With increasing amount of crystalline material the shear viscosity function increases at small shear rates. At the same time marked strain hardening shows up in uniaxial elongation. The rheological phenomena observed during annealing at temperatures in the melting range are interpreted by assuming a network of nonmolten, growing lamellae which are connected by tie chains