Thermorheologically complex self-seeded melts of propylene-ethylene copolymers

A novel approach to study self-nucleation in semicrystalline polymers is presented. Self-nucleation, i.e., the peculiar increase of recrystallization kinetics associated with a range of melting temperatures above or below the melting point, is investigated in parallel with differential scanning calorimetry (DSC) and rheological measurements, for a series of metallocene propene/ethylene random copolymers of varying comonomer content. DSC experiments revealed the exact temperature region where self-nucleation effects are detected in the various samples, while with dynamic viscoelastic results the obedience to the time-temperature superposition principle (TTS) is tested, for both self-nucleated and homogeneous melts. Self-nucleated melts do not obey to the TTS principle, contrary to fully isotropic copolymer melts. Such rheological thermocomplexity constitutes the first physical experimental evidence of the presence of melt heterogeneities, which act as self-nuclei when the melt is cooled and recrystallizes. The degree of thermorheological complexity of the different copolymers is quantified and correlated with the original crystalline content of the copolymer.