Effect of Melt Annealing on the Phase Structure and Rheological Behavior of Propylene-Ethylene Copolymers

V. DE LA TORRE; A. RODRIGUEZ FRIS; M. D. FAILLA; L. M. QUINZANI

POLYMER ENGINEERING AND SCIENCE

JOHN WILEY & SONS INC

Año: 2007 p. 912 - 921

0032-3888

The morphological and rheological properties of a commercial propylene-ethylene copolymer with 16.4 mol% of global ethylene content are investigated. Fractions obtained from this material by solvent extraction were blended with the original polymer.  The series of copolymers with different concentrations of poly(ethylene-co-propylene) prepared in this way were also studied. The morphological study is done analyzing the phase structure of samples exposed to different thermal and mechanical histories using scanning electron microscopy. The linear viscoelastic properties of all the molten polymers were measured using small amplitude oscillatory shear flow and different test sequences. These sequences include dynamic frequency and time sweeps. The copolymers present an initial phase structure form by domains of approximately 1 mm dispersed in a polypropylene matrix. This phase structure changes dramatically with time when the polymers are kept in the molten state due to the coalescence of the domains. For example, regions of more than 10 mm of average diameter are observed in the original copolymer after 90 minutes at 178°C at rest. The morphological results parallel the changes observed in the viscous and dynamic moduli with time during the dynamic time sweeps. The value of the dynamic moduli depends on the mechanical history applied to the samples during the rheological tests. For example, the elastic modulus of the commercial copolymer increases approximately 32% during a dynamic time sweep of 45 minutes using a frequency of 0.1 s-1, while it decreases ~18% when a frequency of 1 s-1 is applied. Moreover, the modulus measured at 0.1 s-1 of samples annealed at rest during 45 or 90 minutes is, respectively, 58 or 100% larger than that of the fresh material.