Entropy change with yielding and fracture of polypropylene

This paper discusses the influences of plastic yielding and fracture on the entropy change in polypropylene having different morphologies, induced by different cooling rates during molding. Compressive tests were performed over various loading rates to analyze the yielding kinetics and Ree-Eyring's model was used to analyze the entropy change in the material. The results suggest that plastic deformation implies more cooperative segmental motions for higher crystalline content, which result in higher values of the pre-exponential factor containing the entropy term. The parameters related to the secondary relaxation process did not undergo any significant change during ageing, suggesting that ageing at room temperature does not affect localized molecular motions. The transition from ductile to brittle fracture involves however the amorphous polymer that evolves with ageing. Physical ageing has thus very little influence on yielding performance, but strongly affects the fracture process of the polymer. Dielectric spectroscopy was conducted in the frequency domain using a broadband dielectric spectrometer. for the lowest and highest crystalline morphologies in both aged and unaged states. The results showed significant changes in the dissipation factors of the primary relaxations of the amorphous polymer with ageing time, for both low and high crystallinity samples. On the other hand, no change in the secondary relaxation was observed in either case. (C) 2009 Elsevier Ltd. All rights reserved.