Simultaneous morphological and rheological measurements on polypropylene: Effect of crystallinity on viscoelastic parameters

Abstract

A rotational rheometer equipped with an optical module is adopted to characterize the evolution of rheological parameters and at the same time the morphology development during isothermal crystallization. This allowed the determination of the effect of crystallinity on the viscoelastic functions of an isotactic polypropylene. A linear multimode Maxwell model was then applied to obtain the modulus and relaxation time of each mode as a function of the measured crystallinity degree. It was found that at low crystallinity levels, the increase of moduli with crystallinity is about the same for all the modes whereas, when crystallinity degree rises, the increase of moduli is higher for the modes characterized by higher relaxation times. Concerning the relaxation times, it was found that the increase due to crystallinity is about the same for all the modes and reaches a factor of about 10 when relative crystallinity degree is close to 10%. The effect of crystallinity on complex viscosity was also assessed by determining a shift factor due to crystallinity. It was observed that the data collected at the lowest frequencies need higher shift factors with respect to the other ones. This was ascribed to the fact that more than one shift factor is needed to describe the effect on moduli, and low frequencies stir modes with higher relaxation times, which undergo a moduli increase larger than modes with shorter relaxation times.