Spherulitic Nucleation and Growth Rates in an iPP under Continuous Shear Flow

The study of polymer crystallization enhanced by flow has attracted much interest because it implies the possibility of controlling the final morphology and the resulting mechanical and functional properties of semicrystalline polymers. An improved understanding of the fundamentals of flow-enhanced crystallization effects can help to tailor advanced processing strategies. Indeed, a complete understanding of the fundamentals of structure development during processing remains a challenge. In this work, the effect of a steady shear flow applied during crystallization on the morphology evolution and on the kinetics of isothermal crystallization of an iPP has been studied experimentally. In particular, measurements of nucleation and growth rates of spherulites during continuous and constant steady shear flow were performed by means of a Linkam shearing cell coupled with an optical microscope. During all the tests carried out in this work, the dominant crystalline structure was fully spherulitic. It was found that nucleation density in quiescent conditions remained constant with time (i.e., no nucleation rate was observed during the test). On the contrary, under shear flow, an increase of nucleation density with time was observed. This increase was found to be essentially linear with time, The linear dependence allowed to calculate a constant nucleation rate, which was found to be dependent on shear rate according to a power law expression whose exponent was found to be about 3. The evolution of crystallinity under shear conditions, calculated combining the results obtained on nucleation and growth rate, was also successfully compared with data obtained in the same conditions by means of in situ wide-angle X-ray diffraction. Finally, an attempt was made to determine scaling rules which can describe the effect of flow on crystallization kinetics.