Rheological scaling and modeling of shear-enhanced crystallization rate of polypropylene

Shear-induced isothermal crystallization of a commercial isotactic polypropylene (iPP) has been investigated by using a rotational rheometer at the steady shear rates ranging from 0.00012 s⁻¹ to 1 s⁻¹, and the temperatures from 135 to 145 °C. Two time scales can be utilized to characterize the crystallization rates: one is the level-upturn onset time of the viscosity; another is that of the normal force. Plotting the onset times against the corresponding onset strain, a common critical value for all the undercooling temperatures can be identified, below which the shear flows have no significant effect on the crystallization rates. Furthermore, we propose a concept of dimensionless onset work; this parameter can make the normalized onset times approximately temperature-invariant in the range of our experiment. Our modeling of the quiescent crystallization is based on the nucleation theory of Ziabicki; the results indicate two-dimensional crystallite growth on pre-existing nuclei. The shear enhanced crystallization is modeled by estimating the excess free energy induced by the flow, and using the rheological model recently proposed by Marrucci, in which the required relaxation times are derived from our rheological measurements. The results imply that the crystallization under the present low shear rates is still two-dimensional crystallite growth on pre-existing nuclei, thus supporting the athermal nucleation theory proposed by Janeschitz-Kriegl. Compared with the experimental data, the modeling is only partially successful. Further improvements encompassing the effects of shear flows on the non-linear increase of the number density of athermal nuclei and on the acceleration of polymer chain disentanglement are needed.