Shear-induced crystallization of isotactic poly(1-butene)

In the present work the kinetics of the isothermal quiescent crystallization of the isotactic Poly(I-butene) (i-PB) is analyzed by means of theological measurements and is used as an internal reference to study the effects of the shear flow. In particular, the flow-induced crystallization of two i-PB samples with different molecular weight is investigated by step-shear experiments at the crystallization temperature, Tc, of 95degreesC. Indeed, with step-shear experiments the effects of the applied shear rate and shear strain on the crystallization kinetics can be separately analyzed. In all the cases the overall-crystallization rate constant, k, defined as the inverse of the half-time of crystallization (t(0.5)) has been evaluated. The results show that a critical shear rate (at a constant shear strain) and a critical shear strain (at a constant shear rate) are necessary to obtain the enhancement of k. These results significantly depend on the molecular weight revealing a stronger sensitivity to flow for the higher molecular weight sample. On the contrary, in the quiescent experiments no evidence of relevant molecular weight effects is detected. The in-situ optical analysis has shown that faster growth rates and a strong enhancement of nucleation density are observed in the step-shear experiments compared to the quiescent case.