Large Strain Requirements for Shear Induced Crystallization of Isotactic Polypropylene

Strain effects on the crystallization of a series of isotactic polypropylenes ($i$PP), with various molecular weights, were studied using rheology, light transmission, birefringence, differential scanning calorimetry, and wide-angle X-ray diffraction. Transmitted light intensity measurements demonstrate that the effect of pre-shear on crystallization rate keeps increasing up to very larger strain levels, much beyond strains that are required to reach steady shear flow (at given \textit{We}). Crystal orientation sets in at a total strain of about \textit{$\gamma $}$_{0}=$600 or higher. WAXD and DSC analyses corroborated the light transmission results. The samples were pre-sheared and then crystallized at constant temperature. Total shear strains \textit{$\gamma $}$_{0}$=200 to 1000 were applied to the $i$PP samples at the beginning of a crystallization experiment, after the samples had reached the crystallization temperature of 145$^{o}$C (under-cooled state). A constant Weissenberg number \textit{We}=1 (\textit{We}, defined as the product of shear rate and a relaxation time) was maintained throughout the study. \textit{We}=1 corresponds to the onset of shear thinning in steady shear. Deborah number values were low, \textit{De}$<<$1, indicating that steady shear flow had been reached in all pre-shearing runs.