Large strain requirements for shear-induced crystallization of isotactic polypropylene

We studied strain effects on the crystallization of a series of isotactic polypropylenes (iPP) of various molecular weights using rheology, rheo-optical measurements, differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD). The samples were pre-sheared and then crystallized both at the same temperature. Transmitted light intensity measurements demonstrate that the effect of pre-shear on crystallization rate keeps increasing with strain up to surprisingly large strain levels, much beyond strains that are required to reach steady shear flow (at given We). Crystal orientation sets in at a total strain of about or higher. WAXD and DSC measurements corroborated the light transmission results. Total shear strains to 1000 were applied to the iPP samples at the beginning of a crystallization experiment, after the samples had reached the crystallization temperature of 145°C (under-cooled state). A constant Weissenberg number We=1 (We is defined as the product of shear rate and a relaxation time) was maintained for all pre-shearing of this study. We=1 corresponds to the onset of shear thinning in steady shear. Deborah number values were low, De<<1, indicating that steady shear flow had been reached in all pre-shearing runs (De is defined as the ratio of relaxation time to pre-shearing time). Further studies are needed at high We as there are indications that strain requirements are much reduced at high We. A fundamental understanding is still missing.