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Flow induced crystallization (FIC) has a large impact on kinetics and morphology of semicrystalline polymers, and can therefore drastically change final properties such as dimensional and thermal stability, modulus and strength. Processing of polymers usually involves flow, so it is important to understand the mechanism of FIC. It is known that oriented precursors formed during flow are the key to FIC but they are not yet well understood. In this study, flow induced precursors are investigated by examining their relaxation, and the effect of comonomer is probed.
Using a commercial isotactic polypropylene and random propylene-ethylene copolymers, a fiber pull-out technique coupled with polarized optical microscope and a hot stage has been used, as in terms of sensitivity is the most powerful indicator of melt perturbations. The distinct morphological zones that happen after shear and relaxation (cylindritic or highly nucleated morphology and classical spherulitic morphology) have been used as an indicator of whether full reequilibration has been attained or not in the sheared melt. The lifetime of precursors, t*, has been associated with the full disappearance of cylindritic morphology. Data of t* obtained in a range of relaxation temperatures were fitted with an Arrhenius-type equation leading to values of apparent activation energy and detaching stem length. The apparent energies of activation Ea for disappearance of precursors and stem detaching length LS were found to decrease with increasing ethylene content. The values of Ea for iPP in this study were somewhat larger than those in the literature. The possible reasons for such differences are discussed. The values of LS were within the range previously found for other semicrystalline polymers.
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