The relaxation process of shear-induced crystal nucleation precursors has been investigated in a temperature range slightly above the melting point using a series of commercial grade isotactic poly(1-butene)s with different molecular weights. Development of transcrystalline morphology from the surface of a fiber pulled through the molten polymer is ascribed to high concentration of ordered clusters promoted by the alignment of chain segments under the high-intensity shear flow at the fiber−melt interface. Sheared melts isothermally crystallized immediately after cessation of flow exhibit a well-pronounced cylindritic morphology, characterized by closely spaced fibrillar branches; on the other hand, prolonged relaxation in the molten state before crystallization leads to classical spherulitic morphology. The lifetime of shear-induced nucleation precursors, t*, has been associated with the complete disappearance of the transcrystalline morphology. It has been found that systems composed of short chains relax much faster than those containing a large fraction of high molecular weight species. When relaxed slightly above the melting point of the tetragonal crystal modification the highest molecular weight samples keep memory of flow-induced structuring during several hours. Temperature has a dominant role on the kinetics of reequilibration of sheared samples:  experimental data of t* obtained in wide range of “relaxation” temperatures can be fitted by an Arrhenius-type equation with an apparent activation energy of around 700 kJ/mol. Results can be justified by considering a network of aligned and ordered polymolecular clusters, originated under the high-intensity shear flow field at the solid−melt interface, whose relaxation involves large scale restructuring.

The dissolution of flow-induced nucleation precursors in isotactic polypropylene is investigated indirectly by means of in situ rheo-SAXS measurements. The progress of crystallization and the evolution of crystal orientation are recorded in isothermal conditions after a controlled shear step followed by an annealing step of different duration at various melt temperatures. The results confirm that the survival time of shear-induced nucleation precursors is extremely large compared to typical rheological relaxation times and it is longer for the precursors originated at higher shear rate. Most important, we show that the effect of flow on the development of oriented morphologies is lost much earlier than that on the overall crystallization kinetics. A schematic model for precursors' dissolution involving gradual transformation from row into point-like nuclei is proposed.

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Rheooptical experiments on narrow isotactic polystyrene (i-PS) fractions have been carried out to investigate temperature and molar mass dependence of the lifetime of shear-induced nucleation precursors. Similar to i-poly(1-butene) and i-polypropylene, the survival of the flow-induced structures lasts very long, even at temperatures well above the measured melting point of the crystals. It has been observed that the decrease of concentration of nucleation precursors follows a first-order kinetics with a strongly temperature-dependent rate constant. The apparent activation energy of the overall relaxation process is around 400 kJ/mol, of the same order of magnitude previously found for other semicrystalline polymers. A comparative analysis of the apparent activation energy data for different polymers, coupled with morphological evidence, suggests that the rate-determining step in the evolution of the system toward the equilibrium state in the melt is the detachment of stems from the lateral surface of flow-induced oriented bundles, present either as isolated entities, in the case of pointlike nuclei, or as rows alternated by disordered nanodomains, in the long threads of the shish. Thanks to the monodisperse character of the investigated samples, the molar mass dependence of lifetime of oriented nucleation precursors has also been established.