Lifetime of Shear-Induced Crystal Nucleation Precursors

Abstract

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.

Thermal stability of shear‐induced precursor structures in isotactic polypropylene by rheo‐X‐ray techniques with couette flow geometry

“…) corroborated the earlier observations, not only in PE but also in a variety of other polymers, such as iPP, 43,44,50,[52][53][54][55][56][57][70][71][72][73][75][76][77][78] isotactic poly-1-butene (iPB), 90 polyethylene terephthalate (PET), 91 polydiethylsiloxane (PEDS), 91 etc. In some report, the quasicrystallographic flowinduced structures are referred to as ''mesophase.''…”

“…In this regard, the studies of Azzurri and Alfonso. 90 and Zuo et al 92 deserve a special mention; in a series of well-designed experiments, they systematically investigated the relaxation behavior of shear-induced precursors in iPB melts of different molecular weight and in bimodal PE blends containing a high molecular weight species of narrow molecular weight distribution, respectively. Both studies showed that the long chains are primarily responsible for the memory of ordered structures.…”

Flow Memory and Stability of Shear-Induced Nucleation Precursors in Isotactic Polypropylene

“…[49][50][51] By performing fiber-pulling experiments, and using the cylindritic morphology as a marker for the presence of high linear nucleation density, it was possible to determine the lifetime of shear-induced nucleation precursors in various semicrystalline polymers at several temperatures. 49,50 The results were in good agreement with those previously reported by Janeschitz-Kriegl et al [40][41][42] Further experiments, performed on i-PBu-1 and i-PS, have shown that the characteristic relaxation time is strongly dependent on the weight-average molar mass of the polymer, both for polydispersed grades and narrow fractions. 50,52 A mechanism for precursor's dissolution, based on the detachment of chain stems from the lateral surface of cylindrical clusters, was proposed to explain the gathered experimental evidence.…”

Flow induced crystallization of LDPE nanocomposites: A rheological and morphological characterization

“…A good level of knowledge has been reached and it is well established that application of flow fields before and/or during polymer solidification results in a large increase of nucleation density and, as a consequence, of the overall crystallization rate [1][2][3]7,[10][11][12][13].…”

“…The application of flow promotes alignment of macromolecular chains along the flow direction, generating oriented nano-ordered aggregates which act as nuclei at the crystallization temperature [4][5][6][12][13][14][15][16][17][18][19][20]. FIC studies have been addressed to investigate the role of molecular parameters (composition [21][22][23], molar mass [24][25][26][27][28][29][30][31], etc) and flow conditions [11][12][32][33][34] on the crystallization kinetics of polymers, mainly of polyolefins.…”