Insights into Formation and Relaxation of Shear-Induced Nucleation Precursors in Isotactic Polystyrene

Abstract

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.

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

“…This behavior differs from what usually observed for other not compatibilized polyolefins [1,2,[11][12][13][14][15] for which, as reported in literature, crystallization kinetics dramatically increases on increasing the shear rate. Temperature (°C) On the contrary, from Figure 4, related to the LDPE:PE-g-MA:D72T 90:9:1 nanocomposite, a slightly different situation can be observed.…”

“…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.…”

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

“…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. 52 Notwithstanding the considerable experimental efforts in this field, some open questions still remain.…”

Morphology evolution and crystalline structure of controlled‐rheology polypropylene in micro‐injection molding

“…It has been reported that the relaxation behaviors of polymer molecules are primarily dependent on the chain length (molecular weight). [45][46][47] Therefore, it is necessary to investigate molecular weight and MWD of CRPP. The MWD curves are shown in Fig.…”