Shear‐Enhanced Nucleation of Isotactic Polypropylene in Limited Space

Abstract

The nucleation rate was measured by directly counting the number of nuclei, which were developed while an isotactic polypropylene melt was flowing under shear in a thin film. The nucleation rate was enhanced with an increased rate of shear, e.g., by a factor of 10 larger at the rate of shear of 14 s 21 compared with the quiescent state, at 1348C. The ratio of the shear-enhanced nucleation rate to the nucleation rate in the quiescent state was larger at a higher temperature of crystallization, i.e., about 10 times at 1348C to 590 times at 1408C. The increase of the nucleation rate under shear flow was explained by a reduction of the lateral and end (fold) surface free energies; the product s s 2 s e decreased to 3.2 Â 10 27 for the sheared melt, from 6.0 Â 10 27 (J m 22 ) 3 for the isotropic state. The free energy reduction was caused by transition of the nucleus formation mode from three-dimensional folded chain nuclei to two-dimensional bundle nuclei, in which chains lie down on the glass substrate, aligning parallel to the flow direction.

Shear flow of molten polymer in melt blowing

“…So, there should be an annular distribution of contour line of shear rate in the fiber cross‐section, because there is the same shear rate with the same radius. Because shear flow was efficient to enhance the crystallization kinetics [23, 24], there might be an annual contour of microcrystallization gradient structure in the fiber cross‐section (see Fig. 5), similar with the sheath‐core structure in the literature [18].…”

Bubble formation in lattice Boltzmann immiscible shear flow

“…[6][7][8] Mesoscopic interparticle potential supplies a unique chance to tackle this question because the shear is easily introduced in the lattice Boltzmann equation and the format of the interparticle potential is suitable for any flow environment. This has not been addressed clearly.…”

“…Many efforts have been devoted to improve the description of bubble formation within or beyond the classical nucleation framework, for example, the bubble and droplet formation without nucleation barriers. [6][7][8] Observations of bubble formation are usually restricted by the resolution of analysis equipment and the capability of sample preparation, which makes it difficult to access the properties of tiny bubbles. The question of whether the shear induces or suppresses nucleation is still a debating topic.…”