Because of twisting correlation of crystallites along the radial direction, polyethylene spherulites are known to develop concentric band pattern. The mechanism of branching and reorientation of lamellar crystallites in the banded spherulites has been examined experimentally by optical and atomic force microscopies associated with quenching and chemical etching. The microscopic observation suggests a branching instability of lamellar crystals at the growth front of the spherulite. We propose a mechanism of consecutive branching and twisting reorientation of branches regulated by the inherent torsional stress expected for the banded spherulites and the branching instability. We have experimentally examined the relationships among the growth rate, the maximum lateral width of crystals at the growth front, and the period of bands for three different molecular weight fractions of polyethylene. The predicted relationship among them holds for the fractions.

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The mechanism of instability-driven branching of lamellar crystallites in the formation of polymer spherulites has been examined experimentally for polyethylene-banded spherulites in terms of the molecular weight dependence by optical and atomic force microscopies associated with quenching and chemical etching. The possibility of instability-driven branching has been suggested by the experimental results of our previous work in terms of the temperature dependences of the band spacing, the lamellar width at the growth front, and the growth rate. The examination of the dependences on the average and distribution of molecular weights enables us to differentiate possible origins of the instability, e.g., compositional gradient with the diffusion of uncrystallized fractions and pressure gradient caused by the density difference between the crystal and the melt. The experimental results suggest the instability driven by the pressure gradient in the melt ahead of the growth front.

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