Polymers under mechanical stress: Deformation of the nanostructure of isotactic polypropylene revealed by scanning force microscopy

Scanning force microscopy (SFM) was used to study the lamellar morphology of α and β spherulites in bulk intruded isotactic polypropylene (iPP) samples deformed under shear stress. The iPP specimens were sheared at a constant shear rate at room temperature. Just beyond yielding, at a shear strain level of 0.5, the lamellar structure of the α-spherulites resembles that for unsheared iPP; few cracks are observed along the interfaces between neighboring α-spherulites: this behavior is attributed to a “cross-hatched” lamellar texture of the α-spherulites. In β-spherulites, at the same shear strain, SFM observations reveal the presence of nanocracks perpendicular to the principal tensile strain axis, going through stacks of crystalline lamellae. Profuse local lamellar kinkings occur along the principal compressive strain axis and few nanocracks parallel to the molecular axis are seen in individual lamellae that lie along the shear direction. Neither particular preferred lamellar orientation nor localization of deformation within shear bands has been detected.

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