Microscopic in situ observation of the plastic deformation of polybutene-1 films under simple shear

Optical micrographs were obtained in situ during the course of simple shear tests operated on thin films of isotactic polybutene-1 (modification I) and the stress-strain curves were recorded simultaneously. A detailed observation of individual spherulites showed that the radial crystallites perpendicular to the major principal tensile axis of stress are separated by bending and that those inclined on this axis participate in the plastic deformation. It is demonstrated that the simple shear test cannot be performed on polymer films without being perturbated by plastic buckling and by a deformation of the material in the grips. These artefacts do not seriously affect the results concerning the microstructure dependence of the yield stress but make questionable the quantitative interpretation of the plastic regime. A geometric criterion is proposed for the design of shear samples which could undergo the plastic shear strain without side effects.