Tensile tests in the environmental scanning electron microscope (ESEM) - Part I: Polypropylene homopolymers

Performing tensile tests in an ESEM enables the simultaneous recording of both the force-elongation diagrams and the microstructures forming at the crack tip. Whereas the former are macroscopic parameters, the latter provide information on the micro-scale and are dependent both on the microstructure of the material itself and the test parameters. Changes in the fracture behaviour during a tensile test can be directly observed and correlated with the respective part of the force-elongation diagram.Neat polypropylene samples with varying MFI values and different types of crystal modification and structure were investigated. Videos of the crack propagation at the crack tip clearly show that irreversible deformation already starts at forces far below the maximum load. The size of the spherulites seems to determine the size of the structures developing at the crack tip. Cyclic tests demonstrate that the respective structures can deform elastically, but they do not heal. Furthermore, relaxation experiments prove that relaxation does not take place at the central fracture region but beyond it. Decreasing the specimen temperature below the glass transition temperature of the material provides information about the change in the microstructures at the crack tip involved therewith.