The tensile deformation process of polypropylene at high pressure

With the application of high pressure there were two transitions in the tensile response of polypropylene, one associated with the crystalline component and the other with the amorphous component. The first was the development of a distinct bend to lesser slope in the stress-strain curve prior to the stress maximum. This is the yield transition which is associated with the onset of crystalline flow. The second transition is a jump in the tensile response at a particular pressure. This was determined to be due to a pressure-induced rubber-to-glass transition of the amorphous component. Comparing the tensile response across these two transitions results in a more complete understanding of the interaction between the amorphous and crystalline components at various stages of tensile deformation prior to the stress maximum.;Specifically, the onset of yield is attributed to the creation of a void volume of sufficient size to allow nucleation of dislocations. Based on the sensitivity of the yield stress to the glass transition pressure, it is suggested that the nucleation site is at the crystalline-amorphous interface. This geometry defines a constrained ductile interlayer which will generate the triaxial tensile state of stress necessary to form a void. The triaxial tensile state of stress required for void formation must overcome the triaxial compressive state of stress due to the hydrostatic pressure and accounts for the linearity observed between the yield stress and pressure in polymers.;Deformation prior to crystalline yield is largely elastic, and is due to chain extension of the tie molecules within the amorphous component. Prior to crystalline flaw the tie molecules are effectively pinned at their entry points into the lamellae, and respond in a manner similar to an elastomeric polymer in the final state of extension. In this case the limiting strain is defined not by specimen fracture but by formation of avoid large enough to allow dislocation nucleation. The pressure dependence of the tensile modulus at finite strains is a direct consequence of the critical tensile stress required for void formation for crystalline yield coupled with the deformation response of the tie molecules which are in the final stage of elastomeric extension.