An investigation of equibiaxial hydrostatic deformation of isotactic polypropylene: Process mechanics, order-disorder transition, crystal texture and deformation mechanism

This thesis studies the development of crystal texture and deformation mechanisms of isotactic polypropylene (i-PP) in the solid state. The strain field is hydrostatic, equibiaxial as produced by uniaxial compression. The central issue addressed is the deformation-induced order-disorder phenomenon. The disordered phase, referred to as the smectic modification of i-PP is stable below 70$\sp\circ$C and its formation is used to probe the deformation mechanism and its kinetics. The biaxial field induces a particular orientation of the smectic phase with respect to the crystal to reveal unique information on the deformation mechanism of i-PP, and the structure (and formation) of the smectic phase. The deformation mechanism of i-PP is explained by a decrystallization-recrystallization process. For deformation at high strain rates and low temperature, the recrystallized phase is the smectic phase which further participates in the deformation process inducing ductility. Thus the deformation undergoes two activated processes: crystal-to-smectic and smectic-to-crystal. Ductility can be enhanced by diminishing the second process. This principle of enhanced ductility induced by a crystal-to-crystal transition, can be generalized to other semicrystalline polymers to maximize crystal orientation for a given draw ratio. The crystal orientation (as measured by WAXS) on compression drawn i-PP revealed a double texture: (i) Fiber texture and (ii) Planar texture. The Planar texture converts to Fiber texture as the deformation proceeds. The reason for the two textures is explained in terms of major slip system: (0k0), $\langle001\rangle$. From this texture analysis a classification of semicrystalline polymers is proposed, based on their slip systems. The proposal states a criteria (and testing method) for selection of polymers for applications requiring biaxial orientation. A theoretical model is developed to simulate the stress-strain behavior of i-PP on uniaxial compression. The model assumes a rigid plastic behaviour of i-PP with Bauschinger effect. This one parameter theory fits the experimental curve (within 5%) up to compression ratios $\u3e$ 7x and temperature range from 30 to 130$\sp\circ$C. Yield stress in compression and tension is obtained in a single experiment from such a fit. The dependence of yield stress and the ratio of the two yield stress on draw temperature is consistent with the order-disorder phenomenon and the deformation mechanism