The effect of thermal treatment on the mechanical performance of biaxially oriented poly(p-phenylene sulfide) by cold roll milling was studied. It was found that the ductility of the cross-rolled material could be further enhanced by annealing at temperatures above Tg and the yield strength could be enhanced by annealing at temperatures below Tg with a partial loss in ductility. Two-dimensional wide-angle X-ray diffraction patterns suggested a reduced crystallinity in the rolled material and showed that the crystal structure of the unrolled material could be partially recovered by annealing above Tg; however, scattering patterns adjacent to the crystalline reflections persisted with heat treatment. Modulated differential scanning calorimetry (MDSC) and broadband dielectric spectroscopy indicated that cross-rolling altered the kinetics of the cold-crystallization process. MDSC and dynamic mechanical analysis measured a reduction in Tg suggesting enhanced molecular mobility as an outcome of cross-rolling. After annealing at temperatures well beyond Tg, a large reduction in elongation to failure was observed, and the material could not cold draw. These results suggest that a two-stage process involving plastic deformation by roll milling followed by annealing treatment may be used to commercially produce high toughness and yield strength sheet products from commercially available resins.

Deformation of polyethylene by bidirectional cold rolling was investigated. The objective was to better understand the evolution of the hierarchical structure with thickness reduction and the effect on mechanical properties. At a 60% thickness reduction, the thickness recovery after rolling peaked, and the density and crystallinity decreased rapidly resulting in greater optical clarity. Tensile specimens were deformed in uniaxial tension, and it was observed that the yield mechanism shifted from necking and whitening to a diffuse yielding process with extent of rolling. Concomitant greater work hardening, fracture stress, and elastic recovery were also observed upon fracture at both 25 degrees C and -40 degrees C. Scanning electron micrographs of the fracture surface revealed discrete buckled microlayers. Furthermore, dynamic mechanical analysis (DMA) revealed the loss tangent peaks associated with the beta and alpha relaxations shifted, suggesting that dilatation rather than compaction was the dominant mode of deformation at 60% thickness reduction.

In this study, we propose a novel two-stage solid-state processing technique to fabricate thin-oriented high-density polyethylene (HDPE) rods with enhanced mechanical properties. This unique solvent-free technology involves biaxial rolling (Stage 1) followed by a uniaxial orientation procedure (Stage 2). The oriented thin HDPE rods exhibit an impressive modulus of 37 GPa. This is a result of a complex, oriented composite structure containing crazes with a network of interlocking fibers. Surprisingly, the high-modulus thin rods were created under relative low uniaxial tensile orientation conditions having draw ratio of eleven. Gel-spun ultra-high-molecular-weight fiber processing requires orientations greater than thirty. A corresponding scaled hierarchical model was proposed to summarize the process-structure-property relationships. This study demonstrates a strategy for creating high-modulus polymeric materials via controlled solid-state processing.