Nature of intergrain bonding upon compaction of ultrahigh-molecular-weight polyethylene reactor powders

Abstract

165 Undoubted advantages of methods for obtaining high-strength filaments and fibers directly from reactor powders (without preliminary melting and/or dissolution) are their environmental friendliness and reduced energy consumption. Studies of the mechanism of transformation of compacted reactor powders (CRPs) into an ultrahigh-strength oriented material point to a decisive role of cohesive bonds between grains. Their formation is caused by deformation, diffusion, and recrystallization in the boundary layers of neighboring grains upon compaction [1‐4]. To some extent, analogous processes inevitably proceed in the interior of grains, although the complete failure of the native structure of reactor powder grains, for example, on remelting, renders the attainment of high draw ratios impossible [5]. The lack of a direct method of identification of cohesive bonds between grains makes it impossible not only to develop theoretical models of deformation of heterogeneous systems but also to control the density of a network of cohesive bonds between grains during optimization of the processes of fabrication of ultrahigh-strength materials based on UHMWPE reactor powders. In this work, we identified for the first time such cohesive bonds between grains in study of the deformation of CRPs in adsorption-active liquid media. At room temperature, CRP samples in which consolidation was intentionally not completed can undergo brittle fracture at small strains [4]. The tensile properties of such materials in air are mainly determined by the physical and cohesive bonds at the intergrain boundaries, where fracture cracks are nucleated. Under these conditions, it is difficult to separate out the contribution of cohesive bonds to the total strength of a heterogeneous system. However, as is known [6], the interfacial surface energy γ sl at the boundary with the medium has a crucial effect on fracture of solids (the Rehbinder effect). By using physically active (adsorption-active) media, the strength of physical bonds between the units of a heterogeneous system can be considerably changed. At the same time, similar media have no direct effect on the strength of chemical bonds. This has allowed us to suggest that deformation in liquid media will provide the possibility to separate the “physical” and “chemical” components of strength in PE CRPs, i.e., to identify intergrain cohesive bonds and determine the density of their network. In this paper, we report the results of our studies in this direction. As distinct from low-molecular-weight compounds, the fracture of polymers is inevitably associated with the displacement and/or rupture of polymeric chains. The rupture of cohesive bonds sharply increases the fracture work, and the relationship between γ sl and