Extension induced phase separation and crystallization in semidilute solutions of ultra high molecular weight polyethylene

We investigate the influence of controlled uniaxial extension on various flow induced phenomena in semidilute solutions of ultra high molecular weight polyethylene. Concentrations range from 9w% to 29w% and the choice of solvent is paraffin oil. The start-up extensional behavior is measured at various Hencky strain rates ε˙ and at two different temperatures (150∘C and 170∘C) well above the melting point. For Hencky strains ε>0.9 the qualitative behavior of the samples differ significantly depending on the imposed conditions and the concentration of the samples. Overall we propose two flow scenarios: Scenario 1 - flow induced phase separation resulting in an unstable bulky filament and Scenario 2 - flow induced phase separation and crystallization resulting in a stable deformation and a smooth strongly strain hardening filament. Scenario 2 is observed only at 150∘C at high ε˙ and high concentrations. Scenario 1, observed at both temperatures, is most pronounced at low rates and/or high concentrations.