Molecular structure and gross melt fracture triggering

Abstract

We report in this paper rheological and rheo-optical experiments on a linear and a star-branched statistical copolymer of styrene butadiene rubber (SBR). The aim of this work is to study the influence of molecular structure on gross melt fracture defect. First, the rheological behavior in shear and in elongation of both materials are investigated. Then, the experimental results obtained by various rheological techniques enable us to compare the level of apparent shear rates and elongational stresses applied to each polymer at the onset of flow instabilities. The critical apparent shear rates and elongational stresses are found to be higher for the star-branched SBR copolymer. Moreover, the shear behavior of solutions of SBR copolymers in toluene is investigated using classical and optical rheometry. These results allow us to compute their stress optical coefficients. Birefringence experiments performed with a specific two-dimensional slit die corroborate axisymmetric observations. The level of the first normal stress difference along the flow axis is higher for the star-branched SBR copolymer than for the linear one before gross melt fracture occurrence. Thus, all experiments illustrate the dependence of gross melt fracture defect on molecular structure.