Disentanglement of polymer melts flowing through porous medium before entering a capillary die

Two random styrene butadiene rubber copolymers, one linear and one star-branched, are successively extruded through a porous medium and a capillary die. Porous media consisting of filters stacks are positioned just at the capillary die entrance. First, the effect of the flow history in different porous media on the corrected pressure in the capillary is investigated. In contrast to a previous work [Done et al. (1983)], no significant decrease on the pressure loss across the capillary die wall is observed. Thus, extruded rods are submitted to elongational flow at the die exit by means of a Gottfert Rheotens apparatus in quasi-isothermal conditions. The melt strength of the polymers is measured to assess the effect of filter stacks on rheological properties. As the response of the extruded polymer melt is characterized by its complex-flow prehistory in the porous media, the filtration is found to decrease the melt strength, whereas the breaking stretching ratio is not affected. It is clearly shown that the magnitude of the variation depends on the filtering rate. As proposed by Done et al. (1983), the decrease in melt strength is also assigned to the disentanglement of macromolecular chains in the porous medium. (C) 2002 The Society of Rheology.