LIQUID-LIQUID PHASE-SEPARATION IN BLENDS OF POLYDISPERSE LINEAR AND BRANCHED POLYETHYLENES

A generalized Flory-Huggins theory is employed to investigate liquid-l iquid phase separation in a blend of polydisperse linear and branched polyethylenes (LPE/BPE). A temperature- and concentration-dependent ch i parameter is used. The temperature- and composition-dependent coeffi cients of chi are obtained by fitting to experimental cloud-point data , and having a detailed knowledge of the molecular weight distribution s of the two components. The value of chi is found to be small and pos itive (3.4 x 10(-4) ((phi BPE) = 0.0) to 2.6 x 10(-4) ((phi PBE) = 1.0 ), T = 423 K) over the temperature composition range of interest, cons istent with an approximately athermal blend with small, non-combinator ial entropic contributions to the free energy of mixing. The magnitude of chi determined here is also in reasonable agreement with values me asured previously on related systems. The best-fit cloud-point curve o btained for the LPE/BPE blend is a closed immiscibility loop, which is consistent with the experimental observations. However, no critical p oint(s) or spinodal(s) are found to exist. It is suggested, based on t he limited data available, that a blend comprising two monodisperse LP E/BPE components, with molecular weights corresponding to the weight-a verage molecular weights of the polydisperse linear and branched polye thylenes, would not undergo liquid-liquid phase separation.