LIQUID-LIQUID PHASE-SEPARATION IN LINEAR LOW-DENSITY POLYETHYLENE

The issue of multiple equilibrium phases in compositionally heterogene ous random copolymers is studied with an ethylene-butene copolymer rep resentative of many linear low-density polyethylenes (LLDPE). This mat erial has a dispersed minority phase (volume fraction f(beta) almost-e qual-to 0.02) of highly branched, amorphous chains. A thermodynamic ca lculation of the equilibrium liquid state is done using the distributi on of chain branching from temperature rising elution fractionation an d the Flory-Huggins interaction parameter chi(AB) for linear and ethyl branched C4H8 repeat units. The calculation indicates that this copol ymer is metastable, between the binodal and spinodal at a melt tempera ture of 150-degrees-C. The predicted volume fraction of the second pha se, f(beta) = 0.016, is in good agreement with experiment. This work i s the first to compare directly the observed and calculated two-phase behaviors in a random copolymer. (C) 1994 John Wiley & Sons, Inc.