RECENT DEVELOPMENTS IN PHASE-SEPARATION OF POLYOLEFIN MELT BLENDS

Saturated hydrocarbon polymers may be differentiated by the relative a mount and placement of methylene, methyl, methine, and quaternary carb on moieties. While it has been known or suspected for some time that p olyolefins of conventional molecular weight (M-omega approximate to 10 0 kg/mol) with dissimilar chemical microstructures are most often immi scible in the liquid state, recent experiments with binary blends of m odel polyolefins have increased greatly our understanding of thermodyn amic interactions between unlike chains. Model systems with methyl (-C H3) and ethyl (-C2H5) short-chain branches give results, expressed as the Flory-Huggins interaction parameter chi, that are nearly universal ; repulsive interactions (chi > 0) are more pronounced at low temperat ures, leading to liquid-liquid phase separation at an upper critical s olution temperature. Phase behavior of more complex systems (with dist ributions of chain microstructures and/or molecular weight ) is genera lly consistent with predictions from model systems. An interesting exc eption is from work at Bristol on blends of Lightly branched ethylene - alpha-olefin copolymers with unbranched polyethylene as the minority species. Here the presence of two liquid phases is inferred under con ditions not expected from model studies; effects of copolymer composit ion and molecular weight are also unusual. Recent theoretical work poi nts to the importance of chain stiffness (established by short-chain b ranching) in determining the thermodynamics of model blends. Nonrandom mixing of chains with different stiffness gives rise to an enthalpic chi, which may be negative under certain conditions. Other limitations of the Flory-Huggins approach to describing blend energetics are cons idered. At present there is no theoretical basis for liquid-liquid pha se separation reported by the Bristol group. (C) 1997 John Wiley & Son s, Inc.