Recent developments in phase separation of polyolefin melt blends

Abstract

Saturated hydrocarbon polymers may be differentiated by the relative amount and placement of methylene, methyl, methine, and quaternary carbon moieties. While it has been known or suspected for some time that polyolefins of conventional molecular weight (Mw ≈ 100 kg/mol) with dissimilar chemical microstructures are most often immiscible in the liquid state, recent experiments with binary blends of model polyolefins have increased greatly our understanding of thermodynamic interactions between unlike chains. Model systems with methyl (−CH3) and ethyl (−C2H5) short‐chain branches give results, expressed as the Flory‐Huggins interaction parameter χ, that are nearly universal; repulsive interactions (χ > 0) are more pronounced at low temperatures, leading to liquid‐liquid phase separation at an upper critical solution temperature. Phase behavior of more complex systems (with distributions of chain microstructures and/or molecular weight) is generally consistent with predictions from model systems. An interesting exception is from work at Bristol on blends of lightly branched ethylene − α‐olefin copolymers with unbranched polyethylene as the minority species. Here the presence of two liquid phases is inferred under conditions not expected from model studies; effects of copolymer composition and molecular weight are also unusual. Recent theoretical work points to the importance of chain stiffness (established by short‐chain branching) in determining the thermodynamics of model blends. Nonrandom mixing of chains with different stiffness gives rise to an enthalpic χ, which may be negative under certain conditions. Other limitations of the Flory‐Huggins approach to describing blend energetics are considered. At present there is no theoretical basis for liquid‐liquid phase separation reported by the Bristol group. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2329–2353, 1997

Is Compressibility Important in the Thermodynamics of Polymer Mixtures?

“…Contrary to the model, values of x SANS obtained in this way are found to vary significantly with blend composition [3][4][5][6][7]. The systems showing these effects have been tentatively classified into two categories [8].…”

“…The systems showing these effects have been tentatively classified into two categories [8]. Isotopic mixtures [3,6], or saturated hydrocarbon mixtures [5,7], which show a parabolic dependence of x SANS on f, will be referred to in this work as "quadratic" blends. Other blends, which are generally comprised of chains with dissimilar chemical structures, show a linear composition dependence of x SANS [6] and will be referred to as "linear" blends.…”

“…The origin of the unexpected composition dependence of x SANS is an unresolved issue. Some of the various proposed possibilities-non-mean-field effects, nonrandom mixing, compressibility effects -have been reviewed recently [5,7,8]. The role of compressibility has been controversial.…”

Morphology and Crystallization Kinetics of Melt Miscible Polyolefin Blends

“…[1] However, over the last two decades there has been some progress in the study of blends of linear and branched polyethylene (LPE and BPE respectively), by using either indirect methods after cooling from the melt to below crystallization temperatures or methods that involved examining the blend in the melt via the contrast achieved by deuteration of one component. [2] Although initially the observation of a two phase Full Paper: Blends of copolymers of ethylene/hexene (9 CH 3 /1 000 C) and ethylene/butene (77 CH 3 /1 000 C) synthesized with metallocene catalysts were prepared by coprecipitation from solution. A phase diagram for this blend had been obtained in a preceding work, where the blends were found to be miscible in the melt with a characteristic upper critical solution temperature (UCST).…”

Probing the Melt Miscibility of a Commercial Polyethylene Blend by Small-Angle Light Scattering

“…Several experimental research groups have studied the phase behavior of polyethylene blends that differ in molecular architecture, polarity, molecular size, and composition, using a host of indirect and direct techniques. Since these studies have been extensively reviewed [1,2] only a summary of key relevant results and citations to recent progress in understanding the phase behavior of polyolefin blends will be given here. The experimental methods used include indirect techniques, such as differential scanning calorimetry (DSC), X-ray scattering methods [3,4], light scattering (LS) [5], Atomic Force Microscopy (AFM) [6], electron microscopy, optical microscopy [7], and rheological techniques [8][9][10][11][12][13].…”