Segregative phase separation

The morphology of a polyethylene blend (a homopolymer prepared from ethylene is a blend of species with different molar mass) after crystallisation is dependent on the blend morphology of the molten system before crystallisation and on the relative tendencies for the different molecular species to crystallise at different temperatures. The latter may lead to phase separation (segregation) of low molar mass species at a relatively fine scale within spherulites this is typical of linear polyethylene. Highly branched polyethylene may show segregation on a larger scale, so-called cellulation. Phase separation in the melt results in spherical domain structures on a large scale. 

In this group can be included the structuration driven by dewetting, evaporative self-organization, block copolymer phase separation, segregation of additives to surfaces, or template guided surface structuration. 

Figure 7.3 depicts the energy F e) of a wetted solid for a wide range of liquid thicknesses, ranging from the microscopic to the macroscopic. We may deduce from the figure that, just as in the case of phase separation (segregation of binary mixtures, liquid-vapor phase transition), there are two mechanisms for the drop to recede. 

The immiscible blends of thermoplastic polymers are assumed to be compatible if the presence of a finely dispersed phase and the resistance to bulk phase separation/segregation result in satisfactory physical and mechanical properties. Compatibility is usually facilitated by copolymers, especially block or graft copolymers, with segments capable of specific interactions at the interface, thus resulting in a decrease of the interfacial tension and promoting mechanical interlocking by the interpenetration and entanglements between the blend components. 

To evaluate the heterogeneity of the phase-separated IPN and the degree of phase separation (segregation degree) various approaches can be used. The most fundamental one is based on SAXS in the form proposed by Bonart and... 

Separation by distillation is dependent on the fact that when a Hquid is partially vaporized the vapor and Hquid compositions differ. The vapor phase becomes enriched ia the more volatile components and depleted ia the less volatile components with respect to its equiUbrium Hquid phase. By segregating the phases and repeating the partial vaporization, it is often possible to achieve the desired degree of separation. One measure of the degree of enrichment or the ease of separation is the relative volatiHty defined as ... 

FIG. 37 10-iJLm X 10-jj.m SPFM images showing the phase separation between the hydrated aluminum sulfate and the excess unreacted sulfuric acid as the humidity is lowered. The aluminum sulfate salt precipitates, forming a solid lentillike deposit. At the lowest RH (32%), segregated acid droplets are surrounded by flat ( 20 A-thick) pancakes of liquid. (From Ref. 85.)... 

Phase separation and segregation are occurring in some hydrothermal systems (Gamo, 1995) which modify the chemistry of initial hydrothermal solution. Von Damm and Bischoff (1987) and Butterfield et al. (1994) obtained high chloride concentration of more than twice of the seawater value for the hydrothermal solution at the North Cleft Segment and the South Cleft Segment of the Juan de Fuca Ridge. 

The only real sources of difference between the additive and comonomer systems would be the potential lack of homogeneity of the polymer blends, which may result either through incomplete blending or segregation by phase separation. Since such nonhomogeneity could lead to irreproducibility in the burning trials, this would be detected by the flammability testing. 

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