Demixed polymer blends

It is well-known that one can reduce the droplet size of a demixed polymer blend by adding a well-chosen block-copolymer at given p and T, and achieve interesting improvements of the blend s mechanical properties. 

Fig. 8 Theoretical liquid-liquid demixing curve (solid line) and the bulk melting temperature (dashed line) of a flexible-polymer blend with one component crystallizable and with athermal mixing. The chain lengths are uniform and are 128 units, the linear size of the cubic box is 64, and the occupation density is 0.9375 [86]...

One practical example of demixing that might be attributed to a difference in crystallizability is the incompatibility in blends of polymers with different stereochemical compositions. The stereochemical isomers contain both chemical and geometrical similarities, but differ in the tendency of close packing. In this case, both the mixing energy B and the additional mixing entropy due to structural asymmetry between two kinds of monomers are small. However, the stereochemical differences between two polymers will result in a difference in the value of Ep. Under this consideration, most experimental observations on the compatibility of polymer blends with different stereochemical compositions [89-99] are tractable. For more details, we refer the reader to Ref. [86]. 

EOS models were derived for polymer blends that gave the first evidence of the severe pressure - dependence of the phase behaviour of such blends [41,42], First, experimental data under pressure were presented for the mixture of poly(ethyl acetate) and polyfvinylidene fluoride) [9], and later for in several other systems [27,43,44,45], However, the direction of the shift in cloud-point temperature with pressure proved to be system-dependent. In addition, the phase behaviour of mixtures containing random copolymers strongly depends on the exact chemical composition of both copolymers. In the production of reactor blends or copolymers a small variation of the reactor feed or process variables, such as temperature and pressure, may lead to demixing of the copolymer solution (or the blend) in the reactor. Fig. 9.7-1 shows some data collected in a laser-light-scattering autoclave on the blend PMMA/SAN [46],... 

Compared to bulk polymer mixtures, the interfacial behaviour of polymer blends is essentially different [341]. The demixing process in thin films is strongly affected by the thin film confinement and the interfacial interactions of the blend components with the confining phases (e.g., substrate and air). Even in the one-phase region of the phase diagram, preferential segregation of the components at one of the interfaces leads to a certain composition profile as a function of the distance from the free surfaces and the substrate plane [342,343]. In the... 

Structuring of polymer films attracts considerable attention, and various radiation sources have been employed to crosslink selectively suitable polymers for, e.g., waveguide fabrication [99], Incompatible polymer blends have been forced into certain demixing morphologies along pre-patterned surfaces [55], Persistent structures could be formed by laser radiation in various nonabsorbing polymer solutions, such as polyisoprene in n-hexane [57, 58],... 

Exclusively mechanically interlocked linear polymer blends, typically, are not thermodynamically phase stable. Given sufficient thermal energy (Tuse>Tg), molecular motion will cause disentanglement of the chains and demixing to occur. To avoid phase separation, crosslinking of one or both components results in the formation of a semi-IPN or full-IPN, respectively. Crosslinking effectively slows or stops polymer molecular diffusion and halts the phase decomposition process. 

An alternative approach is to dissolve an efficient electroluminescent chro-mophore in an inert polymer host in a guest-host system or polymer blend. Unfortunately, phase separation and demixing often limits the amount of low-molar-mass electroluminescent chromophores that can be dissolved in polymer hosts. Therefore, the relative brightness is lower for such guest-host systems. This problem can be overcome by fixing the chromophore chemically to the polymer itself as a pendent group on a side-chain polymer separated by spacer units. Bulk phase separation is then impossible, although microphase separation may still take place. This is illustrated by the structures collated in Table 6.10 for some typical side-chain polymers 53-55. ... 

Kuhn R (1983) Characterization of polymer blends, block copolymers, and graft copolymers by fractionation procedures using demixing solvents. Polym Sci Technol 20 45-58. 

It is known that, in a polymer blend, thermodynamic incompatibility between polymers usually causes demixing of polymers. If the polymer is equilibrated in air, the polymer with the lowest surface energy (hydrophobic polymer) will concentrate at the air interface and reduce the system s interfacial tension as a consequence. The preferential adsorption of a polymer of lower surface tension at the surface was confirmed by a number of researchers for a miscible blend of two different polymers. Based on this concept, surface modifying macromolecules (SMMs) as surface-active additives were synthesized and blended into polymer solutions of polyethersulfone (PES). Depending on the hydro-phobic or hydrophilic nature of the SMM, the membrane surface becomes either more hydrophobic or more hydrophilic than the base polymeric material. ... 

Table 3.13. Polymer blends showing coupling of demixing and crystallization. The routes describing the type of coupling are explained in the text...

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