Metastable blend

At ambient and processing temperatures, elastomers are viscous fluids with persistent transport phenomenon. In immiscible blends, these lead to change in the size and shape of the elastomer phases and migration of the fillers, plasticizers, and curatives from one phase to another. These changes are accelerated by processing and plasticization but retarded by the ultimate vulcanization. Retention of the favorable properties of a metastable blend, which is often attained only at a select interphase morphology and filler/plasticizer distribution, thus requires careful control of both the processing and the vulcanization procedures. 

Polymer blend that exhibits metastable miscibility. 

Note In polymers, because of the low mobility of polymer chains, particularly in a glassy state, metastable mixtures may exist for indefinite periods of time without phase separation. This has frequently led to confusion when metastable miscible polymer blends are erroneously claimed to be miscible. 

Figure 7.17. Predicted phase diagiam for the system (CiiInSe2)i-x(ZnSe)2i (a) stable and metastable equilibrium between dialcopyrite (CH) and zinc-blende (ZB) phases using volume-dependent interaction parameters (b) using a hierarchy of less accurate tqiproximations (<)sorio el al. 1993).

Structural data are available (Table 30) for a range of binary, ternary and quaternary sulfides of manganese, almost invariably Mn", and these set the scene for the structures to be expected in the compounds with the more discrete polyhedra.319 Indeed, the structural pattern is established in the simple binary compound MnS. Whereas, the stable modification of this (a-MnS) is green and has the cubic rock salt structure with [MnS6] octahedra, the well-known flesh-coloured precipitates of the qualitative analysis system are metastable / - and y-modifications, which have [MnS4] tetrahedra with respectively the zinc blende or diamond (cubic) and wurtzite (hexagonal) structures. And so, in the rest of the known solids, there are almost equal numbers of four-coordinate tetrahedra and six-coordinate octahedra with no other polyhedra having been detected. 

Heterogeneous blends with very line scales of segregation may have very broad glass transition regions and good optical clarity. It is a moot point, then, whether such mixtures are compatible. If the components are not truly miscible, the blend is not at equilibrium but the user may not be able to distinguish between a persistent metastable state and true miscibility. 

The structural and rheological properties of emulsions, blends, and foams are of great importance in the food, cosmetics, oil-field, and packaging industries. By definition, such fluids are thermodynamically unstable or at best metastable. Hence, conditions of preparation are of extreme importance to both the scientific study and the engineering of these fluids. 

The better understanding of the mechanisms of stability incomplex dermatological emulsions stabilized by surfactants and amphiphiles has enabled the development of a rapid microscopic method for evaluation of potential emulsifiers. The method is based on the observation that good emulsifier blends that stabilize emulsions by the formation of multilayers of stable gel phase also swell spontaneously in water at ambient temperature and this process can be observed microscopically. Mixtures that do not form gel phase or form metastable gels only after a heating and cooling cycle cannot be observed to swell spontaneously at ambient temperature. ... 

Mercury (I I) sulphide, HgS, exists in both zinc-blende and rock-salt structures, the red, hexagonal cinnabar, s.g. 8.18, being the stable and common form the metastable, black, cubic metacinnabar, s.g. 7.60, is rare. Mercury(I) sulphide is unknown. 

Further ultrahigh pressure structural modifications of zinc-blende BN are considered possible, with a rock salt structure preferred over a /i-Sn structured A metastable E-BN phase is obtained during electron-beam-assisted crystallization of BN. Other metastable phases of BN are reported in the condensation products upon sudden cooling in conditions of a pulsed plasma discharge Disordered structures are reported when mixed plasma/chemical processes are usedd Some compounds of type B N are listed in Table... 

The solution of the concentration profile ( >(z) should be specified for given temperature T, film thickness D, and the average blend composition in this film <(( . The parameters T and <( , important in experiments, might be translated [60] into interaction parameter % and the chemical potential difference Ap.more convenient in calculations. Thus, for say D, %, and Ap known and kept constant, the profile ( >(z) may be obtained (Eq. 50) by varying the reservoir concentration (]>b until the boundary conditions (Eq. 51) are met. If a few solutions exist, the relevant ones are those with minimal overall free energy F (Eq. 49). Such a shooting procedure was developed by Flebbe et al. [60]. A numerical method which starts from an arbitrary assumed profile and modifies its discretized form until conditions equivalent to Eqs. (50), (51) and (53) are met has also been proposed recently by Eggleton [222]. The solutions yielded by this technique may however correspond to metastable states. Concentration profiles in thin films were also evaluated by other theoretical treatments [93,118,177,219,221]. 

PCA of proteins suspended in an organic solvent with dissolved polymer PCA to form metastable polymer blends Hydrophilic silica dispersions... 

The effect of dissolved CO2 on the miscibility of polymer blends and on phase transitions of block copolymers has been measured with spectroscopy and scattering (40). The shifts in phase diagrams with CO2 pressure can be pronounced. Polymer blends may be trapped kinetically in metastable states before they have time to phase separate. Metastable polymer blends of polycarbonate (PC) and poly(styrene-cn-acrylonitiile) were formed with liquid and supercritical fluid CO2 in the PCA process, without the need for a surfactant. Because of the rapid mass transfer between the CO2 phase and the solution phase, the blends were trapped in a metastable state before they... 

Mawson S, Kanakia S, Johnston KP. Metastable pol5mrer blends by precipitation with a compressed fluid antisolvent. Pol5mrer 1997 38 2957-2967. 

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