Phase separation metastable

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... 

Within this general framework there have been many different systems modelled and the dynamical, statistical prefactors have been calculated. These are detailed in [42]. For a binary mixture, phase separating from an initially metastable state, the work of Langer and Schwartz [48] using die Langer theory [47] gives the micleation rate as... 

Another kinetic jjhenomenon where Calm s critical waves can possibly be visualized and studied is the replication of interphase boundaries (IPB) illustrated in Figs. 8-10. Similarly to the replication of APBs. it can arise after a two-step quench of an initially uniform disordered alloy. First the alloy is quenched and annealed at temperature T in some two-phase state that can be either metastable or spinodally unstable with respect to phase separation. Varying the annealing time one can grow here precipitates ("droplets ) of a suitable size /. For sufficiently large /, the concentration c(r) within A-riched droplets is close to the equilibrium binodal value C(,(T ) (thin curve in Fig. 9). 

The validity of mean field theory for N —y oo has striking consequences for the initial stages of phase separation. " In a metastable state slightly inside the coexistence curve, the nucleation free energy barrier is due to spherical droplets with a radius R The free energy excess of a droplet is written in terms of bulk and surface terms " "... 

The introduction of new synthetic techniques has led to the discoveries of many new electronic materials with improved properties [20-22]. However, similar progress has not been forthcoming in the area of heterogeneous catalysis, despite the accumulation of considerable information regarding structure-reactivity correlations for such catalysts [14-19]. The synthetic challenge in this area stems from the complex and metastable nature of the most desirable catalytic structures. Thus, in order to minimize phase separation and destruction of the most efficient catalytic centers, low-temperature methods and complicated synthetic procedures are often required [1-4]. Similar challenges are faced in many other aspects of materials research and, in general, more practical synthetic methods are required to achieve controlled, facile assembly of complex nanostructured materials [5-11]. 

Fig. 25 Annealing time evolution of the difference SAXS intensity in the induction period (a) and the crystallization period (b) for the melt crystallization of PET at 244 °C [18]. This system corresponds to crystallization from the metastable state where a nucle-ation and growth type of primary phase separation first occurs followed by the spinodal decomposition type secondary phase separation...

Careful cooling of pure water at atmospheric pressure can result in water that is able to remain liquid to at least 38 °C below its normal freezing point (0 °C) without crystallizing. This supercooled water is metastable and will crystallize rapidly upon being disturbed. The lower the temperature of the supercooled water, the more likely that ice will nucleate. Bulk water can be supercooled to about — 38 °C (Ball, 2001 Chaplin, 2004). By increasing the pressure to about 210 MPa, liquid water may be supercooled to — 92 °C (Chaplin, 2004). A second critical point (C ) has been hypothesized (Tc = 220 K and Pc = 100 MPa), below which the supercooled liquid phase separates into two distinct liquid phases a low-density liquid (LDL) phase and a high-density liquid (HDL) phase (Mishima and Stanley, 1998 Poole et al., 1992 Stanley et al., 2000). Water near the hypothesized second critical point is a fluctuating mixture of LDL and HDL phases. 

If hexane is used as the low molecular weight liquid, the desired phase separation is observed when precursor mixtures containing 6-15 wt % hexane are cured isothermally at 40 °C. Further discussion of the phase separation behavior requires more detailed consideration of the schematic phase diagram, as presented in Fig. 17, which resembles the real phase diagram shown in Fig. 13. Experimentally it is found, that no phase separation occurs with hexane concentrations equal to or lower than 5 wt %. Hence the critical amount for phase separation, (j)p, is given by the intercept of the binodal line and the imaginary value of Hence no phase separation occurs if is reached before the metastable region is entered. 

At higher concentrations of hexane, the metastable region is entered at lower conversions, hence lower viscosity. As the nucleation starts at the point d the diffusion constant is high, leading to a very fast growth of the separated domains immediately after the start of the phase separation. This allows the system to reach the equilibrium concentration after a rather short period. Consequently, the driving force for the phase separation, given by becomes nearly ze-... 

K is positive, representing the "surface free energy at the boundary between emergent phases. Thus, if (3 f/3c ) > 0 the solution is stable to the small fluctuations applicable to eqn. 9 and phase separation by a random nucleation and growth mechanism can only be initiated by a finite, thermally driven fluctuation. The limit of this metastability (i.e., the spinodal) occurs at (3 f/3c ) 0 and the solution becomes unstable whenever (3 f/3c ) is negative. The... 

Note Mixtures exhibiting metastable miscibility may remain unchanged or they may undergo phase separation, usually by nucleation or spinodal decomposition. 

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