Spinodal phase-demixing

The mechanism of formation of morphology structures in iPP-E-plastomers blends via shear-dependent mixing and demixing was investigated by optical microscopy and electron microscopy. A single-phase stmcture is formed under high shear condition in injection machine after injection, namely under zero-shear environments, spinodal decomposition proceeds and leads to the formation of a bicontinuous phase stmcture. The velocity of spinodal decomposition and the phase separation depend on the molecular stmcture of iPP and E-plastomer components. 

Figure 3.6 shows trajectories for several values of the interfacial tension o0 in the metastable region that are predicted by using a NG model [2], For a0 < 0.01 mN/m, all the trajectories are equivalent (i.e., the value of a0 has no effect on the phase separation process) however, if a0 is higher than 0.1 mN/m, then spinodal decomposition is rapidly attained. For a0 = 0.4 mN/m practically no phase separation occurs by the NG mechanism, and demixing proceeds by SD. 

An increase in curing temperature resulted in UPE gelation, which restricted the secondary phase separation and the coalescence of spinodal decomposition. To be specific the longer the demixing period, the larger or the more interconnected the dispersed phase became. Min et al. [20,21] observed the dual phase morphology at 15wt% polysulfone... 

Fig. 17 Phase diagram of a PDMS/PEMS (16.4/48.1) blend. The dashed lines are the binodal and the spinodal. The phase contrast micrographs show typical demixing patterns for spinodal decomposition and nucleation and growth in the respective regions. The bullets mark the initial sample positions. See text for details. Figure from [112]. Copyright (2007) by the American Chemical Society...

Ternary solutions of immiscible polymers in a low-molecular solvent display wide miscibility gaps. Consequently, they invariably involve demixing above a critical concentration of total polymer by spinodal decomposition and subsequent coarsening processes. When solvent evaporation progresses the enhanced viscosity will slow down the rate of phase separation to a level at which no further phase changes can be observed. 

Spinodal demixing was excluded as a possible mechanism of phase separation during polymerization in solutions containing less than 12% rubber by volume [250]. 

The blend is quenched into the miscibility gap to a temperature that lies above the crystallization/melt coexistence curve for the actual composition, but lies below the crystallization curve for the binodal composition. When the blend is quenched, demixing occurs resulting in two coexisting phases of which one is able to crystallize. The demixing can result in spinodally as well as in binodally decomposed material. 

The demixing of the immiscible components in an initially one-phase melt proceeds rapidly due to a thermodynamic instability, this is spinodal decomposition, or a thermodynamic metastability (i.e., nucleation and growth of a new phase). However, this distinction of the character of the liquid-liquid phase transition is largely academic for the purposes of this work, because in either case the system rapidly produces droplets of a new phase (i.e., for off-critical mixtures). The demixing process itself was not studied in detail in this work. However, further details of this process may be found elsewhere (21). The production of droplets of a new phase in these immiscible systems is extremely rapid compared with the subsequent coarsening of the system to the final morphology. 

Figure 6A is a general representation of the pressure-composition diagrams for systems that display liquid -liquid phase separation. The binodal boundary represents the equilibrium demixing pressures for a monodisperse polymer system. Below the binodal there is another boundary known as the spinodal boundary. The binodal and the spinodal envelopes determine the metastable region (shaded area in between). They... 

For amorphous polymer systems, liquid-liquid-phase separation occurs by nucleation and growth of the polymer lean phase when the him composition enters locally the metastable region between the binodal and the spinodal (unstable with respect to l-l demixing). The unstable polymer solution... 

Demixing, Fig. 3 (a) Order parameter profile V (z) across an interface between two coexisting bulk phases with order parameters i fcoex. the interface being oriented perpendicular to the z-direction and centered at z = 0. (b) Radial order parameter profile for a marginally stable droplet of radius R. In the center of the droplet, the order parameter takes the value of the stable phase at coexistence, — i coex, while for radial distances p oo, ij/ip) — ij/jas< the order parameter of the considered metastable state, f(OT ij/jos close to t coex- (e) Same as (b), but for < ms close to the spinodal, ij/sp. Then, ij/(j> = 0) does not reach — i coex. but rather stays close to the unstable extremum of the free energy/(ij/), ij/ ... 

Vycor and Pyrex are sodium borosilicate glasses in which use is made of spinodal demixing in the glass phase to provide strong resistance against chemical... 

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