Phase separation and spinodal decomposition

The entire region enclosed by the outer dome represents immiscibility. The inner dome is known as the spinodal. The outer dome is known as the binodal. In the region of composition between the binodal and spinodal lines, phase separation occurs by the nucleation and growth mechanism and leads to the formation of dispersed micro-spherical glass particles in the matrix (see also Shelby, 1997). Spinodal decomposition which takes place inside the dotted region is a special type of phase separation. In order to understand this, consider two materials A and B, melts of which 

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One of the primary tasks in the past few decades in polymer science has been to control the structure and properties of multicomponent systems. Since the properties of multicomponent systems depend on their structure, the control and design of these structures is fundamental to produce novel properties. Phase separation and spinodal decomposition are used to design multiphase structures. To imderstand the fundamentals of these phenomena it is necessary to understand thermodynamics, phase transitions, (qv) and critical phenomena in polymer blends (qv) and be able to evaluate quantitatively the degree of miscibility between the polymeric blend components. 

Curve defining the region of composition and temperature for a binary mixture across which a transition occurs from conditions where single-phase mixtures are metastable to conditions where single-phase mixtures are unstable and undergo phase separation by spinodal decomposition. 

In Fig. 1.14, the dotted lines for each curve show the activity of the coexisting phases at chemical equilibrium. Similarly in Fig. 1.16 the dotted line BDF shows the activity of the coexisting phases (5 = 0.185 and 0.815). The coexisting phases, which have the same structure, differ in the concentration of vacancies. This phenomenon is generally called phase separation or spinodal decomposition (it is observed not only in the solid phases but also in the liquid phases), and originates from the sign of the interaction energy... 

In a UCST system, when the temperature is reduced to a final value 7/ that is below the critical temperature Tc, a mixture with a concentration 0 not too far from the critical composition phase separate into two phases whose compositions lie on the opposite sides of the binodal envelope line of Fig. 9-1. The dynamics of the separation process of a single phase into these two phases is controlled by Tf, the composition , the rate of the quench dT/dt, the viscous (or viscoelastic) properties of the phases formed, and the interfacial tension F between the two phases. Although a variety of different kinds of behavior can occur, there are two generic types of phase separation, namely, spinodal decomposition (SD) and nucleation and growth (NG). SD occurs when the mixture is quenched into a part of the phase diagram where the mixture is unstable to small variations in composition, leading to immediate growth of phase-separated domains. When the quenched... 

Another type of morphology development was reported for PP/EPR blends [Inaba et al., 1986]. High molecular weight blends of PP and EPR are immiscible. One-phase blends were prepared by precipitation from a solution. This mixture underwent phase separation via spinodal decomposition (SD), followed by crystallization. However, in this case the spherulites were formed seemingly ignoring the bi-continuous SD-structure. 

Figure 16.11 Emerged bicontinuous structure following a temperature jump from the isotropic melt between the LCST and the melting transition into the LCST gap, which is presumably driven by liquid-liquid phase separation through spinodal decomposition in the 50/50 sPP/EPDM mixture (a) 1000 s and (b) 3000 s.

Figure 16.12 Competition between the liquid-liquid phase separation through spinodal decomposition and the crystalline structure formation in the 50/50 sPP/EPDM blend. The crystallization occurs with the preformed SD networks. The top and bottom rows represent the temporal evolutions of the concentration field and the corresponding crystal order field.

When mixtures with less than 10 wt% were cured isothermally, they were phase separated due to a nucleation and growth mechanism. On the other hand, with more than 20 wt% polysulfone, the blends were phase separated by spinodal decomposition. These different mechanisms are important because in thermoset/thermoplas-tic blending, the fracture toughness is determined by the morphology that is formed as a consequence of the phase separation. 

On the other hand, in some cases, a phase separation mechanism, which is often proposed to explain the formation of organic gels (see Section 8.2.1.2. below), agrees better with experimental results. If the initial homogeneous solution is separated into two immiscible continuous and interconnected liquid phases, the phase separation is named spinodal phase separation or spinodal decomposition. The two phases... 

The time evolution of phase separation by spinodal decomposition is described by a diffusion equation for the density (pure fluid) or species concentration (mixture) [101,103,105]. To illustrate the basic aspects of the theory, we consider the onedimensional, pure fluid case. Generalization to more dimensions, and extension to binary incompressible mixtures involve just a change of notation. For a pure fluid, the diffusion equation reads... 

Fig. 1.4.3 Extent of phase separation by spinodal decomposition vs. time showing the various time regimes, starting with the induction period, followed by an accelerated-growth period, then the coarsening period (Redrawn with permission from Caneba and Saxena, 1996)...

For an early stage of phase separation by spinodal decomposition, it has been shown (Hashimoto et al., 1983 Izumitani and Hashimoto, 1985 Sasaki and Hashimoto, 1984 Snyder, et al., 1983) that... 

Polymer Blends and Phase Separation the spinodal decomposition phenomena d(J / Ct... 

Figure 4.11 Phase separation by spinodal decomposition (O) and nucleation and growth ( ), as observed under a microscope (35).

Fig. 3.21. Structure patterns emerging during phase separation in PS/PBra S-mixtures. left Pattern indicating phase separation by nucleation and growth (0(PS) = 0.8) right Pattern suggesting phase separation by spinodal decomposition (0(PS) = 0.5) [17]...

Figure 3.5 Schematic representation of phase separation mechanisms spinodal decomposition (top) and nucleation and growth (bottom). The last stage at time tj displays an equilibrium state.

The value of (j) at = 0 is the stability limit of the gel, that is, the spinodal line. Figure 5 shows the calculated spinodal lines of the NIPAAm gel at given X Xi values. The hatched area is the unstable region and point C is the critical point. When the gel enters the hatched region due to a sudden temperature change, there will be phase separation by spinodal decomposition. 

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