Spinodal mechanism

Brenner et al. reported an atom-probe field-ion microscope study of decomposition in an Fe-Cr-Co alloy (see Fig. 18.11) [23]. The atom probe allows direct compositional analysis of the peaks and valleys of the composition waves. It is probably the best tool for verifying a spinodal mechanism in metals, because the growth in amplitude of the composition waves can be studied as a function of aging time, with near-atomic resolution. In spinodal alloys, there is a continuous increase in the amplitude of the composition waves with aging time. On the other hand, for a transformation by nucleation and growth, the particles formed earliest generally exhibit a compositional discontinuity with the matrix. 

T. Miyazaki, S. Takagishi, H. Mori, and T. Kozakai. The phase decomposition of iron-molybdenum binary alloys by spinodal mechanism. Acta Metall.. 28(8) 1143-1153, 1980. 

In low molecular weight systems, no hindrance of phase separation during llquid-liquid-demlxlng could occur by the spinodal mechanism. 

Figure 27.3 Blend images showing (a) nucleation and growth and (b) spinodal mechanisms.

Nag] Nagarajan, A., Flinn, P.A., Phase Separation in the Copper-Nickel-Iron System Mossbauer Effect Evidence Against Decomposition by the Spinodal Mechanism , Appl. Phys. Lett., 11 (4), 120-122 (1967) (Experimental, Kinetics, Phase Relations, 10)... 

In the course of blending polymers, the following systems can be formed one-phase systems, two-phase (colloid) systems, or systems in a metastable state of transition from a one-phase into a two-phase system. The properties of polymer mixtures are determined to a great extent by the phase equilibrium in the system formed and their properties can be changed by controUing the processes of phase separation, which occur hy two mechanisms hy nucleation and growth or by the spinodal mechanism. 

The area below the spinodal curve is the region of absolute instability of a polymer blend. The phase separation in this region is controlled by a spinodal mechanism. The region between spinodal and binodal curves is called the metastable region. Phase separation in this region is controlled by a nucleation mechanism. 

That crystallization can occur via a spinodal mechanism in nonisotropic system, such as drawn polyCethylene teraphtalate), is well known (92). The spinodal mechanism is characterized by the following development of the scattering intensity ... 

Thus, the phase separation in polymer alloys, especially the ones proceeding according to the spinodal mechanism, does not reach the stage of a complete separation into two corresponding phases. It follows that the arising structures can be described most adequately with the use of the approach of non-linear thermodynamics and the concept of dissipative structures. Prigogine has demonstrated the so-called dissipative structures, which are stable because of interaction with the environment, and arise far from the equilibrium. 

Figure 6.2 shows the time variation of the amplitude of sinusoidal fluctuations of the composition. The amplitude increases with time and, when the system decomposes into two final phases, the amplitude value becomes formally infinite. From Figure 6.2, in particular, a veiy important consequence ensues concerning the mechanism of formation of transition layers in systems decomposing through the spinodal mechanism. If Co is the starting concentration of one of the components in the system, then the developing phases are characterized by compositions determined by the composition fluctuation amplitudes. 

A general examination leads to determining the following principal differences between a phase decomposition through nucleation and the spinodal mechanism ... 

The experimental data shows that the formation of semi-IPN occixrs according to a spinodal mechanism which is realized in spite of the preceding reaction and, consequently, the non-equilibrium conditions of the process. It was also shown that under these conditions the reaction kinetics determine the beginning of microphase separation, i.e., the thermod5mamics of formation of the system and its thermod3uiamic state are determined by the reaction kinetics. 

Mechanical properties of IPNs will differ, depending on the mechanism of the phase separation. For the case of nucleation, the traditional models may be used, which have been discussed in Chapter 4 for filled polymers. These models allow one to predict the properties of two-phased systems of the matrix-inclusion type. For IPNs which are decomposed according to the spinodal mechanism, the appearance of two interconnected phases and dual-phase continuity should be taken as a basis for calculations. 

For calculations, a matrix was constructed with dimensions of 100x100 elements, which by means of unities and zeroes depicts a two-phase structure in a plane close to interconnected structures formed by the spinodal mechanism of phase separation. 

FIGURE 5.6 Observed microstructures in glass (a) nucleated growth type f iase separaticn (b) phase separation by the spinodal mechanism. After H. Rawson, Properties and Applications of Glass, Fig. JO, Elsevier Science Publishers, Netherlands, 1980.)... 

Various types of IPNs may also be classified by the mechanism of phase separation proceeding during IPN formation. These mechanisms are nucle-ation and growth, and spinodal decomposition. Differences in the conditions of phase separation predetermine the physical and morphological features of IPNs. As a rule, simultaneous IPNs are phase-separated by a spinodal mechanism, and sequential ones via a mechanism of nucleation and growth. Another approach to IPN nomenclature was proposed by Sperling [1], who used the differences in morphological features of IPNs. 

The data on the microphase structure of the semi-IPN show that the phase separation proceeds according to the spinodal mechanism in spite of the simultaneously proceeding chemical reaction. Thus, the process is a nonequilibrium one. The peculiarity of this process is in its two-stage nature. It... 

As will be shown later, the phase separation begins at very low degrees of conversion and is enhanced by increasing MM and voliune fraction of copolymer. The kinetics of IPN formation determines the onset of phase separation and influences strongly this process as a whole, whereas phase separation does not influence the kinetic cmves. This fact may serve as an additional confirmation of the assumption that the phase separation proceeds according to a spinodal mechanism, because in this case the compositions of two evolving phases are very close. 

The incomplete phase separation or phase separation due to the spinodal mechanism leads to the formation of a transition zone or an interphase between two evolved phases. The first attempt to estimate the volume fraction... 

All the ideas connected with the nonequilibrium multiphase structure of IPNs are in good agreement with a comparatively low segregation degree in IPNs (see Chap. 4, this volume). In such a way the whole structure of IPNs may be presented as a mesophase matrix with embedded microphase regions, which represent two evolved phases. Such a structural model coincides with the spinodal mechanism of decomposition. 

The formation of a dual-phase morphology in PEI-epoxy IPNs is explained in the following way [90,171]. Two cases were discussed. First, when phase separation is induced by the curing reaction, PEI-modified epoxy shows bimodal UCST behavior (Fig. 30). At composition (p (volume fraction of component 1) phase separation starts at conversion x when the curing temperature is Ti. Because of the spinodal mechanism of decomposition and due to the lower viscosity of the medium, the system will have macroscale phase separation and domains with volume fraction cp[ and. As the reaction proceeds and the conversion X2 is reached, an abrupt change in the equilibrium composition of the PEI-rich phase occurs from cp l to (p " in a very short time. This abrupt change is similar to the effect of the two-... 

It was shown that the increasing curing temperature increases the degree of segregation, i.e., the completeness of phase separation in the system. For the system under consideration, the phase separation proceeds according to the spinodal mechanism. With an increase in the curing temperature, the rate... 

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