Interfaces spinodal decomposition

The exsolution process in either binodal or spinodal decomposition may lead to coherent or incoherent interfaces between the unmixed phases (figure 3.15). The... 

Fine-scale, spatially periodic microstructures are characteristic of spinodal decomposition. In elastically anisotropic crystalline solutions, spinodal microstructures are aligned along elastically soft directions to minimize elastic energy. Microstructures resulting from continuous ordering contain interfaces called antiphase boundaries which coarsen slowly in comparison to the rate of the ordering transformation. 

Figure 18.7 Interfaces resulting from two types of continuous transformation, (a) Initial structure consisting of randomly mixed alloy, (b) After spinodal decomposition. Regions of B-rich and B-lean phases separated by diffuse interfaces formed as a result of long-range diffusion, (c) After an ordering transformation. Equivalent ordering variants (domains) separated by two antiphase boundaries (APBs). The APBs result from A and B atomic rearrangement onto different sublattices in each domain.

In crystalline solids, only coherent spinodal decomposition is observed. The process of forming incoherent interfaces involves the generation of anticoherency dislocation structures and is incompatible with the continuous evolution of the phase-separated microstructure characteristic of spinodal decomposition. Systems with elastic misfit may first transform by coherent spinodal decomposition and then, during the later stages of the process, lose coherency through the nucleation and capture of anticoherency interfacial dislocations [18]. 

Another peculiar phenomenon is double phase separation in which each of the two phases formed during spinodal decomposition of a mixture of A and B becomes unstable to a second phase separation, in which droplets of B-rich phase appear in the A-rich domain and droplets of A-rich phase appear within the B-rich domains (Tanaka 1994b). This phenomenon is thought to occur when the capillary coarsening process (in which the domain size grows as a oc t) outruns the diffusion process and the A-rich domains are left with a small excess concentration of B over that allowed at bulk equilibrium. This excess of B cannot diffuse to the interface with the A-rich phase as fast as that interface moves away by capillary coarsening. The excess B therefore nucleates into droplets of B-rich phase within the coarsened A domains. The converse occurs in the A-rich domains. 

Fig. 5.a A homogeneous monolayer confined by two external interfaces I and II ordering spinodal waves along z, and morphologies resulting from the spinodal decomposition, b Bilayer equilibrium structure [93]. c Equilibrium structure with two-dimensional domains [60]. d Exemplary transient morphology [94]... 

The variation of the chemical composition of the substrate (not realized in a continuous tunable fashion) leads to drastic modifications of surface fields exerted by the polymer/substrate (i.e.,II) interface [94,97, 111, 114,119]. The substrate may, for instance, change contact angles with the blend phase from zero to a finite value. As a result the final morphology changes from a layered structure of Fig. 5b into a column structure of Fig. 5c [94,114]. On the other hand our very recent experiment [16] has shown that the surface fields are temperature dependent. Therefore, although it has been shown that surface-induced spinodal decomposition yields coexisting bilayer structure (Fig. 5b) at a singular temperature [114,115], that in principle may not be necessary true for other temperatures. This motivated our comparative studies [107] on coexistence compositions determined with two techniques described above interfacial relaxation and spinodal decomposition. 

Coexistence conditions of high polymer mixtures may be determined directly with the advent of the novel approach [74,75] focused on two coexisting phases confined in a thin film geometry and forming a bilayer morphology. Such equilibrium situation is obtained in the course of relaxation of an interface between pure blend components or in late stages of surface induced spinodal decomposition. It is shown that both methods lead to equivalent results [107] (Sect. 2.2.1). 

The usual and therefore most important situation where polymers crystallize is in melts eooled below the point of the fusion of a crystallite of infinite dimensions. Then, crystallization occurs by the nucleation and growth of spherulites. Another crystallization process is sometimes encountered in oriented melts and glasses. In such systems, the crystallization seems to occur at once in the whole sample and not at the interface between the growing crystallites and the amorphous matrix. Despite munerous studies, the crystallization process is not fully understood. Scattering measurements suggest a preliminary spinodal decomposition of the undercooled isotropic melt in phases with and without chain ends and chain defects before the formation of the crystallites [32]. 

The Kinetics of Spinodal Decomposition. Cahn s kinetic theory of spinodal decomposition (2) was based on the diffuse interface theory of Cahn and Hilliard (13). By considering the local free energy a function of both composition and composition gradients, Cahn arrived at the following modified linearized diffusion equation (Equation 3) to describe the early stages of phase separation within the unstable region. In this equation, 2 is an Onsager-type... 

Kammer [1977] considered the interface between two polymers from the basic thermodynamic point of view. He derived a simple relation Vj = A 0g/S, where A 0g is the excess chemical potential of polymer B in the mixture, and S is the molar area of the interface. Near the spinodal decomposition, using Cahn-Hilliard gradient theory, he calculated ... 

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