Bicontinuous structures spinodal decomposition

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. 

Fig. 6.2 Representative micrographs showing macrophase separation (Lowenhaupt and Hellmann 1991) (a) and (b) are bicontinuous structure, typical of those for spinodal decomposition (c) and (d) show discrete domains, consistent with a nucleation and growth process of macrophase separation. The diblock details are as Fig. 6.1, the homopolymer has A/w = 161 kg mol-1. Temperatures and volume fraction of copolymer are indicated.

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.

At some compositions and under some hydrolysis conditions, bicontinuous phases can be obtained (with the silica and polymer phases interpenetrating one another). The mechanism may be spinodal decomposition, occuning either before or after the polymerization. Since the two networks interpenetrate one another, the mechanical properties first exhibited by the material can be very peculiar. In a first deformation, the silica network would give a very high initial modulus, but once this structure is broken, additional deformation cycles would indicate much lower values of the modulus. 

The phase separation process can be divided into two categories (1) nucleation and growth and (2) spinodal decompositions. Spinodal decomposition results in a disordered bicontinuous two-phase structure. The path in the phase diagram traverses the critical point. The morphology is discontinuous. Interfacial tension is a salient consideration. Uniform particle size distribution is possible during spinodal decomposition. 

FIGURE 12.17 Illustration of the preparation of a bimodal mesoporous-macroporous carbon by dual-phase separation. The macropores are formed from the spinodal decomposition of glycolic solvents (a). Bicontinuous structure, framework structure, and the large macropores left by the solvent after annealing and carbonization (b). The carbon walls display large amounts of mesopores (c) templated by the triblock copolymer. (From Liang, C. D. et al., Chemistry of Materials, 21, 2115, 2009. With permission.)... 

The issue is more difficult when the composition of the quenched material lies close to the center of the miscibility gap. In this case, the final two-phase system exhibits a bicontinuous geometry, both phases occupying about the same volume fraction, and thus the structure cannot be modeled by a set ofisolated nano-objects. For the centtal part of the miscibihty gap, a theoretical model named spinodal decomposition was proposed (Calm, 1965). At advanced stages, even after nearly having reached the equilibrimn concentrations, both phases stiU evolve by a coarsening process. 

The dynamics and pattern formation during phase separation processes have been a subject of many experimental and theoretical studies over the past decades as a fascinating example of nonlinear, nonequilibrium phenomena [1,2]. If a binary mixture is rapidly quenched from the single-phase region to the spinodal region of the phase diagram by changing thermodynamic variables, such as temperature and pressure, the mixture becomes thermodynamically unstable and separates, via spinodal decomposition (SD), into two phases. If the volume fraction of one of the phases is close to 0.5 ( isometric case ), the phase-separated structure is implied to be periodic and bicontinuous with the aid of theories [50], experiments [51, 52], and computer simulations [53-56]. 

---