Flow-induced miscibility

Winter et al. [119, 120] studied phase changes in the system PS/PVME under planar extensional as well as shear flow. They developed a lubrieated stagnation flow by the impingement of two rectangular jets in a specially built die having hyperbolic walls. Change of the turbidity of the blend was monitored at constant temperature. It has been found that flow-induced miscibility occurred after a duration of the order of seconds or minutes [119]. Miscibility was observed not only in planar extensional flow, but also near the die walls where the blend was subjected to shear flow. Moreover, the period of time required to induce miscibility was found to decrease with increasing flow rate. The LCST of PS/PVME was elevated in extensional flow as much as 12 K [120]. The shift depends on the extension rate, the strain and the blend composition. Flow-induced miscibility has been also found under shear flow between parallel plates when the samples were sheared near the equilibrium coexistence temperature. However, the effect of shear on polymer miscibility turned out to be less dramatic than the effect of extensional flow. The cloud point increased by 6 K at a shear rate of 2.9 s. ... 

Flow-induced miscibility was also found for blends of poly(ethylene-co-vinyl acetate) and solution chlorinated polyethylene undergoing simple shear flow at... 

This article reviews the phase behavior of polymer blends with special emphasis on blends of random copolymers. Thermodynamic issues are considered and then experimental results on miscibility and phase separation are summarized. Section 3 deals with characteristic features of both the liquid-liquid phase separation process and the reverse phenomenon of phase dissolution in blends. This also involves morphology control by definite phase decomposition. In Sect. 4 attention will be focused on flow-induced phase changes in polymer blends. Experimental results and theoretical approaches are outlined. 

Blends of PI with PB were dynamically sheared at large amplitude (y = 0.8) and frequency CO = 0.63 and 6.3 rad/s [Matsuzaka et al., 1997]. After a temperature jump, the spinodal decomposition (SD) was in-situ observed at the lower frequency, but not at the higher. In the latter case, after stopping the oscillation, a modified SD pattern emerged. The authors postulated that the dynamic flow induced a structure in miscible system, quite different from that that exists in the non-sheared specimens. 

Drop deformation in shear that leads to fibrillation was recently examined using microscopy, light scattering and fluorescence [Kim et al., 1997]. The authors selected to work with systems near the critical conditions of miscibility, thus where the flow affects miscibility and reduces the value of The drop aspect ratio, p, plotted as a function of the capillarity number, K, showed two distinct regimes. For K < K., p was directly proportional to K, whereas for K > K., p followed more complex behavior, with an asymptote that corresponds to flow-induced homogenization. 

The response of heterogeneous systems to a stress field allows them to be placed in two categories (i) those in which stress induces irreversible changes e.g., precipitation, denaturation of protein, crystallization, etc.) and (ii) those in which the changes are reversible. The classification is not perfect, as the type and magnitude of stress field can be cmcial, but it provides a guide in most cases, miscibility in systems (i) is reduced by stress, while in systems (ii) it is increased. In other words, if a system can be irreversibly modified by rheological means, its solubility will be reduced. An excellent review on phase transition in shear flow was recently published [Onuki, 1997]. 

The first observation of shear-induced increase of the LCST was reported for PS/PVME by Mazich and Carr [1983]. The authors concluded that shear stress can enhance miscibility by 2-7°C. Larger effects, AT < 12°C, were reported for the same system in hyperbolic flow [Katsaros et al., 1986]. In a planar extensional flow at 8 = 0.012 - 26 s the phase separated PS/PVME was homogenized at temperatures 3 to 6°C above... 

Flow can impact on the blend s morphology in a number of ways. It can affect either the drops, causing drop deformation to ellipsoids or fibrils, break up or coalescence, or the whole system, inducing then domain segregation, encapsulation, interlayer slip or variations of miscibility. Examples of each of these phenomena abound. 

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