Surface-directed spinodal decomposition

While the static equilibrium behavior of polymer blends in thin film geometry thus is rather well understood, at least in principle, the kinetic behavior (Sects. 2.8,3.3) is much less well understood, since there is a delicate interplay between surface-directed spinodal decomposition, thickness-limited growth of wetting layers, and the hydrodynamic mechanisms of coarsening in this constrained geometry still needs investigation. 

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). 

Puri S (2005) Surface-directed spinodal decomposition. J Phys Condens Matter 17 R1-R42... 

Figure 5.30. Surface-directed spinodal decomposition. In (a) material near the surface has segregated to the surface, leading to the formation of a depletion layer. In (b) material has diffused from regions of low concentration to regions of high concentration ( uphill diffusion ), which deepens the concentration minimum and leads to a secondary maximum.

Surface-directed spinodal decomposition was first observed in an isotopic polymer blend (Jones et al. 1991) thin films of a mixture of poly(ethylene-propylene) and its deuterated analogue were annealed below the upper critical solution temperature and the depth profiles measured using forward recoil spectrometry, to reveal oscillatory profiles similar to those sketched in figure 5.30. Similar results have now been obtained for a number of other polymer blends, including polystyrene with partially brominated polyst)u-ene (Bruder and Brenn 1992), polystyrene with poly(a-methyl styrene) (Geoghegan et al. 1995) and polystyrene with tetramethylbisphenol-A polycarbonate (Kim et al. 1994), suggesting that the phenomenon is rather general. 

In the last two decades a lot of effort has been directed towards phase separation of fluid mixtures confined in thin films [16, 19]. Qualitative evidence for surface-directed spinodal decomposition has been found both experimentally [19, 20], see e. g. Fig. 6, and from numerical Simula-... 

Brown, G. and Chakrabarti, A. (1992) Surface-directed spinodal decomposition in a two-dimensional model. Phys. Rev. A,... 

Puri, S. and Frisch, H.L. (1997) Surface-directed spinodal decomposition ... 

Currently, AFM, neutron reflection (NR) spectroscopy and SIMS are used to examine phase separation in symmetrically segregating thin polymer blend films (ca. 1000 A). Phase separation in the film leads to undulations of the liquid-air interface, provided that the film is sufficiently thin to suppress any surface-directed spinodal decomposition waves. Flattened droplets are formed at a very... 

Jones RAL, Norton LJ, Kramer EJ, Bates FS, Wiltzius P. Surface-directed spinodal decomposition. Phys Rev Lett 1991 66 1326-9. 

Approaches based on phase separation of polymer components and the selective removal of one of the components may also be directed in such a way that fibers without pores yet characterized by fractal surfaces (Fig. le) result. Such types of fibers can be predominantly obtained for specific concentration ranges of the two polymers and for spinodal decompositions. 

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