Polymer anisotropic contraction

Fig. 5.13 Photomechanical effects. Schematic illustration of UV-light-induced bending of a cross-linked liquid-crystalline polymer film containing azobenzene groups. Light is absorbed at the upper surface layer of the film and causes anisotropic contraction. Adapted from Ikeda et al. [51] with permission from Wiley-VCH.

If a water-swollen cross-linked polyelectrolyte gel is inserted between a pair of planar electrodes and a voltage difference is applied, the material can undergo anisotropic contractions and concomitant fluid exudations [197,198], Electrically induced contractions of the gel are caused by transport of hydrated ions and water in the network (electrokinetic phenomena). In fact, when an outer electric field is applied across a gel, both macro- and micro-ions are subjected to electrical forces in opposite directions. However, macro-ions are typically in a stationary phase, being chemically fixed to the polymer network, while counter ions are mobile and are capable of migrating along the electric field, dragging water molecules with them. 

Anisotropic materials have different properties in different directions (1-7). 1-Aamples include fibers, wood, oriented amorphous polymers, injection-molded specimens, fiber-filled composites, single crystals, and crystalline polymers in which the crystalline phase is not randomly oriented. Thus anisotropic materials are really much more common than isotropic ones. But if the anisotropy is small, it is often neglected with possible serious consequences. Anisoiropic materials have far more than two independent clastic moduli— generally, a minimum of five or six. The exact number of independent moduli depends on the symmetry in the system (1-7). Anisotropic materials will also have different contractions in different directions and hence a set of Poisson s ratios rather than one. 

Phase separation controlled by diffusion exchange often results in a skin which is composed of a micellar assembly of nodules, as will be discussed below. When extremely hydrophobic polymers (e.g., modifled-PPO) are cast from dioxane into water (pg = p = p ) a dense polymer layer is formed at the solution s interface that somewhat resembles the type of layer formed by Interfacial polymerization. There is almost no inward contraction of the interfacial skin, and the coagulation process is controlled by diffusion through the dense, interfacial thin film. These result in an anisotropic membrane with a very fine "coral" structure (Figures 9 and 10). 

Relationship Between Nodular and Rejecting Layers. Nodular formation was conceived by Maler and Scheuerman (14) and was shown to exist in the skin structure of anisotropic cellulose acetate membranes by Schultz and Asunmaa ( ), who ion etched the skin to discover an assembly of close-packed, 188 A in diameter spheres. Resting (15) has identified this kind of micellar structure in dry cellulose ester reverse osmosis membranes, and Panar, et al. (16) has identified their existence in the polyamide derivatives. Our work has shown that nodules exist in most polymeric membranes cast into a nonsolvent bath, where gelation at the interface is caused by initial depletion of solvent, as shown in Case B, which follows restricted Inward contraction of the interfacial zone. This leads to a dispersed phase of micelles within a continuous phase (designated as "polymer-poor phase") composed of a mixture of solvents, coagulant, and a dissolved fraction of the polymer. The formation of such a skin is delineated in the scheme shown in Figure 11. 

Ellipsometry at noble metal electrode/solution interfaces has been used to test theoretically predicted microscopic parameters of the interface [937]. Investigated systems include numerous oxide layer systems [934-943], metal deposition processes [934], adsorption processes [934, 944] and polymer films on electrodes [945-947]. Submonolayer sensitivity has been claimed. Expansion and contraction of polyaniline films was monitored with ellipsometry by Kim et al. [948]. Film thickness as a function of the state of oxidation of redox active polyelectrolyte layers has been measured with ellipsometry [949]. The deposition and electroreduction of Mn02 films has been studied [950] below a thickness of 150 nm, the anodically formed film behaved like an isotropic single layer with optical constants independent of thickness. Beyond this limit, anisotropic film properties had to be assumed. Reduction was accompanied by an increase in thickness, which started at the ox-ide/solution interface. 

Coupling n The linking of a side effect to a principal effect. For composites an anisotropic laminate couples the shear to the normal components, while an unsymmetric one couples curvature with extension. Poisson coupling links lateral contraction to axial extension. The joining together of two or more polymer molecules, which contain terminal chemically reactive groups, by reaction with a third, usually small, molecule capable of reaction with the polymer functional groups which are normally the same. 

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