Competing microstructures

While the order parameters derived from the self-diffusion data provide quantitative estimates of the distribution of water among the competing chemical equilibria for the various pseudophase microstructures, the onset of electrical percolation, the onset of water self-diffusion increase, and the onset of surfactant self-diffusion increase provide experimental markers of the continuous transitions discussed here. The formation of irregular bicontinuous microstructures of low mean curvature occurs after the onset of conductivity increase and coincides with the onset of increase in surfactant self-diffusion. This onset of surfactant diffusion increase is not observed in the acrylamide-driven percolation. This combination of conductivity and self-diffusion yields the possibility of mapping pseudophase transitions within isotropic microemulsions domains. 

In the present case, the electron hopping chemistry in the polymeric porphyrins is an especially rich topic because we can manipulate the axial coordination of the porphyrin, to learn how electron self exchange rates respond to axial coordination, and because we can compare the self exchange rates of the different redox couples of a given metallotetraphenylporphyrin polymer. To measure these chemical effects, and avoid potentially competing kinetic phenomena associated with mobilities of the electroneutrality-required counterions in the polymers, we chose a steady state measurement technique based on the sandwich electrode microstructure (19). 

Scheme 3 summarizes this problem with a minimum number of sites and competing processes. In this scheme, two sites, square-well type (X) and spherical-well type (Y), are available for the residence of reactant molecules (A). For the sake of convenience, molecules residing at sites X and Y are labeled Ax and AY. Excitation of these molecules gives rise to A and A. Photoreactivity of molecules excited in each site will be identical if they equilibrate between X and Y before becoming photoproducts. In media with time-independent structures, such as crystals, equilibration requires diffusion of molecules of A in media with time-dependent structures, such as micelles and liquid crystals, equilibration can be accomplished via fluctuations in the microstructure of the reaction cavities as well as translational motion of A (Scheme 4). An additional mechanism for site selective reactions or equilibration of A and A molecules can be achieved via energy migration (e.g., energy hopping, exciton migration, or Forster energy transfer).

Heteropolymers can self-assemble into highly ordered patterns of microstructures, both in solution and in bulk. This subject has been reviewed extensively [1,123-127]. The driving force for structure formation in such systems is competing interactions, i.e., the attraction between one of the monomer species and the repulsion between the others, on the one hand, and covalent bonding of units within the same macromolecule, on the other hand. The latter factor prevents the separation of the system into homogeneous macroscopic phases, which can, under specific conditions, stabilize some types of microdomain structures. Usually, such a phenomenon is treated as microphase separation transition, MIST, or order-disorder transition, ODT. 

In terms of characterizing the microstrac-ture of polymer chains, the two most useful techniques are infrared spectroscopy (IR) and nuclear magnetic resonance (NMR) spectroscopy. Commercial infrared spectrometers were introduced after the end of the second world war and quickly became the workhorse of all polymer synthesis laboratories, providing a routine tool for identification and, to a certain degree, the characterization of microstructure (e.g., the detection of short chain branches in polyethylene). In this regard it can no longer compete with the level of detail provided by modem NMR methods. Nevertheless, IR remains useful or more convenient for certain analytical tasks (and a powerful tool for studying other types of problems). So here we will first describe both techniques and then move on to consider how they can be applied to specific problems in the determination of microstructure. 

In this section, the population balance will be used to model batch and CSTR precipitators where aggregation is a competing growth mechanism. Figure 6.30 is an example of the aggregate microstructure in... 

Dowell, F. In Competing Interactions and Microstructures Statics and Dynamics LeSar, R. Bishop, A. Heffner, R., Eds. Springer-Verlag Berlin, 1988 p. 177. 

Microstructural taxonomy has reached a sufficient level of sophistication that it is possible to identify certain broad classes of microstructure. Before beginning, however, it is necessary to reiterate what this chapter is not. Our primary emphasis will be on metals. The majority of experimental data to be highlighted will be drawn from the metallurgical literature with occasional reference to nonmetallic precipitates within such metals. This is not to say that other material systems are without interesting microstructures. On the contrary, the subject is so vast as to have escaped even a modicum of competency on the part of the author. 

In the opposite case, hv > g, photons produce electron-hole pairs. Accumulation of holes at the oxide surface increases the local potential drop which may cause a fast photocorrosion. Ion migration is enhanced in the thin film, corrosion is enhanced, and altogether a fast dissolution of metal takes place by a photoelectro-chemical process in the passive film. An example is given for Ti [160]. This technique can be used for microstructuring of Ti- or Al surfaces [104]. On the other hand, anodic metal ion dissolution competes with the opposite anodic film forming ITR of oxygen ions. Therefore, in dependence on the special conditions, laser induced oxide growth may overcome pit formation [160]. 

Passivation techniques have to compete with other techniques of surface protection, like phosphating, electrodeposition of paint and others. In high-tech systems, especially in micro and nanotechnology [27], passivation has to compete with other microstructuring techniques, like PVD,... 

Liquid crystallinity and block microphase separation both compete during the minimization of free energy of the system. As we will show later in this review, in the case of a rod-coil diblock copolymer, liquid crystallinity plays a very important role in the microphase separation process and leads to morphologies distinctly different from the conventional spheres, cylinders and lamellar microstructures and include the arrow head, zig-zag, and wavy lamellae phases [40, 41], In the case of SGLC-coil... 

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