Templates and Orientation of Substrates on Template

Part A of the scheme represents the initial state. Two parts of the system are separated by semi-permeable membrane. Polymer (represented by ) is surrounded by molecules of monomer, M, and molecules of solvent, S. Composition of mixed solvent (solvent and monomer) is initially uniform. If the interaction between monomer and polymer is stronger than that between polymer and solvent, the diffusion through the membrane takes place. Monomer molecules are associated with macromolecules while molecules of solvent are displaced to the right part of the vessel. 

The state in which the system reached equilibrium is presented in part B of the diagram. The mixture of solvent, S, polymer, P, and monomer, M, has total volume V° before mixing and V after mixing. According to Aminabhavi and Munk the value q can be calculated. This value expresses extra weight of monomer per gram of polymer in comparison with the amount in the surrounding solution. The equation below can be used for calculation  

In this formula, v and v are the increments of the refractive index for the polymer to the increments measured at constant molarity and at constant chemical potential, respectively. (dn/dOs ) is the refractive index increment of the monomer in pure solvent 

Interferometric measurements combined with equilibrium dialysis show that many monomers are selectively associated with polymers present in three-component solutions. For instance, for methacrylic acid in DMF in the presence of poly(2-vinylpyridine), it was found that q = 0.4. 

The template chain may be regarded as composed of two parts - the first - fully occupied by the monomer units (one monomer unit per one unit of the template) and the second part contains free sites. The friction of occupied sites is 0 and that of free sites (1-9). 

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The formation of bottom-up block copolymer patterns within or on top-down substrate patterns is the basis for so-called templated self-assembly processes, in which long-range order and orientation of microdomain patterns can be imposed by a template or guide . These top-down templates can take a variety of forms including periodic thickness profiles and chemically patterned surfaces. 

From this equation, it is clear that concentration of the solvent, S, influences a number of sites on the template which are occupied by the monomer, M. As the result of monomer units association with the template, the orientation of the substrate takes place and some special type of structure can be created. The structures, in which the monomer is aligned in a regular manner on the polymer template, were described by Chapiro in the case of polymerization of acrylic acid and acrylonitrile and details are described below. The ordered structure increases concentration of monomer at the reaction site, affects distances between pre-oriented monomer molecules, and changes a steric hindrance. This change in structure leads to the change in the kinetics of the polymerization reaction and it is responsible for stereo-control of the propagation step. 

Surfaces. Preference in orientation of the substrates on the surfaces of silica, due to cavity wall-guest interactions, may also lead to some regio- and stereoselectivity. Examples of photoreactions of steriodal enones provided by de Mayo and co-workers illustrate the principles involved [173-175]. In these examples, the surface serves as a template when guest molecules are adsorbed on it. 

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