Cross-linked polymers steric hindrance

It should be pointed out that the characteristics of polymer structure (e.g., porosity, tortuosity, steric hindrance, mesh size, etc.) should be determined in order to calculate the diffusion coefficient of a specific molecule in a particular polymer. For cross-linked polymers, additional polymer properties should be characterized. Even though there are methods to determine these properties, a simple mathematical relationship between the diffusion coefficient of a solute and its molecular weight has been used due to the complexity of the experiment ... 

At very low temperatures, most degrees of freedom are frozen. The detailed chemical structure of the polymer chains does not remarkably influence most of the elastic and thermal properties at these temperatures. (Properties, such as mechanical strength or dielectric loss, may be influenced by the chemical structure because of factors such as steric hindrance and dielectric polarization.) Cross-linking is one structural feature of epoxy resins which might influence low-temperature properties. 

In all cases, the films were obtained by oxidative electropolymerization of the cited substituted complexes from organic or aqueous solutions. The mechanism of metalloporphyrin Him formation was suggested to be a radical-cation induced polymerization of the substituents on the periphery of the macrocycle. As it was reported for the case of polypyrrole-based materials ", cyclic voltammetry and UV-visible spectroscopy with optically transparent electrodes were extensively used to provide information on the polymeric films (electroactivity, photometric properties, chemical stability, conductivity, etc.). Based on the available data, it appears that the electrochemical polymerization of the substituted complexes leads to well-structured multilayer films. It also appears that the low conductivity of the formed films, combined with the cross-linking effects due to the steric hindrance induced by the macrocyclic Ugand, confers to these materials a certain number of limitations such as the limited continuous growth of the polymers due to the absence of electronic conductivity of the films. Indeed, the charge transport in many of these films acts only by electron-hopping process between porphyrin sites. 

Differences between linear, soluble polymers and cross-linked matrix supports in terms of their kinetic aspects were first reported by Andreatta and Rink [24]. In comparison with the corresponding monomeric analogous compound, they found substantial differences for an aminolysis reaction which can be explain by interactions with the polymer and steric hindrance of the reactive sites (Table 1). 

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