Chain flexibility factors

The above spectral densities can be modified for the occurence of chain flexibility, and for the director being oriented at dLD w.r.t. the external BQ field in the L frame. For CD bonds located in the flexible chain, the effect of DF is reduced due to an additional averaging of the time dependent factor (/f g) by conformational transitions in the chain. Consequently, the spectral densities given in Eqs. (60)-(62) are modified by replacing Soc%0(Pm,q) by the segmental order parameter YCD of the C-D bond at a particular carbon site on the chain.146,147 As observed experimentally,148,149 the spectral densities in a flexible chain show a SqD dependence when DF dominate the relaxation rates. The general expression of Jm(co 0LD) due to DF in uniaxial nematic phases is given by... 

The conclusion was reached that the mechanical properties of glass-reinforced unsaturated polyester are influenced by the chemical structure of the spacer groups in the methacrylate functional silane. Effective factors include hydro-phobicity, reactivity of the double bond, chain flexibility of the backbone, and adsorption behavior. 

Polymers for membrane preparation can be classified into natural and synthetic ones. Polysaccharides and rubbers are important examples of natural membrane materials, but only cellulose derivatives are still used in large scale for technical membranes. By far the majority of current membranes are made from synthetic polymers (which, however, originally had been developed for many other engineering applications). Macromolecular structure is crucial for membrane barrier and other properties main factors include the chemical structure of the chain segments, molar mass (chain length), chain flexibility as well as intra- and intermolecular interactions. 

K is a numerical factor which accounts essentially for chain flexibility. The 2H NMR spectrum might be simulated by superposing the contributions of all chains, considered independently [35]. If the macroscopic deformation is transmitted affinely to crosslink junctions, P(R) may be written in the deformed network (the strain being along z) ... 

This study is the first step towards a quantitative prediction of chain flexibility based on conformational analysis. Torsional relaxation of the adjacent bonds is very important. The present approach differentiates between the polymer relaxations in the glassy state and in the melt state. It provides insight into the crystallinity of a system and succeeds in explaining the isomorphic transformations of PDES. Chain flexibility is influenced by at least two types of factors the number of isomeric states available and the torsional freedom in a given state, which is determined by the shape of the potential well. 

Even if this does not reflect the swollen state porosity, it would lead to increased diffusional limitations and a larger specific surface area. The photopolymers probably have a more open pore structure in the swollen state giving the template more rapid access to the sites, which are in this case confined to a smaller surface area. The difference in the conversion of pendant double bonds, and thereby the difference in cross-linking densities between the two types of materials, is probably also a factor that comes into play. An increase in chain flexibility at the sites is likely to cause an increase in the template adsorption-desorption rate coefficients. In this context it is interesting to note that increased rate enhancements were observed upon controlled hydrolysis of the polymer backbone of an imprinted esterase model [73]. 

As Tg marks the onset of molecular motion, a number of factors that affect rotation about links (necessary for movement of polymer chains) will also influence the Tg of a polymer. These include (a) chain flexibility, (b) molecular structure (steric effects), (c) molecular weight, and (d) branching and crosslinking. 

For symmetrical polymers, the chemical nature of the backbone chain is the important factor determining the chain flexibility and hence Tg. Chains made up of bond sequences which are able to rotate easily are flexible, and hence polymers containing -(-CH2-CH2-)-, -(-CH2-0- H2-)-, or -(-Si-O-Si-)- links will have correspondingly low values of Tg. For example, poly(dimethyl siloxane) has one of the lowest Tg values known (-123°C) presumably because the Si-0 bonds have considerable torsional mobility. 

The above discussion clearly indicates that stereo regularity, chain flexibility, polarity, and other steric factors have profound influence on crystallizability and melting points and, hence, as we shall see later, play an important role in the thermal and mechanical behavior of polymers. 

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