Equilibrium flexibility

Equilibrium flexibility has a profound effect on the melting point, T, of a polymer. Since crystallites in turn have a profound effect on the properties, the crystallization PDMS has been a subject of considerable activity 23, 79-82 qjjg melting points of small molecules in PDMS networks have also been reported.  

High flexibility in the equilibrium sense means high conformational randomness in the amorphous state, and thus high entropy of fusion and low melting point. This entropy can be reduced by stretching, in what is called strain-induced crystallization. Such strain-induced crystallization has been studied extensively, both experimentally and theoretically. The crystallites thus generated can be very important since they 

Some ways for making a polymer more rigid.  

A possible reaction for preparing a sesquisiloxane ladder polymer.  

Sketch explaining the increase in melting point with increase in chain rigidity. Reproduced by permission of John Wiley and Sons. 

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Distribution functions for the end-to-end separation of polymeric sulfur and selenium are obtained from Monte-Carlo simulations which take into account the chains geometric characteristics and conformational preferences. Comparisons with the corresponding information on PE demonstrate the remarkable equilibrium flexibility or compactness of these two molecules. Use of the S and Se distribution functions in the three-chain model for rubberlike elasticity in the affine limit gives elastomeric properties very close to those of non-Gaussian networks, even though their distribution functions appear to be significantly non-Gaussian. 

Conformational energy estimates are employed to determine the conformational characteristics of polyfvinyl fluoride) (PVF), polyfluoromethylene (PFM), and polytrifluoroethylene (PTF3). Effects of stereoconfiguration and, in the case of PVF and PTF3, the presence of head-to-head tail-to-tail (HH TT) defect structures are considered. The calculated results are compared to corresponding values found for polyfvinylidene fluoride), polytetrafluoroethylene, and polyethylene, and the equilibrium flexibilities of PVF, PFM, and PTF3 are discussed on this basis. 

The conformational entropies of copolymer chains are calculated through utilization of semiempirical potential energy functions and adoption of the RIS model of polymers. It is assumed that the glass transition temperature, Tg, is inversely related to the intramolecular, equilibrium flexibility of a copolymer chain as manifested by its conformational entropy. This approach is applied to the vinyl copolymers of vinyl chloride and vinylidene chloride with methyl acrylate, where the stereoregularity of each copolymer is explicitly considered, and correctly predicts the observed deviations from the Fox relation when they occur. It therefore appears that the sequence distribution - Tg effects observed in many copolymers may have an intramolecular origin in the form of specific molecular interactions between adjacent monomer units, which can be characterized by estimating the resultant conformational entropy. 

The equilibrium flexibility of PDMS can be characterized by its unperturbed dimensions, in particular its value of the characteristic ratio described in Chapter 2. 

The calculations also predict that polysilane should have a higher equilibrium flexibility than polyethylene,178 and solution characterization techniques could be used to test this expectation. Dynamic flexibility can also be estimated from such energy maps, by determining the barriers between energy minima. Relevant experimental results could be obtained by a variety of dynamic techniques.179... 

Cyclolinear structure of synthesized POCS-4, POCS-5 and POCS-6 polymers was proved by hydro-dynamic study of properties of their solutions [56], The expected results of these investigations should indicate the predominant influence of the equilibrium flexibility of macromolecules and other specific properties of polymeric chains in block on occurrence of the thermotropic mesophase in cyclolinear organosiloxanes. 

Quantitative information on the equilibrium flexibility of actually existing chain molecules permitting the determination of their A (or a) values can be obtained from the experimental study of their conformational, hydrodynamic, optical and other properties in dilute solutions ... 

The diffusion method is much more effective for studying polymers with high equilibrium flexibility. Since the translational diffusion coefficient D of a molecule in solution depends only on the friction coefficient f and is inversely proportional to the latter the dependence on M is the stron r the higher the r dity of chain molecules (in contrast to the dependence of [s] on M). For very rigid-chain polymers at low M this dependence is close to inverse proportionality (Fig. 10) and can be used in the study of molecular characteristics. 

Cotts, P.M., T.M. Swager, and Q. Zhou. 1996. Equilibrium flexibility of a rigid linear conjugated polymer. Macromolecules 29 7323. 

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