Polymer segmental relaxation

More recently, Ingram et al. and Wright et al. independently tried to develop new polymer hosts with secondary structures similar to that of a liquid crystalline state, so that ion transport could occur with a higher degree of freedom in the highly oriented environments and become at least partially decoupled from the polymer segmental relaxations. Ion conductivities approaching liquidlike values have been obtained on the condition that the liquid crystalline state could be maintained. However, the incorporation of these novel polymer materials in electrochemical devices remains to be tested. 

The eoeffieient n is between 0.5 for Fiekian diffusion and 1.0 for relaxation-eontrolled diflusion (diflusion of solvent is mueh faster than polymer segmental relaxation). This relationship is frequently taken literally to calculate diflusion distanee from a measurement of flie ehange of flie linear dimensions of swollen material. 

Trsdng to expain this curious phenomenon, Kubat has assumed that the relaxation occurs in clusters. That is, neither individual atoms in metals nor the polymer segments relax individually, but both kinds of units relax collectively. On the basis of this assumption, Kubat succeeded in developing a general theory of stress relaxation that provides predictions agreeing precisely with experiments (16). 

This occurs when the permeant mobility and polymer segment relaxation rates are similar. A number of models have been proposed to explain the occurrence of these anomalous sorption modes. However none provide a comprehensive explanation and they only describe certain features of a particular type of behaviour. ... 

The Zimm model predicts correctly the experimental scaling exponent xx ss M3/2 determined in dilute solutions under 0-conditions. In concentrated solution and melts, the hydrodynamic interaction between the polymer segments of the same chain is screened by the host molecules (Eq. 28) and a flexible polymer coil behaves much like a free-draining chain with a Rouse spectrum in the relaxation times. 

FIGURE 24.3 Local segmental relaxation times for polymethyltolylsiloxane (PMTS) plotted as a function of TV. (From Casalini, R. and Roland, C.M., Coll. Polym. Set, 283, 107, 2004.)... 

The presence of four kinds of nuclear magnetic resonance (NMR) observable nuclei ( H, uB, 13C, and 29Si) allows poly(m-carborane-siloxane) to be readily investigated using NMR spectroscopy. In addition, H spin-echo NMR relaxation techniques can provide an insight into polymer segmental chain dynamics and therefore useful information on material viscoelastic characteristics. 

The observations above can be rapidly turned into a semi-quantitative theory for star-polymer stress-relaxation [24] which is amenable to more quantitative refinement [25]. The key observation is that the diffusion equation for stress-re-lease, which arises in linear polymers via the passage of free ends out of deformed tube segment, is now modified in star polymers by the potential of Eq. (16). Apart from small displacements of the end, the diffusion to any position s along the arm will now need to be activated and so is exponentially suppressed. Each position along the arm, s, will possess its own characteristic stress relaxation time T(s) given approximately by... 

It was proposed that the temperature dependence of polymer 5 arises from the temperature dependence of the kA step. Specifically, it was suggested that the polymer segments to which the radicals are attached are conformationally stressed. There are two possible modes for the newly formed radicals to relax and become separated They can rotate or recoil away from each other (Scheme 9). These secondary motions of the polymer arise from the relaxation of unfavorable bond conformations that are formed during the polymer casting process. The increased thermal energy facilitates the rotation and recoil relaxation processes, which effectively increases the rate constant for diffusion of the radicals out of the cage, kA. This leads to decreased radical-radical recombination and consequently an increase in photodegradation efficiency. 

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