Relaxation phenomena applications

A spectacular relaxation phenomenon of polymeric materials is exhibited in a field-temperature treatment phenomenon. As was shown above, the polarisation of a polymeric material after application of an electric field increases after an instantaneous polarisation to the relaxed polarisation. If the electric field is subsequently released the polarisation decreases again with time. However, if the polarisation took place at a temperature above the glass transition temperature after which the temperature was decreased to a temperature well below Tg, then depolarisation is not possible, due to the immobilisation of the polymer molecules. In this way a so-called electret maybe formed, the electric counterpart of a magnet. 

The process giving rise to this asymmetry is called the relaxation effect, and the time taken for the ion to build up its new ionic atmosphere is called the relaxation time. As the ion moves, the build up and decay are occurring continuously. Since the process is a relaxation phenomenon, then the build up of the asymmetry in the ionic atmosphere under the influence of the external field and the decay of the asymmetry to the symmetrical ionic atmosphere once the external field is removed will be first order processes. The overall rate constant for a first order relaxation process can be shown to be given by feoveraii = buildup + decay, and the relaxation time is given by t = l/(fcbuiidup + decay)- This applies even if the rate constants for build up and decay are different. This has the consequence that the same relaxation time is applicable to both build up and decay. 

As is well-known, the rate of nucleation is enhanced by the application of a deformation to the polymer melt (2/i). But only recently, a theory of shear induced crystallization could be developed (25). This type of crystallization causes highly oriented boundary layers in injection molded articles (cf.ref. 8). Shear induction is an elastico-viscous relaxation phenomenon. So far, however, this perception did not contribute to a simplification of the situation. 

Many designs incorporate the phenomenon of stress-relaxation. For example, in many products, when plastics are assembled they are placed into a permanently deflected condition, as for instance press fits, bolted assemblies, and some plastic springs. In time, with the strain kept constant the stress level will decrease, from the same internal molecular movement that produces creep. This gradual decay in stress at a constant strain (stress-relaxation) becomes important in applications such as preloaded bolts and springs where there is concern for retaining the load. The amount of relaxation can be measured by applying a fixed strain to a sample and then measuring the load with time. 

It should also be noted that the LIESST phenomenon has been recently observed on these materials [53-55]. This discovery may lead to a new wave of photomagnetic investigations of these bistable materials in view of potential applications. The shape of the relaxation curves after LIESST could be modelled within the framework of a revised ID Ising like model [56]. 

This behavior results from stress relaxation and other viscoelastic phenomena that are typical of TPs. In addition to using heat TPs such as polyolefins, neoprenes, silicones, and other cross-linkable TPs are example of plastics that can be given memory either by radiation or by chemically curing. Fluoroplastics need no such curing. When this phenomenon of memory is applied to fluoroplastics such as TFE, FEP, ETFE, ECTFE, CTFE, and PVF, interesting and useful high-temperature or wear-resistant applications become possible. 

Negative adsorption is a relatively import2mt phenomenon in concentrated disperse systems and in capillaries. It is responsible for the Donnan effect, for the exclusion of electrolytes from concentrated sols, dispersions and capillaries and the ensuing salt-sieving effect, already introduced in chapter I.l. It also plays a role in double layer relaxation as occurs in alternating fields or in particle-particle Interaction. As negative adsorption is a purely electrostatic feature and takes place far from the surface, in all these applications its computation from Polsson-Boltzmann statistics is reliable, especially at high ly l. 

Recently [16] we have shown that water diffusion in the PHB films with 100 pm thick was completed in several tens of minutes, whereupon the films absorbed the limiting equilibrium concentration of water (ca. 1 wt %). Structural relaxation in PHB under humid conditions is finished in longer period of time (nearly 1000 minutes). We have investigated kinetics of release for several tens of days, therefore, to a first approximation, a water transport phenomenon in PHB is not essential. However, long-term kinetics of drug release from PHB films has an intricate form and demands special analysis for both diffusion modeling and drug delivery application. 

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