Polymer relaxation activity, effects

Figure 8.36. The B-relaxation hardly shifts as a function of the measuring frequency due to the presence of a large crystalline phase. The y-relaxation is deary much stronger frequency dependent. An activation energy value of 63 kJ/mole was calculated for the y-relaxation from the slope of this curve. Some typical y-relaxation activation energy values for linear polymers are 63, 54 and 54 kJ/mole for respectively PVC [14], PC and PET [15]. The mechanisms of these y-relaxations are often described as local mode relaxation effects [15]. The same mechanism might also be responsible for the y-relaxation effect in polyketone polymers.

Dielectric relaxation spectra of poly(methyl acrylate) (IfA) and poly(t-butyl acrylate) (tBA) were measured at temperatures above and below Tg, and both a- and 3-relaxation processes were observed. As for the 3-relaxation process, in order to clarify the quantitative relationship between the relaxation mechanism and the polymer structure, the effective dipole moment(Pg) was estimated by a method according to the 2-state transition theory. In the estimation, the average local configuration of the main chain was assumed to be in isotactic form or syndiotactic form. Since samples used were atactic polymers, the authors assume that Pg(atact) = Xi Pe(i) + (1 - X ) Pe(s)> where X denotes the tacticity, i represents isotactic form, and s, sytidiotactic form, respectively. And, the activation energy for the atactic form sample is examined in a similar way. From the results, it can be concluded that the 3-relaxation of samples is attributed to the restricted rotation of the side chain, especially, to the rotation of the first bond-axis connecting the side chain and main chain. 

The only previous report of activation energies for poly (methyl o chloroacrylates) is for the "conventional" free radical polymer which can be assumed to be reasonably syndiotactic (16). The values quoted are 130 kcal/mole for the a relaxation and 26 kcal/mole for the 3 relaxation. The present results (Table II) are in qualitative agreement with these values. In general the activation energies for the relaxations decrease with increasing ester side chain length or bulkiness and the isotactic isomers have a relaxation activation energies about 35-50 kcal/mole lower than the comparable syndiotactic isomers. This effect, as already discussed, is a consequence of the Tg difference between the i somers. 

In molecular doped polymers the variance of the disorder potential that follows from a plot of In p versus T 2 is typically 0.1 eV, comprising contributions from the interaction of a charge carrier with induced as well as with permanent dipoles [64-66]. In molecules that suffer a major structural relaxation after removal or addition of an electron, the polaron contribution to the activation energy has to be taken into account in addition to the (temperature-dependent) disorder effect. In the weak-field limit it gives rise to an extra Boltzmann factor in the expression for p(T). More generally, Marcus-type rates may have to be invoked for the elementary jump process [67]. 

It is mostly used to describe the effect of temperature on the dynamics of relaxation processes in glassy polymers. Rearranging Eq. (8), we obtain the relation between the activation energy AU (in kJ) and the temperature of relaxation, Tr(f), detected at frequency f ... 

As in the case of other polymers containing saturated cyclic side chains the a relaxation is obscured by low-frequency conductive effects. Moreover in the case of PCBuMM, dielectric activity is also observed. Therefore both polymers show important conductivity contributions at high temperatures and low frequency. The conductivity analysis in this case is also performed using the hopping model... 

When adsorbed (from ambient air), water molecules might act as plasticizers and alter the dynamics of polymers. Moreover, water has a strong dipole moment and, consequently, dielectric active relaxation processes, which could partially occlude significant parts of the dielectric spectra of interest. Special attention to this effect has to be paid when the dynamics of thin polymer films is investigated, for example in relation to phenomena like the glass transition, dewetting, pattern formation, surface mobility etc. 

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