Relaxation process with main chain

The y Relaxation. In common with many other polymers (8) both PPO and PS display significant loss maxima below room temperature at the frequencies under consideration. Whereas the process responsible for the a loss is at least qualitatively understood in terms of a main chain relaxation associated with the glass transition, y losses can often only tentatively be attributed to specific mechanisms. In PPO, for example, it does not seem unreasonable to propose that the y loss is associated with librations in the two pendant methyl groups this view is somewhat reinforced by the observation that in the dielectric measurements the relaxational strengths of the y and a loss processes are comparable. As the latter can be well interpreted (6) in terms of a dipolar relaxation of the main chain in which the entire dipolar contributions arise from the methyl groupings, it seems plausible to assume that the same dipoles are responsible for the y loss mechanism. In polystyrene there is a similar... 

The only temperature dependent parameter is snd the activation energy for solutions has been measured as 4.1 kcal mol ( .) Temperature variation of the relaxation of polystyrene main chain bonds has been investigated (] 2) giving an activation energy of 4,1 kcal mol and a room temperature value of 4 X lO" s. The agreement between these two different experimental measurements suggests they are both concerned with the same process. 

Not all five relaxation processes are always simultaneously observed in individual polyacrylate-based materials. In fact, this finding significantly aids detailed assignment of the relaxation processes to particular modes of motion. By changing fragments of molecular architecture of the side chain unit or of the main chain and performing comparative DR studies it is, in principle, possible to identify different relaxation processes with particular dipole moments and their motions, and to study the influence of... 

We turn to the relaxation processes observed in smectic polymers with different attachment of mesogenic groups to the macromolecular backbone and compare dielectric behaviour of smectic and nematic polymers having identical mesogenic groups but different main chain structure. 

The temperature position of the secondary fi relaxation (about 290 K 1 Hz), generally attributed to partial rotations of the side chains COOR, is only slightly affected by the polarity and volume of the substituent R but decreases markedly (by 120 K) on removal of the a-methyl group on the main chain. The experimental data obtained contradict the assumption that there is a certain relationship between this temperature and the glass transition temperature. Nevertheless, we can infer that the pertinent molecular mechanism in polymethacrylates differs from that in polyacrylates, probably due to the different participation of the main chains. The values of the individual contributions to the activation energy were estimated by employing a procedure similar to that used in the y relaxation process, and their sum was found to agree approximately with the experimental values. 

It is interesting to point out the change in the relative heights of the and a peaks between PMMA and CMIM20 (Fig. 129). In the latter polymer, only a small part of the dielectric relaxation happens through the j3 motional processes. The cooperative motions involved in the a transition are required for achieving an important relaxation, whereas it is the opposite for PMMA. Such a behaviour is consistent with the hindrance of main-chain cooperativity by the rigid CMI units. 

In a subsequent investigation, with Roos and Kampschreur (1989), Northolt extended the modified series model to include viscoelasticity. For that an additional assumption was made, viz. that the relaxation process is confined solely to shear deformation of adjacent chains. The modified series model maybe applied to well-oriented fibres having a small plastic deformation (or set). In particular it explains the part of the tensile curve beyond the yield stress in which the orientation process of the fibrils takes place. The main factor governing this process is the modulus for shear, gd, between adjacent chains. At high deformation frequencies yd attains its maximum value, ydo at lower frequencies or longer times the viscoelasticity lowers the value of gd, and it becomes a function of time or frequency. Northolt s relations, that directly follow from his theoretical model for well-oriented fibres, are in perfect agreement with the experimental data if acceptable values for the elastic parameters are substituted. 

Table 21. Parameters of the intramolecular mobility for high-frequency (rh.f. r/) and low-frequency (t Tf) relaxation processes in the main chain and in side chains of PMMA labeled with luminescent markers of different structure... 

By combination of relaxation strength measurements obtained from isochronal scans and molecular mass data, it could be concluded that the relaxation strength of the P process increases with decreasing molecular mass, i. e. with increasing content of polar end groups. It may thus be postulated that the dielectric p relaxation to some extent is associated with the local main-chain motions of the methylene spacer group adjacent to the polar end groups. 

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