P relaxations

Transitions. Samples containing 50 mol % tetrafluoroethylene with ca 92% alternation were quenched in ice water or cooled slowly from the melt to minimise or maximize crystallinity, respectively (19). Internal motions were studied by dynamic mechanical and dielectric measurements, and by nuclear magnetic resonance. The dynamic mechanical behavior showed that the CC relaxation occurs at 110°C in the quenched sample in the slowly cooled sample it is shifted to 135°C. The P relaxation appears near —25°C. The y relaxation at — 120°C in the quenched sample is reduced in peak height in the slowly cooled sample and shifted to a slightly higher temperature. The CC and y relaxations reflect motions in the amorphous regions, whereas the P relaxation occurs in the crystalline regions. The y relaxation at — 120°C in dynamic mechanical measurements at 1 H2 appears at —35°C in dielectric measurements at 10 H2. The temperature of the CC relaxation varies from 145°C at 100 H2 to 170°C at 10 H2. In the mechanical measurement, it is 110°C. There is no evidence for relaxation in the dielectric data. 

The results for the y relaxation, compared with those of the p relaxation, give a deeper insight about the influence of the spacer structure on the viscoelastic... 

Polycarbonate (PC) serves as a convenient example for both, the direct determination of the distribution of correlation times and the close connection of localized motions and mechanical properties. This material shows a pronounced P-relaxation in the glassy state, but the nature of the corresponding motional mechanism was not clear 76 80> before the advent of advanced NMR techniques. Meanwhile it has been shown both from 2H NMR 17) and later from 13C NMRSI) that only the phenyl groups exhibit major mobility, consisting in 180° flips augmented by substantial small angle fluctuations about the same axis, reaching an rms amplitude of 35° at 380 K, for details see Ref. 17). 

Apparently local motions indicating differences in packing are closely related to the mechanical properties of glassy polymers. One of the puzzling features of the P-relaxation in PC as in other glassy polymers 3 6 76 77) is that it often is suppressed if the glass transition temperature is lowered by adding a plasticizer. The material then becomes brittle, which severely limits the applications of such polymers. Such low... 

Koehl P Relaxed specificity in aromatic prenyltransferases. Nat Chem Biol 1(2) 71—72... 

Polymer Melts Coherent Scattering and van Hove Correlation Functions. Part I Dynamics in the p-Relaxation Regime. 

Figure 3. Cross polarization magnetization for the PIP-cured epoxy under the SL (Hartmann-Hahn) condition. The cross polarization contact time is rcp. The decay corresponds to proton T,p relaxation.

Urinary bladder Detrusor Contraction p Relaxation (usually)... 

Ciliary muscle Contraction for near p Relaxation for far vision... 

The mere fact that the reaction proceeds in the glass-like state implies that for the chemical reaction to occur a small-scale local mobility (possibly P-relaxation) is sufficient, whereas the segmental mobility provides for a translational diffusion of the functional groups in the system3,70,71). 

Often, however, there is insufficient molecular mobility in the solid state for asymmetric fluorine placement to significantly increase the dielectric constant. Below the glass transition temperature, only restrained local motions are possible, and below subglass relaxations such as the P relaxation in polyimides, even these limited motions are virtually eliminated, rendering orientation polarization negligible. 

Paramagnetic effects on the l/7 and /T2 relaxation rates of 13C and 31P nuclei of adenylylated glutamine synthetase ([2-13C]AMP-GS) were measured when the two enzyme-bound Mg2+ ions were replaced by Mn2+ (121). Also the paramagnetic effects of Co2 + were measured on the 3 P relaxation rates of adenylylated enzyme. Distances between these metal ions and the l3C and 31P nuclei are summarized in Fig. 26. 

We will discuss some preliminary results, which have been performed recently l01). In Fig. 39a the results for polymer No. 2d of Table 10 are shown, which were obtained by torsional vibration experiments. At low temperatures the step in the G (T) curve and the maximum in the G"(T) curve indicate a p-relaxation process at about 120-130 K. Accordingly the glass transition is detected at about 260 K. At 277 K the nematic elastomer becomes isotropic. This phase transformation can be seen only by a very small step in G and G" in the tail of glass transition region, which is shown in more detail in Fig. 39 b. From these measurements we can conclude, that the visco-elastic properties are largely dominated by the properties of the polymer backbone the change of the mesogenic side chains from isotropic to liquid crystalline acts only as a small disturbance and in principle the visco-elastic behavior of the elastomer... 

As mentioned earlier, we usually encounter two characteristic secondary relaxations in polymethacrylates and polyacrylates (below the glass transition temperature) which are assigned to side-chain motions1,12,13,15 The p relaxation due to partial rotation of... 

The p peak occurs at - 70 °C at 1 Hz. As the only significant dipoles in PET that are mobile are the carboxyl groups, the dielectric p relaxation can be unambiguously attributed to the motion of these groups. Furthermore, the dielectric p peak is quite symmetrical and can be fitted by a Gaussian curve. 

From another point of view, the dielectric increment, Aer> of the p relaxation process is 0.52, which is much smaller than the one calculated for... 

Fig. 13 Dielectric tan 8 peaks in PET associated with the p relaxation process observed at 10kHz ( ), 1 kHz ( ), 100Hz ( ), 10Hz (O) and 1 Hz (A) (from [13])...

In order to get more detailed information about the motions associated with the p relaxation in PET and to understand the differences observed between mechanical and dielectric data, 13 C NMR was used, as well as 2H NMR on PET samples selectively deuterated either on the phenyl rings or on the ethylene glycol units [12]. Due to the higher frequency range corresponding to NMR experiments (105 Hz), the extrapolation of the dielectric results leads to the occurrence of the motions involved in the f3 relaxation around 25 °C, which is effectively observed. 

In addition to the difficulty in extrapolating the dynamic mechanical results over 5 decades in frequency, the complexity of the p relaxation in epoxy resins induces quite a broad range for the temperatures extrapolated at 105 Hz. The calculated values are equal to ... 

The p relaxation can play a very important role in fracture properties. For comparable Tg values, for instance 100°C, amine-crosslinked epoxies are considerably more ductile and tough than unsaturated polyesters. 

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