Y-relaxation

The relaxatioa temperature appears to iacrease with increa sing HFP coateat. Relaxatioa iavolves 5—13 of the chaia carboa atoms. Besides a and y relaxations, one other dielectric relaxation was observed below —150° C, which did not vary ia temperature or ia magnitude with comonomer content or copolymer density (55). The a relaxation (also called Glass 1) is a high temperature transition (157°C) andy relaxation (Glass 11) (internal friction maxima) occurs between —5 and 29°C. 

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. 

Determination of the glass-transition temperature, T, for HDPE is not straightforward due to its high crystallinity (16—18). The glass point is usually associated with one of the relaxation processes in HDPE, the y-relaxation, which occurs at a temperature between —100 and —140° C. The brittle point of HDPE is also close to its y-transition. 

The y relaxation takes place at the lowest temperature, overlaps with the )3 relaxation (Fig. 15), and coincides in location and activation energy with the typical y relaxation of polyethylene [35,36], and also of polyethers [37], and polyesters [38] with three or more consecutive methylene units. It appears, for 3 Hz and tan6 basis, at - 120°C (P7MB) and - 126°C (P8MB), and its location and activation energy (35-45 kJ mol ) agree with the values of a similar relaxation associated with kink motions of polymethylenic sequences. 

The relaxation at the lowest temperature y relaxation) takes place below - I0O°C. The two polymers with shorter spacers (PDEB and PTEB) show weak relaxations overlapped with the )3 ones due to the low tanS values (0.03 and 0.04, respectively). Notwithstanding this, the y relaxation is clearly distinguished when using loss modulus plots, even in the case of PDEB, that shows the weakest maximum (see Fig. 16). For PTTB, tan6 values in the y relaxation interval are of the order of 0.05. 

It is usually considered that the y relaxation arises from crankshaft and kink movements of polymethylenic sequences, but the clear maximum of tanS and loss modulus for the three polybibenzoates here reported leads to the conclusion that the motion responsible of this relaxation also takes place when one of the methylenic... 

On the contrary, the phase structure and the thermal history do not have important effects on the location and intensity of the /3 relaxation. This relaxation is very broad in all the samples and overlaps the y relaxation. The activation energy of the /3 peak is about 85 kJ mol for the three samples, of the same order of magnitude as that of other polyesters [38,40]. Finally, the y relaxation is found in the three samples of PTEB with no remarkable influence of the thermal history. 

In conclusion, the different thermal histories imposed to PTEB have a minor effect on the /3 and y relaxations, while the a. transition is greatly dependent on the annealing of the samples, being considerably more intense and narrower for the specimen freshly quenched from the melt, which exhibits only a liquid crystalline order. The increase of the storage modulus produced by the aging process confirms the dynamic mechanical results obtained for PDEB [24], a polyester of the same series, as well as the micro-hardness increase [22] (a direct consequence of the modulus rise) with the aging time. 

The y relaxation appears around - 120°C (Fig. 19). PTEB and the copolyesters show the y maximum at a temperature that is practically constant, slightly lower than the one for P8MB. The low activation energy values (Table 3) are the usual ones for this relaxation. 

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... 

Table 3 Temperature Location (tan5 basis, 3 Hz) and Activation Energies of the /3 and y Relaxations for Different Samples with Varying Content in Oxyethylene Units (frEo)...

B. The existence of different barriers AUj calculated by empirical formulas or by quantum mechanics yields a spectrum of relaxation processes (a, 0, y relaxation). Hoffman uses an arbitrary rectangular distribution of relaxation processes38. Other authors use a Gaussian distribution for the relaxation processes14. ... 

Gray and McCrum735 used the Hashin-Shtrikman theory to explain the origin of the y relaxation in PE and PTFE, Maeda et al.745 have given exact analyses of several two phase models for semi-crystalline polymers and Buckley755 represented a biaxially oriented sheet of linear polyethylene by a two phase composite model. 

With the exception of local main-chain motions, the above-mentioned types of molecular motions have been investigated on a series of hydrophilic polymethacrylates and polyacrylates by means of dynamic mechanical measurements carried out with a torsional pendulum. For this purpose, the constitution of polymethacrylates was systematically altered and correlated with the dynamic mechanical response spectra. It was established for a series of copolymers of poly(2-hydroxyethyl methacrylate) that the temperature of the y relaxation (140 K 1 Hz), assigned to the motion of 2-hydroxyethyl... 

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. 

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 y transition observed from dynamic mechanical analysis at 1 Hz, is centred around - 150 °C. However, the temperature range available experimentally does not permit observation of the whole y relaxation. The temperature position is independent of the chemical composition of the xTy -y copolyamides. 

P relaxation is observed as a shoulder of the a relaxation. This behavior preclude the possibility to perform an exhaustive analysis of the j3 relaxation [33], The 5 and y relaxations are commonly deconvoluted for the Fuoss-Kirkwood [69] empirical expression ... 

In the case of PCHEM and PCHPM the values of m parameter are compiled on Tables 2.1 and 2.2. These values shows that the y relaxation is broader than the 8 relaxation. This result is indicative that probably the y relaxation involve a more complex molecular motion than 8 relaxation. 

Table 2.1 Parameters of Huoss-Kirkwood equation for 8 and y relaxations of PCHEM. (From ref. [33])...

In this family of polymers, with saturated cyclic side chain the effect of the side chain structure and also the effect of the spacer group on the viscoelastic behaviour is clearly illustrated in Fig. 2.11a and b in which it is possible to observe the variation of tan 8 for five members of the series with temperature for 8 and y relaxations. 

Another way to get information about the relaxational behavior of these materials can be performed by dynamic mechanical calculations. In order to get information about the origin of the secondary y relaxation, molecular dynamic (MD) calculations over the repeating unit were performed. By this way considering the axial and equatorial equilibrium on the cyclohexyl group and the interconversion of these two... 

Therefore, the free energy change against the number of carbon atoms in the side chain is in excellent agreement with the experimental data which means that the number of carbon atoms of the spacer group has an influence on the y relaxation associated with the chair-to-chair conformational change in the cyclohexyl... 

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