Relaxation processes crystallinity

One of the most common methods utilized to characterize the phase behavior of polymer blends employs low amplitude cyclic deformation studies to obtain the elastic and viscoelastic properties. This method, termed dynamic mechanical characterization, yields high resolution of polymer transitions including secondary relaxation processes, crystalline melting transitions and of primary importance, the glass transition. This method maps the data over a broad temperature range to ascertain the phase behavior. 

Such data have been reported for PTFE laminates over frequencies from 10 to 10 ° Hz from - 75°C to 200°C. In a comprehensive study of polytrichloroethylene (60), dielectric properties were reported at frequencies from 0.1 to 8.6 x 10 Hz at temperatures from - 50 to 244°C for specimens of varying degrees of crystallinity. The results were interpreted in terms of relaxation processes, crystallinity, and molecular configuration. Test procedures are described in great detail (60). 

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. 

Studies have considered the effect of crystallinity on the performance of CR adhesives (97), on segmental mobiUty as determined by nqr studies (101), on strain induced property changes (102), and on relaxation processes (103). 

Hayakawa, R. Relaxation processes in crystalline and non-crystalline phases in polymers. Progress in Polymer Science, Japan, Vol. 3. Tokyo Kodansha 1972. 

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... 

In order to determine the content of this noncrystalline line further, we examined in more detail the behavior of the spin-lattice relaxation. Figure 5 shows the partially relaxed spectra in the course of the inversion recovery pulse sequence (180°-t-90°-FIDdd-10s)i2o with varying x values. The magnetization that was recovered for 10 s in the z direction was turned to negative z direction by 180° pulse and the magnetization recovered in z direction for varying x was measured in the xy plane under H DD. The spectra at different steps of the longitudinal relaxation were obtained by Fourier transform and are shown in Fig. 5. In these spectra the contribution from the crystalline components with Tic s of2,560 and 263 s are eliminated due to the lack of time for recovery at each pulse sequence. Therefore, we observed preferentially the relaxation process of the noncrys-... 

Fully deuterated linear poly(ethylene) (PE) has been investigated also via various 2H NMR techniques below Tm, i.e. in the semi crystalline state [83, 84]. The crystallinity ratio was measured as a function of temperature and it was shown that the motions are highly restricted in the amorphous regions of PE. It was shown that the onset of 3 and a transitions (at which mobility appears in the crystalline phase) may be observed by 2H NMR on raising the temperature. This onset of local motions in the crystalline phase is related to the chain relaxation process quoted previously. 

Another example of an application of Eq. (145) is on microcomposite polymer materials. We have performed dielectric measurements of the glass transition relaxation process in a nylon-6,6 sample quenched in amorphous (QN), a crystalline nylon-6,6 sample (CN), and a microcomposite sample (MCN), which is the same crystalline nylon-6,6 but with incorporated kevlar fibers [275,276],... 

One can also see from Table IV that the presence of either the crystalline phase or kevlar fibers in a sample leads to an increase of the cutoff time To, indicating a slowdown of the relaxation process. Their presence also leads to an increase of the (Do value as well. This is a manifestation of a decreasing mobile polymer chain length. 

In polyethylene the ac-relaxation process (see Section 3.4) enables the movement of chains into and out of the crystalline lamellae. Theoretical treatments have demonstrated that it most probably proceeds by propagation of a localized twist (180° rotation) about the chain axis extending over 12 CH2 units (Fig. 6.14). As the twist defect travels along the chain, it rotates and translates the chain by half a unit cell (i.e, by one CH2 unit) - this is termed the c-shear process (Mansfield and Boyd, 1978). The activation energy for this process is about HOkJmoF1, corresponding to the extra energy required to introduce the twist defect into the crystal. Once formed, the twist can freely... 

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