Relaxation alpha

Kakizaki, M., Anada, U. and Hideshima, T., Distribution of free-volume and dielectric alpha-relaxation in glass-forming liquid-mixtures, Jpn. J. Appl. Phys., Parti, 1995, 34, 3593. 

The volumetric, elastic and dynamic properties of internally and externally plasticised PVC were studied and compared with those of unplasticised PVC. The glass transition temperature for the plasticised samples was markedly lowered and this decrease was more important for the externally plasticised ones. The positions of the loss peaks from dielectric alpha-relaxation measurements confirmed the higher efficiency of the external plasticisation. However, the shape of the dielectric alpha-relaxation function was altered only for the internally plasticised samples. The plasticisation effect was linked with a decrease in the intensity of the beta-relaxation process but no important changes in the activation energy of this process were observed. The results were discussed. 47 refs. 

In this study, both the normal mode relaxation of the siloxane network and the MWS processes arising from the interaction of the dispersed nanoclay platelets within the polymer network have been observed. Although it is routine practice to observe the primary alpha relaxation of a polymeric system at temperatures below Tg, in this work it is the MWS processes associated with the clay particles within the polymer matrix that are of interest. Therefore, all BDS analyses were conducted at 40°C over a frequency range of 10 to 6.5x10 Hz. At these temperatures, interfacial polarization effects dominate the dielectric response of the filled systems and although it is possible to resolve a normal mode relaxation of the polymer in the unfilled system (see Figure 2), MWS processes arising from the presence of the nanoclay mask this comparatively weak process. 

In accordance to the data reported in the literature for bulk hyperbranched polyesters [34,35], three relaxation processes are also observed in thin POHOAc films, (Fig. 23) the alpha relaxation process, representing the dynamic glass transition, the beta process, attributed to the relaxation of the ester groups, and the gamma relaxation process, originating from fluctuations of the —OH end groups. The latter two, which are broad and not well-separated from one another, are only distinguishable in the temperature representation of the dielectric spectra (inset, Fig. 23). 

The assignment of the alpha relaxation was additionally confirmed by AC-calorimetric measurements on thin films of POHOAc [31], The effect of confinement on the overall dynamics of POHOAc is given in Fig. 24, showing the dielectric spectra for different film thicknesses, ranging from 310 nm down to 17 nm. While the beta and the gamma relaxations, as local processes, are not affected with decreasing film thickness (except a decrease of the dielectric... 

Figure 23. Dielectric loss e" versus frequency at different temperatures, as indicated, showing the alpha relaxation process for a thin film of hyperbranched polyesters of 17 nm. Inset. Dielectric loss e" versus temperature at 49 kHz, showing the beta and the gamma relaxation processes for the same sample. Dotted lines serve as a guide for the reader.

Using the usual fitting procedure [ 1 ], the dependence of the relaxation rate on the inverse temperature for the alpha and beta relaxation process is extracted (Fig. 25). The dynamic glass transition becomes more than one order of magnitude faster with increasing confinement, corresponding to a shift of 36 K to lower temperatures (Fig. 26). The thickness dependence of both the alpha relaxation time (at a constant temperature of 427 K) and the maximum... 

Figure 26. Relaxation time of the alpha relaxation process (at 427 K) and the maximum temperature position of the alpha peak (at 0.3 Hz) as a function of film thickness. Inset The relaxation time distribution of POHOAc at 510 K for different thicknesses, as indicated.

Figure 27. Dielectric strength of the alpha relaxation process of POHOAc at 500 K as a function of film thickness. Inset Refractive index of POHOAc at X — 630 nm in dependence on the film thickness.

Figure 29. Comparison dielectric spectroscopy versus dilatometry Tg — T" r (determined by dilatometry) versus film thickness, where 7T" f is the value of Ts for the 310-nm-thick sample and T.j — Tlf at 0.6 Hz versus film thickness, where T represents the maximum temperature position of the alpha relaxation peak and 7," i corresponds to the thickness of 310nm.

Figure 33. Normalized dielectric loss versus frequency at 122°C, showing the alpha relaxation process of a thin PS film of 75 nm as prepared, and after different annealing times at 180°C in air, as indicated. Inset. The corresponding relaxation rate in dependence on the annealing time at 180°C in air.

Much more pronounced effects are observed at higher temperatures The alpha relaxation peak of a thin PS film of 75 nm (measured at 122°C) is shifted to higher frequencies after keeping the sample at 180°C in ambient air (Fig. 33). Consequently, the average relaxation rate of the alpha relaxation process increases in time by more than one order of magnitude during this thermal treatment (inset, Fig. 33). 

For thin PS films of 63 nm, using the usual fitting procedure [1], the relaxation rate as a function of inverse temperature is extracted (Fig. 34), after different annealing steps in air and in pure nitrogen. While unchanged after 24 hours at 180°C in a nitrogen atmosphere, the dynamic glass transition becomes one decade faster when the sample is annealed in air. This corresponds to a shift to lower temperatures of the maximum position of the alpha relaxation peak (inset, Fig. 34). 

Figure 34. Relaxation rate versus inverse temperature for a 63-nm-thick PS film as prepared, after 24 hours annealing at 180°C in a pure nitrogen atmosphere, and after 1, 2, 5, and 10 hours annealing at the same temperature in air. Inset The corresponding alpha relaxation at 1.2 kHz in spectra of s" versus temperature.

Figure 39. Relaxation rate of the alpha process versus inverse temperature for different thin PS films, as indicated. Inset. Dielectric loss (normalized) versus temperature at 31 kHz skHz showing the alpha relaxation of the same samples.

It is semicrystalline (50-60%) and melts at 240°C (commercial grade). " It has an alpha relaxation at 140°C, a beta at 90°C, and gamma relaxation at -65°C. Conformation of ECTFE is an extended zigzag in which ethylene and CTFE alternate. The unit cell of ECTFE s crystal is hexagonal. 

Cerveny investigated the development of the dynamic glass transition in styrene-butadiene copolymers by dielectric spectroscopy in the frequency range from 10 to 10 Hz. Two processes were detected and attributed to the alpha- and beta-relaxations. The alpha relaxation time has a non-Arrhenius temperature behavior that is highly dependent on styrene content... 

Broad band dielectric spectra of liquids enable an insight into relaxation processes associated both with the rotational and translational molecular motions. " The latter originates from small residual ionic dopants, always present in liquids. The structural relaxation, called also the alpha relaxation, is often associated with reorientation of entire molecules coupled to the permanent... 

Therefore, in glass formers, the spatial aspect of the dynamic heterogeneity of the alpha relaxation may be described as a consequence of a mobility contrast between an island of high mobility [65] (where local mobility persists even below Tg as described previously in characterizing the beta process cage-rattling motions)... 

Hall DB, Hooker JC, Torkelson JM (1997) Ultrathin polymer films near the glass transition effect on the distribution of alpha-relaxation times as measured by second harmonic generation. Macromolecules 30 667-669... 

Zetsche, A., and E. W. Fischer. 1994. Dielectric studies of the alpha-relaxation in miscible polymer blends and its relation to concentration fluctuations. Acta Polym. 45 168-175. 

Storage modulus, alpha relaxation and creep behavionr in polymers are influenced by electron irradiation. Thus, creep strain increased upon exposure to electron beam irradiation, below 4 MRad. 

The F q, t) is calculated at a value corresponding to the first peak of structure factor S(q). The alpha relaxation time (t ) is obtained usually as the time at which the F q, t) decays by a factor of e. 

LLDPE Storage modulus (alpha relaxation) Effects of beta irradiation 112... 

Several polymers combine excellent ultraviolet resistance with good tensile and elongation at break properties (Table 11.3). The storage modulus, alpha relaxation, and creep in polymers are influenced by electron irradiation. Thus, the creep of some polymers increased upon exposure to electron beam irradiation below 4 Mrad. Neutron/gamma irradiation also had an adverse effect on some polymer properties. Thus, some glass fiber-reinforced plastics lose 20%-40% of their flexural strength after exposure to neutron/gamma irradiation doses above 1 x 10 Gy [3]. 

---