Alpha relaxation 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). 

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.

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 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). 

A DMTA study of polyolefin-clay nanocomposites has shown that alpha, beta and gamma relaxations of the polymer were affected by polymer chain branching and clay exfoliation level [50]. Salmeron Sanchez and co-workers [51] studied the structure of the system obtained after free radical copolymerisation of ethyl acrylate and hydroxyethyl methacrylate comonomers using dynamic-mechanical and calorimetric techniques. Copolymerisation theory states that the free radical copolymerisation reaction of two monomers may give rise to a copolymer with a different chain composition from that of the random mixture corresponding to the original solution. In this system, the dynamic-mechanical spectra suggested there were two main alpha relaxation processes in the copolymers. 

HF High-frequency glassy (alpha) relaxation process... 

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. 

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 25. Relaxation rate of the alpha and beta relaxation process of POHOAc versus inverse temperature for different film thicknesses, as indicated.

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... 

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)... 

For polymers, dielectric spectroscopy is sensitive to fluctuations of dipoles, which are related to the molecular mobility of groups, segments, or the polymer chain as well [38]. The molecular mobility is taken as a probe for structure. The basic quantity is the complex dielectric function e f) = t (f) - it"(f) as a function of the frequency/and the temperature T. s (/) is the real whereas e"(/) is the loss part i = >f ). A relaxation process is indicated by a step-like decrease of s (/) with increasing frequency and a peak in e"(/). From the maximum position of the peak a mean relaxation rate can be deduced, which corresponds to the relaxation time of the fluctuation of the dipole moment of a given structural imit. For details see reference [49]. All shown measurements were carried out isothermally in the frequency range from 10 to 10 Hz by an ALPHA analyzer (NovocontroF). The temperature of the sample is controlled by a Quatro Novocontrol system with stability better than 0.1 K. 

Temperature-modulated differential scanning calorimetry (T-MDSC) applies a thermal modulation in temperature to a conventional DSC mn and determines a dynamic heat capacity from the relationship between the modulation components of temperature and of heat flow. Primary application of this technique has been the measurement of specific heat capacity and the examination of the anomaly in a relaxation process such as alpha process related to the glass transition. An application to the first-order phase transitions of crystallisation and melting of polymer crystals has recently been suggested. The method and typical results are described. 13 refs. 

The inherent radioactive characteristics of the spent nuclear fuel condition determine many of the key processes to be studied. Owing to its energy content, spent fuel relaxes by transferring alpha, beta, and gamma radiation to water when contacting it. This originates what is known as radiolysis reactions. The key processes occurring at the spent fuel water interface are depicted in Fig. 8. 

Figure 9. Temperature dependence of dielectric strengths for DEGMA evidencing the change of slope for the secondary relaxations around Tg The p process is merged under the main alpha process and so is only detected in a limited temperature range (adapted from reference [47]).

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