Dynamic glass transition process

Zentel et al were also first to observe another high-frequency DR process in an unoriented mesomorphic sample. It appears at frequencies above the (5-process and shows a very broad absorption curve. The temperature dependence demonstrates non-Arrhenian behavior, i.e. an increase in the activation energy with decreasing temperature. It was assigned to the micro-Brownian segmental dynamics of the backbone, i.e. the dynamic glass transition process, and termed the a-process. ... 

The relaxation process associated with the dynamic glass transition, the a relaxation, and the P relaxation, as a shoulder of the a relaxation can be observed in all these figures. At low temperatures another relaxation labeled as y relaxation can be observed. In the case of PCHpM, the maximum of the y relaxation is well away from the temperature range. Heijboer and Pineri [36,57] have reported that the maximum for this polymer is at about 100 K for 1 Hz. In the case of PCHpMM and PCOcM, the y relaxation can be observed which may be analyzed by using the Fuoss-Kirkwood (F-K) equation ... 

Figures 2.37 and 2.38, show the isochronal curves of the permittivity and loss factor for P2NBM and P3M2NBM as a function of temperature at fixed frequencies. A prominent relaxation associated with the dynamic glass transition is observed in both polymers. Clearly the effect of the methyl substitution in position 3 of the norbornyl group is to decrease the temperature of this relaxational process.

As expected, two relaxation processes are observed for PIP in the bulk the segmental mode, related to the dynamic glass transition representing the dynamics of the polymer segments, and the normal mode, sensing the chain dynamics (Fig. 9). 

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). 

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... 

The glass transition processes in foods may result from a rapid removal of water from solids. Based on that, e.g., the Tg values of anhydrous polysaccharides are high, and the food materials may decompose at temperatures below Tg (Kokini et al 1994 Roos and Karel, 1991b). The glass temperature transition affects viscosity, stickiness, crispness, collapse, crystallization, and ice formation, and can strongly influence deteriorative reaction rates. This provides a new theoretical and experimental framework for the study of food systems to unify structural and functional aspects of foods, described in terms of water dynamics and glass dynamics. 

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... 

Thus, one has to expect one dielectric relaxation process, assigned to a local motion of the Si-O bond. It corresponds to the dynamic glass transition (a-relaxation) of the bulk polymer. Measured from temperatures between —25 °C to 33 °C this relaxation process shifts from about 1 Hz to about 10 Hz (Fig. 21.4). 

Broadband dielectric spectroscopy enables one to analyse the dynamics of polar groups in polymeric systems. Due to its broad frequency range of more than 10 decades a manifold of different molecular fluctuations can be studied from the dynamic glass transition (spanning already more than 10 decades in times) to secondary relaxations. Additionally one finds in chiral liquid crystals cooperative processes like soft-and Goldstone modes. 

The viscoelastic behaviour of the majority amorphous phase is important in terms of the mechanical properties of the material. Dynamic mechanical (S) and dielectric studies (8) reveal three relaxations, labelled a, p and 7 in order of decreasing temperature. Variations in the strength of these relaxations with systematic changes in the mole fraction of each component have led to the association of the 7 relaxation with the HBA component and the p relaxation with the HNA component, with the a relaxation displaying features typical of a glass transition process. Support for these assignments has been obtained from analysis of the proton NMR second moments (9,10),... 

In side-chain LC polymers, the molecular dynamics of the mesogenic groups is decoupled from the main chain by use of a flexible (aliphatic) spacer. Three relaxation processes typically take place. (1) The p-relaxation which is assigned to librational fluctuation of the mesogen arotmd the long molecular axis. (2) The 5-relaxation process corresponding to librational fluctuation around the short molecular axis. (3) And finally, the a-relaxation process corresponds to the motion of the polymer backbone at the dynamical glass transition. It should be... 

These fluorocyanopolymers were dielectrically characterized in a wide range of frequencies and temperatures. The dominating relaxation process detected in these materials is the a-relaxation, associated with the dynamic glass transition. A VFTH temperature dependence of the relaxation times was found for these fluorocyanopolymers. The polarity-dielectric constant relationship has been established. Actually, the inclusion of CN group into fluorinated units enhances the dielectric increment and makes them potential candidates for film capacitors. 

We have performed an analysis of the entire MoBbauer line shape taking into aecount a density fluctuation correlation function of the Kohlrausch type 0(f) = aexp( r /r) with P = 1/2 and the relaxation time t. We could thus also follow the onset of the structural relaxation a process related to the dynamic glass transition. The relaxation times decrease from about 10 s at to 5 x 10 s at 300 K. Details of these studies especially concerning the behaviour of swollen polymers will be given elsewhere. 

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