Glass transition dynamics

Taking the e (T) peak temperature (Tm) to be representative of the dynamic glass transition temperature of pmn, the inset in Figure 15.6 shows the pressure dependence of Tm. The... 

The dielectric analysis of these systems show, that only one peak can be observed corresponding to the dynamic glass transition. The sub-glass relaxations are very small. 

Dielectric permittivity and loss for both polymers under study can be observed on Figs. 2.17 and 2.18. In both figures a prominent peak corresponding to the dynamic glass transition temperature can be observed, which at low frequencies is overlapped by conductivity effects. Moreover, in both polymers a broad secondary peak is observed at about -50°C. This peak is more prominent in P2tBCHM which is in good... 

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.

Although molecular mobility is severely restricted below the glass transition temperature, the dynamic glass transition temperature (main transition or, conventionally -relaxation) in polymers as it have been described above, is usually accompanied by subglass secondary relaxations labeled as p, y, S, relaxations. The glass transition at low temperatures is assumed to be caused by the cooperative motion of many particles, while the secondary relaxations have a more localized molecular... 

An interesting result with respect to applications obtained with the IPN hydrogels is that these are two- phase systems (two glass transition temperatures), with the hydrophilic domains behaving essentially like the pure hydrophilic component.6,7,9 Thus, the two basic functions of these IPN hydrogels with respect to applications, namely hydrophilicity and mechanical stability, are separately taken over by the two IPN components, the hydrophilic and hydrophobic domains, respectively. Figure 1 shows TSDC and DMA results for the water content dependence of the a relaxation (dynamic glass transition) of PHEA in sequential IPNS prepared from PHEA and poly(ethyl methacrylate) (PEMA) as the hydrophobic component.9 In these IPNs a porous PEMA network was prepared first, and PHEA was then polymerized in the pores. In addition to the... 

Fig. 13 a Optical image (top view) of the sample geometry for a thin PS film of 239 nm after 4 hours at 180 °C in a pure nitrogen atmosphere ( 1 mm x 1 mm) b the same sample after 4 hours at 180 °C in air ( 1 mm x 1 mm) c the relaxation time of the dynamic glass transition vs. inverse temperature for a thin PS film of 63 nm after different annealing times at 180 °C in a pure nitrogen atmosphere and in air. Inlet, time dependence of the sample capacity at 180 °C and 0.1 MHz in a pure nitrogen atmosphere and in air... 

Dynamic Glass Transition in Thin Polystyrene Films... 

For thin polystyrene films annealed for 12 hours at 150 °C in high vacuum (10-6 mbar) and measured in a pure nitrogen atmosphere the dynamic glass transition was characterized using two experimental techniques capacitive scanning dilatometry and Broadband Dielectric Spectroscopy. Data from the first method are presented in Fig. 15a, showing the real part of the complex capacity at 1 MHz as a function of temperature for a thin PS film of 33 nm. 

Similar results were obtained by Broadband Dielectric Spectroscopy (Fig. 17) no shifts in the relaxation time of the dynamic glass transition were detected, even for PS films as thin as 20 nm. 

Fig. 17 Relaxation rate of the dynamic glass transition vs. inverse temperature for different film thicknesses, as indicated. Inlet, dielectric loss vs. temperature at 31 kHz showing the dynamic glass transition of thin PS films for different film thicknesses, as indicated...

TABLE L PREDICTION OF DYNAMIC GLASS TRANSITION TEMPERATURE FROM DSC DATA... 

Table III also shows that E increases with increasing DSC T. This would be expected from restricted segmental mobility of trie high T samples. Lewis iH found that Arrhenius plots of log frequency versus reciprocal dynamic glass transition temperature for restricted and nonrestricted polymers converges to a different point in the frequency/temperature scale. From this finding, equations were derived to predict static T from the dynamic T value and vice versa. ...

Leutheusser 1984 Bentzelius et al. 1984), has stirred both excitement and controversy (Mezei et al. 1987 Richter et al. 1991 Retry et al. 1991 Schonhals et al. 1993 Mezei 1991 Kim and Mazenko 1992). While the details of the mode-coupling theory are beyond the scope of this chapter, the main idea is that at high fluid densities there is a nonlinear feedback mechanism by which fluctuations in the structure (or local density) of the fluid become arrested and cannot relax to equilibrium. The point at which this occurs is then a purely dynamic glass transition. 

In Eq. (4-31), the first three terms describe a simple damped harmonic oscillator the first term is due to molecular accelerations, the second is due to viscous drag, and the third is due to the restoring force. Qq is the oscillator frequency, which is of order 10 sec", and p is a viscous damping coefficient. The crucial term producing the dynamic glass transition is, of course, the fourth term, which has the form of a memory integral, in which molecular motions produce a delayed response. The kernel m(t — t ) is determined self-consistently by the time-dependent structure. One simple choice relating m(s) to the structure is ... 

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... 

Figure 32. Dielectric loss versus frequency at 140°C showing the dynamic glass transition for a thin PS him of 84 nm in a pure nitrogen atmosphere (a) and in air (b).

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). 

Two experimental techniques are employed to investigate the dynamic glass transition for thin polystyrene annealed at least 12 hours at 150°C in an oil-free... 

Measurements by broadband dielectric spectroscopy also reveal no shifts of the dynamic glass transition (inset, Fig. 39). In consequence, the average relaxation rate of the dynamic glass transition remains unchanged for all thicknesses investigated in the present study (Fig. 39). 

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... 

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