Beta relaxation

Dyn ic mechanical analysis was discussed in terms of the nodular morphology concept in crossllnked structures. Beta relaxations in all the cured resins were bimodal in appearance. But, vAiile MPD-cured resins shewed a maximum at 25 C with a smaller shoulder at -40 C, TDA and DAEB-cured resins had maxima at -40 C with a less significant peak at 25 C. For DAIPB and DATBB-cured resins the two peaks were approximately equal in magnitude. The two overlapping peaks at -40 and 25 C were attributed to crankshaft motions in the matrix and nodules. 

The existence of fast fi relaxation is unique to glassy materials and has been attributed to the nature of the packing statistics of the molecules and to intramolecular reorientation of the molecules [20,21,29]. The fast beta relaxation exists below Tg, and its temperature dependence is Arrhenius [20,21,28]. At high temperatures, [1 and slow a relaxations often merge near a temperature termed the crossover temperature Tx, while at lower temperatures they bifurcate [7,20,21,29,30,31]. A recently discovered attribute of supercooled... 

Physical aging affects significantly the mechanical damping in both the alpha and beta relaxation peaks. Figure 19 shows the decrease in the beta loss peak as a func-... 

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. 

Figure 5.2. A schematic representation of typieal alpha and beta relaxations observed from mechanical and dielectric properties. The observed relaxation temperatures increase with increasing frequency.

Fig. 40. Loss tangent data from dynamic mechanical testing showing the effects of physical aging on the broad beta relaxation plateau...

With a simple change of notation the present theory may be used to set the Gilroy and Philips model [82] of structural relaxation processes in amorphous materials and Dyre and Olsen s minimal model for beta relaxation in viscous liquids [83] in the framework of the general theory of stochastic processes. Moreover, the formulation of the theory in terms of kinetic equations as the... 

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... 

Cerveny, S. Bergman, R. Schwartz, G.A. Jacobsson, P. Dielectric alpha- and beta-relaxations in uncured styrene butadiene rubber. Macromole-cular 2002, 35, 337-342. 

Water and low temperature (20-35 C) aging influence the dynamic mechanical properties of a poly(amide-imide). At concentrations below 2 weight percent, water contributes to a low temperature relaxation between -120 and -50 C. Above 2 weight percent the water influences the beta transition. The enthalpy of activation for the beta relaxation is dependent upon aging temperature and time. Aging temperatures closer to the beta transition temperature result in higher activation enthalpies for that dispersion. 

Based on the dielectric and dynamic mechanical data, it appears that water and small polar molecules contribute to three dispersions in this poly(amide-imide). One is the low temperature relaxation between -100 and 0°C. This may be a hydrogen bonded relaxation since the activation enthalpy was 30 kJ/mol. This occurs at concentrations of water ranging between 0 to 4 weight percent. Two, the dielectric relaxation between 0 and 70 C can probably be attributed to conductive contaminants whose mobility is dependent upon a minimum amount of water. Three, at high water concentrations, greater than 2 weight percent, the water/NMP contributes to the beta relaxations observed between 50 and 150 C. 

Fig. 3.2 DMA plot of a PE-UHMW specimen alpha and beta relaxation are given [unpbl].

ECTFE is semicrystalline (50-60%) and melts at 240°C (commercial grade) [72]. It has an alpha relaxation at 140°C, a beta relaxation at 90°C and a gamma relaxation at -65°C. The conformation of ECTFE is an extended zigzag in which ethylene and CTFE alternate. The unit cell of ECTFE s crystal is hexagonal. 

Popli R, Glotin M, Mandelkern L and Benson R S (1984) Dynamic mechanical studies of alpha-relaxation and beta-relaxation of polyethylene, J Polym Sci Polym Phys Ed 22 407-448. 

Garwe F, Schonhals A, Lockwenz H, Beiner M, SchrOter K and Donth E (1996) Influence of cooperative alpha dynamics on local beta relaxation during the development of the dynamic glass transition in poly(n-alkyl methacrylate)s. Macromolecules 29 247-253. 

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