Glass transition asymmetry

Glass transitions involve mainly the onset or freezing of cooperative, large-amplitude motion and can be studied using thermal analysis. Temperature-modulated calorimetry, TMC, is a new technique that permits to measure the apparent, fiequency-dependent heat capacity. The method is described and a quasi-isodiermal measurement method is used to derive kinetic parameters of the glass transitions of poly(ethylene terephthalate) and polystyrene. A first-order kinetics expression can describe the approach to equilibrium and points to the limits caused by asymmetry and cooperativity of the kinetics. Activation energies vary from 75 to 350 kJ/mol, dependent on thermal pretreatment. The preexponential factor is, however, correlated with the activation energy. 

As seen in Figure 3.24, the DSC profile of the P153/PtBS42 blend exhibits very broad glass transition. Such broad transition is well known for miscible blends having a large djmamic asymmetry between the components (see... 

Fig. 101. Left Near-ZF (i.e., a small LF is applied to suppress relaxation effects from nuclear moments) pSR spectra in YjMojO, stated as polarization (A(0 divided by the instrumental asymmetry, A, ), at various temperatures. The inset shows the early time behavior tar below the glass transition temperature (Dunsiger et al. 1996a). Right Near-ZF muon spin relaxation rate 1/T, as a function of temperatuie in TbjTijO,. The inset shows representative pSR spectra, all of which exhibit exponential relaxation. From Gardner et al. (1999).

For pure, fully annealed polymers, the glass transition is approximately symmetrical [5]. For partially miscible systems in which there are interfaces, the transition will be asymmetric and become broadened [5]. This asymmetry and broadening may provide a wealth of information of both practical and theoretical value that has not yet been fully extracted. 

Besides the dynamic heterogeneity discussed above, binary miscible polymer blends can be considered as dynamically asymmetric if the two components have a large difference in the glass transition temperatures. Usually the dynamic asymmetry is defined by A = where x " is the relaxation time of the... 

As the relaxation processes in the glassy state and glass transition region are non-exponential and nonlinear, the theories must take account of the thermal history of glass formation and the asymmetry of the relaxations, which depend on how the system departs from equilibrium. 

Here, we focus our attention on phase separation in complex fluids that are characterized by the large internal degrees of freedom. In all conventional theories of critical phenomena and phase separation, the same dynamics for the two components of a binary mixture, which we call dynamic symmetry between the components, has been implicitly assumed [1, 2]. However, this assumption is not always valid especially in complex fluids. Recently, we have found [3,4] that in mixtures having intrinsic dynamic asymmetry between its components (e.g. a polymer solution composed of long chain-like molecules and simple liquid molecules and a mixture composed of components whose glass-transition temperatures are quite different), critical concentration fluctuation is not necessarily only the slow mode of the system and, thus, we have to consider the interplay between critical dynamics and the slow dynamics of material itself In addition to a solid and a fluid model, we probably need a third general model for phase separation in condensed matter, which we call viscoelastic model . 

Viscoelastic phase separation is expected to be universal in any mixture having asymmetry in elementary molecular dynamics between its components. The possible candidates for dynamic asymmetry are (1) slow dynamics in complex fluids such as polymer solutions and surfactant solutions, coming from their complex internal degree of freedom (e.g., entanglement effects in polyers) and (2) that near-glass transition. We hope that more examples of viscoelastic phase separation will be found in the family of complex fluids in the near future. 

Among the requirements for E/0 devices based on 2nd-order NLO effects is the absence of symmetry, frequently achieved by aligning the dipole moments of chromophores (which are responsible for the absorptions creating the nonlinearities), a process known as "poling". A common method of poling is to include a 2nd-order-NLO-active CP in a polymer matrix, heat the latter to above its glass transition temperature, apply a strong electric field, and then rapidly cool the matrix to lock in the asymmetry. Such methods with CPs have met with limited success The... 

Overall, we have indicated that materials exhibit complicated behavior in the glass transition region. Such effects as time dependence, asymmetry, non-linearity, memory and complex Xg behavior must be clearly explained by any successful theory. 

Figure 9. Fundamental explanation of asymmetry in glass transition phenomena as interpreted by the model.

Our focus has been the elucidation of specific features of the model which give rise to the most important aspects of glass transition behavior. Thus time dependence arises naturally from a consideration of the molecular aspects of the overall phenomena involved. The pronounced nonlinearity and asymmetry of behavior result from the structure dependency of the individual retardation times introduced in the model. In fact, our analysis shows that, on a time scale appropriately compensated to take account of structural dependence, non-linearity and asymmetry vanish. The existence of a multiplicity of recovery times in the model leads to memory, which is observed in real systems. 

The solution of equation (70) depends upon the functionality of the retardation time t. This consequently determines how well actual glassy behavior is described by the one-parameter model. If one assumes that t depends upon temperature alone (in isobaric experiments), then glass formation and the uniform heating through the glass transition range can be qualitatively described. However, this model predicts that isothermal recovery proceeds via a simple exponential decay of S in contradiction with observation. Also a dependence of t on J alone cannot account for the asymmetry of approach depicted in Figure 31. 

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