Some Glass Transition Theories

On the other hand, some phenomenological distributions of relaxation times, such as the well known Williams-Watts distribution (see Table 1, WW) provided a rather good description of dielectric relaxation experiments in polymer melts, but they are not of considerable help in understanding molecular phenomena since they are not associated with a molecular model. In the same way, the glass transition theories account well for macroscopic properties such as viscosity, but they are based on general thermodynamic concepts as the free volume or the configurational entropy and they completely ignore the nature of molecular motions. 

With today s knowledge of glass-transition theory, one might attempt to explain these results in terms of plasticization. All of the studied humectants would be expected to have some plasticization effect on the MCC matrix. Once again, there are two reasonable explanations for the effect of added glycols from each of the two main theories for food stability. These theories were explored further by our research group, as outlined below (Sherwin and Labuza, 2003). 

The thermodynamic theories [7,8] deny the pure kinetic nature of the glass transition and link it directly to thermodynamic quantities like the configurational entropy of the material. Some recent results suggest a correlation between kinetic quantities and thermodynamic parameters [9]. Also recently, this theory was successfully merged with a potential landscape approach [10]. The thermodynamic approach is interesting since it reflects the different configurations that are allowed not only for the whole ensemble but also for the internal conformations... 

Thereby, the features of the a-relaxation observed by different techniques are different projections of the actual structural a-relaxation. Since the glass transition occurs when this relaxation freezes, the investigation of the dynamics of this process is of crucial interest in order to understand the intriguing phenomenon of the glass transition. The only microscopic theory available to date dealing with this transition is the so-called mode coupling theory (MCT) (see, e.g. [95,96,106] and references therein) recently, landscape models (see, e.g. [107-110]) have also been proposed to account for some of its features. 

Simulations [73] have recently provided some insights into the formal 5c —> 0 limit predicted by mean field lattice model theories of glass formation. While Monte Carlo estimates of x for a Flory-Huggins (FH) lattice model of a semifiexible polymer melt extrapolate to infinity near the ideal glass transition temperature Tq, where 5c extrapolates to zero, the values of 5c computed from GD theory are too low by roughly a constant compared to the simulation estimates, and this constant shift is suggested to be sufficient to prevent 5c from strictly vanishing [73, 74]. Hence, we can reasonably infer that 5 approaches a small, but nonzero asymptotic low temperature limit and that 5c similarly becomes critically small near Tq. The possibility of a constant... 

Tg data for the blends are shown in Figure 1. Results obtained by DTA on PMMA-PVdF blends which had been melt extruded and "annealed at room temperature appeared to be anomalous in terms of the theory for glass transition of copolymers or compatible polymer blends (6). These data indicated a limiting value for Tg of ca. 40°-45°C. However, x-ray examination showed that samples with more than ca, 35% PVdF exhibited a crystalline phase, indicating that some of the PVdF had precipitated. When these systems were re-examined by dilatometry... 

Some thermodynamic aspects of the glass transition Free volume, entropy and enthalpy theories. J. Chem. Phys. To be published (1964). 

Before we go into some details of MCT, we briefly mention that there exists another microscopic theory of the glass transition phenomenon, the replica theory [120,121], which is inspired by spin glass theory [122] and which lends some... 

The replica theory is another microscopic theory of the glass transition. Inspired by the spin glass theory,230 it lends some justification to the configurational entropy theory.216 However, details of the evolution of the dynamic susceptibility are less worked out. 

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