Kinetic parameters, glass transition

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... 

The glass transition temperature is thus closely related to kinetic parameters and to the duration of the experiment conducted on the material. Thus, the glass transition temperature is an increasing function of the quenching rate. In practice a variation of about 10-20 K for T may be observed for the same glass (Menetrier, Hojjaji, Estournes and Levasseur, 1991). Note also that for a well defined compound which may be obtained in the form of a glass T and Tq are linked by an empirical... 

Because the AG equation (33) for r reduces exactly to the VFTH equation (34) over the temperature range in which ScT is proportional to 5T, the correspondence between these expressions for r uniquely establishes a relation between the kinetic fragihty parameter D = I/Kg and thermodynamic fragihty St T)T/hT. Specifically, in the temperature regime near the glass transition... 

The glass transition temperature can be measured in a variety of ways (DSC, dynamic mechanical analysis, thermal mechanical analysis), not all of which yield the same value [3,8,9,24,29], This results from the kinetic, rather than thermodynamic, nature of the transition [40,41], Tg depends on the heating rate of the experiment and the thermal history of the specimen [3,8,9], Also, any molecular parameter affecting chain mobility effects the T% [3,8], Table 16.2 provides a summary of molecular parameters that influence the T. From the point of view of DSC measurements, an increase in heat capacity occurs at Tg due to the onset of these additional molecular motions, which shows up as an endothermic response with a shift in the baseline [9,24]. 

In any case, the region of accelerated aging must be limited by physical transitions (melting, glass transition), since the kinetic parameters undergo practically unpredictable discontinuous variations at these transitions, as illustrated by the example of oxidation (amide growth in an amide-cured epoxy) (Fig. 14.22). 

An explicit expression relating kinetic fragility to thermodynamic behavior of supercooled liquids was accomplished for the first time by Mohanty and coworkers [55,56] and independently by Speedy [54], These authors derived an expression for the steepness parameter, a measure of kinetic fragility, from the temperature variation of the relation time or viscosity, with the ratio of excess entropy and heat capacity changes at the glass transition temperature [54-56]. A detailed description of this work will be provided later in the review chapter. 

The value of Tg depends on the rate of the temperature decrease it decreases with decreasing cooling rate, as is shown in Fig. 13.33 where the cooling rate decreases from qi to q3- This alone already shows that the glass transition temperature is not a thermodynamic parameter but kinetically determined. 

Chapter 4(71) focuses on the characterization of sorption kinetics in several glassy polymers for a broad spectrum of penetrants ranging from the fixed gases to organic vapors. The sorption kinetics and equilibria of these diverse penetrants are rationalized in terms of the polymer-penetrant interaction parameter and the effective glass transition of the polymer relative to the temperature of measurement. The kinetic response is shown to transition systematically from concentration independent diffusion, to concentration dependent diffusion, and finally to complex nonFickian responses. The nonFickian behavior involves so-called "Case II" and other anomalous situations in which a coupling exists between the diffusion process and mechanical property relaxations in the polymer that are induced by the invasion of the penetrant (72-78). ... 

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