Asymmetry parameter poly

Asymmetry of disubstitution on a backbone carbon atom generally increases Tg. This effect is taken into account by the structural parameter x9, which equals the number of asymmetrically disubstituted backbone carbon atoms. For example, there are two asymmetrically disubstituted (disubstituted by an -F and a -CF3) backbone carbon atoms in Perfluoropolymer 1 [Figure 6.1(a)], which therefore has x9=2. Asymmetrically disubstituted backbone silicon atoms are not counted in x9. For example, poly[oxy(methylphenylsilylene)] [Figure 6.11(a)] has x9=0. 

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. 

With the above assumptions, RSC were able to compute volume-recovery curves which mimic the major features of the glassy phenomenology. In Table 4 are shown the parameters used in the calculations. Figure 38 shows the asymmetry of the calculated approach curves compared with the experimental data of Kovacs for poly(vinyl acetate). In Figure 39 is shown a plot of logteff calculated from the model compared with the data for poly(vinyl acetate). As was the case for the phenomenological models, the i-effective paradox is not resolved. 

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