Time constants above glass transition temperature

There is growing evidence that t-T superposition is not valid even in miscible blends well above the glass transition temperature. For example, Cavaille et al. [1987] reported lack of superposition for the classical miscible blends — PS/PVME. The deviation was particularly evident in the loss tangent vs. frequency plot. Lack of t-T superposition was also observed in PI/PB systems [Roovers and Toporowski, 1992]. By contrast, mixtures of entangled, nearly mono-dispersed blends of poly(ethylene-a/f-propylene) with head-to-head PP were evaluated at constant distance from the glass transition temperature of each system, homopolymer or blend [Gell et al, 1997]. The viscoelastic properties were best described by the double reptation model , viz. Eq 7.82. The data were found to obey the time-temperature superposition principle. 

The solution of the resulting modified equation takes the form of a sharp front that moves into the polymer, not as the square root of time, but at a constant velocity. Ahead of this fi-ont is a small Fickian precursor, which may be detected by ion beam analysis techniques. This type of diffusion - known as case II diffusion - is very important and widespread for the case of penetration of glassy polymers by solvent. Analogous phenomena will take place when two miscible polymers are brought together, one of which is below its glass transition temperature and one above. Such situations are not difficult to realise in practice (Jabbari and Peppas 1995). 

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