Glass transition theory thermal factors

The kinetic theory of polymers and the TTSP are only valid above the glass transition temperature. However, many feel that the procedure, in a modified form, is valid below the glass-transition temperature but exactly how far below is uncertain. The WLF equation, on the other hand is known to be only valid above the Tg because below this temperature the material can no longer be considered a super cooled liquid. In fact. Ferry, (1980) notes that the slope of the shift factor curve should be discontinuous at the Tg for the same reason that the coefficient of thermal expansion suffers a discontinuity at the Tg. 

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