Structural relaxation in polymers above

Liquid-glass transition occurs when the molecular rearrangements (structural relaxation) necessary to alter the liquid structure in order to maintain the equilibrium 

There is a vast amount of literature on these investigations of many kinds of glass-forming liquids by use of various experimental techniques. The volume of data accumulated over the greater part of the last century until now would make any review a Herculean effort. Fortunately, general patterns have been found in the 

The local segmental relaxation or a-relaxation in amorphous polymers is fitted well by the empirical KWW stretched exponential function (see also Eq. (2.9)), 

The simplest way to see the difference between the temperature dependences of ar,a and is from the data on high molecular weight polyethylene and hydrogenated polybutadiene [155]. These flexible polymers have low TgS and their viscosities have Arrhenius temperature dependences instead of the WLF dependence at rheological-measurement temperatures. The Arrhenius temperature dependence is basically characterized by its activation enthalpy Ea,t]- Polyethylene has Ea,t] = 26.8 kJ mols [156]. On the other hand, NMR measurements of polyethylene at high temperatures in the picosecond range [157] revealed that the local segmental relaxation is best described by an exponential correlation function. 

Although the temperature dependence of r for amorphous polymers is described well by the WLF equation, as shown in Fig. 2.19, this is usually not the case for small-molecular organic glass-formers. Over an extended temperature range with r varying from say 10 U or shorter to 10 s or longer, the temperature dependence is of Arrhenius form at short r, but at longer times it is necessary to use one VFTH equation followed by another in order to describe the temperature dependence fully [158,159]. 

---