Molecular interpretations of time-temperature equivalence

1 Molecular rate processes with a constant activation energy the site nodel theory 

The simplest theories that attempt to deal with the temperature dependence of viscoelastic behaviour are the transition state or barrier theories [9,10]. The site model was originally developed to explain the dielectric behaviour of solids [11,12], but was later applied to mechanical relaxations in polymers [13]. 

The transition probability for a jump from site 1 to site 2 is given by 

Assume that the applied stress o causes a small linear shift in the free energies of the sites such that 

Combining these equations and making suitable approximations gives a rate equation 

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Biomedical Applications. Spin-lattice relaxation times Ti and Tid as well as NMR second moment were employed to study the molecular dynamics of riboflavin (vitamin B-2) in the temperature range 55-350 K. The broad and flat T1 minimum observed at low temperatures is attributed to the motion of two non-equivalent methyl groups. The motion of the methyl groups is interpreted in terms of Haupt s theory, which takes into account the tunneling assisted relaxation. 

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