Molecular Cooperativity in the Glass Transition

The fact that the relaxation times associated with the glass-rubber transition and the secondary relaxations are independent of molecular weight suggests 

The dynamics of polymers may be interpreted by assuming the existence of domains. At high temperature (T Tg), each domain is occupied by a single conformer and the interactions between neighboring segments or conformers are considered to be negligible. Once the potential barrier is crossed, conformers relax independently of their neighbors. Each domain 

Statistical mechanical principles suggest that the conformational entropy Sc of I mol of conformers occupying domains is given by 

This expression indicates that the size z is proportional to the reciprocal of the conformational entropy. 

The probability for a conformer to relax is / = exp(-Ag/i T), where Ag is the free energy barrier, while the probability that the z conformers of the same domain relax simultaneously is given by 

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