COOPERATIVE RELAXATION PROCESSES IN POLYMERS

The basic mode of relaxation in polymer molecules involves the rotatio n of a conformer, with a time scale of the order of picoseconds. This fast relaxation process, however, cannot take place easily in the cond ensed state crowded by densely packed conformers, necessitating the in termolecular cooperativity among them. The domain of cooperativity gro ws at lower temperatures, towards the infinite size at the Kauzman zer o entropy temperature, though the system deviates from the equilibrium as the glass transition intervenes at about 50 degrees C above that t emperature. From the temperature dependence of the domain size, the we ll-known Vogel equation is derived, which we consider is the basic ori gin of the empirical WLF and free volume equations. The molar volume i s a crucial factor in determining molar fi-ee volume. The molecular we ight of a conformer with a density correction, therefore, can be used as a parameter in determining the T-g of liquids and amorphous polymer s. A larger size conformer means a higher glass transition temperature . A conformer at the chain end, on the other hand, has a higher enthal py, i.e., a smaller effective size for that conformer. If a conformer is reacted trifunctionally, the resulting conformer is a combination o f the two conformers and T-g increases, but a further addition of anot her conformer to that branch point reduces the average size of the con formers, so T-g decreases. The model for cooperative relaxation can be directly applied to predicting T(g)s from the chemical structure of p olymers, the kinetics and T(g)s of thermosets during the crosslinking reaction, the distribution of relaxation times from the domain size di stribution at a given temperature, the dynamics of the physical aging process, and other complex behaviors of polymers and liquids near the glass transition temperature. (C) 1997 John Wiley & Sons, Inc.