Correlation between glass transition temperature and chain structure for randomly crosslinked high polymers

The empirical form for the dependence, T-g(n) approximate to T-g(infinity).(1 + alpha/n), of the glass transition temperature T-g on the average number n of repeat units between crosslinks, is generalized for randomly crosslinked high polymers. The new form, T-g(n) approximate to T-g(infinity).[1 + c/(n . N-rot)], is based on a correlation study of data for 77 samples of 10 different sets of resins. The fitting parameter a is resolved into composition-dependent N-rot and composition-independent c terms. N-rot summarizes the average number of rotational degrees of freedom per repeat unit, and is estimated in a straightforward manner from the structure and mol fraction of each repeat unit. The value of c is found from data analysis to be 5 +/- 2. The results of this work are consistent with expectations based on the entropy theory of glasses, and provide improved understanding and predictive ability for the properties of crosslinked polymers. (C) 1996 John Wiley & Sons, Inc.