The Meaning of the Glass Temperature of Random Copolymers and Miscible Polymer Blends

The glass transition temperature of random copolymers and miscible polymer blends exhibits generally a non-additive composition dependence, showing both positive and negative deviations of additivity predicted by 'Gordon-Taylor' like equations, among which the Fox relation represents the simplest additivity rule for the glass temperature of these polymeric systems. It is shown that the real Tg vs. composition behaviour of both copolymers and polymer blends can be adapted by a parameterized third order Tg vs. composition equation. The fitting parameter, K1, of the square concentration term of this equation accounts essentially for the effect of binary hetero-sequences in copolymers and for specific contact hetero-interactions in polymer blends. The fitting parameter of the third order concentration term, K2, is related exclusively to the effects of heterotriad sequences (copolymers) and conformational entropy changes due to hetero-contact formation (polymer blends), respectively. It is shown that the K1 parameter correlates roughly with the difference between the solubility parameters of the components.