GLASS TEMPERATURE OF ETHYLENE PROPYLENE COPOLYMERS - AN APPLICATION OF THE PSEUDOSTEREOCHEMICAL EQUILIBRIUM APPROACH/

The glass-transition temperature of ethylene/propylene statistical cop olymers is evaluated by using the pseudostereochemical equilibrium app roach; i.e., the partition function of a copolymer chain with interact ions between neighboring conformations is formally obtained from a fic titious homopolymer wherein the two comonomer units are in equilibrium , with proper conformational constraints to produce the existing amoun ts of unit pairs. The resulting formalism enables us to avoid the diff icult task of evaluating the geometrical average of the partition func tions of different chains. The Gibbs-DiMarzio theory is extended to of f-lattice models with any stereochemical structure; the rotational-iso meric-state entropy is supplemented with the contribution from fluctua tions of the bond rotation angles around their energy minima. Allowing for rotational fluctuations in the glassy state enables one to accoun t for a larger-than-crystalline entropy through the rms fluctuation an gle, treated as an adjustable parameter. Although additional experimen tal data are awaited, the observed trend of T-g vs composition for pro pylene-rich copolymers is correctly reproduced. It is predicted that c opolymers tending to alternation (r(E)r(p) similar to 0) should have a T-g lower by more than 20 degrees C than those with unit blocks (r(E) r(P) > 1), in qualitative agreement with what is observed for vinylide ne fluoride/hexafluoropropene copolymers.