RUBBER-LIKE NETWORK STRUCTURE FROM TENSILE TESTS ON SWOLLEN SAMPLES ACCORDING TO FLORY LAST THEORY AND APPLICATION TO POLYURETHANES

It is shown that Flory’s last elasticity theory allows calculation of the limit values for the network chain molecular weight starting from mechanical equilibrium measurements of the modulus |f*|. For any case it is possible to evaluate the upper limit Ma corresponding to affine behavior. However, it is not always possible to evaluate the lowest limit: when M is known and the synthesis conditions are controlled, the lowest limit is Mg, which is the chain stoichiometric molecular weight; otherwise, the lowest limit becomes MPh, corresponding to phantom behavior, which, however, can be calculated only when the effective functionality ф of the network cross-links is known. It is also shown that imperfections of the cross-linking reactions can be estimated by comparing the experimental modulus with that of the perfect phantom network characterized by the stoichiometric Ms and фS (eq 15). A practical example of this analysis is proposed for two classes of polyurethanes, polyether diols reacted with toluenediyl diisocyanate (TDI) and polyester diols reacted with TDI. The demonstrations and the applications are made in the case of a swollen system; however, they can be extended to dry systems simply by putting V2 = 1 in the formulas. © 1990, American Chemical Society. All rights reserved.