Effect of molecular weight distribution on viscoelastic properties of polymers

The effect of molecular weight distribution on mechanical properties of concentrated polymer solutions was studied by measuring creep compliance and complex modulus for solutions of binary blends of narrow distribution polystyrenes in chlorinated biphenyl. Two‐step plateaus in the rubbery plateau region, as reported for undiluted polymer blends, were observed for concentrated solutions provided the molecular weights of component polymers differed greatly. The nth order blending law (n = 1, 2, 3) of Bogue et al. was simplified in form to compare with experimental results. As far as the present version of simplification was concerned, the cubic blending law was better than the quadratic one in describing viscoelastic properties of polymer blends. The cubic blending law reproduced closely the magnitudes of viscoelastic functions such as loss modulus and relaxation modulus but it was not able to predict the two‐step plateaus, even when the ratio of molecular weights of components was as large as 10. It also provided a good approximation to reported results on the steady‐state compliance J for undiluted binary polymer blends and present results for solution blends. Since the cubic blending law in the simplified version was consistent with previous results on concentration dependence of the steady‐state compliance for concentrated solutions of narrow distribution polymers, it gave a unified expression for the steady‐state compliance of binary polymer blends and concentrated solutions of narrow distribution polymers.