Effect of polydispersity in polystyrene on stress relaxation

Stress relaxation master curves are presented for twenty‐three polystyrene samples whose weight‐average molecular weights Mw varied from 20,800 to 310,000 and whose heterogeneity indices varied from 1.0 to 15.92. The maximum relaxation times τm, the associated partial modulus Em, and the flow viscosity η were determined for these samples at 115°C. It is shown that at 115°C. the relation log τm (minutes) = −15.10 + 3.4 Mw is obeyed. It is also shown that the quantity αB = τmEm/η correlates with the heterogeneity index of the sample. It is concluded that stress relaxation studies can be used to obtain weight‐average molecular weights and also to provide a qualitative measure of the heterogeneity index.