Rheological Properties of Linear and Branched Polyisoprene

Viscosity η and first normal stress N₁ have been measured as functions of shear rate γ for solutions of linear, four-arm star, and six-arm star polyisoprenes, all with narrow molecular weight distributions. Concentrations in tetradecane ranged from 0.02 g/ml to 0.33 g/ml; molecular weights ranged from 35 000 to 2 000 000. All measurements were made at 25 °C in a Weissenberg Rheogoniometer. The effects of branching on zero shear viscosity t)o, steady state compliance J_e⁰, the characteristic shear rate (marking the onset of shear rate dependence in η) γ₀, and the shape of the η vs. γ master curve were determined. At low concentrations and molecular weights the values of η₀ and J_e⁰ were lower for the branched samples, while at high concentrations and molecular weights the reverse became true and substantial enhancements in η and J_e⁰ were found. On the other hand, the product η₀J_e⁰γ₀ was essentially the same for all samples irrespective of concentration, molecular weight, or branching, and the form of the master curve appeared to be independent of branching. Enhancement factors were determined and compared with results reported by Kraus and Gruver on star-branched polybutadienes. Data on viscosity enhancement for the two polymers reduced to essentially the same curve when correlated in terms of Φ^5/6(ΦM_b/M_c), Φ being the volume fraction of polymer, M_b the molecular weight of the branches, and M_c the characteristic molecular weight for entanglement in the respective undiluted linear polymers. The form of this correlating parameter was arrived at by considerations on the effect of long branches on macromolecular mobility in entangling systems. The interpretation of enhancement in η⁰ and J_e⁰ in relation to the spectrum of relaxation times and their practical effects on η vs. γ behavior are discussed.