Onset of Entanglement in Aqueous Solutions of Pullulan with Narrow Molar Mass Distributions

Abstract

Dynamic viscoelastic behaviors were examined for aqueous solutions of pullulan samples, which are natural polysaccharides with weight-average molar masses (Mw) of 220, 390, and 820 kg mol–1 and narrow molar mass distributions such as Mw/Mn ≤ 1.1, over a concentration range from 5.0 to 467 g L–1 and a temperature range from 1 to 30 °C. The obtained viscoelastic data at different temperatures were superposed to compose master curves for storage and loss moduli (G′ and G″) using the time–temperature superposition principle. The Arrhenius-type temperature dependencies of the shift factors necessary to compose G′ and G″ master curves provided the activation energies (Ev*) of the viscoelastic relaxation times as functions of the concentrations (c) for each pullulan sample. Fundamental viscoelastic parameters such as the average relaxation time (τw), the steady-state compliance (Je), and the zero-shear viscosity (η0) were determined as functions of c for each pullulan sample. Above the critical concentrations (cen ∝ Mw–1) demonstrating the onset of entanglement formation among pullulan molecules, the characteristic relationships, Je–1 ∝ c2, τw ∝ cα–2Mw3.5, and η0 ∝ cαMw3.5 with α = 6.0, were obtained. Although this α value was substantially greater than that for usual synthetic polymers like poly(styrene) dissolved in good solvents where α ∼ 4.5, the Mw exponent of 3.5 for τw and η0 was identical to that of synthetic polymers. The Ev* value was close to the activation energy of the solvent, water viscosity of 19 kJ mol–1, in the dilute regime and abruptly increased with increasing c. Finally, Ev* approached a constant value of ca. 39 kJ mol–1 corresponding to the activation energy necessary for pullulan molecules to release entanglements above cen irrespective of the Mw values. The observed characteristic viscoelastic behaviors of aqueous pullulan solutions are reasonably interpreted by a theoretical model for associating polymers dissolved in good solvents.