Viscoelastic properties of polyacrylamide solutions from creep ringing data

Aqueous polyacrylamide (PAAm) solutions with concentrations ranging from 5 to 15 wt. % and molecular weights (5-6M and 18M) were characterized under creep measurements. The effect of 1M NaCl on the viscoelastic behavior was also studied. As expected, the initial response before the dumping oscillations was independent of the polymer concentration; this early response was correlated to the moment of inertia of the instrument and the geometry. We found that the polymer concentration has a rigorous effect on the damping oscillations. In addition, the present data indicate that reducing the molecular weight of PAAm significantly reduces both the amplitude and frequency of ringing. Furthermore, it was observed that the viscoelastic properties of the solutions are restricted by the presence of the salt. At the same concentrations, the polyethylene oxide solutions showed more elastic behavior including higher ringing frequencies. If the instrument inertia is taken into account, the Jeffreys model provides a satisfying fit to the data. The elastic modulus of 5 and 15 wt. % solutions of 18M PAAm was 465 and 8680 Pa, respectively. The corresponding characteristic creep ringing frequencies ranged between 20 and 77.7 rad s^-1. Large amplitude oscillatory sweep measurements were also conducted. The viscoelastic parameters estimated by both methods were in agreement. Moreover, we found that independent of the magnitude of the applied stress, within or out of the linear viscoelastic region (LVR), the creep data initially coincide during the occurrence of ringing. Overall, the creep ringing method is extremely helpful since the long-term creep compliance and the viscoelastic properties can simultaneously be obtained.