Flow Properties of Polymers
Time-dependent Fluids

Many substances, particularly food, pharmaceutical and personal care products display flow behavior that depends on the shear history. Time-dependent changes in viscosity have also been reported for a number of polymer solutions and melts. Although most polymer melts and solution do not show any measurable time-dependency of viscosity. However, in a few cases, disentanglement of polymer molecules during shear can cause time-dependent effects, particularly when a dynamic physical network (gel) is dynamically formed and broken during stirring.

In most cases, time-dependent changes in viscosity are reversible, but in a few cases, shearing the polymeric material can cause irreversible changes.

It is believed that time-dependent flow behavior is due to the formation of a polymer network which is the result of reversible crosslinks which may be either broken (thixotropy) or formed (anti-thixotropy or negative thixotropy) during stirring. The later case is a rather rare phenomenon. A special case is rheopexy. It describes the gelation (solidification) of (polymeric) systems under gentle agitation (mechanical forces).

The time-dependent flow behavior can best be demonstrated using a cone-and-plate viscometer. The liquid is sheared at a constant velocity gradient (shear rate) for a certain time, for example for a minute at a low shear rate. The shear rate is then gradually increased and the torque measured at different shear rates. After reaching a certain torque, the shear rate is gradually decreased without stopping the measurement. In the case of a time-independent fluid, the two curves will coincide, whereas a hysteresis is evidence for a time-dependent fluid (see figures above).

Two examples of thixotropic liquids are very high molecular weight (MW ≥ 107) poly(methyl methacrylate) and polystyrene in a viscous solvent (η > 1 poise). An example of an anti-thixotropic liquid is polymethacrylic acid (PMA). It shows a 350 fold increase in viscosity when sheared at 10 s-1 for one minute.1 It was concluded that simultaneously occurring hydrophobic and hydrophilic interactions between PMA molecules, that are dynamically formed and broken during shearing, cause anti-thixotropic behavior.

1Seiji Ohoya, Shigeo Hashiya, Kyoji Tsubakiyama, and Togoro Matsuo, Polymer Journal, Vol. 32, No.2, pp 155 (2000)

  • Summary

    Thixotropy

    is the flow-induced decrease in viscosity that has been observed for many polymer solutions.

  • Anti-Thixotropy

    or negative thixotropy is the flow-induced increase in viscosity. It is a comparatively rare phenomenon.

  • Time-dependent flow behavior is the result of reversible crosslinks which may be either broken (thixotropy) or formed (anti-thixotropy) during shearing.

  • Rheological measurements provide important information about the flow and creep properties of polymeric materials. This information might aid in optimizing the processing conditions and the composition of plastics.

  • Rheological measurements are helpful to understand the effect of aging on the viscoelastic properties. They also help to assess the damage caused by heat, oxygen etc. on the structure of the polymers.

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