Modelling Polymer Compression in Flow: Semi-dilute Solution Behaviour
  Rheo-optical measurements on Polyemethyl methacrylate (PMMA) and 4-Butoxycarbonylmethylurethane (4-BCMU) show that these synthetic random coil polymer chains compress in Couette flow at semi-dilute concentrations. All previous models of polymer flow behaviour have assumed that the chains extend. A new model of polymers in flow in semi-dilute solution is presented where the chains compress in accord with the experimental findings. The chains are modeled as simple blobs exposed to a linear shear gradient which results in a compressive hydrodynamic force on the chain. Equating the hydrodynamic and elastic forces predicts chain compression with shear rate. The model also predicts a power law behaviour for polymer shear thinning of -2/3. The predicted power law for the decrease in radius with shear rate is -1/9 in close agreement with the measured value of -0.09+/-0.02 for both polymer systems. 
Dave E. Dunstan
cond-mat.soft