  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.
