  The validity of the affine assumption in model flexible polymer networks is
explored. To this end, the displacements of fluorescent tracer beads embedded
in polyacrylamide gels are quantified by confocal microscopy under shear
deformation, and the deviations of these displacements from affine responses
are recorded. Non-affinity within the gels is quantified as a function of
polymer chain density and cross-link concentration. Observations are in
qualitative agreement with current theories of polymer network non-affinity.
The measured degree of non-affinity in the polyacrylamide gels suggests the
presence of structural inhomogeneities which likely result from heterogeneous
reaction kinetics during gel preparation. In addition, the macroscopic
elasticity of the polyacrylamide gels is confirmed to behave in accordance with
standard models of flexible polymer network elasticity.
