  The extensional rheology of polymeric liquids has been extensively examined
through experiments and theoretical predictions. However, a systematic study of
the extensional rheology of polymer solutions in the semidilute regime, in
terms of examining the effects of concentration and molecular weight, has not
been carried out so far. Prior studies of the shear rheology of semidilute
polymer solutions have demonstrated that their behaviour is distinctively
different from that observed in the dilute and concentrated regimes. This
difference in behaviour is anticipated to be even more pronounced in
extensional flows. In this work, the extensional rheology of linear,
double-stranded DNA molecules, spanning an order of magnitude of molecular
weights (25 to 289 kilobasepairs) and concentrations (0.03 to 0.3 mg/ml), has
been investigated. DNA solutions are now used routinely as model polymeric
systems due to their near-perfect monodispersity. Measurements have been
carried out with a filament stretching rheometer since it is the most reliable
method for obtaining an estimate of the elongational stress growth of a polymer
solution. Transient and steady-state uniaxial extensional viscosities of DNA
dissolved in a solvent under excess salt conditions, with a high concentration
of sucrose in order to achieve a sufficiently high solvent viscosity, have been
determined in the semidilute regime at room temperature. The dependence of the
steady state uniaxial extensional viscosity on molecular weight, concentration
and extension rate is measured with a view to determining if data collapse can
be observed with an appropriate choice of variables. Steady state shear
viscosity measurements suggest that sucrose-DNA interactions might play a role
in determining the observed rheological behaviour of semidilute DNA solutions
with sucrose as a component in the solvent.
