Monodisperse Lambda Dna As A Model To Conventional Polymers: A Concentration-Dependent Scaling Of The Rheological Properties

We have measured the dynamics of completely monodisperse (PDI = 1.0) ultrahigh-molecular-weight linear lambda (lambda) DNA solutions as a function of concentration. Due to the very high molecular weight of the DNA, Mn = Mw > 30 million g/mol, we were able to study the dynamic properties of well-entangled systems even in very dilute (low-concentration) conditions. We report the linear rheology by conducting dynamic oscillatory measurements well into the entanglement regime (11 < C* < 90), where C* is the overlap concentration. The tests are reported in good solvent conditions. Upon comparing our results with previously reported data in the literature by Teixeira et al. [ Macromolecules 2007, 40 (7), 2461-2476] and reproducing their data, we can confirm their measurements to have been conducted in the nonlinear regime. This leads to the conclusion that the lambda DNA exhibits extreme strain sensitivity in the observed dynamics, and this induces the earlier onset of nonlinearity as the angular frequency decreases. The time-concentration superposition (TCS) was found to be valid in the terminal zone, which permitted the evaluation of similar to 9 decades of dynamics in the mastercurve. The concentration dependence of the time-concentration shift factors (vertical and horizontal) was found to be in good agreement with the plateau moduli and the crossover frequency scaling. A concentration dependence of plateau modulus G(N)(0) similar to C-2.29 is obtained from the dynamic tests. The plateau modulus scaling is consistent with the blob model for entangled polymer solutions. The terminal relaxation time shows a change like the unentangled-to-entangled crossover in synthetic polymer solutions from tau(d) similar to C-1.1 and tau(d) similar to C-3.53 at around 1 mg/mL (24C*). A very high concentration dependence of the zero-shear viscosity, eta(0) similar to C5.5, is estimated for the high-concentration samples. We interpret the concentration-dependent scaling to be in an entangled regime observed only in very high molecular weight solutions at sufficiently high concentrations. A Likhtman-McLeish model was used to fit the LVE with the constraint release parameter, c(nu), fixed at 1 and 10. The Likhtman-McLeish model does not seem to capture all of the physical processes in the dynamics, and a good fit was not obtained, particularly for the higher-concentration samples though the fit quality improved with the greater constraint release parameter magnitude. Entanglement density predicted by the Likhtman-McLeish model scaled linearly with the entanglement density calculated by the blob model for solutions. The entangled dynamics is possibly nonreptative as reptation or its derivative models do not predict the observed strong nonlinearity and the high susceptibility of the system to strain.