Extremely early onset of nonlinear viscoelasticity in dynamic shear of ideally monodisperse DNA

This work reports that the rheology of ideally monodisperse lambda ($\lambda$) phage DNA solutions is extremely sensitive to the applied strain in dynamic oscillatory experiments. $\lambda$ DNA exhibits nonlinearity at strains far smaller than what is observed in conventional synthetic polymers. However, it is found that polydisperse calf thymus DNA does not exhibit the extreme strain sensitivity. By mixing samples of monodisperse $\lambda$ DNA with the polydisperse calf thymus DNA, we correlate the molecular weight distribution (or polydispersity) to the onset of nonlinear viscoelasticity. We demonstrate that the strain sensitivity weakens as the polydispersity index increases. This is the first work correlating the inception of nonlinear viscoelasticity in entangled polymer system such as lambda DNA solution to the polydispersity of the samples. The experimental data suggests that ideally monodisperse polymer systems are very sensitive to the applied strain and have a very early transition to the nonlinear regime. We conclude that nominally monodisperse synthetic polymers are not sufficiently monodispersed to exhibit the early onset.