Rheology of Entangled Solutions of Ring-Linear DNA Blends

Bulk rheology measurements on concentrated monodisperse ring-linear DNA mixtures are reported for the first time. The entanglement behavior of the ring-linear DNA mixtures (with 15 and 50% linear chain fractions, ϕ_Lin) over a range of total DNA concentration, ϕ_Tot, from ∼0.8 mg/mL (20C*) to 2 mg/mL (50C*) is reported. A discussion on the current problems in the reported ring dynamics/scaling is included. The concentration-dependent dynamics of the ring-linear DNA mixtures are compared with the dynamics of 100% linear DNA at the same linear chain degree of entanglement, Z. Dynamic oscillatory tests were conducted to understand the bulk rheological behavior of the ring-linear DNA blends. The blends showed a broadening of the rubbery plateau region compared with that of the pure linear counterparts─not reported in any previous works on ring polymers. However, the final plateau moduli, G_N⁰, of the ring-linear mixtures were found to be lower than for the pure linear DNA at the same total concentration. The plateau moduli for the mixtures followed a 2.2-2.3 power law dependence with total concentration, ϕ_Tot, similar to the scaling seen in the 100% linear analogue. G_N⁰ for the blends scaled as G_N⁰ ∼ (ϕ_Tot)^2.2-2.29(ϕ_Lin)^0.7-0.8 for the two linear chain percentages and the range of concentrations studied. The blends at the same total concentration exhibited much higher viscosities relative to the linear counterparts than reported by prior works on synthetic ring melt systems. This is consistent with very long terminal relaxation times most likely due to linear chain threading of these very large macrocycles. The zero shear viscosities for the blends with only 15% linear chain fraction could not be obtained at shear rates as low as 10^-5 s^-1. The Cox-Merz rule was found to hold for the ring-linear blends. Linear chains seem to dictate the dynamics and the entanglement scaling of the blends even at low linear chain fraction of 15%.