Entanglements between Polyelectrolytes in Solutions

How entanglements between ionic polymers are formed remains an open question in polymer science. For some 20 years, it has been known that theoretical predictions for polyelectrolyte entanglement in solution are at odds with experimental evidence, but remarkably little progress has been made on this topic. Here we present a systematic study of the diffusion and rheological properties of polyelectrolytes in solution, and use these results to shed light on the dynamics of polyelectrolyte solutions.

In dilute salt-free and excess salt solutions NaPSS and NaCMC are found adopt rod-like ([η] ∼ N²) and expanded coil ([η] ∼ N^0.77) conformations respectively, in agreement with theoretical expectations. Combining viscosity and single-chain diffusion data for semidilute non-entangled data, we show that the Rouse-Zimm model quantitatively describes polyelectrolyte dynamics in this regime. Further, we find that the local friction coefficient of NaPSS is a strong function of polymer concentration. This means that crossovers in the viscosity-concentration power laws, often used in literature to identify the entanglement crossover (c_e), do not always yield reliable estimates c_e.

While addition of salt leads to a large decrease in the size of polymer chains, it does not, surprisingly, affect the entanglement properties of polyelectrolytes. This finding is at odds with the expectation of packing and scaling models of polymer entanglement and suggests that the density of binary contacts in solution may not affected by the solvent quality. We find that the entanglement crossover and entanglement plateau are unaffected by the concentration of added salt for NaPSS and NaCMC. Based on these observations, we work out the reptation dynamics of polyelectrolytes in salt-free solution, which differ appreciably from earlier models.