Conformation and dynamics of flexible polyelectrolytes in semidilute salt-free solutions

Abstract

We present steady shear rheology data for sodium polystyrene sulfonate (NaPSS) in semidilute unentangled salt-free aqueous solution as a function of polymer concentration (c) and degree of polymerisation (N). The measured terminal modulus (G) agrees with the scaling prediction of G ≃ kTc/N. The specific viscosity varies with the degree of polymerisation as η ∝ N. The observed dependence differs with the linear relation expected from the Rouse model for ideal chains as predicted by de Gennes' scaling model and subsequent theories. Together with the diffusion data of Oostwal et al., our results suggest that chains may follow non-Gaussian statistics beyond the correlation length (ξ). Small angle neutron scattering data on salt-free semidilute solutions at low polymer concentrations partially support this hypothesis but do not confirm it. The electrostatic Kuhn length of NaPSS salt-free solutions is found to be proportional and slightly larger than the correlation length (L ≃ 1.3ξ). This result agrees well with the scaling model of Dobrynin et al. Radii of gyration (R) data as a function of polymer concentration reveal that a concentrated regime (R ∝ c) does not occur for NaPSS in the concentration range considered (c ≲ 4M). We conclude by comparing the predictions of the scaling model of Dobrynin et al. and Muthukumar's double screening theory with viscosity data for different polyelectrolyte-solvent systems. Dobrynin's model provides a better description of experimental observations. Our findings confirm several aspects of current models for polyelectrolyte solutions but some deviations from theory remain.

Diffusion and viscosity of non-entangled polyelectrolytes

“…7,15 Scaling models, following the pioneering work of de Gennes and co-workers [16][17][18][19][20][21] , provide a relatively simple and useful framework to understand polyelectrolyte behaviour, and are particularly successful in predicting the conformational properties of flexible polyelectrolytes in solution. 22 The scaling treatment of polyelectrolyte dynamics on the other hand displays poorer agreement with experimental results. [22][23][24][25][26][27] In salt-free or low ionic strength media, polyelectrolytes adopt highly extended conformations 22,[26][27][28][29][30][31][32][33][34][35] , meaning that their solutions are above the overlap concentration (c * ) for most practical applications.…”

“…22 The scaling treatment of polyelectrolyte dynamics on the other hand displays poorer agreement with experimental results. [22][23][24][25][26][27] In salt-free or low ionic strength media, polyelectrolytes adopt highly extended conformations 22,[26][27][28][29][30][31][32][33][34][35] , meaning that their solutions are above the overlap concentration (c * ) for most practical applications. While the overlap concentration of polyelectrolytes is much lower than that of non-ionic polymers, especially for high degrees of polymerisation (N ), the entanglement concentration (c e ) is only weakly dependent on charge fraction 26,36 .…”

“…As the result, there exists a wide range in the N − c phase space where polyelectrolytes are in the semi-dilute non-entangled regime (c * < c < c e ). 22,26,27 For example, sodium polystyrene sulfonate (NaPSS) with N 2000 displays c e /c * 10 3 in salt-free water. By comparison, c e /c * 10 for polystyrene with equivalent degree of polymerisation in good solvent.…”

“…26 a) lopez@pc.rwth-aachen.de b) christian.mayer@uni-due.de Single chain diffusion measurements provide an important test to the microscopic description of polymer dynamics put forward by various theories 37,38 . In the context of polyelectrolytes, chain diffusion has been studied primarily in the excess salt, zero polymer concentration limit by dynamic light scattering [39][40][41] , as well as by NMR, fluorescence methods and centrifugation in the infinite polymer dilution limit, both in salt-free and excess added salt 22,[42][43][44][45][46][47] . Measurements above c * have received far less attention [48][49][50] , and most experimental data come from two studies by Oostwal and co-workers 49,50 , who investigated the molar mass, polymer concentration and added salt concentration variation of the diffusion coefficient of NaPSS in D 2 O.…”

Oscillatory structural forces between charged interfaces in solutions of oppositely charged polyelectrolytes

“…49 One may again compare the effective diameter d, which is obtained from these crossover concentrations, with the gyration radius R g , which is known from scattering experiments with PSS solutions. 50 One finds the ratio d/R g = 1.4, which is independent of the molecular mass. This number is close to our estimate for PLL, and definitely in the range suggested theoretically.…”