Characterization of Poly(styrenesulfonate) at Minimum Ionic Strength by Electric and Magnetic Birefringence Experiments

(Received 19 September 1994, revised 17 November 1994, accepted 18 January 1995)

Abstract
Results of birefringence experiments on aqueous solutions of charged poly (styrenesulfonate) (PSS) with different molecular weights between 10⁵ and  g/mol in external electric and magnetic fields are presented. Most of our studies are carried out on solutions of minimum ionic strength (down to  M), under conditions where the particles are surrounded by extended counterion clouds. The PSS particles can be nearly completely oriented in high electric fields ( >10⁵ V/m) and the saturation value of the birefringence is obtained. In low electric fields a Kerr behavior is found and we are able to determine the electric anisotropy of a single particle caused by the anisotropic polarization of the counterion cloud. At low particle concentrations c, this cloud (respectively the electric anisotropy) decreases like c^-1/2 with increasing concentration. Rising this concentration once more leads to a decrease of the Kerr constant , where is the persistence length characterizing flexible polymers) faster than c^-1/2 pointing out a starting coiling expressed by a lowering of the persistence length. In the electric birefringence experiment we are limited to particle concentrations between and 5 mg/ml wherefore we complemented our studies by high magnetic field experiments ( 0<B<16.8 T) at higher concentrations ( 10<c<75 mg/ml). The mechanism orienting PSS particles in magnetic fields is much weaker than that in electric fields. Therefore no full alignment of our polyelectrolytes is possible, all measurements are done in the Cotton-Mouton regime. This is the first investigation of the persistence length of poly(styrenesulfonate) at minimum ionic strength over a wide concentration regime.