CHARACTERIZATION OF POLY(STYRENESULFONATE) AT MINIMUM IONIC-STRENGTH BY ELECTRIC AND MAGNETIC BIREFRINGENCE EXPERIMENT

Results of birefringence experiments on aqueous solutions of charged p oly(sytrene-sulfonate) (PSS) with different molecular weights between 10(5) and 1.1 x 10(6) g/mol in external electric and magnetic fields a re presented. Most of our studies are carried out on solutions of mini mum ionic strength (down to approximate to 10(-6) M), under conditions where the particles are surrounded by extended counterion clouds. The PSS particles can be nearly completely oriented in high electric fiel ds (> 10(5) V/m) and the saturation value of the birefringence is obta ined. In low electric fields a Kerr behavior is found and we are able to determine the electric anisotropy Delta alpha(el) (= alpha(el,paral lel to) - alpha(el,perpendicular to)) of a single particle caused by t he anisotropic polarization of the counterion cloud. At low particle c oncentrations c, this cloud (respectively the electric anisotropy) dec reases like c(-1/2) with increasing concentration. Rising this concent ration once more leads to a decrease of the Kerr constant (k proportio nal to Delta alpha(el) . l(p) where l(p) is the persistence length cha racterizing flexible polymers) faster than c(-1/2) pointing out a star ting coiling expressed by a lowering of the persistence length. In the electric birefringence experiment we are limited to particle concentr ations between 5 x 10(-4) and 5 mg/ml wherefore we complemented our st udies by high magnetic field experiments (0 < B < 16.8 T) at higher co ncentrations (10 < c < 75 mg/ml). The mechanism orienting PSS particle s in magnetic fields is much weaker than that in electric fields. Ther efore no full alignment of our polyelectrolytes is possible, all measu rements are done in the Cotton-Mouton regime. This is the first invest igation of the persistence length of poly(styrenesulfonate) at minimum ionic strength over a wide concentration regime.