STATIC LIGHT-SCATTERING AND ELECTRIC BIREFRINGENCE EXPERIMENTS ON SALTFREE SOLUTIONS OF POLY(STYRENESULFONATE)

Static light scattering and electric birefringence measurements on aqu eous solutions of charged poly(styrenesulfonate) (PSS) with different molecular weights between 10(5) and 1.1 x 10(6) g/mol are presented. A ll experiments were performed in the dilute and the semidilute concent ration regime (0.0005 mg/ml < c < 10 mg/ml) at minimum ionic strength (down to almost-equal-to 10(-6) M). The static light scattering experi ments show a single broad peak in the scattered intensity. The scatter ing vectors of these peaks increase with increasing concentrations c a nd scale either with c1/3 or with c1/2 only depending on a relative co ncentration but not on the molecular weight: below about 20 c (1 c* : = overlap concentration of extended chains = 1 particle/(contour lengt h l(c)3) we found a c1/3 dependence of the scattering vector, above 20 c a c1/2 law is valid. A similar behaviour has been observed for rig id rods [1]. Our results are compared with previous light-[2, 3], smal l angle neutron-[4, 5], and small angle X-ray [6] scattering investiga tions. Nearly all of these studies are in a very good agreement with t he c1/3-and c1/2-law. The relaxation of the electric birefringence sig nal after an applied rectangular electric field is monoexponential in nearly all cases. Again the concentration 20 c, independent of the mo lecular weight, seems to be a critical concentration : below 20 c we found a normal (negative) birefringence signal, above an anomal (posit ive) signal. These results are compared with results of electric biref ringence measurements on aqueous solutions of rigid rods [7, 8] and PS S- at different ionic strengths [9-11]. It is claimed that our results can be explained by a small flexibility of strongly elongated PSS- ro ds which increase slightly with increasing molecular weight and distin ctly with raising concentration. The concentration c is shown to be a reasonable quantity to describe polyelectrolyte solutions without add ed salt, independent of the molecular weight, respectively the contour length. By rescaling comparable data published by other authors we ca n establish the critical concentration 20 c, which seems to be univer sally valid.