Static light scattering and electric birefringence experiments on saltfree solutions of poly(styrenesulfonate)

Abstract

Static light scattering and electric birefringence measurements on aqueous solutions of charged poly(styrenesulfonate) (PSS) with different molecular weights between 10 5 and 1.1×10 6 g/mol are presented. All experiments were performed in the dilute and the semidilute concentration regime (0.0005 mg/ml<c<10 mg/ml) at minimum ionic strength (down to ≃10 -6 M). The statir light scattering experiments show a single broad peak in the scattered intensity. The scattering vectors of these peaks increase with increasing concentrations r and scale either with c 1/3 or with c 1/2 only depending on a relative concentration but not on the molecular weight: below about 20 c * (1 c * :=overlap concentration of extended chains=1 particle/(contour length l c ) 3 ) we found a c 1/3 dependence of the scattering vector, above 20 c * a c 1/2 law is valid. A similar behaviour has been observed for rigid rods [1]. Our results are compared with previous light-[2, 3], small angle neutron-[4, 5], and small angle X-ray [6] scattering investigations. Nearly all of these studies are in a very good agreement with the c 1/3 -and c 1/2 -law. The relaxation of the electric birefringence signal after an applied rectangular electric field is monoexponential in nearly all cases. Again the concentration 20 c * , independent of the molecular weight, seems to be a critical concentration: below 20 c * we found a normal (negative) birefringence signal, above an anomal (positive) signal. These results are compared with results of electric birefringence measurements on aqueous solutions of rigid rods [7, 8] and PSS- at different ionic strengths [9-11]. It is claimed that our results can be explained by a small flexibility of strongly elongated PSS-rods which increases slightly with increasing molecular weight and distinctly with raising concentration. The concentration c * is shown to be a reasonable quantity to describe polyelectrolyte solutions without added salt, independent of the molecular weight, respectively the contour length. By rescaling comparable data published by other authors we can establish the critical concentration 20 c * , which seems to be universally valid