Static and Hydrodynamic Dimensions of Flexible Polyions in Solutions
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STATIC ANliD HVDRODVNA C DIMENSIONS OF FLEXIBLE POLYB©N$ IN SOLU•TION•S HEDI MATTOUSSI,° STEVE O'DONOHUE,+ FRANK E. KARASZ Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA 01003 ABSTRACT The variation of the radius of gyration RG and the hydrodynamic size RH are followed as the ionic strength of the medium is monitored for a flexible polyelectrolyte compound in solutions. In addition, a comparison between the thermodynamical and hydrodynamical interaction parameters is given for the present system. flNTRODUCTOIN
It has been known that the presence of charges on the macromolecule may transform, drastically, the properties of polymeric materials [1-3]. For example, phase separation in polymeric gels can be induced by simply changing the ionic strength of these media [4]. Similar features are observed for polymer solutions. For instance, a 0 state can be induced by increasing the counterion excess in these media [5]. Precipitation and phase separation processes have been reported for polyelectrolyte solutions with high ionic strengths [5]. The second virial coefficient A2 and the intrinsic viscosity [11] were also found to strongly depend on the polyion charge, and the medium ionic strength as well, for these solutions [5-7]. These features are governed by the competition between the Coulomb repulsions (between charges in the macromolecules), and the screening effects brought by the counterion excess. These two sources affect both the macroion dimensions and the interaction parameters for flexible chains. They modify only the interaction parameters A2 and k 0 [1] for micelles and biological materials where rigid structures are present: BSA and DNA rods, fc-r instance [6,7]. Nevertheless, these effects do not follow one common behavior for all polyelectrolyte systems [1-9]. In this study we present measurements of the polyion dimensions, static RG and hydrodynamic RH, and the second virial coefficient A2 for a polyelectrolyte compound in solutions. The data are discussed with the implications on the hydrodynamic interaction parameter kD[fl] which accounts for the dependence of the mutual diffusion coefficient DM on the solute concentration c. *Present address: Carnegie Mellon University, Dept. of Chemistry, Pittsburgh, PA 15213 +Present address: Polymer Labs., Essex Road, Church Stretton, Shropshire SY6 6AX, U. K. Mat. Res. Soc. Symp. Proc. Vol. 248. @1992Materials Research Society
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EXPERIMENTAL SECTION We used static and quasielastic light scattering techniques, SALS and QELS, respectively. From the first technique where scattering in the limits of small angles and small solute volume fraction are required, c 0 and c -> 0 in
the Zimm plot) [5]. The scattering wavevector q is a function of the scattering angle 0, the light wavelength X and the medium refractive index (solvent) no: q={4tno/0 }sin(0/2). The QELS experiment where the time dependence of the concentration fluctuations is studied also in the limit of small q, provides the hydrodynamic radius RH from the diffusion c
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