Zusammenfassung
Light and neutron scattering measurements on highly charged polyelectrolyte
solutions have recently provided firm evidence for the existence
of ''domain structures'' containing many chains, even at rather low-polymer
concentrations. In the present paper, we systematically investigate
the influence of counterion charge valency Zc on the scattering properties
of sulfonated polystyrene (PSS) solutions in water with monovalent
and divalent counterions. This study is part of a larger effort to
identify essential factors governing polyelectrolyte domain formation
and the geometric properties of these transient structures. Neutron
scattering measurements indicate that the interchain correlation
length XiD within the domains becomes larger by a factor of 1.5-2
for divalent relative to monovalent counterions. This observation
is consistent with the Manning model estimate of the change in effective
polymer charge density Tau* with Zc and with previous observations
linking Xid from the peak position in the scattering intensity I(q)
with the bare polymer charge density, Tau. Light scattering measurements
of the radius of gyration Rg, XiD of the domains indicate that their
size becomes smaller for divalent counterions and with a reduction
of Tau. We observe that the ''fractal'' dimension of the domains
measured from the low-angle scaling of I(q) depends on the chemical
structure of the polyelectrolyte. Zero average contrast (ZAC) neutron
scattering measurements show that the radius of gyration Rg,c of
individual polyelectrolyte chains is also reduced for divalent counterions,
suggesting that chain rigidity is strongly influenced by Tau*. Charge
valency effects on relaxation times are investigated by dynamic light
scattering. As usual, two diffusive modes are observed in the light
intensity autocorrelation function, G(tau). The ''fast'' mode becomes
slower and the ''slow'' mode becomes faster for the divalent counterion
(Mg2+), relative to the monovalent counterion (Na+). Counterion valence
has a large influence on the structure and dynamics of highly charged
polyelectrolyte solutions through its influence on Tau*
Nutzer