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Polyelectrolytes and Their Counterions Studied by EPR Spectroscopy
Dariush Hinderberger, Max-Planck-Institut für Polymerforschung, Mainz (2004)
In this thesis methods of EPR spectroscopy were used to investigate polyion-counterion interactions in
polyelectrolyte solutions. The fact that EPR techniques are local methods is exploited and by
employing spin-carrying (i.e., EPR-active) probe ions it is possible to examine polyelectrolytes from
the counterions’ point of view.
It was possible to gain insight into i) the dynamics and local geometry of counterion
attachment, ii) conformations and dynamics of local segments of the polyion in an indirect manner,
and iii) the spatial distribution of spin probe ions that surround polyions in solution.
Analysis of CW EPR spectra of dianion nitroxide spin probe Fremy’s salt (FS, potassium
nitrosodisulfonate) in solutions of cationic PDADMAC polyelectrolyte revealed that FS ions and
PDADMAC form transient ion pairs with a lifetime of less than 1 ns. This effect was termed as
dynamic electrostatic attachment (DEA). By spectral simulation taking into account the rotational
dynamics as a uniaxial Brownian reorientation, also the geometry of the attached state could be
characterized. By variation of solvent, the effect of solvent viscosity and permittivity were
investigated and indirect information of the polyelectrolyte chain motion was obtained. Furthermore,
analysis of CW EPR data also indicates that in mixtures of organic solvent/water PDADMAC chains
are preferentially solvated by the organic solvent molecules, while in purely aqueous mixtures the
PDADMAC chain segments were found in different conformations depending on the concentration
ratio R of FS counterions to PDADMAC repeat units.
Broadenings in CW EPR spectra of FS ions were assigned to spin-exchange interaction and
hence contain information on the local concentrations and distributions of the counterions. From
analysis of these broadenings in terms of a modified cylindrical cell approach of polyelectrolyte
theory, radial distribution functions for the FS ions in the different solvents were obtained. This
approach breaks down in water above a threshold value of R, which again indicates that PDADMAC
chain conformations are altered as a function of R.
Double electron-electron resonance (DEER) measurements of FS ions were carried out to
probe the distribution of attached counterions along polyelectrolyte chains. For a significant fraction of
FS spin probes in solution with a rigid-rod model polyelectrolyte containing charged Ru2+-centers, a
bimodal distance distribution was found that nicely reproduced the spacings of direct and nextneighbor
Ru2+-centers along the polyelectrolyte: 2.35 and 4.7 nm. For the system of FS/PDADMAC,
DEER data could be simulated by assuming a two-state distribution of spin probes, one state
corresponding to a homogeneous (3-dimensional) distribution of spin probes in the polyelectrolyte
bulk and the other to a linear (1-dimensional) distribution of spin probes that are electrostatically
condensed along locally extended PDADMAC chain segments. From this analysis it is suggested that
the PDADMAC chains form locally elongated structures of a size of at least ~5 nm.
