EPR STUDY OF AN ASSOCIATIVE POLYMER IN SOLUTION - DETERMINATION OF AGGREGATION NUMBER AND INTERACTIONS WITH SURFACTANTS

It has been well established that hydrophobically end-capped water-sol uble polymers, such as poly(ethylene oxide) didodecyl ether, C(12)EO(2 00)C(12), self-associate in aqueous solutions by virtue of the hydroph obic effect and thus form hydrophobic domains comprising the end-group s. These hydrophobic domains are able to solubilize spin probes which are virtually insoluble in the aqueous phase, enabling them to be inve stigated by EPR. An estimate of the number of polymer end-groups parti cipating in each domain can be obtained by studying the details of the changes in the fine structure of the EPR lineshape as a function of t he average number of spin probes per domain. Using this technique, the aggregation number of C(12)EO(200)C(12) in a 2.5 wt.% solution has be en estimated to be 31 +/- 6 end-groups per hydrophobic domain. The mic roviscosity, (the local viscosity of these domains), estimated from th e rotational correlation time of an aminoxyl spin probe was invariant with polymer concentration in the range 0.25-5 wt.%. The micropolarity , or the 'effective relative permittivity', estimated from the nitroge n hyperfine coupling constant, was also invariant in this concentratio n range. Therefore, the structure of the hydrophobic domains remained similar as the polymer concentration was increased, while the macrosco pic viscosity was known to increase dramatically in this concentration region. A model which incorporate these observations is discussed. Th e hydrophobicity and microviscosity of the hydrophobic domains are als o compared and contrasted with values for surfactant micelles. Interac tions of surfactants with the polymer have been monitored by measuring the spin-probe sensed polarity, as a function of surfactant concentra tion, at two concentrations of polymer. At ratios <3 sodium dodecyl su lfate (SDS) molecules per end-group, the polarity increased from that of the hydrophobic domain towards that of the anionic SDS micelle. On addition of the non-ionic surfactant C(12)E(23), the polarity decrease d from that of the hydrophobic domain toward that of the neutral C(12) E(23) micelle. Addition of both types of surfactants therefore altered the composition and thus the properties of the hydrophobic domains. A t high surfactant concentration, the evaluation of the spin-probe sens ed polarity is more complicated.