Spin-labeled polyelectrolyte gels based on poly(N-isopropylacrylamide). Effects of the network structure and the gel collapse on the EPR spectra

The swelling and collapse of aqueous copolymer gels of N-isopropylacrylamide and 2-acrylamido-2-methylpropanesulfonic acid have been studied by EPR spectroscopy using spin-labeled polymers. The swelling of the gels is strongly affected by added electrolytes and pH of the aqueous solvent. The EPR spectra from aqueous solutions of the linear polymer are motionally narrowed three-line spectra at temperatures below and above the critical temperature. Spectra from the gels are composites, i.e. they contain also a broad slow motional component, and their shapes are dependent on temperature as well as on the cross-link density. The intensities of the EPR spectra are suggested to be affected by the protonation of the radical label. Changes of the spectral intensity with temperature may be used to detect conformational changes of the polymer.