Phase Separation in Sulfonated Poly(ether ether ketone) (SPEEK) Ionomers by Spin Probe ESR: Effect of the Degree of Sulfonation and Water Content

Abstract

We present the effect of the degree of sulfonation (DS) and water content on the phase separation in sulfonated poly(ether ether ketone) (SPEEK) ionomers, using the method of spin probe electron spin resonance (ESR). The ionomer contained the nitroxide radical 4-oxo-TEMPO (TEMPONE) as the spin probe. ESR spectra in the temperature range 110–350 K were measured in order to obtain information on the dynamics of the spin probe, and the local polarity of the probe site in SPEEK membranes with DS = 38%, 58%, and 70%. ESR spectra at 110 K and at 350 K were simulated for each DS value, in samples soaked at ambient temperature and at 60 °C. For all samples the hyperfine splittings for the 14N nucleus, aiso, was in the range 15.4–15.8 G, indicating a hydrophilic environment of the probe, based on the comparison with the aiso value measured for TEMPONE in bulk water at 350 K. The local dynamics of the probe is reflected in the type and number of spectral components as a function of DS and type of soak. At 350 K spe...