ELECTRON-SPIN-RESONANCE STUDY OF POLYMER SELF-ASSEMBLING - CATIONIC SPIN PROBES IN AQUEOUS-SOLUTIONS OF POLY(ETHYLENE-CO-METHACRYLIC ACID) (EMAA) IONOMERS

Aqueous solutions of the ionomer poly(ethylene-co-methacrylic acid) (E MAA) in the concentration range 0.5-23 wt % were studied by the electr on spin resonance (ESR) spin probe method. The spin probes selected fo r this study were 4-(N,N-dimethyl-N-alkyl)ammonium-2,2,6,6 -tetramethy lpiperidine-1-oxyl iodides (CATn) with different alkyl groups: methyl (CAT1), octyl (CAT8), and hexadecyl (CAT16). The main objective of thi s study was to compare the ESR spectra of the CATn probes as a functio n of temperature and ionomer content in order to deduce details on the structure and dynamics of EMAA micelles and on the interface region b etween the aggregates and the solvent. ESR spectra were recorded in th e temperature range 120-360 K, and the line shape variations and the N -14 hyperfine splittings, alpha(N), were key parameters fop the interp retation of the results in terms of the spin probe mobility, and its' location in and bonding to the polymeric aggregates. The detection of two spectral components in the ESR spectra of the probes provided evid ence for the existence of an equilibrium between large multichain aggr egates and unimeric micelles, in accord with previous ESR (based on do xylstearic acid spin probes) and fluorescence (based on pyrene as the luminophore) studies. The results indicated that most spin probe molec ules are bound to large intermolecular micelles; the long alkyl chains of CAT8 and CAT16 penetrate into the interior of the aggregates but e xhibit different dynamics, suggesting that the longer alkyl chain of C AT16 penetrates deeper, into the more viscous regions of the micelle, compared to CAT8. This conclusion implies that the cationic probes wit h different alkyl substituents can be used to map the local viscosity and the viscosity gradient. In the EMAA solutions an isotropic triplet was also detected, as a minor spectral component. Its contribution de pends on the type of probe and on ionomer concentration and ranges fro m approximate to 1% for CAT16 in the 23 wt % EMAA solution to approxim ate to 10-15% (depending on probe concentration) for CAT8 in the 0.5 w t % EMAA solution at 300 K. This component was assigned to spin probes bound to unimeric (intramolecular) micelles. Analysis of the N-14 hyp erfine splittings indicated the formation of an ionic bond between the carboxylic group of the ionomer and the cation of the probes.