Fluorescence studies of self-assembly in aqueous solutions of poly(ethylene-co-methacrylic acid) ionomers

Steady-state and time-resolved florescence measurements were performed on aqueous solutions of poly(ethylene-co-methacrylic acid) (EMAA) ionomers using pyrene (P) and 1,3-bis(1-pyrenyl) propane (P3P) as the luminophores and the nitroxide radical 5-doxylstearic acid methyl ester (5DSE) as the fluorescence quencher. The ionomers contained 7.5 mol % methacrylic acid and were neutralized (90%) with KOH. The fluorescence spectra of P and P3P together with the electron spin resonance (ESR) spectra of 5DSE indicate that all probes are located in an environment of low polarity and high viscosity that was identified as the hydrophobic micellar core of EMAA aggregates. The local polarity was estimated from the intensity ratio of the third to the first vibronic peaks (I-III/I-I) in the fluorescence spectrum of P. The critical micelle concentration (CMC) of the ionomer, CMC = 0.02% (w/w) EMAA, was deduced from the dependence of I-III/I-I on the ionomer concentration. The spectroscopic data (ESR and fluorescence) point to the existence, below the CMC, of intramolecular (unimeric) micelles, which are in equilibrium with large aggregates above the CMC. The. microviscosity of the micellar core, eta, was estimated to be greater than or equal to 230 cP at ambient temperature on the basis of the fluorescence spectra of P3P, from a calibration curve of the intensity ratio of the excimer to monomer emissions, I-E/I-M, vs viscosity in 14 nonaqueous solvents of known viscosities. The fluorescence decay of P in EMAA in the presence of 5DSE as the quencher was analyzed with two kinetic models, the Infelta-Tachiya model based on a Poisson distribution of the quencher and the luminophore in the micelles and the,general approach of dispersive kinetics that introduced the time-dependent rate coefficient, k(qm), for intramicellar quenching. The second model gives a better fit to the experimental data, and the low value of the dispersive parameter, alpha = 0.30 (compared to unity for classical kinetics), indicates a broad distribution of the quenching rate constants. The results obtained in this stud; add important structural-details to the recent models for self-assembly of EMAA ionomers, which were deduced from spin probe ESR spectroscopy and small-angle neutron scattering.