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) ion omers using pyrene (P) and 1,3-bis(1-pyrenyl) propane (P3P) as the lum inophores and the nitroxide radical 5-doxylstearic acid methyl ester ( 5DSE) as the fluorescence quencher. The ionomers contained 7.5 mol % m ethacrylic acid and were neutralized (90%) with KOH. The fluorescence spectra of P and P3P together with the electron spin resonance (ESR) s pectra of 5DSE indicate that all probes are located in an environment of low polarity and high viscosity that was identified as the hydropho bic 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 conce ntration (CMC) of the ionomer, CMC = 0.02% (w/w) EMAA, was deduced fro m the dependence of I-III/I-I on the ionomer concentration. The spectr oscopic data (ESR and fluorescence) point to the existence, below the CMC, of intramolecular (unimeric) micelles, which are in equilibrium w ith large aggregates above the CMC. The. microviscosity of the micella r core, eta, was estimated to be greater than or equal to 230 cP at am bient temperature on the basis of the fluorescence spectra of P3P, fro m 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 v iscosities. The fluorescence decay of P in EMAA in the presence of 5DS E as the quencher was analyzed with two kinetic models, the Infelta-Ta chiya model based on a Poisson distribution of the quencher and the lu minophore in the micelles and the,general approach of dispersive kinet ics that introduced the time-dependent rate coefficient, k(qm), for in tramicellar quenching. The second model gives a better fit to the expe rimental data, and the low value of the dispersive parameter, alpha = 0.30 (compared to unity for classical kinetics), indicates a broad dis tribution of the quenching rate constants. The results obtained in thi s stud; add important structural-details to the recent models for self -assembly of EMAA ionomers, which were deduced from spin probe ESR spe ctroscopy and small-angle neutron scattering.