Fluorescence Probe Studies of Hydrophobic Domains in a Model Hydrophobically Modified Alkali-Swellable Emulsion (HASE) Polymer with C20H41 Groups

The fluorescence and fluorescence decay profiles of pyrene and 1-ethylpyrene in solutions of a hydrophobic alkali-swellable emulsion (HASE) polymer were examined to characterize the association structure formed from the hydrophobic substituents. The HASE polymer was obtained by the copolymerization of ethyl acrylate (EA), methacrylic acid, and a macromonomer containing a C 20 H 41 group separated from the backbone by 32 ethylene oxide units. We examine solutions neutralized with 1 equiv of NaOH, at polymer concentrations where virtually all of the added probe is partitioned into hydrophobic domains of the polymer. Both monomer and excimer emission are observed, and I E / I M increases in proportion to the amount of probe added to the system. Individual monomer fluorescence decay profiles fit well to the traditional micelle Poisson quenching model, but attempts to calculate the hydrophobe aggregation number N R led to values that changed markedly with the ratio of probe to polymer. These results were rationalized in terms of a polymer structure in water containing various hydrophobic domains of different composition. These domains vary from nonionic micelle like structures containing upward of 60−80 C 20 H 41 groups, where the first probes added to the system are located, to mixed structures containing both C 20 H 41 and EA groups from the polymer backbone.