Fluorescence studies of associative behavior of cationic surfactant moieties covalently linked to poly(acrylamide) at the surfactant head or tail

Association properties of hydrophobically modified water-soluble polymers bearing the same (or similar) hydrophobic and charged groups at different relative positions (with respect to the polymer main chain) were compared. Two types of surfactant monomers (surfmers) were synthesized; a cationic surfactant was linked to the p-position of styrene at the surfactant charged head (dimethyldodecyl-4-vinylbenzylammonium chloride (St-QC12)) or at the tail end (13-(4-vinylphenyl)tridecyltrimethylammonium bromide (St-C13Q)). These surfmers were copolymerized with acrylamide (AAm), and the association behavior of the copolymers in water was characterized using different fluorescence techniques. The surfmer contents in the AAm/St-QC12 and AAm/St-C13Q copolymers were fixed to more or less 10 mol%. The surfmer units in the copolymers self-associate to form multipolymer aggregates. An apparent critical micelle concentration (cmc) (i.e. a polymer concentration for the onset of interpolymer association of the surfmer units) estimated from excitation spectra of pyrene probes for AAm/St-QC12 (cmc=2.6×10−4 g/l) was several times lower than that for AAm/St-C13Q (cmc=1.3×10−3 g/l), surfmer units in AAm/St-QC12 showing a stronger tendency for interpolymer association than those in AAm/St-C13Q. The presence of excimer emissions in steady-state fluorescence spectra for the p-substituted styrene residues (phenylene groups) in AAm/St-C13Q indicated that the benzene rings are in close proximity to one another in the polymer aggregates. Steady-state fluorescence spectra and fluorescence quenching by thallium nitrate and 1-nitrohexane indicated that the phenylene groups in AAm/St-C13Q were incorporated in hydrophobic microdomains formed from surfactant hydrocarbon chains, whereas the phenylene groups in AAm/St-QC12 are located near the surface of polymer aggregates in a way that they are sufficiently apart from each other and hence no excimer is formed.