FLUORESCENCE PROBE STUDIES OF POLY(ACRYLIC ACID) CATIONIC SURFACTANT INTERACTIONS FOLLOWING HIGH-SHEAR

Polarity and rigidity (induced by shear) in the local environment of poly(acrylic acid), PAA, in association with a cationic surfactant (alkyltrimethylammonium bromide) in aqueous solution were studied by fluorescence techniques using pyrene and auramine O as probes. The polarity along the PAA is strongly affected by the alkyl tail length of the surfactant. The critical association concentrations (cac's) of polymer-bound surfactants are more than 2 orders of magnitude less than their critical micellar concentrations. The local polarity and cac's are also affected by molecular weight and concentration of PAA; higher molecular weight PAA significantly lowers the polarity, leading to low cac's, whereas higher concentration of PAA tends to increase the cac, after passing through a minimum, suggesting a growing influence of interchain aggregation (of the bound surfactants) with increasing PAA concentrations. Chain rigidity resulting from the shear-induced self-association (through interchain H-bonding) of PAA increases noticeably at low surfactant concentrations but it decreases sharply at higher surfactant concentrations. This suggests that the interchain H-bonding of PAA (induced by shear) is facilitated in a low hydrophobic environment but it undergoes disruption in a highly hydrophobic one. Such behavior is strongly pH dependent with increased rigidity at a relatively low pH.