A fluorescence probe study on the effects of surfactants on cloud points in aqueous poly(N-isopropylacrylamide) solutions

Fluorescence probe methods were used to investigate micelle formation of poly(N-isopropylacrylamide) (PNIPA) with two types of surfactants, anionic sodium n-dodecyl sulfate (SDS) and cationic n-dodecyltrimethylammonium chloride (DTAC), in aqueous solutions using pyrene or 1-pyrenecarboxaldehyde as a fluorescence probe. Two PNIPA samples, one having a hydrophobic chain-end group and the other having a negatively charged hydrophilic chain-end group, were used to investigate the effects of the chain-end group on the formation of the micelles. The critical aggregation concentration, at which surfactant molecules begin to bind to PNIPA chains to form micelles, was observed to be much lower for PNIPA solutions containing SDS than for solutions containing DTAC. This is consistent with previous results showing that the cloud point in PNIPA solutions containing SDS begins to increase from its value in surfactant-free solutions at a much lower concentration of added surfactant than that in solutions containing DTAC. It was also observed that there is a discrepancy in the emission spectra of solutions containing SDS between the two PNIPA samples but not for solutions containing DTAC, indicating that the chain-end groups of PNIPA may affect the microenvironmental polarity in the micelles composed of PNIPA and the surfactants.