Steady-state fluorescence investigation of pyrene-labeled poly(acrylic acid)s in aqueous solution and in the presence of sodium dodecyl sulfate

Commercial poly(acrylic acid) (PAA) samples with M-v = 150 000 and 450 000 were labeled randomly with small amounts (2-3 mol %) of I-pyrenylmethyl (MePy) groups. A sample of PAA (M-v = 450 000) labeled with 2 mol % MePy and modified with 2 mol % n-dodecyl (C-12) groups was also prepared. The photophysical properties of the polymers have been investigated by steady-state fluorescence spectroscopy. The ratio (I-E/I-M) of excimer-to-monomer emission intensities was used to determine the effect of changes in pH and addition of sodium dodecyl sulfate (SDS) on the solution properties of the labeled polymers in the absence and presence of salt (NaCl). Changes in polymer conformation in aqueous solution upon addition of base were revealed by the curve I-E/I-M Vs pH that had an inflection point at pH 4.7, the pK(a) value of labeled PAA. The ratio I-1/I-3 of the emission of MePy linked to PAA was monitored to obtain information on the micropolarity sensed by the pyrene label and how it is affected by external stimuli such as changes in pH and addition of surfactant. SDS interacts with labeled PAA in solution of pH 3 to form a polymer/SDS complex with a critical aggregation concentration (CAC) lower than the critical micelle concentration of SDS. The CAC values decrease further in the presence of NaCl but are not affected significantly by the molecular weight of the parent PAA or the grafted dodecyl along the PAA chain.