Aggregation number determination in aqueous solutions of a hydrophobicallymodified poly(ethylene oxide) by fluorescence probe techniques

In this paper, we report the results of fluorescence probe studies of assoc iation in aqueous solution of a model polymer ODU, which is a C18H37 end-ca pped monodisperse poly(ethylene oxide) with a molecular weight of 35 000 an d essentially 100% end group substitution. The main parameter of interest i s the end group aggregation number N-R, which we determine by the time-reso lved fluorescence quenching technique using pyrene as a probe. Aiming to ex tend this technique to situations where the probe is not solubilized comple tely by the polymer micelles, which happens when the polymer concentration in a solution becomes very low, we modified the standard Poisson quenching model to account for the emission of the probe in the water phase. ODU solu tions undergo phase separation at polymer concentrations between 2.1 g/L an d 20 g/L. The aggregation number in a 2.1 g/L solution was found to be 23. For high concentration polymer solutions, from 3 to 17 wt %, we found that the aggregation number tends to increase with the polymer concentration, wh ereas the pyrene quenching constant decreases.