Aggregation of hydrophobically end-capped poly(ethylene oxide) in aqueous solutions. Fluorescence and light-scattering studies

The association of a monodisperse model associative polymer, hydrophobically end-capped poly(ethylene oxide), denoted C(12)EO(460)C(12), in aqueous solution has been studied. The macroscopic properties have been investigated by theological methods and correlated to properties on the microscopic level, as revealed from fluorescence and light scattering. The C(12)EO(460)C(12) polymer associates in water through its hydrophobic end groups and gives rise to a sharp increase in viscosity at about 2 x 10(-2) g mL(-1) due to formation of large aggregates or networks. The aggregation starts, however, at much lower concentrations. Already at 0.5 x 10(-2) g mL(-1) hydrophobic domains large enough to dissolve pyrene have formed. The aggregation process is very gradual without a distinct cmc. An estimate of the aggregation number of the hydrophobic domains obtained from fluorescence is about 15-30 end groups per micelle. The distribution of relaxation times from DLS shows three important diffusive modes in the concentration range 3 x 10(-3) to 5 x 10(-2) g mL(-1). A fast mode in the dilute region is attributed to unimers or small oligomeric aggregates of C(12)EO(460)C(12). At concentrations near the point where the viscosity rises an intermediate mode appears. This mode is considered to reflect the cooperative motions of the formed network. In the whole concentration range studied a slow mode is present. In the dilute region this mode dissappears completely upon addition of salt and is therefore attributed to the PEO interchain interaction. In the high-concentration region, however, this mode is probably also influenced by the aggregation of the hydrophobic ends of C(12)EO(460)C(12). The validity of these results is discussed and compared with a model considering the gain/loss of free energy upon formation of aggregates (loops, oligomeric aggregates, and micelles).