| In order to quantitatively characterize the association process of associating polymers (APs) at the molecular level, a fluorescence study of pyrene-labeled APs was undertaken. Seven pyrene-labeled poly(N,N-dimethylacrylamide)s (PyPDMAAm), and eleven pyrene-labeled poly(L-glutamic acid)s (PyPGA) were examined. Hydrophobic associations and the encounter mechanism of pyrenes were probed in aqueous, organic, and mixtures of aqueous and organic solvents.; Although no significant thickening behavior was observed for the pyrene-labeled polymers, the analysis of the fluorescence data with the blob model yielded the fraction of aggregated pendants, an important yet elusive measure of the level of association of APs. The diffusion-controlled encounters between pyrenes were analyzed in acetone, DMF, and water. The blob model provided a quantitative description of the encounters occurring between pyrenes attached onto PyPDMAAm and PyPGA. The blob model furnishes the rate constant (kdiff) for diffusive encounters between pyrenes inside a blob volume (Vblob) and the make up of the blob in monomer units (Nblob). kdiff was found to be inversely proportional to the blob volume. Interestingly, kdiff did not vary with the solvent viscosity as should be expected for diffusion-controlled encounters. The parameter Vblob scales as Nblob 3 where ν equals 0.5 in a theta solvent and 0.6 in a good solvent, demonstrating that some of the basic scaling principles applicable to polymer chain size, volume, and dynamics hold remarkably well for this empirically derived blob model. The blob model has been applied to the same polymer undergoing different levels of hydrophobic association. The blob model is expected to become a powerful tool for the study of any polymeric system where the characterization of the diffusional encounters occurring between pendants located along a polymer backbone and the determination of their level of association are required. |
