Probing the Solution Structure of Amphiphilic Polymer Flower-Micelles Using Fluorescence and Hyper-Rayleigh...

Flower micelles are self-assembled structures formed when certain amphiphilic polymer architectures are dissolved in water; they typically carry a core formed by the hydrophobic segments and a shell containing loops of the hydrophilic backbone segments. First, a periodically grafted amphiphilic polymer (PGAP) carrying long PEG2000 segments in the backbone and pendant eicosyl (C20) segments located at periodic intervals, along with a small fraction (∼10%) of pendant clickable propargyl units was prepared. This served as the parent scaffold to covalently install fluorescence probes, namely, N,N-dimethyl naphthalimide and pyrene; the fluorescence spectral features of these covalently linked probes revealed that they reside within the hydrophobic core of the anticipated flower micelles formed by these PGAPs in water. In a second study, a different chromophore possessing a strong dipole, namely, 3,6-dinitrocarbazole, tethered to a hydrophobic dodecyl segment was installed on every repeat unit of the chain to yield a specially designed PGAP that was studied by hyper-Rayleigh scattering (HRS). An unusual variation in the second harmonic intensity (I_2ω), as a function of chloroform-methanol composition, suggested that the conformation of the chain, upon going from a good solvent (chloroform) to a PEG-selective solvent (methanol), traverses through an intermediate state, wherein directionally ordered chromophoric clusters lead to a significant increase in the I_2ω values. This was confirmed by the study of an analogous model surfactant bearing the same chromophore at the tail-end of the hydrophobic alkyl segment; this model surfactant also exhibited a similar increase before decreasing to a final low value, which is expected for a spherically symmetric arrangement of the dipoles. In summary, flower micelle formation by suitably derivatized PGAPs was first confirmed by examining the spectral features of covalently installed polarity-sensitive fluorescent probes; conversely, HRS studies revealed the conformational pathway toward flower micelles in a specifically designed dye-derivatized PGAP.