Experimental study of hydrophobically modified amphiphilic block copolymer micelles using light scattering and nonradiative excitation energy transfer

The micellization behavior of a hydrophobically modified double tagged polystyrene-blockpoly(methaerylic acid) diblock copolymer, PS-N-PMA-A was studied in 1,4-dioxane-H2O mixtures by light-scattering and fluorescence techniques. This polymer was fluorescently tagged by a naphthalene moiety at the junction of the blocks and by anthracene at the end of the PMA block. The behavior of a single-tagged sample, PS-N-PMA, and low-molar-mass analogues were studied for comparison. Multi-molecular polymeric micelles with compact PS cores and PMA shells may be prepared indirectly by dialysis from 1,4-dioxane-rich mixtures as water is a strong selective precipitant for the PS block. In both types of micelles, the naphthalene tags are trapped in a nonpolar and fairly viscous core/shell interfacial region. The hydrophobic anthracene tags in PS-N-PMA-A are at the ends of the water-soluble PMA blocks and tend to avoid the bulk polar solvent, burying themselves into the shell. The collapse of a fraction of the PMA chains is an enthalpy-driven process,. but it is entropically unfavorable, and the distribution of the anthracene tags in the shell is a result of the enthalpy-to-entropy interplay. Measurements of direct nonradiative excitation energy transfer (NRET) were performed. on PS-N-PMA-A to estimate the distribution of the anthracene-tagged PMA ends in the shell. The experimental fluorometric data show that the anthracene tags penetrate deeply into the shell in water-rich solvents, although there is considerable fluctuation in the distance of closest approach to. the excited naphthalene. We find that the collapsed PMA chains and loops in the shell results in the counterintuitive effect that the hydrodynamic radius is significantly increased compared to. the corresponding unmodified PS-N-PMA.