MOLECULAR SOLVATION AND MOBILITY IN POLYMER LIQUID INTERPHASES - A FLUORESCENCE STUDY ON POLYSTYRENE-POLY(ETHYLENE GLYCOL) MICROBEADS/

Porous microbeads of low cross-linked polystyrene (PS) grafted with po ly(ethylene glycol) chains (PEG) are labeled at the free chain ends wi th 3-(1,6-diphenyl-1,3,5-hexatrienyl)propionic acid (DPH-PA) and 1-(di methylamino)naphthalene-5-sulfonic acid (DANS), which probe the polari zability, polarity, and viscosity of their environments. The beads are investigated in a series of pure liquid phases and in acetonitrile/wa ter mixtures by electronic absorption and steady state as well as time -resolved fluorescence spectroscopy. The extent of solvation in the po lymer/liquid interphase is characterized from spectral shifts by intro ducing a solvation fraction which quantifies the relative amount of po lymer and liquid in the solvation shell. The rotational mobility of DP H-PA is derived from time-resolved fluorescence anisotropy measurement s. Depending on the liquid phase, the rotational correlation times var y strongly between tau(R)=500 ps and 100 ns. The shortest tau(R)-value s are found in liquids which are able to solvate both the fluorophore and the polymer, e.g. toluene and acetonitrile. Long correlation times are observed in the presence of aliphatic hydrocarbons, which do not solvate the bead and therefore cannot penetrate into the polymeric fra me, and in presence of water, which solvates PEG but not PS and DPH. I n contrast, water is able to weakly solvate DANS chemically bound to t he polymeric frame.