Porous microbeads of low cross-linked polystyrene (PS) grafted
with poly(ethylene glycol) chains (PEG) are labeled at the free chain ends with
3-(1,6-diphenyl-1,3,5-hexatrienyl)propionic acid
(DPH-PA) and 1-(dimethylamino)naphthalene-5-sulfonic acid (DANS),
which probe the polarizability,
polarity, and viscosity of their environments. The beads are
investigated in a series of pure liquid phases
and in acetonitrile/water mixtures by electronic absorption and steady
state as well as time-resolved
fluorescence spectroscopy. The extent of solvation in the
polymer/liquid interphase is characterized from
spectral shifts by introducing a solvation fraction which quantifies
the relative amount of polymer and
liquid in the solvation shell. The rotational mobility of DPH-PA
is derived from time-resolved fluorescence
anisotropy measurements. Depending on the liquid phase, the
rotational correlation times vary strongly
between τR = 500 ps and 100 ns. The shortest
τR-values 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 frame, and in presence of water, which solvates PEG
but not PS and DPH. In contrast,
water is able to weakly solvate DANS chemically bound to the polymeric
frame.
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