Fluorescence Studies on a Thermoresponsive PNIPAM-Polyfluorene Graft Copolymer

The fluorescence behavior of a PNIPAM-grafted polyfluorene copolymer (PF-PNIPAM) together with the corresponding precursor polymer (9,9-bis­(ω-carboxypentyl)-substituted polyfluorene, PF-COOEt) was investigated by UV–Vis spectroscopy and steady-state and time-resolved fluorescence techniques in DMF and in an acetone:water (1:2) mixture over the temperature range 5 to 70 °C. Temperature dependence plots show a clearly different behavior in the acetone:water (1:2) mixture, which, based on the overall data, is attributed to the occurrence of a lower critical solution temperature (LCST) for PF-PNIPAM, which is found at ∼45 °C, different from the value for PNIPAM chains in water of ∼32 °C. In the acetone:water (1:2) mixture, the weak temperature dependence of the fluorescence decay time components between 20 and 70 °C documents a dominant intra- and interchain hopping-type deactivation mechanism. However, additionally, the thermoresponsive PNIPAM side chains cause aggregate formation at the LCST temperature coupled to abrupt changes of the fluorescence intensity. Dynamic light scattering (DLS) studies for PF-PNIPAM also in the acetone:water (1:2) mixture in the temperature range of 20–55 °C show for T > 40 °C a unimodal size distribution peaking for diameters of >200 nm corresponding to aggregates, contrasting to a bimodal distribution with lower hydrodynamic radii at lower temperatures. The study delivers additional information to the puzzling LCST behavior of PNIPAM-based (co)­polymers, which in PF-PNIPAM impacts the presence of both polyfluorene PF main chains and PNIPAM side chains.