Thermoresponsive Fluorescent Polymers: Influence of Size, Composition, and Architecture

Abstract

Thermoresponsive fluorescent polymers (TFPs) provide a versatile platform for optical temperature sensing by coupling phase transitions with changes in fluorescence. Here, we report TFPs based on poly(N-isopropylacrylamide) (PNIPAM) and poly(ethylene glycol methacrylate) (PEGMA), incorporating fluorescein acrylate (FluA) as a model fluorophore via reversible addition-fragmentation chain transfer (RAFT) copolymerization. Linear and hyperbranched architectures were synthesized, with different compositions to position cloud point temperatures (Tcp) within a physiologically relevant temperature window, and their fluorescence behavior was investigated as a function of temperature, concentration, and pH. Distinct responses were observed across the Tcp, governed by the interplay between polymer collapse, microenvironmental confinement, and chromophore-chromophore interactions. Systems with low fluorescein loading exhibited fluorescence enhancement near the Tcp followed by quenching at higher temperatures, whereas higher loading systems showed predominantly decreased fluorescence intensity upon heating. Because all samples were compared at equal mass concentration, raw emission intensity scaled primarily with fluorophore loading per chain; we therefore interpret intensity differences cautiously and emphasize the temperature-dependent response as the diagnostic readout. Fluorescence intensity was strongly concentration-dependent, and pH studies revealed changes in both fluorescence intensity and emission maxima, with temperature-responsive fluorescence most prominent at pH 7 and moderately retained at pH 9. Rheological measurements further demonstrated changes in complex viscosity across the Tcp, linking macroscopic behavior to structural transitions. Hyperbranched polymers displayed responses distinct from their linear analogues. We summarize these observations as a set of provisional design rules that link each structural variable, such as composition, fluorophore loading, and architecture, to its dominant effect on Tcp, fluorescence, and viscosity. We note that several of these variables co-vary across the present sample set. Trends are therefore interpreted within controlled compositional windows rather than as fully orthogonal factors, and the resulting rules are intended as practical guidelines for designing polymer-based optical thermometers.