Effect of Fluorophobic Character upon Switching Nanoparticles in Polymer Films from Aggregated to Dispersed States Using Immersion Annealing

For nanoparticle (NP) polymer composites, the state of dispersion vs aggregation significantly affects optical, electronic, thermal, and mechanical properties. The switching of NP distribution states thus far was limited to polymer solutions or bulky polymer-grafted NPs. Herein, for the first time, NP distribution states within polymer films are switched by adjusting fluorophobic interactions and the enthalpy of mixing with immersion annealing. The fluorophobic effect is the tendency of fluorinated molecules to strongly phase-separate from non/less fluorinated molecules. A highly fluorophobic homopolymer, poly(perfluorooctyl acrylate) (PFOA), was combined with gold NPs of variable fluorophobic character, prepared using mixtures of small-molecule ligands (xF-NP, where x is the mol % fluorinated ligands). Low-to-moderately fluorophobic F-NPs with PFOA were aggregated after spin coating where film swelling via immersion annealing with moderately fluorophobic trifluoro toluene (TFT) generally led to a dispersed state. In contrast, the highly fluorophobic 100F-NPs were dispersed regardless of immersion annealing. This behavior was attributed to the PFOA acting like a surfactant to enable dispersion of highly fluorophobic NPs in TFT. Since these two distinct behaviors favor nonoverlapping ranges of xF-NP compositions, the NPs with intermediate compositions exhibited limited dispersibility. This fluorophobic switchability could enable time- and chemical-selective sensing of fluorinated compounds in the future.