Bimodal Size Distribution of VO₂Nanoparticles in Hydrophilic Polymer Films for Temperature-Triggered Infrared Transmission Control

This work reports on a synthetic approach for enhanced thermochromic polymer films delivering vanadium oxide nanoparticles (VO2 NPs) in a bimodal particle size distribution: ∼50 and ∼300 nm. Monoclinic VO2 particles are the active ingredients in polymer coatings for temperature-triggered infrared (IR) transmission control. The bimodality promoted the dispersion of VO2 nanoparticles in the hydrophilic polymer coatings poly(4-vinylpyridine) (P4VP) and polyvinylpyrrolidone (PVP). The superior dispersion enhanced the coating’s IR switching efficiency (ΔTIR) (i.e., transmission change due to the temperature-triggered phase transition). The effects of VO2 NP concentration in the polymers as well as coating thickness on the optical properties of the coatings were systematically evaluated. The resulting VO2(M)/P4VP films exhibited good infrared (IR) transmission switching, ΔTIR@2500nm, of 47% with an adequate visible transmittance, Tvis@680nm, of 41%. Hydrophobic VO2/poly(methyl methacrylate) (PMMA) films gave a ΔTIR@2500nm of 30% and a Tvis@680nm of 29%. The superb performance of the VO2(M)/P4VP coating is ascribed to the uniform dispersion of VO2(M) nanoparticles within the matrix.