Vapor-Induced Negative Expansion of Porous Cross-Linked Polymers Observed by Optical Resonance

We report mesoporous cross-linked polymeric microspheres (PMS^1:0.5) that undergo negative expansion upon the uptake of organic vapor. The microspheres were prepared by emulsifying an oil phase containing ethylene glycol dimethacrylate (EGDMA) as a cross-linkable monomer, 2,2-dimethoxy-2-phenylacetophenone (DMPA) as a photoinitiator, (+)-limonene as a template, and Nile Red as a fluorescent dye, followed by photocuring and extensive washing. The resulting PMS^1:0.5 possesses mesopores with a Brunauer-Emmett-Teller surface area of 193 m² g^-1. Upon photoexcitation under a microscope, PMS^1:0.5 displays luminescence due to homogeneously dispersed Nile Red, which is confined within the particle via total internal reflection and exhibits whispering gallery mode (WGM) optical resonance in the photoluminescence spectra. The WGM peaks blue-shift upon exposure to n-hexane vapor, indicating a contraction of PMS^1:0.5 by -2.2% in diameter. Scanning electron micrographs and N₂ sorption isotherms further corroborate the shrinkage of PMS^1:0.5. The slight red-shift at lower vapor concentrations followed by an abrupt blue-shift at higher concentrations suggests that vapor condensation within the mesopores induces an inward Laplace pressure against the pore walls, which exceeds the elastic force of the polymer matrix and results in shrinkage.