Reduction of the Thermal Conductivity of Polyurethanes by Fluorination: Impact of Crystallinity, Atomic Density, and Sound Velocity

The intrinsic thermal conductivity ($$) of polymers ranges between 0.13 W m⁻¹K⁻¹ in amorphous polyvinyl chloride to 60 W m⁻¹K⁻¹ in ultrahigh molecular weight polyethylene. Increasing the amorphous content of polymers to further lower $$ is insufficient as this approach reaches a practical limit at approximately 0.15 W m⁻¹K⁻¹. Inspired by the low $$ and low speed of sound of fluorinated liquids, we explored whether this behavior in liquids can be extended to polymers. We synthesized seven partially fluorinated (9%–17% atomic fraction F) and ten conventional polyurethanes. Fluorinated polyurethanes exhibit a reduction in $$ up to 50% compared to their nonfluorinated counterparts. Microstructural analysis revealed that the fluorinated polyurethanes exhibited reduced crystallinity and increased molecular spacing. Furthermore, we observed a decreased speed of sound in fluorinated polymers by forced Brillouin scattering via a new analysis method that captures weak signals from highly scattering semicrystalline polymers. The lowest thermal conductivity, 0.13 W m⁻¹K⁻¹ at room temperature, was observed in polyurethane synthesized from 2,2,3,3,4,4,5,5-octafluoro-1,6-hexanediol (16F) and isophorone diisocyanate (IPDI). Our study provides deeper insights into the relationship between $$, microstructure, and chemical structure, paving the way to rational design of polymers with thermal conductivity below the lowest limit of conventional amorphous polymers.