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

The intrinsic thermal conductivity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>Λ</mml:mi> <mml:annotation>$\text{\ensuremath{\Lambda}}$</mml:annotation></mml:semantics> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>Λ</mml:mi> <mml:annotation>${\Lambda}$</mml:annotation></mml:semantics> </mml:math> is insufficient as this approach reaches a practical limit at approximately 0.15 W m^-1K^-1. Inspired by the low <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>Λ</mml:mi> <mml:annotation>${\Lambda}$</mml:annotation></mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>Λ</mml:mi> <mml:annotation>${\Lambda}$</mml:annotation></mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>Λ</mml:mi> <mml:annotation>${\Lambda}$</mml:annotation></mml:semantics> </mml:math> , microstructure, and chemical structure, paving the way to rational design of polymers with thermal conductivity below the lowest limit of conventional amorphous polymers.