Materials with exceptionally low thermal conductivity are desirable for thermal insulation and waste heat recovery. While foams and aerogels boast ultralow thermal conductivities akin to air, lack of mechanical stiffness in these soft materials necessitates a paradigm shift in materials design that can offer thermal insulation and mechanical rigidity simultaneously. Here, we show that spun-cast layered hybrid organic-inorganic perovskite thin films, azobenzene ethyl ammonium lead iodides, exhibit a record-low thermal conductivity, down to ∼0.04 watts per meter per kelvin at room temperature, while maintaining mechanical rigidity with an elastic modulus of 7.7 gigapascals that surpasses that of most plastics, foams, and aerogels. This unusual combination of ultralow thermal conductivity and high mechanical rigidity is attributed to the specially engineered organic cations in the layered structure. Our finding highlights the potential of molecular engineering in hybrid layered structures to push the extreme of thermal insulation in dense, rigid solids.

Achieving low thermal conductivity in nonporous polymer materials without compromising mechanical integrity remains a longstanding challenge. Conventional strategies, such as introducing porosity, are inherently limited in dense systems. Here, we demonstrate that thermal transport in polymer–organic filler hybrids is closely associated with the engineered availability of thermally accessible vibrational modes, rather than individual interfacial resistance or porosity. Using PU/organic filler hybrids as a model system, we show that incorporating rigid organic motifs shifts the vibrational density of states toward higher frequencies, where vibrational modes are weakly thermally populated at ambient conditions. This limits the number of heat-carrying channels, leading to suppressed thermal conductivity despite negligible individual interfacial resistance (~10⁻⁹ m² K W⁻¹). These findings provide initial evidence that vibrational mode engineering may offer a promising molecular-level route for reducing thermal conductivity in dense polymer-organic hybrid systems.

Molecular dynamics (MD) simulations were conducted to investigate the impact of hybrid wettability on thermal properties, specifically focusing on the constant volume molar heat capacity and thermal conductivity of Nano-confined liquid (NCL). The simulation domain, maintained at a temperature of 100 K, consisted of a∼ 3 nm—thin film of liquid argon entrapped between two solid copper surfaces with hybrid wettability. Hybrid wettability surfaces were produced by varying the solid–liquid interaction parameter and applying two different wettability factors (μ) to the same surface. In this study, key findings pertaining to the influence of hybrid wettability on heat capacity and thermal conductivity include: (i) The heat capacity of liquid confined within hybrid wettability surfaces surpasses the heat capacity of the liquid in its bulk form. …

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Equilibrium Molecular Dynamics (EMD) simulations are carried out to investigate the effects of wall wettability and nanogap thickness (h) on the constant volume molar heat capacity (Cv) of liquid argon entrapped between two solid copper walls at a fixed temperature of 100 K. Lennard-Jones (LJ) potential is employed to model the interaction between atoms, with nanogap thickness (h) varying between 0.585 nm to 5.85 nm. To characterize interfacial surface wettability as hydrophobic or hydrophilic, the solid-liquid interaction potential has been altered, and its effect on molar heat capacity (Cv) has been evaluated. Simulation results reveal three key findings. As the surface becomes more hydrophobic (i) both maximum heat capacity (Cv,max), and critical nanogap thickness (hc, the nanogap thickness at which maximum heat capacity is obtained) decrease; …

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