Anisotropic Thermal Conductivity in Imine-Linked Two-Dimensional Polymer Films Produced by Interfacial Polymerization

Anisotropic thermal transport was measured in imine-linked two-dimensional polymer (2DP) films that were prepared by interfacial polymerization. Measurements of both in-plane (<i>k</i>_∥) and cross-plane (<i>k</i>_⊥) thermal conductivities relied on preparing free-standing 2DP films that were readily transferred for different measurement configurations. We polymerized two 2DP (Per-PDA and TAPPy-PDA) films at a liquid-liquid interface. These polycrystalline, imine-linked 2DP films are 100-200 nm in thickness and were measured by frequency domain thermoreflectance to extract <i>k</i>_⊥ and a suspended calorimetric platform technique to evaluate <i>k</i>_∥. We find that <i>k</i>_∥ is larger than <i>k</i>_⊥ in both materials at room temperature, leading to anisotropy ratios (<i>k</i>_∥/<i>k</i>_⊥) as high as 2.3. We attribute this behavior to the fact that the stiff, in-plane covalent bonds of 2DPs transport heat more effectively than the flexible, supramolecular cross-plane interactions. Variable-temperature measurements revealed a positive correlation between temperature and thermal conductivity, which we attribute to phonon scattering from grain boundaries and defects in the polycrystalline 2DP films. Molecular dynamics simulations of pristine crystals predict larger thermal conductivities and anisotropy ratios exceeding 7. The simulations suggest that as higher quality 2DP films become available, higher thermal conductivities and anisotropy ratios will also manifest.