Thermal Transport in Two-Dimensional Polymers and Polymer Chains with Tacticity Control

New thermal conductivity studies of two polymer systems will be discussed.

Crystalline two-dimensional polymers (aka covalent organic frameworks) feature in-plane directional covalent bonds and cross-plane van der Waals bonding.¹ We have synthesized polycrystalline thin films of two-dimensional polymers with imine linker groups. Cross-plane and in-plane thermal conductivity measurements, made using frequency domain thermoreflectance and micro-platforms, provide the first experimental proof of anisotropy in thermal transport for these systems. Temperature-dependent measurements in both configurations suggest that phonon scattering due to polycrystalline grain boundaries may be a limitation to thermal conductivity in the film samples. Molecular dynamics simulations of imine-linked two-dimensional polymers confirm that higher in-plane thermal conductivity and higher anisotropy would be expected in pristine single crystals.

Tacticity refers to the arrangement and orientation of pendent groups along a polymer chain. Increased isotacticity is known to promote intermolecular crystallinity in polymers, which may impact their thermal properties.² Isotacticity may also directly impact the thermal conductivity of a single polymer chain by promoting long-range intramolecular crystallinity. Molecular dynamics simulations of polypropylene and polystyrene single chains were performed as a function of tacticity. We find that the thermal conductivity of isotactic and syndiotactic (alternating side group position) chains is higher than atactic chains. These findings suggest that tacticity control may be an important structural feature to consider in developing high thermal conductivity polymers.

(1) Evans, A. M.; Giri, A.; Sangwan, V. K.; Xun, S.; Bartnof, M.; Torres-Castanedo, C. G.; Balch, H. B.; Rahn, M. S.; Bradshaw, N. P.; Vitaku, E.; Burke, D. W.; Li, H.; Bedzyk, M. J.; Wang, F.; Brédas, J.-L.; Malen, J. A.; McGaughey, A. J. H.; Hersam, M. C.; Dichtel, W. R.; Hopkins, P. E. Thermally Conductive Ultra-Low-k Dielectric Layers Based on Two-Dimensional Covalent Organic Frameworks. Nat. Mater. 2021, 20 (8), 1142–1148. https://doi.org/10.1038/s41563-021-00934-3.

(2) Roy, R.; C. Stevens, K.; A. Treaster, K.; S. Sumerlin, B.; H. McGaughey, A. J.; A. Malen, J.; M. Evans, A. Intrinsically Thermally Conductive Polymers. Materials Horizons 2024, 11 (14), 3267–3286. https://doi.org/10.1039/D3MH01796F.