Thermal Transport in Poly(p-phenylene): Anomalous Dimensionality Dependence and Role of - Stacking

For heat conduction along polymer chains, a decreasein the axialthermal conductivity often occurs when the polymer structure changesfrom one-dimensional (1D) to three-dimensional (3D). For example,a single extended aliphatic chain (e.g., polyethylene or poly(dimethylsiloxane))usually has a higher axial thermal conductivity than its double-chainor crystal counterparts because coupling between chains induces stronginterchain anharmonic scatterings. Intuitively, for chains with anaromatic backbone, the even stronger & pi;-& pi; stacking,once formed between chains, should enhance thermal transport acrosschains and suppress the thermal conductivity along the chains. However,we show that this trend is the opposite in poly(p-phenylene) (PPP), a typical chain with an aromatic backbone. Usingmolecular dynamics simulations, we found that the axial thermal conductivityof PPP chains shows an anomalous dimensionality dependence where thethermal conductivity of double-chain and 3D crystal structures ishigher than that of a 1D single chain. We analyzed the probabilitydistribution of dihedral angles and found that & pi;-& pi;stacking between phenyl rings restricts the free rotation of phenylrings and forms a long-range order along the chain, thus enhancingthermal transport along the chain direction. Though possessing a strongerbonding strength and stabilizing the multiple-chain structure, & pi;-& pi;stacking does not lead to a higher interchain thermal conductancebetween phenyl rings compared with that between aliphatic chains.Our simulation results on the effects of & pi;-& pi; stackingprovide insights to engineer thermal transport in polymers at themolecular level.