Molecular Structure Dependence of Out-of-Plane Thermal Diffusivities in Polyimide Films: A Key Parameter for Estimating Thermal Conductivity of Polymers

The dependence of thermal diffusivity along the out-of-plane direction (D ⊥) on molecular structure, chain orientation, and molecular packing was extensively investigated for 21 kinds of aromatic and semiaromatic polyimide (PI) films using temperature wave analysis. The inherent structural character and the orientaion of polymer chains were quantitatively evaluated based on the polarizability anisotropy calculated for the repeating units of PIs using the density functional theory (DFT) and the observed in-plane/out-of-plane birefringences (Δn). Following the findings, a new parameter, Φ⊥, which can be used to estimate the D ⊥ of PIs, is proposed based on modified Lorentz−Lorenz and Vuks equations, and the validity of the parameter was evaluated. The Φ⊥ is readily estimated from the experimental values of in-plane (n || ) and out-of-plane (n ⊥) refractive indices and the Δn of PI films. The positive and linear correlation between D ⊥ and Φ⊥ revealed that not only an increase in the degree of molecular orientations along the out-of-plane direction, but also a densification of molecular packing is effective for an enhancement of D ⊥.