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-perpendicular to) 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 (Delta n). Following the findings, a new parameter, phi(perpendicular to), which can be used to estimate the D-perpendicular to of PIs, is proposed based on modified Lorentz-Lorenz and Vuks equations, and the validity of the parameter was evaluated. The phi(perpendicular to) is readily estimated from the experimental values of in-plane (n) and out-of-plane (n) refractive indices and the n(perpendicular to) of PI films. The positive and linear correlation between D-perpendicular to and phi(perpendicular to) 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-perpendicular to.