Strained Polymer Thermal Conductivity Enhancement Counteracted By Additional Off-Axis Strain

Thin-film (5-20 mu m) polymer dielectrics are a critical component in high energy density capacitors. Understanding thermal transport in these materials is critical to addressing thermally enabled dielectric breakdown, a primary failure mechanism. Here, we measure the anisotropy in thermal conductivity for strained polymer films using frequency-domain thermoreflectance (FDTR), which provides us with unique sensitivity to thermal conductivity in the cross-plane and radial directions using a wide range of imposed modulation frequencies. We find that the anisotropy in the thermal conductivity is significantly enhanced by in-plane polymer alignment with strain in polypropylene films. Interestingly, this enhancement is then reduced by the application of additional strain in the orthogonal direction. We use insights from molecular dynamics simulations and Raman spectroscopy to understand the physical mechanism for this reduction.