In a polymer blend, what effect does phase separation typically have on its thermal conductivity?

When a polymer blend separates into distinct phases, each phase has its own thermal conductivity, and the interfaces between them scatter heat‑carrying phonons. This scattering usually lowers the overall thermal conductivity compared to a perfectly mixed blend because heat cannot travel freely across the boundaries. If the high‑conductivity phase does not form a connected network, the material behaves like a composite with many insulating barriers, and the conductivity drops. For example, a blend of polyethylene (high conductivity) and polypropylene (lower conductivity) that separates into isolated polyethylene droplets will conduct heat less efficiently than a homogeneous mixture, because the droplets are surrounded by a less conductive matrix. Thus, phase separation typically reduces the thermal conductivity of a polymer blend unless the high‑conductivity phase percolates through the material.