Observation of Highly Anisotropic Thermal Expansion of Polymer Films

Whiledimensional change under thermal loading dictates variousdevice failure mechanisms in soft materials, the interplay betweenmicrostructures and thermal expansion remains underexplored. Here,we develop a novel method to directly probe the thermal expansionfor nanoscale polymer films using an atomic force microscope as wellas confining active thermal volume. In a model system, spin-coatedpoly-(methyl methacrylate), we find that the in-plane thermal expansionis enhanced by 20-fold compared to that along the out-of-plane directionsin confined dimensions. Our molecular dynamics simulations show thatthe collective motion of side groups along backbone chains uniquelydrives the enhancement of thermal expansion anisotropy of polymersin the nanoscale limit. This work unveils the intimate role of themicrostructure of polymer films on its thermal-mechanical interaction,paving a route to judiciously enhance the reliability in a broad rangeof thin-film devices.