Observation of highly anisotropic thermal expansion of polymer films

Abstract

While dimensional change under thermal loadingdictates various device failure mechanisms in soft materials, theinterplay between microstructures and thermal expansion remainsunderexplored. Here, we develop a novel method to directly probethe thermal expansion for nanoscale polymer films using an atomicforce microscope as well as confining active thermal volume. In amodel system, spin-coated poly(methyl methacrylate), we find thatthe in-plane thermal expansion is enhanced by 20-fold comparedto that along the out-of-plane directions in confined dimensions.Our molecular dynamics simulations show that the collectivemotion of side groups along backbone chains uniquely drives theenhancement of thermal expansion anisotropy of polymers in 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 abroad range of thin-film devices.