Size effect on the thermal conductivity of ultrathin polystyrene films

Ultrathin polymers have recently been shown to exhibit different chain dynamics and physical properties, as compared to their bulk counterparts. In this work, we measure the effective cross-plane thermal conductivity of polystyrene (PS) thin films using the ultrafast laser-based frequency-dependent time-domain thermoreflectance method. The effective thermal conductivity of PS films is found to increase with decreasing film thickness when the thickness of the PS film is comparable to or smaller than the radius of gyration of bulk PS. By correlating the effective thermal conductivity with the ratio of the film thickness to radius of gyration, we find that this increase is mainly due to the increase of interfacial thermal conductance between PS thin film and the substrate, which likely results from the changing chain structure near the PS/substrate interface.