Thermal Transport in Nanostructured Polymers

In recent years, with the discovery of low-dimensional materials and the advance of fabrications, nano-structuring has opened new possibilities for a variety of novel applications with either increased or reduced thermal conductivity. Nanostructured organic materials or polymer-based materials have received much less attention compared to inorganic materials although there are significant application advantages, such as being easy to process, light-weight, and flexible. Extreme thermal conductivity and fundamental new physics of phonon dynamics and thermal transport might exist in nanostructured polymers, either polymers with aligned chains or hybrid organic-inorganic materials. The objective of this thesis is to investigate the thermal transport in nanostructured polymers by simulation and characterization. First, the effect of the chain parameters of polymers, such as chain orientation, backbone flexibility, monomer type, and molecular weight, on the thermal transport in polymers is investigated through molecular dynamics simulations. Thermal conductivity of amorphous polymers is related with the orientational order parameter (a quantitatively indicator of the chain conformations and alignments) through an exponential relation. Moreover, the thermal conductivity of single extended polymer chain of various polymers, which can be 1-2 orders of magnitude higher than their bulk counterparts, is a strong function of their monomer types and molecular chain lengths. The ultrafast pump-and-probe characterization system is then extended for measuring thermal properties of nanostructured materials, including in-plane and cross-plane thermal conductivity, heat capacity, and interfacial thermal conductance between materials. The measurement of the thermal properties of hybrid organic-inorganic materials enabled by the atomic/molecular layer deposition confirms that the backbone flexibility plays a critical role in the structural morphology and thermal conductivity in these films. These results also suggest that dramatic material difference between organic and inorganic materials may provide a route for producing materials with ultralow thermal conductivity. The effective thermal conductivity of the polystyrene thin films is measured to be increased with decreasing sample thickness compared to the radius of gyration of bulk polystyrene, which indicates a strong polymer chain confinement effect in ultrathin polymer films. The findings in this thesis could be useful for further understanding of nanostructured polymer materials for thermal management and energy conversion