Thermal Properties of Polymers and Hybrid Material Thin Films

Abstract

The technological progress of electronic devices is still enormous. Modern electronic devices already reached nanometer dimensions. Simultaneously, the computing power of these devices increases. This development implies enhancing power and heat densities. The cooling of these nanostructured devices is a challenging task of great interest. A fundamental understanding of heat transport on the nanoscale is necessary for optimizing thermal management. For a better understanding of thermal transport on the nanoscale, systematic investigations of structure-property relationships are required. For this purpose, model systems with precise structural and chemical control are essential. However, such nanostructured materials usually provide only small sample geometries. The small dimensions make thermal conductivity characterization quite challenging. For this work, a photoacoustic (PA) setup was implemented. With this method, it is possible to characterize thin films of a sample, which is supported by a substrate. Thus, small amounts of brittle samples that cannot be processed into freestanding films can be characterized. The capability of the PA method to determine the thermal properties of thin films was demonstrated by the investigation of three different polymer-based materials. The first two materials present the class of polymer nanocomposites (PNCs). Due to the miscibility on the molecular level, these can be also referred to as hybrid materials. The investigated PNCs can be divided by their filler geometry. First, the most straightforward filler geometry is investigated. Spherical Ag nanoparticles (NPs) are functionalized by polystyrene (PS) brushes of different molecular weights. The steric repulsion allows the adjustment of the interparticle distance (IPD) from 2 nm to 16 nm in the resulting PNC material. The ligand exchange used for this purpose is designed to be flexible. Thus, also different material combinations are viable with the presented procedure. Additionally, a new purification method is introduced, the centrifugation at θ-conditions of the ligand. The AgPS PNCs were processed into thin films and characterized regarding optical, mechanical and thermal properties. The individual distribution of the Ag NPs preserves the localized surface plasmon resonance (LSPR). Therefore, a laser can be used to locally heat up the sample within the laser spot. The higher the Ag volume fraction, the more significant the thermoplasmonic effect. Also the mechanical properties, determined by Brillouin light scattering (BLS) measurements, and the thermal conductivity depend on the Ag ratio. However, thermal conductivity could not be predicted by effective medium models without an finite interfacial thermal conductance. Accordingly, our findings suggest, that interfaces have an impact on thermal conductivity in this nanostructured material. The second filler type is characterized by two-dimensional extensions. The in-plane dimensions of these sheets are in the micrometer regime, while their thickness in cross-plane…