title: Thermal Properties of Polymers and Hybrid Material Thin Films
Creators: name: Hummel, Patrick
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: Thermal conductivity
subject: Nanomaterials
subject: organic-inorganic hybrid composites
subject: Polymers
subject: Photoakustische Methode
subject: 540

description: 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 direction is only a few nanometers. When such nanoplatelets are aligned in stacks, their properties are strongly anisotropic, hence, direction-dependent. In a first study, different sizes of graphene oxide (GO) and sodium fluorohectorite platelets were compared regarding their in-plane and cross-plane thermal conductivity. The anisotropy ratio of the synthetic clay was found to be higher, due to the significant higher in-plane thermal conductivity. In a consecutive work, the sodium hectorite was processed into a layered hybrid material with perfect periodicity. The high anisotropy ratio could be further increased, by the enormous drop in cross-plane thermal conductivity. This drop is caused by the tremendous number of hard-soft interfaces in this direction. The measured cross-plane thermal conductivity of 0.09 Wm-1K-1 is extremely low for a completely dense material. The low cross-plane thermal conductivity was observed for all hybrid samples, regardless of the individual basal spacing, which varied from 1.9 nm to 3.8 nm. Despite the extreme polymer confinement and the nanolaminate structure, the density, specific heat, and in-plane thermal conductivity can be described by a classical parallel mixing model. The last material in this work handles the unique class of ampholytic polymers. The implementation of functional groups in a polymer can enhance interchain thermal transport. The investigated ampholytic polymers provide a hydrogen bond (H-bond) donor and acceptor group per repetition unit. The high amount of functional groups ensures a high density of intermolecular H-bonds. These bonds were characterized by infrared (IR) spectroscopy. The carbonyl stretching vibration around 1700 cm-1 was used to deduce the local H-bond environment. For a better resolution, Fourier self-deconvolution (FSD) was applied to this absorption peak. Within the ampholytic polymers, a correlation between the H-bond strength and thermal conductivity was observed. In summary, the results of this thesis underline the impact of interfaces and surface chemistry in polymers and polymer-based hybrid materials on the thermal transport properties. In polymer hybrids, the organic-inorganic interfaces provide significant thermal resistance. In polymers, the bonding strength of interchain interactions can improve thermal transport. Because of the great industrial importance of polymer-based materials, the results of this work are of great interest in the future development of thermal management applications.