Charakterisierung maßgeschneiderter makromolekularer Systeme: Analyse von photoschaltbaren Polymeren und Untersuchung der Wärmeleitfähigkeit von Polymeren und Polymernanokompositen
Characterisation of tailored macromolecular systems: Analysis of photoswitchable polymers and investigation of the thermal conductivity of polymers and polymernanocomposites
by Torsten Fornefeld
Date of Examination:2021-04-13
Date of issue:2021-04-23
Advisor:Prof. Dr. Philipp Vana
Referee:Prof. Dr. Philipp Vana
Referee:Prof. Dr. Marcus Müller
Referee:Prof. Dr. Michael Buback
Referee:Prof. Dr. Burkhard Geil
Referee:Dr. Tim Schäfer
Referee:Prof. Dr. Thomas Zeuch
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Abstract
English
In this work, tailored azobenzene-based polymers were synthesized and characterized with respect to their isomerization kinetics, micro structure and thermal conductivity. Additionally, the thermal conductivity of polymethylmethacrylate (PMMA), PMMA-b-polybutylacrylate (PBA) and a nanocomposite of PMMA and gold nanoparticles was determined to analyze relations between the materials structure and its properties. Originating from azobenzene, methacrylate-based monomers (AzoMA, AzoPMA, AzoHMA and AzoUMA) were prepared, which differ in the length of the linker between the carboxylic group and the azobenzene. The kinetics of the cis to trans isomerization in the photoswitchable monomers and the related polymers, synthesized by reversible addition–fragmentation chain transfer polymerization, was investigated in solution using time resolved UV/Vis spectroscopy. For the case of AzoMA it could be shown, that the activational energy of the isomerization decreases with the dipole moment of the used solvent. The length of the linker showed no influence on the activational energy. However, the reaction rate increases with increasing linker length. The obtained activational energies for the polymers are lower than those for the monomers. Moreover, the reaction rate of the isomerization is higher for the polymers in comparison to the correlating monomers. Thin films of photoswitchable polymers and block copolymers PAzoPMA-b-PBA were investigated using ellipsometry and atomic force microscopy. The film thickness was reversibly switchable. The block copolymers were characterized with regard to their self arranging structure. Time resolved investigation showed the transistion from an non-ordered state to a lamellar morphology of micro phase separation. A comparison of block copolymers with different length of the PBA block showed an increasing width of the PBA phase with increasing block length. Additionally, the parameters of the structure could be tuned by the addition of the correspondent homo polymers. Photoswitchable aggregates of PAzoPMA-b-poly dimethylaminoethylmethacrylate were prepared in propylene carbonate. These were filled with 8,2 wt.-% of crystal violet by photoswitching. The thermal conductivity of different macromolecular systems was determined using transient thermoreflectance. For PAzoPMA, a decrease in thermal conductivity of 60 % by photoswitching was obtained. On the basis of a series of PMMA samples, it was shown, that the thermal conductivity is independent from the chain length between molar masses between 3.6 x 10^3 and 1.6 x 10^5 g/mol. The thermal conductivity of PMMA could be increased by 25 % by the addition of 4.8 wt.-% of PMMA functionalized gold nanoparticles. The microphases of PMMA-b-PBA could be arranged in different orientation using individual annealing methods (thermal and solvent annealing in toluene). The thermal conductivity of the system was found to be anisotropic, with the thermal conductivity across the cylinders being 40 % higher than along the cylinders.
Keywords: polymer science; photo switchable; block copolymer; thermal conductivity