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Tuning Mesoporous Silica Structures via RAFT Polymers: From Multiblock Copolymers as new Templates to Surface Modification

dc.contributor.advisorVana, Philipp Prof. Dr.
dc.contributor.authorSchmidt, Sonja
dc.publisherNiedersächsische Staats- und Universitätsbibliothek Göttingende
dc.titleTuning Mesoporous Silica Structures via RAFT Polymers: From Multiblock Copolymers as new Templates to Surface Modificationde
dc.contributor.refereeVana, Philipp Prof. Dr.
dc.description.abstractengIn this work, new procedures to tailor mesoporous silica thin films are presented. On the one hand, linear multiblock copolymers were utilized as structure directing agents for the synthesis of mesoporous silica thin films. On the other hand, the design of an interfacial architecture using the surface-initiated reversible addition–fragmentation chain transfer (SI RAFT) polymerization was accomplished which altered the transport properties of the mesoporous silica thin films. Linear multiblock copolymer systems were synthesized using polyfunctional RAFT agents. The dependence of their microphase separation behavior on the annealing technique was systematically studied using atomic force microscopy (AFM). Multiblock copolymers containing polystyrene (PS) and poly(n butyl acrylate) (PBA) segments showed cylindrical morphologies using thermal annealing. Thin films of amphiphilic multiblock copolymers consisting of PS and poly(N isopropylacrylamide) (PNIPAM) segments were annealed using selective solvents. Perpendicular PS cylinders arranged in a PNIPAM matrix were found for a volume fraction 0.59 < fPNIPAM < 0.77 using selective solvent mixtures of methanol-toluene or methanol-tetrahydrofuran. The formation of this morphology was independent of the segment number. Different morphologies than those observed using solvent mixtures were achieved using only one type of solvent. Furthermore, the formation of the self-assembled structure was less pronounced when increasing the film thickness. Additionally, it was found that the morphology of triblock copolymers using solvent annealing was inverted via subsequent thermal treatment. The observed inversion was caused by the stronger temperature-dependency of the surface energy of the PS segments. For the first time, these amphiphilic multiblock copolymers were used as structure directing agents in the formation of mesoporous silica thin films. A general process was developed using evaporation-induced self-assembly. The combination of solvent annealing upon film deposition and oxygen plasma treatment to remove the organic template was found to be an effective way to form mesoporous silica thin films. Scanning electron microscopy and AFM were used to visualize the porous structure of the films after template removal. The optimal sol aging time was found to be three days. Longer aging times led to a disappearance of the porous structure caused by condensation of the silicate species. Moreover, an effect of the amount of multiblock copolymers in solution on the organization of the film was observed. Higher amounts of multiblock copolymers promoted the self-assembly of the organic-inorganic interface and led to more pronounced organization. In addition, it was observed that the pore size depends on the number of segments in the multiblock copolymer. Multiblock copolymers with segment numbers greater than three arrange into a linear string of molecular micelles. Therefore, pore sizes smaller than 16 nm are accessible. Single flower-like intramolecular micelles are formed for multiblock copolymers containing less than four segments. Here, pore sizes larger than 16 nm are observed. To completely characterize the films ellipsometry and nitrogen adsorption-desorption measurements were performed. Ellipsometry revealed thicknesses below 40 nm and nitrogen adsorption-desorption isotherms indicated slit-shaped or particle-like interconnected pores. To exploit the influence of surface tethered polymer brushes on the transport properties of mesoporous silica thin film, a hybrid material containing the thermoresponsive PNIPAM was synthesized. Here, mesoporous silica thin films were modified with RAFT agent carrying silica anchor groups. These functionalized mesoporous materials were subsequently used in SI-RAFT of PNIPAM and systematically characterized. Ellipsometry measurements revealed the selective modification of the outer sphere of the mesoporous silica thin film. Furthermore, cyclic voltammetry measurements using redox probes were performed to give a better understanding of the temperature-induced switching ability of PNIPAM as a gating system. The results revealed that cationic probes could be shuttled across or be excluded depending on the temperature. Anionic redox probes were expelled independent of the temperature. The functionalization was proven to be successful and leads to thermosensitive cation-selective mesochannels. The unique transport properties of the hybrid interface could be observed due to the presence of the cooperative interaction between the gating of the PNIPAM layer and the anion entry exclusion exerted by the mesoporous silica
dc.contributor.coRefereeMüller, Marcus Prof. Dr.
dc.contributor.thirdRefereeGeil, Burkhard Prof. Dr.
dc.contributor.thirdRefereeBuback, Michael Prof. Dr.
dc.contributor.thirdRefereeMata, Ricardo Prof. Dr.
dc.contributor.thirdRefereeEhlers, Florian Dr.
dc.subject.engMultiblock copolymersde
dc.subject.engRAFT Polymerizationde
dc.subject.engMesoporous silica structuresde
dc.subject.engMicrophase separationde
dc.subject.engselective mesochannelde
dc.affiliation.instituteFakultät für Chemiede
dc.subject.gokfullChemie  (PPN62138352X)de

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