PT Unknown
AU Altinpinar, S
TI Characterization of Photocleavable Block Copolymer Ultra-Thin Films
PD 07
PY 2018
LA en
AB Block copolymers have been widely studied in recent years because of their ability to form defined nanostructures with different morphologies and adjustable periodicity. This ability is based on the fact that covalently linked polymer blocks are arranged in well-ordered domains due to both positive enthalpy of mixing and small entropy of mixing. This process is called microphase separation and the molecular size of the individual polymer chains determines their separation distance. In addition, macrophase separation can be avoided, leading to thermodynamically stable nanostructured microdomains.
The investigation of highly-ordered thin films is one of the outstanding fields of research in the context of block copolymers since they enable a versatile self-assembled morphology in the range of 5-50 nm by means of a “bottom-up” approach. They promise applications such as polymer membranes or templates for nanostructured materials. Among the block copolymer structures, vertical cylinders have received great attention because of their ability to produce highly-ordered nanopores. Nanopores can be formed from a thin block copolymer film by removing one block by selective etching or by dissolving a polymer block. In this study, polystyrene-block-poly(ethylene oxide) diblock copolymer is used with ortho-nitrobenzyl ester (ONB) as a photocleavable block linker (PS-hν-PEO). The pore material PEO is washed out by a suitable solvent after the two blocks are photocleaved applying mild UV radiation. This results in an arrangement of nanopores which are decorated with functional groups, leading to materials for adsorption or filtration purposes.
In the first step of the work, the efficiency of the photocleavage was investigated in solution by FCS measurements in order to determine the exposure time with UV at 365 nm for a quantitative cleavage of the two polymer blocks.
In the following experiments ultra-thin polymer films are prepared on top of a silicon substrate and the desired arrangement of the films is then generated by solvent vapor annealing. The adjustment of the solvent vapor is of great importance in the treatment in order to manage the forces at the substrate/polymer film interface and to obtain cylinders perpendicular to the sample surface. For the characterization of the thin films, microscopic methods were combined with X-ray scattering methods to represent both the surface and the internal structure of the films.
Further work deals with the removal of the pore material by washing with a suitable solvent. After the films were cleaved with UV, the kinetic behavior of the films was identified towards the two solvents ethanol and ultra-pure water. A washing protocol was prepared for both solvents and the effect of the solvent nature on the internal as well as the surface structure was verified. For this purpose, grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) were performed.
Based on this, the treatment of the polymer film during the washing process was observed in real time in order to draw conclusions about the dynamics of the removal process. In-situ X-ray reflectivity measurements provide statistically significant information about the change in the layer thickness as well as the roughness of the polymer film during pore formation. By in-situ AFM measurements, the structure analysis of the ultra-thin block copolymer films was completed and thus the temporal effect of the washing process was elaborated.
This thesis deals with experimentally advanced analytical techniques, which have been extended with regard to their application in the field of thin films of block copolymer research. As a result, the approach of photocleavable block copolymer thin films was put under scrutinity and evaluated regarding its feasibility.
ER