Advanced Synthesis of polymer brushes and their characterisation using X-Ray scattering

Abstract

Polymer brushes are a special form of polymer thin films and have widely been used to modify the surface properties of various materials. If polymer chains are densely tethered with one end to a supporting surface, intermolecular interactions appear and the chains straighten up to form a brush structure. Functional polymer layers, responsive surface coatings and colloid stabilisation are only a few applications, for which polymer brushes are promising systems. Although experimental research on polymer brushes has intensively been done over the last decades, their theoretical description is more advanced than experimental findings. One explanation to this phenomenon is the nontrivial analysis of the grafting density (numbers of chains per surface area) and the molecular weight of grafted chains, which predominantly define their structure and accordingly the thin film properties. In a majority of all publications on polymer brushes, the focus of characterisation is rather on their performance regarding different applications, than it is on a detailed characterisation of brush length or grafting density. The theoretical physical description of grafted chains has its origins in the pioneering work by Alexander and De Gennes, whose theories led to three different proposed states of polymer chains on surfaces, ranging from pancake, over mushroom to brush structures with increasing number of chains per surface area. Further investigations have been done theoretically and in experimental set ups, but the real structure of grafted chains and their dependencies on grafting density or molecular weight are yet unrevealed. Here the primary aim was a closer look on how to determine the structure of tethered polymer chains, using several characterisation methods, such as Atomic Force Microscopy (AFM), ellipsometry, X-Ray Reflectivity (XRR) and Grazing Incidence Small Angle X-Ray Scattering (GISAXS). As indicator for the transition from polymer brushes to mushroom or pancake structures, roughness correlation of polymer brushes was used, which is the ability of polymer thin films, to copy the roughness profile of underlying substrate surfaces. In this thesis, this phenomenon was found to be dependent on the number of chains per surface area and their length. In the first part of this thesis, a procedure was developed to synthesise polymer brushes homogeneously on silicon wafers via surface initiated ATRP. Silicon surfaces could be functionalised with (3-Aminopropyl)dimethylethoxysilane (APDMES) from gas phase and modified with bromoisobutyryl bromide to obtain an active monolayer of alkyl halide for Atom Transfer Radical Polymerisation (ATRP). Different monomers could be polymerised from this functionalised surface to synthesise polymer brushes. As proof, that diffuse scattering (GISAXS) is a powerful tool to analyse roughness correlation of polymer thin films, different samples were prepared, including polymer brushes and spincoated polymer films and analysed with the mentioned methods. All preparations and synthesis procedures were found to be reproducible with an adjustable layer thickness of the polymer layer. Published results about roughness correlation of spincoated PS films were confirmed with GISAXS, and as proposed, roughness correlation was removed after solvent vapour annealing. GISAXS experiments also proved roughness correlation of all polymer brush samples and even on brush-spin-coated multilayers, which was published for the first time. After roughness correlation was proven to be an intrinsic property of polymer brushes, it could be used to analyse 2D-gradient systems of PMMA brushes. This should answer the question at which grafting density and molecular weight the two interfaces are no longer correlated. Therefore, procedures were developed to gradually reduce the concentration of active bromide end groups on functionalised silicon wafers, by exchanging them for hydroxyl groups. This procedure enables a gradient in grafting density of polymer brushes. Orthogonally to this gradient, a varying molecular weight could be achieved with a dip-coating approach, by modifying the time the ATRP solution covered different parts of the wafer. Such a 2D-wafer was analysed at P03 beamline at PETRA III synchrotron at DESY Hamburg and GALAXY beamline in Julich. With these GISAXS experiments a dependency of roughness correlation on molecular weight and grafting density was shown qualitatively. With the molecular weight from Gel Permeation Chromatography (GPC) measurements and ellipsometric swelling experiments of the brushes in Dichloromethane (DCM) and acetone, brush length and grafting density were estimated. These experiments gave new insights on the properties of polymer brushes and the possibilities to analyse their structure. The thesis concludes in the development of a new light induced ATRP procedure to synthesise polymer brushes with visible light. At first, PMMA brushes were successfully synthesised with UV-light and 10-phenylphenothiazine, which also showed correlated roughness in GISAXS experiments, even without conservative copper catalysed ATRP. With Eosin Y as dye and N,N,N’,N”,N”-Pentamethyldiethylenetriamine (PMDETA) as electron donating molecule, the synthesis of PMMA brushes could be performed, using the same setup as before, but with a wavelength of 530 nm, which was the first time, this has ever been done.