Materials Science and Technology of Polymers at the University of Twente

Engineering of Surfaces with Organometallic Poly[ferrocenylsilanes]

The work described in this thesis deals with the use of organometallic poly(ferrocenylsilanes) (PFS) in surface engineering. Poly(ferrocenylsilanes) have a unique structure incorporating Si and Fe atoms in the main chain. The presence of inorganic elements provides unusual properties to the polymer, such as electrochemical activity and resistance towards reactive ion etching. The objective of the work presented here has been to use these responsive organometallic PFS polymers to fabricate thin films by various techniques, like chemisorption from solution, electrostatic layer-by-layer self assembly, or microcontact printing and to explore the properties and possible applications of these films. In Chapter 2 a general introduction about the use of polymers in surface modifications is given. Due to the redox active behavior of PFS we believed that they could function as electrode modifiers. Therefore the first part of this Chapter discusses the fabrication of polymer modified electrodes, the charge transfer mechanism in these electrodes and their most important applications. In the second part of this Chapter the use of polyelectrolytes for the preparation of thin film multilyer assemblies by the electrostatic layer-by-layer selfassembly is described. The structure-property relations and possible applications of the polyelectrolyte multilayer films are also treated. As microcontact printing was used in the experimental work, the principles of this technique are outlined in the third part of Chapter 2. A literature survey on the materials chemistry of organometallic poly(ferrocenylsilanes) is given in Chapter 3. The synthetic routes to obtain these polymers, their properties and possible applications are presented. Emphasis was given to the electrochemical properties of the PFS polymers as these are relevant to the work presented in this thesis. Chapter 4 presents the synthesis and characterization of end-functionalized poly(ferrocenyldimethylsilanes). Ethylenesulfide and trimethylenesulfide endfunctionalized PFS (ESPFS and TMSPFS) with different degrees of polymerization were prepared via living anionic ring-opening polymerization. Various techniques, such as proton nuclear magnetic resonance spectroscopy (1H-NMR), gel permeation chromathography (GPC), Fourier transform infrared spectroscopy (FTIR) and elemental analysis, were undertaken for the molecular characterization of the polymers synthesized. The fabrication by chemisorption from solution and the characterization of thin films of end-functionalized poly(ferrocenyldimethylsilanes) on gold are discussed in Chapter 5. Different techniques, such as contact angle measurements, FTIR, and x-ray photoelectron spectroscopy (XPS) were used to characterize the prepared films. Tapping mode AFM revealed a globular morphology of the films. Film thicknesses and adsorption kinetics were measured by surface plasmon resonance spectroscopy (SPRS). The electrochemical behavior of the films was studied by cyclic voltammetry (CV), chronocoulometry, chronoamperometry, and differential pulse voltammetry (DPV). According to results obtained by CV the films showed two reversible redox peaks indicating a stepwise oxidation of Fe atoms. This was interpreted by assuming effective electron interaction between closest ferrocene neighbors. Due to these interactions in the first oxidation wave neighbors in the second coordination spheres get oxidized first, which is followed by a full oxidation at higher potentials. Further electrochemical investigations proved that the two one-electron processes had different kinetics. DPV allowed us to determine that a small fraction of Fe atoms at the immediate metal surface was oxidized first at lower potentials. At higher potentials one third of the remaining redox centers was oxidized. A further increase of the potential was required to complete the oxidation. It was shown that the electrochemical behavior of the layers was influenced by the concentration and nature of the electrolyte. Chapter 6 deals with the study of electrochemically induced thickness and morphology changes in the end-functionalized PFS layers on gold. The Chapter begins with the principles of the techniques used, such as electrochemical atomic force microscopy (ECAFM), surface plasmon resonance spectroscopy, spectroscopic ellipsometry and x-ray reflectivity, all combined with electrochemistry. The electrochemical oxidation of poly(ferrocenylsilanes) resulted in charged polymer chains. It was anticipated that repulsive interactions between positively charged Fe atoms will change the conformation of polymer coils, and therefore increase the thickness of the films. ECAFM revealed that the morphology of the films changed as a function of the applied potential. SPRS and spectroscopic ellipsometry, both combined with electrochemistry, showed a dependence of the optical thickness of the films on the oxidation state. These techniques did not allow us to separate the contributions of the geometrical thickness and the refractive index to the overall optical thickness. However, the results suggested that the change in refractive index at different potentials had a minor contribution in comparison with the change in geometrical thickness. X-ray reflectivity combined with electrochemistry proved this finding and showed that the films were indeed thicker in the oxidized state. Chapter 7 describes the use of water soluble poly(ferrocenylsilane) polyanions and polycations for the construction of fully organometallic multilayer assemblies by the electrostatic layer-by-layer self-assembly technique. The film growth was monitored by UV-vis spectroscopy and ellipsometry. The absorbance and multilayer thickness were found to increase with an increasing number of bilayers. Cyclic voltammetry was employed for the electrochemical investigation on the multilayer assemblies. By integrating peak areas, an increasing ferrocene coverage was found with an increasing number of bilayers. PFS polyions were used to prepare patterned multilayer assemblies, which could act as etch masks resulting from the resistance of PFS towards reactive ion etching. In Chapter 8 it is demonstrated that organometallic PFS can be used as inks in soft lithography. Prior to microcontact printing, the PDMS stamp was surface treated to increase its surface energy and to promote the wetting properties by the PFS ink. Oxygen plasma treatment of PDMS stamps caused the formation of an oxide layer on the stamp surface and expansion of the stamp due to the heat generated by the plasma. When cooling to room temperature, the stamp relaxed and buckles formed on its surface. The stamp with the buckles formed could be used to transfer the corresponding patterns to Si by using PFS as inks, and etch the structures into the underlying substrate by reactive ion etching subsequently. Controlled order on multiple lengthscales was thus obtained.