Poly[ferrocenyldimethylsilanes] at the Interface of Chemistry and Material Science Synthesis, Structure-Properties and Thin Film Applications
The work described in this thesis concerns the synthesis, characterization, and properties study of ferrocenyldimethylsilane homopolymers and block copolymers. Due to the presence of iron and silicon in the polymer main chain, these macromolecules possess characteristics that are very distinctive from common organic polymers. The objective of this thesis was to investigate and combine the unprecedented features of these organometallic polymers with block copolymer self-assembly, as discussed in Chapter 1. A brief introduction was presented in Chapter 2 in topics that were relevant for this thesis. Attention was given to crystallization and melting behavior of semi-crystalline polymers. Furthermore, block copolymer self-assembly was described with special interest in thin films of block copolymers, blends of block copolymers with corresponding homopolymer, and conformational asymmetry in block copolymers. The foundation of the techniques that were employed throughout the work was also presented. The anionic ring-opening polymerization of strained dimethylsilylferrocenophane allowed the synthesis of well-defined polymers with targeted molar masses. Chapter 3 describes the anionic synthesis, characterization, and thermal behavior of a range of ferrocenylsilane homopolymers. These semi-crystalline macromolecules displayed a meltingrecrystallization process during heating from the crystallized state. The reorganization process involved the melting of thin lamellae that rapidly crystallized into thicker ones, which melted subsequently at somewhat higher temperatures. The occurrence of this reorganization process was investigated by differential scanning calorimetry experiments that displayed multiple melting transitions during a heating scan from the crystallized state. Results from small-angle x-ray scattering measurements complemented these conclusions. In Chapter 4, the action of reactive ion etching on poly(ferrocenylsilane) was discussed. Treatment with an oxygen plasma resulted in the formation of a thin oxide layer, rich in iron and silicon, at the surface of the polymer. Due to the formation of this non-volatile oxide layer, the etch rate of this organometallic polymer was much lower compared to the etch rate of organic polymers. In addition, the non-volatile oxide layer was also very resistant towards fluorocarbon plasma treatment. It was demonstrated that substrates, like silicon and silicon nitride, were selectively removed by the action of a fluorocarbon plasma while the iron-silicon-oxide remained. Chapter 5 described the synthesis and self-assembly of styrene-block-ferrocenylsilane copolymers. Block copolymers were synthesized by the sequential anionic polymerization of styrene and silylferrocenophane that resulted in near monodisperse materials with tailored compositions and molar masses. Upon phase separation, these block copolymers as well as their blends with homopolymer formed morphologies such as spheres on a bcc lattice, hexagonally packed cylinders, double gyroid, and alternating and perforated lamellae. Rheological experiments were performed to give insight into the order-disorder transitions. Thin films, on solid substrates or freestanding, of a block copolymer of styrene and ferrocenylsilane were studied which was discussed in Chapter 6. This diblock formed a cylindrical morphology in the bulk, consisting of hexagonally packed ferrocenylsilane cylinders in an organic polystyrene matrix. In thin films on solid silicon substrates, the surface free energies and the affinity for the substrate of the two blocks induced a parallel orientation of the domains with respect to the substrate. The films became frustrated when the film thickness was not commensurate with the equilibrium domain spacing. This resulted in a change in the domain spacing. However, if the frustration was large enough, the film formed islands or holes after longer annealing times. In addition, a change in morphology was observed after extensive annealing times, which resulted in the disappearance of the surface relief structures. The microdomain morphology in thin films was visualized by atomic force microscopy, after the selective removal of the organic matrix phase by means of an oxygen plasma etch. A transition was observed from parallel oriented cylinders to hexagonally packed domains when the annealing time was exceeding several days. Asymmetric isoprene-block-ferrocenylsilane copolymers were considered in Chapter 7. Three diblocks were synthesized, consisting of ferrocenylsilane volume fraction of 20, 24, and 28, respectively. All three copolymers exhibited a cylindrical structure in the bulk. Thin films of these copolymers displayed unique morphological changes for each composition as a function of film thickness. The block copolymer consisting of 28 vol% cylinders displayed a wormlike structure for all film thicknesses. When the film thickness was below the domain period, holes were formed so that the remaining film still exhibited the wormlike morphology. The diblock with 24 vol% of the organometallic block also displayed a wormlike structure, unless the film thickness was approximately equal to the domain spacing. In that case also areas containing hexagonally packed domains were observed. For the diblock containing 20 vol% ferrocenylsilane, a film thickness approximately equal to the domain spacing, resulted in a surface morphology completely composed of the hexagonal structure. For thicker films of all three compositions, a wormlike morphology was observed at the surface. The crystallization of the ferrocenylsilane block occurred even at room temperature. This resulted in the slow development of large hedritic features. The use of ferrocenylsilane polymers combined with self-assembly as a strategy towards nanostructuring surfaces was described in Chapter 8. As thin organic-blockorganometallic copolymer films formed lateral patterns on solid substrates, they can be employed to serve as lithographic masks with dimensions of sub-100 nm level. Oxygen reactive ion etching selectively removed the organic material and converted the organometallic material into a non-volatile oxide. A subsequent fluorocarbon plasma treatment etched the unexposed substrate areas whereas the organometallic nanodomains were insensitive towards removal. It was demonstrated that the thin block copolymer films could be used as templates for nanolithography.