Macromolecule-Substrate Interactions in Directed Self-Assembly
From Tailored Block Copolymers with Polyferrocenylsilanes towards Functional Nanoplatforms
For many possible applications of block copolymers, such as high density data storage, the positional control of microdomains is crucial. Understanding the mechanisms governing the ordering in phase separated block copolymers is therefore fundamental for designing usable systems. In this study, organicorganometallic block copolymers were used, in which the organometallic block was poly(ferrocenylsilane) (PFS). The presence of iron in the backbone of these polymers imparts properties such as etch resistance or catalytic activity to the polymers. Therefore, PFS containing block copolymers can directly be used as nanolithographic templates or for the fabrication of a catalytic platform of nanosized domains. Two different approaches were explored and are described in this thesis. The first approach involved the study of the effect of chemical and topographical modifications of a substrate on the morphology of one particular type of diblock copolymer. The second method involved the study of polymersubstrate interactions in diblock copolymers with different chemistry. Chapter 1 and Chapter 2 give a general introduction to the scope of the work and to phase separation in block copolymers. Particular attention was given to ordered morphologies in bulk and in supported films of asymmetric block copolymers, where spherical domains of one phase exist in a matrix of the other. A number of techniques that were used to probe different aspects of order are discussed. These techniques include structure characterization techniques, such as small-angle X-ray scattering and dynamic secondary ion mass spectrometry, as well as image analysis techniques used to quantify lateral order. The correlation between the information obtained by these different techniques, and how this information needs to be interpreted for block copolymers in bulk and in thin films is addressed in Chapter 2. In Chapter 3 through 5 the synthesis, characterization and a thin film study of amorphous, low-Tg block copolymers are described. The synthesis of amorphous PFS polymers and block copolymers, and their thermal properties in the bulk are discussed in Chapter 3. Amorphous PFS was prepared by copolymerization of small amounts of asymmetrically substituted ethylmethyl- silaferrocenophane monomer with dimethylsilaferrocenophane. The corresponding polyisoprene (PI)-b-PFS block copolymers showed no crystallization within the studied periods of time. Time-consuming annealing steps to obtain thermodynamic equilibrium were redundant in these copolymers, which makes them attractive for use in thin film applications. An important thermal transition for the choice of the annealing strategy is the order-disorder transition between an ordered phase-separated system and a disordered melt. The order-disorder temperatures were determined for the synthesized block copolymers and were shown to decrease with an increasing amount of incorporated asymmetrically substituted monomer. This dependence on the EM content was probably due to the slightly more aliphatic character of the PFS phase with increasing amount of EM. The thin film morphology of the PI-b-PFSs is discussed in Chapter 4. On flat substrates, the amorphous copolymers exhibited an intrinsic large-area ordering, extending over more than 1 micron, which is exceptionally large for block copolymers that generally have grain sizes of only a few hundred nanometer. In films of one monolayer of domains, a hexagonal packing of microdomains was found. A dynamic secondary ion mass spectrometry study revealed the structural transition of this hexagonal packing to a BCC morphology with increasing film thickness. The BCC morphology was oriented with the 110 plane parallel to the substrate throughout the entire film. Both blocks of PI-b-PFS were found at the polymer-substrate interface, while in PS-b-PFS the PFS wetted the substrate, rendering the substrate wetting for PI-b-PFS and PS-b-PFS qualitatively different. A study of graphoepitaxial alignment of microdomains (order directed by topographic features on a substrate) with PI-b-PFS is therefore of special interest, since the alignment effect in grapoepitaxy has so far been ascribed to the existence of a brush layer of just one phase at the substrate. Such a study is presented in Chapter 5. In grooves of one monolayer thickness, the position of the domains was fixed despite the neutral wetting condition. The position of the domains was successfully directed at larger distances from the side-walls in linear and hexagonal grooves of up to 1.3 μm width. The hexagonal pits demonstrated 2D alignment. In circular pits, the graphoepitaxial effect was absent. The strain imposed by curved sidewalls was absorbed by expansion or compression of microdomains near the edge. Chapter 6 presents the synthesis, characterization and thin film morphology of a different PFS containing block copolymer, to probe the polymersubstrate interactions in case of different chemistry. PFS-b-polylactide (PLA) was synthesized using a combination of anionic polymerization and catalyzed ring-opening polymerization. The PLA block constituted a distinctly more polar block. The resulting strong substrate-polymer interactions caused the formation of dewetted aggregates of PFS on top of a strongly substrate-adsorbed PLA layer in ultrathin films (< 10 nm). This polymer, the first example of a block copolymer consisting of an organometallic block and a biodegradable block, enabled the fabrication of two different types of patterns, which might serve as templates for arrays of functional nanodomains. In addition, the PLA enables soft etching methods, through the selective hydrolysis of the PLA. The PFS-b-PLA block copolymers were mainly amorphous and exhibited an orderdisorder transition around 135 _C. At higher temperatures, chain scission and transesterfication processes of the PLA block became more important, which influenced the thin film morphology. In Chapter 7 the use of PFS containing block copolymers in a catalytic application was explored. The formation of carbon nanotubes (CNTs) with controlled diameters fabricated from PS-b-PFS templated catalytic arrays is described. Multiwalled carbon nanotubes with a narrow distribution in diameter were obtained. The diameters could be tuned by varying the molar mass of the block copolymer precursor, using acetylene/hydrogen as the feed gas in chemical vapor deposition. The CNT diameters decreased with decreasing domain sizes. Preliminary results on amine end-functionalization of PFS are given as an outlook in Chapter 8. This functionalization enables new routes for the addition of particular chemical functionalities at the PFS chain end or other organic blocks. Also, specific motifs may be introduced at the junction between two blocks, with the aim of enhancing phase-separation.