3 search hits
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Nanostructure formation in thin polymer films
(2004)
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Nicolaus Rehse
- In the first part of this thesis an improved process is presented to prepare laterally structured substrates via hierarchical self organization. A miscut silicon surface annealed at 1400 K under ultra high vacuum conditions is used. The resulting facets are stable against oxidation and form a topographic pattern which can be further modified to a chemical pattern via evaporation of gold on every other facet. By controlling the time of annealing, we create structures with a reproducible mean width ranging from 40 to 400 nm. Despite the rather complex ultra high vacuum treatment and an additional evaporation step, we are able to produce substrates in a relatively short time (36 h). These substrates show a nanometer sized structure over an area of 0.5 cm². The automation of the cleaning process and a controlled heating during the annealing increases the yield of high-quality, stepped substrates. These structures allowed us to study the behavior of ultra-thin polystyrene films on topographically structured substrates. The film thickness of some nanometers is comparable to the radius of gyration of the polymers. The substrate corrugation causes a regular variation of the film thickness. We start with a homogeneous film, which is annealed above the glass transition temperature. During annealing the films are stable or form long polymer nanochannels, which lie in the grooves of the substrate structure. The balance of the radius of gyration and the film thickness controls the stability of the polymer film, while the corrugation only triggers the dewetting. The same behavior is found for films on flat substrates. Here small contaminations nucleate the formation of holes. Evaporation of gold stripes and their modification with self assembled monolayers leads to chemical patterned substrates. This expands the possibilities to manipulate the substrate wettability on the nanometer scale. The second part of the thesis describes the formation of ordered structures in block copolymer films. ABC triblock copolymers show a large variety of morphologies in thin films. We have shown that surface reconstructions play an important role in the structure formation process of these structures. In very thin films, where the film thickness is smaller than the long period of the polymer's micro domains, confinement effects overlap with the surface effects. The component with the lowest surface energy is accumulated at the free surface. It needs a subtle balance between the different surface energies (external fields) and the interaction of the three polymer blocks (internal fields) to create a surface reconstruction. This was shown by variation of the chemistry of the end block and by changing the sequence of blocks in the experiment. To analyze the surface reconstruction we used selective staining along with scanning electron microscopy, selective etching in oxygen plasma in combination with scanning probe microscopy, as well as quantitative TappingMode atomic force microscopy. Surface reconstructions of block copolymers show remarkable similarities with reconstructions of single crystal surfaces. In both cases the driving force for a rearrangement is the decrease in surface free energy of the ideal surface. A second analogy between the lamella forming SBM triblock copolymer and Si(100) is the fact that two non-equivalent layers of matter aligned parallel to the free surface lead to two different terminations at the surface. This shows that the phenomenon of surface reconstructions is not limited to classic crystals. The results of this thesis give new insights in the behavior of polymers at surfaces and in thin films. This gives the opportunity to create or manipulate nanometer sized structures accurately via self assembly, external stimuli, or a combination of both.
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Equilibrium and Dynamic Phase Behavior in Thin Films of Cylinder-Forming Block Copolymers
(2004)
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Armin Knoll
- The equilibrium and dynamic phase behavior in thin films of cylinder-forming block copolymers has been studied. The results range from an extension of an advanced strong segregation theory to the treatment of cylindrical microdomains in a thin film, over the detailed analysis of the phase behavior and the microdomain dimensions of a cylinder forming model system, to the first in-situ measurements of phase transitions and microdomain dynamics in a thin block copolymer film. The first result concerns the characterization of thin films of the model system, a polystyrene-b-polybutadiene-b-polystyrene (SBS) triblock copolymer, by tapping mode scanning force microscopy (TM-SFM). We present a procedure to establish reliably with TM-SFM the true surface topography of a soft polymeric sample. The measurement of an array of amplitude phase distance (APD) curves enables us to distinguish quantitatively between the ¡°real¡± surface topography and lateral differences in tip indentation. We find that conventional TM-SFM height images are not necessarily reflecting the surface topography of the sample. In the case of SBS we find that the surface is flat and that conventional TM-SFM height images only reflect lateral differences of tip indention. A theoretical treatment of the phase behavior in thin films of cylinder forming block copolymers within an advanced strong segregation theory (SST) framework of Olmsted and Milner is developed. Although the theory is strictly valid only for highly stretched chains and does not account for minority domain structures other than cylinders, the predicted results are in good agreement to the experimental results in this thesis. The microdomain structures under consideration are in plane oriented cylinders terminating with either the matrix component CP or a half cylinder CH at the surface and cylinders oriented perpendicular to the film CS. The thin film phase behavior of concentrated solutions of SBS in chloroform is studied experimentally. The stable phases are mapped as a function of film thickness and polymer concentration phi. The variation of phi can be interpreted as a variation of the molecular interactions between the two polymer components and between the components and the boundary surfaces. The preferential attraction of the majority component to the surface, the surface field, causes the cylinders to align parallel to the plane of the film, whenever the thickness fits an integer multiple of cylinder layers. At intermediate thickness the cylinders align perpendicular to the film plane. At higher polymer concentration, i.e., at stronger surface fields, a perforated lamella (PL) of polystyrene forms. The surface field needed for PL formation increases with increasing film thickness. A wetting layer exists underneath all films, which either consists of pinned molecules or of a half lamella. Furthermore the principal microdomain spacings of the thin film microdomain structure are investigated in detail and compared to the SST results. A new image analysis algorithm provides the recognition and the localization of the different structures in the SFM phase contrast images. The microdomain spacings are discussed as a function of various parameters like the film thickness, the polymer concentration and the local curvature of the structure. An examination of the dependence of the spacing in the C¨U structure from the local curvature of the cylinders yields an effect of second order, which can be explained in terms of density conservation of the polymer blocks. The spacing of the CP structure also depends on the local thickness of the thin film. The CP phase is stable in a certain thickness range close to its preferred thickness. Within this range it has to adapt the cylinder "height" to be commensurable with the film thickness. Minimization of the interface between the blocks leads to a smaller or larger lateral cylinder spacing if the cylinder has to stretch or shrink its dimension perpendicular to the plane of the film, respectively. Finally the first in-situ observation of phase transitions in thin block copolymer films is presented. During annealing with a controlled atmosphere of chloroform vapor the film develops islands and holes. The development of the microdomain structure and its spacial fluctuations are captured. The rather high polymer concentration in the thin film results in a time scale of the fluctuations in the order of seconds to minutes. The decreasing film thickness inside of the holes triggers phase transitions from CS to CP to PL according to the phase diagram of the system.
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Complex nanostructures in triblock terpolymer thin films
(2004)
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Sabine Ludwigs
- The thin film phase behavior of poly(styrene)-block-poly(2-vinylpyridine)-block-poly(tert-butyl methacrylate) (PS-b-P2VP-b-PtBMA) triblock terpolymers with volume fractions f(PS) : f(P2VP) : f(PtBMA) = 1 : 1.2 : x, with x ranging from 3.05 to 4, is studied with a combinatorial gradient approach. Gradients in film thickness are prepared via thin film flow coating of dilute solutions in chloroform. Upon controlled annealing in nearly saturated solvent vapor the films form terraces of well-defined step height. The dependence between morphology and film thickness is studied with optical microscopy, tapping mode SFM, and SEM. Though showing different morphologies in the bulk, the same sequence of surface structures is found with increasing film thickness for the whole range of compositions: a disordered phase in the thinnest regions, a liquid-like distribution of upright standing cylinders, cylinders oriented parallel to the film, and finally a hexagonally ordered perforated lamella structure (PL) on the first terrace with a thickness of d = (37+3) nm. Higher terraces also exhibit PL as surface structures. Due to the chemical nature of the block components and the particular stoichiometry of the polymer a wetting layer with a PtBMA-rich top layer is formed next to the substrate. By imposing an additional gradient in substrate surface energy, orthogonal to the gradually increasing film thickness, the perforated lamella is shown to be a stable phase, regardless of the chemical nature of the substrate, which makes this structure and methodology robust for application in nanotechnology. The complex phase behavior observed in thin films is supported by mesoscale computer simulations based on dynamic density functional theory. Thin films of the above mentioned triblock terpolymers are modeled as a melt of A3B4C12 Gaussian chains which is confined in a slit with film thickness H. By adjusting the interaction parameters between the polymer components and the surfaces, the experimentally observed sequence of surface structures as function of the film thickness can be successfully modeled. At well-defined film thickness the perforated lamella structure is formed. In analogy with earlier work on a two-component system these structures are identified as surface reconstructions of the bulk structure. In particular, the core-shell PL can be seen as analogue to the PL surface reconstruction of cylinder-forming AB and ABA systems. The influence of film thickness, surface field, and the interaction parameters between the different polymer components on the phase behavior is also explored. A large spectrum of surface structures is observed in analogy to the experiments. Further attention has been given to the perforated lamella structure. This structure can be visualized as P2VP/PS/P2VP lamellae which are perforated by channels of PtBMA interconnecting between two outer layers of PtBMA. A highly ordered PL structure could be prepared with a very small number of defects over an area of about 12 x 4 µm2. Because of the special functionalities of the triblock terpolymer a rather versatile nanostructure was produced. By selective UV-depolymerization of the PtBMA matrix phase, the PL phase might potentially be used for lithographic applications similar to the case of perpendicularly oriented poly(methyl methacrylate) (PMMA) cylinders in PS-b-PMMA block copolymer thin films. Furthermore, a responsive membrane can be created by selective removal of the matrix phase. The remaining PL has a P2VP shell which might be either switched via the pH-value or loaded with metal components. A polymer-analogous reaction of the matrix phase of the PL to poly(methacrylic acid) via acid-catalyzed hydrolysis leads to a pH-responsive nanostructure without altering the overall structure. With SFM in aqueous environment structural changes of the PL phase are studied as function of the pH-value. Upon changing the pH of the surrounding medium a strong swelling of the original film thickness is observed at pH-values > 6 to a maximum degree of 7.5-fold swelling. This swelling is explained with a conformational change of the matrix phase poly(methacrylic acid). The hexagonal arrangement of the pattern is not affected. The first two blocks PS and P2VP act as skeleton of the PL phase which withstands the mechanical forces exerted on the strongly swollen PMAA. In contrast to the PL phase core-shell cylinders oriented parallel to the interfaces cannot withstand these forces and are solubilised at high pH-values.
