Structure formation in macromolecular systems
Molecular self-assembly and formation of functional structures on surfaces
Nature has followed a large number of pathways in organising molecules for the creation of functional materials and systems. Extraordinary properties and functionalities are introduced by a hierarchical organisation at different length-scales.
Molecular processes of spontaneous self-organization, especially at interfaces, allow the creation of functional and controllable structures. In this context, long-range ordered supra-molecular structures with sub-molecular precision have to be mentioned. The understanding of organizational processes and their precise control over several length scales of time is essential in order to obtain operational building blocks of artificial functional systems.
Structure Formation of Coil-Rod Block Copolymers in Thin Films
The structure formation of a polystyrene-block-poly(γ-benzyl-L-glutamate) (PS-PBLGlu) with an amorphous and a crystallizable, helical segment has been investigated in thin films and compared to the situation in the bulk. Thin films were prepared by spin-coating from dilute tetrahydrofuran (THF) solution and subsequently annealed in saturated THF vapor to achieve a controlled crystallization of PBLGlu. Analysis of the films with scanning force microscopy (SFM) reveals a variety of structures with ordering on several length-scales, ranging from a few nanometers to microns.
(A) SFM height image and results of histographic analysis of a film of PS52-b-PBLGlu104 obtained by spin-coating from a 0.5 mg/mL THF solution and subsequent exposure to saturated THF vapor for 66 hours.
(B) Structure model
Characteristic AFM images obtained from about 40 nm thick films after exposure to vapours of good solvents:
Top: PS52-PBLGlu104
Bottom: PS63-(PBLGlu37)8
Evolution of Multilevel Order in Supramolecular Assemblies
The process of self-assembly at multiple length scales of bis-urea substituted toluene on a Au(111) surface was studied by low temperature scanning tunneling microscopy. Pattern formation is controlled by specific hydrogen bonds between these molecules but also by significantly weaker lateral coupling between the resulting supramolecular polymers and a quasiepitaxial interlocking with the substrate. The ordered assemblies exhibit a tunnel transparency. Our experiments indicate the necessity of multiple interactions of different strengths for obtaining ordered structures with hierarchical levels of organization.
Characteristic AFM-image obtained from a thin film resulting from dip-coating in a 10-4 molar toluene solution. The sample was exposed to toluene vapour at 60°C for 27 hours. | (a) High resolution STM image (5 x10 nm2, 0:4 V, 1.9 nA) with insets of a space filling model of EHUT; (b) schema of the possible commensurability relation between the supramolecular layer and the Au(111) reconstruction. (c) A side view of the adsorbed EHUT layer presented in (b). The scale of the z axis is amplified by a factor of 2. |
| STM image of an EHUT monolayer on Au(111) 20 × 13.5 nm2. The substrate with the herringbone reconstruction is visible in the lower part of the image. EHUT molecules self-organize via H bonds in supramolecular twin rows as depicted by the superimposed molecular model of the previous figure (c). bright features correspond to alkyl chains pointing out of the substrate plane in interaction to form a “zipper” line. The regular orange contrast along the molecule confirms the nonplanar position of the toluene core. | Top view of a section of a double row of EHUT molecules on the gold surface. Gold atoms are displayed in ochre, carbon in light blue, nitrogen in dark blue, oxygen in red, and hydrogen in white. In molecular modeling simulations, the ethyl-hexyl group can arrange so that the ethyl branch is adsorbed and the end of the hexyl branch is pointing away from the surface. |
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