Module 6: Macromolecules at interfaces and structured organic films (PHY6006)
Using the tendency of some macromolecules to order at interfaces to make inexpensive nanostructured coatings and substrates.
Above: Chemical patterning of a polymer film.
Böltau et al. Nature 391 877 (1998).
How can we use the self-assembling properties of macromolecules to make nanostructured surfaces and thin films? New technologies like soft lithography and controlled microphase separation will exploit self-assembly at surfaces to yield inexpensive and simple processing routes to devices such as:
- One-step smart coatings.
- Microfluidics.
- Printed polymer semi-conductor circuits.
- Smart substrates for crystal growth.
- Substrates for controlled interactions with living cells.
Outline syllabus
- Principles of Surface and Interface Chemistry: surface energy and tension, wetting, characterisation of surfaces and interfaces.
- Techniques for Manipulating Surfaces: adsorption of surfactants and macromolecules, physical grafting of macromolecules (including some common routes for chemical grafting), coating surfaces with thin films.
- Structured Coatings: surface segregation and self-assembly in films of blends and copolymers, films by Langmuir-Blodgett and sequential adsorption.
- Non-Lithographic Patterning Methods: microphase separation in copolymers, dewetting processes, microcontact printing, other uses of self-assembly for pattern creation.
Above: TEM of a lamellar structure
(see text right).
M. Geoghegan et al. Polymer 35 2019 (1994).
Surface-induced lamellar structure
The Transmission Electron Micrograph (TEM, left) is of a cross-section of solvent-cast film of a mixture of polystyrene (PS, light) and polybutadiene (PB, dark). The polymers demix during casting, and the surface triggers the direction of demixing. The film consists of alternating layers of polystyrene (PS) and polybutadiene (PB). The scale bar is 1 μm long.
Above: AFM of a triblock polymer.
H. Elbs et al. Macromolecules 32 1204 (1999).
Block copolymers
The self-organised structure of a triblock copolymer has been made visible in the AFM image (shown left) by exposing the polymer to a selective solvent for one of the components. The image is 1 μm wide, and the triblock film is about 25 nm thick.
Above: TEMs of a triblock copolymer;
the image to the left is as cast, and shows disordered material. After exposure
to solvent vapour the blocks separate to give an ordered lamellar structure
(right hand image). Scale bar is 1μm.
Images by: K. Fukunaga, UBE industries, Japan.
A similar triblock copolymer is shown in the TEMs above to order into a lamellar structure when exposed to solvent vapour.
Above: AFM image from the Krausch
group (Bayreuth) of channels in
a polystyrene film overlying a corrugated silicon substrate.
Rehse et al. Eur. Phys. J. E 4 69 (2001).
Topographic patterning
Above: The de-wetting processes can be used to create nano-channels, as shown here (from Rehse et al. Eur. Phys. J. E 4 69 (2001)).
Silicon with 5 nm high corrugations ~350 nm periodicity can be covered with thin polystyrene films (shown above). On annealing polystyrene de-wets the substrate and forms 'nano-channels'.
Above: AFM image from the Steiner
group (Groningen), lateral scale: about
50 μm; height scale: a few nm.
Böltau et al. Nature 391 877 (1998).
Chemical patterning
A chemical pattern on a substrate will strongly influence the morphology of an immiscible mixture placed on top of it, as shown in the AFM image left. In this a polymer film composed of a mixture of polystyrene (PS) and poly(vinyl pyridine) (PVP) has been deposited on a gold surface. In the lower half of the image the gold surface had been covered by a chemical pattern, resulting in an ordered phase separation with a linear structure. In the top part of the image the phase separation is disordered.
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