Professor Sergiy Minko's Work Involves Nanostructured Responsive Materials

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Some applications include smart pores (membranes), responsive colloids and capsules, micro valves and pumps, environment responsive lithography, smart textiles, and hydrophilic/hydrophobic switches.

Responsive materials can be thin polymer films consisting of two different incompatible polymers chemically grafted to the same solid substrate. Such a layer is named a mixed polymer brush. The incompatible polymers build segregated nanoscopic phases. The phase segregation mechanism is strongly affected by the outside conditions (temperature, solvent, pH). Properties of the materials coated with the layer can be strongly alternated by applying external signals. For example, wetting behavior can be switched from hydrophilic to ultrahydrophobic with a self cleaning effect (Figure 1).

Figure 1. (a) Photograph of a water drop deposited onto the responsive surface: the stroboscopic image shows that a water drop jumps and rolls on the ultra-hydrophobic surface obtained after exposure of the sample to toluene. (b) In contrast, exposure to acidic water switches the sample to a hydrophilic state and the water drop spreads on the substrate.

Strong morphological transformations of responsive materials can be used for the fabrication of well ordered nanostructured functional materials for nanoscale devices and system architectures compatible with various operation environments and integrating across multiple length scales.

For example, supermolecular block-copolymer assemblies can be used for the fabrication of well ordered arrays of cylindrical nanodomains or nanopores which can be filled with electro luminescence molecules, metal clusters, or semiconducting particles (Figure 2). These materials are very promising for light emitting devices, sensors, memories providing the development of such challenging areas as nano-electronics, nano-photonics, and nano-magnetics.

Figure 2. Morphological transitions in the block-copolymer film upon an external signal.

The strategy to developing anisotropic materials with hierarchical architecture across multiple length scales will be a revolutionary way to improve products with special functional properties, which very often are difficult to combine. For example, there is a strong need for polymeric materials with a very high level of thermal and electrical conductivity on the one hand, which, however, retain their visco-elastic properties (e.g. sealing elastomers, glues, and printing plates) for electronics, lithographic processes, constructing glues and other materials, on the other hand. Different approaches, based on the application of carbon, metal or metal oxide particles as well as short conductive fibers, require a high proportion of the fillers/fibers in the material to reach the percolation threshold when the inclusions form a continuous (percolated) conductive net. Such approaches have frequently come into a contradiction with desired mechanical properties of the composite materials because of the high filling degree resulting in ridged and brittle behavior of the composites.

The desired percolation structures can be approached at a much lower weight fraction (less than 5%) of the conductive fillers using an appropriate size and shape of the conductive (nano)particles possessing a high surface-to-volume ratio. Furthermore, if the conductive particles are directed through a non-conductive matrix by a continuous minor phase their content can be further reduced.

To this end Professor Minko's approach is based on the application of electro-spun nanofibers decorated with metallic nanoparticles fabricated directly in situ on the fiber surface. The nanofibers serve as carriers for the conductive particulates. In fact the fibers, when introduced in a polymer matrix, form a cobweb-like continuous net of the electro/thermoconductive nanoparticles.

For more information about Professor Sergiy Minko and his research, please call him at 315-268-3807 or send email to sminko@clarkson.edu.