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Dynamically reconfigurable thin polymer films: impact on nano-motion

The need for building nano motors or engines, that could actively and independently manipulate small structures was called by the dramatic growth of nanotechnology over the last 20 years. Although the current abilities to move nano-objects are quite amazing, they still leave a plenty of space for searching a new methods for nano-manipulation. We have recently proposed to use specifically designed polymer systems that can guide the motion of nano-objects using switchable topographical patterns. These polymer systems have to match certain requirements: (i) their topography should be reversibly switched between two different states (e.g., flat and structured) by changing the external conditions, (ii) the topography switching should be accompanied by a change in surface energy, (iii) both processes should occur at the length scale of the size of nano-particles to be moved.

Motion of nano-objects on polymer brushes

One of such polymer systems that we have been investigated to move nano-spheres are diblock-copolymer- and mixed brushes (Figure 1). The polymer brushes consist of long polymer chains, typically several hundreds nanometers in length, with one end covalently attached to a surface. Special case of these polymer systems is so-called copolymer brushes consisting of two or three different polymers, in which phase separation can occur (Figure 1).

Figure 1. Schematic depiction of the two types of polymer brushes: (a) diblock-copolymer brush, (b) mixed brush; (c) and (d) illustration of the nanophase separation into a structured topography for a diblock copolymer brush.

The partial restriction of a motion of the grafted chains together with the tendency of the chains to microphase separate introduces additional complexity resulting in formation of nanopattern on the brush surface (Figures 1c,d). The size, shape and composition of the pattern depend on many parameters such as molecular weight of the polymers, grafting density, volume fraction of a single block, the immiscibility parameter (Flory-Huggins) of polymers, surface free energy of each polymers and environmental conditions. Depending on the quality of solvent used to expose the brush, different states of topography within the brush might be obtained. Remarkable, the switching of the topography morphology between different states can be performed over many cycles, so that one has a flexible thin polymer films sensing environmental changes and reacting through alteration of surface pattern. Motion of the nano-objects is imposed by fluctuation of the topography underneath the object during multiple exposure to different solvents. The change in the topography accompanies by the local change in the surface composition resulting in fluctuation of surface energy on a nano-scale. This imposes dynamically competing surface forces acting on nano-object, resulting in relocation of the object on the surface, as illustrated in Figure 2.

Figure 2. Scheme of a diblock-copolymer brush carrying a nano-sphere. During switching a topography of the brush from patterned (top) to flat (middle) and back to patterned (bottom), it might occur that the nano-sphere “hops” along the surface.

Magnetically switchable polymer nanomembranes

In this project an external stimuli is proposed to use to drive a motion of nano-objects. One example is the realization by introduction of an external magnetic field gradients. Since most polymers are dielectric and non-magnetic, a supporting substance is required responding to the external field and in turn inducing topographical changes in polymer film hosting the nano cargo (Figure 3). For this we use thin membrane made of an elastic polymer. The membrane consists of a thin, cross-linked polymer film, that is filled with magnetic material. This nano-composite system is called Magnetic Particle-Filled Polymer Film (MPFPF). MPFPFs are covalently bonded to surface contact areas on a silicon substrate, that is furnished with micro-channels (Figure 3).

Figure 3. Scheme of a “belt-conveyor” made of MPFPF for transporting of nano-objects.. (a) shows MPFPF covalently attached to a periodically structured substrate. Ferromagnetic nano-particles are shown as red spots. (b) The MPFPF is dilated by the magnetic field gradient, that can be periodically applied. The particles adsorbed on the MPFPF randomly (c) can be driven by means of a dynamically fluctuating force field induced during topography fluctuation