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XX5.07 - Liquid Crystal Networks: From Responsive Surfaces to Switchable Nanopores 
April 23, 2014   10:30am - 11:00am

In-situ photopolymerization of liquid crystalline (LC) monomers in their aligned state has proven to be a valuable technique for the formation of well-ordered polymer networks in a single-step process. Their anisotropic properties led to a variety of applications in optics, electronics and mechanics. The use of light to initiate polymerization enables lithographic approaches for patterning. The LC behavior enables formation of complex morphologies on molecular level. Controlling the director profile of an LC network film in transversal direction gives geometrical morphing upon minor changes in order parameter. Examples of suited profiles of molecular orientation are twisted or splayed director configurations tied up in the network configuration. Reversible order parameter changes can be induced by a variety of means. It can be simply induced by temperature changes resulting in gradients in thermal expansion over the cross-section of the film. But more sophisticated and of interest for applications is a light induced change as a result of the E-Z isomerization of a built-in azo group. In conjunction with the change in order parameter the density of the LC network changes. When this density change is confined to a small volume and the film is attached to a solid, high modulus, substrate this results in localized formation of protrusions. In other words the films have switchable surface topographies which size and shape can either be controlled by mask exposure or by a pre-set director pattern.Rather than composing the monomers of covalent bonds alone, one can chose to replace some bonds by secondary interactions such as hydrogen bridges, thus providing responsive molecularly organized hydrogels. We applied the H-bridge based dimerization of benzoic acid to form nematic LC acrylate monomers. By a controlled and reversible rupture of the H-bridges mechanical responses can be initiated. Photopolymerization of smectic LC monomers lock in structures of different length scales. The first length scale is the resolution of lithography, and goes down to a few micrometers. The second length scale is set by the spacing of smectic layers and is typically a few nanometers. The third length scale is the intermolecular distance in the layers, usually around 1 nm or below. By modifying smectic molecules with H-bridges the smectic periodicity can break-up into separated layers with a well-defined spacing, also in the nanometer range. The nanopores form by breaking the H-bridges at elevated temperatures or by contact with an alkaline solution. The integrity of the film is maintained by copolymerizing with fully covalent smectic crosslinkers.The size of the nanopores can be controlled by changing the pH or by photochemistry at the crosslinking bridges.

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