Topography from Topology: Photoinduced Surface Features Generated in Liquid Crystal Polymer Networks

摘要

Films subsumed with topological defects are transformed into complex, topographical surface features with light irradiation of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs). Using a specially designed optical setup and photoalignment materials, azo-LCN films containing either singular or multiple defects with strengths ranging from |½| to as much as |10| are examined. The local order of an azo-LCN material for a given defect strength dictates a complex, mechanical response observed as topographical surface features. The ability of biological systems to self-assemble materials into pre-programmed shapes in response to external stimuli is inspirational. Emulating the capability of self-assembling, stimuli-responsive materials with tailored functionality (actuation, shape change, surface manipulation, or other property changes) is of paramount interest to fields ranging from biomedical engineering1 to robotics.2 Liquid crystalline phases and defects play a major role in the self-assembly of biological materials, including the plasma membrane, wood, silk, and the insect cuticle.3 Here, we examine the response of complexly patterned photoresponsive liquid crystal polymer networks (LCN). The rich and diverse topographical features reported here are retiscent to shape adaptations and topographical surface manipulation observable in nature and could be useful in a range of applications including haptic displays, optics/photonics, flow control, or even catalysis. Liquid crystalline materials have repeatedly been demonstrated to exhibit a diverse range of stimuli-responsive behavior, such as negative thermal expansion and multi-stage phase transitions.7, 8 The responsive nature of liquid crystal polymers are enabled and controlled by the anisotropic orientation of the mesogenic moieties. Azobenzene-functionalized LCN (azo-LCN) materials have been the subject of considerable recent examinations as adaptive materials9 and shape memory polymers.19 Importantly for the work presented here, the directionality of the strain generated with a stimulus in aligned LCN materials (monodomain, twisted nematic, splay) is dictated by the director orientation of the material. For example, strain generated in a monodomain (nematic) LCN is primarily observed parallel to the alignment of the mesogens.16, 22 Offsetting the orientation of the director profile to the sample geometry (film or cantilever) results in out-of-plane deformation observable as twisting.10, 18, 20, 23 The work presented here was motivated by a series of recent papers24 in which Modes and Warner predict that defect-containing LCNs should exhibit mechanically adaptive responses highly distinguishable to any reports to date.24 The authors clearly show that sheets composed of glassy LCNs subsumed with a central topological defect and resulting director profile spanning the entirety of the film will result in a complex distribution of strain that concentrates at the defect. The clearest depiction of this is the predictions that subjecting glassy LCN materials patterned with a single +1 topological defect to an appropriate stimulus will cause the sheet to spontaneously morph into a cone with the center of the defect as the apex.25 The subsequent papers extended upon these results by introducing the design framework (so-called “grammar and vocabulary”28) surrounding the piece-wise use of topological defects (strengths |m| ≤1) as localized Gaussian curvature building blocks that could be assembled to create pre-programmed, shape-reconfigurable, non-developable surfaces from flat sheets. One of the salient features of liquid crystalline materials is the ability to readily organize the director profile into complex patterns. Most commonly, director patterning is facilitated by the use of photoalignment surfaces based on azobenzene materials. Domain profile patterning with photoalignment has been recently reported, such as in the fabrication of arrays of axial waveplates.3