Abstract
Micro- and nanoscale surface textures, when optimally designed, present a unique approach to improve surface functionalities. Coupling surface texture with shape memory polymers may generate reversibly tuneable surface properties. A shape memory polyetherurethane is used to prepare various surface textures including 2 μm- and 200 nm-gratings, 250 nm-pillars and 200 nm-holes. The mechanical deformation via stretching and recovery of the surface texture are investigated as a function of length scales and shapes. Results show the 200 nm-grating exhibiting more deformation than 2 μm-grating. Grating imparts anisotropic and surface area-to-volume effects, causing different degree of deformation between gratings and pillars under the same applied macroscopic strain. Full distribution of stress within the film causes the holes to deform more substantially than the pillars. In the recovery study, unlike a nearly complete recovery for the gratings after 10 transformation cycles, the high contribution of surface energy impedes the recovery of holes and pillars. The surface textures are shown to perform a switchable wetting function. This study provides insights into how geometric features of shape memory surface patterns can be designed to modulate the shape programming and recovery, and how the control of reversibly deformable surface textures can be applied to transfer microdroplets.
Highlights
There has been much effort to investigate the bulk formulation of SMPs, macroscopic deformation, and the bulk material recovery mechanism[2,11,12,13]
The thermoplastic SMP Tecoflex EG72D used in this work is a cycloaliphatic polyetherurethane block copolymer comprising two phase-segregated components: hard segments composed of methylene bis(p-cyclohexyl isocyanate) and 1,4-butanediol, and soft segments composed of poly(tetramethylene glycol) (Supplementary Fig. S1)[12]
Polyetherurathane-based SMP was thermally imprinted with permanent shape made up of micro- and nanoscale surface textures
Summary
There has been much effort to investigate the bulk formulation of SMPs, macroscopic deformation, and the bulk material recovery mechanism[2,11,12,13]. Chen et al.[15] exploited the shear deformation and recovery of high-aspect-ratio SMP micropillars to manipulate the surface wettability, whereby distinct wettabilities in the tilted state and the recovered vertical state were demonstrated, especially at larger pillar spacing. There is a lack of study on the effects of surface geometries on the deformation and recovery of SMP especially at the micro- and nanometre length scales. While studies of mechanically deformable surface patterns on polymers have been reported[24,25,26], there is a dearth of comprehensive experimentally-verified study on correlating geometric designs of surface textures with pattern deformation and recoverability It remains to be seen how length scale and pattern design of a structured surface influence the pattern deformation/recovery and contribute to the associated responsive functions, and the rational design of responsive surfaces remains a challenge. Rational geometric design of shape memory surface pattern could be applied to create a reconfigurable surface where the wettability could be tuned reversibly
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