Contactless, photoinitiated snap-through in azobenzene-functionalized polymers

Abstract

Photomechanical effects in polymeric materials and composites transduce light into mechanical work. The ability to control the intensity, polarization, placement, and duration of light irradiation is a distinctive and potentially useful tool to tailor the location, magnitude, and directionality of photogenerated mechanical work. Unfortunately, the work generated from photoresponsive materials is often slow and yields very small power densities, which diminish their potential use in applications. Here, we investigate photoinitiated snap-through in bistable arches formed from samples composed of azobenzene-functionalized polymers (both amorphous polyimides and liquid crystal polymer networks) and report ordersof-magnitude enhancement in actuation rates (approaching 10 2 mm/ s) and powers (as much as 1 kW/m 3 ). The contactless, ultra-fast actuation is observed at irradiation intensities <<100 mW/cm 2 . Due to the bistability and symmetry of the snap-through, reversible and bidirectional actuation is demonstrated. A model is developed to elucidate the underlying mechanics of the snap-through, specifically focusing on isolating the role of sample geometry, mechanical properties of the materials, and photomechanical strain. Using light to trigger contactless, ultrafast actuation in an otherwise passive structure is a potentially versatile tool to use in mechanical design at the micro-, meso-, and millimeter scales as actuators, as well as switches that can be triggered from large standoff distances, impulse generators for microvehicles, microfluidic valves and mixers in laboratory-on-chip devices, and adaptive optical elements.photochemistry | elastic instability T ransduction of light into work in photoresponsive polymeric materials does not require contact and can be remotely triggered potentially over long distances. The ease with which light can be manipulated spatio-temporally in intensity, phase, and polarization further distinguish this novel input stimulus in mechanically active systems. Prior demonstrations of photomechanical effects in polymeric materials have used both photothermal (1) and photochemical (2-4) mechanisms. Photochemical routes, in nearly all cases using the photoisomerization of azobenzene, are particularly interesting because they offer broader opportunities for modulating photomechanical responses with irradiation wavelength and polarization.The basis of light-to-work transduction (actuation) in azobenzenefunctionalized polymeric materials is the rate and efficiency of the isomerization/reorientation of the embedded azobenzene chromophores. The extent of azobenzene isomerization (efficiency) is considerably lessened by embedding this photochromic moiety in a polymeric material-evident in the work of Morawetz, which contrasts the kinetics and photostationary state concentration of azobenzene photoisomerization in dilute solution, plasticized, and bulk forms (5). Photoisomerization in polymeric materials becomes less efficient and slower with increase in storage modulus (6). Accordingly, pho...

Azobenzene-functionalized polyimides as wireless actuators

“…Upon irradiation, the force exerted by the films was recorded and reported as stress after normalization of the force by area of the film. The polyimide samples were also examined in a bistable actuator configuration to exploit limit-point instabilities (snapthrough) that generate power densities and rates relevant for end use applications in wireless actuation [32]. Additionally, the snapthrough response provides complementary information useful in further benchmarking the photomechanical responses of the polyimides.…”

“…The photomechanical response of the materials is visualized in cantilever experiments and quantified as photogenerated stress in tensile experiments. Extending upon a recent report [32], photoinitiated snap-through is examined in the azo-PIs to ascertain the influence of molecular design to photomechanical response in this promising geometry.…”

Modeling Defects, Shape Evolution, and Programmed Auto-Origami in Liquid Crystal Elastomers

“…In practice, we add dissipative forces to allow the sample to reach mechanical equilibrium; motion may thus be under-or over-damped. The capability to model momentum-conserving elastodynamics, and not just quasistatic relaxation, allows us to model snap-through shape transitions (Shankar et al, 2013;Smith et al, 2014) and other complex dynamical behavior.…”

Geometrically controlled snapping transitions in shells with curved creases

“…When these shells have multistable configurations, the transition between them is opposed by geometrically enhanced rigidity resulting from the dominant stretching energy. Often, even for relatively small range of deformation, stretching leads to the high forces and rapid acceleration associated with a "snap-through" transition in many natural and man-made phenomena (10)(11)(12)(13)(14)(15)(16)(17). For example, Venus flytraps (Dionaea muscipula) use this mechanism to generate a snapping motion to close their leaves (11), hummingbirds (Aves: Trochilidae) twist and rotate their curved beaks to catch insect prey (14), and engineered microlenses use a combination of bending and stretching energy to rapidly switch from convex to concave shapes to tune their optical properties (12).…”