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Ware T.H.,Air Force Research Lab | Ware T.H.,Azimuth Corporation | White T.J.,Air Force Research Lab
Polymer Chemistry | Year: 2015

The ability to program the local mechanical response of liquid crystalline polymer networks has been shown to generate complex mechanical responses. A facile two-step method to synthesize these anisotropic materials to realize either reversible or irreversible shape change behavior is reported. The first reaction is the addition of a nematic diacrylate to a primary amine to build macromers within a liquid crystal alignment cell. Subsequently, these macromers are crosslinked to trap the order of the liquid crystal into a crosslinked film. In unaligned samples, mechanical reorientation of the nematic director is used to isothermally program shapes at room temperature that can be recovered on heating. Under a load, the mechanically aligned materials exhibit tensile actuation behavior comparable to human skeletal muscle in stroke and specific work capacity. We also report spatially aligned films that reversibly morph from flat to a complex 3D shape with tunable strain from 3% to 55%. This journal is © The Royal Society of Chemistry 2015.

Lee K.M.,Air Force Research Lab | Lee K.M.,Azimuth Corporation | Tabiryan N.V.,BEAM Engineering for Advanced Measurements | Bunning T.J.,Air Force Research Lab | White T.J.,Air Force Research Lab
Journal of Materials Chemistry | Year: 2012

Azobenzene-functionalized polymeric materials have proven capable of shape adaptive responses when irradiated with light. This work focuses on isolating the fundamental differences between the photogenerated mechanical output of glassy, polydomain azobenzene liquid crystal polymer networks (azo-LCN) upon exposure to either UV and blue-green irradiation. Profound differences in the fundamental photochemical mechanism are identified through spectroscopic examination of representative materials before and after irradiation with UV or blue-green light. The photomechanical response is further elucidated in structure-property examination to ascertain the role of crosslink density, azobenzene concentration, and azobenzene connectivity (crosslinked or pendant) on the photomechanical output. © 2012 The Royal Society of Chemistry.

Lee K.M.,Air Force Research Lab | Lee K.M.,Azimuth Corporation | Koerner H.,Air Force Research Lab | Vaia R.A.,Air Force Research Lab | And 2 more authors.
Macromolecules | Year: 2010

We report on the influence of cross-link density of azobenzene-containing liquid crystal polymer networks (azo-LCNs) on the thermomechanical properties and the laser-directed bending of cantilevers consisting of these materials. Cross-link density of azo-LCN was increased by adjusting the length of photocuring from 1 to 120 min. The storage modulus (E′), loss modulus (E″), and glass transition temperature (Tg) of the azo-LCNs increase with cross-link density. Increasing the cross-link density of the polydomain azo-LCN reduces the magnitude of the bending angle of the cantilevers. The relationship between the thermomechanical and photomechanical properties of the polydomain azo-LCN reported here is further elucidated in the examination of laser-directed bending over a wide range of temperature. The temperature dependence of the equilibrium photodriven bending angle is shown to be strongly related to the temperature dependence of the storage modulus for a given azo-LCN sample. Normalizing the temperature dependence of the photomechanical response of the azo-LCN cantilevers by the Tg provides a master curve that can be used to tailor the photomechanical response. © 2010 American Chemical Society.

Lee K.M.,Air Force Research Lab | Lee K.M.,Azimuth Corporation | Bunning T.J.,Air Force Research Lab | White T.J.,Air Force Research Lab
Advanced Materials | Year: 2012

Repeatedly forming temporary shapes can be a limitation to the employment of shape memory polymers. This work utilizes glassy, liquid crystal polymer networks to spontaneously form 3D shapes that are independent of a user. These shapes are autonomously fixed with rapid temperature cycling. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The potential for wireless transduction of input light energy into mechanical outputs has led to a reinvigorated pursuit of photomechanical effects in polymeric materials and composites. We report here on factors influencing the photochemical mechanism (and thus the mechanical output) in monodomain azobenzene-functionalized liquid crystal polymer networks. Through systematic examination of a representative material with both mechanics and spectroscopic characterization the prevalence of the trans-cis and trans-cis-trans mechanisms is elucidated. Furthermore, the role of light intensity in generating heat (photothermal effects) is also reported. © 2012 American Chemical Society.

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