Timothy J. White

Wright-Patterson Air Force Base, Dayton, Ohio, United States

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Publications (96)548.73 Total impact

  • Kyung Min Lee · Vincent P. Tondiglia · Timothy J. White
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    ABSTRACT: The position or bandwidth of the selective reflection of polymer stabilized cholesteric liquid crystals (PSCLCs) prepared from negative dielectric anisotropy ("-Δε") liquid crystalline hosts can be shifted by applying a DC voltage. The underlying mechanism of the tuning or broadening of the reflection of PSCLCs detailed in these recent efforts is ion-facilitated, electromechanical deformation of the structurally chiral, polymer stabilizing network in the presence of a DC bias. Here, we show that these electro-optic responses can also be photosensitive. The photosensitivity is most directly related to the presence of photoinitiator, which is a known ionic contaminant to liquid crystal devices. Measurement of the ion density of a series of control compositions before, during, and after irradiation with UV light confirms that the ion density in compositions that exhibit photosensitivity is increased by irradiation and correlates to not only the concentration of the photoinitiator but also the type. Thus, the magnitude of the electrically tuned or broadened reflection of PSCLC of certain compositions when subjected to DC field is further increased in the presence of UV light. While interesting and potentially useful in applications such as architectural windows, the effect may be deleterious to some device implementations. Accordingly, compositions in which photosensitivity is not observed are identified.
    No preview · Article · Jan 2016 · Soft Matter
  • Timothy J. White · Dirk J. Broer
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    ABSTRACT: Liquid crystals are the basis of a pervasive technology of the modern era. Yet, as the display market becomes commoditized, researchers in industry, government and academia are increasingly examining liquid crystalline materials in a variety of polymeric forms and discovering their fascinating and useful properties. In this Review, we detail the historical development of liquid crystalline polymeric materials, with emphasis on the thermally and photogenerated macroscale mechanical responses-such as bending, twisting and buckling-and on local-feature development (primarily related to topographical control). Within this framework, we elucidate the benefits of liquid crystallinity and contrast them with other stimuli-induced mechanical responses reported for other materials. We end with an outlook of existing challenges and near-term application opportunities.
    No preview · Article · Oct 2015 · Nature Materials
  • Cyrus Mostajeran · Taylor H. Ware · Timothy J. White
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    ABSTRACT: Considerable recent attention has been given to the study of shape formation using modern responsive materials that can be preprogrammed to undergo spatially inhomogeneous local deformations. In particular, nematic liquid crystal polymer networks offer exciting possibilities in this context. In this paper, we discuss the generation of Gaussian curvature in thin nematic sheets using smooth in-plane director fields patterned across the surface. We highlight specific patterns which encode constant Gaussian curvature of prescribed sign and magnitude and present experimental results which appear to support the theoretical predictions. Specifically, we provide experimental evidence for the realization of positive and negative Gaussian curvature in glassy and elastomeric liquid crystal polymer networks through the stimulation of smoothly varying in-plane director fields.
    No preview · Article · Oct 2015
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    ABSTRACT: The spontaneous conversion of a flat film into a 3-D shape requires local programming of the mechanical response. Historically, the ability to locally program the mechanical response of high strain (>30%) liquid crystalline elastomers (LCEs) has been limited to magnetic or mechanical alignment techniques, which limits spatial resolution. Recently, we reported on the preparation of LCEs capable of 55% strain with spatial control of the mechanical response at scales as small as 0.01 mm2. Here, we report a distinct formulation strategy to realize programmable stimulus-response in LCEs. Photopolymerization of thiol-ene/acrylate formulations yields materials that exhibit large reversible strain up to 150%. The photopolymerization reaction is extremely rapid, reducing preparation time from days to minutes. The mechanical behavior of these materials can be tuned by varying cross-link density. Spatial and hierarchical programming of the director profile is demonstrated, enabling 3-D shape change, including twisting ribbons and localized Gaussian curvature.
    No preview · Article · Sep 2015 · ACS Macro Letters
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    ABSTRACT: This communication reports large magnitude (exceeding 1500 nm) and reversible reflection notch tuning in polymer stabilized cholesteric liquid crystals (PSCLCs) formulated with negative dielectric anisotropy (−Δε) hosts upon application of a direct current (DC) field. Selective and repeatable tuning ranges of 100 nm to 400 nm are demonstrated. The reflection changes are dictated by a nonlinear distortion of the pitch across the cell thickness, which is associated with electromechanical displacement of the polymer stabilizing network. The electro-optical control of the selective reflection in −Δε PSCLCs may have potential use in optics, displays, and other commercial products.
    No preview · Article · Aug 2015
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    Full-text · Conference Paper · Jul 2015
  • Kyung Min Lee · Vincent P. Tondiglia · Timothy J. White
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    ABSTRACT: We report on the ability to switch an optical material composed of a polymer stabilized cholesteric liquid crystal (polymer stabilized cholesteric texture, PSCT) between stable transparent (reflective) and scattering modes. The degree of scattering is controllable with the strength of the applied electric field. The mechanism for bistable switching of the PSCT is distinguished from prior examinations by employing electromechanical displacement of a stabilizing polymer network. The stable transparent (reflective) or scattering modes are induced with a variety of driving schemes employing both alternating and direct current fields. The relative degree of scattering can be varied to allow for grayscale control potentially useful in smart window and display applications.
    No preview · Article · Jun 2015 · MRS Communications
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    ABSTRACT: Dynamic control of shape can bring multifunctionality to devices. Soft materials capable of programmable shape change require localized control of the magnitude and directionality of a mechanical response. We report the preparation of soft, ordered materials referred to as liquid crystal elastomers. The direction of molecular order, known as the director, is written within local volume elements (voxels) as small as 0.0005 cubic millimeters. Locally, the director controls the inherent mechanical response (55% strain) within the material. In monoliths with spatially patterned director, thermal or chemical stimuli transform flat sheets into three-dimensional objects through controlled bending and stretching. The programmable mechanical response of these materials could yield monolithic multifunctional devices or serve as reconfigurable substrates for flexible devices in aerospace, medicine, or consumer goods. Copyright © 2015, American Association for the Advancement of Science.
    No preview · Article · Feb 2015 · Science
  • Jeong Jae Wie · Kyung Min Lee · Taylor H. Ware · Timothy J. White
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    ABSTRACT: Three-dimensional shape control is an enabler of dexterous motion in nature. Herein, we report on the thermally initiated out-of-plane (torsional) responses observed in a series of glassy, liquid crystalline polymer networks prepared with a range of cross-link densities. The three-dimensional shape of these materials is strongly dictated by both cross-link density as well as the preparation conditions (polymerization temperature). All of the materials examined herein undergo torsional inversion of the handedness with increasing temperature. The temperature at which the material flattens (crossover between handedness inversion) can be positioned by the polymerization temperature. Monoliths prepared with either multimaterial composition or multidirector orientations are shown to exhibit spatial variation in shape adaptivity.
    No preview · Article · Feb 2015 · Macromolecules
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    ABSTRACT: A different generation of polymer-dispersed liquid crystals (PDLCs) based on a liquid crystalline polymer host is reported wherein the fluid behavior of the reactive mesogenic monomer is an enabler to concentration windows (liquid crystal polymer/liquid crystal) (and subsequent morphologies) not previously explored. These liquid crystal (LC) polymer/LC composites, LCPDLCs, exhibit excellent optical and electro-optical properties with negligible scattering losses in both the ON and OFF states. These systems thus have application in systems where fast phase modulation of optical signal instead of amplitude control is needed. Polarized optical microscopy and high resolution scanning electron microscopy confirm a bicontinuous morphology composed of aligned LC polymer coexisting with a phase separated LC fluid. Operating voltages, switching times, and spectra of LCPDLCs compare favourably to conventional PDLC films. The LCPDLCs exhibit a low switching voltage (4–5 V/μm), symmetric and submillisecond (200 μs) on/off response times, and high transmission in both the as formed and switched state in a phase modulation geometry.
    Full-text · Article · Dec 2014 · Applied Physics Letters
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    ABSTRACT: Front Cover: The synthesis of azobenzene-functionalized polyimide materials and their employment to transduce light into electricity when laminated with the piezoelectric polyvinylidene fluoride (PVDF) is illustrated. Photopiezoelectric conversion is originated within the amorphous azobenzene-functionalized polyimides which subsequently subjects PVDF to stress resulting in an electrical signal. The frequency of the output electric signal can match the frequency of the periodic irradiation. Further details can be found in the article by J. J. Wie, D. H. Wang, V. P. Tondiglia, N. V. Tabiryan, R. O. Vergara-Toloza, L.-S. Tan, and T. J. White* on page 2050. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    No preview · Article · Dec 2014 · Macromolecular Rapid Communications
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    ABSTRACT: We report on the preparation of color-tunable mirrors based on electrically regulated bandwidth broadening of the circularly polarized reflection of polymer-stabilized cholesteric liquid crystals (PSCLCs). A number of improvements relating to the practical implementation of these materials are detailed including color and bandwidth stability, baseline optical properties, and response times. Experimentation reported herein focuses on the contribution of structural chirality, viscoelastic properties of the polymer network architecture, and electro-optic drive schemes. Through the examination of samples prepared in different conditions and compositions, we further elucidate the dominant role of structural chirality as well as the impact of cross-linking of the polymer stabilizing network on the threshold voltage and relative change in bandwidth per voltage (Δ(Δλ)/V). Furthermore, the appearance of nonideal optical properties (scatter and haze) in some samples is shown to be correlated with the polymer/LC compatibility and effectiveness of structural templating. Due to the employment of an electromechanical displacement mechanism, the switching times of the PSCLCs are slower than mechanisms based on liquid crystal reorientation. However, a potential approach is identified to reduce the on and off switching times to approximately 1 s.Keywords: liquid crystals; optical materials; polymer stabilization; color; photonic band gap
    No preview · Article · Oct 2014
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    ABSTRACT: Light is a readily available and sustainable energy source. Transduction of light into mechanical work or electricity in functional materials, composites, or systems has other potential advantages derived from the ability to remotely, spatially, and temporally control triggering by light. Toward this end, this work examines photoinduced piezoelectric (photopiezoelectric) effects in laminate composites prepared from photoresponsive polymeric materials and the piezoelectric polymer polyvinylidene fluoride (PVDF). In the geometry studied here, photopiezoelectric conversion is shown to strongly depend on the photomechanical properties inherent to the azobenzene-functionalized polyimides. Based on prior examinations of photomechanical effects in azobenzene-functionalized polyimides, this investigation focuses on amorphous materials and systematically varies the concentration of azobenzene in the copolymers. The baseline photomechanical response of the set of polyimides is characterized in cantilever deflection experiments. To improve the photomechanical response of the materials and enhance the electrical conversion, the polyimides are drawn to increase the magnitude of the deflection as well as photogenerated stress. In laminate composites, the photomechanical response of the materials in sequenced light exposure is shown to transduce light energy into electrical energy. The frequency of the photopiezoelectric response of the composite can match the frequency of the sequenced light exposing the films.
    No preview · Article · Oct 2014 · Macromolecular Rapid Communications
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    ABSTRACT: A microspectrophotometer was used to measure reflection spectra of cholesteric liquid crystals (CLCs) in cells with interdigitated electrodes as a function of applied voltage in order to probe the spatial variation in behavior in the electrode and gap regions. Complex changes in the optical spectra are observed in the gap regions for cells in which the electric field magnitude changes significantly through the thickness of the cell. This leads to a non-uniform helix unwinding and pitch gradient in the cell. In cells with smaller field gradients, the unwinding occurs in a uniform manner and it is possible, under certain conditions, to distinguish discrete changes in pitch, corresponding to a decrease in the number of half-turns of the helical structure in the cell.
    No preview · Article · Sep 2014 · Molecular Crystals and Liquid Crystals
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    ABSTRACT: Photomechanical effects in polymeric materials directly convert input photonic energy into a macroscopic mechanical output. The photoinitiated mechanical output of these materials is typically dominated by classical mechanics, primarily derived from the material stiffness and sample geometry. Accordingly, large magnitude shape change (e.g., motion) is typically traded for large magnitude force generation. Here, we report on the systematic preparation and comparison of photomechanical effects in a set of isomerically varied linear and cross-linked azobenzene-functionalized materials that demonstrate the critical role of segmental mobility (evident in the magnitude of the beta-transition) to assimilate the typically exclusive properties of large force generation and large shape change in a single material.
    No preview · Article · Sep 2014 · Chemistry of Materials
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    ABSTRACT: Wireless transduction of light into mechanical work manifested as shape-changing surfaces, adaptive structures, or actuators is a topic of considerable recent interest. In the work reported here, the photomechanical responses of a new series of crosslinked azobenzene-functionalized polyimides prepared with increasing backbone rigidity over a range of crosslinker concentrations are examined. The baseline properties of the materials were characterized with X-ray diffraction, thermal analysis, and photomechanical examination including cantilever bending experiments and tensile tests. Increasing the rigidity of the backbone repeat unit reduces the magnitude of the shape change (observed as cantilever deflection) but increases the magnitude of photogenerated stress (in tensile tests). The promise of these materials as wireless actuators was examined in photoinitiated snap-through experiments.
    Full-text · Article · Jul 2014 · Polymer
  • Mariacristina Rumi · Timothy J White · Timothy J Bunning
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    ABSTRACT: We studied the appearance of second- and third-order Bragg reflections in cholesteric liquid crystals (CLCs) in cells where the electric field was perpendicular to the helical axis. Second-order reflections with reflectance values as large as 80% of the first-order one were observed in the gap regions of alignment cells with interdigitated electrodes for CLC mixtures with pitches in the range 0.5-1.0 μm upon application of a field. The characterization was enabled by local probing of the CLC using a microspectrophotometer. LC cells that are transparent in the visible spectrum in the off-state and become colored upon application of a field due the second- or third-order reflection band appearance were demonstrated. The spectral position of the higher-order Bragg reflections can also be tuned by adjusting the magnitude of the electric field.
    No preview · Article · Jun 2014 · Optics Express
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    ABSTRACT: Cholesteric liquid crystals (CLCs) are selectively reflective materials that can exhibit a number of dynamic optical responses. We recently reported on electrically-induced, seven-fold increase in bandwidth in polymer stabilized CLCs (PSCLCs) subjected to DC electric fields. Here, the underlying mechanism of the electrically-controllable bandwidth broadening in PSCLCs is isolated by employing a variety of electro-optic experiments. We conclude that the mechanism is ionic charge trapping by the polymer network which subjects the material system to pitch expansion near the positive electrode and pitch compression near the negative electrode resulting in approximately linear pitch variation throughout the cell thickness
    Full-text · Article · Jun 2014 · Optical Materials Express
  • Jeong Jae Wie · Kyung Min Lee · Timothy J. White
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    ABSTRACT: Thermally and optically fixed shape memory is examined in glassy, azobenzene- functionalized liquid crystalline polymer networks (azo-LCN) in the twisted nematic (TN) geometry. The thermal and optical responses of two materials with a large difference in crosslink density are contrasted. The crosslink density was reduced through the inclusion of a monoacrylate liquid crystal monomer RM23. Reducing the crosslink density decreases the threshold temperature of the thermally-induced shape change and increases the magnitude of the deflection. Surprisingly, samples containing RM23 also allows for retention of a complex permanent shape, potentially due to differentiated thermal response of the pendant and main chain mesogenic units of the azo-LCN material.
    No preview · Article · Jun 2014 · Molecular Crystals and Liquid Crystals
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    ABSTRACT: Stimuli-induced control of the reflection notch of cholesteric liquid crystals (CLCs) is potentially useful in optics, photonics, and displays. This report briefly reviews and describes a series of related results in which we have observed symmetric bandwidth broadening and reflection notch tuning in polymer stabilized CLCs (PSCLCs) upon application of a DC field. The electro-optic responses are typically observed in the presence of polymer stabilization, in formulations based on negative dielectric liquid crystals hosts, and when subjected to DC field.
    No preview · Article · Jun 2014

Publication Stats

2k Citations
548.73 Total Impact Points


  • 2008-2016
    • Wright-Patterson Air Force Base
      Dayton, Ohio, United States
  • 2014
    • United States Air Force
      • Air Force Research Laboratory
      New York, New York, United States
  • 2012-2013
    • Air Force Research Laboratory
      Washington, Washington, D.C., United States
    • Florida A&M University
      • Department of Mechanical Engineering
      Tallahassee, Florida, United States
  • 2006-2010
    • University of Iowa
      • Department of Chemical and Biochemical Engineering
      Iowa City, Iowa, United States