Timothy J. White

Air Force Research Laboratory, Washington, Washington, D.C., United States

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Publications (92)380.29 Total impact

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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.
    Applied Physics Letters 12/2014; 105(23):231122. · 3.52 Impact Factor
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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.
    Macromolecular Rapid Communications 12/2014; 35(24):2045. · 4.93 Impact Factor
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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.
    Macromolecular Rapid Communications 10/2014; · 4.93 Impact Factor
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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.
    Molecular Crystals and Liquid Crystals 09/2014; 595(1):123-135. · 0.58 Impact Factor
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    Polymer 07/2014; · 3.77 Impact Factor
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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.
    Optics Express 06/2014; 22(13):16510-16519. · 3.55 Impact Factor
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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
    Optical Materials Express 06/2014; 24(22):6260-6276. · 2.92 Impact Factor
  • 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.
    Molecular Crystals and Liquid Crystals 06/2014; 596(1). · 0.53 Impact Factor
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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.
    SID Symposium Digest of Technical Papers. 06/2014; 45(1).
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    ABSTRACT: A microspectrophotometer was used to elucidate the local optical properties of cholesteric liquid crystals (CLCs) in cells with interdigitated electrodes as a function of applied voltage. The spectra collected from a spatially selective and micron-sized sampling area allow for new insights into the spectral properties of CLCs in the gaps between patterned electrodes. The microscopic electro-optic response is shown to be highly dependent on the cell thickness and the electrode periodicity. Specifically, the helix unwinding of the CLC superstructure does not always occur uniformly in the sample, as a result of field gradients through the cell thickness: for cells with relatively narrow gaps and electrodes, the redshift occurs initially only in the CLC layers closest to the substrate with the electrodes, leading to broad reflection spectra and different reflection colors depending on which side of the cell is illuminated. Theoretical estimates of the expected shift in the reflection band gap based on the critical field for a given CLC material and the spatial variation of electric field in the cell are found to be in good agreement with the complex behavior observed experimentally. In contrast, in thin cells with wider gaps, the pitch increase affects the whole CLC layer uniformly, because the electric field gradient is small.
    ChemPhysChem 05/2014; 15(7):1311-1322. · 3.35 Impact Factor
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    ABSTRACT: We use both lyotropic liquid crystals composed of prolate micelles and thermotropic liquid crystals made of rod-like molecules to uniformly disperse and unidirectionally align relatively large gold nanorods and other complex-shaped nanoparticles at high concentrations. We show that some of these ensuing self-assembled orientationally ordered soft matter systems exhibit polarization-dependent plasmonic properties with strongly pronounced molar extinction exceeding that previously achieved in self-assembled composites. The long-range unidirectional alignment of gold nanorods is mediated mainly by anisotropic surface anchoring interactions at the surfaces of gold nanoparticles. Polarization-sensitive absorption, scattering, and extinction are used to characterize orientations of nanorods and other nanoparticles. The experimentally measured unique optical properties of these composites, which stem from the collective plasmonic effect of the gold nanorods with long-range order in a liquid crystal matrix, are reproduced in computer simulations. A simple phenomenological model based on anisotropic surface interaction explains the alignment of gold nanorods dispersed in liquid crystals and the physical underpinnings behind our observations.
    Physical Review E 05/2014; 89(5-1):052505. · 2.31 Impact Factor
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    ABSTRACT: Light responsive materials that exhibit wirelessly actuated, multidimensional deformation are excellent candidates for programmable matter applications such as morphing structures or soft robotics. A central challenge to designing adaptive structures from these materials is the ability accurately predict three dimensional deformations. Previous modeling efforts have focused almost exclusively on pure bending. Herein we examine key material parameters affecting light driven flexural-torsional response in azobenzene functionalized liquid crystal polymer networks. We show that a great deal of control can be obtained by specifying material alignment and actuating the material with polarized light. Insight gained from the theoretical framework here lays the foundation for more extensive modeling efforts to combine polarization controlled flexural-torsional deformations with complex geometry, boundary conditions, and loading conditions.
    Soft Matter 02/2014; 10(9):1400-10. · 4.15 Impact Factor
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    ABSTRACT: Azobenzene-functionalized polyimide materials can directly transduce light into mechanical force. Here, we examine the impact of polymer backbone rigidity on the photomechanical response in a series of linear, azobenzene-functionalized polymers. The rigidity of the backbone was varied by the polymerization of five dianhydride monomers with a newly synthesized diamine (azoBPA-diamine). The azobenzene-functionalized linear polymers exhibit glass transition temperatures (Tg) ranging from 276 to 307 °C and maintain excellent thermal stability. The photomechanical response of these materials was characterized by photoinduced cantilever bending as well as direct measurement of photogenerated stress upon exposure to linearly polarized, 445 nm light. Increasing the rigidity of the polymer backbone increases the magnitude of stress that is generated but decreases the angle of cantilever deflection.
    Macromolecules 12/2013; 47(2). · 5.93 Impact Factor
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    ABSTRACT: The photomechanical response and photochemistry of a conventional, unsubstituted azobenzene-functionalized liquid crystalline polymer network (azo-LCN) is contrasted to that of an analogous material prepared with meta-fluorinated azobenzene chromophores. The polydomain azo-LCN materials exhibit nearly identical thermomechanical and optical properties. Photomechanical characterization indicates that the fluorination of the azobenzene chromophore reduces the deflection of cantilevers composed of the materials by 50%, which spectroscopic analysis reveals is due to a reduction in the ability of this material to isomerize and potentially reorient. This work is further confirmation that the underlying photochemistry of azobenzene is a primary contributor to the generation of photomechanical work in these materials. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014
    Journal of Polymer Science Part A Polymer Chemistry 12/2013; · 3.54 Impact Factor
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    ABSTRACT: Tunable reflective material systems are potentially useful in a range of applications including displays, optical filters, photonics, and sensing. Presented here is a systematic study of thermally induced reflection wavelength tuning in structurally chiral nematic gels. The tuning is enabled by a swelling/de-swelling/re-swelling transition driven by order/disorder transitions in the liquid crystal and the liquid crystalline polymer network. By preparing liquid crystal polymer networks from single component liquid crystal monomers we isolate the contribution of chemical chirality and the crosslink density of the liquid crystal polymer network to thermally-induced reflection wavelength tuning. Further elucidating the fundamental relationships between the composition of the polymer network and the resulting optical properties is a key step towards enabling advancement and optimization necessary to realize the utility of this mechanism in devices such as mirrorless lasers.
    J. Mater. Chem. C. 11/2013; 2(1).
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    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 orders-of-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.
    Proceedings of the National Academy of Sciences 11/2013; · 9.81 Impact Factor
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    ABSTRACT: Light can induce rich and diverse topographical transformations of initially flat films composed of photoresponsive, azobenzene-functionalized liquid-crystalline polymer networks. The N-fold symmetry of the topography of the photoinduced response of defect-containing azo-LCN films is examined by T. J. White and co-workers on page 5880. It is found to be inherently coupled to the strength of the defect, confirming that the director profile emanating from the topological defect governs the behavior. The topography of a film subsumed with a +10 defect resting on water after exposure to light is shown here.
    Advanced Materials 11/2013; 25(41):5830. · 14.83 Impact Factor
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    ABSTRACT: A considerable body of knowledge has been developed on the general behavior of cholesteric liquid crystal (CLC) materials in electric fields. One approach that has been reported to achieve tunability in optical filters based on CLCs with a positive dielectric anisotropy and in the planar homogeneous state involves the application of electric fields perpendicular to the axis of the CLC helix. The field leads to a progressive unwinding of the helix and a corresponding red-shift in the position of the reflection band of the CLC. In this work, a microspectrophotometer was employed to probe the spatial heterogeneity of the optical spectra of the CLC in cells with interdigitated electrodes. We will show that a complex behavior of the Bragg reflection band is obtained in the gap between electrodes for certain parameters of cells with interdigitated electrodes as a function of the applied field. This is ascribed to variations in the field magnitude and direction in the cell, which lead to a spatial variation of helix unwinding.
    Proc SPIE 09/2013; 8828:882817.
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    ABSTRACT: On the other hand: Azoarene compounds with axially chiral binaphthyl units of the same and opposite chiral configurations were doped into achiral liquid crystals (LCs). They were found to efficiently induce self-organized helical superstructures, which could be reversibly tuned by light irradiation using trans-cis photoisomerization to change the handedness of the helix in LC hosts.
    Angewandte Chemie International Edition 06/2013; · 11.34 Impact Factor
  • Macromolecular Chemistry and Physics 06/2013; 214(11). · 2.39 Impact Factor

Publication Stats

334 Citations
380.29 Total Impact Points

Institutions

  • 2009–2014
    • Air Force Research Laboratory
      Washington, Washington, D.C., United States
  • 2008–2014
    • Wright-Patterson Air Force Base
      Dayton, Ohio, United States
  • 2013
    • University of Pittsburgh
      • Industrial Engineering
      Pittsburgh, Pennsylvania, United States
  • 2009–2013
    • Kent State University
      Kent, Ohio, United States
  • 2004–2010
    • University of Iowa
      • Department of Chemical and Biochemical Engineering
      Iowa City, Iowa, United States