In holographic Bragg gratings formed by anisotropic photopolymerization of free-radical monomers, an electric field is required to switch the grating between a diffracting and a transmitting state. The voltage necessary to turn the grating completely off is defined as the switching voltage. In this work, we report on the gradual increase of this switching voltage with time after initial fabrication. The switching field increases approximately 60% over a seven-day period for a pentaacrylate/E7 material system. Using dielectric measurements, it is observed that the resistance of the cell does not change over time while the capacitance decreases. FT-IR spectroscopy was also used to explore the continued post-polymerization after fabrication of the gratings. Increases in switching field were observed for samples fully illuminated with light as well as those kept in the dark. We speculate that the rise in the switching voltage is caused by post polymerization of residual reactive moieties located near the polymer/nematic droplet interface. An increase in the amount of interfacial area generated by post-polymerization yields more surface area for the LC molecules to bind; thus a larger field is needed to switch.

We have investigated the dynamics of formation of a reflection hologram in a photosensitive formulation containing pre-polymer and liquid crystal. Kogelnik’s two beam coupling theory of an isotropic material has previously been expanded to account for variations of refractive index Δn in the x, y, and z directions. This theory predicts a non-zero p-polarized coupling coefficient, κp at 45o internal angle, only when a macroscopic anisotropy in the grating is present. A broadband source was used as a probe to monitor the diffraction efficiencies (DE) during exposure for both s- and p-polarized light. The onset of a macroscopic ordering of the liquid crystal is observed at the same time as the onset of scattering. We report here the effects of laser writing power on the temporal evolution of s- and p-polarized diffraction efficiency and ppolarized scattered intensity.

Using conducting-tip atomic force microscopy (C-AFM), we study the spatial distribution of current paths and local electrical properties in carbon nanofiber/polymer nanocomposites. Previous studies of similar systems were hindered by a polymer-rich skin layer that exists at the nanocomposite surfaces. We present an experimental technique using oxygen plasma etching to controllably remove this polymer skin layer. After this treatment, we can directly probe the microscopic transport characteristics of the nanocomposite using C-AFM. The C-AFM results show that the electrical transport is solely carried by the carbon nanofiber (CNF) networks in the nanocomposites. In addition, high-resolution C-AFM maps show nonuniform distribution of current along the length of some CNFs, suggesting the presence of a heterogeneously distributed adsorbed polymer layer around nanofibers. Finally, two probe conductivity measurements in which one electrode (the C-AFM tip) is contacting a single constituent conducting particle were performed to study local conductivity. Results indicate that Ohmic pathways exist in the conducting network of the nanocomposite to the lowest measured nanofiber concentrations. However, non-Ohmic behavior indicating tunneling transport may also be present, especially near the percolation threshold.

Extract HTML view is not available for this content. However, as you have access to this content, a full PDF is available via the ‘Save PDF’ action button. Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005

Aligned carbon nanotubes (CNT's) have been found to form on both the Si and C faces of silicon carbide (SiC) wafers at high temperature. The CNT's form when the SiC wafer is exposed to temperatures in the range 1400-1700 degrees C under moderate vacuum. The CNT's are aligned roughly parallel to the surface. After a half hour at 1700 degrees C under vacuum of 10(-4) torr, a near continuous CNT layer about 250mn thick is formed. The entire surface of the SiC is covered with CNT's including both single and multiwalled tubes, and some graphitic carbon. SEM, TEM, AFM, XPS and Raman scattering measurements have been used to analyse the CNT/SiC structures. The metal catalyst free CNT's on SiC exhibit low density of structural defects and are very straight. The carbon source is believed to be residual carbon from the SiC left on the surface after preferential evaporation of Si. It is speculated that CNT's growth is catalysed by low concentrations of residual oxygen in the chamber during growth. The vacuum conditions can significantly affect CNT's growth. Single wall carbon nanotubes are evident in Raman spectra on the samples grown at 10(-3) Torr, not on these grown at 10(-5) Torr.

It is widely appreciated that electro-optic activity in polymer-dispersed liquid crystals (PDLCs) depends on separation of the polymer and liquid crystal (LC) phases. Since the phase structure develops in a non-equilibrium system, the morphology of the LC domains depends on the details of the chemical and physical processes active during domain formation. The nature of the interface between the polymer and liquid crystal phases is of particular interest. This work discusses the two-phase morphology in an acrylate-based system that develops during polymerization-induced phase separation (PIPS). Using small-angle X-ray scattering (SAXS) and ultra-small-angle X-ray scattering (USAXS), we find that interfaces in PDLCs developed from an acrylate-based recipe are more disordered than generally appreciated. Information gained from SAXS and USAXS is compared to data from scanning electron microscopy (SEM) and transmission electron microscopy (TEM). To elucidate the apparent discrepancies between imaging and scattering, we investigated the effects of SEM sample preparation. We observe significant alteration of the interface morphology due to the leaching of the LC phase.

AC electric field alignment (see Figure) offers a route to composite films with unusual physical properties, in numerous geometries and applicable to a wide range of organically modified layered silicates. Exchangeable organic cations on the aluminosilicate surface are suspected to cause the induced dipole. Orientational, compositional, and translational control of nanoparticles, paralleling efforts in electric field‐trapping and traveling wave applications, could be achieved.

Holographic Bragg transmission gratings formed via the anisotropic phase separation of nanosized LC droplets (H-PDLC) were fabricated using thiol-ene photopolymerization. Using coherent UV laser light and a single prism, electrically switchable transmission gratings in the Bragg regime were written. The performance of the thiol-ene-based gratings were compared with those of multifunctional acrylate-based gratings written under similar conditions. Optical and electro-optical measurements suggest that thiol-ene polymers offer promise as hosts for improved H-PDLC performance. Interesting differences in the diffraction efficiencies for s– and p-probe beams are noted for the two matrices. Morphology studies by TEM and SEM exhibit striking differences in droplet shape and uniformity. These differences are speculated to be due to differences in polymer MW growth due to the step-growth propagation mechanism for thiol-enes as compared to the chain-growth propagation mechanism in multifunctional acrylates. The response times of the thiol-ene gratings were ten times slower than those of acrylates.

Using scattering methods, we determine the morphology of carbon nanotube suspensions over length scales from 1 nm to 50 μm. We find no evidence of rod-like character at any length. Rather, a network structure of aggregated tubes, similar to that seen in dry samples, is found. These observations have significant implications regarding the use of single-walled nanotubes as a composite reinforcing filler since the network structure has significantly lower modulus than fully dispersed tubes. We also show that it is possible to isolate a rod-like fraction from the aggregated suspension using intense sonication, providing a potential route to fully dispersed nanotubes.

Holographic reflection gratings in polymer-dispersed liquid crystals (H-PDLCs) were formed by thiol-ene photopolymerization. Using UV laser light and a single prism, electrically switchable reflection gratings in blue, green, yellow, and red colors were fabricated. Results indicate that thiol-ene polymers function as better hosts for H-PDLC than multifunctional acrylate as matrixes. These differences are the result of a much different temporal structure development caused by fundamental differences in the polymerization propagation mechanism: a step-growth addition mechanism for the thiol-ene system compared to a chain-growth addition mechanism in multifunctional acrylates. Morphology studies by TEM support these conclusions, as striking differences in droplet shape and uniformity are observed. Discrete nematic droplets with a nearly spherical shape were seen. Thiol-ene polymers offer lower switching fields, higher diffraction efficiencies, better optical properties, and higher thermal stabilities. The response times of the thiol-ene gratings were five times slower than those of acrylates.

Holography offers a versatile, rapid and volume scalable approach for making large area, multi-dimensional, organic PBGs; however, the small refractive index contrast of organics prevents formation of a complete band-gap. The introduction of inorganic nanoparticles to the structure provides a possible solution. In contrast to the multiple steps (exposure, development and infiltration) necessitated by lithographic-based holography (e.g. photoresists), holographic photopolymerization of monomer-nanoparticle suspensions enables one-step fabrication of multidimensional organic-inorganic photonic band gap (PBG) structures with high refractive index contrast. The PBGs are formed by segregation of semiconductor nanocrystals during polymerization of the polymer network. Addition of CdSe/ZnS polymerization of the highly cross-linked polymer network. Addition of CdSe/ZnS quantum dots or ZnO nanocrystals to the H-PDLCs formulation results in phase segregation of the nanoparticles into the liquid crystal rich lamellae, producing photonic structures with high diffraction efficiencies that may be modulated by application of an external electric field.

Journal Article Morphology of Holographic Polymer Dispersed Liquid Crystal Reflection Gratings Written in Thiol-ene and Acrylate Polymer Hosts: Part I-Grating Formation Get access DW Tomlin, DW Tomlin UES, Inc, 4401 Dayton-Xenia Road, Dayton, OH 45432 Search for other works by this author on: Oxford Academic Google Scholar LV Natarajan, LV Natarajan Science Applications International Corporation, 4031 Col. Glenn Highway, Dayton, OH 45431 Search for other works by this author on: Oxford Academic Google Scholar VP Tondiglia, VP Tondiglia Science Applications International Corporation, 4031 Col. Glenn Highway, Dayton, OH 45431 Search for other works by this author on: Oxford Academic Google Scholar RL Sutherland, RL Sutherland Science Applications International Corporation, 4031 Col. Glenn Highway, Dayton, OH 45431 Search for other works by this author on: Oxford Academic Google Scholar TJ Bunning TJ Bunning Air Force Research Laboratory, Materials and Manufacturing Directorate, WPAFB, OH 45433 Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 9, Issue S02, 1 August 2003, Pages 382–383, https://doi.org/10.1017/S1431927603441913 Published: 19 July 2003

Electrically switchable photonic crystals are simply and rapidly formed by holographic polymerization-induced phase separation of liquid crystal from a monomer-liquid crystal mixture. We report the fabrication and electro-optical properties of liquid-crystal-filled polymer photonic crystals of orthorhombic F symmetry. Inverse opal and fcc structures can also be obtained. The crystals exhibit electrically switchable Bragg diffraction at ~8-10 V/microm with crystal structure in good agreement with theoretical expectations. These photonic crystals compare favorably with liquid-crystal-imbibed colloidal crystal arrays.

Two-photon holographic photopolymerization was used to form switchable Bragg gratings composed of layers of phase-separated liquid-crystal (LC) domains interspersed with cured, crosslinked polymer. These holographic polymer-dispersed liquid crystals form a periodic structure which diffracts red light due to nanostructured planes ∼250 nm in spacing. These structures were formed by interfering two 90-fs pulses coherently upon a reactive syrup consisting of acrylate monomer, liquid crystal, and a two-photon dye. The large two-photon cross-section allows excitation of the two-photon dye that results in electron transfer between this dye and the monomer. Diffraction efficiencies of approximately 10% were obtained, which can be modulated using an electric field applied across the film. Switching speeds below 1 ms were observed due in part to the small size of the LC domains.