Pamela F. Lloyd

Aminopropyltriethoxysilane (APTES)-functionalized nanoporous polymeric gratings: fabrication and application in biosensing

We have fabricated aminopropyltriethoxysilane (APTES)-functionalized nanoporous polymeric gratings by combining holographic interference patterning and APTES-functionalization of the pre-polymer syrup. The APTES facilitates the immobilization of biomolecules onto the polymeric grating surfaces. The successful detection of multiple biomolecules (biotin, steptavidin, biotinylated anti-rabbit IgG, and rabbit-IgG) indicates that the functionalized nanoporous polymeric gratings can act as biosensing platforms which are label-free, inexpensive, and applicable as high-throughput assays.

Cholesteric liquid crystals: Image contrast in the TEM

Abstract Bright-field image contrast of thermotropic cholesteric liquid crystalline materials in the transmission electron microscope (TEM) is investigated. Possible sources of contrast for these systems are discussed in terms of their molecular anisotropy. A cholesteric side-chain liquid crystalline compound was investigated with TEM, low voltage high resolution scanning electron microscopy (LVHRSEM), and atomic force microscopy (AFM) to determine the origin of the strong contrast observed in these systems using bright-field transmission electron microscopy. Initial contrast of thin microtomed sections, as viewed with TEM low dose techniques and an image intensifier, was much weaker than observed under normal viewing conditions. The periodic steady state contrast typically observed for these materials is the result of beam damage. Furthermore, the surface of microtomed samples (parallel to the cholesteric helical axis) is corrugated with a periodicity of 1/2 the pitch due to a preferred fracture path in ...

Polymer crystallization/melting induced thermal switching in a series of holographically patterned Bragg reflectors

Holographic photopolymerization (H-P) is a simple, fast and attractive means to fabricate one-, two- and three-dimensional complex structures. Liquid crystals, nanoparticles and silicate nano-plates have been patterned into submicron periodical structures. In this article, we report fabrication of a one-dimensional reflection grating structure by patterning a semicrystalline polymer, polyethylene glycol (PEG), in Norland resin (thiol-ene based UV curable resin) matrix using the H-P technique. Sharp notches observed in the reflection grating of this Norland/PEG system indicate a finite Δ present in the system due to spatial segregation of the PEG and Norland resin. The notch position red shifts upon heating and the diffraction efficiency (ratio between diffraction and incident light intensity, DE) increases from ∼20% to 60% for the Norland 65/PEG 4600 grating. This dynamic behavior of the reflection grating is also fully reversible. The unique thermal switching behavior is attributed to the melting/formation of PEG crystals during heating/cooling. By employing different molecular weight PEGs which have different melting temperatures, a series of switching temperatures have been achieved. Since PEG can be easily coupled with a variety of functional groups, this research might shed light on fabricating multifunctional Bragg gratings using the H-P technique.

Coherent diffraction and random scattering in thiol-ene–based holographic polymer-dispersed liquid crystal reflection gratings

Bragg diffraction and random scattering in reflective holographic polymer-dispersed liquid crystal gratings are modeled using a matrix approach for a stack of low-high index layer pairs and an effective medium theory. Scattering is due to both random roughness of layer interfaces and random index variations within the layers. These are related to random liquid crystal droplet size and location as well as random orientation of the symmetry axes of bipolar droplets. Characteristic parameters governing coherent diffraction efficiency and random scattering are obtained partly from experiments, where possible, and partly from calculations based on a model of an effective medium applied to the grating. Calculations of grating transmittance are then compared to experimental transmittance spectra. Effects of scattering, primarily a decrease in baseline transmittance with wavelength and a small reduction in diffraction efficiency at the Bragg wavelength, are found to be due primarily to index inhomogeneities withi...

Nanoporous polymeric photonic crystals by emulsion holography

We report the holographic photopatterning of a microemulsion with a formamide polar phase and an acrylate monomer-containing nonpolar phase to produce polymer structures with periodic nanoscale porosity. Formamide is a highly polar solvent that forms well dispersed, nonaqueous emulsion droplets within the monomer-containing nonpolar phase before holographic patterning. Photochemically initiated polymerization of the nonpolar phase generates ordered structures defined by the holographic interference. Evaporation of the formamide from this lamellar structure yields a multilayered stack that acts as a photonic crystal (PC) with high optical reflectivity and a wide reflection bandwidth. The size of formamide droplets in the photopolymer fluid must be controlled to have a narrow distribution and peak near 60 nm to fabricate PC with high reflectivity. Addition of a sodium dioctyl sulfosuccinate (AOT) surfactant helps to stabilize the formamide microemulsion which further facilitates the formation of ordered nanopores with a uniform size.

Experimental verification of the applicability of the homogenization approximation to rough one-dimensional photonic crystals using a holographically fabricated reflection grating

The theoretical reflectance spectrum of a one-dimensional photonic crystal with large amounts of interfacial roughness has been calculated using a previously proposed method, and compared to the actual experimental reflectivity of the structure. The photonic crystal was fabricated using a simple and fast method involving the holographic exposure of a liquid crystal/photosensitive prepolymer syrup via the self-interference patterns from two laser beams. The calculated reflectance spectrum for this structure matched the experimental one extremely well, giving very similar reflectivity peak positions and intensities. Slight discrepancies between the two reflectance spectra are attributed to either small variations in the microstructure of the reflection grating beyond that which is captured in the transmission electron micrograph, or the dispersion of the polymer which was not taken into account. These results serve as experimental verification of the theory for rough photonic crystals reported previously.

Holographic polymer dispersed liquid crystal (HPDLC) transmission gratings formed by visible light initiated thiol-ene photopolymerization

We report on the initial development of a visible initiator for thiol-ene photopolymerization using the 647 nm radiation from a Krypton ion laser. The photoinitiator system consists of the dye oxazine 170 perchlorate and the co-initiator benzoyl peroxide. Electron transfer occurs between the singlet excited state of the oxazine dye and benzoyl peroxide with subsequent decomposition of the peroxide yielding benzoyl oxy radicals capable of free radical initiation. We demonstrate that this photoinitiation system enables holographic patterning of HPDLC gratings as initial Bragg transmission gratings with a periodicity less than one micron using 647 nm radiation. These gratings were electrically switchable between a diffractive and transmissive state. Morphology studies using bright field transmission electron microscopy (BFTEM) indicate the phase separation of nearly spherical shaped nematic liquid crystal droplets of several hundred nanometers in diameter. This demonstration suggests that reflection gratings can be written using this photoinitiator system and 647 nm radiation which have switchable notch wavelengths approaching 2 microns.

Influence of morphology on the lasing behavior of pyrromethene 597 in a holographic polymer dispersed liquid crystal reflection grating

Interference lithography of polymer dispersed liquid crystals allows rapid, facile fabrication of complex polymeric photonic structures that have an inherent electro-optic component for agile structures. The polymerization mechanism (step-growth or chain growth) strongly influences the morphology of the LC droplet and distribution within the polymer matrix. Using a multi-functional acrylate monomer that undergoes chain growth polymerization leads to asymmetrical LC droplets of random size and distribution, in contrast to the step-growth mechanism of thiol-ene formation where LC droplets form with a nearly uniform size distribution and spherical shape. Thiol-ene holographic polymer dispersed liquid crystals (H-PDLCs) diffraction structures have narrower bandwidth and less baseline scatter than the acrylatebased H-PDLCs. Furthermore, distributed feedback lasers constructed from thiolene-based H-PDLC lasers show marked improvement in the optical and electro-optical properties as evinced by the factor of two decrease in switching voltage and the reduction of lasing threshold from 0.17 mJ cm-2 to 0.07 mJ cm-2. These differences in optical and electro-optic properties directly correlate with the difference in microscale morphology of the H-PDLCs giving insight to the importance of microscale structure on macroscale phenomenon.

The impact of culture medium on the development and physiology of biofilms of Pseudomonas fluorescens formed on polyurethane paint

Microbial biofilms cause the deterioration of polymeric coatings such as polyurethanes (PUs). In many cases, microbes have been shown to use the PU as a nutrient source. The interaction between biofilms and nutritive substrata is complex, since both the medium and the substratum can provide nutrients that affect biofilm formation and biodeterioration. Historically, studies of PU biodeterioration have monitored the planktonic cells in the medium surrounding the material, not the biofilm. This study monitored planktonic and biofilm cell counts, and biofilm morphology, in long-term growth experiments conducted with Pseudomonas fluorescens under different nutrient conditions. Nutrients affected planktonic and biofilm cell numbers differently, and neither was representative of the system as a whole. Microscopic examination of the biofilm revealed the presence of intracellular storage granules in biofilms grown in M9 but not yeast extract salts medium. These granules are indicative of nutrient limitation and/or entry into stationary phase, which may impact the biodegradative capability of the biofilm.

Electrical Control of Shape in Voxelated Liquid Crystalline Polymer Nanocomposites

Liquid crystal elastomers (LCEs) exhibit anisotropic mechanical, thermal, and optical properties. The director orientation within an LCE can be spatially localized into voxels [three-dimensional (3-D) volume elements] via photoalignment surfaces. Here, we prepare nanocomposites in which both the orientation of the LCE and single-walled carbon nanotube (SWNT) are locally and arbitrarily oriented in discrete voxels. The addition of SWNTs increases the stiffness of the LCE in the orientation direction, yielding a material with a 5:1 directional modulus contrast. The inclusion of SWNT modifies the thermomechanical response and, most notably, is shown to enable distinctive electromechanical deformation of the nanocomposite. Specifically, the incorporation of SWNTs sensitizes the LCE to a dc field, enabling uniaxial electrostriction along the orientation direction. We demonstrate that localized orientation of the LCE and SWNT allows complex 3-D shape transformations to be electrically triggered. Initial experiments indicate that the SWNT-polymer interfaces play a crucial role in enabling the electrostriction reported herein.