W. Wade Adams

Crystallization and Morphology of Poly (Aryl Ether Ether Ketone)

Abstract The morphology of poly(aryl-ether-ether-ketone) (PEEK) has been studied using optical microscopy (at room temperature and at elevated temperatures), small-angle light scattering (H v and V v ), transmission electron microscopy (bright field, dark field, and selected area electron diffraction), and wide and small-angle X-ray scattering. As expected, density of nucleation and hence spherulite size depends on melt temperature. Higher melt temperatures give rise to low nucleation density and hence large spherulites. The spherulite growth rate is independent of melt temperature and depends on crystallization temperature. The sign of the spherulite birefringence was determined between room temperature and 320°C by polarizing microscopy and at room temperature by V v light scattering. In this temperature range the spherulites were negatively birefringent. From selected area electron diffraction, the crystal unit cell b -axis is found to align along the radius of the spherulite. The crystallographic (110) plane, which makes an angle of 52.7 degrees with the radial b -axis, appears to be the preferred growth plane. Chain polarizability was also calculated using refined atomic coordinates and the bond polarizabilities. PEEK crystals were more stable in the electron beam, by about an order of magnitude, than polyethylene.

Wafer-scale monodomain films of spontaneously aligned single-walled carbon nanotubes.

The one-dimensional character of electrons, phonons and excitons in individual single-walled carbon nanotubes leads to extremely anisotropic electronic, thermal and optical properties. However, despite significant efforts to develop ways to produce large-scale architectures of aligned nanotubes, macroscopic manifestations of such properties remain limited. Here, we show that large (>cm(2)) monodomain films of aligned single-walled carbon nanotubes can be prepared using slow vacuum filtration. The produced films are globally aligned within ±1.5° (a nematic order parameter of ∼1) and are highly packed, containing 1 × 10(6) nanotubes in a cross-sectional area of 1 μm(2). The method works for nanotubes synthesized by various methods, and film thickness is controllable from a few nanometres to ∼100 nm. We use the approach to create ideal polarizers in the terahertz frequency range and, by combining the method with recently developed sorting techniques, highly aligned and chirality-enriched nanotube thin-film devices. Semiconductor-enriched devices exhibit polarized light emission and polarization-dependent photocurrent, as well as anisotropic conductivities and transistor action with high on/off ratios.

Switchable-focus lenses in holographic polymer-dispersed liquid crystal

Fine-grained polymer dispersed liquid crystals have recently become available for electrically switchable holographic elements. We explore applications of this novel material to switchable focus diffractive lenses. Several fabrication approaches, holographic and non-holographic, are demonstrated and compared with respect to design flexibility, diffraction efficiency, switching dynamic range, and optical quality. It appears possible to shift optical power between widely separated focal points with a modulation ratio 100:1 on a 10 - 50 microsecond(s) time scale.

Effect of processing temperature on the morphology of silk membranes

A concise literature survey concerning the processing and uses of silk membranes is presented in this note together with initial observations of new morphological data for the effect of processing temperature on morphology. Liquid silk from the middle section of the Middle Division of the silk gland of Bombyx mori was cast onto glass plates at 20, 40, 50, 60 and 80 °C. Silk from the anterior and posterior sections was cast at 20 °C. Samples cast at 20 °C exhibit particles, grains, nanofibrils and an irregular morphology. Each exhibits approximately the same dimensions for all the samples. Samples cast above 20 °C do not exhibit the irregular morphology. Samples cast above 50 °C exhibit larger grains and larger, more densely packed nanofibrils. All these changes might result from conversion of the amorphous structure to the β-pleated structure (Silk II). The nanofibrils appear to be self-assembled bio-nanofibrils. Membranes of regenerated fibroin treated with aqueous methanol solution exhibit grains and apparent nanofibrils. Opportunities for further work are pointed out.

Electron beam damage in high temperature polymers

Abstract Electron microscopic studies of polymers are limited due to beam damage. Two concerns are the damage mechanism in a particular material, and the maximum dose for a material before damage effects are observed. From the knowledge of the dose required for damage to the polymer structure, optimum parameters for electron microscopy imaging can be determined. In the present study, electron beam damage of polymers has been quantified by monitoring changes in the diffraction intensity as a function of electron dose. The beam damage characteristics of the following polymers were studied: poly( p -phenylene benzobisthiazole) (PBZT); poly( p -phenylene benzobisoxazole) (PBO); poly(benzoxazole) (ABPBO); poly(benzimidazole) (ABPBI); poly( p -phenylene terephthalamide) (PPTA); and poly(aryl ether ether ketone) (PEEK). Previously published literature results on polyethylene (PE), polyoxymethylene (POM), nylon-6, poly(ethylene oxide) (PEO), PBZT, PPTA, PPX, iPS, poly(butylene terephthalate) (PBT), and poly(phenylene sulphide) (PPS) were reviewed. This study demonstrates the strong dependence of the electron beam resistivity of a polymer on its thermal stability/melt temperature.

Molecular Packing and Crystalline Order in Polybenzobisoxazole and Polybenzobisthiazole Fibers

The structures of poly(p-phenylenebenzobisthiazole) (PBZT) and poly(p-phenylenebenzobisoxazole) (PBO) fibers have been investigated by fiber diffraction techniques. d-spacings were obtained from equatorial and meridional scans recorded on a four-circle diffractometer. Intensity data were derived from x-ray rotation patterns taken on Weissenberg and vacuum cylindrical cameras. Unit cells were found to be monoclinic and non-primitive, each containing two chains per cell of dimensions a = 11.79(2), b = 3.539(5), c = 12.514(9) A, γ = 94.0(2) o for PBZT; and = 11.20(1), b = 3.540(2), c = 12.050(3) A, γ = 101.3(1) for PBO. The fiber axes correspond to c. The conformational torsion angle between the bisthiazole and phenylene units and the orientation of chains within the unit cells were obtained from a ‘linked-atom least-squares’ (LALS)refinement procedure. A packing model is proposed for each polymer in which two independent molecular chains are displaced longitudinally by discrete rather than random increments. Primitive unit cells (Z = 1), besides requiring perfect axial registry of molecular chains, suffer from the occurrence of short intermolecular contacts and are rejected from further consideration.

Morphology and mechanical properties of a phase separated and a molecular composite 30% PBT/70% ABPBI triblock copolymer

Abstract The morphology of a triblock copolymer of 30% rigid-rod poly(p-phenylene benzobisthiazole) (PBT) and 70% semi-flexible coil poly(2,5(6)benzimidazole) (ABPBI) was examined by wide angle X-ray scattering and scanning and transmission electron microscopy. Samples that were vacuum cast from a solution formed a microphase separated film with 0.1 μm particles and platelets of well-oriented 10 nm PBT crystallites in a ductile ABPBI matrix. Fibres were dry-jet/wet-spun from an optically homogeneous solution into a water coagulation bath to inhibit large scale phase separation. Heat-treated fibre contained crystallites of PBT and ABPBI with lateral dimensions no larger than 3 nm, demonstrating that PBT molecular segments were well dispersed and that a rigid-rod, molecular level composite had been achieved. The molecular level dispersion and high orientation in the ‘molecular composite’ fibre resulted in excellent mechanical properties with a modulus of 100 GPa and a tensile strength of 1.7 GPa which were about an order of magnitude greater than for the vacuum cast copolymer film.

Switchable holograms in new photopolymer-liquid-crystal composite materials

Switchable holograms open up the possibility of real-time electro-optical control of diffractive optic components. We have developed a novel photopolymer-liquid crystal material system which allows fast, single-step recording of holograms with diffraction efficiency controllable by conveniently applied electric fields. With the addition of a surfactant to our standard material recipe, we have achieved complete switching of a first-order Bragg diffracted beam into the zero-order with an applied field of approximately 5 V/micrometers and microsecond response time. We have also demonstrated image storage and electro-optical readout with these materials. Low voltage, high resolution scanning electron microscopy studies have confirmed that gratings formed in this material system consist of periodic polymer-dispersed liquid crystal planes. The critical fields for switching and the response times agree very well with a simple liquid crystal shaped-droplet model which we have applied to these gratings.

Development of photopolymer/liquid crystal composite materials for dynamic hologram applications

Switchable holographic gratings are desirable for a wide range of applications in diffractive optics. Liquid crystalline materials look attractive for these devices because of their large field- induced birefringence. The combination of electro-optical liquid crystals with photopolymeric holographic materials offers a unique single system approach to the economical fabrication of switchable holograms. We report on the progress in our development of a novel system where holographic gratings are recorded in a single step process and consist of periodic polymer-dispersed liquid crystal planes. Gratings have been recorded with high diffraction efficiency (approaching 100%) and narrow angular selectivity (<1 degree(s) FWHM). The diffraction efficiency can be controlled electrically or thermally

Effects of eliminating the chain extender and varying the grating periodicity on the morphology of holographically written Bragg gratings

Experimental variables affecting the morphology of holographically written Bragg gratings in polymer dispersed liquid crystalline composite systems are discussed. The spatial anisotropic photo-crosslinking of a multifunctional acrylate monomer results in periodic regions of phase separated LC droplets. These films have many electro-optic applications owing to the fact their refractive index profiles can be modulated. Low-voltage high resolution scanning electron microscopy (LVHRSEM) was used to investigate the morphology of these films due to the very small LC domain sizes formed. Specifically, the morphology is discussed in terms of the rates of LC diffusion parallel to the grating vector, and the relative rates of nucleation and subsequent gelation. Using micrographs, the effect of writing intensity, LC content, and chain extender concentration is examined first in uniformly illuminated (flood lit) samples and then in transmission gratings. Elimination of the chain extender increases the gelation time relative to the nucleation time resulting in larger LC domains. The effect of increasing the Bragg spacing on the phase separation behavior is also examined.