Robert L. Crane

Optical limiting in solutions of diphenyl polyenes

The optical-limiting behavior of a series of trans-alpha, omega-diphenyl polyene compounds was observed in solutions of chloroform. The influence of planarity, the substitution of donor and acceptor groups, and the extent of pi-electron delocalization on the nonlinear thresholds of the diphenyl polyenes in an optical-limiting geometry were examined. A saturation effect of optical-limiting nonlinear thresholds, consistent with the theoretical work, was observed at 10-11 pi-electron bonds. The temporal profiles of the transmitted laser pulses and the power dependence of the nonlinear thresholds as a function of spot size and wavelength were examined. These examinations led to the conclusion that the predominant nonlinear mechanism was quasi-steady-state self-focusing. Nonlinear thresholds an order of magnitude lower, and thus effective n(2) and X(3) values an order of magnitude higher, than the well-known self-focusing medium CS(2) were observed. Our studies demonstrate that this series of polyenes consists of efficient broadband nonresonant optical-limiting materials.

KINETIC STUDY OF THE HELIX TO COIL DARK REACTION OF POLY(SPIROPYRAN-l-GLUTAMATE)*

Abstract— An investigation of kinetics of the helix to coil dark reaction of light adapted poly(spiropyran‐ L‐glutamic acid) (PSLG) dissolved in hexafluoroisopropanol was performed. The reaction was associated with the spiropyran (SP) to merocyanine (MC) ring opening. The ring opening reaction monitored with UV/NIS spectroscopy showed first order kinetics. Chromophore and polypeptide backbone circular dichroism data fit to an expression consistent with a single intermediate series mechanism. By FTIR, we monitored the polypeptide a‐helix amide I, the MC chromophore —C=C — stretch and the protonated unmodified carboxylate C=O stretch bands. During the first step of the series mechanism, changes in the hydrogen bonding of the unmodified carboxylate groups occurred, suggesting breakup of polypeptide aggregates. The second step of the proposed series mechanism was dominated by the helix to coil transition and the ring opening of SP to MC. The CD spectrum of MC in the dark adapted PSLG was red shifted and had a narrower bandwidth than the UV/VIS spectrum. The kinetic and spectroscopic data suggested that a fraction (population I) of the MC chromophores experienced optical activity induced by the chiral polypeptide environment, while the remainder of the MC chromophores (population II) were solvated and enantiomeric.

Crystallization kinetics of AsSbS glass in bulk and thin film form

Abstract The kinetics of crystallization in As2−xSbxS3 glasses on a melt quenched bulk form and a thin film form was studied with a differential scanning calorimeter used in dynamic heating and isothermal mode. Our understanding of the crystallization mechanism was aided by SEM and X-ray diffraction analysis. The crystallization initiated at the surface of the samples, and for this reason, the films crystallized much quicker even though the apparent activation energy was higher in films. Data indicated the difference in the initial glassy state between bulk glasses and films.

Holographic grating formation in poly(spiropyran I-glutamate).

An argon-ion laser interferometer (0.3638 or 0.488 microm) wrote photoreversible holographic grating (fringe spacing 0.39-100 microm) on spin-cast thin films (5 microm thick) of poly(spiropyran l-glutamate).

INVESTIGATION OF LIGHT‐INDUCED CONFORMATION CHANGES IN SPIROPYRAN‐MODIFIED SUCCINYLATED POLY(l‐LYSINE)

Abstract— To determine the maximum range of coupling between side‐chain photochromism and polypeptide conformation change, we modified the carboxylate side chains of succinylated poly(l‐lysine) with a spiropyran to form polypeptide I. The extent of modification was determined to be 35.5%. The spacer group length between the polypeptide a‐carbon and the dye was 12 atoms, providing minimum polypeptide‐dye interaction. Conformation changes were monitored by circular dichroism as a function of light adaptation and solvent composition (hexafluoroisopropanol [HFIP] vs trifluoroethanol [TFE]). Under all solvent compositions, the dark‐adapted dye was in the merocyanine form. Light adaptation by visible light converted the dye to the spiropyran form. When dissolved in TFE, I adopted a helical conformation insensitive to light adaptation. With increasing percentage HFIP, a solvent‐induced helix‐to‐coil transition was observed around 80% (vol/vol) HFIP. At 100% HFIP, both light‐ and dark‐adapted forms of I were in the coil state. Near the midpoint of the solvent‐induced helix‐to‐coil transition, light adaptation caused conformation changes. Applying helix‐to‐coil transition theory, we measured a statistically significant difference in coil segment‐HFIP binding constant for light‐ v.v dark‐adapted solutions (6.38 ± 0.03 M‐1vs 6.56 ± 0.03 M‐1), but not for the nucleation parameter σ (1.2 ± 0.4 10‐3 v.v 1.3 ± 0.3 × 10‐3). The small binding constant difference translated to a light‐induced binding energy difference of 17 cal/mol/monomer. Near the midpoint of the helix‐to‐coil transition, collective interactions between monomer units made possible the translation of a small energy difference (less than RT) into large macromolecular conformation changes. This work parallels similar behavior observed in poly(isocyanate) (Green, M. M. et al. J. Am. Chem. Soc.115, 4941–4942, 1993). The subtle differences in dye‐backbone interaction in I suggested a maximum coupling distance (12 atoms) beyond which polypeptide conformation and dye state are uncoupled.

Preparation of Polypeptide-Dye Multilayers by an Electrostatic Assembly Process

To develop novel optical thin films, we have prepared self-assembled polypeptide films by an electrostatic process. The films were placed on a glass slide previously silanized by an amino silane and given a positive charge by immersion in aqueous acid. Subsequent immersion of the slide in aqueous anionic solutions of either poly(L-glutamic acid), congo red, copper phthalocyanine tetrasulfonic acid or p -nitroaniline-modified poly(L-glutamic acid) resulted in deposition of the anions on the surface. Following anionic immersion, the slides were dipped into a cationic poly(L-lysine) solution. Alternate dipping into anionic and cationic solutions yielded multilayers. The thin films were characterized by optical absorption and circular dichroism. The optical density increased with dipping cycles. Circular dichroism measurements of the thin films showed induced dichroism of the congo red and phthalocyanine-containing films, suggesting formation of a locally ordered dye-polypeptide complex. Solution circular dichroism measurements of the polypeptides indicated a coil conformation, while poly(Lglutamic acid)/poly(L-lysine) complexes showed circular dichroism spectrum characteristic of a β-sheet.

Progress in the Reliability of Bonded Composite Structures

This paper reviews recent research progress in the detection of contamination on composites surfaces before bonding. Results to date indicate that it is possible to use a simple handheld instrument to determine if a composite surface is in such a state that a durable bond can be achieved. This study examined both airborne and contact contamination and found that contact contaminants can originate from unexpected sources. Monitoring of airborne contaminants in various manufacturing locations indicated that discrete contamination events can occur that are potentially detrimental to adhesion.

New smart materials: molecular simulation of nonlinear optical chromophore-containing polypeptides and liquid crystalline siloxanes

In our continuing efforts towards the design of an ordered three-dimensional network of nonlinear-optical chromophore-substituted polymer chains with controlled optical properties (the so-called smart filter), molecular simulations of polypeptide-bound chromophores and siloxane-based liquid crystals are presented. The importance of chromophore substitution on the amount of polypeptide helix destabilization was demonstrated, and similar effects are shown to exist for (L-tyrosine) in this study. Molecular dynamics simulations also indicate that the charged Congo Red substituent destabilizes the polypeptide, showing larger helix bending and backbone dihedral angles variations resulting from interactions with the NLO moiety. Understanding these molecular ordering phenomena is critical to our ability to design new materials with tailored optical properties.

7.15 Adhesive Bonding of Composite Structures

This chapter reviews recent research progress in the two areas that can significantly increase the reliability of adhesive bonding of composite materials. The first area concerns an in situ way of using a high power laser to proof load a bonded joint, which can be used to establish a minimum load bearing capability of a bond. The second area concerns the detection of contamination on a composite surface prior to bonding. Research results indicate that a simple handheld instrument can detect contamination from either assembly personnel or the environment on a composite surface prior to consolidation of the component.

5.10 – Nondestructive Inspection of Composites

This chapter examines the role of radiography in guaranteeing a minimum quality in composite materials and structures. The chapter covers traditional film based radiography and the new solid-state radiography. Because traditional radiographic techniques are not applicable to composites, sections cover modifications necessary to enhance the contrast between composite flaws and the material background. Sections are included detailing how to make and use an images quality indicator – the composite penetrameter. Final sections are devoted to the new solid-state based digital radiography and computed tomography.