Lee R. Cambrea

Synthesis, Characterization, and Cure Chemistry of Renewable Bis(cyanate) Esters Derived from 2-Methoxy-4-Methylphenol

A series of renewable bis(cyanate) esters have been prepared from bisphenols synthesized by condensation of 2-methoxy-4-methylphenol (creosol) with formaldehyde, acetaldehyde, and propionaldehyde. The cyanate esters have been fully characterized by infrared spectroscopy, (1)H and (13)C NMR spectroscopy, and single crystal X-ray diffraction. These compounds melt from 88 to 143 °C, while cured resins have glass transition temperatures from 219 to 248 °C, water uptake (96 h, 85 °C immersion) in the range of 2.05-3.21%, and wet glass transition temperatures from 174 to 193 °C. These properties suggest that creosol-derived cyanate esters may be useful for a wide variety of military and commercial applications. The cure chemistry of the cyanate esters has been studied with FTIR spectroscopy and differential scanning calorimetry. The results show that cyanate esters with more sterically demanding bridging groups cure more slowly, but also more completely than those with a bridging methylene group. In addition to the structural differences, the purity of the cyanate esters has a significant effect on both the cure chemistry and final Tg of the materials. In some cases, post-cure of the resins at 350 °C resulted in significant decomposition and off-gassing, but cure protocols that terminated at 250-300 °C generated void-free resin pucks without degradation. Thermogravimetric analysis revealed that cured resins were stable up to 400 °C and then rapidly degraded. TGA/FTIR and mass spectrometry results showed that the resins decomposed to phenols, isocyanic acid, and secondary decomposition products, including CO2. Char yields of cured resins under N2 ranged from 27 to 35%, while char yields in air ranged from 8 to 11%. These data suggest that resins of this type may potentially be recycled to parent phenols, creosol, and other alkylated creosols by pyrolysis in the presence of excess water vapor. The ability to synthesize these high temperature resins from a phenol (creosol) that can be derived from lignin, coupled with the potential to recycle the composites, provides a possible route to the production of sustainable, high-performance, thermosetting resins with reduced environmental impact.

Renewable thermosetting resins and thermoplastics from vanillin

Two cyanate ester resins and a polycarbonate thermoplastic have been synthesized from vanillin. The bisphenol precursors were prepared by both an electrochemical route as well as by a McMurry coupling reaction. 1,2-Bis(4-cyanato-3-methoxyphenyl)ethene (6) had a high melting point of 237 °C and did not cure completely under a standard cure protocol. In contrast, the reduced version, 1,2-bis(4-cyanato-3-methoxyphenyl)ethane (7) melted at 190 °C and underwent complete cure to form a thermoset material with Tg = 202 °C. 7 showed thermal stability up to 335 °C and decomposed via formation of phenolics and isocyanic acid. A polycarbonate was then synthesized from the reduced bisphenol by a transesterification reaction with diphenylcarbonate. The polymer had Mn = 3588, Mw/Mn = 1.9, and a Tg of 86 °C. TGA/FTIR data suggested that the polycarbonate decomposed via formation of benzodioxolones with concomitant elimination of methane. The results show that vanillin is a useful precursor to both thermosetting resins and thermoplastics without significant modification.

Synthesis, characterization, and cure chemistry of high performance phosphate cyanate ester resins

Three thermosetting cyanate ester resins with phosphate cores have been synthesized from chlorophosphates by a straightforward, three-step approach. A p-substituted bis(cyanate) ester generated by this route (PhosCy) was cured to give a thermoset material with a Tg of 223 °C and a char yield of 47% in air, while the m-substituted version (MPhosCy) yielded a thermoset with a Tg of only 131 °C and a char yield of 65% in air. The low Tg of the MPhosCy thermoset is attributed to a unique intramolecular cyclization of the cyanate ester groups, while the high char yield is attributed to a high temperature cross-linking reaction involving the phosphate core. To further explore this class of materials, a trifunctional phosphate cyanate ester (PhosCy3) was prepared from POCl3. This material was cured to generate a thermoset with an impressive Tg of >360 °C and a char yield of 67% in air. All three hybrid resins can potentially be used in fire-resistant composite materials or as protective surface coatings for conventional polymers.

Refractive index of infrared-transparent polycrystalline alumina

The refractive index of polycrystalline α-alumina prisms with an average grain size of 0.6 μm is reported for the wavelength range 0.9 to 5.0 and the temperature range 293 to 498K. Results agree within 0.0002 with the refractive index predicted for randomly oriented grains of single-crystal aluminum oxide. This paper provides tutorial background on the behavior of birefringent materials and explains how the refractive index of polycrystalline alumina can be predicted from the ordinary and extraordinary refractive indices of sapphire. The refractive index of polycrystalline alumina is described by 𝑛𝑛2 − 1 = (A+B [𝑇𝑇2−𝑇𝑇20]) +Dλ2 /λ2−(λ1+C [𝑇𝑇2−𝑇𝑇20])2 + λ2−λ22 where wavelength λ is expressed in μm, To = 295.15 K, A = 2.07156, B = 6.273× 10-8, λ1 = 0.091293, C = –1.9516 × 10-8, D = 5.62675, and λ2 = 18.5533. The slope dn/dT varies with λ and T, but has the approximate value 1.4 × 10-5 K-1 in the range 296–498 K.

Slow crack growth study of polycrystalline alumina and multispectral zinc sulfide

Samples of fine-grain, transparent polycrystalline alumina (CeraNova Corp) and multispectral zinc sulfide (Cleartran) were tested to determine mechanical strength and slow crack growth parameters. Mechanical strength measurements of coupons were fit to a Weibull equation that describes the material strength and its distribution. Slow crack growth parameters were calculated using the procedure set forth by Weiderhorn.1 This paper describes the derivation of Weibull and slow crack growth parameters from strength measurements over a range of stress rates and how these parameters are used to predict window lifetime under stress. Proof testing is employed to ensure that a window begins its life with a known, minimum strength.

Selective solvent-free chromium detection using cadmium-free quantum dots

Abstract. Currently, the method of choice to test for the presence of chromium in water is to submit samples to a lab for testing. We present a simple field-ready test that is selective for the presence of chromium at concentrations of 100 ppb or greater. The Environmental Protection Agency maximum contaminant level (MCL) for total chromium is 100 ppb. This test uses a simple on/off fluorescent screening employing the use of silver indium sulfide (AgInS2) quantum dots (QDs). These QDs were impregnated into cotton pads to simplify field testing without the need for solvents or other liquid chemicals to be present. The change in fluorescence is instant and can be readily observed by eye with the use of a UV flashlight.

Weibull analysis and window lifetime prediction: a tutorial

Mechanical strength measurements of transparent ceramic window material coupons are customarily fit to a Weibull equation that describes the strength and its distribution. Predictions of window lifetime under stress are commonly based on slow crack growth parameters obtained by measuring the mechanical strength of coupons over a range of constant stress rates. This tutorial paper describes how to derive Weibull and slow crack growth parameters from strength measurements and how to use those parameters to predict window lifetime under stress. Proof testing is employed to ensure that a window begins its life with a known, minimum strength.

Assessment of low-expansion tungstates for thermal-shock-resistant infrared windows

Low-thermal-expansion tungstate materials have the potential to be used as thermalshock- resistant midwave (3-5 μm) infrared windows. Material properties that favor thermal shock resistance are high strength, high thermal conductivity, low elastic modulus, and low thermal expansion. Sapphire, for example, owes its high thermal shock resistance to high strength and high thermal conductivity. In principle, it is possible to obtain even higher thermal shock resistance if a window material with near-zero thermal expansion can be made. This paper assesses recent work on Zr(WO4)2 and Al0.5Sc1.5(WO4)3. It is concluded that multi-phonon absorption in the midwave spectral region limits the optical capabilities of tungstate materials. These materials have more absorption—and therefore, more emission—than aluminum oxynitride in the 4-5 μm wavelength region.

Two-dimensional, periodic mushroomlike nanostructures for SERS applications

Two-dimensional, large-area, periodic mushroomlike metallodielectric nanostructures have been simulated, fabricated, and characterized for biosensing applications. Simulations show high electrical field around the tips of the structure. The fabrication process consists of using holographic lithography to create 2-D periodic nanohole array. Subsequently, oblique metal deposition on the nanohole array results in mushroomlike nanostructure with a cavity underneath the void space. The precise geometry of the nanocavity is dependent on the deposition time (thickness). The periodicity of the array was designed to excite propagating surface plasmon resonance (SPR) modes, while the geometric shape of the nanostructure excites localized plasmons on its edges. The coupling between these two phenomena results in higher electric field and thus higher enhancement factor than conventional nanohole array over the whole substrate area ( > 4 cm2). By analyzing the Raman mode of the adsorbed benzenethiol on the surface, the surface enhanced Raman scattering (SERS) enhancement factor of greater than 106 has been measured. Due to its moderately-high enhancement factor, large-area array, and low-cost fabrication method, this nanostructure can be used for future SERS biosensing applications.

Comparison of novel graded refractive index tapered nanowell design against a triangle array commonly used for surface enhanced raman scattering

A novel plasmonic surface is evaluated as a potential surface for surface enhanced Raman scattering (SERS) experiments. This paper examines the electromagnetic response of an array of gold triangles commonly used for surface enhanced Raman scattering (SERS) with a finite element simulator, and then compares those results with the theoretical performance of a novel surface described herein. The gold triangle array modeled as a standard SERS surface could be fabricated using nanosphere lithography [1]. The new design introduced in this paper utilizes a strongly tapered nanowell shape, which is etched out of a gold/alumina multilayer stack. The nanowell void creates a series of resonators stacked on top of one another, with each metal/insulator/metal combination representing one resonator. Resonators with different void radii, have different refractive indices. Therefore, the taper of the nanowell defines the vertical refractive index gradient, and the taper in this paper was chosen to span both positive and negative refractive indices within the multilayer stack. The tapered nanowell design is shown to have a very strong response, and displays a unique stability with respect to disorder within the array.