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Bis-phenylethynyl polyhedral oligomeric silsesquioxane (bis-PE-POSS) compounds were synthesized and thermally cured yielding crosslinked materials. After curing at 370 °C, thermal decomposition occurs near 600 °C under nitrogen. These materials were synthesized by condensation of a new phenylethynyl-functional dichlorosilane onto tetrasilanol phenyl POSS, yielding two geometric isomers.

The aim of this research was to study the residual burst strength, dynamic structural response, and failure mechanism of composite overwrapped pressure vessel (COPV) subjected to low velocity impact by numerically and experimentally. A three-dimensional composite pressure vessel model was developed using Wound composite module (WCM) plugin of Abaqus. The impact response of composite pressure vessels was calculated based on the Hashin progressive damage criteria. The residual burst strength was predicted using maximum strain failure criteria. The COPV was impact tested and the residual burst strength was calculated. The predicted and testing results were compared, and a good correlation was found.

PMR-15 is a high temperature thermosetting polyimide oligomer widely used in aircraft engines, rocket casings, bushings, and bearings. Unfortunately, PMR-15 is prepared from the mutagenic and hepatotoxic compound methylene dianiline (MDA). To develop a safer alternative to PMR-15, an alternative thermosetting polyimide oligomer (PMR-PCy) was prepared from a less toxic bisaniline, 4,4′-methylenebis-(5-isopropyl-2-methylaniline) (CDA), derived from the renewable aromatic compound p-cymene. The thermoset network prepared from PMR-PCy had a glass transition temperature (Tg) of 323 °C as measured by differential scanning calorimetry (DSC), good thermo-oxidative stability, and water uptake of only 3% after immersion in boiling water for 24 h. The Quantitative Structure Activity Relationship (QSAR)-predicted low toxicity of CDA was confirmed by in vitro testing for mutagenicity, acute toxicity, and aquatic toxicity. CDA was found to be non-mutagenic in the Ames test, had a predicted LD50 of 725 mg kg−1 in rats, and an EC50 of 299.3 mg L−1. These results suggest that CDA is not acutely toxic to humans and has minimal to no aquatic toxicity. The combination of exceptional material properties coupled with the low toxicity of CDA suggest that PMR-PCy may be a viable non-toxic replacement for PMR-15 in a variety of aerospace applications.

A detailed design optimization of a composite over wrapped pressure vessel (COPV) is performed through finite element analysis (FEA) and testing. The design optimization of a type IV polymer-lined composite overwrapped pressure vessel is performed by means of the finite element method using ABAQUS software. A plug-in named WCM in ABAQUS is used that automates the entire process of constructing a COPV FEA model. This work investigates the influence of winding angle, number of layers, and sequence of layers on the burst strength of COPV. Maximum stress, maximum strain, Tsai Hill, Tsai Wu and Hashin progressive failure criteria are used to find the burst strength of the COPV. Comparing all the results obtained using above the failure criteria, it is found that the maximum strain criterion is more conservative, and the Hashin criterion is the least conservative. Digital Image Correlation (DIC) was utilized during burst testing to quantify the failure strain in different directions. The numerical results were compared with the experimental burst testing results and a reasonably good comparison was found.

Because of the unusually high heats of vaporization of room-temperature ionic liquids (RTILs), volatilization of RTILs through thermal decomposition and vaporization of the decomposition products can be significant. Upon heating of cyano-functionalized anionic RTILs in vacuum, their gaseous products were detected experimentally via tunable vacuum ultraviolet photoionization mass spectrometry performed at the Chemical Dynamics Beamline 9.0.2 at the Advanced Light Source. Experimental evidence for di- and trialkylimidazolium cations and cyano-functionalized anionic RTILs confirms thermal decomposition occurs primarily through two pathways: deprotonation of the cation by the anion and dealkylation of the imidazolium cation by the anion. Secondary reactions include possible cyclization of the cation and C2 substitution on the imidazolium, and their proposed reaction mechanisms are introduced here. Additional evidence supporting these mechanisms was obtained using thermal gravimetric analysis-mass spectrometry, gas chromatography-mass spectrometry, and temperature-jump infrared spectroscopy. In order to predict the overall thermal stability in these ionic liquids, the ability to accurately calculate both the basicity of the anions and their nucleophilicity in the ionic liquid is critical. Both gas phase and condensed phase (generic ionic liquid (GIL) model) density functional theory calculations support the decomposition mechanisms, and the GIL model could provide a highly accurate means to determine thermal stabilities for ionic liquids in general.

A homogenous polycarbonate/cyanate ester network has been prepared from a renewable, eugenol-derived bisphenol. The pure polycarbonate exhibited a Tg of 71 °C, Mn = 8360, and polydispersity of 1.88. An 80:20 blend of cyanate ester: polycarbonate was prepared and thermally cured. The presence of the polycarbonate had no significant effect on the cure behavior of the cyanate ester. Small Angle Laser Light Scattering (SALLS) and DSC were used to analyze the blend and no phase separation was observed either during or after cure, suggesting that a homogenous network was generated. TMA of the resulting composite material revealed a single Tg of 132 °C (tan δ), roughly 55 °C lower than the Tg of the pure polycyanurate and 60 °C higher than the polycarbonate. A solvent extraction study showed that the polycarbonate could be quantitatively separated from the thermoset matrix after cure. This result proved that no chemical grafting occured under the cure conditions employed. The excellent miscibility of the polycarbonate and cyanate ester coupled with the efficient cure of the blend to a homogenous network suggests that these types of blends may have applications for fabrication of toughened composite structures.

We report the thermal, mechanical, and diffusion properties of bisphenol E based polycyanurate nanocomposites with three forms of graphene derived from sequential processing of the same carbon nanostructure. Edge-functionalized graphene nanoplatelets (GNP) were converted to graphene oxide (GO), then heated to produce thermally reduced graphene oxide (TRGO). All three reinforcements were individually mixed with the dicyanate ester of bisphenol E (LECy) at low loading levels and cured to form polycyanurate nanocomposites. GNP, with very low oxygen functionality, was incompatible with the cyanate ester, while the highly oxidized GO formed well-dispersed (though not exfoliated) nanocomposites, with the TRGO forming a good dispersion on mixing but phase separating during cure. The addition of GO, and, to a lesser extent, TRGO, resulted in improved mechanical properties, particularly fracture toughness, with the addition of TRGO having a modestly negative effect on the glass transition temperature. Surprisingly, neither GO nor TRGO addition was effective at slowing down the diffusion of water in the polycyanurate, with the addition of both resulting in increased equilibrium moisture uptake. It thus appears that the trade-off between dispersion and the required level of oxygen functionality acts in a manner to frustrate attempts at minimizing the permeation of water by addition of graphene-based reinforcements.

The synthesis and physical properties of new silicon-containing polyfunctional cyanate ester monomers tris(4-cyanatophenyl)methylsilane and tetrakis(4-cyanatophenyl)silane, as well as polycyanurate networks formed from these monomers are reported. The higher cross-linking functionality compared to di(cyanate ester) monomers enables much higher ultimate glass transition temperatures to be obtained as a result of thermal cyclotrimerization. The ability to reach complete conversion is greatly enhanced by cocure of the new monomers with di(cyanate ester) monomers such as 1,1-bis(4-cyanatophenyl)ethane (LECy). The presence of silicon in these polycyanurate networks imparts improved thermochemical stability in air compared to carbon-containing analogs in cases where the thermochemical performance in air is significantly worse than under nitrogen. The water uptake in the silicon-containing networks examined is 4 - 6 wt% after 96 hours of immersion at 85 degrees C, considerably higher than both carbon-containing and/or di(cyanate ester) analogs.

With the goal of improving the moisture resistance of thermosetting polyimides, a thermally stable, polyhedral oligomeric silsesquioxane (POSS) dianiline, which features an aromatic periphery, was co-oligomerized with 4,4 (hexafluoroisopropylidene)diphthalic anhydride (6-FDA), 4,4-diaminodiphenyl ether (ODA), and phenylethynylphthalic anhydride (PEPA) to form thermosetting oligomers of a prescribed number of repeat units. The resultant oligomers were characterized for their chemical architecture, rheological characteristics, and cure behavior. POSS regioisomerism provided a unique opportunity to study the effects of monomer architecture on properties. The type of isomer affects rheological and cure characteristics but was found to have no discernible influence on cured state properties. The benefits of POSS co-oligomerization are 3-fold in that processability, thermal stability, and moisture resistance are each improved. Cured glass transition temperature is reduced, however, for a fixed number of repeat units, due to increased length and molecular weight between cross-links concomitant with POSS co-oligomerization.

The co-cyclotrimerization of two tricyanate ester monomers, Primaset PT-30 and 1,2,3-tris(4-cyanato)propane (FlexCy) in equal parts by weight with Primaset LECy, a liquid dicyanate ester, was investigated for the purpose of exploring synergistic performance benefits. The monomer mixtures formed stable, homogeneous blends that remained in the supercooled liquid state for long periods at room temperature, thereby providing many of the processing advantages of LECy in combination with significantly higher glass transition temperatures (315-360 °C at full cure) due to the presence of the tricyanate-derived segments in the conetwork. Interestingly, the glass transition temperatures of the conetworks after cure at 210 °C, at full cure, and after immersion in 85 °C water for 96 h were all higher than predicted by the Flory-Fox equation, most significantly for the samples immersed in hot water. Conetworks comprising equal parts by weight of PT-30 and LECy retained a "wet" glass transition temperature near 270 °C. The onset of thermochemical degradation for conetworks was dominated by that of the thermally less stable component, while char yields after the initial degradation step were close to values predicted by a linear rule of mixtures. Values for moisture uptake and density in the conetworks also showed behavior that was not clearly different from a linear rule of mixtures. An analysis of the flexural properties of catalyzed versions of these conetworks revealed that, when cured under the same conditions, conetworks containing 50 wt % PT-30 and 50 wt % LECy exhibited higher modulus than networks containing only LECy while conetworks containing 50 wt % FlexCy and 50 wt % LECy exhibited a lower modulus but significantly higher flexural strength and strain to failure. Thus, in the case of "FlexCy", LECy was copolymerized with a tricyanate that provided both improved toughness and a higher glass transition temperature.

A novel processing technique was used to solution cast films of poly(benzimidazo benzophenanthroline), (BBL), and the novel ladder polymer poly(4-aza-benzimidazo benzophenanthroline) (Py-BBL), which were used as cathode materials in Type IV electroactive polymer-based electrochemical capacitors (EPECs). This new processing technique involves co-casting the polymer from solution with a room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIBTI). The new processing technique gave polymer films with superior transport properties and electrochemical stabilities, did not require a break-in period, and yielded higher charge capacity than the standard films. Co-cast films of BBL and Py-BBL were each incorporated into separate Type IV EPECs using poly(3,4-propylene dioxythiophene) (PProDOT) as the anode material. It was found that the PProDOT/BBL capacitors store, on average, about 50% more energy than a comparable PProDOT/Py-BBL EPEC. While PProDOT/BBL films have an energy density advantage at rates (power densities) less than 0.01 kW kg −1 , PProDOT/Py-BBL EPECs are capable of delivering higher energy than the BBL EPECs at rates greater than 0.01 kW kg −1 (550 s per cycle). In fact, PProDOT/Py-BBL devices delivered more than ten times the energy density of PProDOT/BBL devices at 0.5 kW kg −1 (50 s per cycle). The PProDOT/Py-BBL EPECs were cycled for 10,000 cycles at 65% depth of discharge and maintained 96% of the initial energy and power density, whereas the PProDOT/BBL EPECs were cycled under the same conditions and lost more than 35% of the initial energy and power density after only 2300 cycles.

A series of novel aromatic Polyhedral Oligomeric SilSesquioxane (POSS) dianiline molecules has been synthesized for use in the preparation of high temperature aromatic polyimides. A general synthetic strategy was devised to improve the structure, yield, and utility of POSS dianilines over those currently available. Peripheral aromatic functionality was specifically incorporated in order to improve thermal properties and to increase compatibility with aromatic polymers. Silyloxy and/or aromatic functional groups were used to link the aniline groups to the POSS cage in order to ensure that this linkage was not a thermal weak point. Additionally, the stereochemistry of the aniline functionality has been varied to produce both meta- and para- isomers. The new dianiline monomers can be used to produce high molecular weight polyimide polymers. These dianilines have been incorporated both as pendants to the polymer backbone, and in a “bead-on-a-string” fashion by insertion into the polymer main-chain. In addition to producing POSS polyimides with superior thermal stability, these new monomers will allow full characterization and comparison of POSS-polyimides with differing structures in order to delineate the structure-property relationships of POSS and polymer architectures.

The effects of the peripheral architecture on the macroscopic properties of eight symmetric and asymmetric aryl polyhedral oligomeric silsesquioxane (POSS) molecules are described. These POSS materials were synthesized in our laboratory and characterized by single-crystal and powder Xray diffraction techniques, as well as differential scanning calorimetry, thermogravimetric analysis, mass spectrometry, and Fourier transform infrared spectroscopy. The peripheral aryl architecture, although similar in most cases, was found to significantly influence observed thermally induced phase transitions, mass loss during decomposition, and heat capacity. These properties are correlated to POSS assembly, molecular interactions, and the specific attributes of peripheral structure and connectivity to the inorganic core.

Studies of the physical properties of the cocured networks formed from three similar dicyanate ester monomers revealed a number of unexpected variations from simple linear mixing rules. These variations shed light on important synergistic effects in cocured thermosetting networks and their possible causes. The monomers utilized were the dicyanate esters of Bisphenol A (BADCy) and Bisphenol E (LECy) and the silicon-containing analogue of Bisphenol A (SiMCy). The most important of the synergistic effects was a decrease of ∼25% in moisture uptake seen only in conetworks of LECy and SiMCy. For all other systems, a clear relationship between moisture uptake and the number density of cyanurate rings was observed. This relationship generally applies to many types of cyanate esters and gives an indication of the importance of specific sites (as opposed to free volume alone) in moisture uptake. Numerous additional examples of nonlinear mixing relations were observed in the glass transition temperature, density, an...

Four new asymmetric Polyhedral Oligomeric SilSesquioxanes (POSS) with the formula (Aryl)Phenyl7Si8O12, where Aryl = 1-naphthyl, 2-naphthyl, 9-phenanthrenyl, and 1-pyrenyl, have been synthesized in reasonable yield and high purity. These compounds were characterized with 1H, 13C, 29Si NMR and elemental combustion analysis. These compounds possess polycyclic aromatic functionality, which disrupts symmetry to improve solubility in organic solvents and aromatic polymers, without significant impact on thermal stability.

andM. Joseph RobertsNAVAIR Research Department, ChemistryDiVision-NAWCWD, China Lake, California 93555ReceiVed February 9, 2007ReVised Manuscript ReceiVed March 23, 2007Polyhedral oligomeric silsesquioxanes (POSS) are a familyof nanoscale chemical structures that contain a silicon/oxygencore based on (SiO

A new cyanate ester monomer was prepared from bis(4-cyanatophenyl)dimethylsilane (SiMCy) and fully characterized by analytical and spectroscopic techniques. The monomer was found to have a melting point about 20 °C lower than that of the commercial bis(4-cyanatophenyl)dimethylmethane (BADCy) with similar melt viscosity, curing kinetics, and postcure glass transition temperature. Analysis of the single-crystal molecular structure by X-ray diffraction showed that intermolecular packing was dominated by weak hydrogen-bonding attractions between the aromatic rings and the −OCN nitrogen atoms. In contrast, the packing interactions found in BADCy are dominated by dipole−dipole interactions of the OCN groups. These differences may explain the 50% reduction in moisture uptake observed in SiMCy as compared to BADCy during exposure to boiling water. In addition, thermogravimetric analysis revealed that SiMCy exhibited a significantly higher char yield in air than BADCy, presumably due to the formation of silicates ...

A theoretical study has been undertaken to elucidate the three‐dimensional pattern formation during holographic polymer‐dispersed liquid crystal fabrication employing various optical wave interference techniques. Initially miscible mixtures of nematic liquid crystal and reactive multifunctional monomer with a photosensitive initiator were exposed to geometrically arranged interfering beams of light, producing a spatially dependent intensity distribution within the sample. To mimic the spatio‐temporal evolution of periodic photonic structures in three dimensions, the time‐dependent Ginzburg–Landau Model C equations, coupled with spatially variant reaction rate equations, have been solved numerically incorporating the local free energy densities pertaining to isotropic mixing, nematic ordering, and network elasticity. The simulated results reveal some key observations during the formation of electrically switchable photonic crystals with few defects. It appears that the network elasticity term exerts profou...

A series of four new end-capped and hydroxymethyl-functionalized polyimides were prepared. Through a two-step chemical modification process (3-aminopropyl)(hepta-i-butyl) polyhedral oligomeric silsesquioxane (POSS) was covalently attached to the polymer backbone. POSS loading levels as high as 36 wt % could be obtained while maintaining excellent processability and optical clarity of thin films. Concurrent attachment of either a cyanate ester or hydroxyethyl methacrylate (HEMA) group afforded processable POSS-polyimides that underwent thermal curing to yield solvent-resistant films, both having final Tg's of 251 °C. Kinetic analysis of the cure reactions yielded energy of activations of 93 kJ/mol (cyanate ester) and 103 kJ/mol (HEMA). Exposure of a POSS-polyimide containing ∼31 wt % POSS to atomic oxygen displayed no measurable level of erosion relative to a Kapton H standard.

The present article describes the experimental and theoretical observations on the formation of holographic, polymer-dispersed, liquid crystals and electrically switchable, photonic crystals. A phase diagram of the starting mixture of nematic liquid crystal and photo-reactive triacrylate monomer was established by means of differential scanning calorimetry (DSC) and cloud point measurement. Photolithographic patterns were imprinted on the starting mixture of LC/triacrylate via multi-beam interference. A similar study was extended to a dendrimer/ photocurative mixture as well as to a single component system (tetra-acrylate). Theoretical modeling and numerical simulation were carried out based on the combination of Flory-Huggins free energy of mixing and Maier-Saupe free energy of nematic ordering. The combined free energy densities were incorporated into the time-dependent Ginzburg-Landau (Model C) equations coupled with the photopolymerization rate equation to elucidate the spatio-temporal structure growth. The 2-D photonic structures thus simulated were consistent with the experimental observations. Furthermore, 3-D simulation was performed to guide the fabrication of assorted photonic crystals under various beam-geometries. Electro-optical performance such as diffraction efficiency was evaluated during the pattern photopolymerization process and also as a function of driving voltage.

The phase behavior and separation dynamics have been investigated in blends of diglycidyl ether of bisphenol A (DGEBA), curing agent methylene dianiline (MDA), and a reactive liquid rubber (R45EPI) through application of differential scanning calorimetry (DSC), one- and two-dimensional light scattering, and optical microscopy. DSC analysis indicates that the system consists of three reactions: the self-condensations of DGEBA and R45EPI, as well as a cross-reaction between the two constituents. Observation of the dynamics of the 50/25.4/50 DGEBA/MDA/R45EPI system reveals that an initial phase separation is governed by the dominant self-curing reaction of DGEBA, followed by a phase dissolution characterized by a broadening of the interfacial regions catalyzed by a cross-reaction between the two species. A subsequent phase separation occurs at late stages since the copolymerization reaction does not proceed to completion. On the other hand, by changing the ratio of the beginning constituents to 70/25.4/30 DGEBA/MDA/R45EPI, the dissolution phenomena is not observed resulting from an even more dominant DGEBA/MDA condensation reaction. It is demonstrated that alterations in the initial compositional ratio greatly affect the phase separation dynamics of the system.

Experimental observations of the dynamics of phase behavior for blends of reactive constituents, i.e. diglyeityl ether of bisphenol A (DGEBA). caring agent methylene dianiline (MDA), and a reactive liquid rubber (R45EPI), have been theoretically modeled by coupling system thermodynamies governed by a summation of the free energies of mixing and network elasticity with reaction kineties and diffusiong equations. Snap-shots of the termporal evolution of temary phase diagrams have been eatablished based on the self-condensation reactions of DGEBA-MDA and R45EPI as well as a cross-reaction between the woconstiments forming a copolymer. Numerical solution of the proposed mean-field model provides good qualitative agreement with experimental rsults, namely, the observance of phase separation followed by a phase dissolution and subsequent secondary segregation in a 50/24.4/50 DGEBA/MDA/R45EPI mixture, as well as a single gradual phase separation in a 70/25.4/30 mixture. The phase separation dynamies are explained by a competition between thegrowth in molecular weights of the reactive species rendering the systems towards instability, and the formation of copolymer acting to compatibilize the mixtures.

New POSS compounds with high temperature capability and good solubility in various organic solvents have been developed. By placing a single polycyclic aromatic group (naphthyl, phenanthryl, or pyrenyl) on a corner (via a "corner-capping" reaction) of (phenyl)7Si7O9(OH)3, cage symmetry is disrupted, which significantly increases solubility over (phenyl)8Si8O12. Thermal characteristics are retained, which makes these materials suitable for use in high performance polymers. Reasonable yields (29- 95%) were obtained for the reactions.

Progress in the development of a new class of multi-functional polyimides for use in electro-optical devices is reported. These polyimides contain hydroxymethyl-functional side-groups attached to the polymer backbone, allowing for the attachment of a wide variety of molecular species. It is shown that multiple types of organic molecules may be attached to the polymer simultaneously, with a quantitatively controllable distribution, to tailor the physical properties of the material. Methods for cross-linking the polyimides are presented, based on both modification to the backbone and the addition of difunctional additives (such as isocyanates) to solutions of the polymer during spin casting. Processing studies using spectroscopy to track the cross-linking reaction and its effects on organic nonlinear optical materials indicate that the latter method is compatible with poling processes for polymer guest/host systems with high nonlinear optical activities. Further studies using a novel thermomechanical analysis method demonstrate that the cross-linking reactions increase the glass transition temperature and inhibit physical relaxation processes in the cross-linked guest/host films.

A method of chemical synthesis that allows for the facile attachment of a wide variety of chemical compounds, including highly active nonlinear optical chromophores, to polyimides has been developed recently at the Naval Air Warfare Center, Weapons Division. The synthesis of these compounds is presented, along with a discussion of their relevant physical and chemical properties, alone and in comparison to equivalent guest/host materials. Examples of attached chromophores include the well-known Disperse Red 1, along with high-activity chromophores of more recent interest such as FTC and CLD. The synthesis of structures that contain both attached chromophores and chemical functionalities that enable thermal cross-linking of the polyimides is also discussed.