Vandana Vij

Atomic Oxygen Effects on POSS Polyimides in Low Earth Orbit

Kapton polyimde is extensively used in solar arrays, spacecraft thermal blankets, and space inflatable structures. Upon exposure to atomic oxygen in low Earth orbit (LEO), Kapton is severely eroded. An effective approach to prevent this erosion is to incorporate polyhedral oligomeric silsesquioxane (POSS) into the polyimide matrix by copolymerizing POSS monomers with the polyimide precursor. The copolymerization of POSS provides Si and O in the polymer matrix on the nano level. During exposure of POSS polyimide to atomic oxygen, organic material is degraded, and a silica passivation layer is formed. This silica layer protects the underlying polymer from further degradation. Laboratory and space-flight experiments have shown that POSS polyimides are highly resistant to atomic-oxygen attack, with erosion yields that may be as little as 1% those of Kapton. The results of all the studies indicate that POSS polyimide would be a space-survivable replacement for Kapton on spacecraft that operate in the LEO environment.

Effect of Chemical Structure and Network Formation on Physical Properties of Di(Cyanate Ester) Thermosets

Key physical properties of three dicyanate ester monomers, bisphenol A dicyanate (BADCy), bisphenol E dicyanate (LECy), and the dicyanate of a silicon-containing analogue of bisphenol A (SiMCy) were investigated as a function of cyanurate conversion at conversions ranging from approximately 70% to greater than 90% in order to assess the range of applicability of both traditional and more unusual structure-property-process relationships known for cyanate ester resins. A more complete understanding of these relationships is essential for the continued development of cyanate ester resins and their composites for a wide variety of aerospace applications. The degree of cure in each system was determined by differential scanning calorimetry (DSC). The degree of conversion achieved at a given temperature was dependent on the structure of the repeat unit, with SiMCy displaying the highest relative ease of cure. The density at room temperature was found to decrease monotonically with increasing conversion for all monomer types studied. In contrast, the water uptake decreased with increasing cure for all three materials over most or all of the conversion range studied, but leveled off or began to increase with increasing conversion at conversions of approximately 90%. The T(g) decreased after exposure to hot water in resins with greater than 85% conversion, but unexpectedly increased in samples with lower conversions. An investigation of the effect of hot water exposure on network chemistry via infrared spectroscopy indicated that carbamate formation varied with both monomer chemistry and extent of cure, but was greatest for the BADCy polycyanurates. On the other hand, the unreacted cyanate ester band tended to disappear uniformly, suggesting that reactions other than carbamate formation (such as cyclotrimerization) may also take place during exposure to hot water, possibly giving rise to the observed unusual increases in T(g) upon exposure.

Synthesis of Aromatic Polyhedral Oligomeric SilSesquioxane (POSS) Dianilines for Use in High-Temperature Polyimides

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.

Dinitrogen Difluoride Chemistry. Improved Syntheses of cis- and trans-N2F2, Synthesis and Characterization of N2F+Sn2F9−, Ordered Crystal Structure of N2F+Sb2F11−, High-Level Electronic Structure Calculations of cis-N2F2, trans-N2F2, F2N═N, and N2F+, and Mechanism of the trans−cis Isomerization of N2F2†

N(2)F(+) salts are important precursors in the synthesis of N(5)(+) compounds, and better methods are reported for their larger scale production. A new, marginally stable N(2)F(+) salt, N(2)F(+)Sn(2)F(9)(-), was prepared and characterized. An ordered crystal structure was obtained for N(2)F(+)Sb(2)F(11)(-), resulting in the first observation of individual N[triple bond]N and N-F bond distances for N(2)F(+) in the solid phase. The observed N[triple bond]N and N-F bond distances of 1.089(9) and 1.257(8) A, respectively, are among the shortest experimentally observed N-N and N-F bonds. High-level electronic structure calculations at the CCSD(T) level with correlation-consistent basis sets extrapolated to the complete basis limit show that cis-N(2)F(2) is more stable than trans-N(2)F(2) by 1.4 kcal/mol at 298 K. The calculations also demonstrate that the lowest uncatalyzed pathway for the trans-cis isomerization of N(2)F(2) has a barrier of 60 kcal/mol and involves rotation about the N=N double bond. This barrier is substantially higher than the energy required for the dissociation of N(2)F(2) to N(2) and 2 F. Therefore, some of the N(2)F(2) dissociates before undergoing an uncatalyzed isomerization, with some of the dissociation products probably catalyzing the isomerization. Furthermore, it is shown that the trans-cis isomerization of N(2)F(2) is catalyzed by strong Lewis acids, involves a planar transition state of symmetry C(s), and yields a 9:1 equilibrium mixture of cis-N(2)F(2) and trans-N(2)F(2). Explanations are given for the increased reactivity of cis-N(2)F(2) with Lewis acids and the exclusive formation of cis-N(2)F(2) in the reaction of N(2)F(+) with F(-). The geometry and vibrational frequencies of the F(2)N=N isomer have also been calculated and imply strong contributions from ionic N(2)F(+) F(-) resonance structures, similar to those in F(3)NO and FNO.

Space Survivability of Main‐Chain and Side‐Chain POSS‐Kapton Polyimides

Kapton® polyimde (PI) is extensively used in solar arrays, spacecraft thermal blankets, and space inflatable structures. Upon exposure to atomic oxygen (AO) in low Earth orbit (LEO), Kapton® is severely degraded. An effective approach to prevent this erosion is chemically bonding polyhedral oligomeric silsesquioxane (POSS) into the polyimide matrix by copolymerization of POSS‐diamine with the polyimide monomers. POSS is a silicon and oxygen cage‐like structure surrounded by organic groups and can be polymerizable. The copolymerization of POSS provides Si and O in the polymer matrix on the nano level. During POSS polyimide exposure to atomic oxygen, organic material is degraded and a silica passivation layer is formed. This silica layer protects the underlying polymer from further degradation. Ground‐based studies and MISSE‐1 and MISSE‐5 flight results have shown that POSS polyimides are resistant to atomic‐oxygen attack in LEO. In fact, 3.5 wt% Si8O11 main‐chain POSS polyimide eroded about 2 μm during the...

Bio-mimicking self assembly in materials: role of hydrogen and halogen bonding

Hydrogen bonding to an electronegative heteroatom is becoming an increasingly interesting area for chemists, biochemists and biologists. Because of such interactions, proteins can interact with drugs, which has enhanced our understanding of various biological processes within the body. We have made similar attempts to investigate the structures and properties of “big” small molecules, especially those containing silicon using a “macromolecular” approach to deal with data quality and diffraction issues. Halogen bonding in molecules containing silicon and tin is a favored process due to the capability of these atoms to expand their coordination spheres. Such an expansion of the coordination sphere can sometimes result in formation of unexpected crystal structures, which impart outstanding physical properties. If these systems contain fluorocarbons or fluorinated ligands, there are domains of hydrophobicity and hydrophilicity. The challenge is, however, to overcome the low temperature phase transitions to obtain meaningful crystal structures without the destruction of crystal lattice. Data collection employing CCD or GADD detectors utilizing a sealed tube (Copper or Molybdenum) or rotating anode sources and structure solution/refinement strategies will be discussed, time permitting.

Synthesis and Structural Characterization of Nitrogen Containing High Energy Density Materials

Abstract : In this paper we report the crystal structures of the novel N5+ cation in N5+Sb2F11-, which is the first stable polynitrogen species to be discovered in a century since the discovery of the azide ion.