Joseph D. Lichtenhan

Polymer/layered silicate nanocomposites as high performance ablative materials

The ablative performance of poly(caprolactam) (nylon 6) nanocomposites is examined. A relatively tough, inorganic char forms during the ablation of these nanocomposites resulting in at least an order-of-magnitude decrease in the mass loss (erosion) rate relative to the neat polymer. This occurs for as little as 2 wt.% (∼0.8 vol.%) exfoliated mica-type layered silicate. The presence of the layers does not alter the first-order decomposition kinetics of the polymer matrix. Instead, the nanoscopic distribution of silicate layers leads to a uniform char layer that enhances the ablative performance. The formation of this char is only minutely influenced by the type of organic modification on the silicate surface or specific interactions between the polymer and the aluminosilicate surface, such as end-tethering of a fraction of the polymer chains through ionic interaction to the layer surface. Thus, the enhancement in ablative performance should be general for the class of exfoliated layered silicate/polymer nanocomposites.

Structural development during deformation of polyurethane containing polyhedral oligomeric silsesquioxanes (POSS) molecules

A unique polyurethane (PU) elastomer containing inorganic polyhedral oligomeric silsesquioxane (POSS) molecules as molecular reinforcements in the hard segment was investigated by means of wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) techniques. The mechanical properties of POSS modified polyurethane (POSS-PU) were also compared to those of polyurethane without POSS. The crystal structures of two different POSS molecules were first determined by X-ray powder diffraction analysis, yielding a rhombohedral cell with aa 11.57 A ˚, aa 95.58 for octacyclohexyl-POSS (1,3,5,7,9,11,13,15octacyclohexylpentacyclo[9.5.1.13,9.15,15.17,13] octasiloxane) and aa 11.53 A ˚, aa 95.38 for hydrido-POSS (1-[hydridodimethylsiloxy]3,5,7,9,11,13,15-heptacyclohexylpentacyclo [9.5.1.13,9.15,15.17,13] octasiloxane). WAXD results showed that reflection peaks distinct to POSS crystal diffraction were seen in POSS-modified polyurethane, which suggests that POSS molecules formed nanoscale crystals in the hard domain. During deformation, the average size of POSS crystals in POSS-PU was found to decrease while elongation-induced crystallization of the soft segments was observed at strains greater than 100%. The SAXS results showed microphase structure typical of segmented

Thermal and Viscoelastic Property of Epoxy-Clay and Hybrid Inorganic-Organic Epoxy Nanocomposites

The properties of nanostructured plastics are determined by complex relationships between the type and size of the nanoreinforcement, the interface and chemical interaction between the nanoreinforcement and the polymeric chain, along with macroscopic processing and microstructural effects. In this article, we investigated the thermal and viscoelastic property enhancement on crosslinked epoxy using two types of nanoreinforcement, namely, organoion exchange clay and polymerizable polyhedral oligomeric silsesquioxane (POSS) macromers. Glass transitions of these nanocomposites were studied using differential scanning calorimetry (DSC). Small-strain stress relaxation under uniaxial deformation was examined to provide insights into the time-dependent viscoelastic behavior of these nanocomposites. Since the size of the POSS macromer is comparable to the distance between molecular junctions, as we increase the amount of POSS macromers, the glass transition temperature Tg as observed by DSC, increases. However, for an epoxy network reinforced with clay, we did not observe any effect on the Tg due to the presence of clay reinforcements. In small-strain stress relaxation experiments, both types of reinforcement provided some enhancement in creep resistance, namely, the characteristic relaxation time, as determined using a stretched exponential relaxation function increased with the addition of reinforcements. However, due to different reinforcement mechanisms, enhancement in the instantaneous modulus was observed for clay-reinforced epoxies, while the instantaneous modulus was not effected in POSS–epoxy nanocomposites.

Nanoscale reinforcement of polyhedral oligomeric silsesquioxane (POSS) in polyurethane elastomer

A unique class of polyurethane (PU) elastomer containing inorganic molecules (polyhedral oligomeric silsesquioxane, POSS) as molecular reinforcement in the hard segment was investigated by means of wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. WAXD results indicate that POSS molecules form nanoscale crystals showing distinct reflection peaks. The formation of POSS crystals is probably prompted by the microphase separation between solid-like hard segments and rubbery soft segments in PU. The microphase separation of hard and soft segments was observed by SAXS, which shows a long period of 111 A for 34 wt% POSS-PU and 162 A for 21 wt% POSS-PU, and hard segment domains with sizes of about 34 A for both of them. WAXD results from a series of POSS compounds with a corner substituted by a functional group of varying length were compared with POSS-PU, which also confirms the presence of nanoscale POSS crystals in the polymer matrix. © 2000 Society of Chemical Industry

Polyhedral Oligomeric Silsesquioxanes (Poss): Silicon Based Monomers and their Use in the Preparation of Hybrid Polyurethanes

Abstract : A series of Polyhedral Oligomeric Silsesquioxane (POSS) monomers bearing reactive hydroxyl functionalities, suitable for incorporation into step-growth polymers, is described. These monomers are difunctional in nature and are particularly well suited for use as chain extenders in the synthesis of polyurethanes. This work describes the synthesis of these POSS and their incorporation into a series of polyurethanes. Preliminary thermal, mechanical and rheological data for the POSS containing polyurethanes will also be discussed.

POSS® Coatings as Replacements for Solar Cell Cover Glasses

Presently, solar cells are covered with Ce-doped microsheet cover glasses that are attached with Dow Corning DC 93500 silicone adhesive. This general approach has been used from the beginning of space exploration, however, it is expensive and time consuming. Furthermore, as the voltage of solar arrays increases, significant arcing has occurred in solar arrays, leading to loss of satellite power. This problem could be ameliorated if the cover glass extended over the edges of the cell, but this would impact packing density. An alternative idea that might solve these issues and be less expensive and more protective is to develop a coating that could be applied over the entire array. Such a coating must be resistant to atomic oxygen for low earth orbits below about 700 km, it must be resistant to ultraviolet radiation for all earth and near-sun orbits and it must withstand the damaging effects of space radiation. Coating flexibility would be an additional advantage. We have been exploring the use of newly discovered polyoligomericsilsesquioxane (POSSreg) materials with metallic additives for these applications. This technology has several significant advantages: the glass-like composition of POSSreg provides excellent resistance to radiation and VUV and the POSS nano-building blocks can be incorporated into all known plastics using conventional polymerization or compounding techniques that can lead to tailored optically transparent materials with entirely new performance levels. We will report on the results of POSS coatings containing various additives (e.g. organic and metallic). Thick samples (150 mum) are being applied to various substrates and have been exposed to 2 MeV protons up to 10 15 P+/cm2 and UV/VUV irradiation up to 1000 hrs. The 2 MeV protons are absorbed within about 85 mum depth with ~2 mum straggle so the damage is contained entirely within the layer. Results of these tests with several POSSreg matrices will be presented

A conformal coating for shielding against naturally occurring thermal neutrons

Calculations and experimental data on the shielding effectiveness of a new kind of conformal coating against thermal neutrons typical of the natural environment are presented. This coating is shown to be capable of significantly reducing the thermal neutron threat to COTS ICs in a cost-effective manner.

Radiation Tolerant POSS Solar Cell Cover Glass Replacement Material

Recent work with polyhedral oligomeric silsesquioxanes (POSS) modified polymers has identified several enhanced characteristics in the hybrid material, including improved radiation and atomic oxygen durability in simulated space environment and extended duration space flight test experiments. Results are summarized for simulated radiation and oxidizing environment tests on POSS modified epoxy resins polyimide and DC93-500 silicon. Preliminary tests indicate that the addition of POSS into DC93-500 improves resistance to frosting due to oxidization of the surface. Additionally the hybrid material has less surface tack and self-adhesion than the stock material providing improved handling characteristics. Additional tests indicate that the hybrid material has the same radiation tolerance as the stock material when exposed to 2MeV protons. In addition to the DC93-500, a series of epoxy polymers of various composition and formulations have been screened to identify materials suitable for usage as a cover glass replacement material on single crystal or thin film solar cells. A summary of the effects of radiation and oxidizing environment on the transmittance and durability is presented.

Poss® Coatings as Replacements for Solar Cell Cover Glasses

Presently, solar cells are covered with Ce-doped microsheet cover glasses that are attached with Dow Corning DC 93500 silicone adhesive. This general approach has been used from the beginning of space exploration, however, it is expensive and time consuming. Furthermore, as the voltage of solar arrays increases, significant arcing has occurred in solar arrays, leading to loss of satellite power. This problem could be ameliorated if the cover glass extended over the edges of the cell, but this would impact packing density. An alternative idea that might solve these issues and be less expensive and more protective is to develop a coating that could be applied over the entire array. Such a coating must be resistant to atomic oxygen for low earth orbits below about 700 km, it must be resistant to ultraviolet radiation for all earth and near-sun orbits and it must withstand the damaging effects of space radiation. Coating flexibility would be an additional advantage. We have been exploring the use of newly discovered polyoligomericsilsesquioxane (POSSreg) materials with metallic additives for these applications. This technology has several significant advantages: the glass-like composition of POSSreg provides excellent resistance to radiation and VUV and the POSS nano-building blocks can be incorporated into all known plastics using conventional polymerization or compounding techniques that can lead to tailored optically transparent materials with entirely new performance levels. We will report on the results of POSS coatings containing various additives (e.g. organic and metallic). Thick samples (150 mum) are being applied to various substrates and have been exposed to 2 MeV protons up to 10 15 P+/cm2 and UV/VUV irradiation up to 1000 hrs. The 2 MeV protons are absorbed within about 85 mum depth with ~2 mum straggle so the damage is contained entirely within the layer. Results of these tests with several POSSreg matrices will be presented

New Insightl into the Structure-Property Relationships of Hybrid (Inorganic/Organic) Poss™ Thermoplastics

Abstract : The demand for multi-purpose, high-performance polymer systems has resulted in a need for advancing polymer properties beyond what traditional systems can offer. Only through control/alteration at the molecular level can one maximize property enhancements to meet current military and commercial needs. Over the last seven years the Air Force Research Laboratory has developed and incorporated discrete Si-O frameworks (POSS = Polyhedral Oligomeric Silsesquioxanes) into traditional organic polymer systems. This research has resulted in new hybrid inorganic-organic polymer systems with remarkable enhancements in mechanical and physical properties including dramatic increases in both glass transition and decomposition temperatures, reduced flammability, increased moduli and oxidation resistance. We have shown that these enhancements result from the chemical composition (Si-O core) and size (^15 A in diameter) of the POSS frameworks, and can be copolymerized, grafted, or even blended using traditional processing methods. Recently, we have focused our efforts on understanding and controlling the molecular level interactions between POSS frameworks and the polymer matrix. The development of new POSS monomers has allowed us to study how functionality, size, and geometry enhances the bulk properties of these hybrid materials. For example, increasing the solubility of side groups on the POSS framework results in greater POSS-polymer matrix interactions and increased Tg and storage moduli. Recent studies with POSS blends have resulted in increases in hardness, and reductions in dielectric constants and coefficients of thermal expansion. New results in polymer synthesis, characterization, and applications will be discussed with a strong emphasis on the versatility of this new nanotechnology to many polymer systems.