Bruce X. Fu

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

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

Physical gelation in ethylene–propylene copolymer melts induced by polyhedral oligomeric silsesquioxane (POSS) molecules

The rheological behavior of ethylene–propylene (EP) copolymers containing polyhedral oligomeric silsesquioxane (POSS) molecules was investigated by means of wide-angle X-ray diffraction (WAXD), oscillatory shear, stress and strain controlled rheology in the molten state and dynamic mechanical analysis (DMA) in the solid state. WAXD results showed that the majority of POSS molecules in the EP melt were present in the crystal form. Oscillatory shear results showed that the EP/POSS nanocomposites exhibited a solid-like rheological behavior compared with the liquid-like rheological behavior in the neat resin, i.e. POSS caused physical gelation in EP. While POSS exhibited only a minimum effect on the flow activation energy of EP, the high POSS concentration samples were found to induce higher yield stress than the neat resin. This behavior was similar to the Bingham rheology, indicative of a structured fluid. DMA results indicated that the presence of POSS increased the Youngs modulus as well as the Tg of the EP copolymer. These results suggested that two types of interactions contributed to the physical gelation in EP/POSS melts were present: the strong particle-to-particle interactions between the POSS crystals and the weak particle-to-matrix interactions between the POSS crystals and the EP matrix.

Correct determination of crystal lamellar thickness in semicrystalline poly(ethylene terephthalate) by small-angle X-ray scattering

Abstract For the purpose of resolving an uncertainty over the correct determination of the crystalline lamellar thickness in semicrystalline poly(ethylene terephthalate), PET, via small-angle X-ray scattering (SAXS) analysis, a gel-crystallization method from oligomeric poly- (ethylene glycol) solution was used to prepare samples with high crystallinity (57%). By using simultaneous synchrotron SAXS and wide-angle X-ray diffraction (WAXD) measurements, the heating and cooling processes of the gel-crystallized PET sample were monitored. Results support the assignment of the larger thickness value from the SAXS correlation function analysis as the lamellar crystal thickness. Analysis of WAXD 0 1 1 reflection line broadening gives the minimum lamellar thickness (in the chain axis) and verifies the thickness assignment for gel and melt crystallized samples. This assignment is critical as it affects the correct interpretation of the crystallization behavior in semicrystalline polymers of relatively low crystallinity.

Synthesis and Characterization of Segmented Polyurethanes Containing Polyhedral Oligomeric Silsesquioxanes Nanostructured Molecules

Segmented polyurethanes based on diphenylmethane-4,4′-diisocyanate and polytetra-methylene glycol were synthesized using a mixture of polyhedral oligomeric silsesquioxane (POSS)-diol and 1,4-butanediol as chain extenders. The polymers were characterized by differential scanning calorimetry, wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS) and tensile property tests. Microphase separation between the hard and soft segment domains was observed in all the samples by SAXS. The increase of the POSS concentration was found to weaken the microphase separation between the domains and increase the T g of the soft segments. The WAXD results showed that when the POSS concentration was greater than 10 wt%, the 101 diffraction peak from the POSS crystals could be observed, which suggested the formation of POSS nanocrystals in the hard domain. The tensile property tests showed that polyurethanes containing the nanostructured POSS molecules had higher moduli, but lower maximum elongation ratios.

Uniaxial Deformation of Nylon 6–Clay Nanocomposites by In-Situ Synchrotron X-Ray Measurements

An in-situ uniaxial deformation study of nylon 6–clay hybrid (NCH) nanocomposites was carried out using simultaneous wide-angle x-ray diffraction (WAXD) and small-angle x-ray scattering (SAXS) techniques with synchrotron radiation. This nanocomposite system exhibited the partial exfoliated clay morphology. The WAXD patterns along the through-and edge-views from the as-pressed film samples indicated that both clay platelets and nylon 6 crystallites were parallel to the film plane but oriented randomly in the plane. During uniaxial deformation, the orientation of the molecular axis in the crystals increased with strain but decreased with the clay content. This behavior could be attributed to the rotation of clay platelets during deformation, which hindered the orientation of nylon 6 crystals. The corresponding tensile mechanical properties indicated that the elongation at break decreased and the tensile modulus and strength increased with the clay concentration, which has been observed before.

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