Time filter

Source Type

Kulpsville, PA, United States

Drake K.,Drexel University | Drake K.,Greene Tweed and Co. | Mukherjee I.,Drexel University | Mirza K.,Drexel University | And 2 more authors.
Macromolecules | Year: 2011

Polyaryloxydiphenylsilanes have been studied for decades and are known to be stable at high temperatures. Polybiphenyloxydiphenylsilane was synthesized to further study its high-temperature characteristics. However, condensation reactions between dichlorosilanes and biphenol produced polybiphenyloxydiphenylsilanes that underwent uncontrolled cross-linking through silanol end-groups, when heated to high temperatures (around 275 °C). End-capping the polymers with phenoxy groups (phenol as end-capping agent) to prevent cross-linking or with ethynyl containing end-groups to allow for controlled cross-linking resulted in improved thermal stability compared to the uncapped polymer, which was verified using rheology. Two synthetic routes were developed to end-cap the polymer with phenylethynyl containing substituents. Successful end-capping using lithiumphenylacetylide and 4[(4- fluorophenylethynyl)]phenol was verified chemically by 13C NMR, FTIR, and Raman analysis. Capping was further confirmed by the cessation of growth in molecular weight after addition of the capping reagents as measured by GPC, which led to enhanced high-temperature melt stability relative to that of comparable molecular weight uncapped materials as measured by parallel plate rheometry. Exothermic peaks consistent with ethynyl curing reactions were observed by differential scanning calorimetry (DSC) analysis of ethynyl-capped polymers. This work demonstrates how end-capping can be used to control the reactivity and thermal behavior of a polymer that has high-temperature applications. © 2011 American Chemical Society. Source

Drake K.,Drexel University | Drake K.,Greene Tweed and Co. | Mukherjee I.,Drexel University | Mirza K.,Drexel University | And 4 more authors.
Macromolecules | Year: 2013

A new diacetylinic polymer was prepared through polycondensation of 4,4′-buta-1,3-diyne-1,4-diyldiphenol and dichlorodiphenylsilane. This aromatically substituted siloxane polymer contained thermally cross-linkable diacetylene links in the mainchain. FTIR, Raman, and 13C NMR analysis confirmed the diethynyl group was present in the polymer. DSC analysis showed the polymer had a Tg of 130 C, and a strong exothermic cure peak at 260 C. Parallel plate rheological testing through monitoring of changes in viscosity confirmed the polymer cross-linked during heating. After curing above 260 C, the polymer vitrified, with no detectable Tg observed on subsequent reheating. The activation energy of thermally initiated curing of the diacetylene groups was estimated to be 120 ± 17 kJ/mol from DSC data using the Ozawa Flynn Wall method. TGA analysis in nitrogen starting from uncured polymer showed a 5% weight loss temperature of 541 C and a pyrolysis yield of 82% at 800 C. © 2013 American Chemical Society. Source

Mukherjee I.,Drexel University | Drake K.,Drexel University | Drake K.,Greene Tweed and Co. | Berke-Schlessel D.,Drexel University | And 4 more authors.
Macromolecules | Year: 2010

Curcumin has attracted much attention due to its chemopreventive and anti-inflammatory properties. Here we describe the synthesis of poly[(arylenedioxy)(diorganylsilylene)]s via polycondensation between curcumin and various diorganodichlorosilanes. These novel polymers incorporate the β-diketone unit of curcumin as well as the Si-O bond in the backbone. The polymer structure was characterized by means of 1HNMR, FTIR, and elemental analysis, while GPC results showed high molecular weights. Preliminary cell culture results suggest lack of cytotoxicity, which is important for potential applications such as implant and scaffold materials. The T gs of these polymers are in the 24 to 131°C range, tunable by altering the pendant organic groups. The un-cross-linked polymers are stable at 250°C in air. The presence of vinyl groups in the backbone also allows the possibility for thermal cross-linking. DSC and rheology data demonstrate that the materials can cross-link at a temperature above 200°C which suggests the feasibility of melt processing these polymers via a technique wherein a low viscosity polymer is made to flow into a heated mold where it cross-links over time and becomes a rigid thermoset material. © 2010 American Chemical Society. Source

Kim D.-M.,Korea Institute of Ceramic Engineering And Technology | Kim D.-M.,Korea University | Lee S.-H.,Greene Tweed and Co. | Alexander W.B.,Greene Tweed and Co. | And 3 more authors.
Journal of the American Ceramic Society | Year: 2011

We have prepared oxides having various yttrium to aluminum ratios and exposed them to fluorine-based plasma. The etch rate of the aluminum oxide decreased abruptly with the addition of yttrium oxide and then slowly with further addition. The yttrium oxide had a more fluorinated surface than the aluminum oxide, indicating that the etch rate was not determined by the surface fluorination, contrary to the etching of Si-based materials. From the X-ray photoelectron spectroscopy (XPS) analysis of a multication YAlO3 single crystal, a similar tendency was observed, showing a higher ratio of Y-F to Y-O bonding than the ratio of Al-F to Al-O. Angle resolved XPS and depth profiling analysis revealed the presence of a fluorinated layer of a few nanometer thick under a roughly 1 nm carboneous top layer. The etching behavior and surface chemical status of these oxides were discussed in terms of thermodynamic aspects of aluminum and yttrium fluoride. © 2011 The American Ceramic Society. Source

Han J.H.,Greene Tweed and Co. | Khawaja A.,Greene Tweed and Co. | Kilic M.H.,Greene Tweed and Co. | Ingram K.,Greene Tweed and Co.
CAMX 2015 - Composites and Advanced Materials Expo | Year: 2015

As random discontinuous long-fiber (DLF) composites are increasingly finding utility in various aerospace structural applications, less time-consuming and less expensive methods of certifying DLF composite components are being demanded by the aerospace industry. Greene, Tweed (GT) has spent the past few years working on the development of structural analysis methods for DLF composites to reduce individual part testing to decrease expenses for certification. However, establishment of structural analysis methods for DLF composites is a challenging task because DLF structural performance is dependent on fiber orientation that is significantly influenced by the molding process. Structural analysis of DLF parts for performance requires proper processing models and computation tools to predict the fiber orientation in compression molded DLF components. Furthermore, use of flake molding compounds chopped from prepreg tape for DLF composites make the processing simulation modeling more challenging. To date, a few compression molding simulation tools capable of predicting flow pattern, fiber orientation, and fiber length distributions for long-fiber reinforced thermoplastic composites are commercially available. It is critical to identify the prediction errors prior to the use of the simulation results to complement the structural analysis methods for DLF composites. In this study, Moldflow-one of the few compression molding simulation tools-is used to predict fiber orientation in DLF components. Moreover, validation of fiber orientation prediction of Moldflow has been conducted by comparing the simulation results with the CT images of actual specimens. It is observed that the prediction of Moldflow is in reasonable agreement with the fiber orientation obtained from CT images of a component with simple shape and geometry. However, Moldflow fails to predict the fiber orientation of complex shape and small size components that develop high back pressure during packing (after the mold is filled), which changes the fiber orientation tensors. Source

Discover hidden collaborations