Octamethyl Polyhedral Oligomeric Silsesquioxanes Research Articles

Encapsulated Atomic Hydrogen in Octamethyl-POSS Cages: A Pulsed EPR Study.

Encapsulated atomic hydrogen in polyhedral oligomeric silsesquioxane (POSS) cages is a promising candidate for spin-based quantum technologies. Key parameters such as spin relaxation times and magnetic interactions with surrounding electron and nuclear spins can be typically probed with advanced electron paramagnetic resonance (EPR) methods. Here we present a detailed pulsed EPR study of the species H@Si8O12R8 with R=CH3, namely encapsulated atomic hydrogen in the octamethyl POSS derivative. The temperature dependence of the spin-lattice relaxation rate 1/ is analyzed in terms of a Raman process with a Debye temperature of K and a thermally activated process with (552 cm-1), whereas, the phase memory time shows the typical shortening behaviour at K observed for all methyl-containing derivatives. The hyperfine coupling of the cage Si nuclei is measured by hyperfine sublevel correlation (HYSCORE) spectroscopy and is found to fulfil the so-called "matching condition" at the low-field EPR transition. The potential of this paramagnetic molecule to perform one-qubit quantum operations is probed by room-temperature Rabi oscillations.

A "Green" Stirring Plasma Functionalization Strategy for Controllable Oxygen-Containing Functional Groups on Octa-Methyl POSS Microstructure.

The distinctive cage-like structure of polyhedral oligomeric silsesquioxane (POSS) materials makes them highly effective fillers in composite membranes for separation applications. However, realizing their full potential in the application often requires specific surface functionalization with various groups. However, this requirement remains challenging owing to the limitations of wet-chemistry approaches, which frequently result in the generation of hazardous chemical by-products. In this paper, a "green" stirring plasma strategy is presented for the functionalization of octa-methyl POSS sub-micron particles into designable oxygen-containing functional groups using a low-pressure oxygen plasma from combined continuous wave and pulsed (CW+P) modes. Plasma from oxygen gas with CW mode offers highly oxygen-reactive species to continuously etch and activate the surface of the POSS. The resulting pulsed plasma assists in grafting more reactive oxygen species onto the active methyl groups of the POSS to form specific oxygen-containing functional groups including hydroxyl and carboxyl. A precise control of nearly one hydroxyl or one carboxyl group at the corner of the cage structure of the POSS is demonstrated, without damaging the core. Therefore, the plasma process discussed in this work is suggested by the authors as controllable fundamental research for the surface functionalization of sub-micron particles, promoting a more environmentally friendly pathway for the preparation of designable fillers.

Single step synthesis of Janus nano-composite membranes by atmospheric aerosol plasma polymerization for solvents separation

Solvent permeation across membranes is limited due to physical resistance to diffusion from the selective layer within the membrane and to plasticizing effects generated by the solvent molecules onto the polymeric macromolecular matrix. Nano-composite thin film membranes provide promising routes to generate controlled microstructural separation materials with higher selectivities and permeabilities. Here, the fabrication of nano-composite based on octamethyl-polyhedral oligomeric silsesquioxane - hexamethyldisiloxane thin film membranes is demonstrated by aerosol assisted atmospheric plasma deposition onto pre-formed nano-porous membrane supports for the first time. Stable, atomically smooth and continuous solid films with controllable thickness down to 50 nm were achieved. The deposition process allowed for the control of the wettability of the surfaces to water and organic solvents, leading to the generation of hydrophobic but alcohol-philic surfaces. The liquid entry pressure of the films to water was found to be 8 bar from plasma polymerization as oppose to 3 bar for the bare nano-porous support only. In addition, the ideal separation selectivity for ethanol to water, up to 6.5, highlight the impact of both the surface energy and level of cross-linking of the hexamethyldisiloxane nanostructures on the diffusion mechanisms. This new atmospheric plasma deposition strategy opens-up cost-effective and environmentally friendly routes for the design of the smart Janus membrane with customizable properties and performance.

Polyethylene/polyhedral oligomeric silsesquioxanes composites: Electrical insulation for high voltage power cables

Xylene solution blending was tested as a method for producing low-density polyethylene (LDPE)-based nanodielectrics containing 1 wt% and 5 wt% polyhedral oligomeric silsesquioxanes (POS) of different types — octamethyl POS, octaisobutyl POS, and isooctyl POS. The chemical composition, the morphology, the thermal properties and the dielectric properties of LDPE and its composites were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, thermal conductivity measurements, broadband dielectric spectroscopy, progressive-stress breakdown tests, and surface partial discharge tests. The results showed that although xylene solution blending was an effective method for producing LDPE/POS dielectric composites, it appeared ineffective in dispersing POS at the nanoscale. Furthermore, some of the composites presented an improved thermal conductivity, a maintained breakdown strength and an enhanced resistance to corona discharges.

Molecular Dynamics Force-Field Refinement against Quasi-Elastic Neutron Scattering Data.

Quasi-elastic neutron scattering (QENS) is one of the experimental techniques of choice for probing the dynamics at length and time scales that are also in the realm of full-atom molecular dynamics (MD) simulations. This overlap enables extension of current fitting methods that use time-independent equilibrium measurements to new methods fitting against dynamics data. We present an algorithm that fits simulation-derived incoherent dynamical structure factors against QENS data probing the diffusive dynamics of the system. We showcase the difficulties inherent to this type of fitting problem, namely, the disparity between simulation and experiment environment, as well as limitations in the simulation due to incomplete sampling of phase space. We discuss a methodology to overcome these difficulties and apply it to a set of full-atom MD simulations for the purpose of refining the force-field parameter governing the activation energy of methyl rotation in the octa-methyl polyhedral oligomeric silsesquioxane molecule. Our optimal simulated activation energy agrees with the experimentally derived value up to a 5% difference, well within experimental error. We believe the method will find applicability to other types of diffusive motions and other representation of the systems such as coarse-grain models where empirical fitting is essential. Also, the refinement method can be extended to the coherent dynamic structure factor with no additional effort.

Crystallization and mechanical properties of biodegradable poly(p-dioxanone)/octamethyl-polyhedral oligomeric silsesquioxanes nanocomposites via simple solution casting method

In this study, biodegradable poly(p-dioxanone) (PPDO)/octamethyl-polyhedral oligomeric silsesquioxanes (ome-POSS) nanocomposites were fabricated by the simple solution casting method with various ome-POSS loadings. Scanning electron microscopic observations indicate that ome-POSS is well dispersed in the PPDO matrix. Effect of ome-POSS on the isothermal melt crystallization and dynamic mechanical properties of PPDO in the nanocomposites were studied in detail. It shows that the overall crystallization rates are faster in the nanocomposites than in neat PPDO and increase with the increase in ome-POSS loadings; however, X-ray diffraction patterns, POM and the Avrami exponent suggest that the crystal structure and the crystallization mechanism do not change despite the presence of ome-POSS. The mechanical property of PPDO/ome-POSS nanocomposites was enhanced with respect to neat PPDO.

Effects of functional groups on the solubilities of polyhedral oligomeric silsesquioxanes (POSS) in supercritical carbon dioxide

The phase behavior of the binary systems containing CO2 and various hybrid polyhedral oligomeric silsesquioxanes (POSS) with different functional groups were investigated. A high-pressure view cell was used to measure the dew-points or cloud-points at temperatures between 308 and 323K, up to 30MPa. Among the studied POSS structures, methacryl, isooctyl and octaisobutyl POSS were observed to form homogenous solutions with CO2, while octamethyl POSS was found to be insoluble in the supercritical fluid. The solubilities of both methacryl and isooctyl POSS in scCO2 decrease with increasing temperature, while isooctyl POSS exhibits nearly an order of magnitude higher solubility than methacryl POSS at both temperatures. The octaisobutyl POSS-CO2 binary system exhibits a crossover pressure, below which the solubility of the component decreases with increasing temperature. Above this pressure, as the temperature is increased, the solubility of POSS in scCO2 increases. These results show that the functional groups of POSS affect its solubility in scCO2 and the phase behavior of the POSS-CO2 binary system significantly. The scCO2-soluble POSS can be implemented in environmentally benign material processing applications using the supercritical fluid.

Chemically Selective Imaging of Overlapping C–H Stretching Vibrations with Time-Resolved Coherent Anti-Stokes Raman Scattering (CARS) Microscopy

Chemically selective imaging of spectrally overlapping compounds is studied with a time-resolved, femtosecond approach on coherent anti-Stokes Raman scattering (CARS) microscopy taking advantage of time-dependent oscillating CARS amplitude which is sensitive to different chemical components at different time points. Chemically selective imaging is demonstrated for composite material of polypropylene (PP) matrix and om-POSS (octamethyl polyhedral oligomeric silsesquioxane) microparticles having partly overlapping CH stretching vibrations. Inverse Fourier transformation (IFT) was applied to Raman spectra of PP and om-POSS, indicating that the oscillatory structures of the vibrational decays differ markedly and allow selective imaging, with minimally using one time point per spatial point, which is also confirmed by the CARS measurements. CARS decays measured additionally for lipid films of cholesterol and DOPC (1,2-dioleyl-sn-glycero-3-phosphocholine) indicate selective detection of cholesterol at a specific probe delay time. The results of this study show that the tr-CARS technique has potential for chemically selective, nonresonant background free imaging using overlapping vibrations.

Synergistic and antagonistic effects in flame retardancy of an intumescent epoxy resin

Phosphorus‐based products are able to strongly reduce the flammability of epoxy resins. In order to further improve their fire behavior, combinations of a conventional flame retardant (ammonium polyphosphate [APP]) and two other particles have been investigated. The partial substitution of APP by carbon nanotubes reveals an antagonism between these two compounds in terms of heat release rate measured by mass‐loss calorimetry. On the contrary, the combination with octamethyl polyhedral oligomeric silsesquioxane leads to a huge synergistic effect. These effects are mainly attributed to the char structures. Furthermore, the two systems were studied by thermogravimetric analysis, and they exhibit modifications of the degradation temperatures. The existence of different stabilization effects was also revealed. Copyright © 2011 John Wiley & Sons, Ltd.

Preparation and Properties of Biodegradable Poly(l-lactide)/Octamethyl-Polyhedral Oligomeric Silsesquioxanes Nanocomposites with Enhanced Crystallization Rate via Simple Melt Compounding

Biodegradable poly(l-lactide) (PLLA)/octamethyl-polyhedral oligomeric silsesquioxanes (ome-POSS) nanocomposites were prepared via simple melt compounding at various ome-POSS loadings in this work. Scanning and transmission electron microscopy observations indicate that ome-POSS were homogeneously dispersed in the PLLA matrix. Effect of ome-POSS on the nonisothermal crystallization behavior, isothermal melt crystallization kinetics, spherulitic morphology, crystal structure, dynamic mechanical properties, and thermal stability of PLLA in the nanocomposites was investigated in detail. It is found that the presence of ome-POSS enhances both nonisothermal cold and melt crystallization of PLLA in the nanocomposites relative to neat PLLA. The overall isothermal melt crystallization rates are faster in the PLLA/ome-POSS nanocomposites than in neat PLLA and increase with increasing the ome-POSS loading; however, the crystallization mechanism of PLLA remains unchanged. The nucleation density of PLLA spherulites is enhanced, while the crystal structure of PLLA is not modified in the PLLA/ome-POSS nanocomposites. The storage modulus has been apparently improved in the PLLA/ome-POSS nanocomposites with respect to neat PLLA, whereas the glass-transition temperatures vary slightly between neat PLLA and the PLLA/ome-POSS nanocomposites. The thermal stability of PLLA matrix is reduced slightly in the PLLA/ome-POSS nanocomposites.

The use of POSS as synergist in intumescent recycled poly(ethylene terephthalate)

Fire retarded poly(ethylene terephthalate) (PET) has been obtained by the incorporation of octamethyl polyhedral oligomeric silsesquioxane (OMPOSS) and Exolit OP950, a phosphinate-based compound, in recycled PET. The presence of Exolit OP950 only leads to intumescence explaining the improvement of the flame retardancy. The addition of OMPOSS leads to a synergistic effect considerably increasing the fire retarding performances of the polymer in terms of cone calorimetry and limiting oxygen index even if a small thermal stabilisation as well as a very poor dispersion of OMPOSS and OP950 into the matrix has been observed.

Aggregation of POSS Monomers in Liquid Hexane: A Molecular-Simulation Study

Polyhedral oligomeric silsesquioxanes (POSS) are multifunctional molecules that can be employed as building blocks to develop nanocomposite materials whose mechanical properties often improve upon those of traditional polymeric materials. We report here molecular simulation results for the effective potential of mean force between octamethyl POSS monomers and between POSS monomers in which one methyl group has been substituted by a linear alkane chain of nine carbon atoms in liquid normal hexane at 300 and 400 K. The results are discussed and compared to available data for the effective interactions between octamethyl POSS monomers in normal hexadecane. Our results show that the effective short-ranged POSS-POSS attraction is significantly weaker in hexane than it is in hexadecane, perhaps explaining why normal hexane is often the solvent of choice for the preparation of POSS-containing materials. Additionally, we provide results for the radial distribution functions between selected sites in the POSS monomers that can be used both to understand the association between POSS monomers in solution and to parametrize coarse-grained simulation models. Such models will be used to study the formation of POSS-containing supramolecular structures such as lamellae or micelles that are currently not accessible by atomistic simulation and can be compared to experimental observations.