Characterization of low molecular weight components of [(ViMe2SiO1/2)x(PhSiO3/2)y(SiO4/2)z], [(ViMe2SiO1/2)x(SiO4/2)y], and [(SiO4/2)x(HO1/2)y(tBuO1/2)z] silsesquioxanes by electrospray ionization fourier transform mass spectrometry (ESI-FTMS)

This paper describes the characterization of low molecular weight components of four materials using electrospray ionization Fourier transform mass spectrometry (ESI-FTMS). The materials in the current study are [(ViMe₂SiO_1/2)_x(PhSiO_3/2)_y(SiO_4/2)_z] (MTQ), [(ViMe₂SiO_1/2)_x(SiO_4/2)_y] (MQ), and [(SiO_4/2)_x(HO_1/2)_y(^tBuO_1/2)_z] (Q) silsesquioxanes. Accurate mass measurements coupled with knowledge of resin chemistry afforded siloxane composition determination that was used to propose specific structures for the oligomers. Branched or linear (T_nQ_mM_n+2m+2), and monocyclic (T_nQ_mM_n+2m) structures are predominant structures for the low molecular weight species in MTQ. For MQ and Q, more condensed structures, such as partially opened cage structures (Q_mM_2m?6 and Q_mM_2m?8), were identified. The differences between MQ, Q, and MTQ are likely attributed to differences in intrinsic structure and reactivity of T and Q building blocks. The structural information obtained for these oligomeric species will ultimately provide a better understanding of new resin materials and their associated physical properties.     

Continuous desorption rate measurement from a shallow-bed of poly(styrene-divinylbenzene) particles with correction for experimental artifacts

A 0.50 mm high bed, containing ca. 3 mg of the nominally non-porous poly(styrene-divinylbenzene) (PS-DVB) sorbent Hamilton PRP-infinity, is located in a valve. After the bed is pre-equilibrated with a (7/3) methanol/water solution of naphthalene (NA), the valve is switched and (7/3) methanol/water solvent flows continuously through the bed at a high linear velocity. This causes NA to desorb into a constantly refreshed solvent, creating a "shallow-bed" contactor with an "infinite bath" kinetic condition. The effluent from the bed passes through a UV-absorbance detector which generates the observed instantaneous desorption rate curve for NA. The same experiment is performed using the solute phloroglucinol (PG), which is not sorbed by PRP-infinity and serves as an "impulse response function marker" (IRF-Marker). The resulting peak-shaped IRF curve is used in two ways (i.e. subtraction and deconvolution) in order to correct the observed instantaneous rate curve of NA for the following experimental artifacts: hold-up volume of the bed and valve, transit-delay time between the bed and the detector and instrument bandbroadening of the NA zone. The cumulative desorption rate curve, which is a plot of moles NA desorbed versus time, is obtained by integration. It is found to be accurately described by the theoretical equation for homogeneous spherical diffusion. The diffusion coefficient of NA inside the PRP-infinity particles (5.0+/-0.6) x 10(-11) cm2/s, agrees with the literature value that was obtained from the sorption rate of NA into the same particles. This constitutes virtually conclusive evidence for diffusion control of intra-particle kinetics of NA in the PS-DVB matrix of PRP-infinity and related polymers. The influence of both sorbent and solute properties on the method is evaluated.     

Insights into the Oxidation Chemistry of SiOC Ceramics Derived from Silsesquioxanes

We have undertaken a systematic study of the oxidation chemistry for a range of SiOC ceramics derived from silsesquioxane polymeric precursors. This study examines the oxidation for 500 hours at 600, 800, 1000 and 1200°C for four SiOC powders. The material changes upon oxidation were characterized qualitatively by color change and optical microscopy and quantitatively by weight and composition change. In this study we employ a very easy method that uses the weight change upon oxidation and a carbon analysis after oxidation to arrive at the composition of the oxidized SiOC. Combined these qualitative and quantitative techniques have shown that on oxidation at 800 and 600°C the SiOC composition is more rapidly changed to that of silica than oxidation over the same time frame at 1000 or 1200°C. The data indicates that this difference is due to the relative rates of oxidation of the excess carbon versus the Si—C bonds in the SiOC. At lower temperatures initially the carbon oxidation predominates which leads to higher porosity throughout the material and an increase in the surface area with eventually complete oxidation to silica. At higher temperatures the Si—C bond oxidation rate is comparable to the rate of oxidation of carbon. This allows a silica-like surface to build up on the SiOC, which slows all subsequent reactions due to the necessity to diffuse O2 in and COx out of the bulk. Under these oxidation conditions materials that originally contain high amounts of excess carbon are more quickly oxidized to silica than those that contain minimal amounts of excess carbon, as confirmed by elemental analysis and optical microscopy. Regardless of the time or temperature of the oxidation conditions no materials were found to be completely stable to oxidation. SiOC materials with low levels of excess carbon showed the best resistance to change upon oxidation.