Evaluation of the suitability of sampling on Tenax TA and polydimethylsiloxane for the analysis of combustion gases

Two sorbents commonly employed for air sampling were selected for the evaluation of their suitability for the analysis of combustion gases namely Tenax TA as adsorbent and polydimethylsiloxane (PDMS) as absorbent. Target compounds were selected among the gaseous combustion products of polyurethane foam and fire-retarded polystyrene. The combustion gases were generated by burning test materials in the flame of a Bunsen burner. Gaseous combustion products were sampled simultaneously with the two sorbents using a two-way adapter, thereby exposing each sorbent to the same combustion gas atmosphere. Special attention was given to the deterioration encountered in the Tenax TA performance upon repeated combustion gas exposure, limiting its use for sampling reactive atmospheres.     

Field effect of screened charges: electrical detection of peptides and proteins by a thin-film resistor.

For many biotechnological applications the label-free detection of biomolecular interactions is becoming of outstanding importance. In this Article we report the direct electrical detection of small peptides and proteins by their intrinsic charges using a biofunctionalized thin-film resistor. The label-free selective and quantitative detection of small peptides and proteins is achieved using hydrophobized silicon-on-insulator (SOI) substrates functionalized with lipid membranes that incorporate metal-chelating lipids. The response of the nanometer-thin conducting silicon film to electrolyte screening effects is taken into account to determine quantitatively the charges of peptides. It is even possible to detect peptides with a single charge and to distinguish single charge variations of the analytes even in physiological electrolyte solutions. As the device is based on standard semiconductor technologies, parallelization and miniaturization of the SOI-based biosensor is achievable by standard CMOS technologies and thus a promising basis for high-throughput screening or biotechnological applications.     

Rheology of polydimethylsiloxane swollen with supercritical carbon dioxide

Viscosity curves were measured for polydimethyl siloxane (PDMS) melts swollen with dissolved carbon dioxide at 50 and 80°C for shear rates ranging from 40 to 2300 s⁻¹, and for carbon dioxide contents ranging from 0 to 21 wt %. The measurements were performed with a capillary extrusion rheometer modified for sealed, high-pressure operation to prevent degassing of the melt during extrusion. The concentration-dependent viscosity curves for these systems are self-similar in shape, exhibiting low-shear rate Newtonian plateau regions followed by shear-thinning “power-law” regions. Considerable reduction of viscosity is observed as the carbon dioxide content is increased. Classical viscoelastic scaling methods, employing a composition-dependent shift factor to scale both viscosity and shear rate, were used to reduce the viscosity data to a master curve at each temperature. The dependence of the shift factors on polymer chain density and free volume were investigated by comparing the shift factors for PDMS-CO₂ systems to those obtained by iso-free volume dilutions of high molecular weight PDMS. This comparison suggests that the free volume added to PDMS upon swelling with dissolved carbon dioxide is the predominant mechanism for viscosity reduction in those systems. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 523–534, 1997     

Dielectric relaxation of polydimethylsiloxane

Dielectric studies were performed on crystallized and amorphous polydimethylsiloxane which had been characterized by differential thermal analysis and polarized light microscopy. The crystallized specimen displayed one relaxation near 160 K at 1 kHz while the amorphous specimen showed absorption peaks at 155 and 165 K. For the latter material the high-temperature peak was not due to a true relaxation but resulted from crystal nucleation at 160 K and the subsequent growth of spherulites. The low-temperature peak at 155 K resulted from the relaxation associated with the glass transition. A sharp decrease of dielectric constant was observed for both specimens at the melting point (235 K). For the dielectric relaxation associated with the glass transition in crystallized specimens, the values of the dispersion amplitude, the apparent activation energy at 160 K, and the half-width of the absorption curve are 0.43 and 29 kcal/mole, and 5.6 decades, respectively, which are in marked contrast to the corresponding values of 0.82, 18, and 2.2 for amorphous specimens.     

Field sampling technique for speciation of inorganic chromium in rivers

A simple and robust field sampling technique has been developed for the determination of chromium (III) and chromium (VI) in rivers. Water samples, on collection, were immediately passed through microcolumns of activated alumina to isolate and retain the desired species. Microcolumns were then returned to the laboratory and inserted into a FI-ICP-ES system for elution/ quantitation. Field sampling performed at 2 stations in S. Yorkshire over a 1 month period yielded elevated concentrations of chromium (III) (8–20g/1) and chromium (VI) (1.1–4.5 g/1) and, for each data set, the sum of the two fractions matched the total chromium concentration.     

Composites of kaolin and polydimethylsiloxane

Kaolin particles were surface-treated with isobutyltrimethoxysilane (IBTMS), hydrogenated tallow (HT), and a polyisobutyl chain-based stabilizer (SAP) to make composites with polydimethylsiloxane (PDMS). IBTMS did not cover the strong acid sites on the kaolin surface and as a result a cross-linking reaction occurred for silanol-terminated PDMS. The polyisobutyl chain of SAP was found to be incompatible with PDMS and this caused aggregation of the kaolin particles. HT was the most effective at dispersing the particles into silanol-terminated PDMS. The aggregation state of the composites was characterized using rheology and microscopy. Both showed the HT-treated particles were well-dispersed in low molecular weight silanol-terminated PDMS, and they were weakly flocculated in higher molecular weight silanol-terminated PDMS. However, the same particles aggregated when dispersed in methyl-terminated PDMS. It appears the silanol-terminated PDMS acted as costabilizer through interaction with the kaolin surface. Transverse relaxation NMR was used to probe mobility of the PDMS chains in the composites. This showed little dependence on surface treatment, aggregation state, or polymer end groups. For all samples, chain mobility decreased with increasing kaolin concentration.     

Thermal degradation of amino-group-modified polydimethylsiloxane

Degradation behaviors of amino-group-modified polydimethylsiloxane (APS) under nitrogen and air atmosphere were studied by thermogravimetric analysis, pyrolysis–gas chromatography-mass spectrometry, and infrared spectroscopy, and the effect of amino-group content on the thermal stability of the tested APS was investigated. Results showed that the existence of amino-group in APS molecule decreased its thermal stability, and the degradation behavior and mechanism of APS in nitrogen and air atmosphere were different.     

A radiofrequency plasma in a polydimethylsiloxane (PDMS) microchip

This is the first report of an analytical plasma in a polymer (polydimethylsiloxane, PDMS) microchip. The plasma channel has dimensions 2 mm diameter × 50 mm long, is operated at atmospheric pressure in Ar, 27.12 MHz and 70 W, and is viewed axially through a purged fiber optic cable. CF₄ gas at 0.1% in argon yields mainly C₂ emission bands. This PDMS microchip is manufactured easily, inexpensive, and more tolerant to fluorocarbons than microchip plasmas in silica. Based on these initial results, this PDMS microchip plasma could become useful as a sensor for the fluorocarbon gases emitted in semiconductor process or as a gas chromatography (GC) detector for potential application.     

Flow behavior of narrow-distribution polydimethylsiloxane

The flow behavior of α,ω-dihydroxypolydimethylsiloxanes, having a weight-average number-average molecular weight ratio of 1.1–1.2, was studied with a Cannon-Manning viscometer and an Instron rheometer. Comparison of the flow behavior of samples with narrow and broad molecular weight distributions indicated that the onset of non-Newtonian behavior occurred at a much higher shear rate for narrow-distribution polydimethylsiloxanes than for polydisperse polydimethylsiloxanes. A plot of reduced viscosity versus \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma \eta _0 {M \mathord{\left/ {\vphantom {M T}} \right. \kern-\nulldelimiterspace} T} $\end{document} gave two experimental master curves, one for polymer of narrow distribution and the other for polydisperse polymer. The experimental master curve obtained from the narrow-distribution polymer was found to fit the theoretical master curve derived from Graessley's entanglement theory. The viscosity–molecular weight relationship for the higher molecular weight polydimethylsiloxanes was found to be the same for both hydroxydimethylsilyl- and trimethylsilyl-endblocked polymers. However, at low molecular weight, the viscosity–molecular weight curve deviated from linearity because of the association of polydimethylsiloxanols, which apparently is not significant at higher molecular weights. The critical molecular weight of entanglement, M_c, was found to be about 30,000.     

Laser induced surface modification of polydimethylsiloxane as a super-hydrophobic material

In order to render the surface of polydimethylsiloxane (PDMS) super-hydrophobic without changing its bulk properties, a PDMS film without photosensitizer was exposed to CO₂ pulsed laser, at room temperature, as the excitation source. The modified surfaces have been studied by performing scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDXA) and attenuated total reflectance infrared (ATR-IR) spectroscopy. To evaluate the surface property, the water drop contact angle was measured. The dependence of ---Si---O---Si infrared peak intensity, O/Si ratio and water drop contact angle of the treated PDMS as a function of the number of laser pulses were studied. SEM micrographs and water drop contact angle variations show the uniform porosity and super-hydrophobic nature on the surface of PDMS. ATR-FTIR spectra show that the modified PDMS surface contains carbonate groups which enriched the oxygen content of the surface. EDXA analysis shows a higher percentage of oxygen on the surface of the modified PDMS. The hydrophobicity of the samples was found to depend upon the number of laser pulses, but with significant variation between the treated samples. The bulk mechanical properties of PDMS after being laser-treated did not change as shown by dynamic mechanical thermal analysis (DMTA).     

Rheology of trimethylsiloxysilsesquioxanes in polydimethylsiloxane

Properties of the MQ polymer solution in PDMS elastomer in the temperature range 266.7–351.1 K are investigated using the viscometry technique. On the basis of viscosity and density isotherms, the solutions in question are shown to belong to the systems whose viscosity is a function of free fluctuation volume of their constituent components. The viscosity of such systems is demonstrated to conform well to the Fulcher-Tamman law. These properties of the solutions are found to ensure better mechanical and thermal qualities of their vulcanizates as compared to PDMS vulcanizates.     

Modification of polycyanurate networks with protic additives based on polydimethylsiloxane

Hydroxy telechelic polycaprolactone / polydimethylsiloxane / polycaprolactone triblock copolymers were used to modify aromatic polycyanurate networks through a polycondensation-induced phase separation. Model studies revealed that the cyclotrimerization mechanism in the presence of alcohol functions was complex. The additives, which activate the reaction, are finally chemically linked to the matrix. Both thermodynamic and kinetic features result in very finely divided morphologies, with highly interpenetrated phases which contribute to a great improve of the static mechanical properties.     

Polysulfone–polydimethylsiloxane block copolymers

Alternating block copolymers have been synthesized from dihydroxyl-terminated polysulfone and bis(dimethylamine)-terminated polydimethylsiloxane oligomers. The products are soluble, amorphous, and transparent, and display excellent thermal and hydrolytic stability. Elastomeric and rigid compositions can be prepared by varying oligomer molecular weight. Copolymers made with oligomers of ≥ 5000 molecular weight are two-microphase systems which display glass transition temperatures at ?120°C and at +160°C, and therefore have a wide useful temperature range.     

Local mobility in grafted polydimethylsiloxane melts

Spin–lattice relaxation time constants, T₁, were studied for low-molecular-weight linear and grafted polydimethylsiloxane over a wide temperature and frequency range. Quantitative evaluations of proton T₁ measurements indicated two relaxation processes: anisotropic rotation of methyl groups around the Si–C bond (low temperature process) and motions of the PDMS side-chains connected with the glass transition (high temperature process). Additional analyses of the T₁ relaxation dispersion profiles revealed specific local segment fluctuation times, which are characteristic of the coherent motions in the grafted polymer chains.     

Model networks of end-linked polydimethylsiloxane chains

Summary Elastomeric networks of high extensibility were prepared by end-linking mixtures of vinyl-terminated polydimethylsiloxane chains having molecular weights of approximately 600 and 11,000 g mol^–1, with silanes chosen to give junction functionalities ranging from 3 to 8. The resulting bimodal networks were studied in elongation, at 25 °C, to their rupture points, and in swelling equilibrium in benzene at room temperature. The elongation moduli [f ^*] were found to be in satisfactory agreement with previous results obtained by end-linking hydroxyl-terminated polydimethylsiloxane chains. Values of [f ^*] at low and moderate deformations gave relatively low values of the ratio of elasticity constants 2C ₂/2C ₁, which is a measure of the extent to which the elongation changes from approximately affine to nonaffine as the elongation increases. The low values obtained for this ratio are presumably due to diminished interpenetration of configurational domains in the case of very short chains. In spite of its small magnitude, 2C ₂/2C ₁ does show some decrease with increase in , as predicted by the recent molecular theory of rubberlike elasticity developed by Flory. The swelling equilibrium results were also found to be in satisfactory agreement with theory. The elongation moduli increased significantly at high elongations, and the values of the elongation at which the upturn was first discernible were very nearly independent of , This is consistent with the interpretation of this anomalous behaviour in terms of limited chain extensibility. The maximum extensibility generally decreased somewhat with increase in and this caused a decrease in both the ultimate strength and the toughness of the elastomer, as measured by the energy required for rupture.     

Automated sampling with capillaries

A system is described for the analysis of serum contained in capillaries. The capillaries, filled with samples, are placed directly into a moving stream of diluent which flushes the capillaries, carrying the samples into a continuous flow or discrete system of analysis. The capillaries are inserted into holes in a plastic block which is pushed forward sequentially by a drive mechanism. As each capillary comes into line with an entrance tube and exit tube, reagent is pumped through these tubes and through the capillary. As an alternative, a dispenser is attached to the inlet tube, and as each capillary comes into position, a measured amount of liquid is dispensed through the capillary and into a container. The system is applied to continuous flow analysis of phosphate, alkaline phosphatase, uric acid, and creatinine. The construction of an efficient and reliable peristaltic pump is described for the continuous flow system.     

Condensation synthesis of high molecular polydimethylsiloxane

Synthesis approach to high molecular organosilicon polymers by condensation way merely starting from low molecular α,ω-dihydroxyoligomers was developed.     

Anodic stripping coulometry at a thin-film mercury electrode

Primary coulometric versions of d.c. anodic stripping voltammetry, performed in a microcell at a mercury-plated rotating glassy carbon electrode, are applied to the determination of cadmium and lead. With exhaustive preelectrolysis of the sample solution, 5–100 ng of the metal can be determined from the charge contained in the stripping signal; precision and accuracy are in the 5% range. An extrapolation procedure that requires only partial metal deposition is evaluated.     

Microfabrication of polydimethylsiloxane electrospray ionization emitters

Microfabricated polydimethylsiloxane (PDMS) emitters for electrospray ionization mass spectrometry (ESI-MS) were implemented as tips along the edge of the PDMS device by three methods which utilize soft lithography processes. These microfabrication methods for producing PDMS emitters as an integral part of a microfluidic device will facilitate development of more complex microfluidic analysis systems using ESI-MS.