Methylsiloxane Polymers Research Articles

Fate of dimethylsilanediol (DMSD) in soil-plant systems

Dimethylsilanediol (DMSD) is the degradation product of methylsiloxane polymers and oligomers such as volatile cyclic methylsiloxanes (cVMS). To better understand the environmental fate of this key degradation product, we conducted a three-part study on the movement of DMSD in soil. The objective of this third and final study was to determine the fate of DMSD in soil-plant systems under constant irrigation. Soil columns were constructed using two soils with the upper 20 cm layers spiked with 14C-labeled DMSD. Corn seedlings were transplanted into the soil columns and placed in a field plot underneath a transparent cover that prevented rainwater from reaching the soil columns while allowing soil water to be volatilized freely. The soil-plant columns were regularly irrigated with known amounts of DMSD-free plant growth solution to sustain the plant growth. At pre-determined time intervals (15–67 days), the plant and soil columns were sectioned and the distribution of 14Corganosilicon species in the soil profile and plant parts was determined using a combination of Liquid Scintillation Counting and High-Performance Liquid Chromatography-Flow Scintillation Analysis, while soil water loss was determined gravimetrically. It was found that the majority (>92 %) of DMSD initially spiked into the soil was removed from the soil-plant systems. Although DMSD was transported from the soil to the plant, it was subsequently volatilized from the plant via transpiration, with only a small fraction (∼5%) remaining at the conclusion of the experiments. In addition, little non-extractable DMSD was found in the top layer of soil in the soil-plant systems, suggesting that the air-drying of soil is a necessary pre-condition for the formation of such non-extractable silanol residue on topsoil.

Rapid and Clean Covalent Attachment of Methylsiloxane Polymers and Oligomers to Silica Using B(C6F5)3 Catalysis.

The rapid, room-temperature covalent attachment of alkylhydridosilanes (R3Si-H) to silicon oxide surfaces to form monolayers using tris(pentafluorophenyl)borane (B(C6F5)3, BCF) catalysis has recently been described. This method, unlike alternative routes to monolayers, produces only unreactive H2 gas as a byproduct and reaches completion within minutes. We report the use of this selective reaction between surface silanols and hydridosilanes to prepare surface-grafted poly(dimethylsiloxane)s (PDMSs) with various graft architectures that are controlled by the placement of hydridosilane functionality at one end, both ends, or along the chain of PDMS samples of controlled molecular weight. We also report studies of model methylsiloxane monolayers prepared from pentamethyldisiloxane, heptamethyltrisiloxane (two isomers), heptamethylcyclotetrasiloxane, and tris(trimethylsiloxy)silane. These modified silica surfaces with structurally defined methylsiloxane groups are not accessible by conventional silane surface chemistry and proved to be useful in exploring the steric limitations of the reaction. Linear monohydride- and dihydride-terminated PDMS-grafted surfaces exhibit increasing thickness and decreasing contact angle hysteresis with increasing molecular weight up to a particular molecular weight value. Above this value, the hysteresis increases with increasing molecular weight of end-grafted polymers. Poly(hydridomethyl-co-dimethylsiloxane)s with varied hydride content (3-100 mol %) exhibit decreasing thickness, decreasing contact angle, and increasing contact angle hysteresis with increasing hydride content. These observations illustrate the importance of molecular mobility in three-phase contact line dynamics on low-hysteresis surfaces. To calibrate our preparative procedure against both monolayers prepared by conventional approaches as well as the recent reports, a series of trialkylsilane (mostly, n-alkyldimethylsilane) monolayers was prepared to determine the reaction time required to achieve the maximum bonding density using dynamic contact angle analysis. Monolayers prepared from hydridosilanes with BCF catalysis have lower bonding densities than those derived from chlorosilanes, and the reactions are more sensitive to alkyl group sterics. This lower bonding density renders greater flexibility to the n-alkyl groups in monolayers and can decrease the contact angle hysteresis.

Comb-Like Polymethylsiloxanes. Synthesis, Structure and Properties

Comb-like polymethylsiloxanes consisting of a silsesquioxane backbone and dimethylsiloxane side chains were synthesized by means of the “grafting to” method using a linear polyfunctional matrix [SiMe(ONa)O]n and monofunctional oligomers (CH3)3Si[OSi(CH 3)2]nOCOCH3. The effects of architecture of the synthesized methylsiloxane polymers on their physicochemical properties, rheology, and behavior at the air–water interface were studied and compared with those for the linear analogue – polydimethylsiloxane.

Chemical Eradication of Pyrrosia piloselloides (syn Drymoglossum pilosselloides), a Debilitating Fern Epiphyte of Tea and Other Trees

Pyrrosia piloselloides, a common tropical epiphytic fern, harms its host by smothering its growth. In tea, it inhibits the regrowth and renewal of the plucking table. Manual eradication is laborious as the rhizames adhere strongly to the branches and the fern spreads throughout and deep in the canopy. Glufosinate ammonium, at the recommended rate for weeds under shade, can kill the fern without serious damage to the tea host. Efficacy is improved with adjuvants, in particular those that are a blend of petroleum oil, their derivatives and fatty acid esters and those that contain methylsiloxane polymers. The effect of the herbicide is also much more pronounced if the fern is exposed to sunlight at time of application, as during pruning. Keywords: Pyrrosia piloselloides, epiphyte, weed, chemical eradication, tea.

3D Composites Based on Hydroxyapatite-Chitosan-Polysiloxane as a Biomimetic Scaffold Materials

ABSTRACTThree-dimensional (3D) composites of hydroxyapatite, phosphorylated and/or nonphosphorylated chitosan and sulphonated methylsiloxane polymer (HA/P-CHI: CHI/ S-MSP) were prepared by solid-liquid phase separation and solvent sublimation method. The HA, P-CHI and S-MSP were synthesized by chemical continuous wet-calcination, phosphorylation reaction and hydrosilylation-sulphonation reactions, respectively. 3D composites were analyzed by scanning electron microscopy (SEM). Swelling ability, stability and formation of HA particles on the composites immersed in simulated body fluid (SBF) were analyzed. Osteoblasts showed high viability when cultured on composite scaffolds. 3D composites were not degraded after 8 weeks of subcutaneous implantation in rats.

Two-dimensional solid state NMR characterization of physisorbed siloxane polymer (OV-225) on silica

Physisorbed cyanopropyl–methyl–phenyl–methyl–siloxane polymer on a silica surface was characterized by one- and two-dimensional solid state NMR techniques including heteronuclear proton–silicon correlation spectroscopy. Spin–lattice relaxations of protons of the siloxane polymer exhibited only small changes upon anchoring to the silica surface indicating somewhat altered molecular dynamics of proton moieties that contribute to the relaxation process. However, the same relaxation rates of the siloxane polymer’s silica atoms were reduced due to restricted mobility of the polymer. Proton–silicon heteronuclear correlation spectroscopy (HETCOR) revealed strong correlations of silanol protons with both Q 3 and Q 4 sites of the silica surface. In addition, a correlation between methyl protons and the Q 3 site of the silica surface was observed when HETCOR experiments with very small mixing time (5 ms) were performed. The presence of these correlations is indicative of the coherent magnetization transfer mainly through dipolar mechanisms. Since magnetization transfer through the dipolar mechanism is 1/ r 3 dependent, methyl protons must lie in close proximity to the silica surface. Hydrogen bonding of the silica surface’s hydroxyl protons with the bridging oxygen of the siloxane polymer is most likely responsible for positioning the methyl protons closer to the surface. Additional correlations between 29Si nuclei and methylene protons next to cyano group was also observed with mixing time indicating the closer proximity of these protons to the silica surface as well. This juxtaposition of methylene protons is most likely due to hydrogen bonding of the siloxane polymer through the cyano moiety. Furthermore, the hydrogen bonding through the cyano group is most likely to be in parallel orientation to the surface. Finally, the aromatic protons exhibited weak correlations only with Q 3 sites, indicating that these protons must also lie in close proximity of the silica surface.

Thermal decomposition of cyclolinear and cyclic methylsiloxane polymers

AbstractA series of branched‐chain methylsiloxane polymers built up of four‐membered siloxane cycles and linear segments of zero to four siloxy units, and another series consisting only of siloxane cycles of five to eight members were investigated. The samples were pyrolyzed at temperatures increased stepwise from 300 to 800°C. The pyrolysate was analyzed online by gas chromatography and mass spectrometry. In the pyrolysates, series of cyclic, bicyclic, and polycyclic methylsiloxane compounds were found. Quantitative analysis of the pyrolysates shows that their composition reflects the composition and structure of the pyrolyzed polymethylsiloxane. The amount of the cyclic dimethylsiloxanes is related to the length of the linear siloxane segments between the branching points. The amount of bicyclic and polycyclic dimethylsiloxane oligomers in the pyrolysates is related to the average frequency and vicinity of the branching points.

Separation of urinary steroids by gas—liquid chromatography on packed and mixed-bed columns

Gas-liquid chromatography (GLC) is one of the most widely used and effective techniques for group analysis of hormonal steroids. Packed columns with liquid phases OV-1, OV-17 and SE-30 are most often used and are most thermally stable. OV-1 methylsiloxane polymer is a non-selective phase employed for separating steroids according to their molecular weights and shapes. The selective phase OV-225 is a methylphenylpolysiloxane containing 75% of phenyl groups: it is used for further differentiation of functional groups of steroid molecules. We mixed nine parts of OV-1 (1%) and one part of OV-225 (3%) both on Gas-Chrom Q (100-120 mesh) and thus obtained a mixed-bed column, which enabled a higher degree of separation of some clinically important steroid metabolites.

Thermal decomposition of cyclo-linear methylsiloxane polymers

Abstract The pyrolysis products from two cyclo-linear methylsiloxane polymers have been analysed, and formation of the important products interpreted in terms of a simple decomposition mechanism. This mechanism is very similar to that of the polydimethylsiloxane degradation; thus cyclic oligomers are formed from the linear segments, and polycyclic compounds from the cyclic segments of the cyclolinear methylsiloxane polymers.

Pyrolysis gas chromatography of branched-chain methylsiloxane polymers

The catalyzed and non-catalyzed thermal degradation of a branched-chain methyl siloxane polymer system has been investigated by pyrolysis gas chromatography measurements using the pyrolysis unit described in a previous paper. The effects of the catalysts KOH, DMSO and KOH+DMSO, in addition to that of the pyrolysis time, upon the nature and distribution of the products has been examined. The pyrolysis products have been identified and it has been shown that during the course of thermal treatment slow processes occur in the presence of DMSO. for this reason, the mechanism of thermal degradation in the presence of DMSO is different from that of the non-catalyzed and KOH-catalyzed pyrolysis. The slow processes mentioned above are most probably related to the migration of the CH 3 groups along the Si-O-Si skeleton.

Gas-liquid chromatographic determination of human fecal bile acids

A method for the determination of total bile acids in human feces that is suitable for routine application is described and discussed. Bile acids are extracted from freeze-dried feces with acetic acid and toluene, in the presence of the internal standard 23-nordeoxycholic acid. After saponification of the extract, bile acids and the internal standard are methylated and converted by mild chromic acid oxidation into their ketonic derivatives. The resultant mixture of a few stable compounds can be separated and measured quantitatively by gas-liquid chromatography on a methylsiloxane polymer. A reference bile acid mixture including the internal standard is also taken through the entire procedure with each series of samples. It has been demonstrated that, in spite of the omission of the usual purification steps, the method is specific for bile acids.

Gas-liquid Chromatography and Structure-Retention Time Relationship of Anhalonium Alkaloids and Related Bases

Gas-liquid chromatographic separation of 18 anhalonium alkaloids and related bases has been studied employing 1 per cent methylsiloxane polymer (containing about 5 per cent phenyl substitution) as the liquid phase. The GLC behavior of these closely related compounds suggested a structure-retention time relationship. In general, <i>N</i>-monomethylation of primary and secondary amines, <i>O</i>-methylation, or <i>C</i>-monomethylation of the bases studied was found to decrease the retention time, while introduction of hydroxyl, methoxyl, or methylenedioxy group, or an unsaturation, produced an increase in retentivity. These changes in retention time may be attributed to the changes in polarity of amino and/or phenolic hydroxyl groups brought about by substitution in or adjacent to these groups.

Analysis of Methyl Fluorosilanes from Methypolysiloxanes by Gas Chromatography

A gas Chromatographic method has been developed for determining the composition (relative amounts of Me3-Si-, Me2Si=, and MeSi≡ groups; of methylsiloxane polymers. The polymers which cannot be analyzed as such, are depolymerized to methylfluorosilane monomers with boron trifluoride diethyl etherate in an enclosed system. The methylfluorosilane reaction products are then analyzed by gas chromatography. The procedure is modified for siloxanes containing silanic hydrogen.