Siloxane Matrix Research Articles

Study on some alkoxysilanes used for hydrophobation and protection of wood against decay

Two types of silanes, namely aklylalkoxysilanes and arylalkoxysilanes, were used for wood impregnation aiming at hydrophobisation of the ligno-cellulose material. The suggested mixture of alkyltriethoxysilanes and dialkyldiethoxysilanes improved water repellence when compared to conventionally used silanes. An attempt to immobilise boric acid with the siloxane matrix was also done. Decay resistance of the treated samples was tested by soil-jar test using the brown rot fungus Postia placenta. Features of the impregnated wood such as moisture content, swelling, anti-swelling efficacy and weight percentage gain were not significantly related to the mass loss of samples caused by the fungus. Samples impregnated with EtPhSi(OEt) 2 silanes showed improved durability (3.5–11.5% mass loss). Addition of boric acid to two of the silanols showed the best result for durability of wood, i.e. the lowest mass loss of 2.3 and 1.1%. These treatments upgraded the wood to “very durable”, a significant improvement from its natural status, i.e. slightly durable. The studied alkoxysilanes have moderate ability to penetrate the wood cell wall and hence to improve its hydrophobicity. These silanes are promising although more research should be carried out on the degree of oligomer polymerisation in the wood cell wall.

Influence of the composition and structure of epoxy siloxane matrix on the spectral behavior of the nile red dye: I. Sol-gel system based on tetraethoxysilane and a mixture of epoxy resins

The hybrid organic-inorganic materials with fluorescence properties are synthesized in the form of gels and xerogels from tetraethoxysilane and a mixture of epoxy resins by the sol-gel method. The concentration of the introduced Nile Red dye is 1.6 × 10−6M. The fluorescence spectra of the prepared materials are measured and analyzed as a function of the ratio between the initial components forming the sol-gel epoxy siloxane matrices. The assumption regarding the structure of the synthesized hybrids is made invoking the data obtained by Fourier-transform IR spectroscopy and atomic-force microscopy.

Electrochemical behavior of tungsten-containing nanocomposite with siloxane matrix

Electrochemical properties of a thin-film nanocomposite “silicon-carbon matrix-tungsten carbide” deposited onto pyroceramics (“sitall”) substrate are studied by potentiodynamic curves and electrochemical impedance spectroscopy. Transfer coefficients in model redox system [Fe(CN)6]3−/4− are measured. With the decreasing of the films’ electrical resistance, their experiment behavior gradually changed from that of “poor conductor” till nearly metal-like one. In particular, the electrode differential capacitance increases, which is explained by the increase in the number of conducting metal-containing clusters in the film bulk and at the film/electrolyte solution interface. Some specific features of the complex-plane plots of impedance spectra are tentatively explained by the adsorption at the nanocomposite surface elements.

Development of a Chemiresistor Sensor for Polypropylene Degradation Products

AbstractThis paper presents the development of a sensor to detect the oxidative and radiation induced degradation of polypropylene. Recently we have examined the use of crosslinked assemblies of nanoparticles as a chemiresistor-type sensor for the degradation products. We have developed a simple method that uses a siloxane matrix to fabricate a chemiresistor-type sensor that minimizes the swelling transduction mechanism while optimizing the change in dielectric response. These sensors were exposed with the use of a gas chromatography system to three previously identified polypropylene degradation products including 4-methyl-2-pentanone, acetone, and 2-pentanone. The limits of detection 210 ppb for 4-methy-2-pentanone, 575 ppb for 2-pentanone, and the LoD was unable to be determined for acetone due to incomplete separation from the carbon disulfide carrier.

Synthesis and characterization of chiral liquid‐crystalline polysiloxanes containing fluorinated units and sulfonic acid groups

AbstractChiral side‐chain liquid‐crystalline polysiloxanes (PS‐1, PS‐2, PS‐3, PS‐4, PS‐5, PS‐6) bearing fluorinated units and sulfonic acid groups were synthesized with poly(methylhydrogeno)siloxane, cholest‐5‐en‐3‐ol(3β)‐4‐(2‐propenyloxy)benzoate, and 3‐trifluoromethyl‐phenyl 3‐sulfo‐4‐undec‐10‐ enoyloxy‐benzoate. The effects of fluorinated units and sulfonic acid groups on characteristic of liquid‐crystalline properties were studied. PS‐1, PS‐2, and PS‐3 exhibited both smectic and cholesteric mesophases, while PS‐4, PS‐5, and PS‐6 exhibited only cholesteric mesophase. As the polymers contained more fluorinated units and sulfonic acid groups, segregation of the fluorinated segment to the surface and aggregation of hydrogen bonding should occur. Therefore, the highly ordered lamellar mesogen–siloxane matrix systems should be disturbed severely, suggesting that PS‐4, PS‐5, and PS‐6 show no smectic phase. The maximum reflection bands become broad and shifted slightly to long wavelength from PS‐1 to PS‐6. Copyright © 2008 John Wiley & Sons, Ltd.

Hydrodynamic interaction and coalescence of two droplets under large step shear strains

We observed change in distance between two droplets in each step after application of large multi-step shear strains. Experiments were performed using a sliding plate apparatus. Large step shear strains were applied to two polyisobutylene droplets in poly(dimethyl siloxane) matrix in the same plane between the plates. The distance between the two droplets decreases with increasing the total shear strain, which is given by the product of the step strain magnitude and the number of application of the step strains. The two droplets coalesce when the distance becomes less than the diameter of the droplets. The slope for plots of the distance versus the total strain is independent of the step strain magnitude. This indicates that the effect per unit strain on the distance is the same, irrespective of the strain magnitude. It is suggested that a stronger hydrodynamic interaction between the droplets is the main cause for the droplet approach.

Chiral side-chain liquid crystalline polysiloxanes bearing fluorinated mesogens and cholesteryl groups

A series of chiral side-chain liquid crystalline (LC) polysiloxanes bearing fluorinated mesogens were synthesized with a cholesteric LC monomer and a fluorinated nematic LC monomer. They were characterized by use of various experimental techniques, and effect of fluorinated mesogens on characteristic of LC polysiloxanes was studied as well. In photoluminescence spectra, a narrow and a broad peak occur at around 270–317 nm, originated, respectively, from fluorinated phenyl groups and the conjugated xenene structure. The specific rotation analysis of all polymers showed negative values, but absolute values were lower than those of the chiral monomers. All polymers showed smectic LC phase with very wide temperature ranges on heating and cooling cycles. Especially, only polymers bearing more fluorinated component exhibited smectic–cholesteric phase transition when they were heated. As the polymers contained more fluorinated mesogens, segregation of the fluorinated segment to the surface should occur at mesomorphic temperature. The highly ordered lamellar mesogen–siloxane matrix systems should be disturbed severely by separation of fluorinated mesogens, suggesting mesogenic orders transition from lamellar smectic to cholesteric phase.

High resolution deformation and damage detection using fluorescent dyes

We demonstrate the application of fluorescence microscopy to detect nanoscale deformation and damage in polymeric materials. Fluorescent probes were dispersed in a poly-dimethyl siloxane matrix, and were subsequently strained with and without the presence of edge cracks. This technique can reveal cracks that are invisible to white light microscopy (smaller than 250 nm), and is outperformed only by high resolution electron or scanning probe microscopy. The technique may find applications in early stage damage detection in structural health monitoring systems for a wide variety of polymeric materials.

Cobalt–silicon mixed oxide nanocomposites by modified sol–gel method

Cobalt–silicon mixed oxide materials (Co/Si=0.111, 0.250 and 0.428) were synthesised starting from Co(NO 3) 2·6H 2O and Si(OC 2H 5) 4 using a modified sol–gel method. Structural, textural and surface chemical properties were investigated by thermogravimetric/differential thermal analyses (TG/DTA), XRD, UV–vis, FT-IR spectroscopy and N 2 adsorption at −196 °C. The nature of cobalt species and their interactions with the siloxane matrix were strongly depending on both the cobalt loading and the heat treatment. All dried gels were amorphous and contained Co 2+ ions forming both tetrahedral and octahedral complexes with the siloxane matrix. After treatment at 400 °C, the sample with lowest Co content appeared amorphous and contained only Co 2+ tetrahedral complexes, while at higher cobalt loading Co 3O 4 was present as the only crystalline phase, besides Co 2+ ions strongly interacting with siloxane matrix. At 850 °C, in all samples crystalline Co 2SiO 4 was formed and was the only crystallising phase for the nanocomposite with the lowest cobalt content. All materials retained high surface areas also after treatments at 600 °C and exhibited surface Lewis acidity, due to cationic sites. The presence of cobalt affected the textural properties of the siloxane matrix decreasing microporosity and increasing mesoporosity.

Synthesis of maleimide substituted polystyrene–silica hybrid utilizing michael addition reaction

The transparent polymer hybrids were prepared from polystyrene bearing pendant maleimide moieties (16%) and tetraethoxysilane (TEOS) using γ-aminopropyltriethoxysilane (γ-APS) as a crosslinking agent by an in situ sol–gel process by utilizing Michael-addition reaction. Maleimide substituted polystyrene was synthesized by a mild Friedel-Crafts reaction of polystyrene and N-chloromethylmaleimide. Fourier Transform Infrared (FTIR) spectral data confirms the occurrence of Michael-addition reaction between the pendant maleimide moieties of the styrene copolymer and γ-aminopropyltriethoxysilane. The percentage of maleimide substitution was calculated from 1H NMR spectrum. The transparent hybrid shows high solvent resistance at the boiling point of Tetrahydrofuran (THF) since the polystyrene-substituted-maleimide (PS-s-MA) was covalently bonded with siloxane matrix. Thermal properties of the transparent hybrid materials were investigated by Differential scanning calorimeter (DSC) and Thermo gravimetric analysis (TGA) in order to ascertain their glass transition temperature (Tg) and thermal stability behaviour. Morphology and transparency of the organic–inorganic hybrids were confirmed by Scanning electron microscopy (SEM) and optical images. The homogeneity of the polymer hybrids was also examined by nitrogen porosimetry studies.

Recovery of heavy metal ions from aqueous solutions using modified organosilicas

Sorption of Cu(II), Pb(II), Cd(II), and Zn(II) from aqueous solutions on two-component organosilicas was studied as influenced by sorbent composition, contact time, ratio of solid and liquid phases, solution pH, nature and concentration of heavy metal ions, and content of modifying agent. The degrees of sorption of these metal ions from aqueous solutions using organosilicas modified with aluminum(III) added into the siloxane matrix or with copper(II) grafted to the sorbent surface layer were compared.

Dielectric relaxation behavior of poly(acrylonitrile- co-methacrylonitrile) microcapsules dispersed in a silicone matrix

The dielectric relaxation behavior of poly(acrylonitrile- co-methacrylonitrile) dispersed in a cured polydimethyl siloxane (PDMS) matrix as microcapsules was investigated over multiple thermal cycles and at varying concentrations. The copolymer microcapsules contained an isopentane core. In the PDMS matrix this copolymer displayed a pronounced relaxation signal at temperatures above the glass transition of the copolymers due to Maxwell–Wagner–Sillars (MWS) relaxation. The mechanism of MWS relaxation interpreted by the Havriliak–Negami and Kohlrausch–Williams–Watts relaxation functions was found to be very similar to previous studies of neat polyacrylonitrile and its copolymer. The activation energy of the relaxation decreased over successive thermal cycling coincident with a decreasing strength of the relaxation. These observations were attributed to the decreasing concentration of nitrile groups due to intramolecular cyclizations.

Novel Approach for Biofouling-Release Materials with Interpenetrating Polymer Networks

AbstractEnvironmentally friendly organic-inorganic hybrid materials with repellent activity against marine fouling organisms, such as blue mussel, have been developed using interpenetrating polymer networks (IPNs), composed of a 3-dimensional siloxane matrix of tetraethoxysilane (TEOS) and poly(vinylacetate) (PVAc). Facile bioassay using blue mussels allowed screening the adequate proportions of those components. The repellent activity of IPN with PVAc/silica reached approximately 90% at a specific fraction, relative to that of tributyl tin oxide (TBTO). On the other hand, poly(methylmethacrylate)(PMMA) /silica composite did not show marked repellent activity against blue mussel, even though chemical structure of MMA is similar to that of vinylacetate.

Solid state nitric oxide sensor prepared by sol–gel entrapment of iron-diethyldithiocarbamate in a siloxane matrix

In this work we report the synthesis of a solid state nitric oxide (NO) sensor based on the trapping properties of iron-diethyldithiocarbamate. Electron Spin Resonance was used to quantify the number of NO trapped. The sensors were made by entrapping the iron complex within a siloxane matrix using the sol–gel process. Different synthesis parameters such as pH or annealing temperature have been changed and its effect on the sensor properties were analyzed. It is found that the process of solidification can be accelerated using a sensor with small masses, approximately a few micrograms, using an acid environment and aging temperatures above 50°C. The NO trapped in solid states was found to be more stable than in solution, the NO related ESR signal could be observed even after 14 days. The use of dimethyl formamide was observed to increase the ESR signal related to NO trapping, while improving the matrix structure stability. The sensor can be reused and it is able to measure NO concentrations higher than 0.01mM.

Mechanical properties of ceramic matrix composites with siloxane matrix and liquid phase coated carbon fiber reinforcement

In order to evaluate the benefits of continuous liquid phase coating (CLPC) for carbon fibers, coated fibers as well as uncoated fibers were applied in the preparation of unidirectionally reinforced ceramic matrix composites (CMCs) with polysiloxane based matrix. Fibers coated with precursor based ceramic or carbon coatings were transferred into prepregs by continuous fiber impregnation with liquid polysiloxane and filament winding. The wet prepregs were cut to shape, laminated and then pressed and cured in the mold at 150 °C for 1 h. The cured polymeric matrix composites were calcined and densified by subsequent precursor infiltration/calcination cycles. The flexural strength of the CMCs was measured by 4-point bending tests, the microstructure was determined by optical and scanning electron microscopy. The application of CLPC coated fibers led to a significant improvement in composite strength and young's modulus compared to identical reference samples with uncoated carbon fibers.

Spin-On Catalyst: Straightforward and Flexible Route to Substrate-Grown Single Wall Carbon Nanotubes

The synthesis and characterization of single-wall carbon nanotubes (SWNTs) from simple catalytic precursors at 900 °C from methane and hydrogen is demonstrated. The catalyst precursor is comprised of iron nitrate and a commercially available siloxane polymer solution and is applied to various substrates via spin coating or dipping. The siloxane matrix simultaneously provides for the controlled formation of catalytic nanoparticles via phase separation and, upon heating, a smooth, continuous film of SiOx, which prevents catalyst coarsening and agglomeration at growth conditions. Furthermore, the processibility of the polymer enables the formation of complex, 3D structures from which nanotubes can grow. This combination of processing ease, flexibility, and inherent multifunctionality provides facile, controllable growth of single-wall carbon nanotubes on a multitude of substrates and facilitates additional routes toward SWNT-based device fabrication.

Liquid-crystalline elastomers produced by chemical crosslinking agents containing sulfonic acid groups

A series of siloxane-based liquid-crystalline (LC) elastomers were synthesized by using chemical crosslinking agents containing sulfonic acid groups. The crosslink densities of the elastomers were determined by swelling experiments. Two kinds of melting temperature corresponding to crystallization of siloxane matrix and crystallization of LC segments were clearly detected. Some of the polymers exhibited smectic mesophase textures or cholesteric mesophase textures. A proposed model containing LC segment structure and ionic crosslinking lamellar structure separated by siloxane chains was given. The ion aggregated in domains forces the siloxane chains to fold and form an irregular lamellar structure. Ionic aggregates and LC segments may be dispersed each other to form multiple blocks with increasing ionic crosslinking content.

Development and Characterization of Si-Al-O-N-C Ceramic Composites Obtained from Polysiloxane-Filler Mixtures

Manufacturing and microstructure of novel materials in the Si-Al-O-N-C system derived from polymer-filler mixtures were investigated. Silicon (Si) and aluminium (Al) were added to a poly(methyl and phenyl)siloxane matrix to react with the carbon-bearing products that result from the decomposition of the latter during pyrolysis. This was carried out in nitrogen atmosphere up to 1500 degrees for 2h. Silicon Carbide (SiC) particles were used as inert filler embedded in the polymer/active filler mixture. Green bodies of the metal/polysiloxane mixtures were pressed and cured at 170 °C for 30min. Microcrystalline composites, such as SiC, AIN and SiAlON, from the filler reaction products embedded in a silicon oxycarbide glass matrix could be formed with complex geometry through near-net-shape process of polymer/ceramic conversion.

Hydrogen-Bond Interactions in Organically-Modified Polysiloxane Networks Studied by 1D and 2D CRAMPS and Double-Quantum 1H MAS NMR

Hydrogen-bonding interactions, distribution of various hydroxy groups, and surface morphology in organically modified polysiloxane networks were studied by solid-state NMR techniques based on 1H spin-exchange, double-quantum, and 1H−29Si heteronuclear MAS NMR spectroscopy. 1H CRAMPS experiments revealed four main types of OH groups differing in hydrogen-bond strength, order, and dynamics, which are mutually dipolar-coupled (their interatomic distances are not larger than 0.5 nm), however, not involved in fast chemical exchange even in the fully hydrated state at room temperature. The resulting hydrogen-bonding network is inhomogeneous in the entire set of hydroxyl groups. These findings were correlated with the quantum chemical geometry optimization of hydrogen-bonded local structures and subsequent calculations of 1H NMR chemical shifts. 2D 1H spin-diffusion experiments were used to determine the 1H−1H interatomic distances and to probe the average size of OH clusters, which is ca. 1−2 nm. Intimate mixing of strongly hydrogen-bonded OH and methyl groups was confirmed by 2D double-quantum 1H MAS NMR spectra. Hydrogen-bonding strength of various hydroxyl clusters was evaluated with respect to the type of siloxane structure units and geometry of the siloxane matrix by 2D 1H−29Si heteronuclear experiments.

Molecular design of luminescent organic-inorganic hybrid materials activated by europium (III) ions

Abstract Luminescent hybrid materials consisting in rare-earth (Eu 3+ , Gd 3+ ) organic complexes covalently attached to a silica-based network have been obtained by a sol–gel process. Four dicarboxylic acids with different aromatic subunits (dipicolinic acid, 4-phenyl-2,6-pyridinedicarboxylic acid, 4-(phenylethynyl)-2,6-pyridinedicarboxylic acid and 2,6-Bis(3-carboxy-1-pyrazolyl)pyridine) have been chosen as ligands for Ln 3+ ions. They were grafted to 3-aminopropyltriethoxysilane (APTES) to give organically modified alkoxysilanes that were used as molecular precursors for the preparation of hybrid materials. Ln 3+ first coordination sphere, composition of the siloxane matrix and connection between the organic and inorganic parts have been characterized by infrared spectroscopy, by 13 C 29 Si solid-state NMR as well as by elemental analyses. UV excitation in the organic component resulted in strong emission from Eu 3+ ions due to an efficient ligand-to-metal energy transfer. As compared to reference organic molecules, hybrid samples exhibited similar emission properties under UV excitation in addition to mainly unchanged excited states lifetimes. However, by direct excitation of the Eu 3+ - 5 D 0 energy level, the presence of two different site distributions were evidenced in the four hybrid compounds. Emission features related to each of these site distributions and their respective attribution were investigated. Variations in the relative emission intensities were observed according to the nature of the organic chromophore. These variations were discussed in relation to the ATE (Absorption-Transfer-Emission) mechanism and to the relative energy positions of the ligand and the rare-earth ions respectively.