Siloxane Films Research Articles

Monitoring of the Degree of Condensation in Alkoxysiloxane Layers by NIR Reflection Spectroscopy

This paper introduces a novel analytical approach for monitoring the degree of condensation of thin siloxane films, which is potentially suitable for in-line process control during the deposition of such layers, e.g., to polymer films. Near-infrared (NIR) reflection spectroscopy in combination with chemometric methods was used as a process monitoring tool. The state of the formation of the inorganic Si–O–Si network in partially condensed 3-methacryloxypropyltrimethoxysilane batches was analyzed by inverse gated 29Si NMR spectroscopy. Results were expressed in terms of different relative ratios of the Ti species (i.e., structures with different numbers of Si–O–Si units per Si atom). These data were used for calibration of the NIR method, which was applied to thin layers printed on a polymer foil with a thickness of ∼2.2 g m–2. The root-mean-square error of prediction (RMSEP) for the determination of the ratio of the Ti species from the NIR spectra was found to be less than 3%. The error of the reference data from 29Si NMR spectroscopy is 4%, which results in an overall error of 5%. Moreover, the thickness of siloxane layers was determined by this method in a range from 2.5 to 5.5 g m–2 using gravimetry for calibration (prediction error ∼0.3 g m–2).

Bipolar strain sensor based on an ultra-thin film of single-walled carbon nanotubes

A bipolar strain sensor based on an ultra-thin film of single-walled carbon nanotubes (SWNTs) has been fabricated. First, a random network of SWNTs was grown on a Si substrate with thermal oxide by using chemical vapor deposition and then transferred to a transparent poly(dimethyl)siloxane (PDMS) film. A mechanical load was applied by pressing the PDMS-SWNT film with a blunt micrometer tip, and its electrical conductance was found to decrease linearly with increasing pressure. Upward bending of the flexible PDMS-SWNT film was found to yield increases in conductance whereas downward bending of the film was found to result in decreases in the conductance. We modeled the SWNT network on the PDMS film with a two-dimensional percolation system, and found that the increases (decreases) in the conductance of the film upon bending could be explained in terms of stick-density changes in the 2-D percolation system. Finally, because PDMS swells with certain organic vapors, a PDMS-SWNT film can be used as a chemical sensor for volatile organic compounds. Unlike for three-dimensional composites of SWNTs and polymers, the bipolar response upon bending and simple fabrication process for the system introduced here mean that it is an attractive candidate for tactile and motion sensor applications.

Grafting of collagen onto interpenetrating polymer networks of poly(2-hydroxyethyl methacrylate) and poly(dimethyl siloxane) polymer films for biomedical applications

Poly(dimethyl siloxane) (PDMS) films were modified using poly(2-hydroxyethyl methacrylate) (PHEMA) by sequential method of interpenetrating polymer networks (IPNs). Collagen (type I) was also linked onto the modified films via methyl sulfonyl chloride. Collagen reacted with the activated groups to obtain covalently linked protein layers. The sur- face properties of samples were characterised by attenuated total reflectance Fourier transform infrared spectroscopy (ATR- FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and water contact angle measurements. Dynamic Mechanical Thermoanalysis (DMTA) and tensile testing were used to study the physical and mechanical properties of the IPNs. Grafting of collagen on the surface was confirmed using ATR-FTIR and XPS. The results also showed that the hydrophilic- ity of modified samples increased as the content of PHEMA increased. The biocompatibility of the PDMS surface was eval- uated by culture of fibroblast cells (L929). The collagen-linked surfaces showed significant cell adhesion and growth in comparison with unmodified PDMS samples.

Development and ultra-structure of an ultra-thin silicone epidermis of bioengineered alternative tissue.

Burn wound care today has a primary objective of temporary or permanent wound closure. Commercially available engineered alternative tissues have become a valuable adjunct to the treatment of burn injuries. Their constituents can be biological, alloplastic or a combination of both. Here the authors describe the aspects of the development of a siloxane epidermis for a collagen-glycosaminoglycan and for nylon-based artificial skin replacement products. A method to fabricate an ultra-thin epidermal equivalent is described. Pores, to allow the escape of wound exudate, were punched and a tri-filament nylon mesh or collagen scaffold was imbedded and silicone polymerisation followed at 120°C for 5 minutes. The ultra-structure of these bilaminates was assessed through scanning electron microscopy. An ultra-thin biomedical grade siloxane film was reliably created through precision coating on a pre-treated polyethylene terephthalate carrier.

Capacitive response of PDMS-coated IDE platforms directly exposed to aqueous solutions containing volatile organic compounds

In this work we report on the capacitive response of interdigitated electrodes (IDEs) covered with a thin (~50μm) polydimethyl siloxane (PDMS; Sylgard®184) film to (1) the deposition and evaporation of drops of pure volatile organic compounds (VOCs) and to (2) the direct exposure to aqueous solutions of these VOCs (up to 1mM) under continuous flow conditions. The investigated VOCs were chloroform, methyl tert-butyl ether, 1-hexanol, toluene, m-xylene, and n-hexane. It is shown that the capacitive response changes upon absorption of the VOCs are in both cases in line with their polymer affinity and their relative dielectric constants as compared to the one of thin PDMS layers. In addition, the response changes were fully reversible. Chloroform, methyl tert-butyl ether, and 1-hexanol were measured reproducibly with a detection limit of ~0.1mM. For toluene, m-xylene, and n-hexane the response reproducibility was strongly impaired due to their low water solubility and the lowest measured concentrations were in the order of ~0.2mM for the aromatics and ~0.009mM for n-hexane. Within a closed flow cell system, the presence of a bypass circuit and/or a parasitic parallel impedance to the environment (third electrode effect) was observed. In the case of n-hexane and m-xylene additional features of the response profile were observed before reaching equilibrium, revealing the contribution of additional response mechanisms. This could be rationalized by an overshoot of the VOC concentration in the PDMS or by swelling effects.To conclude, this work reports on the capability of direct detection of organic pollutants in water by capacitive PDMS-coated IDE platforms. It points out the challenges in system design and response interpretation, which facilitates the further development of water pollutant sensoring systems.

Multiscale Rough Titania Films with Patterned Hydrophobic/Oleophobic Features

Oxide-based hybrids are promising systems to modulate the surface properties and impart different functionalities. Here, the wettability features of rough titanium dioxide (TiO2) layers derivatized by siloxanes are tailored with respect to both water and nonaqueous solvents. The adopted synthetic procedure is very simple and may be extended to different substrates, as it is based on the direct functionalization of homemade, tailored, TiO2 nanoparticles by different siloxanes, both fluorinated and nonfluorinated. Nanotitania provides a multiscale roughness able to impart superhydrophobicity and its photocatalytic activity can be exploited to obtain surfaces with patterned wettability by photocatalytic lithography. The behavior of the different siloxanes (oleophobicity degree, self-cleaning properties, and kinetics of ultraviolet degradation) is related to the surface energy components of the bare siloxane films, evaluated by Owens–Wendt classical model, and to the structure of the siloxane monolayer at the...

Potassium spin polarization lifetime for a 30-carbon chain siloxane film

The siloxane film derived from the 30-carbon chain triacontyltrichlorosilane (TCTS) is studied as an anti-relaxation coating for atomic vapor cells. The longitudinal spin relaxation lifetime of optically pumped potassium atoms in the presence of TCTS is measured and the average number of non-relaxing atom-wall collisions, or bounces, enabled by the coated surface is determined. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) of TCTS were performed to investigate changes in chemical states and surface morphology of TCTS arising from K atom deposition on the film surface. TCTS was found to give approximately 530 bounces. Following lifetime measurements, K2p signals were clearly observed in XPS spectra. AFM images display non-preferential K deposition on the TCTS surface, however additional AFM studies with a TCTS surface exposed to Rb atoms show deposition occurs along surface defects. In agreement, Rb is found to preferentially deposit along the step edges of an 18-carbon chain monolayer film derived from 1-Octadecene. Finally, AFM indicates a much smoother surface for a tetracontane coating relative to TCTS. The importance of siloxane surface morphology versus film thickness with respect to coating performance is discussed.

Application of PM-IRRAS to study thin films on industrial and environmental samples

Polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS) was employed to analyze two unique samples: (1) an industrially prepared alkoxysilane-pretreated aluminum alloy (AA6111) in the absence and presence of a ~600-nm-thick lubricant coating and (2) a chemical warfare agent simulant, triethyl phosphate (TEP), on glass. For the pretreated aluminum samples, PM-IRRAS spectra were analyzed for three distinct regions; the SiO stretching vibration around 1120 cm(-1), the NH(2) bending mode at ~1600 cm(-1) and the CH stretching region around 2900 cm(-1). Our results showed that increasing the curing temperature (from 55 to 100 °C) improved the overall extent of cross-linking within the siloxane network. In addition, the spectra of lubricant (top coating) and the underlying siloxane layer for the aluminum samples with lubricant were collected for the same sample. Our results show that the nature of the siloxane film remains intact and unaltered after deposition of the lubricant top-coat. For detection of TEP on glass, the band at 1268 cm(-1), corresponding to the P═O vibration, was monitored. A droplet of TEP solution in dichloromethane was deposited on glass. After solvent evaporation had occurred, the intensity of the P═O vibration band was used to construct calibration curves to determine the experimental limit of detection, which was found to be ~200 μg for TEP on glass.

Synthesis of multi-layer zeolite LTA membranes with enhanced gas separation performance by using 3-aminopropyltriethoxysilane as interlayer

A facile strategy for the layer-by-layer synthesis of sandwich-like zeolite LTA membranes by using 3-aminopropyltriethoxysilane (APTES) as modifier is reported. APTES reacts with surface hydroxyls and forms a covalently bonded siloxane interlayer. APTES treatment is helpful to promote nucleation and growth of the zeolite LTA layer on solid surfaces. Further, the covalently bonded self-assembled siloxane film can protect the already formed LTA layer from dissolution and transformation during the next hydrothermal LTA synthesis step, thus facilitating the formation of well intergrown and phase-pure multi-layer zeolite LTA membranes. The multi-layer zeolite LTA membranes display molecular sieving performance, and each additional LTA layer improves the selectivity of the sandwich-like LTA layer stack. In mixed gas separation at 100°C, the mixture separation factors for equimolar mixtures of H2/CO2, H2/N2, H2/CH4 and H2/C3H8, on the single/multi-layer LTA zeolite membranes in the Na+ state are found to be 6.3, 5.6, 4.3 and 6.5 (single-layer), 8.8, 7.2, 5.8 and 12.6 (double-layer), and 12.5, 8.6, 6.5 and 19.3 (triple-layer).

Patterning of TiO<SUB>2</SUB> Particles on Poly(dimethyl siloxane) Films by Using Proton Irradiation and Liquid-Phase Deposition Process

Micropatterning of titanium dioxide (TiO2) on the surface of thin poly(dimethyl siloxane) (PDMS) films was described by means of proton irradiation and liquid-phase deposition (LPD) techniques. The surface of thin PDMS films was irradiated with accelerated proton ions through a pattern mask in the absence or presence of oxygen in order to create hydrophilically/hydrophobically patterned surfaces. The results of the surface analysis revealed that the PDMS films irradiated at the fluence of 1 x 10(15) ions cm-2 in the presence of oxygen showed the highest hydrophilicity. The LPD of TiO2 particles on the patterned PDMS film surface showed a selective deposition of TiO2 on the irradiated regions, leading to well defined TiO2 micropatterns. The crystal structure of the formed TiO2 films was found to be in an anatase phase by X-ray diffraction analysis. This process can be applied for patterning various metal and metal oxide particles on a polymer substrate.

Superhydrophobic polymer‐particle composite films produced using various particle sizes

Hydrophilic alumina (Al2O3) nanoparticles (25, 35, and 150 nm) are dispersed in different concentrations in solutions of a commercial hydrophobic poly(alkyl siloxane) (Silres BS‐290), and the suspensions are sprayed on glass surfaces. Static contact angles (θS), measured on surfaces of siloxane‐nanoparticle composite films that were prepared from dilute dispersions, increase rapidly with particle concentration. Composite films prepared from concentrated dispersions exhibit a maximum, constant θS (at saturation θS is 160°), which is not affected by the size of the particles. These films exhibit also very small contact angle hysteresis (5°), which is also independent of the particle size. Consequently, the same superhydrophobic character can be induced in siloxane films using nanoparticles, which can range from a few up to several tenths of nanometers. However, the particle size and more precisely the particle specific surface area affects dramatically the minimum critical particle concentration, which must be used in the dispersions to induce superhydrophobicity on the surface of the composite films, that is, to achieve θS = 150°. It is shown that critical particle concentration decreases exponentially with specific surface area. This result can be important for manufacturers of superhydrophobic surfaces who are interested in having a good control on the wettability of the composite films. Copyright © 2012 John Wiley & Sons, Ltd.

Electrochemical sensor based on imprinted sol–gel and nanomaterial for determination of caffeine

The development of a novel sensitive molecularly imprinted electrochemical sensor is described for the detection of caffeine. The sensor was prepared onto a glassy carbon electrode modified with multiwall carbon nanotubes (MWCNTs)/vinyltrimethoxysilane (VTMS) recovered by a molecularly imprinted siloxane (MIS) film prepared by sol–gel process. MWCNTs/VTMS was produced by a simple grafting of VTMS on MWCNTs surface by in situ free radical polymerization. The siloxane layer was obtained from the acid-catalyzed hydrolysis/condensation of a solution constituted by tetraethoxysilane (TEOS), methytrimethoxysilane (MTMS), 3-(aminopropyl)trimethoxysilane (APTMS) and caffeine, as a template molecule. The morphology and performance of the imprinted siloxane film was characterized by scanning electron microscopy (SEM) and differential pulse voltammetry (DPV). The MIS/MWCNTs-VTMS/GCE sensor was tested in a solution of the caffeine and other similar molecules. After optimization of the experimental conditions, the sensor showed a linear response range from 0.75 to 40μmolL−1, with a detection limit (LOD) of 0.22μmolL−1. The imprinted sensor was successfully tested to detect caffeine in real samples.

Comparing deposition of organic and inorganic siloxane films by the atmospheric pressure glow discharge

The atmospheric pressure glow discharge burning in argon with small admixture of oxygen and hexamethyldisiloxane is used for the deposition of organosilicon and SiO x -like films on the stainless steel surface. The organosilicon film has high growth rate (85 nm/min) and extremely low surface roughness (0.8 nm), while the SiO x -like film shows relatively low growth rate (40 nm/min) and higher surface roughness (9 nm). The surface wettability, microhardness and the corrosion resistance of the deposited siloxane films are detected. The organosilicon film is hydrophobic (contact angle: 98°) and soft (microhardness: 0.68 GPa), while the SiO x -like film is hydrophilic (contact angle: 45°) and hard (microhardness: 7.85 GPa). The potentiodynamic polarization tests show that by depositing siloxane films on the surface, the corrosion resistance of the stainless steel in the Hanks solution can be largely improved. In particular, the SiO x -like film exhibits better corrosion resistance than the organosilicon film.

Application of plasma polymerized siloxane films for the corrosion protection of titanium alloy

Organosilicon film and SiOx-like film are deposited on titanium alloy (Ti6Al4V) surfaces by atmospheric pressure (~105Pa) dielectric barrier discharge to improve its corrosion resistance in Hanks solution. Hexamethyldisiloxane (HMDSO) is used to be the chemical precursor. The organosilicon film deposited in Ar/HMDSO system has high growth rate (75nm/min) and low surface roughness (3nm), while the SiOx-like film deposited in Ar/O2/HMDSO system has lower growth rate (35nm/min) and slightly higher surface roughness (9nm). The potentiodynamic polarization tests show that both the two siloxane films coated Ti6Al4V samples have more positive corrosion potential and one order of magnitude lower corrosion current density than the substrate, indicating the corrosion resistance of Ti6Al4V can be improved by depositing siloxane film on its surface. In particular, as the surface is more compact and cross-linked, the SiOx-like film has better corrosion resistance than the organosilicon film.

Photocurrents from the Direct Irradiation of a Donor–Acceptor Complex Contained in a Thin Film on Indium Tin Oxide

We describe the performance of a photoelectrochemical cell based on an electron donor–acceptor (EDA) complex formed by the association of tetraphenylborate anion with a viologen dication siloxane film immobilized on an indium tin oxide (ITO) electrode. Visible irradiation of a weak absorbance band of the EDA complex leads to simultaneous photoreduction of the viologen and photooxidation of the tetraphenylborate, forming a viologen monocation radical and tetraphenylborate radical. Spontaneous decomposition of the latter to products incapable of further reaction inhibits reverse electron transfer, permitting harvesting of the redox equivalents stored as the viologen monocation radical at the electrode. Results of statistically designed two-level factorial experiments indicate that the photocurrent increases with light intensity (L) and viologen coverage (P), but decreases with viologen siloxane film age (A). Although the solution BPh4– concentration (F) does not significantly affect the photocurrent response under our experimental conditions, the nature of the electrolyte anion (C) does. Although significant interactions among the L, P, A, and C variables exist, our analyses provide a model that (1) describes the viologen film aging; (2) adequately predicts cell photocurrent responses as functions of levels of the L, P, and, to a somewhat lesser extent, A variables; and (3) identifies areas for improvement and optimization of cell performance.

Rapid ultraviolet-curing of epoxy siloxane films

The ultraviolet (UV) curable epoxy siloxane polymer is shown to cross-link at low UV dosages of 130 mJ/cm 2, making it desirable for use in nanoimprinting and the rapid fabrication of micro/nano-scaled patterns. In this paper, the dielectric and mechanical properties of this UV-cured epoxy siloxane polymer are investigated. The results of these tests show that the rapid UV-cured polymer films have a dielectric constant of 2.7 ± 0.13, leakage current density on the order of 10 −9 A/cm 2 under 1 MV/cm, dielectric strength of greater than 5 MV/cm, and a reduced modulus of elasticity of 6.2 GPa characterized using nanoindentation. These properties indicate that the epoxy siloxane can be used to fabricate layers for functional device applications.

Novel electrochemical sensor for the selective recognition of chlorogenic acid

In this study, a novel sensitive molecularly imprinted electrochemical sensor was constructed for the selective detection of chlorogenic acid (CGA) by deposition of a molecularly imprinted siloxane (MIS) film, prepared by sol–gel process, onto Au bare electrode surface. Initially, a (3-mercaptopropyl)siloxane layer (MSL) was formed on the Au bare surface, followed by a siloxane layer obtained from the acid-catalyzed hydrolysis/condensation of a solution constituted by tetraethoxysilane (TEOS), phenyltriethoxysilane (PTEOS), 3-(aminopropyl)trimethoxysilane (APTMS) and CGA, as a molecular template. After the GCA extraction the MIS imprinted film was electrochemically characterized using differential pulse voltammetry (DPV). The MIS/Au sensor was tested in a solution of the CGA template and other similar molecules. This electrode displayed excellent selectivity towards CGA when compared with structurally similar molecules. Under optimized experimental conditions, the peak current response of the sensor for CGA was linear from 5.0 × 10 −7 mol L −1 to 1.4 × 10 −5 mol L −1, and the detection limit was 1.48 × 10 −7 mol L −1. The MIS/Au sensor was successfully applied for the determination of CGA in coffee and tea samples.

Film morphology, orientation and performance of dodecyl/carboxyl functional polysiloxane on cotton substrates

A novel polysiloxane (RCAS) bearing dodecyl and carboxyl side groups was synthesized by reaction of a dodecyl/amino functionalized polysiloxane with maleic anhydride. Film morphology, molecular orientation and performance of the synthesized polysiloxane on cotton substrates were investigated by field emission scanning electron microscope (FESEM), atomic force microscope (AFM), X-ray photoelectron microscope (XPS) and so on. Affected by the dodecyl and polar carboxyl side groups, RCAS formed a semi hydrophilic, macroscopic smooth but actually uneven siloxane film with many pillar-likes or small humps on the treated substrate surfaces. On the natural cotton surface, RCAS may take such an orientation to form its film that the Si-CH3, Si-C12H25 groups projected outward into air, while the carboxyl groups pointed inward to the substrate, interacting with the hydroxyls of the cotton substrates in ester and hydrogen bonds or twisted away from the negatively charged cotton fiber surface. As a result of such a film-formation, RCAS provided the treated fabric with not only a good wettability of about 22.96s and a whiteness of 88.44°, but also an improved softness as well as thickening handle.

Nanoscale Thermal AFM of Polymers: Transient Heat Flow Effects

Thermal transport around the nanoscale contact area between the heated atomic force microscopy (AFM) probe tip and the specimen under investigation is a central issue in scanning thermal microscopy (SThM). Polarized light microscopy and AFM imaging of the temperature-induced crystallization of poly(ethylene terephthalate) (PET) films in the region near the tip were used in this study to unveil the lateral heat transport. The radius of the observed lateral surface isotherm at 133 °C ranged from 2.2 ± 0.5 to 18.7 ± 0.5 μm for tip-polymer interface temperatures between 200 and 300 °C with contact times varying from 20 to 120 s, respectively. In addition, the heat transport into polymer films was assessed by measurements of the thermal expansion of poly(dimethyl siloxane) (PDMS) films with variable thickness on silicon supports. Our data showed that heat transport in the specimen normal (z) direction occurred to depths exceeding 1000 μm using representative non-steady-state SThM conditions (i.e., heating from 40 to 180 °C at a rate of 10 °C s(-1)). On the basis of the experimental results, a 1D steady-state model for heat transport was developed, which shows the temperature profile close to the tip-polymer contact. The model also indicates that ≤1% of the total power generated in the heater area, which is embedded in the cantilever end, is transported into the polymer through the tip-polymer contact interface. Our results complement recent efforts in the evaluation and improvement of existing theoretical models for thermal AFM, as well as advance further developments of SThM for nanoscale thermal materials characterization and/or manipulation via scanning thermal lithography (SThL).

Corrosion resistance of new epoxy–siloxane hybrid coatings. A laboratory study

Traditional multilayer epoxy/polyurethane type anticorrosive paint systems are widely employed in the protection of steel structures due to their high efficiency against atmospheric corrosion. However, the use of isocyanate in the curing process, and the high volatile organic compound (VOC) content of such systems, makes it necessary to search for new isocyanate-free paints. Hybrid organic–inorganic coatings, such as epoxy–siloxane coatings, represent a step forward in the field of paint coatings for atmospheric corrosion protection. These new isocyanate-free hybrids present low VOC levels – due to the high solid content associated with their low viscosity – along with good heat and UV radiation stability and excellent chemical resistance. With the new polysiloxane inorganic resins it is hoped to improve the anticorrosive behaviour of traditional organic binders. This work assesses the anticorrosive performance of one-layer epoxy–siloxane coatings compared to traditional two-layer epoxy/polyurethane coatings, in paint systems with and without an epoxy or silicate-type zinc-rich primer. The anticorrosive properties of these coatings applied on steel substrates have been evaluated using a wide range of experimental techniques, namely measurement of water vapour and oxygen permeability in free films, 3-year outdoor testing, accelerated corrosion testing (condensing humidity, Kesternich, salt fog and Prohesion), and electrochemical impedance spectroscopy (EIS). Hybrid epoxy–siloxane films show the lowest oxygen and water vapour permeability, capacitance values, and the highest ionic resistance values, along with excellent behaviour in humidity and Kesternich tests, outperforming traditional epoxy/polyurethane paints, while the behaviour of both types of coatings is similar in the salt fog, Prohesion and short-term outdoor tests. This type of paint requires the presence of zinc-rich primers to optimise its anticorrosive behaviour.