Silica-like Layer Research Articles

The surface structure of UV exposed poly-dimethylsiloxane (PDMS) insulator studied by slow positron beam

In this study, the variations in surface structure of Poly (dimethyl siloxane) (PDMS) composite before and after different UV treatment conditions have investigated using slow positron beam. S profiles of the exposed samples show a broader shallow valley in a low energy range (1–2.25keV) suggest that a silica-like layer has formed at the surface during UV-treatment, and its thickness could be estimated as 80nm. Furthermore, corresponding to the range that from ∼180 to ∼360nm from the surface, the height of the peak in S (E) curve for each UV treated sample has increased with longer exposure time, and the peak position tends toward the low energy side. These features may indicate that, formed by chain scission in situ, a loose structure including more Low weighted molecular (LWM) species has been generated beneath the silica-like layer during the oxidation process. The longer the oxidation process lasts, the more LWM species accumulate beneath the silica-like layer. While, the data obtained from contact angle measurement and X-ray photoelectron spectroscopy (XPS), show less structural change has occurred, which could contribute the LWM species migrate to the topmost surface, resulting covered the useful information.

Atmospheric pressure PECVD based on a linearly extended DC arc for adhesion promotion applications

In the aircraft industry, an increasing demand for adhesion promotion layers for titanium based materials is observed to replace polluting and unhealthy substances. Plasma enhanced chemical vapour deposition (PECVD) at atmospheric pressure (AP) seems to be favourable for the deposition of thin silica-like layers. In this work, a linearly extended and scalable DC arc plasma source is used for PECVD of silica layers on Ti6Al4V. The influence of the precursor chemistry is evaluated for HMDSO and TEOS precursors.Using HMDSO, hydrophobic almost stoichiometric SiO2 coatings with a carbon content of 3at.-% to 5at.-% are deposited on the titanium alloy substrate. For TEOS, hydrophilic carbon-free stoichiometric SiO2 coatings are deposited. The dynamic deposition rate for TEOS is 3 times lower than that for HMDSO (up to 640nm•m/min). For the intended coating of slightly curved substrates, the film properties and deposition rates are studied for different working distances from 20mm up to 60mm. For 60mm the deposition rate is reduced by a factor 6 compared to 20mm, also the index of refraction of the film decreases from 1.45 at 20mm distance to 1.39 at 60mm. With increasing distance, the particle size and concentration on the surface increases and leads to less dense structures than in the case of shorter distances.The wedge test is used to evaluate the adhesion properties of the layer. The HMDSO-based coatings exhibit a good long-term durability and a high bonding strength comparable to that of the currently used standard pre-treatment, but without the use of harmful substances.

A simple method for fabrication of filler-free stretchable polydimethylsiloxane surfaces

We propose a simple method to elaborate a filler-free stretchable PDMS surface strong enough to resist to successive elongation/retraction cycles even at high degree of stretching. It consists in creating free radicals on a filler-containing PDMS surface by argon plasma exposure and reacting them with a filler-free PDMS resin during the crosslinking step. Changes of physical and chemical properties upon plasma modification are monitored by FTIR and XPS spectroscopies, contact angle measurements and atomic force microscopy. Electron spin resonance (ESR) is used to identify the nature of radicals involved in interfacial bonding. Although a brittle silica-like layer is created on the filler-containing PDMS surface after plasma treatment, an increase in the PDMS/PDMS interfacial strength is observed and a high interfacial resistance has been found under elongation/retraction (stretching/relaxation) cycles.

Surface wrinkling by chemical modification of poly(dimethylsiloxane)-based networks during sputtering

Wrinkling is an important mechanical phenomenon that generates periodic topographical patterns across a surface. This paper presents experimental evidence that surface wrinkles, which form consequent to thin film magnetron sputtering of either indium tin oxide (ITO) or aluminum on poly(dimethylsiloxane) networks (PDMS-N) made from a commercial Sylgard-184 kit, result from chemical modification of the PDMS-N surface as opposed to extrinsic thermomechanical stresses originating from differential thermal expansion. X-ray photoelectron spectroscopy results reveal that the PDMS-N surface becomes depleted in carbon and concurrently enriched in oxygen relative to silicon due to sputtering. This silica-like surface layer possesses intrinsic compressive stress that leads to wrinkle formation during the first ≈5 seconds of sputtering. The wrinkles maintain their periodicity irrespective of the thickness of the ITO film formed during subsequent deposition. Furthermore, upon removal of the ITO layer, the wrinkles persist with their periodicity unchanged. A narrow sputtering pressure window between 2 and 12 mTorr generates wrinkles. Pressures below this range cannot sustain a radio frequency plasma, while pressures above this range provide sufficient thermalization of kinetic energy as to eliminate the energetic bombardment that modifies the PDMS-N. This study provides a new understanding of the origins of wrinkling in sputtered films on polymeric substrates and creates opportunities to manipulate the topography produced by spontaneous surface wrinkling.

Solar Cells: Photovoltaic Efficiency Enhancement by the Generation of an Embedded Silica‐Like Passivation Layer along the P3HT/PCBM Interface Using an Asymmetric Block‐Copolymer Additive (Adv. Mater. 47/2012)

A new approach to improve the power conversion efficiency of polymer bulkheterojunction solar cells by the formation of an embedded passivation layer is demonstrated. In the Communication on page 6311, Ji-Woong Park and co-workers report a simple method to generate silica-like layers along the three-dimensionally intertwined P3HT/PCBM interface by addition of an asymmetric block copolymer containing a short organo-silica precursor.

Surface charge decay on HTV silicone rubber: effect of material treatment by corona discharges

Surface charge decay on thick flat samples of high temperature vulcanized silicone rubber is studied prior and after ac and dc corona pre-treatments. It is found that the charge decay rate on the material exposed to ac corona becomes much higher and sensitive to moisture content in the surrounding air. These features are associated with an increased surface conductivity and formation of a silica-like layer on the polymeric surface, both resulting from ac corona treatment. In contrast, characteristics of the charge decay on the material exposed to dc corona are found to be similar to that measured on untreated samples.

Organosilicon Coatings Deposited in Atmospheric Pressure Townsend Discharge for Gas Barrier Purpose: Effect of Substrate Temperature on Structure and Properties

Organosilicon plasma polymer and silicalike layers are deposited at different temperatures in a dielectric barrier discharge at atmospheric pressure operating in the Townsend regime. Final properties of these two kinds of layers can be finely tuned by the plasma process conditions. In particular, influence of deposition temperature is investigated when hexamethyldisiloxane based monolayers are deposited on poly(ethylene naphtalate) substrate. Coating chemical structure is tested by means of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Their thickness, topography, and mechanical properties are evaluated by ellipsometry, scanning electron microscopy observation of coatings cross sections, atomic force microscopy, and nanoscratch testing. Permeability of coated polymer is measured for transparent silicalike layers, and the effect of coating structure on the oxygen gas permeability is discussed. The deposition temperature of coatings at 90 °C provides a strong improvement in barrier property compared to room temperature deposition, thanks to a densification of the SiO₂ matrix and to a decrease in the silanol group content.

Photovoltaic Efficiency Enhancement by the Generation of an Embedded Silica‐Like Passivation Layer along the P3HT/PCBM Interface Using an Asymmetric Block‐Copolymer Additive

A new approach to improve the power conversion efficiency of polymer bulk-heterojunction solar cells is demonstrated by generating a silica-like passivation layer embedded along the three-dimensionally intertwined interfaces between the nanoscopic domains of P3HT and PCBM by addition of an aymmetric block copolymer containing a short organo-silica precursor.

Coating of structured catalytic reactors by plasma assisted polymerization of tetramethyldisiloxane

An original process based on cold plasma assisted polymerization of tetramethyldisiloxane (TMDSO) in the presence of O2 was developed to cover substrates showing various shapes (plate and foam). In the frame of catalytic application, this coating has to act as a bonding layer for the deposition of active phase such as VOx/TiO2 well-known for its properties in NOx and volatile organic compounds abatement and in the production of chemical intermediates. Good results were obtained by the deposition of a 5-μm thick polysiloxane film followed by a thermal treatment under air at 650°C and by a remote nitrogen plasma post treatment. This procedure led to a silica-like layer allowing its coating in an aqueous suspension of TiO2. Such a multilayered material can be obtained homogeneously on the whole surface of a sample showing a 3D open geometry like metallic foam. Characterizations of the different steps of the elaborated multilayer material were performed by Fourier transformed infrared spectroscopy, Raman spectrometry, X-ray photoelectron spectroscopy, and electron probe micro-analyzer. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers

Thickness and Elastic Modulus of Plasma Treated PDMS Silica-like Surface Layer

The adhesion of poly(dimethylsiloxane) (PDMS) rubber is largely improved by oxygen plasma surface treatment. The thickness of the silica-like surface layer is characterized by performing transmission electron microscopy imagery on microtome slices of welded plasma treated surfaces. The specific double layer contrast can be considered as equal to twice the thickness of the silica-like layer. The thickness measurements combined with strain-induced elastic buckling instability analysis gives an estimate of the elastic modulus of the silica-like layer equal to 1.5 GPa.

Influence of gas flow dynamics on discharge stability and on the uniformity of atmospheric pressure PECVD thin film

The aim of this paper is to improve the understanding of the mechanisms controlling the uniformity of thin films made by atmospheric pressure plasma enhanced chemical vapour deposition (AP-PECVD). To reach this goal, the influence of the gas flow-rate and injection design on the thin film thickness uniformity is studied through experiments and numerical simulation in the case of silica-like layers deposited from silane and nitrous oxide using a nitrogen Townsend dielectric barrier discharge. Results show that whatever the gas flow-rate, when the gas is injected uniformly parallel to the substrate, the obtained layer is always uniform along the substrate width, while when the gas is injected perpendicularly to the substrate, the width-uniformity of the layers decreases when the gas flow-rate increases. The layer non-uniformity is related to the penetration of gas recirculation into the discharge zone, which was confirmed by computational fluid dynamics. This link is corroborated experimentally by a clear improvement of the deposit uniformity when the discharge cell dimensions are modified in order to reduce the recirculation influence on the discharge. A plausible hypothesis for the layer uniformity dependence on the recirculation is the possible enhancement of powder formation and growth in the recirculation zone: when the particle size is large enough, electrons may attach on the powder inducing electron depletion at the origin of the discharge instabilities.

Surface Functionalization of Silicone Rubber for Permanent Adhesion Improvement

The surface properties of poly(dimethyl siloxane) (PDMS) layers screen printed onto silicon wafers were studied after oxygen and ammonia plasma treatments and subsequent grafting of poly(ethylene -alt-maleic anhydride) (PEMA) using X-ray photoelectron spectroscopy (XPS), roughness analysis, and contact angle and electrokinetic measurements. In the case of oxygen-plasma-treated PDMS, a hydrophilic, brittle, silica-like surface layer containing reactive silanol groups was obtained. These surfaces indicate a strong tendency for "hydrophobic recovery" due to the surface segregation of low-molecular-weight PDMS species. The ammonia plasma treatment of PDMS resulted in the generation of amino-functional surface groups and the formation of a weak boundary layer that could be washed off by polar liquids. To avoid the loss of the plasma modification effect and to achieve stabilization of the mechanically instable, functionalized PDMS top layer, PEMA was subsequently grafted directly or after using gamma-APS as a coupling agent on the plasma-activated PDMS surfaces. In this way, long-time stable surface functionalization of PDMS was obtained. The reactivity of the PEMA-coated PDMS surface caused by the availability of anhydride groups could be controlled by the number of amino functional surface groups of the PDMS surface necessary for the covalent binding of PEMA. The higher the number of amino functional surface groups available for the grafting-to procedure, the lower the hydrophilicity and hence the lower the reactivity of the PEMA-coated PDMS surface. Additionally, pull-off tests were applied to estimate the effect of surface modification on the adhesion between the silicone rubber and an epoxy resin.

Contact Mechanics of UV/Ozone-Treated PDMS by AFM and JKR Testing: Mechanical Performance from Nano- to Micrometer Length Scales

The Young’s modulus of cross-linked poly(dimethylsiloxane) (PDMS) surface was quantitatively investigated as a function of UV/ozone treatment time across different length scales. An AFM was used to probe PDMS surface mechanical properties at the nanometer length scale. The Young’s modulus of each sample was estimated with continuum contact mechanics theory (Sneddon method) using AFM data by employing the hyperboloid tip shape model. A custom-built ATD device (JKR method) was also used which allowed us to simultaneously monitor the load, the contact area, and the relative displacement between (a) a lens (made either of PDMS elastomer or of Si3N4) and (b) surface-treated PDMS films upon loading and unloading on the micrometer to submillimeter length scales. The modulus of PDMS increased with increasing treatment time as observed by AFM as well as by ATD, which we explained by the gradual formation of a silica-like layer. For all specimens tested, the modulus values obtained were highest from AFM, lower from ATD, and lowest from bulk tensile experiments for the same UV/ozone dose. These results demonstrate the effect of the probed length scale of the tests used to assess mechanical performance

SiOx Ultrathin Layer Coverage Effect on the (Photo)catalytic Activities of Rutile TiO2

A two-step method consisting of 1,3,5,7-tetramethycyclotetrasiloxane chemisorption and the postheating has formed a uniform and dense silica-like SiOx ultrathin layer (SiOx-UTL) on the surface of rutile TiO2. Diffuse reflectance infrared Fourier transform and solid-state 29Si NMR spectroscopy showed that three-dimensional Si−O−Si networks grow to become rigid by repeating the treatment. The coverage with the SiOx-UTL eliminates the surface acid sites, leading to an almost complete loss of the photocatalytic activity at the thicknesses above 0.36 ± 0.03 nm. On the basis of a tunneling model, the significant restriction of the photocatalytic activity can be accounted for by the excellent barrier effect of the SiOx-UTL for the photogenerated charge carrier transfer from TiO2 to acceptors in solution.

Stretchable Gold Tracks on Flat Polydimethylsiloxane (PDMS) Rubber Substrate

A process to fabricate stretchable gold tracks on silicone rubber substrates is studied by XPS, static water contact angle measurement, AFM, and SEM. The process involves several steps: removing uncured oligomers by hexane Soxhlet extraction; pre-stretching the substrate; activating the strained silicone surface by an oxygen plasma treatment; coating the strained substrate with 5 nm titanium and 80 nm gold layers; and finally releasing the sample. The plasma treatment creates a thin brittle silica-like layer that temporarily increases the substrate's surface energy. Indeed, the plasma treatment is followed by a hydrophobic recovery. As a consequence, the delay between plasma treatment and metal deposition has to be reduced as much as possible. The silica-like layer can be nicely observed after release. The entire process allows us to obtain stretchable metallized samples that remain conductive even after an excessive deformation leading to electrical failure.

Metallization Process for Polydimethylsiloxane (PDMS) Rubber

AbstractA process to fabricate stretchable and adherent gold tracks on flat silicone rubber substrates is studied by X-ray photoelectron spectroscopy (XPS), static water contact angle measurement, atomic force microscopy (AFM) and scanning electron microscopy (SEM). The process involves several steps: curing flat silicone substrate; removing uncured oligomers by hexane Soxhlet extraction; pre-stretching the substrate; activating the strained silicone surface by an oxygen plasma treatment; coating the strained substrate with 5nm titanium and 80nm gold layers by e-beam evaporation; and finally releasing the sample. The plasma treatment creates a thin brittle silica-like layer that temporarily increases the substrate's surface energy. Indeed the plasma treatment is followed by a hydrophobic recovery. As a consequence, the delay between plasma treatment and metal deposition has to be reduced as much as possible. The silica-like layer can be nicely observed after release. Its thickness is estimated to be around 20nm to 50nm. The entire process allows us to obtain stretchable metallized samples that remain conductive even after an excessive deformation leading to electrical failure.

Micropatterning of Copper on a Poly(ethylene terephthalate) Substrate Modified with a Self-Assembled Monolayer

We have developed a technique for the site-selective electroless deposition of Cu on poly(ethylene terephthalate) (PET) substrate modified with an organic self-assembled monolayer (SAM). The PET substrate was first modified with a silica-like layer by being dip-coated in an acetone solution of 3-aminopropyltrimethoxysilane and treated with UV light. The PET substrate was further modified with thiol groups using a 3-mercaptopropyltrimethoxysilane-SAM and then irradiated by UV light through a photomask to prepare thiol-group regions and OH-group regions. Cu was then deposited on only the thiol-group regions of the substrate by electroless deposition in a neutral solution with no catalysts by using dimethylamineborane as a reducing reagent. This site-selective deposition process can control the deposition conditions by an organic thin film fabricated on a surface-modified PET substrate, and thus can be applied to other low heat-resistant substrates.

Plasma/Corona Treatment of Silicones

There is considerable current interest in plasma and corona treatment of silicones. It is well established that a thin, silica-like layer is created on the surface by such treatments. Much fresh insight has emerged concerning the role of low-molecular-weight materials in the hydrophobic recovery of treated elastomers. Even if these diffusible species are removed before treatment, hydrophobic recovery still occurs above a threshold energy implying that they are produced in situ by the treatment. A particularly significant new direction in the plasma treatment field is plasma polymerization of organosilicon monomers using atmospheric pressure glow discharge (APGD).

A novel process to form a silica-like thin layer on polyethylene terephthalate film and its application for gas barrier

A novel process for polyethylene terephthalate (PET) surface modification with a silica-like thin layer is proposed. 3-Aminopropyltrimethoxysilane was employed to react with acetone to form dimethyliminopropyltrimethoxysilane (DIPTMS) after aging for 10 days at room temperature. After hydrolysis of alkoxy groups in DIPTMS, dimethylimine-modified silica clusters occurred resulting in an increase of the solution viscosity. Consequently, a dense and homogeneous thin layer was easily dip-coated onto a PET film. After heat treatment at 150 °C, a smooth, flexible and transparent silica-like film (about 70 nm thick) was formed via dehydration and condensation. The surface of the PET dramatically changed from hydrophobic (water contact angle: 70±2°) to hydrophilic (45±3°). As an attempt for application, a dense film of lithium metasilicate (Li 2O·2SiO 2· nH 2O) was successfully prepared by dip coating on the modified PET film, which appeared very low oxygen permeability of about 0.17 cm 3/m 2 day atm at 23 °C and 85% humidity.

Hydrophobic recovery of UV/ozone treated poly(dimethylsiloxane): adhesion studies by contact mechanics and mechanism of surface modification

Silicone elastomers (Sylgard 184 and 170), based on poly(dimethylsiloxane) (PDMS), were surface treated by a combined exposure to UV and ozone. The effects of the treatments were analyzed as a function of time elapsed after stopping the treatments using different standard surface characterization techniques, such as water contact angle measurements, XPS and atomic force microscopy (AFM). However, the primary focus of this study was to apply the Johnson–Kendall–Roberts (JKR) contact mechanics approach to investigate PDMS samples prior to and following UV/ozone surface treatment. A gradual formation of a hydrophilic, silica-like surface layer with increasing modulus was observed with increasing UV/ozone exposure. A subsequent hydrophobic recovery after UV/ozone exposure was observed, as indicated by increasing contact angles. This supports the hypothesis that the hydrophobic recovery is mainly caused by the gradual coverage of a permanent silica-like structure with free siloxanes and/or reorientation of polar groups. PDMS containing a homogenously dispersed filler (Sylgard 184), exhibited a decreasing surface roughness (by AFM) when the oxidized surface region “collapsed” into a smooth SiO x layer (final surface roughness <2 nm). PDMS containing heterogeneously distributed, aggregated filler particles (Sylgard 170), exhibited an increasing surface roughness with treatment dose, which was attributed to the “collapse” of the oxidized surface region thus exposing the contours of the underlying filler aggregates (final surface roughness ∼140 nm). A dedicated device was designed and built to study the contact mechanics behavior of PDMS prior to, and following surface treatment. The value of the combined elastic modulus obtained for PDMS lens and semi-infinite flat surface system showed an increase in full agreement with the formation of a silica-like layer exhibiting a high elastic modulus (compared with untreated PDMS). The work of adhesion observed in JKR experiments exhibited an increasing trend as a function of treatment done in agreement with contact angle data. JKR experiments showed hydrophobic recovery behavior as anticipated from contact angle measurements. Single pull-off force measurements by JKR and numerical analysis of full-approach JKR curves were in quantitative agreement regarding practical work of adhesion values.