Thin Plasma Polymer Research Articles

Comparative study of the residual stress development in HMDSN-based organosilicon and silicon oxide coatings

To investigate the stress formation mechanisms in thin plasma polymers, a comparative study of organosilicon (SiNOCH) and silicon oxide (SiOx) coatings in dependence of power input for deposition was conducted. Both coating types were produced in a low-pressure (15 Pa) microwave excited hexamethyldilisazane (HMDSN) plasma. Residual stress values were obtained using a high-throughput, time resolved and in-situ measurement method, including a CCD-camera, a line laser and micro-machined cantilever sensor chips. Both plasma polymer types were shown to form residual stresses with opposite signs. The stress evolution in the coatings revealed a strong dependency on the variation of power input for deposition. The SiOx coating exhibits mostly compressive stresses. Higher power inputs constitute higher ion momentums as well as a higher degree of fragmentation of the monomer. The SiOx coatings were deposited with a high oxygen flow and with a higher average energy of the plasma for all investigated parameter sets than the SiNOCH coating. Therefore, it is conceivable that ion peening is mostly responsible for the compressive stress formation in the SiOx coatings. In contrast to the SiOx coating, the SiNOCH coating can be applied without residual stress. For higher excitation powers, tensile stresses are predominant, most likely due to attractive forces between island or column boundaries and crosslinking.

HMDSO-Based Thin Plasma Polymers as Corrosion Barrier Against NaOH Solution

HMDSO-based films with excellent corrosion barrier properties against strong alkaline solutions were deposited on chemically non-resistant SiOx barrier coatings, which were previously applied on polished gold-coated Si-Wafers and PET films for coating analysis. The plasma process parameters are seen to have a strong influence on the achievable corrosion barrier properties of the plasma polymers. Coatings, which were applied in a pulsed microwave plasma with low mean power input, exhibit a substantially higher resistance against NaOH aqueous solution in electrochemical tests than those applied in higher energy plasmas. An analysis of the coatings revealed that the great difference in chemical resistivity of the investigated coatings can be explained by their chemical composition as well as their nano-porosity and surface topography. XPS measurements indicate that a higher organic content in the films contributes to their chemical resistivity. FTIR measurements showed that an ordered Si-O-Si network with methyl groups, which promote steric shielding, lead to superior corrosion resistance. Furthermore, a correlation of protective performance and nano-porosity was found in cyclic voltammetry measurements. Coatings with good corrosion protection proved to be initially pore free and even after 30 min of exposure to NaOH, an open pore surface of only 2% can be measured. Finally, measurements of the oxygen transmission rate (OTR) of coated PET substrates showed that the barrier of a coating system comprising a non-resistant barrier layer and a protective top coat can withstand up to at least 90 min of exposure to hot NaOH solution without significant loss in barrier performance. After this, the barrier of the system is gradually reduced. To slow down this reduction process, a multilayer approach proved to be effective.

Interaction of endothelial cells with plasma-polymer modified surfaces

Endothelialisation of implantable vascular grafts and stents is directed by the adsorbed protein layer. Chemical and mechanical cues sensed at the biomaterial surface by endothelial cells determine their attachment, survival and proliferation. Given the multiplicity of possible interactions we describe the novel application of live cell imaging, factorial analysis and single molecule imaging to investigate higher order interactions between surface chemistry and adsorbed proteins influencing endothelial cell (EC) adhesion dynamics. EC fates were tracked by time-lapse imaging of cell contact area on plasma polymer modified surfaces. The combinatorial effect of plasma polymer chemistry and adsorbed proteins was characterised by factorial experimental design and analysis. Single molecule imaging and counting was used for the first time to quantify binding of fluorescently labelled albumin to plasma polymerized surfaces: Modified glass surfaces with thin plasma polymer coatings rich in primary amine groups had a high affinity for albumin (~2, 650 molecules/μm2) while plasma polymer surfaces functionalised with hydroxyl groups had very low levels of albumin binding (22–30 molecules/μm2). Plasma polymer coatings rich in primary amine groups also promoted endothelial cell adhesion and was superior to tissue culture plastic. An interaction between albumin, heparin and fibronectin promoted adhesion to amine plasma polymer coated glass, while hydroxyl plasma polymer coated glass prevented EC attachment. Tracking cell-specific interactions with the adsorbed protein layer by time-lapse microscopy is more predictive of in vivo cellular responses to biomaterials compared to studies that only measure protein adsorption.

Thin plasma polymerised coatings for corrosion protection against strong alkaline solutions

Thin plasma polymers were applied on gold- and aluminium substrates using low pressure microwave- and radiofrequency-excited hexamethyldisilazane (HMDSN) plasma. The corrosion resistance properties of these coatings against sodium hydroxide solution (NaOH) was characterised by means of time resolved electrochemical impedance spectroscopy (EIS) and light microscopy. The evaluated resistance values were correlated with coating topography, chemical composition, wetting properties, and morphology with particular focus on porosity. Coating porosity was determined by using cyclic voltammetry (CV) and light microscopy. The topography and chemistry of the coatings were characterised by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The wetting properties were determined by optical contact angle (OCA) measurements. It is shown that the plasma polymer's resistance against NaOH can be greatly increased by lowering the energy input during the deposition process. This can be attributed to the strong correlation between porosity and resistivity: low energy input during plasma deposition leads to the formation of smaller and more uniform particles in the plasma bulk and possibly a Stranski–Krastanov growth of the layers, resulting in a smoother coating topography and lower nano-porosity. A more dense and compact coating morphology leads to a better corrosion protection performance.

Mechanical properties of thin plasma polymer coatings from hexanediol dimethacrylate and relations with their chemical properties

The deposition of thin coatings by plasma processes operated at atmospheric pressure is a versatile process for the deposition of functional coatings. However, there is a need to understand their mechanical properties as a function of plasma deposition process parameters since the durability of the coatings is strongly dependent on these properties.In this work, a new approach to obtaining plasma coatings with a controlled level of cross-linking was developed. The precursor molecule to plasma polymer coatings is hexanediol dimethacrylate (HdiMA), a di-functional methacrylate; the plasma deposition conditions were chosen to strongly decrease the precursor fragmentation, so that the plasma polymerization of this precursor leads to a cross-linked structure through the reaction of methacrylate bonds.Plasma coatings from HdiMA are deposited in different plasma conditions. The chemical structure of plasma polymers is investigated by means of infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). The mechanical properties of the coatings are determined by nanoscratch and atomic force microscopy (AFM) force curves tests. Better mechanical properties are obtained for coatings deposited with a low concentration of aerosol in the plasma and a higher plasma power. This corresponds to coatings exhibiting the higher conversion rate.

Low Pressure Plasma Processing of Collagen Membranes for Anti-Cancer Drug Delivery

For targeting the cancer cells, a low pressure ICP reactor was utilised to fabricate a drug delivery system. On the surface of biocompatible collagen membranes, thin plasma polymer coatings were deposited to encapsulate an anticancer drug carboplatin and impart different biologically active functionalities. The characteristics of the deposited films were analysed by FTIR and XPS and the overall functionalities were tested by in vitro and in vivo experiments on the cancer cells lines on the cancer nodules respectively. Preliminary results of fabricated drug delivery systems showed the inhibition of cancer cell proliferation in vitro as well as tumour growth in mice models by 2.8 folds in mass compared to the control case

Promotion of Adhesion of Green Flame Retardant Coatings onto Polyolefins by Depositing Ultra-Thin Plasma Polymer Films: A Critical Review

Various methods have been used for introducing fire retardant additives into polymers. Deposition of thick fire retardant coatings directly onto polymer substrates is an alternative technique. An important advantage of the coating technique is the preservation of the physical and chemical integrity of the polymer material. Moreover, the fire retardancy of the polymer materials can be achieved following their production. Suitable coating materials are inorganics, intumescent, char-forming, oxygen-diluting, and cooling or radical quenching layers. The most important problem is to achieve sufficient coating thickness to withstand the direct attack of flame and to protect the polymer bulk from pyrolysis, otherwise blistering of coating, caused by emitted pyrolysis gases, is often observed. To avoid blistering of coating, the adhesion between polyolefin substrate and fire retardant coating has to be extraordinarily high. In order to achieve such a high level of adhesion, the polymer surface has to be modified with adhesion-promoting functional groups. The deposition of thin plasma polymers as adhesion-promoting layers with NH2, OH or COOH groups has been the most suited method. These functional groups are able to form covalent bonds and other interactions between the fire-resistant coating and the plasma-modified polyolefin substrate. Additionally, the plasma polymer counteracts the strong mechanical stresses in the laminate on exposure to high temperatures by its flexibility. The thick fire retardant coatings were chosen based on green ecological aspects to avoid flame-initiated emission of toxic or corrosive gases and remains of toxic char.

Wetting hysteresis induced by temperature changes: Supercooled water on hydrophobic surfaces

The state and stability of supercooled water on (super)hydrophobic surfaces is crucial for low temperature applications and it will affect anti-icing and de-icing properties. Surface characteristics such as topography and chemistry are expected to affect wetting hysteresis during temperature cycling experiments, and also the freezing delay of supercooled water. We utilized stochastically rough wood surfaces that were further modified to render them hydrophobic or superhydrophobic. Liquid flame spraying (LFS) was utilized to create a multi-scale roughness by depositing titanium dioxide nanoparticles. The coating was subsequently made non-polar by applying a thin plasma polymer layer. As flat reference samples modified silica surfaces with similar chemistries were utilized. With these substrates we test the hypothesis that superhydrophobic surfaces also should retard ice formation. Wetting hysteresis was evaluated using contact angle measurements during a freeze–thaw cycle from room temperature to freezing occurrence at −7°C, and then back to room temperature. Further, the delay in freezing of supercooled water droplets was studied at temperatures of −4°C and −7°C. The hysteresis in contact angle observed during a cooling–heating cycle is found to be small on flat hydrophobic surfaces. However, significant changes in contact angles during a cooling–heating cycle are observed on the rough surfaces, with a higher contact angle observed on cooling compared to during the subsequent heating. Condensation and subsequent frost formation at sub-zero temperatures induce the hysteresis. The freezing delay data show that the flat surface is more efficient in enhancing the freezing delay than the rougher surfaces, which can be rationalized considering heterogeneous nucleation theory. Thus, our data suggests that molecular flat surfaces, rather than rough superhydrophobic surfaces, are beneficial for retarding ice formation under conditions that allow condensation and frost formation to occur.

Response of Plasma-Polymerized Hexamethyldisiloxane Films to Aqueous Environments.

Thin plasma polymer films were deposited in hexamethyldisiloxane (HMDSO) and HMDSO/O2 low-pressure discharges and their chemical structures analyzed using infrared (IR) spectroscopy and neutron reflectometry (NR). The (plasma-polymerized) ppHMDSO film exhibits hydrophobic, poly(dimethylsiloxane)-like properties, while the retention of carbon groups is reduced by O2 addition, yielding a more inorganic, hydrophilic ppSiOx film. Both films show a minor (vertical) density gradient perpendicular to the substrate, where the exposed film surface seems to be more oxidized, indicating oxidative aging reactions upon contact with air. The hydration and water uptake abilities of the films in aqueous environments were investigated in humid environments using ellipsometry, NR in D2O, and multiple transmission-reflection IR measurements after equilibration of the films in water.

Cured melamine systems as thick fire-retardant layers deposited by combination of plasma technology and dip-coating

Melamine and melamine resins are widely used as fire-retardants for polymer building materials. Cured melamine systems are used in heat-sensitive items, such as furniture and window frames and sills. In this work, differently cured methylated poly(melamine-co-formaldehyde) (cmPMF) resins were used as fire-retardant coverage for poly(styrene) (PS) and poly(ethylene) (PE) building materials. Such polymer layers should have several tenths of micrometers thickness to produce sufficient fire retardancy. These thick layers were produced by dip-coating. To promote sufficient adhesion of such thick coating to the polyolefin substrates, also in the case of high temperatures occurring at fire exposure, the polymer substrates were firstly coated with a few hundred nanometer thick adhesion-promoting plasma polymer layer. Such thin plasma polymer layers were deposited by low-pressure plasma polymerization of allyl alcohol (ppAAl). It was assumed that the hydroxyl groups of ppAAl interact with the melamine resin; therefore, ppAAl was well suited as adhesion promoter for thick melamine resin coatings. Chemical structure and composition of polymer films were investigated using infrared-attenuated total reflectance and X-ray photoelectron spectroscopy (XPS). Peel strengths of coatings were measured. After peeling, the peeled polymer surfaces were also investigated using optical microscopy and XPS the layers for identification of the locus of peel front propagation. Thermal properties were analyzed using TGA (thermo-gravimetric analyses). Finally, the fire-retardant properties of such thick coated polymers were evaluated by exposure to flames.

Amination of NCD Films for Possible Application in Biosensing

Nanocrystalline diamond (NCD) films may be used in biosensing after modification of their surfaces with active functional groups. Here, surface functionalization of O- and H-NCD films with amine groups was made by 2 plasma-based processes: a) RF plasma treatment in ammonia, b) the deposition of thin amine-containing plasma polymer via RF magnetron sputtering of nylon target in Ar/N2 mixture. Amination of the NCD films by plasma treatment in NH3 caused an attachment of 1% of primary amines but destroyed the surface conductivity of H-NCD films. The 5% concentration of NH2 was detected on the NCD covered with the plasma polymer. Moreover, the surface conductivity of H-NCD films reached 10−7 S.

Coating and Functionalization of Carbon Fibres Using a Three‐Step Plasma Treatment

A three‐step plasma treatment—activation, functionalization and polymerization—has been used to deposit a thin plasma polymer with amine groups on carbon fibres (CFs). This plasma polymer has strong adhesion to the CF surface and the amine groups enable strong bonding to a matrix. The CFs were first treated by Ar plasma to activate and clean the surface, followed by O2 plasma to incorporate oxygen‐containing functional groups, and finally a heptylamine thin film was deposited using combined continuous wave and pulsed plasma polymerization. Strong adhesion between the plasma polymer and the CF was observed. The fibre strength was not affected by the treatment.

Improving the mechanical properties of multiwalled carbon nanotube/epoxy nanocomposites using polymerization in a stirring plasma system

Uniform treatment of multiwalled carbon nanotubes by plasma treatment has been investigated using a custom-built stirring plasma system. A thin plasma polymer with high levels of amine groups has been deposited on MWCNTs using a combination of continuous wave and pulsed plasma polymerization of heptylamine in the stirring plasma system. Scanning electron microscopy showed that the plasma polymerization improved the dispersion and interfacial bonding of the MWCNTs with an epoxy resin at loadings of 0.1, 0.3 and 0.5 wt%. The flexural and thermal mechanical properties of plasma polymerized MWCNT/epoxy nanocomposites were also significantly improved while untreated MWCNT/epoxy nanocomposites showed an opposite trend. The epoxy with 0.5 wt% plasma polymerized MWCNTs had the greatest increase in flexural properties, with the flexural modulus, flexural strength and toughness increasing by about 22%, 17% and 70%, respectively.

A Novel Dry Chemical Path Way for Diene and Dienophile Surface Functionalization toward Thermally Responsive Metal–Polymer Adhesion

In this paper, we report a new and easily up-scalable dry chemical method to functionalize with diene and dienophile groups a large range of surfaces, such as metal, polymer, or glass, and we demonstrate the potentiality of this technique to realize thermally responsive adhesion between these materials. A complete and extensive surface chemistry analysis of the grafted surfaces, based on the deposition of an anhydride-rich thin plasma polymer layer by using an atmospheric pressure dielectric barrier discharge (DBD) plasma process, and its subsequent gas phase aminolysis reaction with specific diene or dienophile compound is discussed. The optimization of the assembling condition for these tailored surfaces has led to achieve a Diels-Alder adhesion force up to 0.6 N/mm at ambient temperature, which can be reduced by a factor of 50 when the retro Diels-Alder is ignited at a heating temperature around 200 °C. The study of the failure interface produced after peeling tests is presented and a mechanism of failure is proposed, based on forensic analyses involving surface analytical techniques such as XPS, ToF-SIMS, and SEM combined to AFM analyses for the retrieving of chemical and morphological information.

Aging effects of plasma polymerized ethylenediamine (PPEDA) thin films on cell-adhesive implant coatings

Thin plasma polymer films from ethylenediamine were deposited on planar substrates placed on the powered electrode of a low pressure capacitively coupled 13.56MHz discharge. The chemical composition of the plasma polymer films was analyzed by Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) as well as by X-ray photoelectron spectroscopy (XPS) after derivatization of the primary amino groups. The PPEDA films undergo an alteration during the storage in ambient air, particularly, due to reactions with oxygen. The molecular changes in PPEDA films were studied over a long-time period of 360days. Simultaneously, the adhesion of human osteoblast-like cells MG-63 (ATCC) was investigated on PPEDA coated corundum blasted titanium alloy (Ti-6Al-4V), which is applied as implant material in orthopedic surgery. The cell adhesion was determined by flow cytometry and the cell shape was analyzed by scanning electron microscopy. Compared to uncoated reference samples a significantly enhanced cell adhesion and proliferation were measured for PPEDA coated samples, which have been maintained after long-time storage in ambient air and additional sterilization by γ−irradiation.

Plasma deposited polymers as gas sensitive films

The possibility is presented of producing thin plasma polymers with desired properties by using nanofillers. Composite films are synthesized from a mixture of hexamethyldisiloxane (HMDSO) and detonation nanodiamond particles (DNDs). The chemical structure of the composite consists of DNDs distributed in the polymer matrix. The effect of DNDs on the humidity and ammonia sorptive properties of the polymers obtained is studied by measuring the mass changes as a result of gas sorption by using a quartz crystal microbalance (QCM). The results show that, in view of building a sensing element for measuring humidity, ammonia or other gases, it is possible to maximize the sensor sensitivity to a certain gas by using an appropriate concentration of DNDs in HMDSO. Thus, a high degree of sensor sensitivity, together with short response time and minimum hysteresis, can be achieved. Composites of plasma-polymerized HMDSO with DNDs can be used as gas sensitive layers for the development of quartz resonator sensors.

Organic Plasma-Polymer Thin Film Coatings on Super-Hydrophilic Coated Surface for Increasing the Surface Durability against Scratch without Changing of Surface Wettability

In this study, the SiO polymer coating film was prepared the containing tetraethyl orthosilane (TEOS) solution by the sol-gel method on soda lime glass. After then, the plasma polymer coating was deposited on SiO polymer coated glass by plasma enhanced chemical vapor deposition (PECVD) method at room temperature during 15 seconds. The thiophene monomer was used as organic precursor. It was heat up to 60 oC and bubbled with hydrogen gas which flow rate was 50 sccm. Plasma was ignited by radio frequency (RF, 13.56 MHz) and its power was 10 W. SiO polymer and plasma polymer coated SiO polymer films were investigated by Fourier Transform Infrared (FT-IR), scanning electron microscopy (SEM), ultraviolet-visible (UV-Vis.), water contact angle, the adhesion test, and the pencil hardness test. The IR spectra shows evidence of very thin organic plasma polymer, which could not be measured by SEM cross image. The SEM images show that the morphology of each film was not changed by plasma polymer coating. Low water contact angles showed with both coating. Moreover surface hardness was increased by plasma polymer coating.

Combination of zinc alloy coating with thin plasma polymer films for novel corrosion protective systems on coated steel

Steel sheet used in automotive applications has to be corrosion protected effectively, which is usually realized by zinc or zinc alloy coatings with a thickness range of 5–10μm. Steel sheet for areas of a car body which are exceptionally stressed by corrosion, e.g. cavity flanges or joints, may be protected additionally by a thin weldable organic coating with a thickness of 2–4μm. A very promising approach to a significantly reduced use of resources is the combination of zinc alloy coatings with thin plasma polymer films deposited by means of plasma-enhanced chemical vapour deposition (PECVD). Such plasma polymer films of just a few 100nm thickness show excellent barrier and adhesion properties as well as a high mechanical stability.Within this work thin plasma polymer films were deposited on zinc alloy coated steel substrates using the strip hollow cathode (SHC) method, which was modified for application on grounded substrates. A pulsed DC glow discharge in a mixture of argon and an organosilane precursor was used for the deposition of films with a thickness of 100–500nm.The chemical compositions of the coatings were determined by means of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The morphologies of the coating systems were studied by means of scanning electron microscopy. The performance of the coating systems has been studied in different specific tests of corrosion and processing behaviour. The investigated coating systems show a corrosion resistance comparable to reference samples of electro-galvanised steel sheet with additional organic coating even with a coating thickness less of half of the reference samples.

Surface modified porous substrate by plasma immobilization of allyl alcohol for drug delivery

The objective of this study is to examine an effectiveimmobilization method anchoring a thin functional plasmapolymer as a barrier material onto substrate’s surface forcontrolling drug-releasing behavior by a new composite LT-plasma treatment. The term ‘composite’ means combinationof two kinds of LT-plasma treatment, i.e. plasma polymer-ization and plasma-induced grafting. In general, a prerequisitefor introducing specific functional groups onto a substrate’ssurface requires an ability to make covalent bonds betweenthe substrates’ surface radicals and unsaturated carbon bondsof monomers. If such is the case, thin films deposited byplasma polymerization have merit for this purpose becausethey have surface radicals for further reactions, especiallythe grafting by unsaturated hydrocarbon monomers. As acandidate for a suitable monomer for this purpose, allylalcohol is presumed to play an essential role since it has acarbon double bond in its backbone and, in addition, ahydroxyl group responsible for hydrophilic character. In thiswork, the immobilization of allyl alcohol as a thin film-forming material with hydrophilic functionality onto cellu-lose surface as a substrate is investigated by the compositeplasma treatment, in which the cellulose surface is coatedfirst with thin film by allyl alcohol-plasma-polymer (AAPP)and then grafted with allyl alcohol polymer via covalentbond. Cellulose, (C

Self-organized, gratinglike nanostructures in polymer films with embedded metal nanoparticles induced by femtosecond laser irradiation

The self-organized formation of periodic superstructures in thin plasma polymer films containing noble metal nanoparticles upon femtosecond laser irradiation has been studied in detail. By applying several hundred laser pulses on average per spot, the nanostructure of the metal layer is persistently changed into an approximate line grating with periodical changes in particle size and shape as observed by scanning and transmission electron microscopy. The formation of the structures is only possible within rather narrow ranges of laser intensity and metal content. The orientation of the lines is given by the laser polarization, while their spatial periodicity depends on laser wavelength and incidence angle. These observations give evidence that interference of incident light with light scattered into the film plane is the main mechanism controlling the nanostructure formation. We also discuss the optical spectra of the irradiated regions, in particular the observed dichroism and its relation to the prepared periodic structures.