Short-time plasma surface modification of HDPE powder in a Plasma Downer Reactor - process, wettability improvement and ageing effects

The effectiveness of improving the wettability of HDPE powders within less than 0.1 s by plasma surface modification in a Plasma Downer Reactor is investigated. A correlation is revealed between the XPS results (O/C-ratio) and the wettability (contact angle, polar surface tension by capillary rise method). The O₂-content in the plasma feed gas has been adjusted for best wettability properties. XPS results indicate the formation of CO and COOH functional groups on the powder surface. The O/C-ratio increased from 0.0 (no oxygen on the non-treated powder) up to 0.15 for the plasma treated HDPE powder surface. With pure O₂-plasma treatment, a water contact angle reduction from >90° (no water penetration into the untreated PE powder) down to 65° was achieved. The total surface free energy increased from 31.2 to 45 mN/m. Ageing of treated powders occurs and proceeds mostly within the first 7 days of storage. Contact angle measurements and O1s/O2s intensity ratio data support that ageing is mainly a diffusion-controlled process. Nevertheless, XPS results show the presence of oxygen functional groups even after 40 days, which explains why the powder is still dispersible in water without any addition of surfactants.     

Fluorination of polymethylmethaacrylate with tetrafluoroethane using DC glow discharge plasma

Fluorination of polymer surfaces has technological applications in various fields such as microelectronics, biomaterials, textile, packing, etc. In this study PMMA surfaces were fluorinated using DC glow discharge plasma. Tetrafluoroethane was used as the fluorinating agent. On the fluorinated PMMA surface, static water contact angle, surface energy, optical transmittance (UV-vis), XPS and AFM analyses were carried out. After the fluorination PMMA surface becomes hydrophobic with water contact angle of 107° without losing optical transparency. Surface energy of fluorine plasma-treated PMMA decreased from 35 mJ/cm² to 21.2 mJ/cm². RMS roughness of the fluorinated surface was 4.01 nm and XPS studies revealed the formation of C-CF_x and CF₃ groups on the PMMA surface.     

Asymmetric surface modification of poly(ethylene terephthalate) film by CF4 plasma immersion

Poly(ethylene terephthalate) (PET) films were treated with CF₄ plasma immersion. The samples were processed at different RF powers and treatment time. The surface modification of PET films was evaluated by water contact angle (CA), X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). Decrease in contact angle of both sides of PET films was observed under mild treatment conditions. However, as raising treatment power and/or time, the change in contact angle between the two sides of PET films was different. The relatively hydrophobic and hydrophilic surfaces were being in situ formed on the two sides of PET films, respectively. And the extreme values of water contact angle reached 108.63 and 7.56°, respectively. XPS analyses revealed that there was a substantial incorporation of fluorine and/or oxygen atoms in both side surfaces. The relative chemical composition of the C (ls) spectra's showed the incorporation of non-polar fluorine-based functionalities (i.e. CFCF_n, CF₂ or CF₃ groups) and polar oxygen-based functionalities (i.e. COOH or OH groups) in the surfaces. Correlation between the plasma parameters and the surface modification of PET films is also discussed.     

Surface analysis of oxygen plasma treated poly(p-phenylene benzobisoxazole) fibers

The influence of oxygen plasma treatment on surface properties of poly(p-phenylene benzobisoxazole) (PBO) fibers and aging effect of the oxygen plasma modified PBO fiber surfaces were investigated by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and dynamic contact angle analysis (DCAA), respectively. The results indicated that the oxygen plasma treatment introduced some polar groups to PBO fiber surfaces, enhanced surface roughness and changed surface morphologies of PBO fibers by plasma etching and oxidative reactions. Surface wettability of PBO fibers may be significantly improved by increasing surface free energy of the fibers via oxygen plasma treatment. Aging effect of the oxygen plasma treated PBO fibers showed that the fiber surface wettability degraded in the first several days after the plasma treatment, and it was found to be changeless as the aging time continued as long as 30 days.     

Fluorocarbon nano-coating of polyester fabrics by atmospheric air plasma with aerosol

A fluorocarbon coating was deposited on polyester (PET) woven fabric using pulse discharge plasma treatment by injecting a fluoropolymer directly into the plasma dielectric barrier discharge. The objective of the treatment was to improve the hydrophobic properties as well as the repellent behaviour of the polyester fabric. Plasma treatment conditions were optimised to obtain optimal hydrophobic properties which were evaluated using water contact angle measurement as well as spray-test method at the polyester fabric surface. The study showed that adhesion of the fluoropolymer to the woven PET was greatly enhanced by the air plasma treatment. X-ray photoemission spectroscopy (XPS) analyses revealed chemical surface modifications occurring after the plasma treatments.     

Self-assembled monolayer growth on chemically modified polymer surfaces

We report a study of the self-assembled monolayer (SAM) growth of bis[3(triethoxysilane)propyl]tetrasulfide (Tetrasulfide) on low dielectric constant (low-k) aromatic hydrocarbon SiLK whose surface chemistry was modified using sulfuric acid, He plasma treatment, and N₂ plasma treatment. X-ray photoelectron spectroscopy (XPS) spectra show that there is no detectable growth of Tetrasulfide SAM on untreated SiLK surfaces. After the SiLK surfaces have been treated with sulfuric acid, He plasma, or N₂ plasma, the original chemically inert polymer surfaces are functionalized with polar groups resulting in a significant improvement of their wettability, which is confirmed by their reduction of water droplet contact angles. The introduction of polar functional groups thus facilitates the formation of Tetrasulfide SAM on the polymer surfaces. Atomic force microscopy (AFM) analysis shows an insignificant change in the surface morphology after the growth of Tetrasulfide SAM on the chemically modified SiLK surfaces. Quantitative XPS analysis also showed that Tetrasulfide SAM growth is more prominent on He and N₂ plasma treated surfaces than those treated by sulfuric acid.     

Wettability modification of electrospun poly(?-caprolactone) fibers by femtosecond laser irradiation in different gas atmospheres

The effect of femtosecond laser irradiation in air and in O₂ and CF₄ gas flows on the wettability of electrospun poly(?-caprolactone) fiber tissue scaffolds was studied. Laser power, focus spot size, raster scan spacing and gas atmosphere were varied in experiments. SEM imaging showed the average fiber diameter and surface porosity sizes were both altered by ablation. The micro-scale surface roughness measured by scanning laser profilometry was found to have a non-monotonic relationship to the surface wettability measured by the contact angle of sessile water droplets. In contrast, surface water contact angle continuously decreased with increased oxygen atomic percentage and oxygen-containing group fraction as measured by XPS. Further, the oxygen content was larger for more extensively ablated fiber surfaces, regardless of whether the increased ablation was caused by high laser power, smaller scanning space or smaller defocusing distance. Of the three gas atmospheres, O₂ gas flow was the most favorable environment for increasing surface oxidization, resulting in the largest water contact angle decrease for given laser power. For CF₄ gas flow, the least oxidization occurred, and the magnitude of water contact angle decrease was smallest for treatment at a given laser power.     

Argon ion beam interaction on polyethylene terephthalate surface by a 4 kJ plasma focus device

Polyethylene terephthalate (PET) has a wide variety of applications ranging from making regular bottles to biosensors. However, for many of these applications, surface treatment is needed to improve its surface characteristics such as adhesion to other materials. In this study, we focussed on treating PET foils by dense Ar pulsed plasma produced by a 4.5 kJ Mather-type plasma focus device (20 kV, 40 µf, 115 nH) to examine its ability to make the PET surface hydrophilic. The most common method to examine this characteristic is measuring the water contact angle on a polymer surface. The results show that while the energy and density of plasma in our device are higher compared to other devices, as the exposure time is very low, the device can enhance the wettability of PET film surfaces.     

Surface modification of a biomedical polyethylene terephthalate (PET) by air plasma

In this work, low-pressure air plasma has been used to improve polyethylene terephthalate (PET) surface properties for technical applications. Surface free energy values have been estimated using contact angle value for different exposure times and different test liquids. Surface composition and morphology of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Surface topography changes related with the etching mechanism have been followed by weight loss study. The results show a considerable improvement in surface wettability and the surface free energy values even for short exposure times in the different discharge areas (discharge area, afterglow area and remote area), as observed by a remarkable decrease in contact angle values. Change of chemical composition made the polymer surfaces to be highly hydrophilic, which mainly depends on the increase in oxygen-containing groups. In addition to, the surface activation and AFM analyses show obvious changes in surface topography as a consequence of the plasma-etching mechanism.     

Preparation of Titanium Oxide-containing Organic Film by Dipping Ti(OR)4 and Cold Plasma oxidizing on PET

Low temperature process to prepare titanium oxide film on the surface of PET was investigated in this study. The substrates were pre-treated by oxygen plasma activated procedure, and then spin coating of Ti(OR)₄ precursor solution was carried out to prepare Ti-containing organic films. Finally, O₂ plasma was employed to decompose organic compounds and oxidize Ti to form oxides. Oxygen plasma post treatment can oxidize the organic compounds to form titanium oxide carbon subsequently. From the IR and ESCA analysis could be observed that -CH bond decreases and Ti-O, C-O bonds increase after O₂ plasma treatment. Their surface hydrophilicity was enhanced by UV-irradiation, the degree of water contact angle decreased from 60° to 10°.     

Surface modification of a polyamide 6 film by He/CF4 plasma using atmospheric pressure plasma jet

Polyamide 6 (PA 6) films are treated with helium(He)/CF₄ plasma at atmospheric pressure. The samples are treated at different treatment times. The surface modification of the PA 6 films is evaluated by water contact angle, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The etching rate is used to study the etching effect of He/CF₄ plasma on the PA 6 films. The T-peel strengths of the control and plasma treated films are measured to show the surface adhesion properties of the films. As the treatment time increases, the etching rate decreases steadily, the contact angle decreases initially and then increases, while the T-peel strength increases first and then decreases. AFM analyses show that the surface roughness increases after the plasma treatment. XPS analyses reveal substantial incorporation of fluorine and/or oxygen atoms to the polymer chains on the film surfaces.     

Fabrication and anti-frosting performance of super hydrophobic coating based on modified nano-sized calcium carbonate and ordinary polyacrylate

Nano-sized calcium carbonate (CaCO₃) particles were modified by heptadecafluorodecyl trimethoxysilane under acidic water condition. An ordinary polyacrylate prepared via radical copolymerization of methyl methacrylate, butyl acrylate, acrylic acid and β-hydroxyethyl methacrylate was used as the binder to form hydrophobic coatings with the modified CaCO₃. Super hydrophobic coating with water contact angle of 155° was obtained from modified CaCO₃ and the polyacrylate at their weight ratio of 8/2 by a simple procedure. Based on surface analysis by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS), the super hydrophobicity can be attributed to both the surface microstructure and surface enrichment of fluoroalkyl chains. Due to a low water sliding angle, carbon black powder on super hydrophobic surface was easily removed by rolling water droplet. Furthermore, the anti-frosting performance of different surfaces was investigated, which indicated that the frost formed on superhydrophobic surface was greatly retarded compared with that on bare copper surface. The surface kept super hydrophobicity even after freezing-thawing treatment for 10 times.     

Functionalization of PET and PA6.6 woven fabrics

In the present work as received woven fabrics of polyethylene terephthalate (PET) and polyamide 6.6 (PA6.6) were exposed to a continuous dielectric barrier discharge (DBD), in air at atmospheric pressure, at selected discharge power values and conveyor speeds. The chemical modification of the fabric surface was studied by contact angle analysis, attenuated total reflection (ATR)-FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS). The results confirmed that the treatment changed the fabric surface chemistry, increasing its wettability by polar liquids and its oxygen content. Contact angle results showed different behaviour of the two polymer fabrics toward ageing effects; while PET showed a contact angle increase along the subsequent days of treatment, the PA6.6 fabric maintained its hydrophilicity even 15 days after treatment. The surface morphology analysed by scanning electron microscopy (SEM), did not show any significant difference before and after treatment.     

Influence of aramid fiber moisture regain during atmospheric plasma treatment on aging of treatment effects on surface wettability and bonding strength to epoxy

One of the main differences between a low-pressure plasma treatment and an atmospheric pressure plasma treatment is that in atmosphere, the substrate material may absorb significant amount of water which may potentially influence the plasma treatment effects. This paper investigates how the moisture absorbed by aramid fibers during the atmospheric pressure plasma treatment influences the aging behavior of the modified surfaces. Kevlar 49 fibers with different moisture regains (MR) (0.5, 3.5 and 5.5%, respectively) are treated with atmospheric pressure plasma jet (APPJ) with helium as the carrier gas and oxygen as the treatment gas. Surface wettability and chemical compositions, and interfacial shear strengths (IFSS) to epoxy for the aramid fibers in all groups are determined using water contact angle measurements, X-ray photoelectron spectroscopy (XPS), and micro-bond pull out tests, respectively. Immediately after the plasma treatment, the treated fibers have substantially lower water contact angles, higher surface oxygen and nitrogen contents, and larger IFSS to epoxy than those of the control group. At the end of 30 day aging period, the fibers treated with 5.5% moisture regain had a lower water contact angle and more polar groups on the fiber surface, leading to 75% improvement of IFSS over the control fibers, while those for the 0.5 and 3.5% moisture regain groups were only 30%.     

Biocompatibility of polypropylene non-woven fabric membrane via UV-induced graft polymerization of 2-acrylamido-2-methylpropane sulfonic acid

This work described the graft polymerization of a sulfonic acid terminated monomer, 2-acrylamido-2-methylpropane sulfonic acid (AMPS), onto the surface of polypropylene non-woven (NWF PP) membrane by O₂ plasma pretreatment and UV-induced photografting method. The chemical structure and composition of the modified surfaces were analyzed by FTIR-ATR and XPS, respectively. The wettability was investigated by water contact angle and equilibrium water adsorption. And the biocompatibility of the modified NWF PP membranes was evaluated by protein adsorption and platelet adhesion. It was found that the graft density increased with prolonging UV irradiation time and increasing AMPS concentration; the water contact angles of the membranes decreased from 124° to 26° with the increasing grafting density of poly(AMPS) from 0 to 884.2 μg cm⁻², while the equilibrium water adsorption raised from 5 wt% to 75 wt%; the protein absorption was effectively suppressed with the introduction of poly(AMPS) even at the low grafting density (132.4 μg cm⁻²); the number of platelets adhering to the modified membrane was dramatically reduced when compared with that on its virgin surface. These results indicated that surface modification of NWF PP membrane with AMPS was a facile approach to construct biocompatible surface.     

Surface wettability of atmospheric dielectric barrier discharge processed Armos fibers

Wettability of Armos fibers has been investigated after exposed to dielectric barrier discharge (DBD) plasma, which was performed at atmospheric pressure in air while varying the sample treatment time between 9 and 27 s. Contact angles and surface free energy of the original and plasma-treated fibers were measured with dynamic contact angle analysis (DCAA) to reveal the correlation between the fiber wettability and the surface treatment, including surface composition and topography modifications, which were evaluated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. It was found by XPS analysis that the O/C atomic ratio on Armos fiber surface can be increased from 0.134 to 0.248 after the 18 s exposure to the plasma and many polar functional groups were proved to be incorporated into the surface, which aided good wetting. In addition, AFM observations revealed the changes of fiber surface microstructure, showing significant enhancement of the surface roughness after the treatment, which could also bring the fiber better wettability. What's more, the impacts of fiber surface treatment on its tensile properties were characterized by single fiber tensile strength (SFTS) testing. Results showed that Armos fibers exhibited only slight reductions in their tensile strengths with the great enhancement in fiber surface free energy.     

Ageing effects on the wettability behavior of laser textured silicon

In the present work we investigate the ageing of acid cleaned femtosecond laser textured 〈1 0 0〉 silicon surfaces. Changes in the surface structure and chemistry were analysed by Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS), in order to explain the variation with time of the water contact angles of the laser textured surfaces. It is shown that highly hydrophobic silicon surfaces are obtained immediately after laser texturing and cleaning with acid solutions (water contact angle > 120°). However these surfaces are not stable and ageing leads to a decrease of the water contact angle which reaches a value of 80°. XPS analysis of the surfaces shows that the growth of the native oxide layer is most probably responsible for this behavior.     

Wettability of Chemically Modified Polymers: Experiment and Theory

The wettabilities of four polymers, polytetrafluoroethylene (PTFE), ultra-high molecular weight polyethylene (UHMW-PE), polyetherurethane (PEU) and polydimethylsiloxane (PDMS) have been studied experimentally. Using PBS as the wetting fluid, advancing and receding contact angles have been measured for virgin surfaces and for materials treated in O₂-, Ar-, N₂- and NH₃-plasmas. In the case of PTFE, further surface modi-fication was produced by storage of virgin and plasma treated specimens in air or PBS for one month. These experimental measurements are presented on wettability plots which facilitate interpretation in terms of current theoretical understanding. Wettability plots show contact angle measurements as a function of a parameter designated X _M which quantifies the surface composition as a weighted average of chemically distinct surface domains. The theoretical wettability of smooth surfaces with molecular scale heterogeneities can be drawn as a line (zerohysteresis) on such wettability plots and provides a helpful datum for analysing the hysteretic wetting behaviour of real surfaces in terms of contact line pinning by surface heterogeneities. An experimental method based on analysis of XPS spectra for estimating X _M is presented. This enables wettability plots (contact angles vs. X _M) to be plotted for the four materials studied. These plots elucidate the variation of the wettability of a given material as a function of surface treatment, and the comparative behaviour of different materials.     

Controllable wettability of poly(ethylene terephthlate) film modified by oxygen combined inductively and capacitively coupled radio-frequency plasma

Poly(ethylene terephthalate) (PET) film was modified by using oxygen combined inductively coupled radio-frequency plasma (ICP) and capacitively coupled radio-frequency plasma (CCP) at the radio-frequency (RF) power of 200 W and 100 W, respectively, for a treatment time up to 300 s. The RF plasma modification under the combined ICP and CCP mode with the controllable oxygen plasma density and oxygen ion-flux energy significantly improved the wettability of PET film, due to the creation of the polar functional groups containing oxygen, such as C-O and O-CO, and the increase of the surface roughness. At a low surface roughness, the polar functional groups on the PET film affected both the advancing contact angles and receding contact angles. When the surface roughness increased over a threshold, the advancing contact angles mainly depended on the polar functional groups, and the receding contact angles were particularly dependent on the surface roughness. Therefore, the controllable advancing contact angles and receding contact angles on the plasma-modified PET film were independently determined by plasma functionalization and plasma etching under the combined ICP and CCP mode.     

Influence of He/O2 atmospheric pressure plasma jet treatment on subsequent wet desizing of polyacrylate on PET fabrics

The influence of He/O₂ atmospheric pressure plasma jet (APPJ) treatment on subsequent wet desizing of polyacrylate on PET fabrics was studied in the present paper. Weight loss results indicated that the weight loss increased with an increase of plasma treatment time. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) showed an increased surface roughness after the plasma treatment. SEM also showed that the fiber surfaces were as clean as unsized fibers after 35 s treatment followed by NaHCO₃ desizing. X-ray photoelectron spectroscopy (XPS) analysis indicated that oxygen-based functional groups increased for the plasma treated polyacrylate sized fabrics. The percent desizing ratio (PDR) results showed that more than 99% PDR was achieved after 65 s plasma treatment followed by a 5 min NaHCO₃ desizing. Compared to conventional wet desizing, indicating that plasma treatment could significantly reduce desizing time.