Plasma-modified Polymer Research Articles

Applicability of Electron-Beam and Hybrid Plasmas for Polyethylene Terephthalate Processing to Obtain Hydrophilic and Biocompatible Surfaces.

The applicability of beam-plasma chemical reactors generating cold hybrid plasma for the production of noncytotoxic polymeric surfaces with high hydrophilicity and good biocompatibility with human fibroblast culture and human red blood cells was studied. Oxygen hybrid plasma was excited by the joint action of a continuous scanning electron beam and a capacity-coupled RF-gas discharge. Experiments showed that hybrid plasma treatment caused polar oxygen-containing functional group formation in the surface layer of poly (ethylene terephthalate) films. No thermal or radiative damage in tested polymer samples was found. The plasma-modified polymers turned out to be noncytotoxic and revealed good biocompatibility with human fibroblasts BJ-5ta as well as lower hemolytic activity than untreated poly (ethylene terephthalate). Experiments also demonstrated that no phenomena caused by the electrostatic charging of polymers occur in hybrid plasma because the electron beam component of hybrid plasma eliminates the item charge when it is treated. The electron beam can effectively control the reaction volume geometry as well as the fluxes of active plasma particles falling on the item surface. This provides new approaches to the production of abruptly structured patterns or smooth gradients of functionalities on a plane and 3D polymeric items of complicated geometry.

BEAM-PLASMA REACTORS FOR FUNCTIONAL GRADIENT MATERIALS PRODUCTION

Application and perspectives of beam-plasma chemical rectors generating cold hybrid plasma for production of functionally graded polymeric materials for tissue engineering and regenerative medicine are considered. Oxygen hybrid plasma was produced by joint action of a continuous or intermittent electron beam and a capacity coupled radio-frequency (RF) gas discharge (13.56 MHz) on gaseous media at moderate pressures (~ 1000 Pa). The scanning electron beam (accelerating voltage, 30 kV, and beam current, 1-1.5 mA) was injected through the grid of the active RF electrode; polymeric samples were placed in the gap between electrodes of the RF-discharge system. Electron beam characteristics, namely, scanning mode and power, can instantly and independently control the reaction volume geometry as well as concentrations of active plasma particles. Methods for such control by means of the scanning of intermittent electron beams are described. As a result, the RF discharge can be accurately localized on the desirable zone of the polymeric surface forming abruptly structured patterns or smooth gradients of functionalities, while radiation and thermal damages of polymers are prevented. The approach was tested using polymethylmethacrylate, polyethylenterphtalate, and polycarbonate samples, and the changes in the chemical composition and hydrophilic properties of their surfaces were shown, which are associated with the improvement of biocompatibility of the plasma-modified polymers.

Crazes in Plasma-Modified Polymer Materials

We report on the results of an analysis of the craze-formation center in plasma-modified polymers. It is shown that, as a result of plasma modification, oxidation, destruction, and joining processes occur in the polymer, which create conditions for the formation of craze centers and, hence, of crazes.

Plasma Modification of Poly Lactic Acid Solutions to Generate High Quality Electrospun PLA Nanofibers

Physical properties of pre-electrospinning polymer solutions play a key role in electrospinning as they strongly determine the morphology of the obtained electrospun nanofibers. In this work, an atmospheric-pressure argon plasma directly submerged in the liquid-phase was used to modify the physical properties of poly lactic acid (PLA) spinning solutions in an effort to improve their electrospinnability. The electrical characteristics of the plasma were investigated by two methods; V-I waveforms and Q-V Lissajous plots while the optical emission characteristics of the plasma were also determined using optical emission spectroscopy (OES). To perform a complete physical characterization of the plasma-modified polymer solutions, measurements of viscosity, surface tension, and electrical conductivity were performed for various PLA concentrations, plasma exposure times, gas flow rates, and applied voltages. Moreover, a fast intensified charge-couple device (ICCD) camera was used to image the bubble dynamics during the plasma treatments. In addition, morphological changes of PLA nanofibers generated from plasma-treated PLA solutions were observed by scanning electron microscopy (SEM). The performed plasma treatments were found to induce significant changes to the main physical properties of the PLA solutions, leading to an enhancement of electrospinnability and an improvement of PLA nanofiber formation.

Plasma modification of poly(2-heptadecyl-4-vinylthieno[3,4-d]thiazole) low bandgap polymer and its application in solar cells.

For the first time, we here propose a green methodology to modify a low bandgap polymer for highly efficient solar cells using atmospheric pressure plasma jet or soft plasma operating on different feeding gases (air, Ar and N2). The physical properties of the modified polymer were investigated using conductivity measurements, UV-visible spectroscopy, photoluminescence spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammograms, atomic force microscopy, cathodoluminescence and confocal Raman spectroscopy. Further, we examined the variation of the work function of the polymer before and after plasma treatment using a γ-focused ion beam. Additionally, photovoltaic cells based on the plasma-modified polymer having ITO/PEDOT:PSS/PHVTT (with or without plasma modification):PC71BM/LiF/Al configuration were fabricated and then characterized. We found that the power conversion efficiency (PCE) of the plasma-modified polymer increased dramatically as compared to the control polymer (without plasma treatment). PCE of the control polymer was found to be 4.11%, while after air, Ar and N2 gas plasma treatment the polymer showed PCEs of 4.85%, 4.87% and 5.14% respectively. Thus, plasma treatment not only alters the surface properties, but also modifies the bulk properties (changes in HOMO and LUMO bandgap level). Hence, this work provides new dimensions to explore more about plasma and polymer chemistry.

Grafting of bovine serum albumin proteins on plasma-modified polymers for potential application in tissue engineering

In this work, an influence of bovine serum albumin proteins grafting on the surface properties of plasma-treated polyethylene and poly-l-lactic acid was studied. The interaction of the vascular smooth muscle cells with the modified polymer surface was determined. The surface properties were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, nano-LC-ESI-Q-TOF mass spectrometry, electrokinetic analysis, and goniometry. One of the motivations for this work is the idea that by the interaction of the cell with substrate surface, the proteins will form an interlayer between the cell and the substrate. It was proven that when interacting with the plasma-treated high-density polyethylene and poly-l-lactic acid, the bovine serum albumin protein is grafted on the polymer surface. Since the proteins are bonded to the substrate surface, they can stimulate cell adhesion and proliferation.

Structure and properties of nanosized coatings deposited onto polymers

Results of studies aimed at developing a new approach to measuring stress-strain properties of nanosized solids (strength, yield stress, and the value of plastic deformation at uniaxial tension) are generalized. This approach is based on the analysis of the parameters of microrelief arising upon the deformation of polymer films with thin coatings. It is demonstrated for the first time that the stress-strain properties of aluminum coatings deposited onto Lavsan substrates depend on the level of stresses in the substrate, the value of its deformation, and the thickness of the coating. The evolution of these parameters is related to the strain hardening of metal and the effect of nanostructuring of crystalline materials in the range of small thicknesses. When precious metal (Au, Pt) nanosized films are deposited onto polymers by ion-plasma sputtering, in the course of metal deposition, polymer surface layers interact with cold plasma. Stress-strain properties of polymer surface layers modified by plasma are quantitatively estimated for the first time. The model is proposed that makes it possible to take into account the contribution of the properties of precious metal and plasma-modified polymer surface layer to the strength of the coating.

Plasma-modified polymer surfaces: Characterization using XPS

Amongst all the available surface treatments, the plasma techniques have taken a major importance these last decades, both in fundamental studies and for industrial applications. This paper presents the utility of X-ray photoelectron spectroscopy for the characterization of plasma-modified polymer surfaces. Due to the originality of the reactivity of the plasmas (generation of radicals), and to the surface-selective character of the plasma treatments, some specific uses of XPS are emphasized, such as peak fitting, derivatization, angle-resolved XPS. We also show several selected examples of typical XPS analysis of plasma-treated polymers: ageing, biocompatibility, plasma polymerization, adhesion, surface grafting of polar functions.

Surface characterisation of plasma treated flexible substrates for waveguide-on-flex application

This paper reports the physical and chemical effects of argon, nitrogen and oxygen plasma treatments on the surface properties of flexible circuit materials. Both KaptonHN™ and Vecstar™ films selected as optical flexible substrates under plasma irradiation were studied by atomic force microscope (AFM), contact angles, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The etch rate enhancement of the polymeric materials is more pronounced in oxygen plasma than in argon or nitrogen plasma. The increases of surface functional groups (–COOH, –CNOH) of plasma-modified polymer films are correlated with the increased presence of polar component in surface free energy. While the plasma treatment on KaptonHN™ substrates have been widely discussed in literature, the detailed characterisation of both pristine and different plasma surface modified Vecstar™ films is reported and compared for the first time.

A multitechnique study of preferential protein adsorption on hydrophobic and hydrophilic plasma-modified polymer surfaces

The adsorption process of albumin, lysozyme and lactoferrin was investigated onto polymer surfaces, both hydrophobic and hydrophilic treated by oxygen–plasma. In particular thin films of polyhydroxymethylsiloxane (about 90° of static water contact angle) were converted by oxygen plasma treatments at reduced pressure into hydrophilic SiO x phases (less than 10° of water contact angle). The protein adsorption process was investigated in situ by quartz crystal microbalance with dissipation monitoring in terms of coverage and kinetics mechanism, while chemical structure and topography of the protein adlayers were studied ex situ by angular-resolved XPS and atomic force microscopy, respectively. The plasma surface modification of the polymer film drastically modified the adsorption process of the three proteins, both in terms of kinetics and coverage.

Plasma surface modification of polymers for biomedical use

Polymeric materials can be used in many application areas due to their mechanical (e.g. elastic) characteristics, chemical stability, and their light weight, as well as for their many design possibilities. Even in the fields of medicine and biotechnology many products are completely or partly made of polymers. In contact with biological systems, compatibility of these materials is not always given. To fulfill the requirements for medical applications, the surfaces have to be modified. Plasma techniques are demonstrated as an appropriate tool for the generation of the demanded surface properties. Experimental data are given for surface modification by plasma polymerization, with retention of the functional groups of the monomers used. Qualitative and quantitative characterization of the thin films with respect to the type and density of available groups at the surfaces is presented. Some possible applications of plasma-modified polymers are also discussed.

Chemical derivatization techniques in the determination of functional groups by X-ray photoelectron spectroscopy

The fundamentals of chemical derivatization techniques as applied to the quantitative analysis of ultrathin surface layers of various organic compounds via functional groups were considered. Using X-ray photoelectron spectroscopy as an example of the most informative and almost nondestructive technique for characterization of thin layers, the general requirements imposed on the selective chemical reactions and possible artifacts were discussed. The validity of the technique was illustrated by the examples of analyses of surface layers of organic polymers with known concentrations of surface functional groups, plasma-modified polymers, and carbon fibers. It was noted that selective chemical reactions are successfully used in other techniques for studying material surfaces. The surface analysis of organic materials via functional groups can be performed with a simultaneous increase in the sensitivity of the corresponding spectral technique.

Diamine-plasma treated and Cu(II)-incorporated poly(hydroxyethylmethacrylate) microbeads for albumin adsorption

Poly(2-hydroxyethylmethacrylate) (PHEMA) microbeads prepared by suspension polymerization were treated with diamine-plasmas (i.e. ethylene-diamine (EDA) and hexamethylene diamine (HMDA)) in a glow-discharge reactor in which the exposure time and glow-discharge power were changed between 5 and 30 min and 5 and 20 W, respectively. The amount of nitrogen deposition increased both with increase in exposure time and glow-discharge power. The maximum amounts of nitrogen deposition on the microbeads were 22.3 and 23.4 μmol g-1 with the EDA- and HMDAplasmas. Then, Cu(II) ions were incorporated onto the PHEMA microbeads by chelating with the nitrogen-carrying functional groups. Different amounts of Cu(II) ions (2.4-6.8 mg g-1) were incorporated on the PHEMA microbeads by changing the initial concentration of Cu(II) ions. Bovine serum albumin (BSA) adsorption onto the unmodified PHEMA, diamine-plasma treated PHEMA, and diamine-plasma treated Cu(II)-incorporated PHEMA microbeads was investigated. The nonspecific adsorption of BSA on the unmodified microbeads was very low (0.22 mg BSA g-1). Deposition of nitrogen increased the BSA adsorption (9.3 mg g-1 for EDA-plasma and 12.7 mg g-1 for HMDA-plasma). Cu(II)-incorporation significantly increased the BSA adsorption (154 mg g-1 for EDA-plasma and 178 mg g-1 for HMDA-plasma). Further increases in the albumin adsorption capacities of the polymer microbeads (185 mg g-1 for EDA-plasma and 208 mg g-1 for HMDA-plasma) were observed when human plasma was used. More than 92% of the adsorbed albumin molecules was desorbed in 1 h in the desorption medium containing 0.5 M NaSCN at pH 8.0. Repeated adsorption-desorption cycles showed the feasibility of these plasma-modified polymer microbeads.

Barrier properties of plasma and chemically fluorinated polypropylene and polyethyleneterephthalate

The functionalization, perfuorination or an coating of polypropylene (PP) and polyethyleneterephthalate (PET) surfaces in different plasma atmospheres and in an F 2 + N 2 gas mixture were investigated. The aim of this work was to form a barrier layer against solvent and fuel permeation. Typical test solvents were n-pentane, toluene, tetrachloroethylene and mixtures of n-pentane and methanol. The permeation was gravimetrically measured. The permeation of the solvent mixtures was analyzed by mass spectrometry and Fourier transform IR spectroscopy. The permeation of the solvent mixtures through the plasma-modified polymer samples was associated with increased absorption of polar solvent components in the polymer film. The thickness of the plasma-fluorinated PP surface layer was 10–100 nm and that of chemically fluorinated PP about 5000 nm. Nearly the same barrier efficiency was measured for both plasma- and chemically fluorinated samples.

Quantitative Analysis of Polymer Surface Restructuring

Polymer surfaces can adapt to surrounding media by reorientation of surface segments ; such motions can produce changes in contact angles (CAs). Building on two existing formalisms for time-independent CAs of heterogeneous surfaces, treatments are derived for the analysis of time-dependent CAs, enabling quantitative evaluation of the restructuring of polymer surfaces. In particular, we treat the case of air/water CAs on polymers which contain some polar surface groups that are immobile and others that are mobile and can migrate away from the surface by rotational and/or translational motions of polymer chains. A first-order process is assumed for the rate-limiting reorientation motion. The resulting equations have three parameters, viz., the fraction of the surface area covered by mobile polar groups, the fraction of the surface area covered by immobile polar groups, and the characteristic time constant (lifetime) of the reorientation process. The theoretical descriptions were applied to sets of contact angles measured on various plasma-modified polymer surfaces over extended periods oftime, enabling quantitative comparison of the effects of identical plasma treatments on the surface restructuring of different polymers.

Barrier properties of plasma-modified polypropylene and polyethyleneterephthalate

Plasma treatment changes the solvent absorption and permeation as well as the swelling properties of polymers. Enchanced solvent absorption and swelling are effects of an improved solvent compatibility. The plasma introduces a large number of different groups at the polymer surface depending on the nature of the plasma. Fluorine-containing plasmas can replace hydrogen atoms of the polymer molecule with fluorine atoms. Moreover, fluorine-containing plasma polymer layers can be formed. All these processes reduce the resulting surface free energy, reduce the diffusion length of solvent molecules, and produce a barrier layer. We have studied the formation of solvent barriers by plasma fluorination and by crosslinking by ultraviolet (UV) radiation. Thin foils of polypropylene (PP) and polyethyleneterephthalate (PET) were used as substrates. CF4, SF6, and SOF2 were applied as sources of fluorine atoms. Hexafluoropropene, tetrafluorethylene, and perfluorohexylethylene form plasma polymer layers on the polymer substrates. Test solvents were n-pentane, tetrachloroethylene, dimethylsulfoxide, and mixtures of n-pentane and methanol. The permeation rate of solvents through plasma-modified polymers was measured gravimetrically. Mass spectrometry was applied to analyze the permeating components of the solvent mixtures. Fluorination of surface layers by plasma-chemical (CF4, SF6) means considerably reduces the permeation rate of PP (95% barrier effect) and PET (100%). The preferred permeation of one component of the pentane/methanol mixture is influenced by the polarity of plasma-introduced groups at the polymer surface.

Modification of isotactic polypropylene by a cold plasma or an electron beam and grafting of the acrylic acid onto these activated polymers

AbstractSurface grafting of polymeric materials, such as films and fibers, may improve their reactive surface properties. Polypropylene active sites that should initiate a surface postgrafting can be formed by either nitrogen cold plasma or an electron‐beam irradiation. The plasmamodified polymer surface is cross‐linked and functionalized with primary amino groups and with oxidized groups, and it is almost not degraded. Electron‐beam‐modified polypropylene is also functionalized through an aging reaction, emphasized by a high radical concentration. In both cases, active surface films are susceptible to react with monomers in a postgrafting reaction. The grafting yield and rate of acrylic acid were shown to be dependent on the formed amino group concentration: The hydrophilic character of the modified polypropylene surface enhances the acrylic acid approach. The electron‐beam treatment leads also to a reactive surface that can initiate a grafting reaction. But its yield and rate are dependent on the radical concentration: radicals formed during the irradiation. © 1994 John Wiley & Sons, Inc.

XPS studies of in situ plasma-modified polymer surfaces

X-ray photoelectron spectroscopy (XPS) has been used to study the chemical effects of both inert (argon) and reactive (oxygen, nitrogen, and mixed gas) plasma treatments done in situ on a variety of polymer surfaces. Inert gas plasma treatments introduce no new detectable chemical species onto the polymer surface but can induce degradation and rearrangement of the polymer surface. However, plasma treatments with reactive gases create new chemical species which drastically alter the chemical reactivity of the polymer surface. These studies have also shown that the surface population of chemical species formed after plasma treatment is dependent on both the chemical structure of the polymer and the plasma gas. The effects of direct and radiative energy-transfer processes in a plasma have also been studied. Polymers containing certain functional groups were found to be more susceptible to damage via radiative energy transfer. Ageing studies of plasma-modified polymer surfaces exposed to the atmosphere have shown that the ageing process consists of two distinct phases. The initial phase, which occurs rapidly, involves adsorption of atmospheric contaminants and, in some cases, specific chemical reactions. The second phase, which occurs slowly, is due to surface reorganization.

In SituFTIR Investigations Of Polymer Surface Modification In Downstream Microwave Plasma Etching

AbstractIn situFourier transform infrared (FTIR) reflection-absorption spectroscopy investigations of etching of thin polyimide and poly(methyl methacrylate) films (200–1500Å) in downstream microwave NF3/O2/Ar plasmas are reported. Etch rates and surface chemistry are monitored as a function of gas phase composition, plasma treatment conditions and time. NF3/Ar plasma treatment leads to significant surface fluorination characterized by the formation of aliphatic fluorine compounds (CFx), acyl fluorides, benzoyl fluoride, and polyfluorinated benzene. Addition of oxygen to the etching gas reduces the degree of surface fluorination and modifies the chemical structure. The absorption bands due to CFxstructures decrease gradually while polyfluorinated benzene rings and benzoyl fluoride are absent for NF3/O2mixtures with more than 20% oxygen. The effect of humidity on the plasma-modified polymers is studied by comparing infrared spectra collectedin situwith those after air exposure. For NF3/O2plasma-treated polyimides significant changes are observed while samples fluorinated in NF3show no changes after exposure to air overnight. The FTIR data are supplemented by XPS analysis.

In Situ Ftir Investigations of Polymer Surface Modification in Downstream Microwave Plasma Etching

AbstractIn situ Fourier transform infrared (FTIR) reflection-absorption spectroscopy investigations of etching of thin polyimide and poly(methyl methacrylate) films (200-1500Å) in downstream microwave NF3/O2/Ar plasmas are reported. Etch rates and surface chemistry are monitored as a function of gas phase composition, plasma treatment conditions and time. NF3/Ar plasma treatment leads to significant surface fluorination characterized by the formation of aliphatic fluorine compounds (CFx), acyl fluorides, benzoyl fluoride, and polyfluorinated benzene. Addition of oxygen to the etching gas reduces the degree of surface fluorination and modifies the chemical structure. The absorption bands due to CFx structures decrease gradually while polyfluorinated benzene rings and benzoyl fluoride are absent for NF3/O2 mixtures with more than 20% oxygen. The effect of humidity on the plasma-modified polymers is studied by comparing infrared spectra collected in situ with those after air exposure. For NF3/O2 plasma-treated polyimides significant changes are observed while samples fluorinated in NF3 show no changes after exposure to air overnight. The FTIR data are supplemented by XPS analysis.