The structure and properties of polymer-metallic coating interphase

The structure of the surface layer in polymers (LDPE and PET) decorated with a thin metal (gold and platinum) layer was studied after their deformation under different conditions. It was found that relatively thick coatings debonded from the polymer substrate during tensile drawing. Debonding was observed at low tensile strains (below 20–30%). During the further drawing of a polymer, a regular microrelief typical of deformable “rigid coating on a soft substrate” systems appeared on its surface. This phenomenon is explained by the fact that the debonding metal coating uncovers not the surface of the pure polymer but a certain modified layer, which has a higher elastic modulus than the pure polymer. The formation of this layer is associated with the inclusion of metal atoms into the polymer during the metal decoration by plasma immersion ion deposition. As a result of this inclusion, a modified layer, which has a higher glass transition temperature, a higher elastic modulus, and other mechanical properties, is formed between the coating and the polymer.     

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.     

Correlation between structure and stress-strain characteristics of metallic coatings deposited onto a polymer by the method of ionic plasma sputtering

Data on the strength of coatings based on noble metals (Pt, Au) deposited onto PET films by the method of ionic plasma sputtering are analyzed. In addition to precipitation of the metal, this mode of deposition is accompanied by modification of the surface polymer layer due to its interaction with plasma. As a result, a complex three-layered structure near the polymer surface forms. A new method for estimating the strength of coatings deposited onto polymer supports is advanced. This method makes it possible to analyze stress-strain characteristics of the three-layered systems that emerge owing to deposition of nanoscale layers of noble metals on polymer films via ionic plasma sputtering. The proposed relationships are in fair agreement with the experimental data.     

Low-Frequency Glow Discharge Treatment of Poly(ethylene terephthalate) Films

The alteration of surface characteristics of the PET-E poly(ethylene terephthalate) film by treatment in the cathode fall of an ac (50 Hz) glow-discharge plasma was studied. The plasma-assisted modification leads to surface hydrophilization which is retained for a long period of time. It was found that the discharge treatment induced a negative charge in the polymer surface layer. The charge density created under different treatment conditions was correlated with the contact angle. The thermally stimulated relaxation and depolarization measurements on the original and the modified film showed that charge buildup in the film during the discharge treatment was due to trapping of injected electrons. From the experimental data, it was inferred that the charge states emerged play the determining role in enhancing the surface energy of modified PET films.     

Au nanolayers deposited on polyethyleneterephtalate and polytetrafluorethylene degraded by plasma discharge

Polyethyleneterephtalate (PET) and polytetrafluorethylene (PTFE) foils were modified by plasma discharge. The effect of plasma modification on polymer surface wettability and on properties of gold coatings were studied as a function of time from plasma exposure (aging time) and polymer substrate temperature. Thickness, sheet resistance, and surface topology of gold layers were studied. Aging of the plasma‐exposed samples is accompanied by increase in contact angle, which is explained by rearrangement of the polymer segments in the polymer surface monolayer, and a decrease in the concentration of polar groups. The aging also leads to a decline in surface roughness R_a measured by atomic force microscopy (AFM). Under deposition conditions, comparable thicknesses of deposited Au layers were prepared on pristine PET and plasma‐treated PET and PTFE samples. The thinnest Au layers were evaporated onto pristine PTFE. The sheet resistance decreases with increasing thickness of Au layer. Plasma treatment leads to an increase of PTFE surface roughness, which becomes even more pronounced after Au deposition. A higher roughness shows that the PET samples are deposited with the Au layer at temperatures above the glassy transition temperature T_g. Copyright © 2006 John Wiley & Sons, Ltd.     

Surface characteristics of a self-polymerized dopamine coating deposited on hydrophobic polymer films

This study aims to explore the fundamental surface characteristics of polydopamine (pDA)-coated hydrophobic polymer films. A poly(vinylidene fluoride) (PVDF) film was surface modified by dip coating in an aqueous solution of dopamine on the basis of its self-polymerization and strong adhesion feature. The self-polymerization and deposition rates of dopamine on film surfaces increased with increasing temperature as evaluated by both spectroscopic ellipsometry and scanning electronic microscopy (SEM). Changes in the surface morphologies of pDA-coated films as well as the size and shape of pDA particles in the solution were also investigated by SEM, atomic force microscopy (AFM), and transmission electron microscopy (TEM). The surface roughness and surface free energy of pDA-modified films were mainly affected by the reaction temperature and showed only a slight dependence on the reaction time and concentration of the dopamine solution. Additionally, three other typical hydrophobic polymer films of polytetrafluoroethylene (PTFE), poly(ethylene terephthalate) (PET), and polyimide (PI) were also modified by the same procedure. The lyophilicity (liquid affinity) and surface free energy of these polymer films were enhanced significantly after being coated with pDA, as were those of PVDF films. It is indicated that the deposition behavior of pDA is not strongly dependent on the nature of the substrates. This information provides us with not only a better understanding of biologically inspired surface chemistry for pDA coatings but also effective strategies for exploiting the properties of dopamine to create novel functional polymer materials.     

Hydrogel characteristics of electron-beam-immobilized poly(vinylpyrrolidone) films on poly(ethylene terephthalate) supports

A novel strategy for the preparation of thin hydrogel coatings on top of polymer bulk materials was elaborated for the example of poly(ethylene terephthalate) (PET) surfaces layered with poly(vinylpyrrolidone) (PVP). PVP layers were deposited on PET foils or SiO2 surfaces (silicon wafer or glass coverslips) precoated with PET and subsequently cross-linked by electron beam treatment. The obtained films were characterized by ellipsometry, X-ray photoelectron spectroscopy, infrared spectroscopy in attenuated total reflection, atomic force microscopy (AFM), and electrokinetic measurements. Ellipsometric experiments and AFM force-distance measurements showed that the cross-linked layers swell in aqueous solutions by a factor of about 7. Electrokinetic experiments indicated a strong hydrodynamic shielding of the charge of the underlying PET layer by the hydrogel coatings and further proved that the swollen films were stable against shear stress and variation of pH. In conclusion, electron beam cross-linking ofpreadsorbed hydrophilic polymers permits a durable fixation of swellable polymer networks on polymer supports which can be adapted to materials in a wide variety of shapes.     

Radiofrequency-discharge plasma treatment of heavy ion-irradiated poly(ethylene terephthalate) films

The effects of treatment in a radiofrequency (RF) discharge plasma on the rate of chemical etching of the tracks made by xenon ions (with an energy of ~1 MeV/nucleon) in poly(ethylene terephthalate) (PETP) films were investigated. The influence of plasma treatment conditions on the structure and properties of nuclear track membranes formed by etching was studied. It was found that the RF plasma treatment of heavy ion-bombarded PETP films leads to a decrease in etchability of both tracks and the starting polymer matrix. The changes in track and matrix etchability due to crosslinking of the polymer surface layer were shown to be responsible for the asymmetry of the track membrane structure.     

Nucleation and growth of gas barrier aluminium oxide on surfaces of poly(ethylene terephthalate) and polypropylene: effects of the polymer surface properties

The nucleation and initial stages of growth of aluminium oxide deposited on two different polymer surfaces [poly(ethylene terephthalate), (PET) and amorphous polypropylene, (PP)] have been studied by atomic force microscopy (AFM). The permeation of water vapor and oxygen through the films has been measured. The initial stages of the growth of the oxide consisted of separated islands on the polymer surface. Further growth of oxide depends strongly on the surface morphology and chemical nature of the polymer surface. Growth on PET follows a layer‐by‐layer mechanism that maintains the native surface roughness of the polymer substrate. Growth on PP, however, follows an island mode, which leads to an increase in surface roughness. This may be due to a lack of chemical bonding between the polymer and the arriving metal–oxygen particles. The oxide layer on PET grows more densely than on PP, providing superior barrier to gas permeation. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3151–3162, 2000     

Titanium-supported nickel-copper oxide catalysts for oxidation of carbon(II) oxide

Nickel-copper compositions for catalytic oxidation of carbon(II) oxide to carbon(IV) oxide were prepared by impregnation of oxide films on titanium surface, obtained by plasma electrolytic oxidation followed by annealing. Plasma electrolysis oxide coatings with a layer thickness of 5 to 50 μm were generated using different electrolytes. The compositions were studied by X-ray powder diffraction, X-ray spectral analysis, and electron microscopy, and moisture absorption of the initial plasma electrolytic structures was estimated. A linear correlation was found between the overall concentration of nickel and copper (4 to 25 mol %) in the surface layer of ∼2–5-μm compositions and their catalytic activity. The overall concentration of nickel and copper was found to increase in parallel with the moisture absorption of plasma electrolytic oxidation coatings. Nickel-copper compositions based on plasma electrolytic oxidation coatings generated in a silicate electrolyte displayed the best catalytic, mechanical, and adhesion properties.     

Estimating the strain-strength characteristics of nanothick nonmetallic coatings deposited onto poly(ethylene terephthalate) films

The features of surface structuring during tensile drawing of PET films with deposited nanothick coatings are studied. In contrast to our earlier studies, this investigation is focused on the study of nonmetallic coatings (carbon deposited by vacuum sputtering and modified silica deposited via evaporation from the solution). In both cases, the tensile drawing of the supporting polymer is accompanied by the fragmentation of deposited coatings on PET films and by the development of regular surface patterns. The above features (i) demonstrate the general character of the phenomena occurring during tensile drawing of the polymer films and (ii) have allowed the first quantitative estimation of the strain-strength characteristics of the deposited coatings (modified silica gel) in nanoscale layers.     

Surface interpenetrating networks of polyacrylamide in poly(ethylene terephthalate) as a means of surface modification

Surface interpenetrating network (IPN) polymers are emerging hybrid materials in which the surface of existing polymers can be modified to preserve their chemical structure and bulk properties. A detailed structural characterization of poly(ethylene terephthalate) (PET) thin films on nanoscopically flat silicon wafers has been carried out by Scanning Probe Microscopy (SPM) and X-ray photoelectron spectroscopy (XPS). Examination of the surface of spin-coated annealed PET film by the SPM in tapping mode revealed a two-phase structure. One phase appeared as a dense crystalline fraction of the polymer while the other was identified as amorphous. These findings were supported by Differential Scanning Calorimetry (DSC), which recognized the crystallinity of annealed PET film at 30%. Modification of the PET surface with interpenetrating polyacrylamide (PAM) increased the roughness of the surface with uniform properties. The depth profiling with XPS revealed that PAM interpenetration extended down to 7.2 nm, confirming a three-dimensional character of the polymer modification, with a relative mass concentration of PAM at about 30.7% in the IPN interface.     

Preparation of composite membranes by means of plasma polymerization of tiophene

The structure and the charge transport properties of poly(ethylene terephthalate) track membranes modified in a thiophene plasma were studied. It was found that polymer deposition on the surface of a track membrane via the plasma polymerization of thiophene results in composite membranes that, in the case of the formation of a semipermeable layer, exhibit conductivity asymmetry—rectifying effect—in electrolyte solutions. It was shown that chemical doping with iodine or photo-oxidation of the polymer layer produced in plasma leads to alteration in the electrochemical properties of plasma-modified membranes.     

Angle resolved XPS characterization of cationic polyacrylamides

In many applications surfaces are modified using polymer films and the polymers used are often complex copolymers. In biomedical applications it is critical to determine the surface properties of a substrate as it is these that mediate the cellular interactions. The surface structure of copolymer films can only rarely be established from their bulk composition alone. In this study angle resolved XPS was used to build a model of the structure of copolymer films produced on glass substrates from a family of poly(acrylamide) copolymers containing cationic blocks. The thickness of the copolymer films was demonstrated to be dependent on the concentration of the polymer solution and the ratio of non‐cationic to cationic blocks in the copolymer. The data demonstrated that the cationic blocks of the copolymer preferentially segregated to the glass surface and the non‐cationic poly(acrylamide) blocks preferentially segregated to the air–vacuum interface. A low concentration of the cationic functional groups was present throughout the poly(acrylamide) layer and it was suggested that this resulted from a small fraction of the cationic blocks being pulled into the poly(acrylamide) layer at points along the polymer chain where the two blocks are connected. Evidence of a thin surface hydrocarbon contamination layer was also observed. Copyright © 2009 John Wiley & Sons, Ltd.     

Preparation and characterization of nonfouling polymer brushes on poly(ethylene terephthalate) film surfaces

In this study, a surface grafting of nonfouling poly(ethylene glycol) methyl ether acrylate (PEGMA) on poly(ethylene terephthalate) (PET) was carried out via surface-initiated atom-transfer radical polymerization (SI-ATRP) to improve hemocompatibility of polymer based biomaterials. To do this, the coupling agent with hydroxyl groups for the ATRP initiator was first anchored on the surface of PET films using photochemical method, and then these hydroxyl groups were esterified by bromoisobutyryl bromide, from which PET with various main chain lengths of PEGMA was prepared. The structures and properties of modified PET surfaces were investigated using water contact angle (WAC), ATR-FTIR, X-ray photoelectron spectroscopy (XPS) and Atomic force microscopy (AFM). The molecular weights of the free polymer from solution were determined by gel permeation chromatography (GPC). These results indicated that grafting of PEGMA on PET film is a simple way to change its surface properties. The protein adsorption resistance on the surfaces of PET was primarily evaluated by an enzyme-linked immunosorbent assay (ELISA). The result demonstrated that the protein adsorption could be well suppressed by poly(PEGMA) brush structure on the surface of PET. This work provides a new approach for polymers to enhance their biocompatibility.     

Synthesis of patterned polymer brushes from flexible polymeric films

Surface-initiated polymerizations were carried out from polymeric surfaces of commercially important polyester films, poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN). These plastic films were modified prior to polymerization by plasma oxidation, exposing surface hydroxyl groups, in order to immobilise patterned self assembled monolayers (SAMs) of trichlorosilane initiator, through the soft lithographic method of microcontact printing (μCP). Subsequently, polymerizations were initiated from the surface via controlled atom transfer radical polymerization (ATRP), under aqueous conditions, to create patterned brushes of the thermo-responsive polymer poly(N-isopropyl acrylamide) (PNIPAM). By creating patterned, rather than homogeneous brushes characterization was made possible by atomic force microscopy (AFM).     

Colloid probe AFM investigation of interactions between fibrinogen and PEG-like plasma polymer surfaces

Interaction forces between surfaces designed to be protein resistant and fibrinogen (Fg) were investigated in phosphate-buffered saline with colloid probe atomic force microscopy. The surfaces of the silica probes were coated with a layer of fibrinogen molecules by adsorption from the buffer. The technique of low-power, pulsed AC plasma polymerization was used to make poly(ethylene glycol) (PEG)-like coatings on poly(ethylene teraphthalate) by using diethylene glycol vinyl ether as the monomer gas. The degree of PEG-like nature of the films was controlled by use of a different effective plasma power in the chamber for each coating, ranging from 0.6 to 3.6 W. This produced a series of thin films with a different number of ether carbons, as assessed by X-ray photoelectron spectroscopy. The interaction force measurements are discussed in relation to trends observed in the reduction of fibrinogen adsorption, as determined quantitatively by (125)I radio-labeling. The plasma polymer coatings with the greatest protein-repelling properties were the most PEG-like in nature and showed the strongest repulsion in interaction force measurements with the fibrinogen-coated probe. Once forced into contact, all the surfaces showed increased adhesion with the protein layer on the probe, and the strength and extension length of adhesion was dependent on both the applied load and the plasma polymer surface chemistry. When the medium was changed from buffer to water, the adhesion after contact was eliminated and only appeared at much higher loads. This indicates that the structure of the fibrinogen molecules on the probe is changed from an extended conformation in buffer to a flat conformation in water, with the former state allowing for stronger interaction with the polymer chains on the surface. These experiments underline the utility of aqueous surface force measurements toward understanding protein-surface interactions, and developing nonfouling surfaces that confer a steric barrier against protein adsorption.     

Introduction of Primary Amino Groups on Poly(ethylene terephthalate) Surfaces by Ammonia and a Mix of Nitrogen and Hydrogen Plasma

With the aim of introducing primary amino groups on the surface of poly(ethylene terephthalate) (PET), two methods were compared—the use of ammonia or a combination of nitrogen and hydrogen low-pressure microwave plasma. Several plasma parameters were optimized on the reactor to increase the –NH₂ surface density, which was estimated by colorimetric titration and X-ray photoelectron spectroscopy (XPS). These techniques show that whatever the plasma treatment, almost 2 –NH₂/nm2 are incorporated on PET films. Emission spectroscopy highlighted a correlation between the density of primary amino groups and the ratio between an NH peak intensity and an Ar peak intensity (I_NH/I_Ar). Variation in surface hydrophilicity with aging in air after plasma treatment was monitored with contact angle measurements and showed a hydrophobic recovery. This was confirmed by XPS, which suggests also that surfaces treated by NH₃ plasma are more stable than surfaces treated by N₂/H₂.     

Alteration of Structure and Properties of Thin Polyimide Films and Coatings by Plasma and Thermal Treatment

The influence of alternating radiofrequency-plasma and heat treatment on the structure and bulk properties of polyimide films and coatings with a thickness of 3.5 m was studied. It was shown that plasma treatment leads to the enhancement of the specific surface energy due to the polar component and to an increase in the diffusion coefficient and in the amount of sorbed water. The subsequent thermal treatment leads to the recovery of the polyimide surface and bulk properties to almost their initial magnitudes. It was assumed that these effects are caused by a change in the structure of thin films and coatings and by the buildup of negatively charged particles at the surface and in mesopores of the polymer upon RF plasma treatment and due to annealing upon thermal treatment.     

Low Pressure Radio Frequency Ammonia Plasma Surface Modification on Poly(ethylene terephthalate) Films and Fibers: Effect of the Polymer Forming Process

We investigated the effect of a polyethylene terephthalate (PET) forming process on radiofrequency ammonia plasma surface-treated PET flat films and fibres obtained by melt blowing. Ammonia plasma treatment allowed for the incorporation of amino functionalities on both the film and fibre surfaces, with higher values observed at very short treatment times. This plasma treatment also induced polymer chain scissions which were observed as the formation of hydrophilic nodules that coalesced together and were loosely bound to the underlying polymeric materials. These plasma-induced surface damages were notably more important on the melt-blown PET fibres. Consequently, maximisation of the surface amino groups with minimal polymer chain breaking was achieved using very short plasma treatment times (typically 1 s). We also demonstrated that the polymer forming process must be taken into account when plasma modifications are to be performed on PET, as it may already lead to polymer chain breakings subsequently added to those induced in the plasma environment.