Low Molecular Weight Oxidized Material Research Articles

Mechanism behind polypropylene surface modifications by OH radicals: An experimental study

We quantitatively investigated the roles and mechanisms of OH radicals in polypropylene (PP) surface modification utilizing a previously proposed vacuum ultraviolet photodissociation reactive species supply method. Water contact angle (WCA), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the modifications in surface hydrophilicity and components. The relationship between the WCA decline rate and OH radical density at the PP surface was quantified as (2.1 ± 0.1)[OHs] + (0.5 ± 0.1) degrees/s, where [OHs] is in ppm. The ATR-FTIR and XPS results demonstrated that C–OH, CO, C–O–O, and OCO groups were gradually introduced onto the PP surface at different treatment stages. The formation and behavior of low-molecular-weight oxidized materials (LMWOMs) on the PP surface during OH radical treatment were investigated. A modified layer composed of LMWOMs eventually formed on the PP surface. The physicochemical mechanisms underlying these phenomena at the micro-level were also discussed. A possible surface oxidation model was proposed based on WCA results, surface functional groups, and LMWOMs, indicating that the oxidation of the PP chains by OH radicals was initiated at the topmost surface and advanced in the depth direction.

Quantitative and selective study of the effect of O radicals on polypropylene surface treatment

Abstract Gaining a comprehensive understanding of the mechanisms underlying plasma-polymer surface treatments using atmospheric pressure plasmas can guide their industrial applications. To this end, a novel vacuum ultraviolet photodissociation reactive species supply (VUV PRS) method, which can quantitatively supply the desired type of reactive species to polymer surfaces, was developed in our previous study. In this study, the innovative approach is used to quantitatively study the effects of O radicals on polypropylene (PP) surface treatment. Specifically, water contact angle (WCA), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, and atomic force microscopy (AFM) are used to characterize the surface modification. The results show that the surface treatment is dominated by O radicals under the present experimental conditions. The surface modification rate, characterized by the WCA decline rate, is linearly correlated with the concentration of O radicals, that is (surface modification rate) = (8.1 ± 0.4)[Os] + (0.13 ± 0.02) degrees s−1, where [Os] indicates the density of O radicals at PP surface (in ppm). The ATR-FTIR spectra shows that –OH and C=O functional groups are introduced onto the PP surface after the treatment, which results in a decrease in the WCA of the surface. Atomic force microscopy (AFM) analysis shows that the surface morphology of the treated PP remains almost unchanged. Low molecular weight oxidized materials (LMWOMs) on PP surface are generated when the O radical dose that arrives at the PP surface exceeds a certain threshold. More than 60% of the oxygen-containing functional groups exist as LMWOMs when the WCA saturates. The results presented in this paper indicate that the VUV PRS method has the potential to drive the understanding of plasma-polymer surface interactions at the atomic and molecular levels. Moreover, the measured roles of the O radicals on the polymer surface at atmospheric pressure can guide the industrial application of atmospheric pressure plasma treatment.

Effect of Low Molecular Weight Oxidized Materials and Nitrogen Groups on Adhesive Joints of Polypropylene Treated by a Cold Atmospheric Plasma Jet.

Polypropylene is a typical representative of synthetic polymers that, for many applications including adhesive joints, requires an increase in wettability and chemical surface reactivity. Plasma processing offers efficient methods for such surface modifications. A particular disadvantage of the plasma jets can be the small plasma area. Here, we present a cold atmospheric plasma radio-frequency slit jet developed with a width of 150 mm applied to polypropylene plasma treatment in Ar, Ar/O and Ar/N We identified two main parameters influencing the tensile strength of adhesive joints mediated by epoxy adhesive DP 190, nitrogen content, and the amount of low molecular weight oxidized materials (LMWOMs). Nitrogen functional groups promoted adhesion between epoxy adhesive DP 190 and the PP by taking part in the curing process. LMWOMs formed a weak boundary layer, inhibiting adhesion by inducing a cohesive failure of the joint. A trade off between these two parameters determined the optimized conditions at which the strength of the adhesive joint increased 4.5 times. Higher adhesion strength was previously observed when using a translational plasma gliding arc plasma jet with higher plasma gas temperatures, resulting in better cross linking of polymer chains caused by local PP melting.

Rapid Atmospheric Pressure Ambient Air Plasma Functionalization of Poly(styrene) and Poly(ethersulfone) Foils

Activation of polymeric surfaces, i.e. formation and/or modification of the functional groups on the surface of a material, is essential prior to the further processing of polymers, especially in applications where wettability plays a crucial role. In this study, an atmospheric pressure ambient air plasma treatment of poly(styrene) (PS) and poly(ethersulfone) (PES) foils using diffuse coplanar surface barrier discharge is presented. The plasma treatment for 0.5 s resulted in a decrease of water contact angle from the original value of 83° to 26° for PS and from 76° to 32° for PES. No significant changes in wettability were observed for prolonged treatment times. Better wettability was correlated with decreasing carbon to oxygen ratio resulting from an incorporation of oxygen-containing functional groups C–OH, C=O and O–C=O on the surface. X-ray photoelectron spectroscopy was employed to study details in the changes of the surface chemistry following the plasma exposure. We used atomic force microscopy to study the formation of low molecular weight oxidized material (LMWOM) during the plasma treatment. After dissolving the LMWOM in water, we observed roughening of the plasma-treated surfaces at the nanometre level due to etching induced by plasma treatment.

Cold atmospheric pressure plasma treatment for adhesion improvement on polypropylene surfaces

Cold atmospheric pressure plasma from a plasma jet was used to improve the adhesion properties of polypropylene (PP). Differences in adhesion were at first observed for either air or N2 as process gas, tested on a PP-acrylic pressure sensitive adhesive (PSA) model system. For a deeper understanding changes in the surface chemistry and topography on PP after plasma jet treatment, using several different O2/N2 mixtures, were investigated in a comprehensive surface analytical study. Independent of the gas composition a majority of plasma induced polar groups exist as water soluble low molecular weight oxidized material (LMWOM), located within the first few nanometers. Consequently, the observed changes in the surface chemistry for various process gas compositions are mainly attributed to differences in the formed LMWOM. However, these chemical changes indicated also differences in the interaction mechanism of plasma reactive species with the polymer surface and therefore, in the resulting plasma-polymer etching process. Hence, significant changes in the topography have been observed for different process gas compositions, which could be reliably correlated with the obtained adhesion results.

Surface functionalization of polypropylene using a cold atmospheric pressure plasma jet with gas water mixtures

This work presents a surface analytical study on polypropylene (PP), treated with a cold atmospheric pressure plasma jet (CAPPJ) system. A novel treatment approach to moisten the process gas is followed, enabling high concentrations of admixed water at atmospheric conditions. Experiments were carried out with a systematic increase of water admixtures in the process gas (air/N2). Optical emission spectroscopy (OES) was used to determine chemical changes in the plasma. Contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were performed on PP samples directly after the plasma treatment and an additional subsequent washing step of the surface. The results show that water admixtures significantly change the chemistry and morphology of the treated surfaces. However, these changes heavily depend on the process gas (air or N2). While in the case of N2 process gas water admixtures influenced the chemistry of so called created low molecular weight oxidized material (LMWOM), the formation of LMWOM during the plasma treatment was quenched completely when water was admixed into air process gas instead.

Impact of plasma treatment under atmospheric pressure on surface chemistry and surface morphology of extruded and injection-molded wood-polymer composites (WPC)

The influence of plasma treatment performed at atmospheric pressure and ambient air as process gas by a dielectric barrier discharge (DBD) on the morphological and chemical surface characteristics of wood-polymer composites (WPC) was investigated by applying several surface-sensitive analytical methods. The surface free energy showed a distinct increase after plasma treatment for all tested materials. The analyzing methods for surface topography—laser scanning microscopy (LSM) and atomic force microscopy (AFM)—revealed a roughening induced by the treatment which is likely due to a degradation of the polymeric surface. This was accompanied by the formation of low-molecular-weight oxidized materials (LMWOMs), appearing as small globular structures. With increasing discharge time, the nodules increase in size and the material degradation proceeds. The surface degradation seems to be more serious for injection-molded samples, whereas the formation of nodules became more apparent and were evenly distributed on extruded surfaces. These phenomena could also be confirmed by scanning electron microscopy (SEM). In addition, differences between extruded and injection-molded surfaces could be observed. Besides the morphological changes, the chemical composition of the substrates’ surfaces was affected by the plasma discharge. Infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) indicated the formation of new oxygen containing polar groups on the modified surfaces.

Contact angle measurements on oxidized polymer surfaces containing water-soluble species

Advancing and receding contact angle measurements on polymer surfaces can be performed using a number of different methods. Ballistic deposition is a new method for both rapidly and accurately measuring the receding contact angle of water. In the ballistic deposition method, a pulsed stream of 0.15-μL water droplets is impinged upon a surface. The water spreads across the surface and then coalesces into a single 1.8-μL drop. High-speed video imaging shows that, on most surfaces, the water retracts from previously wetted material, thereby forming receding contact angles that agree with the receding angles measured by the Wilhelmy plate technique. The ballistic deposition method measures the receding angle within one second after the water first contacts the surface. This rapid measurement enables the investigation of polymer surface properties that are not easily probed by other wettability measurement methods. For example, meaningful contact angles of water can be obtained on the water-soluble low-molecular-weight oxidized materials (LMWOM) formed by the corona and flame treatment of polypropylene (PP) films. Use of the ballistic deposition method allows for a characterization of the wetting properties and an estimation of the surface energy components of LMWOM itself. Both corona- and flame-generated LMWOM have significant contact angle hysteresis, almost all of which is accounted for by the non-dispersive (polar) component of the surface rather than by the dispersive component. Surface heterogeneity is thus associated primarily with the oxidized functionalities added to the PP by the corona and flame treatments.

Morphological and chemical analysis of PP film treated by Dielectric Barrier Discharge

PP films have been activated by DBD, with different gases (air, N2, H2, CO2) and at different energy treatments. Then film surfaces were analysed by AFM, contact-angle technique, ink test measurements and XPS, to explore modifications in surface topography, wettability, and oxidation state. After the treatments, it is possible to observe the formation of water-soluble low-molecular-weight oxidized materials (LMWOMs), which dimensions and spatial density are related to treatment parameters. Using AFM data it is possible to analyse the statistical distribution of the bubbles sizes. The analysis of XPS spectra shows the formation of oxidized polar groups on surface (e.g. C-O, C=O, COH) and allows to evaluate the extent and the nature of film oxidation, and how it influences surfaces properties. The relationship between surface chemical and morphological modifications and treatment parameters is pointed out. Then PP sample have been monitored in the time, to determine the ageing effects.

Surface modification of polymeric materials by cold atmospheric plasma jet

• We investigate polymer surface modification by atmospheric pressure plasma jet APPJ. • Jet operation conditions for uniform surface modification were determined. • The APPJ added O atoms to the polymer surface and also enhanced the roughness. • The degree of polymer surface modification by APPJ and DBD were compared. • The APPJ is more efficient in attaching O atoms and produces less polymer fragments. In this work we report the surface modification of different engineering polymers, such as, polyethylene terephthalate (PET), polyethylene (PE) and polypropylene (PP) by an atmospheric pressure plasma jet (APPJ). It was operated with Ar gas using 10 kV, 37 kHz, sine wave as an excitation source. The aim of this study is to determine the optimal treatment conditions and also to compare the polymer surface modification induced by plasma jet with the one obtained by another atmospheric pressure plasma source – the dielectric barrier discharge (DBD). The samples were exposed to the plasma jet effluent using a scanning procedure, which allowed achieving a uniform surface modification. The wettability assessments of all polymers reveal that the treatment leads to reduction of more than 40° in the water contact angle (WCA). Changes in surface composition and chemical bonding were analyzed by x-ray photoelectron spectroscopy (XPS) and Fourier-Transformed Infrared spectroscopy (FTIR) that both detected incorporation of oxygen-related functional groups. Surface morphology of polymer samples was investigated by Atomic Force Microscopy (AFM) and an increase of polymer roughness after the APPJ treatment was found. The plasma-treated polymers exhibited hydrophobic recovery expressed in reduction of the O-content of the surface upon rinsing with water. This process was caused by the dissolution of low molecular weight oxidized materials (LMWOMs) formed on the surface as a result of the plasma exposure.

Study of polypropylene surface modification by air dielectric barrier discharge operated at two different frequencies

In this work, air dielectric barrier discharge (DBD) operating at the line frequency (60Hz) or at frequency of 17kHz was used to improve the wetting properties of polypropylene (PP). The changes in the surface hydrophilicity were investigated by contact angle measurements. The plasma-induced chemical modifications of PP surface were studied by X-ray photoelectron spectroscopy (XPS) and Fourier-transformed infrared spectroscopy (FTIR). The polymer surface morphology and roughness before and after the DBD treatment were analyzed by atomic force microscopy (AFM). To compare the plasma treatment effect at different frequencies the variation of the contact angle is presented as a function of the deposited energy density. The results show that both DBD treatments leaded to formation of water-soluble low molecular weight oxidized material (LMWOM), which agglomerated into small mounts on the surface producing a complex globular structure. However, the 60Hz DBD process produced higher amount of LMWOM on the PP surface comparing to the 17kHz plasma treatment with the same energy dose. The hydrophilic LMWOM is weakly bounded to the surface and can be easily removed by polar solvents. After washing the DBD-treated samples in de-ionized water their surface roughness and oxygen content were reduced and the PP partially recovered its original wetting characteristics. This suggested that oxidation also occurred at deeper and more permanent levels of the PP samples. Comparing both DBD processes the 17kHz treatment was found to be more efficient in introducing oxygen moieties on the surface and also in improving the PP wetting properties.

Surface modification of biaxially oriented polypropylene (BOPP) film using acrylic acid-corona treatment: Part I. Properties and characterization of treated films

In this work, the acrylic acid (AAc)-corona discharge was carried out on biaxially oriented polypropylene (BOPP) films by introducing AAc vapor into the corona region of a normal corona treater. Three different corona energies of 15.3, 38.2 and 76.4kJ/m2 were studied. Surface properties of treated films were compared with those of air-corona treated films prepared with the same corona energies. The change in chemical composition on the film surface was characterized by curve-fitting of the ATR-FTIR spectra. The wettability of treated films, before and after aging in different environments, was observed by water contact angle and surface free energy. The surface morphology of air- and AAc-corona treated films was investigated using SEM and AFM techniques. Adhesion of the treated films to some other substrate was determined with the T-peeling test. It was found that the hydrophilicity of all treated films increased with increasing corona energy. AAc-corona treated films showed greater wettability than did the air-corona treated films and could retain the surface hydrophilicity for more than 90days of aging under ambient conditions. The surface morphology of BOPP films changed after corona treatment into a globular structure. The AAc-corona treated films showed rougher surfaces due to surface oxidation and polymer formation, whereas, air-corona treated films displayed a similar structure but of smaller size due to the formation of low molecular weight oxidized materials (LMWOM) arising from the degradation of BOPP films. AAc-corona treated films showed greater peel strength than did the air-corona treated films.

Surface characteristics analysis of polypropylene treated by dielectric barrier discharge at atmospheric pressure

Polypropylene (PP) samples were treated by Dielectric Barrier Discharge (DBD) in order to modify their surface characteristics. The XPS analysis reveals that the DBD treatment added oxygen atoms to the PP surface. These polar groups cause increase in the wettability as shown by water contact angle measurements. The formation of low-molecular-weight oxidized materials (LMWOMs) in the form of small nodules on the PP surface was observed by atomic force microscopy (AFM). The presence of oxygen polar groups on the PP surface was also confirmed by infrared spectroscopy (FTIR). All analysis were performed before and after rinsing the treated samples in water and showed that the LMWOM can be removed from the surface by polar solvents.

Polymer Surface Modification by Aerosol Based DBD Treatment of Foils

The effect of different nebulized liquids directly introduced into the dielectric barrier discharge (DBD) was compared with simple air DBD treatment of polyethylene foils. Water, alcohols and aqueous solutions of different organic substances (environmentally compatible) and water soluble polymers were applied as aerosols and injected into the DBD zone. The DBD residence time (number of treatment cycles) and the power were varied. The durability of the surface modification effect was studied after removing of Low-Molecular Weight Oxidized Material (LMWOM) by washing the samples with water and ethanol. The modified foils were characterized by XPS and contact angle measurements as a function of the applied plasma conditions. The concentration of functional groups at modified surfaces was estimated by derivatization and subsequent XPS measurement.

Surface modification of acrylate intraocular lenses with dielectric barrier discharge plasma at atmospheric pressure

Surface modification with dielectric barrier discharge (DBD) plasma was carried out at atmospheric pressure (argon as the discharge gas) to improve the biocompatibility of hydrophobic acrylate intraocular lens (IOL). Changes of the plasma-treated IOL surface in chemical composition, morphology and hydrophilicity were comprehensively evaluated by X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and water contact angle (WCA) measurements. The surface biocompatibility of the untreated and plasma-treated IOLs was compared with the adhesion behavior of platelets, macrophages and lens epithelial cells (LECs) in vitro. After DBD plasma treatment, the hydrophilicity of the IOL surface was obviously improved. The changes in WCA with treatment extension may be attributed to both the introduction of oxygen or/and nitrogen-containing polar groups and the increase of surface roughness induced by plasma etching effect. The existence of low molecular weight oxidized material (LMWOM) was proved on the plasmatreated IOL which was caused by the chain scission effect of the plasma treatment. The plasma-treated IOLs resisted the adhesion of platelets and macrophages significantly. The LECs spreading and proliferation were postponed on the IOLs plasma-treated for more than 180 s, with a well maintained epithelial phenotype of LECs. The IOL biocompatibility was improved after the DBD plasma treatment. We speculate that slighter foreign-body reaction and later incidence of anterior capsule opacification (ACO) may be expected after implantation of the argon DBD plasma-treated IOL.

Flame treatment of low-density polyethylene: Surface chemistry across the length scales

The relationship between surface chemistry and morphology of flame treated low-density polyethylene (LDPE) was studied by various characterization techniques across different length scales. The chemical composition of the surface was determined on the micrometer scale by X-ray photoelectron spectroscopy (XPS) as well as with time of flight secondary ion mass spectrometry (ToF-SIMS), while surface wettability was obtained through contact angle (CA) measurements on the millimeter scale. The surface concentration of hydroxyl, carbonyl and carboxyl groups, as a function of the “number” of the flame treatment passes (which is proportional to the treatment time) was obtained. Moreover, a correlation was found with chemical composition and polarity, emphasizing the role of oxygen-containing functional groups introduced during the treatment. Carboxyl functional groups were specifically identified by fluorescent labeling and the results were compared with the ToF-SIMS data. In addition, atomic force microscopy (AFM) was used to evaluate changes in surface topography and roughness on the nanometer to micrometer length scales. After flame treatment, water-soluble low molecular weight oxidized materials (LMWOM), which were generated as products of oxidation and chain scission of the LDPE surface, agglomerated into small topographical mounds that were visible in the AFM micrographs. After rinsing the flame treated samples with water and ethanol, bead-like nodular surface structures were observed. The ionization state of flame treated LDPE surfaces was monitored by chemical force microscopy (CFM). The effective surface p K a values of carboxylic acid (–COOH) obtained by AFM were revealed by chemical force titration curves and the effective surface p K a values were found to be around 6.

Different responses of several kinds of copolymerized polyimide films to ultraviolet irradiation

The surfaces of three kinds of copolymerized polyimide films were irradiated by ultraviolet (UV) light. The chemical composition changes of the films after irradiation were investigated by X-ray photoelectron spectroscopy (XPS), while their topographical changes were studied by atomic force microscopy (AFM). Besides, the surface topographical variation induced changes in friction behavior were recorded on a DF·PM unidirectional friction and wear test rig in a ball on block contact configuration. It was found that photodegradation took place, which induced the chain scission of the polyimide molecules and formation of low molecular weight oxidized material (LMWOM). Due to different photostability of the three kinds of polyimide, different chemical composition changes and forms of “LMWOM” were observed. Also, different friction behavior variations of the three kinds of polyimide films were recorded. It is thought that the polyimide film of PI (PPA + PMDA) with rod-like structures might have stronger photo-resistance, whose form of “LMWOM” was “nodules” and whose friction coefficient increased at 2 h of irradiation possibly due to cross-linking. In the case of PI (OPA + PMDA) and PI (MPA + PMDA) films, they mainly underwent photodegradation, which might soften the surfaces and increased the friction coefficients with increasing irradiation time.

Development of Poly(propylene) Surface Topography During Corona Treatment

AbstractSummary: Atomic force microscopy (AFM), contact‐angle measurements, and X‐ray photoelectron spectroscopy (XPS or ESCA) were used to characterize biaxially oriented poly(propylene) (PP) films modified by exposure to a corona discharge. Surface analysis was performed on PP films modified at various corona energies to explore the changes in surface topography, wettability, and oxidation state resulting from the corona treatment. Even at low corona energies, water‐soluble low‐molecular‐weight oxidized materials (LMWOM) are formed. These LMWOM products agglomerate into small topographical mounds that are visible in the AFM images. For the detection of LMWOM on corona‐treated surfaces, AFM appears to be at least as sensitive as contact‐angle measurements or ESCA. A major advantage of AFM relative to the other surface analytical techniques used to confirm the presence of the LMWOM is that no washing of the surface with water is required in conjunction with the AFM analysis.magnified image

Surface degradation and hydrophobic recovery of polyolefins treated by air corona and nitrogen atmospheric pressure glow discharge

AbstractThe surface degradation and production of low molecular weight oxidized materials (LMWOM) on biaxially oriented polypropylene (BOPP) and low‐density polyethylene (LDPE) films was investigated and compared for two different dielectric barrier discharge (DBD) treatment types, namely air corona and nitrogen atmospheric pressure glow discharge (N2 APGD). Contact angle measurements, X‐ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) analyses were performed in conjunction with rinsing the treated films in water. It is shown that N2 APGD treatments of both polyolefins result in much less surface degradation, therefore, allowing for a significantly higher degree of functionalization and better wettability. Hydrophobic recovery of the treated films has also been studied by monitoring their surface energy (γs) over a period of time extending up to several months after treatment. Following both surface modification techniques, the treated polyolefin films were both found to undergo hydrophobic recovery; however, for N2 APGD modified surfaces, γs ceases to decrease after a few days and attains a higher stable value than in the case of air corona treated films. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1291–1303, 2004

A Comparison of Corona-Treated and Flame-Treated Polypropylene Films

The comparison of corona-treated and flame-treated polypropylene (PP) films provides insight into the mechanism of these surface-oxidation processes. Atomic force microscopy (AFM), contact-angle measurements, and X-ray photoelectron spectroscopy (XPS or ESCA) were used to characterize surface-treated biaxially oriented PP. While both processes oxidize the PP surface, corona treatment leads to the formation of water-soluble low-molecular-weight oxidized materials (LMWOM), while flame treatment does not. Computational modeling of the gas-phase chemistry in an air corona was performed using a zero-dimensional plasma-chemistry model. The modeling results indicate that the ratio of O to OH is much higher in a corona discharge than in a flame. Chain scission and the formation of LMWOM are associated with reactions involving O atoms. The higher ratios of O to OH in a corona are more conducive to LMWOM production. Surface-oxidized PP exhibits considerable thermodynamic contact-angle hysteresis that is primarily caused by microscopic chemical heterogeneity.