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Surface modification of poly(ethylene terephthalate) fibers induced by radio frequency air plasma treatment
Applied Surface Science 211 (2003) 386–397
Surface modification of poly(ethylene terephthalate) fibers induced by radio frequency air plasma treatment Claudia Riccardia, Ruggero Barnia, Elena Sellib,*, Giovanni Mazzoneb, Maria Rosaria Massafrac, Bruno Marcandallic, Giulio Polettid a
Dipartimento di Fisica G. Occhialini, Universita` degli Studi di Milano-Bicocca and INFM, Piazza delle Scienze 3, I-20126 Milan, Italy b Dipartimento di Chimica Fisica ed Elettrochimica, Universita` degli Studi di Milano, via Golgi 19, I-20133 Milan, Italy c Stazione Sperimentale per la Seta, via G. Colombo 83, I-20133 Milan, Italy d Istituto di Fisiologia Generale e Chimica Biologica, Universita` degli Studi di Milano, via Trentacoste 2, I-20133 Milan, Italy Received 29 August 2002; accepted 1 March 2003
Abstract The surface chemical and physical modifications of poly(ethylene terephthalate) (PET) fibers induced by radiofrequency air plasma treatments were correlated with the characteristics of the discharge parameters and the chemical composition of the plasma itself, to identify the plasma-induced surface processes prevailing under different operating conditions. Treated polymer surfaces were characterized by water droplet absorption time measurements and XPS analysis, as a function of the aging time in different media, and by AFM analysis. They exhibited a remarkable increase in hydrophilicity, accompanied by extensive etching and by the implantation of both oxygen- and nitrogen-containing polar groups. Etching was mainly a consequence of ion bombardment, yielding low molecular weight, water soluble oxidation products, while surface chemical modifications were mainly due to the action of neutral species on the plasma-activated polymer surface. # 2003 Elsevier Science B.V. All rights reserved. PACS: 52.75.R; 87.59.V; 52.70 Keywords: RF plasma treatment; Poly(ethylene terephthalate) fibers; Hydrophilicity; Etching; Plasma diagnostics
1. Introduction Appropriate cold plasma treatments are widely employed to modify many surface properties of polymers, such as adhesion, friction, penetrability, wettability, dyeability, and biocompatibility, to adapt them to specific applications [1,2]. Depending on the feedstock gas, rapid, clean, environmentally friendly dry *
Corresponding author. Tel.: þ39-02-503-14237; fax: þ39-02-503-14300. E-mail address: [email protected] (E. Selli).
processes are able to induce physical and chemical surface changes in polymers through several concurrent processes (etching, grafting, polymerization, cross-linking), usually without modifying their original bulk qualities [3,4]. Low temperature plasmas are produced by electrical discharges in low pressure gases. They consist of a mixture of highly reactive species, i.e. ions, radicals, electrons, photons and excited molecules, preserving electrical neutrality. Their chemical composition and physical characteristics are determined also by device parameters, such as vacuum chamber geometry, gas
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00265-4
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pressure, gas flow rate and electrical power input and frequency [4]. Thus, the species mainly responsible for the observed surface modification of polymeric materials and their prevailing action could only be identified by correlating the effects of plasma treatment with plasma operating conditions, after a thorough plasma diagnostics analysis [5–7]. Furthermore, most investigations employed polymer films [8–15], not fibers, although surface modification of textiles by plasma treatment appears to be a still very active field of technological investigation [16–24]. The morphology of the treated polymer fibers, as well as the way in which their crystalline and amorphous phases interact with plasma, may have great importance in practical uses of treated specimens [18]. In the present work poly(ethylene terephthalate) (PET) fabrics were treated to a radiofrequency (RF) air plasma under different operating conditions, with the aim of permanently increasing their hydrophilicy. Indeed, the chemical structure of polyester polymers with its lack of polar groups, such as –COOH and –OH, results in the presence of a low surface free energy and poor wettability. These polar groups may be introduced on the polymer surface by cold plasma treatments employing either oxygenated or inert gases [8–11]. Their introduction, however, is always accompanied by etching and by surface cross-linking [12–14], that
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may affect the durability of plasma-imparted surface modifications. Thus, aiming at identifying the plasmainduced surface processes prevailing under different operating conditions and imparting different properties to the treated PET surfaces, a detailed analysis of how the chemical composition and the physical properties of the plasma itself depend on discharge parameters was carried out in parallel to plasma treatment. The changes of surface chemical composition and topography of specimens plasma-treatment under optimal conditions were also examined, as well as aging phenomena occurring on the surface of differently plasma-treated PET samples. Such investigations yielded valuable information relatively to the processes undergone by the treated polymer surfaces.
2. Experimental 2.1. Materials The thermofixed PET fabric (100.4 g m2) used in this work was washed for 10 min at 40 8C with a water solution containing 0.5 wt.% of an anionic detergent, thoroughly rinsed up to total removal of this latter and dried. Treated and untreated specimens (5:0 cm 5:0 cm) were usually stored at (20 2) 8C and
Fig. 1. Schematic view of the reactor setup.
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(65 2)% relative humidity (conditioned atmosphere). Air used in plasma treatment was a Linde product (purity >99.999 vol.%). 2.2. Plasma reactor The reactor employed for plasma treatments has been described elsewhere [25,26]. A schematic view of the setup is sketched in Fig. 1. The discharge was produced in a T-shaped cylindrical tube (10 cm diameter, 27 cm length). At one cap the vacuum chamber was connected to the pumping system, which allowed to evacuate the reactor up to a residual pressure lower than 106 mbar. After evacuation, the vacuum chamber was filled with air and pressure was kept constant inside the reactor (at a level ranging between 0.05 and 10 mbar, monitored by a Pirani gauge), acting on a leakage valve mounted on the cap opposite to that connected to the pumps. Under such operating conditions, the pumping system sustained a flow rate of about 25 sccm, corresponding to a residence time in the plasma of about 0.2 s with a flow velocity of about 50 cm s1 at a pressure of 0.1 mbar. The discharge was driven by a capacitive RF antenna: a stainless steel cylindrical pin (1 cm diameter, 2 cm length), axially located in the cylindrical vacuum chamber, was fed with a RF voltage with respect to the same chamber, which was grounded. The antenna was then externally connected through a matching network to a 13.56 MHz-RF power supplier. 2.3. Characterization of treated samples The absorption time of demineralized water droplets was measured using a home-made apparatus [27]. Water droplet absorption times were obtained as the average absorption time of four 200 ml water droplets that were deposited through a standardized procedure on different parts of each fabric specimen. X-ray photoelectron spectra were recorded using an M-probe apparatus (Surface Science Instruments), incorporating a monochromatic Al Ka radiation (1486.6 eV), an elliptical spot size of 0:4 mm 1:0 mm and a pass energy of 25 eV, as already reported [27–29]. Scanning electron microscopy (SEM) tests were performed using a Leica Cambridge 440 StereoScan apparatus, equipped with EDXS
X-ray analyser and a Link Analytical QX-2000 microanalysis system. Atomic force microscopy (AFM) studies were carried out on a ThermoMicroscopes AutoProbe CP Research System in the non-contact mode, with an Ultralever type B (ThermoMicroscopes) tip, employing the ThermoMicroscopes Image Processing and Data Analysis software. The scanning rate was 0.8– 1.0 Hz with a 40 nm setpoint. The resonant oscillating frequency was 90 kHz and the mean cantilever spring constant was 3.2 N m1 (nominal values).
3. Results and discussion The choice of the feedstock gas was dictated by the capability of producing reactive species in the plasma phase, that could act as precursors of the hydrophilic groups to be implanted on the polymer surface. Oxygenated gases were thus preferred to inert gases. The prevailing actions of inert gases are expected to be etching and cross-linking of polymer surfaces, besides creating radical species able to combine with atmospheric oxygen, thus indirectly introducing polar groups on the polymer surface [11]. In particular, cold air plasma was employed in the present work, as it was proved to be more efficient than pure oxygen plasma in inducing a permanent hydrophilic modification to the PET fibers. The species prevailing in such plasma under different plasma treatment conditions were identified through a thorough air plasma diagnostics and simulation of the gas phase composition. Moreover, to verify the relevance of etching processes, the polarization of the sample holder within the plasma reactor was varied, with consequent variation in the flux of positive ions and negative ions. 3.1. Plasma diagnostics A suitable diagnostic system was implemented in the reactor, to correlate the external operating parameters with the local properties of the plasma that was directly interacting with the polymer surface. This diagnostics was based on the use of movable electrostatic probes [25]. The analysis of the data led to the conclusion that a diffuse plasma region was produced in our device, surrounding the RF antenna and extending longitudinally for about
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Fig. 2. Longitudinal profile of the electron density ne (full symbols) and temperature Te (open symbols) in the air plasma at input RF power WRF equal to 100 (circles), 50 (squares) and 20 W (triangles). The antenna location is drawn at the bottom as a reference.
10 cm. The plasma parameters were found to be in the following ranges: electron density, ne ffi 107 109 cm3, ion density, ni ffi 1010 1012 cm3, and electron temperature Te ffi 510 eV. Thus, the total charge density was by far dominated by ions, while the electron density was at least two orders of magnitude below the ions density. In electronegative gases, such as air, ions can reach fairly large den-
sities and play a major role in plasma–surface interactions [30]. A typical longitudinal profile of the discharge composition is reported in Fig. 2. The diffuse plasma region produced in the vacuum chamber showed a bell-shaped density profile, that was approximately symmetrical around the RF antenna. By raising the RF power level, both the peak density and the plasma
Fig. 3. Dependence of the electron density ne (squares), the electron temperature Te (diamonds) and the ion density ni (circles) on the RF power, WRF. The data are extracted from measurements at Z ¼ 11 cm in the vacuum chamber (see Fig. 2).
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width increased, while the electron temperature profile remained flat and its value appeared to be almost independent of the input power. Fig. 3 shows plasma parameters that were measured at a fixed position on the axis of the cylindrical vacuum chamber (at 2 cm from the antenna, that is at Z ¼ 11 cm, see Fig. 2) as a function of the RF power, WRF. An almost linear increase of the electron density was reported up to the largest RF powers that were tolerable for the setup. The ion density correspondingly increased, as the square root of WRF, according to the expectations of the plasma modelling of electronegative discharges [30].
Ion fluxes delivered to the material could be estimated from the ion density measurements by means of the generalized Bohm criterion [25,30]. Ions impinged onto the polymer surface with a kinetic energy of a few tens of eV, gained in crossing the voltage fall from the plasma. This energy could be increased by applying a negative potential Vh to the substrate holder. For example, an integrated energy flux of 7.7 J cm2 could be estimated under the optimal operating conditions (P ¼ 0:4 mbar, WRF ¼ 200 W, ttr ¼ 10 min, Z ¼ 14 cm, Vh ¼ 30 V, vide infra). In principle, a positive bias should work in the same way. However, since electrons easily detach from negative ions and
Fig. 4. Gas-phase composition in the discharge region predicted from the simulation of the chemical kinetics dictated from the plasma parameters extracted from the measurements shown in Fig. 2. The density of the majority charged and neutral species is shown as a function of the longitudinal distance in the vacuum chamber. The antenna location is also drawn as a reference point.
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are much lighter than ions, the positive polarization mainly delivered an electron flux to the sample, leading to a less marked surface modification. According to the gas phase model that is described in [30], an almost electronless plasma was produced in our device, with the positive charge components, mainly comprising O2þ, N2þ and NOþ ions, balancing the O negative ions. A typical example of the results of gas phase modelling is shown in Fig. 4. Air dissociation occurred to a limited extent (less than 10%) and mainly led to the production of atomic nitrogen, nitrous oxide NO, and, to a lower extent, atomic oxygen. The production of ozone and other nitrogen oxides was negligible, mainly because of the low pressure and also because of the short residence time in the discharge. Thus, the reactor performance appeared to be suitable both for ion assisted processes, such as sputtering or etching, and for direct grafting of nitrogen and oxygen on the treated polymer surface. 3.2. Hydrophilicity The surface hydrophilicity of treated samples was evaluated through water droplet absorption time measurements which, in the case of fibers, yield much more reliable results than water advancing contact angles [27]. All of the plasma-treated PET specimens
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exhibited a marked increase of hydrophilicity: the absorption time tabs measured immediately after the treatment was always lower than 10 s, to be compared with tabs ffi 350 s for untreated PET. Due to the small tabs values, no clear correlation could be evidenced between them and plasma treatment conditions. The hydrophilicity of treated fibers, however, decreased with the time of aging after treatment. As shown in Fig. 5, the surface hydrophobicity partly recovered at a rather fast rate in the first days after the plasma treatment, becoming progressively slower afterwards. In each case, the water droplet absorption time tended to stabilize to values that were much lower than that typical of untreated PET. Such post-treatment effects, that were clearly dependent on plasma treatment conditions, proved to be a much more useful characterization tool than the tabs values measured immediately after the treatment. The reversibility of plasma-imparted surface properties is a known consequence of the fact that plasma treatments modify a very thin (a few nanometers) polymer surface layer. Treated surfaces may be vulnerable to recovery of their primitive properties through the reorganization of surface polymer chains, tending to reduce the interfacial tension between the polymer surface and the medium in contact with the polymer surface [8,9,11,27,31].
Fig. 5. Water droplet absorption time tabs measured at different aging time of PET specimens treated with air plasma (a) for (&) 1 and (&) 10 min at WRF ¼ 100 W, Z ¼ 14 cm; (b) at (&) WRF ¼ 50 W, (&) WRF ¼ 100 W and (*) WRF ¼ 200 W for 1 min at Z ¼ 16:5 cm.
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Fig. 6. Water droplet absorption time tabs measured at different aging time of PET specimens treated with air plasma with WRF ¼ 200 W and ttr ¼ 10 min, at (*) Z ¼ 12 cm (weight loss ¼ 0.22%) and (~) Z ¼ 14 cm (weight loss 0.05%), or (&) with the sample polarized at 30 V (Z ¼ 14 cm, weight loss 0.82%).
The efficacy of the air plasma treatment in inducing more or less stable wettability on PET fibers was investigated by varying the radiofrequency power of the reactor, WRF, the time of treatment, ttr, the position of the treated sample downstream from the emitting antenna, Z (Z > 9 cm, see Figs. 2 and 4), and the sample holder polarization. The results of such investigation are reported in Figs. 5 and 6. The gas pressure inside the reactor was usually maintained at 0.4 mbar, since it appeared to be the optimal value for sustaining the discharge, in the chosen reactor geometry [30]. Excellent durability of wettability properties was achieved by use of a relatively long treatment time (Fig. 5a) and high RF power (Fig. 5b), while less drastic conditions, although providing a basis for achieving good hydrophilicity immediately after the plasma treatment, did not assure enough stability of the treated surface. According to our plasma diagnostics studies, surface etching or cross-linking phenomena should efficiently compete with the implantation of hydrophilic groups, especially when operating at the smallest Z values and in the presence of sample holder polarization. Indeed, etching and cross-linking have relevant roles in stabilizing plasma-induced modifications of treated polymer surfaces, as outlined by the results reported in Fig. 6. The closer to the antenna the samples were treated (lower Z values), the longer they maintained their high hydrophilicity. Moreover, PET samples that were air plasma treated at 200 W for
10 min at 30 V exhibited excellent, very stable hydrophilicity (Fig. 6). Thus, a negative polarization of the fabric specimen during plasma treatment and the consequent increase of positive ion flux on its surface assured optimal and permanent wettability. Under such operating conditions, besides hydrophilic species implantation (vide infra), considerable surface etching also occurred, as evidenced by their weight loss during the treatment (see the caption of Fig. 6). By contrast, a positive polarization of the sample holder did not produce any increase of PET wettability, with respect to samples that were plasma-treated without polarization, indicating that negatively-charged species (mainly O and electrons, see Figs. 2 and 4) play minor roles in the implantation of polar groups on the polymer surface. 3.3. Surface chemical analysis Table 1 reports XPS results that were obtained immediately after air plasma treatment at 200 W for 10 min at 30 V and exposure to atmospheric air, as well as those obtained after 10 days of aging in water at room temperature or conditioning in air, to be compared with the data obtained with untreated PET. First of all, a considerable amount of nitrogen and a remarkable increase of surface oxygen content were detected on the PET surface after the treatment. Although the percentage of nitrogen did not exhibit
C. Riccardi et al. / Applied Surface Science 211 (2003) 386–397 Table 1 O/C and N/C atom ratios and ratio between oxygen doubly and singly bound to carbon determined by XPS analysis immediately after air plasma treatmenta and after 10 days long storage under water or conditioned air PET
O/Cb
N/C
C¼O/C–Oc
Treated Treated and stored under air Treated and stored under water
0.57 0.51 0.37
0.056 0.062 0.059
0.71 1.65 1.11
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benzene ring should be substituted by hydroxyls and, possibly, by amine groups after the treatment, in line with the observed plasma-induced increase of surface wettability. Moreover, a fourth peak (C4) appears around 287.5 eV in the C (1s) signal, attributable to amide carbon atoms (–CONH– groups) [34]. Their presence on PET surface can only be explained by supposing that nitrogen-containing activated species
a
Treatment conditions: WRF ¼ 200 W, Vh ¼ 30 V, Z ¼ 14 cm, ttr ¼ 10 min. b O/C ratio of untreated PET fibers: 0.25. c Evaluated as the ratio between the areas of the O (1s) bands peaking at ca. 531.8 and 533.8 eV.
any marked variation during aging, the oxygen surface content decreased during aging, especially when in contact with water. Of course, this could not be a consequence of the above-mentioned folding of surface polymer chains, tending to decrease interfacial tension. In fact, oxygen-containing polar moieties would preferentially remain at the interface in contact with polar water. The most plausible explanation of the observed phenomenon is that part of the oxygen present on the surface should be in the form of low molecular weight water soluble species [8,12,32,33], that can be easily dissolved by water and, thus, removed from the treated polymer surface. Such species, at least in part, should remain adsorbed on the PET surface, if aging is carried out in contact with atmospheric air. Information concerning the manner in which oxygen and nitrogen were implanted on the polymer surface during the air plasma treatment can be obtained from the deconvolution of XPS signals. As shown in Fig. 7a, the C (1s) signal of the untreated PET contained three well separated peaks at 284.6, 286.3 and 288.7 eV, corresponding to carbon atoms bound only to carbon or hydrogen in the benzene ring (C1), to methylene carbons singly bound to oxygen (C2) and to ester carbon atoms (C3), respectively [10,12,34]. In the XPS signal that was recorded immediately after the plasma treatment (Fig. 7b), a drastic decrease of the peak at 284.6 eV (C1) due to benzene ring carbon atoms is observed and a marked increase of the C2 band peaking at 286 eV, which can attributed to –C–O– and also to –C–N– groups. This indicates that the hydrogen atoms that are bound to the
Fig. 7. Deconvolution of the C (1s) peak of XPS spectra of PET recorded (a) before plasma treatment; (b) immediately after air plasma treatment (WRF ¼ 200 W, Vh ¼ 30 V, Z ¼ 14 cm, ttr ¼ 10 min); after subsequent 10 days conditioning (c) in air and (d) in water.
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present in the RF air plasma collide with PET surface with enough energy, to cause the scission of the polymer chain at the ester group level, which can be envisaged as a weak point on the polymer backbone [13] (i.e. plasmolysis occurs). Amide groups and hydroxyl groups should then be expected to terminate polymer chain fragments. Plasma-induced polymer chain scission appears to be favored by negative polarization of the sample during the plasma treatment, increasing the amount and the speed of positive species, such as N2þ and NOþ (see Fig. 4). Of course, air plasma treatment also created radical species on the polymer surface, mainly through polymer chain scission or through hydrogen abstraction [35]. These species can combine with atmospheric gases, thus also contributing to increase the amount of polar groups on the treated polymer surface [11,35]. The deconvolution of the C (1s) peak of the XPS spectrum of PET that was aged in air (Fig. 7c) exhibits a restoration of the C1 peak (benzenic carbon atoms bound to hydrogen) and a corresponding decrease of the C2 peak (–C–O– and –C–N– groups), that is consistent with the decreased wettability observed with aging time, due to the folding of surface chains, tending to decrease the interfacial tension in contact with air. In contrast, the C4 peak that is due to the amide groups does not decrease with aging time in contact with air (Fig. 7c), probably because these groups, being less polar than those originating the C2 peak, are less susceptible to surface reorganization. For the sample stored under water, a much more evident restoration of surface hydrocarbons was observed (Fig. 7d) and a parallel decrease of the C2 peak, attributed to carbon atoms singly bound to oxygen. Thus, the removal of low molecular weight oxidation products from the polymer surface, consequent to their dissolution in water, ‘cleans’ (and stabilizes) the treated polymer surface. The C3 peak that is due to –COO– groups seems to remain practically unchanged after plasma treatment and subsequent aging in both media (Fig. 7c and d), probably as a consequence of a balance between two opposite effects, the removal of O–C¼O groups contained in low molecular weight oxidation products (prevailing under water) and the oxidation of carbon atoms, consequent to the interaction of plasma-created activated sites with atmospheric oxygen. These processes obviously have opposite effects on the O/C ratio,
which in fact decreases, especially when storage is carried out under water (Table 1). Finally, also the O (1s) signal in the XPS spectra could be deconvoluted into two peaks, correlated to the presence of both C¼O groups (531.8 eV) and –C– O– groups (533.8 eV) [34]. As shown in Table 1, after the plasma treatment, the ratio between the integrated areas of the two peaks increased more for the airstored sample than for the water-stored sample, confirming that surface carbon atoms undergo oxidation in contact with atmospheric oxygen. 3.4. Surface structure and topography While scanning electron microscopy did not indicate any difference between treated and untreated specimens, as in the case of other plasma treatments on fibers [27,29], much more detailed information on surface topographical modifications was obtained by atomic force microscopy. SEM has limited depth resolution, that does not enable the detection of morphological features of much less than a micrometer [36]. The AFM images that are reported in Fig. 8 are the original scans without any filtering or contrast
Fig. 8. AFM images of (a) untreated and (b) air plasma-treated PET (WRF ¼ 200 W, Vh ¼ 30 V, Z ¼ 14 cm, ttr ¼ 10 min).
C. Riccardi et al. / Applied Surface Science 211 (2003) 386–397 Table 2 Statistical parameters obtained from AFM images of untreated PET fibers and plasma-treateda PET fibers, calculated as mean values along six equally spaced lines in the direction parallel to fibrils (y-direction) or perpendicular to them (x-direction, see Fig. 8)
Untreated y-direction x-direction Plasma-treated y-direction x-direction
Mean roughness (nm)
Mean height (nm)
Arc length (mm)
180 270
980 950
1.9 14
20 39
135 132
1.6 2.7
a Treatment conditions: WRF ¼ 200 W, Vh ¼ 30 V, Z ¼ 14 cm, ttr ¼ 10 min.
enhancement. The surface of untreated PET fibers was characterized by a series of more or less ‘in relief’, elongated fibrils, as shown in Fig. 8a. They were oriented almost in the same direction, with an extension that is larger than a few micrometers. Their size in the vertical dimension was micrometric and their almost regular interspace in the horizontal dimension was around 40 nm. The original topographical aspect of the PET fibers underwent a deep modification after air plasma treatment (Fig. 8b): a more isotropic and uniform surface appeared and the elongated features were greatly reduced. Indeed, the size of vertical features decreased by almost one order of magnitude, so that both the vertical patterns and the horizontal patterns were in the submicrometric scale. Typical surface parameters before plasma treatment and after plasma treatment, calculated by applying a statistical analysis to the scanned images, are reported in Table 2. It is evident that most of the anisotropy of the untreated sample disappeared with plasma treatment. The study of the periodic surface features through a FFT analysis also gave evidence of two different space frequencies for the untreated samples, one on the micrometric and the other in the submicrometric scale. The former (dc: 250 nm) described the elongated structures, whose amplitude dominated over the entire image (Fig. 8a). The smaller features described by the submicrometric scale (25–73 nm) were superimposed on the larger structures. In the plasma treated-sample, the amplitude of the micrometric scale was drastically
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reduced and the smaller, submicrometric scale became dominant. Such a large modification of the surface structure, practically leading to the washing out of the larger scale surface patterns, can be ascribed to etching of the fibrils of the fabric surface. Amorphous regions of the material would be expected to undergo etching preferentially respect to crystalline regions [20,29,33,37]. This might be the origin of roughness on the submicrometric scale [20] and would contribute to a decrease in the mobility of surface polymer chain after the plasma treatment, thus increasing the durability of plasma-imparted surface properties. In conclusion, a completely new surface with different physical properties and chemical properties can be obtained via air plasma treatment. Thus, not only surface chemical modifications, but also topographical changes may contribute independently to the observed modification of the wettability of PET fabrics. 3.5. Prevailing plasma-induced surface processes The main processes occurring at the plasma–fiber interface during our air plasma treatments can be identified by comparing the data collected from plasma diagnostics and modelling with those relative to PET surface modification in wettability, by taking into account also the observed changes in chemical composition and topographical aspects of the samples that had undergone the deepest modification. A first plasma-induced process is surface etching, evidenced by AFM analysis and by the weight loss of treated samples (occurring at a rate up to 1:4 107 g cm2 s1, corresponding to a mean etching rate of 0.45 nm s1). Such values correlate well with the ion density profile shown in Fig. 3, leading to the conclusion that the energy drive is mainly due to ion bombardment, with subsequent scission of the polymer chain. The removal of polymeric material towards the gas-phase reasonably occurs through the formation of low molecular weight compounds, formed upon complete or partial oxidation of polymer fragments. The formation of amide groups and, in general, of oxidated polymer fragments, that are weakly bound to the fiber surface and easily soluble in water, also confirm the occurrence of extensive etching in
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samples that have been treated under high ion bombardment, as they can be envisaged as the by-products of the etching process itself. On the other hand the generalized increase in wettability, observed even in the absence of noticeable etching, i.e. on PET plasma-treated at low WRF and in positions inside the reactor downstream to the plasma (Fig. 5b), as well as the formation of C–O and C–N bonds in the surface layer and the simultaneous decrease of C–C and C–H bonds (Fig. 7a and b), are more indicative of a direct grafting process consequent to the incorporation of oxygen and nitrogen. Indeed, this phenomenon better correlates to the expected flux of radical species rather than to the ion fluxes inside the plasma reactor (see Fig. 4). The ionic species that are present in the air plasma also play a role in the implantation of polar groups on PET surface, by assisting polymer chain breaking and by inducing surface activation. These processes could also be induced by UV irradiation [6,7,35,38]. However, ion fluxes alone should not be sufficient to explain the observed chemical modification of the PET surface. Finally, cross-linking of the activated surface should also occur, restricting the mobility of surface polymer chains [35] and thus increasing the durability of the plasma-imparted surface modifications. The possible preferential etching of the amorphous regions of PET fibers would also produce the same effect. Unfortunately, the two processes could not be easily distinguished one from the other, as characterization techniques such as ATR, XRD and DSC did not provide any information, due to the very thin surface layer involved in plasma-induced surface modification.
4. Conclusions RF air plasma treatment of PET fibers led to an increase in their hydrophilicity. Best and more durable wettability results were obtained under high RF power and under negative polarization of treated samples, i.e. under conditions in which the implantation of polar groups on PET surface, evidenced by XPS analysis, was accompanied by extensive surface etching and possibly cross-linking between activated species.
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C. Riccardi et al. / Applied Surface Science 211 (2003) 386–397 [27] E. Selli, G. Mazzone, C. Oliva, F. Martini, C. Riccardi, R. Barni, B. Marcandalli, M.R. Massafra, J. Mater. Chem. 11 (2001) 1985. [28] E. Selli, P.L. Beltrame, G. Testa, A.M. Bonfatti, E. Rossi, A. Seves, Angew. Makromol. Chem. 257 (1998) 63. [29] E. Selli, C. Riccardi, M.R. Massafra, B. Marcandalli, Macromol. Chem. Phys. 202 (2001) 1672. [30] C. Riccardi, R. Barni, M. Fontanesi, J. Appl. Phys. 90 (2001) 3735. [31] T. Yasuda, T. Okuno, K. Tsuji, H. Yasuda, Langmuir 12 (1996) 1391. [32] C. Ton-That, D.O.H. Teare, P.A. Campbell, R.H. Bradley, Surf. Sci. 433–435 (1999) 278.
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[33] B.D. Beake, J.S.G. Ling, G.J. Leggett, J. Mater. Chem. 8 (1998) 1735. [34] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Boniben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Eden Prairie, Minnesota, 1995. [35] S.H. Wheale, C.P. Barker, J.P.S. Badyal, Langmuir 14 (1998) 6699. [36] R. Mahlberg, H.E.-M. Niemi, F.S. Denes, R.M. Rowell, Langmuir 15 (1999) 2985. [37] M.B. Olde Riekerink, J.G.A. Terlingen, G.H.M. Engbers, J. Feijen, Langmuir 15 (1999) 4847. [38] J. Hopkins, S.H. Wheale, J.P.S. Badyal, J. Phys. Chem. 100 (1996) 14062.
Surface modification of poly(ethylene terephthalate) fibers induced by radio frequency air plasma treatment Claudia Riccardia, Ruggero Barnia, Elena Sellib,*, Giovanni Mazzoneb, Maria Rosaria Massafrac, Bruno Marcandallic, Giulio Polettid a
Dipartimento di Fisica G. Occhialini, Universita` degli Studi di Milano-Bicocca and INFM, Piazza delle Scienze 3, I-20126 Milan, Italy b Dipartimento di Chimica Fisica ed Elettrochimica, Universita` degli Studi di Milano, via Golgi 19, I-20133 Milan, Italy c Stazione Sperimentale per la Seta, via G. Colombo 83, I-20133 Milan, Italy d Istituto di Fisiologia Generale e Chimica Biologica, Universita` degli Studi di Milano, via Trentacoste 2, I-20133 Milan, Italy Received 29 August 2002; accepted 1 March 2003
Abstract The surface chemical and physical modifications of poly(ethylene terephthalate) (PET) fibers induced by radiofrequency air plasma treatments were correlated with the characteristics of the discharge parameters and the chemical composition of the plasma itself, to identify the plasma-induced surface processes prevailing under different operating conditions. Treated polymer surfaces were characterized by water droplet absorption time measurements and XPS analysis, as a function of the aging time in different media, and by AFM analysis. They exhibited a remarkable increase in hydrophilicity, accompanied by extensive etching and by the implantation of both oxygen- and nitrogen-containing polar groups. Etching was mainly a consequence of ion bombardment, yielding low molecular weight, water soluble oxidation products, while surface chemical modifications were mainly due to the action of neutral species on the plasma-activated polymer surface. # 2003 Elsevier Science B.V. All rights reserved. PACS: 52.75.R; 87.59.V; 52.70 Keywords: RF plasma treatment; Poly(ethylene terephthalate) fibers; Hydrophilicity; Etching; Plasma diagnostics
1. Introduction Appropriate cold plasma treatments are widely employed to modify many surface properties of polymers, such as adhesion, friction, penetrability, wettability, dyeability, and biocompatibility, to adapt them to specific applications [1,2]. Depending on the feedstock gas, rapid, clean, environmentally friendly dry *
Corresponding author. Tel.: þ39-02-503-14237; fax: þ39-02-503-14300. E-mail address: [email protected] (E. Selli).
processes are able to induce physical and chemical surface changes in polymers through several concurrent processes (etching, grafting, polymerization, cross-linking), usually without modifying their original bulk qualities [3,4]. Low temperature plasmas are produced by electrical discharges in low pressure gases. They consist of a mixture of highly reactive species, i.e. ions, radicals, electrons, photons and excited molecules, preserving electrical neutrality. Their chemical composition and physical characteristics are determined also by device parameters, such as vacuum chamber geometry, gas
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00265-4
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pressure, gas flow rate and electrical power input and frequency [4]. Thus, the species mainly responsible for the observed surface modification of polymeric materials and their prevailing action could only be identified by correlating the effects of plasma treatment with plasma operating conditions, after a thorough plasma diagnostics analysis [5–7]. Furthermore, most investigations employed polymer films [8–15], not fibers, although surface modification of textiles by plasma treatment appears to be a still very active field of technological investigation [16–24]. The morphology of the treated polymer fibers, as well as the way in which their crystalline and amorphous phases interact with plasma, may have great importance in practical uses of treated specimens [18]. In the present work poly(ethylene terephthalate) (PET) fabrics were treated to a radiofrequency (RF) air plasma under different operating conditions, with the aim of permanently increasing their hydrophilicy. Indeed, the chemical structure of polyester polymers with its lack of polar groups, such as –COOH and –OH, results in the presence of a low surface free energy and poor wettability. These polar groups may be introduced on the polymer surface by cold plasma treatments employing either oxygenated or inert gases [8–11]. Their introduction, however, is always accompanied by etching and by surface cross-linking [12–14], that
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may affect the durability of plasma-imparted surface modifications. Thus, aiming at identifying the plasmainduced surface processes prevailing under different operating conditions and imparting different properties to the treated PET surfaces, a detailed analysis of how the chemical composition and the physical properties of the plasma itself depend on discharge parameters was carried out in parallel to plasma treatment. The changes of surface chemical composition and topography of specimens plasma-treatment under optimal conditions were also examined, as well as aging phenomena occurring on the surface of differently plasma-treated PET samples. Such investigations yielded valuable information relatively to the processes undergone by the treated polymer surfaces.
2. Experimental 2.1. Materials The thermofixed PET fabric (100.4 g m2) used in this work was washed for 10 min at 40 8C with a water solution containing 0.5 wt.% of an anionic detergent, thoroughly rinsed up to total removal of this latter and dried. Treated and untreated specimens (5:0 cm 5:0 cm) were usually stored at (20 2) 8C and
Fig. 1. Schematic view of the reactor setup.
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(65 2)% relative humidity (conditioned atmosphere). Air used in plasma treatment was a Linde product (purity >99.999 vol.%). 2.2. Plasma reactor The reactor employed for plasma treatments has been described elsewhere [25,26]. A schematic view of the setup is sketched in Fig. 1. The discharge was produced in a T-shaped cylindrical tube (10 cm diameter, 27 cm length). At one cap the vacuum chamber was connected to the pumping system, which allowed to evacuate the reactor up to a residual pressure lower than 106 mbar. After evacuation, the vacuum chamber was filled with air and pressure was kept constant inside the reactor (at a level ranging between 0.05 and 10 mbar, monitored by a Pirani gauge), acting on a leakage valve mounted on the cap opposite to that connected to the pumps. Under such operating conditions, the pumping system sustained a flow rate of about 25 sccm, corresponding to a residence time in the plasma of about 0.2 s with a flow velocity of about 50 cm s1 at a pressure of 0.1 mbar. The discharge was driven by a capacitive RF antenna: a stainless steel cylindrical pin (1 cm diameter, 2 cm length), axially located in the cylindrical vacuum chamber, was fed with a RF voltage with respect to the same chamber, which was grounded. The antenna was then externally connected through a matching network to a 13.56 MHz-RF power supplier. 2.3. Characterization of treated samples The absorption time of demineralized water droplets was measured using a home-made apparatus [27]. Water droplet absorption times were obtained as the average absorption time of four 200 ml water droplets that were deposited through a standardized procedure on different parts of each fabric specimen. X-ray photoelectron spectra were recorded using an M-probe apparatus (Surface Science Instruments), incorporating a monochromatic Al Ka radiation (1486.6 eV), an elliptical spot size of 0:4 mm 1:0 mm and a pass energy of 25 eV, as already reported [27–29]. Scanning electron microscopy (SEM) tests were performed using a Leica Cambridge 440 StereoScan apparatus, equipped with EDXS
X-ray analyser and a Link Analytical QX-2000 microanalysis system. Atomic force microscopy (AFM) studies were carried out on a ThermoMicroscopes AutoProbe CP Research System in the non-contact mode, with an Ultralever type B (ThermoMicroscopes) tip, employing the ThermoMicroscopes Image Processing and Data Analysis software. The scanning rate was 0.8– 1.0 Hz with a 40 nm setpoint. The resonant oscillating frequency was 90 kHz and the mean cantilever spring constant was 3.2 N m1 (nominal values).
3. Results and discussion The choice of the feedstock gas was dictated by the capability of producing reactive species in the plasma phase, that could act as precursors of the hydrophilic groups to be implanted on the polymer surface. Oxygenated gases were thus preferred to inert gases. The prevailing actions of inert gases are expected to be etching and cross-linking of polymer surfaces, besides creating radical species able to combine with atmospheric oxygen, thus indirectly introducing polar groups on the polymer surface [11]. In particular, cold air plasma was employed in the present work, as it was proved to be more efficient than pure oxygen plasma in inducing a permanent hydrophilic modification to the PET fibers. The species prevailing in such plasma under different plasma treatment conditions were identified through a thorough air plasma diagnostics and simulation of the gas phase composition. Moreover, to verify the relevance of etching processes, the polarization of the sample holder within the plasma reactor was varied, with consequent variation in the flux of positive ions and negative ions. 3.1. Plasma diagnostics A suitable diagnostic system was implemented in the reactor, to correlate the external operating parameters with the local properties of the plasma that was directly interacting with the polymer surface. This diagnostics was based on the use of movable electrostatic probes [25]. The analysis of the data led to the conclusion that a diffuse plasma region was produced in our device, surrounding the RF antenna and extending longitudinally for about
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Fig. 2. Longitudinal profile of the electron density ne (full symbols) and temperature Te (open symbols) in the air plasma at input RF power WRF equal to 100 (circles), 50 (squares) and 20 W (triangles). The antenna location is drawn at the bottom as a reference.
10 cm. The plasma parameters were found to be in the following ranges: electron density, ne ffi 107 109 cm3, ion density, ni ffi 1010 1012 cm3, and electron temperature Te ffi 510 eV. Thus, the total charge density was by far dominated by ions, while the electron density was at least two orders of magnitude below the ions density. In electronegative gases, such as air, ions can reach fairly large den-
sities and play a major role in plasma–surface interactions [30]. A typical longitudinal profile of the discharge composition is reported in Fig. 2. The diffuse plasma region produced in the vacuum chamber showed a bell-shaped density profile, that was approximately symmetrical around the RF antenna. By raising the RF power level, both the peak density and the plasma
Fig. 3. Dependence of the electron density ne (squares), the electron temperature Te (diamonds) and the ion density ni (circles) on the RF power, WRF. The data are extracted from measurements at Z ¼ 11 cm in the vacuum chamber (see Fig. 2).
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width increased, while the electron temperature profile remained flat and its value appeared to be almost independent of the input power. Fig. 3 shows plasma parameters that were measured at a fixed position on the axis of the cylindrical vacuum chamber (at 2 cm from the antenna, that is at Z ¼ 11 cm, see Fig. 2) as a function of the RF power, WRF. An almost linear increase of the electron density was reported up to the largest RF powers that were tolerable for the setup. The ion density correspondingly increased, as the square root of WRF, according to the expectations of the plasma modelling of electronegative discharges [30].
Ion fluxes delivered to the material could be estimated from the ion density measurements by means of the generalized Bohm criterion [25,30]. Ions impinged onto the polymer surface with a kinetic energy of a few tens of eV, gained in crossing the voltage fall from the plasma. This energy could be increased by applying a negative potential Vh to the substrate holder. For example, an integrated energy flux of 7.7 J cm2 could be estimated under the optimal operating conditions (P ¼ 0:4 mbar, WRF ¼ 200 W, ttr ¼ 10 min, Z ¼ 14 cm, Vh ¼ 30 V, vide infra). In principle, a positive bias should work in the same way. However, since electrons easily detach from negative ions and
Fig. 4. Gas-phase composition in the discharge region predicted from the simulation of the chemical kinetics dictated from the plasma parameters extracted from the measurements shown in Fig. 2. The density of the majority charged and neutral species is shown as a function of the longitudinal distance in the vacuum chamber. The antenna location is also drawn as a reference point.
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are much lighter than ions, the positive polarization mainly delivered an electron flux to the sample, leading to a less marked surface modification. According to the gas phase model that is described in [30], an almost electronless plasma was produced in our device, with the positive charge components, mainly comprising O2þ, N2þ and NOþ ions, balancing the O negative ions. A typical example of the results of gas phase modelling is shown in Fig. 4. Air dissociation occurred to a limited extent (less than 10%) and mainly led to the production of atomic nitrogen, nitrous oxide NO, and, to a lower extent, atomic oxygen. The production of ozone and other nitrogen oxides was negligible, mainly because of the low pressure and also because of the short residence time in the discharge. Thus, the reactor performance appeared to be suitable both for ion assisted processes, such as sputtering or etching, and for direct grafting of nitrogen and oxygen on the treated polymer surface. 3.2. Hydrophilicity The surface hydrophilicity of treated samples was evaluated through water droplet absorption time measurements which, in the case of fibers, yield much more reliable results than water advancing contact angles [27]. All of the plasma-treated PET specimens
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exhibited a marked increase of hydrophilicity: the absorption time tabs measured immediately after the treatment was always lower than 10 s, to be compared with tabs ffi 350 s for untreated PET. Due to the small tabs values, no clear correlation could be evidenced between them and plasma treatment conditions. The hydrophilicity of treated fibers, however, decreased with the time of aging after treatment. As shown in Fig. 5, the surface hydrophobicity partly recovered at a rather fast rate in the first days after the plasma treatment, becoming progressively slower afterwards. In each case, the water droplet absorption time tended to stabilize to values that were much lower than that typical of untreated PET. Such post-treatment effects, that were clearly dependent on plasma treatment conditions, proved to be a much more useful characterization tool than the tabs values measured immediately after the treatment. The reversibility of plasma-imparted surface properties is a known consequence of the fact that plasma treatments modify a very thin (a few nanometers) polymer surface layer. Treated surfaces may be vulnerable to recovery of their primitive properties through the reorganization of surface polymer chains, tending to reduce the interfacial tension between the polymer surface and the medium in contact with the polymer surface [8,9,11,27,31].
Fig. 5. Water droplet absorption time tabs measured at different aging time of PET specimens treated with air plasma (a) for (&) 1 and (&) 10 min at WRF ¼ 100 W, Z ¼ 14 cm; (b) at (&) WRF ¼ 50 W, (&) WRF ¼ 100 W and (*) WRF ¼ 200 W for 1 min at Z ¼ 16:5 cm.
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Fig. 6. Water droplet absorption time tabs measured at different aging time of PET specimens treated with air plasma with WRF ¼ 200 W and ttr ¼ 10 min, at (*) Z ¼ 12 cm (weight loss ¼ 0.22%) and (~) Z ¼ 14 cm (weight loss 0.05%), or (&) with the sample polarized at 30 V (Z ¼ 14 cm, weight loss 0.82%).
The efficacy of the air plasma treatment in inducing more or less stable wettability on PET fibers was investigated by varying the radiofrequency power of the reactor, WRF, the time of treatment, ttr, the position of the treated sample downstream from the emitting antenna, Z (Z > 9 cm, see Figs. 2 and 4), and the sample holder polarization. The results of such investigation are reported in Figs. 5 and 6. The gas pressure inside the reactor was usually maintained at 0.4 mbar, since it appeared to be the optimal value for sustaining the discharge, in the chosen reactor geometry [30]. Excellent durability of wettability properties was achieved by use of a relatively long treatment time (Fig. 5a) and high RF power (Fig. 5b), while less drastic conditions, although providing a basis for achieving good hydrophilicity immediately after the plasma treatment, did not assure enough stability of the treated surface. According to our plasma diagnostics studies, surface etching or cross-linking phenomena should efficiently compete with the implantation of hydrophilic groups, especially when operating at the smallest Z values and in the presence of sample holder polarization. Indeed, etching and cross-linking have relevant roles in stabilizing plasma-induced modifications of treated polymer surfaces, as outlined by the results reported in Fig. 6. The closer to the antenna the samples were treated (lower Z values), the longer they maintained their high hydrophilicity. Moreover, PET samples that were air plasma treated at 200 W for
10 min at 30 V exhibited excellent, very stable hydrophilicity (Fig. 6). Thus, a negative polarization of the fabric specimen during plasma treatment and the consequent increase of positive ion flux on its surface assured optimal and permanent wettability. Under such operating conditions, besides hydrophilic species implantation (vide infra), considerable surface etching also occurred, as evidenced by their weight loss during the treatment (see the caption of Fig. 6). By contrast, a positive polarization of the sample holder did not produce any increase of PET wettability, with respect to samples that were plasma-treated without polarization, indicating that negatively-charged species (mainly O and electrons, see Figs. 2 and 4) play minor roles in the implantation of polar groups on the polymer surface. 3.3. Surface chemical analysis Table 1 reports XPS results that were obtained immediately after air plasma treatment at 200 W for 10 min at 30 V and exposure to atmospheric air, as well as those obtained after 10 days of aging in water at room temperature or conditioning in air, to be compared with the data obtained with untreated PET. First of all, a considerable amount of nitrogen and a remarkable increase of surface oxygen content were detected on the PET surface after the treatment. Although the percentage of nitrogen did not exhibit
C. Riccardi et al. / Applied Surface Science 211 (2003) 386–397 Table 1 O/C and N/C atom ratios and ratio between oxygen doubly and singly bound to carbon determined by XPS analysis immediately after air plasma treatmenta and after 10 days long storage under water or conditioned air PET
O/Cb
N/C
C¼O/C–Oc
Treated Treated and stored under air Treated and stored under water
0.57 0.51 0.37
0.056 0.062 0.059
0.71 1.65 1.11
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benzene ring should be substituted by hydroxyls and, possibly, by amine groups after the treatment, in line with the observed plasma-induced increase of surface wettability. Moreover, a fourth peak (C4) appears around 287.5 eV in the C (1s) signal, attributable to amide carbon atoms (–CONH– groups) [34]. Their presence on PET surface can only be explained by supposing that nitrogen-containing activated species
a
Treatment conditions: WRF ¼ 200 W, Vh ¼ 30 V, Z ¼ 14 cm, ttr ¼ 10 min. b O/C ratio of untreated PET fibers: 0.25. c Evaluated as the ratio between the areas of the O (1s) bands peaking at ca. 531.8 and 533.8 eV.
any marked variation during aging, the oxygen surface content decreased during aging, especially when in contact with water. Of course, this could not be a consequence of the above-mentioned folding of surface polymer chains, tending to decrease interfacial tension. In fact, oxygen-containing polar moieties would preferentially remain at the interface in contact with polar water. The most plausible explanation of the observed phenomenon is that part of the oxygen present on the surface should be in the form of low molecular weight water soluble species [8,12,32,33], that can be easily dissolved by water and, thus, removed from the treated polymer surface. Such species, at least in part, should remain adsorbed on the PET surface, if aging is carried out in contact with atmospheric air. Information concerning the manner in which oxygen and nitrogen were implanted on the polymer surface during the air plasma treatment can be obtained from the deconvolution of XPS signals. As shown in Fig. 7a, the C (1s) signal of the untreated PET contained three well separated peaks at 284.6, 286.3 and 288.7 eV, corresponding to carbon atoms bound only to carbon or hydrogen in the benzene ring (C1), to methylene carbons singly bound to oxygen (C2) and to ester carbon atoms (C3), respectively [10,12,34]. In the XPS signal that was recorded immediately after the plasma treatment (Fig. 7b), a drastic decrease of the peak at 284.6 eV (C1) due to benzene ring carbon atoms is observed and a marked increase of the C2 band peaking at 286 eV, which can attributed to –C–O– and also to –C–N– groups. This indicates that the hydrogen atoms that are bound to the
Fig. 7. Deconvolution of the C (1s) peak of XPS spectra of PET recorded (a) before plasma treatment; (b) immediately after air plasma treatment (WRF ¼ 200 W, Vh ¼ 30 V, Z ¼ 14 cm, ttr ¼ 10 min); after subsequent 10 days conditioning (c) in air and (d) in water.
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present in the RF air plasma collide with PET surface with enough energy, to cause the scission of the polymer chain at the ester group level, which can be envisaged as a weak point on the polymer backbone [13] (i.e. plasmolysis occurs). Amide groups and hydroxyl groups should then be expected to terminate polymer chain fragments. Plasma-induced polymer chain scission appears to be favored by negative polarization of the sample during the plasma treatment, increasing the amount and the speed of positive species, such as N2þ and NOþ (see Fig. 4). Of course, air plasma treatment also created radical species on the polymer surface, mainly through polymer chain scission or through hydrogen abstraction [35]. These species can combine with atmospheric gases, thus also contributing to increase the amount of polar groups on the treated polymer surface [11,35]. The deconvolution of the C (1s) peak of the XPS spectrum of PET that was aged in air (Fig. 7c) exhibits a restoration of the C1 peak (benzenic carbon atoms bound to hydrogen) and a corresponding decrease of the C2 peak (–C–O– and –C–N– groups), that is consistent with the decreased wettability observed with aging time, due to the folding of surface chains, tending to decrease the interfacial tension in contact with air. In contrast, the C4 peak that is due to the amide groups does not decrease with aging time in contact with air (Fig. 7c), probably because these groups, being less polar than those originating the C2 peak, are less susceptible to surface reorganization. For the sample stored under water, a much more evident restoration of surface hydrocarbons was observed (Fig. 7d) and a parallel decrease of the C2 peak, attributed to carbon atoms singly bound to oxygen. Thus, the removal of low molecular weight oxidation products from the polymer surface, consequent to their dissolution in water, ‘cleans’ (and stabilizes) the treated polymer surface. The C3 peak that is due to –COO– groups seems to remain practically unchanged after plasma treatment and subsequent aging in both media (Fig. 7c and d), probably as a consequence of a balance between two opposite effects, the removal of O–C¼O groups contained in low molecular weight oxidation products (prevailing under water) and the oxidation of carbon atoms, consequent to the interaction of plasma-created activated sites with atmospheric oxygen. These processes obviously have opposite effects on the O/C ratio,
which in fact decreases, especially when storage is carried out under water (Table 1). Finally, also the O (1s) signal in the XPS spectra could be deconvoluted into two peaks, correlated to the presence of both C¼O groups (531.8 eV) and –C– O– groups (533.8 eV) [34]. As shown in Table 1, after the plasma treatment, the ratio between the integrated areas of the two peaks increased more for the airstored sample than for the water-stored sample, confirming that surface carbon atoms undergo oxidation in contact with atmospheric oxygen. 3.4. Surface structure and topography While scanning electron microscopy did not indicate any difference between treated and untreated specimens, as in the case of other plasma treatments on fibers [27,29], much more detailed information on surface topographical modifications was obtained by atomic force microscopy. SEM has limited depth resolution, that does not enable the detection of morphological features of much less than a micrometer [36]. The AFM images that are reported in Fig. 8 are the original scans without any filtering or contrast
Fig. 8. AFM images of (a) untreated and (b) air plasma-treated PET (WRF ¼ 200 W, Vh ¼ 30 V, Z ¼ 14 cm, ttr ¼ 10 min).
C. Riccardi et al. / Applied Surface Science 211 (2003) 386–397 Table 2 Statistical parameters obtained from AFM images of untreated PET fibers and plasma-treateda PET fibers, calculated as mean values along six equally spaced lines in the direction parallel to fibrils (y-direction) or perpendicular to them (x-direction, see Fig. 8)
Untreated y-direction x-direction Plasma-treated y-direction x-direction
Mean roughness (nm)
Mean height (nm)
Arc length (mm)
180 270
980 950
1.9 14
20 39
135 132
1.6 2.7
a Treatment conditions: WRF ¼ 200 W, Vh ¼ 30 V, Z ¼ 14 cm, ttr ¼ 10 min.
enhancement. The surface of untreated PET fibers was characterized by a series of more or less ‘in relief’, elongated fibrils, as shown in Fig. 8a. They were oriented almost in the same direction, with an extension that is larger than a few micrometers. Their size in the vertical dimension was micrometric and their almost regular interspace in the horizontal dimension was around 40 nm. The original topographical aspect of the PET fibers underwent a deep modification after air plasma treatment (Fig. 8b): a more isotropic and uniform surface appeared and the elongated features were greatly reduced. Indeed, the size of vertical features decreased by almost one order of magnitude, so that both the vertical patterns and the horizontal patterns were in the submicrometric scale. Typical surface parameters before plasma treatment and after plasma treatment, calculated by applying a statistical analysis to the scanned images, are reported in Table 2. It is evident that most of the anisotropy of the untreated sample disappeared with plasma treatment. The study of the periodic surface features through a FFT analysis also gave evidence of two different space frequencies for the untreated samples, one on the micrometric and the other in the submicrometric scale. The former (dc: 250 nm) described the elongated structures, whose amplitude dominated over the entire image (Fig. 8a). The smaller features described by the submicrometric scale (25–73 nm) were superimposed on the larger structures. In the plasma treated-sample, the amplitude of the micrometric scale was drastically
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reduced and the smaller, submicrometric scale became dominant. Such a large modification of the surface structure, practically leading to the washing out of the larger scale surface patterns, can be ascribed to etching of the fibrils of the fabric surface. Amorphous regions of the material would be expected to undergo etching preferentially respect to crystalline regions [20,29,33,37]. This might be the origin of roughness on the submicrometric scale [20] and would contribute to a decrease in the mobility of surface polymer chain after the plasma treatment, thus increasing the durability of plasma-imparted surface properties. In conclusion, a completely new surface with different physical properties and chemical properties can be obtained via air plasma treatment. Thus, not only surface chemical modifications, but also topographical changes may contribute independently to the observed modification of the wettability of PET fabrics. 3.5. Prevailing plasma-induced surface processes The main processes occurring at the plasma–fiber interface during our air plasma treatments can be identified by comparing the data collected from plasma diagnostics and modelling with those relative to PET surface modification in wettability, by taking into account also the observed changes in chemical composition and topographical aspects of the samples that had undergone the deepest modification. A first plasma-induced process is surface etching, evidenced by AFM analysis and by the weight loss of treated samples (occurring at a rate up to 1:4 107 g cm2 s1, corresponding to a mean etching rate of 0.45 nm s1). Such values correlate well with the ion density profile shown in Fig. 3, leading to the conclusion that the energy drive is mainly due to ion bombardment, with subsequent scission of the polymer chain. The removal of polymeric material towards the gas-phase reasonably occurs through the formation of low molecular weight compounds, formed upon complete or partial oxidation of polymer fragments. The formation of amide groups and, in general, of oxidated polymer fragments, that are weakly bound to the fiber surface and easily soluble in water, also confirm the occurrence of extensive etching in
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samples that have been treated under high ion bombardment, as they can be envisaged as the by-products of the etching process itself. On the other hand the generalized increase in wettability, observed even in the absence of noticeable etching, i.e. on PET plasma-treated at low WRF and in positions inside the reactor downstream to the plasma (Fig. 5b), as well as the formation of C–O and C–N bonds in the surface layer and the simultaneous decrease of C–C and C–H bonds (Fig. 7a and b), are more indicative of a direct grafting process consequent to the incorporation of oxygen and nitrogen. Indeed, this phenomenon better correlates to the expected flux of radical species rather than to the ion fluxes inside the plasma reactor (see Fig. 4). The ionic species that are present in the air plasma also play a role in the implantation of polar groups on PET surface, by assisting polymer chain breaking and by inducing surface activation. These processes could also be induced by UV irradiation [6,7,35,38]. However, ion fluxes alone should not be sufficient to explain the observed chemical modification of the PET surface. Finally, cross-linking of the activated surface should also occur, restricting the mobility of surface polymer chains [35] and thus increasing the durability of the plasma-imparted surface modifications. The possible preferential etching of the amorphous regions of PET fibers would also produce the same effect. Unfortunately, the two processes could not be easily distinguished one from the other, as characterization techniques such as ATR, XRD and DSC did not provide any information, due to the very thin surface layer involved in plasma-induced surface modification.
4. Conclusions RF air plasma treatment of PET fibers led to an increase in their hydrophilicity. Best and more durable wettability results were obtained under high RF power and under negative polarization of treated samples, i.e. under conditions in which the implantation of polar groups on PET surface, evidenced by XPS analysis, was accompanied by extensive surface etching and possibly cross-linking between activated species.
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