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Applied Surface Science 305 (2014) 292–300

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

urface modification of cellulosic substrates via atmospheric pressurelasma polymerization of acrylic acid: Structure and properties

ose Garcia-Torresa,∗, Dioulde Syllab, Laura Molinaa, Eulalia Crespoa,ordi Motaa, Llorenc Bautistaa

Advanced Materials group, R&D Department, Leitat Technological Center, c/Innovacio 2, 08225 Terrassa, Barcelona, SpainElectrodep, Physical Chemistry Department, University of Barcelona, c/Marti i Franques, 1, 08028 Barcelona, Spain

r t i c l e i n f o

rticle history:eceived 4 December 2013eceived in revised form 19 February 2014ccepted 8 March 2014vailable online 19 March 2014

a b s t r a c t

Surface chemical modification of cellulose-based substrates has been carried out by atmospheric pres-sure plasma enhanced chemical vapor deposition (AP-PECVD) of acrylic acid. The structure/propertiesrelationship of the samples was studied as a function of the plasma experimental conditions. Acrylic acidmonomer/helium ratio and treatment speed clearly influences the wettability properties of the papersubstrate: advancing contact angle values were reduced to the half if compare to non-treated paper. Sur-face morphology of the films did not greatly vary at short polymerization times but fibers were covered

eywords:crylic acidellulose-based substratestmospheric pressure plasma enhancedhemical vapor depositionoly(acrylic acid) filmsydrophilic character

by a poly(acrylic acid) film at longer times. FTIR and XPS techniques allowed detecting the retention ofcarboxylic acid groups/moieties. The possibility to quickly design architectures with tunable carboxylicfunctions by modifying the plasma processing parameters is shown.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

The nature of the surface of bulk materials has a high impact onheir final properties because of the vast majority of phenomenaake place at the interface level. Therefore, controlling the surfacetate of the material may widen their application window [1,2]. Inhis sense, the preparation of coatings different than the bulk mate-ial or the nanostructuration/patterning of the materials surface areommonly ways to modify surface characteristics. The introduc-ion of chemical groups of complementary polarity (as in Lewiscid–base chemistry) or reactivity (electrophilic or nucleophilicharacters) is attracting more and more interest because it allowsuning material surface functionality which opens the door to newechnological applications (i.e. sensor technology, life sciences, etc.)3,4].

A number of surface functionalization procedures are currentlyvailable like covalent bonding, adsorption, electrostatic inter-

ction, oxidation, etc. [5]. Among those, plasma processes haveeceived increase attention because they allow surface modifica-ion of materials attributing to different properties but without

∗ Corresponding author. Tel.: +34 699813563.E-mail address: garcia.torres.jm@gmail.com (J. Garcia-Torres).

ttp://dx.doi.org/10.1016/j.apsusc.2014.03.065169-4332/© 2014 Elsevier B.V. All rights reserved.

affecting their bulk. Up to this day, plasma deposition has beensuccessfully used to obtain coatings with different chemically reac-tive moieties (primary amine (–NH2), carboxyl (–COOH), hydroxyl(–OH) groups, etc.) [6–9]. Moreover, this technique allows not onlythe deposition on a wide variety of substrates [7,10] but also tuningthe surface properties by varying the experimental conditions[11,12]. All these characteristics together with the unique proper-ties of the obtained coatings (i.e. the films are generally amorphous,free from pinholes, highly cross-linked, resistant to heat and cor-rosion and very adhesive to a variety of substrates) makes themideal candidates to be applied in a wide number of fields: mechan-ics, optics, electronics, biotechnology, biomedicine, etc. [13,14] byproviding with new functionalities such as hydrophilicity, adhe-sion, biocompatibility, conductivity, anti-fogging, anti-fouling, andlubrication [15].

Besides these well-known examples, state-of-the-art applica-tions are continuously being developed especially in the industrialdomain. In the framework of these industrial applications, plasmapolymerization of acrylic acid has been studied in order to obtaincoatings with a high density of carboxylic acid (–COOH) groups

[16,17]. Such COOH-dense surfaces can have important industrialapplications: (1) improve adhesion and aging resistance of bondedcomponents in the automotive industry [18] and (2) activate poly-olefin surfaces and thus replace conventional treatments in the

J. Garcia-Torres et al. / Applied Surface Science 305 (2014) 292–300 293

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Fig. 1. Chemical structures of (a) acrylic acid

ackaging sector [19]. Acrylic acid has been selected because itseasibility to give COOH-rich surfaces after plasma polymeriza-ion [16]. Different plasma techniques have been used to growoly(acrylic acid) films, the most common which enables a goodontrol of the surface chemistry is radiofrequency low pressurelow discharge [20,21]. The main drawbacks of these plasmas arehe need for expensive vacuum systems and the low depositionates. For all these reasons, atmospheric pressure plasma-enhancedhemical vapor deposition (AP-PECVD) is receiving increasingttention not only from a scientific but also from a technologicalnd industrial point of view as an alternative to grow such plasma-olymer films [22].

Poly(acrylic acid) coatings prepared by plasma polymerizationave been studied over a variety of substrates (glass, metals or poly-ers) [16]. However, cellulose has never been used to polymerize

crylic acid by AP-PECVD. Our interest resides on the use of cellu-ose because of the excellent properties it shows: biocompatiblity,iodegradability, high modulus and high strength, flexibility andood stability, low cost and abundance in nature. All these proper-ies make this biopolymer an excellent candidate to find potentialpplications in optics [23], electronics [24], magnetic [25], mechan-cs [26], catalysts [27], packaging [28] and biomedicine [29–31].

oreover and in the context of a sustainable society, there is strongotivation to replace petroleum-based polymers with polymers

rom renewable resources like cellulose.Therefore, the objective of the present work is the surface

hemical modification of cellulose-based substrates (Fig. 1) bylasma polymerization with carboxyl groups in order to mod-

fy their surface chemical structure and thus their properties (i.e.ettability properties). Acrylic acid (Fig. 2) is used as a monomer

o prepare plasma polymerized films on paper substrate usingtmospheric pressure-plasma enhanced chemical vapor depositionAP-PECVD) technique. AP-PECVD method was selected attend-ng the high-treatment speeds and the lack of vacuum conditionsn order to evaluate the feasibility of this work to be applied atndustrial scale. Acrylic acid was chosen for this study because

t is known that this monomer can be easily polymerized byonventional polymerization processes [16]. Different depositiononditions (applied potential, treatment speed, monomer/carrieras ratio, etc.) have been employed. The resulting film structure and

mer, (b) poly(acrylic acid) and (c) cellulose.

properties have been characterized: morphology (FE-SEM), chem-ical structure (XPS, ATR-FTIR) and wettability (dynamic contactangle).

2. Materials and methods

2.1. Materials

Acrylic acid (AA) (with a 99% purity) was purchased fromSigma–Aldrich and used without further purification. Helium wasused as a carrier and was purchased from Air Liquid. Premium Officepaper 100% recyclable, 80 g/m2, EU Ecolabel PT/11/02 was used assubstrate.

2.2. Plasma treatments

Paper samples were coated in continuous mode using anAtmospheric Pressure Glow Discharge (APGD) equipment, modelPLATEX 600 – LAB VERSION, from the Italian company Grinp, S.r.l.The two-planar electrode equipment operates at low frequency(20–45 kHz) to partially ionize gases and/or vapors of precursors.To obtain plasma-polymerized acrylic acid coatings, the dischargeis fed with helium loaded with acrylic acid vapor. The gas mixtureis introduced in the electrode gap through the upper and lowerelectrodes (electrodes distance = 1.2 mm) allowing the treatmentof both sides of the substrate. In this work the monomer/gas car-rier ratio (0.1/0, 0.1/1 and 0.1/3 l/min), power of discharge (1–2 kW)and treatment speed (1–10 m/min) have been investigated. Occa-sionally, the treatment speed was zero in order to have a thickercontinuous film with the objective to compare the properties ofthe cellulosic-based substrate under the different treatments. Onthe other hand, the electrode temperature (160 ◦C) and distancebetween electrodes (1.2 mm) were kept constant throughout theexperiments. The deposition was carried out at atmospheric pres-sure and at room temperature.

2.3. Characterization

Wettability properties have been analyzed using water con-tact angle (WCA) measurements. In order to measure the dynamic

294 J. Garcia-Torres et al. / Applied Surface Science 305 (2014) 292–300

Fig. 2. Influence of the power of discharge on the contact angle for differentacrylic acid/He ratios: (a) 0.1/0 l/min, (b) 0.1/1 l/min and (c) 0.1/3 l/min. Treatmentspeed = 2 m/min.

Fig. 3. Influence of the treatment speed on the contact angle of the plasma-polymerized acrylic acid obtained from the mixture with the highest He content(0.1 l/min acrylic acid and 3 l/min He) at different discharge powers: (a) 1000 W and

(b) 2000 W.

contact angle of plasma-treated samples, a Krüss K100 MK2 ten-siometer was employed. Wilhelmy method has been applied on20 mm × 20 mm samples. An average value of contact angle hasbeen calculated by measuring four replicates of each sample. Allmeasurements were determined at 65 ± 2% of relative humidityand 20 ± 0.5 ◦C of temperature (ISO 139:2005). WCA were mea-sured immediately after the deposition process, except when theinfluence of time on contact angle was studied.

Surface morphology was examined with a Hitachi H-4100FE field emission scanning electron microscope (FE-SEM) afterdepositing a carbon nanocoating to make the samples conductive.Chemical structure characterization of poly(acrylic acid) film-coated papers have been done by ATR-FTIR and XPS spectroscopies.ATR-FTIR (Nicolet 710 FTIR – Diamond crystal, Pike®, MiracleTM)spectra were recorded between 500 and 4000 cm−1. Mean spectraof 32 scans and normalization to the maximum peak have beencarried out using IR solution software (Shimadzu, Japan). X-rayphotoelectron spectroscopy (XPS) measurements were performed

with a PHI 5600 multitechnique system. The binding energies (BE)of the XPS signals of all species have been corrected by assumingC1s signal at 285.0 eV [32].

J. Garcia-Torres et al. / Applied Surface Science 305 (2014) 292–300 295

Fig. 4. Images of water droplets onto cellulose-based substrates where the varia-tion of WCA is observed depending on the treatment: (a) untreated, WCA = 123.0◦ ,(b) plasma-treated cellulose substrates: treatment speed = 2 m/min, acrylic acid/Herca

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atio = 0.1/3 l/min and discharge power = 2 kW, WCA = 95.1◦ and (c) plasma-treatedellulose substrates: treatment speed = 1 m/min, acrylic acid/He ratio = 0.1/3 l/minnd discharge power = 1 kW, WCA = 60.3◦ .

. Results and discussion

.1. Influence of the plasma polymerization conditions on theettability properties

Chemical nature of the paper substrate surface was modifiedy grafting carboxylic groups through acrylic acid atmospheric-ressure plasma enhanced chemical vapor deposition. Differentperational parameters (discharge power, treatment speed andonomer/carrier gas ratio) were modified in order to anchor

varying number of –COOH groups and thus modify sur-ace/wettability properties. The surfaces of the plasma polymersre first studied using WCA measurements because it is the eas-est and quickest method to examine surface properties. WCA oncrylic acid plasma-polymerized films can provide information onhe hydrophilicity of the samples. Consequently it gives and indica-ion on the surface modification by the plasma polymers and thusstablish some relationship with the polymerization conditions.

Firstly, the influence of the acrylic acid/Helium ratio and theischarge power on the wetting properties of the treated paper isnalyzed. Fig. 2 shows the variation of the advancing WCA withhe applied power. As it can be observed, the discharge poweras no influence on the contact angle when no helium is usedFig. 2a). However, under the presence of a small proportion ofelium, WCA decreases but only at high discharge powers (2 kW)Fig. 2b). A decrease in the WCA was observed at lower dischargeowers when the proportion of He in the fed mixture increased

Fig. 2c). The reason could be an activation process of the gas car-ier at the higher He content which made easier the polymerizationf acrylic acid monomer on the substrate: the higher the He con-ent, the higher the activation and the better the polymerization.

Fig. 5. Effect of aging on WCA of different plasma polymerized cellulose-basedsubstrates.

Moreover, no important differences were detected when dischargepower increased up to 2 kW. In order to attribute the decrease ofthe contact angle to the polymerization of acrylic acid onto the cel-lulosic substrate instead a simple activation process by He, someblank samples were exposed to the same conditions than before butonly with the gas carrier (no acrylic acid monomer was fed to theplasma equipment). The contact angles measured were higher thanthose reported for acrylic acid plasma-polymerized substrates, dis-carding only a conventional activation process rather than surfacefunctionalization.

Other important parameter that may affect the wettability of thefilms is the treatment speed. Treatment speed is related to the timethe substrate resides between the two electrodes, in other words,the lower the treatment speed is the higher the plasma treatmenttime is. Fig. 3 shows how the treatment speed can influence on thecontact angle of the tested films prepared with the higher He con-tent. As it can be observed, the wettability of the paper decreasesas the treatment speed increases, which could be attributed to alower amount of carboxylic acid groups present onto the surface. Itis interesting to remark at this point that a significant decrease inthe WCA was achieved despite the high treatment speed, which isinteresting for industrial applications where high treatment ratesare required. When the treatment speed approached to zero (orthe polymerization time was almost infinite (t ∼ ∞)) water contactangles of approximately 12◦ were measured. This is in agreementwith the formation of a continuous poly(acrylic acid) film sinceWCA of around 9◦ are typical for this polymer [33]. In view of theseresults, it is expected that as residence time increases or treatmentspeed decreases a more continuous and thicker poly(acrylic acid)film (in the nm-range) is obtained.

Based on the previous results, the films with the better wett-ability properties were obtained from the mixture acrylic acidmonomer/helium ratio 0.1/3 l/min at a treatment rate of 1 m/minand a discharge power of 1 kW, leading to WCA of 60.3◦ (±2.5◦).

Fig. 4 shows the images of water droplets onto untreated andplasma-treated cellulose-based substrates. As it can be observed,water droplet shows a more spherical shape on untreated sam-ples (Fig. 4a). Meanwhile, the droplet starts losing (Fig. 4b) orcompletely loses (Fig. 4c) its spherical shape when paper isplasma-polymerized under different conditions. This result indi-cates that substrate surface is clearly modified in a different

manner depending on the polymerization conditions. A consider-able decrease in the contact angle of water droplets was detected,going from around 120◦ for untreated paper to around 60◦ for

296 J. Garcia-Torres et al. / Applied Surface Science 305 (2014) 292–300

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ig. 6. SEM images of untreated (a,e) and plasma-treated cellulose-based substrate m/min and (d,h) 0 m/min.

lasma-polymerized cellulose-based materials. This decrease isainly ascribed to the incorporation of polar groups (i.e. car-

oxyl groups) which favored the wetting of paper surface byater.

Finally, the variation of contact angle with time was also stud-ed in order to detect any time-dependent process occurring afterlasma treatment (i.e. aging). While a small WCA decrease wasbserved 14 days after polymerization which could be attributed

acrylic acid/He ratio 0.1/3 l/min at different treatment speeds: (b,f) 1 m/min, (c,g)

to some cross-linking, no further WCA variation was observed even6 months later indicating that polymerization with acrylic acidmonomer was very effective as no aging was detected (Fig. 5). Itis important to highlight that the lowest WCA observed (60.3◦) in

the as-deposited sample decreased to 53.7◦ fourteen days after theplasma polimerization treatment. The observed decrease in WCAduring the first days could be attributed to some cross-linkingleading to more hydrophilic materials. In this sense, AP-PECVD

J. Garcia-Torres et al. / Applied Surface Science 305 (2014) 292–300 297

F rized(

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ig. 7. ATR-FTIR spectra of (a) non-treated cellulosic-substrate, (b) plasma-polymed) acrylic acid monomer.

echnique not only allows obtaining cellulosic-based substratesith low contact angles but also treated samples which are stable

hroughout time. These are very positive results from an indus-rial point of view because it allows obtaining low WCA and stablen time treated paper substrates avoiding at the same time usingacuum and thus, expensive equipments.

.2. Surface characterization

Surface morphology of paper substrates was observed by FE-EM in order to analyze the influence of the plasma polymerizationn it. Fig. 6 shows the FE-SEM images of non-treated and plasma-olymerized paper substrates showing the lowest water contactngles. In these images, cellulose fibers as well as the inorganicharge typically added to printing papers are observed. No notice-ble differences were detected between non-treated and plasmareated substrates at the high treatment speeds (1–10 m/min) whenxamined at low magnifications (Fig. 6a–c). Meanwhile, higheroughness seems to be observed after plasma polymerization atigher magnifications (Fig. 6e–g) and attributed to some etchinguring plasma treatment thus favoring the anchorage of carboxylicroups. These results are in agreement with those presented bymorosi et al. [34]. Finally, when the polymerization time was dra-atically increased (t ≈ ∞) (Fig. 6d and h) cellulose fibers were

artially covered with a poly(acrylic acid) layer avoiding to observehe fibrillar structure. These results are in accordance with thosereviously presented in which WCA values typical of poly(acryliccid) films (12◦) were measured.

cellulosic substrate at 1 m/min, (c) plasma-polymerized substrate at 0 m/min and

The chemical structure of the plasma-polymerized cellulosicsubstrates was studied by ATR-FTIR. As it was expected, no signifi-cant differences between the spectra of the non-treated (Fig. 7a)and plasma-treated (Fig. 7b) substrates were observed whenthe samples were prepared at a treatment speed in the range1–10 m/min (short times). The ATR-FTIR spectra in these samplesshow the typical transitions for cellulose: (1) the broad band inthe region 3500–3000 cm−1 corresponds to the stretching vibrationmodes of O H groups, (2) the band in the region 3000–2750 cm−1 isattributed to symmetric stretching vibration modes of C H, (3) thepeak at 1420 cm−1 is related to O H in plane bending vibrations,(4) the peaks at wavenumbers 1169 and 1120 cm−1 are attributedto the asymmetric and symmetric C O C stretching vibrationsof the pyranose ring, respectively and (5) the peak at 1052 cm−1

corresponds to C O stretching vibration. When comparing thosespectra with that of the poly(acrylic acid) plasma polymerized ontocellulose at zero treatment speed (or when the residence timebetween the electrodes increased infinitely) some bands associatedto poly(acrylic acid) were detected (Fig. 7c). The broad bands in thewavenumber range from 3500 cm−1 to 2750 cm−1 are related to thestretching vibration modes of the O H group of the COOH unit andto vibrations of the CH unit [35–37]. But most important for the fol-lowing discussion is the stretching vibration of the carbonyl group(C O) that appears at 1703 cm−1, a clear fingerprint of poly(acrylicacid). When comparing these results with those obtained for acrylic

acid monomer (Fig. 7d), it possesses absorption bands at 1636 and980 cm−1 due to the stretching vibration modes C C and CH2,respectively, which are bands typically of the monomer [34]. Thesebands disappeared upon plasma polymerization, thereby signifying

298 J. Garcia-Torres et al. / Applied Surface Science 305 (2014) 292–300

Fig. 8. General XPS survey spectra of cellulose-based susbtrates (a) non-treated, (b)0r

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Fig. 9. High-resolution XPS C1s peaks of (a) non-treated and plasma-treated cel-lulose substrates (b) 0.1/3 l/min acrylic acid/He ratio, 1000 W, 1 m/min and (c)0.1/3 l/min acrylic acid/He ratio, 1000 W, 0 m/min.

.1/3 l/min acrylic acid/He ratio, 1000 W, 1 m/min and (c) 0.1/3 l/min acrylic acid/Heatio, 1000 W, 0 m/min.

he reaction of the carbon–carbon double bond to yield poly(acryliccid).

In order to analyze more precisely the chemical composition ofhe cellulose-based substrates treated at the short deposition timesPS measurements were required. This spectroscopic technique

an provide atomic composition information of the topmost sur-ace layers of the deposited films and generates two types of data.he survey scans lend information on the ratios of the different ele-ents present at the surface meanwhile; the high-resolution peaks

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an render information on the molecular environment of each ele-ent. Fig. 8 shows the general survey of non-treated and acrylic

cid plasma polymerized surfaces at different scan rates and there-ore, different residence times of the paper in the plasma cloud. Thetomic composition of the films is displayed, observing that thexygen amount on the films increases with increasing residenceime as a consequence of a higher incorporation of oxygen atomsoming from carboxyl groups.

To obtain further insight into the chemical bonds present onhe surface of the plasma-polymerized films, curve fitting of theigh-resolution C1s peak can be performed. Fig. 9 shows the C1seaks of the substrate as well as of the poly(acrylic acid) coatingst different residence times. As it can be observed, the C1s peakf the substrate (Fig. 9a) is a single and symmetric peak centeredt 285.0 eV. However, as the cellulose is plasma-polymerized ahoulder to the main peak or a double peak develops as the poly-erization time increases (Fig. 9b and c). The C1s envelope of

he deposited films can be decomposed into different peaks: theeak at 285.0 eV corresponding to C C and C H bonds, a peakt 286.5 due to C OH or C O C functional groups and a peak at89.1 eV which can be attributed to carboxylic acid ( COOH) and/orster ( COOR) groups [34]. Fig. 9b and c shows that the peak at89.1 eV increases with increasing plasma time exposure, suggest-

ng a higher retention of carboxylic acid and/or ester groups in thelm. Some authors have distinguished the contribution of the acid

orm from that of the ester by performing derivatization exper-ments with trifluoroethanol on plasma polymers deposited onolyethylene substrates [38,39]. Their investigations have shownhat the contribution of the carboxylic acid function corresponds to

ore than 90% of the component at 289.1 eV, therefore, this peakan be mainly associated with carboxylic acid groups [38,39].

. Summary

In the present work, coatings made from acrylic acid via AP-ECVD processes were prepared. Depending on the conditions ofhe process, the wettability properties were significantly modified:he helium content in the fed mixture as well as the treatmentpeed greatly influenced on the contact angle. Contact angle val-es of approximately 60◦ were obtained after plasma-polymerizedaper substrates as a consequence of surface functionalization witholar groups. These results indicate that AP-PECVD is a promis-

ng technology for the treatment of cellulosic-based substratesith poly(acrylic) acid with real industrial scalability. ATR-FTIR

echnique has allowed the identification of poly(acrylic) acid filmsfter long plasma polymerization times as the characteristic fea-ure of the polymer were observed in the spectrum meanwhile;PS technique allowed detecting the retention of –COOH groupst the shortest residence times. Moreover, surface morphology ofellulosic fibers was slightly roughened by the plasma process.ummarizing, plasma polymerization parameters have to be care-ully adjusted in order to control the film characteristics such asettability or morphology.

cknowledgements

This work has been achieved thanks to the support of ACC1ÓGeneralitat de Catalunya) in the framework of the BIP projectECCOL11-1-0001 co-financed by FEDER fundings.

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