Wettability Interpretation of Oxygen Plasma ModifiedPoly(methyl methacrylate)

Jinan Chai, Fuzhi Lu, Baoming Li, and Daniel Y. Kwok*

Nanoscale Technology and Engineering Laboratory, Department of Mechanical Engineering,University of Alberta, Edmonton, Alberta, T6G 2G8, Canada

Received April 27, 2004. In Final Form: September 14, 2004

Poly(methyl methacrylate) (PMMA) has been modified via a dc pulsed oxygen plasma for differenttreatment times. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS),optical profilometer, zeta potential, and advancing contact angle measurements. The measured advancingcontact angles of water decreased considerably as a function of discharge. Several oxygen-basedfunctionalities (carbonyl, carboxyl, carbonate, etc.) were detected by XPS, while zeta potential measurementsconfirmed an increase in negative charge for the treated PMMA surface. Evaluating the correlation betweenthe concentration of polar chemical species and zeta potential, we found that increase in surface hydrophilicityresults from the coeffect due to incorporation of oxygen functional groups and creation of charge states.The electrical double layer (EDL) effect was also considered in contact angle interpretation by introducingan additional surface tension term into Young’s equation. We also found that EDL contribution to thesolid-liquid interfacial tension is negligible and can be safely ignored for the systems considered here.

I. Introduction

Polymers and composites are widely used for technicalproducts due to their low cost, flexibility, and ease ofmanufacturing. However, their surface properties oftendo not meet the demands regarding wettability, biocom-patibility, and adhesion. For example, poly(methyl meth-acrylate) (PMMA) is distinguished by its excellent opticalproperty, high molding precision, and cost-effectiveness;its relatively low surface energy may have a negative effecton the adhesion of coatings and biocompatibility. Con-sequently, it is very difficult to wet and bond PMMA toadhesiveswithoutmodifying its surface in order to improveitswettability characteristics.Hence,an additional surfacemodification is required to achieve the desired properties,while maintaining their bulk characteristics.1 Plasmatreatment of polymer surfaces is a well-establishedtechnique for many technical applications because of itsunique ability to modify polymer surfaces without affectingtheir bulk properties. It was shown that substantialchanges in the chemical functionality, surface state,wettability, and bondability of polymer surfaces can beachieved by plasma treatment.2-6

Plasma containing electrons, ions, radicals, and neutralmolecules strongly interacts with polymer surfaces, andas a result, chemical and physical modifications occur onthe surfaces. The effect of plasma surface modificationmainly depends on the combination of the type of gas andchemical structure of the polymer.3,7,8 Among various

plasma treatments, oxidative (oxygen and air) plasmatreatment was found to be an effective tool for improvingwettability. It is well-known that when a polymer is treatedwith O2 plasma, polar functional groups containing oxygenare introduced into its surface, leading to a hydrophilicsurface.7,9 However, most work has neglected the influ-ence of surface charges on the change in wettability;relatively fewer studies claimed the relationship betweenthe surface charge and the wettability of the modifiedsurfaces.10 So far, the cause of the wettability differenceduring plasma treatment is not completely understood;the structural state and chemical modification of a sur-face and its wettability can be complex, and much effortis being directed toward understanding the wettingprocess.11

In the present study, a dc pulsed O2 plasma treatmentwas carried out on PMMA surfaces with different treat-ment times to increase wettability. A dc pulsed dischargepresents the following advantages over radio frequency(rf) and dc plasmas: it saves energy as the dischargevoltage is zero during the afterglow and reduces arcingwhich is detrimental to the growing layer.12 A low-ratedynamic contact angle technique was employed based onaxisymmetric drop shape analysis (ADSA) to investi-gate the wettability and surface tension of the PMMAsurfaces. The chemical structure and charge propertiesof treated PMMA surfaces were characterized usingX-ray photoelectron spectroscopy (XPS) and zeta poten-tial measurements, respectively. The topographical modi-fication was examined by an optical profilometer. Theeffect of chemical structure and surface charge on thewettability modification and their relationships will bediscussed.

II. Materials and MethodsA. Material Preparation. PMMA was purchased from

Polysciences (Warrington, P.A., catalog no. 04553, MW ) 75 000)

* To whom correspondence should be addressed. E-mail:daniel.y.kwok@ualberta.ca.

(1) Li, R.; Ye, L.; Mai, Y. W. Composites 1997, 28A, 73.(2) Yasuda, H. J. Macromol. Sci. 1976, A10, 383.(3) Liston, E. M.; Martinu, L.; Wertheimer, M. R. J. Adhes. Sci.

Technol. 1993, 7, 1091.(4) Oehr, C. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 208,

40.(5) Hegemann, D.; Brunner, H.; Oehr, C. Nucl. Instrum. Methods

Phys. Res., Sect. B 2003, 208, 282.(6) Collaud, M.; Groening, P.; Nowak, S.; Schlapbach, L. J. Adhes.

Sci. Technol. 1994, 8, 1115.(7) Egitto, F. D.; Matienzo, L. J. IBM J. Res. Dev. 1994, 38, 423.(8) Grace, J. M.; Gerenser, L. J. J. Dispersion Sci. Technol. 2003, 24,

305.

(9) Coates, D. M.; Kaplan, S. L. MRS Bull. 1996, Chapter IV.(10) Drachev, A. I. High Energy Chem. 2003, 37, 297.(11) Wilson, D. J.; Pond, R. C.; Williams, R. L. Interface Sci. 2000,

8, 389.(12) Hugon, R.; Henrion, G.; Fabry, M. Meas. Sci. Technol. 1996, 7,

553.

10919Langmuir 2004, 20, 10919-10927

10.1021/la048947s CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 11/06/2004

as 200 µm beads. A 2% PMMA/chloroform solution was preparedusing chloroform (Fisher Chemicals; C297-4, 99.9%) as thesolvent. Silicon wafers ⟨100⟩ (Wafer World, West Palm Beach,FL; test grade) were selected as the substrate for the polymercoating. They were obtained as circular disks of 10 cm diameterand were cut into rectangular wafer surfaces of about 2.5 cm ×4.5 cm. Each slide was then soaked in chromic acid for at least24 h, rinsed with doubly distilled water, and dried under a heatlamp before the polymer coating. The PMMA films were preparedby spin coating at a velocity of 2000 rpm for 60 s. A homogeneousthin layer of less than 1 µm of PMMA formed on the slide surfaceafter the solution spread and chloroform evaporated.

B. Plasma Treatment of Surface. The experimental setupused for plasma treatment is a home-built system presented inFigure 1. The reactor consists of a cylindrical stainless steelchamber (Kurt J. Lesker, 10 in. in diameter and 12 in. in height)with a symmetrical plate-to-plate configuration of electrodes (thestainless steel plate is 8 cm in diameter) where the interelectrodedistance was fixed at 4 cm. The upper electrode was grounded,and the lower one was negatively polarized. A Velonex 360 high-frequency pulse generator delivers a dc pulse with a pulse widthof 10 µs and frequency of 1 kHz. The negatively pulsed voltagemagnitude was 1000 V during the treatment, and the pulseswere externally triggered by an Agilent 33120A functionalgenerator. The voltage and current waveforms were visualizedby a digital oscilloscope (Tetronix TDS3014B).

The PMMA slides were installed on the lower plane electrodeand treated with O2 plasma. The reactor was evacuated to a basepressure of 2×10-5 Torr, and then oxygen gas (Praxair, industrialgrade) was introduced into the chamber. The gas flow rate iscontrolled by a mass flow controller (MKS, M100B), and thechamber is pumped by a Balzers turbomolecular pump (TPH330)and a Leybold mechanical dry pump (EcoDry M15). The operatingpressure is maintained at 0.2 Torr with a gas flow rate at 10sccm during the treatment procedure. The PMMA slides weretreated between 1 and 50 s.

C. Determination of Zeta Potential. Zeta potentials weredetermined with home-built devices based on streaming potentialand streaming current measurements.13 The measuring systemconsists of a parallel-plate microchannel, a liquid flow loopincluding a driving pump and flow rate meter, a pressure

transducer, and an electrometer (Keithley, 6517A). Here, mi-croscope glass slides (Fisher, catalog no. 12-550A) instead ofsilicon wafers were selected for PMMA coating and plasmamodification to assemble the rectangular microchannel. The twoparallel plates were separated by thin plastic shims (Small PartsInc., FL) as spacers for the channel walls. Details of themeasurement setup can be found elsewhere.14

As is well-known,15 the ú potential can be obtained from eithera streaming potential Us or streaming current Is measurementaccording, respectively, to

or

where η is the dynamic viscosity of the fluid; KB is the specificbulk conductivity of the fluid and Kσ is the surface conductivity;ε0 is the permittivity of vacuum and εr is the relative dielectricpermittivity of the streaming solution; L, b, and h are the length,width, and height of the channel, respectively. The width andlength of the microchannel were measured by using a precisiongauge (Mitutoyo, Japan, model CD-6B) with an accuracy of 1µm. For laminar flow, the dependence of the volume flux Q ina slit channel on its geometry is given by

Hence, the slit channel height h can be calculated from themeasured fluid flow Q and known pressure differences p:

(13) Werner, C.; Konig, U.; Augsburg, A.; Arnhold, C.; Korber, H.;Zimmermann, R.; Jacobasch, H. J. Colloids Surf., A 1999, 159, 519.

(14) Lu, F.; Yang, J.; Kwok, D. Y. J. Phys. Chem. B 2004, 108, 14970.(15) Hunter, R. J. Foundations of Colloid Science; Oxford University

Press: New York, 2001.

Figure 1. A schematic experimental setup for the oxygen plasma treatment.

ú(Us) )η(KB + 2Kσ/h)

ε0εr

dUs

dp(1)

ú(Is) ) ηLε0εrbh

dIs

dp(2)

Q ) pbh3

12ηL(3)

10920 Langmuir, Vol. 20, No. 25, 2004 Chai et al.

The streaming potential and streaming current values for a givenchannel height h can be used to calculate the surface conductivityKσ:16

D. Contact Angle Measurements. The wettability of PMMAsurfaces was examined by measurements of contact angles. Theinterpretation of contact angles in terms of solid surface tensionsrelies on the validity of Young’s equation,

which interrelates the Young contact angle with the interfacialtensions of the liquid-vapor γlv, solid-vapor γsv, and solid-liquidγsl interfaces; θY is the Young contact angle (i.e., a contact anglewhich can be used in conjunction with Young’s equation). Theexperimental advancing contact angle θa can be expected to bea good approximation of θY if the surfaces are smooth.17

Advancing contact angles of water on PMMA surfaces weremeasured at ambient temperature by using ADSA-P (axisym-metric drop shape analysis-profile). ADSA-P is a technique todetermine liquid-fluid interfacial tensions and contact anglesfrom the shape of axisymmetric menisci, that is, from sessile aswell as pendant drops.18,19 Assuming that the experimental dropis Laplacian and axisymmetric, ADSA-P finds a theoretical profilethat best matches the drop profile extracted from an image of areal drop, from which the surface tension, contact angle, dropvolume, surface area, and three-phase contact radius can becomputed. The strategy employed is to fit the shape of anexperimental drop to a theoretical drop profile according to theLaplace equation of capillarity, using surface/interfacial tensionas an adjustable parameter. The best fit identifies the correctsurface/interfacial tension from which the contact angle can bedetermined by a numerical integration of the Laplace equation.Details of the methodology and experimental setup can be foundelsewhere.17,20

With respect to the low-rate dynamic contact angle measure-ments by ADSA-P, liquid was supplied to the sessile drop frombelow the wafer surfaces using a motorized syringe device. Ahole of about 1 mm diameter in the center of each wafer surfacewas made by using a diamond drill bit (Abrasive Technology,catalog no. 261.8C) in order to facilitate such a procedure. Toavoid leakage between a stainless steel needle and the hole, Teflontape was wrapped around the end of the needle before insertingit into the hole.

In actual experiments, an initial liquid drop of about 0.3 cmradius was carefully deposited, covering the hole on the sur-face. This is to ensure that the drop will increase axisym-metrically in the center of the image field when liquid is suppliedfrom the bottom of the surface and will not hinge on the lip ofthe hole. Then, deionized water was pumped by a motorizedsyringe from below at a velocity of the three-phase contact lineof less than 1 mm/min. A sequence of pictures of the growingdrop was then recorded by a Linux computer typically at a rateof 1 picture every 1 s, until the three-phase contact radius wasabout 0.5 cm or larger. For each low-rate dynamic contact angleexperiment, at least 50 and up to 200 pictures were normallytaken. Since ADSA-P determines the contact angle and the three-phase contact radius simultaneously for each picture, theadvancing dynamic contact angles as a function of the three-phase contact radius (i.e., location on the surface) can be obtained.

This method has an additional advantage: the quality of thesurface is observed indirectly in the measured contact angles. Ifa solid surface is not very smooth, irregular and inconsistentcontact angle values will be seen as a function of the three-phasecontact radius. When the measured contact angles are essentiallyconstant as a function of surface location, the mean contact anglefor a specific rate of advancing can be obtained by averaging thecontact angles, after the three-phase contact radius reaches0.4-0.5 cm.

E. X-ray Photoelectron Spectroscopy Analysis. XPS isan ideal technique for studying plasma-induced chemistry atthe modified polymer surface because of its surface sensitivity(1-10 nm), quantification of results, and the information obtainedabout the nature of chemical bonding from the binding energy(BE) values.21,22 In this study, XPS spectra and atomic concen-trations of the sample surfaces were acquired by a Kratos Axis165 photoelectron spectrometer using a monochromatic alumi-num X-ray source (Al KR 1486.6 eV) operating at 210 W (15 kV,14 mA). The analyzed surface area was 1 mm in diameter, andthe photoelectron takeoff angle (with respect to the surfacenormal) was held at 90°. The instrument was calibrated to the4f7/2 peak of gold at EB ) 84.0 eV. The base pressure was 2 ×10-10 Torr with an operating pressure of 2 × 10-9 Torr. Chargecompensation was accomplished using low energy electrons.Standard operating conditions for good charge compensation are-2.8 V bias voltage, -1.0 V filament voltage, and filament currentof 2.1 A. The hemispherical analyzer functioned at a constantpass energy of 20 eV for high-resolution spectra and at 150 eVfor quantitative analysis.

Preliminary data analysis and quantification were performedusing the VISION software provided by the manufacturer. TheC1s and O1s envelopes were fitted using asymmetrical 70%Gaussian and 30% Lorentzian component profiles after subtrac-tion of baseline using the linear background. BEs were deter-mined by reference to the C1s BE (284.6 eV) prior to peak fitting.The samples were cut to a size of ∼1 × 1 cm2 and clamped to thesample holder with metal clamps directly before introductioninto the instrument. The samples were plasma-treated in ourlaboratory and then transferred to another laboratory in adifferent department for analysis. Thus, minute exposure of thetreated substrates to atmosphere is unavoidable.

F. Topographical Observation by an Optical Profilo-meter. Surface morphology was observed by means of an opticalprofilometer (Zygo, New View 5000). This three-dimensionalsurface structure analyzer employs scanning white light forimaging and allows measurements of the microstructure andtopography of surfaces. Such a device consists of a microscope,an interferometric objective, and a high-resolution camera. Theincoming light is split into two inside an interferometer with onebeam going to an internal reference surface and the other to thesample. After reflection, the beams recombine inside the inter-ferometer, undergoing constructive and destructive interferenceand producing light and dark fringe patterns; a precision verticalscanning transducer and a camera generate a three-dimensionalinterferogram of the surface, which is processed by the computerand transformed by frequency domain analysis to result in aquantitative 3-D image.

III. Results and DiscussionA. Contact Angle and Solid Surface Tensions.

Figure 2 displays a typical experimental result of ADSA-Pwater contact angle of untreated PMMA. As can be seenin this figure, there was an initial increase in the contactangle at essentially constant three-phase contact radiusas the drop volume increased. This is a result of the factthat even carefully putting an initial liquid drop fromabove on a solid surface can result in a contact anglesomewhere between advancing and receding. The contactangles are essentially constant, as the drop volume Vfurther increases and hence the three-phase contact radiusR. Increasing the drop volume in this manner ensures the

(16) Werner, C.; Zimmermann, R.; Kratzmuller, T. Colloids Surf., A2001, 192, 205.

(17) Kwok, D. Y.; Neumann, A. W. Adv. Colloid Interface Sci. 1999,81, 167.

(18) Rotenberg, Y.; Boruvka, L.; Neumann, A. W. J. Colloid InterfaceSci. 1983, 93, 169.

(19) Cheng, P.; Li, D.; Boruvka, L.; Neumann, A. W. Colloids Surf.1990, 43, 151.

(20) Kwok, D. Y.; Leung, A.; Lam, C. N. C.; Wu, A. L. R.; Neumann,A. W. J. Colloid Interface Sci. 1998, 206, 44.

(21) Briggs, D.; Seah, M. P. Practical Surface Analysis; Wiley:Chichester, 1990.

(22) Idage, S. B.; Badrinarayanan, S. Langmuir 1998, 14, 2780.

h ) (12ηLQpb )1/3

(4)

Kσ ) L2b

Is

Us-

KB

2h (5)

γlv cos θY ) γsv - γsl (6)

Oxygen Plasma Modified PMMA Langmuir, Vol. 20, No. 25, 2004 10921

measured θ to be an advancing contact angle. Themeasured contact angles were essentially constant as Rincreases. This indicates good surface quality of thesurfaces used. The averaged contact angle of 72.5° isconsistent with the literature values.20,23

Advancing contact angles for water against differenttreatment times for plasma-modified PMMA surfaces areshown in Figure 3. The applied voltage was maintainedat 1 kV during the discharge treatment. Measurementswere made immediately after the treatment. The contactangle decreased rapidly with treatment time and remainedessentially constant after about 5 s of treatment. Thecalculation of solid surface tension γsv from the contactangle of a liquid of surface tension γlv starts with Young’seq 6 and an equation of state,17

where â takes the value of 0.000 105 7 (m2/mJ)2. Notethat a γlv value of 72.70 mJ/m2 for water is used. Combiningeqs 6 and 7 yields

where the value of γsv can be determined from a pair ofexperimentally determined γlv and θ; the corresponding

γsl value can then be calculated from either eq 6 or eq 7.Table 1 shows the computed γsv and γsl values for untreatedand oxygen plasma treated PMMA corresponding tothe advancing contact angles of water. From the table,the untreated PMMA surface exhibits a contact angle of72.5° and is associated with a solid-vapor surface tensionγsv value of 40.1 mJ/m2. After a discharge treatment of 1s, the contact angle of PMMA decreased rapidly to 61.5°as a result of surface modification. For a treatment timeof 2 s, the contact angle decreased from 61.5° to 56.4°.With respect to the solid surface tension, there is anapparent increase from 46.9 mJ/m2 for 1 s treatment timeto 52.8 mJ/m2 for 5 s, indicating the influence of treatmenttime on contact angles and hence surface tensions. Table1 also shows the remaining contact angles for othertreatment times longer than 5 s; no significant change inthe contact angle and hence the solid surface tension isapparent.

B. Zeta Potential Measurements and Evalua-tion. After completion of the streaming potential andstreaming current measurements of each channel for aspecific liquid and flow rate, the averaged values from atotal of 10 readings for p and Is were used in eq 2 todetermine the zeta potential ú. Figure 4 shows thedependence of the zeta potential upon the treatment timeof oxygen plasma discharge. It can be seen that a strongincrease in ú takes place within the first 5 s; after that,the curve flattens out, reaching a value of about -86 mV.The shape of the curve is also similar to that of contactangle in Figure 3.

Zeta potential depends on the surface charge densityand double layer thickness of different electrolyte solu-tions. The surface charge density σ can be related to thezeta potential by solving the nonlinear Poisson-Boltz-

(23) Eaton, P. J.; Graham, P.; Smith, J. R.; Smart, J. D.; Nevell, T.G.; Tsibouklis, J. Langmuir 2000, 16, 7887.

Figure 2. Low-rate advancing contact angles of water onuntreated PMMA.

Figure 3. Experimental advancing contact angles of wateron O2 plasma modified PMMA with different treatmenttimes.

γsl ) γlv + γsv - 2xγlvγsv(1 - â(γlv - γsv)2) (7)

γlv(1 + cos θY) ) 2xγlvγsv(1 - â(γlv - γsv)2) (8)

Table 1. Experimental Advancing Water Contact Anglesand Calculated Surface Tensions of O2 Plasma TreatedPMMA without Consideration of the EDL Effect Using

Equation 8a

treatment (s) contact angle (deg) γsv (mJ/m2) γsl (mJ/m2)

0 72.5 ( 0.6 40.1 18.21 61.5 ( 0.6 46.9 12.22 54.2 ( 0.7 51.2 8.75 51.5 ( 1.4 52.8 7.6

10 51.2 ( 1.4 53.0 7.450 51.0 ( 1.4 53.1 7.4

a A γlv value of 72.7 mJ/m2 for water was employed for thecalculation. Error bars are standard deviations.

Figure 4. Experimental zeta potential ú of O2 plasma modifiedPMMA with different treatment times.

10922 Langmuir, Vol. 20, No. 25, 2004 Chai et al.

mann equation.24 In this case, the zeta potential is givenby

where T is the absolute temperature, e is electron charge,and s is a dimensionless surface charge:

where κ-1 is the Debye length. Combining eqs 9 and 10,one obtains

The zeta potential and corresponding interfacial chargedensity of untreated and 50 s plasma-treated PMMA indifferent solutions are given in Table 2. The plasmatreatment obviously causes an increase in the value ofzeta potential and interfacial charge density for allsolutions due to a strong electrical double layer (EDL).

C. Surface Chemistry. The fitted high-resolution C1score-level photoemission spectra of untreated PMMAtogether with a representation of the chemical structureof the monomers are presented in Figure 5. The peak labelsin the spectra correspond to the carbon atoms havingdifferent environments in the monomers which can bedecomposed into four distinct components:25 C1s(1) cali-brated at a binding energy of 284.6 eV corresponds tocarbon atoms involved in C-C and C-H bonds of thePMMA, C1s(2) at 285.2 eV corresponds to the quaternary

carbon atom in R-position to the ester group, C1s(3) at286.3 eV is the carbon atom involved in the methoxy groupof the ester chemical function, and C1s(4) at 288.6 eV isthe carbon atom of the carboxylic group (O-CdO). Thefits were based on reference measurements of Kratos26

and were performed by keeping the binding energies andwidths (fwhm, full width at half-maximum) of the peaksidentical to the literature values, while varying theintensities. The binding energies and the relative area ofthe fitted peaks are given in Table 3.

When the samples were treated with O2 plasma, thestructure of the C1s spectra varied. The differences in theshape of the C1s spectra reveal strong modifications ofthe elemental composition of the PMMA surface (cf. Figure6). Decomposition of the C1s peak reveals the increasedarea corresponding to polar functional groups and thepresence of two new components: C1s(5), located at 289.8eV, corresponds to carbonate groups O2CdO; and C1s(6)located at 287.2 eV is attributed to free carbonyl groupsCdO (Figure 6). The percentages of different bondingcontributions, estimated from the ratios of the peak areaof Figure 6, are given in Table 4. It can be seen that thecomponent C1s(3) increased rapidly from 19.42% at 0 sto 27.88% at 1 s and then decreased. The C1s(4) componentdecreased slowly from 17.63% to 14.71% upon 10 sexposure but increased to 20.29% at 50 s treatment.Starting with 1.51% for 1 s plasma treatment, thecomponent C1s(5) increased to 4.87% (10 s) with con-comitant decrease in the C1s(3) and C1s(4) componentsand then decreased to 1.46% at 50 s treatment. Up to 5s of plasma exposure, the C1s(6) component becomesapparent and continuously increases to 7.61% upon 50 sexposure. Thus, the first stage of surface modification couldbe the increase of C1s(3) and then the formation of C1s(5)and C1s(6). It is difficult to ensure whether the formationof C1s(5) and C1s(6) is due to breakdown of C1s(3) andC1s(4) or to independent occurrence. Prediction of the rate

(24) Attard, P.; Antelmi, D.; Larson, I. Langmuir 2000, 16, 1542.(25) Amor, S. B.; Baud, G.; Jacquet, M.; Nanse, G.; Fioux, P.; Nardin,

M. Appl. Surf. Sci. 2000, 153, 172.(26) High resolution XPS spectra of polymers; www.kratos.com; K.

A. Co.: Manchester, 1997.

Figure 5. (a) C1s XPS spectra with fitted peaks and (b) chemical structure of PMMA.

Table 2. Experimental Values of ú and σ for Untreatedand 50 s Treated PMMA in Different Electrolyte

Solutions

sample solution ú (mV) σ (µC/cm2)

water -35.9 -0.0089untreated 0.1 mM KCl -13.4 -0.031

1 mM KCl -7.5 -0.055water -82.5 -0.028

50 s treated 0.1 mM KCl -48.5 -0.1301 mM KCl -20.9 -0.157

ú )2KBT

eln(s/2 + x1 + s2/4) (9)

s ) eεε0KBTκ

σ (10)

σ )2εε0KBTκ

esinh eú

2KBT(11)

Table 3. Fitting Parameters for the C1s Envelope ofPMMA

peak no. BE (eV) fwhm (eV) area (%)

1 284.6 0.95 36.42 285.2 0.95 23.03 286.3 1.10 22.04 288.6 0.85 18.6

Oxygen Plasma Modified PMMA Langmuir, Vol. 20, No. 25, 2004 10923

of formation for each component is also difficult due toexperimental uncertainties and overlap of native PMMApeaks with oxidized species in the core-level spectra.

Nevertheless, Figure 7 reveals that the calculatedoxygen content in PMMA increases continuously as afunction of discharge exposure time. It also appears thatthe main effects of modification occur during the first 5s of the treatment; the percentage of oxygen then increasesslowly with time. It is interesting to find that the curvein Figure 7 (started from 5 to 50 s treatment) is steeper

Figure 6. C1s XPS spectra of plasma-treated PMMA.

Table 4. Relative Area of Different Components in theC1s Envelope for O2 Plasma Treated PMMA

treatment(s)

C1s(1)(%)

C1s(2)(%)

C1s(3)(%)

C1s(4)(%)

C1s(5)(%)

C1s(6)(%)

0 35.43 27.53 19.42 17.63 0 01 31.12 21.91 27.88 17.54 1.51 02 30.03 24.47 26.76 16.90 1.85 05 26.86 24.97 23.85 16.82 3.52 3.98

10 27.98 24.64 23.02 14.71 4.87 4.7750 27.87 20.74 22.03 20.29 1.46 7.61

Figure 7. Oxygen percentage of PMMA after plasma exposurewith different treatment times.

10924 Langmuir, Vol. 20, No. 25, 2004 Chai et al.

than those observed in Figures 3 and 4. This indicatesthat XPS could measure a different surface than do contactangle and zeta potential measurements. The latter twocharacterize the properties of the outermost layers ofPMMA surfaces, while XPS also provides chemicalinformation from sublayers of the samples due to thecreation of photoelectrons at a depth below the surfaces.We speculate that the oxidative reactions could extend tothe sublayer of the PMMA surface after prolonged plasmatreatment while modification of the chemical structure ofthe outermost surface remains saturated.

D. Discussion. The wettability improvement of PMMAsurfaces by plasma treatment as well as other polymershas been reported in the literature.27 This enhancementis caused by the increase in surface free energy. So far,

how plasma treatment changes the surface energy of apolymer surface is not completely understood. Most workrelated the wettability change to incorporation of oxygenspecies by plasma activation.28-30

The activation mechanism is believed to be the creationof free radicals on the polymeric material’s surfacemolecules and then subsequent coupling of these freeradicals with active species from the oxygen plasmaenvironment.9 In the case of a dc pulsed plasma conditionset for our experiments, each pulsed duration was 10 µswith a discharge frequency of 1 kHz. Thus, the effectiveglow duration is extremely short when compared to theeffective afterglow duration; the duty factor (ratio betweendischarge duration and pulsed voltage period) is deter-

(27) Briggs, D.; Rance, D. G.; Kendall, C. R.; Blythe, A. R. Polymer1980, 21, 895.

(28) Groning, P.; Coen, M. C.; Schlapbach, L. Chimia 2001, 55, 171.(29) France, R. M.; Short, R. D. Faraday Trans. 1997, 93, 3173.(30) Johansson, B.; Larsson, A.; Ocklind, A.; Ohrlund, A. J. Appl.

Polym. Sci. 2002, 86, 2618.

Figure 8. A possible oxidation scheme of O2 plasma treated PMMA.

Oxygen Plasma Modified PMMA Langmuir, Vol. 20, No. 25, 2004 10925

mined to be ∼1%. It is also understood that the lifetimeof oxygen radicals is longer than those of oxygen ions andelectrons, as electron-ion recombination is much fasterthanradical recombination.Hence, the ioneffect onsurfacemodification can be safely neglected and is in agreementwith those reported elsewhere.31,32 Therefore, the long lifesinglet state molecular O2(1∆) and the atomic species inground-state O(3P) are expected to be the main species toreact with the PMMA surface.33,34 First, hydrogen isabstracted from the C-H bond in PMMA by atomic oxygenyielding a free radical site; then, the singlet-state mo-lecular oxygen O2(1∆) can couple with the free radicalcreating a peroxy radical. These peroxy radicals arestabilized by hydrogen transfer (from the same or aneighboring chain) to form a hydroperoxide and a newradical site. There is a multiplicity of oxidation mecha-nisms that can be proposed in PMMA since there are atleast three potential sites for initiating the oxidationreaction by a C-H bond rupture and different oxidativereactions take place simultaneously while competing witheach other. Figure 8 presents possible examples ofoxidation schemes for the formation of carboxyl, carbonyl,and carbonate groups which were observed in high-resolution XPS spectra (cf. Figure 6 and Table 4).

It is true that the plasma-modified surface will becomehydrophilic due to the incorporation of polar functionalgroups on account of the propensity of water molecules toform hydrogen bonds. However, the change of surfaceenergy can be ascribed to many other effects such assurface charge and roughness, in addition to chemicalstructure. Coen et al. reported the creation of a conductivesurface layer on polymers by low-pressure plasma treat-ments,35 while Gil’man et al. related the charge densityvariation to the wettability of plasma-modified polymersurfaces.36 Nevertheless, the zeta potential effect on thesolid surface tension of polymeric materials after plasmatreatment remains unclear in the literature. From Figures3 and 4, we observed an obvious relationship betweenzeta potential and contact angle of water on O2 plasmatreated PMMA.

The increased in negative zeta potential of modifiedPMMA surfaces in water can come about in two ways:37

(1) by the generation of charge states due to plasmatreatment (i.e., the surface is precharged) and (2) by theionization or dissociation of created new functional groups

(e.g., the dissociation of surface carboxylic groups [-COOHf -COO- + H+]). Obviously, the influence of plasmaprecharge on the EDL is a complicated process. Both ofthe above mechanisms can lead to a more negativelycharged surface, causing the increase in the streamingcurrent and streaming potential.

When a water droplet is in contact with a solid surface,the droplet will spread until a free energy minimum isreached and is determined by the interaction between theliquid and surface. When the surface has a zeta potentialú, a charge density σ builds up in the EDL of the liquidphase, causing the solid-liquid interfacial tension to bedifferent from that without the EDL. Using Lippmann’sequation, a solid-liquid interfacial tension contributedby the EDL effect γsl

EDL can be taken as15

where σ and φ are the surface charge density and surfacepotential in the EDL, respectively. After inserting thecharge density from eq 11 and integrating eq 12 from φ

) 0 to φ ) ú, γslEDL can be given by

Thus, in the presence of an EDL effect, γsl as it appearsin Young’s eq 6 should be written as

where γsl0 and γsl

EDL are the solid-liquid interfacial ten-sions without and due to the EDL effect, respectively.Hence, a modified Young’s equation can be written as

It is now instructive to rewrite the equation of stateapproach eq 7 as

The use of γsl0 instead of γsl represents the fact that the

equation of state relation did not consider any additional(unexpected) effect on the solid-liquid interfacial tensionsuch as that due to EDL interactions. Combining eq 16with the modified Young’s eq 15 yields

As γslEDL can be determined from eq 13 by means of the

experimental ú potential values in Figure 4, the γsv valuefor each solid-liquid pair with known γlv and θ can becalculated from the above equation of state eq 17. Once

(31) Clement, F.; Held, B.; Soulem, N.; Martinez, H. Eur. Phys. J.:Appl. Phys. 2003, 21, 59.

(32) Narushima, K.; Tasaka, S.; Inagaki, N. Jpn. J. Appl. Phys. 2002,41, 6506.

(33) Lianos, L.; Parrat, D.; Hoc, T. Q.; Duc, T. M. J. Vac. Sci. Technol.,A 1994, 12, 2491.

(34) Clement, F.; Held, B.; Soulem, N.; Guimon, C. Eur. Phys. J.:Appl. Phys. 2002, 18, 135.

(35) Coen, M. C.; Groening, P.; Dietler, G.; Schlapbach, L. J. Appl.Phys. 1995, 77, 5695.

(36) Gil’man, A. B.; Rishina, L. A.; Drachev, A. I.; Shibryaeva, L. S.High Energy Chem. 2001, 35, 151.

(37) Israelachvili, J. N. Intermolecular and Surface Forces; AcademicPress: London, 1985.

Table 5. Calculated γslEDL from Equation 13, γsv from Equation 17, γsl

0 from Equation 15, and γsl from Equation 14 forVarious PMMA Surfaces with Different Treatment Timesa

treatment(s)

γlv cos θ(mJ/m2)

γslEDL

(mJ/m2)γsv

(mJ/m2)γsl

0

(mJ/m2)γsl

(mJ/m2)∆γlv cos θ(mJ/m2)

∆γslEDL

(mJ/m2)∆γsv

(mJ/m2)∆γsl

0

(mJ/m2)

0 21.86 0.00153 40.09065 18.23086 18.22933 0 0 0 01 34.67 0.00279 46.85026 12.16361 12.16082 12.81 0.00126 6.75961 -6.067252 42.53 0.00716 51.22101 8.70175 8.69459 20.67 0.00563 11.13036 -9.529115 45.26 0.00837 52.80551 7.55707 7.54870 23.40 0.00684 12.71486 -10.6737910 45.55 0.00910 52.97995 7.43496 7.42586 23.69 0.00757 12.88930 -10.7959050 45.75 0.00960 53.09609 7.35410 7.34450 23.80 0.00807 13.00544 -10.87676

a Their variations in reference to the respective zero time (t ) 0 s) values are also shown.

γslEDL ) ∫σ dφ (12)

γslEDL )

8n∞KBTκ (cosh eú

2KBT- 1) (13)

γsl ) γsl0 - γsl

EDL (14)

γlv cos θY ) γsv - (γsl0 - γsl

EDL) (15)

γsl0 ) γlv + γsv - 2 xγlvγsv(1 - â(γlv - γsv)

2) (16)

γlv(1 + cos θY) - γslEDL ) 2xγlvγsv(1 - â(γlv - γsv)

2)(17)

10926 Langmuir, Vol. 20, No. 25, 2004 Chai et al.

γsv is known, the values of γsl0 can be determined by means

of either eq 15 or eq 16. The computed γsv, γslEDL, γsl

0 , andγsl and their variations using those at zero time asreferences are shown in Table 5. Direct comparison of thevalues of γsl between Tables 1 and 5 indicates that theassumption used in the equation of state approach inneglecting the EDL effect on contact angles is reasonable

as the contribution of the EDL effect to the solid-liquidinterfacial tension γsl

EDL is extremely small, causingnearly no variation in the calculated γsv and γsl values, atleast for the systems considered here. We conclude thatit is not the EDL that makes the plasma-modified PMMAsurfaces hydrophilic; rather, the main contribution ofhydrophilicity results from the variation of the totalsurface tensions γsv and γsl.

Surface roughness may also affect the wettability ofsurfaces. On very rough surfaces, contact angles aregenerally larger than those on smooth and chemicallyidentical surfaces.17 Here, we employed an optical pro-filometer to investigate the surface topography of thePMMA samples before and after plasma treatment. Figure9 indicates that the roughness of the surface is about 6nm and does not increase appreciably with time up to 10s of plasma treatment. The root-mean-square (rms)average of the measured height deviation for the sampleupon 50 s of treatment increased from 6 to 16 nm. It isapparent that prolonged treatment time can result in anincreased etching effect via oxygen plasma. Based on theresults in Figure 3, variation of 10 nm in roughness doesnot have a significant effect on the wettability: the contactangle remains nearly 51.0° as the treatment time changedfrom 10 to 50 s.

Conclusions

We have modified the surfaces of PMMA by means ofa dc pulsed oxygen plasma. Polar functional groups suchas carbonyl and carbonate groups were introduced intoPMMA, and their presence was confirmed by XPS analysis.The water contact angle of PMMA surfaces increasedrapidly within the first 5 s of plasma treatment andremained nearly unchanged after prolonged treatmentup to 50 s. We also found that variation of the zeta potentialfor the plasma-treated PMMA surfaces behaves similarlyto that of contact angles. The increase in the negativezeta potential and surface charge density suggest thatcharge state and dissociation of polar functional groupswere created on the surfaces when in contact with liquid.The plasma modification results presented here correlatedwell with wettability, zeta potential, and chemical struc-ture of the treated PMMA. We also concluded that thecontribution of the EDL effect to the solid-liquid inter-facial tension was negligible for the systems considered.

Acknowledgment. This research was supportedfinancially by the Natural Sciences and EngineeringResearch Council (NSERC) of Canada through a Discoveryand Research Tools and Instrument grant, the CanadaResearch Chair Program (CRCP), the Alberta IngenuityEstablishment Fund, and the Canada Foundation forInnovation (CFI). J.C. acknowledges financial supportfrom Alberta Ingenuity through a studentship fund.

LA048947S

Figure 9. Surface topography of plasma-treated and untreatedPMMA.

Oxygen Plasma Modified PMMA Langmuir, Vol. 20, No. 25, 2004 10927