201 (2007) 4854–4857www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

PAA polymerized downstream of a high pressure DBD plasma

Qiang Chen ⁎, Yabo Fu, Yuefei Zhang, Yuanjing Ge

Laboratory of Plasma Physics and Materials, Beijing Institute of Graphic Communication, Daxing, Beijing, 102600, China

Available online 30 October 2006

Abstract

The aim of this work is to develop functional materials by using a high pressure dielectric barrier discharge (DBD). As an exploration thepolymers were deposited downstream of the DBD plasma. For this purpose the plasma polymer obtained from a purely organic compound, acrylicacid (AA), for the carboxyl group film as example was characterized. The properties of the coating were determined by the contact anglemeasurement, Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). It revealed that the contact angle could beminimized ca. 3° in high pressure plasma polymerization film, and the density of the carboxyl group was retained as high as ca. 33.3% at 2200 Paworking pressure but depending on the working pressure.© 2006 Published by Elsevier B.V.

PACS: 52. 75. Hn; 52. 77. Fv; 52. 80. Hc; 52. 80.WKeywords: High pressure DBD; Functional group; Downstream

1. Introduction

With the quick and eager development of assay technologies,especially in bio-assay, it requires the development of small, fastand easy-to-use devices as well as various new strategies fortoxicant, harmful reagent, heavymetals such asHg2+, Cd2+, Pb2+,Cu2+and Zn2+ and glucose as well as DNA diagnostics [1–3].

In general there are two key issues for sensitive sensor: thereceiver and the signal transduction system. The conventionalchemical or physical immobilized receiver now is challenged byplasma polymerization films which demonstrate a significantrole in the assay composition due to their specificity, speed,simplicity and reusability. For example the wide-scale plasmapolymerization functional groups of carboxylic and amino toimmobilize antibodies for antigen–antibody assay were ex-ploited in many laboratories [4–7].

Generally, for a high density of functional groups the filmswere polymerized in a small power of continuous wave (CW)plasma [8] or in the prolonged plasma-off time pulsed plasma inlow pressure, mostly in radio frequency (RF) low pressureplasma where the high purity of functional groups is indeedinsurable [9]. But the discharge stability, the surface uniformity,roughness, the film stabilization and the simple process still

⁎ Corresponding author. Tel.: +86 10 6026 1099; fax: +86 10 6026 1108.E-mail address: chenqiang@bigc.edu.cn (Q. Chen).

0257-8972/$ - see front matter © 2006 Published by Elsevier B.V.doi:10.1016/j.surfcoat.2006.07.217

need to be exploited [10,11]. It is well-known that the gaseousdischarge at a high pressure, besides simplifying the plasmaequipment and cutting the budget in the process, can provideplenty of activation radicals depositing the polymer in a highrate. But in high pressure, the intrinsic characteristics of thedischarge transferring into filamentary or stream discharge,which depend on the gas flow rate, discharge pressure, and gasas well as power sources, still need to pay more attention forobtaining a uniform and homogeneous film. However, down-stream, a great number of long life radicals dominates thus thepossibility to form a uniform and homogeneous film with a highretention of functional groups maybe realize.

In this paper, we presented a high pressure plasma in whichthe controversy of surface uniformity and high density offunctional groups is solved by polymerizing process down-stream of the DBD plasma through the use of a grounded gridelectrode. The properties of film polymerized downstream werecharacterized by the contact angle, Fourier transform inferred(FTIR) spectroscopy and X-ray photoelectron spectroscopy(XPS), which were also presented in this paper.

2. Experimental

Polymerization was carried out downstream of a highpressure DBD discharge reactor as Fig. 1 shows, in which theplasma is driven by 100 kHz high frequency power and, the

Fig. 3. The FTIR spectra of PAA polymerized downstream of the DBD plasmain the different working pressures (KBr wafer, 3.6 kV discharge voltage).

Fig. 1. The schematic diagram of plasma setup for high pressure polymerization.

4855Q. Chen et al. / Surface & Coatings Technology 201 (2007) 4854–4857

distances between the power electrode and the grounded gridelectrode are 2 mm, and 3 mm for the substrate to the groundedgrid electrode downstream, respectively. Before putting in thechamber, substrates were cleaned in acetone ultrasonically for5 min, and then dried through nitrogen flowing. Furthermore inthe chamber the substrates were cleaned again by Ar plasma for5 min prior to the monomer flowing into the reactor.

In polymerizing process, commercial acrylic acid was used asprecursor, which is not further purified after buying fromAldrich. The polymerizations were performed at 300 Pa, 500 Pa,1000 Pa, 2000 Pa and 2200 Pa working pressures. It is noted thatin the FTIR analysis (Shimadzu, FTIR-8400/8900, Japan) theKBr wafers were used after the film was polymerized on thesurfaces. But the Si substrate was employed in the XPS studied,where the XPS data were obtained using a VG CLAM2 spec-troscopy (Vacuum Generations, East Grinstead, U.K.) utilizingAl Ka radiation at 1486.6 eV with a pass energy of 20 eV givinga resolution of 0.2 eV for C1s. The contact angle measurementof a distilled water droplet of 2 μL lying on the surface wascarried out by syringe in JY-200, China, on a slide glass justafter the film was polymerized. All angles were averaged for 5measurements.

3. Results

Fig. 2 shows the relationship of the contact angles of plasmapolymerization poly(acrylic acid) (PAA) on the slide glass

Fig. 2. The influence of working pressure on the contact angle in plasmapolymerization PAA surfaces.

substrates to the working pressures. The remarkable depen-dence of the contact angles on the polymerization pressure canbe noticed, i.e. greatly decreasing with the increase of workingpressure. The water contact angle was 14° at 300 Pa workingpressure whereas it reached only ca. 4° at a higher pressure suchas 2000 Pa.

Fig. 4. The C1s XPS spectra of PAA polymerized downstream with differentworking pressures (3.6 kV discharge voltage).

Table 1C1s XPS analyses of film polymerized at high pressures on Si wafers

Peak number 5 4 3 2 1

EB (eV) 288.1 286.8 289.4 285.5 285.0Composition (at.%)

Theory [16] – – 25 25 50300 Pa 0.01 6.71 27.3 14.8 51.31100 Pa 0.01 9.76 33.4 10.7 46.02200 Pa 0.01 11.1 33.3 12.4 43.2

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The contact angle measurements are consistent with the FTIRspectra. Fig. 3 shows PAA films were indeed polymerized onthe substrates downstream of the DBD plasma at high pres-sures. The main bands of the PAA film were observed at3400 cm−1 for O–H stretching, 2900 cm−1 for –CHx sym-metrical stretching and 1720 cm−1 for C_O stretching. Thefigure also illustrates that the main peak at 3000–3500 cm−1

for COOH/R stretching was significant in the spectra but be-coming narrower and narrower when working pressures weredecreasing, where the peak at 1720 cm−1 for C_O stretchingwas always weakening. A closer investigation of the FTIRspectra showed that in the 2000 Pa working pressure the mostfunctional group –COOH/R was retained from the parentmonomer after comparing the spectrum with the acrylic acidprecursor, the main peaks are much similar.

XPS analysis clearly clarifies that the high density of the –COOH/R group was retained in high pressure polymerizationfilms after the Gaussian fitted C1s and O1s spectra as Fig. 4 andTable 1 show. According to Refs. [12,13], the high resolution ofC1s atom was deconvoluted at 285 eV for hydrocarbon (C–C/H),at 285.5 eV for carbon in aβ-shift bond to carboxylate, at 286.8 eVfor an alcohol or ether carbon (C–OH/R), at 288.1 eV for carbonyl(C=O) and at 289.4 eV for carboxylic acid or ester (COOH/R),and O1s atomwas deconvoluted at 532.3 eV for the C_O groupsand 533.9 eV for theC–Ogroups. It is found that the proportion ofcarbon in a COOH/R environment reached a maximum at 33.5%in 1100 Pa working pressure and steadied at higher workingpressures. Comparison with the results in small power CW orprolonged plasma-off time pulsed RF plasma [13], the concen-tration of the –COOH/R functional group is obviously higher.

4. Discussion

In biological strategy the objective is to produce a surfacecontaining a high density of –COOH functional groups for thecovalent immobilization of biomolecules with a super hydro-philicity or a polar surface which is considered one of the majorreasons for the cells' or bacteria's adhesion/absorption, growth,proliferation [14].

Not like in a small power CW plasma or pulsed plasmapolymerization, the polymerization of high density functionalgroups in this experiment may benefit from both the highpressure and downstream of the DBD plasma. From the inverserelationship of plasma power and the density of the functionalgroup in CW plasma, it could be summarized that positive ionsmay contribute directly to polymer growth, whereas in pulsedplasma the significant role played by radical chemistry or ionic

reaction is not yet clear even if most functional groups grew onsurfaces at a long plasma-off time. At high pressure, however,especially downstream of the DBD plasma it seems to beundoubted that radical chemistry dominates film growth due tothe major composition of active radicals in this area. Besides,high pressure causes a short mean free path and a relativelysmall ionization rate, downstream the ions generated in thedischarging zone shall be neutralized in the ground grid beforearriving at the substrates. Therefore in general the polymeriza-tion reaction is mostly attributed to the long life radicals, i.e.radical chemistry either in the gas phase or in the gas–solidinterface contributes to the polymer growth downstream. Thehigh pressure leading to the PAA chemical structure depen-dence on the working pressure is dominantly indicated in FTIRspectra. The peak at 3400 cm−1 broadening in the higher pres-sure such as in 2000 Pa attributes to the small/no fragmentized –COOH/R group retaining in the chemical chain whereas in300 Pa working pressure the relatively high ionization rateobtained a narrow peak at 3400 cm−1 and higher density of C–C/H formed on the surface, which decrease the proportion ofacid in the film.

5. Conclusion

Downstream of the DBD plasma source PAA coatings fromacrylic acid has been polymerized in a high pressure. It is foundthat –COOH/R functional material PAA can be perfectlypolymerized on the substrate and forms a contact angle ca. 4°super hydrophilicity, which the chemical structures changedwith the working pressures. It can be noticed that the polymerdeposited at high pressure was favorable to retain more func-tional groups on the surface. It can be concluded that highpressure polymerization downstream of the DBD plasma canprovide an alternative for biological strategy with an intensityfunctionized surface.

Acknowledgements

This study was supported by a Grant from the Nature ScienceFoundation Committee (NSFC) in China (10475010). Theauthors would also like to thank Dr. Nan Jiang, Institute ofPhysics, Chinese Academy of Sciences, for the kind help inoffering the plasma setup.

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