Plasma polymerization of acrylic acid onto polystyrene by cyclonic plasma at atmospheric pressure

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Plasma polymerization of acrylic acid onto polystyrene by cyclonic plasma at atmospheric pressure

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Plasma polymerization of acrylic acid onto polystyrene by cyclonic plasma at atmospheric pressure

Yi-Jan Chang, Chin-Ho Lin, and Chun Huang*

Department of Chemical Engineering and Materials Science, Yuan Ze University, Chungli 32003, Taiwan

*E-mail: [email protected]

Received March 17, 2015; accepted August 18, 2015; published online November 16, 2015

The cyclonic atmospheric-pressure plasma is developed for chamberless deposition of poly(acrylic acid)film from argon/acrylic acid mixtures. The photoemission plasma species in atmospheric-pressure plasma polymerization was identified by optical emission spectroscopy (OES). The OES diagnosis data and deposition results indicated that in glow discharge, the CH and C2 species resulted from low-energy electron-impact dissociation that creates deposition species, but the strong CO emission lines are related to nondeposition species. The acrylic acidflow rate is seen as the key factor affecting thefilm growth. Thefilm surface analysis results indicate that a smooth, continuous, and uniform surface of poly(acrylic acid)films can be formed at a relatively low plasma power input. This study reveals the potential of chamberlessfilm growth at atmospheric pressure for large-area deposition of poly(acrylic acid)films. ©2016 The Japan Society of Applied Physics

1. Introduction

In manufacturing, low-temperature, fast, and environmentally friendly thin-film deposition is a vital process. Plasma polymerization has become an effective process for thin-film deposition in various academic=industrial areas^1,2) and for advances in the proliferation of interfaces on different composite materials.³^–⁵⁾ This is because of the specific interaction of plasma-polymerized films with polymeric substrates, and the good adherence to various substrates.

Besides the advantages of plasma-polymerizedﬁlms, it is a challenging task to achieve large-area deposition with the desired chemical surface features. The capability of atmospheric-pressure plasma in large-area plasma polymerization to achieve these requirements has been demonstrated.^6,7) However, because of the high gas temperature in the discharge, the usage of atmospheric-pressure plasma discharges has yet to be completely realized compared with low- pressure plasma discharges. As a consequence, low-temperature atmospheric-pressure plasma polymerization systems are required.⁸^–¹⁰⁾

During the past decade, a variety of low-temperature atmospheric-pressure plasmas have been developed, includ- ing those in a dielectric barrier discharge, atmosphere plasma torch, corona discharge, and atmospheric-pressure plasma jet.¹¹^–¹³⁾ Increasing the plasma size or extending the discharge gap between the electrodes to make atmospheric plasma more suitable has received much attention¹⁴^–¹⁶⁾ in these plasma systems. In this work, we describe a new type of atmospheric-pressure plasma with a low-temperature nature that makes it useful for depositing plasma-polymerized film on a heat sensitive polymer surface. This plasma is cyclonic in shape, which is suitable for low-cost continuous roll coating. Acrylic acid is a common monomer used in the formation of carboxylic-group containing-films in biointer- face-related applications. In this study, the cyclonic atmospheric-pressure plasma was used to deposit poly(acrylic acid) films. We will present experimental results on the formation of plasma polymerized acrylic acid (pp-AAc) by cyclonic atmospheric-pressure plasma. Atmospheric-pressure pp-AAc thinfilms will be formed using argon as carrier gas, aiming at large-area deposition. After the description of the experimental setup, results concerning the surface character-

istics of the thin ﬁlms obtained by the static contact angle method (CA), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and atomic forced microscopy (AFM) will be presented.

2. Experimental procedure

The cyclonic atmospheric-pressure plasma system shown in Fig. 1(a) was used to deposit pp-AAc thinfilms. This type of plasma is based on two rotating plasma jets that create a discharge cyclone to enlarge the deposited area. The double- pipe discharge jets used for the cyclonic atmospheric-pressure plasma polymerization are similar to that reported in Refs.17 and18. This atmospheric-pressure plasma cyclone is obtained in a gas compartment with two discharge jets placed a certain distance apart inside the gas compartment. The operational mode of cyclonic plasma discharge mainly depends on the rotation speed control. An electricalfield is applied to ignite the plasma glow discharge using a 13.56 MHz RF power supply. The capacitively coupled RF plasma source power is operated in the continuous mode (Advanced Energy DRESSLER CESAR 1310). The sample is mounted on an X–Y movable table in order to simulate in-line processing at variable line speeds. A diffusing glow gradually fills the reactor region. This indicates that the luminous gas phase can also be stabilized by applying the enhanced cyclonic gas flow to the discharge. The rapid cyclonic gasflow quenches thermal instabilities by the convective removal of energy dissipated in the discharge. Figure 1(b) shows the infrared thermal imaging of cyclonic atmospheric-pressure plasma.

The sensitivity of the infrared thermal measuring technique is 150 mK at a 298 K scene temperature. The radiant temperature proﬁle of cyclonic atmospheric-pressure plasma measured by infrared thermal analysis was similar to those measured by thermocouple thermometer analysis and is shown in Fig. 1(b) at low temperature (326–348 K). Argon at a gas ﬂow rate (15000 sccm) is introduced from the top of the plasma system and passes through the gas compartment as the ionization gas.

Liquid acrylic acid of 99% purity (Sigma-Aldrich) was directly introduced into the plasma gas compartment in a downstream region, near the outlet nozzle. Theﬂow rate was adjusted from 32 to 80 sccm.

The thickness of the cyclonic atmospheric-pressure pp- AAc thin ﬁlms was measured using an optical thin-ﬁlm

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thickness detector at a wavelength of 632.8 nm. The current and voltage waves of cyclonic atmospheric-pressure plasma were determined using an oscilloscope (TDS 1002B). The electrical power provided to the discharge was calculated using the Lissajous diagram.¹⁹⁾ The discharge power is directly proportional to the area of the ellipse in the diagram, and can be calculated as

Pave¼VmaxImax

2 cos:

The major plasma diagnostic apparatus of atmospheric- pressure plasma is an optical emission spectroscope (OES).

This equipment consists of both instrumentation and spectrum analysis software and was supplied by Hong-Ming Technology. The observable spectral range was 250–850 nm with a resolution of 2 nm. The water contact angles of plasma polymerizedfilms were measured by projecting an image of a sessile droplet resting on the film surface with a contact angle goniometer model 100SB by Sindatek Instruments. The chemical structure of cyclonic atmospheric-pressure plasma- polymerized acrylic acid films was characterized using the

FTIR spectrometer (Perkin-Elmer Spectrum 100). Each spectrum was obtained from an average of 256 scans in the range of 650–4000 cm⁻¹ at a resolution of 4 cm⁻¹. The surface morphology of cyclonic atmospheric-pressure pp- AAc films were examined by SEM. SEM analysis was performed with a JEOL JSM-5600 scanning electron spectroscope. A tungsten filament was used as the electron source. A 5 keV accelerator voltage was used for scanning the sample surfaces. The surface roughness of cyclonic atmospheric-pressure pp-AAcfilms were examined by AFM, using a SPM-9500 scanning probe microscope.

3. Results and discussion

The applied voltage–current (V–I) waveforms of cyclonic atmospheric-pressure plasma for various plasma power levels are shown in Fig. 2(a). Obviously, the number of micro- discharge pulses at a high RF plasma power is much higher than that at a lower RF plasma power. The number of microdischarges is directly related to the pollutant decomposition via low-temperature plasma. The Lissajousﬁgures of the discharge are shown in Fig. 2(b) and can also be used to (a)

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Fig. 1. (Color online) (a) Schematic diagram of cyclonic atmospheric-pressure plasma system, and (b) the temperature proﬁle of infrared thermal imaging.

(a)

-300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300

Voltage (V)

Times (s)

Current (mA)

150W

-1750 -1500 -1250 -1000 -750 -500 -250 0 250 500 750 1000 1250 1500 1750

-1.50E-007 -1.00E-007 -5.00E-008 0.00E+000 5.00E-008 1.00E-007 1.50E-007 -1.50E-007 -1.00E-007 -5.00E-008 0.00E+000 5.00E-008 1.00E-007 1.50E-007

-300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300

Times (s)

Voltage (V) Current (mA)

75W

-1750 -1500 -1250 -1000 -750 -500 -250 0 250 500 750 1000 1250 1500 1750

-300 -200 -100 0 100 200 300

-1500 -1000 -500 0 500 1000 1500

Current (mA)

150 W 125 W 100 W 75 W 50W

Voltage (V) (b)

Fig. 2. (Color online) (a) Voltage and current waveforms. (b) Lissajous characteristic curves for cyclonic atmospheric-pressure plasma.

Jpn. J. Appl. Phys.55, 01AB05 (2016) Y.-J. Chang et al.

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distinguish the variances of the discharge characteristics for the mentioned plasma power input effect. The shapes of the Lissajousfigures containing multiple pulses are conventional parallelograms, and the discharge transition (corresponding to the perpendicular edges of the Lissajous figures) shows a region where the data points form a vertical line, corresponding to the peak in the current waveforms. The area of this parallelogram was equal to the energy dissipated during one period of the voltage.²⁰⁾ Figure 2(b) shows that more than 90% of the input power was coupled into the cyclonic atmospheric plasma when the input power was 150 W and the calculated power was 141 W.

To detect the photo emitting species, and thus indirectly measure the chemical composition of the glow discharge of the cyclonic atmospheric-pressure plasma deposition system, OES was used as a plasma diagnostic. Optical emission analysis of the cyclonic atmospheric-pressure plasma shows that the dominant argon emission lines are observed at 700–800 nm, as shown in Fig. 3(a).²¹⁾The emission lines of the molecular nitrogen bands, between 300–400 nm, are also seen in the optical emission spectrum. When the acrylic acid was introduced as the monomer into the cyclonic plasma system, an obvious color change (purple–pink) was observed, as shown in Fig. 3(a). OES analysis of atmospheric-pressure plasma polymerization shows that this color change corre- sponds to the possible deposition of Ar⁺, N2+ and plasma polymer forming species, such as CH and C2 species. This

result indicates that the films are generated mainly by the decomposition of the acrylic acid (AA) monomer in the plasma. Figure 3(b) shows the wide-scan optical emission spectra (300–800 nm) for various AA monomer flow rate input to the plasma system. It was also detected that, when the monomer was added into the cyclonic plasma, the whole optical emission intensity, which was attributed to argon, was significantly quenched, as shown in Fig. 3(b). Meanwhile, three new lines located at 431, 486, and 514 nm, which were attributed to CH and C₂species. Considering that the opening of the C=C bond requires only 2.74 eV, whereas 3.61, 3.64, and 7.55 eV are correspondingly required for C–C bond, C–O bond, and C=O bond dissociation, it is easier to break the C=C bonds.²²⁾ OES analysis specified that the atmospheric- pressure plasma polymerization proceeds primarily via an opening of the C=C bonds, which leads to active species with a high retention of the carboxylic acid groups that are grafted on the surfaces of polystyrene. Figure 4 illustrates the acrylic acid monomerflow rate dependence on the specific emission intensities of cyclonic atmospheric-pressure plasma polymerization. In Fig. 4, the normalized emission intensities of CH and C₂ emission lines are shown. The rising emission intensities can be considered to correspond to the depositing contribution in the plasma and it is stabilized with increasing monomerflow rate as well. It suggests that, as expected, the dissociation of acrylic acid in the plasma is enhanced when the monomer input is increased.

Table I shows Yasuda parameters of the deposited= polymerized acrylic acid thin ﬁlm by cyclonic atmospheric

Ar Ar+AAc

Intensity (arb.unit)

Wavelength (nm)

C₂,CO C₂ OH

N₂ N₂

N₂ C₂

CCN₃CH

Ar

N₂ N₂ N₂ OH

O O Ar

(a)

300 400 500 600 700 800

32 sccm 48 sccm 64 sccm 80 sccm

Ar

Intensity (arb.unit)

Wavelength (nm)

(b)

Fig. 3. (Color online) Optical emission spectra from cyclonic atmospheric-pressure plasma.

0 1 2 3 4 5

I/I Ar-763 nm (arb.unit)

AAc flow rate (sccm)

C₂-385 nm C₃-405 nm C₂-461-471 nm C₂-515 nm

(a)

32 48 64 80

0.0 0.5 1.0 1.5 2.0

CN-419 nm CH-431 nm CO-561 nm

AAc flow rate (sccm) I/I Ar-763 nm (arb.unit)

(b)

Fig. 4. (Color online) Optical emission intensity dependence of C₂lines, C₃lines, CH lines, CN lines, and CO lines on AA monomer gasﬂow rate.

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pressure plasma. The identical effects of plasma power input and monomer concentration on plasma polymerization can be combined by taking into account the Yasuda parameter W=FM, where W stands for plasma power,F for monomer flow rate, and M for the molecular weight of the monomer.^23,24) W=FM is an apparent input energy per unit of monomer molecule and is therefore considered to be proportional to the concentration of polymer-forming=activated species in the plasma. It can be said that each monomer can gain further energy when the plasma power is increasing at a constant monomer flow rate. From this point of view, the progress of plasma polymerization can be evaluated using Yasuda parameters:

FMðg=minÞ ¼ flow rateðcm²=minÞ

24453:135ðcm²=molÞ72:06g=mol: Table I reveals the energy-deﬁcient domain of the cyclonic atmospheric pressure plasma of argon=acrylic acid mixtures, and the plasma polymerization behavior are found to be virtually identical. The energy-deﬁcient domain is observed upon an increase in the monomer rate and plasma power with increasing FM and W=FM, where more polymer-forming species are created with increasing energy density.

Figures 5(a) and 5(b) show the static contact angles of deionized water and diiodomethane of the cyclonic atmospheric-pressure plasma pp-AAcﬁlm on polystyrene (PS).

It can be seen that the deposited PS has a static water contact angle of 92°, which makes it hydrophobic for biomedical use.

In contrast, the cyclonic atmospheric-pressure plasma pp- AAc PS showed improved hydrophilicity with a static water contact angle lower than 40°, which is due to the plasma polymer deposition on the PS surface. For improving hydrophilicity of PS, a higher plasma power was more eﬀective than higher monomer gasﬂow rate as seen in Fig. 5.

Table II shows a summary of surface energies of pp-AAc surfaces. The surface energies of cyclonic atmospheric- pressure plasma pp-AAc ﬁlms exhibit stable high values (70–78 mJ=m²). These results show that cyclonic atmospheric-pressure plasma pp-AAc ﬁlm was deposited on the PS surface and enhances the hydrophilicity of PS.

The cyclonic atmospheric-pressure plasma pp-AAc films were characterized as hydrophilic and transparent films. The film growth can be clarified from the radical generation, i.e., with higher monomerflow rate more radicals are produced, which leads to an increase in thefilm growth. In this case, the number of deposition-forming radicals in the plasma grows with an increase in the monomer.²⁵⁾ For this reason, the

production of deposition-forming radicals contributing to the deposition would enhance theﬁlm growth and hydrophilicity.

The FTIR analysis of the plasma-polymerized heat-sensitive PS substrates provided further evidence for the presence of high retention of carboxylic acid group, as shown in Fig. 6.

It shows FTIR spectra of a blank PS substrate and cyclonic atmospheric-pressure plasma pp-AAc PS. The peaks that appeared at 1,370 cm⁻¹, and 1,455 cm⁻¹ are due to CH₃ bending band, and CH₂ bending band. These absorption peaks belong to the polystyrene substrates.²⁶⁾ It is apparent that many new absorption bands appeared for cyclonic atmospheric-pressure plasma polymerized ﬁlms. The stron-

Table I. The deposition parameters of poly(acrylic acid)ﬁlm.

Power (W)

Flow rate (sccm)

FM (g=min)

W=FM (MJ=kg⁻¹)

100 32 0.0943 63.6

100 48 0.1414 42.4

100 64 0.1886 31.8

100 80 0.2357 25.5

75 80 0.2357 19.1

100 80 0.2357 25.5

125 80 0.2357 31.8

150 80 0.2357 38.2 ⁰

10 20 30 40 50 60 70 80 90 100

Water Diiodomethane

AAc flow rate (sccm)

Untreated PS (Diiodomethane) Untreated PS (Water)

Contact angle (deg.)

(a)

32 48 64 80

75 100 125 150

0 10 20 30 40 50 60 70 80 90 100

Water Diiodomethane

Power (W)

Untreated PS (Diiodomethane) Untreated PS (Water)

Contact angle (deg.)

(b)

Fig. 5. (Color online) Static contact angles of deionized water and diiodomethane of atmospheric-pressure plasma pp-AAc thinﬁlms on PS.

(a) Monomer gas eﬀect. (b) Plasma power eﬀect.

Table II. Surface free energy, polar composition and nonpolar composition of diﬀerent parameters of poly(acrylic acid)ﬁlm.

Power (W)

Flow rate (sccm)

Surface energy (mJ=m²)

Polar component

(mJ=m²)

Dispersive component (mJ=m²)

100 32 70.5 25.6 44.9

100 48 69.5 25.4 44.1

100 64 72 28.8 43.2

100 80 72.7 29.2 43.5

75 80 70.6 27.0 43.6

100 80 72.7 29.2 43.5

125 80 78.0 33.0 45.0

150 80 78.1 34.0 44.1

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gest absorption in the spectrum at 1713 cm⁻¹(C=O stretch) is indicative of the presence of a high level of the carboxylic acid group.²⁷⁾In addition, a broad absorption bands appeared at 3400–3600 cm⁻¹ and was assigned to the OH stretching vibrations that originate from the carboxylic groups. The acrylic acid monomer absorption due to alkene absorption bands at 1636–1642 (C=C stretch), 986–995 (trans- CH=wag) and 912 cm⁻¹ (CH₂=wag) was not apparent, which indicated that the opening of the C=C bonds occurred during plasma polymerization process. This result is in agreement with OES measurements that have shown that higher monomer input induces more monomer fragmentation in the plasma polymerization; therefore, less retention of the carboxylic groups is observed in the plasma-polymerized ﬁlm.

SEM offers a direct method for identifying the surface morphology of cyclonic atmospheric-pressure plasma pp- AAc thin films. Figure 7 shows the SEM images of PS and pp-AAc PS. Cyclonic atmospheric-pressure plasma polymerized acrylic acidfilms show continuous and smooth surface morphologies. The SEM result indicates that cyclonic atmospheric-pressure plasma results in less ion bombardment damage in the plasma polymerization. This result supports the implication that dissociation precedes ionization in atmospheric-pressure plasma polymerization. AFM analyses were used to examine the change in the surface morphology on cyclonic atmospheric pressure plasma pp-AAc PS. The vertical view of the films in Fig. 8 shows that cyclonic atmospheric-pressure plasma pp-AAc PS obtained had an

4000 3500 3000 2500 2000 1500 1000 80 sccm

48 sccm

Transmittance (%)

Wavenumber (cm-¹)

OH

Untreated PS

C=O C-O-C

(a)

4000 3500 3000 2500 2000 1500 1000 150W

100W

Transmittance (%)

Wavenumber (cm-¹)

OH

Untreated PS

C=O C-O-C

(b)

Fig. 6. (Color online) FTIR spectrum of atmospheric-pressure plasma pp- AAc thinfilms on PS. (a) Monomer gas effect. (b) Plasma power effect.

(a)

(b)

Fig. 7. SEM images of (a) untreated PS and (b) cyclonic atmospheric pressure pp-AAc PS.

(a)

(b)

Fig. 8. (Color online) AFM images of (a) untreated PS and (b) Cyclonic atmospheric pressure pp-AAc PS.

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increasingly rougher surface than the unpolymerized PS. This result reaﬃrms that poly(acrylic acid) thinﬁlm was success- fully deposited on the PS surface.

4. Conclusions

In this paper, the possibility of depositing poly(acrylic acid) thin film using low-temperature cyclonic plasma created under atmospheric pressure has been reported. The relation- ship between the polymerized acrylic acid film growth and atmospheric pressure plasma was examined. In this cyclonic plasma technique, there is a synergy between plasma reactivity and temperature, both work to deposit poly(acrylic acid) films. The energetic character of the plasma discharge was demonstrated by the specific distribution of excited species and temperatures in the cyclone. Less thermal effect can be considered to be work in this cyclonic plasma polymerization system. In particular, increasing the acrylic acid monomerflow rate resulted in increasedfilm formation.

The atmospheric-pressure pp-AAc ﬁlms have hydrophilic features with high surface energy. High peak intensities of the C=O and OH groups in theﬁlm structure were found by FTIR analysis. Therefore, it is concluded that this cyclonic atmospheric-pressure plasma method represents a practicable technique for low-temperature deposition.

Acknowledgment

The authors are thankful for the support from the Ministry of Science and Technology under grant 103-2221-E-155-065 and MOST 104-2221-E-155-050.

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