Surface modification of polystyrene with atomic oxygen radical anions-dissolved solution

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Applied Surface Science 254 (2008) 4191–4200 www.elsevier.com/locate/apsusc

Surface modification of polystyrene with atomic oxygen radical anions-dissolved solution Lian Wang a, Lifeng Yan a, Peitao Zhao b, Yoshifumi Torimoto c, Masayoshi Sadakata d, Quanxin Li a,* a

Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, PR China b Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Science, Hefei, Anhui, PR China c Oxy Japan Corporation, 7# Floor, Miya Building, 4-3-4, Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan d Department of Environmental Chemical Engineering, Kogakuin University, 2665-1, Nakano-machi Hachioji-shi, Tokyo 192-0015, Japan Received 8 August 2007; received in revised form 31 December 2007; accepted 1 January 2008 Available online 16 January 2008

Abstract A novel approach to surface modification of polystyrene (PS) polymer with atomic oxygen radical anions-dissolved solution (named as O water) has been investigated. The O water, generated by bubbling of the O (atomic oxygen radical anion) flux into the deionized water, was characterized by UV-absorption spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. The O water treatments caused an obvious increase of the surface hydrophilicity, surface energy, surface roughness and also caused an alteration of the surface chemical composition for PS surfaces, which were indicated by the variety of contact angle and material characterization by atomic force microscope (AFM) imaging, field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), and attenuated total-reflection Fourier transform infrared (ATR-FTIR) measurements. Particularly, it was found that some hydrophilic groups such as hydroxyl (OH) and carbonyl (C O) groups were introduced onto the polystyrene surfaces via the O water treatment, leading to the increases of surface hydrophilicity and surface energy. The active oxygen species would react with the aromatic ring molecules on the PS surfaces and decompose the aromatic compounds to produce hydrophilic hydroxyl and carbonyl compounds. In addition, the O water is also considered as a ‘‘clean solution’’ without adding any toxic chemicals and it is easy to be handled at room temperature. Present method may suit to the surface modification of polymers and other heatsensitive materials potentially. # 2008 Elsevier B.V. All rights reserved. Keywords: Polystyrene; O water; Surface modification; Hydrophilicity

1. Introduction Polymers have become important materials in various technological applications, as they have many unique advantages such as their high flexibility for the fabrication and patterning with different shapes and thicknesses, low density, and low manufacturing cost etc. Polystyrene (PS) is one of the important polymers and has been used as biomedical material and optical material [1–3] etc. However, one of troublesome problems for PS is its hydrophobicity and low surface energy that induces poor adhesion and coating

* Corresponding author. Tel.: +86 551 3601118; fax: +86 551 3606689. E-mail address: [email protected] (Q. Li). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.035

properties [4]. Accordingly, surface modification is necessarily performed to make the hydrophobic surfaces into hydrophilic ones. Many methods for surface modification of polystyrene have been explored such as gas plasma treatment [5,6], chemical vapor deposition [7], ion-beam bombardment [8,9], ultraviolet (UV) treatment (laser and lamp) [10–12], and the alteration of surface roughness at nm-scale [13,14], etc. Among these methods, plasma treatments, ion-beam bombardment, and UV treatments are attractive for their high efficiency. The increase of surface roughness at nm-scale via topographical cues is also used as a possible type of surface modification, which can increase the surface hydrophilicity and adhesion of PS [13,14]. Chemical treatments seem to be more flexible in surface modification of polymer. However, some chemical ways are needed to perform at higher temperature and/or in

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strongly acidic or basic conditions, in the presence of a large quantity of volatile organic solvents [15]. Active oxygen species such as hydroxyl radical (OH), superoxide (O2), and peroxide radical (OOH) have strong chemical reactivity [16,17]. For example, it is reported that active oxygen species produced by the combination of ozone aeration and UV irradiation have been used for improving the hydrophilicity and adhesion properties of polymer surface significantly [18–20]. O is also an important active oxygen species and has very strong oxidation ability in chemical reactions [21,22]. O is a monovalent anion (or monovalent negative ion) through the attachment of an electron to atomic oxygen (O). At the same time, O is also considered as a radical because it has an unpaired electron in its outmost orbit. O may be one of the most active oxygen species and therefore has various potential applications such as the chemical synthesis [23,24] and the thin film oxidation [25,26]. The conventional method for generating O is the attachment of a free low-energy electron to atomic oxygen or is through negative ion/molecule reactions that can occur in following processes: plasma process, electron impact process, or laser irradiation on molecules in gas phase. Atomic oxygen radical anions generated by above methods are generally accompanied by the formation of other ion species. A new approach to generate pure and sustainable O flux has been developed by our research group [27–29], where O can be emitted from the anionic storage-emission material of [Ca24Al28O64]4+4O (C12A7-O). Recently, we have also synthesized the various derivatives of C12A7-O such as C12A7-OH [30,31] and C12A7-H [32,33]. More recently, we also found that the C12A7-O material or the modified ones would be practically used in a one-step synthesis of phenol from benzene [23], the reduction of NO [34], a fast inactivation of microorganisms [35], and the dissociation and oxidation of bio-oil [36]. Present work aims to investigate a new approach to surface modification of PS polymer with O water and to provide a flexible and environmentally friendly method for the surface modification of polymer. The O water was characterized by UV-absorption spectroscopy and EPR spectroscopy. The hydrophilicity, roughness, morphological alteration, and surface chemical composition of the modified samples were indicated by contact angle, AFM, FESEM, XPS, and ATRFTIR measurements. Furthermore, on the basis of these studies, the mechanism of the surface modification was discussed. 2. Experimental 2.1. Preparation of PS samples PS beads (Mw = 240,000) were purchased from Aldrich Chemical Co., UK and without further purification. PS beads were heated at 180 8C on a glass substrate [18]. The molten PS sandwiched between glass substrates was pressed to form a slice of 1–1.5 mm thickness. After being cooled at room temperature, the PS slice would be separated from the glass substrates.

2.2. Preparation of the O water The approach for generating pure and sustainable O flux has been developed previously [27,28], where O anions are emitted from the anionic storage-emission material of C12A7O. The C12A7-O material was synthesized by the solid-state reactions of CaCO3 and g-Al2O3 under flowing dry oxygen environment. The powders of CaCO3 and g-Al2O3 with the average particle diameter of 20–30 mm were mixed and grained at a molar ratio of CaCO3:g-Al2O3 = 12:7. Then, the samples were temperature-programmed to 1350 8C and sintered for 10 h under flowing dry O2 environment. Finally, the sintered samples were cooled to room temperature naturally. The structure of C12A7-O is characterized by a positive charged lattice framework [Ca24Al28O64]4+ including 12 sub-nanometer sized cages with a free space of about 0.4 nm in diameter [37]. The concentration of O stored in the bulk of C12A7-O was about 2.0  1020 cm3 [37]. On the other hand, the O stored in the bulk can be emitted into the gas phase and formed the gaseous O flux by heating the sample. Emission current density of O (i.e., beam intensity) strongly depends on the surface temperature of the C12A7-O materials [29]. The increase of the surface temperature will lead to the increase of anion diffusion rate in the material bulk and desorption rate of anions from the materials, which can be realized by using a heater, oven, or focused IR lamp. Generally, the surface temperature is set up ranging from 600 to 800 8C, depending on the beam intensity required. To increase the emission amount of O, the O flux was produced from two O generators (length: 120 mm, width: 120 mm, thickness: 8 mm) made by Oxy Japan Co. (Japan) in this work. The O generator is made up of three parts: a C12A7-O-coated film (50  10 mm), a ceramic support, and a Fe–Cr alloy filament heater embedded in ceramic layer (110 V/ 220 V, 750 W). The O emitted from the surface of the O generator was investigated by an anionic time-of-flight (TOF) mass spectrometer. The detailed conditions of TOF mass spectrometer in this contribution are the same as in our previous work [29,31]. The emission current of O was modulated by the output power of the O generator. The emission current of O was detected by a Keithley model 6485 picoammeter (USA). The O water was prepared by bubbling of the O flux into the deionized water. The apparatus setup for producing the O water was schematically illustrated in Fig. 1. The apparatus consists essentially of two O generators installed in the emission chamber, a carrier gas feeding/controlling section, and the collection system of O water, together with an anionic TOF mass spectrometer. The O water was prepared by bubbling of the O flux into the deionized water for a given duration, where the O flux was emitted from two O generators. In order to decrease the loss of O flux (e.g., quenched by the tube), the O flux was carried by a fast flowing inert gas (5 l/min of argon) and pumped by a minitype air pump (APN-110kv-1; Iwaki Co., Ltd, Japan). The flow rate of inert gas was indicated by a flow meter. In the O water preparation system of this study, the O generators worked at 750 8C and

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deionized water immediately after treatment. In addition, to clarify the effect of H2O2 in the resultant O water on polymer modification, the treatment with the diluted H2O2 in the same concentration with that in the O water was used as a comparison solution. 2.4. Surface characterization methods

Fig. 1. Schematic setup used for generating O water. The O flux was formed from two O generators, which was analyzed by an anionic time-of-flight mass spectrometer. The O water was prepared by bubbling of the O flux into the deionized water. The reference numerals in the figure: (1) carrier gas (argon), (2) carrier gas inlet, (3) O emission chamber, (4) C12A7-O-coated film (50  10 mm), (5) ceramic support, (6 and 7) O generator, (8) Fe–Cr alloy filament heater, (9 and 10) cooling-water system, (11) ion extraction electrode, (12) picoammeter, (13) O flux inlet, (14) deionized water, (15) glass bottle, (16) tail gas outlet, (17) minitype air pump, (18) skimmer, (19) ion accelerate electrode, (20) anion flight section, (21) tandem micro-channel plates, (22) amplifier, (23) digital oscilloscope, (24) computer-controlled counter system, (25) molecular pump.

the output power was 750 W. The flow rate of inert gas was 5 l/ min. The preparation duration of O water was generally 2–5 h. Electron paramagnetic resonance (EPR) measurements were performed to investigate the O concentration dissolved in the O water. EPR spectroscopy was conducted at 9.1 GHz (Xband) using a Bruker ER-200D spectrometer at 77 K. The gvalues of the O are about gxx = gyy = 2.036 and gzz = 1.994 [37,38]. Absolute O concentration dissolved in the deionized water was determined from the second integral of the EPR spectrum using CuSO45H2O as a standard [37]. A little amount of H2O2 was also generated in the O water, which could be measured by an UV spectrometer (UV-2401 PC; Shimadzu Co., Japan) with the absorption band of 190– 350 nm (maximum absorption at about 206 nm). The H2O2 concentration was measured by the absorbance at 206 nm and calibrated with 0.3 mmol/l H2O2 standard solution. 2.3. O water treatment To remove possible surface contaminants, the PS samples (20 mm  20 mm) used for studying the effects of O water were cleaned via acetone (about 2 min) and alcohol (about 2 min) in an ultrasonic bath, and then washed for three times with copious deionized water (about 30 min). It was confirmed that there was no significant influence on the surface chemical composition, surface functional groups, and morphological alteration of PS during the above pre-treatment process. The cleaned PS sample was then placed in a quartz tube with stopper (200 mm in length, 25 mm in inside diameter) containing 50 ml new-made O water. The PS samples were immersed in the new-made O water (typically, O concentration: 0.03  0.01 mmol/l; H2O2 concentration: 0.2  0.1 mmol/l) for a given duration. All treatments were conducted at 25.0  1.0 8C. The treated PS sample was rinsed with

The surface hydrophilicity and surface energies of the sample were quantified by measuring the water and formamide contact angles on the substrate of PS at room temperature with a contact angle meter (Model JY-82, China) by the sessile drop method. The deionized water and formamide, with the surface tension of 72.2 and 58.2 mN/m, respectively, were used as polar and non-polar solutions. The droplets (0.03 ml of the deionized water or the formamide) were dropped at five different sites on each sample, and the values of the contact angles were averaged. The values of contact angles were used to calculate the surface energies by combining a geometric mean approach and the Young’s equation [39,40], qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi D þ 2 gPgP ð1 þ cos uÞg L ¼ 2 g D (1) g S L S L where u is the measured contact angle of test liquid on the polymer surface and gL is the surface tension of the test liquid with its known dispersion component (g D L ) and polar compoP nent (g PL ). g D S and g S are the unknown dispersion and polar components of surface energy (gS) of the polymer surface, respectively. The polar force (g PL ) and the dispersion force (g D L) of the deionized water were 50.2 and 22.0 mN/m respectively, and g PL and g D L of the formamide were 18.6 and 39.6 mN/m, P respectively [40]. The g D S and g S of PS surfaces are estimated by solving the two equations set up for the measured contact angles of deionized water and formamide following Eq. (1). Surface roughness of sample was detected with an atomic force microscope (AFM, Nanoscope Sa, Digital Instruments) in tapping mode. The root-mean-square (RMS) Rq of the sample was calculated from the images obtained from the AFM [41]. For statistical analysis, roughness data were calculated as averages of five measurements obtained from each sample. In order to examine the morphological alteration, the surface of PS was coated with about 3 nm gold and was observed with a JSM-6700F field emission scanning electron microscope (FESEM) (JEOL, Tokyo, Japan). The surface elements were analyzed by X-ray photoelectron spectroscopy (XPS) (ESCALAB-250; Thermoelectron Co., USA). Mg Ka X-ray was used in XPS analysis. The background pressure of the analytical chamber was 1.0  109 Torr. The analyzed surface was 1 mm in diameter and the take-off angle of photoelectrons was 908 from the PS surface. Preliminary data analysis and quantification were performed using XPSPEAK 4.1 software. All binding energies were calibrated by reference to the centre of CC peak at 285.0 eV for PS [42]. Atomic concentration data were determined using evaluation of relative peak areas. For statistical analysis, these relative peak areas were calculated from five XPS spectra obtained from different locations of each sample.

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Functional groups on PS surfaces were examined by the ATR-FTIR spectra with Bruker Equinox 55 FTIR spectrometer using the attachment for the attenuated total-reflection method. 3. Results and discussion 3.1. Characteristics of O generator and O water The O water was prepared by bubbling of the O flux into the deionized water. To increase the emission amount of O, the O flux was produced from two O generators (total emission area: 288 cm2) that were coated with the anion storageemission material of C12A7-O. The anionic species emitted from the surface of the O generator were measured by an anionic TOF mass spectrometer. Fig. 2(a) shows a typical mass spectrum when output power of the O generator was 750 W.

The dominant peak was the mass number of 16, which corresponded to O. The emission purity of O, defined by the ratio of the O intensity to total one, was more than 95%. The emission current of O was modulated by the output power of the O generator. The maximum emission current of O is about 220  18 mA detected by a picoammeter when the O generator is at an output power of 750 W. Thus, the O generator can provide stable and pure O flux. EPR measurements were performed to investigate the concentration of O dissolved in the O water. As shown in Fig. 2(b), the spectrum was attributed to O based on its g value [37,38]. The O concentration in the O water was about 0.03  0.01 mmol/l when the O water was prepared by bubbling of the O flux (the output power of O generator was 750 W) into the deionized water (50 ml) for 2 h. In addition, no peak was observed in the EPR spectra for the deionized water (not shown). It was also found that a little amount of H2O2 had been formed, which was analyzed via UV-absorption spectroscopy and calibrated with a standard solution of 0.3 mmol/l H2O2 (Fig. 2(c)). Absolute concentration of H2O2 was determined by the absorbance at 206 nm, which was compared with that of the standard diluted H2O2. The H2O2 concentration in the O water was about 0.2  0.1 mmol/l when the O water was prepared by bubbling of the O flux (the output power of O generator was 750 W) into the deionized water (50 ml) for 2 h. According to the above investigation, it was observed that the active oxygen species such as O and H2O2 existed in the O water. The O water, containing O of 0.03  0.01 mmol/l and H2O2 of 0.2  0.1 mmol/l, was used for investigating the surface modification of PS samples in present work. The pristine PS, pre-treated PS, and PS treated via 0.3 mmol/l H2O2 were generally used for comparisons. 3.2. Effects of O water on surface properties of PS

Fig. 2. (a) Typical TOF mass spectrum from the C12A7-O-coated O generator at the output power of 750 W. (b) EPR spectrum used to investigate the O concentration dissolved in the O water. The symbol 400 in the figure stand for the amplified time of the EPR signal. (c) UV absorption spectra of a standard solution of 0.3 mmol/l H2O2 (dotted line) and the O water (solid line), respectively. The O water used in (b) and (c) was prepared by bubbling of the O flux (the output power of O generator was 750 W) into the deionized water (50 ml) for two hours.

3.2.1. Effects of O water on hydrophilicity and surface energy Surface hydrophilicity of the PS surfaces was investigated by the contact angle measurements before and after the O water treatment. The contact angles of PS were determined by the sessile drop technique using deionized water and formamide solution. The dispersion force (g D S ) and the polar force (g PS ) of the PS surfaces were calculated from the contact angles of water (polar) and formamide (non-polar) based on Eq. (1). The polar water contact angles (Fig. 3(a)), non-polar formamide contact angles (Fig. 3(b)) and the surface energies (Table 1) of the treated PS sample were measured as a function of treatment time. For the PS samples treated via the O water, the contact angle of the PS surface monotonously decreased with increasing of the treatment time. Both the water contact angle and the formamide contact angle of PS obviously decreased by 9.18 (from 88.38 to 79.28) and 9.78 (from 65.18 to 55.48) for 48 h treatments, respectively, indicating an obvious increase in the hydrophilicity of PS surface after treatment. Furthermore, it was also found that the contact angle of O-

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P energies (gS), the dispersion force (g D S ) and the polar force (g S )  of the PS polymers treated by O water and H2O2 for different treatment time were presented in Table 1. An increase in the surface energy was found for the O-water-treated PS samples. The mechanism of the alteration of contact angle and surface energy via the O water treatments may be due to the following reasons: (1) formation of hydrophilic functional groups, (2) change of surface roughness, and (3) morphological alteration on the PS surface, etc. In order to clarify the O water effects on the increase of hydrophilicity, we investigated surface roughness, morphological alteration, surface composition and surface chemical bonds of the treated PS surfaces by the AFM, FESEM, XPS, and ATR-FTIR measurements.

Fig. 3. (a) The polar water contact angles and (b) non-polar formamide contact angles of the PS surface treated by the O water were measured as a function of treatment time.

water-treated PS remained relatively unchanged during the 15day period of aging (not shown), which indicated that there were no appreciable changes in the modified surface properties of the samples during storage. As treated by the H2O2 solution with low concentration (0.3 mmol/l), no significant variation in the water contact angle and formamide contact angle of the PS surface was observed under present experimental conditions (Table 1). The surface energies of the pristine PS sample and the treated samples were estimated by contact angles. The surface Table 1 Contact angles and surface energies of the pristine PS surface, the PS surfaces treated by the O water and H2O2 solution for different treatment time, respectively PS samples

Time (h)

Pristine 

Contact angle (8)

Surface tension (mN/m)

Water

Formamide

g PS

gD S

gS

88.3

65.1

2.7

29.7

32.4

O -water-treated

4 8 12 24 36 48

87.1 86.3 84.5 82.5 81.4 79.2

64.2 63.0 61.2 60.2 58.1 55.4

3.1 3.2 3.7 4.6 4.6 5.2

29.5 30.3 30.6 29.7 31.3 32.3

32.6 33.5 34.3 34.3 35.9 37.5

H2O2-treated

12 24 36 48

86.5 88.0 89.0 87.5

63.2 65.0 65.2 64.4

3.1 2.8 2.4 2.9

30.3 29.5 30.3 29.7

33.4 32.3 32.7 32.6

3.2.2. Effects of O water on surface roughness and morphological alterations In order to reveal the correlation between surface hydrophilicity and surface roughness, the influence of the O water on the surface roughness of the PS surface was investigated by AFM measurements. The measured area was 30 mm  30 mm for all AFM images. Fig. 4(a–e) present representative AFM images of the pristine PS surface, pre-treated PS surface, PS surfaces treated by O water for 24 and 48 h, and PS surface treated by H2O2 for 48 h, respectively. The pristine, pre-treated, and H2O2-treated PS surfaces showed relative smoothness, whereas the surface of the O-water-treated PS sample became relative roughness (Fig. 4(c) and (d)). Table 2 shows the rootmean-square (RMS) Rq in each case, which was calculated from the corresponding AFM profile data [41]. The Rq of the pristine PS surface was about 183.4 nm. For pre-treated or H2O2-treated PS, no significant variation in the roughness of the PS surface was observed. The Rq values of the O-water-treated PS samples increased to 337.2 and 388.0 nm for the treatment time of 24 and 48 h, respectively. The AFM results indicated that the PS surface became rougher after the O water treatment. The morphological alterations of the treated PS surfaces were also observed by FESEM. The magnification of all FESEM micrographs was 20,000. Fig. 5(a) shows the FESEM image of the pristine PS samples where low density of defects presented. The FESEM image of the H2O2-treated samples appeared similar to those of the pristine samples (Fig. 5(d)). Fig. 5(b) and (c) refer to typical morphology of the PS surfaces treated by O water for 24 and 48 h, respectively. In contrast to the pristine PS surface, it is noticed that numerous defects and small voids appeared on the O-water-treated PS surfaces, which indicated that PS surface became rougher and accorded to the AFM observations. The small voids and defects distributed randomly with different shapes, which might arise from reactions between the active oxygen species and the PS constitutes followed by main-chain scission during the O water treatment. 3.2.3. Formation and destroy of chemical bonds by O water The variety in contact angle may be due to the formation of hydrophilic functional groups on the PS surface via the O

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Fig. 4. Representative AFM images of the PS (a) pristine, (b) pre-treated, (c) O-water-treated, 24 h, (d) O-water-treated, 48 h, (e) H2O2-treated, 48 h, respectively.

water treatment. Therefore, we investigated the surface elements and functional groups of the treated and untreated PS surfaces by XPS and ATR-FTIR measurements. The relative content (at.%) of surface elements was examined by the relative intensity of the C1s and O1s peaks obtained from the wide-scan XPS spectra, which is shown in Table 3. Based on the quantitative elemental composition analysis, relative oxygen content measured on the pristine PS surface was 4.7%. No significant variation in the oxygen content of pre-treated or H2O2-treated PS surface was observed. The small amount of oxygen observed on the pristine, pre-treated, or H2O2-treated PS surfaces may be due to depositing through atmospheric

oxidative degradation during the sample preparation or storage in the containing oxygen environment [43]. However, it was noticed that a remarkable increase of oxygen content was observed for the O-water-treated PS surface and changed to 11.5 and 13.1% for 24 and 48 h treatment, respectively. The above results suggest that oxygen-containing functional groups would be introduced onto the O-water-treated PS surfaces during the treatment. Fig. 6(a–e) shows the C 1s core-level XPS spectra of the pristine PS surface, pre-treated PS surface, PS surfaces treated by O water for 24 and 48 h, and PS surface treated by H2O2 for 48 h, respectively. One strong profile at lower binding energy

Table 2 Variations of roughness (Rq) of the pristine PS, pre-treated PS, PS surfaces treated by the O water for 24 and 48 h, and PS surface treated by the H2O2 for 48 h, respectively Pristine

Rq (nm)

183.4  20.1

Pre-treated

186.4  21.3

Treated by O water

Treated by H2O2

24 h

48 h

48 h

337.2  32.6

388.0  35.4

180.6  23.7

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Fig. 5. SEM images of the PS (a) pristine, (b) O-water-treated, 24 h, (c) O-water-treated, 48 h, (d) H2O2-treated, 48 h, respectively.

(near 285.0 eV) appeared in the C1s spectra for the pristine, pre-treated, and H2O2-treated PS, which could be deconvolved into three component Gaussian peaks: C–C/C–H at the binding energy of 285.0 eV, C–O at 286.6 eV, and a weak dilatory profile near 291.5 eV assigned to p–p* shake-up transitions in the aromatic ring of PS [44,45]. As shown in Fig. 6(c) and (d), the C1s core-level spectra for the O-water-treated PS were different from those of the pristine, pre-treated, or H2O2-treated PS surfaces. A longer trail from 286 to 290 eV was added into the strong band of 285.0 eV, which would be fitted by four components: C–C/C–H at 285.0 eV, C–O at 286.6 eV, C O bond at 288.0 eV, and O–C O bond at 289.1 eV [44,45]. The above results clearly showed that the new function groups such as the C O bond and O–C O bond were introduced onto the PS surfaces after the O water treatment. A more quantitative analysis of the variation of functional groups (evaluation of relative areas of these peaks) before and after the treatments is shown in Table 4. The relative contents of the C O bond, O–C O bond, and C–O bond on the O-watertreated PS surfaces increased with the increase of treatment time. Additionally, the p–p* shake-up at 291.5 eV after O water treatment reproducibly became weaker than that of the pristine PS. The intensity of p–p* shake-up is sensitive and generally proportional to the amount of unsaturated aromatic ring. Weaker shake-up peak reflects less aromatic rings on the

PS surfaces [9]. It was speculated that part of the aromatic rings on PS surfaces were damaged during the O water treatment, leading to a decrease of relative content of the unsaturated aromatic rings on the O-water-treated PS surfaces. The above XPS measurements revealed that oxygencontaining functional groups such as the C O bond and O– C O bond were introduced onto the PS surface by the O water treatment. We further investigated the functional groups in more detail by using ATR-FTIR measurements. The ATR-FTIR spectra of the pristine PS surfaces (Fig. 7(a)) mainly consist of three basic bands: from 3200 to 3000 cm1 and from 1640 to 1420 cm1, C–H and ring stretching mode of the aromatic ring; and from 3000 cm1 to 2700 cm1, CH2 stretching mode of the principal chain [45]. Similar ATR-FTIR characteristic band of PS was also observed for the H2O2-treated PS sample (Fig. 7(d)). As shown in Fig. 7(b) and (c), besides the characteristic bands of PS, weak and reproducible absorption peaks near 3445 and 1730 cm1 were identified on the Owater-treated PS surfaces, which were assigned to the OH [18,45] and C O groups [18], respectively. The emergence of C O groups on the O-water-treated PS surfaces may be due to the ring opening and oxidation of PS aromatic rings, which accords to the XPS results mentioned above. The above ATRFTIR results further confirmed that hydrophilic functional groups such as OH and C O groups were introduced onto the PS surfaces after the O water treatment.

Table 3 Atomic contents (atomic %) of O and C on the pristine PS surface, pre-treated PS surface, PS surfaces treated by the O water for 24 and 48 h, and PS surface treated by the H2O2 for 48 h, respectively Element

O C

Pristine

4.7  0.3 95.3  0.3

Pre-treated

4.9  0.3 95.1  0.3

Treated by O water

Treated by H2O2

24 h

48 h

48 h

11.5  0.7 88.5  0.7

13.1  0.6 86.9  0.6

4.8  0.4 95.2  0.4

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Fig. 6. Representative C 1s core-level XPS spectra of the PS (a) pristine, (b) pre-treated, (c) O-water-treated, 24 h, (d) O-water-treated, 48 h, (e) H2O2-treated, 48 h, respectively.

3.3. Mechanism of surface modification Present results showed that the O water treatment caused an obvious decrease in the contact angle and an increase in surface energy of the PS sample, indicating an increase in the surface hydrophilicity after the treatment. The modification

mechanism of PS polymers via the O water treatment was discussed in the following sections. It is well known that O is high reactive radical in the anion chemistry [21,22]. O has strong oxidation power, particularly in low-temperature oxidation of hydrocarbons [21–24]. O can dissolve in the deionized water by aerating the O flux, as observed by present

Table 4 Relative intensity (% area) of various functional groups (chemical bonds) measured by the C1s peaks from the pristine PS surface, pre-treated PS surface, PS surfaces treated by the O water for 24 and 48 h, and PS surface treated by the H2O2 for 48 h, respectively Functional groups

CC/CH CO C O OC O p–p*

Pristine

Pre-treated

94.6  0.3 2.3  0.2

94.4  0.3 2.5  0.1

3.1  0.1

3.1  0.1

Treated by O water

Treated by H2O2

24 h

48 h

48 h

82.8  0.6 10.0  0.5 3.5  0.4 1.8  0.3 1.9  0.2

81.3  0.5 10.3  0.6 4.4  0.3 2.2  0.4 1.8  0.1

94.5  0.4 2.4  0.2

3.1  0.1

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O þ H2 O !  OH þ OH

(2)

O þ H2 O ! O ðH2 OÞ

(3)

reactions between the active oxygen species and PS surfaces were added to the aromatic rings. Furthermore, the surface features of PS would be changed by the acetone solution, especially under higher temperature (>280 8C) and higher pressure conditions [49,50]. According to the XPS and AFM measurements (Fig. 4(b) and 6(b); Tables 2–4), however, there was no significant influence on the surface chemical composition, surface functional groups, and morphological alteration of PS during the present pre-treatment process. We also tested H2O2 (0.3 mmol/l) effect on surface modification of PS sample. It was found that the H2O2 had neglectable effect on the contact angle and morphological alteration of PS surface. We therefore speculate that the active oxygen species such as O and OH may play an important role in the modification processes. The emergence of C O groups observed by XPS and ATRFTIR measurements might due to ring opening and oxidation of aromatic rings on the PS surface. The introduction of hydrophilic OH and C O groups by the O water treatments is most likely to account for the reduction of the contact angle and the increase of hydrophilicity of PS surface. To increase the modification effect of O water on polymer, the O generator as well as the preparation procedure of O water should be further modified.

O ðH2 OÞ þ H2 O ! OH ðH2 OÞ þ  OH

(4)

4. Conclusions

Fig. 7. ATR-FTIR spectra of the PS (a) pristine, (b) O-water-treated, 24 h, (c) O-water-treated, 48 h, (d) H2O2-treated, 48 h, respectively.

EPR measurements. Part of the O anions dissolved in water may react with H2O and O2, producing active oxygen species through a series of reactions as follows [46–48]:





O þ OH ! HO2 

O þ HO2





! O2



(5) þ OH



(6)

O þ O2 ! O3 

(7)

2 OH ! H2 O2

(8)

We have observed that a small amount of H2O2 appeared in the O water. The presence of H2O2 may arise from the reaction between hydroxyl radicals (Eq. (8)). However, the characteristics of the O water are not fully understood up to now. To clarify other active oxygen species such as OH, HO2 and O(H2O) cluster occurred or not in the O water, it needs further work including laser induced fluorescence (LIF) measurements and laser-detachment spectroscopy as well as dynamic simulation. Work towards this goal is in progress. In our previous study, it was found that O could decompose and oxidize benzene via the hydrogen abstraction reaction on the C12A7-O surface (i.e., C6H6 + O ! C6H5 + OH + e) [23]. OH is also one of the strong reactive species and undergoes hydrogen abstraction and addition reactions [19,16]. The PS contains aromatic rings and hydrogen atoms, consisting of numerous repeated linked units of

. Present

investigation shows that the hydrophilic groups of hydroxyl (OH) and carbonyl (C O) groups can be introduced onto the PS surfaces. In general, the active oxygen species (e.g., O and  OH) might react with the aromatic ring molecules on the PS surfaces and/or decompose the aromatic compounds to produce hydrophilic compounds such as hydroxyl and carbonyl groups. Accordingly, the hydrophilic hydroxyl groups formed by the

This work presents a novel approach for the surface modification of the PS polymer via the O water treatment. The surface properties of the modified PS, the active species in the O water, as well as possible modification mechanism have been studied, which were summarized as below. It was found that the O water treatment caused an obvious decrease of contact angles and increase of surface energy, leading to the increase of surface hydrophilicity. The water and formamide contact angles on treated PS surface gradually decreased by about 98, and the surface energy increased from 32.4 to 37.5 mN/m for 48 h treatment, respectively. The surface roughness increased from 183.4 to 388.0 nm for 48 h treatment via the AFM and FESEM investigation. The O water treatment also resulted in a remarkable increase of the oxygen content on the treated PS surface from 4.7 to 13.1% for 48 h. Especially, the hydrophilic OH and C O functional groups, analyzed by XPS and ATR-FTIR, were formed after the O water treatment, which was most likely to account for the increase of surface hydrophilicity. The O water would be considered as a ‘‘clean solution’’ without adding any toxic chemicals and it is easy to be handled at room temperature. Present approach, potentially, may be applied to the surface modification of polymers and other heatsensitive materials. Comparing with some known modification methods (e.g., plasma and UV treatments, etc.), however, the modification efficiency (evaluated by the alteration of contact angle per minute) induced by the present O water was still low, which would be attributed to lower concentration of active species in the O water. It needs to further increase the concentration of active species by modifying the O generator as well as the preparation procedure of the O water.

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