SURFACE MODIFICATION OF POLYSTYRENE ...

Advanced Materials Research Vols. 264-265 (2011) pp 1532-1537 Online available since 2011/Jun/30 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.264-265.1532

SURFACE MODIFICATION OF POLYSTYRENE BEADS BY UV/OZONE TREATMENT A.N. Yusilawati1, M. Maizirwan1,a, I. Sopyan2, M.S. Hamzah3, K.H. Ng4 and C.S. Wong5 1

Bioprocess and Molecular Engineering Research Unit, 2Biomedical Engineering Research Group, 3

Halal Industry Research Centre, Faculty of Engineering, IIUM Gombak, 50728 Kuala Lumpur, Malaysia 4

School of Arts and Science, Tunku Abdul Rahman College, 50932 Kuala Lumpur, Malaysia

5

Plasma Research Laboratory, Physics Department, University of Malaya, 50603 Kuala Lumpur, Malaysia a

Email address: [email protected]

Key words: polymer, polystyrene, UV/ozone treatment

Abstract. It is known that polystyrene must be chemically modified to make its surface amenable to covalent cross-linking with protein. The aim of this study was to set up a UV/Ozone system and investigate the effects of UV/Ozone treatment on polystyrene surface. Microsize polystyrene beads with an average size of 150 µm in diameter were treated with and without distilled water at the same treatment time, ozone flow-rate and UV intensity. The treated beads were analyzed by ATRFTIR, SEM, EDX and hydrophilicity measurement. The results show that the hydrophilicity of the surface of polystyrene beads was increased after the UV/ozone treatment and the introduction of carbonyl, carboxyl and hydroxyl groups on the polystyrene beads surface was also confirmed. It was demonstrated that the UV/Ozone system was effective for treatment of polystyrene bead and the best result was obtained without distilled water. Introduction Several surface modification techniques have been developed to improve wetting, adhesion, and printing of polymer surfaces by introducing a variety of polar groups, with little attention to functional group specifically. These include wet chemical [1-2], UV irradiation [3, 4], plasma treatment [5-7], flame treatment [8] and UV/Ozone treatment [9]. Each of these techniques has advantages and disadvantages over the others. In this paper, surface modification by UV/Ozone treatment has been investigated. The UV/Ozone treatment has been shown to be a highly successful method for the controlled modification of polymers for applications ranging from adhesion improvement to the production of surfaces for enhanced cell attachment. The UV/Ozone treatment has previously been used to increase the surface oxygen, surface polarity, and wettability for a number of polymers in studies related to adhesion and wetting. It has many advantages over the alternative methods of surface modification, which include: no vacuum system required and able to give precise control over the modification process. The absence of wet chemistry means that there will be no residual solvent or other contaminant [9, 10]. Furthermore, the technique itself does not require sophisticated apparatus, it is easy to use, and the oxidation is controllable [10]. Ozonation treatment with UV light can easily be carried out in various gases, solvents and solution media at room temperature without a vacuum system and is suitable for heatunstable materials such as organic substrates [9,11,12].

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Polystyrene (PS) is a widely used polymer for cell culture because of its favorable properties such as low specific weight, high chemical resistance, good mechanical flexibility and biocompatible [13]. Although PS is found to be a suitable biomaterial for cell attachment, however, the adhesion of most animal cell lines on unmodified PS surfaces is found to be generally poor [14, 15]. This can be attributed to the strong adsorption of albumin from serum containing cell culture media [16]. Thus, several studies to modify the surface structure of PS have been done to improve the responsivity of this polymer for cell attachment, spreading and migration [4,17,18]. The surface for cell culture must be hydrophilic and correctly charged before the adhesion of cells can occur. For instant vertebrate cells possess unevenly distributed negative surface charges and can be cultured on surfaces that are either negatively or positively charged [19, 20]. Experimental System Setup. The system for the surface modification of the PS beads was built around the system shown in Fig. 1 similar to that reported by Takuoro et al. [4] but with slight modification. The ozonizer system (designed and fabricated at Plasma Research Laboratory, University of Malaya, Malaysia) was used to convert oxygen (O2) into ozone (O3) gas. The outlet tube of the ozonizer was connected to the inlet of a Dreschel bottle head (sinter porosity grades 1), obtained from Quickfit® via a fine valve for precise flow control while the outlet tube was open to the atmosphere via olive oil to absorb the residual ozone. The assembled system was placed into a compartment containing two high intensity low-pressure mercury grid lamps which generate UV light at 254 nm. For the treatment with distilled water, the Dreschel bottle was placed on an incubator shaker (Jeio Tech, Korea) which has been placed in the UV compartment.

Bottle UV Box

Oxygen Gas

UV lamp Tube

Rubber Tube

Ozonizer

(a)

Shaker

(b)

Fig.1. (a) Picture of the UV/Ozone treatment system, (b) Schematics of the UV/Ozone treatment system. Sample Treatment. Before use the PS beads were washed in ethanol solution to remove any contaminant present on the surface. After several washing cycles the solvent was removed by vacuum filtration and the PS beads were allowed to dry. Distilled water (100 ml) was placed in a Dreschel bottle and was aerated with ozone (300 ppm) at 1.0 litre per min flow rate for 1 hr. It was irradiated from both sides with two 6 W low-pressure mercury lamps (UV lamps) at wavelength of 254 nm and a constant radiation of 22 mW cm-2 at a distance of 10cm from both UV lamps. Ozone was produced by silent discharge of oxygen (purity of 99.9%). The temperature and pH value of distilled water are 22.0o C and 5.8 respectively. PS beads were dispersed into the distilled water in the bottle and were treated by ozone aeration combined with UV irradiation and were shaked at 180 rpm to obtain homogenous treatment. For treatment without water, the sample was directly exposed to UV and ozone for 1 hour at the same flowrate.

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Sample Characterization. Evaluation of the UV/Ozone treated PS beads was carried out by measuring the FT-IR (Perkin Elmer) spectra using the attachment for the attenuated total reflection method (ATR/IR) to determine the existence of functional group. The structures and morphologies of untreated sample, sample treated without distilled water and sample treated with distilled water were observed using SEM (JEOL JSM 5600). Changes in oxygen percentage of the PS beads were investigated using Energy Dispersive X-ray spectroscopy (EDS). Results and discussion Fourier Transform Infrared Spectroscopy (FTIR). Figure 2 shows changes in ATR/IR spectra of untreated the PS beads as compared to those treated by UV/Ozone with and without water. An additional absorption peak at 1740.34 cm-1 signifying the presence of the carbonyl group (C=O) and carboxylic group (COOH) was observed for all the treated samples. These functional groups were introduced by the reaction of untreated beads with radical species generated in the system and they play a vital role in hydrophilicity. It had been reported that these functional groups were found to increase with increasing treatment time [4,21,22].

Fig. 2. ATR/IR spectra of (a) PS treated with UV/Ozone in water, (b) PS treated with UV/Ozone, (c) PS treated with Ozone, and (d) Untreated PS. From the spectra, it was observed that treatment of PS beads by UV/ozone system without distilled water was able to introduce higher amount of aldehyde group compared to treatment with distilled water and ozone only. It was shown in the FTIR spectra that the carbonyl group was reduced when treated with distilled water. This was supported by Davidson et al. [23] where they reported that the surface oxygen concentration was reduced when washing with water. It was reported in previous studies [21,22] that UV/ozone treatment introduced OH functional group at peak around 3450cm-1. Result in Fig.2 indicated very small amount of OH functional group for both PS beads modified with ozone and UV/Ozone treatment and this effect was found to diminish when adding water in the system. Scanning Electron Microscopy (SEM). Surface roughness is an important parameter to determine the effect of surface treatment as it may affect many applications. Compared to untreated PS(Fig. 3(a)), SEM results show that the PS surface roughness is increased and their structure is changed after surface treatment as demonstrated in Figs. 3(b)-3(d). The surface roughness of the PS bead was increased most significantly after treatment with ozone only (Fig. 3(b)), followed by UV/ozone treatment (Fig. 3(c)) and UV/ozone treatment with water (Fig. 3(d)). These results suggested that


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the implementation of UV into ozone system could introduce aldehyde and crboxyl groups with less degradation and roughening of the surface. This is because UV irradiation may result in a cleavage of chemical bonds, creation of free radical and subsequent oxidation of irradiated materials [4] thus causing the surface roughness to increase [10].

(a)

Fig. 3. SEM images at 1000X magnification.

Energy Dispersive X-Ray Spectroscopy (EDX). Table 1 shows measurement of oxygen content on PS bead surface before and after surface treatment. The negative percentage of oxygen obtained for untreated PS indicates no oxygen element in the chemical structure and higher percentage of elemental carbon. This can be inferred from the original chemical structure of PS which contains only carbon and hydrogen elements [9]. From the data, it was shown that the percentage of oxygen element was higher for PS sample treated by UV/ozone without water (4.04%), followed by treatment by ozone only (3.26%) and finally UV/ozone with water (2.13%). These results are in agreement with FTIR spectra in Fig.2 which show the introduction of carbonyl and carboxyl group after treatment. Moreover, the percentage of atomic oxygen was also found to increase with increasing percentage of elemental oxygen. On the other hand, the percentage of carbon element (d) (c) was also increased after treatment. It was 7.96% for untreated PS, 15.65% for sample treated by UV/ozone with water, 18.79% sample treated by ozone only and 20.25% for sample treated by UV/ozone without water. This might be due to the opening of aromatic ring in PS structure which releases the carbon element. No peak from element other than carbon and oxygen was found after UV/ozone treatment indicating only reaction between PS surface and oxygen has occurred [24]. Table 1. EDX data showing oxygen concentration on PS beads. Element Carbon

Oxygen

Treatment Untreated PS Ozone UV/Ozone UV/Ozone/water Untreated PS Ozone UV/Ozone UV/Ozone/water

Element [%] 7.96 18.79 20.35 15.65 -6.11 3.26 4.04 2.13

Atomic[ %] 235.95 88.47 87.02 90.75 -135.95 11.53 12.98 9.25

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Reaction with Water. Direct observation was done to determine the reaction of PS beads with water. Fig. 4(i) shows that untreated PS beads are hydrophobic in nature and tend to agglomerate and appear only on the surface of the water. Fig. 4(ii) and Fig. 4(iii) clearly show the best treatment method with the system where the beads are completely dispersed in distilled water indicating a higher wettability of surfaces of the PS beads. Hydrophilicity of the PS beads was attributed to oxygen containing functional group [10, 25]. On the other hand, a Fig. 4(iv) show only some of the PS beads can attach to the water and the other half is hydrophobics chemically. This clearly indicates that the PS beads have not been completely treated due to non-homogenous mixture during shaking with distilled water [22]. Furthermore, it was reported that low molecular weight oxidized materials formed on the surface of the PS beads treated by UV/ozone treatment may be removed when washed with water [10].

Fig.4. Reaction of PS beads with distilled water (i) untreated sample, (ii) treated by ozone only, (iii) treated by UV/ozone without water, (iv) treated by UV/ozone with water. Conclusion We examined the effect of surface modification of PS beads utilizing ozone/UV treatment with and without aqueous distilled water to introduce aldehyde and carboxylic group onto the bead surface. ATR/IR measurements verified that both PS beads treated with and without water acquired hydroxyl (OH), carbonyl (C=O) and carboxylic (COOH) groups after the treatment. However, treatment of PS beads in the absence of water results in higher amount of hydrophilic functional group produced. The treatment of PS beads using UV/Ozone system needs no chemical reagents. It is simple to implement and involved low cost apparatus. Acknowledgements Thanks to laboratory assistants from Animal Tissue Engineering Laboratory and Metallurgy of IIUM and KTAR for their excellent work. This work was financially supported by the Ministry of Science and Technology (MOSTI) Malaysia.


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Advances in Materials and Processing Technologies II doi:10.4028/www.scientific.net/AMR.264-265 Surface Modification of Polystyrene Beads by UV/Ozone Treatment doi:10.4028/www.scientific.net/AMR.264-265.1532