Surface Functionalization of a Fluoropolymer by Ion Beam-induced Graft Polymerization of 4-Vinyl Pyridine

Transcription

1 Journal of Radiation Industry () : 1~ (010) Surface Functionalization of a Fluoropolymer by Ion Beam-induced Graft Polymerization of -Vinyl Pyridine Chan-Hee Jung, In-Tae Hwang, Jae-Hak Choi* and Young-Chang Nho Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup 80-18, Korea Abstract - The surface functionalization of a fluoropolymer by ion beam-induced graft polymerization was described in this research. The surface of poly(tetrafluoroethylene) (PTFE) films were irradiated by a 10 kev H + ions, and -vinyl pyridine (VP) as a functional monomer was then thermally graft polymerized on the irradiated surface. The surface properties of poly(-vinyl pyridine) (PVP)-grafted PTFE films were investigated in terms of grafting degree, wettability, chemical structure, and morphology. The results revealed that the surface of PTFE films was successfully functionalized by ion beam-induced graft polymerization of VP. Key words : Surface functionalization, Ion beam-induced graft polymerization, Poly(tetrafluoroethylene), -Vinyl pyridine INTRODUCTION Surface modification of a polymer to obtain desirable surface properties is one of the major scientific challenges for the manufacture of high performance polymers. For many applications, such as the adhesion of polymers on metals and alloys, wettability, biocompatibility, and so on, various methods have been developed to modify the surface properties, including chemical treatment, corona discharge, grafting, coating, plasma treatment, and ion implantation. Among them, grafting has numerous advantages over other methods to improve surface properties of polymers (Borcia et al. 008; Chen et al. 008). Grafting is simple and useful to introduce polymer chains with high density at exact localization and it can provide a long-term chemically stable grafted polymer chain (Gupta et al. 00; Ito et al. 00; Uhlmann et al. 00). Graft polymerization is mediated by an active site as a starting point. Many different methods, such as the use of * Corresponding author: Jae-Hak Choi, Tel , Fax , . jaehakchoi@kaeri.re.kr a chemical initiator, plasma discharge, ozone treatment, UV radiation, and high energy radiation, have been developed to create active sites on the surface (Choi et al. 008). Among them, high energy radiation-induced graft polymerization is a well-established technique that does not require any initiators and additives. A grafting reaction with electron beams and gamma rays was extensively studied (Brack et al. 00; Nasef et al. 00). However, grafting induced by ion beam has rarely been carried out. Ion beambased process is one of the attractive techniques because it is a surface-specific modification technique without detrimentally affecting the bulk properties. Moreover, it is also controllable, reproducible, and clean, and the dimensions of a material are not affected due to a low processing temperature. For these merits, ion beam-based process is suitable to modify the surface of polymeric materials to obtain desirable properties (Massa et al. 00; Choi et al. 009). In this study, we reported the surface graft polymerization of -vinyl pyridine (VP) on a poly(tetrafluoroethylene) (PTFE) film by ion irradiation to functionalize its surface. The surface functionalization and surface property were 1

2 Chan-Hee Jung, In-Tae Hwang, Jae-Hak Choi and Young-Chang Nho investigated by using a grafting degree measurement, contact angle measurement, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and scanning electron microscopy (SEM). MATERIALS AND METHODS 1. Materials A poly(tetrafluoroethylene) film with 100 μm thickness purchased from Universal corporation was used as a substrate material for radiation-induced graft polymerization. The functional monomer, -vinyl pyridine, was purchased from Aldrich and used without further purification.. The surface activation of PTFE by ion irradiation PTFE films were ultrasonically washed with ethanol and dried under a vacuum before used. PTFE films were irradiated with 10 kev H + ions at various flunences ranging from to ions cm -. The pressure in the implanter s target chamber was 10 - ~10 - Torr. The beam current density was kept less than 1.0 μma cm - to prevent a thermal effect on the sheets. The irradiated films were stored in air for h for further oxidation.. Surface graft polymerization The irradiated PTFE films were put into glass tubes containing aqueous solution containing 0, 0, and 80 vol% of -vinyl pyridine and, subsequently, purged with nitrogen gas to remove oxygen. Graft polymerization was performed by placing the tubes at 80 C in a water bath for 1 to 18 h. After the graft polymerization, the resulting films were thoroughly washed with deionized water to remove the homopolymers. The resulting poly(-vinyl pyridine) (PVP)-grafted PTFE films were finally dried under vacuum at C. The grafting degree of poly(-vinyl pyridine) grafted onto the PTFE surface was measured by an Acid Orange 7 (AO7) staining method (Combellas et al. 007). The grafted films were immersed in a 0. mm AO7 solution at ph 10 and then placed for h at room temperature. The resulting stained films were thoroughly washed with an excess amount of a sodium hydroxide solution (ph 10) to remove the free AO7 molecules on the surface. Afterwards, the AO7 molecules complexed on the grafted films were detached in a 0.l mol NaOH base solution. The grafting degree was calculated from the calibration plot of the optical density versus AO7 concentration assuming the 1 : 1 stoichiometry of the binding between the AO7 and the pyridine group. The optical densities of the resulting solution were measured at 80 nm using a UV/Vis spectrophotometer (MQX 00 model, Bio- Tek Instruments).. Surface characterization The surface chemical structure was analyzed by using an attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Bruker, Tensor 7). The water contact angle measurement was performed by using a contact angle analyzer (Phoenix 00, Surface Electro Optical Company). The surface morphology was observed by using a scanning electron microscopy (SEM, JEOL JSM-90). RESULTS AND DISCUSSION The surface graft polymerization of VP on PTFE films was performed under various conditions to optimize the surface functionalization of PTFE films and the grafting degree of PVP grafted onto the PTFE surface was determined by a AO7 staining-based colorimetric method. Fig. 1 shows the grafting degree of PVP on the surface as a function of the fluence. The grafting degree increased to.7 μg cm - with increasing the fluence to 10 1 ions cm -, above Fluence (ions cm - ) Fig. 1. The grafting degree as a function of the fluence: [-vinyl pyridine]=80 vol% and grafting reaction time=1 h.

3 Ion Beam-induced Surface Graft Polymerization,0 cm -1,9 cm -1 1,9 cm -1 1, cm -1 1,0 cm -1 Absorbance (c) (b) (a) Reaction time (h) Fig.. The grafting degree as a function of the reaction time: the fluence= 10 1 ions cm - and [-vinyl pyridine]=80 vol% Wavenumber (cm -1 ) Fig.. The ATR-FTIR spectra of the control (a), irradiated (b), and PVP-grafted PTFE films (c). 10 Irradiated PTFE Contact angle ( ) PVP-grafted PTFE Monomer concentration (%) Fig.. The grafting degree as a function of monomer concentration: the fluence= 10 1 ions cm - and reaction time=1 h. which it decreased. This result could be explained by the fact that the ion irradiation at a relatively lower fluence than 10 1 ions cm - generates higher amounts of active species such as peroxide or hydroperoxide on the surface due to proper oxidation, but carbonization by defluorination predominantly occurs at a higher fluence, resulting in the reduction of active species on the irradiated PTFE. The effect of the grafting reaction time on the grafting degree was also investigated and the results are shown in Fig.. The grafting degree initially increased with increasing the grafting reaction time up to 1 h, after which it leveled off. This result can be interpreted as follows. With increasing the reaction time up to 1 h, the graft polymerization of VP Fluence (ions cm - ) Fig.. The Contact angles of the control, irradiated, and PVPgrafted PTFE films as a function of the fluence. initiated by radicals generated from active species such as peroxide and hydrogen peroxide on the irradiated PTFE surface was predominant, resulting in an increase in the grafting degree. However, for the reaction time above 1 h, the grafting degree was not much increased because all the formed peroxides on the PTFE surface were almost consumed and the monomers were almost polymerized after the certain grafting time. The effect of the monomer concentration on the grafting degree is present in Fig.. The grafting degree increased with increasing the concentration of VP up to 80 vol%. Therefore, from these data, it is clear that the surface graft

4 Chan-Hee Jung, In-Tae Hwang, Jae-Hak Choi and Young-Chang Nho (a) (b) (c) Fig.. The SEM images of the control (a), irradiated (b), and PVP-grafted PTFE films (c): The insets in (b)-(c) magnify the dotted rectangles in each respective figure. polymerization with the 80 vol% concentration of VP on the PTFE irradiated at a fluence of 10 1 ions cm - for 1 h achieved the maximum grafting degree in this grafting system. The ATR-FTIR spectra of the control, irradiated, and PVP-grafted PTFE films are shown in Fig.. The PTFE irradiated with Ar ions at 10 1 ions cm - and the PVPgrafted PTFE prepared with the PTFE treated at the same fluence were used for the analysis. As shown in the control PTFE spectrum of Fig. (a), the characteristic bands corresponding to the stretching vibration of the -CF present in the control PTFE appeared at 1,11 and 1,1cm -1. The irradiated film does not show significant changes in the main spectrum compared to the control PTFE as shown in the control PTFE spectrum of Fig. (a) and (b). On the other hand, the PVP-grafted PTFE fim in Fig. (c), the characteristic bands corresponding to the chemical structure of PVP were identified at,0 (the stretching vibration of aromatic CH ),,9 (the stretching vibration of aliphatic CH ), and 1,9, 1,, and 1,0 cm -1 (the stretching vibrations of C=C and C=N) (Melendez-Ortiz et al. 009). These results confirm that PVP has been successfully grafted onto the surface of the PTFE film. The changes in the water contact angle of the control, irradiated, and PVP-grafted PTFE films as a function of the fluence are shown in Fig.. In comparison with that of the control PTFE film, 10, the contact angles of the irradiated PTFE films were gradually decreased up to 9 with an increasing fluence. In the case of the PVP-grafted PTFE films, the contact angle further decreased up to about 70. This result implies that hydrophtlic PVP was successfully grafted on the hydrophobic PTFE surface, which improved the wettability of the PTFE surface. The changes in the surface morphology of the PTFE after ion implantation and graft polymerization of VP was investigated by SEM observation and are shown in Fig.. As shown in Fig. (a), the surface of the control PTFE had a few scar like poles but was rather smooth on the whole surface. After ion irradiation, the surface morphological changes were not observed in comparison to that of the control PTFE as shown in Fig. (b). However, after surface graft polymerization, the surface morphology of the PTFE was much rougher as present in Fig. (c). This result revealed that the PVP was successfully grafted on the irradiated PTFE surface. CONCLUSIONS In this study, the surface functionalization of PTFE was successfully performed by ion beam-induced graft polymerization of a functional monomer, -vinyl pyridine. The results of the ATR-FTIR, contact angle measurement, and SEM observation confirmed that the surface graft polymerization of -vinyl pyridine was successfully carried out on the irradiated PTFE. The grafting degree was dependant on the fluence, monomer concentration, and grafting reaction time. The surface graft polymerization using 80 vol% vinyl pyridine on the irradiated PTFE films at a fluence of 10 1 ions cm - for 1 h was the optimum graft polymerization condition to achieve the maximum grafting degree. Further work is underway to immobilize metal nanoparticles on PVP-grafted PTFE surfaces for electronic applications.

5 Ion Beam-induced Surface Graft Polymerization ACKNOWLEDGMENTS This research was supported by the Nuclear R&D program through the National Research Foundation funded by the Ministry of Education, Science, and Technology, Korea. REFERENCES Borcia C, Borcia G and Dumitrascu N Relating plasma surface modification to polymer characteristics. Appl. Phys. A 90:07-1. Brack HP, Padeste C, Slaski M, Alkan S and Solak HH. 00. Preparation of micro- and nanopatterns of polymer chains grafted onto flexible polymer substrates. J. Am. Chem. Soc. 1: Chen Y and Koberstein FT Fabrication of block copolymer mobnolayers by adsorption from supercritical fluids: A versatile concept for modification and functionalization of polymer surfaces. Langmuir : Choi J-H, Kim D-K and Jung C-H Preparation of patterned polymer brushes by radiation-induced grafting. J. Korean Phys. Soc. : Choi J-H, Ganesan R, Kim D-K, Jung C-H, Hwang I-T, Nho YC, Yun JM and Kim JB Patterned immobilization of biomolecules by using ion irradiation-induced graft polymerization. J. Polym. Sci. Part A: Polym. Chem. 7: 1-1. Combellas C, Kanoufi F and Nunige S Surface modification of halogenated polymers. 10. Redox catalyst induction of the polymerization of vinylic monomer. application to the localized graft copolymerization of poly(tetrafluoroethylene) surfaces by vinylic monomers. Chem. Mater. 19: Gupta B and Anjum N. 00. Plasma and radiation-induced graft modification of polymers for biomedical applications. Adv. Polym. Sci. 1:-1. Ito N, Mase A, Seko N, Tamada M, Sakata E and Kogi Y. 00. Surface treatment of poly(tetrafluroroethylene) and perfluoroethylene-propylene by radiation grafting. J. Appl. Phys. :9-9. Massa G, Mazzei R, Bermudez GG, Filevich A and Smolko E. 00. Grafting of acrylic acid onto polypropylene films irradiated with argon ions. Nucl. Instrum. Methods. B : 7-7. Melendez-Ortiz HI, Bucio E and Burillo G Radiationgrafting of -vinylpyridine and N-isopropylacrylamide onto polypropylene to give novel ph and thermo-sensitive films. Radiat. Phys. Chem. 78:1-7. Nasef MM and Hegazy ESA. 00. Preparation and applications of ion exchange membranes by radiation-induced graft copolymerization of polar monomers onto non-polar films. Prog. Polym. Sci. 9:99-1. Uhlmann P, Ionov L, Houbenov N, Nitschke M, Grundke K, Motornov M, Minko S and Stamm M. 00. Surface functionalization by smart coatings: Stimuli-responsive binary polymer brushes. Prog. Org. Coat. : Manuscript Received: November, 010 Revision Accepted: November 10, 010