Surface modification of polyethylene terephthalate by plasma treatment

Surface modification of polyethylene terephthalate by plasma treatment

Radiation Measurements 40 (2005) 746 – 749 www.elsevier.com/locate/radmeas Surface modification of polyethylene terephthalate by plasma treatment N.L...

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Radiation Measurements 40 (2005) 746 – 749 www.elsevier.com/locate/radmeas

Surface modification of polyethylene terephthalate by plasma treatment N.L. Singha,∗ , Anjum Qureshia , Nilam Shaha , A.K. Rakshitb , S. Mukherjeec , A. Tripathid , D.K. Avasthid a Physics Department, M.S. University of Baroda, Vadodara 390002, India b Chemistry Department, M.S. University of Baroda, Vadodara 390002, India c Institute for Plasma Research, Gandhinagar 382044, India d Nuclear Science Centre, Aruna Asaf Ali Marg, New Delhi 110067, India

Received 27 August 2004; accepted 19 January 2005

Abstract The surfaces of polyethylene terephthalate (PET) films have been treated by reactive gas (N2 ) plasma to determine the effects of low-energy ions on the surface modification of PET. The surface morphology of the PET was studied by the microhardness tester, atomic force microscope (AFM), FTIR spectroscopy as a function of fluence. DSC studies of pristine and irradiated samples are also reported here. It is observed that the hardness of the film increases significantly as the fluence increases. The bulk hardness of the film was measured at loads greater than 400 mN. The increase follows a linear trend with fluence and has been explained in terms of cross-linking as observed from FTIR spectra. AFM shows that the average roughness (Ra) of the film surface decreases from 51–6 nm after plasma treatment as fluence increases. DSC thermograms indicate changes in the melting properties of the system. © 2005 Elsevier Ltd. All rights reserved. Keywords: Polyethylene terephthalate; Nitrogen plasma; Microhardness; AFM; FTIR; DSC

1. Introduction Plasma treatment of polymer has been utilized broadly in surface modification to increase material adhesion and improve compatibility, etc. (Chen Qiang, 2002). It involves the interaction of the plasma-generated excited species with a solid interface and results in a physical and/or chemical modification of the first few molecular layers of the surface, while maintaining the properties of the bulk. It is known that

∗ Corresponding author. Tel.: +91 265 2783924; fax: +91 265 2787556. E-mail address: [email protected] (N.L. Singh).

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by conventional ion beam and plasma treatment, the polymer samples can obtain resistance to chemical attacks, in addition, there is an increase in surface hardness and a metal oxide layer is formed on the surface. Polyethylene terepthlate (PET) is widely used for different applications: packaging, decorative coatings, capacitors, magnetic tape, etc. Sakudo et al. (2003) developed a new technique to enhance the barrier characteristics of PET film against CO2 and O2 gases. Plasma-based ion implantation of nitrogen was found to be able to change a polymer surface into diamond-like carbon (DLC). Yoshida et al. (2003) investigated the characteristics of carbon films formed on PET by plasma source ion implantation using CH4 and C2 H2 . They found that surface modification with CH4 is a viable alternative method for reducing the oxygen transmission rates. Ueda et al. (2003)

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studied the treatment of PET by aluminum plasma for oxidation protection. Wang et al. (2004) studied the C2 H2 plasma immersion ion implantation deposition on PET to improve its blood compatibility. Lee et al. (1997) found substantial improvements in the hardness and wear resistance after implanting a range of ions into various polymers. Previous research shows that the interactions of inert and reactive gas plasma on the PET surface usually stabilize the polymer against oxidation, etc. In this paper we have studied the mechanical property (i.e. microhardness) and surface morphology by means of Vickers’ microhardness indentation and atomic force microscopy (AFM) respectively, and thermal properties (i.e. type of thermal reactions on application of heat) by means of differential scanning calorimetry (DSC). The change in intensities of functional groups with fluence was studied by the analysis of FTIR spectra. Fig. 1. Plot of hardness (Hv ) versus applied load (P ) for pristineand plasma-treated PET films.

2. Experimental detail Three pieces of PET [(C10 H8 O4 )n; density 1.4 g/cm3 ] each of thickness 230 m and of size 2 × 2 cm were cut from commercially available sheets (Garware group, India). These films were exposed to N2+ plasma of current density of the order of 10 mA/cm2 during the pulse-on time, given a total fluence of 1013 , 1014 , and 1015 ions/cm2 . The variation in total fluence was obtained by extending the plasma exposure. The base pressure was of the order of 10−3 bar and the operating pressure was of the order of 10−1 bar (Mukherjee et al., 2002). A Carl Zeiss microscope and accessories were used to investigate Vickers’ microhardness and surface morphology of pristine and implanted samples. The surfaces morphology of pristine and irradiated surfaces was obtained using AFM in the tapping mode. A Perkin Elmer thermal analyzer calibrated through the melting points of indium and tin carried out DSC measurements. Structural changes were studied using FTIR spectroscopy in the wave number range 500–4000 cm−1 with resolution of 4 cm−1 wave numbers.

3. Results and discussion The projected range of 35 keV nitrogen plasma in PET was calculated to be 110 nm using the SRIM-2000 code (Ziegler, 2000). 3.1. Micro hardness Fig. 1 shows the plot of the Vickers’ microhardness (Hv ) versus applied load (P ) at different fluences. The microhardness indentations were carried out on the surface of the pristine and irradiated films at room temperature under the different applied loads from 100 to 1000 mN and at a constant loading time of 30 s.

Fig. 2. FTIR spectra of pristine- and plasma-treated PET films.

It is evident that the Hv value increases with the load up to 400 mN and then saturates beyond the load of 400 mN. Hardness can be defined as resistance to indenter penetration, or as the average pressure under the indenter, calculated as the applied load divided by the projected area of contact incorporating the plastic component of displacement. The hardness is known to be influenced by surface effects. Particularly at low penetration depths, the strain hardening modifies the true hardness of the material. At the higher loads, beyond 400 mN, the interior of the bulk specimen is devoid of surface effects. Hence the hardness value at higher loads represents the true value of the bulk and is consequently independent of the load. The hardness is found to increase as fluence increases. This may be attributed to cross-linking phenomena (Lee et al., 1997). It is also observed from the FTIR spectra (Fig. 2) that there is no change in overall structure of the polymer, but a minor change in intensity

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Fig. 3. AFM photomicrographs of pristine- and plasma-treated PET films. (a) AFM image of untreated PET film. (b) AFM image of plasma (N2 ) treated PET film at fluence 1013 ions/cm2 . (c) AFM image of plasma (N2 ) treated PET film at fluence 1014 ions/cm2 . (d) AFM image of plasma (N2 ) treated PET film at fluence 1015 ions/cm2 .

has been observed. The wave number region: from 3600 to 2500 cm−1 , where the C–H and O–H stretching vibrations of different types of H-bonds occur; 1680–1430 cm−1 typical for vibration modes of the aromatic rings, for stretching vibrations of double bonds, and for CH2 bending vibrations. Amorphization of the crystalline fraction of the polymer and scission of the main chain at para position of disubstituted benzene rings are assigned to the bands at 1471 and 1504 cm−1 , respectively (Steckenreiter et al., 1997). Significant changes are observed in the wave number of the range 1400–1800 cm−1 . This might be due to the breakage of few bonds in the structure as well as enhancement of few functional groups in the structure. 3.2. Atomic force microscopy The surface morphology of nitrogen plasma-treated PET measured by AFM on 5 × 5 m2 area is shown in Fig. 3. The surface of pristine PET exhibits rectangular structures and pinnacle-like structures can be observed in the image of plasma-treated PET film. Each AFM image is analyzed in terms of average surface roughness (Ra). The roughness values are 51.61, 33.06, 17.96, and 6.56 nm for pristine and irradiated samples at the fluences of 1013 , 1014 , and 1015 ions/cm2 , respectively. It is observed that after nitrogen plasma implantation, the roughness of the surface decreases and the surface becomes significantly smoother, and smoothness increases as fluence increases. This reveals that sputtering effects work for the surface smoothing.

Fig. 4. DSC thermograms of the pristine and plasma-treated PET films.

3.3. Differential scanning calorimetry Fig. 4 shows the DSC curves of pristine and irradiated PET samples. The endothermic transformation of a pristine sample takes place in a temperature range from 248.5 to 262.5 ◦ C, with a peak at 258 ◦ C. The low-temperature part of the curve is related to the relaxation behavior of the complex morphology of the melt-crystalline system. The melting enthalpy of a pristine sample calculated by integration in the above temperature range is H = 14.317 J/g. From this value, the degree of crystallinity can be evaluated through the relation X = H /H0 , where H0 is the melting enthalpy of completely crystalline polymer, equal to be 104 J/g (Biswas et al., 1999). Accordingly, the crystalline fraction of pristine PET is 0.137. It is observed that the temperature of the melting peak shifted to higher temperature. A sharp melting peak at the temperature of 265 ◦ C is observed for an ion fluence of 1015 ions/cm2 as compared with the pristine melting peak at 258 ◦ C. It is seen that there is no appreciable change in melting temperature and melting enthalpy up to the fluence of 1014 ions/cm2 . It seems that the system remains reasonably organized but gets disorganized with some residual energy when a fluence of 1015 ions/cm2 is used.

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4. Conclusions Plasma treatment has been found to increase Vickers’ hardness of the PET films. The increase in hardness with the fluence may be attributed to cross-linking that occurs, whereby, two free dangling bonds on neighboring chains unite. The surface roughness decreases as the fluence increases as observed from AFM studies. The differential calorimetric measurements of plasma-treated PET shows significant changes in its melting property.

Acknowledgements Authors are thankful to the Institute for Plasma Research, Gandhinagar for providing the plasma implantation facility, Nuclear Science Centre (NSC), New Delhi, for providing an AFM and Reliance Industries Ltd. Hazira, Surat for providing the DSC equipment.

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Lee, E.H., Rao, G.R., Mansur, L.K., 1997. Hardness enhancement and crosslinking mechanisms in polystyrene irradiated with high energy ion-beams. Mater. Sci. Forum 248–249, 135–146. Mukherjee, S., Chakraborty, J., Gupta, S., Raole, P.M., John, P.I., Rao, K.R.M., Manna, I., 2002. Low and high energy plasma immersion ion implantation for modification of material surfaces. Surf. Coat. Technol. 156, 103–109. Sakudo, N., Mizutani, D., Ohmura, Y., Endo, H., Yoneda, R., Ikenaga, N., Takikawa, H., 2003. Surface modification of PET film by plasma-based ion implantation. Nucl. Instrum. Methods B 206, 687–690. Steckenreiter, T., Balanzat, E., Fuess, H., Trautmann, C., 1997. Chemical modifications of PET swift heavy ions. Nucl. Instrum. Methods B 131, 159–166. Ueda, M., Tan, I.H., Dallaqua, R.S., Rossi, J.O., Barroso, J.J., Tabacniks, M.H., 2003. Aluminum plasma immersion ion implantation in polymers. Nucl. Instrum. Methods B 206, 760–766. Wang, J., Pan, C.J., Kwok, S.C.H., Yang, P., Chen, J.Y., Wan, G.J., Huang, N., Chu, P.K., 2004. Characteristics and anticoagulation behavior of polyethylene terepthalate modified by C2 H2 plasma immersion ion implantation-deposition. J. Vac. Sci. Technol. A22, 170–175. Yoshida, M., Watanabe, S., Takagi, T., Shinohara, M., Lee, J.W., 2003. Investigation of diamond-like carbon formed on PET film by plasma-source ion implantation using C2 H2 and CH4 . Nucl. Instrum. Methods B 206, 712–716. Ziegler, J.F., 2000. SRIM-2000, the stopping range of ions in matter. IBM Research, New York, USA.