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
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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)
N.L. Singh et al. / Radiation Measurements 40 (2005) 746 – 749
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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