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Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer
Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
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Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer Sanjeev Kumar Gupta a,b, Rashi Gupta a, Paramjit Singh c, Vikas Kumar a, Manoj Kumar Jaiswal d, S.K. Chakarvarti e, Rajesh Kumar a,⇑ a
University School of Basic & Applied Sciences, Guru Gobind Singh Indraprastha University, New Delhi 110078, India Department of Physics, Aggarwal College, Ballabhgarh, Faridabad 121004, India Department of Applied Science, Guru Nanak Dev Engineering College, Ludhiana 141006, India d Department of Physics, Shaheed Rajguru College of Applied Sciences for Women, University of Delhi, New Delhi 110096, India e Centre for R&D, Manav Rachana International University, Faridabad 121004, India b c
a r t i c l e
i n f o
Article history: Received 24 July 2016 Received in revised form 11 December 2016 Accepted 7 February 2017 Available online xxxx Keywords: Ion beam Kapton-H polymer X-ray diffraction UV–visible FTIR
a b s t r a c t The optical, structural and chemical properties of polyimide Kapton-H polymer thin film samples were modified by irradiation with 100 MeV O7+ ions (in the fluence range of 1 1011 to 5 1012 ions/cm2) and the modifications of these properties were observed by UV–visible (UV–Vis) spectroscopy, X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy respectively. The band gap energy of the polymer decreased considerably with discrete increment of the ion fluence (different fluence for each sample) and effective change for the sample irradiated at a fluence of 5 1012 ions/cm2 was observed from that of pristine sample. The amorphous nature of the polymer was observed to be decreased with increase of ion fluence. The vibrations of C„C appeared at mid fluences but the stretching vibrations of OAH bond disappeared at these fluences due to the high LET of the oxygen ions. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Kapton (registered trademark of Dupont), is a material of significant commercial importance having high performance, particularly in high radiation environment. It belongs to polyimides which are a class of thermally stable polymers based on stiff aromatic backbones. The ability of Kapton to maintain its excellent physical, electrical, and mechanical properties over a wide temperature range (from 269 °C to 400 °C) [1] has opened new design and application areas to plastic films. Kapton does not melt or burn as it has the highest UL-94 flammability rating: V-0. It has excellent chemical resistance [2]. At high temperature it offers good resistance to radiation effects, so used as a thermal blanket in the outer surface of satellite structures [3]. Low out-gassing rate makes it suitable insulator for ultra high vacuum environment. With a good combination of thermal conductivity, dielectric strength and availability in thin films, it is well suited material in cryogenics. Kapton films are available in a number of variants such as Kapton-H, HN, VN, FN and others. Out of these variants Kapton-H was the first film of its type and is a general purpose film
⇑ Corresponding author. E-mail address: [email protected] (R. Kumar).
which has a repeating structure of C22H10O5N2 (shown in fig. below)
Swift heavy ion (SHI) irradiation of polymers in the last few decades has emerged as one of the most important and effective techniques to induce modifications in the physico-chemical properties of polymers up to desired extent and tailor them as useful materials for different applications in commercial, scientific, medical and domestic fields [4–5]. The SHIs deposit huge amount of energy in the target polymeric material due to high value of linear energy transfer (LET) and the deposited energy is nonhomogeneous due to differential nature of energy lost by charged particles (i.e. dE/dX) in the material medium [6]. The deposited energy forms the latent tracks or defects along its path resulting in free radical formation, scission and cross-linking of the polymeric chains, bond cleavage, bond formation, carbon cluster
http://dx.doi.org/10.1016/j.nimb.2017.02.011 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: S.K. Gupta et al., Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.02.011
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formation, etc. [7–10] These damages, which are confined to beam diameter only, can be utilized to improve various properties, such as surface, optical, structural, chemical, electrical and free volume, etc. [11–15]. The modifications in the properties of polymers depend on ion beam parameters, such as energy, fluence, charge and mass of the impinging ion as well as thickness, structure and chemical composition of the polymer under irradiation [16]. The effects of linear energy transferred by swift heavy ions on the micro-structural properties of electrochemically synthesized polymer composite thin films are well documented in the literature [17,18] and it was found that the surface morphologies of the polymer changed significantly due to cross-linking or chain- scissoring of polymeric chains. So the irradiation of polymers leads to significant changes in physico-chemical properties of a polymer. A brief look at the existing literature reveals that many studies have been carried out in the last decade for modifications in the mechanical, structural and electrical properties of the polyimide Kapton-H polymer due to energetic ions irradiations. Kucheyev et al., studied the mechanical properties of Kapton-H such as hardness and observed that its Young’s modulus increased and tensile strength decreased after irradiating the polymer with 1H, 4 H, and 12C ions of energy lying between 1.5 MeV and 3.8 MeV at ion fluences of the order of 1011 ions cm2. They reported that these modifications depend linearly upon ion fluence and super linearly upon electronic energy loss [11].Variations in dielectric, optical, and structural properties of Kapton-H polymer irradiated to 80 MeV O6+ ion beam at fluences of 1011 to 1013 ions cm2 were observed by Prasher et al. [19]. The dielectric constant has been found to increase with increase of ion fluence whereas the optical band gap energy shows slight decrease in its value at higher ion fluences as reported by Kudaikulova et al. [20]. The stability and reflectivity of silver coated polyimide films were found to be improved up to 98% when prepared by heterogeneous chemical modification process. Such metalized reflectors find applications as solar energy concentrators with high electroconductivity to dissipate charges, which remarkably reduce weight and fragility of mirrors, providing good flexibility of packing for subsequent deployment in space applications. The inelastic mean free path, stopping power, and energy-loss straggling of electron, proton, and a-particle beams in a broad incident energy range in Kapton-H polymer have been calculated by de Vera et al. and they observed that irradiation with energetic ion beams offers a possibility to modify both the structural and the functional properties of polymer [3]. Effect of high temperature from 325 to 400 °C on optical properties and refractive index of Kapton-H has also been studied [21]. A continuous red shift of the absorption edge with rise in heating temperature with a corresponding decrease in the optical band gap was noticed. The refractive index of the heat treated samples was also found to be increased with increase in temperature. In the present work we intend to report the modifications induced in the physico-chemical properties of polyimide KaptonH polymer by irradiating it with oxygen ions (O7+) of energy 100 MeV at different fluences varying from 1 1011 ions/cm2 to 5 1012 ions/cm2. Many studies regarding the modifications in various properties of Kapton-H polymer with various light and heavy ions having different energies (low to medium) and different fluences are reported but no study has been carried out so far upon Kapton H polymer by irradiation with the oxygen ions to the best of our knowledge. So, it would be quite interesting to further explore the polymer in terms of modifications induced, if any, by oxygen ions (O7+) at specified energy and fluences. The aim of the present work is to understand the mechanism of modifications in the optical, structural and chemical properties of the polymer at different fluences in a better way and tailor them more efficiently as per requirements.
2. Experimental methods 2.1. SRIM (stopping and range of ions in matter) calculations Using SRIM code [22], the parameters such as the electronic energy loss (Se), nuclear energy loss (Sn) and projected range of 100 MeV oxygen ions in Kapton-H polymer were calculated which come out to be 53.6 eV/Å, 2.84 102 eV/Å and 122.74 mm respectively. 2.2. Materials and irradiation Thin sheets of Kapton-H polyimide of thickness 75 mm were commercially purchased from Goodfellow, United Kingdom. Kapton-H is a semi-crystalline polymer having triclinic crystal structure with tensile modulus around 2.5 GPa, density 1.42 g/ cm3 and glass transition temperature between 360 °C and 410 °C. The samples were used as received and no further studies were carried out to measure the thickness and density of the samples. These sheets were cut in 1 cm 1 cm size and irradiated by 100 MeV O7+ in the Material Science Beam Line of the 15 UD Pelletron accelerator facility at Inter University Accelerator Centre (IUAC), New Delhi, India. The fluences were taken in the range of 1 1011 ions/cm2 to 5 1012 ions/cm2. The beam current was 0.5 pnA and high vacuum of the order of 5.33 104 Pa was maintained in the target chamber during the bombardment. The beam was scanned in X-Y plane in order to irradiate the samples uniformly throughout the whole target area. The whole irradiation experiment was carried out at room temperature. 2.3. Characterization techniques 2.3.1. Optical studies UV–visible spectroscopy is an important technique to study and analyse the modifications in the optical properties such as optical band gap and the creation of conjugated double or triple bonds in the material. When the light in the ultraviolet and visible region is absorbed by a polymeric material, it results in the transition of electrons from lower energy states to higher energy states in the molecular orbital. The difference in energy of the two states of the orbital is called the optical band gap energy (Eg). This band gap energy gets modified when the material is exposed to energetic heavy ions. Any modification in the values of band gap energy results in the modification of the optical conductivity of the polymeric material. The UV–visible spectroscopy is also used to estimate the shift in the absorption edge of the polymeric samples upon irradiation, which is attributed to the formation of conjugated system of bonds. In the present study, Kapton-H polymer samples were studied by UV–visible spectroscopy in the range of 200–800 nm using Hitachi U-3300 spectrophotometer at IUAC, New Delhi, India. With the help of UV–visible absorption spectra of pristine and irradiated samples, the parameters such as optical band gap energy (Eg) (using Tauc’s relation) [23–24] for direct transitions were calculated by the following equation:
ðahmÞ ¼ Bðhm Eg Þ
n
ð1Þ
Here a is the optical absorption coefficient, B is the band tailing parameter and its value depends on the transition probability. The value of n characterizes the transition processes in K-space, which is taken to be equal to 1/2 for direct allowed transition (direct band gap calculation). The number of carbon atoms per conjugation length (N) [25–26] of the polymeric chain was calculated with the help of following equation
N ð34:3=Eg Þ2
ð2Þ
Please cite this article in press as: S.K. Gupta et al., Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.02.011
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2.3.2. Structural studies X-ray diffraction (XRD) is the technique used to study the modifications in the structural properties of a polymer upon irradiation. By analysing the XRD patterns, we can observe the shift in the diffraction peaks, if any, of the polymeric samples upon irradiation and therefore can estimate the change in lattice parameters. Also by observing the change in the values of FWHM (i.e. Full Width at Half Maximum), the change in the intensity of diffraction peaks can be obtained which results in the modification of the amorphous nature of the polymer upon irradiation. In the present study, Kapton-H polymer samples were studied by the X-ray diffraction (XRD) method using Brooker AXS system (scan speed of 1°/min) at IUAC, New Delhi, India. With the help of XRD patterns obtained for pristine and irradiated samples the crystallite size (L) of the pristine and oxygen ion irradiated polymer samples were calculated using following equation given by Scherrer [27]
L ¼ Kk=ðb cos hÞ
ð3Þ
Here K is a constant of proportionality (called the Scherrer constant), b is the full width at half maximum (FWHM) of the peak (in radian), k is the wavelength of the X-rays used (1.54 Å in case for Cu- Ka radiation) and h is the angle which is calculated by taking ½ of 2h value. 3. Results and discussion 3.1. UV–visible studies UV–visible spectra of pristine and oxygen ion irradiated samples of Kapton-H polymer are shown in Fig. 1. The optical absorption edges of irradiated samples shifted towards the red end (longer wavelength) of the visible spectrum, which increased with the increase of ion fluence. Fig. 2 shows the variation of absorbance w. r. t. oxygen ion fluence at a particular wavelength of 467 nm. The curve clearly indicates that the absorbance increases with increase in ion fluence. Similar results are reported by Kudo et al. [28] in their study of Kapton-H of thickness 12.5 lm bombarded with proton, helium beams, C, Si, Ar and Fe ion beams of energy 6 MeV per nucleon. They attributed the shift in the absorbance edge to the LET values of the ions in the polymer. More is the value of LET more is the shift of absorption edges towards higher wavelength of the spectrum. This shift was due to the degradation of CAC bond of the imide ring, which resulted into the cyclicization and condensation at high LET radiation. Some other researchers
Fig. 2. Graph between Absorbance vs Fluence of pristine and O7+ ion irradiated Kapton-H polymer samples at wavelength 467 nm.
also reported a shift of absorbance edge towards higher wavelength in Kapton-H polymer bombarded with ionising radiations at different energies [1,29]. According to them this shift in absorbance edge may be attributed to formation of conjugated system of double and triple bonds [30] and increase in conjugation length due to the creation of free electrons or free radicals. The direct consequence of this shift of absorbance edge is the increase in the energy of valence band thus causing the band gap between the valence and the conduction band to decrease, hence increasing the conductivity of the polymer. These absorption bands (in the of range 200–800 nm) are associated with p–p⁄ electronic transitions [31] and may be attributed to the presence of chromophores like aromatic rings such as phenyl rings having C@C and C@O double bonds [21].The optical band gap for direct transitions was calculated by using Eq. (1) and the values are tabulated in Table 1 for pristine as well as oxygen ion irradiated samples respectively. It was observed from the data that the band gap energy for ion irradiated polymer samples decreased from 2.64 eV to 2.58 eV i.e. 2.28% as compared to the pristine sample. The decrease in the band gap energy is quite significant within the typical calculation error (±0.01%). Many researchers reported the decrease in the band gap of different polymers when irradiated with radiation of different types [32–36]. Another effect of shift of absorbance edge towards higher wavelength is the increase in the number of carbon atoms per conjugation length (N). As shown in Table 1, the value of N for irradiated Kapton-H polymer samples increased from 169 to 176 (approx. 4.1%). The increase in the values of N is quite significant within the typical calculation error (±1.0%). 3.2. X- ray diffraction studies The XRD patterns of oxygen ion irradiated and pristine Kapton-H polymer samples are shown in Fig. 3. The XRD- patterns Table 1 Calculated values of Direct band gap energy (Eg) and number of carbon atoms per conjugation length (N) for pristine and O7+ ion irradiated Kapton-H polymer samples. Typical errors in the calculated values of Eg and N are about ±0.01 eV and ± 1 respectively.
Fig. 1. UV–visible spectra of pristine and O7+ ion irradiated Kapton-H polymer samples.
Fluence (ions/cm2)
Eg (eV)
N
Pristine 1 1011 5 1011 1 1012 5 1012
2.64 2.62 2.62 2.61 2.58
169 171 171 173 176
Please cite this article in press as: S.K. Gupta et al., Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.02.011
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S.K. Gupta et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx Table 3 Approximate characteristics of the vibration and corresponding band position for Kapton-H polymer [40–42,28]. Band position (cm1)
Assignment
734 1079 1167 1230,1367 1599, 1515 1703 1783 3380 3550 560
C@O bending CAOAC bending CAC bending CANAC stretching vibrations C@C aromatic C@O symmetrical stretching C@O asymmetrical stretching NH stretching OH symmetric stretching CAC skeletal vibrations
Fig. 3. XRD patterns of pristine and O7+ ion irradiated Kapton-H polymer samples.
Table 2 Calculated values of peak width (FWHM) and crystallite size (L) for pristine and O7+ion irradiated Kapton-H polymer samples. Typical errors in the calculated values of FWHM and L are 0.001 radians and 0.1 Å respectively. Fluence (ions/cm2)
FWHM (radians)
L (Å)
Pristine 1 1011 5 1011 1 1012 5 1012
0.028 0.031 0.032 0.032 0.026
52.17 50.74 45.34 46.21 54.68
Fig. 4. FTIR spectra of pristine and O7+ ion irradiated Kapton-H polymer samples.
conforms to the patterns showed by Ahmed et al. [29]. For the pristine sample the diffraction peaks occur at 2h–15.08°, 23.1° and 27.58° indicating the amorphous nature of the polymer [5]. In the irradiated samples identical peaks were observed with a minor change in the peak position. The new positions of the peaks were 14.55°, 21.76° and 26.88° respectively indicating change of lattice parameters of irradiated samples. Also a small variation in the peak intensity of the irradiated samples was observed. The intensity of the diffraction peaks for ion fluences from 1 1011 to 1 1012 ions/cm2 showed a slight decrease. The decrease in intensity of the diffraction peaks is obtained from the increase in FWHM (from 0.028 to 0.032 radians), as shown in Table 2 suggesting a decrease in the ordered behaviour of the polymeric chains due to chain scission at the mentioned fluences, which leads to the increase in amorphization of the polymer. At the fluence of 5 1012 ions/ cm2 the value of FWHM decreased marginally (from 0.028 to 0.026 radians) resulting in the ordered behaviour of the polymer (Table 2) due to cross linking, resulting in an increase in the crystalline nature of the polymer. Also the average crystallite size of the irradiated samples decreased from 52.17 for the pristine sample to 45.34 at fluence of 5 1011 ions/cm2 confirming the chain scission of the polymeric chains and an increase in the value of crystallite size from 52.17 to 54.68 at the highest fluence confirming the cross-linking of the polymer chains at this fluence. Similar results are also reported in different polymer samples due to radiation exposure in which the crystalline nature of the polymer material varies with the variation of ion fluence or gamma dose [37–39]. 3.3. FTIR studies The chemical modifications in the irradiated Kapton-H polymer samples were studied by Fourier transform infrared (FTIR)
Fig. 5. Graph between%T and Fluence of pristine and O7+ ion irradiated Kapton-H polymer samples for wave numbers 2344 cm1 and 2830 cm1.
spectroscopy using Perkin-Elmer FTIR spectrophotometer in the range 4000–400 cm1 in transmission mode at Variable Energy Cyclotron Centre (VECC), Kolkata, India. The FTIR spectrum of pristine Kapton-H polymer sample conforms to the spectrum reported by Garg and Quamara; Guenther et al.; Sun et al.; Kudo et al. [40– 42,28]. In the pristine spectrum of polyimide Kapton-H, primarily groups were assigned viz. imide group ((C@O)2NAC) at 1117 cm1, ether group (ACAOACA) at 1167 cm1 and carbonyl group (C@O) at 1776 cm1 and the band between 3450 and 3650 corresponds to AOH and C@O stretching vibrations. Various other approximate characteristics of the vibrations and corresponding
Please cite this article in press as: S.K. Gupta et al., Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.02.011
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wave number values (band position) of the pristine sample which show the characteristic absorption bands of the imide and aromatic groups are represented in Table 3. Fig. 4 represents the FTIR spectra of pristine and oxygen ion irradiated samples of Kapton-H polymer. In the spectra a new band was seen to be developed between 2297 and 2387 cm1 (correspond to C„C or C„N) at the fluences of 1 1011 and 5 1011 ions/cm2 as shown by the circled part of the spectra. The absorption band at position 2830 cm1 (OAH stretching) seems to be disappeared at the ion fluence of 5 1011 ions/cm2. The new developments can be explained with the help of Fig. 5, which represents the plots between %T (transmittance) and the different ion fluences at some specific wavenumbers 2344 cm1 and 2830 cm1 respectively. It is clear from the plot (black curve) that, at the wavenumber 2344 cm1, the transmittance decrease or in turn the corresponding absorbance increase significantly at the fluences of 1 1011 ions/cm2 and 5 1011 ions/cm2 showing the formation of new band, probably due to increased cross-linking of the polymeric chains. Similarly, the increase in the transmittance (or decrease in absorbance) from the plot (red curve) of Fig. 5, at the wavenumber 2830 cm1 indicates the disappearance of band probably due to increased scission of the polymeric chains. Absorption bands at other wave numbers showed an insignificant variation in their intensity. 4. Conclusions The increase in the number of carbon atoms per conjugation length (N) due to the formation of conjugated system of double and triple bonds and the corresponding decrease in band gap energy attributed to the formation of free radicals confirm that the effect of ion irradiation induces cross linking in the polymeric chains and improves the optical properties of the Kapton-H polymer, particularly at the highest fluence. Modifications in the structural properties of the Kapton-H polymer are attributed to change in the lattice parameters due to the shift in the positions of the diffraction peaks. A slight decrease in the intensity of the diffraction peaks at lower and intermediate fluences indicate an increase in the amorphous character of the polymer. At the highest fluence the intensity of the diffraction peaks increased slightly suggesting an increase in the crystalline nature of the polymer. The Kapton-H polymer also showed some chemical modifications upon ion irradiation due to the developement of new absorption bands at 2297–2387 cm1 (C„C or C„N bending) at intermediate fluences and disappearance of absorption band at 2830 cm1 (OAH stretching) at higher fluence. Also decrease in intensity of existed absorption bands was observed at different ion fluences. Acknowledgements The authors are thankful to the staff at IUAC, New Delhi, India for providing the ion irradiation facilities and characterization techniques such as XRD and UV–vis spectroscopy. The authors also extend their gratitude to Dr. A. K. Himanshu, Scientist-D at Variable Energy Cyclotron Center, Kolkata, India for providing the FTIR facility.
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Please cite this article in press as: S.K. Gupta et al., Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.02.011
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer Sanjeev Kumar Gupta a,b, Rashi Gupta a, Paramjit Singh c, Vikas Kumar a, Manoj Kumar Jaiswal d, S.K. Chakarvarti e, Rajesh Kumar a,⇑ a
University School of Basic & Applied Sciences, Guru Gobind Singh Indraprastha University, New Delhi 110078, India Department of Physics, Aggarwal College, Ballabhgarh, Faridabad 121004, India Department of Applied Science, Guru Nanak Dev Engineering College, Ludhiana 141006, India d Department of Physics, Shaheed Rajguru College of Applied Sciences for Women, University of Delhi, New Delhi 110096, India e Centre for R&D, Manav Rachana International University, Faridabad 121004, India b c
a r t i c l e
i n f o
Article history: Received 24 July 2016 Received in revised form 11 December 2016 Accepted 7 February 2017 Available online xxxx Keywords: Ion beam Kapton-H polymer X-ray diffraction UV–visible FTIR
a b s t r a c t The optical, structural and chemical properties of polyimide Kapton-H polymer thin film samples were modified by irradiation with 100 MeV O7+ ions (in the fluence range of 1 1011 to 5 1012 ions/cm2) and the modifications of these properties were observed by UV–visible (UV–Vis) spectroscopy, X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy respectively. The band gap energy of the polymer decreased considerably with discrete increment of the ion fluence (different fluence for each sample) and effective change for the sample irradiated at a fluence of 5 1012 ions/cm2 was observed from that of pristine sample. The amorphous nature of the polymer was observed to be decreased with increase of ion fluence. The vibrations of C„C appeared at mid fluences but the stretching vibrations of OAH bond disappeared at these fluences due to the high LET of the oxygen ions. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Kapton (registered trademark of Dupont), is a material of significant commercial importance having high performance, particularly in high radiation environment. It belongs to polyimides which are a class of thermally stable polymers based on stiff aromatic backbones. The ability of Kapton to maintain its excellent physical, electrical, and mechanical properties over a wide temperature range (from 269 °C to 400 °C) [1] has opened new design and application areas to plastic films. Kapton does not melt or burn as it has the highest UL-94 flammability rating: V-0. It has excellent chemical resistance [2]. At high temperature it offers good resistance to radiation effects, so used as a thermal blanket in the outer surface of satellite structures [3]. Low out-gassing rate makes it suitable insulator for ultra high vacuum environment. With a good combination of thermal conductivity, dielectric strength and availability in thin films, it is well suited material in cryogenics. Kapton films are available in a number of variants such as Kapton-H, HN, VN, FN and others. Out of these variants Kapton-H was the first film of its type and is a general purpose film
⇑ Corresponding author. E-mail address: [email protected] (R. Kumar).
which has a repeating structure of C22H10O5N2 (shown in fig. below)
Swift heavy ion (SHI) irradiation of polymers in the last few decades has emerged as one of the most important and effective techniques to induce modifications in the physico-chemical properties of polymers up to desired extent and tailor them as useful materials for different applications in commercial, scientific, medical and domestic fields [4–5]. The SHIs deposit huge amount of energy in the target polymeric material due to high value of linear energy transfer (LET) and the deposited energy is nonhomogeneous due to differential nature of energy lost by charged particles (i.e. dE/dX) in the material medium [6]. The deposited energy forms the latent tracks or defects along its path resulting in free radical formation, scission and cross-linking of the polymeric chains, bond cleavage, bond formation, carbon cluster
http://dx.doi.org/10.1016/j.nimb.2017.02.011 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: S.K. Gupta et al., Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.02.011
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formation, etc. [7–10] These damages, which are confined to beam diameter only, can be utilized to improve various properties, such as surface, optical, structural, chemical, electrical and free volume, etc. [11–15]. The modifications in the properties of polymers depend on ion beam parameters, such as energy, fluence, charge and mass of the impinging ion as well as thickness, structure and chemical composition of the polymer under irradiation [16]. The effects of linear energy transferred by swift heavy ions on the micro-structural properties of electrochemically synthesized polymer composite thin films are well documented in the literature [17,18] and it was found that the surface morphologies of the polymer changed significantly due to cross-linking or chain- scissoring of polymeric chains. So the irradiation of polymers leads to significant changes in physico-chemical properties of a polymer. A brief look at the existing literature reveals that many studies have been carried out in the last decade for modifications in the mechanical, structural and electrical properties of the polyimide Kapton-H polymer due to energetic ions irradiations. Kucheyev et al., studied the mechanical properties of Kapton-H such as hardness and observed that its Young’s modulus increased and tensile strength decreased after irradiating the polymer with 1H, 4 H, and 12C ions of energy lying between 1.5 MeV and 3.8 MeV at ion fluences of the order of 1011 ions cm2. They reported that these modifications depend linearly upon ion fluence and super linearly upon electronic energy loss [11].Variations in dielectric, optical, and structural properties of Kapton-H polymer irradiated to 80 MeV O6+ ion beam at fluences of 1011 to 1013 ions cm2 were observed by Prasher et al. [19]. The dielectric constant has been found to increase with increase of ion fluence whereas the optical band gap energy shows slight decrease in its value at higher ion fluences as reported by Kudaikulova et al. [20]. The stability and reflectivity of silver coated polyimide films were found to be improved up to 98% when prepared by heterogeneous chemical modification process. Such metalized reflectors find applications as solar energy concentrators with high electroconductivity to dissipate charges, which remarkably reduce weight and fragility of mirrors, providing good flexibility of packing for subsequent deployment in space applications. The inelastic mean free path, stopping power, and energy-loss straggling of electron, proton, and a-particle beams in a broad incident energy range in Kapton-H polymer have been calculated by de Vera et al. and they observed that irradiation with energetic ion beams offers a possibility to modify both the structural and the functional properties of polymer [3]. Effect of high temperature from 325 to 400 °C on optical properties and refractive index of Kapton-H has also been studied [21]. A continuous red shift of the absorption edge with rise in heating temperature with a corresponding decrease in the optical band gap was noticed. The refractive index of the heat treated samples was also found to be increased with increase in temperature. In the present work we intend to report the modifications induced in the physico-chemical properties of polyimide KaptonH polymer by irradiating it with oxygen ions (O7+) of energy 100 MeV at different fluences varying from 1 1011 ions/cm2 to 5 1012 ions/cm2. Many studies regarding the modifications in various properties of Kapton-H polymer with various light and heavy ions having different energies (low to medium) and different fluences are reported but no study has been carried out so far upon Kapton H polymer by irradiation with the oxygen ions to the best of our knowledge. So, it would be quite interesting to further explore the polymer in terms of modifications induced, if any, by oxygen ions (O7+) at specified energy and fluences. The aim of the present work is to understand the mechanism of modifications in the optical, structural and chemical properties of the polymer at different fluences in a better way and tailor them more efficiently as per requirements.
2. Experimental methods 2.1. SRIM (stopping and range of ions in matter) calculations Using SRIM code [22], the parameters such as the electronic energy loss (Se), nuclear energy loss (Sn) and projected range of 100 MeV oxygen ions in Kapton-H polymer were calculated which come out to be 53.6 eV/Å, 2.84 102 eV/Å and 122.74 mm respectively. 2.2. Materials and irradiation Thin sheets of Kapton-H polyimide of thickness 75 mm were commercially purchased from Goodfellow, United Kingdom. Kapton-H is a semi-crystalline polymer having triclinic crystal structure with tensile modulus around 2.5 GPa, density 1.42 g/ cm3 and glass transition temperature between 360 °C and 410 °C. The samples were used as received and no further studies were carried out to measure the thickness and density of the samples. These sheets were cut in 1 cm 1 cm size and irradiated by 100 MeV O7+ in the Material Science Beam Line of the 15 UD Pelletron accelerator facility at Inter University Accelerator Centre (IUAC), New Delhi, India. The fluences were taken in the range of 1 1011 ions/cm2 to 5 1012 ions/cm2. The beam current was 0.5 pnA and high vacuum of the order of 5.33 104 Pa was maintained in the target chamber during the bombardment. The beam was scanned in X-Y plane in order to irradiate the samples uniformly throughout the whole target area. The whole irradiation experiment was carried out at room temperature. 2.3. Characterization techniques 2.3.1. Optical studies UV–visible spectroscopy is an important technique to study and analyse the modifications in the optical properties such as optical band gap and the creation of conjugated double or triple bonds in the material. When the light in the ultraviolet and visible region is absorbed by a polymeric material, it results in the transition of electrons from lower energy states to higher energy states in the molecular orbital. The difference in energy of the two states of the orbital is called the optical band gap energy (Eg). This band gap energy gets modified when the material is exposed to energetic heavy ions. Any modification in the values of band gap energy results in the modification of the optical conductivity of the polymeric material. The UV–visible spectroscopy is also used to estimate the shift in the absorption edge of the polymeric samples upon irradiation, which is attributed to the formation of conjugated system of bonds. In the present study, Kapton-H polymer samples were studied by UV–visible spectroscopy in the range of 200–800 nm using Hitachi U-3300 spectrophotometer at IUAC, New Delhi, India. With the help of UV–visible absorption spectra of pristine and irradiated samples, the parameters such as optical band gap energy (Eg) (using Tauc’s relation) [23–24] for direct transitions were calculated by the following equation:
ðahmÞ ¼ Bðhm Eg Þ
n
ð1Þ
Here a is the optical absorption coefficient, B is the band tailing parameter and its value depends on the transition probability. The value of n characterizes the transition processes in K-space, which is taken to be equal to 1/2 for direct allowed transition (direct band gap calculation). The number of carbon atoms per conjugation length (N) [25–26] of the polymeric chain was calculated with the help of following equation
N ð34:3=Eg Þ2
ð2Þ
Please cite this article in press as: S.K. Gupta et al., Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.02.011
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2.3.2. Structural studies X-ray diffraction (XRD) is the technique used to study the modifications in the structural properties of a polymer upon irradiation. By analysing the XRD patterns, we can observe the shift in the diffraction peaks, if any, of the polymeric samples upon irradiation and therefore can estimate the change in lattice parameters. Also by observing the change in the values of FWHM (i.e. Full Width at Half Maximum), the change in the intensity of diffraction peaks can be obtained which results in the modification of the amorphous nature of the polymer upon irradiation. In the present study, Kapton-H polymer samples were studied by the X-ray diffraction (XRD) method using Brooker AXS system (scan speed of 1°/min) at IUAC, New Delhi, India. With the help of XRD patterns obtained for pristine and irradiated samples the crystallite size (L) of the pristine and oxygen ion irradiated polymer samples were calculated using following equation given by Scherrer [27]
L ¼ Kk=ðb cos hÞ
ð3Þ
Here K is a constant of proportionality (called the Scherrer constant), b is the full width at half maximum (FWHM) of the peak (in radian), k is the wavelength of the X-rays used (1.54 Å in case for Cu- Ka radiation) and h is the angle which is calculated by taking ½ of 2h value. 3. Results and discussion 3.1. UV–visible studies UV–visible spectra of pristine and oxygen ion irradiated samples of Kapton-H polymer are shown in Fig. 1. The optical absorption edges of irradiated samples shifted towards the red end (longer wavelength) of the visible spectrum, which increased with the increase of ion fluence. Fig. 2 shows the variation of absorbance w. r. t. oxygen ion fluence at a particular wavelength of 467 nm. The curve clearly indicates that the absorbance increases with increase in ion fluence. Similar results are reported by Kudo et al. [28] in their study of Kapton-H of thickness 12.5 lm bombarded with proton, helium beams, C, Si, Ar and Fe ion beams of energy 6 MeV per nucleon. They attributed the shift in the absorbance edge to the LET values of the ions in the polymer. More is the value of LET more is the shift of absorption edges towards higher wavelength of the spectrum. This shift was due to the degradation of CAC bond of the imide ring, which resulted into the cyclicization and condensation at high LET radiation. Some other researchers
Fig. 2. Graph between Absorbance vs Fluence of pristine and O7+ ion irradiated Kapton-H polymer samples at wavelength 467 nm.
also reported a shift of absorbance edge towards higher wavelength in Kapton-H polymer bombarded with ionising radiations at different energies [1,29]. According to them this shift in absorbance edge may be attributed to formation of conjugated system of double and triple bonds [30] and increase in conjugation length due to the creation of free electrons or free radicals. The direct consequence of this shift of absorbance edge is the increase in the energy of valence band thus causing the band gap between the valence and the conduction band to decrease, hence increasing the conductivity of the polymer. These absorption bands (in the of range 200–800 nm) are associated with p–p⁄ electronic transitions [31] and may be attributed to the presence of chromophores like aromatic rings such as phenyl rings having C@C and C@O double bonds [21].The optical band gap for direct transitions was calculated by using Eq. (1) and the values are tabulated in Table 1 for pristine as well as oxygen ion irradiated samples respectively. It was observed from the data that the band gap energy for ion irradiated polymer samples decreased from 2.64 eV to 2.58 eV i.e. 2.28% as compared to the pristine sample. The decrease in the band gap energy is quite significant within the typical calculation error (±0.01%). Many researchers reported the decrease in the band gap of different polymers when irradiated with radiation of different types [32–36]. Another effect of shift of absorbance edge towards higher wavelength is the increase in the number of carbon atoms per conjugation length (N). As shown in Table 1, the value of N for irradiated Kapton-H polymer samples increased from 169 to 176 (approx. 4.1%). The increase in the values of N is quite significant within the typical calculation error (±1.0%). 3.2. X- ray diffraction studies The XRD patterns of oxygen ion irradiated and pristine Kapton-H polymer samples are shown in Fig. 3. The XRD- patterns Table 1 Calculated values of Direct band gap energy (Eg) and number of carbon atoms per conjugation length (N) for pristine and O7+ ion irradiated Kapton-H polymer samples. Typical errors in the calculated values of Eg and N are about ±0.01 eV and ± 1 respectively.
Fig. 1. UV–visible spectra of pristine and O7+ ion irradiated Kapton-H polymer samples.
Fluence (ions/cm2)
Eg (eV)
N
Pristine 1 1011 5 1011 1 1012 5 1012
2.64 2.62 2.62 2.61 2.58
169 171 171 173 176
Please cite this article in press as: S.K. Gupta et al., Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.02.011
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S.K. Gupta et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx Table 3 Approximate characteristics of the vibration and corresponding band position for Kapton-H polymer [40–42,28]. Band position (cm1)
Assignment
734 1079 1167 1230,1367 1599, 1515 1703 1783 3380 3550 560
C@O bending CAOAC bending CAC bending CANAC stretching vibrations C@C aromatic C@O symmetrical stretching C@O asymmetrical stretching NH stretching OH symmetric stretching CAC skeletal vibrations
Fig. 3. XRD patterns of pristine and O7+ ion irradiated Kapton-H polymer samples.
Table 2 Calculated values of peak width (FWHM) and crystallite size (L) for pristine and O7+ion irradiated Kapton-H polymer samples. Typical errors in the calculated values of FWHM and L are 0.001 radians and 0.1 Å respectively. Fluence (ions/cm2)
FWHM (radians)
L (Å)
Pristine 1 1011 5 1011 1 1012 5 1012
0.028 0.031 0.032 0.032 0.026
52.17 50.74 45.34 46.21 54.68
Fig. 4. FTIR spectra of pristine and O7+ ion irradiated Kapton-H polymer samples.
conforms to the patterns showed by Ahmed et al. [29]. For the pristine sample the diffraction peaks occur at 2h–15.08°, 23.1° and 27.58° indicating the amorphous nature of the polymer [5]. In the irradiated samples identical peaks were observed with a minor change in the peak position. The new positions of the peaks were 14.55°, 21.76° and 26.88° respectively indicating change of lattice parameters of irradiated samples. Also a small variation in the peak intensity of the irradiated samples was observed. The intensity of the diffraction peaks for ion fluences from 1 1011 to 1 1012 ions/cm2 showed a slight decrease. The decrease in intensity of the diffraction peaks is obtained from the increase in FWHM (from 0.028 to 0.032 radians), as shown in Table 2 suggesting a decrease in the ordered behaviour of the polymeric chains due to chain scission at the mentioned fluences, which leads to the increase in amorphization of the polymer. At the fluence of 5 1012 ions/ cm2 the value of FWHM decreased marginally (from 0.028 to 0.026 radians) resulting in the ordered behaviour of the polymer (Table 2) due to cross linking, resulting in an increase in the crystalline nature of the polymer. Also the average crystallite size of the irradiated samples decreased from 52.17 for the pristine sample to 45.34 at fluence of 5 1011 ions/cm2 confirming the chain scission of the polymeric chains and an increase in the value of crystallite size from 52.17 to 54.68 at the highest fluence confirming the cross-linking of the polymer chains at this fluence. Similar results are also reported in different polymer samples due to radiation exposure in which the crystalline nature of the polymer material varies with the variation of ion fluence or gamma dose [37–39]. 3.3. FTIR studies The chemical modifications in the irradiated Kapton-H polymer samples were studied by Fourier transform infrared (FTIR)
Fig. 5. Graph between%T and Fluence of pristine and O7+ ion irradiated Kapton-H polymer samples for wave numbers 2344 cm1 and 2830 cm1.
spectroscopy using Perkin-Elmer FTIR spectrophotometer in the range 4000–400 cm1 in transmission mode at Variable Energy Cyclotron Centre (VECC), Kolkata, India. The FTIR spectrum of pristine Kapton-H polymer sample conforms to the spectrum reported by Garg and Quamara; Guenther et al.; Sun et al.; Kudo et al. [40– 42,28]. In the pristine spectrum of polyimide Kapton-H, primarily groups were assigned viz. imide group ((C@O)2NAC) at 1117 cm1, ether group (ACAOACA) at 1167 cm1 and carbonyl group (C@O) at 1776 cm1 and the band between 3450 and 3650 corresponds to AOH and C@O stretching vibrations. Various other approximate characteristics of the vibrations and corresponding
Please cite this article in press as: S.K. Gupta et al., Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.02.011
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wave number values (band position) of the pristine sample which show the characteristic absorption bands of the imide and aromatic groups are represented in Table 3. Fig. 4 represents the FTIR spectra of pristine and oxygen ion irradiated samples of Kapton-H polymer. In the spectra a new band was seen to be developed between 2297 and 2387 cm1 (correspond to C„C or C„N) at the fluences of 1 1011 and 5 1011 ions/cm2 as shown by the circled part of the spectra. The absorption band at position 2830 cm1 (OAH stretching) seems to be disappeared at the ion fluence of 5 1011 ions/cm2. The new developments can be explained with the help of Fig. 5, which represents the plots between %T (transmittance) and the different ion fluences at some specific wavenumbers 2344 cm1 and 2830 cm1 respectively. It is clear from the plot (black curve) that, at the wavenumber 2344 cm1, the transmittance decrease or in turn the corresponding absorbance increase significantly at the fluences of 1 1011 ions/cm2 and 5 1011 ions/cm2 showing the formation of new band, probably due to increased cross-linking of the polymeric chains. Similarly, the increase in the transmittance (or decrease in absorbance) from the plot (red curve) of Fig. 5, at the wavenumber 2830 cm1 indicates the disappearance of band probably due to increased scission of the polymeric chains. Absorption bands at other wave numbers showed an insignificant variation in their intensity. 4. Conclusions The increase in the number of carbon atoms per conjugation length (N) due to the formation of conjugated system of double and triple bonds and the corresponding decrease in band gap energy attributed to the formation of free radicals confirm that the effect of ion irradiation induces cross linking in the polymeric chains and improves the optical properties of the Kapton-H polymer, particularly at the highest fluence. Modifications in the structural properties of the Kapton-H polymer are attributed to change in the lattice parameters due to the shift in the positions of the diffraction peaks. A slight decrease in the intensity of the diffraction peaks at lower and intermediate fluences indicate an increase in the amorphous character of the polymer. At the highest fluence the intensity of the diffraction peaks increased slightly suggesting an increase in the crystalline nature of the polymer. The Kapton-H polymer also showed some chemical modifications upon ion irradiation due to the developement of new absorption bands at 2297–2387 cm1 (C„C or C„N bending) at intermediate fluences and disappearance of absorption band at 2830 cm1 (OAH stretching) at higher fluence. Also decrease in intensity of existed absorption bands was observed at different ion fluences. Acknowledgements The authors are thankful to the staff at IUAC, New Delhi, India for providing the ion irradiation facilities and characterization techniques such as XRD and UV–vis spectroscopy. The authors also extend their gratitude to Dr. A. K. Himanshu, Scientist-D at Variable Energy Cyclotron Center, Kolkata, India for providing the FTIR facility.
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Please cite this article in press as: S.K. Gupta et al., Modifications in physico-chemical properties of 100 MeV oxygen ions irradiated polyimide Kapton-H polymer, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.02.011