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100 MeV Ag7+ ion induced physicochemical changes in isotactic polypropylene
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NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 1793–1798 www.elsevier.com/locate/nimb
100 MeV Ag7+ ion induced physicochemical changes in isotactic polypropylene N.L. Mathakari a, D. Kanjilal b, V.N. Bhoraskar a, S.D. Dhole a,* a
Microtron Accelerator Laboratory, Department of Physics, University of Pune, Ganeshkhind, Pune 411007, Maharashtra, India b Inter University Accelerator Center, New Delhi, India Received 25 September 2007 Available online 22 November 2007
Abstract Thin films of isotactic polypropylene having 500 lm thickness were irradiated with 100 MeV Ag7+ ions at the fluences varying from 1011 to 5 1012 ions/cm2. The properties such as chemical, optical, structural and surface morphology were characterized by techniques namely FTIR, UV–visible, photoluminescence, XRD, SEM and contact angle method. The FTIR spectra show the scissioning of C–H and C–C bonds, whereas, in photoluminescence, the intensities of the peaks at 440 and 480 nm in emission spectra and at 236 nm in excitation spectra observed to be decreased with increase in ion fluence. This may be due to the decomposition of luminescent centers. The UV–visible spectra also show a remarkable red shift from 218 to 367 nm and the subsequent large reduction in the optical band gap from 5.37 to 3.39 eV. This attributes to the carbonization or graphitization of polypropylene. On the contrary, the intensities of XRD peaks, particularly the peak due to 1 1 0 planes, shows sufficient enhancement which signifies overall increase in crystallinity. This ascribes to relief in the local strain on the crystallites due to scissioning of tie molecules in the amorphous zones. The contact angle has increased from 78° to 97° which reveals the absence of hydrophilic functional groups, carbonization and surface roughening. The result is also supported by SEM analysis. Ó 2007 Elsevier B.V. All rights reserved. PACS: 61.80.Lj; 61.82.Pv; 34.50.Bw; 68.49.Uv; 61.80.Nz; 68.37.Hk; 62.20.Qp; 33.50.Dq Keywords: SHI irradiation; Polypropylene; FTIR; UV–visible; XRD; Photoluminescence spectroscopy; SEM; Contact angle; Hydrophobicity; Surface roughening; Oxidation
1. Introduction From last few decades, studies related to irradiation effects on polymers have emerged as an important area of basic as well as applied research. Since irradiation of polymers can modify their structure and properties due to scissioning and crosslinking, some of the irradiation-based polymer processing methods have emerged such as radiation assisted diffusion, growth of polymer nano-composites, polymer blends, polymer grafting etc [1,2]. Systematic investigations on effects of a wide range of radiations on various polymers are thus important for provid*
Corresponding author. Tel.: +91 20 25692678; fax: +91 20 25691684. E-mail address: [email protected] (S.D. Dhole).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.10.039
ing useful information for radiation processing of polymers. The radiation-induced physicochemical changes are very specific to type of polymer, radiation and its environment [3]. Amongst the range of low to high-energy radiations, Swift heavy ions (SHI) in particular are of special interest from basic as well as application point of view. This is because in contrary to ‘‘classical” radiations such as gamma rays and electrons, SHI irradiation is characterized by highest known linear energy transfer (LET) and largest deposited energy density along the trajectory of ion and its range [4,5]. Thus the polymers, which are difficult to process by chemical methods, can be easily modified using SHI irradiation. This leads to significant physicochemical changes, a few of which include radiochemical alterations such as unsaturation or formation of double
N.L. Mathakari et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1793–1798
bonds, evolution of gases, formation of carbonaceous clusters, chemically etchable latent tracks, change in intrinsic free volume, creation of defects (such as free radicals or impurities), amorphisation, phase transitions etc. As a result, SHI irradiation of polymers have shown great potential for the fields such as microelectronics, biomedical, device technology, nano-materials and material science [4–6]. Polypropylene, though simple in chemical structure, is a versatile, low cost, chemically stable and lightweight polymer. It offers attractive mechanical, electrical and thermal properties due to its relative higher degree of crystallinity and is used in many applications such as radiation-sterilized medical and pharmaceutical components, food packaging materials and cosmetics [7,15]. There are number of reports available on the effects of low as well as high-energy radiations on polypropylene, out of which, the gamma irradiation [7,9] and high energy electron irradiation [10–12] shows significant chemical and morphological changes. A literature survey indicates that SHI irradiation of polypropylene showed decrease in band gap, loss in crystallinity and formation of carbonyl and hydroxyl groups for 86 MeV Ni7+, 3.6 and 5.4 MeV C+ [13,15], whereas decrease in melting and crystallization temperature for 80 MeV Si7+ and 120 MeV Ag9+ [14]. However, to modify the polymer, energy loss of SHI is very important and specific to its nature, energy and environment. The effects of such parameters of SHI irradiation on surface properties of polypropylene have not been paid much attention. Therefore, in this paper, the effects of 100 MeV Ag7+ ions on chemical, optical, structural and surface properties of polypropylene have been studied in order to confirm the relative contributions of carbonization, conjugation and oxidation. 2. Experimental Thin films of isotactic polypropylene having approximately 500-lm thickness were obtained by compression molding technique. In this technique, the granules of isotactic polypropylene were compressed at a temperature of 185° C and 5 103 pounds. The films thus molded were then cooled in ambient atmosphere. Samples having dimensions 1 cm 1 cm 500 lm were cut from these films. These samples were then irradiated by 100 MeV Ag7+ ions from 15 UD pelletron accelerator at Interuniversity Accelerator Center, New Delhi, India. The fluence was varied from sample to sample in the range of 1 1011 to 5 1012 ions/cm2. The irradiation was carried out in high vacuum (10 8 torr) and at room temperature. The energy loss (LET) of 100 MeV Ag7+ ions in polypropylene as calculated from SRIM, TRIM code is 7.306 MeV/lm and the range is estimated to be 24.82 lm. The pre and post-irradiated samples were then subjected to various characterizations. FTIR spectroscopy was used to investigate the effects of radiation on scissioning and/or crosslinking of various bonds and to verify the possibility of
formation of various irradiation-induced functional groups, especially the carbonyl and hydroxyl ones. UV–visible spectroscopy was used to study the changes in typical optical properties such as band gap and absorbance in UV–visible region. Photoluminescence spectroscopy (PL) in emission as well as excitation mode was carried out to investigate the changes in luminescent properties of polypropylene, particularly as regards to SHI irradiationinduced oxidation, unsaturation and defects. The peaks recorded in emission spectra were further confirmed in excitation spectra. XRD was used to investigate the irradiation-induced changes in various crystalline properties such as relative crystallinity, crystal symmetry and size of crystallites. The surface morphology was investigated using SEM analysis. Finally, the effects of SHI irradiation on surface texture and chemistry of polypropylene were studied using contact angle method. 3. Results and discussion 3.1. FTIR analysis Fig. 1 shows the typical FTIR spectra of pristine and ion-irradiated polypropylene at different fluences. Y scale in the figure is appended for comparison. In the spectra, absorbance in the region 2850–2950 cm 1 corresponds to CH3, CH2, CH stretching, while the peaks in the region 1450–1480 cm 1 are due to CH3 and CH2 bending. Various weak, medium and strong peaks in the region 809 to 1377 cm 1 correspond to CH3, CH2, CH bending, wagging, twisting and C–C stretching. The peaks observed near 1170 cm 1, 999–977 cm 1 confirm the isotactic category of polypropylene used in this study [8,9]. It can be observed that the absorbance at various positions rapidly decreases with the fluence and it clearly indicates the scissioning of various C–H and C–C bonds due to 100 MeV Ag7+ ion irradiation. The scissioning of bonds is observed to become apparent at the minimum fluence of 1 1011 itself and the 450 % Transmittance (Appended)
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Fig. 1. FTIR spectra of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at different fluences: spectra recorded a few days after irradiation.
N.L. Mathakari et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1793–1798
3.2. UV–visible spectroscopy Fig. 3 shows UV–visible spectra of pristine and Ag7+ ion-irradiated polypropylene at different fluences. It can be observed that the absorption edge rapidly shifts with the fluence towards higher wavelength. The maximum red shift is from pristine value of 218 nm to 367 nm at the highest fluence of 5 1012 ions/cm2. This corresponds to
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Fig. 3. UV–visible spectra of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at different fluences.
decrease in the optical band gap from 5.37 to 3.39 eV, which is almost 68%. Moreover, the absorbance at all the wavelengths is observed to be increased with the ion fluence. The large reduction of optical band gap can be attributed to formation of carbon enriched clusters due to partial evolution of hydrogen after irradiation. The residual carbon left after the evolution of hydrogen, may have graphite-like form which results in to the decrease in optical band gap [13,16,17]. Further, the increase in absorbance at all the wavelengths in UV–visible regime for different fluences can be attributed to change in color of the films after irradiation. It was observed that the pristine polypropylene film was colorless and after irradiation it successively became yellowish, reddish and finally brownish with increase in fluence. This strongly attributes to carbonization, irradiation-induced defects and chromophoric groups behaving as color centers. 3.3. XRD analysis Fig. 4 shows the XRD spectra of pristine and ion-irradiated polypropylene at various fluences. The spectra are (110)
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bond structure is found to be successively and rigorously damaged in the fluence range from 5 1011 to 5 1012 ions/cm2. Moreover, the peaks near 1170 cm 1, 999–977 cm 1 are also found to be decreased in intensity with the ion fluence. This indicates that the isotactic arrangements of polymer chains are distorted after irradiation. However, the typical peaks around 1725 cm 1 and 3400 cm 1 which correspond to carbonyl (C@O) and hydroxyl (OH) groups respectively, are not noticeably observed in the FTIR spectra. This shows that polypropylene irradiated by 100 MeV Ag7+ in high vacuum has not undergone oxidation process. This attributes to the presence of high vacuum during irradiation, insignificant concentration of free radicals and carbonization on the surface of polypropylene. As the energy loss of 100 MeV Ag7+ ions in polypropylene is considerably high, the possibility of partial scissioning of bonds (that is required for production of free radicals involving unpaired electrons) seems to be insignificant. Secondly, the carbonization may have reduced the free volume on irradiated polypropylene surface [16]. The FTIR analysis of same irradiated samples was repeated after four months, in order to investigate the post-irradiation effects as regards to oxidation of polypropylene. These spectra are shown in Fig. 2. It is found that, particularly at low fluences, the C–H bonds are partially recovered and peak corresponding to carbonyl groups are also marginally observed at 1705 cm 1. This indicates the possibility of crossliniking and oxidation at low fluences in the post-irradiation period.
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Fig. 2. FTIR spectra of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at different fluences: spectra recorded four months after irradiation.
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Fig. 4. XRD spectra of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at different fluences.
N.L. Mathakari et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1793–1798
appended for comparison. The XRD spectra confirms the known a-monoclinic form of polypropylene and the prominent peaks observed at 2h = 14°, 17°, 18.5° and 22° correspond to (1 1 0), (0 4 0), (0 3 0) and (0 4 1) planes, respectively [18]. It is observed from the figure that there is a successive growth in the intensity of crystalline peaks with the ion fluence. In particular, the height of (1 1 0) peak markedly increases with the fluence. This mainly attributes to local relief in the strain on the crystallites due to breaking of tie molecules existing in amorphous zones [12]. Secondly, the molecules that acquire extra mobility due to scissioning may displace from amorphous to the crystalline zones [19]. As a result, the crystalline zones may enrich in atoms while the amorphous zones may deplete. Moreover, the significant increase in overall crystallinity may also indicate that the relative fraction of crosslinked network contributes to amorphous content of the polypropylene, and it decreases due to scissioning.
3.4. Photoluminescence spectra An idealized polypropylene is not known to be luminescent; however the presence of impurities, additives, defects, unsaturation and chromophores may cause photoluminescence [20,21]. Fig. 5 shows the photoluminescence spectra of pristine and Ag7+ ion-irradiated polypropylene at different fluences. The excitation wavelength used in this case was 236 nm. The emission spectra show a strong peak at 443 nm i.e. 2.81 eV and a weak peak at 484 nm i.e. 2.57 eV. These peaks were further confirmed by recording the excitation spectra by taking emission wavelength as 443 nm. Fig. 6 shows the corresponding excitation spectra which confirms the excitation peak at 236 nm. It is observed that, the intensity of both the peaks in emission spectra decrease with the increase in ion fluence. This shows that polypropylene becomes successively less luminescent after ion irradiation. This attributes to the disappearance of luminescent centers such as impurities, 1000 900 Intensity (Arb. Units)
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Fig. 6. Photoluminescence (excitation) spectra of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at different fluences.
defects, unsaturation and chromophores in the polypropylene. [20,21]. As SHI leads to scissioning of various bonds and evolution of gases, the luminescent centers may decompose and then released during irradiation, which results in to the decrease in intensity of the emission peaks. Further, the absence of the effects such as oxidation and conjugation and the decrease in optical band gap also seems to be responsible for decrease in the PL intensity. 3.5. Contact angle measurements Wettebility of the polymer surface is governed by its roughness and surface chemistry. In general, surfaces involving polar groups such as carbonyl or hydroxyl are known to be hydrophilic. Thus the contact angle measurements can clarify the changes in the surface texture and chemistry after SHI irradiation. Fig. 7 shows the contact angles of a 25 ll drop of distilled water on the surface of pristine and Ag7+ ion-irradiated polypropylene at the maximum fluence of 5 1012 ions/cm2. It is observed that the contact angle increases from pristine value of 78° to 97°. This increase in the contact angle is almost 24% and it clearly shows that polypropylene surface has become significantly hydrophobic after irradiation. This attributes to the absence of carbonyl and hydroxyl groups which are known to be polar, excessive surface roughening and carbonization (graphite is non polar) [22–24]. Moreover, the trapped air between water droplet and the surface due to increased surface roughness may reduce the liquid–solid contact area resulting in increased hydrophobicity [25]. The surface roughening is supported by SEM analysis.
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Fig. 5. Photoluminescence (emission) spectra of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at different fluences.
Fig. 8 shows the SEM images of pristine and 100 MeV Ag+7 ion-irradiated polypropylene at different fluences. It can be observed that the degree of roughness/imperfection increases with the ion fluence. The surface of ion-irradiated
N.L. Mathakari et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1793–1798
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Fig. 7. Photographs of a 25 ll water drop on the surface of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at the fluence of 5 1012 ions/cm2.
Fig. 8. SEM images of pristine 100 MeV Ag7+ ion-irradiated polypropylene at different fluences.
polypropylene is observed to be consisting of defects in the form of voids, blisters, globules and tracks of varying sizes. This can be mainly attributed to uneven evolution of gases from the surface of polypropylene during ion irradiation. The scissioning of bonds due to ion irradiation results in the erosion/sputtering of atoms on the surface of polypropylene. The rate of evolution of gases may have different yields at different positions on the surface. This also results in change free volume near the polypropylene surface, due which it is observed to become successively rough with the ion fluence [26–28]. 4. Conclusion Polypropylene has shown significant modifications in the surface and bulk properties after 100 MeV Ag7+ ion irradiation. The FTIR spectra show scissioning of C–H and C–C bonds, however, oxidation effects are not noticeably observed. Whereas, UV–visible spectra have shown 68% reduction in the band gap and also increase in the absorbance. This mainly attributes to the carbonization. XRD spectra show significant increase in overall crystallinity which indicates the increase in the fraction of crystalline at the expense of amorphous zones. The PL spectra show decrease in the intensity of emission as well as excitation peaks which reveal the disappearance of luminescent centers due to SHI irradiation. Contact angle is found to be increased with the fluence which indicates the increase in hydrophobic properties, due to absence of oxidation effects, excessive surface roughening and carbon enrichment on the surface. The surface roughening is confirmed in SEM analysis. This study thus leads to a definite conclusion that 100 MeV Ag7+ ion irradiation can induce several desirable changes in the structure and properties of poly-
propylene namely, prominent increase in overall crystallinity, significant decrease in optical band gap and increase in the hydrophobicity of the surface. References [1] M.R. Cleland, L.A. Parks, S. Cheng, Nucl. Instr. and Meth. B 208 (2003) 66. [2] Andrzej G. Chmielewski, Mohammad Haji-Saeid, Shamshad Ahmed, Nucl. Instr. and Meth. B 236 (2005) 44. [3] A. Cahpiro, Nucl. Instr. and Meth. B 105 (1995) 5. [4] E. Balanzt, N. Betz, S. Bouffard, Nucl. Instr. and Meth. B 105 (1995) 46. [5] J. Jagielski, A. Turos, D. Bielinski, A.M. Abdul-Kader, A. Piatkowska, Nucl. Instr. and Meth. B 261 (2007) 690. [6] D. Fink, P.S. Alegaonkar, A.V. Petrov, M. Wilhelm, P. Szimkowiak, M. Behar, D. Sinha, W.R. Fahrner, K. Hoppe, L.T. Cahdderton, Nucl. Instr. and Meth. B 236 (2005) 11. [7] Harish B. Thorat, C.S. Prabhu, K. Suresh Kumar, M.V. Pandya, J. Appl. Polym. Sci. 65 (1997) 2715. [8] M.P. Macdonald, I.M. Ward, Polymer 2 (1961) 341. [9] D. Sinha, T. Swu, S.P. Tripathy, R. Mishra, K.K. Dwivedi, D. Fink, Rad. Eff. Deffect Sol. 158 (2003) 531. [10] R. Mishra, S.P. Tripathi, K.K. Dwiwedi, D.T. Khathing, S. Ghosh, M. Muller, D. Fink, Radiat. Measure. 33 (2001) 845. [11] H.M. Abdel-Hamid, Solid State Electron. 49 (2005) 1163. [12] R.M. Radwan, J. Phy. D: Appl. Phys. 40 (2007) 374. [13] Lakhwant Singh, Ravinder Singh, Nucl. Instr. and Meth. B 225 (2004) 478. [14] S. Chawla, A.K. Ghosh, S. Ahmad, D.K. Avasthi, Nucl. Instr. and Meth. B 224 (2006) 248. [15] A. Saha, V. Chakraborty, S.N. Chintalapudi, Nucl. Instr. and Meth. B 168 (2000) 245. [16] V. Svorcik, V. Rybka, O. Jankovski, V. Hnatowicz, J. Appl. Polym. Sci. 61 (1996). [17] D. Fink, W.H. Chung, M. Wilhelm, Rad. Eff. Deffect Sol. 133 (1995) 209. [18] Kushal Sen, Praveen Kumar, J. Appl. Polym. Sci. 55 (1995) 857. [19] N.V. Bhat, M.M. Nate, M.B. Kurup, V.A. Bambole, S. Sabharwal, Nucl. Instr. and Meth. B 237 (2005) 585.
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[20] Ohki Y. Toyoda, T. Hama, Y. Wei Wei Massines, F. in: Proceedings of 6th International Conference on Properties and Applications of Dielectric Materials, June 21–26, 2000, IEEE, 1, 2000, p. 439. [21] G. Teyssedre, P. Tiemblo, F. Massines, C. Laurent, J. Phys. D: Appl. Phys. 29 (1996) 3137. [22] T. Lippert, T. Nakamura, H. Niino, A. Yabe, Appl. Surf. Sci. 109/110 (1997) 227. [23] A. Hozumi, N. Shirahata, Y. Nakanishi, S. Asakura, A. Fuwa, J. Vac. Sci. Tech. A22 (4) (2004) 1309, Jul/Aug.
[24] T. Uchida, N. Shimo, H. Sugimura, H. Masuhara, J. Appl. Phys. 76 (8) (1994) 4872. [25] H.Y. Kwong, M.H. Wong, Y.W. Wong, K.H. Wong, Appl. Surf. Sci. 253 (2007) 8841. [26] A.M. El-Naggar, H.C. Kim, L.C. Lopez, G.L. Wilkes, J. Appl. Polym. Sci. 37 (1989) 1655. [27] R.M. Abdul Majeed, V.S. Purohit, S.V. Bhoraskar, A.B. Mandale, V.N. Bhoraskar, Rad. Eff. Deffect Sol. 161 (8) (2006) 495. [28] Sobu Thomas, B.R. Gupta, S.K. De, Polym. Degrad. Stabil. 18 (1987) 189.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 1793–1798 www.elsevier.com/locate/nimb
100 MeV Ag7+ ion induced physicochemical changes in isotactic polypropylene N.L. Mathakari a, D. Kanjilal b, V.N. Bhoraskar a, S.D. Dhole a,* a
Microtron Accelerator Laboratory, Department of Physics, University of Pune, Ganeshkhind, Pune 411007, Maharashtra, India b Inter University Accelerator Center, New Delhi, India Received 25 September 2007 Available online 22 November 2007
Abstract Thin films of isotactic polypropylene having 500 lm thickness were irradiated with 100 MeV Ag7+ ions at the fluences varying from 1011 to 5 1012 ions/cm2. The properties such as chemical, optical, structural and surface morphology were characterized by techniques namely FTIR, UV–visible, photoluminescence, XRD, SEM and contact angle method. The FTIR spectra show the scissioning of C–H and C–C bonds, whereas, in photoluminescence, the intensities of the peaks at 440 and 480 nm in emission spectra and at 236 nm in excitation spectra observed to be decreased with increase in ion fluence. This may be due to the decomposition of luminescent centers. The UV–visible spectra also show a remarkable red shift from 218 to 367 nm and the subsequent large reduction in the optical band gap from 5.37 to 3.39 eV. This attributes to the carbonization or graphitization of polypropylene. On the contrary, the intensities of XRD peaks, particularly the peak due to 1 1 0 planes, shows sufficient enhancement which signifies overall increase in crystallinity. This ascribes to relief in the local strain on the crystallites due to scissioning of tie molecules in the amorphous zones. The contact angle has increased from 78° to 97° which reveals the absence of hydrophilic functional groups, carbonization and surface roughening. The result is also supported by SEM analysis. Ó 2007 Elsevier B.V. All rights reserved. PACS: 61.80.Lj; 61.82.Pv; 34.50.Bw; 68.49.Uv; 61.80.Nz; 68.37.Hk; 62.20.Qp; 33.50.Dq Keywords: SHI irradiation; Polypropylene; FTIR; UV–visible; XRD; Photoluminescence spectroscopy; SEM; Contact angle; Hydrophobicity; Surface roughening; Oxidation
1. Introduction From last few decades, studies related to irradiation effects on polymers have emerged as an important area of basic as well as applied research. Since irradiation of polymers can modify their structure and properties due to scissioning and crosslinking, some of the irradiation-based polymer processing methods have emerged such as radiation assisted diffusion, growth of polymer nano-composites, polymer blends, polymer grafting etc [1,2]. Systematic investigations on effects of a wide range of radiations on various polymers are thus important for provid*
Corresponding author. Tel.: +91 20 25692678; fax: +91 20 25691684. E-mail address: [email protected] (S.D. Dhole).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.10.039
ing useful information for radiation processing of polymers. The radiation-induced physicochemical changes are very specific to type of polymer, radiation and its environment [3]. Amongst the range of low to high-energy radiations, Swift heavy ions (SHI) in particular are of special interest from basic as well as application point of view. This is because in contrary to ‘‘classical” radiations such as gamma rays and electrons, SHI irradiation is characterized by highest known linear energy transfer (LET) and largest deposited energy density along the trajectory of ion and its range [4,5]. Thus the polymers, which are difficult to process by chemical methods, can be easily modified using SHI irradiation. This leads to significant physicochemical changes, a few of which include radiochemical alterations such as unsaturation or formation of double
N.L. Mathakari et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1793–1798
bonds, evolution of gases, formation of carbonaceous clusters, chemically etchable latent tracks, change in intrinsic free volume, creation of defects (such as free radicals or impurities), amorphisation, phase transitions etc. As a result, SHI irradiation of polymers have shown great potential for the fields such as microelectronics, biomedical, device technology, nano-materials and material science [4–6]. Polypropylene, though simple in chemical structure, is a versatile, low cost, chemically stable and lightweight polymer. It offers attractive mechanical, electrical and thermal properties due to its relative higher degree of crystallinity and is used in many applications such as radiation-sterilized medical and pharmaceutical components, food packaging materials and cosmetics [7,15]. There are number of reports available on the effects of low as well as high-energy radiations on polypropylene, out of which, the gamma irradiation [7,9] and high energy electron irradiation [10–12] shows significant chemical and morphological changes. A literature survey indicates that SHI irradiation of polypropylene showed decrease in band gap, loss in crystallinity and formation of carbonyl and hydroxyl groups for 86 MeV Ni7+, 3.6 and 5.4 MeV C+ [13,15], whereas decrease in melting and crystallization temperature for 80 MeV Si7+ and 120 MeV Ag9+ [14]. However, to modify the polymer, energy loss of SHI is very important and specific to its nature, energy and environment. The effects of such parameters of SHI irradiation on surface properties of polypropylene have not been paid much attention. Therefore, in this paper, the effects of 100 MeV Ag7+ ions on chemical, optical, structural and surface properties of polypropylene have been studied in order to confirm the relative contributions of carbonization, conjugation and oxidation. 2. Experimental Thin films of isotactic polypropylene having approximately 500-lm thickness were obtained by compression molding technique. In this technique, the granules of isotactic polypropylene were compressed at a temperature of 185° C and 5 103 pounds. The films thus molded were then cooled in ambient atmosphere. Samples having dimensions 1 cm 1 cm 500 lm were cut from these films. These samples were then irradiated by 100 MeV Ag7+ ions from 15 UD pelletron accelerator at Interuniversity Accelerator Center, New Delhi, India. The fluence was varied from sample to sample in the range of 1 1011 to 5 1012 ions/cm2. The irradiation was carried out in high vacuum (10 8 torr) and at room temperature. The energy loss (LET) of 100 MeV Ag7+ ions in polypropylene as calculated from SRIM, TRIM code is 7.306 MeV/lm and the range is estimated to be 24.82 lm. The pre and post-irradiated samples were then subjected to various characterizations. FTIR spectroscopy was used to investigate the effects of radiation on scissioning and/or crosslinking of various bonds and to verify the possibility of
formation of various irradiation-induced functional groups, especially the carbonyl and hydroxyl ones. UV–visible spectroscopy was used to study the changes in typical optical properties such as band gap and absorbance in UV–visible region. Photoluminescence spectroscopy (PL) in emission as well as excitation mode was carried out to investigate the changes in luminescent properties of polypropylene, particularly as regards to SHI irradiationinduced oxidation, unsaturation and defects. The peaks recorded in emission spectra were further confirmed in excitation spectra. XRD was used to investigate the irradiation-induced changes in various crystalline properties such as relative crystallinity, crystal symmetry and size of crystallites. The surface morphology was investigated using SEM analysis. Finally, the effects of SHI irradiation on surface texture and chemistry of polypropylene were studied using contact angle method. 3. Results and discussion 3.1. FTIR analysis Fig. 1 shows the typical FTIR spectra of pristine and ion-irradiated polypropylene at different fluences. Y scale in the figure is appended for comparison. In the spectra, absorbance in the region 2850–2950 cm 1 corresponds to CH3, CH2, CH stretching, while the peaks in the region 1450–1480 cm 1 are due to CH3 and CH2 bending. Various weak, medium and strong peaks in the region 809 to 1377 cm 1 correspond to CH3, CH2, CH bending, wagging, twisting and C–C stretching. The peaks observed near 1170 cm 1, 999–977 cm 1 confirm the isotactic category of polypropylene used in this study [8,9]. It can be observed that the absorbance at various positions rapidly decreases with the fluence and it clearly indicates the scissioning of various C–H and C–C bonds due to 100 MeV Ag7+ ion irradiation. The scissioning of bonds is observed to become apparent at the minimum fluence of 1 1011 itself and the 450 % Transmittance (Appended)
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100 Pristine
50 0 4000 3500 3000 2500 2000 1500 1000 Wavenumber (1/cm)
500
Fig. 1. FTIR spectra of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at different fluences: spectra recorded a few days after irradiation.
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3.2. UV–visible spectroscopy Fig. 3 shows UV–visible spectra of pristine and Ag7+ ion-irradiated polypropylene at different fluences. It can be observed that the absorption edge rapidly shifts with the fluence towards higher wavelength. The maximum red shift is from pristine value of 218 nm to 367 nm at the highest fluence of 5 1012 ions/cm2. This corresponds to
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0.0 200
300
218 232
400
500
329 355 367 nm
600
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Wavelength (nm)
Fig. 3. UV–visible spectra of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at different fluences.
decrease in the optical band gap from 5.37 to 3.39 eV, which is almost 68%. Moreover, the absorbance at all the wavelengths is observed to be increased with the ion fluence. The large reduction of optical band gap can be attributed to formation of carbon enriched clusters due to partial evolution of hydrogen after irradiation. The residual carbon left after the evolution of hydrogen, may have graphite-like form which results in to the decrease in optical band gap [13,16,17]. Further, the increase in absorbance at all the wavelengths in UV–visible regime for different fluences can be attributed to change in color of the films after irradiation. It was observed that the pristine polypropylene film was colorless and after irradiation it successively became yellowish, reddish and finally brownish with increase in fluence. This strongly attributes to carbonization, irradiation-induced defects and chromophoric groups behaving as color centers. 3.3. XRD analysis Fig. 4 shows the XRD spectra of pristine and ion-irradiated polypropylene at various fluences. The spectra are (110)
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% Transmittance (Appended)
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4.5
Absorbance (Arb. units)
bond structure is found to be successively and rigorously damaged in the fluence range from 5 1011 to 5 1012 ions/cm2. Moreover, the peaks near 1170 cm 1, 999–977 cm 1 are also found to be decreased in intensity with the ion fluence. This indicates that the isotactic arrangements of polymer chains are distorted after irradiation. However, the typical peaks around 1725 cm 1 and 3400 cm 1 which correspond to carbonyl (C@O) and hydroxyl (OH) groups respectively, are not noticeably observed in the FTIR spectra. This shows that polypropylene irradiated by 100 MeV Ag7+ in high vacuum has not undergone oxidation process. This attributes to the presence of high vacuum during irradiation, insignificant concentration of free radicals and carbonization on the surface of polypropylene. As the energy loss of 100 MeV Ag7+ ions in polypropylene is considerably high, the possibility of partial scissioning of bonds (that is required for production of free radicals involving unpaired electrons) seems to be insignificant. Secondly, the carbonization may have reduced the free volume on irradiated polypropylene surface [16]. The FTIR analysis of same irradiated samples was repeated after four months, in order to investigate the post-irradiation effects as regards to oxidation of polypropylene. These spectra are shown in Fig. 2. It is found that, particularly at low fluences, the C–H bonds are partially recovered and peak corresponding to carbonyl groups are also marginally observed at 1705 cm 1. This indicates the possibility of crossliniking and oxidation at low fluences in the post-irradiation period.
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0 500
Fig. 2. FTIR spectra of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at different fluences: spectra recorded four months after irradiation.
10
20
30 40 2θ (Deg)
50
60
Fig. 4. XRD spectra of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at different fluences.
N.L. Mathakari et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1793–1798
appended for comparison. The XRD spectra confirms the known a-monoclinic form of polypropylene and the prominent peaks observed at 2h = 14°, 17°, 18.5° and 22° correspond to (1 1 0), (0 4 0), (0 3 0) and (0 4 1) planes, respectively [18]. It is observed from the figure that there is a successive growth in the intensity of crystalline peaks with the ion fluence. In particular, the height of (1 1 0) peak markedly increases with the fluence. This mainly attributes to local relief in the strain on the crystallites due to breaking of tie molecules existing in amorphous zones [12]. Secondly, the molecules that acquire extra mobility due to scissioning may displace from amorphous to the crystalline zones [19]. As a result, the crystalline zones may enrich in atoms while the amorphous zones may deplete. Moreover, the significant increase in overall crystallinity may also indicate that the relative fraction of crosslinked network contributes to amorphous content of the polypropylene, and it decreases due to scissioning.
3.4. Photoluminescence spectra An idealized polypropylene is not known to be luminescent; however the presence of impurities, additives, defects, unsaturation and chromophores may cause photoluminescence [20,21]. Fig. 5 shows the photoluminescence spectra of pristine and Ag7+ ion-irradiated polypropylene at different fluences. The excitation wavelength used in this case was 236 nm. The emission spectra show a strong peak at 443 nm i.e. 2.81 eV and a weak peak at 484 nm i.e. 2.57 eV. These peaks were further confirmed by recording the excitation spectra by taking emission wavelength as 443 nm. Fig. 6 shows the corresponding excitation spectra which confirms the excitation peak at 236 nm. It is observed that, the intensity of both the peaks in emission spectra decrease with the increase in ion fluence. This shows that polypropylene becomes successively less luminescent after ion irradiation. This attributes to the disappearance of luminescent centers such as impurities, 1000 900 Intensity (Arb. Units)
800
443 nm i.e. 2.81 eV
Pristine 11
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484 i.e. 2.57 eV
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236 nm i.e. 5.27 eV
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2
12
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1x10 ions/cm 11 2 5x10 ions/cm 12 2 1x10 ions/cm 5x10 ions/cm
300 200 100 0 210 220 230 240 250 260 270 280 290 300 Wavelength (nm)
Fig. 6. Photoluminescence (excitation) spectra of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at different fluences.
defects, unsaturation and chromophores in the polypropylene. [20,21]. As SHI leads to scissioning of various bonds and evolution of gases, the luminescent centers may decompose and then released during irradiation, which results in to the decrease in intensity of the emission peaks. Further, the absence of the effects such as oxidation and conjugation and the decrease in optical band gap also seems to be responsible for decrease in the PL intensity. 3.5. Contact angle measurements Wettebility of the polymer surface is governed by its roughness and surface chemistry. In general, surfaces involving polar groups such as carbonyl or hydroxyl are known to be hydrophilic. Thus the contact angle measurements can clarify the changes in the surface texture and chemistry after SHI irradiation. Fig. 7 shows the contact angles of a 25 ll drop of distilled water on the surface of pristine and Ag7+ ion-irradiated polypropylene at the maximum fluence of 5 1012 ions/cm2. It is observed that the contact angle increases from pristine value of 78° to 97°. This increase in the contact angle is almost 24% and it clearly shows that polypropylene surface has become significantly hydrophobic after irradiation. This attributes to the absence of carbonyl and hydroxyl groups which are known to be polar, excessive surface roughening and carbonization (graphite is non polar) [22–24]. Moreover, the trapped air between water droplet and the surface due to increased surface roughness may reduce the liquid–solid contact area resulting in increased hydrophobicity [25]. The surface roughening is supported by SEM analysis.
200 100 0 400
3.6. SEM analysis 425
450
475
500
525
550
Wavelength (nm)
Fig. 5. Photoluminescence (emission) spectra of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at different fluences.
Fig. 8 shows the SEM images of pristine and 100 MeV Ag+7 ion-irradiated polypropylene at different fluences. It can be observed that the degree of roughness/imperfection increases with the ion fluence. The surface of ion-irradiated
N.L. Mathakari et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1793–1798
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Fig. 7. Photographs of a 25 ll water drop on the surface of pristine and 100 MeV Ag7+ ion-irradiated polypropylene at the fluence of 5 1012 ions/cm2.
Fig. 8. SEM images of pristine 100 MeV Ag7+ ion-irradiated polypropylene at different fluences.
polypropylene is observed to be consisting of defects in the form of voids, blisters, globules and tracks of varying sizes. This can be mainly attributed to uneven evolution of gases from the surface of polypropylene during ion irradiation. The scissioning of bonds due to ion irradiation results in the erosion/sputtering of atoms on the surface of polypropylene. The rate of evolution of gases may have different yields at different positions on the surface. This also results in change free volume near the polypropylene surface, due which it is observed to become successively rough with the ion fluence [26–28]. 4. Conclusion Polypropylene has shown significant modifications in the surface and bulk properties after 100 MeV Ag7+ ion irradiation. The FTIR spectra show scissioning of C–H and C–C bonds, however, oxidation effects are not noticeably observed. Whereas, UV–visible spectra have shown 68% reduction in the band gap and also increase in the absorbance. This mainly attributes to the carbonization. XRD spectra show significant increase in overall crystallinity which indicates the increase in the fraction of crystalline at the expense of amorphous zones. The PL spectra show decrease in the intensity of emission as well as excitation peaks which reveal the disappearance of luminescent centers due to SHI irradiation. Contact angle is found to be increased with the fluence which indicates the increase in hydrophobic properties, due to absence of oxidation effects, excessive surface roughening and carbon enrichment on the surface. The surface roughening is confirmed in SEM analysis. This study thus leads to a definite conclusion that 100 MeV Ag7+ ion irradiation can induce several desirable changes in the structure and properties of poly-
propylene namely, prominent increase in overall crystallinity, significant decrease in optical band gap and increase in the hydrophobicity of the surface. References [1] M.R. Cleland, L.A. Parks, S. Cheng, Nucl. Instr. and Meth. B 208 (2003) 66. [2] Andrzej G. Chmielewski, Mohammad Haji-Saeid, Shamshad Ahmed, Nucl. Instr. and Meth. B 236 (2005) 44. [3] A. Cahpiro, Nucl. Instr. and Meth. B 105 (1995) 5. [4] E. Balanzt, N. Betz, S. Bouffard, Nucl. Instr. and Meth. B 105 (1995) 46. [5] J. Jagielski, A. Turos, D. Bielinski, A.M. Abdul-Kader, A. Piatkowska, Nucl. Instr. and Meth. B 261 (2007) 690. [6] D. Fink, P.S. Alegaonkar, A.V. Petrov, M. Wilhelm, P. Szimkowiak, M. Behar, D. Sinha, W.R. Fahrner, K. Hoppe, L.T. Cahdderton, Nucl. Instr. and Meth. B 236 (2005) 11. [7] Harish B. Thorat, C.S. Prabhu, K. Suresh Kumar, M.V. Pandya, J. Appl. Polym. Sci. 65 (1997) 2715. [8] M.P. Macdonald, I.M. Ward, Polymer 2 (1961) 341. [9] D. Sinha, T. Swu, S.P. Tripathy, R. Mishra, K.K. Dwivedi, D. Fink, Rad. Eff. Deffect Sol. 158 (2003) 531. [10] R. Mishra, S.P. Tripathi, K.K. Dwiwedi, D.T. Khathing, S. Ghosh, M. Muller, D. Fink, Radiat. Measure. 33 (2001) 845. [11] H.M. Abdel-Hamid, Solid State Electron. 49 (2005) 1163. [12] R.M. Radwan, J. Phy. D: Appl. Phys. 40 (2007) 374. [13] Lakhwant Singh, Ravinder Singh, Nucl. Instr. and Meth. B 225 (2004) 478. [14] S. Chawla, A.K. Ghosh, S. Ahmad, D.K. Avasthi, Nucl. Instr. and Meth. B 224 (2006) 248. [15] A. Saha, V. Chakraborty, S.N. Chintalapudi, Nucl. Instr. and Meth. B 168 (2000) 245. [16] V. Svorcik, V. Rybka, O. Jankovski, V. Hnatowicz, J. Appl. Polym. Sci. 61 (1996). [17] D. Fink, W.H. Chung, M. Wilhelm, Rad. Eff. Deffect Sol. 133 (1995) 209. [18] Kushal Sen, Praveen Kumar, J. Appl. Polym. Sci. 55 (1995) 857. [19] N.V. Bhat, M.M. Nate, M.B. Kurup, V.A. Bambole, S. Sabharwal, Nucl. Instr. and Meth. B 237 (2005) 585.
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