Swift heavy ion induced modification in makrofol-KG ... - NOPR

Indian Journal of Pure & Applied Physics Vol. 48, March 2010, pp. 166-171

Swift heavy ion induced modification in makrofol-KG polycarbonate Rajesh Kumar1*, Paramjit Singh1, S Asad Ali2, Anil Sharma1, S A Khan3, R G Sonkawade3 & Rajendra Prasad2,4 1

University School of Basic and Applied Sciences, Guru Gobind Singh Indraprastha University, Delhi 110 403

2

Department of Applied Physics, Z H College of Engineering & Technology, Aligarh Muslim University, Aligarh 202 002 3

Inter University Accelerator centre, New Delhi 110 067

4

Vivekananda College of Technology & Management, Aligarh 202 002 *E-mail: [email protected] Received 2 December 2009; accepted 22 January 2010

Swift heavy ion ( SHI )induced modifications in Makrofol-KG, a bisphenol a polycarbonate irradiated by 100 MeV Si8+ ion beam have been studied by UV-visible and FTIR spectroscopy and dielectric constant measurements. The absorbance in UV and visible range increases with ion fluence and the absorption edge shifts towards the visible region, indicating the carbonization of the material. Dielectric constant increases with ion fluences, implying an improvement in the orientational polarization of molecules. The FTIR spectra obtained after the irradiation exhibit an overall reduction of the intensities of the typical vibrational bands of pristine PC and the appearance of new bands. Keywords: Makrofol-KG polycarbonate, Si ions, Radiation effects, UV-Vis, FTIR spectroscopy, Dielectric constant

1 Introduction When an energy rich ion penetrates a solid, the material along the trajectory of the ion beam is modified. Atoms are pushed out of their normal positions, many are split into pieces and ordered structures such as that of the crystal are destroyed. In this process, latent track is created by the ion, the diameter and length of this track depend on the type of the ion and its energy as well as on the structure and chemical composition of the irradiated material. If the radiation dose is so high that ion tracks overlap, the physical and chemical properties of the material can also be altered on a macroscopic scale to such an extent that it can be considered a new material with new properties. Various modifications in polymeric materials have been observed due to irradiation of polymers with energetic heavy ions1,2. This happens due to very high value of the electronic stopping power or high linear energy transfer (LET) of the ions which induces an unusual density of the electron hole pairs close to ion path. Energetic heavy ions create cylindrical track with complex damage structures such as radical formation, main chain scission, intermolecular cross-linking, creation of triple bond and unsaturated bond and loss of volatile fragments3,4. The effects of ion irradiations are mostly due to electronic excitations and ionization. Makrofol-KG, a bisphenol a polycarbonate (PC) is widely used for ion track recording and to prepare

track etched membrane as micro filters. Now PC particle track-etched membranes (nano-PTM) with pore shape and size, very well controlled5,6 within diameters from 10 to 100nm have been produced. These membranes are used for the manufacturing of nano tube and nano wires7,8. Swift Heavy Ion degradation of polymers has been analyzed by various researchers9-12 in a wide range of energies. The sensitivity of a polymer13 to the registration of particle tracks is closely related to its sensitivity to the formation of chain scission under irradiation. It provides strong evidence that chain scission is of primary importance in the track formation process in track storing materials. Various studies point out that carbonaceous clusters are forward along latent tracks of energetic ions in polymers3,14-15. Formation of these carbonaceous clusters can be studied from the absorption edge of ultraviolet-visible (UV-Vis) spectra which give an idea about the value of optical band gap (Eg). Fourier Transform Infrared (FTIR) spectroscopy in conjunction with the UV-Vis results enable us to understand the structural changes in irradiated polymer. In the present paper, modifications in optical, chemical and electrical properties of Makrofol-KG PC induced by 100 MeV Si8+ ions have been investigated by UV-Vis and FTIR spectroscopy and dielectric constant measurements. The energy of projectile was chosen so that the ions could easily

KUMAR et al.: SWIFT HEAVY ION INDUCED MODIFICATION POLYCARBONATE

pass through the PC sample. Thus, the modifications are mostly due to electronic energy loss. 2 Experimental Details 60 µm thick Makrofol-KG bisphenol A PC films, manufactured by a castic process were obtained from Bayer AG, Lever Kussen, Germany. The chemical structure of Makrofol-KG polycarbonate is given as: CH3 O

C CH3

O C O

n

The samples of size (1.5×1.5 cm2) were irradiated under normal incidence at the 15 UD Pelletron accelerator, Inter University Accelerator Center, New Delhi, India. Irradiation was performed with 100 MeV Si8+ ions under high vacuum. Five identical samples were mounted on vacuum shielded vertical sliding ladder and irradiated in the General Purpose Scattering Chamber. The fluence varied from 1010-1012 ions/cm2. To expose the whole target area, the beam was scanned in the X-Y plane. The range (81.85 µm), as estimated by SRIM-2003 of the incident ion is more than the thickness of the PC film. Ion beam induced modifications have been analyzed using UV-Visible spectrophotometer (SHIMADZUUV-160) in the range 200-800 nm. The Fourier Transform Infrared (FTIR) Spectroscopy was performed in transmission mode using NICOLET-550 FTIR spectrometer. The spectra were recorded in the wave number range 4000-400 cm−1. Hawlett-Packard LCR meter (model no. 4284), a device to measure capacitance, inductance and resistance over the frequency range 100 Hz-1 MHz was used for dielectric constant and dielectric loss studies. 3 Results and Discussion In the present study, significant changes have been observed in optical, chemical as well as in dielectric response of Makrofol -KG PC after irradiation. In our previous study16 on CR-39 (DOP) PC and polyamide nylon-6 using 70 MeV C+5 ion irradiation, considerable changes in FTIR and XRD were observed. The projected range of 100 MeV Si+8 ions in Makrofol-KG PC is around 81.85µm which is more than two times the thickness of the polymer under these conditions, ionization processes are very important.

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3.1 Optical response

The results of optical absorption studies with UV-Visible Spectrophotometer carried out on virgin and irradiated samples are shown in Fig. 1. The optical absorption spectrum (Fig. 1a) shows a sharp decrease with increasing wavelength up to 300 nm followed by a plateau, region. Figure 1(b-g) shows the optical spectrum for Makrofol-KG PC samples after irradiation to the fluences of 1×1010, 3×1010, 1×1011, 3×1011, 6×1011 and 1×1012 ions/cm2. It is evident from Fig. 1 that the optical absorption increases with increasing fluence and there is a shift of this absorption from the UV-Vis towards the visible region for irradiated samples. Some earlier studies17,18 have reported increased absorbance in polymers bombarded with MeV heavy ions. This behaviour may be attributed to the formation of extended systems of conjugate bonds as a result of the beam induced bond cleavage and reconstruction17. The shift of absorption edge of UV-Visible spectra towards the visible region can be correlated with optical band gap (Eg) by Tauc’s expression14: ω2ε(λ) = (ћω – Eg)2

…(1)

where ε(λ) is the optical absorbance, ω is the frequency and λ is the wavelength. The intersection of the extrapolated spectrum with the abscissa of the plot {ε (λ)/λ}1/2 versus 1/λ yields the gap wavelength (λg) from which energy gap is derived as : Eg = hc / λg For a linear structure, the number of carbon atoms per conjugation length N is given as14:

Fig.1 — Optical absorption spectra of Makrofol-KG polycarbonate irradiated with 100 MeV Si8+ ion beam

INDIAN J PURE & APPL PHYS, VOL 48, MARCH 2010

168 N= 2βπ/Eg

…(2)

where 2β gives the band structure energy of a pair of adjacent π sites. The value of β is taken to be −2.9 MeV as it is associated with π-π* optical transitions in −C=C− structure. Eq. (2) can be applied, as the shift of the absorption edge can be attributed to an increase of the conjugation length without formation of new lengthy linear conjugated structures. Table 1 presents the values of Eg and the corresponding number of carbon atoms per conjugations length. The value of Eg decreases with increase of fluence. Figure 2 shows the variation in band gap energy (Eg) with fluence on a log scale. The optical band gap energy remains almost constant up to a dose of 3×1011 ions/cm2 after which it decreases gradually with increase in dose (Fig. 2). The band gap of the pristine PC is found to be 4.360 eV which starts decreasing for PC irradiate to a dose of 3×1011 ions/cm2 and reaches a minimum of 2.508 eV for the PC irradiated to a dose of 1012 ions/cm2. The radicals Table 1 — Optical band gap energy (Eg) and number (N) of carbon atoms or conjugation length Fluence (ions/cm2)

Absorption edge (λg) (nm)

Band gap energy (eV)

N

0 1×1010 3×1010 1×1011 3×1011 6×1011 1×1012

285.10 291.80 291.86 294.03 315.42 418.20 495.48

4.360 4.299 4.259 4.227 3.941 2.972 2.508

~4 ~4 ~4 ~4 ~4 ~6 ~7

Fig. 2 — Variation in band gap energy with irradiation fluences

contribute to the polymeric restructuring process which leads to conductivity14 as confirmed by Sinha et al.19 that the free radical formation takes place in PADC (acrylics) by gamma irradiation at higher doses. The decrease in the present case can be correlated to the formation of free radicals. 3.2 Dielectric constant and dielectric loss

The dielectric constant of the samples was determined by measuring the capacitance of the samples. Simultaneously the loss factor was also measured. Capacitance (Cp) and dielectric loss (tan δ) measurements were carried out using a parallel plate configuration of electrodes on both sides of PC film using a Hawlett-Packard LCR meter. The effect of ion irradiation on the dielectric properties of polymers has been studied earlier20-23. Figure 3 shows the dielectric response of Si8+ irradiated PC samples compared to the pristine samples. It is evident from Fig. 3 that the polymer samples show dielectric dispersion. Dielectric constant remains almost constant in the range 400-800 kHz. It appears to increase with increasing dose. In low frequency region, the change arises due to carrier polarization indicating that the involved charge carriers move by discontinuous hopping movements between localized sites24. In the higher frequency region, the dielectric response is primarily due to the lattice contributions. The increase of dielectric constant with fluence implies a gradual improvement in the orientation of PC macromolecules and increase in orientational polarization with increase in ion dose. Dole and Chanchard20 have attributed the increase in dielectric constant values on irradiation to the increase in rigidity of the polymer. Figure 4 shows the plot of dielectric loss (tanδ) versus frequency for the pristine and Si8+ irradiated samples. It is observed that the loss factor increases as the fluence increases and tanδ has positive values indicating the dominance of inductive behaviour. The nature of the frequency dependence of dielectric response for pristine and irradiated samples reveals the presence of a low frequency dispersion which can be related to inter facial polarization25. Due to the presence of overlapping tracks of heavy ions, some of the polymer bonds are broken. As a result, some carbon rich clusters are formed in the polymers matrix3. This means a favorable situation in the polymer matrix to develop built in potential barriers in the depletion regions separating any two nearly clusters. The resulting dipoles in these capacitive regions,thus,give rise to interfacial polarization.


Fig. 3 — Frequency variation of dielectric constant for Si8+ ions irradiated Makrofol-KG polycarbonate

Fig. 4 — Frequency variation of dielectric loss tangent in case of Si8+ ions irradiated Makrofol-KG polycarbonate

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3.3 FTIR spectroscopy

The vibration modes of chemical bonds are characterized by the absorption bands26. Figure 5 shows the various absorption bands of the PC foils irradiated to different fluences of Si ions alongwith the absorption bands of the virgin film. The infrared absorption peaks of functional groups27 of PC are present in spectra. Fink and coworkers28 used low energy Ar ions to carry out IR studies on polycarbonate whereas Varda Rajulu et al.29 have used 60 MeV Si ion beam for modifying the chemical structure of polymers, like polypropylene, polyimide and polymethyl methacrylate/polystyrene blend. In the pristine foil in the absence of the absorption band around 3500 cm−1 shows the absence of terminal hydroxy group indicating high molecular weight of the polymer under study. This polymer is synthesized by transestrification of biphenyl carbonate with bisphenol A with the elimination of phenol as side product. Therefore, it is expected that the initial concentration of hydroxyl group will monotonously decreases with the increase in the chain length of the polymer. There is almost no change in the spectra up to the irradiation to the fluence of 1×1011 on irradiation to the fluence of 6×1011 and 1×1012 ions/cm2 and the intensity of the peak corresponding to 3500 cm−1 with the absorption intensity of the band at1770 cm−1 representing C=O stretch changed with the ion fluence. This indicates that chain scission may be taking place at the carbonate site with probable elimination of carbon dioxide/carbon monoxide and formation of hydroxy group. H atom required for its formation coming perhaps from the isopropyl group as the absorption around 2970 cm−1 which arise due to CH3 symmetric stretch which decrease in intensity with increase in ion fluence. The corroboration of chain scission can be deduced from the decrease in the intensity of absorption bands around 1160 cm−1 attributed to carbonate C−O stretch. The intensity of absorption bands at 830 and 1018 cm−1 decreases. This corresponds to para out-of-plane aromatic C H wag of two adjacent H atoms and para in plane aromatic C−H bends, lends credence to the fact that substantial changes are taking place in the environment around the phenyl ring which perhaps is affecting the wagging nature. The reduction in absorbance observed with ion fluence and the resulting phenomena are in agreement with the result obtained by Gagnadre et al27. It indicates that bond breaking of functional groups of

the PC film occurs under irradiation. Since ether is the skeleton of macromolecules in PC films, its breaking suggests a decrease of the molecule weight. Contrary to the bands mentioned above, the bands at 1600 cm−1 and 3500 cm−1 show increasing absorbance with the ion fluence. The increase of the hydroxy bond suggests an increase of the end group of macromolecules. This is consistent with the results observed by Liick30, which indicate that under irradiation the weight of the macromolecular decreases and the etchability of the irradiated PC film increases. Alkyne formation in FTIR analysis is an important feature of the polymer degradation by SHI bombardment. It can be seen from Fig. 5 that a broad peak develops around 3295 cm−1 after irradiation and increases with the fluence. The bond may be due to the characteristic υ≡CH of the alkyne end group R−C≡H. It has been observed in many polymers irradiated by SHI.

Fig. 5 — FTIR spectra of Makrofol-KG Polycarbonate irradiated with 100 MeV Si8+ ion beam (a) Virgin, (b) 1×1010, (c) 3×1010 (d) 1×1011, (e) 3×1011, (f) 6×1011 and (g) 1×1012 ions/cm2


4 Conclusions The results of our measurements indicate that Makrofol-KG polycarbonate shows substantial modification in its optical, dielectric and chemical characteristics when it is bombarded with swift heavy ions. With increasing fluence, the absorbance in UV and visible region increases and the absorbation edge shifts towards the visible region, indicating the carbonization of the material. The transition π→π* has a significant role in the latent track formation in Makrofol-KG PC for medium range energy. An increase in dielectric constant values with increasing ion fluence has been observed at all measured frequencies for PC irradiated with 100 MeV Si ions, implying an improvement in the orientational polarization of the molecules with the applied electric field. The value of tanδ has positive values indicating the dominance of inductive behaviour. The role of chemical modification comes out in terms of breaking of the cleavaged C−O single bond of carbonate and possible formation of phenolic O−H bend. Acknowledgement The authors wish to thank Dr D K Avasthi and the staff of the Inter University Accelerator Centre, New Delhi, for their help during irradiation The authors also thank University Grant Commission (UGC), New Delhi, for providing financial assistance to carry out this research work. References 1 Balanzat E, Bouffard S, Le Moel A & Betz N, Nucl Instr & Meth B, 91 (1994) 140. 2 Sun Y M, Zhu Z Y, Wang Z G, Liu J, Jin Y F, Hou M D, Wang Y, & Duan J G, Nucl Instr & Meth B, 212 (2003) 211. 3 Fink D, Klett R, Chaddertan L T, Cardosa J, Montiel R, Vazquez M H & Karanovich A, Nucl Instr & Meth B, 111 (1996) 303. 4 Picq V, Ramillon J M & Balanzat E, Nucl Instr & Meth B, 146 (1998) 496. 5 Ferain E & Legras R, Nucl Instr & Meth B, 174 (2001) 116. 6 Ferain E, Radiat Meas, 34 (2001) 585.

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7 Piraux L, Dubois S & Demoustier-champagne S, Nucl Instr & Meth B, 131 (1997) 357. 8 Jerome C, Demoustier-champagne S, Legras R & Jerome R, Chem Eur J, 6 (17) (2000) 3089. 9 Ferain E, Legras R, Nucl Instr & Meth B, 82 (1993) 539. 10 Steckenreiter T, Balanzat E, Fuess H & Trautman C, Nucl Instr & Meth B, 151 (1999) 161. 11 Chipra M I & Reifes-Romero J, Nucl Instr & Meth B, 185 (2001) 77. 12 Zhu Z, Sun Y, Liu C, Liu J & Jin Y, Nucl Instr & Meth B, 193 (2002) 271. 13 Sullivan D O, Price P B, Knoshita K & Wilson C G, Electrochem J, 129 (1982) 811. 14 Fink D, Chug W H, Klett R, Schmoidt A, et al., Radiat Eff & Defects Solids, 133 (1995) 193. 15 Davenas J, Thevenasd P, Boiteux G, Fallavier M & Lu X L, Nucl Instr & Meth B, 46 (1990) 317. 16 Rajesh Kumar, Rajendra Prasad, Vijay Y K, Acharya N K, Verma K C & Udayan De, Nucl Instr & Meth B, 212 (2003) 221. 17 Farenza L S, Papaleo R M, Hallen A, Araujo M A, Livi R P, & Sundqvist B U R, Nucl Instr & Meth B, 105 (1995) 134. 18 Saha A, Chakraborty V & Chintalpudi S N, Nucl Instr & Meth B, 168 (2000) 245. 19 Sinha D, Phukan T, Tripathy S P, Mishra R & Dwievedi K K, Radiat Meas, 34 (2001) 109. 20 Dole P & Chauchard J, Angew Makromol Chem, 79 (1996) 273. 21 Chailan J F, Boiteux G, Chauchard J & Seytre G, Nucl Instr & Meth B, 131 (1997) 172. 22 Martinez-Pardoma E, Cardoso J, Vazquez H & Aquilar M, Nucl Instr & Meth B, 140 (1998) 327. 23 Phukan T, Kanjilal D, Goswami T D & Das H L, Nucl Instr & Meth B, 155 (1999) 116. 24 Jonschen A K, Nature, 276 (1977) 673. 25 Kaplan M L, Forrest S R, Schimdt P H & Venkatesan T, J Appl Phys, 55 (3) (1984) 7 32. 26 Noda I, Dowery A W & Marcott C, in Mark (Ed) J E, Physical Properties of Polymers Handbook, (AIP Press, New York), 1996. 27 Gagnadre C, Decossas J L & Vareille J C, Nucl Instr & Meth B, 73 (1993) 48. 28 Fink D, Muller M, Chadderton L T, Cannington P H, Ellimam R G & McDonaldt D C, Radiat Eff Def Solids, 132 (1994) 313. 29 Varada Rajulu A, Lakshminarayana Reddy R, Avasthi D K & Ashokan K, Radiat Eff & Defects Solids, 152 (2000) 57. 30 Lück H B, Nucl Instr & Meth B, 200 (1982) 517.