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Study of modifications in Lexan polycarbonate induced by swift O6+ ion irradiation
Nuclear Instruments and Methods in Physics Research B 268 (2010) 1813–1817
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Study of modifications in Lexan polycarbonate induced by swift O6+ ion irradiation S. Asad Ali a,*, Rajesh Kumar b, F. Singh c, P.K. Kulriya c, Rajendra Prasad a,d a
Department of Applied Physics, Z. H. College of Engineering and Technology, Aligarh Muslim University, Aligarh 202 002, India Department of Physics, University School of Basic and Applied Sciences, G.G. S.I.P. University, Delhi 110 403, India c Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India d Vivekananda College of Technology and Management, Aligarh 202 002, India b
a r t i c l e
i n f o
Article history: Available online 25 February 2010 Keywords: Lexan polycarbonate 95 MeV O6+ ion irradiation UV–Vis FTIR
a b s t r a c t Swift Heavy Ion (SHI) irradiation of the polymeric materials modifies their physico-chemical properties. Lexan polycarbonate films were irradiated with 95 MeV oxygen ions to the fluences of 1010, 1011, 1012, 1013 and 2 1013 ions/cm2. Characterization of optical, chemical, electrical and structural modifications were carried out by UV–Vis spectroscopy, FTIR spectroscopy, Dielectric measurements and X-ray Diffraction. A shift in the optical absorption edge towards the red end of the spectrum was observed with the increase in ion fluence. The optical band gap (Eg), calculated from the absorption edge of the UV–Vis spectra of these films in 200–800 nm region varied from 4.12 eV to 2.34 eV for virgin and irradiated samples. The cluster size varied in a range of 69–215 carbon atoms per cluster. In FTIR spectra, appreciable modification in terms of breaking of the cleavaged C–O bond of carbonate and formation of phenolic O–H bond was observed on irradiation. A rapidly decreasing trend in dielectric constant is observed at lower frequencies. The dielectric constant increases with fluence. It is observed that the loss factor increases moderately with fluence and it may be due to scissoring of polymer chains, resulting in an increase in free radicals. A sharp increase in A.C. conductivity in pristine as well as in irradiated samples is observed with frequency and is attributed to scissoring of polymer chains. XRD analyses show significant change in crystallinity with fluence. A decrease of 9.02% in crystallite size of irradiated sample at the fluence of 2 1013 ions/cm2 is observed. Ó 2010 Elsevier B.V. All rights reserved.
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 a so called, latent track is created by the ion. The diameter and length of this track depends 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 ions [1,2]. 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
* Corresponding author. E-mail address: [email protected] (S. Asad Ali). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.02.081
radical formation, main chain scission, intermolecular cross-linking, creation of triple bond and unsaturated bond and loss of volatile fragments [3,4]. Lexan polycarbonate (PC) is widely used for ion track recording and to prepare track-etched membranes as micro filters. Now PC particle track-etched membranes (nano-PTM) with pore shape and size very well controlled within diameters from 10 to 100 nm [5,6] have been produced. These membranes are used for the manufacturing of nano tubes and nano wires [7,8]. Swift Heavy Ion degradation of polymers has been analyzed by various researchers [9–12] in a wide range of energies. The sensitivity of a polymer [13] 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 forwarded along latent tracks of energetic ions in polymers [3,14,15]. Formation of these carbonaceous clusters can be studied from the absorption edge of ultraviolet–visible (UV–Vis) spectra which gives an idea about the value of optical band gap (Eg). Fourier Transform Infra Red (FTIR) spectroscopy in conjunction with the UV–Vis results enable to understand the structural changes in irradiated polymer.
S. Asad Ali et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1813–1817
In the present work, modifications in optical, chemical, structural and electrical properties of Lexan PC induced by 95 MeV O6+ ions have been investigated by UV–Vis, FTIR spectroscopy, Xray diffraction (XRD) and dielectric constant measurements. The energy of projectile was chosen so that the ions could easily pass through the PC sample. Thus the modifications are mostly due to electronic energy loss. 2. Experimental
5.5
4.5 4.0
Thick Lexan PC (225 lm) films were obtained from Farben Fabri-ken Bayer A.G., Germany. The chemical structure of Lexan Polycarbonate is given as:
3.5 3.0
CH3
e
2.0 d
1.5
c
1.0
CH3 C
f
2.5
0.5
O
a - Pristin 10 2 b - 1x10 ions/cm 11 2 c - 1x10 ions/cm 12 2 d - 1x10 ions/cm 13 2 e - 1x10 ions/cm 13 2 f - 5x10 ions/cm
5.0
Absorbance(a.u.)
1814
b a
0.0 300
O C O n
The samples of size (1.5 1.5 cm2) were irradiated under high vacuum with 95 MeV O6+ ions from 15 UD Pelletron accelerator at Inter University Accelerator Centre (IUAC), New Delhi, India to the fluences of 1010, 1011, 1012, 1013 and 2 1013 ions/cm2. To expose the whole target area the beam was scanned in the X–Y plane. Ion beam induced modifications have been characterized by various techniques. UV–Vis spectrum was recorded by UV–Vis spectrophotometer (SHIMADZU-UV-160) in the range 200– 800 nm. FTIR Spectroscopy was performed in transmission mode using NICOLET-550 FTIR spectrometer. The spectra were recorded in the wave number range of 4000–400 cm1. XRD measurements for the polymer films were carried out using D8 Advanced Bruker diffractometer with Cu-Ka radiation (k = 1.541838 Å) at room temperature by taking 0.020 step size. The cathode was maintained at 30 kV. Diffraction patterns were recorded in the range 20° 6 2h 6 80o. Hewlett–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 Lexan PC after irradiation. 3.1. Optical response The results of optical absorption studies with UV–Vis Spectrophotometer carried out on virgin and irradiated samples are presented in Fig. 1. Fig. 1(a) depicts the UV–Vis spectrum of pristine sample. It shows a sharp decrease with increasing wave length up to 300 nm followed by a plateau region. Fig. 1(b–e) show the optical spectrum for Lexan PC samples after irradiation to the fluences of 1010, 1011, 1012, 1013 and 2 1013 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 studies [16,17] have reported increased absorbance in polymers bombarded with MeV heavy ions. This behavior may be attributed to the formation of extended systems of conjugate bonds as a result of the beam induced bond cleavage and reconstruction [16].
400
500
600
700
800
Wavenumber (nm) Fig. 1. Optical absorption spectra of Lexan polycarbonate polymer irradiated with 95 MeV O6+ ion beam.
The shift of absorption edge of UV–Vis spectra towards the visible region can be correlated with optical band gap (Eg) by Tauc’s expression [14].
x2 eðkÞ ¼ ðhx Eg Þ2
ð1Þ
where, e(k) is the optical absorbance, x is the frequency and k is the wavelength. The intersection of the extrapolated spectrum with the abscissa of the plot {e(k)/k}1/2 vs 1/k yields the gap wavelength (kg) from which energy gap can be derived
Eg ¼ hc=kg
ð2Þ
For a linear structure the number of carbon atoms per conjugation length N is given by [18].
N¼
2bp eV Eg
ð3Þ
here, 2b is the band structure energy of a pair of adjacent p sites and its value is taken as 2.9 eV as it is associated with p ? p* optical transitions in –C = C structure. As the shift of the absorption edge can be attributed to an increase of the conjugated structures, Eq. (3) can be used in the present study. The values of absorption edge (kg) and the corresponding optical band-gap (Eg) and number of carbon atoms per conjugation length (N) are given in the Table 1. The value of Eg decreases with increase in the fluence. The optical band gap energy of the pristine PC is found to be 4.12 eV. It reduces to 2.34 eV at 2 1013 ions/cm2. The radicals contribute to the polymeric restructuring process which leads to conductivity [14] as confirmed by Sinha et al. [19] that the free radical formation takes place in PADC (Acrylics) by gamma irradiation at higher
Table 1 Variation of absorption edge (kg), energy gap (Eg) and number of carbon atoms N per conjugation length(N) in pristine and O6+ ion irradiated samples of Lexan (PC) to different flounces. Fluence (ions/cm2)
Absorption edge kg (nm)
Band gap energy Eg (eV)
N
Unirradiated 1010 1011 1012 1013 2 1013
301.8 312.3 349.2 385.8 493.0 530.2
4.12 3.98 3.56 3.22 2.52 2.34
5 5 5 6 7 8
1815
S. Asad Ali et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1813–1817
doses. The decrease in present case can be correlated to the formation of free radicals.
600 450
2 x1 0
13
2
io n s / c m
300 150
3.2. FTIR spectroscopy
3.3. Structural response Fig. 3 shows the X-ray diffraction patterns of the pristine and irradiated Lexan samples. From the figure it can be observed that
2x10
13
Transmittance (%)
1x10
13
1x10
12
1x10
11
1x10
10
Pristine
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 2. FTIR spectra of Lexan PC films: pristine and irradiated with 95 MeV O6+ ions to different fluences.
13
10
750
Intensity (a.u.)
The vibration modes of chemical bonds are characterized by the absorption bands [20]. Fig. 2 shows various absorption bands of the PC foils irradiated to different fluences of O6+ ions along with the absorption bands of the pristine film. The Infrared absorption peaks of functional groups of PC [21] are present in spectra. In the pristine foil the absence of the absorption band around 3500 cm1 shows the absence of terminal hydroxyl group indicating high molecular weight of the polymer under study. This polymer is synthesized by transestrification of diphenyl 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 decrease with the increase in the chain length of the polymer. There is almost no change in the spectra of samples irradiated to the fluence of 1011 ions/cm2. On irradiation to the fluence of 1012–1013 ions/cm2 the intensity of the peak corresponding to 3500 cm1 along with the absorption intensity of the band at 1770 cm1 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 hydroxyl group. H atom required for its formation coming perhaps from the isopropyl group as the absorption around 2970 cm1 which arises due to CH3 symmetric stretch decreases 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 cm1 attributed to carbonate C–O stretch. The intensity of absorption bands at 830 and 1018 cm1 decreases and corresponds to para out of plane aromatic C–H wag of two adjacent H atoms and para in plane aromatic C–H bends and 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.
1000 0 750 500 250 1000 0
1 0
12
2
io n s / c m
io n s / c m
2
500 250 1000 0 750 500 250 10000
10
1 0
750
11
io n s / c m
10
io n s / c m
2
2
500 250 1000 0 P r is tin e
750 500 250 0
Fig. 3. X-ray diffraction patterns for Lexan PC films: pristine and irradiated with 95 MeV O6+ ions to different fluences.
Table 2 FWHM and crystallite size of pristine and 95 MeV O6+ ion irradiated Lexan PC films. Fluence (ions/cm2)
2h (°)
FWHM (b)
Crystallite size L (Å)
Unirradiated 1010 1011 1012 1013 2 1013
16.98 16.18 16.94 16.92 16.76 17.96
4.513 4.607 4.634 4.676 4.720 4.967
17.79 17.41 17.32 17.17 17.00 16.18
the X-ray diffraction patterns of the samples can be characterized by main halo. The profile of the halo shows that that the Lexan polymer is partly crystalline polymer with a dominant amorphous phase The average crystal size, more commonly known as particle size L is related to b, the full width at half maximum (FWHM) of the peak (in radian) by the well known Scherer formula L = k/(b cosh) [22], where k is the wavelength of the X-rays used. The crystallite size corresponding to the diffraction peaks of the pristine and irradiated polymer samples were calculated using Scherer equation and the results are presented in Table 2. Calculations show that L is 17.79 Å for the virgin or unirradiated polymer and 16.18 Å for the sample irradiated to 2 1013 ions/ cm2. Peak at the position 2h = 16.98o shifted to 2h = 17.96o at the highest fluence of 2 1013 ions/cm2. As the ion fluence increases, crystallite size decreases at the highest fluence of 2 1013 ions/ cm2, a reduction of nearly 9.06%. It can be interpreted that on irradiation, a process of ‘‘end-linking” is possible during scission. At least one of the fragments may link to the main chain of neighboring molecule to give a branched molecule of higher molecular weight [23]. Physical properties of an end-linked polymer would be essentially identical to those of cross-linked polymer. The free radicals produced due to scission may cause branching that destroys crystallanity.
3.4. Electrical response The dielectric response of material provides information about the orientational translational adjustment of mobile charges
S. Asad Ali et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1813–1817
present in the dielectric medium in response to an applied electric field. The most important property of dielectric materials is the ability to be polarized under the action of the field. The dielectric loss behavior of polymer films is very important because of their possible applications for insulation, isolation and passivation in micro-electronic circuits [24]. In general polymers are insulators and commonly used in insulation of electric wires. However, certain classes of polymers have been discovered and used as semiconductor and capacitors with unusual electrical properties. The dielectric polarization may be judged in terms of the dielectric constant and the dissipation factor (loss angle or tand). 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 (tand) measurements were carried out using a parallel plate configuration of electrodes on both sides of PC film using a LCR meter (Hewlett–Packard 4284) in the frequency range of 1–1000 kHz at room temperature. The measured values of capacitance then have been converted into the dielectric constant (e0 ) by using the formula
e0 ¼ Cd=eo A
11.8
11.6
11.4
11.2
11.0
10.8 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Log frequency (Hz) Fig. 5. Plot of dielectric constant versus log frequency for pristine and irradiated with 95 MeV O6+ ions of Lexan PC.
ð5Þ
Pristine 10 2 10 ions/cm 11 2 10 ions/cm 12 2 10 ions/cm 13 2 10 ions/cm 13 2 2x10 ions/cm
0.040 0.035 0.030 Tan δ
where, f is the frequency and tand is the dielectric loss. Fig. 4 shows the dependence of conductivity of Lexan PC films on log frequency at room temperature for pristine and irradiated samples. A sharp increase in conductivity has been observed. It is also observed that conductivity increases as fluence increases. The increase in conductivity due to irradiation may be attributed to scissoring of polymers chains. When a.c. field of sufficiently high frequency is applied to a metal polymer/metal structure, it may cause a net polarization which is out of phase with the field. This results in a.c. conductivity. Fig. 5 shows that the dielectric constant at lower frequencies shows a rapidly decreasing trend. The dielectric constant decreases also at higher frequencies but slowly. It also decreases with fluence. As the frequency increases the charge carriers migrate through the dielectric and get trapped against a defect site and induce an opposite charge in its vicinity. At these frequencies for the ions, the polarization of trapped and bound charges can not take place and hence the dielectric constant decreases.
0.025 0.020 0.015 0.010 0.005 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Log frequency (Hz) Fig. 6. Variation of tand with log frequency for pristine and 95 MeV O6+ ions irradiated Lexan PC.
27
Pristine 10 2 10 ions/cm 11 2 10 ions/cm 12 2 10 ions/cm 13 2 10 ions/cm 13 2 2x10 ions/cm
24 -1
12.0
0.045
ra:c: ¼ 2pf tan d eo er
-6
Pristine 10 2 10 ions/cm 11 2 10 ions/cm 12 2 10 ions/cm 13 2 10 ions/cm 13 2 2x10 ions/cm
12.2
ð4Þ
where, d is the thickness of polymer film, A is the area of the electrode plates and eo is the permittivity of free space. A.C. conductivity ra.c. is calculated by the relation given below
Conductivity x10 (ohm.cm )
12.4
Dielectric constant (ε ')
1816
21
It is observed from the Fig. 6 that the loss factor increases moderately with fluence. The increase in loss factor with fluence may be due to scissoring of polymer chains, resulting in an increase of free radicals.
18 15 12
References
9 6 3 0 2
3
4
5
6
Log frequency ( in Hz) Fig. 4. AC conductivity versus log frequency plot for pristine and 95 MeV O6+ ions irradiated Lexan PC.
[1] E. Balanzat, S. Bouffard, A. Le Moel, N. Betz, Nucl. Instrum. Methods B 91 (1994) 140. [2] Y.M. Sun, Z.Y. Zhu, Z.G. Wang, J. Liu, Y.F. Jin, M.D. Hou, Y. Wang, J.G. Duan, Nucl. Instrum. Methods B 212 (2003) 211. [3] D. Fink, R. Klett, L.T. Chaddertan, J. Cardosa, R. Montiel, M.H. Vazquez, A. Karanovich, Nucl. Instrum. Methods B 111 (1996) 303. [4] V. Picq, J.M. Ramillon, E. Balanzat, Nucl. Instrum. Methods B 146 (1998) 496. [5] E. Ferain, R. Legras, Nucl. Instrum. Methods B 174 (2001) 116. [6] E. Ferain, Radiat. Meas. 34 (2001) 585. [7] L. Piraux, S. Dubois, S. Demoustier-champagne, Nucl. Instrum. Methods B 131 (1997) 357.
S. Asad Ali et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1813–1817 [8] C. Jerome, S. Demoustier-champagne, R. Legras, R. Jerome, Chem. Eur. J. 6 (17) (2000) 3089. [9] E. Ferain, R. Legras, Nucl. Instrum. Methods B 82 (1993) 539. [10] T. Steckenreiter, E. Balanzat, H. Fuess, C. Trautman, Nucl. Instrum. Methods B 151 (1999) 161. [11] M.I. Chipra, J. Reifes-Romero, Nucl. Instrum. Methods B 185 (2001) 77. [12] Z. Zhu, Y. Sun, C. Liu, J. Liu, Y. Jin, Nucl. Instrum. Methods B 193 (2002) 271. [13] D.O. Sullivan, P.B. Price, K. Knoshita, C.G. Wilson, J. Electrochem. 129 (1982) 811. [14] D. Fink, W.H. Chung, R. Klett, A. Schmoldt, J. Cardoso, R. Mntiel, H. Vazquez, L. Wang, F. Hosoi, H. Omichi, P. Goppelt-Langer, Radiat. Eff. Defects Solids 133 (1995) 193. [15] J. Davenas, P. Thevenasd, G. Boiteux, M. Fallavier, X.L. Lu, Nucl. Instrum. Methods B 46 (1990) 317.
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[16] L.S. Farenza, R.M. Papaleo, A. Hallen, M.A. Araujo, R.P. Livi, B.U.R. Sundqvist, Nucl. Instrum. Methods B 105 (1995) 134. [17] A. Saha, V. Chakraborty, S.N. Chintalpudi, Nucl. Instrum. Methods B 168 (2000) 245. [18] J. Robertson, E.P. O’Reilly, Phys. Rev. B 35 (1987) 2946. [19] D. Sinha, T. Phukan, S.P. Tripathy, R. Mishra, K.K. Dwievedi, Radiat. Meas. 34 (2001) 109. [20] I. Noda, A.W. Dowery, C. Marcott, in: J.E. Mark (Ed.), Physical Properties of Polymers Handbook, AIP Press, New York, 1996. [21] C. Gagnadre, J.L. Decossas, J.C. Vareille, Nucl. Instrum. Methods B 73 (1993) 48. [22] P. Scherrer, Gott. Nachar 2 (1918) 98. [23] S. Nouh, N. El-Mehdi, A. Morsy, M. Morsy, Radiat. Meas. 39 (2005) 471. [24] Z.M. Dang, Y. Shen, C.W. Nan, Appl. Phys. Lett. 81 (2002) 4814.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Study of modifications in Lexan polycarbonate induced by swift O6+ ion irradiation S. Asad Ali a,*, Rajesh Kumar b, F. Singh c, P.K. Kulriya c, Rajendra Prasad a,d a
Department of Applied Physics, Z. H. College of Engineering and Technology, Aligarh Muslim University, Aligarh 202 002, India Department of Physics, University School of Basic and Applied Sciences, G.G. S.I.P. University, Delhi 110 403, India c Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India d Vivekananda College of Technology and Management, Aligarh 202 002, India b
a r t i c l e
i n f o
Article history: Available online 25 February 2010 Keywords: Lexan polycarbonate 95 MeV O6+ ion irradiation UV–Vis FTIR
a b s t r a c t Swift Heavy Ion (SHI) irradiation of the polymeric materials modifies their physico-chemical properties. Lexan polycarbonate films were irradiated with 95 MeV oxygen ions to the fluences of 1010, 1011, 1012, 1013 and 2 1013 ions/cm2. Characterization of optical, chemical, electrical and structural modifications were carried out by UV–Vis spectroscopy, FTIR spectroscopy, Dielectric measurements and X-ray Diffraction. A shift in the optical absorption edge towards the red end of the spectrum was observed with the increase in ion fluence. The optical band gap (Eg), calculated from the absorption edge of the UV–Vis spectra of these films in 200–800 nm region varied from 4.12 eV to 2.34 eV for virgin and irradiated samples. The cluster size varied in a range of 69–215 carbon atoms per cluster. In FTIR spectra, appreciable modification in terms of breaking of the cleavaged C–O bond of carbonate and formation of phenolic O–H bond was observed on irradiation. A rapidly decreasing trend in dielectric constant is observed at lower frequencies. The dielectric constant increases with fluence. It is observed that the loss factor increases moderately with fluence and it may be due to scissoring of polymer chains, resulting in an increase in free radicals. A sharp increase in A.C. conductivity in pristine as well as in irradiated samples is observed with frequency and is attributed to scissoring of polymer chains. XRD analyses show significant change in crystallinity with fluence. A decrease of 9.02% in crystallite size of irradiated sample at the fluence of 2 1013 ions/cm2 is observed. Ó 2010 Elsevier B.V. All rights reserved.
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 a so called, latent track is created by the ion. The diameter and length of this track depends 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 ions [1,2]. 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
* Corresponding author. E-mail address: [email protected] (S. Asad Ali). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.02.081
radical formation, main chain scission, intermolecular cross-linking, creation of triple bond and unsaturated bond and loss of volatile fragments [3,4]. Lexan polycarbonate (PC) is widely used for ion track recording and to prepare track-etched membranes as micro filters. Now PC particle track-etched membranes (nano-PTM) with pore shape and size very well controlled within diameters from 10 to 100 nm [5,6] have been produced. These membranes are used for the manufacturing of nano tubes and nano wires [7,8]. Swift Heavy Ion degradation of polymers has been analyzed by various researchers [9–12] in a wide range of energies. The sensitivity of a polymer [13] 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 forwarded along latent tracks of energetic ions in polymers [3,14,15]. Formation of these carbonaceous clusters can be studied from the absorption edge of ultraviolet–visible (UV–Vis) spectra which gives an idea about the value of optical band gap (Eg). Fourier Transform Infra Red (FTIR) spectroscopy in conjunction with the UV–Vis results enable to understand the structural changes in irradiated polymer.
S. Asad Ali et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1813–1817
In the present work, modifications in optical, chemical, structural and electrical properties of Lexan PC induced by 95 MeV O6+ ions have been investigated by UV–Vis, FTIR spectroscopy, Xray diffraction (XRD) and dielectric constant measurements. The energy of projectile was chosen so that the ions could easily pass through the PC sample. Thus the modifications are mostly due to electronic energy loss. 2. Experimental
5.5
4.5 4.0
Thick Lexan PC (225 lm) films were obtained from Farben Fabri-ken Bayer A.G., Germany. The chemical structure of Lexan Polycarbonate is given as:
3.5 3.0
CH3
e
2.0 d
1.5
c
1.0
CH3 C
f
2.5
0.5
O
a - Pristin 10 2 b - 1x10 ions/cm 11 2 c - 1x10 ions/cm 12 2 d - 1x10 ions/cm 13 2 e - 1x10 ions/cm 13 2 f - 5x10 ions/cm
5.0
Absorbance(a.u.)
1814
b a
0.0 300
O C O n
The samples of size (1.5 1.5 cm2) were irradiated under high vacuum with 95 MeV O6+ ions from 15 UD Pelletron accelerator at Inter University Accelerator Centre (IUAC), New Delhi, India to the fluences of 1010, 1011, 1012, 1013 and 2 1013 ions/cm2. To expose the whole target area the beam was scanned in the X–Y plane. Ion beam induced modifications have been characterized by various techniques. UV–Vis spectrum was recorded by UV–Vis spectrophotometer (SHIMADZU-UV-160) in the range 200– 800 nm. FTIR Spectroscopy was performed in transmission mode using NICOLET-550 FTIR spectrometer. The spectra were recorded in the wave number range of 4000–400 cm1. XRD measurements for the polymer films were carried out using D8 Advanced Bruker diffractometer with Cu-Ka radiation (k = 1.541838 Å) at room temperature by taking 0.020 step size. The cathode was maintained at 30 kV. Diffraction patterns were recorded in the range 20° 6 2h 6 80o. Hewlett–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 Lexan PC after irradiation. 3.1. Optical response The results of optical absorption studies with UV–Vis Spectrophotometer carried out on virgin and irradiated samples are presented in Fig. 1. Fig. 1(a) depicts the UV–Vis spectrum of pristine sample. It shows a sharp decrease with increasing wave length up to 300 nm followed by a plateau region. Fig. 1(b–e) show the optical spectrum for Lexan PC samples after irradiation to the fluences of 1010, 1011, 1012, 1013 and 2 1013 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 studies [16,17] have reported increased absorbance in polymers bombarded with MeV heavy ions. This behavior may be attributed to the formation of extended systems of conjugate bonds as a result of the beam induced bond cleavage and reconstruction [16].
400
500
600
700
800
Wavenumber (nm) Fig. 1. Optical absorption spectra of Lexan polycarbonate polymer irradiated with 95 MeV O6+ ion beam.
The shift of absorption edge of UV–Vis spectra towards the visible region can be correlated with optical band gap (Eg) by Tauc’s expression [14].
x2 eðkÞ ¼ ðhx Eg Þ2
ð1Þ
where, e(k) is the optical absorbance, x is the frequency and k is the wavelength. The intersection of the extrapolated spectrum with the abscissa of the plot {e(k)/k}1/2 vs 1/k yields the gap wavelength (kg) from which energy gap can be derived
Eg ¼ hc=kg
ð2Þ
For a linear structure the number of carbon atoms per conjugation length N is given by [18].
N¼
2bp eV Eg
ð3Þ
here, 2b is the band structure energy of a pair of adjacent p sites and its value is taken as 2.9 eV as it is associated with p ? p* optical transitions in –C = C structure. As the shift of the absorption edge can be attributed to an increase of the conjugated structures, Eq. (3) can be used in the present study. The values of absorption edge (kg) and the corresponding optical band-gap (Eg) and number of carbon atoms per conjugation length (N) are given in the Table 1. The value of Eg decreases with increase in the fluence. The optical band gap energy of the pristine PC is found to be 4.12 eV. It reduces to 2.34 eV at 2 1013 ions/cm2. The radicals contribute to the polymeric restructuring process which leads to conductivity [14] as confirmed by Sinha et al. [19] that the free radical formation takes place in PADC (Acrylics) by gamma irradiation at higher
Table 1 Variation of absorption edge (kg), energy gap (Eg) and number of carbon atoms N per conjugation length(N) in pristine and O6+ ion irradiated samples of Lexan (PC) to different flounces. Fluence (ions/cm2)
Absorption edge kg (nm)
Band gap energy Eg (eV)
N
Unirradiated 1010 1011 1012 1013 2 1013
301.8 312.3 349.2 385.8 493.0 530.2
4.12 3.98 3.56 3.22 2.52 2.34
5 5 5 6 7 8
1815
S. Asad Ali et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1813–1817
doses. The decrease in present case can be correlated to the formation of free radicals.
600 450
2 x1 0
13
2
io n s / c m
300 150
3.2. FTIR spectroscopy
3.3. Structural response Fig. 3 shows the X-ray diffraction patterns of the pristine and irradiated Lexan samples. From the figure it can be observed that
2x10
13
Transmittance (%)
1x10
13
1x10
12
1x10
11
1x10
10
Pristine
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 2. FTIR spectra of Lexan PC films: pristine and irradiated with 95 MeV O6+ ions to different fluences.
13
10
750
Intensity (a.u.)
The vibration modes of chemical bonds are characterized by the absorption bands [20]. Fig. 2 shows various absorption bands of the PC foils irradiated to different fluences of O6+ ions along with the absorption bands of the pristine film. The Infrared absorption peaks of functional groups of PC [21] are present in spectra. In the pristine foil the absence of the absorption band around 3500 cm1 shows the absence of terminal hydroxyl group indicating high molecular weight of the polymer under study. This polymer is synthesized by transestrification of diphenyl 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 decrease with the increase in the chain length of the polymer. There is almost no change in the spectra of samples irradiated to the fluence of 1011 ions/cm2. On irradiation to the fluence of 1012–1013 ions/cm2 the intensity of the peak corresponding to 3500 cm1 along with the absorption intensity of the band at 1770 cm1 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 hydroxyl group. H atom required for its formation coming perhaps from the isopropyl group as the absorption around 2970 cm1 which arises due to CH3 symmetric stretch decreases 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 cm1 attributed to carbonate C–O stretch. The intensity of absorption bands at 830 and 1018 cm1 decreases and corresponds to para out of plane aromatic C–H wag of two adjacent H atoms and para in plane aromatic C–H bends and 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.
1000 0 750 500 250 1000 0
1 0
12
2
io n s / c m
io n s / c m
2
500 250 1000 0 750 500 250 10000
10
1 0
750
11
io n s / c m
10
io n s / c m
2
2
500 250 1000 0 P r is tin e
750 500 250 0
Fig. 3. X-ray diffraction patterns for Lexan PC films: pristine and irradiated with 95 MeV O6+ ions to different fluences.
Table 2 FWHM and crystallite size of pristine and 95 MeV O6+ ion irradiated Lexan PC films. Fluence (ions/cm2)
2h (°)
FWHM (b)
Crystallite size L (Å)
Unirradiated 1010 1011 1012 1013 2 1013
16.98 16.18 16.94 16.92 16.76 17.96
4.513 4.607 4.634 4.676 4.720 4.967
17.79 17.41 17.32 17.17 17.00 16.18
the X-ray diffraction patterns of the samples can be characterized by main halo. The profile of the halo shows that that the Lexan polymer is partly crystalline polymer with a dominant amorphous phase The average crystal size, more commonly known as particle size L is related to b, the full width at half maximum (FWHM) of the peak (in radian) by the well known Scherer formula L = k/(b cosh) [22], where k is the wavelength of the X-rays used. The crystallite size corresponding to the diffraction peaks of the pristine and irradiated polymer samples were calculated using Scherer equation and the results are presented in Table 2. Calculations show that L is 17.79 Å for the virgin or unirradiated polymer and 16.18 Å for the sample irradiated to 2 1013 ions/ cm2. Peak at the position 2h = 16.98o shifted to 2h = 17.96o at the highest fluence of 2 1013 ions/cm2. As the ion fluence increases, crystallite size decreases at the highest fluence of 2 1013 ions/ cm2, a reduction of nearly 9.06%. It can be interpreted that on irradiation, a process of ‘‘end-linking” is possible during scission. At least one of the fragments may link to the main chain of neighboring molecule to give a branched molecule of higher molecular weight [23]. Physical properties of an end-linked polymer would be essentially identical to those of cross-linked polymer. The free radicals produced due to scission may cause branching that destroys crystallanity.
3.4. Electrical response The dielectric response of material provides information about the orientational translational adjustment of mobile charges
S. Asad Ali et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1813–1817
present in the dielectric medium in response to an applied electric field. The most important property of dielectric materials is the ability to be polarized under the action of the field. The dielectric loss behavior of polymer films is very important because of their possible applications for insulation, isolation and passivation in micro-electronic circuits [24]. In general polymers are insulators and commonly used in insulation of electric wires. However, certain classes of polymers have been discovered and used as semiconductor and capacitors with unusual electrical properties. The dielectric polarization may be judged in terms of the dielectric constant and the dissipation factor (loss angle or tand). 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 (tand) measurements were carried out using a parallel plate configuration of electrodes on both sides of PC film using a LCR meter (Hewlett–Packard 4284) in the frequency range of 1–1000 kHz at room temperature. The measured values of capacitance then have been converted into the dielectric constant (e0 ) by using the formula
e0 ¼ Cd=eo A
11.8
11.6
11.4
11.2
11.0
10.8 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Log frequency (Hz) Fig. 5. Plot of dielectric constant versus log frequency for pristine and irradiated with 95 MeV O6+ ions of Lexan PC.
ð5Þ
Pristine 10 2 10 ions/cm 11 2 10 ions/cm 12 2 10 ions/cm 13 2 10 ions/cm 13 2 2x10 ions/cm
0.040 0.035 0.030 Tan δ
where, f is the frequency and tand is the dielectric loss. Fig. 4 shows the dependence of conductivity of Lexan PC films on log frequency at room temperature for pristine and irradiated samples. A sharp increase in conductivity has been observed. It is also observed that conductivity increases as fluence increases. The increase in conductivity due to irradiation may be attributed to scissoring of polymers chains. When a.c. field of sufficiently high frequency is applied to a metal polymer/metal structure, it may cause a net polarization which is out of phase with the field. This results in a.c. conductivity. Fig. 5 shows that the dielectric constant at lower frequencies shows a rapidly decreasing trend. The dielectric constant decreases also at higher frequencies but slowly. It also decreases with fluence. As the frequency increases the charge carriers migrate through the dielectric and get trapped against a defect site and induce an opposite charge in its vicinity. At these frequencies for the ions, the polarization of trapped and bound charges can not take place and hence the dielectric constant decreases.
0.025 0.020 0.015 0.010 0.005 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Log frequency (Hz) Fig. 6. Variation of tand with log frequency for pristine and 95 MeV O6+ ions irradiated Lexan PC.
27
Pristine 10 2 10 ions/cm 11 2 10 ions/cm 12 2 10 ions/cm 13 2 10 ions/cm 13 2 2x10 ions/cm
24 -1
12.0
0.045
ra:c: ¼ 2pf tan d eo er
-6
Pristine 10 2 10 ions/cm 11 2 10 ions/cm 12 2 10 ions/cm 13 2 10 ions/cm 13 2 2x10 ions/cm
12.2
ð4Þ
where, d is the thickness of polymer film, A is the area of the electrode plates and eo is the permittivity of free space. A.C. conductivity ra.c. is calculated by the relation given below
Conductivity x10 (ohm.cm )
12.4
Dielectric constant (ε ')
1816
21
It is observed from the Fig. 6 that the loss factor increases moderately with fluence. The increase in loss factor with fluence may be due to scissoring of polymer chains, resulting in an increase of free radicals.
18 15 12
References
9 6 3 0 2
3
4
5
6
Log frequency ( in Hz) Fig. 4. AC conductivity versus log frequency plot for pristine and 95 MeV O6+ ions irradiated Lexan PC.
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