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Physico-chemical modifications induced in Makrofol-N polycarbonate by swift heavy ions
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 251 (2006) 163–166 www.elsevier.com/locate/nimb
Physico-chemical modifications induced in Makrofol-N polycarbonate by swift heavy ions Rajesh Kumar a
a,*
, H.S. Virk b, K.C. Verma c, Udayan De c, A. Saha d, Rajendra Prasad
a
Department of Applied Physics, Z.H. College of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India b Variable Energy Cyclotron Centre, 1/AF, Bidhan Nagar, Kolkata 700064, India c 360/71, Mohali, SAS Nagar, Chandigarh 160071, India d UGC-DAE Consortium for Scientific Research, Kolkata 700098, India Received 29 January 2006; received in revised form 24 May 2006 Available online 9 August 2006
Abstract Optical, chemical and thermal properties of the polymers can be modified by swift heavy ion (SHI) irradiation. Fifty microns thick films of Makrofol-N were irradiated with 70 MeV C5+ ions to the fluences of 107, 108 and 9.3 · 1011 and 9.3 · 1012 ions/cm2 from Pelletron accelerator at Nuclear Science Centre, New Delhi, India. The characterization of the radiation induced modifications has been carried out with FTIR, UV–Vis and differential scanning calorimetry (DSC). No change has been observed at lower fluences of 107 and 108 ions/cm2. At higher fluences, slight shift in the optical absorption edge towards the red end of the spectrum has been observed with increase in fluence. FTIR and thermal data show changes at higher fluences. 2006 Elsevier B.V. All rights reserved. Keywords: Makrofol-N polycarbonate; C5+ Ion irradiation; Ion beam modification; FTIR; UV–Vis; Differential scanning calorimetry
1. Introduction Various modifications in polymeric materials have been observed due to irradiation of polymers with energetic heavy ions [1,2]. The high value of the electronic stopping power, (dE/dx)e, induces a high density of the electron–hole pairs close to the ion path. So, the energetic heavy ions can create cylindrical tracks, form radicals, cause main chain scission, intermolecular cross-linking, creation of triple bonds and unsaturated bonds, and cause loss of volatile fragments [3,4]. The particular bonds which are broken, may be identified by FTIR analysis of the virgin and irradiated samples. The absorption in the UV and visible region by the polymer shows a change in optical density with increasing fluence and the absorption may shift towards the higher or lower wavelength side depending on the physical conditions of the irradiation. Fink et al. [5] have carried out IR studies on
polycarbonate samples using low energy argon ions, whereas Srivastva et al. [1] have used 100 MeV silicon beam for the study of chemical, optical and thermal modifications. In the present work, modifications brought about by 70 MeV C5+ ions in the optical, chemical and thermal properties of Makrofol-N polycarbonate have been investigated. Makrofol-N is a bisphenol-A polycarbonate. It is transparent, having high compact strength and wide applications. Polycarbonates are essentially those polymers in which dihydric or polyhydric phenols are joined through carbonate linkages. These polymers are considered to be toughest of all the thermo-plastics. Chemical and structural modifications due to C-ion irradiation have been observed in our previous studies [6,7] on CR-39 (DOP) and Polyamide Nylon-6. The structure of Makrofol-N polycarbonate is given as CH3 O
*
Corresponding author. Tel./fax: +91 571 2742932. E-mail address: drrajesh04@rediffmail.com (R. Kumar).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.06.003
C CH3
O C O
n
164
R. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 163–166 Table 1 Optical gap energy (Eg) and number of carbon atoms (N) or conjugation length Fluence (ions/cm2)
Absorption edge (kg) (nm)
Band gap energy (eV)
N
0 9.3 · 1011 9.3 · 1012
310.49 ± 9.31 337.18 ± 10.11 340.08 ± 10.20
4.01 ± 0.12 3.69 ± 0.11 3.66 ± 0.10
5 5 5
2. Experimental
Fig. 1. Optical absorption spectra of Makrofol-N (PC) irradiated with a 70 MeV C5+ ion beam.
Makrofol-N polycarbonate (PC) films, manufactured by a castic process, were obtained from Bayer AG, Lever Kussen, Germany. Fifty microns thick PC sheets were irradiated, under a vacuum of 4 · 106 torr with ion beam of 70 MeV C5+ to the fluences of 107, 108, 9.3 · 1011 and 9.3 · 1012 ions/cm2 from Pelletron accelerator at the Nuclear Science Centre, New Delhi, India. The
Fig. 2. FTIR spectra of Makrofol-N polycarbonate irradiated with 70 MeV C5+ ion beam: (a) virgin (b) 9.3 · 1011 (c) 9.3 · 1012 ions/cm2.
R. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 163–166
165
irradiation was carried out at room temperature. The ion beam was defocused using a magnetic scanning system so that an area of 1.5 · 1.5 cm2 could be uniformly irradiated. The beam current was kept low (5 pnA) to suppress thermal decomposition. The thickness of the film was less than the projected range so as to avoid ion implantation. 3. Results and discussion 3.1. UV–Visible spectroscopy The formation of new bonds have been studied by UV– Visible spectroscopy performed in the wavelength range 200–500 nm by SHIMADZU, UV-1601 PC, (Japan), UV–Visible Spectrophotometer. No change was observed at lower fluences of 107 and 108 ions/cm2. The results of absorption studies with UV–Visible spectrophotometer carried out on virgin and irradiated samples at higher fluences (9.3 · 1011 and 9.3 · 1012 ions/cm2) are illustrated in Fig. 1. A shift of this absorption from the UV–Visible towards the visible region has been observed for irradiated samples. With increasing dose to 9.3 · 1011 and 9.3 · 1012 ions/cm2 the absorption peak appears to broaden. This is generally thought to be caused by the formation of conjugated bonds i.e. possible formation of carbon clusters. The absorption bands in the investigated range of wavelength are associated to the p–p* electron transition [8–10]. This type of transition occurs in the unsaturated centers of the molecules i.e. in compounds, containing double or triple bonds and also in aromatics. The excitation of p electron requires small energy and hence transition of this type occurs at longer wavelengths. The optical band edge can be correlated to the optical band gap, Eg by Tauc expression [11] 2
x2 e2 ¼ ð hx E g Þ ;
ð1Þ p
where e2 (k) is the optical absorbance. A plot of e2/k versus 1/k gives the value of Eg. The intersection of the extrapolated spectrum with the abscissa yields the gap wavelength kg from which the gap energy is derived to be Eg = hc/kg. For a linear structure the number of carbon atoms per conjunction length N is given by [4] N ¼ 2bp=Eg ;
ð2Þ
where 2b is the band structure energy of a pair of adjacent p sites and b is taken to be 2.9 eV as it is associated with p ! p* optical transitions in AC@CA structure. As the shift of the absorption edge can be attributed to an increase of the conjugated structures, Eq. (2) is used in the present study. The values of Eg and corresponding number of carbon atoms per conjugation length are presented in Table 1. The value of Eg is seen to decrease with increasing fluence. 3.2. FTIR spectroscopy The nature of chemical bonds of the polymers can be studied through the characterization of the vibration modes
Fig. 3. DSC curves of virgin and irradiated polycarbonate films: (a) virgin, (b) 9.3 · 1011 ions/cm2, (c) a run with empty sample holders (background).
determined by infrared spectroscopy [12–14]. The FTIR spectra of the virgin and irradiated to higher fluences, Makrofol-N PC depicted in Fig. 2 shows various absorption bands of the PC films irradiated to two different fluences along with those of the virgin film. On irradiation a peak around 3500 cm1 appeared. The absence of this absorption band in the pristine PC film shows the absence of terminal hydroxyl group, indicating that the molecular weight of the polymer film is very high. The synthesis of this polymer requires transestrification of diphenyl carbonate with bisphenol A, with the elimination of phenol as a side product. Thus it is expected that the initial concentration of hydroxyl group will monotonously decrease with the
166
R. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 163–166
increase in the chain length of the polymer. In addition to the appearance of a peak around 3500 cm1 there is a decrease due to irradiation, in the intensity of the band at 1775 cm1 which represents C@O strech, with the ion fluence. This indicates that chain scission may take place at the carbonate site with probable elimination of carbon dioxide/carbon monoxide and formation of hydroxyl group. As in UV–Visible, no change was observed in FTIR spectra at lower fluences. 3.3. Differential scanning calorimetry (DSC) The DSC thermograms were obtained for 23–200 C heating at 5 C/min and are shown in Fig. 3. Our DSC result for unirradiated Makrofol-N PC in Fig. 3 shows a dip at around 150 C, 147.9 C being identified as the glass transition temperature (Tg) in an earlier work [1] for unirradiated sample of Lexan PC. This feature disappears even at the lower fluence (9.3 · 1011 ions/cm2) irradiation. 4. Conclusion In the present irradiation with 70 MeV C5+ ions at fluences in the range 107–1013 ions/cm2, modification in the optical, chemical and thermal characteristics have been observed at higher fluences. No changes in optical, chemical and thermal characteristics of Makrofol-N PC have been observed due to irradiation with 70 MeV C5+ ions to the lower fluences of 107 and 108 ions/cm2. On irradiation to higher fluences of 9.3 · 1011 and 9.3 · 1012 ions/ cm2 a shift from UV–Visible towards visible region and decrease in the energy gap and modification in chemical and thermal characteristics have been observed. To our knowledge, there have not been reports of such studies of such heavy ion irradiation in Makrofol-N polycarbonate. Acknowledgements The authors are thankful to Dr. A.K. Sinha, Director, UGC-DAE Consortium for Scientific Research, Kolkata
for providing UV–Vis facilities and constant encouragement. Thanks are also due to Dr. D.K. Avasthi for fruitful discussions and to the staff of the Nuclear Science Centre, New Delhi, India for their help during irradiation. One of the author, Prof. Rajendra Prasad wishes to thank, All India Council of Technical Education, Government of India for providing Emeritus Fellowship to carry out this work. Financial assistance provided by Department of Science and Technology (D.S.T.), Government of India to Dr. Rajesh Kumar as Young Scientist (Award No. SR/FTP/ PS-31/2004) is gratefully acknowledged. References [1] A. Srivastva, T.V. Singh, S. Mule, C.R. Rajan, S. Ponrathnam, Nucl. Instr. and Meth. B 192 (2002) 402. [2] E. Balanzat, S. Bouffard, A. Cassani, E. Dooryhee, L. Protin, J.P. Grandin, J.L. Doualan, J. Margerie, Nucl. Instr. and Meth. B 91 (1994) 134. [3] V. Picq, J.M. Ramillon, E. Balanzat, Nucl. Instr. and Meth. B 146 (1998) 496. [4] D. Fink, R. Klett, L.T. Chadderton, J.M. Cardosa, R. Montiel, H. Vezquez, A. Karanovich, Nucl. Instr. and Meth. B 111 (1996) 303. [5] D. Fink, M. Muller, L.T. Chadderton, P.H. Cannington, R.G. Elliman, D.C. McDonaldt, Radiat. Eff. Defects Solids 152 (2000) 15. [6] Rajesh Kumar, Rajendra Prasad, Y.K. Vijay, N.K. Acharya, K.C. Verma, Udayan De, Nucl. Instr. and Meth. B 212 (2003) 221. [7] H.S. Virk, Nucl. Instr. and Meth. B 111 (1996) 303. [8] N. Betz, A. Le Moel, E. Balanzat, J.M. Ramillon, J. Lamotte, P. Gallas, G. Jaskierowicz, J. Polym. Sci. Pol. Phys. Ed. B 32 (1994) 1493. [9] L. Calcagno, G. Compagnin, G. Foti, Nucl. Instr. and Meth. B 65 (1992) 413. [10] A.M. Cruzman, J.D. Carlson, J.E. Bares, P.P. Pronko, Nucl. Instr. and Meth. B 7–8 (1985) 468. [11] J. Tauc, R. Grigorovici, A. Vaneu, Phys. Status Solidi 15 (1966) 627. [12] J. Davenas, X.L. Xu, G. Boiteux, D. Sage, Nucl. Instr. and Meth. B 39 (1989) 754. [13] I. Noda, A.W. Dowrey, C. Marcott, in: J.E. Mark (Ed.), Physical Properties of Polymers Handbook, AIP Press, New York, 1996. [14] J.A. Dean, Handbook of Organic Chemistry, McGraw Hill Publication Company, New York, 1987.
Nuclear Instruments and Methods in Physics Research B 251 (2006) 163–166 www.elsevier.com/locate/nimb
Physico-chemical modifications induced in Makrofol-N polycarbonate by swift heavy ions Rajesh Kumar a
a,*
, H.S. Virk b, K.C. Verma c, Udayan De c, A. Saha d, Rajendra Prasad
a
Department of Applied Physics, Z.H. College of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India b Variable Energy Cyclotron Centre, 1/AF, Bidhan Nagar, Kolkata 700064, India c 360/71, Mohali, SAS Nagar, Chandigarh 160071, India d UGC-DAE Consortium for Scientific Research, Kolkata 700098, India Received 29 January 2006; received in revised form 24 May 2006 Available online 9 August 2006
Abstract Optical, chemical and thermal properties of the polymers can be modified by swift heavy ion (SHI) irradiation. Fifty microns thick films of Makrofol-N were irradiated with 70 MeV C5+ ions to the fluences of 107, 108 and 9.3 · 1011 and 9.3 · 1012 ions/cm2 from Pelletron accelerator at Nuclear Science Centre, New Delhi, India. The characterization of the radiation induced modifications has been carried out with FTIR, UV–Vis and differential scanning calorimetry (DSC). No change has been observed at lower fluences of 107 and 108 ions/cm2. At higher fluences, slight shift in the optical absorption edge towards the red end of the spectrum has been observed with increase in fluence. FTIR and thermal data show changes at higher fluences. 2006 Elsevier B.V. All rights reserved. Keywords: Makrofol-N polycarbonate; C5+ Ion irradiation; Ion beam modification; FTIR; UV–Vis; Differential scanning calorimetry
1. Introduction Various modifications in polymeric materials have been observed due to irradiation of polymers with energetic heavy ions [1,2]. The high value of the electronic stopping power, (dE/dx)e, induces a high density of the electron–hole pairs close to the ion path. So, the energetic heavy ions can create cylindrical tracks, form radicals, cause main chain scission, intermolecular cross-linking, creation of triple bonds and unsaturated bonds, and cause loss of volatile fragments [3,4]. The particular bonds which are broken, may be identified by FTIR analysis of the virgin and irradiated samples. The absorption in the UV and visible region by the polymer shows a change in optical density with increasing fluence and the absorption may shift towards the higher or lower wavelength side depending on the physical conditions of the irradiation. Fink et al. [5] have carried out IR studies on
polycarbonate samples using low energy argon ions, whereas Srivastva et al. [1] have used 100 MeV silicon beam for the study of chemical, optical and thermal modifications. In the present work, modifications brought about by 70 MeV C5+ ions in the optical, chemical and thermal properties of Makrofol-N polycarbonate have been investigated. Makrofol-N is a bisphenol-A polycarbonate. It is transparent, having high compact strength and wide applications. Polycarbonates are essentially those polymers in which dihydric or polyhydric phenols are joined through carbonate linkages. These polymers are considered to be toughest of all the thermo-plastics. Chemical and structural modifications due to C-ion irradiation have been observed in our previous studies [6,7] on CR-39 (DOP) and Polyamide Nylon-6. The structure of Makrofol-N polycarbonate is given as CH3 O
*
Corresponding author. Tel./fax: +91 571 2742932. E-mail address: drrajesh04@rediffmail.com (R. Kumar).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.06.003
C CH3
O C O
n
164
R. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 163–166 Table 1 Optical gap energy (Eg) and number of carbon atoms (N) or conjugation length Fluence (ions/cm2)
Absorption edge (kg) (nm)
Band gap energy (eV)
N
0 9.3 · 1011 9.3 · 1012
310.49 ± 9.31 337.18 ± 10.11 340.08 ± 10.20
4.01 ± 0.12 3.69 ± 0.11 3.66 ± 0.10
5 5 5
2. Experimental
Fig. 1. Optical absorption spectra of Makrofol-N (PC) irradiated with a 70 MeV C5+ ion beam.
Makrofol-N polycarbonate (PC) films, manufactured by a castic process, were obtained from Bayer AG, Lever Kussen, Germany. Fifty microns thick PC sheets were irradiated, under a vacuum of 4 · 106 torr with ion beam of 70 MeV C5+ to the fluences of 107, 108, 9.3 · 1011 and 9.3 · 1012 ions/cm2 from Pelletron accelerator at the Nuclear Science Centre, New Delhi, India. The
Fig. 2. FTIR spectra of Makrofol-N polycarbonate irradiated with 70 MeV C5+ ion beam: (a) virgin (b) 9.3 · 1011 (c) 9.3 · 1012 ions/cm2.
R. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 163–166
165
irradiation was carried out at room temperature. The ion beam was defocused using a magnetic scanning system so that an area of 1.5 · 1.5 cm2 could be uniformly irradiated. The beam current was kept low (5 pnA) to suppress thermal decomposition. The thickness of the film was less than the projected range so as to avoid ion implantation. 3. Results and discussion 3.1. UV–Visible spectroscopy The formation of new bonds have been studied by UV– Visible spectroscopy performed in the wavelength range 200–500 nm by SHIMADZU, UV-1601 PC, (Japan), UV–Visible Spectrophotometer. No change was observed at lower fluences of 107 and 108 ions/cm2. The results of absorption studies with UV–Visible spectrophotometer carried out on virgin and irradiated samples at higher fluences (9.3 · 1011 and 9.3 · 1012 ions/cm2) are illustrated in Fig. 1. A shift of this absorption from the UV–Visible towards the visible region has been observed for irradiated samples. With increasing dose to 9.3 · 1011 and 9.3 · 1012 ions/cm2 the absorption peak appears to broaden. This is generally thought to be caused by the formation of conjugated bonds i.e. possible formation of carbon clusters. The absorption bands in the investigated range of wavelength are associated to the p–p* electron transition [8–10]. This type of transition occurs in the unsaturated centers of the molecules i.e. in compounds, containing double or triple bonds and also in aromatics. The excitation of p electron requires small energy and hence transition of this type occurs at longer wavelengths. The optical band edge can be correlated to the optical band gap, Eg by Tauc expression [11] 2
x2 e2 ¼ ð hx E g Þ ;
ð1Þ p
where e2 (k) is the optical absorbance. A plot of e2/k versus 1/k gives the value of Eg. The intersection of the extrapolated spectrum with the abscissa yields the gap wavelength kg from which the gap energy is derived to be Eg = hc/kg. For a linear structure the number of carbon atoms per conjunction length N is given by [4] N ¼ 2bp=Eg ;
ð2Þ
where 2b is the band structure energy of a pair of adjacent p sites and b is taken to be 2.9 eV as it is associated with p ! p* optical transitions in AC@CA structure. As the shift of the absorption edge can be attributed to an increase of the conjugated structures, Eq. (2) is used in the present study. The values of Eg and corresponding number of carbon atoms per conjugation length are presented in Table 1. The value of Eg is seen to decrease with increasing fluence. 3.2. FTIR spectroscopy The nature of chemical bonds of the polymers can be studied through the characterization of the vibration modes
Fig. 3. DSC curves of virgin and irradiated polycarbonate films: (a) virgin, (b) 9.3 · 1011 ions/cm2, (c) a run with empty sample holders (background).
determined by infrared spectroscopy [12–14]. The FTIR spectra of the virgin and irradiated to higher fluences, Makrofol-N PC depicted in Fig. 2 shows various absorption bands of the PC films irradiated to two different fluences along with those of the virgin film. On irradiation a peak around 3500 cm1 appeared. The absence of this absorption band in the pristine PC film shows the absence of terminal hydroxyl group, indicating that the molecular weight of the polymer film is very high. The synthesis of this polymer requires transestrification of diphenyl carbonate with bisphenol A, with the elimination of phenol as a side product. Thus it is expected that the initial concentration of hydroxyl group will monotonously decrease with the
166
R. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 163–166
increase in the chain length of the polymer. In addition to the appearance of a peak around 3500 cm1 there is a decrease due to irradiation, in the intensity of the band at 1775 cm1 which represents C@O strech, with the ion fluence. This indicates that chain scission may take place at the carbonate site with probable elimination of carbon dioxide/carbon monoxide and formation of hydroxyl group. As in UV–Visible, no change was observed in FTIR spectra at lower fluences. 3.3. Differential scanning calorimetry (DSC) The DSC thermograms were obtained for 23–200 C heating at 5 C/min and are shown in Fig. 3. Our DSC result for unirradiated Makrofol-N PC in Fig. 3 shows a dip at around 150 C, 147.9 C being identified as the glass transition temperature (Tg) in an earlier work [1] for unirradiated sample of Lexan PC. This feature disappears even at the lower fluence (9.3 · 1011 ions/cm2) irradiation. 4. Conclusion In the present irradiation with 70 MeV C5+ ions at fluences in the range 107–1013 ions/cm2, modification in the optical, chemical and thermal characteristics have been observed at higher fluences. No changes in optical, chemical and thermal characteristics of Makrofol-N PC have been observed due to irradiation with 70 MeV C5+ ions to the lower fluences of 107 and 108 ions/cm2. On irradiation to higher fluences of 9.3 · 1011 and 9.3 · 1012 ions/ cm2 a shift from UV–Visible towards visible region and decrease in the energy gap and modification in chemical and thermal characteristics have been observed. To our knowledge, there have not been reports of such studies of such heavy ion irradiation in Makrofol-N polycarbonate. Acknowledgements The authors are thankful to Dr. A.K. Sinha, Director, UGC-DAE Consortium for Scientific Research, Kolkata
for providing UV–Vis facilities and constant encouragement. Thanks are also due to Dr. D.K. Avasthi for fruitful discussions and to the staff of the Nuclear Science Centre, New Delhi, India for their help during irradiation. One of the author, Prof. Rajendra Prasad wishes to thank, All India Council of Technical Education, Government of India for providing Emeritus Fellowship to carry out this work. Financial assistance provided by Department of Science and Technology (D.S.T.), Government of India to Dr. Rajesh Kumar as Young Scientist (Award No. SR/FTP/ PS-31/2004) is gratefully acknowledged. References [1] A. Srivastva, T.V. Singh, S. Mule, C.R. Rajan, S. Ponrathnam, Nucl. Instr. and Meth. B 192 (2002) 402. [2] E. Balanzat, S. Bouffard, A. Cassani, E. Dooryhee, L. Protin, J.P. Grandin, J.L. Doualan, J. Margerie, Nucl. Instr. and Meth. B 91 (1994) 134. [3] V. Picq, J.M. Ramillon, E. Balanzat, Nucl. Instr. and Meth. B 146 (1998) 496. [4] D. Fink, R. Klett, L.T. Chadderton, J.M. Cardosa, R. Montiel, H. Vezquez, A. Karanovich, Nucl. Instr. and Meth. B 111 (1996) 303. [5] D. Fink, M. Muller, L.T. Chadderton, P.H. Cannington, R.G. Elliman, D.C. McDonaldt, Radiat. Eff. Defects Solids 152 (2000) 15. [6] Rajesh Kumar, Rajendra Prasad, Y.K. Vijay, N.K. Acharya, K.C. Verma, Udayan De, Nucl. Instr. and Meth. B 212 (2003) 221. [7] H.S. Virk, Nucl. Instr. and Meth. B 111 (1996) 303. [8] N. Betz, A. Le Moel, E. Balanzat, J.M. Ramillon, J. Lamotte, P. Gallas, G. Jaskierowicz, J. Polym. Sci. Pol. Phys. Ed. B 32 (1994) 1493. [9] L. Calcagno, G. Compagnin, G. Foti, Nucl. Instr. and Meth. B 65 (1992) 413. [10] A.M. Cruzman, J.D. Carlson, J.E. Bares, P.P. Pronko, Nucl. Instr. and Meth. B 7–8 (1985) 468. [11] J. Tauc, R. Grigorovici, A. Vaneu, Phys. Status Solidi 15 (1966) 627. [12] J. Davenas, X.L. Xu, G. Boiteux, D. Sage, Nucl. Instr. and Meth. B 39 (1989) 754. [13] I. Noda, A.W. Dowrey, C. Marcott, in: J.E. Mark (Ed.), Physical Properties of Polymers Handbook, AIP Press, New York, 1996. [14] J.A. Dean, Handbook of Organic Chemistry, McGraw Hill Publication Company, New York, 1987.