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Gamma irradiation induced chemical and structural modifications in PM-355 polymeric nuclear track detector film
Nuclear Instruments and Methods in Physics Research B 290 (2012) 59–63
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Gamma irradiation induced chemical and structural modifications in PM-355 polymeric nuclear track detector film Vijay Kumar a,⇑, R.G. Sonkawade b, A.S. Dhaliwal a a b
Department of Physics, Sant Longowal Institute of Engineering and Technology, Longowal District, Sangrur, Punjab 148 106, India School of Physical Sciences, BBA University (A Central University), Lucknow 226 025, India
a r t i c l e
i n f o
Article history: Received 1 August 2012 Received in revised form 29 August 2012 Available online 11 September 2012 Keywords: PM-355 Gamma rays Raman UV–Visible XRD
a b s t r a c t This experimental study investigated the modification of chemical and structural properties of PM-355 films by irradiation of 1.25 MeV 60Co c-radiations at doses ranging from 0–675 kGy. The induced modifications were followed by micro-Raman and X-ray diffraction (XRD) spectroscopy. Further, the induced modifications were confirmed by UV–Visible spectroscopy. Raman spectra show that the films are highly disordered at the highest gamma dose. The XRD pattern of PM-355 shows the decreasing intensity of peak positions with an increase in the gamma dose, which suggests the loss of crystallinity of the films due to irradiation. Observed results indicate the formation of a disordered system in the irradiated films. Furthermore, the crystallite size for pristine and bombarded sample has been calculated. Moreover, interchain distance, micro strain, interplanar distance, dislocation density and distortion parameters were calculated. The analysis revealed a significant increase in micro strain, dislocation density and distortion parameters with an increase of gamma dose, which is in line with the Raman analysis. With increasing c-dose, the value of the direct and indirect band gap found to decrease. To the best of our knowledge, this work is the first to show the simultaneous existence of direct as well as the indirect band gap in PM-355 polymer. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Solid state nuclear track detectors (SSNTDs) have been extensively used for the detection of ions. Nowadays, ion track membranes, also known as nuclear track filters, have emerged as the main offshoot from SSNTDs. Many applications of SSNTDs have been developed, including biological filters, detection of light ions, dosimetry to use in ion track etching, magnetic nano wires as magneto resistive sensors and much more [1–4]. As advanced technology keeps on developing every day, the demand for polymers having improved properties is continuously on the rise due to their use of various, scientific and technological applications [5]. Irradiation plays a prominent role in modification of properties of polymers significantly. Ionizing radiation passing through matter, deposit energy in the material and cause irreversible changes in the macromolecular structure of the target material. The most prominent effects of radiation involve a change of phase in the absorbing material due to the scissions or cross linking formation. At the same time, it is also likely possible to observe the creation of chemical bonds between different molecules (intermolecular cross linking) in the main chain. Most of the efforts have been directed towards the effects of radiations on the track etch and bulk etch rates of PM-355 poly⇑ Corresponding author. E-mail address: [email protected] (V. Kumar). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.08.029
meric detectors [6–8]. In fact; many authors have used such detectors basing only on approximate calibration data, which presented a dependence of track diameters on energy of selected particles and etching time for the detectors [9–13]. The PM-355 detectors have used for the detection of light as well as heavy ions, including protons, deuterons, He, C, N, O, S, etc. [14–16]. Recently; PM-355 detectors have been used for the assessment of solar ultra-violet radiation dosimetry [17]. The polymer PM-355 has the same chemical composition as the solid state nuclear track detector CR-39 or PADC that find diverse applications [5,18]. From the above discussion, it was concluded that there is a lacuna in the systematic study on the optical, chemical, structural and thermal properties of PM-355 after irradiation. Very few attempts have been carried out in the study of optical, thermal, chemical and structural properties of PM-355 SSNTDs as a function of radiation doses [19,20]. Moreover, to be best of our knowledge a few reports have been carried out on the use of gamma radiation for the modifications of optical and structural properties of PM355 SSNTDs [21]. Interestingly, as for many other polymeric nuclear track detectors (viz. CR-39, Makrofol KG, PET, etc.), the optical, chemical, structural, and thermal properties may be greatly affected by irradiation [22–25]. Because of the potential usefulness of PM-355, a study was carried out on the chemical and structural modifications induced by gamma irradiation. The aim of this paper is to investigate the change in chemical and structural properties of PM-355 film after gamma irradiation.
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V. Kumar et al. / Nuclear Instruments and Methods in Physics Research B 290 (2012) 59–63
An attempt has been made to correlate the results with the earlier reported data on the effects of irradiation on different polymeric materials in order to make the present investigation more informative. 2. Experimental aspects Small pieces of size (1 1 cm2) and thickness 250 lm were cut from the commercially available sheet of the PM-355 detector having density of 0.9 gm/cm3. This target assembly was then taken for gamma irradiation. Detectors were irradiated using 1.25 MeV gamma radiation (the average energy of the two 60Co photons) source of 60Co in the radiation chamber with dose rate 7.328 kGy/h at IUAC, New Delhi. Doses from 150 to 675 kGy were used in the present study. The nature of changes induced by gamma irradiation were carried out using UV–Visible spectrophotometer in the range of 200– 700 nm to see the dose dependence variation in optical band gap energy. The micro RAMAN investigation has been carried out using Renishaw InVia Raman. The structural studies were carried out by X-ray diffractometer with Cu Ka radiation (1.54 A0) for a range of Bragg’s angle 2h (10 < h < 60) at the scan rate of 1°/min.
another strong intensity band at 2962 cm1 is due to asymmetric CH2. The bands at 1288 and 1451 cm1 corresponds to the C–O stretch mode and –C–H bending mode, respectively. The other small intensity band at 1749 cm1 is due to C@O stretching. After highest gamma dose (i.e. 675 kGy) irradiation, it is clear from the figure that the intensity of the band at 2964 cm1 reduced drastically. Similar reduction in the band intensity at 2958 cm1 was observed by Sharma et al. for nitrogen ion implanted CR-39 films [27]. According to them, this indentifies the reduction of hydrogen content in the surface of implanted CR-39 films. Interestingly, the bands at 1288, 1455, and 1749 cm1 are all disappeared as a result of gamma irradiation, which indicates the change in chemical structure of PM-355 due to irradiation. The intensity of Raman band at 1124 cm1 is higher than the pristine film. The enhancement in Raman band intensities after irradiation was reported by various authors [28,29]. They suggested that this may be due to the carbonization of the track area and nucleation of the carbonrich clusters. Hence, it can be concluded that gamma irradiation leads to the decrease in peak intensity and breaking of polymer chains, which may introduce micro strain in the PM-355 polymer [30]. 3.2. UV–Visible spectroscopy
3. Results and discussion 3.1. Raman spectra Raman spectra of pristine and highest irradiated PM-355 films have been shown in Fig. 1 against Raman shift in cm1. The relative changes in the bonds have been estimated from the relative increase or decrease in Raman intensity of the peaks associated with the functional groups present the polymer. It is clear from the figure that the Raman intensity of most of the peaks increased, and some peaks disappeared, suggesting that some structural changes occur in the polymer film after gamma irradiation. In the pristine PM-355 film, a number of peaks at 1124, 1288, 1455, 1749 and 2962 cm1 are observed confirming its fundamental structure, which is similar to CR-39 polymer [26,27]. The highest intensity band at 1124 cm1 is assigned to C–O–C stretching, whereas
Fig. 2 shows the change in the absorption spectra of PM-355 upon gamma exposure to a dose range of 0–675 kGy. From this figure, it is noticeable that the absorption edge shifted towards the higher wavelength region, which indicates a decrease in the optical band gap energy [22,24,31–33]. As it can be seen, the absorbance intensity increases linearly with gamma dose, and the fine-structure peaks are easily distinguishable after 470 kGy. The shift to the absorption edge as a result irradiation, inferred the bond rupturing leading to scission, free radical formation, cross linking, etc., resulting in the formation of new bonds. Furthermore, absorbance is apparently more affected by irradiation at wavelengths 325–350 nm. The changes observed could be due to unsaturation and the presence of carbonyl and hydroxyl compounds. Not only this, we observe broadening of the absorption edge after irradiation, i.e. broadening of peak at FWHM. The reason of
Fig. 1. Raman spectra of the pristine and irradiated PM-355 film at highest gamma dose.
V. Kumar et al. / Nuclear Instruments and Methods in Physics Research B 290 (2012) 59–63
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the best of our knowledge no one reported the simultaneous existence of the direct and indirect band gap in case of PM-355. At the same time, very limited reports have been found in literature on the simultaneous existence of direct and indirect band gaps in polymers [31,34,35]. Pristine PM-355 is transparent and has very little absorption. Upon visual examination, the colorless surface of PM-355 became yellowish as the gamma dose gradually increased [31,36,37]. For a linear structure, the number of carbon atoms per conjugation length N is given by [31]:
N ¼ 2pb=Eg ;
Fig. 2. UV–Visible spectra of PM-355.
Table 1 The variation of optical band gap energy of both the polymer, along with the number of carbon atoms (N) per conjugated length. Dose (kGy)
0 150 300 470 630 675
Optical band gap energy (eV)
Number of carbon atoms (N)
Indirect
Direct
Direct
Indirect
3.7 ± 0.03 3.6 ± 0.04 3.5 ± 0.05 3.4 ± 0.06 3.1 ± 0.09 3.0 ± 0.02
4.6 ± 0.03 4.6 ± 0.06 4.5 ± 0.06 3.6 ± 0.07 3.5 ± 0.01 3.4 ± 0.08
5 5 5 5 6 6
4 4 4 5 5 5
broadening of absorption edge is the formation of an extended system of conjugated bonds, i.e. the formation of defects after irradiation. In the studied range of wavelength, the maximum absorption is caused by the p–p electronic transitions [31] and the cause at the back is the smaller energy requirement for excitation by p electrons. These transitions need an unsaturated group in the molecules (i.e. compound containing double or triple bonds and in aromatics) to provide the p electrons. The broadening of the peaks can also be attributed to the production of gamma radiation induced defects, which may further results in the formation of new energy level leading to the peak broadening. Such defects may result in the formation of new energy levels leading to the broadening of peaks. From the absorption spectra, the direct and indirect band gap, (ahm)2 and (ahm)1/2 were plotted as a function photon energy (hm), respectively. Zaki [31] clearly explained the concept of direct and indirect band gaps in CR-39 SSNTD after gamma irradiation. The Tauc’s expression is given by [31].
aðhtÞ ¼ Bðht Eg Þn =ht;
ð1Þ
where a is the absorption coefficient, hm is the photon energy, and Eg is the value of the optical energy gap. The plots between (ahm)2 and (ahm)1/2 verses (hm) are not presented in this report. The values of extrapolated intercept for direct and indirect band gaps for pristine and irradiated samples along with their standard errors are enlisted in Table 1. From Table 1, it can be seen that the value of the direct and indirect band decreases after gamma irradiation [31,34]. This result reflects the formation of defects in the PM-355 detector structure after gamma irradiation. However, the values of the indirect band gap have been found to be lower than the corresponding values for the direct band gap as depicted in Table 1. To
ð2Þ
where N is the number of carbon atoms per conjugated length, 2b gives the band structure energy of a pair of adjacent P sites. The value of b is taken to be 2.9 eV as it is associated with p–p optical transition in the –C@C– structure. The values of carbon atoms (N) per conjugation length are given in Table 1. If the PM-355 is to be used as a dosimeter, it is important to investigate the effect of dose rate at which the radiation dose is transferred to the detector. The dependence of optical band gap energies on gamma dose is shown in Fig. 3. A significant dependence of gamma dose on optical band gap was found. These observations have important implication for the use of PM-355 detectors as a dosimeter using well reported protocols. It is clear from Fig. 2 that the absorption edges to lay within the wavelength region 305–360 nm. Fig. 4 shows the variation of absorbance difference with the gamma dose at a characteristic wavelength of 323.5 nm. Absorbance difference for the gamma irradiated samples with respect to pristine one is highest at 323.5 nm. So we have chosen this wavelength as the characteristic’s wavelength. Similar observation was also reported in literature [33,35]. They reported that absorbance difference of pristine and irradiated samples represents the measure of the induced UV absorbance. It was also reported that the gamma irradiation enhances the UV absorbance which is a direct outcome of the formation of new chemical species as a result of energy transfer by the incident of gamma rays [33,35]. 3.3. X-ray diffraction Qualitative XRD analysis was conducted on pristine and irradiated films to have a preliminary idea of the effect of gamma dose on the structure of PM-355. X-ray diffraction spectra of pristine and gamma irradiated PM-355 are shown in Fig. 5. XRD spectrum of pristine sample shows that it is a partly crystalline polymer with dominant amorphous phase. From the Fig. 5, we see that the X-ray diffraction patterns of the samples are characterized by halos extending in the 2h range 10–600. Any change in peak intensity after irradiation shows some significant changes in the structure of the materials after irradiation. Table 2 clearly shows that integral intensity decrease gradually with an increase of gamma dose. A probable explanation for the decrease in intensity can be attributed to the decrease in amount of crystalline phase in the samples, and the crystalline structure has been destroyed. This could be attributed to the cross linking of molecular chains, which change the regularly arranged crystallites into non arranged ones by forming new bonds between the neighbouring chains [19]. Not only this, we have observed increase in full width at half maxima from the diffraction pattern. This behavior is associated with the decrease in crystallinity of the detector [38]. In order to get the qualitative estimates of the modification occurring in the PM-355 detector due to gamma irradiation, area of the diffraction peaks calculated from simple Gaussian fittings to the data [39,40] as a function of gamma doses, is plotted as shown in Fig. 6. It is clear from figure that the area decreases after
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V. Kumar et al. / Nuclear Instruments and Methods in Physics Research B 290 (2012) 59–63 Table 2 XRD parameters of gamma irradiated PM-355 SSNTDs. Gamma dose (kGy)
2h (deg.)
FWHM b (deg.)
Integral intensity, I (a.u.)
Gaussian fit area (a.u.)
0.0 150 300 470 630 675
21.7 21.1 20.5 20.4 22.2 21.7
9.0 9.4 9.1 10.0 11.3 18.0
696 605 555 652 526 356
4337 4109 4013 4562 3874 3216
Fig. 3. Optical band gap energy as a function of absorbed gamma doses.
Fig. 6. Variation of the diffraction peaks area as a function of gamma dose.
Furthermore, the crystallite size (L), interchain distance (r), interplanar distance (d), micro strain (e), dislocation density (d) and distortion parameters (g) were calculated as follows [35,41–44]:
k b cos h 5 k r¼ 8 sin h k d¼ 2 sin h b cos h e¼ 4 1 d ¼ 2 ; and L b g¼ tan h L¼
Fig. 4. Characteristics absorbance difference vs. gamma doses at 323.5 nm.
Fig. 5. X-ray spectra of PM-355.
irradiation, resulting in the decrement of crystallinity thereby leading to disorderness in the polymer structure.
ð3Þ ð4Þ ð5Þ ð6Þ ð7Þ ð8Þ
where k = 1.54 nm is the wavelength of the Cu Ka X-ray radiation used, b is the FWHM of the diffraction peak and h is the Bragg angle (in radians), k is the Scherrer constant (usually taken as unity), L is crystallite size (A0), R is the interchain separation (A0), d is the interplanar distance (A0), e is the micro strain, d is the dislocation density and g is the distortion parameters. The structural parameters viz. L, r, e, g and d are calculated with respect to the prominent peak of the pristine and irradiated films and depicted in Table 3. It can be seen that the crystallite size decreases significantly with an increase in the gamma dose. The crystallite size revealed a similar trend to that of integral intensity where both shows decrease after irradiation. However, the decrease in peak intensity suggests an evolution of polymer toward a more disordered state and also a change in crystallite size. Particularly, in case of solid polymeric materials, the exact nature of the interrelation between spacing, crystallite size and the degree of disorder is yet to the ascertained in proper perspective [45]. However, no data are available in literature to substantiate this. Interchain and interplanar distances were marginally changed
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increase of gamma dose. Thus, the decreases in optical band gap energy are in line with the results of Raman and XRD studies.
Table 3 Structural parameters. Gamma dose (kGy)
2h (deg.)
FWHM b (deg.)
r (A0)
d (A0)
e
0.0 150 300 470 630 675
21.7 21.1 20.5 20.4 22.2 21.7
9.0 9.4 9.1 10.0 11.3 18.0
5.11 5.26 5.41 5.44 5.0 5.11
4.09 4.21 4.33 4.35 4.00 4.09
2.20 2.31 2.23 2.46 2.77 4.41
1/d2 (1017)
g (%)
C.S., L (A0)
Acknowledgments
10.0 10.0 10.3 12.4 15.8 41.6
46.9 50.4 50.3 55.5 57.5 93.9
9.98 9.55 9.85 8.96 7.95 4.9
Authors are highly thankful to Dr. S P Lochab, IUAC New Delhi, India, for providing gamma irradiation facility. Special thanks are due to Prof. Ravi Kumar, Department of Material Science and Engineering, National Institute of Technology, Hamirpur, India, for providing XRD, Raman and UV–Visible facilities. References
Fig. 7. The variation of structural parameters with the gamma dose.
because the angle of the peak (h) did not vary significantly after irradiation. The micro strain, distortion parameters and dislocation density increases with an increase of gamma dose, which is due to the mismatching of the atoms or ions. Fig. 7 shows the variation of gamma dose with structural parameters (viz. micro strain, dislocation density and distortion parameters, respectively). It is clear from figure that structural parameters increase with an increase of gamma dose. These results suggest that the faulting probability of the PM-355 polymer sheet increase after irradiation. The microRaman spectral data also support the XRD analysis. XRD and Raman spectra show decrease in the peak intensity after gamma irradiation, which supports the shift towards amorphous nature. Therefore, these results are in agreement.
4. Conclusion Based on the present results, the chemical and structural properties of the PM-355 polymer are affected by the gamma irradiation. Raman spectra show the decrease in the peak intensity. UV– Visible spectral studies of pristine and gamma irradiated PM-355 polymer films reveals the simultaneous existence of the direct and indirect band gap; an observation which is being reported for the first time to the best of our knowledge. The values of an indirect band gap found to be lower than the corresponding values of the direct band gap. The XRD spectra used to calculate structural parameters viz. crystallite size, interchain separation, interplanar distance, micro strain, dislocation density and distortion parameters of the pristine and irradiated sheets. The XRD results show changes in crystallite size upon gamma irradiation. Interchain and interplanar distances were marginally changed. Lattice and micro strain increase with an
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Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Gamma irradiation induced chemical and structural modifications in PM-355 polymeric nuclear track detector film Vijay Kumar a,⇑, R.G. Sonkawade b, A.S. Dhaliwal a a b
Department of Physics, Sant Longowal Institute of Engineering and Technology, Longowal District, Sangrur, Punjab 148 106, India School of Physical Sciences, BBA University (A Central University), Lucknow 226 025, India
a r t i c l e
i n f o
Article history: Received 1 August 2012 Received in revised form 29 August 2012 Available online 11 September 2012 Keywords: PM-355 Gamma rays Raman UV–Visible XRD
a b s t r a c t This experimental study investigated the modification of chemical and structural properties of PM-355 films by irradiation of 1.25 MeV 60Co c-radiations at doses ranging from 0–675 kGy. The induced modifications were followed by micro-Raman and X-ray diffraction (XRD) spectroscopy. Further, the induced modifications were confirmed by UV–Visible spectroscopy. Raman spectra show that the films are highly disordered at the highest gamma dose. The XRD pattern of PM-355 shows the decreasing intensity of peak positions with an increase in the gamma dose, which suggests the loss of crystallinity of the films due to irradiation. Observed results indicate the formation of a disordered system in the irradiated films. Furthermore, the crystallite size for pristine and bombarded sample has been calculated. Moreover, interchain distance, micro strain, interplanar distance, dislocation density and distortion parameters were calculated. The analysis revealed a significant increase in micro strain, dislocation density and distortion parameters with an increase of gamma dose, which is in line with the Raman analysis. With increasing c-dose, the value of the direct and indirect band gap found to decrease. To the best of our knowledge, this work is the first to show the simultaneous existence of direct as well as the indirect band gap in PM-355 polymer. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Solid state nuclear track detectors (SSNTDs) have been extensively used for the detection of ions. Nowadays, ion track membranes, also known as nuclear track filters, have emerged as the main offshoot from SSNTDs. Many applications of SSNTDs have been developed, including biological filters, detection of light ions, dosimetry to use in ion track etching, magnetic nano wires as magneto resistive sensors and much more [1–4]. As advanced technology keeps on developing every day, the demand for polymers having improved properties is continuously on the rise due to their use of various, scientific and technological applications [5]. Irradiation plays a prominent role in modification of properties of polymers significantly. Ionizing radiation passing through matter, deposit energy in the material and cause irreversible changes in the macromolecular structure of the target material. The most prominent effects of radiation involve a change of phase in the absorbing material due to the scissions or cross linking formation. At the same time, it is also likely possible to observe the creation of chemical bonds between different molecules (intermolecular cross linking) in the main chain. Most of the efforts have been directed towards the effects of radiations on the track etch and bulk etch rates of PM-355 poly⇑ Corresponding author. E-mail address: [email protected] (V. Kumar). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.08.029
meric detectors [6–8]. In fact; many authors have used such detectors basing only on approximate calibration data, which presented a dependence of track diameters on energy of selected particles and etching time for the detectors [9–13]. The PM-355 detectors have used for the detection of light as well as heavy ions, including protons, deuterons, He, C, N, O, S, etc. [14–16]. Recently; PM-355 detectors have been used for the assessment of solar ultra-violet radiation dosimetry [17]. The polymer PM-355 has the same chemical composition as the solid state nuclear track detector CR-39 or PADC that find diverse applications [5,18]. From the above discussion, it was concluded that there is a lacuna in the systematic study on the optical, chemical, structural and thermal properties of PM-355 after irradiation. Very few attempts have been carried out in the study of optical, thermal, chemical and structural properties of PM-355 SSNTDs as a function of radiation doses [19,20]. Moreover, to be best of our knowledge a few reports have been carried out on the use of gamma radiation for the modifications of optical and structural properties of PM355 SSNTDs [21]. Interestingly, as for many other polymeric nuclear track detectors (viz. CR-39, Makrofol KG, PET, etc.), the optical, chemical, structural, and thermal properties may be greatly affected by irradiation [22–25]. Because of the potential usefulness of PM-355, a study was carried out on the chemical and structural modifications induced by gamma irradiation. The aim of this paper is to investigate the change in chemical and structural properties of PM-355 film after gamma irradiation.
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V. Kumar et al. / Nuclear Instruments and Methods in Physics Research B 290 (2012) 59–63
An attempt has been made to correlate the results with the earlier reported data on the effects of irradiation on different polymeric materials in order to make the present investigation more informative. 2. Experimental aspects Small pieces of size (1 1 cm2) and thickness 250 lm were cut from the commercially available sheet of the PM-355 detector having density of 0.9 gm/cm3. This target assembly was then taken for gamma irradiation. Detectors were irradiated using 1.25 MeV gamma radiation (the average energy of the two 60Co photons) source of 60Co in the radiation chamber with dose rate 7.328 kGy/h at IUAC, New Delhi. Doses from 150 to 675 kGy were used in the present study. The nature of changes induced by gamma irradiation were carried out using UV–Visible spectrophotometer in the range of 200– 700 nm to see the dose dependence variation in optical band gap energy. The micro RAMAN investigation has been carried out using Renishaw InVia Raman. The structural studies were carried out by X-ray diffractometer with Cu Ka radiation (1.54 A0) for a range of Bragg’s angle 2h (10 < h < 60) at the scan rate of 1°/min.
another strong intensity band at 2962 cm1 is due to asymmetric CH2. The bands at 1288 and 1451 cm1 corresponds to the C–O stretch mode and –C–H bending mode, respectively. The other small intensity band at 1749 cm1 is due to C@O stretching. After highest gamma dose (i.e. 675 kGy) irradiation, it is clear from the figure that the intensity of the band at 2964 cm1 reduced drastically. Similar reduction in the band intensity at 2958 cm1 was observed by Sharma et al. for nitrogen ion implanted CR-39 films [27]. According to them, this indentifies the reduction of hydrogen content in the surface of implanted CR-39 films. Interestingly, the bands at 1288, 1455, and 1749 cm1 are all disappeared as a result of gamma irradiation, which indicates the change in chemical structure of PM-355 due to irradiation. The intensity of Raman band at 1124 cm1 is higher than the pristine film. The enhancement in Raman band intensities after irradiation was reported by various authors [28,29]. They suggested that this may be due to the carbonization of the track area and nucleation of the carbonrich clusters. Hence, it can be concluded that gamma irradiation leads to the decrease in peak intensity and breaking of polymer chains, which may introduce micro strain in the PM-355 polymer [30]. 3.2. UV–Visible spectroscopy
3. Results and discussion 3.1. Raman spectra Raman spectra of pristine and highest irradiated PM-355 films have been shown in Fig. 1 against Raman shift in cm1. The relative changes in the bonds have been estimated from the relative increase or decrease in Raman intensity of the peaks associated with the functional groups present the polymer. It is clear from the figure that the Raman intensity of most of the peaks increased, and some peaks disappeared, suggesting that some structural changes occur in the polymer film after gamma irradiation. In the pristine PM-355 film, a number of peaks at 1124, 1288, 1455, 1749 and 2962 cm1 are observed confirming its fundamental structure, which is similar to CR-39 polymer [26,27]. The highest intensity band at 1124 cm1 is assigned to C–O–C stretching, whereas
Fig. 2 shows the change in the absorption spectra of PM-355 upon gamma exposure to a dose range of 0–675 kGy. From this figure, it is noticeable that the absorption edge shifted towards the higher wavelength region, which indicates a decrease in the optical band gap energy [22,24,31–33]. As it can be seen, the absorbance intensity increases linearly with gamma dose, and the fine-structure peaks are easily distinguishable after 470 kGy. The shift to the absorption edge as a result irradiation, inferred the bond rupturing leading to scission, free radical formation, cross linking, etc., resulting in the formation of new bonds. Furthermore, absorbance is apparently more affected by irradiation at wavelengths 325–350 nm. The changes observed could be due to unsaturation and the presence of carbonyl and hydroxyl compounds. Not only this, we observe broadening of the absorption edge after irradiation, i.e. broadening of peak at FWHM. The reason of
Fig. 1. Raman spectra of the pristine and irradiated PM-355 film at highest gamma dose.
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the best of our knowledge no one reported the simultaneous existence of the direct and indirect band gap in case of PM-355. At the same time, very limited reports have been found in literature on the simultaneous existence of direct and indirect band gaps in polymers [31,34,35]. Pristine PM-355 is transparent and has very little absorption. Upon visual examination, the colorless surface of PM-355 became yellowish as the gamma dose gradually increased [31,36,37]. For a linear structure, the number of carbon atoms per conjugation length N is given by [31]:
N ¼ 2pb=Eg ;
Fig. 2. UV–Visible spectra of PM-355.
Table 1 The variation of optical band gap energy of both the polymer, along with the number of carbon atoms (N) per conjugated length. Dose (kGy)
0 150 300 470 630 675
Optical band gap energy (eV)
Number of carbon atoms (N)
Indirect
Direct
Direct
Indirect
3.7 ± 0.03 3.6 ± 0.04 3.5 ± 0.05 3.4 ± 0.06 3.1 ± 0.09 3.0 ± 0.02
4.6 ± 0.03 4.6 ± 0.06 4.5 ± 0.06 3.6 ± 0.07 3.5 ± 0.01 3.4 ± 0.08
5 5 5 5 6 6
4 4 4 5 5 5
broadening of absorption edge is the formation of an extended system of conjugated bonds, i.e. the formation of defects after irradiation. In the studied range of wavelength, the maximum absorption is caused by the p–p electronic transitions [31] and the cause at the back is the smaller energy requirement for excitation by p electrons. These transitions need an unsaturated group in the molecules (i.e. compound containing double or triple bonds and in aromatics) to provide the p electrons. The broadening of the peaks can also be attributed to the production of gamma radiation induced defects, which may further results in the formation of new energy level leading to the peak broadening. Such defects may result in the formation of new energy levels leading to the broadening of peaks. From the absorption spectra, the direct and indirect band gap, (ahm)2 and (ahm)1/2 were plotted as a function photon energy (hm), respectively. Zaki [31] clearly explained the concept of direct and indirect band gaps in CR-39 SSNTD after gamma irradiation. The Tauc’s expression is given by [31].
aðhtÞ ¼ Bðht Eg Þn =ht;
ð1Þ
where a is the absorption coefficient, hm is the photon energy, and Eg is the value of the optical energy gap. The plots between (ahm)2 and (ahm)1/2 verses (hm) are not presented in this report. The values of extrapolated intercept for direct and indirect band gaps for pristine and irradiated samples along with their standard errors are enlisted in Table 1. From Table 1, it can be seen that the value of the direct and indirect band decreases after gamma irradiation [31,34]. This result reflects the formation of defects in the PM-355 detector structure after gamma irradiation. However, the values of the indirect band gap have been found to be lower than the corresponding values for the direct band gap as depicted in Table 1. To
ð2Þ
where N is the number of carbon atoms per conjugated length, 2b gives the band structure energy of a pair of adjacent P sites. The value of b is taken to be 2.9 eV as it is associated with p–p optical transition in the –C@C– structure. The values of carbon atoms (N) per conjugation length are given in Table 1. If the PM-355 is to be used as a dosimeter, it is important to investigate the effect of dose rate at which the radiation dose is transferred to the detector. The dependence of optical band gap energies on gamma dose is shown in Fig. 3. A significant dependence of gamma dose on optical band gap was found. These observations have important implication for the use of PM-355 detectors as a dosimeter using well reported protocols. It is clear from Fig. 2 that the absorption edges to lay within the wavelength region 305–360 nm. Fig. 4 shows the variation of absorbance difference with the gamma dose at a characteristic wavelength of 323.5 nm. Absorbance difference for the gamma irradiated samples with respect to pristine one is highest at 323.5 nm. So we have chosen this wavelength as the characteristic’s wavelength. Similar observation was also reported in literature [33,35]. They reported that absorbance difference of pristine and irradiated samples represents the measure of the induced UV absorbance. It was also reported that the gamma irradiation enhances the UV absorbance which is a direct outcome of the formation of new chemical species as a result of energy transfer by the incident of gamma rays [33,35]. 3.3. X-ray diffraction Qualitative XRD analysis was conducted on pristine and irradiated films to have a preliminary idea of the effect of gamma dose on the structure of PM-355. X-ray diffraction spectra of pristine and gamma irradiated PM-355 are shown in Fig. 5. XRD spectrum of pristine sample shows that it is a partly crystalline polymer with dominant amorphous phase. From the Fig. 5, we see that the X-ray diffraction patterns of the samples are characterized by halos extending in the 2h range 10–600. Any change in peak intensity after irradiation shows some significant changes in the structure of the materials after irradiation. Table 2 clearly shows that integral intensity decrease gradually with an increase of gamma dose. A probable explanation for the decrease in intensity can be attributed to the decrease in amount of crystalline phase in the samples, and the crystalline structure has been destroyed. This could be attributed to the cross linking of molecular chains, which change the regularly arranged crystallites into non arranged ones by forming new bonds between the neighbouring chains [19]. Not only this, we have observed increase in full width at half maxima from the diffraction pattern. This behavior is associated with the decrease in crystallinity of the detector [38]. In order to get the qualitative estimates of the modification occurring in the PM-355 detector due to gamma irradiation, area of the diffraction peaks calculated from simple Gaussian fittings to the data [39,40] as a function of gamma doses, is plotted as shown in Fig. 6. It is clear from figure that the area decreases after
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V. Kumar et al. / Nuclear Instruments and Methods in Physics Research B 290 (2012) 59–63 Table 2 XRD parameters of gamma irradiated PM-355 SSNTDs. Gamma dose (kGy)
2h (deg.)
FWHM b (deg.)
Integral intensity, I (a.u.)
Gaussian fit area (a.u.)
0.0 150 300 470 630 675
21.7 21.1 20.5 20.4 22.2 21.7
9.0 9.4 9.1 10.0 11.3 18.0
696 605 555 652 526 356
4337 4109 4013 4562 3874 3216
Fig. 3. Optical band gap energy as a function of absorbed gamma doses.
Fig. 6. Variation of the diffraction peaks area as a function of gamma dose.
Furthermore, the crystallite size (L), interchain distance (r), interplanar distance (d), micro strain (e), dislocation density (d) and distortion parameters (g) were calculated as follows [35,41–44]:
k b cos h 5 k r¼ 8 sin h k d¼ 2 sin h b cos h e¼ 4 1 d ¼ 2 ; and L b g¼ tan h L¼
Fig. 4. Characteristics absorbance difference vs. gamma doses at 323.5 nm.
Fig. 5. X-ray spectra of PM-355.
irradiation, resulting in the decrement of crystallinity thereby leading to disorderness in the polymer structure.
ð3Þ ð4Þ ð5Þ ð6Þ ð7Þ ð8Þ
where k = 1.54 nm is the wavelength of the Cu Ka X-ray radiation used, b is the FWHM of the diffraction peak and h is the Bragg angle (in radians), k is the Scherrer constant (usually taken as unity), L is crystallite size (A0), R is the interchain separation (A0), d is the interplanar distance (A0), e is the micro strain, d is the dislocation density and g is the distortion parameters. The structural parameters viz. L, r, e, g and d are calculated with respect to the prominent peak of the pristine and irradiated films and depicted in Table 3. It can be seen that the crystallite size decreases significantly with an increase in the gamma dose. The crystallite size revealed a similar trend to that of integral intensity where both shows decrease after irradiation. However, the decrease in peak intensity suggests an evolution of polymer toward a more disordered state and also a change in crystallite size. Particularly, in case of solid polymeric materials, the exact nature of the interrelation between spacing, crystallite size and the degree of disorder is yet to the ascertained in proper perspective [45]. However, no data are available in literature to substantiate this. Interchain and interplanar distances were marginally changed
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increase of gamma dose. Thus, the decreases in optical band gap energy are in line with the results of Raman and XRD studies.
Table 3 Structural parameters. Gamma dose (kGy)
2h (deg.)
FWHM b (deg.)
r (A0)
d (A0)
e
0.0 150 300 470 630 675
21.7 21.1 20.5 20.4 22.2 21.7
9.0 9.4 9.1 10.0 11.3 18.0
5.11 5.26 5.41 5.44 5.0 5.11
4.09 4.21 4.33 4.35 4.00 4.09
2.20 2.31 2.23 2.46 2.77 4.41
1/d2 (1017)
g (%)
C.S., L (A0)
Acknowledgments
10.0 10.0 10.3 12.4 15.8 41.6
46.9 50.4 50.3 55.5 57.5 93.9
9.98 9.55 9.85 8.96 7.95 4.9
Authors are highly thankful to Dr. S P Lochab, IUAC New Delhi, India, for providing gamma irradiation facility. Special thanks are due to Prof. Ravi Kumar, Department of Material Science and Engineering, National Institute of Technology, Hamirpur, India, for providing XRD, Raman and UV–Visible facilities. References
Fig. 7. The variation of structural parameters with the gamma dose.
because the angle of the peak (h) did not vary significantly after irradiation. The micro strain, distortion parameters and dislocation density increases with an increase of gamma dose, which is due to the mismatching of the atoms or ions. Fig. 7 shows the variation of gamma dose with structural parameters (viz. micro strain, dislocation density and distortion parameters, respectively). It is clear from figure that structural parameters increase with an increase of gamma dose. These results suggest that the faulting probability of the PM-355 polymer sheet increase after irradiation. The microRaman spectral data also support the XRD analysis. XRD and Raman spectra show decrease in the peak intensity after gamma irradiation, which supports the shift towards amorphous nature. Therefore, these results are in agreement.
4. Conclusion Based on the present results, the chemical and structural properties of the PM-355 polymer are affected by the gamma irradiation. Raman spectra show the decrease in the peak intensity. UV– Visible spectral studies of pristine and gamma irradiated PM-355 polymer films reveals the simultaneous existence of the direct and indirect band gap; an observation which is being reported for the first time to the best of our knowledge. The values of an indirect band gap found to be lower than the corresponding values of the direct band gap. The XRD spectra used to calculate structural parameters viz. crystallite size, interchain separation, interplanar distance, micro strain, dislocation density and distortion parameters of the pristine and irradiated sheets. The XRD results show changes in crystallite size upon gamma irradiation. Interchain and interplanar distances were marginally changed. Lattice and micro strain increase with an
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