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Spectroscopic and thermal studies of 8 MeV electron beam irradiated HPMC films
Nuclear Instruments and Methods in Physics Research B 267 (2009) 2385–2389
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Spectroscopic and thermal studies of 8 MeV electron beam irradiated HPMC films Sangappa a,*, S. Asha a, T. Demappa b, Ganesh Sanjeev c, P. Parameswara d, R. Somashekar d a
Department of Studies in Physics, Mangalore University, Mangalagangotri 574 199, India Department of Studies in Polymer Science, University of Mysore, Sir M.V.P.G. Center, Mandya 571 402, India Microtron Centre, Mangalore University, Mangalagangotri 574 199, India d Department of Studies in Physics, University of Mysore, Manasagangotri, Mysore 570 006, India b c
a r t i c l e
i n f o
Article history: Received 23 December 2008 Received in revised form 13 April 2009 Available online 23 April 2009 PACS: 61.80.Fe 74.25.Gz 81.70.Pg
a b s t r a c t Radiation-induced changes in hydroxypropyl methylcellulose (HPMC) films under electron irradiation were investigated and correlated with dose. Polymer samples were irradiated in air at room temperature by an electron beam accelerator in the range of 0–100 kGy. Various properties of the irradiated films were studied using a Ultraviolet–Visible spectrophotometer and Fourier transform infrared spectroscopy and thermogravimetric analysis. Electron irradiation was found to induce changes in the physical, chemical and thermal properties, depending on the irradiation dose. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: HPMC Electron irradiation Band gap Thermal properties
1. Introduction In radiation chemistry, polymers are classified into two types: scission polymers and cross-linking polymers. Most biopolymers are classified as scission polymers [1,2]. Recent developments have proved however, that a variety of biopolymers could be crosslinked by irradiation with high energy radiation and hydroxypropyl methylcellulose (HPMC) tends to radiation cross-linking [3–5]. HPMC is a biopolymer which has become a successful alternative material for two-piece capsules and is on the market. HPMC is also being adopted as a film coating or a sustained-release tablet material in the pharmaceutical field [6]. HPMC is a widely used cellulose film-forming agent for conventional tablet film coatings [7]. Due to the brittleness of plain HPMC films, electron irradiation is included in HPMC coating formulations. The functions of electron irradiation include softening films, reducing brittleness, increasing flexibility and ductility. Irradiation can increase the segmental mobility of HPMC, resulting in decreasing crystallinity of the samples. Optical absorption is a useful method for investing the induced transition and providing information about the band structure and optical energy gap in the materials. The principle of this technique * Corresponding author. Tel.: +91 8242287363; fax: +91 824 2287367. E-mail address: [email protected] ( Sangappa). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.04.007
is that the photons with energies higher than the band-gap energy will be absorbed [8]. The transition type, which is responsible for absorption, is also an important quantity to be defined for material characterization. Electrons are mostly used to affect the physical and chemical properties of the polymer films [9]. The study of the chemical and physical parameters such as zero-point energy, equilibrium vibration, anharmonicity constant and force constant is of great importance especially when the polymer properties are being modified by ionizing radiation. Thermogravimetric analysis (TGA) gives thermal decomposition kinetics of materials by monitoring the weight loss of the sample in a chosen atmosphere (nitrogen) as a function of temperature. TGA is best known for its ability to provide information on the bulk composition of the samples. Modifications of the optical, structural and thermal properties of pure and electron beam irradiated HPMC films have been studied here. 2. Experimental details 2.1. Preparation of the films The polymer HPMC used in this work was obtained in powder form from Fine Chemicals Ltd., India with an approximate molecular weight of 2,00,000 Dalton. The HPMC films were prepared by the casting method as follows [10]. HPMC powder was dissolved
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Sangappa et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2385–2389
in distilled water and then heated gently with a water bath for complete dissolution. The solution was left to reach a suitable viscosity, after which they were cast into glass dishes and left to dry in a dry atmosphere at room temperature. Samples were transferred to a desiccator to avoid moisture. The thickness of the obtained films was 60 lm.
Irradiation was done at the Microtron Center, Mangalore University, under the conditions listed in Table 1. The dose delivered was measured with an alanine dosimeter close to the sample. The samples were subjected to various integral doses of 0, 25, 50, 75 and 100 kGy.
2.0
Absorbance
2.2. Electron beam irradiation
2.4
1.6
a b c
1.2
d 0.8
2.3. Polymer characterization HPMC polymer films were characterized by different spectroscopic techniques such as a Ultraviolet–Visible (UV–VIS) spectrophotometer and Fourier transform infrared (FTIR) spectroscopy, as well as TGA. UV–VIS absorption spectra were measured in the wavelength range 200–900 nm at room temperature. FTIR spectra were recorded in transmission mode with spectrophotometer having a resolution of 4 cm1 in the wave number range 500– 4000 cm1. The samples were weighed in microbalance and crimped in aluminum pans. TGA of these samples was done with nitrogen as the flushing gas. The temperature range scanned was 25–750 °C at rate of 10 °C/min. The thermogram, i.e. a plot of weight percent as a function of temperature, was used to study the variation in the thermal stability of the polymer. Samples of 3.575–7.623 mg were used for the measurement. 3. Results and discussion 3.1. Absorption spectra analysis UV–VIS spectroscopy gives information about the optical bandgap energy (Eg) so is an important tool. Absorption of light energy by polymeric materials in the ultraviolet and visible regions involves promotion of electrons in r, g and n-orbitals from the ground state to higher energy states which are described by molecular orbitals [11]. The electronic transitions (?) that are involved in the ultraviolet and visible regions are of the following types r ? r*, n ? r*, n ? p*, and p ? p*. Many of the optical transitions which result from the presence of impurities have energies in the visible part of the spectrum. Consequently the defects are referred to as color centers [12]. Energetic beam interaction with polymers generates damage which leads to the formation of new defects and new charge states. The UV–VIS spectrophotometric scans of pure and electron beam irradiated polymer films were measured in the wavelength range 200–600 nm. Electronic absorption spectra of HPMC films irradiated at various integral doses are given in Fig. 1. For the pure
Table 1 Specifications of the electron beam accelerator and irradiation conditions. 1. 2. 3. 4. 5. 6. 7. 8.
Beam energy Beam current (pulsed) Pulse repetition rate Pulse width Distance source to sample Dose range Atmosphere Temperature
8 MeV 20 mA 50 Hz 2.2 ls 30 cm 0–100 kGy Air 24 °C
0.4 200
300
400
500
600
Wavelength (nm) Fig. 1. The electronic absorption spectra for HPMC films, a – pure, b – 25 kGy, c – 50 kGy and d – 75 kGy.
sample it is clearly observed that unirradiated HPMC has weak absorption band at 275 nm in the studied wavelength range. On the other hand, the observed spectra of electron beam irradiated films have absorption band shifts towards higher wavelength. This band is assigned to the n ? g* transition of the methyl groups of HPMC. The intensity of the absorption band changes with increasing dose, while its position slightly shifted towards the higher wavelength side. The intensity increases linearly with increasing dose. This may be because electron irradiation of HPMC causes the creation of carbonyl group, as observed in FTIR results, but in lower amounts at lower doses. This lowers the energy required for the optical transition so the absorption moves towards longer wavelengths. The carbonyl group modifies the optical transitions and hence the optical band gap. 3.1.1. Optical band gap The microstructure modifications produced by irradiation cause a change in the molecular structure of the polymer, so modification within the optical band gap is expected. The information about the optical band gap is accessible from the absorption edge of the UV– VIS spectra of the irradiated polymers. The absorption coefficient, a, of virgin and irradiated films can be calculated from the optical absorption spectrum using the relation [13]:
aðhmÞ ¼ 2:303A=X;
ð1Þ
where X is the film thickness in cm and A = log (I0/I) where I0 and I are the intensity of the incident and transmitted beams respectively. The optical band gap was determined from the analysis of the spectral dependence of the absorption near the absorption edge. The absorption coefficient for non-crystalline materials has the following frequency dependence [14–16]:
aðhmÞ ¼ Aðhm Eg Þr =hm;
ð2Þ
where hm is the energy of the incident photons, Eg is the value of the optical energy gap between the valence and conduction band. Here r represents an index that can take any of the values 1/2, 3/2, 2 or 3 depending on the type of transition responsible for the absorption. The simplest way to deduce the type of transition is to examine the value of r, which fits ht to aht with a straight line relationship [17]. In our case, r = ½ which means an allowed direct transition.
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4.3
240
1/2
(αhν)
160
Optical band gap Eg (eV)
a b c d
200
120 80 40 0 2
3
4
5
6
4.2 4.1 4.0 3.9 3.8 3.7
7
hν (eV)
0
10
20
30
40
50
60
70
80
Electron Dose (kGy)
Fig. 2. The plots of (ahm)½ versus (hm) for electron irradiated HPMC films, a – pure, b – 25 kGy, c – 50 kGy and d – 75 kGy.
Fig. 3. Variation of optical band gaps of electron irradiated HPMC polymer films. Table 2 The dependence of Eg on irradiation dose. Eg (eV) 4.24 4.10 3.99 3.73
The allowed direct transition simply occurs vertically from the top of the valence band to the bottom of the conduction band, while non-vertical transitions are normally forbidden [8]. The factor A depends on the transition probability and can be assumed to be constant within the optical frequency range. The usual method for the determination of the value of Eg involves plotting (ahm)½ against (hm). Plots of (ahm)½ versus (hm) near the absorption edge for HPMC films with different doses produce a linear fit over a wide range, as shown in Fig. 2. Extrapolating the straight regions of these relations to the (hv) axis yields the corresponding forbidden band width, Eg, shown in Table 2. The value of Eg decreased with dose. The value of Eg for the unirradiated film was 4.24 eV, and it was found that energy gap is inversely proportional to the dose. Fig. 3 shows the variation in Eg with dose. It is clear that Eg decreases with dose. This may be due to cross-linking of the polymeric chains. It was revealed by the crystallinity study that the crystallinity of the samples decreases with dose. Since the molecular motion of the polymer chain is regulated in the crystalline area the radicals generated in there cannot meet other radicals to cross-link by recombination reaction [18]. On the other hand, in an amorphous area the radicals can encounter each other to make cross-linking reactions by micro-Brownian molecular motion of the polymer chain. Based on this idea, it is expected that the crystallinity of polymer decreases with dose [19]. 3.2. FT-IR spectral analysis The FTIR absorbance spectra for the virgin and the electron irradiated HPMC polymer samples are shown in Fig. 4. They show broad bands at 3469.3 cm1 due to a stretching vibration in the hydroxyl group. The films are stable up to irradiation of 50 kGy, after which they are degraded due to deformation into monomers such as ester and methyl groups. During irradiation of HPMC from 0 to 50 kGy the films undergo modification and degradation. On the microscopic
3479
2935
d
1731 1535 1225
909 1071 1005
Transmittance (%)
Dose (kGy) 0 25 50 75
2915 2859
1726 1534
c
1225
912 1007 3452
2861
1725
b
1534 1224 910 1009
2877 2904
1728
a
1529 1226
910 1077 1009
3500
3000
2500
2000
1500
1000
-1
Wavenumber (cm ) Fig. 4. FT-IR spectra of the virgin and electron beam irradiated HPMC films, a – pure, b – 25 kGy, c – 50 kGy and d – 75 kGy.
level, the polymer degradation is characterized by macromolecular chain splitting, creation of low mass fragments and production of free radicals, such as 1372–1380 CH3 radicals. In a polymer, an electron beam of low energy may be used to induce effects such as chain scission and cross-linking. The nature of chemical bonds in polymers can be studied through the characterization of the vibration modes by infrared spectroscopy [20]. Here we have calculated IR calculations such as zero-point energy, anharmonicity constant, fundamental and first overtone transition of C–O bonds and force constant of the chain about this bond. For a pure HPMC sample the normal modes of e0 bonds are 700, 1307 and 2306 cm1. The zero-point energy of these bonds is calculated using the following relation [21]:
¼
4 1 X ti : hc 2 i¼1
ð3Þ
The zero–point energy for a pure sample is 0.311 eV. We have also extended this calculation to electron irradiated HPMC films. The
Sangappa et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2385–2389
zero-point energy increased for higher doses and after 50 kGy it starts to decrease, and it was 0.314 eV at 75 kGy, 0.312 eV at 100 kGy. For pure HPMC the fundamental and first overtone transition of C–O bonds are centered at 1928 and 3804 cm1 respectively. The equilibrium vibration frequency of the chain about this bond is calculated using the following relations [21]:
Frequency of the fundamental transition ¼ 1928 cm e ð1 2xe Þ; ¼t
1
ð4Þ
Frequency of the first overtone ¼ 3804 cm1 e ð1 3xe Þ: ¼ 2t
110 100 90 80
Weight loss (%)
2388
70 60 50
ð5Þ
30
Solving the above equations we have obtained the equilibrium e . The calculated equilibrium vibration for pure HPMC vibration t is 1980 cm1. For irradiated samples this value slightly increases and after 75 kGy it was decreased. And zero-point energy is given by the following relation [21]:
20
eo ¼ 1=2te ð1 1=2xe Þ;
0 kGy 25 kGy 50 kGy 75 kGy
40
10 0 100
200
2e ; k ¼ 4p2 lc2 t
ð7Þ
where l is the reduced mass of the system, c is the velocity of light e is the equilibrium vibration. From the Table 3 the force conand t stant for pure HPMC is 1586 N/m. After irradiation it was increased and it reaches 1687 N/m for 75 kGy irradiated sample and it was 1569 N/m for 100 kGy. Using the above calculations we have quantified the IR data given in Table 3. From this we can say there is not much change in the structural properties of 8 MeV electron beam irradiated HPMC films, even though there were changes in microstructural parameters and crystallinity of the samples from wide angle X-ray scattering studies [22]. The minor changes in the peaks of irradiated samples may be due to the breaking of one or two bonds in the structure, but this will not change the overall structure of the polymer. 3.3. Thermogramatric analysis study Thermograms obtained for the virgin and the irradiated HPMC films are shown in Fig. 5 and exhibit three distinct regions. For the virgin HPMC film, in the first region, starting from room temperature up to 300 °C the weight loss is due to water vaporization (drying). The weight change was not significant and the sample was thermally stable. In the second, rather narrow region from 300 to 322 °C the film experienced a great weight loss because of the thermal decomposition. About 75% of the sample decomposed into volatiles. After 322 °C the film was completely decomposed. In the case of electron beam irradiated films the thermograms showed three distinct regions. In the 25 kGy irradiated film, the
400
500
600
o
ð6Þ
e is the equilibrium vibration, xe is the anharmonicity conwhere t stant. Using the above relation we have calculated the zero-point energy as 983.5 cm1 for pure HPMC film and it increases for higher doses. We have also calculated the force constant for all the samples using the following relation [21]:
300
Temperature ( C) Fig. 5. TGA thermograms of virgin and electron beam irradiated HPMC films.
first region, starting from room temperature up to 310 °C the weight loss is due to water vaporization. The weight change is not significant and the irradiated film is thermally stable. In the second region from 310 to 360 °C, the film experienced a great weight loss because of the thermal decomposition. About 50% of the sample decomposed into volatiles. At 400 °C about 87% decomposed and at 500 °C about 90% and at 600 °C 92% decomposed. The 50 kGy irradiated sample also showed the three regions and at 346 °C, about 50% of the sample decomposed. At 400 °C about 85% decomposed, at 500 °C about 86% decomposed and at 600 °C, about 88% decomposed into volatiles. In the 75 kGy dose irradiated sample also showed the three regions and at 346 °C, about 50% of the sample decomposed. It was observed at 400 °C about 82% decomposed, at 500 °C about 84% decomposed and at 600 °C, about 85% decomposed into volatiles. As the irradiation dose increased the cross-linking of the polymer increased. The weight loss (Table 4) decreases as irradiation dose increases. In the 25 kGy irradiated sample at 400 °C about 87% decomposed, at 500 °C about 90% decomposed and at 600 °C, 92% decomposed into volatiles. This indicates that the thermal stability of the HPMC polymer films increases with dose. Table 4 Temperature of decomposition at different weight loss (%) of HPMC before and after 8 MeV EB irradiation. Irradiation dose (kGy)
Unirradiated 25 50 75
Temperature (°C) 100
200
300
400
500
600
12.8 10.00 09.00 09.00
14.71 14.00 12.00 11.00
22.64 22.00 19.00 18.00
– 89.00 84.00 82.00
– 91.00 86.00 84.00
– 92.00 88.00 85.00
Table 3 IR data of pure and electron beam irradiated HPMC films. Samples (kGy)
Zero-point energy (E0) (eV)
An harmonicity Constant xe 103
e Þ cm1 Equilibrium vibration frequency ðt
e0 (cm1)
Force constant k (N/m)
0 25 50 75 100
0.311 0.317 0.317 0.314 0.312
13.2 22.4 14.8 25.1 10.0
1980.0 2010.0 1978.6 2042.5 1969.4
983.5 949.2 981.9 1008.4 979.7
1586 1635 1583 1687 1569
Sangappa et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2385–2389
optical energy gap, which could be due to the degradation of HPMC and the formation of defects and clusters. We have also quantified the physical properties electron irradiated and pure polymer films, in terms of zero-point energy, anharmonicity constant, fundamental and first overtone transition and force constant. From the IR data there is not much change in the structural properties of irradiated films. From the TGA curves and DT thermograms it is concluded that structural changes occurred after irradiation. An increasing dose resulted in an increase in thermal stability, and there was a 10–15% weight loss due to water vaporization. The melting temperature increased with dose and the decomposition temperature also increased with dose because of the cross-linking of HPMC due to irradiation.
DTG (a. u.)
-10
-15 Pure 25 kGy 50 kGy 75 kGy
-20
-25
2389
100 kGy
References -30 50
100
150
200
250
300
350
400
450
500
o
Temperature ( C) Fig. 6. DTG thermograms of virgin and electron beam irradiated HPMC films.
Table 5 DT information of virgin and EB irradiated HPMC films. Sample (kGy)
Tm (°C)
Td (°C)
0 25 50 75
332.5 337.9 339.9 337.7
– 381.7 382.5 383.5
From the DTG thermograms (Fig. 6) the virgin sample has melting temperature at 332 °C and this increases with dose (Table 5). In the 25 kGy sample the melting temperature was 337.9 °C, and the decomposition temperature was 381.7 °C. Increasing decomposition temperature with dose leads to the cross-linking in HPMC polymer samples. 4. Conclusions Modification of the optical properties of HPMC was carried out using electron beam irradiation. Irradiation leads to a decrease in
[1] A. Charlesky, J. Polym. Sci. 15 (1995) 263. [2] J.F. Saeman, M.A. Millett, E.J. Lauton, Ind. Eng. Chem. (Anal.) 44 (1952) 2848. [3] B. Fei, R.A. Wach, H. Mitomo, F. Yoshii, T. Kume, J. Appl. Polym. Sci. 78 (2000) 278. [4] R.A. Wach, H. Mitomo, H.F. Yashii, T. Kume, Macromol. Mater. Eng. 287 (2002) 285. [5] K. Terao, N. Nagasawa, H. Nishida, Y. Fursawa Mori, F. Yoshii, T. Dobashi, J. Biomater. Sci. Poly. Edn. 14 (2003) 1197. [6] V.M. Lehtola, J.T. Hainamaki, P. Nikupaavo, J.K. Yliruusi, Drug Dev. Ind. Pharm. 21 (1995) 675. [7] M.E. Aulton, A. Abdul-Razzak, Drug Dev. Ind. Pharm. 7 (6) (1981) 649. [8] A.A. Abdel-Fattah, H.M. Abdel-Hamid, R.M. Radwan, Nucl. Instr. and Meth. B 196 (2002) 279. [9] R.M. Radwan, J. Phys. D: Appl. Phys. 40 (2007) 374. [10] A. Tamansi, H.M. Zidan, A.H. Oraby, M.E. Dorgham, J. Phys. D: Appl. Phys. 31 (1998) 3428. [11] R. Dyer John, Applications of Absorption Spectroscopy of Organic Compounds, Prentice-Hall Inc., NJ, 1994. [12] A.K. Srivastava, H.S. Virk, J. Polym. Mater. 17 (2000) 325. [13] M.D. Migahed, N.A. Bakr, J. Polym. Mater. 11 (1994) 129. [14] H.M. Zidan, J. Appl. Polym. Sci. 88 (2003) 104. [15] N.F. Mott, N.F. Devis, Electronic Process in Non-Crystalline Materials, second ed., Oxford University Press, 1979. [16] J. Tauc, in: F. Abeles (Ed.), Optical Properties of Solid, North-Holland, Amsterdam, 1972, p. 277. [17] M.A. Gaffar, A. Abu El-Fadl, Cryst. Res. Technol. 34-7 (1999) 915. [18] Mayo Uenoyama, Satoshi Shukushima, Hiroshi Hayami, Seiichi Nishimoto, SEI Tech. Rev. 54 (2002) 61. [19] Sangappa, T. Demappa, Mahadevaih, S. Ganesh, S. Divakara, Manjunatha Pattabi, R. Somashekar, Nucl. Instr. and Meth. B 266 (2008) 3975. [20] J. Davenas, X.L. Xu, G. Boiteux, D. Sage, Nucl. Instr. and Meth. B 39 (1989) 754. [21] G. Aruldhas, Molecular Structure and Spectroscopy, Prentice-Hall of India, New Delhi, 2004. [22] Sangappa, T. Demappa, Mahadevaiah, S. Ganesh, S. Divakara, R. Somasheka, J. Appl. Polym. Sci. 109 (2008) 3983.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Spectroscopic and thermal studies of 8 MeV electron beam irradiated HPMC films Sangappa a,*, S. Asha a, T. Demappa b, Ganesh Sanjeev c, P. Parameswara d, R. Somashekar d a
Department of Studies in Physics, Mangalore University, Mangalagangotri 574 199, India Department of Studies in Polymer Science, University of Mysore, Sir M.V.P.G. Center, Mandya 571 402, India Microtron Centre, Mangalore University, Mangalagangotri 574 199, India d Department of Studies in Physics, University of Mysore, Manasagangotri, Mysore 570 006, India b c
a r t i c l e
i n f o
Article history: Received 23 December 2008 Received in revised form 13 April 2009 Available online 23 April 2009 PACS: 61.80.Fe 74.25.Gz 81.70.Pg
a b s t r a c t Radiation-induced changes in hydroxypropyl methylcellulose (HPMC) films under electron irradiation were investigated and correlated with dose. Polymer samples were irradiated in air at room temperature by an electron beam accelerator in the range of 0–100 kGy. Various properties of the irradiated films were studied using a Ultraviolet–Visible spectrophotometer and Fourier transform infrared spectroscopy and thermogravimetric analysis. Electron irradiation was found to induce changes in the physical, chemical and thermal properties, depending on the irradiation dose. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: HPMC Electron irradiation Band gap Thermal properties
1. Introduction In radiation chemistry, polymers are classified into two types: scission polymers and cross-linking polymers. Most biopolymers are classified as scission polymers [1,2]. Recent developments have proved however, that a variety of biopolymers could be crosslinked by irradiation with high energy radiation and hydroxypropyl methylcellulose (HPMC) tends to radiation cross-linking [3–5]. HPMC is a biopolymer which has become a successful alternative material for two-piece capsules and is on the market. HPMC is also being adopted as a film coating or a sustained-release tablet material in the pharmaceutical field [6]. HPMC is a widely used cellulose film-forming agent for conventional tablet film coatings [7]. Due to the brittleness of plain HPMC films, electron irradiation is included in HPMC coating formulations. The functions of electron irradiation include softening films, reducing brittleness, increasing flexibility and ductility. Irradiation can increase the segmental mobility of HPMC, resulting in decreasing crystallinity of the samples. Optical absorption is a useful method for investing the induced transition and providing information about the band structure and optical energy gap in the materials. The principle of this technique * Corresponding author. Tel.: +91 8242287363; fax: +91 824 2287367. E-mail address: [email protected] ( Sangappa). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.04.007
is that the photons with energies higher than the band-gap energy will be absorbed [8]. The transition type, which is responsible for absorption, is also an important quantity to be defined for material characterization. Electrons are mostly used to affect the physical and chemical properties of the polymer films [9]. The study of the chemical and physical parameters such as zero-point energy, equilibrium vibration, anharmonicity constant and force constant is of great importance especially when the polymer properties are being modified by ionizing radiation. Thermogravimetric analysis (TGA) gives thermal decomposition kinetics of materials by monitoring the weight loss of the sample in a chosen atmosphere (nitrogen) as a function of temperature. TGA is best known for its ability to provide information on the bulk composition of the samples. Modifications of the optical, structural and thermal properties of pure and electron beam irradiated HPMC films have been studied here. 2. Experimental details 2.1. Preparation of the films The polymer HPMC used in this work was obtained in powder form from Fine Chemicals Ltd., India with an approximate molecular weight of 2,00,000 Dalton. The HPMC films were prepared by the casting method as follows [10]. HPMC powder was dissolved
2386
Sangappa et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2385–2389
in distilled water and then heated gently with a water bath for complete dissolution. The solution was left to reach a suitable viscosity, after which they were cast into glass dishes and left to dry in a dry atmosphere at room temperature. Samples were transferred to a desiccator to avoid moisture. The thickness of the obtained films was 60 lm.
Irradiation was done at the Microtron Center, Mangalore University, under the conditions listed in Table 1. The dose delivered was measured with an alanine dosimeter close to the sample. The samples were subjected to various integral doses of 0, 25, 50, 75 and 100 kGy.
2.0
Absorbance
2.2. Electron beam irradiation
2.4
1.6
a b c
1.2
d 0.8
2.3. Polymer characterization HPMC polymer films were characterized by different spectroscopic techniques such as a Ultraviolet–Visible (UV–VIS) spectrophotometer and Fourier transform infrared (FTIR) spectroscopy, as well as TGA. UV–VIS absorption spectra were measured in the wavelength range 200–900 nm at room temperature. FTIR spectra were recorded in transmission mode with spectrophotometer having a resolution of 4 cm1 in the wave number range 500– 4000 cm1. The samples were weighed in microbalance and crimped in aluminum pans. TGA of these samples was done with nitrogen as the flushing gas. The temperature range scanned was 25–750 °C at rate of 10 °C/min. The thermogram, i.e. a plot of weight percent as a function of temperature, was used to study the variation in the thermal stability of the polymer. Samples of 3.575–7.623 mg were used for the measurement. 3. Results and discussion 3.1. Absorption spectra analysis UV–VIS spectroscopy gives information about the optical bandgap energy (Eg) so is an important tool. Absorption of light energy by polymeric materials in the ultraviolet and visible regions involves promotion of electrons in r, g and n-orbitals from the ground state to higher energy states which are described by molecular orbitals [11]. The electronic transitions (?) that are involved in the ultraviolet and visible regions are of the following types r ? r*, n ? r*, n ? p*, and p ? p*. Many of the optical transitions which result from the presence of impurities have energies in the visible part of the spectrum. Consequently the defects are referred to as color centers [12]. Energetic beam interaction with polymers generates damage which leads to the formation of new defects and new charge states. The UV–VIS spectrophotometric scans of pure and electron beam irradiated polymer films were measured in the wavelength range 200–600 nm. Electronic absorption spectra of HPMC films irradiated at various integral doses are given in Fig. 1. For the pure
Table 1 Specifications of the electron beam accelerator and irradiation conditions. 1. 2. 3. 4. 5. 6. 7. 8.
Beam energy Beam current (pulsed) Pulse repetition rate Pulse width Distance source to sample Dose range Atmosphere Temperature
8 MeV 20 mA 50 Hz 2.2 ls 30 cm 0–100 kGy Air 24 °C
0.4 200
300
400
500
600
Wavelength (nm) Fig. 1. The electronic absorption spectra for HPMC films, a – pure, b – 25 kGy, c – 50 kGy and d – 75 kGy.
sample it is clearly observed that unirradiated HPMC has weak absorption band at 275 nm in the studied wavelength range. On the other hand, the observed spectra of electron beam irradiated films have absorption band shifts towards higher wavelength. This band is assigned to the n ? g* transition of the methyl groups of HPMC. The intensity of the absorption band changes with increasing dose, while its position slightly shifted towards the higher wavelength side. The intensity increases linearly with increasing dose. This may be because electron irradiation of HPMC causes the creation of carbonyl group, as observed in FTIR results, but in lower amounts at lower doses. This lowers the energy required for the optical transition so the absorption moves towards longer wavelengths. The carbonyl group modifies the optical transitions and hence the optical band gap. 3.1.1. Optical band gap The microstructure modifications produced by irradiation cause a change in the molecular structure of the polymer, so modification within the optical band gap is expected. The information about the optical band gap is accessible from the absorption edge of the UV– VIS spectra of the irradiated polymers. The absorption coefficient, a, of virgin and irradiated films can be calculated from the optical absorption spectrum using the relation [13]:
aðhmÞ ¼ 2:303A=X;
ð1Þ
where X is the film thickness in cm and A = log (I0/I) where I0 and I are the intensity of the incident and transmitted beams respectively. The optical band gap was determined from the analysis of the spectral dependence of the absorption near the absorption edge. The absorption coefficient for non-crystalline materials has the following frequency dependence [14–16]:
aðhmÞ ¼ Aðhm Eg Þr =hm;
ð2Þ
where hm is the energy of the incident photons, Eg is the value of the optical energy gap between the valence and conduction band. Here r represents an index that can take any of the values 1/2, 3/2, 2 or 3 depending on the type of transition responsible for the absorption. The simplest way to deduce the type of transition is to examine the value of r, which fits ht to aht with a straight line relationship [17]. In our case, r = ½ which means an allowed direct transition.
2387
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4.3
240
1/2
(αhν)
160
Optical band gap Eg (eV)
a b c d
200
120 80 40 0 2
3
4
5
6
4.2 4.1 4.0 3.9 3.8 3.7
7
hν (eV)
0
10
20
30
40
50
60
70
80
Electron Dose (kGy)
Fig. 2. The plots of (ahm)½ versus (hm) for electron irradiated HPMC films, a – pure, b – 25 kGy, c – 50 kGy and d – 75 kGy.
Fig. 3. Variation of optical band gaps of electron irradiated HPMC polymer films. Table 2 The dependence of Eg on irradiation dose. Eg (eV) 4.24 4.10 3.99 3.73
The allowed direct transition simply occurs vertically from the top of the valence band to the bottom of the conduction band, while non-vertical transitions are normally forbidden [8]. The factor A depends on the transition probability and can be assumed to be constant within the optical frequency range. The usual method for the determination of the value of Eg involves plotting (ahm)½ against (hm). Plots of (ahm)½ versus (hm) near the absorption edge for HPMC films with different doses produce a linear fit over a wide range, as shown in Fig. 2. Extrapolating the straight regions of these relations to the (hv) axis yields the corresponding forbidden band width, Eg, shown in Table 2. The value of Eg decreased with dose. The value of Eg for the unirradiated film was 4.24 eV, and it was found that energy gap is inversely proportional to the dose. Fig. 3 shows the variation in Eg with dose. It is clear that Eg decreases with dose. This may be due to cross-linking of the polymeric chains. It was revealed by the crystallinity study that the crystallinity of the samples decreases with dose. Since the molecular motion of the polymer chain is regulated in the crystalline area the radicals generated in there cannot meet other radicals to cross-link by recombination reaction [18]. On the other hand, in an amorphous area the radicals can encounter each other to make cross-linking reactions by micro-Brownian molecular motion of the polymer chain. Based on this idea, it is expected that the crystallinity of polymer decreases with dose [19]. 3.2. FT-IR spectral analysis The FTIR absorbance spectra for the virgin and the electron irradiated HPMC polymer samples are shown in Fig. 4. They show broad bands at 3469.3 cm1 due to a stretching vibration in the hydroxyl group. The films are stable up to irradiation of 50 kGy, after which they are degraded due to deformation into monomers such as ester and methyl groups. During irradiation of HPMC from 0 to 50 kGy the films undergo modification and degradation. On the microscopic
3479
2935
d
1731 1535 1225
909 1071 1005
Transmittance (%)
Dose (kGy) 0 25 50 75
2915 2859
1726 1534
c
1225
912 1007 3452
2861
1725
b
1534 1224 910 1009
2877 2904
1728
a
1529 1226
910 1077 1009
3500
3000
2500
2000
1500
1000
-1
Wavenumber (cm ) Fig. 4. FT-IR spectra of the virgin and electron beam irradiated HPMC films, a – pure, b – 25 kGy, c – 50 kGy and d – 75 kGy.
level, the polymer degradation is characterized by macromolecular chain splitting, creation of low mass fragments and production of free radicals, such as 1372–1380 CH3 radicals. In a polymer, an electron beam of low energy may be used to induce effects such as chain scission and cross-linking. The nature of chemical bonds in polymers can be studied through the characterization of the vibration modes by infrared spectroscopy [20]. Here we have calculated IR calculations such as zero-point energy, anharmonicity constant, fundamental and first overtone transition of C–O bonds and force constant of the chain about this bond. For a pure HPMC sample the normal modes of e0 bonds are 700, 1307 and 2306 cm1. The zero-point energy of these bonds is calculated using the following relation [21]:
¼
4 1 X ti : hc 2 i¼1
ð3Þ
The zero–point energy for a pure sample is 0.311 eV. We have also extended this calculation to electron irradiated HPMC films. The
Sangappa et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2385–2389
zero-point energy increased for higher doses and after 50 kGy it starts to decrease, and it was 0.314 eV at 75 kGy, 0.312 eV at 100 kGy. For pure HPMC the fundamental and first overtone transition of C–O bonds are centered at 1928 and 3804 cm1 respectively. The equilibrium vibration frequency of the chain about this bond is calculated using the following relations [21]:
Frequency of the fundamental transition ¼ 1928 cm e ð1 2xe Þ; ¼t
1
ð4Þ
Frequency of the first overtone ¼ 3804 cm1 e ð1 3xe Þ: ¼ 2t
110 100 90 80
Weight loss (%)
2388
70 60 50
ð5Þ
30
Solving the above equations we have obtained the equilibrium e . The calculated equilibrium vibration for pure HPMC vibration t is 1980 cm1. For irradiated samples this value slightly increases and after 75 kGy it was decreased. And zero-point energy is given by the following relation [21]:
20
eo ¼ 1=2te ð1 1=2xe Þ;
0 kGy 25 kGy 50 kGy 75 kGy
40
10 0 100
200
2e ; k ¼ 4p2 lc2 t
ð7Þ
where l is the reduced mass of the system, c is the velocity of light e is the equilibrium vibration. From the Table 3 the force conand t stant for pure HPMC is 1586 N/m. After irradiation it was increased and it reaches 1687 N/m for 75 kGy irradiated sample and it was 1569 N/m for 100 kGy. Using the above calculations we have quantified the IR data given in Table 3. From this we can say there is not much change in the structural properties of 8 MeV electron beam irradiated HPMC films, even though there were changes in microstructural parameters and crystallinity of the samples from wide angle X-ray scattering studies [22]. The minor changes in the peaks of irradiated samples may be due to the breaking of one or two bonds in the structure, but this will not change the overall structure of the polymer. 3.3. Thermogramatric analysis study Thermograms obtained for the virgin and the irradiated HPMC films are shown in Fig. 5 and exhibit three distinct regions. For the virgin HPMC film, in the first region, starting from room temperature up to 300 °C the weight loss is due to water vaporization (drying). The weight change was not significant and the sample was thermally stable. In the second, rather narrow region from 300 to 322 °C the film experienced a great weight loss because of the thermal decomposition. About 75% of the sample decomposed into volatiles. After 322 °C the film was completely decomposed. In the case of electron beam irradiated films the thermograms showed three distinct regions. In the 25 kGy irradiated film, the
400
500
600
o
ð6Þ
e is the equilibrium vibration, xe is the anharmonicity conwhere t stant. Using the above relation we have calculated the zero-point energy as 983.5 cm1 for pure HPMC film and it increases for higher doses. We have also calculated the force constant for all the samples using the following relation [21]:
300
Temperature ( C) Fig. 5. TGA thermograms of virgin and electron beam irradiated HPMC films.
first region, starting from room temperature up to 310 °C the weight loss is due to water vaporization. The weight change is not significant and the irradiated film is thermally stable. In the second region from 310 to 360 °C, the film experienced a great weight loss because of the thermal decomposition. About 50% of the sample decomposed into volatiles. At 400 °C about 87% decomposed and at 500 °C about 90% and at 600 °C 92% decomposed. The 50 kGy irradiated sample also showed the three regions and at 346 °C, about 50% of the sample decomposed. At 400 °C about 85% decomposed, at 500 °C about 86% decomposed and at 600 °C, about 88% decomposed into volatiles. In the 75 kGy dose irradiated sample also showed the three regions and at 346 °C, about 50% of the sample decomposed. It was observed at 400 °C about 82% decomposed, at 500 °C about 84% decomposed and at 600 °C, about 85% decomposed into volatiles. As the irradiation dose increased the cross-linking of the polymer increased. The weight loss (Table 4) decreases as irradiation dose increases. In the 25 kGy irradiated sample at 400 °C about 87% decomposed, at 500 °C about 90% decomposed and at 600 °C, 92% decomposed into volatiles. This indicates that the thermal stability of the HPMC polymer films increases with dose. Table 4 Temperature of decomposition at different weight loss (%) of HPMC before and after 8 MeV EB irradiation. Irradiation dose (kGy)
Unirradiated 25 50 75
Temperature (°C) 100
200
300
400
500
600
12.8 10.00 09.00 09.00
14.71 14.00 12.00 11.00
22.64 22.00 19.00 18.00
– 89.00 84.00 82.00
– 91.00 86.00 84.00
– 92.00 88.00 85.00
Table 3 IR data of pure and electron beam irradiated HPMC films. Samples (kGy)
Zero-point energy (E0) (eV)
An harmonicity Constant xe 103
e Þ cm1 Equilibrium vibration frequency ðt
e0 (cm1)
Force constant k (N/m)
0 25 50 75 100
0.311 0.317 0.317 0.314 0.312
13.2 22.4 14.8 25.1 10.0
1980.0 2010.0 1978.6 2042.5 1969.4
983.5 949.2 981.9 1008.4 979.7
1586 1635 1583 1687 1569
Sangappa et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2385–2389
optical energy gap, which could be due to the degradation of HPMC and the formation of defects and clusters. We have also quantified the physical properties electron irradiated and pure polymer films, in terms of zero-point energy, anharmonicity constant, fundamental and first overtone transition and force constant. From the IR data there is not much change in the structural properties of irradiated films. From the TGA curves and DT thermograms it is concluded that structural changes occurred after irradiation. An increasing dose resulted in an increase in thermal stability, and there was a 10–15% weight loss due to water vaporization. The melting temperature increased with dose and the decomposition temperature also increased with dose because of the cross-linking of HPMC due to irradiation.
DTG (a. u.)
-10
-15 Pure 25 kGy 50 kGy 75 kGy
-20
-25
2389
100 kGy
References -30 50
100
150
200
250
300
350
400
450
500
o
Temperature ( C) Fig. 6. DTG thermograms of virgin and electron beam irradiated HPMC films.
Table 5 DT information of virgin and EB irradiated HPMC films. Sample (kGy)
Tm (°C)
Td (°C)
0 25 50 75
332.5 337.9 339.9 337.7
– 381.7 382.5 383.5
From the DTG thermograms (Fig. 6) the virgin sample has melting temperature at 332 °C and this increases with dose (Table 5). In the 25 kGy sample the melting temperature was 337.9 °C, and the decomposition temperature was 381.7 °C. Increasing decomposition temperature with dose leads to the cross-linking in HPMC polymer samples. 4. Conclusions Modification of the optical properties of HPMC was carried out using electron beam irradiation. Irradiation leads to a decrease in
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