The samples of CR-39 polymer (TASTRAK, Bristol, England) were annealled thermally at various
temperatures ranging from 100oC to 180oC for 1 hour, in air. FTIR spectroscopy reveals the structural degradation
of CR-39 polymer due to thermal annealing above its glass transition temperature. Optical band gap of pristine
and thermally annealed samples has been determined using UV-Visible absorption spectra. It has been observed
that the optical band gap decreases continuously as a result of annealing.Department of Physics, Kurukshetra University,Kurukshetra-136 119, Haryana, Indi

Aggarwal, Sanjeev

Gupta, Renu

Kumar, S

Nidhi, Nidhi

Sharma, Tanu

IACS Institutional Repository

Indian J. Phys. 83 (7), 921-926 (2009)© 2009 IACSEffect of thermal annealing on optical properties of CR-39polymeric track detectorNidhi, Renu Gupta, Tanu Sharma, Sanjeev Aggarwal and S Kumar*Department of Physics, Kurukshetra University,Kurukshetra-136 119, Haryana, IndiaE-mail : profshyam@gmail.comAbstract : The samples of CR-39 polymer (TASTRAK, Bristol, England) were annealled thermally at varioustemperatures ranging from 100oC to 180oC for 1 hour, in air. FTIR spectroscopy reveals the structural degradationof CR-39 polymer due to thermal annealing above its glass transition temperature. Optical band gap of pristineand thermally annealed samples has been determined using UV-Visible absorption spectra. It has been observedthat the optical band gap decreases continuously as a result of annealing.Keywords : CR-39 polymer, thermal annealing, UV-Visible spectroscopy, optical band gap, FTIR spectroscopy.PACS Nos. : 78.30.jw, 78.30.-j, 78.40.-q1.  IntroductionIt is well known that thermal annealing of polymeric materials changes their morphologyat temperatures higher than their corresponding glass transition temperature (Tg). BelowTg, polymeric chains remain almost immobile and retain their original morphology. However,at annealing temperatures above Tg, energy gained by polymeric chains, enhances [1,2]the inter-chain interactions which results [1] in morphological changes in the polymer.These changes inside the annealed polymers lead to alter [3] their electrical, optical,mechanical properties etc. A systematic study in this direction is significant for the utilizaitonof polymers in the various scientific and technological applications. Further, the opticalperformance of a polymer is significant for the applications like optical sensors, LED's,waveguides, antireflective coatings, optical lenses etc. [4–6].CR-39 is high grade amorphous optically transparent plastic and is widely used as anuclear track detector besides many other applications [7,8]. The chemical structure of* Corresponding Author922 Nidhi, Renu Gupta, Tanu Sharma, Sanjeev Aggarwal and S KumarData\IJP\July 2009\09 Nidhi.p65repeating unit of CR-39 is shown in Figure 1.Figure 1. Monomer structure of CR-39 polymer.In the present work, the effect of thermal annealing on the absorption and structuralbehaviour of CR-39 polymer has been studied through UV-Visible and FTIR spectroscopy.Such studies may contribute towards the better understanding of track formation process.2. Experimental detailsSamples of CR-39 polymer (TASTRAK, Bristol, England) were cut from a flat polishedsheet of thickness 1 mm. The heat treatment was carried out at varying temperaturesranging from 100oC to 180oC in air on different specimens of CR-39 for one hour each.Virgin and heat treated CR-29 samples were subjected to spectral studies in the ultravioletand visible region. These studies were carried out by using Shimadzu Double beam DoubleMonochromator UV-Visible Spectrophotometer (UV-2550) in the wavelength range of 190–900 nm with resolution of 0.5 nm. All the spectra were recorded by mounting the samplesin the integrating sphere assembly ISR-240A attached with the Spectrophotometer, keepingair as reference. The Fourier transform infrared spectroscopy was performed in thetransmission mode using Abb Bomen FTIR Spectrometer in the range 2000–600 cm–1.3. Results and discussionsFigure 2 presents the UV-Visible absorption spectra of CR-39 virgin and thermally annealedsamples at different temperatures ranging from 100–180oC for 1 hour.Figure 2. UV-Visible absorption spectra of CR-39 polymer : (a) virgin and thermally annealed at (b) 100oC,(c) 140oC, (d) 160oC, (e) 170oC and (f) 180oC.Optical DensityWavelength (nm)Effect of thermal annealing on optical properties of CR-39 polymeric track detector 923Data\IJP\July 2009\09 Nidhi.p65It is obvious from Figure 2 (curves 'a' to 'f') that thermal annealing results in the shiftingof absorption edge in virgin CR-39 polymer towards higher wavelengths. Such a red shiftmay be attributed to the fact that due to thermal annealing, energy gained by the hostpolymer changes the polymer morphology. Thermal treatment at temperatures above Tg,results [9–13] in bond rupture, reorientation of dipole moments, cross-linking, stericrearrangement of polymer chains etc. All these processes are responsible [13,14] for thered shift of absorption edge. Another possibility for red shift may be the increase inconjugation [14] with increasing annealing temperature resulting in a decreased HOMO-LUMO gap.3.1.Determination of optical band gap :From the UV-Visible absorption spectra, the optical band gap of virgin and thermally annealedCR-39 polymer has been determined. For this purpose, the values of  were plottedas a function of photon energy ( ) [4,15], taking into account the data corresponding tothe linear portion of the fundamental absorption edge of the UV-Visible absorption spectra.Optical band gap has been determined from the fundamental absorption edge of theabsorption spectra using the relation [6–8] :(1)where hν  is the incident photon energy,  is absorption coefficient, and Eg is the opticalband gap.Figure 3 presents these plots for virgin and thermaly annealed CR-39 samples. Fromthe intercept of the best fit lines on  axis, optical band gap has been determined andthe results are presented in Table 1.Table 1. Optical band gap values for virgin and thermallly annealed CR-39 polymer.Sl. Temperature Optical band RegressionNo. gap (eV) coefficient 'R'1. Virgin 3.81 0.992. 100oC 3.64 0.993 140oC 3.53 0.994 160oC 2.50 0.995 170oC 2.25 0.996 180oC 2.01 0.99The regression coefficients have been found to be nearly 0.99 for the fitted straightlines. From the repeated measurements, the standard error in the quoted values of opticalband gap has been estimated to be less than 3%.924 Nidhi, Renu Gupta, Tanu Sharma, Sanjeev Aggarwal and S KumarData\IJP\July 2009\09 Nidhi.p65It is obvious from Table 1 that the optical band gap goes on decreasing continuouslyfrom 3.81 eV in virgin CR-39 sample to 2.01 eV in thermally annealed sample at 180oC.Such a lowering in optical band gap may be correlated to the morphological changes inthe polymeric sample as a result of thermal annealing.Figure 3. Plots of  versus photon energy ( ) for optical band gap (eV) in CR-39 polymer : (a)virgin and thermally annealed at (b) 100oC, (c) 140oC, (d) 160oC, (e) 170oC and (f) 180oC.FTIR spectra of virgin and thermally annealed CR-39 samples have been shown inFigure 4. In virgin CR-39 samples (curve 'a', Figure 4) a number of sharp intense bands at783, 1227 and 1734 cm–1 have been observed confirming its monomer structure [16]. Thestrongest intensity band at 1227 cm–1 is due to C-O-C stretching, whereas other strongintensity bands at 783 and 1734 cm–1 can be attributed to the =C-H bending and C=Ostretching vibrations. The other small intensity bands at 1450 and 1398 cm–1 originatesfrom C-H bending mode whereas the band at 1136, 1091 and 1021 cm–1 are due to C-Ostretching mode vibrations and the bands at 955 and 870 cm–1 are due to =C-H out ofplane bending mode vibrations.After thermal annealing at 100oC (curve 'b', Figure 4), it is clearly seen that the intensitiesof the transmission bands at 783, 870, 955, 1021, 1091, 1136, 1227, 1398, 1450, 1734cm–1 have been increased. This clearly indicates that reduction in the absorption intensitiesof these bands. The reduction reveals that the sample undergoes serious degradationunder thermal annealing [17]. On further annealing upto 170oC (curve 'c', Figure 4), theband at 850 and 955 and 1091 cm–1 are nearly eliminated. This might be attributed tobreakage of chemical bonds and the formation/emission of low molecular gases and radicalsdue to thermal annealing indicating the change in chemical structure of CR-39 due tothermal annealing [18]. The transmission intensities of the bands 783, 1227 and 1734765432100 2 4 6E( )a  virginb  100oCc  140oCd  160oCe  170oCf  180oCf e dcb aEffect of thermal annealing on optical properties of CR-39 polymeric track detector 925Data\IJP\July 2009\09 Nidhi.p65Figure 4. FTIR spectra of CR-39 polymer : (a) virgin and thermally annealed at (b) 100oC, (c) 170oC.Hence the amorphisation and degradation resulted due to thermal annealing in the CR-39 polymer samples as revealed by FTIR above may be responsible for the reduction inband gap. The decrease in optical band gap as a result of thermal annealing has alsobeen quoted by several other groups in different polymers [2,11,14,19].cm–1 is further increased indicating reduction in the absorption of these bands whichleads to the amorphisation of the specimen.1009080701009590858075908070605040(Transmission)||||||||||||||||||1800 1600 1400 1200 1000 800 600Wave Number (cm–1)'a' Virgin'b' 100oC'c' 170oC926 Nidhi, Renu Gupta, Tanu Sharma, Sanjeev Aggarwal and S KumarData\IJP\July 2009\09 Nidhi.p654. ConclusionsIt is concluded that there is a decrease in optical band gap in CR-39 polymer as a resultof thermal annealing at temperatures above Tg. This decrease in optical band gap can becorrelated with the changes in the structure of CR-39 as seen using FTIR studies.References[1] H H Jellinek (ed.) Aspects of Degradation and Stabilization of Polymers (New York : ElsevierScientific Publishing Company) (1978)[2] M A El-Shahawy Poly. Degr. Stab. 57 157 (1997)[3] A Tager Physical Chemistry of Polymers (Moscow : Mir Publishers) (1978)[4] C J Mathai, S Saravanan, M R Anantharaman, S Venkitachalam and S Jayalekshmi J. Phys. D :Appl. Phys. 35 2206 (2002)[5] K Nagagawa J. Appl. Polymer Sci. 41 2049 (1990)[6] Y Koike, E Nihei, N Tanio and Y Ohtsuka Appl. Opt. 29 2686 (1990)[7] D L Henshaw Physics in Technology 13 266 (1982)[8] S A Najjar, R K Bull and S A Durrani Nucl. Tracks. 3 169 (1979)[9] Anupam, S Kumar and N Lal Nucl. Tracks Radiat. Meas. 18 335 (1991)[10] F Yakaphanoglu, M Arslam, M Kucukislamoglu and M Zengin Solar Energy 79 96 (2005)[11] R Singh, J Kumar, R K Singh, A Kaur, K N Sood and R C Rastogi Polymer 46 9126 (2005)[12] W Lincoln Hawkins (ed.) Polymer Stabilization (USA : Wiley Interscience) (1972)[13] J Liu, T F Guo and Y Yang J. Appl. Phys. 91 1595 (2002)[14] J Hou, C Yan, C He and Y Li Chem. Commun. 8 71 (2006)[15] T Sharma, S Aggarwal, S Kumar, V K Mittal, P C Kalsi and V K Manchanda J. Mat. Sci. 42 1127(2007)[16] D L Pavia, G M Lampman and G S Kriz Introduction to Spectroscopy 2nd ed. (New York : HarcourtBrace College Publishers) (1994)[17] Youmei Sun, Zhiyong Zhu, Zhiguang Wang, Yunfan Jin, Jie Liu, Mingdong Hou and Qinxiang ZhangNucl. Instr. Meth. B209 188 (2003)[18] N L Singh, A Sharma and D K Avasthi Nucl. Instr. Meth. Phys. Res. B206 1120 (2003)[19] K Bhowmick and J R White J. Mat. Sci. 37 5141 (2002)

Effect of thermal annealing on optical properties of CR-39 polymeric track detector

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