Nouh, S A

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Indian J. Phys. 73A (6), 757-764  (1999)U P  A- an intcniational journalThermal and dielectric invesiigation on the effect of UV radiation on LR-115 detectorS A NouhDepartment of Physics, Faculty of Science. Ain Shams University, Cairo, Egypt.Received /  April 1999, accepted 24 June 1999A bstract : Non-isothermal studies were carried out using thcrmt)gravimctry (TG) and differential thermogravimetry (DTG) measurements to obtain the thermal activation energy of decomptisition for LR-115 solid state nuclear track detector before and after exposure to different UV energy fluences ranging from 11.3 to 113 kJ/cm^ Thermal gravitational analysis (TGA) indicated that both unirradiated and irradiated LR115 samples decomposed in one main breakdown stage. Kinetic analysis has been earned out to determine the thermal activation energy of decomposition, a decrease in activation energies was observed on increasing the exposure to UV Also, the variation of the transition temperatures with the UV energy fluence have been determined using differential scanning calorimeter (DSC). In addition, dielectric measurements have been made in the frequency range 100 Hz-IOO kHz. The measurements were earned out in the temperature range 25-^ 90®C The dielectric constant e and dielectric loss showed an increase on increasing the temperature up to 90°C Also the frequency dependence of e' is charactenzed by a primary relaxation process. Moreover, the effect of UV radiation on e' and £*' have been measured at room temperature 25“C and at 100 kHz. The results indicated that both ef and e" showed an increase on increasing the UV energy fluence due to the degradation caused by the UV light.Keywords : UV radiation. Thermal properties. Dielectric properties and LR-115 detector PACS Nos. : 61.82.Pv, 61 lO.Nz, 78.30.Jw1. IntroductionA lthough the com m on cellulose derivatives are among the most investigated o f all high polymers, w e have been prom pted to undertake another study lor several reasons. First, cellulose derivatives appear to be almost the only unbranched polymers readily available in a broad range o f  m olecular weights. Secondly, the study o f the thertnal properties o f© 1999 lACS758 SANouhpolym ers is im portant to check their thermal stability and thus their suitability for service. Third, due to their potential application in high-energy density solid-state batteries, the tem perature and frequency dependencies o f the dielectric properties o f polymers have been studied.Numerous publications have dealt with the temperature and frequency dependencies o f the dielectric properties o f polym ers [1-5]. Also the effect o f UV radiation on the physical and optical properties o f polymers has been studied [6,7]. N ouh et al. [8] studied the effect o f non ionizing radiations on the degree o f ordering o f LR-115 detector and on the thermal properties of some polymers.This paper deals with the investigation o f the effect o f UV radiation on the thermal and dielectric properties of L R -115 polym er aiming to (1) know the mechanism of thermal degradation o f LR-115 polym er to be able to choo.se the ways o f stabilizing it and, consequently, o f prolonging the service life o f articles made from LR-115 polym er, (2) obtain optim um  dielectric insulating characteristics for engineering applications, (3) introduce the basis which may be used in constructing a simple sensor for irradiation.2. ExperimentalA) Samples:LR -115 .sheets used in this study were manufactured by Kodak Pathe (France). It consists of a sensitive cellulose nitrate layer o f 12 pm  thickness on a 100 pm  thick polyester support and its density is 1.42 gm/cm^.B) Irradiation facilities :All sam ples were exposed to different UV energy fluences using a Hg lamp of 50 W, at 50/60 Hz, with an illumination flux o f 3.14 W/cm^. All the measurements were carried out 24 h after UV exposure.C) Experimental apparatus:The thermal behavior was investigated using differential scanning calorim eter (DSC) and thermal gravitational analysis (TGA) type Shimadzu-50. a -A l203 pow der was used as a reference for DSC measurements. Thermal experiments were carried out on all sam ples at different heating rates (5, 10, 15 and 20°C/min) with N 2 as a  carrier gas at a flow rate o f 30 cm-’/m in. The values obtained for transition tem peratures w ere m easured with a basic accuracy o f ±0.1°C.A ll the d ie lec tric  m easurem ents w ere carried  ou t using R C L bridge T ype PM6304/031 programmable automatic RCL m eter m easures in the range from 50Hz to  100 kHz. The tem peratures were controlled using an ultratherm ostate. The capacitance was measured with a  basic accuracy o f 0.2 to 0.3%. Also, a  capacitance accuracy test was m ade ■ by connecting a stable capacitor to the instrum ent and readina the disolav to verifv that theThermal and dielectric investigation etc 7593. Results and Discussion1. Thermal properties:A) Transition tem peratures:T herm al characterization  gives a  fairly good idea about the changes taking place in po lym ers. T he D SC  therm ogram s m easured in the tem perature range from room tem perature up to 320°C for unirradiated and iriadiaied LR-115 samples are shown in F igure 1. A ll the therm ogram s were characterized by the appearance of two peaks, one endotherm ic peak due to the melting temperature and one exothermic peak due to theFigure 1. DSC thermograms measured in the temperature range from room temperature up to 32()°C foi unirradiated and irradiated LR-115 samplescrystallization  tem perature T^. The glass transition temperature could be detected lor som e sam ples. The values obtained for these transition tem peratures arc g iven  in Table 1. T he values indicated that both the crystallization and melting temperatures exhibited the sam e trends with the radiation dose, where, they showed an increa.se in m agnitude up to a m axim um  value around 45.2 kJ/cin^ of irradiation followed by a decrease on increasing the UV energy fluence due to the thermal degradation caused by the UV light.760 S A NouhTable 1. Transition temperatures and values of thermal activation energy of decomposition for LR-115 (the values obtained for T^ ,, and are ±0.1 ®C).UV energy fluence (kJ/cm^)TgCC) TcCC) V C ) £«(eV)0.0 88.2 208.8 256.8 3.66113 — 209.0 257.0 3.4845.2 — 210.0 257.2 3.4190.4 90.1 209.8 256.9 3.20113.0 90.2 208 9 2.55.6 2.98B) Activation energy o f decomposition :Evaluation o f the activation energy o f decom position is useful for studying the thermal stab ility  o f  m aterials. Therm al grav itational analysis (TG A ) provides quantitative inform ation on w eight change process while differential therm al gravitational (DTG) provides rate of weight loss (dW/dT), TGA measurements were performed on all irradiated and unirradiated LR-115 sam ples at different heating rates (5, 10, 15 and 20®C/min). Figure 2 shows TGA and DTG thermogram for unirradiated L R -115 sample as an example.Figure 2. TGA and DTG thermogram for unirradiated LR> 115 sample (as an example).It is c lea r that LR -115 de tec to r decom poses in one w eight loss stage. V arious ihcrmogravimetric methods have been proposed for the measurement o f activation energies.Thermal and dielectric investigation etc 761T he m ethod proposed by O zaw a [9] has been used in the present work. In this method, TG curves obtained a t different heating rates are required where the following equation is v a l id :log <b, +  0 .4567  E/RTi = log d>2 +p. 4567 E/RT2 = where r ,  is the tem perature at which the sample if decomposed at a heating rate d>,.A ccording to the above equation, a plot o f the logarithms of the heating rates against the reciprocal o f  absolute tem perature, leads t0 a straight line relationship in the range where the decom posed ratios are equal. Hence, fce activation energy of decomposition £„ can be evaluated  from  the slope o f the line. T ie  values of thermal activation energy of decom position were calculated for all irradiated and unirradiated LR-115 samples using TG A  and D TG curves and are given in Table 1. The values obtained indicates that E„ decreases in m agnitude on increasing the UV energy fluence. This may be attributed to the thermal degradation, where, under prolonged heating, there occurring a random breaking of bonds and som etim es the detachm ent o f low -m olecular products because of reactions of side groups w ithout any appreciable change in the initial molecular weight.2. Dielectric properties :A) The frequency dependence o f the dielectric constant and dielectric loss:The frequency (f) dependence o f the dielectric constant e ' o f unirradiated and irradiated L R -115 sam ples at room  tem perature (25°C) is shown in Figure 3. The graph shows that e'lO» « 15117i« 20 JOUFigure 3. The dependence of dielectric constant ( e') measured at a temperature of 25®C on the frequency for unirradiated and irradiated LR-115 samples.decreases alm ost linearly with frequency (/) in the frequency range from 4 to 20 kHz. The strong low -frequency  dispersion  that characterizes the frequency dependence ot762 SANouhdielectric constant e ' (Figure 3) is to be ascribed to a dipole-orientation relaxation process [10]. This process norm ally involves a rapid increase in dielectric constant as the measurement frequency is reduced. The variation o f dielectric loss e"  with frequency for ail the unirradiated and irradiated LR -115 samples is shown in Figure 4. From the figure itF ig u re  4 . T h e  d ep en d en ce  o f  d ie lec tric  loss { )  m easu red  at a  tem p era tu re  o f  25“C on the frequency  for un irrad ia ted  and irrad ia ted  LR-115 sam plesis clear that e "  decreases almost linearly in the frequency range 4 -2 0  kHz. While, on increasing the frequency up to 100 kHz, a large drop in e"' values was observed. For this reason, the values of e "  at 100 kHz for all the L R - 1 15 samples are given in Table 2 andT a b le  2. V a lu es o f  e an d  f "  m easu red  a t 100 k H z and  25®C for L R - 1 15 (the va lues o b ta in ed  fo r e* and  e' are  ±  0 .2  to  0 .3% ).U V  en erg y  fluence  (k J /cm ^ )r' e"0.0 8.53 0fX )911.3 8.61 0  01745 .2 8 .90 0 .0459 0  4 9 .23 0 .065113.0 9 .25 0  065only the straight portion (4 -20  kHz) is shown in Figure 4. The dielectric loss relaxation obtained in Figure 4 is alm ost due to the micro-Brownian m otions of the chain (glass- rubber relaxation) in the non crystalline regions o f the polymer (11).B) Temperature dependence o f the dielectric constant and dielectric loss :The temperature dependencies of the dielectric constant and dielectric loss of unirradiatcd LR -115 sample at 100 kHz are given in Figuiie 5. From the figure it is clear that the rale of increase of e '  and e"  with temperature is sl<?w since it depends on the degree of polymerThermal and dielectric investigation etc 7^ 3Temp^ralure/cF ig u r e  5 . T h e  d ep e n d e n c e  o f  d ie le c tn c  co n s tan t ( f ' )  an d  d ie le c tn c  loss ( £ " )  m e a su re d  a t 10 0  k H z  o n  the  tem p era tu re  fo r  u n irrad ia ted  L R -1 ! 5 detecto r.hardness. The higher the hardness the higher the rate of increase, as the temperature causes loosening up of the rigid structure, resulting in an increase in dipole orientation and hence an increase in These results show that L R -1 1 5  plastic detector possess better dielectric properties at temperatures higher than room temperature and this property is useful for electrical insulating purposes. Also the interpretation of Figure 5 can be given as follows :At temperatures above the glass transition temperature, the motion of long- chain molecules occurs as a result of the cooperative movement of segments of the chain. The rate of segmental motion can be estimated from the relaxation time of the dielectric relaxation process associated with the micro-brownian motion of the main chain. This rate can be thought of as a function of the free volume fraction which varies with temperature.C) The effect o f UV energy fluence on the dielectric constant and dielectric loss :The U V  energy fluence dependencies of the dielectric constant and dielectric loss for L R -  115  detector at 100 kHz and 2 5 °C  are given in Table 2. The values obtained indicated that both and showed almost an increase on increasing the exposure to UV. This increase may be due to degradation, where at degradation, the standard chains and greater number of chain ends lead to weakening and often to embrittlement, even the material may have become somewhat softer. Thus the increase in £ with the physical degradation is related to the increase in the degree of soft segments and the loss in hard segments.764 S i4 Nouh4. ConclusionsFrom the above study one can draw the following conclusions ;(1) Exposing the L R - llS  polym er to UV causes a random  breaking o f bonds w hich results in a decrease in therm al activation energy o f decom position. The TG A  measurements indicated that when LR-1 IS polym er is heated, its thermal stability is usually appraised from the loss o f mass and its molecular weight sharply diminishes which is due to the degradation o f the m olecular chains.(2) The values o f  the dielectric constant and dielectric loss o f  LR-1 IS detector are dependent on the frequency, temperature and UV energy fluence.(3) LR-1 IS detector was found to possess better insulating properties at tem peratures h igher than room  tem perature and thus it can be used for electrical insulating purposes.AcknowledgmentThe author is extrem ely grateful to doctor L  A W ahab (National C enter for R adiationResearch and Technology, Cairo, Egypt) for her help in the DSC and TGA measurements.H er supports are gratefully acknowledged.References[ 1 ] C Fanggao, G A Saunders, E F Lambson, R N Hampton, G Carini, and G £>imarco /  Polym. ScL 34B 425(1996)(2J J Abajo. J G La Campa, A Alegna and J M Echave J. Polym. ScL 35B 203 (1997)[3] R L Moreira, P S Gregoiit and M Latour Phase Transitions 14 243 (1989)[4] G T Davis T Furukawa, A J Lovinger and M G Broadhurst MacromoUcuUs 15 329 (1982)[5] S ikeda, H Koninami, K Koyama and Y Wada Ferroelectrics 76 421 (1987)16) S A Nouh Egyptian J. Biophys 2 37 (1996)[7] S A Nouh Radial Meas, 21 499 (1997)m  S A Nouh and A A El Hagg Indian J. Phys. 72A 269 (1998)[9] T Ozawa Bull. Chem. Soc. (Japan) 38 1881 (1965) f 10] C Porter and R Boyd Macromolecules 4 589 (1971)[II] N McCrum, B Read and G Williams An Elastic and Dielectric Effects in Polymeric Solids (New York : Wiley) (1967)

Thermal and Dielectric Investigation on the Effect of UV Radiation on LR-115 Detector

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Thermal and Dielectric Investigation on the Effect of UV Radiation on LR-115 Detector

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