Nuclear Instruments and Methods in Physics Research B 269 (2011) 2792–2797

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Nuclear Instruments and Methods in Physics Research B

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Investigation of thermally stimulated properties of SHI beam irradiatedpolycarbonate/polystyrene double layered samples

Bhupendra Singh Rathore a,b, Mulayam Singh Gaur a,⇑, Kripa Shanker Singh b

a Department of Physics, Hindustan College of Science & Technology, Farah, Mathura 281122, UP, Indiab Department of Physics, R.B.S. College, Agra 282002, UP, India

a r t i c l e i n f o

Article history:Received 29 March 2011Received in revised form 3 September 2011Available online 8 September 2011

Keywords:Double layeredIon beamTSDCDSCTGA

0168-583X/$ - see front matter � 2011 Elsevier B.V.doi:10.1016/j.nimb.2011.09.003

⇑ Corresponding author.E-mail addresses: bsrathorephy@gmail.com (B.S

rediffmail.com (M.S. Gaur).

a b s t r a c t

The double layered samples of polycarbonate/polystyrene (PC/PS) have been prepared by solvent castingmethod and irradiated with 55 MeV C5+ beam at different ion fluences range from 1 � 1011 to1 � 1013 ion/cm2. The effect of swift heavy ion (SHI) beam in interfacial phenomena, phase change,dielectric relaxation, degradation temperature, stability, charge storage and transport mechanism ofPC/PS pristine and irradiated double layered samples have been investigated by thermally stimulated dis-charge current (TSDC), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).TSDC show a, b-relaxation peaks shifted to the lower temperatures side with increase of fluence. The acti-vation energy and relaxation time decrease, while the depolarization current and charge releasedincrease with increase in the ion fluences. DSC curve show the glass transition temperature (Tg) and heatcapacity decreases with increase in the ion fluences. The TGA characteristics represent the thermal sta-bility, which is found to be decreased with increase in the ion fluences.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Polystyrene and polycarbonate thin films show a lot of advanta-ges for use in capacitors and sensors as a functional polymer withexcellent electrical and dielectric properties as well as improvedchemical resistance. The polystyrene capacitor is not for use inhigh frequency circuits, because they are constructed like a coil.They are used well in filter circuits or timing circuits which runat several hundred kHz or less. Polycarbonate thin film used as adielectric, when stable, low-loss component is needed, polycarbon-ate film capacitor is best suited. Polycarbonate would normally bespecified for frequency-sensitive circuits such as filters and timingfunctions. It is also a good general-purpose dielectric for higherpower use [1,2]. However, PC/PS double layered sample is notyet been reported for application as a capacitor and sensor.

Swift heavy ion beam irradiation is an effective technique forthe modification of various properties of the polymeric materialsuch as electrical, optical and mechanical properties, etc. The dou-ble layered polymeric samples were playing a very important rolein the field of electrical properties of polymeric insulating materi-als [3] such as interfacial phenomena or inter phase separation ascharge-carrier transport and storage. Polymeric double layeredsamples were used in capacitors, micro electromechanical systems

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. Rathore), mulayamgaur@

(MEMS) devices, energy storage devices for memory backups, dig-ital communication, electric vehicles, integrated optic polymer de-vices, etc. [4,5]. The polymer–polymer interface acts as a charge-carrier trapping site. Therefore, it is essential to study the effectof double layered polymeric samples interface on charge-carriertransport and storage because most practical insulators are heter-ogeneous/homogeneous polymeric systems [6–8].

A number of poly(alkyl acrylate) and poly(methyl acrylate) sys-tems were irradiated by ion beam to study the glass transition tem-perature, thermal stability and thermally stimulated relaxationproperties and subsequent cross linking [9–17]. However, these ef-fects are rarely reported in case of double layered polymeric system.

The interfacial phenomena as charge-carrier transport and stor-age mechanism of polymeric double layered samples can be stud-ied with the help of thermally stimulated discharge current (TSDC),differential scanning calorimetry (DSC) and thermogravimetricanalysis (TGA) techniques have been used to study of polymer/polymer, polymer/nanocomposites, polymer/mica and polymer/oil interfaces [18–26].

The selection of polymeric material is very important parameterto investigate the thermal, optical and mechanical properties ofpolymeric double layered samples. In this study we are selectedpolycarbonate (PC) and polystyrene (PS) because, there are manysimilarities in PC and PS such as both are amorphous, transparent,light weight, high stability and wide band gap materials [27–29].Therefore, the combinations of PC and PS have been selected toprepare polymeric double layered samples.

Fig. 1. TSDC characteristics of pristine and SHI beam irradiated PC/PS doublelayered samples polarized at 423 K with polarizing field of 50 kV/cm.

Fig. 3. TSDC characteristics of pristine and SHI beam irradiated PC/PS doublelayered samples polarized at 423 K with polarizing field of 150 kV/cm.

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The chemical processes that happen during and after the ionimplantation are not explained sufficiently in case of single layeras well as double layered polymer samples. It is shown thatcross-linking unsaturated bond formation and chain scissoringincluding gas evolution take place during carbon ion bombardmentin polymers. Therefore, in this study, PC/PS double layered sampleswere irradiated with 55 MeV C5+ beams using 15 UD Pelletron atInter University Accelerator Center (IUAC), New Delhi, India. Theeffects of ion beam induced changes have been investigated byusing TSDC, DSC and TGA thermal techniques.

Fig. 4. TSDC characteristics of double layered (pristine) samples at 423 K withpolarizing fields of 50, 100 and 150 kV/cm.

2. Experimental

2.1. Sample preparation

Polycarbonate (PC) and polystyrene (PS) pellets were procuredfrom Redox, India. Thin films of the polycarbonate and polystyrenewere separately prepared by the solvent casting method. Thedichloromethane (DCM) and benzene are used as a solvent for PCand PS respectively. The 5 g of PS pellets were dissolved in 50 mlof benzene (solvent) at 323 K and stirred for 3 h. The transparentsolution of PS was poured onto an optically plane glass plate at aroom temperature for 24 h. Then the PS film was peeled off fromthe glass plate and similar method was used for preparation ofPC film [30,31]. The PS and PC films were further dried at roomtemperature with outgassing of 10�5 torr for a further period of24 h to remove volatile residual solvent. For double layered sam-ple, PC and PS films were compressed together by compressionmolding machine (Model-CE 100DDS) at a temperature of 373 Kand pressure of 2500 lb in.�2 (17.25 mPa).

Fig. 2. TSDC characteristics of pristine and SHI beam irradiated PC/PS doublelayered samples polarized at 423 K with polarizing field of 100 kV/cm.

2.2. Ion beam irradiation

The double layered polymeric samples (30 lm thickness) of thesize (1 � 1 cm2) were mounted on the copper ladder and irradiatedwith 55 MeV C5+ ion beams at different fluences range from1 � 1011 to 1 � 1013 ion/cm2 using the 15 UD pelletron facility inthe general purpose scattering chamber (GPSC) under high vacuumof 5 � 10�6 torr at Inter-University Accelerator Centre (IUAC), NewDelhi. The beam current was kept low to suppress thermal decom-position and was monitored intermittently with the help of a Far-aday cup.

2.3. Thermally stimulated discharge current (TSDC)

To investigate the thermo-electrets behavior of pristine and ionbeam irradiated polymeric double layered samples with the help of

Fig. 5. TSDC characteristics of double layered (1 � 1011 ion/cm2) samples at 423 Kwith polarizing fields of 50, 100 and 150 kV/cm.

Fig. 6. TSDC characteristics of double layered (1 � 1013 ion/cm2) samples at 423 Kwith polarizing fields of 50, 100 and 150 kV/cm.

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TSDC. TSDC is a very useful technique to understand the behaviorof dielectric relaxation, glass transition temperature (Tg), chargestorage and transport mechanism of electrons/holes of the poly-meric samples, this techniques was largely described in detail else-where [32–35]. For good ohmic contact, both the surface of thepolymeric samples were vacuum aluminized using vacuum equip-ment (VEQCO) Delhi, India. Vacuum coating unit with penning andpirani pressure gauges, ST-A6P3; over central circular area of diam-eter 0.8 cm, both sides vacuum aluminized polymeric sampleshave been used for TSDC. The sample holder forming aluminum– polymeric sample – aluminum system was placed in an ‘Ambas-sador’ oven which is programmed to linear rise of temperature.Pristine and ion beam irradiated polymeric double layered sampleswere polarized by subjecting with the desired polarization fields ordc fields (EP) of 50, 100 and 150 kV/cm at a constant temperature

Fig. 7. Activation energy, relaxation time and charge released Vs ion fluences, for pristifields of 50, 100 and 150 kV/cm.

(TP) 423 K (above the glass transition temperature Tg) for 1 h. Thepolymeric double layered samples were then cooled to the roomtemperature for 1/2 h in the presence of dc fields (EP) then the sam-ples were short-circuiting the electrodes for about 15 min to elim-inate the surface stray charges accumulated during polarization.The depolarization currents of polymeric samples were measuredby electrometer (Kethley-6514) at a linear heating rate of 3 K/min. The depolarization’s current were recorded in the tempera-ture range from 300 to 443 K.

2.4. Differential scanning calorimetry (DSC)

DSC (TA-Instruments model 2910) was used to investigate theglass transition temperature (Tg) of pristine and ion beam irradi-ated polymeric double layered samples. The scanning rate of heat-ing and cooling was performed at a 10 K/min in a nitrogenatmosphere with the temperature range from 300 to 700 K for allmeasurements. The pristine and ion beam irradiated polymericdouble layered samples were cut into small pieces and the piecesput into the empty aluminum pans and weighed in micro-balanceup to an accuracy of 10 ppm .The mass of the polymeric samplewas kept in 10–12 mg and aluminum pans used with empty panwere weighed to a mass of 0.002 mg.

2.5. Thermogravimetric analysis (TGA)

TGA (TA-Instruments model Q500) was used to investigate thestability/decomposition temperature of pristine and ion beam irra-diated polymeric double layered samples. The pristine and ionbeam irradiated polymeric double layered samples were cut into

ne and SHI beam irradiated PC/PS double layered samples at 423 K with polarizing

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small pieces and the pieces put into the empty aluminum pans andweighed in micro-balance up to an accuracy of 10 ppm. The massof the polymeric sample and empty aluminum pans were 10–12 mg and 0.002 mg, respectively. These studies were carried outin a nitrogen atmosphere with scanning rate of heating and coolingwas 10 K/min and the temperature range from 300 to 700 K for allmeasurements.

3. Results and discussion

Figs. 1–6 illustrate the thermally stimulated depolarization cur-rent (TSDC) spectra of pristine and ion beam irradiated polymericdouble layered samples of different ion fluences and various polar-ization fields (i.e. EP = 50, 100 and 150 kV/cm) at 423 K.

The TSDC curves of pristine and ion beam irradiated polymericsamples show two peaks. The first peak was observed in the tem-perature region 405–420 K and another peak was observed in thetemperature region 364–374 K. The first peak is also known as a-relaxation (i.e. higher temperature side) and second peak is knownas b-relaxation (i.e. lower temperature side). Since a-relaxation isfound in the glass transition temperature region of PC and b-relax-ation is observed in the glass transition temperature region of PS.The a and b-relaxation are also observed by DSC in same temper-ature region as observed by TSDC. These results confirmed thatdouble layered polymeric sample have good arrangement of PCand PS. From Figs. 1–3 show a and b-relaxation peaks are shifted

Fig. 8. DSC characteristics pristine and SHI beam

to the lower temperature side and depolarization currents are in-crease with ion fluences due to the scissoring of the polymerchains. Figs. 4–6 illustrate the different polarization fields effectsat 423 K on pristine and ion beam irradiated polymeric films. It isclear from Figs. 4–6 that a and b-relaxation peaks are shifted tothe higher temperature side due to the space-chargedepolarization.

The TSDC is due to the recombination of electron and holes,which is supported by the observed behavior of charge released.The charge release is found to be increases with increase of ionbeam fluence. The ion implantation in double layered samplesmodified the interface due to changes of macroscopic propertiesof polymer surfaces through ion-beam induced chemical modifica-tions on a molecular level. Therefore, the hole concentration is as-sumed to be increased. The hole concentration corresponding tothe resistivity and trap density [36] of sample in the temperaturerange of this experiment is decreases (i.e. 1018–1020 cm�3) as ob-served in present study. The trap density is observed to be approx-imately 1019 cm�3. This value is similar as reported by Schwarzet al. [37]. For simplicity we also assume that the recombinationcoefficient is the same for electron–hole recombination as for elec-tron trap capture. Under these conditions, it is statistically impos-sible for a detrapped electron to contribute to a thermallystimulated depolarization current (TSDC) instead of recombiningwith a hole. Hence, the measured TSDC is due to detrapped holes.The plot of current I versus temperature T are typical TSDC curve.

irradiated PC/PS double layered samples.

Fig. 9. TGA characteristics pristine and SHI beam irradiated PC/PS double layeredsamples.

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The activation energy, relaxation time and charge released ver-sus ion fluences shown in Fig. 7(a)–(f). The activation energy andrelaxation time decrease while charge released increase with ionfluence due to irradiation of the carbon ion beam.

The activation energy Ea of the traps engaged in the TSDC pro-cedure have been calculated. Two methods for evaluating E wereused. First, we used the Bucci–Fieschi–Guidi (BFG) method [38]to calculate the relaxation time of holes by taking advantage ofthe whole TSDC curve:

sðTÞ ¼1b

R1t IðtÞdtIðtÞ ð1Þ

where s is the relaxation time of the holes, b is the heating rate and Iis the measured discharge current. By integrating of the TSDC curveusing Eq. (1) and considering the Arrhenius relation:

s ¼ s0 expEa

kT

� �ð2Þ

where Ea is the activation energy, and k is Boltzmann’s constant, itwas calculated from the slope of ln t against 1/T that the averageactivation energy (Ea) of a and b-relaxation peaks are about 1.02and 0.40 eV, respectively.

The relaxation time corresponding to TSDC peaks s was com-puted using Eq. (1), which are decreases with increase in ion flu-ence. The decrease of s is correlated with the decrease of Tm � Tp,where Tm is the peak temperature and Tp is polarization tempera-ture. It is actually with the modification of the type of dipolesaligned by the field due to ion implantation. The relaxation timevaries from 1.1074 � 10�16 to 1.0762 � 10�16 s for a peak and0.0615 � 10�16 to 0.0297 � 10�16 s for b peak. This variation ofthe relaxation time describes (i) the distribution of the relaxationtimes, and (ii) the increase of the molecular motion with increaseof ion fluence. It must be emphasized that there is the significantvariation for activation energy and relaxation time s, proving awide distribution of relaxation times. Although the variation ofrelaxation time is in good agreement with Arrhenius law underly-ing the fact that there are mono-energetic processes, s changes lessfrom one relaxation process to the nearby one, but it changes sig-nificantly for multiple relaxation process as observed in pristineand ion beam irradiated double layered samples.

In order to discuss the fundamental aspects for application ofdouble layered sample of PC and PS, the equivalent frequencywas calculated from TSDC peak. Using the mean relaxation timeof 1.0932 � 10�16 and 0.0471 � 10�16 s, the equivalent frequencyof the TSDC experiment was calculated by using the following rela-tion [39–44].

f ¼ 12pts

ð3Þ

The frequency is equal to in order of 10�6 Hz for a peak and bpeak. This value of frequency is less than the pristine sample of PSand PC. Therefore, the double layered sample is low frequencydielectric and useful for capacitor and sensor application [43,45,46].

Differential scanning calorimetry measurements in exothermicpattern on pristine and ion beam irradiated sample showed twoglass transition temperatures, one corresponding to Tg of polysty-rene and other corresponding to polycarbonate (Fig. 8). Fig. 8 alsopresent that the derivative of heat flow against temperature showsthe two sharp peaks. The position of peak shifted towards lowertemperature side with increase in fluence. The glass transitiontemperature (Tg) was well observed by first order derivative of heatflow curves of the pristine and ion beam irradiated polymeric dou-ble layered samples of different ion fluences are represent inFig. 8(a)–(f).

The heat capacity decreasing with ion fluence was observedmay be due to the formation of disordered polymeric material byion beam irradiation. Our results also indicate that the glass tran-sition temperatures (Tg) are decrease with increase in the ion flu-ences as compare to pristine sample. These results indicate on anincrease in the content of amorphous phase with increase in theion fluences. The decrease in the glass transition temperatures(Tg) and heat capacity are generally due to the increase in molecu-lar mobility as a result of the scissoring of the polymer chains [47–50]. The Tg of pristine PS and PC is reported that the about 363 and415 K, respectively, however, after formation of double layer the Tg

is slightly reduced.In double layered samples the first thermal transition occurring

at about 373 K for the low molecular weight (LMW) PS and atabout 414 K for the high molecular weight (HMW) PC to a glasstransition. The value of the LMW polystyrene and HMW polycar-bonate Tg is in good agreement with the value reported in litera-ture. Secondly, the narrow transition occurring between 372–368 K (b-relaxation) and 414–406 K (a-relaxation) are tentativelyassigned to a phase-disordering transition caused by high energyion beam.

Thermogravimetric analysis (Fig. 9) shows the thermal stability/weight loss of pristine and ion beam irradiated polymeric doublelayered samples. The first order derivative curve (i.e. DTA) of theTGA thermograms is shown in Fig. 9. TGA analysis show the ther-mal stability decrease from 722 K (pristine) to 634 K (1 � 1011 ion/cm2) to 639 K (1 � 1013 ion/cm2) of the polymeric samples.

The TGA traces of the new molecules in ion beam irradiateddouble layered samples as compare to pristine sample. All of themolecules showed low thermal stability, the thermal stability ofdouble layered samples is gradually decreases with increase of

B.S. Rathore et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 2792–2797 2797

fluence. Most of the weight loss occurred in the temperature rangeof 600–750 K, and almost no further weight change occurred above750 K. The high energy ion beam degraded the bulky phenyl groupof PC in double layered samples, therefore, weight loss is signifi-cantly affected in ion beam irradiated double layered samplesand showing poor thermal stability. It is further understand thatthe decrease in thermal stability in polymeric double layered sam-ples can be attributed to chain-scissoring in the polymer due tohigh energy ion beam irradiation.

4. Conclusion

TSDC show the a, b-relaxation peaks shifted to the lower tem-peratures sides, activation energy and relaxation time decreaseswhile the depolarization current and charge released decreaseswith increases in the ion fluences due to the chain scissoring/crosslinking is the amount of electronic energy loss. The ion-beam in-duced chemical modifications on a molecular level enhance thehole concentration in ion beam irradiated samples. Therefore, ob-served TSDC is not only due to the electronic charge carriers butalso due to the recombination of electron and holes, which isresponsible for TSDC peaks. The decrease in the glass transitiontemperature (Tg) from about 413 to 405 K (a-relaxation) and371–368 K (b-relaxation) samples as evident from the DSC thermalanalysis, due to the chain-scissoring in the polymeric samples bythe ion beam irradiation. Thermogravimetric analysis show the de-crease in the thermal stability and degradation temperature of irra-diated polymeric double layered samples as compare to pristinesample due to incorporation of ions into the double layeredsamples.

Acknowledgements

The authors (B.S. Rathore and K.S. Singh) gratefully acknowl-edge the financial support of Inter University Accelerator Center(IUAC), New Delhi-India for pursuing of fellowship under the UFUPproject. We are also thankful to Dr. Fouran Singh, Inter-UniversityAccelerator Centre (IUAC), New Delhi-India for their help duringirradiation.

Authors are highly grateful to Prof. (Retd) Ranjit Singh, Depart-ment of Physics, Rani Durgavati University Jabalpur (MP) India forhis valuable suggestion.

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