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Vacuum 86 (2011) 306e310

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Vacuum

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Thermal properties of ion beam irradiated polycarbonate films

Bhupendra Singh Rathore a,b, M.S. Gaur a,*, K.S. Singh b

aDepartment of Physics, Hindustan College of Science & Technology, Farah, Mathura (U.P.) 281122, IndiabDepartment of Physics, R.B.S. College, Agra (U.P.) 282002, India

a r t i c l e i n f o

Article history:Received 2 February 2011Received in revised form11 July 2011Accepted 11 July 2011

Keywords:SHIGlass transition temperatureTSDCActivation energyRelaxation timePolling field

* Corresponding author. Tel.: þ919412159116; fax:E-mail addresses: bsrathorephy@gmail.com (B.

rediffmail.com (M.S. Gaur).

0042-207X/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.vacuum.2011.07.044

a b s t r a c t

Semicrystalline polycarbonate (PC) films were irradiated under vacuum at room temperature by Cþ5

beam of 55 MeV energy at fluences of 3 � 1011 and 3 � 1013 ions cm2. Ion induced modifications werestudied by X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry(DSC) and thermally stimulated discharge current (TSDC) techniques. With increasing ion fluencesignificant loss of average crystallite size, percentage crystallinity, stability and glass transition (Tg)temperature were observed. TSDC shows that activation energy, released charge, a-relaxation peak, andcharge carrier mobility decrease while relaxation time and peak current increase.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Polycarbonate resins can be divided in two structural classes:aliphatics, which are not extensively used as thermoplastics, andaromatics, which have notable applications as engineering ther-moplastics. bisphenol A polycarbonate (BPAPC) is the most widelyused aromatic polycarbonate. It is a condensation polymer inwhichbenzene rings plus quaternary carbon atoms form bulky stiffmolecules that promote rigidity and strength. The bulky chainscrystallize with great difficulty, so the polymer is normally amor-phous with excellent characteristics such as transparency, tough-ness, thermal stability and dimensional stability. These propertiesgave it uses in many applications like compact discs, riot shields,vandal-proof glazing, baby feeding bottles, electrical components,safety helmets and headlamp lenses.

Over the years, there have been numerous studies on modifi-cation physical properties of PC using SHI at surface and bulk level.Our special attention is to study the thermally stimulated proper-ties at molecular level. The possibility of reaction involved, whenion beam propagated at the surface of polymer will depend uponfluence rate of SHI as well as the oxygen diffusion through thematerial. Different pathways that permit explaining the chemicalmodifications induced by SHI have described the ageing mecha-nism of PC. The first is an oxidative degradation. It involves

þ91562 2850274.S. Rathore), mulayamgaur@

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hydrogen abstraction from the gem dimethyl side-chain leading tothe formation of hydroperoxides that are converted into alcohols,ketones, carboxylic acids, etc. These oxidation reactions, which alsoproduce chain scissions and cross-linking are proposed as respon-sible for the change in mechanical and thermal properties of thematerial. The second mechanism involves successive rearrange-ments of the structure, which account for the modification ofproperties at molecular level. This reaction does not necessitateoxygen and can be easily observed in the case of irradiations carriedout in vacuum or in an inert atmosphere. This last reaction comesfrom a direct absorption of the shortest wavelengths of irradiationby the aromatic carbonate chromophore [1e5].

Swift heavy ion beam play an important role in the field ofpolymer science due to their modification in various propertiessuch as physical, chemical, optical and structural properties [6e8].There are many applications of ion beam irradiated polymer likeagriculture, ecology, microelectronics, sensors and nanotechnology[9e15]. The effect of SHI beam irradiation on polycarbonate hasbeen reported by Srivastava et al. [16]. Spectroscopic investigationon ion beam by Puglisi et al. [17], structural studies by Migahedet al. [18], dielectric studies by Kumar et al. [19] and many authorsreported other irradiation source such as electron, proton, gamma,X-ray and UV source [20e24] for the modification of physicalproperties of polymer. But the thermal properties of ion beamirradiated organic polymers and other irradiation source are notmuch reported.

In the present paper we have investigated the effect of Cþ5 beamof 55 MeV with fluence of 3 � 1011 and 3 � 1013 ions cm�2 on the

Fig. 1. X-ray diffraction pattern of PC pristine and carbon ion beam irradiated PC films.

Fig. 2. TGA pattern of PC pristine and carbon ion beam irradiated PC films.

Fig. 3. DTA pattern of PC pristine and carbon ion beam irradiated PC films.

B.S. Rathore et al. / Vacuum 86 (2011) 306e310 307

average crystallite size, percentage crystallinity, stability, glasstransition temperature, various dielectric relaxation properties ofpolycarbonate pristine/irradiated films using the X-ray diffraction(XRD), thermogravimetric analysis (TGA), differential scanningcalorimetry (DSC) and thermally stimulated discharge current(TSDC) techniques.

2. Experimental

Polycarbonate (PC) used in the present study was procured byRedox, India. 25 mm thick PC films were prepared by solvent castingmethod. The PC (5 gms) pellets was dissolved in 50 ml ofdichloromethane (DCM) at 60 �C. The PC solutionwas stirred for 4 hto become homogeneous and transparent. The prepared PC solutionwas poured onto an optically plane glass plate and the solvent wasthen allowed to evaporate inside an oven at room temperature for24 h to yield PC film. The PC films were further dried at roomtemperature with outgassing of 10�5 mbar for a further period of24 h to remove volatile residual solvent. PC films were mounted ona vacuum shielded vertical sliding copper ladder and films of size1�1 cm2were irradiated by Cþ 5 beamwith 55MeV energy using inthe general purpose scattering chamber (GPSC) and 15 UD Pelletronat Inter University Accelerator Center (IUAC), New Delhi, India. Thebeam current was 1 pnA and films were irradiated at fluence of3 � 1011 and 1 � 1013 ions cm�2. All irradiations were performed invacuum (10�6 mbar) at ambient temperature. X-ray diffraction(XRD) patterns were recorded using the Cu Ka (l¼ 1.541Å) radiationfrom the Brukar AXS D8 diffractometer with scan speed of 1� min�1.The diffraction angle (2q) has been varied from 5 to 40� with stepsize of 0.02. The thermal properties of the PC pristine and SHI beamirradiated films were determined by thermogravimetric analysis(TGA) (TGAQ500, TA instrument) under a nitrogen atmospherewitha heating rate of 10 K/min from 310 to 475 K and DifferentialScanning Calorimetry (DSC) is used to examine the glass transitions

Table 1XRD data of Pristine and ion beam irradiated PC films.

PC Percentagecrystallinity

Crystallite size(Å)

Inter-chain separation(Å)

Pristine 52.24 17.76 3.363 � 1011 50.30 16.09 3.323 � 1013 45.66 14.11 3.31

temperature, for instance, it allows to determining the glass transi-tion temperature (Tg). Moreover, the enthalpy relaxation due tophysical ageing can be determined from DSC measurements.Differential scanning calorimetrymeasurements were carried out on

Fig. 4. DSC thermograms pattern of PC pristine and carbon ion beam irradiated PCfilms.

Fig. 5. TSDC thermograms recorded at 423 K with polarizing field of 100 kV/cm. Fig. 7. TSDC thermograms recorded at 423 K with polarizing field of 200 kV/cm.

Fig. 8. TSDC thermograms of PC (Pristine) at 423 K.

B.S. Rathore et al. / Vacuum 86 (2011) 306e310308

pristine PC and irradiated films using a DSC 2910 MDSC instrumentat 10 K min�1 heating rate with a nitrogen atmosphere andtemperature range from 345 to 460 K. The mass of the sample waskept in 10e15 mg and aluminium pans used with empty pan wereweighed to a mass of 0.002 mg. The TSDC technique is based onmeasuring a current due to the depolarization of the sample bythermal activation. The thermally charged sample is placed in an‘Ambassador’ oven which is programmed to rise linearly intemperature. It is short-circuited through an electrometer (ModelDPM-111, Scientific Equipments Roorkee, India,). In the presentwork, TSDC measurements were performed on PC pristine andirradiated films of (1 �1) cm2 dimensions. Aluminium electrodes of0.8 cm in diameter were deposited under vacuum on both sides ofeach sample. Before measurements, d.c. fields or ‘polling’ fields (i.e.Ep ¼ 100, 150 and 200 kV cm�1) were applied to the samples atpolling temperature of Tp ¼ 150 �C for 2 h, then cooled under fieldand short-circuited during 15 min. TSDC spectra were recorded ata heating rate of 3 K min�1.

3. Results and discussion

3.1. X-ray diffraction (XRD) pattern

Fig. 1 shows the X-ray diffraction pattern of semicrystallinenature of PC pristine and carbon ion beam irradiated films withdifferent fluence of 3 � 1011 and 3 � 1013 ions cm�2. It shows the

Fig. 6. TSDC thermograms recorded at 423 K with polarizing field of 150 kV/cm.

presence of a broad high-angle asymmetric scattering peakstretching from 2q ¼ 5e40�. PC shows four peaks one strong andthree small peaks at about 2q ¼ 16.58� 19.67�, 26.64� and 29.50�

respectively. Nathawat et al. [25] reported the similar results in PC.Slight shifts in the peaks position to lower angle side withincreasing the ion fluence have been observed. The percentage

Fig. 9. TSDC thermograms of PC (3 � 1011 ions/cm2) at 423 K.

Fig. 10. TSDC thermograms of PC (3 � 1013 ions/cm2) at 423 K.

B.S. Rathore et al. / Vacuum 86 (2011) 306e310 309

crystallinity of the polymer films are calculated by the followingrelation [26]:

B ¼ AA0 � 100% (1)

where A is the total area of the peaks and A0 is the total area under thediffraction pattern. Generally intensity and full-width at half-maximum (FWHM) increases then crystallite size decreases. Theaveragecrystallite sizeL,havebeencalculatedbyScherer formula[27];

L ¼ klbcos q

(2)

where b is the FWHM of the peak (in radian) and k is the shapefactor whose value is equal to 0.9. Assuming K ¼ 0.9 in the aboveequation.

The average inter-chain separation is calculated from XRDresults using the following relation [28];

R ¼ 58

�l

sin q

�(3)

The variation of crystallite size, percentage crystallinity andaverage inter-chain separation are shown in Table 1. After irradia-tion the average crystallite size, percentage crystallinity andaverage inter-chain separation decrease for the prominent peak(First peak). The average crystallite size, percentage crystallinityand average inter-chain separation decreases due to the main chainscission and cross-linking with increases of ion fluence.

3.2. Thermogravimetric analysis (TGA)

Fig. 2 shows the TGA curve of PC pristine and SHI beam irradi-ated films indicate that the thermal stability decreases with

Table 2TSDC parameter of Pristine and ion beam irradiated PC films.

Polarization onfield (kV cm�1)

Fluence(ions cm�2)

Peak current(pA)

Peak temperaturTm (K)

100 Pristine 69.92 417.843 � 1011 72.04 412.803 � 1013 80.62 405.36

150 Pristine 71.92 419.913 � 1011 75.50 417.893 � 1013 88.04 407.86

200 Pristine 80.94 426.003 � 1011 82.70 422.803 � 1013 97.92 413.52

increase of ion fluence as compression to pristine PC film. It has alsobeen observed that thermal stability of the irradiated filmdecreases with increase of fluence.

DTG curves of PC pristine and SHI beam irradiated films shownin Fig. 3. The decomposition temperature of PC pristine is 740 Kwhich is slightly higher than that of PC (3 � 1011 ions cm�2) and PC(3 � 1013 ions cm�1). Thus, the thermal stability of pristine PC issuperior to that of irradiated PC films.

3.3. Differential scanning calorimetry (DSC)

It has been observed that the structure of polycarbonate and glasstransition temperature (Tg) is significantly affected by SHI. Fig. 4shows the DSC thermograms of PC pristine and SHI irradiatedfilms. The pristinematerial showed a glass transition temperature ofabout 414 K which is in agreement with values reported in theliterature [29]. The data corresponding to second heating cycle wasused for comparison to avoid any previous thermal history effects. Agradual decrease in the glass transition temperature from about 414to 404 K was observed with the increase in the ion fluence from3� 1011 ions cm�2 and 3� 1013 ions cm�2. This decrease in the glasstransition temperature is generally due to the decrease in molecularmobility as a result of the scissoring of the polymer chains. Atextremely high fluence the cross-linking reaction arising from theinteraction between the scissioned species generates a highly cross-linked systemwhich does not exhibit any segmental mobility whichis reflected in the form of absence of glass transition temperature.

3.4. Thermally stimulated discharge current (TSDC)

TSDC spectra of PC pristine and SHI irradiated films withdifferent poling fields of 100, 150 and 200 kV cm�1 at 423 K areshown in Figs. 5, 6 and 7. The PC pristine and SHI irradiated filmsshow single peak (i.e. a-relaxation peak at glass transition region)at different ion fluences. Generally, the origins of TSDC in insulatingpolymers are dipolar polarization, space charge polarization andionic/electronic polarization etc. The dimerization of the carbonategroup, generation of new shallow/deep traps in the form ofconjugated bonds, an increase in water absorptibity and cross-linking between scissioned species are responsible for modifica-tion of thermally stimulated discharge current [30]. The presentresult shows only a-relaxation process is responsible for TSDC andno any new radiation induced relaxation processes is identified.However, Goyal et al. [31] recently observed the many relaxationprocess in polyetherimide caused by irradiation. The a-relaxationpeak of pristine and irradiated films of PC are distinguish infollowing way:

1. In irradiated samples peak shifted to the lower temperatureside and TSDC current increases with ion fluences at samepolling field (i.e. 100, 150 and 200 kV cm�1).

e Activation energy(eV)

Charge released(�10�12 coul.)

Relaxation ontime (s)

0.0856 421.89 764.05 � 10�16

0.0311 410.20 3415.11 � 10�16

0.0200 313.37 8095.22 � 10�16

0.1135 476.09 226.64 � 10�16

0.0440 454.40 2574.62 � 10�16

0.0304 374.93 7538.51 � 10�16

0.1563 583.06 103.33 � 10�16

0.0760 573.12 2359.53 � 10�16

0.0441 530.16 6629.73 � 10�16

B.S. Rathore et al. / Vacuum 86 (2011) 306e310310

2. However generally, in pristine and irradiated PC, peak shiftedto the higher temperature side and magnitude of currentincreases with increases the polling field as shown inFigs. 8e10.

The TSDC parameters such as activation energy, charge releasedand relaxation time are calculated by using the relations reported inthe literature [32]. Table 2 shows that the calculated value of acti-vation energy, charge released and relaxation time for PC pristineand ion beam irradiated films at different polling fields and fluences.After irradiation activation energy, charge released, peak tempera-ture and mobility decreases while relaxation time and peak currentincreases with increases of ion fluence. Generally a-relaxation peakshifted to the lower temperature side, activation energy, chargereleased decreases and relaxation time increases with ion fluence isdue to the scissioning/cross-linking of the polymer chains.

The TSDC is also a powerful technique to determine the glasstransition temperature of polymeric material as reported in liter-ature [33]. The observed TSDC peak in pristine and irradiatedsample is a direct evidence of glass transition temperature and wellagreed with the Tg observed by DSC.

It has been found that irradiation increases the shallow anddeep traps and dc electrical conductivity due to creation of newcharge carriers in the irradiated sample. These charge carriers arepositively charged and act as vacancies or holes, therefore, thecalculated value of mobility from the TSDC peak increases withincrease of irradiation and is evidence that large numbers ofvacancies are created at the surface and bulk of the polymer due toirradiation, which are responsible for current TSDC.

4. Conclusions

The irradiated polycarbonate films by 55 MeV Carbon (Cþ5)beam, has been investigated using XRD, TGA, DSC and TSDC to thestudy of thermal properties of polymer. XRD shows that the averagecrystallite size and percentage crystallinity is decreases, TGA showthe thermal stability decreases with increase of ion fluence, DSCshows the decrease of glass transition temperature due to irradia-tion. TSDC results revealed the decrease in glass transitiontemperature after irradiation indicates the correlation with theresults obtained from DSC. It has been concluded that the amor-phous nature of PC film increases with ion fluences due toscissoring/cross-linking of the polymer chains.

Acknowledgement

We are thankful to Prof. Ranjit Singh (Retd. from Department ofPhysics, Rani Durgavati University, Jabalpur, India) for his valuablesuggestions. The author’s (B.S.Rathore) gratefully acknowledge thefinancial support of Inter University Accelerator Center, New Delhi

India for pursuing of fellowship under the UFUP project. We arealso thankful to Dr. Fouran Singh, IUAC, New Delhi for their helpduring irradiation. Thank are also to Dr. A. M. Awasthi andMr. S. Bhardwaj, UGC-DAE, Indore (M.P.)-India, for providing DSCfacility.

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