polystyrene double layered samples

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

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Investigation of thermally stimulated properties of SHI beam irradiated polycarbonate/polystyrene double layered samples Bhupendra Singh Rathore a,b, Mulayam Singh Gaur a,⇑, Kripa Shanker Singh b a b

Department of Physics, Hindustan College of Science & Technology, Farah, Mathura 281122, UP, India 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 2011 Received in revised form 3 September 2011 Available online 8 September 2011 Keywords: Double layered Ion beam TSDC DSC TGA

a b s t r a c t The double layered samples of polycarbonate/polystyrene (PC/PS) have been prepared by solvent casting method and irradiated with 55 MeV C5+ beam at different ion fluences range from 1  1011 to 1  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 of PC/PS pristine and irradiated double layered samples have been investigated by thermally stimulated discharge 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 activation energy and relaxation time decrease, while the depolarization current and charge released increase with increase in the ion fluences. DSC curve show the glass transition temperature (Tg) and heat capacity decreases with increase in the ion fluences. The TGA characteristics represent the thermal stability, which is found to be decreased with increase in the ion fluences. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Polystyrene and polycarbonate thin films show a lot of advantages for use in capacitors and sensors as a functional polymer with excellent electrical and dielectric properties as well as improved chemical resistance. The polystyrene capacitor is not for use in high frequency circuits, because they are constructed like a coil. They are used well in filter circuits or timing circuits which run at several hundred kHz or less. Polycarbonate thin film used as a dielectric, when stable, low-loss component is needed, polycarbonate film capacitor is best suited. Polycarbonate would normally be specified for frequency-sensitive circuits such as filters and timing functions. It is also a good general-purpose dielectric for higher power use [1,2]. However, PC/PS double layered sample is not yet been reported for application as a capacitor and sensor. Swift heavy ion beam irradiation is an effective technique for the modification of various properties of the polymeric material such as electrical, optical and mechanical properties, etc. The double layered polymeric samples were playing a very important role in the field of electrical properties of polymeric insulating materials [3] such as interfacial phenomena or inter phase separation as charge-carrier transport and storage. Polymeric double layered samples were used in capacitors, micro electromechanical systems ⇑ Corresponding author. E-mail addresses: [email protected] (B.S. Rathore), mulayamgaur@ rediffmail.com (M.S. Gaur). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.09.003

(MEMS) devices, energy storage devices for memory backups, digital communication, electric vehicles, integrated optic polymer devices, etc. [4,5]. The polymer–polymer interface acts as a chargecarrier trapping site. Therefore, it is essential to study the effect of double layered polymeric samples interface on charge-carrier transport and storage because most practical insulators are heterogeneous/homogeneous polymeric systems [6–8]. A number of poly(alkyl acrylate) and poly(methyl acrylate) systems were irradiated by ion beam to study the glass transition temperature, thermal stability and thermally stimulated relaxation properties and subsequent cross linking [9–17]. However, these effects are rarely reported in case of double layered polymeric system. The interfacial phenomena as charge-carrier transport and storage mechanism of polymeric double layered samples can be studied with the help of thermally stimulated discharge current (TSDC), differential scanning calorimetry (DSC) and thermogravimetric analysis (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 parameter to investigate the thermal, optical and mechanical properties of polymeric double layered samples. In this study we are selected polycarbonate (PC) and polystyrene (PS) because, there are many similarities 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 to prepare polymeric double layered samples.

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

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

The chemical processes that happen during and after the ion implantation are not explained sufficiently in case of single layer as well as double layered polymer samples. It is shown that cross-linking unsaturated bond formation and chain scissoring including gas evolution take place during carbon ion bombardment in polymers. Therefore, in this study, PC/PS double layered samples were irradiated with 55 MeV C5+ beams using 15 UD Pelletron at Inter University Accelerator Center (IUAC), New Delhi, India. The effects of ion beam induced changes have been investigated by using TSDC, DSC and TGA thermal techniques.

2. Experimental

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

2.1. Sample preparation 2.2. Ion beam irradiation Polycarbonate (PC) and polystyrene (PS) pellets were procured from Redox, India. Thin films of the polycarbonate and polystyrene were separately prepared by the solvent casting method. The dichloromethane (DCM) and benzene are used as a solvent for PC and PS respectively. The 5 g of PS pellets were dissolved in 50 ml of benzene (solvent) at 323 K and stirred for 3 h. The transparent solution of PS was poured onto an optically plane glass plate at a room temperature for 24 h. Then the PS film was peeled off from the glass plate and similar method was used for preparation of PC film [30,31]. The PS and PC films were further dried at room temperature with outgassing of 105 torr for a further period of 24 h to remove volatile residual solvent. For double layered sample, PC and PS films were compressed together by compression molding machine (Model-CE 100DDS) at a temperature of 373 K and pressure of 2500 lb in.2 (17.25 mPa).

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

The double layered polymeric samples (30 lm thickness) of the size (1  1 cm2) were mounted on the copper ladder and irradiated with 55 MeV C5+ ion beams at different fluences range from 1  1011 to 1  1013 ion/cm2 using the 15 UD pelletron facility in the general purpose scattering chamber (GPSC) under high vacuum of 5  106 torr at Inter-University Accelerator Centre (IUAC), New Delhi. The beam current was kept low to suppress thermal decomposition and was monitored intermittently with the help of a Faraday cup. 2.3. Thermally stimulated discharge current (TSDC) To investigate the thermo-electrets behavior of pristine and ion beam irradiated polymeric double layered samples with the help of

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

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(TP) 423 K (above the glass transition temperature Tg) for 1 h. The polymeric double layered samples were then cooled to the room temperature for 1/2 h in the presence of dc fields (EP) then the samples were short-circuiting the electrodes for about 15 min to eliminate the surface stray charges accumulated during polarization. The depolarization currents of polymeric samples were measured by electrometer (Kethley-6514) at a linear heating rate of 3 K/ min. The depolarization’s current were recorded in the temperature range from 300 to 443 K. 2.4. Differential scanning calorimetry (DSC) Fig. 6. TSDC characteristics of double layered (1  1013 ion/cm2) samples at 423 K with polarizing fields of 50, 100 and 150 kV/cm.

TSDC. TSDC is a very useful technique to understand the behavior of dielectric relaxation, glass transition temperature (Tg), charge storage and transport mechanism of electrons/holes of the polymeric samples, this techniques was largely described in detail elsewhere [32–35]. For good ohmic contact, both the surface of the polymeric samples were vacuum aluminized using vacuum equipment (VEQCO) Delhi, India. Vacuum coating unit with penning and pirani pressure gauges, ST-A6P3; over central circular area of diameter 0.8 cm, both sides vacuum aluminized polymeric samples have been used for TSDC. The sample holder forming aluminum – polymeric sample – aluminum system was placed in an ‘Ambassador’ oven which is programmed to linear rise of temperature. Pristine and ion beam irradiated polymeric double layered samples were polarized by subjecting with the desired polarization fields or dc fields (EP) of 50, 100 and 150 kV/cm at a constant temperature

DSC (TA-Instruments model 2910) was used to investigate the glass transition temperature (Tg) of pristine and ion beam irradiated polymeric double layered samples. The scanning rate of heating and cooling was performed at a 10 K/min in a nitrogen atmosphere with the temperature range from 300 to 700 K for all measurements. The pristine and ion beam irradiated polymeric double layered samples were cut into small pieces and the pieces put into the empty aluminum pans and weighed in micro-balance up to an accuracy of 10 ppm .The mass of the polymeric sample was kept in 10–12 mg and aluminum pans used with empty pan were weighed to a mass of 0.002 mg. 2.5. Thermogravimetric analysis (TGA) TGA (TA-Instruments model Q500) was used to investigate the stability/decomposition temperature of pristine and ion beam irradiated polymeric double layered samples. The pristine and ion beam irradiated polymeric double layered samples were cut into

Fig. 7. Activation energy, relaxation time and charge released Vs ion fluences, for pristine and SHI beam irradiated PC/PS double layered samples at 423 K with polarizing fields of 50, 100 and 150 kV/cm.

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small pieces and the pieces put into the empty aluminum pans and weighed in micro-balance up to an accuracy of 10 ppm. The mass of the polymeric sample and empty aluminum pans were 10– 12 mg and 0.002 mg, respectively. These studies were carried out in a nitrogen atmosphere with scanning rate of heating and cooling was 10 K/min and the temperature range from 300 to 700 K for all measurements. 3. Results and discussion Figs. 1–6 illustrate the thermally stimulated depolarization current (TSDC) spectra of pristine and ion beam irradiated polymeric double layered samples of different ion fluences and various polarization fields (i.e. EP = 50, 100 and 150 kV/cm) at 423 K. The TSDC curves of pristine and ion beam irradiated polymeric samples show two peaks. The first peak was observed in the temperature region 405–420 K and another peak was observed in the temperature region 364–374 K. The first peak is also known as arelaxation (i.e. higher temperature side) and second peak is known as b-relaxation (i.e. lower temperature side). Since a-relaxation is found in the glass transition temperature region of PC and b-relaxation is observed in the glass transition temperature region of PS. The a and b-relaxation are also observed by DSC in same temperature region as observed by TSDC. These results confirmed that double layered polymeric sample have good arrangement of PC and PS. From Figs. 1–3 show a and b-relaxation peaks are shifted

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to the lower temperature side and depolarization currents are increase with ion fluences due to the scissoring of the polymer chains. Figs. 4–6 illustrate the different polarization fields effects at 423 K on pristine and ion beam irradiated polymeric films. It is clear from Figs. 4–6 that a and b-relaxation peaks are shifted to the higher temperature side due to the space-charge depolarization. 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 ion beam fluence. The ion implantation in double layered samples modified the interface due to changes of macroscopic properties of polymer surfaces through ion-beam induced chemical modifications on a molecular level. Therefore, the hole concentration is assumed to be increased. The hole concentration corresponding to the resistivity and trap density [36] of sample in the temperature range of this experiment is decreases (i.e. 1018–1020 cm3) as observed in present study. The trap density is observed to be approximately 1019 cm3. This value is similar as reported by Schwarz et al. [37]. For simplicity we also assume that the recombination coefficient is the same for electron–hole recombination as for electron trap capture. Under these conditions, it is statistically impossible for a detrapped electron to contribute to a thermally stimulated depolarization current (TSDC) instead of recombining with a hole. Hence, the measured TSDC is due to detrapped holes. The plot of current I versus temperature T are typical TSDC curve.

Fig. 8. DSC characteristics pristine and SHI beam irradiated PC/PS double layered samples.

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The activation energy, relaxation time and charge released versus ion fluences shown in Fig. 7(a)–(f). The activation energy and relaxation time decrease while charge released increase with ion fluence due to irradiation of the carbon ion beam. The activation energy Ea of the traps engaged in the TSDC procedure have been calculated. Two methods for evaluating E were used. First, we used the Bucci–Fieschi–Guidi (BFG) method [38] to calculate the relaxation time of holes by taking advantage of the whole TSDC curve: 1

R1

sðTÞ ¼ b

t

IðtÞdt IðtÞ

ð1Þ

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

s ¼ s0 exp



Ea kT

 ð2Þ

where Ea is the activation energy, and k is Boltzmann’s constant, it was calculated from the slope of ln t against 1/T that the average activation energy (Ea) of a and b-relaxation peaks are about 1.02 and 0.40 eV, respectively. The relaxation time corresponding to TSDC peaks s was computed using Eq. (1), which are decreases with increase in ion fluence. The decrease of s is correlated with the decrease of Tm  Tp, where Tm is the peak temperature and Tp is polarization temperature. It is actually with the modification of the type of dipoles aligned by the field due to ion implantation. The relaxation time varies from 1.1074  1016 to 1.0762  1016 s for a peak and 0.0615  1016 to 0.0297  1016 s for b peak. This variation of the relaxation time describes (i) the distribution of the relaxation times, and (ii) the increase of the molecular motion with increase of ion fluence. It must be emphasized that there is the significant variation for activation energy and relaxation time s, proving a wide distribution of relaxation times. Although the variation of relaxation time is in good agreement with Arrhenius law underlying the fact that there are mono-energetic processes, s changes less from one relaxation process to the nearby one, but it changes significantly for multiple relaxation process as observed in pristine and ion beam irradiated double layered samples. In order to discuss the fundamental aspects for application of double layered sample of PC and PS, the equivalent frequency was calculated from TSDC peak. Using the mean relaxation time of 1.0932  1016 and 0.0471  1016 s, the equivalent frequency of the TSDC experiment was calculated by using the following relation [39–44].

f ¼

1 2pt s

ð3Þ

The frequency is equal to in order of 106 Hz for a peak and b peak. This value of frequency is less than the pristine sample of PS and PC. Therefore, the double layered sample is low frequency dielectric and useful for capacitor and sensor application [43,45,46]. Differential scanning calorimetry measurements in exothermic pattern on pristine and ion beam irradiated sample showed two glass transition temperatures, one corresponding to Tg of polystyrene and other corresponding to polycarbonate (Fig. 8). Fig. 8 also present that the derivative of heat flow against temperature shows the two sharp peaks. The position of peak shifted towards lower temperature side with increase in fluence. The glass transition temperature (Tg) was well observed by first order derivative of heat flow curves of the pristine and ion beam irradiated polymeric double layered samples of different ion fluences are represent in Fig. 8(a)–(f).

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

The heat capacity decreasing with ion fluence was observed may be due to the formation of disordered polymeric material by ion beam irradiation. Our results also indicate that the glass transition temperatures (Tg) are decrease with increase in the ion fluences as compare to pristine sample. These results indicate on an increase in the content of amorphous phase with increase in the ion fluences. The decrease in the glass transition temperatures (Tg) and heat capacity are generally due to the increase in molecular 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 and 415 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 at about 414 K for the high molecular weight (HMW) PC to a glass transition. The value of the LMW polystyrene and HMW polycarbonate Tg is in good agreement with the value reported in literature. Secondly, the narrow transition occurring between 372– 368 K (b-relaxation) and 414–406 K (a-relaxation) are tentatively assigned to a phase-disordering transition caused by high energy ion beam. Thermogravimetric analysis (Fig. 9) shows the thermal stability/ weight loss of pristine and ion beam irradiated polymeric double layered samples. The first order derivative curve (i.e. DTA) of the TGA thermograms is shown in Fig. 9. TGA analysis show the thermal 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 irradiated double layered samples as compare to pristine sample. All of the molecules showed low thermal stability, the thermal stability of double layered samples is gradually decreases with increase of

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fluence. Most of the weight loss occurred in the temperature range of 600–750 K, and almost no further weight change occurred above 750 K. The high energy ion beam degraded the bulky phenyl group of PC in double layered samples, therefore, weight loss is significantly affected in ion beam irradiated double layered samples and showing poor thermal stability. It is further understand that the decrease in thermal stability in polymeric double layered samples can be attributed to chain-scissoring in the polymer due to high energy ion beam irradiation. 4. Conclusion TSDC show the a, b-relaxation peaks shifted to the lower temperatures sides, activation energy and relaxation time decreases while the depolarization current and charge released decreases with increases in the ion fluences due to the chain scissoring/cross linking is the amount of electronic energy loss. The ion-beam induced chemical modifications on a molecular level enhance the hole concentration in ion beam irradiated samples. Therefore, observed TSDC is not only due to the electronic charge carriers but also due to the recombination of electron and holes, which is responsible for TSDC peaks. The decrease in the glass transition temperature (Tg) from about 413 to 405 K (a-relaxation) and 371–368 K (b-relaxation) samples as evident from the DSC thermal analysis, due to the chain-scissoring in the polymeric samples by the ion beam irradiation. Thermogravimetric analysis show the decrease in the thermal stability and degradation temperature of irradiated polymeric double layered samples as compare to pristine sample due to incorporation of ions into the double layered samples. Acknowledgements The authors (B.S. Rathore and K.S. Singh) gratefully acknowledge the financial support of Inter University Accelerator Center (IUAC), New Delhi-India for pursuing of fellowship under the UFUP project. We are also thankful to Dr. Fouran Singh, Inter-University Accelerator Centre (IUAC), New Delhi-India for their help during irradiation. Authors are highly grateful to Prof. (Retd) Ranjit Singh, Department of Physics, Rani Durgavati University Jabalpur (MP) India for his valuable suggestion. References [1] T. Williams, The Circuit Designer’s Companion, Elsevier, Jardan Hill, Oxford OX2 8DP, Burlington, 2005. [2] A.J. Peyton, V. Walsh, Analog Electronics with OP-Amps: A Source Book of Practical Circuits, Cambridge University press, New York, USA, 1993. [3] Y. Suzuoki, G. Cai, T. Mizutani, M. Ieda, Jpn. J. Appl. Phys. 21 (1982) 1759. [4] M. Kryezewski, Polym. Sci. Polym. Symp. 50 (1975) 359. [5] C.A. Mead, Appl. Phys. 33 (1961) 646.

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[6] J.V. Turnhout, in: G.M. Sessler (Ed.), Electrets, Springer-Verlag, Berlin Heidelberg, 1980. [7] P. Shukla, M.S. Gaur, J. Appl. Polym. Sci. 114 (2009) 222. [8] R.H. Page, M.C. Jnrich, A. Sen, R.J. Twieg, J.D. Swalen, G.C. Bjorklund, G.C. Wilson, J. Optics. Sot. Am. 7 (1990) 1239. [9] R. Kumar, H.S. Virk, K.C. Verma, Udayan De, A. Saha, R. Prasad, Nucl. Instr. and Meth. B 251 (2006) 163. [10] G. Goyal, M. Garg, J.K. Quamara, Rad. Eff. Defects Solids 163 (2008) 131. [11] T. Prabha, J.K. Quamara, Rad. Eff. Defects Solids 160 (2005) 187. [12] G. Goyal, J.K. Quamara, Rad. Eff. Defects Solids 163 (2008) 933. [13] R. Kumar, S.A. Ali, P. Singh, U. De, H.S. Virk, R. Prasad, Nucl. Instr. and Meth. B 269 (2011) 1755. [14] R.M. Papaléo, M.A. de Araújo, R.P. Livi, Nucl. Instr. and Meth. B 65 (1992) 442. [15] Y. Takahashi, M. Yoshida, Asano, M. Notomi, T. Nakagawa, Nucl. Instr. and Meth. B 217 (2004) 435. [16] N.L. Singh, A. Qureshi, F. Singh, D.K. Avasthi, Mater. Sci. Eng. A 457 (2007) 195. [17] M.R. Rizzatti, M.A.D. Araújo, R.P. Livi, Nucl. Instr. and Meth. B 91 (1994) 450. [18] T. Tanaka, S. Hayashi, K. Shibayama, J. Appl. Phys. 48 (1977) 3478. [19] S. Tanaka, S. Hayashi, Hirabayashi, K. Shibayama, J. Appl. Phys. 49 (1978) 2490. [20] M.S. Gaur, B.S. Rathore, P.K. Singh, A.P. Indolia, A.M. Awasthi, S. Bhardwaj, J. Thermal Anal. Calor. 101 (2010) 315. [21] T. Mizutani, M. Ieda, S. Ochiai, M. Ito, J. Electrostatics 12 (1982) 427. [22] V.M. Gun’ko, M.V. Borysenko, P. Pissis, A. Spanoudaki, N. Shinyashiki, I.Y. Sulim, T.V. Kulik, B.B. Palyanytsya, Appl. Surf. Sci. 253 (2007) 7143. [23] P. Shukla, M.S. Gaur, Polym. Int. 58 (2009) 1314. [24] J. Vanderschueren, J. Gasiot, in: P. Braunlich (Ed.), Thermally Stimulated Current, Springer-Verlag, Berlin, 1979. [25] P. Hedvig, Dielectric Spectroscopy of Polymers, Adam Hilger Ltd., Bristol, 1977. [26] R. Chen, Y. Krish, Analysis of Thermally Stimulated Processes, Oxford, 1981. [27] A. Rudin, The Elements of Polymer Science and Engineering, Academic Press, An imprint of Elsevier, USA, 1999. [28] A. Ram, Fundamental of Polymer Engineering, A Division of plenum Publishing Corporation, New York, 1997. [29] C.A. Danilels, Polymer Structure and Properties, Technomic Publishing Corporation, 1989. [30] B.S. Rathore, M.S. Gaur, K.S. Singh, Vacuum 86 (2011) 306. [31] B.S. Rathorea, M.S. Gaur, F. Singh, K.S. Singh, Rad. Eff. Defects Solids, 2011. http://dx.doi.org/10.1080/10420150.2011.586034. [32] G. Teyssedre, S. Mezghani, A. Bernes, C. Lacabanne, American Chemical Society, Washington,1997. [33] A.K. Kalkar, S. Kundagol, Polym. Int. 48 (1999) 85. [34] V. Mishra, R. Nath, Polym. Int. 49 (2000) 699. [35] J.P. Ibar, Fundamentals of Thermal Stimulated Current and Relaxation Map Analysis, SLP Press, New Canaan, 1993. [36] W. Shockley, Electrons and Holes in Semiconductors, Van Nostrand Reinhold, New York, 1950. [37] R. Schwarz, F. Wang, M.B. Chorin, S. Grebner, A. Nikolov, F. Koch, Thin Solid Films 255 (1995) 23. [38] A.G. Milnes AG, Deep Impurities in Semiconductors, Wiley, New York, 1997. [39] J.V. Turnhout JV, Thermally Stimulated Discharge of Polymer Electrets, Elsevier, Amsterdam, 1975. [40] P. Hedvig, Dielectric Spectroscopy of Polymers, Adam Hilger, Bristol, 1977. [41] P. Pissis, A.A. Konsta, L. Apekis, D.D. Diamanti, C. Christodoulides, J. Non-Cryst. Solids 131/133 (1991) 1174. [42] R. Neagu, N.M. Mendes, D.K. Das-Gupta, R.M. Neagu, R. Igreja, J. Appl. Phys. 82 (1997) 2488. [43] G. Teyssedre, P. Demont, C. Lacabanne, J. Appl. Phys. 79 (1996) 9258. [44] P. Fischer, P. Röhl, J. Polym. Sci. 14 (1976) 543. [45] E.R. Neagu, J.N.M. Mendes, Jpn. J. Appl. Phys. 40 (2001) 810. [46] G. Teyssedre, P. Demont, C. Lacabanne, in: J.P. Runt, J.J. Fitzgerald (Eds.), Dielectric Spectroscopy of Polymeric Materials, American Chemical Society, Washington, DC, 1997. [47] J.A. Dean, McGraw Hill Publication Company, New York, 1987. [48] P. Chartoff, A.K. Sincar, in: E. Tori (Ed.), Thermal Characterization of Polymeric Materials, Academic Press, 1997. [49] D. Trifonova, J. Varga, G.J. Vanesco, Polym. Bull. 41 (1998) 341. [50] A.Y. Goldman, C.J. Copsey, Resource Innovation 3 (2000) 302.