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Conducting blends of polyaniline and aromatic main-chain liquid crystalline polymer, XYDAR SRT-900
Synthetic Metals 123 (2001) 69±72
Conducting blends of polyaniline and aromatic main-chain liquid crystalline polymer, XYDAR SRT-900 Zhihua Lua, Chaobin Hea,*, T.-S. Chunga,b a
Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602, Singapore Department of Chemical and Environmental Engineering, National University of Singapore 10 Kent Ridge Crescent, Singapore 119260, Singapore
b
Received 14 June 2000; received in revised form 25 August 2000; accepted 22 September 2000
Abstract Conductive polyaniline/fully aromatic main-chain liquid crystalline polymer, XYDAR SRT-900 ®lms have been prepared by solution blend of polyaniline (PANI) emeraldine salt and the liquid crystalline polymer. The polymer blends exhibit excellent mechanical properties and good conductivity at moderate PANI concentration. Moreover, polymer ®lms have optical transparent property at concentration lower than 10 wt.%. The morphology of the blends has been investigated using scanning electronic microscopy and X-ray scattering. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Liquid crystalline polymer; Conductive blends; XYDAR SRT-900
1. Introduction There is an increasing interest in electrically conductive polymeric materials. These materials have showed promising commercial viability in technological applications such as re-chargeable batteries, conductive coating or adhesive, plastics welding, chemical sensors, electromagnetic shielding, electrical and electrochemical devices, light emitting diodes (LEDs), anodic passivation, corrosion prevention of metals and electrochromic displays [1,2]. In order to achieve the speci®c properties, most of these are made combining conductive inorganic materials or intrinsically conductive polymers (ICPs) and matrix polymeric materials. Polyaniline (PANI) is one of the most promising ICPs due to its relatively high environmental and thermal stability and simple and economical production. Conductive polyaniline has been combined with a number of matrix polymers such as poly(ethyleneoxide) [3], poly(ethyene terephthalate) [4], poly(vinylchloride) [5,6], polystyrene [7], polyvinylalcohol [8], poly(methyl methacrylate) [9], and the like to achieve conductive composites. Liquid crystalline polymers (LCPs) are an important class of high performance materials with good thermal stability, excellent mechanical property and chemical resistance.
* Corresponding author. Tel.: 65-8748145; fax: 65-8727528. E-mail address: [email protected] (C. He).
However, very little have been done on the conductive liquid crystalline polymers mainly due to their intrinsic high melting temperatures. The conductive LCPs are derived mainly from solution blends. Recently, high modulus electrical conducting polyaniline composite ®bers have been prepared from air-gap spinning of lyotropic PANI/poly(pphenylene terephthalamide) [10]. In this research, transparent or semi-transparent conductive LCP ®lms are obtained from solution casting of and XYDAR SRT-900 LCP containing PANI complex (PANIS). The effect of the concentration of polyaniline on the conductivity, mechanical property and thermal stability of the resulting polymer composites is studied and the morphology and crystallinity of the polymer composites are also investigated. 2. Experimental section 2.1. Materials Conductive polyaniline emeraldine (PANIS) doped with sulfonic acid (s 6 S/cm, Mw > 15,000, particle size 3±100 mm) was purchased from Sigma±Aldrich company. Liquid crystal polymer, XYDAR SRT-900 is a copolymer of 4-hydroxybenzoic acid, 4,40 -biphenol and terephthalic acid and was obtained from Amoco Performance Products, Inc.
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 7 0 - 1
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Z. Lu et al. / Synthetic Metals 123 (2001) 69±72
Fig. 1. Structure of fully aromatic main-chain liquid crystal polymer, XYDAR SRT-900.
2.2. Preparation of blends films In a typical procedure, LCP polymer, XYDAR SRT-900 was dissolved in 3,5-bistri¯uromethylphenol at a concentration of 10 mg/ml and conductive polyaniline was added and dispersed in the solution. The mixture was cast on glass and dried in air. The resulting ®lm was further dried at 808C for 3 h and 1208C in vacuo for 2 h. 2.3. Dynamical mechanical analysis Dynamical mechanical test was performed using a DMA 2980 Analyser (TA instruments). Film samples were heated at a heating rate of 38C/min in air and the frequency of 1 Hz was used.
and terephthalic acid with mole ratio of 2:1:1 as shown in Fig. 1, i.e. XYDAR SRT-900. There are only a few solvents for the wholly aromatic main chain LC polymers. One of them is 3,5-bistri¯uromethylphenol. We found that it is also a good solvent for sulfonic acid doped polyaniline and high quality conductive LCP ®lm can be prepared from the solution. The ®lms are transparent if the PANIS is less than 10 wt.% in the blends. The mechanical properties of PANIS/XYDAR blends were evaluated by dynamic mechanical measurements. Figs. 2 and 3 show the storage modulus and loss modulus of the blends and the matrix liquid crystal polymer in the range of 30±1008C, respectively. The 5 wt.% PANIS blend exhibits a storage modulus of 7±8 GPa that is comparable to its matrix polymer of about 9 GPa. Further increase of PANIS content
2.4. Measurement of the electrical conductivity Electrical conductivity measurements were performed on the ®lm cast from solution using the usual four-probe technique under laboratory conditions. A Keithley 2000 electrometer was used as a voltmeter. 2.5. Thermal stability analysis TGA curves were obtained through Perkin Elmer TGA 7, which is equipped with a TAC 7/DX thermal analysis controller. The sample was heated at a rate of 208C/min in air. The point at which 5% weight of the polymer was lost was chosen as the decomposition temperature. 2.6. Wide angle X-ray diffraction
Fig. 2. Storage modulus of XYDAR polymer and PANIS/XYDAR blends in air with a heating rate of 38C/min and a frequency of 1 Hz.
Wide angle X-ray diffraction was carried out using a Philips X'Pert X-ray diffractometer and ®ltered Cu±Ka Ê ). The diffractometer was operated radiation (l 1:5418 A at 45 kV and 40 mA. The diffraction pattern of the ®lm was obtained by scanning the sample in a range of 2y 5 50 at a sampling step of 0.058 (2y). 2.7. Scanning electronic microscopy study Scanning electronic microscopy study was carried out using a Philips XL30 electronic microscope. 3. Results and discussion Liquid crystal polymer used in the present study is a random copolymer of 4-hydroxybenzenoic acid, 4,40 -biphenol
Fig. 3. Loss modulus of XYDAR polymer and PANIS/XYDAR blends in air with a heating rate of 38C/min and a frequency of 1 Hz.
Z. Lu et al. / Synthetic Metals 123 (2001) 69±72
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Fig. 4. Electrical conductivity of PANIS/XYDAR blends vs. PANIS loading.
Fig. 6. X-ray diffraction patterns of XYDAR and PANIS/XYDAR blends.
leads to a signi®cant drop of storage modulus. The storage modulus of ®lm containing 20 wt.% of PANIS is about 3 GPa and with a conductivity of 4 10 3 S/cm. The loss modulus of PANIS blends dramatically decreases with increasing the contents of conducting polyaniline. PANIS/ XYDAR blend containing 10 wt.% PANIS has a loss modulus of only 0.2 GPa while the matrix polymer without PANIS has a loss modulus in excess of 1.6 GPa at 308C. This can be attributed to the rigid chain character of PANIS. Fig. 4 shows that the conductivity of the blends dramatically increases with increasing the content of the PANIS. The percolation concentration for this conductive system seems at less than 5 wt.% of PANI. The conductivity of 0.004 S/cm can be achieved with 20 wt.% of PANIS. Further increase of PANI concentration would not lead to a signi®cant increase of conductivity as a conductive network has been completed. Higher conductivity up to 0.01 S/cm can be achieved using more PANIS but the mechanical properties deteriorate. The thermal stability of PANIS and XYDAR is signi®cantly different. TGA thermograph of PANIS, XYDAR and their blend are presented in Fig. 5. PANIS is stable blow 2008C. A signi®cant weight loss starts at 2208C, and the weight loss of 5% occurs at 2658C. It loses 30% of its weight
between 220 and 3408C. It suggests that this weight loss be due to a dopant loss. This thermal behavior of the PANIS is similar to that for other typical sulfonic acid doped aniline [11]. The polyaniline backbone degrades above 3408C. XYDAR is much stable than polyaniline due to its fully aromatic main-chain polymer structure. Its 5% weight loss temperature is 5108C. The overall TGA curve shape of PANIS/XYDAR blend is similar to a combination curve of its components, indicating that PANIS is a dominant factor affects the thermal stability of PANIS/XYDAR composites. The conductive PANIS/XYDAR blends were evaluated using wide angle X-ray scattering diffraction. Fig. 6 shows the X-ray diffraction signatures of the polymer composites with different PANI concentrations. The pure XYDAR ®lm exhibits a high degree of crystallinity. Its exhibits two main re¯ections, at 19.88, 21.48 and a shoulder peak at 19.18 (2y). For pure PANI-pTSA, the X-ray scattering is dominated by three main re¯ections at 2y 15 , 208, and 258, respectively. The structure of the crystalline plan is orthorombic [12]. For the PANIS/XYDAR blends, X-ray scattering signatures show re¯ections at 2y 12:9 , 13.78, 15.08, 19.38, and 20.98, which corresponds well to the scatterings from pure PANI-pTSA and XYDAR. The degree of cystallinity of PANIS/XYDAR blends, however, is lower than that of XYDAR, showing that PANIS may inhibit crystallization of liquid crystal polymers. The PANIS/XYDAR blend's morphology was studied with SEM. The micrographs of blends with 5 and 10 wt.% PANIS are presented in Fig. 7. Small particles with size in a range of 0.1±0.5 mm are observed in the 5 wt.% blend while PANIS particles with size of 1±2 mm are observed in 10 wt.% blend. It suggests that high conductivity of conductive polymer blends be related to high level of dispersability and structuring of the conductive polyaniline particles within the matrix polymer. The SEM micrographs of PANIS/XYDAR blends show that the dispersal level of the PANIS decreases with increasing the PANIS content. Structuring of doped polyaniline thus plays a more important role in achieving the desired conductivity of polymer blends.
Fig. 5. TGA thermograms of PANIS, XYDAR and PANIS/XYDAR blend containing 20 wt.% PANIS.
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Fig. 7. SEM micrographs of PANIS/XYDAR blend with 10 wt.% PANIS (left) and PANIS/XYDAR blend with 5 wt.% PANIS (right).
4. Conclusions
References
Conductive polyaniline/wholly aromatic main-chain liquid crystal polymer ®lms have been prepared by solution casting of mixture of polyaniline emeraldine salt and mainchain liquid crystal polymer, XYDAR SRT-900. The resulting blend ®lms exhibit excellent mechanical properties and suitable conductivity. The resulting conductive ®lm also exhibit good optical property. The storage modulus of the conductive LC polymer blends can be in excess of 3 GPa with a conductivity of 0.004 S/cm. Morphology studies indicate good dispersability of polyaniline emeraldine salt in XYDAR LC polymer matrix.
[1] J. Anand, S. Palaniappan, D.N. Sathyanarayana, Prog. Polym. Sci. 23 (1998) 993. [2] V.G. Kulkarni, ACS Symp. Ser. 735 (1999) 174. [3] L.S. Yang, Z.Q. Shan, P.M. Hou, W.X. Chen, L. Liu, J. Power Sources 44 (1993) 499. [4] S. Mitzakoff, P. De, A. Macro, Eur. Polym. J. 35 (1999) 1791. [5] L.W. Shacklette, C.C. Han, M.H. Luly, Synth. Met. 57 (1993) 3532. [6] Y. Roichman, M.S. Silverstein, A. Siegmann, M. Narkis, J. Macromol. Sci. Phys. B38 (1999) 145. [7] Y. Cao, G.M. Treacy, P. Smith, A.J. Heeger, Synth. Met. 57 (1993) 3526. [8] M. Morita, J. Polym. Sci. Polym. Phys. 32 (1994) 231. [9] P. Juvin, M. Hasik, J. Fraysse, A. Pron, I. Kulszewicz-Bajer, J. Appl. Polym. Sci. 74 (1999) 471. [10] C.H. Hsu, H. Shih, S. Subramoney, A.J. Epstein, Synth. Met. 101 (1999) 677. [11] M. Zilberman, G.I. Titelman, A. Siegmann, Y. Haba, M. Narkis, J. Appl. Polym. Sci. 66 (1997) 243. [12] M. Zilberman, A. Siegmann, M. Narkis, J. Macromol. Sci. Phys. B37 (1988) 301.
Acknowledgements The authors acknowledge ®nancial support from Institute of Materials Research and Engineering, Singapore.
Conducting blends of polyaniline and aromatic main-chain liquid crystalline polymer, XYDAR SRT-900 Zhihua Lua, Chaobin Hea,*, T.-S. Chunga,b a
Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602, Singapore Department of Chemical and Environmental Engineering, National University of Singapore 10 Kent Ridge Crescent, Singapore 119260, Singapore
b
Received 14 June 2000; received in revised form 25 August 2000; accepted 22 September 2000
Abstract Conductive polyaniline/fully aromatic main-chain liquid crystalline polymer, XYDAR SRT-900 ®lms have been prepared by solution blend of polyaniline (PANI) emeraldine salt and the liquid crystalline polymer. The polymer blends exhibit excellent mechanical properties and good conductivity at moderate PANI concentration. Moreover, polymer ®lms have optical transparent property at concentration lower than 10 wt.%. The morphology of the blends has been investigated using scanning electronic microscopy and X-ray scattering. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Liquid crystalline polymer; Conductive blends; XYDAR SRT-900
1. Introduction There is an increasing interest in electrically conductive polymeric materials. These materials have showed promising commercial viability in technological applications such as re-chargeable batteries, conductive coating or adhesive, plastics welding, chemical sensors, electromagnetic shielding, electrical and electrochemical devices, light emitting diodes (LEDs), anodic passivation, corrosion prevention of metals and electrochromic displays [1,2]. In order to achieve the speci®c properties, most of these are made combining conductive inorganic materials or intrinsically conductive polymers (ICPs) and matrix polymeric materials. Polyaniline (PANI) is one of the most promising ICPs due to its relatively high environmental and thermal stability and simple and economical production. Conductive polyaniline has been combined with a number of matrix polymers such as poly(ethyleneoxide) [3], poly(ethyene terephthalate) [4], poly(vinylchloride) [5,6], polystyrene [7], polyvinylalcohol [8], poly(methyl methacrylate) [9], and the like to achieve conductive composites. Liquid crystalline polymers (LCPs) are an important class of high performance materials with good thermal stability, excellent mechanical property and chemical resistance.
* Corresponding author. Tel.: 65-8748145; fax: 65-8727528. E-mail address: [email protected] (C. He).
However, very little have been done on the conductive liquid crystalline polymers mainly due to their intrinsic high melting temperatures. The conductive LCPs are derived mainly from solution blends. Recently, high modulus electrical conducting polyaniline composite ®bers have been prepared from air-gap spinning of lyotropic PANI/poly(pphenylene terephthalamide) [10]. In this research, transparent or semi-transparent conductive LCP ®lms are obtained from solution casting of and XYDAR SRT-900 LCP containing PANI complex (PANIS). The effect of the concentration of polyaniline on the conductivity, mechanical property and thermal stability of the resulting polymer composites is studied and the morphology and crystallinity of the polymer composites are also investigated. 2. Experimental section 2.1. Materials Conductive polyaniline emeraldine (PANIS) doped with sulfonic acid (s 6 S/cm, Mw > 15,000, particle size 3±100 mm) was purchased from Sigma±Aldrich company. Liquid crystal polymer, XYDAR SRT-900 is a copolymer of 4-hydroxybenzoic acid, 4,40 -biphenol and terephthalic acid and was obtained from Amoco Performance Products, Inc.
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 7 0 - 1
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Z. Lu et al. / Synthetic Metals 123 (2001) 69±72
Fig. 1. Structure of fully aromatic main-chain liquid crystal polymer, XYDAR SRT-900.
2.2. Preparation of blends films In a typical procedure, LCP polymer, XYDAR SRT-900 was dissolved in 3,5-bistri¯uromethylphenol at a concentration of 10 mg/ml and conductive polyaniline was added and dispersed in the solution. The mixture was cast on glass and dried in air. The resulting ®lm was further dried at 808C for 3 h and 1208C in vacuo for 2 h. 2.3. Dynamical mechanical analysis Dynamical mechanical test was performed using a DMA 2980 Analyser (TA instruments). Film samples were heated at a heating rate of 38C/min in air and the frequency of 1 Hz was used.
and terephthalic acid with mole ratio of 2:1:1 as shown in Fig. 1, i.e. XYDAR SRT-900. There are only a few solvents for the wholly aromatic main chain LC polymers. One of them is 3,5-bistri¯uromethylphenol. We found that it is also a good solvent for sulfonic acid doped polyaniline and high quality conductive LCP ®lm can be prepared from the solution. The ®lms are transparent if the PANIS is less than 10 wt.% in the blends. The mechanical properties of PANIS/XYDAR blends were evaluated by dynamic mechanical measurements. Figs. 2 and 3 show the storage modulus and loss modulus of the blends and the matrix liquid crystal polymer in the range of 30±1008C, respectively. The 5 wt.% PANIS blend exhibits a storage modulus of 7±8 GPa that is comparable to its matrix polymer of about 9 GPa. Further increase of PANIS content
2.4. Measurement of the electrical conductivity Electrical conductivity measurements were performed on the ®lm cast from solution using the usual four-probe technique under laboratory conditions. A Keithley 2000 electrometer was used as a voltmeter. 2.5. Thermal stability analysis TGA curves were obtained through Perkin Elmer TGA 7, which is equipped with a TAC 7/DX thermal analysis controller. The sample was heated at a rate of 208C/min in air. The point at which 5% weight of the polymer was lost was chosen as the decomposition temperature. 2.6. Wide angle X-ray diffraction
Fig. 2. Storage modulus of XYDAR polymer and PANIS/XYDAR blends in air with a heating rate of 38C/min and a frequency of 1 Hz.
Wide angle X-ray diffraction was carried out using a Philips X'Pert X-ray diffractometer and ®ltered Cu±Ka Ê ). The diffractometer was operated radiation (l 1:5418 A at 45 kV and 40 mA. The diffraction pattern of the ®lm was obtained by scanning the sample in a range of 2y 5 50 at a sampling step of 0.058 (2y). 2.7. Scanning electronic microscopy study Scanning electronic microscopy study was carried out using a Philips XL30 electronic microscope. 3. Results and discussion Liquid crystal polymer used in the present study is a random copolymer of 4-hydroxybenzenoic acid, 4,40 -biphenol
Fig. 3. Loss modulus of XYDAR polymer and PANIS/XYDAR blends in air with a heating rate of 38C/min and a frequency of 1 Hz.
Z. Lu et al. / Synthetic Metals 123 (2001) 69±72
71
Fig. 4. Electrical conductivity of PANIS/XYDAR blends vs. PANIS loading.
Fig. 6. X-ray diffraction patterns of XYDAR and PANIS/XYDAR blends.
leads to a signi®cant drop of storage modulus. The storage modulus of ®lm containing 20 wt.% of PANIS is about 3 GPa and with a conductivity of 4 10 3 S/cm. The loss modulus of PANIS blends dramatically decreases with increasing the contents of conducting polyaniline. PANIS/ XYDAR blend containing 10 wt.% PANIS has a loss modulus of only 0.2 GPa while the matrix polymer without PANIS has a loss modulus in excess of 1.6 GPa at 308C. This can be attributed to the rigid chain character of PANIS. Fig. 4 shows that the conductivity of the blends dramatically increases with increasing the content of the PANIS. The percolation concentration for this conductive system seems at less than 5 wt.% of PANI. The conductivity of 0.004 S/cm can be achieved with 20 wt.% of PANIS. Further increase of PANI concentration would not lead to a signi®cant increase of conductivity as a conductive network has been completed. Higher conductivity up to 0.01 S/cm can be achieved using more PANIS but the mechanical properties deteriorate. The thermal stability of PANIS and XYDAR is signi®cantly different. TGA thermograph of PANIS, XYDAR and their blend are presented in Fig. 5. PANIS is stable blow 2008C. A signi®cant weight loss starts at 2208C, and the weight loss of 5% occurs at 2658C. It loses 30% of its weight
between 220 and 3408C. It suggests that this weight loss be due to a dopant loss. This thermal behavior of the PANIS is similar to that for other typical sulfonic acid doped aniline [11]. The polyaniline backbone degrades above 3408C. XYDAR is much stable than polyaniline due to its fully aromatic main-chain polymer structure. Its 5% weight loss temperature is 5108C. The overall TGA curve shape of PANIS/XYDAR blend is similar to a combination curve of its components, indicating that PANIS is a dominant factor affects the thermal stability of PANIS/XYDAR composites. The conductive PANIS/XYDAR blends were evaluated using wide angle X-ray scattering diffraction. Fig. 6 shows the X-ray diffraction signatures of the polymer composites with different PANI concentrations. The pure XYDAR ®lm exhibits a high degree of crystallinity. Its exhibits two main re¯ections, at 19.88, 21.48 and a shoulder peak at 19.18 (2y). For pure PANI-pTSA, the X-ray scattering is dominated by three main re¯ections at 2y 15 , 208, and 258, respectively. The structure of the crystalline plan is orthorombic [12]. For the PANIS/XYDAR blends, X-ray scattering signatures show re¯ections at 2y 12:9 , 13.78, 15.08, 19.38, and 20.98, which corresponds well to the scatterings from pure PANI-pTSA and XYDAR. The degree of cystallinity of PANIS/XYDAR blends, however, is lower than that of XYDAR, showing that PANIS may inhibit crystallization of liquid crystal polymers. The PANIS/XYDAR blend's morphology was studied with SEM. The micrographs of blends with 5 and 10 wt.% PANIS are presented in Fig. 7. Small particles with size in a range of 0.1±0.5 mm are observed in the 5 wt.% blend while PANIS particles with size of 1±2 mm are observed in 10 wt.% blend. It suggests that high conductivity of conductive polymer blends be related to high level of dispersability and structuring of the conductive polyaniline particles within the matrix polymer. The SEM micrographs of PANIS/XYDAR blends show that the dispersal level of the PANIS decreases with increasing the PANIS content. Structuring of doped polyaniline thus plays a more important role in achieving the desired conductivity of polymer blends.
Fig. 5. TGA thermograms of PANIS, XYDAR and PANIS/XYDAR blend containing 20 wt.% PANIS.
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Z. Lu et al. / Synthetic Metals 123 (2001) 69±72
Fig. 7. SEM micrographs of PANIS/XYDAR blend with 10 wt.% PANIS (left) and PANIS/XYDAR blend with 5 wt.% PANIS (right).
4. Conclusions
References
Conductive polyaniline/wholly aromatic main-chain liquid crystal polymer ®lms have been prepared by solution casting of mixture of polyaniline emeraldine salt and mainchain liquid crystal polymer, XYDAR SRT-900. The resulting blend ®lms exhibit excellent mechanical properties and suitable conductivity. The resulting conductive ®lm also exhibit good optical property. The storage modulus of the conductive LC polymer blends can be in excess of 3 GPa with a conductivity of 0.004 S/cm. Morphology studies indicate good dispersability of polyaniline emeraldine salt in XYDAR LC polymer matrix.
[1] J. Anand, S. Palaniappan, D.N. Sathyanarayana, Prog. Polym. Sci. 23 (1998) 993. [2] V.G. Kulkarni, ACS Symp. Ser. 735 (1999) 174. [3] L.S. Yang, Z.Q. Shan, P.M. Hou, W.X. Chen, L. Liu, J. Power Sources 44 (1993) 499. [4] S. Mitzakoff, P. De, A. Macro, Eur. Polym. J. 35 (1999) 1791. [5] L.W. Shacklette, C.C. Han, M.H. Luly, Synth. Met. 57 (1993) 3532. [6] Y. Roichman, M.S. Silverstein, A. Siegmann, M. Narkis, J. Macromol. Sci. Phys. B38 (1999) 145. [7] Y. Cao, G.M. Treacy, P. Smith, A.J. Heeger, Synth. Met. 57 (1993) 3526. [8] M. Morita, J. Polym. Sci. Polym. Phys. 32 (1994) 231. [9] P. Juvin, M. Hasik, J. Fraysse, A. Pron, I. Kulszewicz-Bajer, J. Appl. Polym. Sci. 74 (1999) 471. [10] C.H. Hsu, H. Shih, S. Subramoney, A.J. Epstein, Synth. Met. 101 (1999) 677. [11] M. Zilberman, G.I. Titelman, A. Siegmann, Y. Haba, M. Narkis, J. Appl. Polym. Sci. 66 (1997) 243. [12] M. Zilberman, A. Siegmann, M. Narkis, J. Macromol. Sci. Phys. B37 (1988) 301.
Acknowledgements The authors acknowledge ®nancial support from Institute of Materials Research and Engineering, Singapore.