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clay nanocomposite
Materials Letters 59 (2005) 648 – 651 www.elsevier.com/locate/matlet
The influence of irradiation on morphology evolution and flammability properties of maleated polyethylene/clay nanocomposite Hongdian Lua, Yuan Hua,*, Junfeng Xiaoa, Qinghong Konga, Zuyao Chenb, Weicheng Fana a
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui, 230026, PR China b Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026, PR China Received 27 June 2004; received in revised form 21 October 2004; accepted 28 October 2004 Available online 14 November 2004
Abstract The morphology and flammability properties of g-ray irradiated maleated polyethylene (PE)/clay nanocomposite were investigated using X-ray diffraction (XRD), transmission electron microscope (TEM) and Cone calorimeter. The exfoliation of clay in the nanocomposites before and after irradiation has been verified by XRD and TEM studies. TEM images demonstrated that the morphology of the nanocomposite evolved via three stages under irradiation: ideally face–face ordered exfoliation followed by a completed disordered dispersion of stacked layers and relative ordered distribution of stacked layers. Cone calorimetry results exhibited that the improvement in heat release rate for irradiated materials were suppressed by the nanodispersion of clay layers, especially at high irradiation dose level. D 2004 Elsevier B.V. All rights reserved. Keywords: Polyethylene; Nanocomposite; Irradiation
1. Introduction The polymer nanocomposites prepared by melt intercalation of polymer chains into the galleries of organically modified clay have been extensively investigated because they often exhibit a wide range of improved properties over their bulk counterparts [1–4]. The clay commonly used for the produce of nanocomposites is montmorillonite (MMT), which consists of two-dimensional layers where two fused silica tetrahedral sheets sandwiching an edge-shared octahedral sheet of either alumina or magnesia. As far as we are aware, apart from that used for fabrication of nanocomposites, MMT has been used as engineering barriers in storages of radioactive wastes due to its stable crystal structure which can accumulate high doses of irradiation without distinct damage.[5,6] Therefore, it can be expected that the
* Corresponding author. Tel./fax: +86 551 3601664. E-mail address: [email protected] (Y. Hu). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.10.057
penetration of g-ray in the polymeric matrix will be hindered by the nanodispersion of MMT layers. Polyethylene (PE) is an important engineering resin, widely used in the wire and cable industries. For the past decades, works on various aspects of PE/clay nanocomposites have been done expect to investigate their structure– property relationships [7–11]. Furthermore, irradiationinduced intermolecular cross-linking of polymers in the presence of high-energy irradiation (gamma or electron beam) has been employed to upgrade their properties.[12– 14] However, in contrast to the significant amount of reports on the development of polyethylene exposed by irradiation and on the PE/clay nanocomposites, articles on the influence of irradiation on the structure and properties of PE/clay nanocomposites are lacking. It is known that it is difficult to fabricate PE-based nanocomposites, and maleic anhydride-grafted PE (MAPE) is often introduced to promote the interaction between PE chains and the organically modified montmorillonite (OMT). For simplicity, in the present work, the pure maleic anhydride-grafted polyethylene/clay nanocomposite was prepared by direct melt intercalation. The resulting nano-
H. Lu et al. / Materials Letters 59 (2005) 648–651
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composite was then irradiated with 60Co g-ray for model study. The aim of this article is to investigate the influence of irradiation on the morphology evolution and flammability characteristics of nanocomposites.
2. Experimental section 2.1. Materials and preparation The maleated polyethylene (MAPE) with approximately 1.0 wt.% maleic anhydride was supplied by Haier Kehua, China. Organophilic montmorillonite (OMT), which has been ion-exchanged with hexanecyl trimethyammounium bromide ions (C16) was kindly provided by Keyan Chemistry. MAPE (dried) and 5 wt.% OMT (dried) were premixed then blended by a two-roll mill (XK-160, JiangShu, China) at 165 8C. The resulting nanocomposite was then compressed molded into sheets (1 and 3 mm thickness). All these specimens were irradiated at room temperature in a 2.221015 Bq 60Co g-ray source with 67.5 Gy/min irradiation dose rate in air atmosphere. 2.2. Morphology studies X-ray diffraction (XRD) patterns were performed on the 1-mm-thick film with a Japan Rigaku D/Max-Ra rotating anode X-ray diffractometer equipped with a Cu Ka tube and Ni filter (k=0.1542 nm). Transmission electron microscopy (TEM) images were obtained on a Jeol JEM-100SX transmission electron microscope with the acceleration voltage of 100 kV. The nanocomposite specimen was cut at room temperature using an ultramicrotome (Ultracut-1, UK) with a diamond knife from an epoxy block, with the films of the nanocomposite embedded. 2.3. Cone calorimeter studies Cone calorimetry experiments were evaluated, following the procedure defined in ISO 5660, on 3-mm-thick 100100 mm2 plates at an incident heat of 35 kW/m2. The cone data reported are reproducible to within about F10%.
3. Results and discussion 3.1. XRD characteristics and morphology evolution The XRD patterns of MAPE/OMT composites with different irradiation doses are displayed in Fig. 1. The results demonstrated that all composites show the same feature. The disappearance of the d 001 peak derived from the interlayer spacing of MMT reveals that almost complete exfoliation of clay had taken place. The nanoscale structure
Fig. 1. XRD patterns of MAPE and 5 wt.% OMT nanocomposites with different irradiation doses.
of the exfoliated nanocomposites elucidated by TEM is shown in Fig. 2 (a–d). For each sample, the clay layers are fine dispersed throughout MAPE matrix; however, the morphology evolves as irradiation dose increases. Fig. 2a shows that the clay layers are well aligned at the whole area with some disorder at the local in the nonirradiated sample, which is caused by the directional mechanical shear on the extrusion process. The ideally face–face ordered exfoliation with homogeneous dispersion of clay layers are observed throughout the 50-KGy irradiated sample (Fig. 2b). The probable explanation may be that the species such as free radicals and ions generated initially under irradiation have high mobility, they may diffuse in and/or out from the clay galleries. This diffusivity state will go on for quite a long time due to the low experimental irradiation dose rate, which may be favorable of the improvement of the steric interaction between clay layers. This improvement is conducive to making ordered exfoliated structure within polymer since the periodicity of the clay layers having anisotropic is originated from the steric interaction.[7] On the other hand, those species mainly macromolecular cations may be terminated by the surfaces of negatively charged silicate layers to form dangling polymer chains. This results in a new conformation at polymer-clay interface, which may partly contribute to the formation of the ordered exfoliated structure. With increasing irradiation to 100 KGy doses, a morphological conversion occurred, where the orientation of silicate layers are completely destroyed, and the stacked clay layers randomly distribute in MAPE matrix, as shown in Fig. 2c. There are two possible ways of the stack of the initially well-dispersed clay layers in the matrix under irradiation. One is that the cross-linking of polymer chains which attached onto the layers take place effectively overwhelms the steric interaction between clay layers resulting in the clay stack. Another may be that the recession of the polymer chains from the clay galleries with
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Fig. 2. TEM micrographs of MAPE and 5 wt.% OMT nanocomposites with irradiation doses: (a) nonirradiated, (b) 50, (c) 100 and (d) 150 KGy.
dewetted clay layers left, which also lead to the clay stack again each other. This is because the driving force of the intercalation between maleated polymer and layered silicates originates from strong hydrophilic interaction between maleic anhydride group and polar silicates [15], whereas the maleic anhydride groups will produce a ring opening followed by the formation of carbonylated volatile products under irradiation. However, the cross-linked networks generated in noncrystallite regions of PE restrict the congregation of these stacks, keeping them randomly disperse throughout the matrix. Fig. 2(d) shows the TEM micrograph of the sample with 150 KGy doses. The distribution of silicate layers is likely to be located at between Fig. 2a and c. This is because the scission and oxidation of polymer chains is considerably intensified at high doses. [12] The destruction of polymer chains may help to enhance the steric interaction, contrarily, the cross-linked networks restrict the mobility of silicate
layers. As a result, relative ordered structure is formed. Similar features of TEM micrograph for the 200-KGy irradiated sample were not shown here. 3.2. Flammability The reduction of heat release rate (HRR), particular peak HRR has been found to be the most clear-cut evidence for the efficiency of a flame retardant. The comparison of the HRR behavior of pure MAPE and its nanocomposite before and after irradiated is presented in Fig. 3. The MAPE and MAPE/OMT samples show the similar features, for the irradiated samples slightly decrease the ignition delay time and enhance the peak HRR compared to their corresponding nonirradiated sample. This is in contrast to the expectation since the cross-linkings easily form conjugated systems and aromatic rings during combustion, which results in the increased char formation in the condensed phase and thus
Fig. 3. The HRR curves with different irradiation doses for (A) MAPE and (B) MAPE and 5 wt.% OMT nanocomposite.
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site have been investigated. The morphology evolution of the irradiated nanocomposite strongly depends on the relative magnitude of these competing processes between the steric interaction of clay layers and the development of intermolecular cross-linking. The improvement in HRR for irradiated material was efficiently suppressed by the nanodispersion of clay layers, especially at high irradiation dose level.
Acknowledgements Fig. 4. The changes of peak HRR for MAPE and MAPE and 5 wt.% OMT nanocomposite with increasing irradiation doses.
makes combustion become difficult.[15] The possible explanation is that the irradiated sample has undergone unavoidable chain scission forming more flammable low molecular weight material, meanwhile, the amount of char derived from the cross-linked products is not high enough to cause fire retardancy [16]. Although the peak HRR in Fig. 3(A) and (B) for irradiated samples is higher than their corresponding nonirradiated sample, the changes of their peak HRR with increasing irradiation doses are different. Fig. 4 shows the change of peak HRR with irradiation doses for pure MAPE and MAPE/OMT nanocomposite. The peak HRR values for MAPE show a rapid increase followed by a gradual reduction and a sharp increase as irradiation doses is raised. For MAPE/OMT nanocomposite, the values initially increase fast then almost level off at higher doses. These results indicate that the irradiation-resistant properties of nanocomposites are superior to that of pure polymer because the nanodispersion of clay layers hinders the gray penetration and decreases the formation of low molecular weight material generated by irradiation-induced chain scission, which makes the nanocomposites efficiently suppress the HRR improvement, especially decreasing the radiolytic damage of the polymer at high dose levels.
4. Conclusion Morphology evolution and flammability properties of the gamma-irradiated maleated polyethylene/OMT nanocompo-
The work was financially supported by the National Natural Science Foundation of China (No. 50476026 and 50323005), the China NKBRSF project (No. 2001CB409600).
References [1] X. Liu, Q. Wu, Polymer 42 (2001) 10013. [2] Y. Kijima, A. Usuki, M. Kawasumi, A. Okada, T. Kurauchi, T. Kurauchi, J. Mater. Res. 8 (1993) 1185. [3] R.S. Sinha, K. Yamada, M. Okamoto, A. Ogami, K. Ueda, Chem. Mater. 15 (2003) 1456. [4] J.W. Gilman, C.L. Jackson, A.B.R.H. Morgan Jr., Chem. Mater. 12 (2000) 1866. [5] A. Negron, S. Ramos, A.L. Blumenfeld, G. Pacheco, J.J. Fripiat, Clays Clay Miner. 50 (2002) 35. [6] R. Pushkareva, E. Kalinichenko, A. Lytovchenko, A. Pushkarev, V. Kadochnikov, M. Plastynina, Appl. Clay Sci. 21 (2002) 117. [7] C.M. Koo, M.J. Kim, M.H. Choi, S.O. Kim, I.J.J. Chung, Appl. Polym. Sci. 88 (2003) 1526. [8] K.H. Wang, I.J. Chung, M.C. Jang, J.K. Keum, H.H. Song, Macromolecules 35 (2002) 5529 – 5535. [9] M. Alexandre, P. Dubois, T. Sun, J.M. Garces, R. Jerome, Polymer 43 (2002) 2123 – 2132. [10] T.G. Gopakumar, J.A. Lee, M. Kontopoulou, J.S. Parent, Polymer 43 (2002) 5483. [11] K.H. Wang, M.H. Choi, C.M. Koo, Y.S. Choi, I.J. Chung, Polymer 42 (2001) 9819. [12] V.G. Barkhudaryan, Polymer 41 (2000) 575. [13] A. Singh, Radiat. Phys. Chem. 56 (1999) 375. [14] S. Chattopadhyar, T.K. Chaki, A.K. Bhowmick, Radiat. Phys. Chem. 59 (2000) 501. [15] J. Li, B. Li, X.C. Zhang, Polym. Degrad. Stab. 71 (2001) 375 – 380. [16] A.I. Balabanovich, S.V. Levchik, G.F. Levdhik, W. Schnabel, C.A. Wikie, Polym. Degrad. Stab. 64 (1999) 191 – 195.
The influence of irradiation on morphology evolution and flammability properties of maleated polyethylene/clay nanocomposite Hongdian Lua, Yuan Hua,*, Junfeng Xiaoa, Qinghong Konga, Zuyao Chenb, Weicheng Fana a
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui, 230026, PR China b Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026, PR China Received 27 June 2004; received in revised form 21 October 2004; accepted 28 October 2004 Available online 14 November 2004
Abstract The morphology and flammability properties of g-ray irradiated maleated polyethylene (PE)/clay nanocomposite were investigated using X-ray diffraction (XRD), transmission electron microscope (TEM) and Cone calorimeter. The exfoliation of clay in the nanocomposites before and after irradiation has been verified by XRD and TEM studies. TEM images demonstrated that the morphology of the nanocomposite evolved via three stages under irradiation: ideally face–face ordered exfoliation followed by a completed disordered dispersion of stacked layers and relative ordered distribution of stacked layers. Cone calorimetry results exhibited that the improvement in heat release rate for irradiated materials were suppressed by the nanodispersion of clay layers, especially at high irradiation dose level. D 2004 Elsevier B.V. All rights reserved. Keywords: Polyethylene; Nanocomposite; Irradiation
1. Introduction The polymer nanocomposites prepared by melt intercalation of polymer chains into the galleries of organically modified clay have been extensively investigated because they often exhibit a wide range of improved properties over their bulk counterparts [1–4]. The clay commonly used for the produce of nanocomposites is montmorillonite (MMT), which consists of two-dimensional layers where two fused silica tetrahedral sheets sandwiching an edge-shared octahedral sheet of either alumina or magnesia. As far as we are aware, apart from that used for fabrication of nanocomposites, MMT has been used as engineering barriers in storages of radioactive wastes due to its stable crystal structure which can accumulate high doses of irradiation without distinct damage.[5,6] Therefore, it can be expected that the
* Corresponding author. Tel./fax: +86 551 3601664. E-mail address: [email protected] (Y. Hu). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.10.057
penetration of g-ray in the polymeric matrix will be hindered by the nanodispersion of MMT layers. Polyethylene (PE) is an important engineering resin, widely used in the wire and cable industries. For the past decades, works on various aspects of PE/clay nanocomposites have been done expect to investigate their structure– property relationships [7–11]. Furthermore, irradiationinduced intermolecular cross-linking of polymers in the presence of high-energy irradiation (gamma or electron beam) has been employed to upgrade their properties.[12– 14] However, in contrast to the significant amount of reports on the development of polyethylene exposed by irradiation and on the PE/clay nanocomposites, articles on the influence of irradiation on the structure and properties of PE/clay nanocomposites are lacking. It is known that it is difficult to fabricate PE-based nanocomposites, and maleic anhydride-grafted PE (MAPE) is often introduced to promote the interaction between PE chains and the organically modified montmorillonite (OMT). For simplicity, in the present work, the pure maleic anhydride-grafted polyethylene/clay nanocomposite was prepared by direct melt intercalation. The resulting nano-
H. Lu et al. / Materials Letters 59 (2005) 648–651
649
composite was then irradiated with 60Co g-ray for model study. The aim of this article is to investigate the influence of irradiation on the morphology evolution and flammability characteristics of nanocomposites.
2. Experimental section 2.1. Materials and preparation The maleated polyethylene (MAPE) with approximately 1.0 wt.% maleic anhydride was supplied by Haier Kehua, China. Organophilic montmorillonite (OMT), which has been ion-exchanged with hexanecyl trimethyammounium bromide ions (C16) was kindly provided by Keyan Chemistry. MAPE (dried) and 5 wt.% OMT (dried) were premixed then blended by a two-roll mill (XK-160, JiangShu, China) at 165 8C. The resulting nanocomposite was then compressed molded into sheets (1 and 3 mm thickness). All these specimens were irradiated at room temperature in a 2.221015 Bq 60Co g-ray source with 67.5 Gy/min irradiation dose rate in air atmosphere. 2.2. Morphology studies X-ray diffraction (XRD) patterns were performed on the 1-mm-thick film with a Japan Rigaku D/Max-Ra rotating anode X-ray diffractometer equipped with a Cu Ka tube and Ni filter (k=0.1542 nm). Transmission electron microscopy (TEM) images were obtained on a Jeol JEM-100SX transmission electron microscope with the acceleration voltage of 100 kV. The nanocomposite specimen was cut at room temperature using an ultramicrotome (Ultracut-1, UK) with a diamond knife from an epoxy block, with the films of the nanocomposite embedded. 2.3. Cone calorimeter studies Cone calorimetry experiments were evaluated, following the procedure defined in ISO 5660, on 3-mm-thick 100100 mm2 plates at an incident heat of 35 kW/m2. The cone data reported are reproducible to within about F10%.
3. Results and discussion 3.1. XRD characteristics and morphology evolution The XRD patterns of MAPE/OMT composites with different irradiation doses are displayed in Fig. 1. The results demonstrated that all composites show the same feature. The disappearance of the d 001 peak derived from the interlayer spacing of MMT reveals that almost complete exfoliation of clay had taken place. The nanoscale structure
Fig. 1. XRD patterns of MAPE and 5 wt.% OMT nanocomposites with different irradiation doses.
of the exfoliated nanocomposites elucidated by TEM is shown in Fig. 2 (a–d). For each sample, the clay layers are fine dispersed throughout MAPE matrix; however, the morphology evolves as irradiation dose increases. Fig. 2a shows that the clay layers are well aligned at the whole area with some disorder at the local in the nonirradiated sample, which is caused by the directional mechanical shear on the extrusion process. The ideally face–face ordered exfoliation with homogeneous dispersion of clay layers are observed throughout the 50-KGy irradiated sample (Fig. 2b). The probable explanation may be that the species such as free radicals and ions generated initially under irradiation have high mobility, they may diffuse in and/or out from the clay galleries. This diffusivity state will go on for quite a long time due to the low experimental irradiation dose rate, which may be favorable of the improvement of the steric interaction between clay layers. This improvement is conducive to making ordered exfoliated structure within polymer since the periodicity of the clay layers having anisotropic is originated from the steric interaction.[7] On the other hand, those species mainly macromolecular cations may be terminated by the surfaces of negatively charged silicate layers to form dangling polymer chains. This results in a new conformation at polymer-clay interface, which may partly contribute to the formation of the ordered exfoliated structure. With increasing irradiation to 100 KGy doses, a morphological conversion occurred, where the orientation of silicate layers are completely destroyed, and the stacked clay layers randomly distribute in MAPE matrix, as shown in Fig. 2c. There are two possible ways of the stack of the initially well-dispersed clay layers in the matrix under irradiation. One is that the cross-linking of polymer chains which attached onto the layers take place effectively overwhelms the steric interaction between clay layers resulting in the clay stack. Another may be that the recession of the polymer chains from the clay galleries with
650
H. Lu et al. / Materials Letters 59 (2005) 648–651
Fig. 2. TEM micrographs of MAPE and 5 wt.% OMT nanocomposites with irradiation doses: (a) nonirradiated, (b) 50, (c) 100 and (d) 150 KGy.
dewetted clay layers left, which also lead to the clay stack again each other. This is because the driving force of the intercalation between maleated polymer and layered silicates originates from strong hydrophilic interaction between maleic anhydride group and polar silicates [15], whereas the maleic anhydride groups will produce a ring opening followed by the formation of carbonylated volatile products under irradiation. However, the cross-linked networks generated in noncrystallite regions of PE restrict the congregation of these stacks, keeping them randomly disperse throughout the matrix. Fig. 2(d) shows the TEM micrograph of the sample with 150 KGy doses. The distribution of silicate layers is likely to be located at between Fig. 2a and c. This is because the scission and oxidation of polymer chains is considerably intensified at high doses. [12] The destruction of polymer chains may help to enhance the steric interaction, contrarily, the cross-linked networks restrict the mobility of silicate
layers. As a result, relative ordered structure is formed. Similar features of TEM micrograph for the 200-KGy irradiated sample were not shown here. 3.2. Flammability The reduction of heat release rate (HRR), particular peak HRR has been found to be the most clear-cut evidence for the efficiency of a flame retardant. The comparison of the HRR behavior of pure MAPE and its nanocomposite before and after irradiated is presented in Fig. 3. The MAPE and MAPE/OMT samples show the similar features, for the irradiated samples slightly decrease the ignition delay time and enhance the peak HRR compared to their corresponding nonirradiated sample. This is in contrast to the expectation since the cross-linkings easily form conjugated systems and aromatic rings during combustion, which results in the increased char formation in the condensed phase and thus
Fig. 3. The HRR curves with different irradiation doses for (A) MAPE and (B) MAPE and 5 wt.% OMT nanocomposite.
H. Lu et al. / Materials Letters 59 (2005) 648–651
651
site have been investigated. The morphology evolution of the irradiated nanocomposite strongly depends on the relative magnitude of these competing processes between the steric interaction of clay layers and the development of intermolecular cross-linking. The improvement in HRR for irradiated material was efficiently suppressed by the nanodispersion of clay layers, especially at high irradiation dose level.
Acknowledgements Fig. 4. The changes of peak HRR for MAPE and MAPE and 5 wt.% OMT nanocomposite with increasing irradiation doses.
makes combustion become difficult.[15] The possible explanation is that the irradiated sample has undergone unavoidable chain scission forming more flammable low molecular weight material, meanwhile, the amount of char derived from the cross-linked products is not high enough to cause fire retardancy [16]. Although the peak HRR in Fig. 3(A) and (B) for irradiated samples is higher than their corresponding nonirradiated sample, the changes of their peak HRR with increasing irradiation doses are different. Fig. 4 shows the change of peak HRR with irradiation doses for pure MAPE and MAPE/OMT nanocomposite. The peak HRR values for MAPE show a rapid increase followed by a gradual reduction and a sharp increase as irradiation doses is raised. For MAPE/OMT nanocomposite, the values initially increase fast then almost level off at higher doses. These results indicate that the irradiation-resistant properties of nanocomposites are superior to that of pure polymer because the nanodispersion of clay layers hinders the gray penetration and decreases the formation of low molecular weight material generated by irradiation-induced chain scission, which makes the nanocomposites efficiently suppress the HRR improvement, especially decreasing the radiolytic damage of the polymer at high dose levels.
4. Conclusion Morphology evolution and flammability properties of the gamma-irradiated maleated polyethylene/OMT nanocompo-
The work was financially supported by the National Natural Science Foundation of China (No. 50476026 and 50323005), the China NKBRSF project (No. 2001CB409600).
References [1] X. Liu, Q. Wu, Polymer 42 (2001) 10013. [2] Y. Kijima, A. Usuki, M. Kawasumi, A. Okada, T. Kurauchi, T. Kurauchi, J. Mater. Res. 8 (1993) 1185. [3] R.S. Sinha, K. Yamada, M. Okamoto, A. Ogami, K. Ueda, Chem. Mater. 15 (2003) 1456. [4] J.W. Gilman, C.L. Jackson, A.B.R.H. Morgan Jr., Chem. Mater. 12 (2000) 1866. [5] A. Negron, S. Ramos, A.L. Blumenfeld, G. Pacheco, J.J. Fripiat, Clays Clay Miner. 50 (2002) 35. [6] R. Pushkareva, E. Kalinichenko, A. Lytovchenko, A. Pushkarev, V. Kadochnikov, M. Plastynina, Appl. Clay Sci. 21 (2002) 117. [7] C.M. Koo, M.J. Kim, M.H. Choi, S.O. Kim, I.J.J. Chung, Appl. Polym. Sci. 88 (2003) 1526. [8] K.H. Wang, I.J. Chung, M.C. Jang, J.K. Keum, H.H. Song, Macromolecules 35 (2002) 5529 – 5535. [9] M. Alexandre, P. Dubois, T. Sun, J.M. Garces, R. Jerome, Polymer 43 (2002) 2123 – 2132. [10] T.G. Gopakumar, J.A. Lee, M. Kontopoulou, J.S. Parent, Polymer 43 (2002) 5483. [11] K.H. Wang, M.H. Choi, C.M. Koo, Y.S. Choi, I.J. Chung, Polymer 42 (2001) 9819. [12] V.G. Barkhudaryan, Polymer 41 (2000) 575. [13] A. Singh, Radiat. Phys. Chem. 56 (1999) 375. [14] S. Chattopadhyar, T.K. Chaki, A.K. Bhowmick, Radiat. Phys. Chem. 59 (2000) 501. [15] J. Li, B. Li, X.C. Zhang, Polym. Degrad. Stab. 71 (2001) 375 – 380. [16] A.I. Balabanovich, S.V. Levchik, G.F. Levdhik, W. Schnabel, C.A. Wikie, Polym. Degrad. Stab. 64 (1999) 191 – 195.