- Email: [email protected]
Grafting of a LLDPE using gamma irradiation
Nuclear Instruments and Methods in Physics Research B 236 (2005) 338–342 www.elsevier.com/locate/nimb
Grafting of a LLDPE using gamma irradiation E. Catarı´ a, C. Albano
a,b,*
, A. Karam
a,*
, R. Perera c, P. Silva d, J. Gonza´lez
c
a
d
Centro de Quı´mica, Laboratorio de Polı´meros Instituto Venezolano de Investigaciones Cientı´ficas (IVIC), Venezuela b Universidad Central de Venezuela, Facultad de Ingenierı´a, Escuela de Ingenierı´a Quı´mica, Venezuela c Departamento de Meca´nica, Universidad Simo´n Bolı´var, Venezuela Centro de Fı´sica, Laboratorio de Fı´sica de la Materia Condensada, Instituto Venezolano de Investigaciones Cientı´ficas (IVIC), Caracas, Venezuela Available online 23 May 2005
Abstract In this investigation, the grafting of a commercial linear low-density polyethylene (LLDPE) with different concentrations of diethyl maleate (DEM, 5 and 15 wt.%) was carried out at different absorbed doses from a cobalt-60 source of gamma rays (0, 15, 30, 50, 100, 200 kGy). This process was performed in a decalin solution at 10% w/v to obtain a homogeneous dispersion of the monomer into the polyethylene matrix. The grafting degree was estimated by means of FTIR using a calibration curve reported in literature. Thermal properties of the functional polymers were studied by thermogravimetric analysis (TGA). Melt flow index (MFI) values were also taken. The results found indicate that the grafting degree increases as the concentration of DEM in the reaction mixture and the absorbed doses are increased upto 100 kGy, as expected. However, the behavior at higher doses is attributed to secondary reactions such as long-chain branching and/or crosslinking, which are faster than radical reactions responsible for the grafting of the DEM onto the polymeric chain. This fact was ascertained by the decrease of the MFI values as the applied irradiation was increased, irrespective of the quantity of DEM used in the grafting reaction. Therefore, in order to obtain a high grafting degree, the absorbed dose should be estimated carefully. Initial degradation temperatures of the grafted PEs decreased when the gamma irradiation dose was higher than 100 kGy. This indicates that the thermal stability decreases as higher doses are applied to the material, which is associated to branching and crosslinking. The grafting degree never exceeded 0.3 mol%, which demonstrates the low efficiency of the functionalization procedure here presented. Ó 2005 Elsevier B.V. All rights reserved. PACS: 81.05.Lg; 61.80.x Keywords: Grafting; Gamma irradiation; Functionalization; LLDPE; Characterization; Diethyl maleate
*
Corresponding authors. Tel.: +58 212 5041636; fax: +58 212 5041350 (C. Albano). E-mail addresses: [email protected], [email protected] (C. Albano), [email protected] (A. Karam).
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.03.273
E. Catarı´ et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 338–342
1. Introduction Polyolefins are considered among the most important thermoplastics due to their industrial interest, good handling, low cost and wide range of properties. However, unmodified polyolefins are rarely miscible with other polymers and their incompatibility has been proved even among themselves due to their different molecular weights and chain branching contents. For instance, polyethylenes and polypropylene are highly immiscible among themselves and with polar polymers, and their blends have two different phases with poor adhesion between them and hence, poor mechanical properties [1,2]. Functionalization or grafting of polyethylenes with polar functional groups is an important method for obtaining new materials of special physical–chemical properties. The incorporation of polar functional groups like carbonyls (C@O) into the polymeric chain provides specific sites for interactions such as hydrogen and covalent bonding which improves the properties of adhesion and compatibility of the polymer with other polymers and materials [3]. These characteristics make such grafted polyethylenes materials of great value at industrial level, because they could be used as compatibilizers in polymer blending and as raw materials for insulating, food packaging and production of electronic components. Many functional monomers have been used and different peroxides have been studied as initiators of the grafting reaction [1,3–6]. Among those monomers, the use of maleic anhydride (MA), acrylic acid (AA) and diethyl maleate (DEM) have been reported [7–9]. On the other hand, the use of gamma irradiation as the mean to initiate the reaction in the grafting of polymers with some monomers has also been studied [10–13]. However, the use of gamma rays to graft DEM onto polyethylenes has not been reported yet. Hence, the functionalization of a linear low-density polyethylene (LLDPE) with diethyl maleate using gamma irradiation as the initiator agent was studied in this work. The DEM concentrations employed were 0, 5 and 15 wt.% and the absorbed doses were 0, 15, 30, 50, 100 and 200 kGy, in order to induce different grafting degrees and to establish their rela-
339
tionship with the product melt flow index (MFI) values and thermal properties.
2. Experimental A commercial linear low-density polyethylene (LLDPE) with a melt flow index (MFI) of 4.4 dg/ min, supplied by Resilin, C.A., a blend of cis- and trans-decahydronaphtalene (decalin) 99%, supplied by Riedel de Hae¨n, and ethanol and n-hexane for washing were employed. Diethyl maleate, manufactured by Aldrich Chemical Company Inc. was used as the functionalization monomer. The LLDPE functionalization was performed in a 10% w/v solution of the polyethylene in decalin, using different concentrations of DEM (0, 5 and 15 wt.%) and absorbed doses (0, 15, 30, 50, 100 and 200 kGy) from a cobalt-60 source of gamma rays, using a dose rate of 5 kGy/h in air and room temperature. After the irradiation of the solution was carried out, it was precipitated in n-hexane and washed several times with n-hexane and ethanol to eliminate the decalin and the unreacted DEM. The product thus obtained was dried in a vacuum oven at 60 °C for 18 h, approximately. The functionalization or grafting degree (GD) was determined by FTIR in a Nicolet Magna-IR 560 spectrometer, measuring the absorption band areas at 1740 cm1 (C@O of the DEM), and at 720 cm1 (A1740 cm1 =A720 cm1 ) and 1460 cm1 (A1740 cm1 =A1460 cm1 ) characteristics of the polyethylene. Relative areas of these absorption bands are proportional to the concentration of carbonyl groups (C@O) in the polymer. The grafting degree was established by means of a IR–NMR calibration curve reported in literature [4]. Additionally, the grafting efficiency was also estimated. The absorbance ratios A965 cm1 =A1460 cm1 (proportional to the concentration of trans-vinylene unsaturations) and A909 cm1 =A1460 cm1 (proportional to the concentration of terminal vinyl unsaturations) were determined as well, to investigate the likelihood of modification of the chemical structure of the polymer. Melt flow index (MFI) values were taken at 190 °C and 2.16 kgf according to the standard procedure reported in ASTM D-123. Thermal
E. Catarı´ et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 338–342
analyses were performed in a thermogravimetric analyzer (Mettler Toledo TGA/SDTA 821), under nitrogen from 25 °C to 500 °C at a heating rate of 10 °C/min. The initial decomposition temperature was determined from the onset of the derivatives and the activation energy was established from the thermograms using the Reich–Stivala method [14].
LLDPE 15 % DEM
0.20 GD (mol DEM/100 CH2 )
340
0.15 0.10 0.05 0.00 0
50
100 Absorbed Dose (kGy)
1740cm
3. Results and discussion FTIR spectroscopy allows studying the different modifications in the chemical structure of the polymers, due to its high sensitivity to the specific identification of functional groups. Hence, the assignment of the characteristic bands of both, polymer under study (LLDPE) and the functional monomer employed (DEM) is easy. The grafting degrees were determined using the IR–NMR calibration curves reported by Rosales et al. [4] for DEM grafted polyethylenes via reactive extrusion. Figs. 1 and 2 show an increase in the grafting degree of the LLDPE with the radiation for all the concentrations of DEM studied as a consequence of the increased amount of energy given to the system, which made the grafting process more favorable. However, a plateau is reached at the approximated value of 0.12 mol DEM/100 CH2. This fact indicates that at the higher doses, the irradiation also induces more secondary reactions such as long-chain branching and/or cross-
-1
/1460cm
-1
1740cm
-1
150
/720cm
200
-1
Fig. 2. Grafting degrees as a function of the absorbed dose in the functionalization of a LLDPE with 15 wt.% of DEM.
linking in the LLDPE, which compete with the main grafting reaction. Fig. 3 depicts the dependence of the functionalization degree on the irradiation dose at different DEM concentrations. From there, it is evident that the grafting degrees depend on the DEM concentration added and on the radiation dose employed. The grafting efficiency (GE), defined as the ratio of the grafting degree to the used DEM concentration is displayed in Fig. 4. It is clear that the GE increases as the DEM content is decreased, probably due to the fact that the amount of DEM added is small and it is grafted in higher proportions than higher concentrations (15 wt.%). On the other hand, the GE gradually increases with the
G.D. (mol DEM/100 CH2)
Grafting Degrees
LLDPE 5 % DEM GD (mol DEM/100 CH2)
0.20 0.15 0.10 0.05
0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 15(A)
15(B)
30(A)
30(B)
50(A)
50(B) 100(A) 100(B) 200(A) 200(B)
Absorbed Dose kGy
0.00 0
50
100
150
200
Absorbed Dose (kGy) -1
1740cm
/1460cm
-1
-1
1740cm
-1
/720cm
Fig. 1. Grafting degrees as a function of the absorbed dose in the functionalization of a LLDPE with 5 wt.% of DEM.
0% DEM
5% DEM
15% DEM
Fig. 3. Grafting degrees at different absorbed doses as a function of the DEM concentration in the functionalization of a LLDPE: (A) calculated from the (A1740 cm1 =A720þ730 cm1 ) peak area ratios; (B) calculated from the (A1740 cm1 =A1460 cm1 ) peak area ratios.
E. Catarı´ et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 338–342
Table 2 Initial degradation temperatures (Ti) in °C of the different samples (D: absorbed doses in kGy)
G.E. (G.D./mol DEM)
Grafting Efficiencies 4000 3500 3000 2500 2000 1500 1000 500 0 15(A)
15(B)
30(A)
30(B)
50(A)
341
50(B) 100(A) 100(B) 200(A) 200(B)
D (kGy) DEM (wt.%)
0
15
30
50
100
200
0 5 15
393 * *
390 375 370
396 394 391
390 374 374
365 361 361
366 366 366
* Not measured.
Absorbed Dose kGy
15% DEM
5% DEM
Fig. 4. Grafting efficiencies at different absorbed doses as a function of the DEM concentration in the functionalization of a LLDPE: (A) calculated from the (A1740 cm1 =A720þ730 cm1 ) peak area ratios; (B) calculated from the (A1740 cm1 =A1460 cm1 ) peak area ratios.
radiation dose, due to the higher concentration of free radicals produced during the irradiation at higher doses. Table 1 exhibits the MFI values of the functionalized products. Those values could only be taken at radiation doses up to 100 and 50 kGy, for the 0% DEM and the DEM concentrations used, respectively, because the samples had a very high viscosity and did not flow. In the polymer without DEM this behavior is attributed to long-chain branching and/or crosslinking as a consequence of free radicals reactions induced by the irradiation process, whereas in the functionalized products, this behavior is ascribed not only to the grafting of the monomer but to long-chain branching and/or crosslinking as well. Similar results were found by Albano et al. [15] in their studies on irradiated polyethylenes. In fact, terminal vinyl groups were consumed and trans-vinylene groups increased to a minor extent as a result of coupling reactions during the grafting of LLDPE. AccordTable 1 MFI values (dg/min) of the samples (D: absorbed doses in kGy) D (kGy) DEM (wt.%)
0
15
30
50
100
200
0 5 15
4.404 * *
3.408 3.393 3.182
2.630 1.824 2.545
1.805 1.530 1.737
0.342 – –
– – –
* Not measured. – Did not flow.
Table 3 Degradation activation energies (Ea) in kJ/mol of the different samples (D: absorbed doses in kGy) D (kGy) DEM (wt.%)
0
15
30
50
100
200
0 5 15
315 * *
313 295 261
324 321 289
316 283 279
257 220 217
259 256 247
* Not measured.
ing to Lachtermacher and Rudin [16], chain-extension reactions account for a decrease in the amount of terminal vinyls. Allylic radicals are intermediate species in chain-coupling reactions involved in the modification and /or grafting reactions of LLDPE, yielding trans- or cis-unsaturations. Long-branch formation and crosslinking result mainly from coupling allylic radicals, which cause a decrease in the terminal unsaturations and an increase in internal unsaturations. Tables 2 and 3 display the initial degradation temperatures (Ti) and activation energies (Ea) of the different samples, respectively. As it can be seen, Ti decreases as the DEM concentration is increased. This tendency is stressed out at low absorbed doses. On the other hand, Ti and Ea decrease as the absorbed dose is increased for all the samples but those irradiated at 30 kGy, where a steady Ti value was observed. At that dose, the grafting and crosslinking rates could be producing some sort of stabilization in the grafted product.
4. Conclusions An increase in the grafting degree of the LLDPE with the DEM concentration and absorbed dose
342
E. Catarı´ et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 338–342
was obtained as a consequence of the increased amount of energy given to the system, which made the grafting process more favorable. At the higher doses, the irradiation also induces more secondary reactions such as long-chain branching and/or crosslinking in the LLDPE, which compete with the main grafting reaction. In general, Ti and Ea decrease as the absorbed dose and DEM concentrations are increased.
Acknowledgment The authors acknowledge the financial support from FONACIT through grant G-2001000817.
References [1] M. Aglietto, R. Bertani, G. Ruggeri, F. Ciardelli, Makromol. Chem. 193 (1992) 179. [2] L.A. Utracki, R.A. Weiss, in: Multiphase Polymers: Blends and Ionomers, American Chemical Society, USA, 1989.
[3] N.C. Liu, W.E. Baker, K.E. Russell, J. Appl. Polym. Sci. 41 (1980) 2285. [4] C. Rosales, R. Perera, M. Ichazo, J. Gonzalez, H. Rojas, A. Sanchez, A. Diaz-Barrios, J. Appl. Polym. Sci. 70 (1998) 161. [5] C. Rosales, L. Marquez, J. Gonzalez, R. Perera, H. Rojas, Polym. Eng. Sci. 36 (1996) 2247. [6] K. Ganzeveld, L. Janssen, Polym. Eng. Sci. 32 (1992) 467. [7] M. Santos, Reactive Extrusion. Principles and Practice, Hanser Publishers, New York, 1992. [8] S. Al Malaika, Reactive Modifiers for Polymers, Blackie Academic & Professional, London, 1997. [9] G. Moad, Prog. Polym. Sci. 24 (1999) 81. [10] M. DoleThe Radiation Chemistry of Macromolecules, Vols. I–II, Academic Press, New York, 1972. [11] A. Chapiro, Radiation Chemistry of Polymeric Systems. High Polymers, Vol. XV, John Wiley & Sons, New York, 1962. [12] E. Bucio, G. Cedillo, G. Burillo, T. Ogawa, Polym. Bull. 46 (2001) 115. [13] Y. Miwa, K. Yamamoto, M. Sakaguchi, S. Shimada, Macromolecules 34 (2001) 2089. [14] L. Reich, S. Stivala, Thermochim. Acta 24 (1978) 9. [15] C. Albano, R. Perera, P. Silva, Y. Sa´nchez, Polym. Bull. 51 (2003) 135. [16] M. Lachtermacher, A. Rudin, J. Appl. Polym. Sci. 58 (1995) 2077.
Grafting of a LLDPE using gamma irradiation E. Catarı´ a, C. Albano
a,b,*
, A. Karam
a,*
, R. Perera c, P. Silva d, J. Gonza´lez
c
a
d
Centro de Quı´mica, Laboratorio de Polı´meros Instituto Venezolano de Investigaciones Cientı´ficas (IVIC), Venezuela b Universidad Central de Venezuela, Facultad de Ingenierı´a, Escuela de Ingenierı´a Quı´mica, Venezuela c Departamento de Meca´nica, Universidad Simo´n Bolı´var, Venezuela Centro de Fı´sica, Laboratorio de Fı´sica de la Materia Condensada, Instituto Venezolano de Investigaciones Cientı´ficas (IVIC), Caracas, Venezuela Available online 23 May 2005
Abstract In this investigation, the grafting of a commercial linear low-density polyethylene (LLDPE) with different concentrations of diethyl maleate (DEM, 5 and 15 wt.%) was carried out at different absorbed doses from a cobalt-60 source of gamma rays (0, 15, 30, 50, 100, 200 kGy). This process was performed in a decalin solution at 10% w/v to obtain a homogeneous dispersion of the monomer into the polyethylene matrix. The grafting degree was estimated by means of FTIR using a calibration curve reported in literature. Thermal properties of the functional polymers were studied by thermogravimetric analysis (TGA). Melt flow index (MFI) values were also taken. The results found indicate that the grafting degree increases as the concentration of DEM in the reaction mixture and the absorbed doses are increased upto 100 kGy, as expected. However, the behavior at higher doses is attributed to secondary reactions such as long-chain branching and/or crosslinking, which are faster than radical reactions responsible for the grafting of the DEM onto the polymeric chain. This fact was ascertained by the decrease of the MFI values as the applied irradiation was increased, irrespective of the quantity of DEM used in the grafting reaction. Therefore, in order to obtain a high grafting degree, the absorbed dose should be estimated carefully. Initial degradation temperatures of the grafted PEs decreased when the gamma irradiation dose was higher than 100 kGy. This indicates that the thermal stability decreases as higher doses are applied to the material, which is associated to branching and crosslinking. The grafting degree never exceeded 0.3 mol%, which demonstrates the low efficiency of the functionalization procedure here presented. Ó 2005 Elsevier B.V. All rights reserved. PACS: 81.05.Lg; 61.80.x Keywords: Grafting; Gamma irradiation; Functionalization; LLDPE; Characterization; Diethyl maleate
*
Corresponding authors. Tel.: +58 212 5041636; fax: +58 212 5041350 (C. Albano). E-mail addresses: [email protected], [email protected] (C. Albano), [email protected] (A. Karam).
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.03.273
E. Catarı´ et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 338–342
1. Introduction Polyolefins are considered among the most important thermoplastics due to their industrial interest, good handling, low cost and wide range of properties. However, unmodified polyolefins are rarely miscible with other polymers and their incompatibility has been proved even among themselves due to their different molecular weights and chain branching contents. For instance, polyethylenes and polypropylene are highly immiscible among themselves and with polar polymers, and their blends have two different phases with poor adhesion between them and hence, poor mechanical properties [1,2]. Functionalization or grafting of polyethylenes with polar functional groups is an important method for obtaining new materials of special physical–chemical properties. The incorporation of polar functional groups like carbonyls (C@O) into the polymeric chain provides specific sites for interactions such as hydrogen and covalent bonding which improves the properties of adhesion and compatibility of the polymer with other polymers and materials [3]. These characteristics make such grafted polyethylenes materials of great value at industrial level, because they could be used as compatibilizers in polymer blending and as raw materials for insulating, food packaging and production of electronic components. Many functional monomers have been used and different peroxides have been studied as initiators of the grafting reaction [1,3–6]. Among those monomers, the use of maleic anhydride (MA), acrylic acid (AA) and diethyl maleate (DEM) have been reported [7–9]. On the other hand, the use of gamma irradiation as the mean to initiate the reaction in the grafting of polymers with some monomers has also been studied [10–13]. However, the use of gamma rays to graft DEM onto polyethylenes has not been reported yet. Hence, the functionalization of a linear low-density polyethylene (LLDPE) with diethyl maleate using gamma irradiation as the initiator agent was studied in this work. The DEM concentrations employed were 0, 5 and 15 wt.% and the absorbed doses were 0, 15, 30, 50, 100 and 200 kGy, in order to induce different grafting degrees and to establish their rela-
339
tionship with the product melt flow index (MFI) values and thermal properties.
2. Experimental A commercial linear low-density polyethylene (LLDPE) with a melt flow index (MFI) of 4.4 dg/ min, supplied by Resilin, C.A., a blend of cis- and trans-decahydronaphtalene (decalin) 99%, supplied by Riedel de Hae¨n, and ethanol and n-hexane for washing were employed. Diethyl maleate, manufactured by Aldrich Chemical Company Inc. was used as the functionalization monomer. The LLDPE functionalization was performed in a 10% w/v solution of the polyethylene in decalin, using different concentrations of DEM (0, 5 and 15 wt.%) and absorbed doses (0, 15, 30, 50, 100 and 200 kGy) from a cobalt-60 source of gamma rays, using a dose rate of 5 kGy/h in air and room temperature. After the irradiation of the solution was carried out, it was precipitated in n-hexane and washed several times with n-hexane and ethanol to eliminate the decalin and the unreacted DEM. The product thus obtained was dried in a vacuum oven at 60 °C for 18 h, approximately. The functionalization or grafting degree (GD) was determined by FTIR in a Nicolet Magna-IR 560 spectrometer, measuring the absorption band areas at 1740 cm1 (C@O of the DEM), and at 720 cm1 (A1740 cm1 =A720 cm1 ) and 1460 cm1 (A1740 cm1 =A1460 cm1 ) characteristics of the polyethylene. Relative areas of these absorption bands are proportional to the concentration of carbonyl groups (C@O) in the polymer. The grafting degree was established by means of a IR–NMR calibration curve reported in literature [4]. Additionally, the grafting efficiency was also estimated. The absorbance ratios A965 cm1 =A1460 cm1 (proportional to the concentration of trans-vinylene unsaturations) and A909 cm1 =A1460 cm1 (proportional to the concentration of terminal vinyl unsaturations) were determined as well, to investigate the likelihood of modification of the chemical structure of the polymer. Melt flow index (MFI) values were taken at 190 °C and 2.16 kgf according to the standard procedure reported in ASTM D-123. Thermal
E. Catarı´ et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 338–342
analyses were performed in a thermogravimetric analyzer (Mettler Toledo TGA/SDTA 821), under nitrogen from 25 °C to 500 °C at a heating rate of 10 °C/min. The initial decomposition temperature was determined from the onset of the derivatives and the activation energy was established from the thermograms using the Reich–Stivala method [14].
LLDPE 15 % DEM
0.20 GD (mol DEM/100 CH2 )
340
0.15 0.10 0.05 0.00 0
50
100 Absorbed Dose (kGy)
1740cm
3. Results and discussion FTIR spectroscopy allows studying the different modifications in the chemical structure of the polymers, due to its high sensitivity to the specific identification of functional groups. Hence, the assignment of the characteristic bands of both, polymer under study (LLDPE) and the functional monomer employed (DEM) is easy. The grafting degrees were determined using the IR–NMR calibration curves reported by Rosales et al. [4] for DEM grafted polyethylenes via reactive extrusion. Figs. 1 and 2 show an increase in the grafting degree of the LLDPE with the radiation for all the concentrations of DEM studied as a consequence of the increased amount of energy given to the system, which made the grafting process more favorable. However, a plateau is reached at the approximated value of 0.12 mol DEM/100 CH2. This fact indicates that at the higher doses, the irradiation also induces more secondary reactions such as long-chain branching and/or cross-
-1
/1460cm
-1
1740cm
-1
150
/720cm
200
-1
Fig. 2. Grafting degrees as a function of the absorbed dose in the functionalization of a LLDPE with 15 wt.% of DEM.
linking in the LLDPE, which compete with the main grafting reaction. Fig. 3 depicts the dependence of the functionalization degree on the irradiation dose at different DEM concentrations. From there, it is evident that the grafting degrees depend on the DEM concentration added and on the radiation dose employed. The grafting efficiency (GE), defined as the ratio of the grafting degree to the used DEM concentration is displayed in Fig. 4. It is clear that the GE increases as the DEM content is decreased, probably due to the fact that the amount of DEM added is small and it is grafted in higher proportions than higher concentrations (15 wt.%). On the other hand, the GE gradually increases with the
G.D. (mol DEM/100 CH2)
Grafting Degrees
LLDPE 5 % DEM GD (mol DEM/100 CH2)
0.20 0.15 0.10 0.05
0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 15(A)
15(B)
30(A)
30(B)
50(A)
50(B) 100(A) 100(B) 200(A) 200(B)
Absorbed Dose kGy
0.00 0
50
100
150
200
Absorbed Dose (kGy) -1
1740cm
/1460cm
-1
-1
1740cm
-1
/720cm
Fig. 1. Grafting degrees as a function of the absorbed dose in the functionalization of a LLDPE with 5 wt.% of DEM.
0% DEM
5% DEM
15% DEM
Fig. 3. Grafting degrees at different absorbed doses as a function of the DEM concentration in the functionalization of a LLDPE: (A) calculated from the (A1740 cm1 =A720þ730 cm1 ) peak area ratios; (B) calculated from the (A1740 cm1 =A1460 cm1 ) peak area ratios.
E. Catarı´ et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 338–342
Table 2 Initial degradation temperatures (Ti) in °C of the different samples (D: absorbed doses in kGy)
G.E. (G.D./mol DEM)
Grafting Efficiencies 4000 3500 3000 2500 2000 1500 1000 500 0 15(A)
15(B)
30(A)
30(B)
50(A)
341
50(B) 100(A) 100(B) 200(A) 200(B)
D (kGy) DEM (wt.%)
0
15
30
50
100
200
0 5 15
393 * *
390 375 370
396 394 391
390 374 374
365 361 361
366 366 366
* Not measured.
Absorbed Dose kGy
15% DEM
5% DEM
Fig. 4. Grafting efficiencies at different absorbed doses as a function of the DEM concentration in the functionalization of a LLDPE: (A) calculated from the (A1740 cm1 =A720þ730 cm1 ) peak area ratios; (B) calculated from the (A1740 cm1 =A1460 cm1 ) peak area ratios.
radiation dose, due to the higher concentration of free radicals produced during the irradiation at higher doses. Table 1 exhibits the MFI values of the functionalized products. Those values could only be taken at radiation doses up to 100 and 50 kGy, for the 0% DEM and the DEM concentrations used, respectively, because the samples had a very high viscosity and did not flow. In the polymer without DEM this behavior is attributed to long-chain branching and/or crosslinking as a consequence of free radicals reactions induced by the irradiation process, whereas in the functionalized products, this behavior is ascribed not only to the grafting of the monomer but to long-chain branching and/or crosslinking as well. Similar results were found by Albano et al. [15] in their studies on irradiated polyethylenes. In fact, terminal vinyl groups were consumed and trans-vinylene groups increased to a minor extent as a result of coupling reactions during the grafting of LLDPE. AccordTable 1 MFI values (dg/min) of the samples (D: absorbed doses in kGy) D (kGy) DEM (wt.%)
0
15
30
50
100
200
0 5 15
4.404 * *
3.408 3.393 3.182
2.630 1.824 2.545
1.805 1.530 1.737
0.342 – –
– – –
* Not measured. – Did not flow.
Table 3 Degradation activation energies (Ea) in kJ/mol of the different samples (D: absorbed doses in kGy) D (kGy) DEM (wt.%)
0
15
30
50
100
200
0 5 15
315 * *
313 295 261
324 321 289
316 283 279
257 220 217
259 256 247
* Not measured.
ing to Lachtermacher and Rudin [16], chain-extension reactions account for a decrease in the amount of terminal vinyls. Allylic radicals are intermediate species in chain-coupling reactions involved in the modification and /or grafting reactions of LLDPE, yielding trans- or cis-unsaturations. Long-branch formation and crosslinking result mainly from coupling allylic radicals, which cause a decrease in the terminal unsaturations and an increase in internal unsaturations. Tables 2 and 3 display the initial degradation temperatures (Ti) and activation energies (Ea) of the different samples, respectively. As it can be seen, Ti decreases as the DEM concentration is increased. This tendency is stressed out at low absorbed doses. On the other hand, Ti and Ea decrease as the absorbed dose is increased for all the samples but those irradiated at 30 kGy, where a steady Ti value was observed. At that dose, the grafting and crosslinking rates could be producing some sort of stabilization in the grafted product.
4. Conclusions An increase in the grafting degree of the LLDPE with the DEM concentration and absorbed dose
342
E. Catarı´ et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 338–342
was obtained as a consequence of the increased amount of energy given to the system, which made the grafting process more favorable. At the higher doses, the irradiation also induces more secondary reactions such as long-chain branching and/or crosslinking in the LLDPE, which compete with the main grafting reaction. In general, Ti and Ea decrease as the absorbed dose and DEM concentrations are increased.
Acknowledgment The authors acknowledge the financial support from FONACIT through grant G-2001000817.
References [1] M. Aglietto, R. Bertani, G. Ruggeri, F. Ciardelli, Makromol. Chem. 193 (1992) 179. [2] L.A. Utracki, R.A. Weiss, in: Multiphase Polymers: Blends and Ionomers, American Chemical Society, USA, 1989.
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