electric anisotropy in high density polyethylene + - iopscience
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This content was downloaded from IP address 125.231.64.24 on 28/11/2021 at 07:25
E. Vigueras-Santiago, S. Hernández-López, M.A. Camacho-López and O. Lara- Sanjuan
Laboratorio de Investigación y Desarrollo de Materiales Avanzados (LIDMA). Facultad de Química, UAEM. Paseo Colón esq. con Paseo Tollocan, s/n. C.P. 50000, Toluca, Estado de México, México.
E-mail: [email protected]
Abstract. High density polyethylene + carbon black composites with electrical anisotropy was studied. Electrical anisotropy was induced by uniaxial mechanical deformation and injection moulding. We show that anisotropy depends on the carbon black concentration and percentage deformation. Resistivity had the highest anisotropy resistivity around the percolation threshold. Perpendicular resistivity showed two magnitude orders higher than parallel resistivity for injected samples, whereas resistivity showed an inverse behaviour for 100% tensile samples. Both directions were set respect to the deformation axe. Anisotropy could be explained in terms of the molecular deformation (alignment) of the polymer chains as a response of the deformation process originating a redistribution of the carbon black particles in both directions. Alignment of the polymer chains was evidenced by polarized Raman spectroscopy.
1. Introduction Actually, the study and synthesis of novel conductive polymer composite (CPC) with carbon black (CB) are of the great interest due to its broad practical applications [1]. In particular is possible to modify the electrical resistivity in at least 11 magnitude orders without important changes in the CPC density [2]. Furthermore, they have the ability of significantly change their electrical resistivity by the stimulus of external parameters as temperature, solvents, pressure, AC electric field, etc [1,3]. Percolation theory is the most cited description for explaining the electric conduction in this type of systems [4]. However, such a description doesn’t consider fundamental questions related with physical or chemical interaction among conductive particles and polymer matrix which take place during the CPC preparation. The optimum develop of these materials is an experimental work for each pair polymer-conductive particles. Is well known that both: appropriated disaggregating and dispersion of the CB particles produce the building of conduction networks [5]. Then the electrical properties of the PCP depend of the processing parameters [5]. In this work we are interested in study the changes in electrical resistivity of high density polyethylene-based CPC with high structure CB. For such propose, CPC in a width composition range of CB were prepared. The main goal is to show the possibility of controllably induces electrical anisotropy in this type of CPC [6,7]. The anisotropy could be explained in terms of a change in spatial configuration of the CB network chains during the uniaxial polymer matrix stretching, induced mainly by the molecular orientation of HDPE polymer chains along of the tensile axe [8,9].
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing Journal of Physics: Conference Series 167 (2009) 012039 doi:10.1088/1742-6596/167/1/012039
c© 2009 IOP Publishing Ltd 1
2. Experimental section HDPE YUZEX® - 8800 and Vulcan XC72-CB donated by Cabot, Co. were used as received. CPCs were obtained by melting mixing using a home made Banbury micromixer at 175°C, 55 rpm for 30 min at compositions of 1 to 20% wt/wt. CPCs were processed to cylinders of 0.5 cm radio and 0.5 cm high by thermomechanic molding following the methodology described in [3]. Silver paste SPI was deposited on both faces of the cylinders as electrical contacts. The electrical resistivity was measured using a Keithley electrometer 6517A . The electrical resistivity error was 6%.
For the anisotropy study, CPCs were processing to bars of 5 x 5 x 50 mm by injection molding. After, these bars were stretched by tension at a deformation rate of 1 cm/mm at 80°C using home made equipment. Molecular orientation as a deformation function was monitored by polarized light microRaman spectroscopy, using a Micro Raman Labram HR-800 de Jobin Yvon-Horiba, a He-Ne ( =632.8 nm) polarized laser and an Olympus BX4 microscope. The relative intensities ( ri ) of the symmetric C-C vibrations from the main chain, with a typical frequency on 1129 cm-1 were compared with the respective asymmetric ones which appear at 1060 cm-1 for compressed, injected and stretched HDPE samples.
3. Results and discussion The resistivity changes as a function of CB composition are shown in Figure 1. The squared curve corresponds to cylinder samples. The circle and triangle symbol curves correspond to the injected bars samples which were measured in both directions: parallel (circles) and perpendicular (triangles), as set up in Figure 2. According to our results, we could observe that conductive networks building depend on the processing method. In particular, resistivity values for injected (bar) samples were higher than those of compressed (cylinder) samples as shown in Figure 1. Differences between both directions of injected samples and thermomechanical molded samples could due to a slight orientation of the HDPE chains during injection processing.
T en
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ax e
Figure 1. Influence of the processing method (compression molding and injection) on the HDPE+CB CPC´s resistivity.
Figure 2. Configuration set up for resistivity measurements: a) parallel resistivities were measured trough the transversal sections perpendicular to tensile axe b) perpendicular resistivities were evaluated between 2 lateral opposite bar faces.
Figure 3 shows the electrical resistivity as a function of the deformation percentage for a 15% CB- CPC bar. Perpendicular direction of a no stretched sample had 2 magnitude order higher resistivity than parallel one (109 and 107 cm, respectively, Figure 1). As it was stretched the resistivity
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing Journal of Physics: Conference Series 167 (2009) 012039 doi:10.1088/1742-6596/167/1/012039
2
diminished at least 5 magnitude orders in both directions, but an inversion of the resistivity values was detected at 100% of deformation At his point, a parallel direction is more resistive than perpendicular one (104 and 102 cm, respectively). For deformations larger than 100%, the resistivity lightly increases for both directions until they reach almost the same value at 250% deformation.
The explanation for those facts are that as the deformation percentage increases, the polymer chains increases their orientation modifying at the same time the electrical network configuration of CB conductive particles. Then electrical resistivity of a stretched bar is modified in several magnitude orders in both directions. The resistivity behavior of HDPE+CB CPC under stretched deformation presented a different behavior in comparison with rubber-based CPC as polybutadiene, polyisoprene and silicon, for example, where an increasing in the length is accomplished of a size diminishing in the lateral direction. Consequently an increase of resistivity along of the parallel direction is observed while along the perpendicular direction the resistivity decreases. For a stretched HDPE+CB CPC, a number of the contacts in the conductive pathway increase in both directions. This could be consequence of the better packing of the chains polymer along the stretching axe diminishing the volume surrounded the particles and bringing near the CB particles (more dense material). This effect could increases the contacts among them and the possibility to build more conductive networks in both directions.
Raman spectroscopy allowed evidencing the polymer chain orientation after the injection and tension processes. Comparing the intensities of the symmetric C-C vibrations which appear at 1129 cm-1 with the asymmetric ones at 1060 cm-1 was possible to establish an orientation difference as shown in Figure 4 [10]. For the samples processed by compression molding, the Raman intensity correlation was 33.1ri in any direction, curve(c) in Figure 4. Whereas for the injected samples that correlation shown lightly variations: 1.36 and 1.17 for parallel and perpendicular directions, respectively. For the tensed sample variation on relationship between symmetric and asymmetric C-C vibrations were significant as shown in Figure 4.
Figure 3. Resistivity dependence of deformation percentage for a 15%CB wt sample.
Figure 4. a) and b) Raman spectra of a 100% stretched HDPE, a) parallel, b) perpendicular and c) HDPE unoriented sample
4. Conclusions In this work we showed that polymer composites of HDPE + CB are materials with a possible electrical anisotropy. Such anisotropy can be induced by uniaxial deformation. The electrical anisotropic effects were higher for CB concentrations near to threshold percolation, detecting differences between parallel and perpendicular directions of at least 2 magnitude orders. The explanation of the electrical anisotropy is described in terms of the percolation theory criteria,
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing Journal of Physics: Conference Series 167 (2009) 012039 doi:10.1088/1742-6596/167/1/012039
3
considering that the CB particles connectivity is modified and improved by the packing and the alignment of the polymer chains into the HDPE semicrystalline matrix.
5. Acknowledgements Financial support by CONACyT-NSF Ref: J110.403/2007 is gratefully acknowledged.
References [1] Sartore L, Penco M, Della Sciucca S, Borsarini G, and Ferrari V 2005 New carbon black
composite vapor detectors based on multifunctional polymers Sensors and Actuators B: Chemical 111
[2] Donnet J B, Bansal R. L, Wang M.-J (Eds.) 1993 Carbon Black New York Dekker [3] San Juan-Farfán R, Hernández-López S, Martínez-Barrera G, Camacho-López M.A,Vigueras-
Santiago E 2005 Electrical characterization of polystyrene-carbon black composites. Phys. Stat. Sol. (c) 2 3762
[4] Stauffer D, Aharony A 1994 Introduction to percolation Taylor and Francis, London [5] Zhang W, Dehgheni – Sanij A A 2007 Carbon Based Conductive Polymer Composites. Matter
Sci. 42 3408 [6] Carmona F, El Amarte A 1987 Anisotropic electrical conductivity in heterogeneus solids wiyh
cylindrical conducting inclusions Physical Review B 35 3284 [7] Skal A, Grebnev I 1992 Percolation and transport coefficients of biaxial anisotropic composites
J. Phys. Condens. Matter. 4 1521 [8] Peterlin A 1971 Molecular model of drawing polyethylene and polypropylene J. Mater. Sci. 6
490 [9] Yui H, Wu G, Sano H, Sumita M, Kino K 2006 Morphology and electrical conductivity of
injection molded polypropylene /carbon black composites with addition of high density polyethylene Polymer 49 3599
[10] Citra M J, Chase D B, Ikeda R M, Gardner K H 1995 Molecular orientation of high density polyethylene fibers characterized by polarized raman spectroscopy Macromolecules 28 4007
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing Journal of Physics: Conference Series 167 (2009) 012039 doi:10.1088/1742-6596/167/1/012039
4
View the article online for updates and enhancements.
-
-
-
-
-
This content was downloaded from IP address 125.231.64.24 on 28/11/2021 at 07:25
E. Vigueras-Santiago, S. Hernández-López, M.A. Camacho-López and O. Lara- Sanjuan
Laboratorio de Investigación y Desarrollo de Materiales Avanzados (LIDMA). Facultad de Química, UAEM. Paseo Colón esq. con Paseo Tollocan, s/n. C.P. 50000, Toluca, Estado de México, México.
E-mail: [email protected]
Abstract. High density polyethylene + carbon black composites with electrical anisotropy was studied. Electrical anisotropy was induced by uniaxial mechanical deformation and injection moulding. We show that anisotropy depends on the carbon black concentration and percentage deformation. Resistivity had the highest anisotropy resistivity around the percolation threshold. Perpendicular resistivity showed two magnitude orders higher than parallel resistivity for injected samples, whereas resistivity showed an inverse behaviour for 100% tensile samples. Both directions were set respect to the deformation axe. Anisotropy could be explained in terms of the molecular deformation (alignment) of the polymer chains as a response of the deformation process originating a redistribution of the carbon black particles in both directions. Alignment of the polymer chains was evidenced by polarized Raman spectroscopy.
1. Introduction Actually, the study and synthesis of novel conductive polymer composite (CPC) with carbon black (CB) are of the great interest due to its broad practical applications [1]. In particular is possible to modify the electrical resistivity in at least 11 magnitude orders without important changes in the CPC density [2]. Furthermore, they have the ability of significantly change their electrical resistivity by the stimulus of external parameters as temperature, solvents, pressure, AC electric field, etc [1,3]. Percolation theory is the most cited description for explaining the electric conduction in this type of systems [4]. However, such a description doesn’t consider fundamental questions related with physical or chemical interaction among conductive particles and polymer matrix which take place during the CPC preparation. The optimum develop of these materials is an experimental work for each pair polymer-conductive particles. Is well known that both: appropriated disaggregating and dispersion of the CB particles produce the building of conduction networks [5]. Then the electrical properties of the PCP depend of the processing parameters [5]. In this work we are interested in study the changes in electrical resistivity of high density polyethylene-based CPC with high structure CB. For such propose, CPC in a width composition range of CB were prepared. The main goal is to show the possibility of controllably induces electrical anisotropy in this type of CPC [6,7]. The anisotropy could be explained in terms of a change in spatial configuration of the CB network chains during the uniaxial polymer matrix stretching, induced mainly by the molecular orientation of HDPE polymer chains along of the tensile axe [8,9].
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing Journal of Physics: Conference Series 167 (2009) 012039 doi:10.1088/1742-6596/167/1/012039
c© 2009 IOP Publishing Ltd 1
2. Experimental section HDPE YUZEX® - 8800 and Vulcan XC72-CB donated by Cabot, Co. were used as received. CPCs were obtained by melting mixing using a home made Banbury micromixer at 175°C, 55 rpm for 30 min at compositions of 1 to 20% wt/wt. CPCs were processed to cylinders of 0.5 cm radio and 0.5 cm high by thermomechanic molding following the methodology described in [3]. Silver paste SPI was deposited on both faces of the cylinders as electrical contacts. The electrical resistivity was measured using a Keithley electrometer 6517A . The electrical resistivity error was 6%.
For the anisotropy study, CPCs were processing to bars of 5 x 5 x 50 mm by injection molding. After, these bars were stretched by tension at a deformation rate of 1 cm/mm at 80°C using home made equipment. Molecular orientation as a deformation function was monitored by polarized light microRaman spectroscopy, using a Micro Raman Labram HR-800 de Jobin Yvon-Horiba, a He-Ne ( =632.8 nm) polarized laser and an Olympus BX4 microscope. The relative intensities ( ri ) of the symmetric C-C vibrations from the main chain, with a typical frequency on 1129 cm-1 were compared with the respective asymmetric ones which appear at 1060 cm-1 for compressed, injected and stretched HDPE samples.
3. Results and discussion The resistivity changes as a function of CB composition are shown in Figure 1. The squared curve corresponds to cylinder samples. The circle and triangle symbol curves correspond to the injected bars samples which were measured in both directions: parallel (circles) and perpendicular (triangles), as set up in Figure 2. According to our results, we could observe that conductive networks building depend on the processing method. In particular, resistivity values for injected (bar) samples were higher than those of compressed (cylinder) samples as shown in Figure 1. Differences between both directions of injected samples and thermomechanical molded samples could due to a slight orientation of the HDPE chains during injection processing.
T en
si le
ax e
Figure 1. Influence of the processing method (compression molding and injection) on the HDPE+CB CPC´s resistivity.
Figure 2. Configuration set up for resistivity measurements: a) parallel resistivities were measured trough the transversal sections perpendicular to tensile axe b) perpendicular resistivities were evaluated between 2 lateral opposite bar faces.
Figure 3 shows the electrical resistivity as a function of the deformation percentage for a 15% CB- CPC bar. Perpendicular direction of a no stretched sample had 2 magnitude order higher resistivity than parallel one (109 and 107 cm, respectively, Figure 1). As it was stretched the resistivity
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing Journal of Physics: Conference Series 167 (2009) 012039 doi:10.1088/1742-6596/167/1/012039
2
diminished at least 5 magnitude orders in both directions, but an inversion of the resistivity values was detected at 100% of deformation At his point, a parallel direction is more resistive than perpendicular one (104 and 102 cm, respectively). For deformations larger than 100%, the resistivity lightly increases for both directions until they reach almost the same value at 250% deformation.
The explanation for those facts are that as the deformation percentage increases, the polymer chains increases their orientation modifying at the same time the electrical network configuration of CB conductive particles. Then electrical resistivity of a stretched bar is modified in several magnitude orders in both directions. The resistivity behavior of HDPE+CB CPC under stretched deformation presented a different behavior in comparison with rubber-based CPC as polybutadiene, polyisoprene and silicon, for example, where an increasing in the length is accomplished of a size diminishing in the lateral direction. Consequently an increase of resistivity along of the parallel direction is observed while along the perpendicular direction the resistivity decreases. For a stretched HDPE+CB CPC, a number of the contacts in the conductive pathway increase in both directions. This could be consequence of the better packing of the chains polymer along the stretching axe diminishing the volume surrounded the particles and bringing near the CB particles (more dense material). This effect could increases the contacts among them and the possibility to build more conductive networks in both directions.
Raman spectroscopy allowed evidencing the polymer chain orientation after the injection and tension processes. Comparing the intensities of the symmetric C-C vibrations which appear at 1129 cm-1 with the asymmetric ones at 1060 cm-1 was possible to establish an orientation difference as shown in Figure 4 [10]. For the samples processed by compression molding, the Raman intensity correlation was 33.1ri in any direction, curve(c) in Figure 4. Whereas for the injected samples that correlation shown lightly variations: 1.36 and 1.17 for parallel and perpendicular directions, respectively. For the tensed sample variation on relationship between symmetric and asymmetric C-C vibrations were significant as shown in Figure 4.
Figure 3. Resistivity dependence of deformation percentage for a 15%CB wt sample.
Figure 4. a) and b) Raman spectra of a 100% stretched HDPE, a) parallel, b) perpendicular and c) HDPE unoriented sample
4. Conclusions In this work we showed that polymer composites of HDPE + CB are materials with a possible electrical anisotropy. Such anisotropy can be induced by uniaxial deformation. The electrical anisotropic effects were higher for CB concentrations near to threshold percolation, detecting differences between parallel and perpendicular directions of at least 2 magnitude orders. The explanation of the electrical anisotropy is described in terms of the percolation theory criteria,
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing Journal of Physics: Conference Series 167 (2009) 012039 doi:10.1088/1742-6596/167/1/012039
3
considering that the CB particles connectivity is modified and improved by the packing and the alignment of the polymer chains into the HDPE semicrystalline matrix.
5. Acknowledgements Financial support by CONACyT-NSF Ref: J110.403/2007 is gratefully acknowledged.
References [1] Sartore L, Penco M, Della Sciucca S, Borsarini G, and Ferrari V 2005 New carbon black
composite vapor detectors based on multifunctional polymers Sensors and Actuators B: Chemical 111
[2] Donnet J B, Bansal R. L, Wang M.-J (Eds.) 1993 Carbon Black New York Dekker [3] San Juan-Farfán R, Hernández-López S, Martínez-Barrera G, Camacho-López M.A,Vigueras-
Santiago E 2005 Electrical characterization of polystyrene-carbon black composites. Phys. Stat. Sol. (c) 2 3762
[4] Stauffer D, Aharony A 1994 Introduction to percolation Taylor and Francis, London [5] Zhang W, Dehgheni – Sanij A A 2007 Carbon Based Conductive Polymer Composites. Matter
Sci. 42 3408 [6] Carmona F, El Amarte A 1987 Anisotropic electrical conductivity in heterogeneus solids wiyh
cylindrical conducting inclusions Physical Review B 35 3284 [7] Skal A, Grebnev I 1992 Percolation and transport coefficients of biaxial anisotropic composites
J. Phys. Condens. Matter. 4 1521 [8] Peterlin A 1971 Molecular model of drawing polyethylene and polypropylene J. Mater. Sci. 6
490 [9] Yui H, Wu G, Sano H, Sumita M, Kino K 2006 Morphology and electrical conductivity of
injection molded polypropylene /carbon black composites with addition of high density polyethylene Polymer 49 3599
[10] Citra M J, Chase D B, Ikeda R M, Gardner K H 1995 Molecular orientation of high density polyethylene fibers characterized by polarized raman spectroscopy Macromolecules 28 4007
XIX Latin American Symposium on Solid State Physics (SLAFES XIX) IOP Publishing Journal of Physics: Conference Series 167 (2009) 012039 doi:10.1088/1742-6596/167/1/012039
4