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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
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.
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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
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