To suggested that some sort of a tunneling

To
enhance the electrical conductivity, carbonaceous fillers are regularly used in
the production of the polymer composites. Current carbonaceous fillers include
graphite, carbon black (CB), carbon fiber (CF), and carbon nanotubes (CNTs)

Many
researchers worked on conductive carbon-polymer composites.

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Sichel et al.
reported that conductivity depends on carbon content of composite. The
mechanism of electrical conduction depends on the gaps between conducting
regions rather than on the detailed physical properties of carbon black. They
made carbon-polymer composites by dispersing carbon blackCB into a polymer
matrix. The compounding is has been done by adding the carbon blackCB to the
polymer, mixing at temperatures above the glass transition temperature and
subjecting the mixture to high shears rates in an extruder. They noted that
over a wide range of compositions the conductivity of the composite is limited
not by the properties of the carbon but by the gaps between carbon spheres or
aggregates of spheres. It has been suggested that some sort of a tunneling
mechanism is responsible for electric conduction. Sichel, 1982.

Miyasaka et al.
found a relation between electrical conductivity and carbon content of
composite. They measured electrical conductivity of carbon particle-filled
polymers as a function of carbon content to find a break point of the
relationships between the carbon content and the conductivity. It has been
known that the electrical conductivity of insulating polymers filled with
conducting particles, such as metal and carbon powder, discontinuously
increases at some content of the filler. They used six kinds of polymers- Polypropylene,
Low density polyethylene, Poly methyl methacrylate, Polystyrene, Styrene
butadine rubber, Natural rubber and a carbon filler. Each polymer except PMMA
was mixed with a given amount of carbon filler in a two-roller mill for 20
minutes at temperatures ranging from 80 to 2250C. The electrical
conductivity of the samples varied over a wide range from 10-18 to 1
S cm-1. Miyasaka, 1982.

Planes
et al.  reported that the mixture of
different types of conductive filler (graphite, CB, CFs)  is a very promising way to improve the
electrical behavior: a well-chosen combination of graphite and others, even
less conductive, carbonaceous fillers (CB, CFs, CNTs), may improve the
electrical conduction as compared to graphite alone. They also noted the
heterogeneity in the electrical properties of the carbon-polymer composite
sheets. They suggested that a coupled optimization of the formulation and
process is necessary to develop highly conductive polymer composites. They
found with these heavily filled materials, the exact formulation impacts the
electrical conductivity less than the processing conditions. Planes, 2015.

Zhang et al.
did a review on carbon-polymer composites behavior. They found that CB
particles and CFs are the most commonly used conductive components to
incorporate conduction to polymer composite. The reason for this is that CB
particles have a much greater tendency to form a conductive network due to
their chain- like aggregate structures compared with other conducting additives
such as metal powder. Whilst CFs may be considered as chain-like aggregates of
carbon particles having long chain length. They suggested lot of processing
parameters to achieve high conductivity. Processing parameters must be tailored
in order to eliminate the processing induced degradation of CB and CF; low
viscosity and multiphase polymer matrices favor an increase of conductivity;
for carbon black, small particle size, high structure, anisometric shapes and
high micro-porosity are preferred; for carbon ?ber, moderate ?ber length and
high aspect ratio aid the improvement of conductivity; additionally high
compatibility of polymer matrix with conductive ?ller and uniformity of
dispersion of conductive ?ller in polymer matrices contribute positively to the
conductive property. Zhang, 2007.

Percolation phenomena

Many conductive polymer composites exhibit percolation characteristic. The
curve of conductivity versus ?ller concentration has S-shape, which clearly
demonstrates a relative narrow ?ller loading range during which a small
increase in loading will result in a drastic increase in conductivity. This
change implies some sudden changes in the dispersing state of conductive
particles, i.e. the coagulation of particles to form networks which facilitates
the electrical conduction through the composites. Put in another way, the
composite exhibits an insulator to conductor transition. The critical amount of
?ller necessary to build up a continuous conductive network and accordingly to
make the material conductive is referred to as the percolation threshold. For
the production of conductive polymer composites, the ?ller content must be as
low as possible but still allows the composite to ful?ll its electrical
requirements. Otherwise the mixture processing becomes dif?cult, the mechanical
properties of the composite are poor and the ?nal cost is high. The way of
decreasing the percolation threshold of conductive ?ller concentration in
polymeric matrices, are mainly based on the use of additives, the optimization
of processing conditions, as well as the size, distribution and porosity of
?ller Zhang, 2007.

Polymer
– Carbon aerogel composite

To
make carbon reinforced composite, carbon aerogel is using intensively in recent
years. A thermoplastic polymer-carbon aerogel composite material patented in
2010 by Williams et al. they blended a polymer material named MXD6 which is
thermoplastic with carbon aerogel. The composite material has better thermal
insulation stability, flexibility and less brittleness at then MXD6. They mixed
aerogel with the polymer in melt extrusion. Williams, 2010.

The mechanical
properties and toughness of epoxy polymers modified by Hsieh et al with aerogel
content of 0-0.5 wt% and Hsieh et al reported the stiffness and strength of the
carbon aerogel reinforced polymers increased with the carbon aerogel content.

The
composites were prepared and studied by incorporating carbon aerogels in an
epoxy polymer. A sonication process was performed in the sample preparation so
as to achieve good aerogel dispersion in the epoxy resin. Calculated amount of
the epoxy resin was ?rst put in an oven at 500C for 15 min. At this
stage, the viscosity of the resin was similar to that of water. Then, the epoxy
resin was removed from the oven, different weight percentages (0.0–0.5 wt %) of
the carbon aerogels were added to the epoxy resin and mixed by an ultrasonic
probe. The sonication process was conducted for 20 min. A stoichiometric amount
of the curing agent was added into the aerogel/epoxy mixture, and then the
mixture was stirred thoroughly at room temperature using a magnetic stirrer at
450 rpm for 20 min, and degassed at 600C and -1 atm using a vacuum
oven. Finally, the carbon aerogel/epoxy mixture was poured into a release-agent
coated steel mold to manufacture plates from which bulk specimens could be
machined. The specimen plates were cured at 800C for 4 h and then
post-cured at 1400C for 8 h. Hsieh, 2015.

Carbonaceous
filler reinforced PLA composite

In
recent years, conductive composites containg PLA are gaining importances
because of PLA has renewable source and has great potential as a substitute for
petroleum derivatives.

Hsieh
et al. successfully made PLA-Carbon fiber (CF) composite which has high
strength, high electrical properties and good thermal behaviors by melt
compounding. They used a modifier named Maleic anhydride grafted
styrene-ethylene-butylene-styrene block copolymer (SEBS-g-MA) during their
combination to improve their interfacial compatibility. They first mixed PLA,
CF and modifier at a single screw extruder, then cooled and dried in oven at
800C for 12 hours and then extruded at 2000C. CF was at constant amount of 12%
of weight while PLA was at different amount according to amounts of modifier.
The electrical conductivity of the composites that contain 8 wt% of SEBS-g-MA
was found 2.2×10-5 S/cm. Hsieh, 2016.

Kuan
et al. achieved high electrical conductivity at PLA + MWCNT composite which was
prepared by them. Before compounding, MWCNT was modified by maleic
anhydride and then MWCNT dispersion enhanced by covalent or hydrogen bonding
between MWCNT and PLA. Composite made by compounding
in twin screw extruder at 1500C -1900C temperature range. As they found higher
electrical conductivity in low crystalline PLA composite than highly
crystalline PLA composite, they also claimed that degree of crystallinity can
influence on the electrical property of composite. Kuan 2008.

Villmow
et al evaluated the influences of extrusion conditions on the dispersion of
MWCNT in PLA matrix. They found that high rotation speed enhances dispersion
and distribution of MWCNT but temperature profile showed less influence. They
also claimed that high electrical conductivity depends on dispersion of carbon
filler. They found electrical resistivity ranging from 60 to 300 ohm cm with
different amounts of MWCNT. Villmow, 2008.

Wu
et al. prepared different amounts of MWCNT reinforced PLA composite by direct
melt compounding at a rheometer at 1700C and 50 rpm for 8 minutes. They claimed
that purified MWCNTs retards the decomposition rate of PLA by increasing
decomposition temperature. Wu, 2008. Tian et al. proposed a 3D printing based
fabrication process of contionus fiber reinforced PLA composite. Tian, 2016.

3.4.
Ionic Liquid

Generally ILs
defines as low-melting organic salts which are liquid below 100 °C. An ionic
liquid (IL) is a salt in the liquid state.  These are largely made of ions and
short-lived ion pairs. Any salt that melts without decomposing or vaporizing
usually yields an ionic liquid.  Ionic
liquid has many practical applications. Ionic liquids are good solvents
and possess electrically conducting Yang, 2010.

Ionic liquids
are cations like imidazolium, pyridinium, quaternary ammonium, and anions like
halogen, triflate, tetrafluoroborate and hexafluorophosphate containing salts.
This salts exist in the liquid state at relatively low temperatures. These  are non-flammability, non-combustibility, have
high thermal stability, relatively low viscosity, wide temperature ranges for
being liquids and high ionic conductivity and they are lack of significant
vapor pressure Su, 2012.

Recently,
ionic liquids have been extensively evaluated as environmentally friendly or
”green” alternatives to conventional organic solvents for a broad range of
organic synthetic applications. To replace the volatile and relatively toxic
organic solvents, Imidazolium-based ionic liquids have been increasingly used
as green solvents in homogeneous and heterogeneous catalysis, materials
science, nano materials, lithium ion batteries, and separation technology Su, 2012.

Recently
ionic liquid of imidazolium-type is widely used as the best material for carbon
dispersion in various liquid media. Ionic liquids based on imidazolium
cations usually show the highest ionic conductivity (~1 and 10–1 S/m) Yang,
2010.

 

3.5.
Polymer – Carbon – Ionic Liquid composite

Peng et al.
presented a review on the imidazolium containing IL assisted fabrication of
CNTs and nanocomposites based on Graphene with a variety of polymers. They observed
that MWCNTs modified by organic molecules, polystyrene (PS), hyper-branched
poly (urea-urethane), biodegradable poly(?-caprolactone) (PCL),
azobenzene-containing polyurethane and ILs are well soluble in water or organic
solvents. After functionalization MWCNTS can be uniformly dispersed and well
bonded to the polymer matrix. Peng, 2013.

Zhou et al.
have reported that an Imi-IL of 1-butyl-3-methylimidazolium hexafluorophosphate
(BmimPF6) can dispersed graphene sheets with the help of poly (1-vinyl-3-butylimidazolium
chloride) poly (VbimCl). Zhou, 2010.  Imi-ILs functionalized graphene sheets are
homogenously embedded in the poly (methyl methacrylate) (PMMA) matrix and thus
contribute to high electrical conductivity Yang, 2010.

The production
of stable homogeneous suspensions of CNTs in Imi-ILs do not need external
dispersants. This information emerges as a powerful strategy toward
manufacturing of polymer composites. Fukushima, 2005.

Mandal et al.
have reported that, in the PVDF matrix, functionalized MWCNTs by an Imi-IL of
3-aminoethyl imidazolium bromide dispersed by solution casting or
melt-blending. The PVDF composites show a very low percolation threshold (0.05
wt %) and a large increase in the conductivity was reported because of the
improved dispersion and interface in the presence of the anchored Imi-ILs. Mandal,
2013. In a recent work reported by Pandey et al. reported was successfully
fabricated a graphene-based all-solid-state supercapacito by a gel electrolyte consisting
of BmimBF4 and PVDF-HFP. Pandey, 2012.

By directly
mixing pristine CNTs with hydrocarbon polymers, it is impossible to produce the
homogeneous composites Mitchell, 2002. Functionalized MWCNTs with
1,2-dimethyl-3-hexadecylimidazolium tetrafluoroborate (DhimBF4) by melt
blending at 185 °C in a twin-crew mini-extruder was prepared by Bellayer et al.
The composite processing temperature is higher than the liquid-crystalline
transition temperature of DhimBF4. DhimBF4-modified MWCNTs were grounded into
powders after cooling and then added to the PS matrix at 195 °C. It was found
DhimBF4-modified MWCNTs were uniformly dispersed in or even individually
exfoliated into the PS matrix. Therefore, DhimBF4 acts as a compatibilizer to
improve the affinity of MWCNTs for PS. Bellayer, 2005.

Zhao et al
reported IL exhibited four functions in PMMA composite. Those are-

1.         IL acted as binder to improve the
dispersion of MWNTs in the PMMA matrix,

2.         IL acted as efficient plasticizer to
decrease the glass transition temperature of PMMA matrix.

3.         IL is a good processing aid and
lubricant during melt processing and thus can decrease the melt viscosity of
composite.

4.         To improve electrical conductivity, IL
takes the role of a dopant.

IL has like
extremely low volatility, excellent thermal stability and wide liquid/phase
region. For these reasons, IL has some potential technical advantages over
traditional plasticizers. IL is a good lubricants (processing aid) due to
having excellent electrical and thermal conductivity. Molecular friction and
the entanglement of molecular chains in the polymer composite also decreased by
the used IL. IL-modified nanocomposites also showed significantly higher
electrical conductivity and lower percolation threshold than nanocomposite
without IL. The IL also improved the dispersion of nanotubes to form an
interconnected network structure between dispersed nanotubes. Therefore, it can
be concluded that IL acted as a bridge for electron transfer between the carbon
nanotubes Zhao, 2012.

It was reported
that composites of IL with polymer can be formed and successfully used as
catalytic membrane. Snedden, 2003.

Kaasika et al.
reported about an actuator material, that consist of carbon aerogel,
1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and
poly(vinylidene-co-hexafluoropropylene) (PVdF(HFP)). Actuators were made by
using layer-by-layer casting method and they work as a bending actuators. They
prepared the actuator electrodes was dissolving PVdF(HFP) pellets in
Dimethylacetamide (DMAc). EMIBF4 and carbon aerogel powder were mixed in 1.5 ml
DMAc and treated with ultrasonic probe using 50% of power and cycle period of
0,5 for five minutes. Afterwards the polymer mixture was added to carbon
aerogel and IL suspension. After that the whole mixture was treated with
ultrasonic probe and was poured out into the mold made of Teflon.  The mold was placed into vacuum oven at 100
mbar and 22 C. Separator was prepared from EMIBF4 and PVdF(HFP) with weight
ratio 1/1 by dissolving in a mixture of 2 ml MP and 500 mg PC. This mixture was
heated up to 70 C and stirred with a magnetic stirrer for 24 h. After the
stirring it was poured out into the Teflon mold. After the solvent has evaporated
the solid electrode and membrane films were obtained. Kaasika, 2011.

 

3.6. Rheological
properties of polymer

Rheology
is a branch of physics which deals with the de- formation and flow of matter
under stress. It is particularly concerned with the particular properties of
matter that determine its behavior when a mechanical force apply on it.

Rheology
has been used as semi- quantitative tools in polymer science and engineering
for many years. The relationship between the structure and rheology of a
polymer is of practical interest for two reasons: firstly, rheological
properties are very sensitive to certain aspect of structure and they are
simpler to use than analytical methods, such as nuclear magnetic resonance.
Secondly, it is the rheological properties that govern the flow behaviour of
polymers when they are processed in the molten state. Agboola, 2011.

Polymer
rheology testing is the study of how the stress in a polymer or force applied
is related to deformation and flow of the polymer. Rheology generally accounts
for the behavior of non-Newtonian fluids, by characterizing the minimum number
of functions that are needed to relate stresses with rate of change of strain
or strain rates.

While
rhetorical properties are performed, the polymer is in the melt phase or has
been dissolved in a solvent. Intertek