View
2
Download
0
Category
Preview:
Citation preview
Indian Journal of Engineering & Materials Sciences
Vol. 20, February 2013, pp. 68-72
Preparation of three-component conducting polymer composite using nucleate
doping technique
Niharika Srivastavaa,b
, Y Singha & R A Singh
b*
aDepartment of Chemistry, Udai Pratap College, Varanasi 221 002, India bMolecular Electronics Laboratory, Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 010, India
Received 25 January 2010; accepted 22 October 2012
This paper reports a new route for the preparation of conducting network in insulating polymer matrix. In existing
techniques solubility of conducting component is an essential condition. Reported technique facilitates making composite of
conducting polymer, which is insoluble in normal solvents, with percolation threshold of 0.5 wt%. Mechanism of formation
of the three component hybrid may be described as when silica which acted as seed is dispersed in insulating polymer, the
in-situ polymerized conducting polymer having high aspect ratio is grown over the silica surface connecting each other and
making a continuous network. The star like growth of the conducting polymer can be seen under optical microscope.
Keywords: Conducting polymer composite, DLA model, Percolation, Nucleate doping
Organic molecular materials, charge-transfer complexes
and conducting polymers, offer a multidisciplinary
challenge to chemists, physicists, material scientists and
electronic engineers and have a great potential for
commercialization. Organic materials cover full range of
materials from insulators to superconductors1,
diamagnetic to ferromagnetic2 and optical to non-linear
optical3,4
(NLO) properties. But the mechanical and
processbility issues need considerable attention in these
materials5. Charge-transfer complexes are usually
soluble in organic solvents, so they have better
processbility but have very little mechanical strength.
On contrary the conducting polymers are self sufficient
on mechanical account but being almost insoluble
practically unprocessble. A common solution to both the
problem is making their composite with conventional
polymers. Conventional polymers form matrix in which
organic conductive fillers are embedded giving
mechanical strength, retaining the electrical properties;
at the same time processbility of insulating polymer can
also be exploited for fabrication of devices.
Owning the potential of the composite material
several methods have been proposed for the
preparation of these materials. These include simple
grinding6, zone casting
7, two-step reticulate doping
8
and electrochemical method9 blending method
10,11,
grafting reaction12-14
and two-phase synthesis15,16
.
Reticulate doping method is one of the best among
these methods. In reticulate doping method, the
microcrystals are grown during film casting17
. In
appropriate conditions the crystallization of the
charge-transfer complex leads to formation of a
conductive network of crystallites in the bulk of
polymer, which makes the film conductive at as low
as 0.3 wt% level of loading, due to high aspect ratio18
.
Tracz et al.19
have summarized main parameters
controlling the charge-transfer complex crystallization
in reticulate doping process. Reticulate doping is a
good method for making composite, where both the
components are easily soluble in same solvent, but
this procedure cannot be used if either of the
components is insoluble or are soluble in two
different immiscible solvents.
In this paper, we report a new technique for preparing
nucleate doped composite of conducting polymers
(which are not soluble in ordinary solvents) with high
aspect ratio. In this method, we disperse silica in
insulating polymer and let the conducting polymer grow
over it. The silica acts as seed and the polymer having
high aspect ratio was grown connecting each other and
making a continuous network.
Experimental Procedure
Silica gel-G (TLC grade, Sulab’s chemical) was
dispersed in PVA (S D Fine chemicals, Mol.wt
(1,25,000)) (in triple distilled water) (2% w/v) with
continuous stirring on magnetic stirrer so that silica
____________
*Corresponding author
(E-mail: rasingh@bhu.ac.in; prrasingh@gmail.com)
SRIVASTAVA et al.: CONDUCTING POLYMER COMPOSITE
69
particle could uniformly distribute in solution.
PANI/silica hybrid was prepared by simple chemical
polymerization route. The aniline monomer was
purchased from (Qualigen AR) and used after
distillation. Aniline (0.05 M) in 0.5 M HCl was added
to the silica gel dispersion and kept under continuous
stirring in ice bath for one hour. Ammonium
persulphate (0.05 M) (Qualigen) was added very
slowly to start polymerization. After completion of
polymerization the solution turned into violet color.
Required amount of the solution was added in the
petri dish and left for solvent evaporation at room
temperature. The concentration of monomer and
oxidant was kept low to slow down the rate and
degree of polymerization so that polymerization could
take place on the surface rather than in bulk. The
samples for BET surface area measurement and SEM
measurement were taken after one hour from the start
of the polymerization and then the system was left for
slow evaporation of the solvent. The solution slowly
turned into gel and finally into thin film after
complete evaporation of solvent. After around
4-5 days the resultant hybrid has shown
well-dispersed structure of PANI over silica in poly
(vinyl-alcohol) matrix having high aspect ratio. The
composite thus formed have been characterized by
various techniques. FTIR was recorded in KBr
medium on Jasco 5300. TEM photos were taken using
JEOL-JEM 100SX at 100 kV acceleration voltage, the
samples for the TEM experiments were prepared by
suspending dried samples in absolute ethanol. A drop
of the sample suspension was allowed to dry on a
copper grid (400 mesh) coated with a carbon film.
SEM images were taken by employing FEI - Quata
200 at 5 kV acceleration voltage. Optical
microscopies were done on Lietz Labour Lux-D with
built-in-camera; BET surface area was measured on
Micromeritics-Gemini 2375 surface area analyzer.
XRD were recorded on Bruker X-ray diffractometer
D8 using Cu-Kα radiation and the electrical
measurements were done using LCZ meter-Keithley
3330. The contacts for electrical measurements were
made by silver paint and the dimension of the sample
was 1 cm × 1 cm × 0.1 cm.
Results and Discussion
The silica used in current study has irregular faces,
which can be seen in the TEM image (Fig. 1). These
faces acted as nucleating point for the growth of
polyaniline (PANI), which could be seen in SEM
images, taken just after start of polymerization (Fig. 2).
In Fig. 2a, silica particles dispersed in poly
(vinyl-alcohol) (PVA) matrix can be seen, whereas
Fig. 2b shows coating of PANI over silica faces as a
thinner lager. The dispersion of silica particles in poly
(vinyl-alcohol) has been studied by Boisvert et al.20
in
details. They observed nice microphase separation
and well defined surface-to-surface distance within
the silica clusters. These mono-dispersed silica
particles act as nucleus for the growth of polyaniline
in our experiment. When the monomer was added to
the silica/PVA dispersion under continuous stirring
the annilinium ion got adsorbed on the surface of
silica particles which on further slow addition of
oxidizing agent polymerization started leading to the
formation of long chain of polyaniline. It can be seen
apparently in SEM (Fig. 2b) images that nucleation
started from different faces of silica. This was further
confirmed by the BET surface area measurement. The
surface area of bare silica was found to be 90.7 m2/g
where as after PANI coating it was increased up to
106.10 m2/g.
The interaction between polyaniline and silica
through hydrogen bonding has already been
reported21
. Weak ineractions between silica and
polyaniline has been confirmed by IR analysis. After
4-5 days star like morphology of PANI/silica hybrid
growing in PVA matrix was observed, though they
were fewer in number. When the sample was left for
at least 10-15 days under slow evaporation the
solution slowly tuned into thin film through the
formation of gel. We observed many more prominent
stars. The optical micrograph of the growth is given in
Fig. 1 Transmission electron micrograph (TEM) of bare silica
particle used as seed for the growth of polyaniline in poly
(vinyl-alcohol) matrix (inset shows individual particles)
INDIAN J. ENG. MATER. SCI., FEBRUARY 2013
70
Fig. 3. Figure 3a shows the pattern of the growth on a
single particle, whereas Fig. 3b shows the
interconnectivity of the stars. These stars have very
high aspect ratio (~12). The aspect ratio of an image
describes the proportional relationship between its
width and its height. The star like morphology in
silica/PANI hybrid is reported elsewhere20
. But those
star structures were small enough so that they could
be seen under TEM only. In the present study the
PVA matrix provides support to the polyaniline
forlarger growth giving high aspect ratio. This growth
in general can be explained on the basis of diffusion-
limiting-aggregation (DLA)22
. DLA has generated
considerable interest in the percolation model. The
model is quite simple, this starts with a seed, which is
silica particle in the present case. Perhaps the success
of this technique is because of the homogeneous
dispersion of seed (silica) in PVA matrix. In absence
of these seed particles the conducting seed generated
in insulating polymer generally get aggregated at
fewer places resulting in molecularly dispersed
composite23
. Another particle is then allowed to walk
at random (i.e. diffuse) from far away until it arrives
at one of the site adjacent to the occupied site, and so
forth. A large cluster may be formed in this way.
The assignment for the main IR peaks in silica,
poly (vinyl-alcohol), polyaniline and the composite
are given in Table 1. Bare silica shows bands and
peaks between 800 cm-1
- 1200 cm-1
due to various
symmetric and asymmetric vibrations of Si-O-Si24
.
Another peak which was assign to silica is at 505 cm-1
due to Si-O-Si-O deformation25
. The peak at 3393 cm-1
in silica is assigned to the –OH stretching vibration
Fig. 2 Scanning electron micrograph of silica particle dispersed in
poly (vinyl-alcohol) matrix (a) at beginning and (b) after one hour
Fig. 3 Optical Micrograph of silica/PVA/PANI composite
(0.5 wt% PANI) showing (a) star like growth of polyaniline on
silica seed in poly (vinyl-alcohol) matrix and (b) interconnectivity
of the individual stars
SRIVASTAVA et al.: CONDUCTING POLYMER COMPOSITE
71
which is due to adsorbed water26
. Poly (vinyl-alcohol)
shows peaks at 1442 cm-1
, 2930 cm-1
and 3415 cm-1.
These peaks are assigned to C=H of vinyl group, C-H
stretching vibration and –OH groups, respectively27,28
.
The peaks at 1597 cm-1
and 1495 cm-1
in polyaniline
are due to quinonoid and benzenoid forms
respectively29
, since conducting polyaniline is partially
oxidized state having 50% quinonoid and 50%
benzenoid structures. In the composite we can see all
these peaks with a slight shifting in the vibrational
band positions which may be due to weak interactions
like hydrogen bonding and vander Waals forces etc,
which hold the components of the composites together.
The XRD patterns of the pure polymer, silica, and
the composite are given in Fig. 4. It is clear from the
XRD patterns that, while polyaniline is amorphous,
silica shows short range ordering. The peaks in
2θ = 20-30 are already reported for silica30,31
. The
XRD peaks matches well with hexagonal lattice
reported for silica (PDF 12-708) having peaks at 2θ =
26.35 (100%), 35.92 (10%), 41.68 (18%), 44.99
(12%) and 49.44 (60%). The composite shows
reduced intensity of the peaks, which confirms the
adherence of polyaniline over silica. Further it is clear
from the IR and XRD data that the composite formed
has only physical interactions and no chemical
interaction exist between the components.
The electrical conductance of the composites was
measured at three frequencies (1 kHz, 10 kHz &
100 kHz). The plot of specific conductance and
loading of polyaniline is given in Fig. 5. The
percolation threshold as low as 0.5 wt% (at all the
three frequencies used in current investigation) of
PANI (showing 4 order increase in conductance) has
been observed in these composites. This is only
slightly higher than 0.3 wt%17
measured in reticulate
doping and significantly lower than other reported
(mainly dispersion type) composites of PANI and
PVA, where this data lies at about 24 wt%27
. Since the
earlier reported PANI/PVA composites are molecular
dispersion they don’t show typical percolation
behavior, but the conductivity of those composites
increases gradually with increased loading of PANI.
Whereas in present case we see sudden increase of
conductance at 0.5 wt% loading level and after that
the plot shows a plateau. It can also be seen in Fig. 5
Table 1 FTIR assignments of silica/PVA/PANI composite
Source Assignment
Silica PVA PANI Composite
Si-O-Si,
(stretch, sym)
802 cm-1 --- --- 800 cm-1
Si-O-Si,
(stretch, asym)
1128 cm-1 --- --- 1111 cm-1
Si-O-Si-O
deformation
505 cm-1 --- --- 464 cm-1
Vinyl group
(C=H),
--- 1442 cm-1 --- 1450 cm-1
-OH, stretch 3393 cm-1 3415 cm-1 --- 3425 cm-1
C-H stretch --- 2930 cm-1 --- 2920 cm-1
Quinonoid
imine (-C=N-),
stretch
--- --- 1597 cm-1 1581 cm-1
Benzenoid
amine (-C-N-),
Stretch
--- --- 1495 cm-1 1477 cm-1
Fig. 4 X-ray diffraction pattern of (a) polyaniline, (b) silica/
PVA/PANI composite and (c) silica gel
Fig. 5 Variation of specific conductance of silica/PVA/PANI
composite as a function of loading of polyaniline at various
frequencies (a) 1 kHz, (b) 10 kHz and (c) 100 kHz
INDIAN J. ENG. MATER. SCI., FEBRUARY 2013
72
that these composites show prominent frequency
dependence of the conductivity. The conductance at
lower frequency is less and it keeps on increasing
gradually as the frequency increases. Frequency
dependence of conductance shows the existence of the
capacitive component, which exists due to charging of
the heterojunctions. This confirms our assumption
that individual stars grow and then connect each other
to make continuous conducting network.
Conclusions In conclusion we can say that a three-component
nucleate doping method can easily be used for the
fabrication of conducting composites of practically
insoluble components, with low percolation threshold
and better processibility. The success of proposed
technique is because of the homogeneous dispersion
of seed (silica) in PVA matrix. In absence of these
seed particles the conducting seed generated in
insulating polymer generally get aggregated at fewer
places resulting in molecularly dispersed composite.
Another particle is then allowed to walk at random
(i.e. diffuse) from far away until it arrives at one of
the lattice site adjacent to the occupied site, and so
forth. A large cluster may be formed in this way. The
present work demonstrated the superiority of the
three-components composite over individual two-
component composites.
References 1 Fabre J M, J Solid State Chem, 168 (2002) 367.
2 Crayston J A, Devine J N & Walton J C, Tetrahedron, 56
(2000) 7829.
3 Jiang M H & Fang Q, Adv Mater, 11 (1999) 1147.
4 Sanchez C, Lebeau B, Chaput F & Boilot J P, Adv Mater, 15
(2003) 1969
5 Margolis J M, Conductive polymer & plastics, (Chapman and
Hall Ltd., London), 1989.
6 Malhotra V P, Chandra R, Garg R K & Motsara S, Indian J
Chem, 33A (1994) 595.
7 Tracz A, ElShafee E, Ulanski J, Jeszka J K & Kryszewski M,
Synth Met, 37 (1990) 175.
8 Ulanski J, Polanowski P, Tracz A, Hofmann M, Dorman E &
Laukhina E, Synth Met., 94 (1998) 23.
9 Depaoli M-A, Waltman R J, Diaz A F & Bargon J, J Chem
Soc Chem Commun, (1984) 1015.
10 Ismail Y A, Shin S R, Shin K M, Yoon S G, Shon K, Kim S I
& Kim S J, Sens Actuat B, 129 (2008) 834.
11 Thanpitch T, Sirivat A, Jamieson A M & Rujiravanit R,
J Appl Polym Sci, 116 (2010) 1626.
12 Marcasuzaa P, Reynaud S, Ehrenfeld F, Khoukh A &
Desbrieres J, Biomacromolecules, 11 (2010) 1684.
13 Scaffaroa R, Lo Re G, Dispenza C, Sabatino M A &
Armelao L, React Funct Polym, 71 (2011) 1177–1186
14 Fonseca L C, Faez R, Camilo F F & Bizeto M A,
Microporous Mesoporous Mater, 159 (2012) 24–29.
15 Dallas P, Stamopoulos D, Boukos N, Tzitzios V, Niarchos D
& Petridis D, Polymer, 48 (2007) 3162.
16 Huang J & Kaner R B, J Am Chem Soc, 126 (2004) 851.
17 Jeszka J K, Ulanski J & Kryszewski M, Nature, 289 (1981) 390.
18 Ulanski J, Tracz A & Kryszewski M, J Phys D: Appl Phys,
18 (1985) 451.
19 Tracz A & Kryszewski M, Makromol Chem Suppl, 15(1989) 219.
20 Persello J, Boisvert J P, Guyard A & Cabane B, J Phys Chem
B, 108 (2004) 9678.
21 Wang Y, Wang X, Li J, Mo Z, Zhao X, Jing X & Wang F,
Adv Mater, 13 (2001) 1582.
22 Witten T A & Sander L M, Phys Rev B, 27 (1983) 5686.
23 Srivastava D N & Singh R A, Synth Met, 114 (2000) 361.
24 Pol V G, Srivastava D N, Palchik O, Palchik V, Slifkin M A,
Weiss A M & Gedanken A, Langmuir, 18 (2002) 3352.
25 Jang S H, Han M G & Im S S, Synth Met, 110 (2000) 17.
26 Kim D S, Park H B, Rhim J W & Lee Y M, J Memb Sci, 240
(2004) 37.
27 Srivastava D N & R. A. Singh, Mol Mater, 11 (1999) 223.
28 Zhang Z & Wan M, Synth Met, 128 (2002) 83.
29 Kan J, Lv R & Zhang S, Synth Met, 145 (2004) 37.
30 Mathai C J, Saravanan S, Anantharaman M R,
Venkitachalam S & Jayalekshmi S, J Phys D: Appl Phys, 35
(2002) 2206.
31 Vidal O, Robert M C, Arnoux B & Capelle B, J Cryst
Growth, 196 (1999) 559.
Recommended