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.

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