Arab Journal of Nuclear Science and Applications, 47(1), (41-52) 2014
41
Effect of Ni Nano particles on Thermal, Optical and Electrical
Behaviour of Irradiated PVA/AAc Films
Dalia E. Hegazy, M. Eid and M. Madani National Center for Radiation Research and Technology, Nasr City, Cairo, Egypt
Received: 25/12/2013 Accepted: 20/1/2014
ABSTRACT
Solid electrolyte polymeric materials based on poly(vinyl alcohol-co-acrylic
acid) P(VA-co-AAc) containing Ni nano particles were prepared using electron
beam induced crosslinking and reduction of Ni ions technique at 30 kGy. The
synthesized composites were analysed by UV-visible spectroscopy,
thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).
The activation energy of decomposition of the electrolytes increased with increasing
salt concentration. The optical band energy gap is estimated using UV–visible
spectra and it decreases with increasing dopant concentration. The DSC studies
indicate that the dopant changes the thermal behavior of PVA/ AAc like glass
transition temperature. Dielectric behavior was studied in the frequency range of
300 Hz- 5 MHz at room temperature. The a.c. conductivity was found to increase
with the increase of dopant concentration as well as frequency. This improved
properties of P(PVA/AAc)-Ni nano composite suggested to be used in optoelectronic
devices.
Keywords: radiation; nano composites; dielectric; thermal analysis and optical band
gap.
INTRODUCTION
Polymeric materials have attracted the scientific and technological researchers, because of their
wide applications. This is mainly due to the lightweight, good mechanical strength, optical properties
and makes them to be multifunctional materials(1). In recent years, studies of electrical and optical
properties of the polymer have attracted much attention in view of their application in electronic and
optical devices. Electrical properties constitute one of the most convenient and sensitive methods for
studying the physical mechanisms that determine this prop- polymer structure aiming to understand
the nature of the charge transport prevalent in these materials, while the optical properties are aimed at
achieving better reflection, antireflection, interface and polarization properties.
The optical absorption spectra of polymers provide essential information about the band
structure and the energy gap in crystalline, semi-crystalline, and non-crystalline polymers. The
electrical and optical properties of polymers can be suitably modified by the addition of dopants.
Moreover, these polymers are traditionally considered as an excellent host material. The field of
polymer additives has attracted strong interest in today’s materials research, in view of this it is very
important to note that the dopant modifies the structure of the polymer and hence its properties. Since,
the change in polymer properties are mainly depends on the nature of the dopant and the way in which
it interacts with the polymer, as it achieve impressive enhancements of the polymer properties as
compared with the pure polymers(2, 3).
The development of nano science and nano technology has allowed us to create new nano-sized
materials having unique electronic and optical properties quite different from those of their bulk
state(4). In various electronic and optical devices the size-dependent properties of the nano materials
were used(5). The optical properties of a metallic nano particle depend mainly on its surface Plasmon
Arab Journal of Nuclear Science and Applications, 47(1), (41-52) 2014
42
resonance, and it is well known that the plasmon resonant peaks and line widths are sensitive to the
size(6) and shape of the nano particles(7), the metallic species(8) and the surrounding medium(9). The
optical properties of metal nano particles embedded in a dielectric medium have also been a subject of
immense interest in recent years because of their novel characteristics.
These studies shows, the changes in the properties like electrical and optical behavior, etc. of
the
polymer due to doping. PVA is normally a poor electric conductor; it can become conductive upon
doping with some dopant. The conducting nature of doped PVA is thought to be due to the high
physical interactions between polymer chains and dopant via hydrogen bonding with hydroxyl groups
as well as the complex formation(10- 12). The optical absorption spectroscopy (UV–visible,
thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) is an established tool
to investigate the effect of dopant on the microstructure of the polymer, particularly on the band
structure and electronic properties including the energy gap Eg. Additions of the dopants to a polymer
modify the energy band gap Eg, which depends on the type and magnitude of the defect concentration
caused by the dopant. Hence, these modifications give information on the optical, electronic, thermal
and microstructural behavior of the polymer.
EXPERIMENTAL
1. Materials:
The chemicals used in this study were fully hydrolyzed Polyvinyl alcohol (PVA) (average Mw~
14000 and purity 98%) and anhydrous acrylic acid (AAc) (linear formula: CH2=CHCOOH, Molecular
Weight: 72.06, density 1.051 g/mL at 25 °C and purity 99%) were obtained from Merck, Germany.
NiSO4 and isopropyl alcohol were supplied from El-Nasr Co. for Chemical Institutes, Cairo, Egypt.
All chemicals were used as received.
2. Preparation of Ni nano particles/ PVA-AAc films:
PVA solution 6 wt% was prepared by dissolving PVA in 85oC double-distilled water with
stirring for 2 h. Acrylic acid monomer dissolved in double-distilled water at room temperature was
added to the PVA solution to form PVA/AAc, 50/50 w/w%, mixture solutions with stirring till
homogeneous mixing at 85 oC. Different contents of NiSO4.5H2O (10, 20, 50, 100, 150 and 250 m
mol) were added followed by adding 0.3 mL of isopropyl alcohol as scavenger of hydroxyl radical.
The mixture solutions were poured into glass dishes and then irradiated using 1.5 MeV electron beam
accelerator (Model ICT) at NCRRT, Cairo, Egypt, at a dose of 30 kGy.
The experimental details, sample preparation procedures and samples characterization have been
described in a previous work(13).
UV–vis absorption and transmittance spectra of the prepared samples were recorded using ad
ouble-beam spectrophotometer (JASCO model V-550, Japan). Thermogravimetric analysis (TGA)
were performed using Shimadzu–50 instrument (Japan) at heating rate of 10ºC / min under flowing
nitrogen (20 ml/min) from ambient temperature to 500ºC.
The glass transition temperature of the samples was investigated by using a Perkin-Elmer DSC7
differential scanning calorimeter. The experiments were carried out in nitrogen atmosphere using
about 7 mg sample sealed in aluminium pans. The samples were heated from room temperature to
135°C. The heating rate was 10°C/min in all cases.
Dielectric measurements were performed by impedance spectroscopy using a Hioki LCR meter,
3531 Z Hi-Tester, Japan, operating at a frequency range (42 Hz–5 MHz), with impedance accuracy
ranges from 0.15% up to 4%. The bridge connected via a standard interface (RS-232C interface,
Hioki, Japan) to a Pentium personal computer for instrument control and data processing. Silver
Arab Journal of Nuclear Science and Applications, 47(1), (41-52) 2014
43
electrode 10 mm in diameter were used on both side of the samples. The capacitance (C) and the loss
tangent (tan ) were obtained directly from the bridge from which `, `` and ac were calculated. The
permittivity (`), dielectric loss (``) and ac conductivity (ac) for the denoted samples were
conducted over a frequency range from 100 Hz to 5 MHz at room temperature.
RESULTS AND DISCUSSION
1. Preparation of P(PVA/AAc) Ni nanoparticles:
Equal volumes of (6 wt%) PVA and (50 wt%) AAc were prepared by dissolving in double-
distilled water at 85oC and room temperature respectively. The two solutions were mixed then
different contents of NiSO4.5H2O of (10, 20, 50, 100, 150 and 250 m mol) were added followed by
adding 0.3 ml of isopropyl alcohol as scavenger of hydroxyl radical. The mixtures were poured into
glass dishes then irradiated by electron beam irradiation at 30 kGy. The prepared copolymers were
characterized using different techniques; optical properties, thermal degradation, thermal parameters
and dielectric analysis.
2. Optical characterization:
Study of the optical absorption edge in the UV-region has proved to be a very useful method for
elucidation of optical transitions and electronic band structure of the materials(14). The law of
absorption of light I=I0 exp (-αd) was used to calculate the absorption coefficient according to the
Davis and Mott(15) model where α() is the absorption coefficient which can be calculated from the
optical absorption spectrum using the relation:
1
303.2)(
Ad (1)
where d is the sample thickness, is the wavelength of the incident photons and A is defined by A=
log (I0/I) where I0 and I are the intensities of the incident and transmitted beams, respectively.
The UV/VIS Spectrtophotometric scans were measured in the wavelength range 200–900 nm
for all samples. Figure 1 (a, b) shows the variation of absorption coefficient and optical transmittance
with wavelength for different films, respectively. Figure 1 (a) shows the optical transmittance for the
doped and un-doped (PVA/ AAc) nano composite. It can be seen that the transimitance percent
increase with wavelength. The transmitance percent affect on the concentration of Ni salt in where the
trancemetance gradually with increasing salt content. It is usually presumed that the transmittance of
the films decreases with grain size in the visible region of spectrum due to light scattering on their
rough surfaces(16). The un-loaded (PVA/AAc) shows high trancematance percent, it reaches 42% at
700nm.
Another observation about these transmission spectra is that after adding nano-Ni in polymer
matrix, an absorption peak starts emerging at 410nm with its intensity continuously increasing with
increasing concentration of the dopant. The appearance of this peak in the visible region is due to the
surface plasmon resonance (SPR) nature of the Ni nanoparticles embedded in a dielectric medium(17).
Figure 1(b) shows the changes in adsorption coefficient with wavelength for Ni-doped and undoped
P(PVA/AAc) nano composite.
Further, it is apparent from Fig. 1 (b) that the observed absorption edge at 307 nm, in undoped
P(PVA/ AAc) due to n→* transitions [17], shifts gradually towards the higher wavelength side with
increasing concentration of Ni nanoparticles as dopant. The absorption edge shifts depend on the
composition of the films, indicating a change in the optical band gap of the films. This shift in the
absorption edge towards the higher wavelength side suggests a reduction in the band gap value(18).
Arab Journal of Nuclear Science and Applications, 47(1), (41-52) 2014
44
Wave length, nm
300 400 500 600 700T
ran
mit
ance
(%
)
0
10
20
30
40
50
60PVA/ AAc
10 mmol
20 mmol
50 mmol
100 mmol
150 mmol
200 mmol
Wave length, nm
300 350 400 450 500 550 600
Ab
sorp
tio
n c
oef
fici
ent
(),
cm
-1
0
5
10
15
20PVA/ AAc
10 mmol
20 mmol
50 mmol
100 mmol
150 mmol
200 mmol
(a) (b)
Fig. 1 (a): Absorption coefficient and (b) transmittance plots of the various films.
The band gap of the films was estimated using the fundamental absorption, which corresponds
to electron excitation from the valence band to conduction band(18). The band gap Eg is the value of
optical energy gap between the valence band and the conduction band. The band gap of the films was
calculated using the following equation(14) :
h
Ehn
g )()( 0
(2)
Where h is the energy of incident photons, o is a constant related to the extent of the band tailing,
and n is the power, which characterizes the transition process in the K-space. Specifically, n is 1/2;
3/2; 2 and 3 for transitions direct allowed, direct forbidding, indirect allowed and indirect forbidden,
respectively. o depends on the transition probability and can be assumed to be constant within the
optical frequency range. The dependence of (h)1/n and photon energy (h) was plotted for the
studied films using different values of n, the best fit was obtained for n= 2. This indicates that the
transition energy for electrons is indirect in K-space and interactions with lattice vibrations (phonons)
take place. Figure 2 shows the plots of (h)1/2 vs. h for various films. The Eg values were
determined by extrapolation the linear portion of the curves until they intercept the photon energy
axis. The band gap of the pure P(PVA/ AAc) was calculated to be 3.4 eV and it found to be decreases
by increasing Ni content in the matrix as shown in table (1). Very similar trend in optical band gap for
copper nano particles doped PVA films was observed(19).
Arab Journal of Nuclear Science and Applications, 47(1), (41-52) 2014
45
h, eV
1 2 3 4 5 6 7
h
0
2
4
6
8
10
12PVA/ AAc
10 mmol
20 mmol
50 mmol
100 mmol
150 mmol
200 mmol
Fig. )2(: (h)1/2 vs. (h) plots of the various films.
Table (1): Analysis of the thermograms and the energy of activation (Ea) for the thermal
decomposition of for Ni/ PVA-AAc at 30 kGy.
NiSO4 content (mmol) Eg, eV
0 3.4
10 3.3
20 2.9
50 1.6
100 1.3
150 1.2
200 1.1
It has been well quoted in literature that PVA, more generally, exhibits the indirect band
transition between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) band edges(20, 21). The gradual decrease in the value of Eg by increasing Ni
concentration may be attributed due to the formation of chemical bonding between polymer chains
and Ni nanoparticles responsible for the generation of localized states (charge transfer complexes,
CTCs) between the HOMO and LUMO energy bands making the lower energy transitions feasible(22).
The reduction in energy gap value in P(PVA/AAc) after embedding Ni nanoparticles make them
efficient materials for optoelectronic devices. This is because of the fact that such devices require the
band gap tunability.
3. Thermal decomposition kinetics:
The application of dynamic TG methods holds great promise as a tool for unraveling the
mechanisms of physical and chemical processes that occur during polymer degradation. The thermal
stability of the polymer composites plays a crucial role in determining the limit of their working
Arab Journal of Nuclear Science and Applications, 47(1), (41-52) 2014
46
temperature and the environmental conditions for uses, which are related to their thermal
decomposition temperature and decomposition rate. Since the thermal decomposition has a direct
relationship with the stability of the compounds, TGA was used to measure the energy of activation Ea
of the prepared samples to determine its stability. Ea for the thermal decomposition of P(PVA-AAc)-
Ni nanocomposite was determined according to Horowitz and Metzger method(23). In this method, a
plot of ln{ln[(W0- Wf)/(Wt- Wf )]} against θ ( Fig. 3) gives a straight lines with a slope of 23
/10 sa RTE , in which the activation energy can be calculated from the slope. W0 and Wf are the
initial and final weights of the samples, respectively. Wt is the weight of the sample at time t and
θ = T- Ts (3)
where T is the temperature of the sample and Ts is the reference temperature defined as the
temperature where
[(Wt- Wf)/(W0 - Wf )] = 1/e (4)
where R denotes the gas constant (R= 8.314 JK-1mol-1). The activation energies calculated for all
prepared films and the results are presented in Table (2). It can be seen that the activation energy for
the thermal decomposition of the prepared samples increases steeply with increasing the Ni nano
particle concentrations in the polymer matrix. The values of the activation energy for thermal
decomposition reflected the improvement of the thermal stability of the prepared composites. The
increase in thermal stability of Ni nano particles embedded P(PVA-AAc), as indicated by the results
of the TGA study, can be due to two reasons. Ni nano particles have higher thermal stability as
compared to the polymeric chains of the hydrogel. Hence, the presence of Ni nano particles in the
system may make it thermally more stable. The other reason for this change could be the reduced
mobility of the polymeric chains due to entrapment of metal salt / metal forming a complex with the
hydroxyl group of the polymer chains and thus decreasing heat transfer process for decomposition of
polymer composites. The char residue value found to be increased systematically with increasing Ni
nano particle embedded in P(PVA/AAc) chains in Table (2).
The improvement in thermal stability may be due to restriction of polymer chain motion. In
order to ascertain this possibility, the differential scanning calorimetric studies were carried out for
P(PVA/AAc) and P(PVA/AAc)-Ni nano particles with various compositions to notice the possible
change in the glass transition temperature (Tg). Figure 4 shows typical plot of the DSC curves for pure
P(PVA/AAc) and P(PVA/AAc)-Ni nano particles which have been recorded on the heating run from
298K to 403 K. The glass transition of P( PVA/AAc) is found to be 345 K it is in agreement with the
reported value(24). The values for glass transition temperatures (Tg) for P(PVA/AAc) and
P(PVA/AAc)-Ni nano particles with various concentrations are presented in Table 2. The transition is
preferably attributed to the glass
Arab Journal of Nuclear Science and Applications, 47(1), (41-52) 2014
47
, K
-150 -100 -50 0 50
ln{ln
[(w
0-
wf)
/ (w
t- w
f)]}
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4 PVA/ AAc
10 mmol
20 mmol
50 mmol
100 mmol
150 mmol
200 mmol
Fig. )3(: Plot of ln{ln[(W0- Wf)/(Wt- Wf )]} against θ for Ni/ PVA-AAc.
Table (2): Analysis of the thermograms, energy of activation (Ea) for the thermal
decomposition and Tg of Ni/ PVA-AAc at 30 kGy.
NiSO4 content
(mmol)
Char residue
at 600oC (%) activation energies Ea (kJ/mol) Tg
0 2.7 12.7 346
10 11.9 12.8
20 14.5 14.8 339
50 19.4 11.9
100 22.2 11.5 335
150 25.2 9.94 330
250 25.6 10.4
transition (Tg) relaxational process resulting from micro-Brownian motion of the main-chain
backbone. The significant shift in the glass transition temperature is noticed in case of all composites
containing metal salts. When the salts are reduced to metallic form in the polymer matrix, the
composites thus formed also exhibit decrease in Tg. Glass transition temperature (Tg) however,
depends on the nature of dopant, its capability to trap free radicals formed during the polymer
degradation and found to be dependent of dopant concentration. The Tg values decreases gradually
with increasing Ni content. The significant decrease in the glass transition temperature can be due to a
decrease in the number of hydroxyl groups available for hydrogen bonding caused by an increase of
the metallic form ions which hinders other hydrogen bonding formation from weakening of physical
network and decreasing Tg (25). The nanoparticles (metal salts / metal oxides / metals) when added to
the polymer might have obstructed polymer chain mobility. This supports our observation that the
Arab Journal of Nuclear Science and Applications, 47(1), (41-52) 2014
48
addition of nanofillers to a polymer increases the thermal stability(26). Khanna et al. observed
improved in the thermal stability of composites prepared by doping nano Ag nano particles in PVA(27).
Temperature (K)
320 340 360 380 400
Exo
(a)
(b)
(c)
(d)
Fig. 4. DSC curves for PVA/AAc with different concentration of NiSO4
(a) 0.00, (b) 20, (c) 100 and (d) 150 mmol.
4. Dielectric analysis:
The permittivity (`), or dielectric constant, and dielectric loss factor (``) can be used to
characterize molecular relaxations. The dielectric constant (') was evaluated from the capacitance
measurement using equation (5)
0
`C
C and
d
AC 00 (5)
where Co is the vacuum capacitance of any configuration of electrodes and C is the capacitance with
an isotropic material filling the space, 0 (=8.85×10-12 F/m) is the permittivity of free space, A is the
cross-sectional area of the sample and d is its thickness. While the dielectric loss (``) was obtained
from the equation, with tan being measured
`
`̀
tan
(6)
Figure 5 depicts the variation of real part of dielectric permittivity ( ` ) with frequency for
various composites. The measurements were made isothermally at room temperature (300 K) in the
Arab Journal of Nuclear Science and Applications, 47(1), (41-52) 2014
49
frequency range from 300 Hz to 5 MHz. At low frequencies, permittivity attained higher values, in all
cases, which diminished rapidly with frequency. From the plots it is clear that ` decreases
monotonically with increasing frequency and attains a constant value at higher frequencies. Similar
behaviour was observed in other materials(28). This is because, for polar materials, the initial value of
the dielectric permittivity is high, but as the frequency of the field is raised the value begins to drop
which could be due to the dipoles not being able to follow the field variation at higher frequencies and
also due to the polarization effects(29). The low frequency dispersion region is attributed to the charge
accumulation at the electrode–electrolyte interface, i.e., charge carriers being blocked at the
electrodes. At higher frequencies the periodic reversal of the electric field occurs so fast that there is
no excess ion diffusion in the direction of the field. Hence ` decreases with the increase of the
frequency in all the samples of PVA polymer electrolytes. Also it is found that the value of `
increases gradually due to the increase of Ni nano particles in the polymer matrix. Enhanced values of
( ` ), especially at low frequencies, can be attributed to increased conductivity, and/or interfacial
polarization, and/or electrode polarization. Electrode polarization is related to the build up of space
charges at the specimen-electrode interfaces and is characterized by very high values of both real and
imaginary part of dielectric permittivity(30). Interfacial polarization results from the accumulation of
unbounded charges, at the interfaces of the constituents, where they form large dipoles.
Frequency (Hz)
1e+3 1e+4 1e+5 1e+6 1e+7
Die
lect
ric
per
mit
tivit
y (`
)
0
20
40
60
80
100
120
140
160
180
PVA/AAc
10 mmol
20 mmol
50 mmol
100 mmol
150 mmol
250 mmol
Fig. )5:( The permittivity against frequency for various samples at room temperature.
Its intensity is connected to the extent of the existing interfacial area within the composite
system, giving thus indirect evidence of the achieved distribution of nanoinclusions(31). The imaginary
part of complex permittivity `` (dielectric loss factor) vs frequency for various samples are shown in
Fig. 6. It is clear that, at low frequency range, `` decreases exponentially with frequency and attains
a constant value at higher frequencies. These results suggest that dc conductivity process is more
significant than interfacial polarization in these materials(32). The higher value of `` at low frequency
range is due to the mobile charges within the polymer backbone. While the increases in `` value with
dopant can be understood in terms of electrical conductivity, which is associated with the dielectric
loss.
Arab Journal of Nuclear Science and Applications, 47(1), (41-52) 2014
50
Frequency (Hz)
1e+3 1e+4 1e+5 1e+6 1e+7
Die
lect
ric
loss
(`
`)
0
20
40
60
80
100
120
PVA/AAc
10 mmol
20 mmol
50 mmol
100 mmol
150 mmol
250 mmol
Fig. (6) :Variation of dielectric loss against frequency for various samples at room
temperature.
The ac conductivity (ac) results introduce a better understanding of the conduction process.
The frequency dependence of ac at romm temperature are plotted in Fig. 9 for various samples. The
ac conductivity was determined by the relation:
`` oac (7)
As shown in figure 7 the Conductivity appears to be frequency dependent. In the low frequency
range, conductivity tends to acquire constant values approaching its dc value, while after a critical
value varies exponentially with frequency(33). This type of behaviour is common in disordered solids,
appears to be in accordance with the so-called ‘ac universality law’, and is considered as a strong
indication for charge migration via the
hopping mechanism(34). The conductivity increases manually with increasing salt concentration. The
local contact regions, between conductive particles and polymer matrix, control the overall
conductance of the system.
Arab Journal of Nuclear Science and Applications, 47(1), (41-52) 2014
51
Frequency (Hz)
1e+3 1e+4 1e+5 1e+6 1e+7
a.c.
co
nd
uct
ivit
y (
s/m
)
1e-7
1e-6
1e-5
1e-4
1e-3
PVA/AAC
10 mmol
20 mmol
50 mmol
100 mmol
150 mmol
250 mmol
Fig. )7( :Variation of log conductivity with log frequency for various samples.
CONCLUSIONS
Conductive polymer electrolytes have been prepared from Ni nano particles embedded
poly(vinyl alcohol) and poly(acrylic acid) using electron beam irradiation. The characteristic
properties of the prepared samples have been studied using DSC, TGA, UV-vis spectroscopy and
dielectric spectroscopy. The activation energy of the thermal decomposition was calculated according
to Horowitz and Metzger method. The activation energy for the thermal decomposition of the
prepared samples increases steeply with increasing the Ni nano particle concentrations in the polymer
matrix which reflects the improvement of the thermal stability of the prepared composites.Also the
conductivity increased manually with increasing salt concentration.
The reduction in optical energy gap value in P(PVA/AAc )after embedding Ni nanoparticles
make them efficient materials for optoelectronic devices. Also the conductivity increases manually
with increasing salt concentration.
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