Variation in viscosity and ion conductivity of a polymer-salt complex exposed to gamma irradiation

Article (PDF Available)inPramana 74(2):271-279 · February 2010with37 Reads
DOI: 10.1007/s12043-010-0026-7
We study changes in microstructure and resulting changes in the properties of PEO(1 − x)-NH4ClO4(x) samples where x = 0.18, when irradiated with gamma doses varying up to 50 kGy. Viscosities of aqueous solutions of the irradiated samples give an idea of the change in molecular weight and show correlation with ion conductivity. On the whole, there is a chain scission on irradiation, though there is evidence of some cross-linking at higher doses. The ion conductivity shows a strong increase for an irradiation of 35 kGy. DSC studies indicate a decrease in crystallinity with gamma dose. KeywordsGamma irradiation-polymer electrolyte-viscosity-ion conductivity
° Indian Academy of Sciences Vol. 74, No. 2
journal of February 2010
physics pp. 271–279
Variation in viscosity and ion conductivity
of a polymer–salt complex exposed to gamma
, S K DE
Physics Department, Condensed Matter Physics Research Centre, Jadavpur University,
Kolkata 700 032, India
Material Science Department, Indian Association for the Cultivation of Science,
Jadavpur, Kolkata 700 032, India
Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Kolkata 700 064, India
Corresponding author. E-mail: sujata
MS received 18 May 2009; revised 29 September 2009; accepted 27 October 2009
Abstract. We study changes in microstructure and resulting changes in the properties
of PEO(1 x)–NH
(x) samples where x = 0.18, when irradiated with gamma doses
varying up to 50 kGy. Viscosities of aqueous solutions of the irradiated samples give an
idea of the change in molecular weight and show correlation with ion conductivity. On
the whole, there is a chain scission on irradiation, though there is evidence of some cross-
linking at higher doses. The ion conductivity shows a strong increase for an irradiation of
35 kGy. DSC studies indicate a decrease in crystallinity with gamma dose.
Keywords. Gamma irradiation; polymer electrolyte; viscosity; ion conductivity.
PACS Nos 61.82.Pv; 66.30.Dn; 47.57.Ng; 81.70.Pg
1. Introduction
When polymers are exposed to high energy radiations such as γ-rays, there may
be cross-linking and/or scission of the long polymer chains. Both pro cesses may
occur simultaneously, but usually one dominates [1,2]. This is manifested in the
different physical and electrical properties of solid polymer electrolytes (SPE) as
well [3]. Since polymer electrolytes are extensively used in practical devices and
appliances [4], it is helpful to study the effect of irradiation on them. The risk
of degradation on appliances exposed to radiation environment in nuclear reactors
or outer space has to be considered as well as the possibility of improvement of
properties on controlled irradiation. So identifying radiation-resistant materials as
well as radiation-sensitive materials is necessary. We report a study on one such
well-known polymer complex. Some preliminary results were reported in [5].
Sujata Tarafdar et al
Polyethylene oxide complexed with x fraction by weight of ammonium perchlo-
rate, PEO(1 x)–NH
(x), is a well studied SPE [6–8]. We have irradiated the
SPE films with gamma doses from 10 to 50 kGy. The changes produced on irradi-
ation are studied using DSC, FTIR, viscosity in aqueous solution and impedance
spectroscopy (IS). The change in average molecular weight produces a variation in
viscosity of the aqueous solution [9]. The change in crystallinity shows up in DSC
measurements. We try to correlate these results with the observed change in ion
conductivity measured by impedance spectroscopy.
We also try to infer details about microstructure from the plots of conductivity vs.
frequency in a double logarithmic scale. A power law is obtained at high frequencies,
the exponent of which gives information about spatial and temporal disorders in
the microstructure.
2. Experimental
2.1 Sample preparation
Polyethylene oxide from B.D.H., England (Mol Wt. = 6.105), methanol (99.9%
pure) and ammonium perchlorate (Fluka, 99.5%) are used for sample preparation.
The polymer films are prepared by the solution casting technique, with methanol
as the solvent. Fraction x by weight of NH
and (1 x) fraction of PEO were
dissolved in methanol. The suspension was stirred for 14–16 h at room temperature
C). Films were cast on a petri dish, dried in air for 4–5 days and then vacuum
dried for 3–4 days. Thickness of the film was about 250 µm. We thus prepared
films of total mass 2 g and composition: PEO(1 x)–NH
(x), where x = 0.18.
2.2 Gamma irradiation
The samples were irradiated in a conventional gamma chamber, which uses a
source with a dose rate of 60 Gy/min or 6 krad/min. Samples were exposed to
doses 10, 12, 15, 17, 20, 25, 30, 35, 40, 45 and 50 kGy. DSC has been done on all
these samples, as well as on the unirradiated sample. Other measurements have
been done on some of the samples.
2.3 FTIR
Fourier transform infrared spectroscopy (FTIR) was done on the pristine and ir-
radiated samples on Shimodzu FTIR8400S, at the Physics Department, Jadavpur
University, India.
2.4 Viscosity measurement
The unirradiated sample and samples irradiated with doses 10, 25, 35 and 45 kGy,
were dissolved in distilled water to produce solutions of 7.65 g/l concentration.
272 Pramana J. Phys., Vol. 74, No. 2, February 2010
Polymer–salt complex exposed to gamma irradiation
Relative viscosity was measured by Ostwald viscometer at the Centre for Surface
Sciences, Jadavpur University, India at room temperature (33
2.5 Impedance spectroscopy
Impedance spectroscopy (IS) was done on an Agilent 4192A impedance analyzer,
in the frequency range 5 Hz to 13 MHz. IS has been done on samples irradiated
with 0, 10, 20, 30, 35 and 40 kGy gamma dose, at temperatures 20–50
C in air.
Measurements were done in the cooling cycle. Heating cycle results are unreliable
showing irregular variation. This is probably because surface irregularities prevent
good contact between the sample and electrode. On heating to about 40–50
C the
samples soften ensuring proper contact. Irradiation makes the samples brittle and
they have to be handled carefully.
2.6 DSC
Differential scanning calorimetry (DSC) has been done on the unirradiated and
irradiated samples using Pyris Diamond (Perkin Elmer) set-up for TG/DTA, at
the Metallurgy Department, Jadavpur University, India. Heating was done at the
rate of 10
C/min under nitrogen atmosphere.
3. Results
Viscosity and ion conductivity show correlated bahaviour, one being high when
the other is low. Though the viscosity is measured for a dilute aqueous solution,
it is seen that the change produced by irradiating the solid polymer electrolyte
is retained in solution. DSC results are also consistent with these findings. We
present the results below and discuss the implications.
3.1 FTIR
Figure 1 shows the results of infrared absorption frequencies for doses 0, 10, 35
and 50 kGy. Thicknesses are nearly equal and salt concentration is 18% for all
the samples studied, and so intensities of the peaks in the graphs can be directly
compared. There are evident changes with gamma irradiation. The most prominent
change is in the peak at 1150 cm
(marked by an arrow), which has been attributed
to C–C bond stretching and C–O–C asymmetric stretching [6]. This peak has an
intensity of about 1.3 (in arbitrary units) in the unirradiated sample, it falls to
about 1.1 at 10 and 35 kGy, rises again to 1.2 at a dose of 50 kGy. This can most
obviously be correlated to main chain scission for low doses, where such bonds
decrease in number. The subsequent increase at higher dose indicates cross-linking.
So results from DSC, viscosity and FTIR are in agreement.
Pramana J. Phys., Vol. 74, No. 2, February 2010 273
Sujata Tarafdar et al
Figure 1. FTIR spectra for gamma doses of 0, 10, 35 and 50 kGy are shown.
Intensity of the peak at 1150 cm
, marked by the arrow, is identified as C–C
or C–O–C asymmetric stretching. It shows a decrease with irradiation up to
35 kGy, followed by an increase at 50 kGy.
3.2 Viscosity in solution
A simple way to probe the change in average molecular weight produced by ir-
radiation, is to measure the viscosity of a solution in a suitable solvent [1]. It
must be kept in mind however, that the viscosity is not simply proportional to the
arithmetic mean of the molecular weights of all macromolecules. Higher molecu-
lar weight chains contribute more to the viscosity. So a properly weighted mean
is required [1]. Nevertheless, the viscosity of solutions of similar concentration in
the same solvent for samples irradiated with different doses, does give an idea
of whether the molecular weight is increasing or decreasing. Viscosity η measured
relative to water at 33
C (η
= 0.7491 cP) decreases from the unirradiated sam-
ple, reaching a minimum around the dose 25 kGy, then increases somewhat up to
45 kGy.
The variation in viscosity of the polymer–salt complexes seems to indicate that
initially chain scission is the dominating process with cross-linking catching up
later. Polymer solutions are non-Newtonian at higher concentration, and so mea-
surement at different shear rates and different concentrations will probably give
more information.
274 Pramana J. Phys., Vol. 74, No. 2, February 2010
Polymer–salt complex exposed to gamma irradiation
Ion-conductivity 10 S/cm
0 10 20 30 40
Gamma dose (kGy)
Relative viscosity
Figure 2. The relative viscosity in aqueous solution and the DC ion conduc-
tivity of the unirradiated sample and samples irradiated with 10, 25, 53 and
45 kGy gamma dose are shown. Conductivity is low, when viscosity is high.
3.3 IS results
The DC conductivity σ was extracted from the Cole–Cole plots. DC ion conductiv-
ity shows a non-monotonic variation like the viscosity. There is first a decrease at 10
kGy dose, followed by an increase with a maximum around 35–40 kGy. Compared
to the unirradiated sample, there is an overall increase in σ on irradiation. Figure
2 compares the variation in ion conductivity and viscosity with gamma dose.
Frequency variation of the real part of the ion conductivity also shows that the
initial doses of 10–20 kGy produces a decrease in σ at all frequencies, which is more
than compensated by the subsequent increase in σ at still higher doses.
The frequency variation of the real part of σ is shown in figure 3 for different
doses at 40
C. The change on irradiating the sample to 10–20 kGy is quite striking.
Besides the magnitude being lower, the cross-over from the dispersionless variation
to a power-law variation at higher frequencies occurs at a much lower frequency
marked as A, compared to the cross-over marked B for the samples irradiated
at higher doses. At gamma doses higher than 20 kGy, the cross-over points are
again closer to the zero dose value. The details of the frequency dependence of
the AC conductivity for disordered structures such as glasses and polymers, con-
tains information about the microstructure and/or relaxation times of the sample
[10–12]. The frequency range where the conductivity remains more or less constant,
indicates that the charge carriers see an average structure, i.e. in the time corre-
sponding to one cycle they cover distances on which the sample is homogeneous.
At higher frequencies, there is usually a cross-over to a regime with a power-law
Pramana J. Phys., Vol. 74, No. 2, February 2010 275
Sujata Tarafdar et al
Figure 3. The real part of the AC conductivity at different frequencies shows
an initial electrode-dependent regime, followed by a dispersionless DC regime.
At still higher frequencies a power-law regime is observed. A and B indicate
the cross-over frequencies for 20 and 30 kGy respectively.
frequency variation [13]. Here, the charge carriers remain confined in small units
which may have a self-similar structure [12]. Such behaviour is attributed to a long-
tailed distribution of relaxation times [10]. Since fractal morphology is observed
in PEO(1 x)–NH
(x) [14], we assume in this case, spatial self-similarity at
smaller length scales. So we infer that the microstructural units in the samples
irradiated to 10–20 kGy are larger than in the unirradiated or higher irradiated
samples, which is manifested in cross-over at a lower frequency. More measure-
ments covering wider frequency ranges are necessary for a complete analysis of this
3.4 DSC results
DSC results show a systematic variation with irradiation dose, and an interesting
observation is the presence of two minima for certain doses. Positions of the minima
shift with irradiation dose, indicating a change in melting points. The results are
shown in figure 4. More detailed results are to be found in [15]. The unirradiated
DSC curve shows two minima, the deepest is at 71.5
C and there is a less prominent
minimum at 60.5
C. This indicates the presence of two different crystalline species
with different melting points. As dose increases, the deeper trough becomes less
prominent, until at 17 kGy, the two minima are almost equal in depth. On further
increase in dose, the higher temperature trough almost vanishes, now the only
prominent minimum is at 58.5
C. So finally only one of the crystalline species
survives the radiation, the other probably becomes amorphous.
276 Pramana J. Phys., Vol. 74, No. 2, February 2010
Polymer–salt complex exposed to gamma irradiation
Figure 4. DSC traces for the unirradiated and gamma-irradiated samples.
Only doses of 17 and 50 kGy are shown for clarity. Presence of two crystalline
sp ecies with different melting point is clear. At 50 kGy irradiation, only one
Figure 5. The total area enclosed by the troughs in the DSC traces decreases
with gamma dose. The results follow an exponential curve quite well, as shown
in the figure.
The area from baseline within the DSC curve has been calculated for all doses. A
graph of the total area against radiation dose is shown in figure 5. As dose increases
the area decreases monotonically, indicating a decrease in total crystallinity. The
effect of gamma irradiation is found to be different from earlier results of ion beam
irradiation on similar samples [16].
The overall observation is that the area within the dip in the absorbed heat
vs. temperature curve, progressively decreases with increasing gamma dose. This
Pramana J. Phys., Vol. 74, No. 2, February 2010 277
Sujata Tarafdar et al
indicates a reduction in the total crystallinity [15]. Decrease in area of the DSC
curve with temperature is shown in figure 5, with an exponential fit to the curve.
Assuming that the area is proportional to the crystallinity χ, this variation can be
understood as follows. If a dose dD produces a proportional change dχ in χ, we
may write
dχ = dD, (1)
where c is a constant. This leads to
χ = A exp(cD), (2)
A being determined by the initial crystallinity.
DSC results are consistent with viscosity results, since we normally expect scission
of linear chains to lead to lower crystallinity [17].
4. Discussion and conclusions
A large number of reports on irradiation effects in polymers and particularly solid
polymer electrolytes have appeared [3,18–21]. Ferloni et al [19] have irradiated
aqueous solutions of PEO
. They found an improvement in ion conduc-
tivity without deterioration in mechanical strength, which occurs on irradiating
dry samples [19]. Plasticizers and modifiers are added for further improvement
[22]. Singh et al [17] found a large enhancement in σ on electron beam irradiation
of low molecular weight PEG(x)–LiClO
, which they attributed to chain scission
and reduction in crystallinity. Song et al [18] considered cross-linking to be respon-
sible for large enhancement in σ for PEG(x)–LiClO
. So there are issues not yet
Earlier work on irradiation of the same polymer electrolyte, PEO–NH
x = 12%, did not show significant improvement in conduction properties [8]. How-
ever at that salt concentration σ is very low for the pristine sample. The present
work is done on samples where x = 18%, and σ is the highest for the unirra-
diated sample. The overall finding is that ion conduction improves with gamma
irradiation dose and this is related to the decrease in crystallinity observed in DSC
studies. However, the improvement is not monotonic. The variation of viscosity
and conductivity with gamma dose, show an inverse correlation which is realistic.
The conductivity is minimum, when the viscosity is maximum. For maximum con-
ductivity, the viscosity is not exactly minimum, but low over the range where σ
is high. The DSC result of more or less monotonic decrease in total crystallinity,
seems not perfectly in agreement with the arguments above, but the presence of
multiple minima in DSC needs a closer scrutiny and analysis. Detailed study of the
dynamic IS results promises to throw more light on changes in microstructure by
The authors thank D Das and Sandeep Chaudhuri (UGC-DAE) for irradiating the
samples. They are grateful to Prof. S P Moulik for encouraging discussions and
278 Pramana J. Phys., Vol. 74, No. 2, February 2010
Polymer–salt complex exposed to gamma irradiation
B Naskar for help with the viscosity measurements. Finally, UGC-DAE is gratefully
acknowledged for providing financial assistance.
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    • "The details depend on the particular polymer and may be beneficial or detrimental for a definite application. For example PEO based polymer films with ammonium perchlorate show enhanced ion-conductivity on irradiation (Nanda et al., 2010; Tarafdar et al., 2010; Sinha et al., 2008; Ghosal et al., 2014; Saha et al., 2015). This shows that controlled gamma irradiation may be used to treat such films, improving their performance as solid electrolytes. "
    [Show abstract] [Hide abstract] ABSTRACT: Solid polymer electrolytes with gelatin as host polymer are subjected to gamma irradiation with dose varying from 0 to 100 kGy. Two sets of samples are studied, one with and one without addition of lithium perchlorate as ionic salt. The effect of varying plasticizer content, salt fraction and radiation dose on the impedance is measured. The dc (direct current) ion-conductivity is determined from impedance spectroscopy results. It is shown that relative to the unirradiated sample, the room temperature dc ion-conductivity decreases in general on irradiation, by an order of magnitude. However on comparing results for the irradiated samples, a dose of 60 kGy is seen to produce the highe