Original Article
Conductivity and dielectric studies of Li3þ-irradiated PVP-based polymer electrolytes
High Performance Polymers 2018, Vol. 30(8) 978–985 ª The Author(s) 2018 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0954008318780494 journals.sagepub.com/home/hip
Divya Singh1, D Kanjilal2, GVS Laxmi2, Pramod K Singh1, SK Tomar3 and Bhaskar Bhattacharya1,4
Abstract Poly(vinylpyrrolidone) (PVP) complexed with sodium iodide (NaI) is synthesized to investigate the ionic conductivity of alkaline-based polymer electrolytes. In this article, we report the modification of electrical properties of a new ionconducting polymer electrolyte, namely, PVP complexed with NaI. Modification of polymer electrolyte was carried out before and after the exposure of films by bombarding them at different fluences with respect to Li3þ ion beam at 60 MeV. The preparation and detailed characterization of PVP:NaI is being reported. Further, a correlation with conductivity and dielectric constant has also been established. The modulation in the conductivity is explained in terms of number of charge carriers (n) and its mobility (m), which confirms the behavior of the polymer electrolyte as an alternative strategy to improve the conductivity. Keywords PVP, conductivity, irradiation, cross-linking, dielectric constant
Introduction Polymer electrolytes are of great interest from past so many decades and have been useful for electromechanical device applications. They behave like solids mechanically but consequently, their conductivity closely resembles to liquid state.1,2 The main advantages of polymer electrolytes are satisfactory mechanical properties, ease of fabrication as thin films and an ability to form good electrode/electrolyte contact, and so on. Ion-conducting solid polymer electrolytes are the most attention seeking and fancy materials today, because of their potential applications in various electrochemical devices, such as dye-sensitized solar cells, supercapacitors, rechargeable batteries, sensors, and so on.1–8 For electrochemical applications, the compatibility of electrolyte and electrode is must and therefore, semicrystalline films are obtained which are mechanically strong, but conductivity is quite weak. Thus, we need to modify these electrolytes to achieve strong conductivity as well. Swift ion interaction with the electron-conducting and -insulating polymers has been studied so far, but interaction with ion-conducting polymer is almost not well-thoughtout. The aim is to amend the chain length and boost the conductivity, for which a newer class of polymer has been preferred as a host polymer. Poly(vinylpyrrolidone) (PVP),
also commonly called polyvidone or povidone, is a polymer made from the monomer N-vinylpyrrolidone. PVP added to iodine forms a complex called povidone–iodine that possesses disinfectant properties and used where iodide is frequently added in electrolyte.9 It is nontoxic, biodegradable, biocompatible, soluble in polar solvents, and economic. It also has excellent wetting properties that make it a decent coating material or an additive to coatings. The ion radiation modifies certain properties of materials, which has become one of the sought-after techniques. Ion radiations are due to their ability to produce cross-linking, a
1
Material Research Laboratory, School of Basic Sciences & Research, Sharda University, Greater Noida, Uttar Pradesh, India 2 Inter University Accelerator Centre, Aruna Asaf Ali Marg, Near Vasant Kunj, New Delhi, India 3 Institute of Engineering and Technology, JK Lakshmipat University, Jaipur, Rajasthan, India 4 Department of Physics, MMV, Banaras Hindu University, Varanasi, Uttar Pradesh, India Corresponding author: Bhaskar Bhattacharya, Material Research Laboratory, School of Basic Sciences & Research, Sharda University, Greater Noida 210310, Uttar Pradesh, India. Email:
[email protected]
Singh et al. process whereby polymer chains link together in a threedimensional network, thereby making the material strong. The intentional degradation of polymer materials is another application of ion radiations. In the present article, a new solid polymer electrolyte of PVP complexed with sodium iodide (NaI) has been studied, for which conductivity, dielectric constant, number of charge carriers and mobility has been calculated, before and after to the exposure with different ion beam and at different fluences. The prime focus of the work is to study the changes in the conductivity and dielectric constant of the polymer electrolyte films due to ion beam exposure.10 Knowledge about the changes in the property of these electrolytes will help one using them in electrochemical devices where ion exposure is possible. This can also be used as one of the tools for in situ reduction of crystallinity/amorphicity of the polymer electrolyte matrix within a device like solar cell.
Experiment PVP (weight-average molecular weight ¼ 130,000 g/mol, Sigma Aldrich, USA) and NaI (CDH, Qualikems Laboratory Reagents, India) were used as procured. Selfsupporting films of polymer salt complex were being obtained by solution cast technique.11 The desired amount of polymer and salt were dissolved in anhydrous acetonitrile (Qualigens, Fisher Scientific, India). The cation to the monomer ratio was kept fixed at [Naþ]:[VP] ¼ 0.065. Then, the solution was stirred approximately 3–4 h or until complete dissolution occurs at room temperature. This polymer shows an excellent wetting property. The polymer salt complex was poured in polypropylene petri dish. After that, the solvent could evaporate slowly. Freestanding films of different compositions of PVP were obtained and further exposed to high-energy nuclear beam. About 10–100 mm thickness of the film were being prepared. Later, these films were vacuum dried to avoid traces of the solvent.12 For improved changes, these films were then exposed to lithium ion (Li3þ) at 60 MeV at Inter University Accelerator Centre (Delhi) using Pelletron accelerator (a tandem Van de Graaff-type accelerator). Prior to the exposure, the charge state of the ions and its energy were calculated by Stopping Range of Ions in Matter13 program and matched with the film thickness. The doses of the ions varied from 109 to 1012 by changing the beam current and time of exposure. The exposed samples were then characterized for their possible changes in structural and electrical properties compared to pristine. Li3þ ion has been chosen due to the fact the ionic radii will be small. Therefore, the local amorphization of the polymer film due to the ions will result in small (local) pockets. Therefore, the effect shall be clear. It has already been observed14 that an ion of higher radii (C5þ) results to faster amorphization of the films—due to the formation of relatively larger (local) pockets.
979 The conductivity values of the samples, before and after irradiation, were calculated from the bulk resistance values that were obtained from alternating current complex impedance spectroscopic data over a frequency range from 1– 100 Hz to 1 MHz using CH instrument workstation (model 604 D, CH instruments, USA). The films were sandwiched between two stainless steel discs and hold in spring-loaded sample holder.15 The ionic conductivity () was calculated using the following formula l 1 l ¼G ¼ A Rb A where is the ionic conductivity, G is conductance (G ¼ 1=Rb , where Rb is the bulk resistance), l is thickness of the sample and A is area of sample The dielectric constant of the film was evaluated using the impedance data. To understand the information regarding the ion polarization, molecular motion and their dielectric relaxation behavior, dielectric spectroscopy technique has been used. Cole and Cole first reported the frequencydependent dielectric studies, and thereafter, it was extended to characterize a wide variety of solid materials including superionic solids.16 The variation in the number of charge carriers and their mobility has been evaluated using the impedance data and the dielectric constant values at different frequencies. It may be noted that there is no direct method of measuring the mobility of charge carriers in polymer electrolyte matrix. Therefore, these two parameters have been derived from the dielectric constant values, as detailed in subsequent part of the article (“Number of charge carriers” and “Mobility” sections).
Results and discussion Conductivity behavior A typical plot of conductivity versus composition of PVP with NaI at different flux has been shown. Figure 1 shows the increasing nature in conductivity with concentration, indicating these electrolytes exhibit completely amorphousness. The enhanced polymer nature was identified from further electrical data. Obviously, it could be observed that the conductivity for all the composition increases drastically after ion beam exposure. Also, the overall conductivity increases significantly as the dose increases. However, for 70:30 composition, the conductivity showed more than two orders increase with increase in fluence but, at very high fluence, it started decreasing.17 The plot showing the variation in conductivity of PVP:NaI at different fluences has been shown in Figure 1. The electrical conductivity is affected by salt concentration and by dose of ion beam (Li3þ) variation. As observed in Figure 1, the ionic conductivity () increases with the initial increase in the conductivity from its pristine value that is attributed to the local amorphization due to low dose
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Figure 1. Effect of lithium ion (Li3þ) irradiation on the conductivity of the polymer electrolyte (PVP:NaI) measured at room temperature. PVP: poly(vinylpyrrolidone); NaI: sodium iodide.
of the ion beam. Formation of amorphous pockets in the polymer matrix leads to availability of more paths for the ions to migrate. This implies the release of free cations and enhanced conductivity. The first dip in the conductivity isotherm is attributed to charge association. The charge carriers that were made free due to amorphization may form higher charge multiplates and hence decrease the conductivity. This explanation is being supported by our calculation of number of charge carriers. Further increase in the conductivity is imputed to the chain scissoring effect due to ions. The increase in conductivity can be either due to increase in mobility or due to increase in the number of charge carriers (or both). For a defined composition, the number of charge carriers remains fixed. The “cage” of crystalline polymer coil18 needs to be broken to release the charge carriers trapped therein and to increase the chain flexibility. This can only happen if the long polymer chain is cut into multiple smaller parts. The chain scissoring has also been reported in the literature.12 It is quite visible in the Figure 1 that at high fluence, conductivity increases up to maxima. This means that due to the long exposure of ions on the polymer matrix, the bond breaks that can hike the conductivity values to maximum. Finally, with high nuclear beam on the polymer electrolyte, cross-linking process dominates and hence the crystallinity starts increasing. As a result, conductivity drops down. This modulation in conductivity could be confirmed by the dielectric study that is being further confirmed by our calculation for number of charge carriers and its mobility. Formation of amorphous pockets in the polymer matrix leads to availability of more paths for the ions to migrate. This implies the release of free cations and conductivity is enhanced.16 The first dip in the conductivity isotherm is attributed to charge association. The charge carriers that were made free due to amorphization may form higher
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Figure 2. Variation in dielectric constant of PVP:NaI with Li3þ at 60 MeV at different fluence. PVP: poly(vinylpyrrolidone); NaI: sodium iodide.
charge multiplates and hence decrease the conductivity.19 Further increase in the conductivity is imputed to the chain scissoring effect due to ions. It is quite visible in Figure 1 that at high fluence, conductivity increase up to maxima. This means that due to the long exposure of ions on the polymer matrix, the bond breaks that can hike the conductivity values to maximum.20 Finally, with high nuclear beam on the polymer electrolyte, cross-linking process dominates and hence the crystallinity starts increasing. As a result, conductivity drops down. This means the increase in conductivity becomes flatter on further addition of NaI to the polymer. This is due to the formation of ionic aggregates. These ionic aggregates impede the conduction process and decrease the conductivity. Finally, the slow increase in the conductivity is due to the electronic conduction in the matrix.12 This modulation in conductivity could be confirmed by the dielectric study, which is further confirmed by our calculation for the number of charge carriers and its mobility.
Dielectric studies Dielectric constant of the films was calculated using the impedance data at 1 MHz frequency (Figure 2). Thus, the modulation in conductivity values, as discussed above, are found to be in accordance with the dielectric constant and have shown the further damaged and stable configuration of the samples. This reveals the idea regarding the change in the crystallinity due to cutting and cross-linking of chain, that is, the formation of new segments. Smaller segments find more freedom. The sharp increase in the dielectric constant is indicative of the enhanced scission and rearrangements of chain. At the same time, the formation of higher
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epsilon' X103
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00
Figure 3. Variation in dielectric relaxation " and " with frequency.
charge multiplates is obvious because the chain starts to recrystallize itself. Therefore, partial ordering occurs, and hence, the dielectric constant values decrease as the conductivity drops down, as shown below. The initial increase as discussed above in conductivity is due to possible scissoring of the chains by the ions. It is wellknown that small segments find more freedom, and hence, segmental motion is higher.21 Due to enhanced scissoring and rearrangement of the chains, the dielectric constant decreases and attains almost a constant value. It has been observed that the chain segment starts partially ordering as the salt concentration is increased. With increase in the concentration of both the salt and the fluence, the sharp increase is at 1012 fluence (ion/cm2) and at 80:20 (PVP:NaI) is due to the enhanced scission and rearrangement of the chains. The sharp increase in dielectric constant can be attributed to the percolation threshold obtained at 80 wt% of PVP:NaI in increasing dose concentration, which is also seen in conductivity spectra. With further increase finally, the cross-linking process dominates, and crystalline properties increases, and dielectric value decreases almost to its pristine value. The variation in dielectric constant in PVP-based electrolytes was also found to be similar to that for the Poly(ethylene oxide) (PEO)-based electrolytes. However, the peak in the dielectric constant isotherms in PVP system is found at higher fluence values compared with PEO. This has direct correlation with the conductivity results and can be justified by the glass transition temperature (Tg) values for these two polymeric systems. Higher the value of Tg, more energy is required to get the system amorphized and hence the peak shifts toward higher fluence.
Dielectric relaxation Complex dielectric function (" ) of the materials dependent 0 00 on frequency is given by " ¼ " i" . The " value
reflects the molecular relaxation and transport processes of the material.22,23 The real part of dielectric permittivity 0 (" ) has same significance as that of ordinary dielectric constant, that is, it measures the elastically stored energy in the material during each cycle of applied alternating field, and the energy returned to the field at the end of each 0 cycle. The higher the value of " , the better is the electrical conductivity. When an electrical field is applied to a material, the dipoles in the material show the tendency to orient them in the direction of the applied field.24 However, the mobilization of the dipole depends on the ductility of the 00 materials. The imaginary part of dielectric permittivity (" ) corresponds to the dielectric loss factor due to the conduction of ionic species in the material when an electrical field is applied. At higher temperatures, dipoles can orient easily whereas a highly cross-linked material finds difficulty in orientation. The delayed response to a stimulus in a system is the called relaxation. The orientation involves a characteristic time called relaxation time .25 These parameters are calculated using the following formulae and as shown in Figure 3 against frequency 00
z " ¼ !CZ 2 0
and
0
"00 ¼
z !CZ 2
From the data of dielectric properties (real and imaginary), it is quite evident that dielectric constant (real and imaginary) decreases with increase of frequency and saturates at higher frequencies.26 The real and imaginary parts 0 00 of dielectric constant (" and " ) rise sharply at low frequencies, and this behavior have been attributed to the occurrence of electrode polarization and space charge effects.27 The presence of this polarization effect indicates
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the non-Debye type of behavior, and the periodic reversal of the electric field at high frequencies does not allow any excess ion diffusion in the direction of the field leading to decrease in polarization resulting in a drop in dielectric constant. 00 As a result, the power law dispersion in " is observed, and it does not reveal any peak in the measured frequency range.28
Number of charge carriers To estimate the contribution of the number of free charge carriers in the total conductivity, we used the dielectric constant data of the films. The conductivity () of the polymer electrolyte is given as the product (nqm) of the charge carrier density n, charge of the carrier q and its mobility m. Therefore, any increase in either of the parameter n or m will result in the increase of the conductivity . To estimate the contribution of the number of free charge carriers in the total conductivity, we have used the dielectric constant data of the films. The details of the formulation for the calculation and its supporting theory can be seen elsewhere.29–31 Therefore, we have calculated the number of charge carriers responsible for such kind of behavior. Figure 4 shows the number of charge carriers at different compositions and at different doses. Further insight into the conductivity mechanism is given by the contribution of the number of charge carriers in the total conductivity we have used. This finding indicates while it is relatively easy for the free ion carriers to move across the domains to have long-range conductivity given by Vittadello et al.32 2 34 DC kT 6 7 n ¼ 4qffiffiffiffiffiffiffiffiffiffiffiffi 5 "0 "0s 2 2 "0! e d 0 "0 1 "0 "s !x s
where DC is the conductivity at high frequency, "0 is the vacuum permittivity, k is the Boltzman constant, e is the 0 electronic charge, "s is the real permittivity at high fre0 quency, and "! is the real permittivity at frequency !. The effect of conductivity/dielectric constant on number of charge carriers is directly reflected. The maximum free charge carriers can be seen at 70:30 concentrations at 109fluence (ions/cm2). This confirms our above statement of local amorphization, realignment, and polymer composite matrix. Also, this behavior can be directly related to the conductivity and dielectric studies as discussed above. The number of charge carriers increases abruptly at a small concentration and gradually as the higher salt concentration increases.32 This shows the similar variation as that of conductivity. The initial increase is due to the complete dissolution of the salt in the polymer matrix as the concentration of the salt is low. Similarly, due to ion exposure, the initial increase in the conductivity and in number of charge carriers from its pristine value is attributed to the local
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Figure 4. Variation in number of charge carriers in PVP þ NaI with Li3þ at 60 MeV with varying fluence. PVP: poly(vinylpyrrolidone); NaI: sodium iodide.
amorphization due to low dose of the beam. The formation of local amorphous regions leads to the availability of more pathway for the ions to migrate; therefore, the release of free cations and hence enhanced carrier concentration, which matches the conductivity data. The first decrease in the variation of number of charge concentration is at 70:30% by weight of PVP:NaI polymer electrolyte is attributed to the charge association or the formation of the higher charge multiplates. The charge carriers that were made free due to amorphization may from higher charge multiplates and hence decrease the carrier concentration/conductivity.
Mobility To support our findings, one more important parameter was calculated. The mobility of the charge carriers was evaluated by the following equation to support the conductivity data. Mobility of the charge carrier DC ne where DC is the conductivity, n is the concentration of charge carrier, and e is the electronic charge. The conductivity enhancement may or may not be due to the relative change in mobility of charge carriers. Either it can depend on number of charge carriers or mobility or both. Figure 5 shows the variation of mobility with increasing fluence for the system. In this system, mobility shows a sharp increase upon irradiation. The initial increase is followed by a peak and then mobility drops down. The initial increase can be attributed to the scissoring of the chains by the ions. It is well-known that small segments find more freedom, and hence, segmental motion is higher. Therefore, the mobility of the ions gets enhanced. After that it has been seen the chain segments start partially ordering and therefore the mobility decreases. With increase in fluence, due to the m ¼
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Figure 5. Variation in mobility in PVP þ NaI with Li 3þ at 60 MeV with varying fluence. PVP: poly(vinylpyrrolidone); NaI: sodium iodide.
enhanced scission and rearrangement of the chains, mobility of the carriers changes which has a correlation with the dielectric constant and conductivity of the samples. Further increase in fluence results in further cross-linking process and a drop in the mobility is observed.
Polarized optical microscopy Change in the surface morphology is one of the fundamental properties to study the structure of the polymer electrolyte matrix. The clear majority of crystallizable polymers form spherulites as the dominant morphological entity during transient stages of crystallization.33,34 As the name suggests, they are spherically symmetric arrays of lamellar crystals immersed in amorphous material. These are commonly seen under polarized optical microscope (POM) in a polymer matrix.33–35 The figure below gives the POM images of the pristine samples. The optical micrographs (POM) of the sample were recorded using POM (BA310 POL, Motic, BA310 MOTIC, India), as shown in Figure 6
((a) and (b)). The most prominent feature in these micrographs are the spherulites that are (ideally) spherical aggregates ranging from submicroscopic in size to millimeters in diameter in extreme cases. Spherulites are recognized due to their characteristic appearance under the polarizing microscope, where they are circular birefringent areas possessing a dark Maltese cross pattern.36 The thickness of the lamellae is governed by the average fold period in the constituent molecules.35 The spherulites are crystalline pockets in the matrix, whereas the space between them that appears black/dark under POM is amorphous region. It is observed that PVP:NaI film (Figure 6(a)) shows well-ordered patches that confirm its semicrystalline nature. Due to exposure of ions on PVP:NaI matrix (Figure 6(b)), the patch sizes randomly distributed, and crystallinity seems perturbed. The damage in crystallinity measurements (ordered pattern) showed further improvement in amorphicity (non-ordered pattern), where different sizes rough patches distributed randomly within polymer matrix. It is believed that amorphous regions (non-ordered pattern) are responsible for enhanced conductivity and hence our optical micrographs support ionic conductivity data as well. The damage in crystallinity (ordered pattern) showed further improvement in amorphicity (non-ordered pattern. It is believed that amorphous regions (non-ordered pattern) provide path for the motion of ions in the polymer electrolytes.37 Our optical micrographs indicate that with increase in dose, the ionic conductivity of the samples should increase. This speculation is confirmed by the conductivity measurements discussed in subsequent sections. Therefore, ions can migrate in the amorphous region. The dominance of darker portion which is amorphous was also confirmed by many other scientists38,39
Conclusion Present investigation furnishes a study of PVP with NaI polymer electrolyte at various irradiation fluences of 60 MeV Li3þ beam. The solid polymer electrolyte film consisting of PVP doped with NaI salt was successfully prepared and characterized. It was observed that addition of
Figure 6. POM of pristine and exposed with Li 3þ ion beam. POM: polarized optical microscope.
984 NaI and with increasing fluence enhances the ionic conductivity and a conductivity maximum was obtained at 80:20% weight value of 7.56 104 S/cm. The dielectric data well match with the impedance data, and the total conductivity of the polymer electrolyte films was found to increase with increasing fluencies. The jump in dielectric is attributed to the possible crystallization of the matrix, whereas the dip is due to the number of local amorphous sites. Values of dielectric constant and loss show frequency dispersions at low frequencies and show low values at high frequencies. The variation in conductivity is explained in terms of changes in number of charge carriers and mobility. More studies are still under process in our laboratory to explore this behavior.
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Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding The author(s) disclosed receipt of financial support for the research, authorship, and/or publication of this article: This work was supported by Inter University Accelerator Centre, New Delhi in terms of Beam Facilities and funded under project of Department of Science and Technology (SR/S2/CMP-0065/2010), Government of India.
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