Quasi-Solid Semi-Interpenetrating Polymer Networks as Electrolytes

Aug 15, 2014 - Nimai Bar†‡§ and Pratyay Basak†‡§ ... Inorganic & Physical Chemistry Division, Council of Scientific & Industrial Research−...
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Quasi-Solid Semi-Interpenetrating Polymer Networks as Electrolytes: Part III. Probing the Mechanism of Ionic Charge Transport Employing Temperature-Step Electrochemical Impedance Spectroscopy Nimai Bar†,‡,§ and Pratyay Basak*,†,‡,§ †

Nanomaterials Laboratory, Inorganic & Physical Chemistry Division, Council of Scientific & Industrial Research−Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad-500 007, Andhra Pradesh, India ‡ CSIR − Network Institutes for Solar Energy (CSIR-NISE), Hyderabad-500 007, Andhra Pradesh, India § Academy of Scientific and Innovative Research (AcSIR), Hyderabad-500 007, Andhra Pradesh, India S Supporting Information *

ABSTRACT: The correlated ion-transport mechanism and its dependence on microscopic phase separation for a new class of quasi-solid semi-IPN electrolytes is probed in considerable detail using temperature-step electrochemical impedance spectroscopy. The response of electrolyte matrices under alternating current perturbation is comprehensively analyzed using a simulated model fit to extract pertinent information relevant to the phase composition and homogeneity, contribution of each phase, interfacial charge-transfer resistance, phase entanglement zones, bulk relaxation time for ionic hopping mechanisms, coupled segmental motions, rate of site reorganization that dictates successful hopping events, and estimates of ionic transport numbers. The normalized complex plane Nyquist plots (ρ′ versus ρ″) show two well-defined regions for bulk (in mid- and high-frequency regions) and electrode− electrolyte interfacial impedance (in low-frequency regions). Rigorous analysis indicates the presence of three microscopic phases in the matrix bulk (pure poly(ethylene oxide)− polyurethane (PEO-PU), pure poly(ethylene glycol) dimethyl ether (PEGDME), and PEOPU/PEGDME mixed phase) along with the charge-transfer resistance (Rct) which contribute to the bulk resistance. Spectroscopic plots of complex impedance against frequency (Z″ versus log f) depict Debye peaks, providing an estimate of the bulk relaxation time (τpeak). Profiles depicting the real component of conductivity (σ′(ω)) as a function of frequency (log f) follow a modified universal power law where the simulated fit results reveal vital information on the site relaxation rates, cumulative favorability for successful hopping events, and predominant charge carrier type. The behavior of the dielectric contributions provides insights into the various ion polarization processes dominant in the high-, mid-, and low-frequency windows of the sweep. These trends were further correlated with our prior evaluation of the physico-chemical properties of the semi-IPN matrices to propose a rational physical model for these complex systems.

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INTRODUCTION Polymer−salt complexes have been the subject of intense investigations for the last few decades, ever since Wright1 and Armand2 put forward the concept of their use as solid polymer electrolytes (SPEs). The sluggish dynamics associated with the macromolecular medium posts the key challenge in their practical applicability. Over the years, attempts by several research groups have considerably enriched the field to narrow down the prerequisites of such systems, while numerous studies have been directed toward improving the matrix properties of the host polymers and understanding the mode of ion conduction in these highly disordered systems.3−22 Ionic conductivity in these systems occurs primarily in the amorphous regions and is associated with both the charge migration of ions between co-ordination sites (ionic hopping) and the polymeric chain segmental motions (micro-Brownian motions). Even though many models proposed by notable researchers describe adequately most of the transport properties in polymer electrolytes, they are not entirely based on a © 2014 American Chemical Society

microscopic treatment; hence, local mechanistic information is sometimes lost. Ratner et al.4,12,23 put forth the dynamic bond percolation theory that explains the charge migration in polymeric systems in terms of a renewal of hopping probabilities. Theoretical models have also been extended employing Monte Carlo (MC) simulations and the molecular dynamics (MD) approach on both crystalline and amorphous poly(ethylene oxide) (PEO)− salt complexes to explain the mechanism responsible for ionic conductivity.24−29 In a combination of theoretical modeling and complex conductivity measurements for PPO/(LiClO 4) x complexes, Furukawa et al.30,31 proposed that the conductivity is primarily assisted via two correlated phenomena. Those associated with dielectric relaxations of dipoles caused by the segmental dynamics of the polymer host are observable Received: June 30, 2014 Revised: August 14, 2014 Published: August 15, 2014 20807

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lithium perchlorate (LiClO4) (Aldrich), N,N-dimethylaniline (DMA) (Rankem), tetrahydrofuran (THF) (Rankem), and acetonitrile (CH3CN) (S. D. Fine-Chem Ltd., India). Polyethylene glycols and the solvents (THF, CH3CN) used were dried prior to synthesis. Synthesis of Semi-IPN Electrolyte Matrices.43 The process of preparing a typical semi-IPN electrolyte matrix involves forming an isocyanate-terminated prepolymer by reacting castor oil (−OH value, ∼2.7) with a diphenylmethane-4,4′-diisocyanate (MDI) in requisite amount for 1 h using THF as the solvent and nitrogen as inert atmosphere (stage I). Thereafter, the reaction vessel containing the isocyanate-terminated prepolymer is charged with the polyether macromonomer (PEG, Mn ∼ 4000) and room-temperature catalyst N,N-dimethylaniline to initiate the formation of the polymer networks, component I (stage II). Concurrently, the component II, i.e., PEGDME (Mn ∼ 500) having nonreactive end group in the preferred weight percent is added within the system to intimately entangle with the growing polymer network. The incorporation of electrolyte salt of desired concentration, dissolved in a 1:1 solvent mixture of THF/ CH3CN, is also achieved at this stage. The reaction mixture is degassed and vigorous mixing is continued for another 30 min, under inert atmosphere, to obtain a uniformly homogeneous viscous mix of an electrolyte composition. Finally, the viscous polymer solution is casted onto a Teflon Petri dish and dried at room temperature for 24 h followed by curing at higher temperature and inert atmosphere to ensure the completion of isocyanate reaction (at 80 °C for 48 h) and obtain the quasisolid semi-IPN electrolyte matrix. The free-standing films so obtained have an average thickness in the range of ∼0.06−0.08 cm. The synthesized semi-IPN samples are coded as P4K-U/P2 in the text with the corresponding composition of component I and component II provided in parentheses as (60:40), (50:50), (40:60), and (30:70) indicating the respective weight percentage. Compositions beyond 70 wt % of PEGDME visibly lacked structural integrity and phase homogeneity and hence were not considered in this study. The total −NCO/− OH ratio was maintained at 1.2 with EO/Li mole ratio of LiClO4 = 30. Synthesis of Semi-IPN Electrolyte Matrices with Variable Chain Length between Cross-Links and Entanglements. The semi-IPN architecture was tailored to study the effect of chain length between cross-links and degree of entanglements by either varying the average molecular weight of the macromonomer used in component I (PEG) or component II (PEGDME) during synthesis while keeping all the other parameters and the electrolyte (LiClO4) used same. The different molecular weights (Mn) of PEG used in the present study are 400, 1000, 2000, 4000, 10 000, and 14 000 in combination with PEGDME of Mn = 500. The semi-IPNs of (30:70) weight compositions so formed are specifically coded as P0.4K-PU/P2, P1K-PU/P2, P2K-PU/P2, P4K-PU/P2, P10K-PU/P2, and P14K-PU/P2, respectively. Similarly, the different molecular weights of PEGDME (component II) used in the present study are 250, 500, 1000, and 2000 in combination with PEG of Mn = 4000 in component I. The sample compositions are accordingly designated as P4K-PU/ P1, P4K-PU/P2, P4K-PU/P3, and P4K-PU/P4, respectively. The code for general representation in the text is PxK-PU/Py semi-IPNs, where “xK” and “y” signify the respective macromonomer or polymer used.

predominantly at high frequencies, whereas the contributions from the local ionic motions can be discerned at lowerfrequency ranges. Using detailed electrochemical impedance analysis on simple model systems aided by equivalent circuits and universal power law fittings, Di Noto et al.32−35 demonstrated further experimental evidence in support of these observations. The suitability of semi-interpenetrating polymer networks (semi-IPNs) as polymer electrolyte matrices has been successfully demonstrated in our continuing research efforts.36−44 In a significant breakthrough, bulk ionic conductivities of 10−4−10−3 S cm−1 in the temperature range of 20−80 °C could be achieved for a new class of quasi-solid semi-IPNs notably without the use of any external plasticization.43,44 Though macroscopic phase separation of the two constituent components can be effectively arrested because of inherent entanglements and complementary miscibility parameters, sufficient indirect evidence of the presence of local microscopic domains for pure and/or intimately mixed constituents could be perceived.43 The primary objective put forth in this study is to understand the effect of microphase separation on events such as (i) the microscopic molecular processes related to the charge-transport mechanism, (ii) the correlated motions of ions along with chain dynamics, (iii) the polarization mechanisms, and (iv) the associated relaxation time in these semi-IPN systems. In contrast to previous reports, in which low molecular weight polymers below the entanglement limits were used, the semi-IPNs investigated in the present study pose a challenge because of their intrinsic complexity. This effort is an attempt toward explaining the charge-transport mechanism by suitably modifying the models proposed by Furukawa and Di Noto to account for the complexities inherent in these semi-IPN matrices. Herein, we present our interesting findings on this new series of quasi-solid semi-IPN electrolytes (polyethylene glycolpolyurethane/polyethylene glycol dimethyl ether (PxK-PU/ Py)) investigated in considerable detail using temperature-step electrochemical impedance spectroscopy (EIS). Rigorous analysis has been carried out on the experimental data, and an attempt is made to isolate the different microscopic contributions to the overall electrical behavior in these complex systems. The conductivity of electrolyte matrices elucidated by ac-impedance spectroscopy study was analyzed using simulated model fits to extract pertinent information relevant to the phase composition and homogeneity, contribution of each phase, interfacial charge-transfer resistance, phase entanglement zones, bulk-relaxation time associated with ionic hopping events, correlated segmental motions, site-relaxation times, and ionic transport numbers. These trends supported by our prior evaluation of the physico-chemical properties of the matrices provide for the establishment of a robust and rational physical model for these complex systems. The comprehensive analysis augments our earlier understanding of the system and can aid us in appropriately tailoring these semi-IPNs to suit specific applications.

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EXPERIMENTAL SECTION Materials. All the chemicals used were of reagent grade. The chemicals were castor oil (CO) (BSS grade), diphenylmethane-4,4′-diisocyanate (MDI) (Merck), poly(ethylene glycol) (PEG, Mn ∼ 400, 1000, 2000, 4000, 10 000, and 14 000) (Aldrich), poly(ethylene glycol) dimethyl ether (PEGDME, Mn ∼ 250, 500, 1000, and 2000) (Aldrich), 20808

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Electrochemical Impedance Spectroscopy. The alternating current (ac) electrochemical impedance measurements were carried out on a Zahner Zennium electrochemical workstation controlled by Thales Operational Software. The system was interfaced with a thermostated oven equipped with parallel test channels independently connected to spring-loaded Swagelok cells to test the samples at identical conditions (see Figure SI-9 in Supporting Information). The synthesized semiIPN electrolyte samples were vacuum-dried overnight before carrying out the electrical measurements. Punched circular discshaped polymer films or quasi-solid samples of surface area ∼0.95 cm2 and thickness ∼0.06−0.08 cm were sandwiched between two 316 stainless steel blocking electrodes with a Teflon spacer of appropriate dimension and loaded in the Swagelok assembly. The spring and Teflon spacer ensured the application of the same amount of spring pressure during the sample mounting and throughout the test. The sample holders were placed in the controlled heating chamber to carry out the variable temperature impedance measurements over a range of ∼20−90 °C at an interval of ∼5−7 °C during heating. The temperature was measured with an accuracy better than ±0.1 °C using a K-type thermocouple placed in close proximity with the sample. Samples were equilibrated at each temperature for 30 min prior to acquiring the frequency sweep impedance data. All data were collected following a frequency sweep through 1 Hz to 4 MHz at an alternating potential with a root mean square amplitude of ±10 mV across the open-circuit voltage (OCV) of the assembled cells. No corrections for thermal expansion of the cells were carried out. The model and simulated fits for the normalized Nyquist plots were achieved using Zman 2.0 analysis software. The real part of the impedance was appropriately normalized for the cell dimensions, and ionic conductivity (σ′(ω) (S cm−1)) was determined. Statistical weighting is necessary because the data spans occasionally over several orders of magnitude, both for ionic conductivity (σ) and for angular frequency (ω). Analysis of temperature dependence of the electrochemical spectroscopy data was done by nonlinear least-squares fits (NLSF) using Microcal OriginPro 8.5 software. The maximum error associated with all the simulated fits is within ±1%. Polarization Studies. The direct current (dc) polarization tests were carried out on the same set-up as described in the above section at three temperatures of interest (∼30, 50, and 80 °C). The decay of current in response to a constant voltage (1 V) was measured as a function of time (6 h). All the synthesized samples were vacuum-dried overnight at 80 °C before carrying out the characterizations.

effect of composition on microphase separation (restricted inhomogeneity/cocontinuity); isolate the contribution of each phase, interfacial charge-transfer resistance, phase entanglement zones, Debye relaxation time, associated segmental motions, rate of successful hopping events, and site-reorganization times; and estimate ionic transport numbers in these semi-IPN systems. Impedance spectroscopy is a powerful probe for studying the structure−conductivity correlations in glassy or solid polymeric materials.4,38,45 In an ac-impedance experiment, the response of the sample to a small sinusoidal perturbation (applied voltage or current) is measured. Contrary to the dc-conductivity measurements in which only the bulk resistance (Rb) is measured, in the case of an ac-perturbation, both opposition of the system to allow the flow of charge (|Z|= Vmax/Imax, analogous to resistance) and the phase lag (θ) or delay in response determine the complex impedance (Z*) of the material at any given frequency.45 The terminal pair acimpedance measurement method was adopted using Swagelok cells, and the experiments were performed in the temperature window of 296−363 K applying ±10 mV across OCV with a frequency sweep of 1 Hz to 4 MHz for the studies. Equivalent Circuit Model for the Semi-IPNs. Evaluation of electrochemical impedance (Z) and the complex plane Nyquist plots to extract relevant information employing software simulation of the response with an equivalent circuit is occasionally used because it is simple and fast and can help to provide a complete physical picture of the system.45−48 Detailed analysis of the normalized Nyquist plots for our semi-IPN electrolyte matrix is initiated by first creating a feasible and realistic model of the bulk using a combination of distributed elements that best represents the complex system. Most of the circuit components selected for the model are common electrical elements, such as, resistors (R), capacitors (C), inductors (L), and constant phase elements (Q).47,48 In the equivalent circuit analog, resistors represent the conductive pathways for charge transfer, such as the resistance of the electrolyte to ion transport or the resistance of a conductor to electron transport. Resistors are also used to represent the resistance associated with the charge-transfer processes at the electrode surface. Impedance of a perfect resistor is ideally independent of frequency (ZR = R). Capacitors and inductors are usually associated with space−charge polarization region, such as charge build-up at phase/grain boundaries, the electrochemical double layer and adsorption−desorption processes that occur at an electrode−electrolyte interface, respectively. For inductors, impedance increases with increasing frequency (ZI = jωL), while for a capacitor impedance decreases with increasing frequency (ZC = 1/jωC). In most practical cases, capacitive elements are nonideal because of nonuniform current distribution and hence are substituted by constant phase elements (ZQ = 1/(jω)αY0, where the admittance Y0 = C and exponent α = 1 for an ideal capacitor). The electrode−electrolyte interface primarily is a electrochemical junction of dissimilar materials occasionally with a considerable difference in their work function and behaves as a capacitive element. This is conventionally denoted as Warburg impedance, and depending on whether the element is infinite or finite (bound),the Warburg impdeance is represented by either W (Z = 1/Y0(jω)1/2) or WO (Z = tanh (B(jω)1/2)/ Y0(jω)1/2), respectively.45−47 Figure 1 presents the cartoon illustration of a plausible physical model of the real semi-IPN matrix under study

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RESULTS AND DISCUSSION The formation of a semi-IPN matrix effectively arrests macroscopic phase separation of the two constituent components because of inherent entanglements and complementary miscibility parameters, as demonstrated in our previous reports.43,44 Nevertheless, sufficient indirect evidence toward the presence of local microscopic domains for pure and/or intimately mixed constituents could be perceived.43 Phase separation is understood to have profound effects on events such as (i) the microscopic molecular processes related to the charge-transport mechanism, (ii) the correlated motions of ions along with chain dynamics, (iii) the polarization mechanisms, and (iv) the associated relaxation time.38,45 A detailed investigation is hence undertaken using temperaturestep electrochemical impedance spectroscopy to understand the 20809

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transition temperature does shift toward the lower temperature, ca. −71 °C, as expected for a sufficiently plasticized P4K-PU/ P2 semi-IPN matrix (see Figure SI-7 in Supporting Information). Nevertheless, clear evidence for the presence of crystallized domains is indicated by two endothermic peaks indicated by Tm1 and Tm2, which were otherwise absent in pure P4K-PU network (component I). The broad lower melting temperature peak observed, Tm1 ∼ at 3 °C, is understandable owing to the polydispersed oligomer PEGDME (P2) forming intramolecular H-bonds. This finding implies the existence of a small amount of exclusive crystalline PEGDME-rich domains (microscopic phase separation) within the constrained confinements of the P4K-PU network. The second endothermic peak, Tm2 at ∼35 °C is significantly shifted to lower temperature compared to the ∼58 °C for the pure PEG macromonomer (P4K). This is attributed to a mixed interface formed by the entangled PEG of P4K-PU network and PEGDME chains that facilitates intermolecular H-bonding. Enhanced miscibility of the two components indicated by a steady decrease in glass transition temperatures (Tg) coupled with noticeable decrease in the degree of crystallinity (%χ) on increasing the weight % of PEGDME supports our contention of intimately mixed phases being present at the molecular level (see Figures SI-7 and SI-8 and Table SI-1 in Supporting Information). It is therefore only logical to presume the coexistence of a pure P4K-PU phase within the system, in which the hard segments restrict close interaction between the two constituents. As detailed in the schematic representation (Figure 1), the four resistances (R1, R2, R3, and Rct) coupled with four constant phase elements (Q1, Q2, Q3, and Qdl) are chosen to account for the resistive and capacitive contribution of the pure P4K-PU, pure P2, intimately mixed P4K-PU/P2 phases along with the overall chargetransfer resistance. A semi-infinite/bound Warburg (WO) resistance for the electrode−electrolyte interface appropriately connected with the distributed elements in a combination of parallel and series assembly yields an equivalent circuit that reasonably mimics the different conduction pathways (phases) expected to be present in these semi-IPN electrolyte matrices. In the present study, simulation of experimental data is carried out presuming the inductance of the connecting wires

Figure 1. A schematic representation of triphasic semi-IPN solid polymer electrolyte matrix and a plausible equivalent circuit represented by the distributed electrical elements. A, B, and C and A′, B′, and C′ represent the occupied and vacant sites for Li+-ion coordination, respectively.

wherein three domains (phases) of individual and intimately mixed components are shown to coexist. Justification for this reasonable assumption comes from our earlier studies on the thermal properties by differential scanning calorimetry (DSC) which indicated the presence of distinctive phases in these semi-IPNs that show morphological transitions with change in composition and temperature.43 Initial DSC studies revealed that with incorporation of PEGDME in the network, the glass

Figure 2. Complex plane Nyquist and Bode plots for a typical semi-IPN electrolyte matrix depicting both the experimental data points and the simulated fits generated by equivalent circuit fitting using Zman 2.0 analysis software. The bottom right shows the relative residuals that validates the goodness of the fit and the proposed physical model of the complex semi-IPN matrix. 20810

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Figure 3. Nyquist plots (ρ′ versus −ρ″) of selected solid polymer electrolytes (SS/PE/SS) complexes at various temperatures and compositions of electrolyte matrix (a) P4K-PU/P2/LiClO4 (60:40) and (b) P4K-PU/P2/LiClO4 (50:50) at five temperatures (296, 306, 313, 320, and 327 K); (c) for the different macromonomer molecular weight used in the PU networks; and (d) for variation of oligomer molecular weight like P4K-PU/P1, P4K-PU/P2, P4K-PU/P3, and P4K-PU/P4. The red lines indicate the fitted data to the normalized experimental impedance data collected.

proposed. The semi-IPN polymeric system under study is mainly polyether-based compositions in which one component, methoxy (-OMe) capped polymer (PEGDME), interpenetrates into a polyurethane (PU) network. The resulting spectrum is apparently a depressed semicircle but asymmetric in shape because of convolution of three different contributions, as indicated in the physical model. Though, the matrix bulk contains three individual component contributions; because of heavy overlap with one another, the resulting bulk resistance (Rb) is always slightly lower than the cumulative value of individual isolated resistance. The observation is consistent with a typical case of phase-separated materials which offers similar environment and relaxation times for the solvated ions.45−47 The higher resistance can be attributed to pure PEO-PU (RPEO‑PU) component, lowest resistance to the pure PEGDME (RPEGDME) component, and the moderate resistance to the intimately mixed phase of PEO-PU/PEGDME (RPEO‑PU/PEGDME). The linear spike observed for the imaginary component in the low-frequency region has contributions from both the charge-transfer resistance (Rct) and the bound Warburg (WO) resistance. Temperature affects the frequency range of the profiles, as is quite evident from Figure 3a,b. With an increase in temperature, the bulk ionic conductivity increases and the highfrequency semicircle gradually diminishes in size and disappears at >327 K. Thus, the simulated analysis of the data are restricted or limited within the range of 296−327 K. The fitting parameters of the equivalent circuit model (RPEO‑PU (pure), RPEGDME (pure), RPEO‑PU/PEGDME (mixed phase), Rct, and Rb) and corresponding capacitive components of constant phase

have negligible contribution in the equivalent circuit model. After a normalization of impedance spectrum prior to any data treatment, the parameters were iterated about initial values using nonlinear least-squares fitting with statistical weighting to obtain the best fit to the data. The fitting proceeded through several iterations in Zman 2.0 analysis software. Figures 2 and 3 show typical Nyquist impedance plots for the semi-IPN polymer electrolytes. It is clearly seen that there is a depressed semicircle in the mid/high- frequency range and an inclined straight line in the low-frequency range for each sample. These plots deviate from an ideal impedance spectrum that usually exhibits a standard semicircle in the high-frequency section and a vertical spike in the lower-frequency region. The highfrequency range carries information related to the conduction process in the bulk while the linear region in low-frequency range is attributed to the effect of blocking electrodes where diffusion-related capacitance at the electrode−electrolyte interface comes into play. The deformed semicircle is dependent on the nonideal behavior of the matrix, relaxation time associated with ionic hopping, morphology of the polymeric film, and the surface-roughness of the electrode. The impedance depends on the frequency of the potential perturbation. For all compositions, a significant degree of depression at the center along with an asymmetry has been observed, and these effects reveal the nonclassical nature of the material and distribution of relaxation time. A typical simulation for both Nyquist and Bode plots is appropriately demonstrated in Figure 2 along with the relative residuals of the data treatment to visually ascertain the goodness of fit and hence the validity of the physical model 20811

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Table 1. Estimated Values for Individual Contribution of Distributed Elements Representing the Microphase Separation Isolated by Equivalent Circuit Simulation of the Semi-IPN Electrolyte Compositionsa sample name

RPEO‑PU (ohms)

RPEGDME (ohms)

RPxK‑PU/Py (ohms)

Rct (ohms)

Rb (ohms)

CPEO‑PU (F)

CPEGDME (F)

CPxK‑PU/Py (F)

(60:40) (50:50) (40:60)b (30:70)

3.67 1.02 6.00 1.66

× × × ×

104 104 103 103

4.08 4.00 1.00 9.85

× × × ×

103 103 103 102

2.46 5.00 5.00 1.01

× × × ×

104 103 103 103

1.00 9.00 1.13 8.67

× × × ×

104 103 104 102

6.30 1.75 1.17 3.40

× × × ×

104 104 104 103

8.78 2.86 1.63 3.69

× × × ×

10−12 10−11 10−11 10−14

1.88 1.71 8.85 6.45

× × × ×

10−7 10−6 10−7 10−12

1.55 1.34 7.77 6.45

× × × ×

10−11 10−11 10−11 10−12

P0.4K-PU/P2 (30:70) P1K-PU/P2 (30:70) P2K-PU/P2 (30:70) P4K-PU/P2 (30:70)b P10K-PU/P2 (30:70) P14K-PU/P2 (30:70)

5.97 4.64 3.81 1.66 4.04 6.50

× × × × × ×

103 103 103 103 103 103

3.01 2.34 2.00 9.85 2.00 2.90

× × × × × ×

103 103 103 102 103 103

4.00 2.47 3.80 1.01 3.00 4.00

× × × × × ×

103 103 103 103 103 103

4.00 2.00 4.80 8.67 1.00 9.70

× × × × × ×

103 103 103 102 103 102

1.20 8.50 8.20 3.40 8.70 1.20

× × × × × ×

104 103 103 103 103 104

8.29 4.94 2.16 2.08 5.98 1.23

× × × × × ×

10−14 10−13 10−13 10−14 10−11 10−11

2.00 1.34 3.49 3.49 6.66 2.24

× × × × × ×

10−8 10−8 10−8 10−8 10−9 10−8

3.33 1.66 1.12 7.77 9.48 2.29

× × × × × ×

10−11 10−12 10−10 10−14 10−12 10−10

P4K-PU/P1 P4K-PU/P2 P4K-PU/P3 P4K-PU/P4

2.30 1.66 3.30 1.40

× × × ×

104 103 103 104

1.23 9.85 2.00 9.90

× × × ×

103 102 103 103

1.01 1.01 2.05 1.20

× × × ×

104 103 103 104

1.00 8.67 1.00 1.00

× × × ×

104 102 104 104

4.82 3.40 7.10 3.45

× × × ×

104 103 103 104

1.30 3.69 4.10 2.09

× × × ×

10−12 10−14 10−13 10−12

3.60 6.45 1.90 6.13

× × × ×

10−12 10−12 10−8 10−9

8.90 6.45 4.90 5.59

× × × ×

10−12 10−12 10−12 10−11

P4K-PU/P2 P4K-PU/P2 P4K-PU/P2 P4K-PU/P2

(30:70) (30:70)b (30:70) (30:70)

a

The bulk resistance (Rb), resistive and capacitive contribution of the pure and mixed phases for different weight ratio of constituents, and variation of macromonomer and oligomer chain length are obtained from the fits to the experimental data collected at 306 K. bThe repeated presentation of the P4K-PU/P2 (30:70) data is for ease of comparison within the sets of composition variations.

Figure 4. Variation of dielectric constant (ε) with frequency at different temperatures for P10K-PU/P2 semi-IPN electrolyte matrix: (a) real component of dielectric constant (ε′) against log f and (b) the imaginary component of dielectric constant (ε″) versus log f at different temperatures. The spectra show three distinct zones in the frequency response region, indicative of different polarization processes involved.

Dielectric Polarization and Debye Relaxation. The study of dielectric relaxation in solid polymer electrolytes is an approach for obtaining information regarding the characteristics of ionic and molecular interactions.45 The dielectric constant (ε) is a measure of reduction of coulomb interaction between the ion pairs in the polymer electrolyte matrix. This provides valuable information, such as characteristics of the ionic− molecular interaction of the polymer electrolyte and the understanding of ion-transport behavior as well. The variation of the dielectric constant with temperature is different for polar and nonpolar polymers. In general, for polar polymers, the dielectric constant increases with increasing temperature. But in the case of nonpolar polymers, the dielectric constant is independent of temperature. The frequency dependence of dielectric constant ε (real and imaginary components) at different temperatures for a semi-IPN electrolyte P10K-PU/ P2/LiClO4 sample is presented in Figure 4. It can be clearly seen that the values of ε′ and ε″ decrease with increase in frequency. Herein, two distinct phenomenon can be clearly observed in Figure 4: the decrease of dielectric permittivity in

elements (C PEO−PU , C PEO‑PU/PEGDME (mixed phase), and CPEGDME) are listed in Table 1 for comparative evaluation. The bulk resistance (Rb) as well as the individual contributions strongly depend on the matrix compositions, macromonomer chain length, and oligomeric entanglements, as exemplified in the plots in Figure 3c,d. It has been observed that upon increasing the amount of the second component (PEGDME) into the polymer electrolytes, the resistive contribution decreases for all three microscopic components and follows the same trend as the bulk resistance.43 The corresponding capacitance values, however, do not follow any trend upon increasing temperature and/or compositions. Nevertheless, it was observed that the capacitance for the PEO-PU matrix is invariably the lowest; mixed phase (PEO-PU/PEGDME) have intermediate values, and the PEGDME phase shows the highest capacitance for all the compositions studied. This confirms preferential solvation of ions and excellent ion pair separation in the amorphous domains of low molecular weight PEGDME oligomers used, as was also indicated in our detailed Fourier transfrom infrared (FTIR) spectroscopy studies.44 20812

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Figure 5. Imaginary component of complex impedance (−Z″(ω)) versus log f of (a) P4K-PU/P2/LiClO4 (60/40) and (b) P4K-PU/P2/LiClO4 (50/50) at five temperatures (296, 306, 313, 320, and 327 K). Debye plots for (c) different compositions of P4K-PU/P2 (60:40, 50:50, 40:60, 30:70) and (d) variation of oligomeric molecular weights within electrolyte matrices (P4K-PU/P1, P4K-PU/P2, P4K-PU/P3, and P4K-PU/P4). The red lines indicate the Lorentzian fits for these Debye plots.

Lorentzian peaks which shift to higher frequencies with increasing temperature. The Debye peak is observed when the condition ωτ = 1 is satisfied, where ω is the angular frequency (ω = 2πf) and τpeak is the Debye relaxation time. Figure 5a−d shows that the frequency of maxima for these Lorentzian shapes depends on the polymer electrolyte matrix composition as well as the temperature of study. When the temperature is increased, the intensity of the peak decreases considerably and the peaks shift toward higher frequency. Fitting these profiles by Lorentzian functions allowed us to measure the bulk relaxation time (τpeak). Similarly, the variation of loss tangent with frequency sweep can also indicate the presence of dipolar relaxation and the effect of both temperature and composition (see Figure SI-11 in Supporting Information). It was clearly evidenced that with the increase in amorphous nature of the materials, a concomitant increase in the number of free ionic charge carriers occurs in the bulk. This coupled with increased segmental motion of polymer chains is translated into enhanced ion-transport properties and faster dynamics reflected as a peak shift toward higher frequency. Mechanisms of Ionic Transport and Site Reorganization. The semi-IPNs are by their nature highly disordered systems. Nevertheless, the major inferences that can be undoubtedly drawn postanalysis of the complex plane and spectroscopic plots are (a) correlated ionic hopping coupled with segmental motions has a predominant role to play, (b) thermal activation facilitates the ion-transport process, and (c)

the lower-frequency region slowly decreases (region I), sharply falls in the moderate frequency region (region II), and then almost attains a constant value in the higher-frequency region (region III). The behavior is indicative of space−charge (interfacial) polarization often observed for materials having multiple phases of dissimilar conductivity in the bulk.38,45−47 When an electric field is applied to these quasi-solid polymer electrolyte films, the charge moves through the PEGDME phase, which is a relatively more conducting phase, but is interrupted as the charge comes across the higher resistivity PEO-PU phase. The decrease in dielectric permittivity with increasing frequency can be associated with the inability of dipoles to rotate rapidly, leading to a lag between frequency of oscillating dipole and that of the applied field. To simplify, the dipoles are unable to follow the applied field. The ε′ lies in the range of 102−104 at room temperature. Such high dielectric constant values in our sample can be attributed to the good interactions between the Lewis acidic sites of the ethylene oxide unit (−CH 2 −CH2 −O−) and the ions of the lithium perchlorate indicating excellent ion pair separation. When the temperature is raised, the dielectric constant also increases because of facilitation in dipole orientation within the semi-IPN matrix of P10K-PU/P2/LiClO4. In all cases, the increase in dielectric constant implied the increase in the number of free ions. The plots of −Z″(Ω) versus the logarithms of frequency (log f), conventionally known as Debye plots,45 reveal typical 20813

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Figure 6. Real component of conductivity σ′(ω) versus log f for (a) the P10K-PU/P2 (30:70) doped with LiClO4 at different temperatures and (b) the P14K-PU/P2 (30/70) doped with LiClO4 at various temperatures. The red lines are the fit to the modified power law for the experimental data. Three distinct zones are can be clearly identified: electrode−electrolyte interface (I), the frequency-independent plateau region (II), and the highfrequency dispersion due to correlated motions of ions along with the polymer−chain dynamics (III).

Coulomb cage and a geometrical relaxation caused by host network adjustment.32,33,49,50 In particular, intra- and interchain hopping events between an occupied site, A to either A′, B′, and/or C′ sites are expected to take place on two distinct time scales: the first is associated with instantaneous hops and the second with the time involved in host medium reorganization.23,55,56 The determination of the parameters τ1 and p are therefore of great significance for understanding the conductivity behavior of electrolytes in terms of ion migration. The applicability of the UPL equation was verified on the σ′(ω) profiles of the PEO-PU/PAN/LiClO4 semi-IPN systems, using nonlinear least-squares fitting (NLSF) method. The real component of conductivity (σ′(ω)) was calculated from the impedance data using the equation33,45 σ′(ω) = Z′(ω)/ k[(Z′(ω))2 + (Z″(ω))2], where k is the cell constant in centimeters. Representative spectra of σ′(ω) profiles for the PEO-PU/PAN/LiClO4 semi-IPN systems are depicted in Figure 6. Frequency dependence of the conductivity spectrum exhibits three distinctive regions (a) low-frequency dispersion, (b) an intermediate frequency plateau, and (c) an extended dispersion at high frequency. The variation of conductivity in the low-frequency region is attributed to the polarization effects at the electrode and electrolyte interfaces. The flat significantly low conductivity zone in this low-frequency regime is indicative of the effects of a bound Warburg (WO). As the frequency reduces, more and more charge accumulation occurs at the electrode and electrolyte interface, leading to a significant drop in conductivity. In the intermediate frequency, conductivity is found to be almost frequency-independent (plateau region) and is equal to the bulk conductivity (σ0 = σdc). In the highfrequency region, the conductivity shows frequency dispersion beyond a critical frequency, ωp (percolation frequency), that is dependent on the material and temperature and progressively increases. This steep increase in σ′(ω) at high frequencies was attributed to correlated ionic motions in the solid polymer electrolyte bulk materials. Evidently, charge migration processes in a polymer electrolyte depend significantly on a successful hop from the sites coordinating an ion to an empty site which could possibly accept the ion. The host medium reorganization can be attributed to both ionic correlation events between the polymer complexed cations and anions, and the geometrical reorganiza-

space−charge/interfacial polarization has a significant contribution in these electrolytes. Furukawa and Di Noto have demonstrated that the frequency-dependent behavior of the real component of conductivity can provide pertinent clues in elucidating ion-transport mechanisms in disordered systems such as glasses and polymers.31,45,49,50 The frequency response of conductivity exhibits a high degree of universality and reflects essential features of the transport mechanisms. The universal behavior in the framework of jump−relaxation models, proposed by Jonscher, is applicable to a wide class of materials and can be analyzed by a universal power law (UPL) equation.51−53 The UPL equation is expressed as σ′(ω) = σ(0) + Aω, where, σ(0) ≅ σdc, the prefactor A = σ(0)/ (ωp)n, and n is the frequency exponent. Both σ(0) and A are thermally activated quantities and can be represented by an equivalent expression:33,34 σ′(ω) = σ(0)[1 + (τω)p]. According to Crammer et al.,54 τ is a time related to τ1, where τ1 is the initial site relaxation time of ionic hopping, p = τ1/τ* the power law exponent according to the jump−relaxation model which takes account of the Coulomb interaction between mobile ions, and τ* the initial back-hop relaxation time of the ionic transport process. The charge migration process can be physically interpreted as follows. For each ion in the bulk material, it is necessary to predict the existence of at least two types of coordination sites as indicated in Figure 1. For example, A, B, and C are the occupied sites of the polymeric segments having transient cross-links with Li+-ions, while A′, B′, and C′ are the corresponding vacant sites in the microphase-separated physical model represented for the semi-IPNs under study. As the ion hops from an occupied to a vacant site, say, A to A′, at t = 0, the effective potential minimum remains with A and two distinct relaxation phenomena can occur. On one hand, the ion can hop back to A, giving no contribution to the overall conductivity.32,33 In this event, the effective potential of the sites remains unchanged and the kinetics is regulated by the time constant (τ*) related to back hop rate, rb (τ* = 1/rb). On the other hand, to accommodate the ion in the new environment, the coordination site is characterized by a new absolute potential minimum. In this case, site A′ can relax with a kinetics regulated by τ1 (the site relaxation time) related to rate of successful forward jump, rf (τ1 = 1/rf). It is suggested that this latter process occurs through two distinct processes, i.e., a shift of the 20814

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Figure 7. (a) Representative plot depicting the power law exponents (p) as a function of temperature for composition variation in the semi-IPN system using different macromonomer chain length. The significantly lower p-values are indicative of preferentially cationic transport in these semiIPN matrices. (b) Typical log τ versus 1000/T plots of relaxation time (τpeak) associated with successful ionic hopping for the samples. The dotted lines in panel b represent the Arrhenius fit.

estimated dc-conductivity (σ0). Direct current polarization studies using SS-blocking electrodes for these semi-IPNs have revealed predominantly ionic conductivity (>0.92) at ambient temperatures (see Figure SI-14 in Supporting Information). The significantly lower values of p estimated in the case of semi-IPN compositions, Figure 7a and Table 2, could be

tion of the coordination cage, accommodating the ion after the hop. Depending on the class of polymer electrolytes, two types of intra- and interchain hopping events, classified as the cationic and anionic hopping, can occur in the bulk material, both of which contribute to the overall cationic migration.32 Because p = back-hop rate/site-relaxation, the fact that 0 < p < 1 observed at ambient temperatures imply that the back-hop is essentially slower than the site relaxation. In a back-hop, the hopping ion can jump back to its initial site, which is caused by the Coulomb repulsive interactions. The site-relaxation is the shift of a site potential minimum to the position of the hopping ion, which is caused by a rearrangement of neighboring ions. In a previous report, Basak et al.38 clearly demonstrated and simulated two separate fits for the UPL equation to explain the presence of distinct binary phases and different relaxation times associated with the PEO-PU and PAN phases, respectively. Although the present semi-IPN system has binary constituents and contributions from three microscopic components are observed for Nyquist plots, the analysis of spectroscopic plots of the real component of bulk conductivity revealed only one relaxation time. It is evident from the heavily overlapped Nyquist plots that the jump−relaxation times are essentially similar in all three microscopic phases present, implying excellent compatibility. The primary and secondary components of the synthesized systems composed of polyethers perceptibly contributes to this behavior, signifying a very conducive pathway for the ionic transport across the phase boundaries; thus, the system behaves macroscopically as a single-component system. It is also observed that the frequency at the dispersion region deviating from the dc-conductivity plateau is defined as the characteristics frequency (ωp), known as the hopping rate, at which σω= 2σ0. The relation between the dc-conductivity and hopping rate is given by σ0 = kωp, where k is the empirical constant which depends on the concentration of mobile ions and the conduction mechanism. The frequency dispersion region starts to decrease and disappears with increase in temperature. Thus, the hopping rate (ωp) at which the relaxation effects begin to appear moves toward the higher frequency with increasing temperature. The increase in the frequency-independent conductivity zone strongly indicates the long-range transport of the mobile lithium ions in response to the electric field, where only successful jumps contribute to the

Table 2. Parameters σ(0) = σdc, Relaxation Time (τ), and Frequency Exponent (p) Obtained from the Modified Universal Power Law Fits for the PxK-PU/Py (30:70) SemiIPNs with LiClO4, EO/Li = 30 as an Electrolyte at 306 Ka sample name P4K-PU/P2 (60:40) P4K-PU/P2 (50:50) P4K-PU/P2 (40:60) P4K-PU/P2 (30:70) P0.4K-PU/P2 (30:70) P1K-PU/P2 (30:70) P2K-PU/P2 (30:70) P4K-PU/P2 (30:70) P10K-PU/P2 (30:70) P14K-PU/P2 (30:70)

σ(0)PxK‑PU/Py (S cm−1) 1.51 5.28 8.44 2.50 7.78 1.06 1.18 2.50 1.09 7.51

× × × × × × × × × ×

10−5 10−5 10−5 10−4 10−5 10−4 10−4 10−4 10−4 10−5

τPxK‑PU/Py (s)

pPxK‑PU/Py

2.83 × 10−8 1.74 × 10−8 6.18 × 10−10 − 1.48 × 10−8 2.7 × 10−8 1.60 × 10−8 − 2.3 × 10−8 6.68 × 10−8

0.5 0.68 0.35 − 0.55 0.53 0.59 − 0.83 0.88

a

The dash (−) for the selected compositions indicates UPL equation fit could not be ascertained because of the lack of ample data in the relevant region.

attributed to a higher rate of successful jumps and is also reflected in their significantly higher conductivity. It also strongly indicates that cationic (Li+) charge transport is favored in these semi-IPN matrices at ambient temperatures. That the fraction of cationic transference number steadily decreases with an increase in temperature is quite evident from the general trend observed from Figure 7a and Table 3. As evidenced, the trends show a steady increase in the p values as the temperature increases. The conductivity relaxation time determined independently from both Debye profiles and UPL fits (σ′(ω) versus log f) are in good agreement and indicative of overall successful ionic hopping events. The appreciably shorter relaxation time (Table 2) indicates faster chain dynamics in the polymer electrolyte matrices. A thermally activated process, these estimated relaxation times show typical Arrhenius behavior, τ = τ0 exp(−ΔEa/kT) (Figure 7b and Table 3). 20815

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Table 3. Parameters σ(0) = σdc, Relaxation Time (τ), and Frequency Exponent (p) Obtained from the Modified Universal Power Law Fits for the PxK-PU/P2 (30:70) Semi-IPNs with LiClO4, EO/Li = 30 as Electrolyte at Different Temperatures sample name

temperature (K)

σ(0)PxK‑PU/P2 (S cm−1)

P0.4K-PU/P2 (30:70)

296 306 313 320 327 334 296 306 313 320 327 334 296 306 313 320 327 334 296 306 313 320 327 334 296 306 313 320 327 334

5.2 × 10−5 8.4 × 10−5 1.28 × 10−4 1.85 × 10−4 2.22 × 10−4 3.16 × 10−4 7.2 × 10−5 1.1 × 10−4 1.8 × 10−4 2.47 × 10−4 3.3 × 10−4 4.38 × 10−4 1.02 × 10−4 1.42 × 10−4 2.12 × 10−4 3.2 × 10−4 4.23 × 10−4 5.89 × 10−4 1.04 × 10−4 1.47 × 10−4 1.98 × 10−4 3.68 × 10−4 4.71 × 10−4 6.52 × 10−4 5.56 × 10−4 7.61 × 10−4 1.3 × 10−4 2.31 × 10−4 3.13 × 10−4 4.13 × 10−4

P1K-PU/P2 (30:70)

P2K-PU/P2 (30:70)

P10K-PU/P2 (30:70)

P14K-PU/P2 (30:70)

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CONCLUSIONS The semi-IPNs can effectively arrest the macroscopic phase separation of the two constituent components because of inherent entanglements and complementary miscibility parameters of the two polyether constituents; however, sufficient evidence for the presence of local microscopic domains for pure and/or intimately mixed constituents could be ascertained. Although the semi-IPN electrolytes investigated in the present study posed a challenge because of their intrinsic complexity, a plausible physical model could be successfully proposed and rationalized based on direct and indirect evidence observed in their physico-chemical and electrochemical characteristics. The effect of the microphase separation on the microscopic molecular processes related to the charge-transport mechanism, the correlated motions of ions along with chain dynamics, the polarization mechanisms associated with the phase boundaries/ electrode−electrolyte interfaces, and the associated relaxation time in these semi-IPN systems could be successfully elucidated following detailed electrochemical impedance analysis and simulated fits. The normalized complex plane Nyquist plots (ρ′ versus ρ″) show two well-defined regions for bulk (in midand high-frequency regions) and electrode−electrolyte interfacial impedance (in low-frequency regions). Rigorous analysis of the asymmetric depressed semicircular region indicates presence of three microscopic phases in the matrix bulk (pure PEO-PU, pure PEGDME, and PEO-PU/PEGDME mixed phase) along with the charge-transfer resistance (Rct) which contributes to the bulk resistance. Although the matrix bulk

τPxK‑PU/P2 (s) 3.99 2.02 1.25 9.1 7.01 5.04 5.76 2.94 1.28 7.51 4.9 2.61 2.67 1.57 1.24 9.12 7.52 3.4 4.04 2.28 1.65 1.18 9.25 6.33 6.9 4.9 2.95 2.05 1.5 1.1

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−8 10−8 10−8 10−9 10−9 10−9 10−8 10−8 10−8 10−9 10−9 10−9 10−8 10−8 10−8 10−9 10−9 10−9 10−8 10−8 10−8 10−8 10−9 10−9 10−8 10−8 10−8 10−8 10−8 10−8

pPxK‑PU/P2 0.51 0.68 0.7 0.67 0.59 0.57 0.73 0.75 0.78 0.82 0.9 1.17 0.61 0.59 0.75 0.90 0.99 0.96 0.78 0.83 0.94 1.14 1.16 1.25 0.72 0.81 0.98 1.2 1.3 1.41

contains three individual component contributions, because of heavy overlap with one another, the resulting bulk resistance (Rb) is always slightly lower than the cumulative value of individual isolated resistance. The observation is consistent with a typical case of phase-separated materials which offers similar environment and relaxation times for the solvated ions. Nevertheless, the temperature dependence of the conductivity behavior of isolated phase contribution is also seen to follow behavior similar to that of the bulk matrix. The estimate of capacitive contributions implies preferential solvation of ions in the oligomeric PEGDME domains. Dielectric permittivity profiles (ε′ and ε″) indicated two characteristic frequency responses indicative of both space−charge and electrode− electrolyte polarization processes occurring in the systems. Spectroscopic plots of complex impedance against frequency (Z″ versus log f) depict Debye peaks providing an estimate of the bulk relaxation time (τpeak), and chain dynamics are observed to be faster with increase in temperature and optimal compositions. Profiles depicting real component of conductivity (σ′(ω)) as a function of frequency (log f) follow a modified universal power law where the simulated fit results reveal vital information on the site-relaxation rates, cumulative favorability for successful hopping events, and predominant charge carrier type. Effect of correlated ion and segmental motions are quite apparent in the high-frequency domain beyond the critical frequency. The relaxation rates indicate thermally activated processes at appreciably fast time scales (microsecond to nanosecond) for the matrix, whereas the estimated low values 20816

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(9) Allcock, H. R.; Sunderland, H. J.; Ravikiran, R.; Nelson, J. M. Poly(phosphazene) with Novel Architectures: Influence on Physical Properties and Behaviors as Solid Polymer Electrolytes. Macromolecules 1998, 31, 8026−8035. (10) Jean-Franois, N.; Alessandro, G.; Cheradame, H.; Jean-Pierre, C. A. Influence of Lithium Perchlorate on Properties of Poly Ether Networks: Specific Volume and Glass Transition Temperature. Macromolecules 1988, 21, 1117−1120. (11) Hawker, C. J.; Chu, F.; Pomery, P. J.; Hill, D. J. T. Hyperbranched Poly(ethylene glycol)s: A New Class of IonConducting Material. Macromolecules 1996, 29, 3831−3838. (12) Druger, S. D.; Nitzan, A.; Ratner, M. Dynamic Bond Percolation Theory: A Microscopic Model for Diffusion in Dynamically Disordered System. I. Definition and One Dimensional Case. J. Chem. Phys. 1983, 79, 3133−3142. (13) Shi, J.; Vincent, C. A. The Effect of Molecular Weight on Cation Mobility in Polymer Electrolytes. Solid State Ionics 1993, 60, 11−17. (14) Andreev, Y. G.; Bruce, P. G. Polymer Electrolytes Structure and Its Implication. Electrochim. Acta 2000, 45, 1417−1423. (15) Li, J.; Pratt, L. M.; Khan, I. M. Poly(ethylene oxide)/ Poly(2vinylpyridine)/Lithium Perchlorate Blends as Solid Polymer Electrolytes: Composition/Property/Structure Interrelationship. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 1657−1663. (16) Acosta, J. L.; Enrique, M. Ionic Conductive Polymer System Based on Polyether and Poly(phosphazene) Blends. J. Appl. Polym. Sci. 1996, 60, 1185−1191. (17) Munichandraiah, N.; Sivasankar, G.; Scanlon, L. G.; Marsh, R. A. Characterization of PEO-PAN Hybrid Solid Polymer Electrolytes. J. Appl. Polym. Sci. 1997, 65, 2191−2199. (18) Watanabe, M.; Sanui, K.; Ogata, N.; Kobayashi, T.; Ohtaki, Z. Ionic Conductivity and Mobility in Network Polymers from Poly(propylene oxide) Containing Lithium Perchlorate. J. Appl. Phys. 1985, 57, 123−128. (19) Florjanczyk, Z.; Krawiec, W.; Wieczorek, W.; Siekierski, M. High Conducting Solid Electrolyte Based on Poly(ethylene oxide-copropylene oxide). J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 629− 635. (20) Allcock, H. R.; O’Connor, S. J. M.; Olmeijer, D. L.; Napierala, M. E.; Cameron, C. G. Poly(phosphazene) Bearing Branched and Linear Oligoethylene Oxide Side Groups as Solid Solvent for Ion Conduction. Macromolecules 1996, 29, 7544−7552. (21) Zhang, Z.; Fang, S. Ionic Conductivity and Physical Stability Study of Gel Network Polymer Electrolytes. J. Appl. Polym. Sci. 2000, 77, 2957−2962. (22) Ichikawa, K.; Dickinson, L. C.; MacKnight, W. J.; Watanabe, M.; Ogata, N. Ionic Motion in Network Polymers Containing Lithium Perchlorate. Polymer 1992, 33, 4699−4704. (23) Druger, S. D.; Ratner, M. A.; Nitzan, A. Generalized Hopping Model for Frequency-Dependent Transport in a Dynamically Disordered Medium, with Applications to Polymer Solid Electrolytes. Phys. Rev. B: Condens. Matter Mater. Phys. 1985, 31, 3939−3947. (24) Neyertz, S.; Brown, D.; Thomas, J. O. Molecular Dynamics Simulation of Crystalline Poly(ethylene oxide). J. Chem. Phys. 1994, 101, 10064−10073. (25) Neyertz, S.; Brown, D.; Thomas, J. O. Molecular Dynamics Simulation of the Crystalline phase of Poly(ethylene-oxide) SodiumIodide, PEO3NaI. Electrochim. Acta 1995, 40, 2063−2069. (26) Neyertz, S.; Brown, D. Computer Simulation Study of the Chain Configurations in Poly(ethylene oxide)-Homolog Melts. J. Chem. Phys. 1995, 102, 9725−9735. (27) Lin, B.; Boinske, P. T.; Halley, J. W. Molecular Dynamics Model of the Amorphous Regions of Polyethylene Oxide. J. Chem. Phys. 1996, 105, 1668−1681. (28) Smith, G. D.; Yoon, D. Y.; Jaffe, R. L.; Colby, R. H.; Krishnamoorti, R.; Fetters, L. J. Conformations and Structures of Poly(ethylene) Melts from Molecular Dynamics Simulations and Small-Angle Neutron Scattering. Macromolecules 1996, 29, 3462− 3469.

of power law exponents (p) at ambient temperatures strongly indicate a very favorable salt-solvation environment, and preferential charge transport mediated by Li+-ions (cationic contribution) is high. The trends supported by our prior evaluation of the physico-chemical properties of the matrices establishes a robust and rational physical model for these complex systems. The comprehensive analysis consolidates our earlier understanding of the system and will aid in appropriately tailoring these semi-IPNs to suit specific applications.

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ASSOCIATED CONTENT

S Supporting Information *

Details on the semi-IPNs probed by mid-FTIR, scanning electron microscopy, differential scanning calorimetry, and the experimental setup; additional complex plane and spectroscopic plots for impedance, dielectric permittivity, loss tangent as well as conductivity along with their respective fitting profiles; and dc-polarization tests. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 040-27193225/27191386. Fax: +91-40-27160921. Email: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.B. acknowledges Council of Scientific and Industrial Research (CSIR), India for financial assistance in the form of a senior research fellowship (SRF). P.B. acknowledges the strong support of DST-Ramanujan Fellowship (GAP-0248), MNRE-CSIR TAPSUN Project on Dye Sensitized Solar Cells (DyeCell: GAP-0366), and CSIR TAPSUN Project on Innovative Solutions for Solar Energy Storage (StoreSolar: NWP-0056) for the grants received. The authors sincerely appreciate the encouragements and considerable help received from Dr. S.V. Manorama, Dr. K.V.S.N. Raju, Dr. R.K. Rana, and Dr. R. Narayan during the course of this investigation.

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dx.doi.org/10.1021/jp506481u | J. Phys. Chem. C 2014, 118, 20807−20818