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BMPyr.TFSI - With improved electrical, optical, thermal and structural properties
Solid State Ionics 310 (2017) 166–175
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Development of ion conducting ionic liquid-based gel polymer electrolyte membrane PMMA/BMPyr.TFSI - With improved electrical, optical, thermal and structural properties
MARK
Safna Hussan K.Pa,⁎, Mohamed Shahin Thayyila, S.K Deshpandeb, Jinitha T.Vc, Jayant Kolted a
Department of Physics, University of Calicut, Malappuram 673635, Kerala, India UGC-DAE Consortium for Scientific Research, Mumbai Centre, BARC, Mumbai, India c Department of Chemistry, University of Calicut, Malappuram 673635, Kerala, India d School of Physics and Materials Science, Thapar University, Patiala, Punjab, India b
A R T I C L E I N F O
A B S T R A C T
Keywords: Ionic liquid-based gel polymer electrolyte SEM DSC Broadband dielectric spectroscopy Optical properties
Ionogel membranes are materials of colossal intrigue worldwide because of their utilization as excellent substitutes of the liquid electrolytes or as separators in ionic gadgets including batteries, fuel cells, supercapacitors, energy units, and so on. In the present work, transparent ionogel composites of PMMA/1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl imide) (100/0, 90/10, 80/20, 70/30) were prepared to be used for optical devices. The impact of varying amount of ionic liquid in the PMMA matrix in weight ratios was compared with the undoped polymer blend. The prepared membrane was characterized for thermal, conductivity, optical and for morphological examinations. In addition, the current voltage characteristics of prepared films have been performed at room temperature. The addition of ionic liquid to the polymer matrix improves the properties by having better thermal stability, enhanced luminescence intensity and making the membrane more amorphous. Among these, PMMA with 20% having a glass transition temperature of 44.95 °C exhibits superior performance with the fairly smooth transparent surface, increased flexibility, enhanced thermal stability, highest ionic conductivity, the maximum capacitance of 0.403 ∗ 10− 6Fcm− 2, non ohmic behaviour and photoluminescence property.
1. Introduction In spite of the realities that the electrodes are the limiting factors in terms of overall capacity, energy density and cyclability, electrolytes plays a crucial role in determining the current density, power density, time stability and safety of the battery in electrochemical devices [1]. Accordingly, as of late solid-state electrolytes are of incredible demand since it overcomes all the restrictions of its liquid nature like leakage, gas formation due to solvent decomposition, volatile and thermal instability [2]. Hence, the researchers centered around the area of the dry solid electrolyte by entrapping different conducting materials in polymer matrix or by polymerization of such materials [2,3]. In these circumstances, ion conducting polymer electrolytes has received worldwide attention due to their intrinsic properties like thin film forming ability, transparency, flexibility, high ionic conductivity and wide electrochemical window [3]. This type of material engineering broadened the array of applications like lithium batteries, solar cells, supercapacitors, fuel cells, sensors, solid state batteries, capacitors, etc.,
⁎
Corresponding author. E-mail address: [email protected] (S.H. K.P).
http://dx.doi.org/10.1016/j.ssi.2017.08.012 Received 29 May 2017; Received in revised form 2 August 2017; Accepted 13 August 2017 0167-2738/ © 2017 Elsevier B.V. All rights reserved.
with high-temperature stability and improved safety. Gel polymer electrolytes consist of a polymeric framework as the host, an organic solvent as the plasticizer, and a conducting electrolytic salt either organic or inorganic. Recent studies have described versatile ionic liquid (IL) electrolytes would be an excellent substitute for liquid electrolytes [4] since it have a wide range of conductivities from 0.1–740 mS/cm [5]. Especially IL with 1-butyl-3-methylpyridinium [BMPy]+ and trihexyl(tetradecyl) phosphonium [P6,6,6,14]+ cations exhibits higher conductivities. Thus immobilization of IL in the polymer host will be a better choice for solid state electrolyte due to their appealing feature of freestanding film and high thermal stability properties [4] and allows us to design easily and cheaply with modularity and reliability in electrochemical devices [9]. Despite the fact that polymers are the best stage to immobilize ILs, only a few polymers and fewer ILs are used till now. But the versatility of both IL and polymer chemistry allows us to develop an infinite number of ionogel [6]. Thus the field of ionogel membranes is yet to explore, since the different combination of IL guest to the polymer host
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added to PMMA solution to reduce its brittleness. This composite solution was gently stirred for 2 h at a temperature of 40 °C and kept in vacuum oven for 1 day at 40 °C.
may result in chaotic outcomes to the optoelectronic world. PMMA is one among the promising representatives of polymeric materials with a wide application in optoelectronic devices, electronic gadgets as dielectric organic thin film transistors. Yet at the same time, it has to satisfy various constraints in regards to amorphous nature, stability against degradation, limits on charge traps, high breakdown potential, band offsets processability and reproducibility [7]. Extensive work has been carried out to tune the properties such as morphology, optical, dielectric and aging behaviour of PMMA films. Yet, the better outcomes were gotten from ionogel with the marvelous compatibility of different organic salts with poly(methyl methacrylate) (PMMA) [4–17]. In the present study, ionic liquid based gel polymer electrolyte membrane PMMA/1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl imide) were prepared with different wt ratio (100/0, 90/10, 80/ 20, 70/30) and detailed investigations were carried out using SEM, XRD, ATR-FTIR and UV–visible spectrophotometer, DSC, TGA, photoluminescence and finally the transport properties were studied using broadband dielectric spectroscopy.
2.4. Characterization The attenuated total reflectance-fourier transform infrared (ATRFTIR) spectra of the ionogel films were recorded by JASCO spectrometer at room temperature. The measurements were carried out in the wave number range from 400 to 4000 cm− 1 The X-ray-diffraction pattern of the prepared films were recorded in the range of 2θ from 20° to 80° at a speed rate of 10° per minute using Rigaku powder diffractometer fitted with a curved crystal monochromator and attenuated total reflectance, which is equipped with Ni-filtered CuKα radiation (λ = 1.5418 Å). 2.5. Scanning electron microscope A scanning electron microscope (SEM, SU6600HI-2102-0003) operated at an accelerating voltage of 5 KV was used to observe the immiscible IL's and grain boundaries at the microscopic level. The specimens were sputter coated with a thin gold layer prior to SEM in order to prevent charging of specimen and to increase the signal to noise ratio.
2. Materials and methods 2.1. Materials 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl)imide, 99% [BMPyr.TFSI], propylene carbonate, 99% [PC], and polymethylmethacrylate, > 98% [PMMA] were purchased from SigmaAldrich, USA. PMMA is the main constituent of the polymer blend acting as host polymer.
2.6. Thermal studies
The geometry of ionogel was optimized using ONIOM/DFT: TDDFT:SM level of theory with 631G ++:631G+: PM6 basis set and shown in Fig. 1.
Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA) were carried out to investigate the thermal stability of the ionogel; which is inevitable in electrochemical applications. The ionogel membrane was exposed to a heating rate of 5 °C/min in the range from −70 °C to 130 °C using Perkin Elmer DSC 8500, provided information about glass transition temperature (Tg), notwithstanding enthalpy for the process [20]. Thermal gravimetric analysis (TGA) using Perkin Elmer (Pyris 1 TGA) from 30 °C to 500 °C at 5 °C/min gave an indication of mass loss and thermal degradation on heating.
2.3. Synthesis of PMMA/BMPyr.TFSI ionogel films
2.7. Broadband dielectric spectroscopy
A solution casting in the glass case was used to prepare the ionogel polymer electrolyte membrane. The solutions of PMMA/BMPyr.TFSI (100/0, 90/10, 80/20, 70/30) were prepared by doping BMPyr.TFSI into the PMMA-chloroform solution. A drop of propylene carbonate is
The dielectric and conductivity measurements were done at ambient pressure in a wide frequency window [0.01 Hz to 10 MHz] for different temperatures using Novocontrol Broadband dielectric spectrometer in which the samples were sandwiched between two gold-plated copper
2.2. Computational calculation
Fig. 1. The optimized chemical structure of ionogel using ONIOM calculation approach with Gaussian 09 software.
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electrodes of the spectrometer. The temperature was controlled using Novocontrol Quatrocryosystem with dry nitrogen flow to get temperature stability better than 0.1 K. In addition Current Voltage characteristics of PMMA/BMPyr.TFSI (100/0, 90/10, 80/20, 70/30) have been performed at room temperature using Keithley 2635 B.
Table 1 Harmonic vibrations of ionogel films PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 obtained from ATR-FTIR and their assignments. Wavenumber (cm− 1) 100:0
Wavenumber (cm− 1) 90:10
Wavenumber (cm− 1) 80:20
Wavenumber (cm− 1) 70:30
Assignments
511
511
750 841
512 569 616 749 839
615 749 839
615 748
985
986
985
986
1055
1054
1054
1181 1136
1181 1139
1181 1137
1352
1351
1350
1435 1721 2952
1438 1723 2953
1449 1724 2952
TFSI anion TFSI anion TFSI anion CeH Bending C]C Bending/ Alkenes C]C Bending/ Alkene S]O Stretching C-O Stretching S]O Stretching CeO Stretching/ Alkyl aryl ether Out of phase vibrations of SO2 CeH Bending/ Aldehyde group CeO Stretching Unsaturated ester Chelate hydrogen bonding
2.8. Optical properties The electronic spectral study is a useful tool to measure transmission and reflectance measurements of the ionogel films to deduce information about energy band structure and optical constants of the solids [21]. The transparency and absorbance of the films from different PMMA/BMPyr.TFSI (100/0, 90/10, 80/20, 70/30) were measured by a UV–visible spectrophotometer (Jasco 550 spectrophotometer) at a wavelength ranging from 200 to 800 nm with films dimension approximately 1 × 1 cm2. Photoluminescent (PL) spectra were taken by Perkin Elmer (LS55) spectroscopy using a He - Cd laser with the excitation using two wavenumbers corresponding to the high absorption energies found out from the UV–visible spectroscopy.
1142 1238
1386 1436 1723 2949
3. Results and discussion Transparent ionogel membrane of PMMA/BMPyr.TFSI with varying thickness a) 100:0 (0.232 ± 0.001 mm) b) 90:10 (0.228 ± 0.001 mm) c) 80:20 (0.225 ± 0.001 mm) d) 70:30 (0.220 ± 0.001 mm) were developed using glass casting technique.
Stretching) and 1181 cm− 1 (S]O Stretching) confirms the successful BMPyr.TFSI guest loading in the PMMA matrix. Moreover, the CeO stretched vibration is disappeared in the ionogel compared to pristine PMMA. A more detailed analysis on its frequency location is out of the scope of this paper; however, it is related to the distribution population of BMPyr.TFSI ionic liquid. The addition of IL decreases the intensity of all these peaks and the further presence of some additional peaks at 512–560 cm− 1, 610 cm− 1, 653–655 cm− 1 corresponds to TFSI− anion indicating the presence of IL in the ionogel membrane [23]. The out of phase vibrations of SO2 were seen at 1350–1352 cm− 1 and the SNS vibrations of TFSI− were observed at 749 cm− 1 [23]. The bands were seen at 2580–3200 cm− 1 could be due to chelate hydrogen bonding emerging from entombing and additionally intra hydrogen bonding between PMMA and BMPyr.TFSI. The absence of bands corresponding to C]O group of PMMA helped us to decipher that the ionogel were shaped by the interaction of BMPyr.TFSI and PMMA by variable hydrogen bonding without any additional bond breakage or bond formation [23].
3.1. ATR-FTIR spectroscopy ATR-FTIR spectroscopy was used to get evidence of BMPyr.TFSI doped ionic liquid in the host PMMA polymer matrix. Fig. 2 presents spectra of loaded ionogel films of PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30. The characteristics peaks and their assignments tabulated in Table 1. All spectra in the Fig. 2 exhibits the characteristics peaks at 751 cm− 1 (CeH Bending), 841 cm− 1 (C]C Bending/Alkenes), 985 cm− 1 (C]C Bending/Alkene), 1238 cm− 1 (CeO Stretching/Alkyl aryl ether), 1386 cm− 1 (CeH Bending/Aldehyde group), 1436 cm− 1 (CeO Stretching), 1723 cm− 1 (C]O stretching/α,β-unsaturated ester), and 2951 cm− 1 (CH Stretching/Alkane groups) which corresponds to the presence of isotactic PMMA [22]. The detection of a band centered on 512 cm− 1 (TFSI anion), 569 cm− 1 (TFSI anion), 616 cm− 1 (TFSI anion), 1055 cm− 1 (S]O
3.2. XRD analysis XRD is a useful tool to investigate the change in crystalline structure in terms of degree of crystallization, orientation, crystal size, lattice strain etc. The X-ray diffraction profile of the PMMA/BMPyr.TFSI ionogel films were shown in Fig. 3. The manifestation of the reflections and diffuse scattering from the XRD pattern of pure PMMA film reveals its semi-crystalline nature due to the presence of a broad crystalline peak at 26–33° while dominating the amorphocity at 14°–21°. It is so certain from the Fig. 3 that yet, XRD patterns of the PMMA/ BMPyr.TFSI composite membranes showed the attributes of pristine PMMA, the intensity of the crystalline peak is reduced to some extent. The degree of crystallinity is calculated as the ratio of the integrated intensity under the crystalline bands to the integrated intensity under the complete XRD pattern [12]. From the inset table in Fig. 3, one can state that the semi- crystalline behaviour of the PMMA is lessened after blending it with different wt% of BMPyr.TFSI ionic liquid by dominating amorphocity [12].
Fig. 2. Vibrations of the ionogel films PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 obtained from ATR-FTIR.
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Table 2 The value of glass transition temperature associated with specific heat capacity obtained from DSC curves. PMMA/BMPyr.TFSI wt/wt%
Tg °C
ΔTg%
ΔCp(J/ g °C)
% Change of Δ Cp
100:0 90:10 80:20 70:30
69.8 50.92 44.95 41.11
– − 24.06 − 11.72 − 8.54
0.305 0.26 0.242 0.216
– − 7.80 − 6.92 − 10.74
Fig. 3. X-ray diffractograms of ionogel films PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30. The inset table shows the percentage changes of crystallinity, in which If-Intensity of fundamental band, Is - intensity of secondary band, CrI - Crystallinity Index.
3.3. Thermal studies of ionogel membranes This section deals with the thermal investigation of ionogel membranes in terms of DSC and TGA analysis.
Fig. 5. Thermograms for prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) PMMA b)PMMA/PC c)PMMA/PC :10IL d)PMMA/PC:20IL e)PMMA/PC:30IL f) BMPyr.TFSI using Thermal Gravitometry Analysis.
3.3.1. Differential Scanning Calorimetry The DSC thermograms as shown in Fig. 4 shows only one single glass transition peak (Tg) which emphasizes the miscibility of the ionogel. It is found that Tg decreases with increase in the weight ratio of ionic liquid in the polymer matrix which is in good agreement with the Fox Flory [24] prediction. It is interesting to note that the peaks were broadened on the addition of ionic liquid, which may be ascribed due to the formation of hydrogen bonding. Relatively, small values of Δ Cp at Tg (Table 2) for pristine PMMA and the ionogel membrane confirm that the thermodynamics is more or less same across Tg for the investigated samples. Slight decrease in Δ Cp at Tg on doping of ionic liquid BMPyr.TFSI while retaining the strong nature may be attributed to slight increase of fragility according to Angell's criteria [25,26].
3.3.2. Thermal gravimetry study TGA thermograms of the ionogels were shown in Fig. 5. All ionogel membranes investigated are showing stability up to 450 °C and showing weight loss on further heating. As against the neat PMMA, mass loss ascribed is approximately 9% and it may be due to the degradation of the fraction of the polymer formed by head to head linkages [27] at lower temperatures around 100 °C are suppressed by the addition of a drop of PC, which could be due to the penetration of the PC to the molecular phase of PMMA and stabilizes the domain [28]. Be that as it may, PC didn't make any variation to the temperature of complete degradation of the pure PMMA at ∼ 400 °C [18,19]. Later due to the insensitive nature of IL's [2], as well as the thermal stability of BMPyr.TFSI (∼475 °C) [29] the complete degradation of the pure PMMA by random chain scission mechanism [27] was bypassed in the ionogel membranes. Thus the incorporation of BMPyr.TFSI the degradation temperature was found to be slightly shifted to higher temperatures around ∼ 450 °C as it had reported PMMA with BFO nanoparticles [14] and clay composites [27].
3.4. Morphology The SEM studies showed that the samples are quite amorphous and miscible in nature without any boundaries and particles lead to the conclusion that BMPyr.TFSI was completely assorted to the PMMA matrix [30]. Ionogel membranes have a fairly smooth surface and lack visible boundary between BMPyr.TFSI and PMMA matrix emphasizing perfect miscibility of BMPyr.TFSI in the polymer matrix in all concentrations of our investigation. The fade mesoscopic structures in Fig. 6b may be due to fast external drying of solvents [7]. Fig. 4. Thermograms for prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 using Differential Scanning Calorimetry.
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Fig. 6. SEM images of ionogel films PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 C) 80:20 D) 70:30.
3.5. Electrical properties of ionogel membranes
Table 3 The fit parameter obtained from the model equivalent circuit using Zview software for ionogel membranes.
This section reveals the dielectric behaviour of ionogel composed with PMMA and BMPyr.TFSI. Since ionic liquid consists of positive and negative ions, the resulting ionogel have ionic conductivity rather than electrical conductivity due to the hoping of ions.
3.5.1. Impedance analysis The Nyquist plots unfolding the impedance behaviour of the ionogel at 308 K and shown in Fig. 7. The lack of high-frequency semicircle in the ionogel confirms lower electrolyte resistance resulting quick transportation of ions with alternating electric potential between positive and negative electrodes in the arc fields. This impedance behaviour of the ionogels was fitted using zview software with an equivalent circuit as shown in the inset scheme of Fig. 7. The model is in good agreement with the experimental values. From the fit parameters the pseudocapacitance values were calculated and tabulated in the Table 3, in which CPE is a constant phase element in the equivalent circuit, CPE-T is the pseudocapacitance which is called Q and CPE-P related to the depressed semicircle in the Nyquist plot depicted as ‘n’, then the pseudo capacitance [31] can be calculated in the order of 10− 6 Fcm− 2 from the equation [32].
C=R
PMMA/ BMPyr.TFSI wt/wt%
R1
CPE1-T
CPE1-P
CPE2-T
CPE2-P
100:0 90:10 80:20 70:30
0.68574 0.01141 0.00318 0.00637
0.00137 0.08594 0.06555 0.16845
0.93809 0.92789 1.273 0.91583
0.00130 0.16075 0.25098 0.34732
0.96964 0.95871 0.88824 0.94338
C=R
1−n 1 n Qn
0.00087 0.05016 0.40358 0.08986
3.5.2. Conductivity relaxation The relaxation dynamics and conductivity mechanism of ionogel membranes were investigated using broadband dielectric spectroscopy. All the ionogel membranes show conductivity relaxation which could not be tracked in the dielectric data due to masking of the ionic conductivity as shown in Fig. 8; the conductivity due to the hopping of ions concealed the relaxation due to molecular dipolar contribution. In such situations, modulus formalism can be applied [10–17]. Modulus is nothing but the inverse of dielectric permittivity. M∗ ε∗ = 1, where M∗ the complex dielectric modulus and ε∗ is complex dielectric permittivity [40]. It is evidently found the discussion in the Section 3.3.1 is right by diminishing the semicircle in the cole-cole plot due to the presence of conductivity, the Fig. 8 revealed that the hopping of ions are more in the ionogel membrane with 20% BMPyr.TFSI and increasing the weight percentage of BMPyr.TFSI decreases the degree of freedom of ion. The different relaxation behaviour of all ionogel membrane in modulus formalism [33–39] was shown in Fig. 9, since the conductivity masked the structural relaxation in the dielectric window. Consequently, the conductivity relaxation of all GPE's was fitted with Havelik Nigami Equation
1−n 1 n Qn
From the fit parameters and calculated pseudocapacitance, it is clear that the capacitive behaviour is high for the ionogel with 20% BMPyr.TFSI. By increasing the amount of BMPyr.TFSI may decrease the movement of ions since the cation and anion are bulky in nature which may restrict the hopping of ions due to lack of space between them.
Fig. 7. Nyquist Plot obtained for prepared ionogel membranes.The inset shows an electric equivalent circuit used to model the impedance curve in which CPE1 is the geometrical capacitance of electrolyte, CPE2 is the capacitance of the double layer formed at electrode-electrolyte interference and R is resistance.
Fig. 8. Cole–Cole plot obtained for prepared ionogel membranes.
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Fig. 9. Conductivity relaxation in the modulus formalism obtained for prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30.
′
M ∗ (iω) = M
(iω) + iM" (iω) =
= M∞ ⎡1 − ⎢ ⎣
∫0
∞
3.5.3. Transport properties It is valuable to measure the electrical conductivity of PMMA/ BMPyr.TFSI ionogel membrane with varying wt ratio of ILs close to room temperature in a wide frequency window (10− 1 to 107), since the ionic conductivity contribution can be used to extract the information on the charge transport mechanism for each film in terms of the real component of complex conductivity.
1 ε ∗ (iω)
dφ dtexp (−iωt ) ⎛− ⎞ ⎤ ⎝ dt ⎠ ⎥ ⎦
The observed two relaxations linearly depend on temperature indicating that both are secondary relaxations since the glass transition temperature is very high. It is very difficult and time-consuming to measure alpha relaxation for such a strong polymer matrices. Among the observed secondary relaxations β process arised due step-wise rotation of the side chain around the carboxyl group accompanied by a small angle wagging motion of short parts of the backbone [41] and γ process occurred above β process may be due to localized motions of ethyl units at the end of the alkyl group in the side chains [42] in the ionogel. Among them, β process precisely depends on temperature, while the other is comparatively less dependent on temperature and shows Arrhenius behaviour.
′
σ ∗ (ω) = σ
(ω) + iσ " (ω)
The electrical conductivity of PMMA/BMPyr.TFSI ionogel membrane consists of three distinctive regions; a low frequency dispersion due to electrode electrolyte or space charge polarization effect [43,44], an intermediate plateau almost frequency independent and can be considered as dc conductivity σdc and finally frequency dependent conductivity. But it is observed that PMMA is lack of the intermediate plateau of dc conductivity, it showed only the electrode electrolyte polarization as well as frequency dependent conductivity. This universal dynamic responses of ionic conductivity can be fitted with Jonscher's power law [40–42].
EA ⎞ τβ = τ∞ exp ⎛ ⎝ RT ⎠
σac = σ0 + AωS
The relaxation in the pristine PMMA shows less shift towards higher frequency side on increasing temperature, which emphasizes that the relaxation may be due to dipolar fluctuations produced due to side chains of PMMA which are temperature independent and confirms that the lack of melt of the membrane. But the incorporation of IL increases the temperature dependency of the relaxation and thus the relaxation peaks moves fastly to higher frequency side compared to neat polymer. This may be due to the increased thermal stability and decreased fragility of the film. On addition of IL the molecules side chains of PMMA gets more degree of freedom by decreasing the viscosity of molecule. It is observed that the relaxation get wider temperature dependence on adding ionic liquid indicating.
Fig. 10 shows the temperature-dependent behaviour of dc conductivity for all composition of IL in PMMA/BMPyr.TFSI GPE's, observing that the conductivity increases with increasing temperature for all compositions. As well as it is observed that conductivity increases with increase in weight ratio's of IL till 20%, above that it remains constant, which indicates the reducing degree of freedom due to the bulky nature of ILs. Further, it is noticed that the dc conductivity and temperature relationship between (log σ and 1000/T) obey Arrhenius law.
−Ea ⎞ σdc = σ0 exp ⎛ ⎝ KT ⎠ where σ0 is pre-exponential factor, Ea is the conduction activation 171
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Fig. 11(a). No hysteresis was found in the J-V characteristics for the forward and reverse sweep direction eliminates the possibility of deep traps in the PMMA films [7]. The increase in the leakage current is observed as the concentration of BMPyr.TFSI IL increases. The leakage current for pristine PMMA film is 7.50 × 10− 9 A/cm2 at 4 kV/cm. It increases to 5.05 × 10− 6 A/cm2 for 30% doped ionic liquid. This increase in the leakage current and the increase in the conductivity of the films may be attributed from the hopping of ions in the ionogel membranes. Due to increased conductivity, the polymer chains have sufficient energy to push against hydrostatic pressure and create a space for a molecular motion to occur. The asymmetric modulation of current transport dynamics in pure PMMA indicates a current generation at 0 V applied field. This could be due to the contact charging between the polymer film and the metal surface. However, further investigation is needed to find out the exact mechanism happening at the metal–film interface. At 10% IL doped PMMA films; it shifts to higher field side. For 20% and 30% IL doped PMMA films, symmetric J-E curves are observed. Fig. 11(b) depicts the log (J) vs. log (E) plots at the low electric field, which represents the space charge limited conduction (SCLC) mechanism. Pure PMMA and 10% IL doped PMMA has slope below one. This indicates that samples are insulating without any conduction mechanism in the sample. As the ionic liquid concentration increases to 20% and 30%, samples show slope ~ 0.90. This value is close to one, and we can conclude that samples exhibit non ohmic conduction mechanism due to the presence of IL.
Fig. 10. Temperature dependence of dc conductivity of PMMA/BMPyr.TFSI ionogel membrane PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30.
Table 4 Activation energy, Ea. and Log (σ0) for various weight ratios of prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30. PMMA/BMPyr.TFSI wt/wt%
Log (σ0)
EA(eV)
90:10 80:20 70:30
11.53 12.55 11.37
1.43 1.38 1.30
3.6. Optical properties 3.6.1. Transparency The transmittance of the prepared ionogel membranes was examined using UV–visible spectroscopy and shown in the Fig. 12 Even though, the transparency of PMMA was retained in all the membranes; on the addition of IL decreases the transparency while retaining it in the range of 80–90% [46]. It demonstrates most astounding transparency around 90%. for 80:20 wt% of PMMA/BMPyr.TFSI blend which might be because of the miscibility of BMPyr.TFSI in PMMA. All the films show low transparency in the range between 200 and 350 nm may be due to high energy absorption assigned to the πeπ* bond [46] present in the polymer as shown in Fig. 12.
energy, K is the Boltzmann constant, and T is the temperature in Kelvin are calculated and tabulated in Table 4. It is interesting to note that PMMA membrane has attained dc conductivity σdc of around 10− 8 by incorporation of BMPyr.TFSI IL in the polymer matrix. 3.5.4. I–V Characteristics The leakage current characteristics of ionogel membranes PMMA/ BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 are shown in
Fig. 11. (a) The leakage current characteristics of prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 measured at room temperature. (b) Leakage current density for ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 fitted by space charge limited conduction (SCLC) mechanism.
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Fig. 14. Direct and indirect band gaps of prepared ionogel membranes PMMA/ BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30.
Fig. 12. UV–Vis transmittance spectrum of prepared ionogel membranes PMMA/ BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 showing transparency of the film.
where α is the absorption coefficient, A is optical absorbance of the film and t is the thickness of the film. The disparity of absorption edge of PMMA/BMPyr.TFSI with varying weight ratio of IL content were depicted in the inset of Fig. 14. The slight doping of IL especially 90:10 wt ratio in PMMA/BMPyr.TFSI polymer composites inhibit a shift on the absorption edge towards low energy values owing to transfer of charge. The band gap of the solid film plays an important role in optoelectronic devices since it determines the energy required to transfer an electron from the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) in order to make the film optically active. The smaller the band gap higher will be its charge conduction [21]. The optical band gap can be determined from the photonic absorbance values obtained from the UV–Visible spectral studies as well as some theoretical calculations using Density function theory. The optical band gaps of PMMA/BMPyr.TFSI composite films involving both direct and indirect electronic transitions are depicted in Fig. 14 and corresponding values are tabulated in Table 5. Here it is more optimistic to consider indirect band gap rather than direct due to the absence of valence and conduction band as like seen in metal doped membranes. It is very interesting to note the anomalous behaviour upon IL doping, It is found that 90:10 wt ratio of PMMA/ BMPyr.TFSI film shows reduced band gap of 4.41 eV compared to all other films, which reflects the role of BMPyr.TFSI IL in modifying the electronic structures of PMMA films through IL-induced polaronic and defect effects [21]. The decrease in the optical band gap also reflects the increase in the degree of disorder in the film. The IL-induced structural defects may lead to the creation of localized metastable state leading to the decreased band gap energy for 90:10 wt ratio of PMMA/ BMPyr.TFSI film, as the wt ratio increases the ion hopping in the polymer matrix might be decreased due to spatial freedom.
3.6.2. Absorbance The optical constants like energy band structure, refractive index, optical conductivity etc., of solids are important parameters that determine its aptness to optical devices. The Fig. 13 shows room temperature absorption spectra for the prepared PMMA/BMPyr.TFSI wt/wt % ionogel membranes in the range 200–800 nm. In the UV region, all the samples retain the characteristics of PMMA by exhibiting three absorption bands at the position 223, 288 and 333 nm. The strong band seen at 223 nm for all samples might be assigned to n → π* transition due to the presence of C]O group; meanwhile the other two absorption may be assigned to π → π* transition due to the unsaturated groups [46] (C]O, C]C) in the polymer [45,47]. The absorption coefficient of each ionogel membrane with different weight ratio of IL was calculated using the equation shown below
α=
(2.303)∗A t
3.6.3. Photoluminescence In the emission spectrum, two fixed wavelengths were used to excite Table 5 The absorption coefficient, direct and in direct band gap of prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30.
Fig. 13. The optical absorbance of prepared ionogel membranes PMMA/BMPyr.TFSI wt/ wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 as a function of wavelength. (Inset shows absorption coefficient of prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 as a function of photon energy).
173
PMMA/BMPyr.TFSI wt/wt%
α (cm− 1)
Direct Eg (eV)
Indirect Eg (eV)
100:0 90:10 80:20 70:30
7056 4451 1881 2321
3.92 3.68 4.03 4.03
4.74 4.41 4.84 4.84
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Fig. 15. Fluorescence emission spectrum obtained for prepared ionogel membranes. Prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% I) 100:0 II) 90:10 III) 80:20 IV) 70:30.
comparable to the task specific ionogel formed by doping europium in the PMMA matrix [10].
the molecules in the prepared ionogel membranes since all the samples showed absorptions at two wavelengths in the UV range as discussed in the Section 3.6.2 and showed in the Fig. 13. It is observed that for each excitation frequency, it has two emission spectra, one for fluorescence and other for phosphorescence. Since the adjustment in the energy for fluorescent emission is by and large not as much as that for absorption, the membrane's fluorescence spectrum is moved to higher wavelengths than its absorption range [48] as shown in Fig. 15. The intensity of fluorescence is proportional to the amount of radiation absorbed by the sample, P0 − PT, and the fluorescence quantum yield is given by [48]
4. Conclusion Ions conducting ionic liquid-based gel polymer electrolyte membranes (GPEs) were prepared by entrapping BMPyr.TFSI ionic liquid in PMMA polymer matrix in different wt ratio's. The incorporation of BMPyr.TFSI to the polymer produces more amorphous and polar membranes contrasted with pure PMMA film, as it was deduced from ATR-FTIR and XRD spectroscopes. The ionogel membranes displayed good chemical stability and were thermally stable up to 450 °C, due to the insensitive nature of BMPyr.TFSI. The ac impedance results indicated that the addition of IL increases conductivity till 20%, after that it is found to decrease due to the bulky nature of ions leads to lack of space for ion transportation in the membranes. Further the CurrentVoltage show non ohmic behaviour at lower voltage with an appreciable leakage current compared to the pristine PMMA polymer indicating its conducting response. The optical properties of PMMA/ BMPyr.TFSI ionogel films designate that, with the insertion of BMPyr.TFSI in PMMA matrix, films show high optical transmittance and reduced band gap of 4.41 eV for 90:10 wt ratio compared to all other films, consequently it showed enhanced luminescence intensity.
If = Kφf (P0 − PT ) where K is a constant related to the efficiency of collecting and detecting the fluorescent emission, Φf is the fraction of excited state molecules returning to the ground state by fluorescence. The intensity of the emission increases when 10% IL then decreases by increasing the concentration of IL which is in great concurrence with the power of absorption radiation. The addition of IL may decrease the molecular gap and allow molecules to interact, subsequently resulting in lowering of the energy levels. It is quite possible that emission from the higher excited state may get quenched due to cross relaxation. The observed fluorescence may be due to the molecule's lowest energy absorption assigned to π → π* transition due to the unsaturated groups (C]O, C]C) in the polymer although some due to n → π* transition due to the presence of C]O group show weak fluorescence [47]. It is well evidently seen from the HOMO-LUMO structures obtained from the ONIOM calculation. The geometrical structures obtained for the HOMO-LUMO orbitals are shown in Fig. 16. In short, very strong orange emission was observed in the PMMA/ BMPyr. TFSI ionogel polymer electrolyte membrane which is somewhat
Acknowledgement Authors thankfully acknowledge fruitful discussions with Benjamin Hudson Baby Dhakar Nilesh, Bhabhina.N.M, Nijisha P, Subair N.P, Soufeena P.P, Vijisha K Rajan, Jemshihas A.P. KPSH and MST gratefully acknowledges financial assistance from UGC-DAE under collaborative research scheme (Sanction Order No-UDCSR\MUM\AO/CRS-M-2 I 0/ 174
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Fig. 16. HOMO-LUMO orbitals of ionogel model obtained from the ONIOM calculation using Gaussian 09.
2015/501 dated 06/01/2015). KPSH further acknowledges UGC-MANF for fellowship with sanction number MANF201718KER78598. MST further acknowledges financial assistance from KSCSTE (SRS, SARD), UGC (MRP) and DST FIST.
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Development of ion conducting ionic liquid-based gel polymer electrolyte membrane PMMA/BMPyr.TFSI - With improved electrical, optical, thermal and structural properties
MARK
Safna Hussan K.Pa,⁎, Mohamed Shahin Thayyila, S.K Deshpandeb, Jinitha T.Vc, Jayant Kolted a
Department of Physics, University of Calicut, Malappuram 673635, Kerala, India UGC-DAE Consortium for Scientific Research, Mumbai Centre, BARC, Mumbai, India c Department of Chemistry, University of Calicut, Malappuram 673635, Kerala, India d School of Physics and Materials Science, Thapar University, Patiala, Punjab, India b
A R T I C L E I N F O
A B S T R A C T
Keywords: Ionic liquid-based gel polymer electrolyte SEM DSC Broadband dielectric spectroscopy Optical properties
Ionogel membranes are materials of colossal intrigue worldwide because of their utilization as excellent substitutes of the liquid electrolytes or as separators in ionic gadgets including batteries, fuel cells, supercapacitors, energy units, and so on. In the present work, transparent ionogel composites of PMMA/1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl imide) (100/0, 90/10, 80/20, 70/30) were prepared to be used for optical devices. The impact of varying amount of ionic liquid in the PMMA matrix in weight ratios was compared with the undoped polymer blend. The prepared membrane was characterized for thermal, conductivity, optical and for morphological examinations. In addition, the current voltage characteristics of prepared films have been performed at room temperature. The addition of ionic liquid to the polymer matrix improves the properties by having better thermal stability, enhanced luminescence intensity and making the membrane more amorphous. Among these, PMMA with 20% having a glass transition temperature of 44.95 °C exhibits superior performance with the fairly smooth transparent surface, increased flexibility, enhanced thermal stability, highest ionic conductivity, the maximum capacitance of 0.403 ∗ 10− 6Fcm− 2, non ohmic behaviour and photoluminescence property.
1. Introduction In spite of the realities that the electrodes are the limiting factors in terms of overall capacity, energy density and cyclability, electrolytes plays a crucial role in determining the current density, power density, time stability and safety of the battery in electrochemical devices [1]. Accordingly, as of late solid-state electrolytes are of incredible demand since it overcomes all the restrictions of its liquid nature like leakage, gas formation due to solvent decomposition, volatile and thermal instability [2]. Hence, the researchers centered around the area of the dry solid electrolyte by entrapping different conducting materials in polymer matrix or by polymerization of such materials [2,3]. In these circumstances, ion conducting polymer electrolytes has received worldwide attention due to their intrinsic properties like thin film forming ability, transparency, flexibility, high ionic conductivity and wide electrochemical window [3]. This type of material engineering broadened the array of applications like lithium batteries, solar cells, supercapacitors, fuel cells, sensors, solid state batteries, capacitors, etc.,
⁎
Corresponding author. E-mail address: [email protected] (S.H. K.P).
http://dx.doi.org/10.1016/j.ssi.2017.08.012 Received 29 May 2017; Received in revised form 2 August 2017; Accepted 13 August 2017 0167-2738/ © 2017 Elsevier B.V. All rights reserved.
with high-temperature stability and improved safety. Gel polymer electrolytes consist of a polymeric framework as the host, an organic solvent as the plasticizer, and a conducting electrolytic salt either organic or inorganic. Recent studies have described versatile ionic liquid (IL) electrolytes would be an excellent substitute for liquid electrolytes [4] since it have a wide range of conductivities from 0.1–740 mS/cm [5]. Especially IL with 1-butyl-3-methylpyridinium [BMPy]+ and trihexyl(tetradecyl) phosphonium [P6,6,6,14]+ cations exhibits higher conductivities. Thus immobilization of IL in the polymer host will be a better choice for solid state electrolyte due to their appealing feature of freestanding film and high thermal stability properties [4] and allows us to design easily and cheaply with modularity and reliability in electrochemical devices [9]. Despite the fact that polymers are the best stage to immobilize ILs, only a few polymers and fewer ILs are used till now. But the versatility of both IL and polymer chemistry allows us to develop an infinite number of ionogel [6]. Thus the field of ionogel membranes is yet to explore, since the different combination of IL guest to the polymer host
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added to PMMA solution to reduce its brittleness. This composite solution was gently stirred for 2 h at a temperature of 40 °C and kept in vacuum oven for 1 day at 40 °C.
may result in chaotic outcomes to the optoelectronic world. PMMA is one among the promising representatives of polymeric materials with a wide application in optoelectronic devices, electronic gadgets as dielectric organic thin film transistors. Yet at the same time, it has to satisfy various constraints in regards to amorphous nature, stability against degradation, limits on charge traps, high breakdown potential, band offsets processability and reproducibility [7]. Extensive work has been carried out to tune the properties such as morphology, optical, dielectric and aging behaviour of PMMA films. Yet, the better outcomes were gotten from ionogel with the marvelous compatibility of different organic salts with poly(methyl methacrylate) (PMMA) [4–17]. In the present study, ionic liquid based gel polymer electrolyte membrane PMMA/1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl imide) were prepared with different wt ratio (100/0, 90/10, 80/ 20, 70/30) and detailed investigations were carried out using SEM, XRD, ATR-FTIR and UV–visible spectrophotometer, DSC, TGA, photoluminescence and finally the transport properties were studied using broadband dielectric spectroscopy.
2.4. Characterization The attenuated total reflectance-fourier transform infrared (ATRFTIR) spectra of the ionogel films were recorded by JASCO spectrometer at room temperature. The measurements were carried out in the wave number range from 400 to 4000 cm− 1 The X-ray-diffraction pattern of the prepared films were recorded in the range of 2θ from 20° to 80° at a speed rate of 10° per minute using Rigaku powder diffractometer fitted with a curved crystal monochromator and attenuated total reflectance, which is equipped with Ni-filtered CuKα radiation (λ = 1.5418 Å). 2.5. Scanning electron microscope A scanning electron microscope (SEM, SU6600HI-2102-0003) operated at an accelerating voltage of 5 KV was used to observe the immiscible IL's and grain boundaries at the microscopic level. The specimens were sputter coated with a thin gold layer prior to SEM in order to prevent charging of specimen and to increase the signal to noise ratio.
2. Materials and methods 2.1. Materials 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl)imide, 99% [BMPyr.TFSI], propylene carbonate, 99% [PC], and polymethylmethacrylate, > 98% [PMMA] were purchased from SigmaAldrich, USA. PMMA is the main constituent of the polymer blend acting as host polymer.
2.6. Thermal studies
The geometry of ionogel was optimized using ONIOM/DFT: TDDFT:SM level of theory with 631G ++:631G+: PM6 basis set and shown in Fig. 1.
Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA) were carried out to investigate the thermal stability of the ionogel; which is inevitable in electrochemical applications. The ionogel membrane was exposed to a heating rate of 5 °C/min in the range from −70 °C to 130 °C using Perkin Elmer DSC 8500, provided information about glass transition temperature (Tg), notwithstanding enthalpy for the process [20]. Thermal gravimetric analysis (TGA) using Perkin Elmer (Pyris 1 TGA) from 30 °C to 500 °C at 5 °C/min gave an indication of mass loss and thermal degradation on heating.
2.3. Synthesis of PMMA/BMPyr.TFSI ionogel films
2.7. Broadband dielectric spectroscopy
A solution casting in the glass case was used to prepare the ionogel polymer electrolyte membrane. The solutions of PMMA/BMPyr.TFSI (100/0, 90/10, 80/20, 70/30) were prepared by doping BMPyr.TFSI into the PMMA-chloroform solution. A drop of propylene carbonate is
The dielectric and conductivity measurements were done at ambient pressure in a wide frequency window [0.01 Hz to 10 MHz] for different temperatures using Novocontrol Broadband dielectric spectrometer in which the samples were sandwiched between two gold-plated copper
2.2. Computational calculation
Fig. 1. The optimized chemical structure of ionogel using ONIOM calculation approach with Gaussian 09 software.
167
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electrodes of the spectrometer. The temperature was controlled using Novocontrol Quatrocryosystem with dry nitrogen flow to get temperature stability better than 0.1 K. In addition Current Voltage characteristics of PMMA/BMPyr.TFSI (100/0, 90/10, 80/20, 70/30) have been performed at room temperature using Keithley 2635 B.
Table 1 Harmonic vibrations of ionogel films PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 obtained from ATR-FTIR and their assignments. Wavenumber (cm− 1) 100:0
Wavenumber (cm− 1) 90:10
Wavenumber (cm− 1) 80:20
Wavenumber (cm− 1) 70:30
Assignments
511
511
750 841
512 569 616 749 839
615 749 839
615 748
985
986
985
986
1055
1054
1054
1181 1136
1181 1139
1181 1137
1352
1351
1350
1435 1721 2952
1438 1723 2953
1449 1724 2952
TFSI anion TFSI anion TFSI anion CeH Bending C]C Bending/ Alkenes C]C Bending/ Alkene S]O Stretching C-O Stretching S]O Stretching CeO Stretching/ Alkyl aryl ether Out of phase vibrations of SO2 CeH Bending/ Aldehyde group CeO Stretching Unsaturated ester Chelate hydrogen bonding
2.8. Optical properties The electronic spectral study is a useful tool to measure transmission and reflectance measurements of the ionogel films to deduce information about energy band structure and optical constants of the solids [21]. The transparency and absorbance of the films from different PMMA/BMPyr.TFSI (100/0, 90/10, 80/20, 70/30) were measured by a UV–visible spectrophotometer (Jasco 550 spectrophotometer) at a wavelength ranging from 200 to 800 nm with films dimension approximately 1 × 1 cm2. Photoluminescent (PL) spectra were taken by Perkin Elmer (LS55) spectroscopy using a He - Cd laser with the excitation using two wavenumbers corresponding to the high absorption energies found out from the UV–visible spectroscopy.
1142 1238
1386 1436 1723 2949
3. Results and discussion Transparent ionogel membrane of PMMA/BMPyr.TFSI with varying thickness a) 100:0 (0.232 ± 0.001 mm) b) 90:10 (0.228 ± 0.001 mm) c) 80:20 (0.225 ± 0.001 mm) d) 70:30 (0.220 ± 0.001 mm) were developed using glass casting technique.
Stretching) and 1181 cm− 1 (S]O Stretching) confirms the successful BMPyr.TFSI guest loading in the PMMA matrix. Moreover, the CeO stretched vibration is disappeared in the ionogel compared to pristine PMMA. A more detailed analysis on its frequency location is out of the scope of this paper; however, it is related to the distribution population of BMPyr.TFSI ionic liquid. The addition of IL decreases the intensity of all these peaks and the further presence of some additional peaks at 512–560 cm− 1, 610 cm− 1, 653–655 cm− 1 corresponds to TFSI− anion indicating the presence of IL in the ionogel membrane [23]. The out of phase vibrations of SO2 were seen at 1350–1352 cm− 1 and the SNS vibrations of TFSI− were observed at 749 cm− 1 [23]. The bands were seen at 2580–3200 cm− 1 could be due to chelate hydrogen bonding emerging from entombing and additionally intra hydrogen bonding between PMMA and BMPyr.TFSI. The absence of bands corresponding to C]O group of PMMA helped us to decipher that the ionogel were shaped by the interaction of BMPyr.TFSI and PMMA by variable hydrogen bonding without any additional bond breakage or bond formation [23].
3.1. ATR-FTIR spectroscopy ATR-FTIR spectroscopy was used to get evidence of BMPyr.TFSI doped ionic liquid in the host PMMA polymer matrix. Fig. 2 presents spectra of loaded ionogel films of PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30. The characteristics peaks and their assignments tabulated in Table 1. All spectra in the Fig. 2 exhibits the characteristics peaks at 751 cm− 1 (CeH Bending), 841 cm− 1 (C]C Bending/Alkenes), 985 cm− 1 (C]C Bending/Alkene), 1238 cm− 1 (CeO Stretching/Alkyl aryl ether), 1386 cm− 1 (CeH Bending/Aldehyde group), 1436 cm− 1 (CeO Stretching), 1723 cm− 1 (C]O stretching/α,β-unsaturated ester), and 2951 cm− 1 (CH Stretching/Alkane groups) which corresponds to the presence of isotactic PMMA [22]. The detection of a band centered on 512 cm− 1 (TFSI anion), 569 cm− 1 (TFSI anion), 616 cm− 1 (TFSI anion), 1055 cm− 1 (S]O
3.2. XRD analysis XRD is a useful tool to investigate the change in crystalline structure in terms of degree of crystallization, orientation, crystal size, lattice strain etc. The X-ray diffraction profile of the PMMA/BMPyr.TFSI ionogel films were shown in Fig. 3. The manifestation of the reflections and diffuse scattering from the XRD pattern of pure PMMA film reveals its semi-crystalline nature due to the presence of a broad crystalline peak at 26–33° while dominating the amorphocity at 14°–21°. It is so certain from the Fig. 3 that yet, XRD patterns of the PMMA/ BMPyr.TFSI composite membranes showed the attributes of pristine PMMA, the intensity of the crystalline peak is reduced to some extent. The degree of crystallinity is calculated as the ratio of the integrated intensity under the crystalline bands to the integrated intensity under the complete XRD pattern [12]. From the inset table in Fig. 3, one can state that the semi- crystalline behaviour of the PMMA is lessened after blending it with different wt% of BMPyr.TFSI ionic liquid by dominating amorphocity [12].
Fig. 2. Vibrations of the ionogel films PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 obtained from ATR-FTIR.
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Table 2 The value of glass transition temperature associated with specific heat capacity obtained from DSC curves. PMMA/BMPyr.TFSI wt/wt%
Tg °C
ΔTg%
ΔCp(J/ g °C)
% Change of Δ Cp
100:0 90:10 80:20 70:30
69.8 50.92 44.95 41.11
– − 24.06 − 11.72 − 8.54
0.305 0.26 0.242 0.216
– − 7.80 − 6.92 − 10.74
Fig. 3. X-ray diffractograms of ionogel films PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30. The inset table shows the percentage changes of crystallinity, in which If-Intensity of fundamental band, Is - intensity of secondary band, CrI - Crystallinity Index.
3.3. Thermal studies of ionogel membranes This section deals with the thermal investigation of ionogel membranes in terms of DSC and TGA analysis.
Fig. 5. Thermograms for prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) PMMA b)PMMA/PC c)PMMA/PC :10IL d)PMMA/PC:20IL e)PMMA/PC:30IL f) BMPyr.TFSI using Thermal Gravitometry Analysis.
3.3.1. Differential Scanning Calorimetry The DSC thermograms as shown in Fig. 4 shows only one single glass transition peak (Tg) which emphasizes the miscibility of the ionogel. It is found that Tg decreases with increase in the weight ratio of ionic liquid in the polymer matrix which is in good agreement with the Fox Flory [24] prediction. It is interesting to note that the peaks were broadened on the addition of ionic liquid, which may be ascribed due to the formation of hydrogen bonding. Relatively, small values of Δ Cp at Tg (Table 2) for pristine PMMA and the ionogel membrane confirm that the thermodynamics is more or less same across Tg for the investigated samples. Slight decrease in Δ Cp at Tg on doping of ionic liquid BMPyr.TFSI while retaining the strong nature may be attributed to slight increase of fragility according to Angell's criteria [25,26].
3.3.2. Thermal gravimetry study TGA thermograms of the ionogels were shown in Fig. 5. All ionogel membranes investigated are showing stability up to 450 °C and showing weight loss on further heating. As against the neat PMMA, mass loss ascribed is approximately 9% and it may be due to the degradation of the fraction of the polymer formed by head to head linkages [27] at lower temperatures around 100 °C are suppressed by the addition of a drop of PC, which could be due to the penetration of the PC to the molecular phase of PMMA and stabilizes the domain [28]. Be that as it may, PC didn't make any variation to the temperature of complete degradation of the pure PMMA at ∼ 400 °C [18,19]. Later due to the insensitive nature of IL's [2], as well as the thermal stability of BMPyr.TFSI (∼475 °C) [29] the complete degradation of the pure PMMA by random chain scission mechanism [27] was bypassed in the ionogel membranes. Thus the incorporation of BMPyr.TFSI the degradation temperature was found to be slightly shifted to higher temperatures around ∼ 450 °C as it had reported PMMA with BFO nanoparticles [14] and clay composites [27].
3.4. Morphology The SEM studies showed that the samples are quite amorphous and miscible in nature without any boundaries and particles lead to the conclusion that BMPyr.TFSI was completely assorted to the PMMA matrix [30]. Ionogel membranes have a fairly smooth surface and lack visible boundary between BMPyr.TFSI and PMMA matrix emphasizing perfect miscibility of BMPyr.TFSI in the polymer matrix in all concentrations of our investigation. The fade mesoscopic structures in Fig. 6b may be due to fast external drying of solvents [7]. Fig. 4. Thermograms for prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 using Differential Scanning Calorimetry.
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Fig. 6. SEM images of ionogel films PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 C) 80:20 D) 70:30.
3.5. Electrical properties of ionogel membranes
Table 3 The fit parameter obtained from the model equivalent circuit using Zview software for ionogel membranes.
This section reveals the dielectric behaviour of ionogel composed with PMMA and BMPyr.TFSI. Since ionic liquid consists of positive and negative ions, the resulting ionogel have ionic conductivity rather than electrical conductivity due to the hoping of ions.
3.5.1. Impedance analysis The Nyquist plots unfolding the impedance behaviour of the ionogel at 308 K and shown in Fig. 7. The lack of high-frequency semicircle in the ionogel confirms lower electrolyte resistance resulting quick transportation of ions with alternating electric potential between positive and negative electrodes in the arc fields. This impedance behaviour of the ionogels was fitted using zview software with an equivalent circuit as shown in the inset scheme of Fig. 7. The model is in good agreement with the experimental values. From the fit parameters the pseudocapacitance values were calculated and tabulated in the Table 3, in which CPE is a constant phase element in the equivalent circuit, CPE-T is the pseudocapacitance which is called Q and CPE-P related to the depressed semicircle in the Nyquist plot depicted as ‘n’, then the pseudo capacitance [31] can be calculated in the order of 10− 6 Fcm− 2 from the equation [32].
C=R
PMMA/ BMPyr.TFSI wt/wt%
R1
CPE1-T
CPE1-P
CPE2-T
CPE2-P
100:0 90:10 80:20 70:30
0.68574 0.01141 0.00318 0.00637
0.00137 0.08594 0.06555 0.16845
0.93809 0.92789 1.273 0.91583
0.00130 0.16075 0.25098 0.34732
0.96964 0.95871 0.88824 0.94338
C=R
1−n 1 n Qn
0.00087 0.05016 0.40358 0.08986
3.5.2. Conductivity relaxation The relaxation dynamics and conductivity mechanism of ionogel membranes were investigated using broadband dielectric spectroscopy. All the ionogel membranes show conductivity relaxation which could not be tracked in the dielectric data due to masking of the ionic conductivity as shown in Fig. 8; the conductivity due to the hopping of ions concealed the relaxation due to molecular dipolar contribution. In such situations, modulus formalism can be applied [10–17]. Modulus is nothing but the inverse of dielectric permittivity. M∗ ε∗ = 1, where M∗ the complex dielectric modulus and ε∗ is complex dielectric permittivity [40]. It is evidently found the discussion in the Section 3.3.1 is right by diminishing the semicircle in the cole-cole plot due to the presence of conductivity, the Fig. 8 revealed that the hopping of ions are more in the ionogel membrane with 20% BMPyr.TFSI and increasing the weight percentage of BMPyr.TFSI decreases the degree of freedom of ion. The different relaxation behaviour of all ionogel membrane in modulus formalism [33–39] was shown in Fig. 9, since the conductivity masked the structural relaxation in the dielectric window. Consequently, the conductivity relaxation of all GPE's was fitted with Havelik Nigami Equation
1−n 1 n Qn
From the fit parameters and calculated pseudocapacitance, it is clear that the capacitive behaviour is high for the ionogel with 20% BMPyr.TFSI. By increasing the amount of BMPyr.TFSI may decrease the movement of ions since the cation and anion are bulky in nature which may restrict the hopping of ions due to lack of space between them.
Fig. 7. Nyquist Plot obtained for prepared ionogel membranes.The inset shows an electric equivalent circuit used to model the impedance curve in which CPE1 is the geometrical capacitance of electrolyte, CPE2 is the capacitance of the double layer formed at electrode-electrolyte interference and R is resistance.
Fig. 8. Cole–Cole plot obtained for prepared ionogel membranes.
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Fig. 9. Conductivity relaxation in the modulus formalism obtained for prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30.
′
M ∗ (iω) = M
(iω) + iM" (iω) =
= M∞ ⎡1 − ⎢ ⎣
∫0
∞
3.5.3. Transport properties It is valuable to measure the electrical conductivity of PMMA/ BMPyr.TFSI ionogel membrane with varying wt ratio of ILs close to room temperature in a wide frequency window (10− 1 to 107), since the ionic conductivity contribution can be used to extract the information on the charge transport mechanism for each film in terms of the real component of complex conductivity.
1 ε ∗ (iω)
dφ dtexp (−iωt ) ⎛− ⎞ ⎤ ⎝ dt ⎠ ⎥ ⎦
The observed two relaxations linearly depend on temperature indicating that both are secondary relaxations since the glass transition temperature is very high. It is very difficult and time-consuming to measure alpha relaxation for such a strong polymer matrices. Among the observed secondary relaxations β process arised due step-wise rotation of the side chain around the carboxyl group accompanied by a small angle wagging motion of short parts of the backbone [41] and γ process occurred above β process may be due to localized motions of ethyl units at the end of the alkyl group in the side chains [42] in the ionogel. Among them, β process precisely depends on temperature, while the other is comparatively less dependent on temperature and shows Arrhenius behaviour.
′
σ ∗ (ω) = σ
(ω) + iσ " (ω)
The electrical conductivity of PMMA/BMPyr.TFSI ionogel membrane consists of three distinctive regions; a low frequency dispersion due to electrode electrolyte or space charge polarization effect [43,44], an intermediate plateau almost frequency independent and can be considered as dc conductivity σdc and finally frequency dependent conductivity. But it is observed that PMMA is lack of the intermediate plateau of dc conductivity, it showed only the electrode electrolyte polarization as well as frequency dependent conductivity. This universal dynamic responses of ionic conductivity can be fitted with Jonscher's power law [40–42].
EA ⎞ τβ = τ∞ exp ⎛ ⎝ RT ⎠
σac = σ0 + AωS
The relaxation in the pristine PMMA shows less shift towards higher frequency side on increasing temperature, which emphasizes that the relaxation may be due to dipolar fluctuations produced due to side chains of PMMA which are temperature independent and confirms that the lack of melt of the membrane. But the incorporation of IL increases the temperature dependency of the relaxation and thus the relaxation peaks moves fastly to higher frequency side compared to neat polymer. This may be due to the increased thermal stability and decreased fragility of the film. On addition of IL the molecules side chains of PMMA gets more degree of freedom by decreasing the viscosity of molecule. It is observed that the relaxation get wider temperature dependence on adding ionic liquid indicating.
Fig. 10 shows the temperature-dependent behaviour of dc conductivity for all composition of IL in PMMA/BMPyr.TFSI GPE's, observing that the conductivity increases with increasing temperature for all compositions. As well as it is observed that conductivity increases with increase in weight ratio's of IL till 20%, above that it remains constant, which indicates the reducing degree of freedom due to the bulky nature of ILs. Further, it is noticed that the dc conductivity and temperature relationship between (log σ and 1000/T) obey Arrhenius law.
−Ea ⎞ σdc = σ0 exp ⎛ ⎝ KT ⎠ where σ0 is pre-exponential factor, Ea is the conduction activation 171
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Fig. 11(a). No hysteresis was found in the J-V characteristics for the forward and reverse sweep direction eliminates the possibility of deep traps in the PMMA films [7]. The increase in the leakage current is observed as the concentration of BMPyr.TFSI IL increases. The leakage current for pristine PMMA film is 7.50 × 10− 9 A/cm2 at 4 kV/cm. It increases to 5.05 × 10− 6 A/cm2 for 30% doped ionic liquid. This increase in the leakage current and the increase in the conductivity of the films may be attributed from the hopping of ions in the ionogel membranes. Due to increased conductivity, the polymer chains have sufficient energy to push against hydrostatic pressure and create a space for a molecular motion to occur. The asymmetric modulation of current transport dynamics in pure PMMA indicates a current generation at 0 V applied field. This could be due to the contact charging between the polymer film and the metal surface. However, further investigation is needed to find out the exact mechanism happening at the metal–film interface. At 10% IL doped PMMA films; it shifts to higher field side. For 20% and 30% IL doped PMMA films, symmetric J-E curves are observed. Fig. 11(b) depicts the log (J) vs. log (E) plots at the low electric field, which represents the space charge limited conduction (SCLC) mechanism. Pure PMMA and 10% IL doped PMMA has slope below one. This indicates that samples are insulating without any conduction mechanism in the sample. As the ionic liquid concentration increases to 20% and 30%, samples show slope ~ 0.90. This value is close to one, and we can conclude that samples exhibit non ohmic conduction mechanism due to the presence of IL.
Fig. 10. Temperature dependence of dc conductivity of PMMA/BMPyr.TFSI ionogel membrane PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30.
Table 4 Activation energy, Ea. and Log (σ0) for various weight ratios of prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30. PMMA/BMPyr.TFSI wt/wt%
Log (σ0)
EA(eV)
90:10 80:20 70:30
11.53 12.55 11.37
1.43 1.38 1.30
3.6. Optical properties 3.6.1. Transparency The transmittance of the prepared ionogel membranes was examined using UV–visible spectroscopy and shown in the Fig. 12 Even though, the transparency of PMMA was retained in all the membranes; on the addition of IL decreases the transparency while retaining it in the range of 80–90% [46]. It demonstrates most astounding transparency around 90%. for 80:20 wt% of PMMA/BMPyr.TFSI blend which might be because of the miscibility of BMPyr.TFSI in PMMA. All the films show low transparency in the range between 200 and 350 nm may be due to high energy absorption assigned to the πeπ* bond [46] present in the polymer as shown in Fig. 12.
energy, K is the Boltzmann constant, and T is the temperature in Kelvin are calculated and tabulated in Table 4. It is interesting to note that PMMA membrane has attained dc conductivity σdc of around 10− 8 by incorporation of BMPyr.TFSI IL in the polymer matrix. 3.5.4. I–V Characteristics The leakage current characteristics of ionogel membranes PMMA/ BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 are shown in
Fig. 11. (a) The leakage current characteristics of prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 measured at room temperature. (b) Leakage current density for ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 fitted by space charge limited conduction (SCLC) mechanism.
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Fig. 14. Direct and indirect band gaps of prepared ionogel membranes PMMA/ BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30.
Fig. 12. UV–Vis transmittance spectrum of prepared ionogel membranes PMMA/ BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 showing transparency of the film.
where α is the absorption coefficient, A is optical absorbance of the film and t is the thickness of the film. The disparity of absorption edge of PMMA/BMPyr.TFSI with varying weight ratio of IL content were depicted in the inset of Fig. 14. The slight doping of IL especially 90:10 wt ratio in PMMA/BMPyr.TFSI polymer composites inhibit a shift on the absorption edge towards low energy values owing to transfer of charge. The band gap of the solid film plays an important role in optoelectronic devices since it determines the energy required to transfer an electron from the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) in order to make the film optically active. The smaller the band gap higher will be its charge conduction [21]. The optical band gap can be determined from the photonic absorbance values obtained from the UV–Visible spectral studies as well as some theoretical calculations using Density function theory. The optical band gaps of PMMA/BMPyr.TFSI composite films involving both direct and indirect electronic transitions are depicted in Fig. 14 and corresponding values are tabulated in Table 5. Here it is more optimistic to consider indirect band gap rather than direct due to the absence of valence and conduction band as like seen in metal doped membranes. It is very interesting to note the anomalous behaviour upon IL doping, It is found that 90:10 wt ratio of PMMA/ BMPyr.TFSI film shows reduced band gap of 4.41 eV compared to all other films, which reflects the role of BMPyr.TFSI IL in modifying the electronic structures of PMMA films through IL-induced polaronic and defect effects [21]. The decrease in the optical band gap also reflects the increase in the degree of disorder in the film. The IL-induced structural defects may lead to the creation of localized metastable state leading to the decreased band gap energy for 90:10 wt ratio of PMMA/ BMPyr.TFSI film, as the wt ratio increases the ion hopping in the polymer matrix might be decreased due to spatial freedom.
3.6.2. Absorbance The optical constants like energy band structure, refractive index, optical conductivity etc., of solids are important parameters that determine its aptness to optical devices. The Fig. 13 shows room temperature absorption spectra for the prepared PMMA/BMPyr.TFSI wt/wt % ionogel membranes in the range 200–800 nm. In the UV region, all the samples retain the characteristics of PMMA by exhibiting three absorption bands at the position 223, 288 and 333 nm. The strong band seen at 223 nm for all samples might be assigned to n → π* transition due to the presence of C]O group; meanwhile the other two absorption may be assigned to π → π* transition due to the unsaturated groups [46] (C]O, C]C) in the polymer [45,47]. The absorption coefficient of each ionogel membrane with different weight ratio of IL was calculated using the equation shown below
α=
(2.303)∗A t
3.6.3. Photoluminescence In the emission spectrum, two fixed wavelengths were used to excite Table 5 The absorption coefficient, direct and in direct band gap of prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30.
Fig. 13. The optical absorbance of prepared ionogel membranes PMMA/BMPyr.TFSI wt/ wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 as a function of wavelength. (Inset shows absorption coefficient of prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% a) 100:0 b) 90:10 c) 80:20 d) 70:30 as a function of photon energy).
173
PMMA/BMPyr.TFSI wt/wt%
α (cm− 1)
Direct Eg (eV)
Indirect Eg (eV)
100:0 90:10 80:20 70:30
7056 4451 1881 2321
3.92 3.68 4.03 4.03
4.74 4.41 4.84 4.84
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Fig. 15. Fluorescence emission spectrum obtained for prepared ionogel membranes. Prepared ionogel membranes PMMA/BMPyr.TFSI wt/wt% I) 100:0 II) 90:10 III) 80:20 IV) 70:30.
comparable to the task specific ionogel formed by doping europium in the PMMA matrix [10].
the molecules in the prepared ionogel membranes since all the samples showed absorptions at two wavelengths in the UV range as discussed in the Section 3.6.2 and showed in the Fig. 13. It is observed that for each excitation frequency, it has two emission spectra, one for fluorescence and other for phosphorescence. Since the adjustment in the energy for fluorescent emission is by and large not as much as that for absorption, the membrane's fluorescence spectrum is moved to higher wavelengths than its absorption range [48] as shown in Fig. 15. The intensity of fluorescence is proportional to the amount of radiation absorbed by the sample, P0 − PT, and the fluorescence quantum yield is given by [48]
4. Conclusion Ions conducting ionic liquid-based gel polymer electrolyte membranes (GPEs) were prepared by entrapping BMPyr.TFSI ionic liquid in PMMA polymer matrix in different wt ratio's. The incorporation of BMPyr.TFSI to the polymer produces more amorphous and polar membranes contrasted with pure PMMA film, as it was deduced from ATR-FTIR and XRD spectroscopes. The ionogel membranes displayed good chemical stability and were thermally stable up to 450 °C, due to the insensitive nature of BMPyr.TFSI. The ac impedance results indicated that the addition of IL increases conductivity till 20%, after that it is found to decrease due to the bulky nature of ions leads to lack of space for ion transportation in the membranes. Further the CurrentVoltage show non ohmic behaviour at lower voltage with an appreciable leakage current compared to the pristine PMMA polymer indicating its conducting response. The optical properties of PMMA/ BMPyr.TFSI ionogel films designate that, with the insertion of BMPyr.TFSI in PMMA matrix, films show high optical transmittance and reduced band gap of 4.41 eV for 90:10 wt ratio compared to all other films, consequently it showed enhanced luminescence intensity.
If = Kφf (P0 − PT ) where K is a constant related to the efficiency of collecting and detecting the fluorescent emission, Φf is the fraction of excited state molecules returning to the ground state by fluorescence. The intensity of the emission increases when 10% IL then decreases by increasing the concentration of IL which is in great concurrence with the power of absorption radiation. The addition of IL may decrease the molecular gap and allow molecules to interact, subsequently resulting in lowering of the energy levels. It is quite possible that emission from the higher excited state may get quenched due to cross relaxation. The observed fluorescence may be due to the molecule's lowest energy absorption assigned to π → π* transition due to the unsaturated groups (C]O, C]C) in the polymer although some due to n → π* transition due to the presence of C]O group show weak fluorescence [47]. It is well evidently seen from the HOMO-LUMO structures obtained from the ONIOM calculation. The geometrical structures obtained for the HOMO-LUMO orbitals are shown in Fig. 16. In short, very strong orange emission was observed in the PMMA/ BMPyr. TFSI ionogel polymer electrolyte membrane which is somewhat
Acknowledgement Authors thankfully acknowledge fruitful discussions with Benjamin Hudson Baby Dhakar Nilesh, Bhabhina.N.M, Nijisha P, Subair N.P, Soufeena P.P, Vijisha K Rajan, Jemshihas A.P. KPSH and MST gratefully acknowledges financial assistance from UGC-DAE under collaborative research scheme (Sanction Order No-UDCSR\MUM\AO/CRS-M-2 I 0/ 174
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Fig. 16. HOMO-LUMO orbitals of ionogel model obtained from the ONIOM calculation using Gaussian 09.
2015/501 dated 06/01/2015). KPSH further acknowledges UGC-MANF for fellowship with sanction number MANF201718KER78598. MST further acknowledges financial assistance from KSCSTE (SRS, SARD), UGC (MRP) and DST FIST.
[23]
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