Ionically conducting and photoresponsive liquid crystalline terpolymers: Towards multifunctional polymer electrolytes

TL;DR: In this paper, AMF and NFKA would like to thank the Royal Academy of Engineering for the award of the Newton Research Collaboration Programme grant NRCP1516/4/61.

About: This article is published in European Polymer Journal.The article was published on 2018-12-01 and is currently open access. It has received 22 citations till now.

Summary

1. INTRODUCTION

• The development of electrolytes with enhanced transport properties is paramount to improving the performance of electrochemical devices for energy conversion and storage1.
• Achieving simultaneous high proton selectivity and low solvent contents remains an unsolved challenge.
• Thermotropic liquid crystals, LCs, offer alternative mechanisms to control the morphology of polymeric electrolytes in different applications 14.
• In these and similar materials, the fluidity of the mesophases can further promote local mobility, as occurs in ionic liquids and polymerised ionic liquids 26-29, but with the potential of anisotropic control of the ion transport.

Synthesis and materials.

• The 10-MeOAzB/AMPS/MMA terpolymers, 1, were prepared over a broad range of compositions of the corresponding monomers: (i) the mesogenic side chains, 10-(4- methoxyazobenzene -4’-oxy)decyl , 10-MeOAzB, (ii) polar groups for solvating ions, acrylamido-2-methyl-1-propanesulfonic acids, AMPS, and (iii) methyl groups, MMA, as non-mesogenic/non-ionic structure modifying units.
• AMPS and MMA were commercially available from Sigma-Aldrich and used without further purification.
• 10-MeOAzB was prepared according to a procedure described in detail elsewhere 46-48.
• The 10-MeOAzB/AMPS/MMA terpolymers were prepared using a one-pot conventional free radical polymerisation under an inert atmosphere and appropriate amounts of the different monomers30.
• Further details of the synthetic procedure are provided in the Supplementary Information.

Characterisation techniques.

• The chemical structures of all the terpolymers and their intermediates were verified using 1H NMR, and Fourier transform infrared, FT-IR, spectroscopies.
• 1H-NMR spectra were measured using a Gemini 300 analyser in either CDCl3 or DMSO-d6.
• Concentrations were calculated on the basis of the compositions of the terpolymers assessed using 1H-NMR spectroscopy.
• Samples were heated above their clearing point inside the cell, and then measured in isothermal steps on cooling to room temperature.

3. RESULTS AND DISCUSSION

• Specifically, the authors calculated the composition in terms of the relative integrals of the 7-8 ppm signals, assigned to the phenyl azobenzene protons (4H) of 10-MeOAzB, the ~2.7 ppm singlet, assigned to the methylene groups (2H) adjacent to the sulfonic acid groups in AMPS, and the peak at ~3.6 ppm, associated with the methyl groups (3H) of MMA.
• The FT-IR spectra of the terpolymers contain bands characteristic of the functional groups in the individual components, including the carbonyl stretching bands from the ester groups in 10-MeOAzB and MMA (~1730 cm-1) together with the C=O stretching band (~1670 cm-1) and the N-H stretching and bending signals (~3400 and ~1550 cm-1, respectively) from the amide groups in AMPS31.
• The molecular weights of all the terpolymers were measured by gel permeation chromatography/size exclusion, GPC/SEC, and are also summarised in Table 1.

Thermal stability.

• The thermal stability of the terpolymers was assessed by thermogravimetric analysis, TGA, and the weight loss curves, TG, and corresponding derivative curves, DTG, are shown in Figures 1(a) and 1(b), respectively.
• Thermal degradation of the P10-MeOAzB homopolymer occurs through two main weight loss processes, which the authors attribute to the breakage of the more labile groups in the side chains, between 300ºC and 400ºC, followed by decomposition of the polymer backbone, between 400 ºC and 500ºC.
• In general terms, the terpolymers display a main weight loss around 400ºC, as well as an additional process at about 300ºC, see Figure electrolytes.
• Terpolymers with high AMPS contents (samples #6 to #8) display additional processes below 300ºC, which could have contributions from the release of residual solvent trapped within the polymer structure 6, 30, 49.

Phase behaviour.

• The transitional properties of the terpolymers are listed in Table 2.
• The nematic phase is observed over a very narrow temperature range on cooling from the isotropic melt, and prior to the formation of the smectic A phase 50.
• The terpolymers tend to exhibit higher glass transitions than P10-MeOAzB, presumably reflecting the less flexible poly(methyl methacrylate), PMMA (Tg > 100ºC 51), and PAMPS (Tg~124ºC 30) backbones.
• The data listed in Table 2 indicate that MMA has a stronger destabilising effect on liquid crystalline behaviour and the sample with the highest MMA concentration, A41/S11/M48, exhibits the lowest TLCI.

Phase structure.

• Figure 3 shows the X-ray diffraction patterns of the terpolymers obtained at room temperature after cooling slowly from the isotropic or liquid crystalline phase.
• The nature of the interactions involving the AMPS units may be investigated using FT-IR spectroscopy, and the spectra of the terpolymers measured at room temperature are shown in Figure 5.
• The presence of sulfonate groups is also consistent with the increase in the IR contribution around 1034 cm-1 at higher AMPS concentrations, assigned to the 𝑆𝑂3 − st. vibration 31, see inset in Figure 5.
• This suggests that the AMPS units are squeezed between alternating smectic layers that are further stabilised by micro-phase separation between polar and non-polar domains, see Figure 4(b).

Light responsive behaviour.

• The authors now turn their attention to the photo-responsive nature of the 10-MeOAzB/AMPS/MMA terpolymers and discuss the dependence of the UV-vis spectra of ~10-5 M THF polymer solutions following UV radiation.
• The UV-vis spectra of the P10-MeOAzB homopolymer contains two main absorption regions associated with the E-isomer of the azobenzene unit, Figure 6(a): an intense band centred at ~362 nm, corresponding to the π−π* transition, and a weak absorption at ~450 nm, associated with the symmetric forbidden n - π* transition 61.
• The maximum at 362 nm may be assigned to contributions from non-aggregated E-azobenezene units 62.
• As expected, reducing the azobenzene content weakens the UV signals for the terpolymers, but otherwise their spectra are essentially identical, suggesting that the azobenzene moieties respond similarly to light exposure.
• The π−π* absorbance band in their UV-spectra decreases dramatically in intensity after exposure, whereas a slight increase in the intensity of the 450 nm region is observed, see Figure 6(b).

Temperature-dependent conductivity.

• The authors now discuss the temperature dependence of the ionic conductivities for the 10- MeOAzB/AMPS/MMA terpolymers, measured using electrochemical impedance spectroscopy, EIS, in isothermal steps on cooling the samples from the isotropic melt.
• According to the FT-IR spectra, a number of these sulfonic acid groups may be deprotonated in the 10-MeOAzB/AMPS/MMA terpolymers, suggesting that the conductivity in these samples could be driven by a combination of hydrogen bonding and ionic charges.
• The lowest activation energies were obtained for A09/S85/M06, whose Arrhenius plot displays a combination of two linear ranges, with the intersect coinciding with the glass transition of the terpolymer, 𝑇𝑔~70℃.

4. CONCLUSIONS

• The 10-MeOAzB/AMPS/MMA terpolymers form smectic phases stabilised by microphase separation between polar and non-polar regions, even at considerably low concentrations of 10-MeOAzB mesogenic units.
• European Polymer Journal 2018; 109:124-132 14. Martinez-Felipe A. Liquid crystal polymers and ionomers for membrane applications.

Figures

Table 1. Molecular weights, polydispersities, Mw/Mn, and number average degrees of polymerization, DP, for the 10-MeOAzB/AMPS/MMA terpolymers, 1. Also listed are the feed compositions used in the copolymerisations. X, Y and Z are the molar percentages of 10-MeOAzB, AMPS and MMA, respectively, assessed experimentally.

Figure 8. Arrhenius plots of the dc conductivity measured for the 10- MeOAzB/AMPS/MMA terpolymers. Dotted arrows indicate the glass transition temperatures and solid arrows, clearing temperatures, TLCI, taken from Table 2. Dotted lines represent the linear ranges for A09/S85/M06.

Figure 2. DSC traces corresponding to the second heating scans of the 10- MeOAzB/AMPS/MMA terpolymers. Curves have been shifted arbitrarily along the Y-axis.

Table 2. Transition temperatures and associated enthalpy, HLCI, and entropy, SLCI/R, changes for P10-MeOAzB and the 10-MeOAzB/AMPS/MMA terpolymers.

Table 3. Activation energies for the conductivity process in the 10- MeOAzB/AMPS/MMA terpolymers, Ea(dc), calculated in the isotropic and smectic ranges.

Figure 5. FT-IR spectra of the terpolymers, highlighting the contributions from the AMPS units. Inset shows the 𝑆𝑂3 stretching band, which overlaps the stretching vibration band of the ether linkage between the azobenzene groups and alkyl chains in 10-MeOAzB, arC-O-alC st. The Y-axis displays IR absorbance (a.u.) with the spectra shifted arbitrarily along this axis.

Figure 4. Proposed structural models for the smectic phases shown by (a) P10-MeOAzB and (b) the 10-MeOAzB/AMPS/MMA terpolymers containing equal amounts of the three components, showing coexisting smectic and acid domains. Dotted arrows indicate potential ionically conducting pathways.

Figure 1. (a) Thermogravimetric curves, TG, and (b) derivative thermogravimetric curves, DTG, corresponding to the P10-MeOAzB homopolymer and the 10- MeOAzB/AMPS/MMA terpolymers. Curves have been shifted arbitrarily along the Yaxes.

Figure 7. Double logarithmic plots of the real component, ’, of the complex conductivity of A29/S36/M35, *=’+ i ’’, as a function of the frequency, measured in isothermal steps (ºC) on cooling, and estimation of dc at T=150ºC. Dotted arrow in the inset shows the spike in the impedance Nyquist plot, indicative of dc conductivity.

Figure 6. (a) UV-vis spectra of selected terpolymers, including P10-MeOAzB for reference (dotted curve); (b) time-dependent UV-vis spectra of A29/S36/M35, showing its thermal trans-to-cis isomerisation after light exposure at 365 nm. The dotted curve in (b) corresponds to the original spectrum prior to exposure, and arrows indicate the cis-to-trans thermal relaxation with time, t.

Figure 3. XRD diffraction patterns obtained at room temperature on cooling from the liquid crystal/isotropic phases of the 10-MeOAzB/AMPS/MMA terpolymers. (a) Effect of increasing MMA content in samples #1 to #4; (b) increasing AMPS content in samples #5 to #8. Arrows indicate decreasing mesogenic content. Y-axes correspond to the diffractogram intensity, a.u., and curves have been shifted arbitrarily along this axis.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Ionically conducting and photoresponsive liquid crystalline terpolymers: towards multifunctional electrolytes" ?

The authors have prepared a series of new ionically conducting electrolytes consisting of side-chain liquid crystal terpolymers with mesogenic azobenzenes, sulfonic acid groups and methyl methacrylates.