Letter pubs.acs.org/macroletters
Ultrastretchable Iono-Elastomers with Mechanoelectrical Response Carlos R. López-Barrón,*,† Ru Chen,‡ and Norman J. Wagner‡ †
ExxonMobil Chemical Company, Baytown Technology and Engineering Complex, Baytown, Texas 77520, United States Center for Neutron Science, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States
‡
S Supporting Information *
ABSTRACT: The emerging technologies involving wearable electronics require new materials with high stretchability, resistance to high loads, and high conductivities. We report a facile synthetic strategy based on selfassembly of concentrated solutions of end-functionalized PEO106−PPO70− PEO106 triblock copolymer in ethylammonium nitrate into face-centered cubic micellar crystals, followed by micelle corona cross-linking to generate elastomeric ion gels (iono-elastomers). These materials exhibit an unprecedented combination of high stretchability, high ionic conductivity, and mechanoelectrical response. The latter consists of a remarkable and counterintuitive increase in ion conductivity with strain during uniaxial extension, which is reversible upon load release. Based on in situ SAXS measurements of reversible crystal structure transformations during deformation, we postulate that the origin of the conductivity increase is a reversible formation of ion nanochannels due to a novel microstructural rearrangement specific to this material.
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bucky gels.11,20,31 This letter reports a novel strategy for facile synthesis of transparent, conductive soft elastomers via selfassembly of an amphiphilic block copolymer in IL followed by micellar cross-linking. Scheme 1 shows the two-step synthesis route of the conductive elastomers, consisting of self-assembly of Pluronic F127 diacrylate (F127-DA) in partially deuterated ethylammonium nitrate (dEAN), followed by UV cross-linking (see Supporting Information, SI). As recently reported,32 the amphiphilic block copolymer F127-DA forms spherical micelles in dEAN with the solvophobic PPO center block segregated in the micelle core and the solvophilic PEO end blocks forming the corona. At the composition (24 wt %) and temperature (40 °C) studied here, the micelles self-assemble into a face-centered cubic (FCC) lattice (Scheme 1). Upon photocuring, the lattice structure is unaltered, as indicated by the nearly identical smallangle neutron scattering (SANS) profiles of the solution before and after cross-linking (inset in Scheme 1). As described elsewhere,32 the solid lines in the inset of Scheme 1 are best fits to a FCC lattice model with paracrystalline distortion, for which the fitting parameters are the micelle radius R and the nearest neighbor distance D.33,34 The fitting yields R = 4.8 nm and D = 28 nm for the solution before cross-linking and R = 4.6 nm and D = 28 nm after cross-linking.32 Even though the structure remains unchanged after crosslinking, the mechanical response of the F127-DA/dEAN solution is greatly modified. The un-cross-linked solution behaves as a “hard gel”, with very long relaxation time and a
tructural materials with high conductivity and high stretchability are becoming an important area of research due to emerging technologies involving stretchable electronics.1 Applications of these materials include stretchable batteries,2,3 wearable sensors,4−9 and integrated circuits.10−12 Fabrication of these materials requires the use of hybrid technology that mixes metals, polymers, and conductive materials, to tie the system together electronically. Some of the most commonly explored strategies include composite materials with “wavy”1,10,12,13 or fractal14 designs, micro- or nanostructured metal embedded in elastomeric matrices,15 biphasic solid−liquid metal thin films embedded in elastomeric substrates,16,17 composite films of carbon nanotubes or graphene and metals,18,19 and polymeric materials doped with dispersions of carbon nanotubes in ionic liquids (bucky gels) which are subsequently coated with elastomeric substrates.11,20 Clearly, the strategies to manufacture these materials involve sophisticated and complex integration of elastomeric substrates with micro- or nanostructured organic or inorganic electronic materials via multistep and often costly processes. It follows that a simplified manufacturing process of stretchable conductor materials would be of great value to scale-up these new stretchable electronic technologies. Alternatively, ionic liquids (ILs) have been used as conductive media in stretchable conductors. Flexible conductive gels (ion gels) can be produced by in situ polymerization of vinyl monomers in ILs which results in flexible conductive gels21−29 or by self-assembly of end-functionalized triblock copolymers followed by cross-linking.30 Using the latter strategy, Gu and co-workers reported solid ion gels with strain-to-break values of ∼350%. A conductive elastomer with limited strain-to-break values (2 wt %,52 we can assume that the non-negligible amount of F127-DA chains that remain in solution can form covalently bonded bridges between micelles during cross-linking. A number of these “bridged” intermicellar cross-links will connect micelles that are relatively far apart, i.e., not in the [111] plane. Tensile stress applied to this morphology will result in elongation of the bridges (which are initially coiled) and rearrangement of the FCC lattice grains into a lower energy configuration, namely, HCP layers. Therefore, slip occurs between [111] FCC planes that are reinforced by the “adjacent” intermicellar cross-links. This slip leads to minimal distortion of the intermicellar hexagonal arrangement, as evidenced by SAXS (Figure 3). The formation of HCP layers perpendicular to the 1−2 plane of deformation produces ion channels between layers, as illustrated in Figure 4. This configuration results in reduction in tortuosity to ion transport in the stretching direction (1) as compared to the initial configuration of randomly oriented FCC grains, which is a plausible mechanism for the measured decrease in electrical resistance upon stretching (Figure 2). Furthermore, during stretching, mechanical energy is stored in the elongated bridges, which remain connected to the same micelles that were originally in a different configuration. Therefore, these bridges act as a microstructure-memory device; namely, the bridges return to their initial random coil conformation when the stress is released, such that the micelles are pulled back into their original positions. This explains the reversibility of the FCC → HCP transition observed by SAXS (Figure 3). This also explains the increase in electric resistance upon unloading as a consequence of the increase in tortuosity when the randomly oriented FCC grain morphology is recovered. Clearly, not all the cross-link points survive the stretching, as reflected by the permanent set measured (Figure 1). Further research is warranted to fully understand the mechanism of layering and micelle orientation. For instance, it is not clear why the layer stacking direction is perpendicular to the 1−2 plane of deformation. In summary, we prepare ultrastretchable soft iono-elastomers by sequential self-assembly and chemical cross-linking of a micellar ion gel. These materials provide a combination of high
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00790. Experimental procedure (PDF) Video showing stretching of iono-elastomer in Linkam (AVI) Video showing stretching of iono-elastomer in SER (AVI) Demonstration of iono-elastomer conductivity (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Carlos R. López-Barrón: 0000-0002-9620-0298 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We wish to thank Byeongdu Lee for his assistance during the SAXS measurements. The Advanced Photon Source (Argonne National Laboratory) is acknowledged for the beam time allocated on the beamline 12-ID-B. N.J.W. and R.C. acknowledge support of cooperative agreements #70NANB12H239 and 70NANB15H260 from NIST, U.S. Department of Commerce. R.C. acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant No. 1247394. 1337
DOI: 10.1021/acsmacrolett.6b00790 ACS Macro Lett. 2016, 5, 1332−1338
Letter
ACS Macro Letters
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DOI: 10.1021/acsmacrolett.6b00790 ACS Macro Lett. 2016, 5, 1332−1338
