Zeebe_et_al._2008_Science[1]

Zeebe_et_al._2008_Science[1] - CREDIT: ADAPTED FROM TIMOTHY...

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Such ion gels have shown superior performance as gate dielectrics in organic thin-film transistors ( 5 ). For many organic semiconductors, device perfor- mance is constrained by the num- ber of charge carriers rather than their mobility; the high capaci- tance of the ion gel boosts the carrier density in the semicon- ductor channel. Further, the high ionic mobility enables switching speeds that are orders of magni- tude higher than with conven- tional polymer electrolytes ( 6 ). Similar ion gels could form the basis of electromechanical actuators ( 7 , 8 ); differential ion migration in response to an applied electric field leads to differential gel swelling and thus to bending. A possible route to accentu- ate this effect would be to polymerize the cations into the B blocks, thus immobilizing some or all of that charge ( 9 , 10 ); the much more mobile anions could then generate a highly asymmetric swelling. The same molecular architecture also holds promise for gas separation. Ionic liquids strongly prefer to dissolve CO 2 and SO 2 over, for example, N 2 and CH 4 . Because transport through an ionic liquid is so facile, it is possi- ble to achieve combinations of selectivity and throughput comparable to those of the best materials currently available. However, a functional gas separation membrane must withstand a substantial pressure drop. The ionic liquid could be literally blown out of the simple ion gel. A polymerized ion gel should not suffer from this drawback, because the attraction between ions would far outweigh the external pressure. Direct polymerization of organic cations has recently been achieved ( 9 , 10 ). By incorporating an appropriate difunctional monomer, Bara et al. have pre- pared and evaluated cross-linked films for the separation of CO 2 from CH 4 or N 2 ( 9 ), with promising results. Applications to other technologies such as fuel cell membranes and lithium battery sepa- rators often require much greater mechanical rigidity and high-temperature stability while retaining high ionic mobility along a given axis. Here, the ability of block polymers to self-assemble into well-defined nanostruc- tures with long-range order holds the key ( 11 ). For example, macroscopic orientation of block polymer cylinders has been achieved by various strategies, including application of flow fields and electric fields and by prepara- tion of suitably treated underlying substrates (see the second figure, top panel) . However, it remains difficult to achieve macroscopic orientation and perfection of the resulting membrane, which is impor- tant for some applications. Alternatively, a network stru- cture such as the double gyroid is isotropic, obviating the need for orientation (see the second figure, bottom panel). Unfor- tunately, this structure can only be achieved under limited combinations of copolymer compositions, molar masses, and processing conditions. Use of multiblock polymers, such as ABC terpolymers, allows network materi- als to be prepared over much wider ranges of
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This note was uploaded on 09/23/2011 for the course CHEM 380 taught by Professor Staff during the Spring '11 term at S.F. State.

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Zeebe_et_al._2008_Science[1] - CREDIT: ADAPTED FROM TIMOTHY...

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