Thermoreversible ion gels with tunable melting temperatures from triblock and pentablock copolymers

We demonstrate a way to tune the critical gelation temperature (T-gel) for thermoreversible ion gels. Thermoreversible ion gels were prepared previously through the self-assembly of poly(N-isopropyl acrylamide-b-ethylene oxide-b-N-isopropyl acrylamide) (PNIPAm-PEO-PNIPAm) triblock copolymers in a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). The resulting T-gel was 17 degrees C, as judged by the intersection of the dynamic moduli (G' and G) at a fixed low frequency on increasing the temperature. These gels are of interest in various applications, including gate dielectric materials in organic field effect transistors. However, it is desirable to bring the T-gel significantly above room temperature in order to realize device applications. By incorporating solvatophobic blocks of polystyrene (PS) into the PNIPAm-PEO-PNIPAm triblock copolymers to produce well-defined PNIPAm-PS-PEO-PS-PNIPAm pentablock copolymers, we are able to adjust the T-gel of the resulting ion gels over a significant range (17-48 degrees C). We propose a melting mechanism for the PNIPAm-PS-PEO-PS-PNIPAm/[EMIM][TFSI] ion gels which involves two separate processes: (1) a thermodynamically controlled dissociation of the PNIPAm aggregates and (2) a dynamic exchange of PS blocks in and out of the PS micellar cores. The exchange dynamics of PS blocks in transient PS-PEO-PS/[EMIM][TFSI] ion gels were also studied, which are orders of magnitude slower than expected from intrinsic dynamics of bulk PS. This dynamic difference can be explained in the framework of hindered diffusion.