Poly(N-isopropylacrylamide) (PNIPAM) is a temperature-responsive polymer with broad applications. Upon temperature increase, hydrogen bonds between amide groups in PNIPAM and water break; hydrophobic interactions occurring within PNIPAM backbone chains prevail and cause its transition from the expanded coil to globule. Inspired by this, in this work, we achieve temperature-induced assembly of reduced graphene oxide (rGO) flakes in water through surface modification with polydopamine (PDA). Dopamine can spontaneously reduce GO to rGO and polymerizes into PDA clusters that assemble on the rGO flakes. The rGO/PDA supramolecular system mimics the amphiphilic structure of PMIPAM: rGO/PDA flakes are stable in water due to hydrogen bonding between water and PDA; meanwhile, rGO provides sp<sup>2</sup> domains as the hydrophobic backbone. Upon heating to as low as 30<sup> o</sup>C, the hydrogen bonds break, and rGO flakes start to separate from the aqueous phase and stack together due to hydrophobic and π-π interactions. Results from transmission electron microscopy and UV-vis show that we achieved temperature-induced self-assembly in this rGO-based supramolecular system. The self-assembled flakes can be re-dispersed through sonication and re-assemble again upon heating. The films made with rGO/PDA that assembled responding to heat showed significantly higher electrical conductivity than films made with original, un-heated rGO/PDA. Moreover, gelation can happen due to the formation of networks between rGO flakes under heating. Overall, this work introduces a new strategy to design temperature-responsive graphene-based materials that avoid complex grafting and multiple steps of polymerization. While previous temperature-responsive GO systems always included temperature-responsive polymers bound to the GO flakes that acted as inert substrates, here, the GO flakes are a crucial component in the responsiveness of the supramolecular system.

Poly(N-isopropylacrylamide) inspired temperature-induced assembly of reduced graphene oxide through surface chemistry design

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