Ion-Conducting\nThermoresponsive Films Based on Polymer-Grafted\nCellulose Nanocrystals

Abstract

Mechanically\nrobust, thermoresponsive, ion-conducting nanocomposite\nfilms are prepared from poly­(2-phenylethyl methacrylate)-grafted cellulose\nnanocrystals (<i><b>MxG</b></i><b>-CNC-</b><i><b>g</b></i><b>-PPMA</b>). One-component nanocomposite\nfilms of the polymer-grafted nanoparticle (PGN) <i><b>MxG</b></i><b>-CNC-</b><i><b>g</b></i><b>-PPMA</b> are imbibed with 30 wt % imidazolium-based ionic liquid to produce\nflexible ion-conducting films. These films with 1-hexyl-3-methylimidazolium\nbis­(trifluoromethylsulfonyl)­imide (<i><b>MxG</b></i><b>-CNC-<i>g</i>-PPMA/[H]</b>) not only display remarkable\nimprovements in toughness (>25 times) and tensile strength (>70\ntimes)\nrelative to the corresponding films consisting of the ionic liquid\nimbibed in the two-component CNC/PPMA nanocomposite but also show\nhigher ionic conductivity than the corresponding neat PPMA with the\nsame weight percent of ionic liquid. Notably, the one-component film\ncontaining 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide\n(<b><i>MxG</i>-CNC-<i>g</i>-PPMA/[E])</b> exhibits temperature-responsive ionic conduction. The ionic conductivity\ndecreases at around 60 °C as a consequence of the lower critical\nsolution temperature phase transition of the grafted polymer in the\nionic liquid, which leads to phase separation. Moreover, holding the <i><b>MxG</b></i><b>-CNC-</b><i><b>g</b></i><b>-PPMA/[E]</b> film at room temperature for 24 h\nreturns the film to its original homogenous state. These materials\nexhibit properties relevant to thermal cutoff safety devices (e.g.,\nthermal fuse) where a reduction in conductivity above a critical temperature\nis needed.