Thermoresponsive Poly(<i>N</i>‑isopropylacrylamide)\nGrafted from Cellulose Nanofibers <i>via</i> Silver-Promoted\nDecarboxylative Radical Polymerization

Abstract

A family\nof thermoresponsive poly­(<i>N</i>-isopropylacrylamide)\n[PNIPAM]-grafted cellulose nanofibers (CNFs) was synthesized <i>via</i> a novel silver-promoted decarboxylative polymerization\napproach. This method relies on the oxidative decarboxylation of carboxylic\nacid groups to initiate free radicals on the surface of CNFs. The\npolymerization reaction employs relatively mild reaction conditions\nand can be performed in a one-step, one-pot fashion. This rapid reaction\nforms a CC bond between CNF and PNIPAM, along with the formation\nof free polymer in solution. The degree of functionalization (DF)\nand the amount of PNIPAM grafted can be controlled by the Ag concentration\nin the reaction. Similar to native bulk PNIPAM, PNIPAM-grafted CNFs\n(PNIPAM-g-CNFs) show remarkable thermoresponsive properties, albeit\nexhibiting a slight hysteresis between the heating and cooling stages.\nGrafting PNIPAM from CNFs changes its cloud point from about 32 to\n36 °C, influenced by the hydrophilic nature of CNFs. Unlike physical\nblending, covalently tethering PNIPAM transforms the originally inert\nCNFs into thermosensitive biomaterials. The Ag concentration used\ndoes not significantly change the cloud point of PNIPAM-g-CNFs, while\nthe cloud point slightly decreases with fiber concentration. Rheological\nstudies demonstrated the sol–gel transition of PNIPAM-g-CNFs\nand revealed that the storage modulus (<i>G</i>′)\nabove cloud point increases with the amount of PNIPAM grafted. The\nnovel chemistry developed paves the way for the polymerization of\nany vinyl monomer from the surface of CNFs and carbohydrates. This\nstudy validates a novel approach to graft PNIPAM from CNFs for the\nsynthesis of new thermoresponsive and transparent hydrogels for a\nwide range of applications.