Mechanically Strong and Thermally Responsive Cellulose Nanofibers/Poly(N-isopropylacrylamide) Composite Aerogels

Abstract

Cellulose nanofibers (CNFs)/poly(N-isopropylacrylamide) (PNIPAm) composite aerogels were successfully fabricated from a CNF aqueous suspension containing PNIPAm solute via lyophilization. PNIPAm was synthesized through free radical polymerization, and CNFs were individualized from filter paper cellulose fibers using 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) mediated oxidation pretreatment followed by mechanical nanofibrillation. It was discovered that the incorporation of CNFs could remarkably improve the structural integrity of composite aerogels by preventing them from shrinking during lyophilization. The structure and properties of the obtained aerogels were comprehensively analyzed with various techniques, including infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption analysis, compression testing, and water contact angle (CA) measurements. Due to the synergistic effects from the two components, the composite aerogels exhibited attractive thermoresponsive properties with exceptionally high compressive strength. The ultimate compressive strength for the composite aerogel (A4) at 70% strain reached 0.227 MPa, more than 12 times higher than that of the neat CNF aerogel. The CA measurements demonstrated that the hydrophilicity/hydrophobicity of CNF/PNIPAm composite aerogels could be switched at a certain temperature. The CA sharply increased from 0 to 97° when the temperature was increased from 20 to 35 °C, exhibiting strong temperature-dependent water absorption behaviors.