Effect of surface charge content in the TEMPO-oxidized cellulose nanofibers on morphologies and properties of poly(N-isopropylacrylamide)-based composite hydrogels

•TEMPO-oxidized cellulose nanofibers (TO-CNF) were isolated from bamboo pulp.•Poly(N-Isopropylacrylamide)/TO-CNF composite hydrogels were synthesized.•The composite hydrogels exhibited improved swelling and compression properties.•The compression strength and modulus were proportional to the carboxylate contents.

Owing to the higher aspect ratio and presence of negatively-charged carboxylate groups that would favorably interact with hydrophilic amide groups in poly(N-isopropylacrylamide) (PNIPAM) molecules, TEMPO-oxidized cellulose nanofibers (TO-CNF) is a kind of promising candidates to improve both swelling and mechanical properties of PNIPAM hydrogels. In this work, a series of TO-CNFs were isolated from inexpensive and abundant bamboo pulp to clarify effects of carboxylate contents (0.5–1.8 mmol/g) on the surfaces of TO-CNF on microstructure as well as compression and swelling properties of PNIPAM/TO-CNF composite hydrogels. An increase in the carboxylate content on the surfaces of TO-CNFs resulted in a higher degree of nanofibrillation. With increasing carboxylate content in the added TO-CNFs, the pore size of the composite hydrogels progressively became larger. It was found that the lower critical solution temperature (LCST) of PNIPAM hydrogels was independent of presence of TO-CNF and its carboxylate contents, while the equilibrium swelling ratio of PNIPAM/TO-CNF composite hydrogels below LCST were largely influenced by carboxylate contents of added TO-CNFs. The incorporation of rigid TO-CNF markedly strengthened the three-dimensional network structure of PNIPAM hydrogels, and compressive properties of the composite hydrogels were found to continuously increase with the carboxylate content in the TO-CNF. When carboxylate content of TO-CNF was 1.8 mmol/g, both compressive strength and modulus of the composite hydrogel reached 79.60 kPa and 0.97 MPa, respectively, which were 300% and 900% higher than those of pure PNIPAM one. Thus, TO-CNF appeared to be a “green” nanofiller that can simultaneously improve swelling and mechanical properties of PNIPAM hydrogels.