Characterization of cellulose nanofibrils-pnipaam composite hydrogels at different carboxyl contents

As part of the research on fabricating functional materials based on cellulose, this study characterized composite hydrogels prepared by combining cellulose nanofibrils (CNF) from 2,2,6,6-tetramethylpiperidinyl-1-oxyl(TEMPO)- mediated oxidation with poly(N-isopropylacrylamide) (PNIPAAm) via in situ polymerization above the lower critical solution temperature. We manipulated the surface carboxylate content level of CNFs by adjusting the amount of sodium hypochloride (NaClO) used as oxidant for the reaction to understand the effects of the carboxylate content on the characteristics of the CNF/PNIPAAm composite hydrogels. As expected, the CNF surface charge increased with an increase in the oxidant amount, which resulted in greater transparency owing to a better CNF dispersion in the polymer suspension. The surface charge level of CNF also influenced the compression strength of CNF/PNIPAAm hydrogels, which was supported by the rheological behavior of CNF. Namely, a lower surface charge level of CNF produced tight interconnections between the fibrils due to an entanglement of the remaining un-converted hydroxyl groups via hydrogen bonds within fibrillated CNFs. Meanwhile, at a higher surface charge level, the finer dispersed CNF at higher electrostatic repulsion slightly decreased fibril to fibril interactions. Fourier transform infrared (FTIR) spectra and differential scanning calorimetry (DSC) results indicated that the CNFs were physically bonded onto PNIPAAm in the hydrogels. These results showed that the surface charge level of the CNF had a great impact on the characteristics of the CNF/PNIPAAm hydrogels, indicating that the selection of a proper level of CNF surface charge and polymerization temperature would be important for developing temperature-responsive hydrogels.