(165r) Preparation and Evaluation of Cellulose Nanofiber Reinforced Thermoresponsive Hydrogel

In recent years, various researchers have reported the development of thermoresponsive hydrogels using poly N-isopropylacrylamide (PNIPAM), which is one of the thermo-responsive polymers, with the aim of application such as drug delivery and heavy metal ion adsorption. However, one of the problems of such a thermoresponsive gel is low mechanical strength. To form an interpenetrating polymer network (IPN) structure, in which two or more kinds of polymers penetrate each other to form a network structure, is an effective approach to improve the strength of hydrogels.

Therefore, in this study, we prepared an IPN gel composed of PNIPAM and cellulose nanofiber (CNF), and evaluated the effects of CNF addition and preparation condition on the mechanical strength and the thermoresponsive properties of the resulting CNF reinforced thermoresponsive hydrogel.

In the experiment, for the mixing of NIPAM, CNF and polymerization initiator, two temperatures of of 0 ^oC and 22 ^oC were tried. As the CNF concentration of the initial CNF dispersion, 1.0 wt% and 3.0 wt% were used. In terms of crosslinking, two types of the semi-crosslinking and the full-crosslinking were tested. In the semi-crosslinking, while the PNIPAM component was crosslinked using N, N’-methylenebisiacrylamide (MBA), the CNF component was not crosslinked in the composite gel. On the other hand, in the full-crosslinking condition, after the crosslinking of the PNIPAM component was completed, the CNF component was also crosslinked by immersing the composite gel in the hydrochloric acid of pH 1.5. The resulting composite gels are referred to as ["mixing temperature" "cross-linked state" "CNF concentration"]. For example, a semi-IPN gel prepared under the condition of the CNF concentration of 3.0 wt% at 22 °C is referred to as 22_semi_3.

Strain-stress curve testing was carried out using a uniaxial tensile tester (STB-1225S, A&D Co., Ltd., Tokyo, Japan). The results show that the breaking stress of the CNF reinforced thermoresponsive gels, e.g., 22_full_3 and 0_semi_3 is approximately 150 kPa, which is nearly three times larger than that of the pure PNIPAM gel. It is also observed that the gels prepared by mixing at 0 °C tend to show higher tensile value than the gels mixed at 22 °C. Finally, repeated shrinking/swelling experiments through solution temperature change between 25 and 40 °C shows that the CNF reinforced thermoresponsive gel successfully exhibits improved durability for the repeated shrinking/swelling operations.