The water absorption behavior of the cellulose reinforced chitosan films was fol­lowed by gravimetrically exposing the films to the moisture at relative humidity (RH) from 55 to 100% or by immersing in deionized water and sometimes the mechanical properties dependence up on moisture absorption. The water uptake at equilibrium of nanocomposite films in water was reduced from 71 to 40% linearly with CNC loading up to 30 wt.%.40 A similar trend in the reduction in the water up­take was also observed with even 4 times shorter nanocrystals and the longer NFCs are used to prepare nanocomposites when the films were incubated in water and at RH 75%, respectively.36 The reduction in water uptake can be attributed to the in­crease in highly crystalline cellulose which lesser hydrophilic than chitosan matrix and a strong interfacial interaction between them. Due to the reduced water uptake, the moisture-dependent mechanical properties chitosan-cellulose nanocomposite films were less affected in comparison of neat chitosan. However, the water uptake can be still reduced by covalently linking the cellulose and chitosan as represented in Fig. 16.4 that compares the unmodified CNC and surface modified CNCs.37

Contrarily, the cellulose-chitosan nanocomposites systems can be tuned to ab­sorb higher amount of water for biological and sanitary applications, when required. The in-situ grafting of acrylic acid onto chitosan in presence of CNCs (10% w/w) has led to a super absorbent hydrogel materials. The superabsorbent materials ex­hibit an improved the swelling capacity in ca.100 units, from 381 to 486 with faster equilibrium. In this case, the averaged dimension of pores was increased by the in­corporation of CNCs to increase the porous capacity.46 These hydrogels can be used as functional pH/ionic materials.