Modulating nanocellulose hydrogels and cryogels strength by crosslinking and blending

The mechanical strength of hydrogels and cryogels made from carboxylated nanocellulose (NC) fibers can be modulated by chemically/physically crosslinking and blending with different fibrils (shorter/longer). Nanocellulose hydrogels are produced by oxidizing Bleached Eucalyptus Kraft (BEK) pulp followed by high pressure mechanical treatment. Polyethyleneimine (PEI) and hexamethylenediamine (HMDA) were selected to crosslink nanocellulose hydrogels physically and chemically, respectively. Shorter cellulose nanocrystals (CNC) and longer microcrystalline cellulose (MCC) fibrils were blended with the TEMPO oxidized nano/micro fibers to produce hydrogels of controlled properties. Nanocellulose cryogels were prepared from these hydrogels by a two steps process of freezing and lyophilization. The mechanical properties of nanocellulose hydrogels and cryogels were modulated by controlling the type and density of crosslinking as well as by blending with nano- or microfibrils. Chemical crosslinking (HMDA) increases the hydrogel elastic compression moduli (G′) but does not significantly affect the compressive strength of the cryogel. SAXS reveals the HMDA crosslinked hydrogel to be structurally homogeneous. Physical crosslinking with high molecular weight PEI increases the storage modulus (G′) of nanocellulose hydrogels. Blending a carboxylated nanocellulose fiber suspension with CNC significantly increases the cryogel compressive strength. Nanocellulose gel exhibits tuneable mechanical strength and absorption capacity from the crosslinking/blending strategy (type and density); this improved fundamental knowledge of the fiber-crosslinker and fiber-cellulose crystal interactions enables greater control and tunability of the properties of hydro- and cryogels for personal and infant care products, as agricultural water retention aids, and for biosensor applications.