Chapter 15 - Lignin in nanocomposite hydrogels

Abstract

Hydrogels, which essentially are natural or synthetic polymeric networks, have been known for more than a century. Hydrogels are highly absorbent and possess high degree of flexibility, due to high (>90%) water content. The nanocomposite hydrogels are modern-day or advanced version of conventional hydrogels, which are nanomaterial-filled, hydrated, polymeric networks that exhibit higher elasticity and strength relative to traditionally made hydrogels. These three-dimensional polymeric networks are hydrophilic and developed in association with the nanoparticles embedded in the polymer matrix. The presence of nanoparticles not only enhances the mechanical properties but also acts as filler and reinforcing agent to the system. Mostly inorganic such as clay, metal, and metal oxide are utilized in the development of the nanocomposite hydrogels. However, most of them are toxic and have unfavorable effects on the environment. Numerous investigations suggested that the use of lignin nanoparticles derived from plants can be helpful in the development of more sustainable and eco-friendly hydrogels. The lignin nanoparticles have a high surface area, can improve the stability in aqueous dispersion, and can be easily produced in distinct shapes and sizes. The fabrication of hydrogels with synthetic polymers, such as poly(vinyl alcohol) and polyacrylamide in association with lignin nanoparticles (as a cross-linker), gathered the attention of various researchers. Contrary to the synthetic polymers, recently numerous biopolymers such as cellulose, starch, alginate, chitosan, xanthan gum as hydrogels matrix along with lignin nanoparticles. These hydrogels exhibit higher thermal and mechanical stability, in addition to higher degree of swelling properties.