Organic solvent-free and high-productive fabrication of lignin nanospheres via soft templating and their mechanical enhancement on PVA hydrogels

Lignin nanoparticles, emerged as novel porous and functional materials, have attracted great attentions during the past decades. However, use of organic solvents and tedious solvent exchange processes restricted their scaleup production due to the concerns on environmental protection and sustainability. Here, we presented an organic solvent-free strategy to fabricate spherical lignin nanoparticles via soft templating and sol-gel-hydrothermal process in alkaline water media for the first time, in which technical soda lignin was precursor and both PEO-PPO-PEO triblock copolymers (P123) and sodium oleate (SO) acted as soft templates. Assisted by the P123/SO templates, lignin nanospheres (LHCS) were obtained with an average diameter of 190 nm, good dispersity and higher yield (63.29%, wt%). For valorization of prepared samples, lignin nanospheres were coated with tannin acid (TA@LHCS) and introduced into polyvinyl alcohol (PVA) hydrogels for enhancement their mechanical properties. The mechanical strength of the hydrogel was improved by the addition of TA@LHCS, endowing hydrogel with superior stretchability and compressibility. The maximum tensile strength of TA@LHCS enhanced PVA hydrogel reached 303.1 kPa, which was improved by three times. Therefore, this work provided a promising strategy for fabricating lignin nanoparticles via a facile method and broadened their valorizations in enhancing hydrogel strength.