Controllable self-assembly of cellulose nanospheres through phosphoric acid triggered dissolution-regeneration and degradation

Phosphoric acid has been utilized as a favorable alternative to strong acids for the production of cellulose nanospheres (CNS) in recent years, partly owing to the reduced reliance on mechanical assistance. In the present study, phosphoric acid hydrolysis was applied to synthesize CNS from natural cotton pulp. Compared to reported long-time hydrolysis over 12 h, reduced time of 4 h is achieved for CNS production. Particle size from 530 nm to 1.3 μm was further controlled by changing the hydrolysis time in 4–11 h. Powdered sample was obtained after freeze-drying. CNS prepared in this work exhibits a cellulose II structure. Crystallinity index of the samples locates in 70–75 % which is higher than the reported 43–60 % for the acid-hydrolyzed medical cotton. Moreover, compared to the sulphuric-acid hydrolyzed CNS with higher crystallinity, thermal stability of the CNS generated from phosphoric-acid hydrolysis is significantly greater. A cooperative dissolution-regeneration and degradation is proposed to induce CNS self-assembly. Initial cellulose microfibrils are completely dissolved as exposed to phosphoric acid. Partial chains aggregate as a result while the remaining chains assemble onto the aggregates in a layer-by-layer manner. Acid degradation to cellulose with time affects length of the molecular chains. CNS size is controlled accordingly.