Control of the aqueous solubility of cellulose by hydroxyl group substitution and its effect on processing

Abstract

Native cellulose is insoluble in water, despite the high number of hydrogen bonding sites per chain, as molecules preferably hydrogen bond to each other, preventing its use in industrial applications. The modification of cellulose has received considerable recent attention, motivated by the move away from conventional petroleum-based, water-soluble polymers, however, a systematic analysis of the effects of modification is rare. Herein a detailed study of hydroxypropyl (HP)- and (2-hydroxypropyl) trimethylammonium chloride-modified cellulose, with degrees of substitution (DS) determined by NMR, establishes modification-property relationships. TEM, small-angle X-ray scattering and rheology demonstrated that increasing DS gradually changes the aqueous solubility, resulting in the formation of different morphologies, including micron-sized aggregates, needle-like cellulose nanoparticles (CNPs) and solvated molecules. It was found that aqueous dispersions with DSHP of 50%, assigned to a ‘sweet spot’ in cellulose modification, are suitable for fiber formation. It is shown that this state of the material can be easily detected by rheo-optical methods based on birefringence. Using structural analysis, molecular dynamics simulation and fiber-spinning results, it is proposed that co-existing CNPs and cellulose molecules, interacting via H-bonding, form a network which orients under shear, acting as a precursor for fiber formation from aqueous solutions.