Experimental research on the solubility of cellulose in different ionic liquids

Solubilities of cellulose in several ionic liquids were investigated by using Thermal Platform Polarizing Microscope (TPPM) and mechanical stirring. The results showed that all the ionic liquids used could dissolve cellulose except 1-ethyl-3-methylimidazolium bromide ([EMIM]Br). The dissolution process of cellulose experienced five steps including infiltration, limited swelled, partially dissolved, dissociation and completely dissolved. Ionic liquids with the cellulose-dissolving temperatures from low to high in sequence was 1-ethyl-3-methylimidazolium acetate ([EMIM]Ac) < 1-butyl-3-methylimidazolium acetate ([BMIM]Ac) < 1-allyl-3-methylimidazolium chloride ([AMIM]Cl) < 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) < 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) < 1-benzyl-3-methylimidazolium chloride ([BZMIM]Cl), while the solubility reduced in turn and the melting points or viscosities of the ionic liquids increased in the same order. Addition of some N,N-dimethyl sulfoxide to the solvents promoted ionic liquids to dissolve cellulose quickly. When dissolution process was applied by mechanical stirring, cellulose partially swelled and looked like "Lamb skewers" before dissolved. The dissolution mechanism was discussed.