Preparation and morphology of different types of cellulose spherulites from concentrated cellulose ionic liquid solutions

Abstract

Spherulites of microcrystalline cellulose (MCC) have been prepared by using the vapor precipitation procedure with proper humidity at various temperatures from concentrated microcrystalline cellulose/1-allyl-3-methylimidazolium chloride (MCC/AMIMCl) solutions, for which AMIMCl is an ionic liquid, a good solvent for dissolving MCC. Four different types of MCC spherulites have been investigated by using polarizing optical microscopy (POM), scanning electron microscopy (SEM) and wide-angle X-ray diffraction (WAXD) technique. POM observations reveal four types of MCC spherulites, i.e., negative and positive banded spherulites, and negative and positive non-banded spherulites, depending on MCC concentration and crystallization temperature (T_c). For banded spherulites, both the band spacing and sizes of spherulites evidently increase with increasing T_c for each MCC/AMIMCl solution. The sizes of spherulites increase with increasing MCC concentration at a given T_c. The findings imply that MCC concentration plays a key role in MCC chain reorganizations into positive or negative spherulites, while the crystallization temperature mainly affects the MCC crystalline lamellar twisting for the formation of banded spherulites. SEM observation reveals that the formation of negative and positive banded spherulites is due to different lamellar twisting directions and the formation of non-banded spherulites is due to the formed radiating fibrillar textures. WAXD profiles confirm that all the four types of MCC spherulites formed are in the crystalline form of cellulose II family, exhibiting more intense and sharper diffraction peaks than those of cellulose II family obtained by the dissolution/precipitation process.