Freezing-Induced Crystalline Transition in Chitosan Microgels and thereby Self-Association between Particles

Abstract

Crystalline aggregates are widely present in various organisms. Regulation of nanostructures is of vital importance to the revelation of life secrets as well as material fabrication. As a representative, chitosan displays polymorphic structures. In tendon chitosan, four polymer chains are packed into an orthorhombic cell, where the antiparallel neighboring chains are linked by two sets of N2···O6 hydrogen bonds, and water molecules are connected with N2, O3, and O5 as well as O6 atoms via hydrogen bonds to stabilize the hydrated structure. The transition from hydrated to anhydrous allomorphs, however, usually involved high-temperature annealing or multistep concentrated acid hydrolysis, which weakened the sustainability of chitosan. To address these issues, we proposed a more convenient strategy to achieve the polymorphic transitions under mild conditions. By subjecting chitosan microgels (CSM) to normal freezing treatment, the hydrated aggregates could be transferred into the anhydrous form. The X-ray diffraction (XRD) results clearly demonstrated that both the (020) and (200) diffraction peaks centered at 2θ = 10 and 19.8° gradually disappeared, while the (110) peak at 2θ = 14.8° indicated the occurrence of anhydrous crystals. Upon freezing, the water molecules that were inserted into the hydrated crystals were removed. Meanwhile, under the effects of N2···O6 intermolecular hydrogen bonding and hydrophobic association, the adjacent polymer chains reorganized into new microcrystalline domains. Moreover, the CSM particles exhibited a strong tendency for self-association during this process. By introducing CSM into the oil/water mixture system to form a Pickering emulsion, the resultant macroporous materials could be used as the oil absorbent through subsequent lyophilization. This work provides a different way to facilely regulate the microcrystalline structure of chitosan in a controllable manner, facilitating the construction of more functional materials.