叔丁醇冷冻干燥法制备纳米纤维素气凝胶

Fabrication of nano-cellulose aerogels by freeze drying using tert-butyl alcohol as displacement solvent

以纳米纤维素为原料,采用"CaCl_2溶液促进物理凝胶法"制备水凝胶,选用叔丁醇溶液为置换溶剂并采用"多步法"完成溶剂置换,最后通过冷冻干燥法制备纳米纤维素气凝胶。通过扫描电子显微镜(SEM)、全自动比表面积与孔隙度分析仪和热重分析仪(TG)对所制备的纳米纤维素气凝胶进行微观形貌、比表面积、孔径分布及热稳定性进行表征分析。结果表明:叔丁醇冷冻干燥法制备的纳米纤维素气凝胶是具有层状的以中孔和大孔为主的多孔材料,其比表面积可达174.3 m2/g,收缩率仅为7.86%,平均孔径约为18.4 nm。随着纤维素质量分数的增加,纳米纤维素气凝胶的吸附量和比表面积增大,孔隙度增加,收缩率逐渐减小;纳米纤维素气凝胶具有与微晶纤维素和纳米纤维素相似的热稳定特性。CaCl_2溶液通过改变原始溶胶体系的电荷分布而使粒子更易相互靠近聚集形成凝胶,落入其中的纳米纤维素颗粒会保持其落入瞬间的完整状态。

Highly porous nano-cellulose aerogels were prepared through a spontaneous physical gelation route by using calcium chloride solution in combination with tert-butyl alcohol solvent displacement and freeze drying technology. According to colloidal stability theory, the existence of calcium chloride could change the original charge distribution of the sol system to improve the gel formation process of the nano particles. Replacing water with tert-butyl alcohol could reduce capillary force, thus reducing the damage to the gel network structure during the drying process as well as lowering the shrinkage rate of aerogel to maintain its three dimensional network structure. The microstructure, specific surface area, pore size distribution, and thermal degradation of resulting aerogel were characterized with scanning electron microscope, micrometrics analyzer and thermo gravietric analyzer, respectively. The prepared nano-cellulose aerogels were white solid and the shrinkage of nano-cellulose aerogels were neglectable. Nano-cellulose aerogel of 3.5% mass ratio exhibited the highest specific surface area up to 174.3 m^2/g, and an average pore size of 18.4 nm and weak shrinkage of 7.86%. The isotherm adsorption-desorption curves of nano-cellulose aerogels were of Type II（IUPAC classification）. The distribution of pore size indicated the meso and macro pores, which was consistent with the results from nitrogen physisorption isotherms. The pore volume, specific surface and the porosity of nano-cellulose aerogels increased while the shrinkage decreased with the increasing cellulose mass ratio. Nano-cellulose aerogel showed a similar thermal degradation behavior as MCC（microcrystalline cellulose）and NCC（nanocrystalline cellulose）, while the onset degradation temperature of aerogel was lower than those of MCC and NCC.