College of Chemistry and Molecular Sciences, Wuhan University , Wuhan 430072, China.
State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, China.
J Phys Chem B. 2017 Mar 2;121(8):1793-1801. doi: 10.1021/acs.jpcb.6b10829. Epub 2017 Feb 16.
To develop a facile approach for the dissolution of cellulose, a novel solvent (9.3 wt % NaOH/7.4 wt % thiourea aqueous solution) was used, for the first time, to dissolve cellulose within 5 min at 8 °C. The results of NMR and Raman spectra demonstrated that stable thiourea···OH complexes were formed through strong hydrogen bonds in NaOH/thiourea at room temperature. Moreover, the strength of the hydrogen bonds in thiourea···OH complexes was much higher than that in urea···OH complexes, and the number of thiourea···OH complexes increased significantly in 9.3 wt % NaOH/7.4 wt % thiourea compared to that in 9.5 wt % NaOH/4.5 wt % thiourea, which dissolved cellulose at -5 °C, leading to the dissolution of cellulose at a relatively high temperature (8 °C). The cellulose that dissolved at such a high temperature was metastable. The results of dynamic light scattering and transmission electron microscope experiments confirmed that the extended cellulose chains and their aggregates coexisted in the dilute cellulose solution. Interestingly, stiff cellulose chains could be self-assembled in parallel to form nanofibers in the metastable cellulose solution, from which cellulose microspheres consisting of nanofibers could be easily produced by inducing heating. This work not only proposed a novel method for the dissolution of cellulose in aqueous system at temperatures over 0 °C but also opened up a new pathway for the construction of nanofibrous cellulose materials.
为了开发一种简便的纤维素溶解方法，首次使用了一种新型溶剂（9.3wt%NaOH/7.4wt%硫脲水溶液），在 8°C 下仅需 5 分钟即可溶解纤维素。NMR 和拉曼光谱的结果表明，室温下在 NaOH/硫脲中形成了稳定的硫脲···OH 配合物，通过强氢键相互作用。此外，硫脲···OH 配合物中的氢键强度远高于脲···OH 配合物中的氢键强度，并且在 9.3wt%NaOH/7.4wt%硫脲中硫脲···OH 配合物的数量比在-5°C 下溶解纤维素的 9.5wt%NaOH/4.5wt%硫脲中显著增加，这导致纤维素在相对较高的温度（8°C）下溶解。在如此高的温度下溶解的纤维素是亚稳的。动态光散射和透射电子显微镜实验的结果证实，在稀纤维素溶液中存在伸展的纤维素链及其聚集体共存。有趣的是，刚性的纤维素链可以在亚稳纤维素溶液中自组装成平行排列的纳米纤维，通过诱导加热，可以很容易地从其中制备由纳米纤维组成的纤维素微球。这项工作不仅提出了一种在 0°C 以上的水溶液中溶解纤维素的新方法，而且为构建纳米纤维纤维素材料开辟了新途径。
