低醚化羟乙基纤维素的氢键网络结构演变及其在NaOH溶剂的溶解机制

Cellulose, as the most abundant renewable resource in the world, is hard to melt or dissolve in common solvents on account of the large amounts of network connecting hydrogen bonds. Hydroxyethyl cellulose (HEC) with low molar substitution which exhibits excellent solubility in NaOH aqueous solution could be obtained by etherification reaction of highly polymerized cellulose, providing a new method to prepare cellulose-based fibers. However, the unclear dissolution mechanism of HEC in NaOH solvent hinders the impovements of properties of HEC solutions and mechanical properties of HEC fibers. Here, we intend to focus on the structural evolution of hydrogen bonds, disclosing the dissolution mechanism of HEC in NaOH solvent by combined experiments and simulation methods. Firstly, the chemical structure and supramolecular structure of HEC will be deeply and systematically studied, in order to understand the effect of substitution groups on hydrogen bonding interactions. Secondly, the possible interactions between HEC molecules and solvent will be investigated, and the influence of factors including HEC structure and dissolving conditions on the dissociation and rebuilding process of hydrogen bonds during dissolution would be revealed. Next, the dissolution process of HEC would be simulated, the interactions between HEC molecules and solvent will be analyzed at the molecular level, and the dissolving model of HEC would be established. Finally, the relationship between dissolution mechanism of HEC in NaOH solvent and the dissolving process together with the solution properties will be discussed, which is beneficial to predict and regulate the dissolution of HEC as well as the solution properties. This work could lay the foundation for the efficient dissolution and high performance processing of cellulose derivatives, and also provide important references for the study on dissolution mechanism of cellulose.

纤维素是自然界储量最丰富的天然高分子材料，但复杂的氢键网络结构使其难以溶解或进行熔融加工。高聚合度纤维素经改性后得到在NaOH溶剂中溶解良好的低醚化羟乙基纤维素（HEC），为制备纤维素基纤维提供了新途径，但尚未清晰的溶解机制为优化溶液性能进而提升纤维力学性能制造了障碍。针对此，本项目拟采用实验和模拟的方法从氢键结构演变入手探讨HEC在NaOH溶剂中的溶解机理：深入解析HEC结构，明确改性后HEC氢键结构的弱化机制；研究溶解中HEC与NaOH溶剂的相互作用及氢键结构的解离与重建过程，揭示影响氢键演变的重要因素及其作用规律；在分子尺度下模拟HEC的溶解过程并研究HEC与溶剂的氢键作用，建立溶解模型。在阐明溶解机理基础上，弄清其与HEC溶解及溶液性能的关系，实现对溶解工艺及溶液性能的预测和调控。本项目的实施将为改性纤维素的高效溶解及高性能化纺丝加工奠定基础，并为研究纤维素溶解机理提供重要参考。

纤维素储量丰富，具有良好的生物相容性和生物可降解性，但复杂的氢键网络和致密的结晶结构使其难以溶解或进行熔融加工。开发纤维素绿色溶剂或加工方法，是实现纤维素的清洁加工及其高值化转化的有效途径。将高聚合度纤维素进行低程度醚化改性，可改善纤维素在NaOH溶剂中的溶解能力，但尚不清晰的溶解机制，为再生制品力学性能的提升制造了难题。本项目选取具有工业化潜力的低醚化羟乙基纤维素（HEC）为主要研究对象，以NaOH为水溶液为溶剂，通过实验和模拟的方法相结合，研究了HEC在NaOH溶剂中的溶解行为，分析了取代基团种类及数目、NaOH浓度关键因素对聚合物溶解和溶液性能的影响规律，阐明了其溶解机理。所取得的主要研究成果包括：1）采用液相法制备了结构相对均匀、溶解性更佳的HEC和羟丙基纤维素（HPC），发现随着侧链长度和取代基团体积的增大，氢键作用减弱，结晶度逐渐降低；2）随取代程度增加，HEC或HPC溶解性能和溶液稳定性改善，但膜制品力学性能下降；相同取代程度下，HEC比HPC溶解所需要的温度更低，溶液中高分子聚集更明显，溶液更易出现凝胶，但所得膜制品力学性能、亲水性和保水性相对较优；NaOH浓度范围在8%-12%之间时溶液稳定性好，膜制品力学性能优良；3）醚化改性过程可降低纤维素内氢键作用，低温下8 wt% NaOH溶液内NaOH水化物尺寸合适、结构稳定，直接与HEC大分子的羟基产生氢键作用，进一步破坏HEC分子间氢键，并以缔合物的结构包覆在HEC大分子外侧，羟基氧（O3、O6）、取代基氧对Na+的相互作用最强，羟基氧（O2、O3、O6）对OH－的相互作用最强，醚键与Na+和OH-相互作用最弱。本项目所开展的研究可以进一步推动纤维素衍生化溶解和纺丝工艺的发展，为制备高性能纤维素基纤维奠定基础；同时，也可为寻找纤维素更强溶剂及研究纤维素在碱性溶剂中的溶解机理提供重要参考。