木质素-丙烯腈共聚物纤维的制备及其热解成碳机理研究

Development of lignin-based carbon fibers is an important research subject for achieving the high-value utilization of lignin and the sustainable development of carbon fibers. However, because of the existence of a large number of hydroxyl groups, there is a strong hydrogen bonding force among the lignin molecules, which results in great difficulty in the fiber spinning and its lack of the necessary plasticity and toughness. Thus, it is hardly to prepare carbon fibers, having relatively high strength, from the pure lignin fiber. In this subject, as a new design, polyacrylonitrile chain segments, which have good spinnability and thermal stability, are grafted to the hydroxyl groups of lignin through esterification reaction and free radical copolymerization reaction. The laws of the esterification reaction and free radical copolymerization reaction of lignin will be studied in depth, and the lignin-acrylonitrile copolymer fiber with a higher content of lignin and a uniform and dense structure will be prepared by means of our existing control technique for the wet-spun coagulation structure. Then, the microstructure and chemical composition of the intermediate products of the fibers during pyrolysis under different temperature, atmosphere and tension fields will be investigated, and the behaviors of heat release, weight loss and small molecules decomposition of the fibers will be also studied by several thermal analysis techniques. The mechanism of pyrolysis and carbonization of the lignin-acrylonitrile copolymer fiber will be revealed for exploring the methods for controlling the microstructure of the carbon fibers. This subject is expected to prepare the lignin-based carbon fibers with satisfactory mechanical properties, and lay a foundation of theory and technology for the achievement of high efficient utilization of lignin in the carbon fiber field.

开发木质素基碳纤维是实现木质素高值化利用和碳纤维可持续发展的重要研究课题。但是，由于木质素分子含有大量羟基，分子间存在很强的氢键作用力，导致其纤维纺丝成型困难并且缺乏必要的塑性和韧性，因而无法用来制得具有较高强度的碳纤维。为此，本项目拟采用新的研究思路，利用酯化反应将木质素的羟基转变为不饱和酯基，再利用自由基共聚反应接枝具有良好可纺性和热稳定性的聚丙烯腈分子链段，通过研究木质素酯化反应和自由基共聚反应规律，并结合本团队已有的湿法纺丝凝固结构控制技术，实现具有较高木质素含量和均匀致密结构的木质素-丙烯腈共聚物纤维的可控制备。通过研究其在不同温度、气氛和张力场下中间热解产物的微观结构和化学组成，并模拟热解过程的放热、失重和小分子逸出行为，揭示其热解成碳机理，探索碳纤维微观结构的调控方法，以期制得具有良好力学性能的木质素基碳纤维，从而为实现木质素在碳纤维领域的高效利用奠定理论和技术基础。

木质素作为一种具有较高碳元素含量和较高碳化收率的可再生资源，被认为是制备碳纤维的理想原料。但是，木质素是一种三维网状聚合物，分子量较低且分布很宽。因此，很难制得连续的木质素纤维。文献报道可以通过与聚丙烯腈共混或者共聚来改善木质素的可纺性。但在共混体系中，木质素易流失在溶剂中，导致纤维内部形成孔洞缺陷；而在共聚改性中，木质素的酚羟基会阻碍自由基共聚反应的进行，导致木质素的实际利用率很低。本项目提出了一种新的研究思路，首先利用酯化反应将木质素的酚羟基转变为不饱和酯基，然后再与丙烯腈进行自由基共聚反应。第一步酯化反应消耗了木质素的部分酚羟基，减弱了其对自由基共聚反应的阻碍作用，同时还引入了不饱和碳碳双键，为第二步自由基共聚反应提供了活性位置。所制得的木质素-丙烯腈共聚物具有良好的可纺性。采用该共聚物和湿法纺丝工艺制得了连续纤维。经热稳定化和碳化处理后，制得了结构致密的碳纤维。本项目深入研究了木质素的酯化反应和自由基共聚反应规律，实现了具有较高木质素含量的木质素-丙烯腈共聚物的可控制备。另外，在研究中还发现木质素会阻碍氰基自由基环化反应。为此，本项目制备了木质素-丙烯腈-衣康酸三元共聚物。衣康酸共聚单体的引入改变了氰基环化反应机理，从而克服了木质素的阻碍作用，促进了热稳定化的进行，最终制得了具有良好力学性能的碳纤维。本项目解决了木质素可纺性差和热稳定化困难的两大关键问题，为实现木质素在碳纤维制备中的高效利用奠定了重要的理论和技术基础。本项目在ACS Sustainable Chemistry & Engineering，Polymer Degradation and Stability，Journal of Materials Science，高分子材料科学与工程，高分子通报等学术期刊上发表论文8篇。发表会议论文1篇、会议墙报2篇。申请发明专利2项，其中1项已获得授权。撰写专著章节1章。培养毕业硕士生3名。