PAN纤维共聚单体作用机理的光电离质谱研究

本工作以丙烯腈/衣康酸（IA）二元共聚原丝以及丙烯腈/IA/丙烯酸甲酯（MA）三元共聚原丝为研究对象，利用热重分析仪（TG）和热解-同步辐射真空紫外光电离质谱（Py-SVUV-PIMS）对其热稳定化过程进行研究。TG结果表明，氮气气氛下，二元共聚原丝（PAN/IA）和三元共聚原丝（PAN/IA/MA）分别呈现三阶段和两阶段的热分解过程，其中PAN/IA的第一和第二阶段均对应PAN线型分子链的断裂，但前者是由自由基环化反应放热引发的，后者则是由正常温度下的热分解所致，而单体MA的加入显著抑制了自由基环化反应，使得PAN原丝的热稳定化能够以单体IA诱导的离子型环化反应为主较平缓地进行，相应的热失重过程也由两阶段转变为了单阶段。Py-SVUV-MS的实验结果表明，两种共聚PAN原丝在程序升温过程中会生成包括含氮小分子、丙烯腈单体及低聚物、成环化合物在内的三类主要热解产物，对比各类产物的生成趋势和产量，推断单体MA通过降低PAN结构的规整度，使其无定形化，从而增加环化反应的引发点，促进PAN原丝向稳定的预氧丝转变，同时由典型热解产物——甲基丙烯腈的生成路径可知，MA本身并不参与PAN大分子的环化反应，属于中性共聚单体。空气气氛下，CO₂的产量差异也间接证明了MA单体能够有效提升PAN纤维的固碳能力，采用三元共聚方法制得的PAN纤维热稳定性能更佳。

Mechanism Study of Comonomers for PAN Fiber Using Photoionization Mass Spectrometry

Polyacrylonitrile (PAN) copolymers are important precursors for making high performance carbon fibers. Two kinds of carbon fiber (CF) precursors based on acrylonitrile and two commonly used comonomers (itaconic acid and methyl acrylate, IA and MA) were prepared in this work via dipolymerization and ternary polymerization, respectively. The stabilization processes of these two PAN precursors were studied with thermogravimetry (TG) analysis and pyrolysis synchrotron vacuum ultraviolet photoionization mass spectrometry (Py-SVUV-PIMS). Given the TG results acquired under nitrogen atmosphere, three thermal decomposition stages were identified for the copolymer of PAN/IA, while only two stages were observed for the terpolymer of PAN/IA/MA. Both the first and the second stages for PAN/IA can be attributed to the scissions of the linear molecular chains of PAN. But they were induced in two different ways. As many studies already reported, with the addition of IA, both of radical and ionic cyclization reactions were proceeding simultaneously during the thermal stabilization process of PAN/IA. So, the first decomposition stage of PAN/IA was probably caused by the liberated heat of the radical cyclization reactions, which showed up in advance of the normal decomposition stage. In contrast, the decomposition of PAN/IA/MA at the first stage was conducting with a mild rate since the radical cyclization process was greatly restrained by the comonomer of MA. The mass spectra of decomposition products of these two CF precursors at isothermal temperatures were measured using Py-SVUV-PIMS. Thanks to the tunability of the photon energy of SVUV-PIMS, almost all of the pyrolysis products can be identified. Major pyrolysis products were divided into three categories, which are composed of nitrogen-containing compounds, acrylonitrile monomers and oligomers, and cyclized compounds. Temperature-evolved profiles of selected species under nitrogen and air atmosphere were also obtained during the programmed heating processes. Based on the formation profiles and yields of specified products, it can be concluded that the ordered molecular chain of PAN was damaged by the addition of MA and was finally converted into several amorphous segments. Since cyclization reactions tend to be initiated in amorphous regions of PAN, the thermal stabilization process of PAN can be greatly improved with the help of MA. Meanwhile, a relatively higher amount of methacrylonitrile was produced during the programmed heating process of PAN/IA/MA under air atmosphere. According to this, a possible formation mechanism of methacrylonitrile was proposed, which suggests that MA did not participate in the cyclization of PAN. In another word, MA is proved to be a neutral PAN comonomer. Furthermore, by comparing the evolution profiles of HCN, H₂O and CO₂ during the heating processes of these two CF precursors, we can conclude that the PAN fiber has a better carbon storage capability with the addition of both IA and MA.