【作者】 张烨；

【导师】 朱波；

【作者基本信息】 山东大学 ， 材料学， 2023， 博士

【摘要】 碳纤维(CFs)是一种由碳原子共价连接而成的先进纤维材料,其特征性的蜂窝状石墨晶格结构使纤维在轻质、稳定的基础上拥有十分优异的力学、电、热等性能。然而,实际生产过程中纤维分子结构的无序演化及复杂生产工艺引入的纤维结构缺陷使目前制备的CFs石墨结构往往不够理想,具体体现在石墨域尺寸小、晶面间距大、取向劣化、连续性差等,这使CFs的各项性能与其理论预测值存在较大的差距,进而限制了其实际应用。针对上述问题,本文以静电纺丝制备的纳米级CFs即碳纳米纤维(CNFs)为研究对象,探索了通过碳质填料优化纤维石墨结构的方案,创造性地提出了通过原位聚合技术促进填料在纤维前驱体聚丙烯腈(PAN)基质中快速、均匀分散的工艺路线,在此基础上开发了纳米石墨(NG)、纳米六方氮化硼(BN)、氧化石墨烯(GO)、氧化洋葱状纳米碳(fOLC)及自组装石墨化碳氮化合物等多种维度的纳米填料,系统研究了填料引入对聚合物前驱体、初纺原丝、预氧丝及最终CNFs结构的影响,具体内容如下:(1)开发了 NG与BN作为三维纳米填料以优化CNFs的结构与性能。研究发现利用原位聚合可以快速实现填料在PAN基质中的均匀分散,PAN在填料的吸引作用下包覆于其表面形成核壳状纳米复合材料,将其溶解后可直接电纺成复合纳米纤维,NG与BN在纤维中分布均匀且沿纤维轴存在一定取向,其对纤维基质的吸引不仅提高了纤维结晶度,同时促使纤维内PAN晶域沿纤维轴组装成规整连续的原纤结构。此外,纳米填料对PAN腈基的吸引与富集还有助于加快纤维预氧化反应速度并提高预氧化反应程度。在碳化过程中,NG与BN可以通过成核及模板作用促进初纺纳米纤维内的原纤状PAN晶域转化为最终的石墨化晶域,复合CNFs结晶度得以提升。在此基础上,NG进一步促进了 CNFs的石墨化并构建起良好的填料-纤维相互作用,而BN由于与纤维基质之间成分的不一致及晶格结构的不匹配而导致其衍生的纤维晶域排列松散且尺寸减小。基于CNFs晶体结构的优化及纳米填料的弥散强化作用,NG掺杂的CNFs强度和模量较纯CNFs分别最高提升约99%和52%,BN掺杂的CNFs强度及断裂伸长率较纯CNFs分别最高提升约87%和86%。(2)基于三维NG对CNFs结构与性能的优化效果,将其剥离成更薄的二维GO后继续研究碳质纳米填料对CNFs的增强作用。研究发现原位聚合仍可以实现GO在PAN基质内的快速、均匀分散,GO可以起到与NG类似的作用以促进初纺纳米纤维内PAN分子链的结晶与原纤化,同时通过成核及模板作用导致最终CNFs的结晶度、石墨化程度及石墨域的面内尺寸出现一定的提高,进而使复合CNFs强度与模量显著增强。然而,与NG不同的是厚度更小的GO纳米片可以实现更均匀地弥散分布并与纤维基质产生更充分的相互作用,因此仅需0.5 wt%添加量的GO便可以使复合CNFs断裂强度实现与1 wt%添加量NG作用下同等程度的提高(约98%),同时前者还可以使纤维模量及断裂伸长率有更大程度的提高(分别最高可达约92%及6%)。(3)沿着“填料尺寸的减小可加深其与纤维基质相互作用”的思路进一步探究了零维球形碳质纳米填料fOLC对CNFs结构与性能的优化作用。研究表明通过原位聚合同样可以实现更小尺寸的fOLC在PAN基质中良好的分散效果。与NG及GO相比,fOLC纳米级的弯曲晶面不利于PAN分子链的有序排列,同时弥散的纳米填料与PAN基质的相互作用也限制了聚合物分子链的自由迁移,因此聚合产物及初纺纳米纤维中PAN基质的结晶度均下降。尽管如此,纺丝过程中fOLC对PAN分子链的吸引与富集仍可以导致初纺纤维内PAN晶域的原纤化组装。在后续热处理过程,拥有相对完善石墨晶格结构的fOLC可通过成核及模板作用促进纤维基质的石墨化并与纤维石墨域建立良好的相互作用,配合刚性填料的弥散强化效果,复合CNFs强度能提高约65%。此外,与前述NG及GO不同的是fOLC由于尺寸的减小而更广泛地弥散分布在纤维基质中,其通过提供众多成核位点而导致纤维晶粒细化,同时纳米填料凭借极小的球形结构也能够与纤维晶域进行更一致的协同运动,进而在纤维基质变形时产生持续增韧效果,这使CNFs断裂伸长率能够提高约41%。(4)考虑到前述纳米填料的高昂成本及团聚倾向仍是制约其应用的主要因素,设计了两种低成本且可以在纺丝溶剂二甲基甲酰胺(DMF)中自组装的2,6-二氨基吡啶(DAP)/三聚氰酸(CA)及2,4,6-三氨基嘧啶(TAP)/巴比妥酸(BA)体系。在静电纺丝过程中,两种体系的单体可以与PAN在DMF中共溶并于纤维基质内原位自组装以分别形成超分子纳米填料DC/DMF及TB/DMF。此工艺下纳米填料在PAN分子链间均匀地形成且通过丰富的极性官能团与PAN分子链产生相互作用。在超分子填料的影响下复合初纺纳米纤维中PAN的结晶度得到提高且PAN晶域排列形成规整的原纤结构。随后,在预氧化及热解过程中,纳米纤维内DC/DMF、TB/DMF通过自组装形成的超分子结构逐渐转化为石墨化结晶结构,同时其可以作为结点键接PAN环化形成的梯形分子,进而形成尺寸更大的共轭芳香平面。石墨化纳米填料在碳化过程中进一步通过成核及模板作用促进CNFs的结晶与石墨化,同时由于极小的尺寸使填料在纤维内弥散分布,其可以通过提供众多成核位点而导致纤维晶粒细化。基于复合CNFs晶体结构的优化,纤维强度、模量及断裂伸长率得到全面提高。当添加量为5 wt%时DC/DMF较TB/DMF可以在纤维基质内实现更充分的弥散并导致CNFs晶体结构有更显著的优化,因此前者掺杂的复合CNFs综合力学性能获得更大提升,其强度、模量与断裂伸长率分别较纯CNFs 提高约 108%、37%及 55%。更多还原

【Abstract】 Carbon fibers(CFs)are advanced fiber materials made of covalently linked carbon atoms,and their characteristic honeycomb graphitic structure enables the fibers to be lightweight and stable with excellent mechanical,electrical,and thermal properties.However,in the actual production,the disordered fiber structure evolution and the fiber structure defects introduced by the complex production processes make the graphitic structure of the prepared CFs often less than ideal,which is reflected in the small crystal size,large interplanar spacing,poor crystal orientation and continuity.As a result,there is a large gap between the actual properties of CFs and their theoretical predictions,which in turn greatly limits their applications.To address the above problems,this paper took the electrospun nanoscale CFs,i.e.,carbon nanofibers(CNFs),as the research object and carried out a systematic study on the application of carbonaceous nanofillers to optimize the graphitic structure and mechanical properties of CNFs.For this purpose,the in-situ polymerization method was applied to rapidly and uniformly disperse nanofillers in the precursor polyacrylonitrile(PAN).On this basis,the multi-dimensional nanofillers,such as nano-graphite(NG),nano-hexagonal boron nitride(BN),graphene oxide(GO),functionalized onion-like carbon(fOLC)and self-assembled graphitic carbon nitride compounds,were developed.The effects of the added nanofillers on the structure of polymer precursors,as-spun nanofibers,stabilized nanofibers and final CNFs were systematically investigated.The main research contents are as follows:(1)NG and BN were developed as three-dimensional nanofillers to optimize the structure and properties of CNFs.It was found that the rapid and uniform dispersion of nanofillers in PAN matrix could be achieved by in-situ polymerization,and PAN was coated on the surface of nanofillers to form core-shell nanocomposites under the attraction of nanofillers.The nanocomposites could be directly electrospun into composite nanofibers after dissolution,and it was shown that NG and BN nanosheets could be uniformly distributed in the fiber matrix with a certain orientation along the fiber axis.In this case,the attraction effect of nanofillers not only improved the crystallinity of PAN matrix but also assembled the PAN crystal domains into regular and continuous fibril structures along the fiber axis.Moreover,the attraction and enrichment of the nitrile groups in PAN chains by the nanofillers also helped to promote the fiber stabilization reaction.During the carbonization process,NG and BN were confirmed to promote the conversion of fibrillar PAN crystals into final graphitic domains through nucleating and templating effects,and thus the crystallinity of composite CNFs could be enhanced.Meanwhile.NG further promoted the graphitization of fiber crystal domains and built up effective filler-fiber interactions.while BN led to looser and finer crystal domains in the fiber matrix for its composition inconsistency and slightly mismatched lattice structure with fiber matrix limiting the templating effect of nanofillers.Based on the optimization of fiber graphitic structures as well as the diffusion reinforcement effect of rigid nanofillers,the NG-doped CNFs showed significantly improved tensile strength and Young’s modulus(up to about 99%and 52%larger than pure CNFs.respectively),while the BN-doped CNFs exhibited greater strength and elongation at break(up to about 87%and 86%larger than pure CNFs,respectively).(2)The three-dimensional NG was further exfoliated into two-dimensional GO to continue the study of the reinforcement effects of carbonaceous nanofillers on CNFs.It was found that in-situ polymerization could also achieve rapid and uniform dispersion of GO in PAN matrix.On this basis,GO could play a similar role as NG to promote the crystallization of PAN and assemble the PAN crystals into fibrillar structures within as-spun nanofibers,and ultimately lead to enhanced crystallinity,graphitization degree,and in-plane size La of the graphitic domains in CNFs.As a result,the strength and modulus of composite CNFs were significantly improved.In this case,compared with NG,the thinner GO could be more uniformly distributed within the nanofibers and more fully interact with the fiber matrix,so that only 0.5 wt%of GO was needed to achieve the same level of increase(～98%)in the strength of composite CNFs as that of 1 wt%of NG,while the former further resulted in a greater increase in the modulus and elongation at break of CNFs(up to about 92%and 6%larger than pure CNFs.respectively).(3)Following the idea that the reduction of nanofiller size can deepen its interaction with fiber matrix,the zero-dimensional spherical fOLC was further used to reinforce CNFs.The rapid and uniform dispersion of nanoscale fOLC nanofillers in PAN matrix could be achieved by in-situ polymerization.It was found that the curved crystalline surface of fOLC and its extensive interaction with the PAN molecular chains restricted the crystallization of the polymer matrix in as-spun nanofibers.Nevertheless,the attraction and enrichment of the nanofillers on the PAN molecular chains could lead to the assembly of PAN crystal domains into regular and oriented fibrillar structures.During the subsequent heat treatment,the well-developed graphitic structure of fOLC could promote the graphitization process of fiber matrix through nucleating and templating effects,and the nanofillers also built up effective interactions with the induced fiber graphitic domains.On this basis,together with the diffusion reinforcement effect of rigid nanofillers,the strength of composite CNFs could be increased by up to about 65%.Besides,unlike the aforementioned NG and GO,fOLC could be more widely dispersed in the fiber matrix due to its reduced particle size,which led to the refinement of fiber graphitic grains by providing numerous nucleation sites.Meanwhile,the nanoscale spherical structure of nanofillers also allowed for the synergistic movement between nanofillers and fiber crystal domains,which in this case produced the continuous toughening effect on the fiber matrix during its deformation process.As a result,the elongation at break of composite CNFs could be significantly increased by up to about 41%.(4)Considering that the cost and dispersibility of the aforementioned carbonaceous nanofillers are still the main constraints on their application,we designed two systems of 2,6diaminopyridine(DAP)/cyanuric acid(CA)and 2.4,6-triaminopyrimidine(TAP)/barbituric acid(BA)that could self-assemble in the spinning solvent dimethylformamide(DMF).The monomers of these two systems were co-dissolved with PAN in DMF and self-assembled within the fiber matrix during the electrospinning process to form supramolecular nanofillers DC/DMF and TB/DMF.respectively.In this case,the nanofillers could be uniformly formed between the PAN molecular chains and interact with PAN through abundant polar functional groups.Under the influence of supramolecular nanofillers,the crystallinity of PAN in composite as-spun nanofibers was enhanced and the PAN crystal domains were arranged to form regular and oriented fibrillar structures.Subsequently,during stabilization and pyrolysis processes,the supramolecular structure of DC/DMF and TB/DMF within nanofibers was gradually transformed into graphitic crystalline structures,and meanwhile,the pyrolytic nanofillers could act as the junction to bond the ladder molecules formed by the cyclization of PAN.As a result,the conjugated aromatic domains with larger sizes in composite nanofibers were obtained.On this basis,the graphitic nanofillers could further improve the crystallinity and graphitization degree of CNFs through the nucleating and templating effects,and the widely distributed nanofillers also resulted in fiber grain refinement by providing numerous nucleation sites.Based on the optimization of the fiber graphitic structure,the strength,modulus and elongation at break of composite CNFs were comprehensively improved.In this case,since DC/DMF could achieve more uniform dispersion within the fiber matrix,it had better optimization effects on the crystal structure of CNFs.Therefore,the comprehensive mechanical properties of DC/DMF-doped composite CNFs showed greater enhancement,and a 5 wt%addition of DC/DMF could make the strength,modulus and elongation at break of composite CNFs increase by about 108%,37%and 55%,respectively,compared with pure CNFs.更多还原