【作者】 杨杰；

【导师】 包永忠； 潘鹏举；

【作者基本信息】 浙江大学 ， 化学工程， 2014， 博士

【摘要】 多孔炭材料具有孔隙结构丰富和比表面积大等特点,应用广泛。多级多孔炭(Hierarchical porous carbons, HPCs)是包含了微孔、中孔和/或大孔的新型多孔炭材料,结合了微孔炭比表面积大与中/大孔炭孔隙尺寸大等优点,在超级电容器、大分子尺寸物质吸附分离和催化剂负载等方面具有良好的应用前景。目前,HPCs的制备方法主要有多模板法、催化活化法、有机凝胶碳化法和共混聚合物碳化法等,大多存在制备工艺复杂,对孔结构控制有限等问题,制约了它们的应用。本文提出一种自模板(self-templating)直接碳化制备高比表面积、高孔容HPC的新方法,即通过活性自由基聚合构建由可碳化形成含微孔碳骨架的偏氯乙烯(VDC)聚合物和可热解聚合物组成的嵌段共聚物,选择合适的可热解聚合物和嵌段共聚物组成而形成微相分离结构,由可热解聚合物的热解形成中孔/大孔,最终得到具有微孔、中孔和/或大孔的HPCS。以聚乙二醇(PEG)、聚丙烯酸丁酯(PBA)、聚丙烯酸(PAA)和聚苯乙烯(PS)为热解聚合物,通过可逆加成断裂链转移聚合(RAFT)制备由VDC聚合物和以上可热解聚合物组成的嵌段共聚物。发现以2-(十二烷基三硫代碳酸酯基)-2-异丁酸(TTCA)为链转移剂(CTA)可以实现以VDC为主单体的RAFT溶液聚合,反应具有活性聚合的特性；但由于VDC聚合易向单体链转移,导致聚合得到的VDC聚合物分子量较低且分子量分布较宽。通过加入热解聚合物的相应单体(丙烯酸丁酯、丙烯酸和苯乙烯)进行再引发反应,制备了PVDC-b-PBA、PVDC-b-PAA和PVDC-b-PS共聚物；利用PEG与TTCA酯化合成大分子CTA,制备了PVDC-b-PEG-b-PVDC共聚物；利用S,S’-双(2-甲基-2-丙酸基)三硫代碳酸酯合成了PS-b-PVDC-b-PS共聚物,并考察了嵌段共聚物平均分子量及分子量分布,发现溶液聚合得到的嵌段共聚物的分子量较低且受链长限制。鉴于溶液聚合的不足,以两亲性大分子RAFT试剂PAA-b-PS-TTCA为乳化剂,实现了以VDC为主单体的RAFT乳液聚合,考察了乳液中和方式、乳化剂结构和浓度以及乳液固含量对聚合动力学的影响,并对聚合成核机理进行了探讨。发现RAFT乳液聚合速率远大于RAFT溶液聚合,聚合产物具有高分子量(Mn=25kg/mol)且分子量分布较窄,反应具有良好的可控性；以VDC聚合物乳胶粒子为种子,进一步通过RAFT种子乳液法制备了高分子量的PVDC-b-PS共聚物。采用原子力显微镜(AFM)、透射电镜(TEM)和小角X光散射(SAXS)进行嵌段共聚物的微相结构分析。由于热解聚合物嵌段与VDC聚合物嵌段的热力学不相容,嵌段共聚物都具有微相分离结构,PVDC-b-PEG-b-PVDC徼相分离尺寸较小,约为10～20nm,PVDC-b-PAA共聚物微相分离尺寸约为30～70nm,PVDC-b-PS共聚物微相分离尺寸约为20～100nm。热重分析仪(TGA)和差示扫描量热仪(DSC)对嵌段共聚物的热性能分析表明,嵌段共聚物都有两个明显的转变温度,分别对应PVDC和热解聚合物嵌段的玻璃化转变,并随着VDC聚合物嵌段的减少和热解聚合物嵌段的增加,热焓有相应的变化。碳化过程中,VDC聚合物嵌段和PBA、PS嵌段可独立分解,但与PEG、PAA嵌段分解温度较近,其中PAA嵌段不能完全热解,生成少量残碳。采用扫描电镜和比表面积分析仪对嵌段共聚物基多孔炭结构进行了表征,结果表明,以设计的嵌段共聚物碳化后均具有多级多孔结构,并可通过调节两种结构嵌段的比例,获得不同孔径范围的多级孔结构。PVDC-b-PEG-b-PVDC共聚物基多孔炭的中孔含量较低,最大比表面积可达1242m2/g,孔容达0.49cm3/g,中孔率达14.5%。PVDC-b-PBA共聚物基多孔炭最大比表面积达957m2/g,孔容达0.52cm3/g,中孔率达44.2%。PVDC-b-PAA碳化后PAA不能完全分解对孔结构有影响,最大比表面积可达1093m2/g,孔容达0.51cm3/g,中孔率达22.6%。PS可完全降解,多孔炭具有高比表面积和较高中孔率。PVDC-b-PS共聚物基多孔炭最大比表面积达1220m2/g,孔容达0.92cm3/g,中孔率达57.5%; PS-b-PVDC-b-PS共聚物基多孔炭大比表面积达839m2/g,孔容达0.42cm3/g,中孔率达54%；种子乳液法PVDC-b-PS共聚物基多孔炭最大比表面积达1226m2/g,孔容达1.86cm3/g,中孔率达77.9%。采用不同结构的多级多孔炭制备了电容器电极,采用循环伏安法和恒电流法进行HPC材料的电化学性能测试,发现多级孔结构对材料电性能起到关键作用,利用微孔-中/大孔结构,既可以充分利用多孔炭的高比表面积,增大电容,又可以加快电解质离子迁移,快速达到电位平衡,保持比电容对大放电倍率的稳定性,提高电极性能。当电流密度为0.5A/g时,PVDC-b-PEG-b-PVDC共聚物基、PVDC-b-PAA共聚物基、PVDC-b-PS共聚物基、PS-b-PVDC-b-PS共聚物基和种子乳液法PVDC-b-PS共聚物基多孔炭的比电容分别最大达到180F/g、233F/g、216F/g、218F/g和216F/g。中孔结构明显改善了材料在大电流下性能,随着电流密度和扫描速率的增加,中孔较多的多孔炭电容衰减较少。更多还原

【Abstract】 Porous carbons exhibit a great number of pores and great specific surface area, and have wide applications in many fields. Hierarchical porous carbons (HPCs) possess a multimodal pore size distribution of micro-, meso-and/or macro-pores, and combine the feature of high surface area of the micro-porous carbons and the large pore diameter of meso-/macro-porous carbon. Thus, HPCs have excellent application properties as they used in the electrodes for electrochemical double layer capacitors, catalyst supports, and macromolecules adsorption and separation. Up to now, HPCs have been generally prepared by the methods of multi-templating carbonization, catalytic activation, carbonization of polymer blend and organic gel. However, needing of complicate process and low controllability on pore structure have limited their applications.This dissertation presented a facile, novel self-template approach to fabricate HPCs with high surface area and large pore volume by direct carbonization of micro-phase separated block copolymers composed of poly(vinylidene chloride)(PVDC) and pyrolyzable polymer blocks. Micro-porous carbon skeleton would formed by the carbonization of PVDC block, and the properly selected pyrolyzable blocks would transfer to the mesopores or meso-/macro-pores after their pyrolysis. Thus, porous carbons exhibited a multimodal pore size distribution of micro-, meso-and/or macro-pores would be obtained.A series of block copolymers comprising the PVDC block and the pyrolyzable block including polyethylene glycol (PEG), polybutyl acrylate (PBA), polyacrylic acid (PAA) and polystyrene (PS) were synthesized via reversible addition fragmentation-chain transfer (RAFT) polymerization. RAFT copolymerization of VDC was successfully carried out using2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (TTCA) as a chain transfer agent (CTA). The kinetics studies showed the obvious living/controlling radical polymerization behavior. The significant transfer reaction of macro-radicals to VDC monomer was considered to address the relatively broad polydispersity index (PDF) and low molecular weights. PVDC-b-PBA, PVDC-b-PAA and PVDC-b-PS copolymers were synthesized via reinitiation reactions of corresponding monomers.PVDC-b-PEG-b-PVDC copolymers were synthesized via RAFT polymerization of VDC using PEG-TTCA macro-CTA prepared by the esterification of PEG and TTCA. PS-b-PVDC-b-PS was synthesized by using S,S’-bis(α,α’-dimethyl-α"-acetic acid)-trithiocarbonate (BDMAT) as CTA. The variations of the molecular weight of block copolymers with conversion indicated that the chain growth depended on the molecular weight of macro-CTAs and the phase separation during the solution RAFT copolymerization of VDC, and the molecular weight of block copolymers were relatively low in the solution copolymerization.Considering disadvantages of the solvent polymerization, an amphiphilic PAA-b-PS-TTCA copolymers containg trithiocarbonate reactive groups were used in the ab initio RAFT emulsion copolymerization of VDC. Effects of macro-RAFT agent structure and concentration, neutralization policy of PAA-b-PS-TTCA macro-RAFT agent, initiator type and polymerization temperature on the kinetics and controllability of polymerization, the stability and particle size distribution of latexes were investigated. It was found that the RAFT emulsion copolymerization of VDC showed greater polymerization rates than the solution polymerization, and PVDC with high molar masses (25kg-mol-1) and low PDI could be obtained. The determined molecular weights of PVDC were increased continuously and were in good agreement with the corresponding theoretical values, indicating well controllability of polymerization. The as-prepared PVDC latexes were further used as seeds in the emulsion polymerization of styrene, enabling the preparation of novel PVDC-b-PS copolymers with a high molar mass and a relatively low PDI.The micro-phase separation of block copolymers were investigated by using atomic force microscopy (AFM), transmission electron microscope (TEM) and small-angle x-ray scattering (SAXS) methods. Due to the thermodynamic incompatibility between pyrolyzable polymer block and PVDC block, all block copolymers showed micro-phase separation structures. The dimensions of the micro-dispersed phase in PVDC-b-PEG-b-PVDC, PVDC-b-PAA and PVDC-b-PS copolymers were10～20nm,30-70nm and20～100nm, respectively. The differential scanning calorimetry (DSC) results indicated the block copolymers exhibited two glass transition temperatures corresponding to the glass transition temperatures of PVDC and pyrolyzable polymer block. The thermogravimetric analysis (TGA) indicated that PVDC block and PBA (or PS) block were decomposed independently, while the degradation temperatures of PEG and PAA blocks were closed to that of PVDC block. Furthermore, PAA block showed an incomplete pyrolysis at even higher temperature, which might jam the pores of the corresponing porous carbons.The micrographs and microstructure parameters of the carbons prepared from the block copolymers were characterized by field emission scanning electron microscopy and N2absorption/desorption analysis. The results indicated that hierarchical porous structures could be accomplished via well-designed self-templates, Multimodal pore size dimensions could also be obtained via adjusting ratio of two blocks structures. The carbons prepared from PVDC-b-PEG-b-PVDC copolymers exhibited a maximum specific surface area (SBET) of1242 m2/g, a maximum total pore volume (Vtotai) of0.49cm3/g and a low mesoporosity of14.5%. The carbons prepared from PVDC-b-PBA copolymers exhibited a maximum SBET of957m2/g, a maximum Vtptal of0.52cm3/g and a mesoporosity of44.2%. PVDC-b-PAA based carbon exhibited a maximum SBET of1093m2/g, a maximum Votal of0.51cm3/g and a mesoporosity of22.6%. Due to complete pyrolysis of PS block, the carbons with higher SBET and mesoporosity could be obtained from PVDC-b-PS copolymers. PVDC-b-PS copolymers based carbon exhibited a maximum SBET of1220m2/g, a maximum Vtotai of0.92cm3/g and a mesoporosity of57.5%. PS-b-PVDC-b-PS copolymers based carbon exhibited a maximum SBET of839m2/g, a maximum Vtotal of0.42cm3/g, and a mesoporosity of54%. The carbon prepared from the seeded emulsion polymerized PVDC-b-PS copolymer exhibited a maximum SBET of1226m2/g, a maximum Vtotai of1.86cm3/g, and a mesoporosity of77.9%.The electrochemical performances of as-prepared HPCs used as supercapacitor electrodes were studied via galvanostatic cycling and cyclic voltammetry. The results showed that the hierarchical porous structures played a key role in electrochemical performances. The specific capacitances of the electrodes were increased with the specific surface areas of HPCs. The presence of larger pore size in HPCs could improve the ion transportation, keep the stability of specific capacitance under high current density and enhance electrodes performance. The highest specific capacitance values of252F/g,233F/g,216F/g,218F/g and216F/g were obtained at0.5A/g for HPCs prepared from PVDC-b-PEG-b-PVDC. PVDC-b-PAA, PVDC-b-PS, PS-b-PVDC-b-PS and seed emulsion polymerized PVDC-b-PS copolynmers, respectively. With increase of current density and scan rate, larger mesoporosity led to less decrease of specific capacitance, which indicated meso-pore structure could obviously enhance performance under high current.更多还原