【作者】 袁剑民；

【导师】 陈小华；

【作者基本信息】 湖南大学 ， 材料物理与化学， 2009， 博士

【摘要】 碳纳米管作为一种独特的纳米结构材料,具有出众的物理性能和机械性能,比如极高的塑性模量、优异的导电和导热性能,这使其成为复合材料设计关注的焦点。正因为如此,最近掀起了研究碳纳米管增强聚合物基复合材料的热潮。但是,碳纳米管固有的表面惰性和不溶解性,使它难分散在溶液和聚合物基体中,在某种程度上来说,这阻碍了碳纳米管在聚合物复合材料领域的应用。研究发现,对碳纳米管进行表面化学改性,是使碳纳米管在溶液和聚合物基体中实现较好分散的有效途径。因此,本研究工作首先选择恰当的碳纳米管纯化处理工艺,使碳纳米管表面生成大量的活性基团(如羟基、羧基);再利用这些活性基团与有机小分子(或聚合物单体)反应,在碳纳米管表面接枝有机分子链;然后用接枝改性的碳纳米管,通过溶液混合法或熔融混合法制备成复合材料。探讨了纯化方法和改性过程对碳纳米管性能的影响,并对复合材料的性能进行了研究,发现了一些重要的规律,可以为聚合物-碳纳米管复合材料的研究和生产提供实验指导。第一,首次采用氯氧化法提纯多壁碳纳米管(MWCNTs),此方法主要由氯水处理和氨水处理两个步骤组成,它同时具备气相氧化法、液相氧化法和酸处理法三者的优点。因此从应用的角度来看,此方法简单、能够满足工业上大规模纯化碳纳米管的需求。透射电镜(TEM)观察发现:经过氯氧化纯化处理后,原始碳纳米管表面的无定形碳层、纳米碳颗粒和催化剂颗粒被除去,而且封闭的端口被打开。X-ray能量散射光谱分析法研究表明:原始碳纳米管的催化剂元素Ni、Mo和Mg含量分别为2.37、3.58和0.91wt%;而纯化碳纳米管(p-MWCNTs)中未检测到Mg和Mo,Ni的含量也降到了0.15wt%。此外,氯氧化法纯化使p-MWCNTs表面生成了大量羟基、羧基和C-Cl,这大大提高了p-MWCNTs在溶剂(如:水、乙醇、丙酮、氯仿和二甲基甲酰胺)中的分散能力。因此,可以以这些基团为基础,通过化学反应对p-MWCNTs表面进行改性。第二,以p-MWCNTs表面的羟基为基础,与4-氯甲基苯乙烯反应,在碳纳米管表面接枝苯乙烯基团。然后使苯乙烯基接枝改性碳纳米管(s-MWCNTs)参与苯乙烯的聚合反应,在其表面接枝聚苯乙烯(PS)分子链,得到PS接枝改性碳纳米管(g-MWCNTs)。研究显示:g-MWCNTs表面的PS层厚度大约5nm,接枝率为20wt%。经过接枝改性后,g-MWCNTs与弱极性溶剂(甲苯、四氢呋喃)的相容性提高。接着通过溶液混合法制备了PS-p-MWCNT、PS-s-MWCNT和PS-g-MWCNT三种复合材料。扫描电镜(SEM)观察发现:由于PS层的包裹,g-MWCNTs与PS基体的相容性大大提高,因此能在基体中比较均匀的分散;随着碳纳米管含量的增加,PS-g-MWCNT复合材料的断裂方式由脆性断裂逐渐转向韧性断裂。机械性能和热性能研究表明:随着碳纳米管含量的增加,PS-p-MWCNT、PS-s-MWCNT和PS-g-MWCNT三种复合材料的冲击强度、拉伸强度和热稳定温度都呈上升趋势;由于具有良好的界面相容性和均匀分散的碳纳米管,在三种复合材料中,PS-g-MWCNT复合材料的机械性能和热性能最优。当碳纳米管含量为1.5wt%时,PS-g-MWCNT复合材料的冲击强度和拉伸强度,较纯PS分别提高了38.8%和33.6%,耐热温度也提高了25℃。第三,在p-MWCNTs存在的条件下,微波辐射引发苯乙烯的聚合,得到PS改性的碳纳米管(m-MWCNTs)。研究发现m-MWCNTs外壁的PS包裹层厚度约为3nm,占m-MWCNTs总质量的10%;改性机理为:在微波辐射下,p-MWCNTs表面的基团(-H、-CH2OH、-CH2Cl和-CH3)被活化并参与苯乙烯的聚合反应,从而在碳纳米管表面共价接枝PS层。分散性实验证明微波辐射得到的共价结合PS层能够与MWCNTs牢固粘接,不会被溶剂洗脱;而在其它实验条件相同,改用传统加热方式来改性MWCNTs时,PS只能与MWCNTs非共价结合,并且可以被溶剂洗脱。然后用溶液混合法制备出碳纳米管含量为20 wt%的PS-p-MWCNT色母料和PS-m-MWCNT色母料。接着在挤出机和注塑机(工业生产用)上熔融混合色母料和纯PS,制备出PS-p-MWCNT复合材料和PS-m-MWCNT复合材料。SEM观察发现:在PS-m-MWCNT复合材料中,碳纳米管被两种PS层包裹,一种是改性过程中形成的接枝PS层,另一种是存在于接枝PS层和PS基体之间的PS中间层,其平均厚度为80nm;而在PS-p-MWCNT复合材料中,没有检测到类似的PS中间层。通过TEM的进一步研究证明:在PS-m-MWCNT复合材料中,碳纳米管呈单根分散状态并沿着成型时的熔体注射方向取向;而在PS-p-MWCNT复合材料中,虽然碳纳米管也是单根分散在PS基体中,但是其排列方式是杂乱无章的。由于接枝PS层和PS中间层的存在,使碳纳米管和PS基体的相容性提高、界面粘接强度增加,因此PS-m-MWCNT复合材料的机械性能和热性能都优于PS-p-MWCNT复合材料。例如,当碳纳米管含量为0.32wt%时,PS-m-MWCNT复合材料的冲击强度较纯PS提高了150%,而PS-p-MWCNT复合材料仅提高50%。第四,为了证明碳纳米管表面改性技术具有普遍适应性,还研究了聚碳酸酯(PC)-碳纳米管复合材料的制备及其性能。首先通过溶液混合回流法制备了PC色母料,目的是利用马来酸酐(MAH)中双键和酸酐基的反应活性,将PC分子链共价接枝到碳纳米管表面,改善碳纳米管和PC基体的相容性。然后用熔融混合法,在双螺杆挤出机和注塑机上制备了公斤级PC-MWCNT复合材料。SEM研究显示:碳纳米管是以棒状的聚集体形式均匀分散在PC基体中,此聚集体由少数几根碳纳米管组成,其直径大约500nm;而且,棒状聚集体沿着注塑成型时的熔体注射方向取向。在碳纳米管的诱导下,PC分子链在其附近规整排列,从而在PC-MWCNT复合材料内部形成大量微晶区,这使复合材料的热稳定性能大大提高。当碳纳米管含量0.32 wt%时,PC-MWCNT复合材料的冲击强度较纯PC提高了60%,热稳定温度较纯PC提高了30℃。总之,以上研究说明:以纯化碳纳米管表面活性基团(羟基、羧基、活泼氢等)为基础进行化学改性,这种工艺是可行的;碳纳米管经过改性后,可以提高其在溶液和聚合物基体中的分散性能,增强碳纳米管-聚合物界面的粘接强度,从而大大提高聚合物-碳纳米管复合材料的综合性能。更多还原

【Abstract】 Carbon nanotubes (CNTs) are unique nanostructured materials with remarkablephysical and mechanical properties, such as high elastic modulus, as well asremarkable thermal and electrical conductivity, making them a very attractivecandidate in composite material formulations. For this reason a current flurry ofresearch is focused on the manufacturing of nanotube reinforced polymer matrixcomposites. However, dispersion of CNTs in solvents and polymer matrixes is anobstacle for their further applications due to the inert surfaces and the poor solubility.Some researches reported that a well dispersion of CNTs in solvents and polymermatrixes could be achieved through chemical modification of the CNT surfaces.Therefore, a proper purification was firstly carried out to achieve a large number ofactive groups (e.g.–OH and -COOH) on the CNT surfaces in this study. Based onthese active groups, CNT surfaces can be grafted and modified by organic chainsthrough reacting with organic molecules or polymer monomers. By using the modifiedCNTs, polymer-CNT composites were prepared via solution-mixing or melt-mixingapproaches. The effects of purification process and modification techniques on theCNTs were studied, and the properties of the polymer-CNT composites wereexamined and analyzed in this study. Some important rules about the polymer-CNTcomposites were found, which was instructive for the basic research and industrialmanufacture.Firstly, chlorine oxidation, composed of chlorine water treatment and ammoniawater treatment, was used to purify multiwalled carbon nanotubes (MWCNTs). Fromthe point view of practical application, this purification, comprising the merits ofgas-phase oxidation, liquid-phase oxidation and acid treatment, is easily carried outand can satisfy the need of purifying MWCNTs on a large scale in industry.Transmission electron microscope (TEM) observation showed that amorphous carbonon the outer-walls of pristine MWCNTs, carbon nanoparticles, and catalysts of metaloxides were eliminated via purification. Moreover, the closed tips of the pristineMWCNTs were opened. The contents of catalyst elements of Ni, Mo, and Mg in thepristine MWCNTs were 2.35, 3.58, and 0.91 wt%, respectively, which were estimatedby analysis of energy dispersive X-ray spectroscopy. However, there were no Mg andMo elements in the p-MWCNTs, and the Ni content was also dropped to 0.15 wt%. Importantly, the dispersion ability of MWCNTs in polar solvents such as water,ethanol, acetone, chloroform, and dimethylformamide, was greatly improved due to alarge number of carboxyl, hydroxyl groups, and C-Cl bonds, introduced on the CNTsurfaces after purification of chlorine oxidation. Therefore, p-MWCNTs could befurther modified via chemical reactions on the basis of these groups.Secondly, on the basis of the hydroxyl, p-MWCNT surfaces were modified withstyryl through reacting with 4-chloromethylstyrene. The modified MWCNTs withsurface styryl (s-MWCNTs) could participate in the polymerization of styrene, whichresulted in the covalent bonding of PS chains on the MWCNTs (g-MWCNTs). It wasfound that the PS layer was approximately 5 nm and 20 wt% of the g-MWCNTs. Aftermodified by PS, the MWCNTs were more compatible with poor polar solvent such astoluene and tetrahydrofuran. PS-p-MWCNT, PS-s-MWCNT, and PS-g-MWCNTcomposites were then prepared by solution-mixing approach. Scanning electronmicroscope (SEM) observation exhibited that because of the coating of PS layer, thecompatibility between the MWCNTs and PS matrix was greatly enhanced, resulting inthe homogeneous dispersion of g-MWCNTs. With the increase of MWCNT content,the fracture mechanism of the PS-g-MWCNT composite was transformed from brittlefracture to elastic one. Studying on the mechanical and thermal properties of thecomposits proved that with the increase of MWCNT content, the impact strength,tensile strength, and temperature of thermal stablility of PS-p-MWCNT,PS-s-MWCNT, and PS-g-MWCNT composites were all enhanced. With a betterinterfacial compatibility and homogeneous MWCNT dispersion, the PS-g-MWCNTcomposite had the better mechanical properties and thermal properties, compared toPS-p-MWCNT and PS-s-MWCNT composites. When the MWCNT content was 1.5wt%, the impact strength and tensile strength of PS-g-MWCNT composite wereincreased to 138.8% and 133.6% of neat PS, respectively. And the temperature ofthermal stability of PS-g-MWCNT composite was enhanced by 25 oC, compared toneat PS.Thirdly, p-MWCNTs were modified by PS via the polymerization of styrene undermicrowave irradiation. It was found that the PS layer (called PS coat layer in thefollowing) on the modified MWCNTs (m-MWCNTs) was approximately 3 nm and 10wt% of the m-MWCNTs. The mechanism of the modification was that the groups (e.g.,-H, -CH2OH, -CH2Cl and -CH3) on the p-MWCNTs were actived by microwaveirradiation and participated the polymerization of styrene, which resulted in theconvalent bond between PS and MWCNT surfaces. The PS coat layer, covalently bonding on the MWCNT surfaces, could not be washed off by solvents, proved bydispersion tests. Whereas, in the case of a control experiment conducted throughconventional heating, the PS was noncovalently bonding on MWCNT surfaces andcould be washed off by solvents. Then, the PS-p-MWCNT and PS-m-MWCNTmasterbatches with MWCNT content of 20 wt% were prerared by solution-mixing.And PS-p-MWCNT and PS-m-MWCNT composites were manufactured viamelt-mixing of masterbatches with neat PS on industrial extruder and injectionmoulding machine. In PS-m-MWCNT composite, the MWCNTs were enwrapped bytwo PS layers, which proved by SEM. One was the PS coat layer achieved duringmodification. The other was a PS middle layer lying between the PS coat layer and PSmatrix, whose thickness was approximately 80 nm. On the contrary, there was not PSmiddle layer in PS-p-MWCNT composite. TEM observations of the PS-m-MWCNTcomposite showed that the MWCNTs were individually dispersed in PS matrix andoriented along the injection direction of PS melt during moulding. In PS-p-MWCNTcomposite, the MWCNTs were also individually dispersed, but they were disordered.The mechanical and thermal properties of PS-m-MWCNT composite were better thanthat of PS-p-MWCNT composite, due to the influence of PS coat layer and PS middlelayer, improving the compatibility and interfacial adhersion between MWCNTs andPS matrix. For instance, when the MWCNT content was 0.32 wt%, thePS-m-MWCNT composite had a 250% increase of impact strength as compared toneat PS, but the PS-p-MWCNT composite had only a 150% increase.Fourthly, to confirm the fact that modification of MWCNT surfaces is universal forother polymer matrix, the preparation and properties of polycarbonate (PC)-MWCNTcomposite were also studied. In order to covalently modify MWCNT surfaces with PCchains, based on the reaction activity of C=C bonds and anhydride groups in maleicanhydride (MAH), and improve the compatibility between MWCNTs and PC matrix, aPC masterbatch was prepared via solution-mixing and reflux reaction of PC,MWCNTs, and MAH. Using industrial extruder and injection moulding, kilogramscale of PC-MWCNT composites were prepared via melt-mixing. SEM observationdisplayed that the claval aggregations of approximately 500 nm diameter, composedof several MWCNTs, were homogeneously dispersed in matrix and oriented along theinjection direction of PC melt during moulding. Because of the inducing effect ofMWCNTs, PC chains were regularly arrayed near the MWCNT surfaces and a largeamount of microcrystalline domains were formed, improving the thermal stability ofPC-MWCNT composite. Therefore, when the MWCNT content was 0.32 wt%, the impact strength of PC-MWCNT composite were increased to 160% of neat PS, thetemperature of thermal stability was enhanced with 30 oC.In conclusion, the above researches prove that it is feasible to chemically modifyMWCNT on the basis of the active groups (e.g., -H, -CH2OH, -CH2Cl and -CH3) onp-MWCNT surfaces. After modification, the dispersion ability of MWCNTs insolvents and polymer matrices can be improved, and interfacial adhersion betweenMWCNTs and polymer matrices can be enhanced, resulting in the improvement ofcomprehensive properties of polymer-MWCNT composites.更多还原