【作者】 陈江溪；

【导师】 夏海平；

【作者基本信息】 厦门大学 ， 高分子化学与物理， 2007， 博士

【摘要】 本论文研究了在先驱体转化法制备高性能细直径SiC纤维的方法中,有机硅高分子先驱体合成的基础科学问题,包括如何从聚二甲基硅烷（PDMS）合成可控分子结构的聚碳硅烷（PCS）、开发了一种新的含铝元素聚碳硅烷（PACS）合成方法以及运用此方法合成了几种新型含杂元素的有机硅高分子。本论文共分为五章,第一章为绪论,阐述了高性能细直径SiC纤维的丰要应用背景及其主要的制备方法。本章重点介绍了先驱体法制备高性能细直径SiC纤维的研究历史与现状,并在此基础上提出本论文的研究设想与目的。本论文第二章为实验部分。第三章总结了本论文所掌握的PCS合成规律,提出了PCS分子结构可控合成的裁剪——重拼模型,并分别用反门控去偶²⁹Si NMR、UV以及GPC表征证实了模型的正确性,从而在理论上解释了如何调整PDMS的裂解重排工艺,进而达到合成可控PCS分子结构的目的。根据裁剪——重拼模型,PCS的分子结构与裂解重排工艺的升温速率快慢和保温温度高低有密切的关系。较慢的升温速率及较低的反应温度可以得到线形度较高的PCS;而较快的升温速率及较高的反应温度则得到线形度较差的PCS。因此,基于裁剪——重拼模型,本论文确定了直接从PDMS合成具有代表性可纺丝PCS的合成工艺条件。与此同时,本章运用裁剪——重拼模型解释了MarkⅡ和Ⅲ型PCS存在的优点,并借鉴管理学上的方法提出了PCS质量的过程控制原则。此外,本论文还发现不同分子量的PCS在某种溶剂中的溶解度是不同的,并研究和发现可以溶解PCS分子量从小到大顺序的溶剂分别为:甲醇＜二甲基甲酰胺＜乙二醇单甲醚＜无水乙醇＜乙二醇单乙醚＜异丙醇＜甲酸乙酯＜乙酸甲酯＜正丙醇;异丙醇＜丙酮＜正丁醇＜二氧六环＜丁酮＜乙二醇二甲醚＜乙醚。因此,这一套溶剂体系可以被用来调节PCS的分子量及分子量分布。最后,本章分析了PCS分子结构对不熔化过程可能造成的影响。本论文的第四章开发了一种全新的先驱体PACS合成方法,具体是由PDMS与乙酰丙酮铝于密闭反应釜中一步反应直接制得PACS。这一方法不仅解决了乙酰丙酮铝易升华的问题,而且还可以简单地通过投料量来准确控制PACS中的铝含量。采用此方法所制备的低铝含量（<2wt%）PACS比相同条件下制备出来的PCS表现出更好的纺丝性能。为此,本章探讨了PACS的合成机理。认为在没有乙酰丙酮铝存在时,由PDMS裂解重排形成PCS的过程中,Si自由基会形成具缠结点作用的SiC₄基团;而在乙酰丙酮铝存在时,部分Si自由基会先形成（Si—O）_nAl基团,这相当于替代了本应形成的SiC₄基团,从而相对改善了缠结作用,最终使PACS的纺丝性能比PCS提高。然而,当PACS的铝含量过高时,即乙酰丙酮铝投料量太多时,则PACS分子结构含有更多的（Si—O）_nAl基团,而过多的（Si—O）_nAl基团将使PACS的纺丝性能下降,甚至变成不熔。因此,PACS中的铝含量应控制在2wt%以内,即可以保证PACS具有良好的纺丝性能,也有利于高性能Sic纤维的制备。此外,由于PDMS与乙酰丙酮铝反应,断裂形成的小分子自由基碎片会与Si自由基继续反应形成含有Si—C的基团。此基团所起的作用也是缠结点的作用,不利于PACS纺丝性能的提高;而且小分子的存在会导致反应体系压力升高,对于PACS的大量生产是不利的,因此采用能承受一定的压力以使反应物为液态和避免乙酰丙酮铝升华,又能以适当的方式排出脱落的小分子碎片的反应釜是有必要的。本章研究发现了PACS的数均分子量随保温时间,保温温度以及乙酰丙酮铝投料量的增加而增大。而且不同分子量的PACS分子在特定溶剂中的溶解度不同,溶解能力从小到大顺序为:甲醇<95%乙醇<乙二醇单甲醚<二甲基甲酰胺<无水乙醇<丙酮<乙酸甲酯<异丙醇＜甲酸乙酯,因此可以利用合适的溶剂体系来调节PACS的分子量及分子量分布。本论文第五章借鉴PACS的合成方法,开展了含钇元素聚碳硅烷（PYCS）和含锆元素聚碳硅烷（PZCS）的制备初探,并进行了FT-IR,¹H、¹³C、²⁹Si NMR结构表征,推测其反应机理与PACS的合成机理类似。然而,采用此方法合成含铁元素聚碳硅烷（PFCS）时,发现PFCS的工艺稳定性很差。本章研究了PFCS作为先驱体,采用Nicalon纤维的制备工艺,将PFCS熔融纺丝,再经氧化交联,及惰性气氛烧成的方法初探SiC（Fe）纤维的制备,发现其具有较好的力学性能,并且可以耐较高的温度（1000℃）。然而,先驱体PFCS的合成工艺稳定性较差,因此该方法不适合于应用在吸波纤维的制备。本章还探索了采用PACS的合成方法将三聚氰胺与PDMS混合直接制备含氮元素聚碳硅烷（PNCS）的方法,并初步探讨了PNCS的合成机理。然而由于采用三聚氰胺的合成工艺太危险,而且原丝样品经空气氧化后,没有氮元素保留在纤维内,表明原来设计的将PNCS经熔融纺丝,空气氧化,制备Si-N-O透波纤维的方案不可行。此外,经初探发现,三苯基硼嗪及其衍生物可以作为催化剂,催化合成无氧MarkⅢ型的PCS;也可以作为原料合成含B及N的PCS。该工作值得进一步深化研究。更多还原

【Abstract】 SiC fiber is one of the most promising candidates as the reinforcing fiber of ceramic matrix composites（CMCs） for high temperature applications,because of its high tensile strength,thermal and oxidation resistance.In this dissertation,different precursors for fine diameter continuous SiC-based fibers were prepared and studied.In chapter 3,the control of structure formation of polycarbosilane（PCS） synthesized from polydimethylsilane（PDMS） was studied.It was found that the molecular structure of PCS was strongly dependent on the reaction temperature and heating rate.Higher reaction temperature and fast heating rate tended to give PCS with highly cross-linked skeleton,while lower reaction temperature and slow heating rate tended to result in PCS with linear backbone.A scissor-couple rearrangement model was proposed for the formation of the PCS skeleton,which was confirmed by²⁹Si NMR,GPC and UV analysis.On the base of this model,PCS with controllable molecular structure and good spinnability can be obtained.Moreover,when vacuum heating is used to remove the low molecular weight fraction in the as-synthesized PCS to increase its softening point,the linearity,hence spinnability of the as-synthesized PCS is reduced,because of the continuing reaction during this process.This problem can be avoided by solvent extraction.The ability of solvents to exact low molecular weight fraction of PCS is in the ascending order of methanol,N,N-dimethylformamide,2-methoxyethanol,ethanol,2-ethoxyethanol, 2-propanol,ethyl formate,methyl acetate and n-propanol;iso-propanol,acetone, n-butanol,1,4-dioxane,butanone,1,2-dimethoxyethane and ether.Through solvent extraction of the as-synthesized PCS,PCS with optimal molecular weight can be obtained.The good spinnability of MarkⅡand MarkⅢPCS was also explained by scissor-couple rearrangement model in chapter 3. In chapter 4,a new method to prepare polyaluminocarbosilane（PACS） directly from the one-pot reaction of PDMS with Al（acac）₃ in autoclave was developed.In this closed system,the sublimation of Al（acac）₃ is prevented and all of Al in Al（acac）₃ are converted into PACS.Therefore,the content of Al can be readily controlled quantitatively.On the base of the FT-IR,¹H NMR,¹³C NMR,²⁹Si NMR and ²⁷Al MAS NMR analysis,the reaction mechanism was proposed as follows:PDMS dissociated during pyrolysis to generate silicon free radicals,and then they reacted with Al（acac）₃ to give PACS containing（Si-O）_nAl groups（n=4,5,6）.Meanwhile,these reactions resulted in the cleavage of O-C and/or O=C bonds in Al（acac）₃.Some of free radical fragments generated by this cleavage continued to react with the silicon free radicals and were incorporated into the structure units of PACS;the rest of them may convert into small oxygen-containing compounds,which were removed in the subsequent processing after the reactions.The low molecular weight fraction in the as-synthesized PACS can be also removed by solvent exaction.The ability of solvents to exact low molecular weight fraction of PACS was found in the ascending order of methanol,95%ethanol,2-methoxyethanol, N,N-dimethylformamide,ethanol,acetone,ethyl formate,2-propanol and methyl acetate.In chapter 5,polyyttrocarbosilane（PYCS）,polyzirconocarbosilane（PZCS）, polyferrocarbosilane（PFCS） and nitrogen-containing polycarbosilane（PNCS） were successfully prepared using the method similar to the synthesis of PACS.The reaction mechanism of PYCS and PZCS were supported to be similar to PACS.However,it was found that the reproducibility of preparation of PFCS was very poor.Meanwhile,the reaction mechanism of PNCS synthesized from melamine and PDMS was studied by FT-IR,¹H,¹³C and ²⁹Si NMR analysis and described in chapter 5.Preparation of SiC（Fe） fiber had been attempt by using PFCS as polymer precursor, producing ceramic fibers with high tensile strength in this chapter.However,attempt to produce Si-N-O fiber was given up,because it failed to produce Si-N-O ceramic by oxidation of PNCS in air.Si-N-O ceramic was not obtained because of the loss of nitrogen during the oxidation curing of PNCS in air.Si-B-N-C polymer precursor was prepared by the the reaction of triphenylborazine with PDMS.It was found that triphenylborazine can serve as the catalyst to prepare MarkⅢPCS.No oxygen was incorporated into the resultant precursor.It can serve as the starting materials to prepare Si-B-N-C ceramics or fiber.更多还原