【作者】 刘育红；

【导师】 井新利；

【作者基本信息】 西安交通大学 ， 化学工程， 2008， 博士

【摘要】 高性能热固性树脂的开发是近年来的一个研究热点。热固性树脂高度的交联结构赋予其许多优异的性能。但是较高的交联密度也使得热固性树脂的韧性较差。目前已经有许多增韧热固性树脂的方法,其难点在于如何保证在提高韧性的同时不损失其他的性能,如模量或玻璃化转变温度。在此,我们以酚醛树脂为研究对象,通过在酚醛树脂体系中引入一种新型结构的聚合物——超支化硼酸酯(HB),来提高酚醛树脂的耐热性能和韧性。超支化聚合物是一类高度支化具有三维椭球状立体构造的大分子。由于具有低粘度、高流变性、良好的溶解性以及大量的末端官能团等一系列独特的物理化学性质,自上世纪80年代以来,超支化聚合物受到了广泛的关注。目前关于超支化聚合物的研究还存在以下三个问题:(1)单体获得困难,难以大规模制备,限制了超支化聚合物的功能化研究和商业化应用;(2)超支化分子量分布较宽,结构可控性差;(3)超支化末端官能团的功能化研究尚处在起步阶段,其具有的优势还远远没有开发。根据上述研究背景,我们设计并制备了端基为酚羟基的超支化硼酸酯(HBp)和硼酸羟基的超支化硼酸酯(HBb)用以改性酚醛树脂。这类HB的单体容易获得,制备过程易于放大,同时HB还具有较高的耐热性能。在此基础上对HB的制备过程、结构性能以及HB改性酚醛树脂的固化过程和相态结构的研究取得以下创新性成果。1.通过A 2 +B3的方法,以商品化的单体间苯二酚和硼酸为原料,可以制备新型骨架的HB。研究发现,以FeCl 3为催化剂,采用分阶段加入带水剂的方法,即:在90℃采用苯为带水剂,在165℃采用二甲苯为带水剂,可以促进聚合反应的进行。同时,研究发现三阶段升温模式:90℃,165℃以及202℃下各反应8h,然后在220～230℃下熔融聚合2h,对抑制环化物的形成以及控制HB的分子量及其分布是有效的。HB具有良好的溶解性能,能够溶于常见的有机溶剂。HBp和HBb的Tg分别为232℃和221℃;T5 %所对应的温度分别为428℃和445℃,在800℃下的氮气氛中,它们的重量保持率分别为74.2%和71.3%,表现出较高的耐热性能。2.采用溶液共混法制备HB改性酚醛树脂。FTIR和DSC分析表明:HB与酚醛树脂具有良好的相容性。通过对HB改性酚醛树脂的固化过程的研究发现,HB的羟基能够对酚醛树脂的固化反应起到催化作用,使得酚醛树脂在较低的温度下聚合,但是HB的加入对酚醛树脂起到稀释作用,使得改性树脂的固化速率降低。在酚醛树脂中加入重量百分比为10%的HBp和HBb,酚醛树脂的T5 %分别提高38℃和59℃,而改性树脂在800℃的重量保持率分别提高了9.4%和11.1%。通过对HB改性酚醛树脂炭化物的组成以及结构的研究发现,改性酚醛树脂耐热性能的提高不仅仅归结于硼促石墨化程度的提高,而且还是因为产生了B4C。HB改性树脂的断面形貌分析表明,改性酚醛树脂主要是通过裂纹锚定、“瞬间原位大取向”和“多重裂延”的协调作用来实现增韧的。同时,HB改性酚醛树脂/碳布复合材料的断面研究发现,纤维和树脂之间形成了良好的界面结合,破坏不仅仅发生在界面,而且也有树脂基体身的破坏,破坏形式表现为界面和树脂两种破坏方式的综合作用。3.HB改性苯并噁嗪树脂的固化过程和固化机理的研究发现:HB中的酚羟基能够引发噁嗪环在较低的温度下开环,而开环反应产生的酚羟基促进了噁嗪环进一步的开环,使得改性苯并噁嗪树脂能够在较低的温度下开环固化,有利于降低能耗,而且固化过程表现为自催化机理。在苯并噁嗪树脂中加入5%的HB,固化产物的T5 %和T1 0%有所降低(约为15℃),这主要是由于树脂结构中酚-氧键的裂解导致的。而改性树脂在800℃下的重量保持率的提高是由于树脂交联密度的增加阻止了固化物中胺的分解挥发,同时改性树脂中的硼元素在热解过程中在炭化物表面形成隔热防护层,也对热解起到了抑制作用,这都赋予了固化物较高的耐热性能。改性苯并噁嗪树脂的力学性能研究发现,HB的加入不仅没有降低苯并噁嗪树脂的模量和玻璃化转变温度,而且有助于树脂韧性的提高。在苯并噁嗪树脂中加入HBb后,改性树脂从均相结构变为非均相结构。同时,HB的支化结构对裂纹起到了约束作用,有助于外力传递,从而避免外力对局部造成的破坏,树脂的韧性得到提高。4.通过“场”“流”分析法,建立“温度场”与“浓度流”的关系,可以获得改性树脂固化程度与时间的关系以及某时刻的固化速率常数。同时,研究发现,在等温固化条件下适宜的初始固化温度(160℃),而在非等温条件下三阶段升温固化模式(120℃/4h+160℃/2h+220℃/4h)对控制改性树脂的相态结构和提高改性树脂的韧性有较大的贡献。再次,通过对改性树脂增韧机理的研究发现:超支化聚合物的活性端基、核壳结构和众多的支链结构对改性苯并噁嗪树脂起到桥联约束和裂纹钉锚作用,这是超支化结构对改性树脂韧性提高的主要原因。更多还原

【Abstract】 High performance thermosets have been received abiding attention since the last century. Many of their favorable properties, including high glass transition temperature, high modulus are directly related to the underlying microstructure of high crosslinking density and rigid molecular network. However, this structure feature also results in an inherent brittleness of the materials. Different methods have been carried out in the past decades to increase the toughness of high performance thermosets, the difficulty lies in how to improve the toughness without sacrifice to such desired properties as modulus and glass transition temperature. Hyperbranched polymers offer a promising alternative for the toughening of high performance thermosets. Because of their highly branched molecular architecture, hyperbranched polymers have such unique physical and chemical properties as good rheology and solubility, lower solution and melt viscosity when compared to linear analogs. At presently, there are three main problems on the research of hyperbranched polymers: (1) The scarcity of the monomers made the bulk synthesis approach especially important from the view point of functional research and commercial applications. (2) Hyperbracnhed polymer has a broader polydispersity and a lower structural controllability in comparison with that of linear polymer. (3) The functionality of the end groups of hyperbranched polymer is in its infancy and there are may undiscovered superiorities. In this paper, we present such an approach to toughening phenolic resin with model hyperbranched polyborates(HB) that were synthesized with readily available monomers and have good thermal stability and scalability. Through characterization and study on the structure of HB and the curing process of blends of HB modified phenolic resins, we obtained the following results.1. Novel hyperbranched polyborates(HB) terminated with phenolic hydroxyl(HBp) and boric hydroxyl(HBb) functional end groups were successfully prepared via an A 2 +B3 method from the easily available reagents of 1,3-benzenediol and boric acid catalyzed with iron trichloride. The polymerization could be promoted by azeotropic distillation of water with benzene or xylene at 90℃and 165℃, respectively. It was found that the three stages synthesis process that is the polymerization was undertaking at 90℃, 165℃and 202℃for 8h, respectively was an ideal method both to decrease polydispersity and to suppress cyclization. HB have significantly higher glass transition temperatures(Tg> 220℃) and good solubility in a variety of common organic solvents. Thermogravimetric analysis exhibited the thermal stability of HB with T5 %of 428℃and 445℃and weight residues of 74.2% and 71.3% at 800℃in nitrogen atmosphere for HBp and HBb, respectively.2. HB modified phenolic resins were prepared by blending HB with phenolic resin in acetone solvent. Both Fourier transformed infrared spectrum and differential scanning calorimetry analysis revealed that the blends have good compatibility. It was found that HB modified phenolic resins have both a lower initial curing temperature and curing velocity in comparison with phenolic resins due to the dilute effect of HB. The T5 %s and the weight residues at 800℃under nitrogen atmosphere for HBp and HBb modified phenolic resins were improved 38℃, 59℃and 9.4%, 11.1%, respectively. The carbonization process strudy revealed that the improved thermal stability was due to the formation of B4C more than the boron acceleration of graphitization. Though it is difficult to accurately clarify and quantify the different states of boron atoms due to the lower boron content(0.4~0.8%), the presence of such a small amount of boron atom, different from that of physical mixing boron atoms, has great effect on enhancement of thermal stability and crystalline size of carbonized PR. The fracture surface study of HB modified phenolic resins revealed that the crack pining, the instantaneous in situ tropism and the multiple cracks are the main mechanisms that contributed to the improvement of the toughness. It was also found from the HB modified phenolic resins/carbon cloth composites that the fractures were the synergy of both the interfaces destroy and the resins destroy.3. The ring-opening temperature of polybenzoxazine was decreased because of the catalysis of phenolic hydroxyls of HB. Whereas the new phenolic hydroxyls produced by the ring-opening of benzoxazine promoted the ring-opening again. Though, the pyrolysis of phenoxy bonds made both T5 % and T1 0%of HB modified polbenzoxazine decreased to some extent, the weight residues of HB modified polybenzoxazines at 800℃under nitrogen atmosphere were improved in that the increased crosslinking density prevented the volatilization of amine of the cured product. On the other hand, boron atoms can form a protect coating on the surface of the carbonized materials, which can restrain the pyrolysis. The dynamic mechanical analysis revealed that the toughness was improved without the sacrifice of modulus and glass transition temperature. Morphologies of polybenzoxazine became two separated phases after adding HBb. The branched structures of HB not only can inhibit the propagation of the cracks but also can contribute to the transfer of stress, which jointly resulted an increased toughness by avoiding the local destroy.4. The“temperature field”and the“concentration flow”were built up by using the“field-flow”method. The obtained relation between both curing degree and velocity with time provide basis information for the control of phase structures. It was found that the initial curing temperature of 160℃was ideal for the enhancement of the toughness under isothermal curing conditions. While under anisothermal conditions, that’s the samples were cured under 120℃, 160℃and 220℃for 4h, respectively, the morphologies were benefit for the increase of toughness. Moreover, the reactive feature of end groups, the core-shell structure and a numerous branched chains of HB can bridge and pin the cracks which were main toughening mechanisms of the unique hyperbranched structure.更多还原