三聚氰胺—甲醛（MF）气凝胶的制备及其改性的研究
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摘要
本文简要综述了三聚氰胺-甲醛(MF)有机气凝胶的基本特性、制备方法、表征手段和其国内外研究概况;通过系统实验,改进了MF气凝胶制备工艺,探索了具有激光惯性约束聚变(ICF)实验应用前景的改性及掺杂MF气凝胶的制备方法。主要工作及相应结果如下:
1)采用溶胶-凝胶工艺、溶剂置换和CO_2超临界流体干燥方法制备出性能优良、具有纳米网络结构的MF气凝胶;深入研究了反应物总浓度,凝胶温度变化对凝胶时间的影响,对凝胶过程中粒子及团簇的生长进行了分析,对凝胶化过程的理论分析进行了验证;通过在溶胶中加入碱的方法改进了MF气凝胶的制备工艺,缩短了凝胶时间,降低了凝胶密度;通过Materials-Studio上Dmol3以及半经验的Vamp模块进行模拟计算,得到升高温度可加快凝胶速度的结论,并选用二甲基亚砜(DMSO)代替水进行MF凝胶的制备,极大地缩短了凝胶时间。对不同方法制备的MF气凝胶进行了表征:FT-IR显示三种不同方法制备的MF气凝胶化学结构相差无几,仅是高温溶剂法所制备的气凝胶由于反应温度较高的影响,使得其红外图的C-H键伸缩振动峰较强。采用HRTEM,FESEM和BET法对所制备的气凝胶的多孔性进行了表征,其中常规法所制备的气凝胶骨架结构较一致,且孔径分布较均一,而加碱法和耐高温溶剂法制备的气凝胶效果次之。
2)通过在溶胶-凝胶过程中向MF溶胶体系中加入间苯二酚-甲醛预聚体的方法结合CO_2超临界流体干燥技术制备出具有纳米网络结构的酚改性MF气凝胶。结果表明此改性MF气凝胶,在凝胶化过程中所需时间较纯MF气凝胶短。用总反应浓度为5%的溶胶,经过溶胶-凝胶过程和超临界流体干燥过程,最终制得密度约为55mg/cm3的酚改性MF气凝胶样品。并通过改变不同的三聚氰胺/间苯二酚(M/R)值,制备出四种不同的改性MF气凝胶。其湿凝胶和气凝胶的光学照片均显示出随M/R值的增大,其颜色越紧接纯MF,透明性也降低。FT-IR法的分析结果表明改性气凝胶化学结构相差无几;采用HRTEM,FESEM和BET的分析结果表明随着M/R值的增加,气凝胶的骨架结构和孔的结构改变:气凝胶的骨架由纳米颗粒堆积成链球状的空间网络结构逐渐变为类似于MF气凝胶的纳米粒子堆积成树枝状分杈型结构,同时孔径也逐渐增大,SBET降低,孔容减少。改性MF气凝胶的TGA图谱分析则显示出,改性MF气凝胶的分解温度较MF气凝胶低,随着体系内三聚氰胺含量的增加,分解温度越紧接MF气凝胶。
3)采用在溶胶-凝胶过程中向MF溶胶体系中加入钛溶胶的方法结合CO2超临界流体干燥技术制备出具有纳米网络结构的TiO2掺杂MF气凝胶。在凝胶过程中,发现掺入MF体系内且最终可形成稳定凝胶的钛溶胶的量是一定,超过此量将容易使得溶胶内的钛溶胶粒子发生团聚、沉淀,得不到稳定的、分散均匀的掺钛MF凝胶。通过对SEM、TEM照片和BET的图谱分析,掺钛后的MF气凝胶仍具有多孔三维网络状骨架结构。从TEM照片中可以明显看出有晶体存在于骨架上,通过EDS分析,这些晶体就是我们掺入的钛元素形成的氧化物晶体,对TEM照片内的晶体面间距的测量结果显示晶体的面间距与锐钛矿型的(1 0 1)的面间距相差不大。对样品的XRD分析虽然得到与标准锐钛矿型的TiO2所一致的峰,但是由于其与MF体系的三嗪环的峰在相同的位置,所以并不能肯定此晶型就为锐钛矿型。对于MF体系内的掺入的氧化钛的晶型有待进一步分析和表征。
This thesis brief summarized the fundamental properties, preparation methods, characterization techniques and recent achievements of melamine-formaldehyde (MF) aerogels. Based on a series of experiments, the preparation process of MF aerogel was improved, and we also developed the preparation methods of phenol and metal oxide doped MF aerogels which have potential application in inertia constraint fusion (ICF) experiments. The investigation and corresponding results mainly includes:
MF aerogels with nano network structure were prepared by sol-gel process, solvent exchange and supercritical drying. The effect of reactant concentration and gelation temperature to gelation time has been analyzed. The development of particles or clusters derived from reactants was continuously observed to verify the sol-gel theory. The alkali was added to sol to improve the preparation process of MF aerogel. The results indicated that the gelation time of MF sol was shortened and the density of MF aerogel was decreased. The Dmol3 and semi-empirical Vamp modules of Materials-Studio software were employed to simulate the optimal preparation conditions. The simulation result revealed that the increase of temperature can accelerate the gelation reaction rate and the gelation time can be significantly shortened by the substitution of water with dimethyl sulfoxide (DMSO) as a solvent in gelation process. The results of Fourier transform infrared spectroscopy (FTIR) spectrums showed that there is no obvious difference in chemical structure of MF aerogels prepared by different methods, except the sample prepared in DMSO. Due to the effect of higher temperature, this sample presents a stronger stretching vibration peak of C-H bond. The porosities of the MF aerogels were determined by high resolution transmission electron microscopy (HRTEM), field emission scanning electron microscope (FESEM) and Brunmaner-Emmett-Teller (BET) methods. The results indicated that the aerogel prepared by traditional method was superior to samples prepared by other two methods in structure homogeneity.
Phenol modified MF aerogels with nano network structure were prepared by adding resorcin to MF sol system, followed by CO_2 supercritical drying process. The results indicated that phenol modified MF aerogels had shorter gelation time than that of pure MF aerogel. Phenol modified MF aerogels with the density of 55mg/cm~3 were obtained from the system with the total reactant concentration of 5%, and the effect of the mole ratio of melamine and resorcin (M/R) was investigated. It was found that the modified MF aerogel changes to opaque as M/R increased, while its apparent color was close to that of pure MF aerogel. The results of FT-IR spectrums showed that the modified aerogels with different mole ratio of M/R had the similar chemical structure. The results of HRTEM, FESEM and BET indicated that the network structure of the modified aerogels depended on the mole ratio of the M/R. As M/R increased, the network structure of modified MF aerogel gradually transformed to branching structure, which increases the pore size and reduces the BET surface area (SBET) and pore volume. The thermogravimeric analysis (TGA) suggested that the thermal stability of the phenol modified MF areogels was improved with M/R increasing.
Titanium dioxide (TiO_2) doped MF aerogels were prepared by introducing TiO_2 sol to MF sol. It was found that TiO_2 sol had a saturation capacity. Beyond the value, TiO_2 colloid particles were easy to agglomerate, which leads to the formation of unstable and non-uniform TiO_2-doped MF gel. The results of HRTEM, FESEM and BET indicated that TiO_2-doped MF aerogels had a nano three-dimensional network structure. From the HRTEM micrograph, it was obvious that there were tiny crystalline dispersed in MF aerogel framework, and its EDS spectra revealed that the crystalline was TiO_2. The XRD analysis results showed that titanium doped MF aerogel possessed the same diffraction angle of MF. Apparently, only the XRD result can not provide the tangible evidence to confirm the crystal structure of TiO_2, further testing experiment is necessary to characterize this TiO_2-doped MF system.
1)采用溶胶-凝胶工艺、溶剂置换和CO_2超临界流体干燥方法制备出性能优良、具有纳米网络结构的MF气凝胶;深入研究了反应物总浓度,凝胶温度变化对凝胶时间的影响,对凝胶过程中粒子及团簇的生长进行了分析,对凝胶化过程的理论分析进行了验证;通过在溶胶中加入碱的方法改进了MF气凝胶的制备工艺,缩短了凝胶时间,降低了凝胶密度;通过Materials-Studio上Dmol3以及半经验的Vamp模块进行模拟计算,得到升高温度可加快凝胶速度的结论,并选用二甲基亚砜(DMSO)代替水进行MF凝胶的制备,极大地缩短了凝胶时间。对不同方法制备的MF气凝胶进行了表征:FT-IR显示三种不同方法制备的MF气凝胶化学结构相差无几,仅是高温溶剂法所制备的气凝胶由于反应温度较高的影响,使得其红外图的C-H键伸缩振动峰较强。采用HRTEM,FESEM和BET法对所制备的气凝胶的多孔性进行了表征,其中常规法所制备的气凝胶骨架结构较一致,且孔径分布较均一,而加碱法和耐高温溶剂法制备的气凝胶效果次之。
2)通过在溶胶-凝胶过程中向MF溶胶体系中加入间苯二酚-甲醛预聚体的方法结合CO_2超临界流体干燥技术制备出具有纳米网络结构的酚改性MF气凝胶。结果表明此改性MF气凝胶,在凝胶化过程中所需时间较纯MF气凝胶短。用总反应浓度为5%的溶胶,经过溶胶-凝胶过程和超临界流体干燥过程,最终制得密度约为55mg/cm3的酚改性MF气凝胶样品。并通过改变不同的三聚氰胺/间苯二酚(M/R)值,制备出四种不同的改性MF气凝胶。其湿凝胶和气凝胶的光学照片均显示出随M/R值的增大,其颜色越紧接纯MF,透明性也降低。FT-IR法的分析结果表明改性气凝胶化学结构相差无几;采用HRTEM,FESEM和BET的分析结果表明随着M/R值的增加,气凝胶的骨架结构和孔的结构改变:气凝胶的骨架由纳米颗粒堆积成链球状的空间网络结构逐渐变为类似于MF气凝胶的纳米粒子堆积成树枝状分杈型结构,同时孔径也逐渐增大,SBET降低,孔容减少。改性MF气凝胶的TGA图谱分析则显示出,改性MF气凝胶的分解温度较MF气凝胶低,随着体系内三聚氰胺含量的增加,分解温度越紧接MF气凝胶。
3)采用在溶胶-凝胶过程中向MF溶胶体系中加入钛溶胶的方法结合CO2超临界流体干燥技术制备出具有纳米网络结构的TiO2掺杂MF气凝胶。在凝胶过程中,发现掺入MF体系内且最终可形成稳定凝胶的钛溶胶的量是一定,超过此量将容易使得溶胶内的钛溶胶粒子发生团聚、沉淀,得不到稳定的、分散均匀的掺钛MF凝胶。通过对SEM、TEM照片和BET的图谱分析,掺钛后的MF气凝胶仍具有多孔三维网络状骨架结构。从TEM照片中可以明显看出有晶体存在于骨架上,通过EDS分析,这些晶体就是我们掺入的钛元素形成的氧化物晶体,对TEM照片内的晶体面间距的测量结果显示晶体的面间距与锐钛矿型的(1 0 1)的面间距相差不大。对样品的XRD分析虽然得到与标准锐钛矿型的TiO2所一致的峰,但是由于其与MF体系的三嗪环的峰在相同的位置,所以并不能肯定此晶型就为锐钛矿型。对于MF体系内的掺入的氧化钛的晶型有待进一步分析和表征。
This thesis brief summarized the fundamental properties, preparation methods, characterization techniques and recent achievements of melamine-formaldehyde (MF) aerogels. Based on a series of experiments, the preparation process of MF aerogel was improved, and we also developed the preparation methods of phenol and metal oxide doped MF aerogels which have potential application in inertia constraint fusion (ICF) experiments. The investigation and corresponding results mainly includes:
MF aerogels with nano network structure were prepared by sol-gel process, solvent exchange and supercritical drying. The effect of reactant concentration and gelation temperature to gelation time has been analyzed. The development of particles or clusters derived from reactants was continuously observed to verify the sol-gel theory. The alkali was added to sol to improve the preparation process of MF aerogel. The results indicated that the gelation time of MF sol was shortened and the density of MF aerogel was decreased. The Dmol3 and semi-empirical Vamp modules of Materials-Studio software were employed to simulate the optimal preparation conditions. The simulation result revealed that the increase of temperature can accelerate the gelation reaction rate and the gelation time can be significantly shortened by the substitution of water with dimethyl sulfoxide (DMSO) as a solvent in gelation process. The results of Fourier transform infrared spectroscopy (FTIR) spectrums showed that there is no obvious difference in chemical structure of MF aerogels prepared by different methods, except the sample prepared in DMSO. Due to the effect of higher temperature, this sample presents a stronger stretching vibration peak of C-H bond. The porosities of the MF aerogels were determined by high resolution transmission electron microscopy (HRTEM), field emission scanning electron microscope (FESEM) and Brunmaner-Emmett-Teller (BET) methods. The results indicated that the aerogel prepared by traditional method was superior to samples prepared by other two methods in structure homogeneity.
Phenol modified MF aerogels with nano network structure were prepared by adding resorcin to MF sol system, followed by CO_2 supercritical drying process. The results indicated that phenol modified MF aerogels had shorter gelation time than that of pure MF aerogel. Phenol modified MF aerogels with the density of 55mg/cm~3 were obtained from the system with the total reactant concentration of 5%, and the effect of the mole ratio of melamine and resorcin (M/R) was investigated. It was found that the modified MF aerogel changes to opaque as M/R increased, while its apparent color was close to that of pure MF aerogel. The results of FT-IR spectrums showed that the modified aerogels with different mole ratio of M/R had the similar chemical structure. The results of HRTEM, FESEM and BET indicated that the network structure of the modified aerogels depended on the mole ratio of the M/R. As M/R increased, the network structure of modified MF aerogel gradually transformed to branching structure, which increases the pore size and reduces the BET surface area (SBET) and pore volume. The thermogravimeric analysis (TGA) suggested that the thermal stability of the phenol modified MF areogels was improved with M/R increasing.
Titanium dioxide (TiO_2) doped MF aerogels were prepared by introducing TiO_2 sol to MF sol. It was found that TiO_2 sol had a saturation capacity. Beyond the value, TiO_2 colloid particles were easy to agglomerate, which leads to the formation of unstable and non-uniform TiO_2-doped MF gel. The results of HRTEM, FESEM and BET indicated that TiO_2-doped MF aerogels had a nano three-dimensional network structure. From the HRTEM micrograph, it was obvious that there were tiny crystalline dispersed in MF aerogel framework, and its EDS spectra revealed that the crystalline was TiO_2. The XRD analysis results showed that titanium doped MF aerogel possessed the same diffraction angle of MF. Apparently, only the XRD result can not provide the tangible evidence to confirm the crystal structure of TiO_2, further testing experiment is necessary to characterize this TiO_2-doped MF system.
引文
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