酚醛树脂和羟甲基化蜜胺在碱性水溶液中在85℃水浴中反应5d经溶胶-凝胶过程形成了水凝胶.水凝胶经丙酮置换产生酮凝胶,而后经超临界二氧化碳干燥生成有机气凝胶.有机气凝胶在氮气氛中800℃下裂解3h形成了炭气凝胶.采用小角X-射线散射技术和散射理论对上述四种凝胶的微结构进行了分析.结果发现:水凝胶粒子是单分散的,被水溶胀并在0.15nm处有强的散射峰,表明具有纳米尺度的空间周期结构,这种结构是通过不稳态纳米尺度相变--旋节微相分离产生的.由丙酮置换产生的酮凝胶也是单分散的,被丙酮稍微溶胀并在0.15nm具有肩峰,表明水凝胶在溶剂置换过程中发生轻微团聚产生的大粒子在较小角处发生强烈的散射致使强峰演化成肩峰.有机气凝胶和炭气凝胶都是多分散的,在小角处没有散射峰,表明在超临界干燥过程中粒子的团聚继续发生导致多分散性并掩盖了纳米尺度的空间周期结构致使散射峰消失.有机气凝胶中存在的扩散界面层在其裂解过程中消失,这可能是由于在裂解过程中扩散界面层向粒子迁移以降低界面能所致.从小角X-射线散射数据用Shull-Roess法得到的有机气凝胶和炭气凝胶的回转半径分布比较可知,在裂解过程中,粒子的团聚长大、质量损失和致密化共同作用的结果使得炭气凝胶的回转半径分布比有机气凝胶稍宽,最可几回转半径稍微减小.
Sol-gel polymerization of phenolic resole and methylolated melamine in basic aqueous solution at 85℃ for 5 days led to the formation of hydrogels, which, after water in the hydrogels was exchanged with acetone, were dried under supercritical carbon dioxide to produce organic aerogels. Carbon aerogels were obtained by pyrolyzing these organic aerogels at 850℃ for 3h in nitrogen. Microstructure evolution during the preparation was followed by small angle X-ray scattering. It is found that hydrogel is monodispersed, swelled by water, and has a sharp peak in the scattering intensity profile, indicating a spatial periodical nanostructure formed by nanoscale phase separation via spinodal decomposition. After solvent exchanging, the acetone gel thus obtained is also monodispersed, but is partly de-swelled, and has a shoulder-like scattering peak. This is a result of a substantial increase in scattering intensity near the lower limit of the scattering vector, q caused by domain growth of the scattering entity. After supercritical carbon dioxide drying, the organic aerogels produced are polydispersed and de-swelled completely by removal of the solvent. The peak in the scattering intensity profile disappears completely by further growth of domains as a result of the structural relaxation from an unstable to a more stable state, induced by an increase in instability of the gel formed by supercritical drying. Moreover, a diffuse interface layer characterized by negative deviation from the Porod law is found in organic aerogels. After pyrolysis, the polydispersion is retained in the carbon aerogel, however, the diffuse interface layer disappears due to migration of the layer to particles to reduce the interface energy. Also in this process, joint actions of further growth in domain size and volume reduction by mass loss and densification slightly broaden the distribution of the radius of gyration of scattering entities, and shift the most probable peak radius of gyration to the small size direction.
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