Stabilizing Surfactant Templated Cylindrical Mesopores in Polymer and Carbon Films through Composite Formation with Silica Reinforcement

Abstract

A facile approach to maintain the periodic mesostructure of cylindrical pores in polymer-resin and carbon films after thermal template removal is explored through the reactive coassembly of resol (carbon precursor) and tetraethylorthosilicate (silica precursor) with triblock copolymer Pluronic F127. Without silica, a low porosity, disordered film is formed after pyrolysis despite the presence of an ordered mesostructure prior to template removal. However for silica concentration greater than 25 wt %, pyrolysis at 350 °C yields a mesoporous silica−polymer film with well-defined pore mesostructure. These films remain well ordered upon carbonization at 800 °C. In addition to the mesostructural stability, the addition of silica to the matrix impacts other morphological characteristics. For example, the average pore size and porosity of the films increase from 3.2 to 7.5 nm and 12 to 45%, respectively, as the concentration of silica in the wall matrix increases from 0 to 32 wt %. The improved thermal stability of the ordered mesostructure with the addition of silica to the matrix is attributed to the reinforcement of the mechanical properties leading to resistance to stress induced collapse of the mesostructure during template removal.

Roll-to-roll fabrication of high surface area mesoporous carbon with process-tunable pore texture for optimization of adsorption capacity of bulky organic dyes

“…The inclusion of silica in the material provides for mechanical reinforcement of the framework [25,32] and its removal after carbonization introduces a significant micropore population. The influence of the TEOS:resol (w/w) ratio (2, 2.5, and 3) on the final OMC mesostructure was investigated at a fixed mass ratio of ethanol/F127/resol of 5/1.6/1, which correspond to 18, 22 and 25 wt% TEOS in solution.…”

“…With tri-constituent self assembly, the condensation of silica oligomers drives self assembly during solvent evaporation [32], unlike the limited ordering that arises for resol-Pluronic alone [34][35][36]. Despite the initial development of the mesostructure on solvent evaporation, the crosslinking temperature impacts the ultimate ordered structure of the OMCs.…”

“…For OMC powders fabricated with Pluronic and resol, 120 °C has been reported as the optimal crosslinking temperature [38]. However, the silica in the framework will assist in stabilizing the structure [32], so a lower temperature may improve the ordering similar to a prior report using a two stage crosslinking process where initially 100 °C is used to promote the ordering, followed by a much higher temperature (150 °C) to crosslink the resol [37]. This competition between ordering and crosslinking likely provides the origins of the bestordered structure for OMC-2.5-5-100.…”

“…The extent of ordering impacts the mechanical properties [39], which in turn are critical to the extent of contraction of the pores during calcination. [40] These effects likely are responsible for the smaller domain spacing for OMC-2.5-5-140. Despite the large silica content, the crosslinking temperature for the resol is an important factor in determining the final structure of these OMCs.…”

Development of SiO2/C and SiC/C Composites Featuring Aerogel Structures

“…1 Formation of RF-SiO 2 aerogel and its transformation to SiC aerogel [12] 第 5 期 何 飞, 等: 具有气凝胶结构特征的 C/SiO 2 和 C/SiC 复合材料研究进展 451 基于上述考虑, 当采用正硅酸乙酯(tetraethoxysilane, TEOS)为先驱体水解得到 SiO 2 溶胶, 加入到事先配 制的 RF 溶胶中, 可以获得均匀的混合溶胶。 混合溶 胶经凝胶、干燥后获得 RF/SiO 2 气凝胶, 随后在惰性 气体保护下热解得到 C/SiO 2 气凝胶材料 [11][12][19][20] 。 TEOS 加入 RF 溶胶能明显缩短体系的凝胶时间, 但 是随着 TEOS 含量的逐渐增加, 气凝胶孔径逐渐减 小, 孔内毛细张力增大, 气凝胶易于开裂 [21] 。Chen 等 [22][23] 针对该混合溶胶体系, 采用一步酸催化快速 合成法(one-pot acid-catalyzed rapid synthesis route) 制备了 RF/SiO 2 气凝胶。在该方法中, HCl 取代 Na 2 CO 3 作为酸性催化剂, 可以提高芳香族链亲电 取代(electrophilic aromatic substitution)的活性, 从 而 加 速 芳 香 族 链 之 间 亚 甲 基 桥 联 的 形 成 ; 乙 腈 (CH 3 [22] 间 [24,27] 。在该溶胶体系中, APTES 除了作为硅源先 驱体参与水解缩聚反应形成 Si-O-Si 网络结构以外, 还可作为"内部催化剂(internal catalyst)"促进 R 和 F 之间的缩聚反应。这是由于 APTES 中氨基的水合 作用(hydration of amino groups)在 APTES 水解产物 中形成了环状的分子间氢键, 为反应提供了良好的 通路, 从而加速溶胶-凝胶过程 [27] 。RF/SiO 2 气凝胶 经高温烧结转变成 C/SiO 2 气凝胶后(高 APTES 含量 时复合材料的骨架主要由 SiO 2 构成; 低 APTES 含 量时骨架则主要由 C 构成 [15] ), 经 1500℃处理转变 为 SiC 气凝胶后, 在 650℃空气环境下具有良好的 抗氧化能力 [25] 。以 APTES 和 TEOS 作为混合硅源 先驱体合成的 RF/SiO 2 气凝胶, APTES 在促进溶胶-凝胶反应过程的同时, 还增大了样品的密度。经热 处理后, 由于 APTES 中氨丙基(aminopropyl)基团的 存在, 引起 RF/SiO 2 气凝胶有更多的质量损失和体 积收缩 [26] 。APTES 中的 Si-C 键加速了碳热还原反 应的进行, 导致 α-SiC 的形成 [26] 。除此以外, 若用 NaOH 溶液将 C/SiO 2 气凝胶中的 SiO 2 蚀刻去掉, 则 可获得高比表面积的碳气凝胶 [28] 。 当热解温度提高至 1200~1500℃, RF/SiO 2 气凝 胶或 C/SiO 2 气凝胶经碳热还原反应转变为 SiC 或 SiC/C 气凝胶, 总的反应方程如式(1)所示 [12,20] : SiO 2 (s)+3C(s)→SiC(s)+2CO(g) (1) 在这一过程中, SiC 首先在先驱体表面形核(nuclei), 随后长大成纳米晶须(nanowhiskers)。其间相互贯穿 孔的存在为气体产物(如 SiO、CO 和 CO 2 )的扩散提 供了通路, 加速了这一反应的进行 [12] 。C 与 SiO 2 的 比例和 C/SiO 2 复合材料的致密度是影响 SiC 反应动 力学和最终 SiC 微观结构的重要因素。图 3 给出了 不同结构 SiC 的形成过程示意图 [28] [28] 。除此以外, 采用低温镁热反应 (low-temperature magnesiothermic reaction)可使 C/SiO 2 材料在 700℃的低温下转变成 SiC [23] 。通过 Fig. 3 Schematic illustration of SiC formation [28] 高硬度、好的抗热震能力、高热导和高稳定性、低 膨胀系数和大的带隙, 可用于增强相、催化剂、高 能和高频电子材料、光电材料、抗辐射材料、氢分 离膜载体和吸波器件等 [12] 。 采用共聚法制备 C/SiO 2 和 C/SiC 气凝胶, 除了 以 R 和 F 的聚合物为先驱体之外, 碳的来源还可能是 间苯三酚-F(Phloroglucinol-Formaldehyde, PF) [29] 、可 溶性酚醛树脂(resol) [30] 、苯酚-R-F(phenol-...…”