Department of Chemistry and Biochemistry, Seton Hall University , South Orange, New Jersey 07079, United States.
Langmuir. 2014 Dec 16;30(49):14797-807. doi: 10.1021/la5031763. Epub 2014 Dec 5.
Methyl-terminated poly(dimethylsiloxanes) (PDMSs) are typically considered to be inert and not suitable for surface functionalization reactions because of the absence of readily hydrolyzable groups. Nevertheless, these siloxanes do react with silica and other oxides, producing chemically grafted organic surfaces. Known since the 1970s and then forgotten and recently rediscovered, this reaction provides a versatile yet simple method for the covalent functionalization of inorganic surfaces. In this work, we have explored the reactions of linear methyl-terminated and cyclic PDMS and bis-fluoroalkyl disiloxanes for the surface functionalization of mesoporous silica (Dpore ≈ 30-35 nm). The optimal reaction conditions included 24 h of contact of neat siloxane liquids and silica at 120-250 °C (depending on the siloxane). A study of the reactions of silicas with different extents of hydration demonstrated the critical role of water in facilitating the grafting of the siloxanes. The proposed reaction mechanism involved the hydrolysis of the adsorbed siloxanes by the Lewis acidic centers (presumably formed by water adsorbed onto surface defects) followed by the coupling of silanols to the surface to produce grafted siloxanes. For rigorously dehydrated silicas (calcination ∼1000 °C), an alternative pathway that did not require water and involved the reaction of the siloxanes with the strained siloxane rings was also plausible. According to FTIR and chemical analysis, the reactions of bis-fluoroalkyl disiloxanes and cyclic PDMS (D3-D5) produced covalently-attached monolayer surfaces, and the reactions of high-MM methyl-terminated PDMS produced polymeric grafted silicas with a PDMS mass content of up to 50%. As evidenced by the high contact angles of ∼130°/100° (adv/rec) and the negligible amount of water adsorption over the entire range of relative pressures, including saturation (p/p0 → 1), the siloxane-grafted porous silicas show uniform, high-quality hydrophobic surfaces. An overall comparison of siloxanes with classical silane coupling agents (i.e., silanes with readily hydrolyzable functionalities such as chloro, amino, etc.) demonstrated that the reactions of siloxanes produced surfaces of similar quality and, although requiring higher temperatures, used noncorrosive, less hazardous reagents, thereby providing an environmentally benign alternative to the chemical functionalization of metal oxide surfaces.
甲基封端的聚二甲基硅氧烷（PDMS）通常被认为是惰性的，不适合表面功能化反应，因为它们缺乏易水解的基团。然而，这些硅氧烷确实会与二氧化硅和其他氧化物反应，生成化学接枝的有机表面。这种反应自 20 世纪 70 年代以来就已经为人所知，但后来被遗忘，最近又被重新发现，它为无机表面的共价功能化提供了一种通用而简单的方法。在这项工作中，我们研究了线性甲基封端和环状 PDMS 以及双氟烷基二硅氧烷在介孔二氧化硅（Dpore≈30-35nm）表面功能化中的反应。最佳反应条件包括在 120-250°C（取决于硅氧烷）下将纯硅氧烷液体与二氧化硅接触 24 小时。对具有不同水合程度的二氧化硅的反应研究表明，水在促进硅氧烷接枝方面起着关键作用。提出的反应机制涉及路易斯酸性中心（可能是由吸附在表面缺陷上的水形成的）吸附的硅氧烷的水解，然后硅醇与表面偶联生成接枝硅氧烷。对于严格脱水的二氧化硅（煅烧约 1000°C），也存在一种不需要水的替代途径，涉及硅氧烷与应变硅氧烷环的反应。根据傅里叶变换红外光谱（FTIR）和化学分析，双氟烷基二硅氧烷和环状 PDMS（D3-D5）的反应生成了共价键合的单层表面，而高 MM 甲基封端 PDMS 的反应生成了具有高达 50% PDMS 质量含量的聚合物接枝硅。高接触角（adv/rec≈130°/100°）和整个相对压力范围内（包括饱和（p/p0→1））水吸附量可忽略不计，表明硅氧烷接枝多孔硅具有均匀、高质量的疏水性表面。硅氧烷与经典硅烷偶联剂（即具有易水解官能团的硅烷，如氯、氨基等）的总体比较表明，硅氧烷的反应生成了具有相似质量的表面，尽管需要更高的温度，但使用了腐蚀性较小、危害较小的试剂，从而为金属氧化物表面的化学功能化提供了一种环境友好的替代方法。
