Deng Jianjun, Hottle John R, Polidan Joseph T, Kim Hyong-Jun, Farmer-Creely Catherine E, Viers Brent D, Esker Alan R
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA.
Langmuir. 2004 Jan 6;20(1):109-15. doi: 10.1021/la035240h.
A trisilanol derivative of polyhedral oligomeric silsesquioxanes (POSS), trisilanolisobutyl-POSS, has recently been reported to form stable monolayers at the air/water interface. Moreover, the trisilanolisobutyl-POSS monolayer undergoes a nonequilibrium structural transition (collapse) around a surface pressure of Rho approximately 18 mN.m(-1). This paper explores the mono- and multilayer properties of POSS molecules at the air/water interface by the Wilhelmy plate technique and Brewster angle microscopy. Surface concentrations are controlled by four mechanisms: (1) compression at a constant rate, (2) stepwise compression followed by surface pressure relaxation to an "equilibrium" value, (3) successive additions of spreading solution followed by relaxation to a stable surface pressure value, and (4) hysteresis loops to test the reversibility of the structural transitions. Results show that both an increasing compression rate and a decreasing temperature lead to an increase in the surface pressure of the structural transition, which is consistent with the formation of solidlike multilayer domains during the collapse process. For the case of compression at a constant rate, small domains initially form and later aggregate to form large solid masses. Cessation of compression allows these large solid masses to relax into equilibrium ringlike structures with a lower surface pressure, Rho approximately 13 mN.m(-1). In contrast, if the film is expanded rapidly, these large solidlike domains relax into "spaghetti" like networks with a residual surface pressure that depends on the initial amount of the solidlike collapsed phase. Finally, successive addition and stepwise compression isotherm experiments lead to different and time-dependent morphologies. Understanding these surface properties of POSS molecules affords an excellent opportunity to design and study POSS/polymer blends for coating applications where POSS molecules with rigid inorganic cores, soft organic coronae, and dimensions comparable to polymeric monolayers can serve as perfectly monodisperse nanofillers.
最近有报道称，多面体低聚倍半硅氧烷（POSS）的三硅醇衍生物——三硅醇异丁基-POSS，能在空气/水界面形成稳定的单分子层。此外，三硅醇异丁基-POSS单分子层在表面压力约为18 mN·m⁻¹时会发生非平衡结构转变（塌陷）。本文通过威尔海姆平板技术和布鲁斯特角显微镜研究了POSS分子在空气/水界面的单分子层和多分子层性质。表面浓度由四种机制控制：（1）以恒定速率压缩；（2）逐步压缩，随后将表面压力松弛至“平衡”值；（3）连续添加铺展溶液，随后松弛至稳定的表面压力值；（4）通过滞后回线测试结构转变的可逆性。结果表明，压缩速率的增加和温度的降低都会导致结构转变表面压力的增加，这与塌陷过程中形成类固态多分子层区域一致。对于以恒定速率压缩的情况，最初会形成小区域，随后聚集形成大的固体团块。停止压缩会使这些大的固体团块松弛成具有较低表面压力（约13 mN·m⁻¹）的平衡环状结构。相反，如果薄膜快速膨胀，这些大的类固态区域会松弛成“意大利面条”状网络，其残余表面压力取决于类固态塌陷相的初始量。最后，连续添加和逐步压缩等温线实验会导致不同的、随时间变化的形态。了解POSS分子的这些表面性质为设计和研究用于涂层应用的POSS/聚合物共混物提供了绝佳机会，其中具有刚性无机核心、柔软有机冠层且尺寸与聚合物单分子层相当的POSS分子可作为完美的单分散纳米填料。
