Fundamentals of Silicon Chemistry: Molecular States of Silicon-Containing Compounds

Silicon and its compounds have made possible the design of new materials, which, from computers to space travel, have helped to shape the technology of our 20th century. Conversely, the demands of new technology have stimulated the fast development of silicon chemistry as part of the “renaissance” of inorganic chemistry. This article uses selected examples of predominantly organosilicon compounds to discuss in simplified terms the measurement and assignment of suitable spectroscopic “molecular fingerprints” as well as the resulting benefit for the preparative chemist. The comparison of “equivalent” states of “chemically related” molecules is emphasized, based on perturbation arguments and supporting quantum-chemical models. Special attention is given to the relation between structure and energy, which allows us to understand and to predict the connectivity between and the spatial arrangement of silicon “building blocks”, the energy-dependent electron distribution over the effective nuclear potentials of a molecular framework, and, especially, the partly considerable effects of “silicon substituents” on molecular properties. Future-directed extensions and applications include polysilane band structures, Rydberg states of chromophores containing silicon centers, redox reactions and ion-pair formation of silicon-substituted π systems, and molecular dynamic phenomena in solution or on thermal fragmentation in the gas phase. The main objective is a set of clear and practical rules for interpreting measurements and planning experiments.     

Reactions of Elementary Silicon

Elementary silicon is normally treated “dry” with gaseous reactants in a fluidized bed. The apparent inertness of ordinary silicon is due to surface passivation. Non-passivated silicon reacts as a strongly electropositive element. Finely divided silicon is pyrophoric and forms surface compounds with many substances (e.g. alcohol, water, and metal or organic halides) even at room temperature. An alternative to the “dry” process is a new suspension process, in which finely divided activated silicon, preferably suspended in its liquid reaction products (possibly in the presence of the soluble catalysts) reacts at elevated pressures, generally with liquids. The reactions in suspension can be easily controlled, and al the silicon is consumed. Polysilanes often are the only initial products, and these react further in the homogeneous liquid phase to give monosilanes. For example, dissolution of ordinary silicon in methanol leads smoothly to methoxysilanes, which can add to unsaturated hydrocarbons.     

On the Theory of Phase Transitions in Magnetorheological Suspensions

The “condensation” of the particles of magnetorheological suspensions (MRSs) under the action of an external magnetic field is theoretically studied. The analysis demonstrated that the formation of rather long linear chain aggregates preceded the condensation of particles into the dense phase. The phase transition of the “gas-liquid” type proceeded in a particle ensemble via the “condensation” of such chains caused by their magnetic interaction. In thin MRS layers oriented perpendicular to the external magnetic field, the scenario of phase transition differs from that observed in systems with infinite volume. After the completion of the phase transition, the ensemble of dense domains discretely distributed in dilute dispersion is formed in thin layers rather than two simple connected phases with different particle concentrations, as it takes place in infinite media. Under fairly strong external magnetic fields, the transition of the “gas-solid” type occurs in a system of particles.     

Fumed silica synthesis. Influence of hydrogen chloride on the fumed silica particle formation process

Interaction of silicon dioxide molecule up to the small silica protoparticle and primary particle formation, and influence of hydrogen chloride (HCl), water (H₂O) and inert molecules (nitrogen N₂) “under flame condition” and “after cooling” on structure and properties of fumed silica were investigated by quantum-chemical simulations. An attempt to give definition of surface concept on atomic level was made.     

The effect of end-capping reagent on liquid chromatographic performance

Luminescence and photoacoustic spectroscopies are used to evaluate the nature of octadecylated silica surfaces as a function of trimethylsilyl “end-capping” reagent. These differences are compared to the chromatographic behavior of similar phases. It is concluded that end-capping with hexamethyldisilazane (HMDS) yields a phase with greater chemical heterogeneity, though lower average polarity, than phases end-capped with trimethylchlorosilane (TMCS). Some of these differences are attributed to the production of basic sites on the surface when HMDS is used. This property may make HMDS more suitable than TMCS as an end-capping reagent for phases used in the separation of basic solutes.     

Dependence of the Capacity Ratio Upon Type of the Excess Adsorption Isotherm

Abstract

Model calculations of the capacity ratio for binary mobile phase “1+2” and the excess adsorption isotherm of 1-st solvent have been made by assuming nonideality of mobile and surface phases and energetic heterogeneity of the adsorbent surface. These calculations make posible the study of correlation between shapes of the folloving functions: the capacity ratio and the excess adsorption isotherm as functions of the mobile phase composition.     

The effect of supramolecular structure on deformation and relaxation transitions in ethylene-1-hexene copolymers crystallized from solution

Structure and mechanical properties of three samples based on ethylene-1-hexene copolymers prepared by crystallization from decalin solutions were studied. The rate of decrease in the true yield stress was shown to depend on temperature. This behavior was assumed to be due to the involvement in deformation of the “intermediate phase” that is located at the lamellar surface between the crystalline and amorphous phases.     

Studies of silicon nanocluster ligand coating by solid-state NMR

Silicon nanoclusters were studied by ²⁹Si and ¹³C MAS NMR (magic angle spinning) spectroscopy. We for the first time confirmed the cleavage of ordinary ether C—O bonds of the solvent in the process of the synthesis of nanoclusters and the “binding” of the decomposition products to the surface of silicon nanoparticles as ligands. The applicability of MAS NMR spectroscopy in the studies of silicon nanocluster ligand coating and in the determination of the processes leading to the formation of the nanoparticle ligand shell was demonstrated.     

Strongly stretched polymer brushes

Polymer “brushes” are formed when long-chain molecules are somehow attached by one end at an interface with a relatively small area per chain. Such adsorbed brushes in the presence of solvent may be used to modify surface properties, stabilize colloidal particles, etc. Strongly segregated block copolymer phases, or interfacial layers of such “polymeric surfactants” may also be modeled in terms of “melt brushes,” (i.e., brushes without solvent). In both cases, when chain attachments are crowded on the interface, the chains stretch out to avoid neighboring chains. The resulting physical state has properties markedly different from polymer solutions, gels, or weakly adsorbed polymer layers. When the chains are strongly stretched, their statistical mechanics become simpler, as fluctuations around the set of most probable conformations are suppressed. This makes possible many pencil-and-paper calculations of brush properties, including bending and compressional moduli, and detailed knowledge of the chain conformations. As a recent example, I will describe calculations of phase diagrams of strongly segregated block copolymers including bicontinuous double-diamond phases. © 1994 John Wiley & Sons, Inc.     

Dependence of growth of the phases of multiphase binary systems on the diffusion parameters

A mathematical model of the diffusion interaction of a binary system with several phases on the equilibrium phase diagram is presented. The theoretical and calculated dependences of the layer thickness of each phase in the multiphase diffusion zone on the isothermal annealing time and the ratio of the diffusion parameters in the neighboring phases with an unlimited supply of both components were constructed. The phase formation and growth in the diffusion zone during “reactive” diffusion corresponds to the equilibrium state diagram for two components, and the order of their appearance in the diffusion zone depends only on the ratio of the diffusion parameters in the phases themselves and on the duration of the incubation periods. The dependence of phase appearance on the incubation periods, annealing time, and difference in the movement rates of the components across the interface boundaries was obtained. An example of the application of the model for processing the experimental data on phase growth in a two-component three-phase system was given.     

New phase supports for partition chromatography of biopolymers

Liquid-liquid partition chromatography of bio-polymers requires aqueous two-phase systems for reasons of sample solubility and stability. Such aqueous two-phase systems form when thermodynamically incompatible polymers are co-dissolved in water. The most common polymer combination providing a two-phase system at reasonably low polymer concentrations is the combination of poly(ethylene glycol) (“PEG”) and dextran (“DX”), detected and introduced for the separation of biopolymers and cells by Albertsson about 30 years ago. The application of this powerful system for liquid-liquid partition chromatography requires support materials with surfaces able to immobilize selectively one of the two aqueous phases. This phase immobilisation may be achieved by exploiting incompatibilities between the polymers dominating in the phases and the hydrated support surface. Examples involving diol-modified silica and polyacrylamide coated diol-silica as support materials in aqueous PEG-DX and PEG-salt systems are presented. The application of such systems for the separation of biopolymers is demonstrated.     

Preparation and characterization of ultrathin dual-layer ionic liquid lubrication film assembled on silica surfaces

A novel ultrathin dual-layer film, which contained both bonded and mobile phases in ionic liquids (ILs) layer, was fabricated successfully on a silicon substrate modified by a self-assembled monolayer (SAM). The formation and surface properties of the films were analyzed using ellipsometer, water contact angle meter, attenuated total reflectance Fourier transform infrared spectroscopy, multi-functional X-ray photoelectron spectroscopy, and atomic force microscope. Meanwhile, the adhesive and nanotribological behaviors of the films were evaluated by a homemade colloidal probe. A ball-on-plate tribometer was used to evaluate the microtribological performances of the films. Compared with the single-layer ILs film deposited directly on the silicon surface, the as-prepared dual-layer film shows the improved tribological properties, which is attributed to the special chemical structure and outstanding physical properties of the dual-layer film, i.e., the strong adhesion between bonded phase of ILs and silicon substrate via the chemical bonding with SAM, the interlinked hydrogen bonds among the molecules, and two-phase structure composed of steady bonded phase with load-carrying capacity and flowable mobile phase with self-replenishment property.     

“Grafting through” polymerization involving surface-bound monomers

“Grafting through” polymerization represents copolymerization of free monomers in solution and polymerizable units bound to a substrate. Free polymer chains are formed initially in solution and can incorporate the surface-bound monomers, and thereby, get covalently bonded to the surface during the polymerization process. As more growing chains attach to the surface-bound monomers, an immobilized polymer layer is formed on the surface. We use a combination of computer simulation and experiments to comprehend this process for monomers bound to a flat impenetrable substrate. We concentrate specifically on addressing the effect of spatial density of the surface-bound monomers on the formation of the surface-attached polymers. We employ a lattice-based Monte Carlo model utilizing the bond fluctuation model scheme to provide molecular-level insight into the grafting process. For experimental validation, we create gradients of density of bound methacrylate units on flat silicon wafers using organosilane chemistry and carry out “grafting through” free radical polymerization initiated in bulk. We report that the proximity of the surface-bound polymerizable units promotes the “grafting through” process but prevents more free growing chains to “graft through'' the polymerizable units. The “grafting through” process is self-limiting in nature and does not affect the overall density of the surface-bound polymer layer, except in case of the highest theoretical packing density of surface-bound monomers. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 263–274     

Limonene in Arizona liquid systems used in countercurrent chromatography. II Polarity and stationary-phase retention

The previous article in this series described the physico-chemical properties and chemical compositions of the two phases of the limonene–ethyl acetate–ethanol–water biphasic liquid system. This system was designed to be a “green” version of the so-called Arizona (AZ) scale of heptane–ethyl acetate–methanol compositions in which the heptane–ethyl acetate volume ratio is exactly the same as the methanol–water ratio. The first major difference between the standard and “green” AZ systems is the difference in upper and lower phase densities. The higher density of limonene compared with heptane greatly reduces the density difference of the “green” system: half the compositions have a density difference lower than 0.06 g mL^?1, precluding their use in hydrodynamic CCC columns. The other major difference is the phase polarity. The better distribution of ethanol between the upper organic and lower aqueous phases of the “green” AZ scale renders them more polar than their counterparts in standard heptane-based compositions. The test solutes aspirin and coumarin have higher distribution constants in the “green” AZ compositions. It is revealed that a hydrostatic column is suitable for use with all “green” compositions, with very good phase retention and limited driving pressure at high flow rates. A hydrodynamic column only functioned at limited flow rates with polar compositions of sufficient phase-density difference. The CCC chromatograms obtained with different compositions and columns are shown, and their peak position and sharpness discussed.     

New polymer systems exhibiting microphase separation transition with the formation of nano-microstructures

The concept of microphase separation was up to now widely applied mainly to the conformational transitions in block-copolymer solutions and melts. However, recently it became obvious that this concept has a much more general meaning. It was shown that microphase separation transition can be observed in random copolymers, interpenetrating polymer networks, polyelectrolyte mixtures, poor solvent polyelectrolyte solutions, ionomer solutions and melts, polymer blends and solutions with nonlocal entropy of mixing. In all these examples the emerging microdomain structures correspond to the nanometer scale, therefore the study of these effects can lead to the new ways of obtaining polymer materials with controlled nano-microstructure. In this presentation the review of our recent findings on microphase separation in some of the above-mentioned systems will be presented. 1. The problem of microphase separation in the systems containing weakly charged polyelectrolytes (polyelectrolyte mixtures and poor solvent polyelectrolyte solutions) will be considered. From the methodic point of view, it will be shown that this problem can be solved by direct minimization of the free energy, without the use of “weak segregation” or “strong segregation” assumptions which are common in the theory of block-copolymers. The final phase diagrams exhibit wide macroscopic phase separation regions, which is their main difference from the corresponding phase diagrams for block-copolymer systems. The formation of microdomains is thus coupled with macroscopic phase separation: in most of the cases microdomain structure is formed in one of the coexisting phases after macroscopic phase separation takes place [1] - [2]. 2. The formation of the multiplet structure in ionomer melts and solutions can be also considered as the microphase separation in the random copolymer system with the formation of the “micelles” (or clusters) of ionic links. The parallels with micelle formation in block-copolymer systems can be established if one considers a new “superstrong segregation regime” for block-copolymer microstructures. This regime can be indeed observed for diblock copolymers with one ionomeric and one neutral block [3]. 3. The microphase separation transition in ordinary polymer blends and solutions is also possible. The conditions for this effect are: (i) significant entropic contribution to polymer/polymer or polymer/solvent miscibility, (ii) the nonlocal character of this contribution with a high value of the nonlocality radius. It is argued that one can expect that the entropy nonlocality radius increases in the vicinity of the glass transition for the blend or polymer solutions (in the latter case solvent molecules act like “poor solvent plasticisers”). Computer simulation data supporting the theoretical prediction of microphase separation transition in these systems will be presented [4] - [5].     

All‐Nanoporous Hybrid Membranes: Redefining Upper Limits on Molecular Separation Properties

New membrane‐based molecular separation processes are an essential part of the strategy for sustainable chemical production. A large literature on “hybrid” or “mixed‐matrix” membranes exists, in which nanoparticles of a higher‐performance porous material are dispersed in a polymeric matrix to boost performance. We demonstrate that the hybrid membrane concept can be redefined to achieve much higher performance if the membrane matrix and the dispersed phase are both nanoporous crystalline materials, with no polymeric phase. As the first example of such a system, we find that surface‐treated nanoparticles of the zeolite MFI can be incorporated in situ during growth of a polycrystalline membrane of the MOF ZIF‐8. The resulting all‐nanoporous hybrid membrane shows propylene/propane separation characteristics that exceed known upper‐bound performance limits defined for polymers, nanoporous materials, and polymer‐based hybrid membranes. This serves as a starting point for a new generation of chemical separation membranes containing interconnected nanoporous crystalline phases.     

Synthesis and characterization of oligo(oxyethylene)/carbosilane copolymers for thermoset elastomers via ADMET

Acyclic diene metathesis (ADMET) polymerization has been used to synthesize latent reactive processable elastomers constructed of carbosilane and polyether segments. Two types of latent modes have been introduced: “chain‐internal” and “chain‐end” sites through the use of labile silicon methoxy functionalities. These latent reactive groups are inert when exposed to metathesis conditions allowing formation of the linear copolymer; subsequently exposure to moisture triggers hydrolysis of the methoxy groups and formation of a chemically crosslinked thermoset. The thermoset's mechanical response can be potentially varied from plastic to elastic behavior, depending on the ratio of carbosilane and oligooxyethylene monomers employed. Different lengths of glycols and numbers of methylene groups between them in the polymer backbone have been investigated to explore structure/property relationship. Polymers composed of oligooxyethylenes with eight methylene groups in between them exhibited fully amorphous character, while the ones with up to 20 methylene groups between glycol units showed their semicrystalline nature. The concentration of “chain‐internal” and “chain‐end” crosslink sites enhances strength; modification to the run length and structure of the soft phase enhances elasticity. Resultant materials have been subjected to mechanical tests using Instron; generated stress/strain curves have shown plastic and elastic behavior. Depending on the composition obtained samples have shown moduli from 0.3 to 115 MPa, tensile strengths from 0.6 to 10 MPa and elongations from 20 to 700%. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3992–4011, 2008     

表面光电压测量中的最佳相位

对基于锁相放大技术的表面光电压测量中的“最佳相位"的物理意义及影响因素进行了系统的理论分析及实验验证.结果表明,对于通常所采用的金属-绝缘体-半导体（MIS）“三明治"测量结构,其等效阻抗会很大程度地影响到最佳相位.对于体相材料,“最佳相位"仅与测量条件有关;对于纳米材料等体系,光照可以改变MIS结构的等效电阻(Rins)、等效电容(Cins),从而将引起最佳相位角的显著变化,使“最佳相位”与所研究材料的特性直接关联起来.因此,该手段有可能成为一种新的纳米材料光电检测方法.     

Synthesis and property of shish-kebab type liquid crystalline polymers with chiral carbons

New polymers with chiral carbons and with rod-like mesogenic units being stringed at waist (as the “kebabs”) by the main-chain (as the “shish” or “skewer”) were synthesized and studied. All the chiral polymers are optically highly active and have strong tendency of nematic phase formation.