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Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Chemistry of Organofunctional Silanes and Applications in Construction Industry Franc Švegl AMANOVA d.o.o. Tehnološki park 18, SI-1000 Ljubljana, Slovenia E-mail: [email protected] Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Table of Contents Introduction Chemistry of Organofunctional Silanes – Silicon Esters Hydrolysis – Condensation POSS frameworks Applications in Construction Industry Silicon Silicon vs. Carbon Chemistry Silane coupling agents - general concept Silane coupling agents - thermoset composites Silane coupling agents – mineral fillers Hydrophobicity, hydrophilicity and silanes Silanes and corrosion Hybrid materials Aerogels Silanes and cement Conclusions Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Introduction - Silicon Silicon shows a rich variety of chemical properties and can be found from such bulk commodities as concrete, clays and ceramics, through more chemically modified systems such as soluble silicates, glasses, and glazes to recent industries based on silicone polymers and solid-state electronics devices. Concrete Ceramics Clays Soluble silicates Silicon Solid-state electronics devices Glasses Silicone polymers Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Introduction – Silicon preparation Group IV Preparation: - reduction of high-purity silica or quartz with carbon in an electric arc furnace (over 1900 °C): SiO2 + C → Si + CO2 metallurgical grade silicon at least 98% pure SiO2 + 2 C → Si + 2 CO Using this method, silicon carbide (SiC) may form – it can be eliminated by keeping concentration of SiO2 high: 2 SiC + SiO2 → 3 Si + 2 CO. Crystallization-Purification: -crystallization by Czochralski method, -float-zone silicon (FZ-Si), -dissolution of metallurgical silicon powder in an acid -zone melting, polycrystalline silicon -conversion to a silicon compound that can be more easily purified by distillation and then conversion of that compound back into pure silicon Si + 2 Cl2 → SiCl4 SiCl4 + 2 Zn 2 HSiCl3 or Si + 3 HCl → HSiCl3 + SiCl4 + H2 → 950 °CSi + 2 ZnCl2 (DuPont process – ultra pure silicon) 1150 °C → Si + 2 HCl + SiCl4 (Siemens process – high-purity silicon) 3 SiCl4 + Si + 2 H2 → 4 HSiCl3 face-centered cubic structure 4 HSiCl3 → 3 SiCl4 + SiH4 SiH4 → Si + 2 H2 (Renewable Energy Corporation fluidized bed technology using silane) Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Introduction – Silicon as raw material The direct synthesis of methylchlorosilanes, known as the Rochow reaction is the reaction of methyl chloride with elemental silicon in the presence of copper catalyst. hydrolitic reaction reduction The direct process is the most widely used basic technique in the silicone industry. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Introduction – Silicon vs. Carbon In their most stable state, silicon and carbon will both conveniently bond to four other atoms. Silicon-based chemicals exhibit significant physical and chemical differences compared to carbon-based analogous. Prominent differences in bond strength: silicon does not form double or -bonds as readily as carbon does; the -Si-Si- bond (230 kJ mol-1) is weak in comparison to the -C-C- bond (356 kJ mol-1), the -Si-O- bond (368 kJ mol-1) is stronger than the corresponding -C-O- σ-bond (336 kJ mol-1). In general, bonds with electronegative elements are stronger with silicon than with carbon. In particular, the silicon-fluorine bond is extremely strong (582 kJ mol-1). Bond strength Electronegativity C – C 347 kJ mol-1 Si – H 323 kJ mol-1 C – H 410 kJ mol-1 Si – C 326 kJ mol-1 (by Pauling) Si 1,90 H 2,20 C 2,55 Si – Cl 398 kJ/mol-1 C – Cl 330 kJ mol-1 Cl 3,16 Si – O 464 kJ mol-1 C – O 360 kJ mol-1 O 3,44 Si – F 582 kJ mol-1 C – F 488 kJ mol-1 F 3,98 Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Bond strength Si – Si 230 kJ mol-1 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Introduction – Silicon vs. Carbon nucleophile (F-, RO-, HO-, etc.) Silicon has a lower electronegativity (1.9) as carbon (2.5) or oxygen (3,4) and consequently, -C-Si- bonds and –Si-O- bonds are polarized, rendering the silicon open to attack by nucleophiles (F-, Cl-, RO-, HO-, …). Silicon is bigger atom as carbon therefore the bonds between silicon and other atoms are in general longer than the equivalent bonds between carbon and the corresponding atoms. The increased bond lengths between silicon and other atoms in comparison to the corresponding systems involving carbon enables hard nucleophiles (in particular F-) to react at sterically hindered silicon centers. partly polarized Bond length Si – C 1,89 Å C–C 1,54 Å In comparison, bonds of silicon to H are relatively weak (i.e. hydrides like triethylsilane (Et3SiH) are reducing agents since the Si–H bond is relatively weak (~323 kJ mol-1)). The carbon-silicon bond is strong enough (~326 kJ mol-1) for trialkyl silyl groups to survive a wide variety of synthetic transformations, but it is weak enough to be selectively cleaved when required using mild conditions. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Introduction - Silicon vs. Carbon Chemistry While carbon forms an extensive series of compounds with hydrogen, the list of silicon analogs is limited. Compounds between silicon and hydrogen are called hydrides and can be viewed as analogs of alkanes. The first silicon hydrides were made in 1857 by Friedrich Wöhler and Heinrich Buff who reacted Al/Si alloys with aqueous HCl. The compounds prepared were analysed by Charls Friedel and Albert Landenburg in 1867 shown to be SiH4 and SiHCl3. The first homologue, Si2H6, was prepared by Henri Moissan and Samuel Smiles in 1902 from the protolysis (HCl) of magnesium silicide (Mg2Si). The thermal instability and great chemical reactivity of the compounds precluded further advances until Alfred Stock developed his greaseless vacuum techniques and first began to study them as contaminants of his boron hydrides in 1916. He proposed the names silanes and boranes by analogy with alkanes. Silanes are branched or unbranced chains (up to n=8). All are colorless volatile liquids or gases at room temperature. They are extremely reactive and spontaneously ignite or explode in air. There is a steady increase in the boiling point with increasing molecular weight and chain length, and trisilane is the first member of the series that is a liquid at room temperature. Instability increases with increasing number of Si-Si bonds, and only gas forms of SiH4 (silane) and Si2H6 (disilane) are stable at room temperature. Silane general formula SinH2n+2 chemical stability increase SiH4 silane (g) H3Si-SiH3 disilane (g) H H3Si-Si-SiH3 trisilane (liq) H etc.. n=8 Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 b.p. increase Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Introduction - Silicon vs. Carbon Chemistry A silane that contains at least one carbon-silicon bond (e.g. CH3-Si-) is known as an organosilane or organofunctional silane. In general, the organofunctional group will act chemically the same in the organosilicon compound as it does in the carbon-based compound. For example, the distance of the amine, or other organofunctional group, from silicon will determine whether the silicon atom affects the chemistry of the organofunctional group. If the organic spacer group is e.g. a propylene linkage (-CH2CH2CH2-), then the organic reactivity in the organofunctional silane will be similar to organic analogs in carbon chemistry. silane silicon ester organosilane Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Introduction - Naming Silicon Compounds Silane SiH4 is the simplest hydride and provides the basis of nomenclature for all silicon chemistry. Compounds are named as derivatives of silane with substituents prefixe. Two or more substituents are listed alphabetically with substituted organic moieties being named first, followed by simple organic fragments. Alkoxy substituents are named next, followed by acyloxy, halogen and pseudohalogen groups, e.g. ethylmethylethoxysilane, C2H5(CH3)SiH(OC2H5), and (3-chloropropyl) methylchlorosilane, ClCH2CH2CH2SiH(CH3)Cl. Silanols Silazanes Cyclic silanes Silanes Chlorosilanes (silylamines) SiH4 H3Si-OH H3SiNHSiH2NHSiH3 silanol silane SiCl4 H3Si-SiH3 tetrachlorosilane disilane HSiCl3 silane diol H trichlorosilane Ph3Si-ONa H3Si-Si-SiH3 H trisilane etc.. n=8 trisilazane H2Si-(OH)2 sodium 1,1,3,3-tetramethyldisilazane triphenylsilanolate methyldichlorosilane CH3-SiHCl2 Silicon group as a unit H3Si- cyclohexasilane (R = H) Shorthand notation for the metylsiloxanes and polymethylsiloxanes silyl Siloxanes H H3Si-O-SiH3 -Si- disiloxane H Me3Si-O-SiMe3 H3SiO- siloxanyl hexamethyldisiloxane Me3Si- trimethylsilyl Me3SiO- silylene M Me -O-Si-O- Me O -O-Si-OD O -O-Si-OO Me Me3Si-O-SiMe3 T MM Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Q Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Chemistry of Organofunctional Silanes Silicon Esters – alkoxy derivatives of silanes Silicon esters are silicon compounds that contain an oxygen bridge from silicon to an organic group. Tetraalkoxysilanes Substituted trialkoxysilanes OR group: R’ group: methoxy -OCH3 (tetramethoxysilane - TMOS), ethoxy -OC2H5 (tetraethoxysilane -TEOS), or acetoxy –OOCCH3 (tetraacethoxysilane) -H, -Cl, -CH3, -C2H5, phenyl, or organofunctional group, such as amino, methacryloxy, epoxy, isocyanate, etc. Applications of tetraalkoxysilanes cover a broad range. These compounds are classified roughly to whether the Si-OR bond is expected to remain intact or to be hydrolyzed in the final application. Applications in which the Si-OR bond is hydrolyzed include: binders for foundry-mold sands used in investment and thin-shell castings, binders for refractories, resins, coatings, sol-gel glasses, crosslinking agents, and adhesion promoters. Applications in which Si-OR bond remains intact include heat transfer and hydraulic fluids. Substitution of one or more organic groups which can be either inert or reactive brings to the compound new functionality. Methyl- and phenyltrialkoxysilanes are primarily used in the production of silicone resins and coatings. Longer chain materials, eg. propyl-, isobutyl-, and octyltrialkoxysilanes, are used in hydrophobic coatings, primarily for masonry and concrete. Many organosilanes with reactive functional group are used as coupling agents. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: hydrolysis - esterification Hydrolysis Alkoxy silanes react readily with water by reaction called hydrolysis. The hydrolysis reaction, through the addition of water, replaces alkoxide group (OR) with hydroxyl group (OH-) which becomes attached to the silicon atom. Depending on the amount of water and catalysts present, hydrolysis may go to completion, so that all OR groups are replaced by OH, or stop while the silicon is only partially hydrolyzed. Esterification Because water and alkoxides are immiscible, a mutual solvent such as an alcohol is utilized. It should be emphasized, however, that the addition of solvents and certain reaction conditions may promote reverse reactions like esterification, in which an alcohol molecule displaces a hydroxyl group to produce an alkoxide ligand plus water as a by-product. R’-Si(OR)3 hydrolysis + nH2O R’-Si(OR)(3-n)(OH)n + nROH esterification Transesterifiaction Transesterification is a reaction in which an alcohol displaces an alkoxide group to produce an another alcohol molecule. It often occurs when alkoxides are hydrolyzed in alcohols containing different alkyl groups and in multicomponent systems that employ several alkoxides with differing alkoxide substituents. R’’OH + R’-Si(OR)3 → R’-Si(OR)2OR’’ + ROH Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: hydrolysis mechanisms Although hydrolysis can occur without addition of an external catalyst, it is most rapid and complete when they are employed. The hydrolysis reaction can be catalyzed by acids and bases (mineral acids (HCl) and ammonia are most generally used, however, other catalysts as acetic acid, KOH, amines, KF, and HF can be used). The rate and extent of the hydrolysis reaction is most influenced by the strength and concentration of the acid- or base catalyst. All strong acids and bases behave similarly, whereas weaker acids and bases require longer reaction times and higher concentration of catalyst to achieve the same extent of reaction. Acid-Catalyzed Mechanism Si – OR + H+ + Si – O-R H SN2 -Si R’ R’ R’ Under acidic conditions, it is likely that an alkoxide group is protonated in a rapid first step. Electron density is withdrawn from the silicon atom, making it more electrophilic and thus more susceptible to attack from water. This results in the formation of a penta-coordinate transition state with significant SN2-type character (bimolecular nucleophilic substitution). The water molecule attacks from the rear and acquires a partial positive charge. The positive charge of the protonated alkoxide is correspondingly reduced, making alcohol better leaving group. The transition state decays by displacement of an alcohol and inversion of the silicon tetrahedron. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: hydrolysis mechanisms Under basic conditions the hydroxyl anion attacks the silicon atom. Again, an SN2-type mechanism has been proposed in which the -OH displaces -OR with inversion of the silicon tetrahedron. The SN2** and SN2* mechanisms involve a stable 5-coordinated intermediate state, which decays through a second transition state in which any of the surrounding ligands can acquire a partial negative charge. Hydrolysis occurs only by displacement of an alkoxide anion, which may be aided by hydrogen bonding of the alkoxide anion with the solvent. Because the silicon acquires a formal negative charge in the transition state, SN2** or SN2*, mechanisms are quite sensitive to inductive as well as steric effects. Base-Catalyzed Mechanism OH- + R’-Si(OR)3 R’ R’ R’-Si(OR)2(OH) + RO- R’ Base-catalyzed hydrolysis of silicon alkoxides proceeds much more slowly than acid-catalyzed hydrolysis at an equivalent catalyst concentration. Basic alkoxide oxygens tend to repel the nucleophile, -OH. However, once an initial hydrolysis has occurred, following reactions proceed stepwise, with each subsequent alkoxide group more easily removed from the monomer then the previous one. Therefore, more highly hydrolyzed silicones are more prone to attack. Although hydrolysis in alkaline environments is slow, it still tends to be complete and irreversible. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: factors effecting hydrolysis Steric Effects Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds (Pauli or Born repulsion), and this may affect the molecule's preferred shape (conformation) and reactivity. Steric effects occur as: hindrance or steric resistance - the size of groups within a molecule prevents chemical reactions that are observed in related smaller molecules; steric shielding –a charged group on a molecule is seemingly weakened or spatially shielded by less charged (or oppositely charged) atoms, including counterions in solution (Debye shielding); steric attraction - molecules have shapes that are optimized for interaction with one another; chain crossing - a random coil molecule can't change from one conformation to a closely related shape by a small displacement; steric repulsions - different parts of molecular system repeal one another. Steric factors exert the greatest effect on the hydrolitic stability of organoxysilanes. Any complication of alkoxy group retards the hydrolysis of alkoxysialnes, but hydrolysis rate is lowered the most by branched alkoxy groups. The retarding effect of the bulkier ethoxide group is clearly evident for the acid hydrolysis of tetraalkoxysilanes. steric Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: factors effecting hydrolysis Inductive Effects The inductive effect is a transmission of charge through a chain of atoms in a molecule by electrostatic induction. The electron cloud in a σ-bond between two unlike atoms like Si and O or C is not uniform and is slightly displaced towards the more electronegative of the two atoms. This causes a permanent state of bond polarization, where the more electronegative atom has a slight negative charge (δ-) and the other atom has a slight positive charge (δ+). If the electronegative atom is then joined to a chain of atoms, the positive charge is relayed to the other atoms in the chain. This is the electron-withdrawing inductive effect, also known as the -I effect. The alkyl groups are less electron-withdrawing than hydrogen and are considered as electron-releasing. They exert an electron releasing character indicated by the +I. inductive Electron withdrawing substituents such as –OH or –OSi help to stabilize the negative charge on silicon, causing the hydrolysis rate to increase with the extent of OH substitution, whereas electronproviding substituents cause the hydrolysis rate to decrease. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: factors effecting hydrolysis Solvent effects Solvents are added to prevent liquid-liquid phase separation during the initial stages of the hydrolysis reaction and to control the concentrations of silicate and water that influence the hydrolysis and condensation kinetics. Solvents may be classified as polar or nonpolar and as protic (containing removable or labile proton) or aprotic. With regard to solvating power the following characteristics are important: • polarity, • dipole moment and • the availability of protons. Brinker and Scherer, Sol-Gel SCience, 1990 The polarity largely determines the solvating ability for polar or nonpolar species. More polar solvents (eg. water, alcohol or formamide) are normally used to solvate polar, silicate species and surfaces. Less polar solvents such as dioxane or tetrahydrofuran (THF) may be used in alkyl-substituted or incompletely hydrolyzed systems. Ether alcohols such as methoxyethanol or ethoxyethanol exhibit both polar and nonpolar character and are useful when a distribution of polar and nonpolar species is present in a solution. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: factors effecting hydrolysis Solvent effects The dipole moment, m, of a solvent determines the length over which the charge on the species can be felt by surrounding species. The lower the dipole moment, the larger this length becomes. The availability of labile protons determines whether anions or cations are solvated more strongly through hydrogen bonding. Because hydrolysis is catalyzed either by hydroxyl (pH > 7) or hydronium ions (pH < 7), solvent molecules that hydrogen bond to hydroxyl ions or hydronium ions reduce the catalytic activity under basic or acidic conditions, respectively. Therefore, aprotic solvents that do not form hydrogen bond to hydroxyl ions have the effect of making hydroxyl ions more nucleophilic, whereas protic solvents make hydronium ions more electrophilic. Hydrogen bonding may also influence the hydrolysis mechanisms. Hydrogen bonding of the solvent may facilitate inversion of silicon tetrahedron by activating poor leaving groups. The availability of labile protons also influences the extent of the reverse reactions, reesterification or siloxane bond alcoholysis or hydrolysis. Aprotic solvents do not participate in reverse reactions such as reesterification or hydrolysis, because they lack sufficiently electrophilic protons and are unable to be deprotonated to form sufficiently strong nucleophiles (eg. OH- or OR-) necessary for these reactions. Therefore compared to alcohol or water, aprotic solvents such as a THF or dioxane are considerably more inert. Reverse reactions Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: condensation Two partially hydrolyzed molecules can link together in a condensation reaction. Subsequent condensation reactions, involving the silanol groups (Si-OH), produce siloxane bonds (Si-O-Si) plus the by-products small molecules of water or alcohol. Under most conditions, condensation commences before hydrolysis is complete. alcohol condensation R’(OR)2Si-OR + HO-Si(OR)2R’ R’(OR)2Si-O-Si(OR)2R’ + ROH alcoholysis water condensation R’(OR)2Si-OH + HO-Si(OR)2R’ R’(OR)2Si-O-Si(OR)2R’ + H2O hydrolysis Typical sequence of condensation products is monomer, dimer, linear trimer, cyclic trimer, cyclic tetramer, and higher order rings. This sequence of condensation requires both depolymerization (ring opening) and the availability of monomers which are in solution equilibrium with the oligomeric species and/or are generated by depolymerization (reverse reactions of acoholysis and hydrolysis). Alcoholysis and hydrolysis of siloxane bonds provide a means for bond breakage and reformation allowing continual restructuring of the growing polymers. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: condensation mechanisms As with hydrolysis, condensation can proceed without catalyst, however, their use in organosiloxanes is highly helpful. Furthermore, the same type catalysts are employed: generally those compounds which exhibit acidic or basic characteristics. Base-catalyzed condensation The most widely accepted mechanism for the basic-catalyzed condensation reaction involves the attack of a nucleophilic deprotonated silanol on a neutral silicate species. This reaction pertains above the isoelectric point of silica (> pH 1,5 – 4,5, depending on the extent of condensation of the silicate species), where surface silanols may be deprotonated depending on their acidity. The acidity of silanols depends on other substituents on the silicon atom. When basic OR and OH are replaced with OSi, the reduced electron density on Si increases the acidity of the protons on the remaining silanos. This favors reactions between larger, more highly condensed species, which contain acidic silanols, and smaller, less weakly branched species. The condensation rate is maximized near neutral pH where significant concentrations of both protonated and deprotonated silanols exist. A minimum rate is observed near the isoelectric point. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: condensation mechanisms Acid-catalyzed condensation The acid-catalyzed condensation mechanism involves a protonated silanol species. Protonation of the silanol makes the silicon more electrophilic and thus more susceptible to nucleophilic attack. The most basic silanol species (silanols contained in monomers or weakly branched oligomers) are the most likely to be protonated. Therefore, condensation reactions may occur preferentially between neutral species and protonated silanos situated on monomer end groups of chains. The condensation of alkylsilanetriol involves protonated silanol. The proposed reaction mechanisms for both base- and acid-catalyzed condensation involve penta- or hexacoordianted transition states or intermediates. Because of this, the condensation reaction kinetics will be influenced by both steric and inductive factors. bimolecular intermediate hexacoordinated silicon atom bimolecular intermediate pentacoordinated silicon atom Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: factors affecting condensation Steric, inductive and solvent effects Steric effects Substituents attached to silicon that reduce steric crowding in the transition state or intermediate will enhance the condensation kinetics. Bulky groups attached to silicon or perhaps partial condensation of the silicon involved in the condensation reaction are expected to retard the condensation process. Inductive effects Replacement of more electron-providing OR groups with progressively more electron-withdrawing OH and OSi groups stabilizes the negative charge on the anionic nucleophile involved in the base-catalyzed condensation reaction and therefore should enhance the kinetics. Extensive hydrolysis and condensation should destabilize the positively charged intermediate or transition state involved in the acid-catalyzed condensation reaction and thus retard the condensation kinetics. In organosilanes organic substituents influence the acidity of silanols involved in condensation. Electronproviding alkyl groups reduce the acidity of the corresponding silanol. This should shift the isolelectric point toward higher pH values. Conversely, electron-withdrawing groups (OH or OSi) increase the silanol acidity, and the condensation rate for oligomeric species occurs at about pH 2. Solvent effects Depending on the pH, either protonated or deprotonated silanos are involved in the condensation mechanism. Because protic solvents hydrogen bond to nucleophilic deprotonated silanos and aprotic solvents hydrogen bond to electrophilic protonated silanols, protic solvents retard base-catalyzed condensation and promote acid-catalyzed condensation. Aprotic solvents have the reverse effect. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ TEOS: hydrolysis and condensation to silica Silicon dioxide never forms directly during hydrolysis. According to Iler, polymerization of monomer Si(OH)4 occurs in three stages: polymerization of monomer to form particles, Si(OH)4 growth of particles, linking of particles into chains, and then networks that extend throughout the liquid medium, thickening it into a gel. The condensation takes place in such a fashion as to maximize the number of Si-O-Si bonds and minimize the number of terminal hydroxyl groups through internal condensation. Thus rings are quickly formed to which monomers add, creating three-dimensional particles. These particles condense to the most compact state leaving OH groups on the outside. The three-dimensional particles serve as nuclei. Further growth occurs by an Ostwald ripening mechanism whereby particles grow in size and decrease in number as highly soluble small particles dissolve and reprecipitate on larger, less soluble nuclei. Growth stops when the difference in solubility between the smallest and largest particles becomes only a few ppm. Due to greater solubility, growth continues to larger sizes at higher temperatures, especially above pH 7. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ TEOS: hydrolysis and condensation to silica Iler divides the polymerization process into three approximate pH domains < pH 2, pH 2-7, and >pH 7. pH 2 appears as a boundary, since the point of zero charge (PZC), where the surface charge is zero, and the isoelectric point (IEP), where the electrical mobility of the silica particles is zero, both are in the range pH 1-3. pH 2 represents a metastability region where the observed gel times are quite long. Below pH 2, the polymerization rate is proportional to H+ conc. pH 7 appears as a boundary because both the silica solubility and dissolution rates are maximized at or above pH 7 and because the silica particles are appreciably ionized above pH 7 so that particle growth occurs without aggregation or gelation. Since the gel times decrease steady between pH 2 and pH 6. It is generally assumed that above the IEP the condensation rate is proportional to OH- conc. Above pH 7 all the condensed species are more likely to be ionized and therefore mutually repulsive, growth occurs primarily by the addition of monomers to more highly condensed particles rather than by particle aggregation. Due to the greater solubility of silica and the greater size-dependence of solubility above pH 7, growth of the primary particles continues by Ostwald ripening. Particles grow rapidly to a size that depends mainly on temperature. Higher temperatures produce larger particles due to greater silica solubility. In the absence of salt, no chaining or aggregation occur, because the particles are mutually repulsive. The addition of salt reduces the thickness of the double layer at given pH, dramatically reducing the gel times. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: condensed structures R‘ = CH3, C2H5, etc... R = -OR‘, aryl, alkyl, -SH, COOH, -SO3H, -NCO, -NR‘‘, methacryloyl, halogen, epoxy, styryl, etc... f=2 (SiO2)n silica f=4 Rn-Si(OR‘)4-n n = 1..4 Polyhedral Oligosilsesquioxanes (POSS) [R-SiO1.5]n 6 < n < 12 polyhedral oligomers Oligo- and polysiloxanes f=3 f=3 f=3 f=2 f=3 [R-SiOm]n n=∞ m > 1.5 insoluble gels f=3 [R-SiOm]n n<∞ m > 1.5 soluble amorphous polymers Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: sol-gel processing The complete hydrolysis of tetratalkoxysilanes under highly controlled conditions usually without the presence of fillers, is associated with sol-gel technology. Sol-gel is a method for preparing specialty metal oxide glasses and ceramics by hydrolyzing a chemical precursor or mixture of chemical precursors that pass sequentially through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel) state before being dehydrated to a glass or ceramic. The use of sol-gel processing has increased dramatically since 1980. A variety of techniques have been developed to prepare fibers, microspheres, thin films, fine nano powders, and special monoliths. Applications for this technology include protective coatings, catalysts, piezoelelctric devices, waveguides, lenses, high strength ceramics, superconductors, insulating materials, and nuclear waste encapsulation. The flexibility of sol-gel technology allows unique access to multicomponent oxide systems and low temperature process regimes. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Silicon Esters: other reactions The Si-OR bond undergoes a variety of reactions apart from hydrolysis and condensation. The reactivity of the Si-OR bond is in many cases analogous to the Si-Cl bond, except that the reactions are more sluggish. These reactions become increasingly more sluggish with greater bulk and steric screening of the alkoxy group. Interesting reactions of Si-OR group are as follows: Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Polyhedral Oligo- and Heterosilsesquioxanes Polyhedral oligosilsequioxanes (POSS) are three-dimensional structures derived from the hydrolytic condensation of trifunctional organosilicon monomers (RSiX3 e.g., RSiCl3 and RSi(OMe)3 ). These reactions can produce a wide variety of interesting products, ranging from small oligomers and discrete clusters to complex resins and T-gels. Fully-condensed silsesquioxanes have the idealized empirical formula [RSiO3/2]n, whereas incompletely-condensed silsesquioxanes possess reactive Si-OH groups which are potentially capable of forming additional Si-O-Si linkages via elimination of water. Many hydrolytic condensation reactions produce synthetically useful quantities of fully-condensed POSS frameworks containing 6, 8, 10 and/or 12 Si atoms. T6 T8 T10 Fully-condensed [RSiO3/2]n frameworks can form in both polar and non-polar solvents under both acidic and basic conditions. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 T12 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Polyhedral Oligo- and Heterosilsesquioxanes A related family of polyhedral Si/O clusters can be prepared via base-catalyzed equilibration of tetrafunctional silicon monomers, such as Si(OEt)4. These clusters are functionalized silicates (i.e., [(SiO2)n(SiO4)m]4m-) rather than silsesquioxanes (i.e., [RSiO3/2]), but they exhibit many similarities to fullycondensed POSS frameworks. For example, base-catalyzed equilibrium of Si(OMe)4, of Si(OEt)4, silicic acid, or SiO2 can be performed under conditions where the major Si-containing species in solutions is [Si8O20]8-. A wide variety of fully-condensed POSS and spherosilicate frameworks can be prepared from commercially available silane monomers. These easily synthesized frameworks have many potential uses and serve as the basic starting materials for more elaborate derivatives. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Hydridosilsesquiosanes Polyhedral Hydridosilsesquioxane cages Polyhedral hydridosilsesquioxanes like H8Si8O12, H10Si10O15 and several larger [HSiO3/2]n frameworks are of special interest because they are easy to synthesize in pure form and commercially available. They serve as precursors to functionalized silsesquioxanes and spherosilicates because of the ease with which Si-H bonds can be synthetically manipulated. The oxidation of H8Si8O12 and other hydridosilsesquioxanes can be accomplished with variety of reagents. Most of these reactions appear to involve initial attack of nucleophile on a framework Si-H groups. Hydroxide is an effective catalyst for the oxidation of Si-H to Si-OH by water. Si8O12H8 Hydrosilylation is one of the most important and general methods for forming Si-C bonds, and it has been used extensively to produce octafunctional silsesquioxanes from H8Si8O12 and α-olefins. The reaction is quite general, but it often produces mixtures of products resulting from both α-addition and β-addition to the olefin. In addition, hydrosilylation reaction of H8Si8O12 with vinyl silanes are also complicated by vinyl/H exchange. Hydrosilylation Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Incompletely-condensed POSS frameworks The hydrolytic condensation of RSiX3 produces high yields of compounds that are intermediates along the way to fully condensed POSS frameworks. These incompletely condensed POSS frameworks have a wide range of applications. condensation + partial hydrolysis (c-C6H11)SiCl3 (c-C6H11)7Si7 O7(OH)3 (c-C6H11)8Si8 O9(OH)2 incompletely condensed incompletely condensed (c-C6H11)6Si6O9 completely condensed [RSiO3/2]n framework Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Incompletely-condensed POSS frameworks and heterosilsesquioxanes Heterosilsesquioxanes are formally derived by substitution of a main-group, transition-metal or f-element atom for one or more Si atoms in a silsesquioxanes. When the heteroatoms are metal atoms, the term metallsilsesquioxane is often used. A wide range of heterosilsesquioxanes are available via functionalization of incompletely condensed silsequioxanes. There are five general methods for attaching heteroatoms to an incompletely condensed silsesquioxane framework: (1) reaction of SiOH groups with metal alkyl complexes; (2) metathesis of SiOH groups for less acidic alkoxide or amide ligands; (3) base-assisted (e.g. Et3N) reaction of SiOH groups with active metal halides; (4) reaction of Me4Sb stabilized silsesquioxanes with metal halides; and (5) reaction of Tl-stabilzed silsequioxanes with metal halides or triflates. Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ POSS frameworks as … Soluble models (for silica, silica supported catalysts, aluminosilicates, etc.): Because of their high Si/O content and short range structural similarities to known SiO2 polymorph, silsesquioxanes and metallasilsesquioxanes have been used extensively as models for silica surfaces, silica supported catalysts, aluminosilicates and monodisperse titanium containing catalysts. Precursors to hybrid inorganic/organic materials: Incompletely condensed frameworks have been used as comonomers for new families of silsequioxane based polymers and as building blocks for network solids. Fully condensed POSS frameworks have been used for an even broader spectrum of applications, including photocurable resins, liquid crystals, elektroactive films, and building blocks for catalytically active organometallic gels. One of the most promising applications for silsesquioxanes is the use of fully condensed POSS frameworks possessing one potentially polymerizable pendant group as comonomers in traditional thermoplastic resins. These comonomers, which are synthesized via corner capping reactions of trisilanols can dramatically improve performance characteristics of many resin systems. Lichtenhan has pioneered the use of POSS monomers in advanced thermoplastics and developed an entire chemical tree of monomers suitable for polymerization and grafting reactions. Both the pool of readily available POSS monomers and the range of applications for these monomers are expanding rapidly. X = H, Cl, OH, OSiMe2H, vinyl, allyl, ethylnorbornenyl, glycidyl, p-styryl, (CH2)3O-methacryl, (CH2)3Cl, (CH2)3CN, (CH2)2CO2Me R = c-C6H11 or c-C5H9 Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013 Spezielle Aspekte bauchemischer Materialien "Siliciumorganische Werkstoffe im Bauwesen“ Organofunctinal Silane Applications Adhesives and Sealants Precision casting Sol-Gel, Hybrid materials Aeroges Spin-On RTV CVD Organofunctional silanes Cements and Ceramics Water Repellents and Surface Protection Glass Frosting Hydraulic and Heat-Transfer Flluids Paints, Inks and Coatings Bonded Phases Technische Universität München, Lehrstuhl für Bauchemie, Prof. Dr. Johann Plank, November 22th 2013