Physics and Chemistry of Hybrid Organic-Inorganic Materials Lecture 8: Polysilsesquioxanes

Why make hybrid materials? Best Inorganic: •Thermal stability •Modulus •Strength •Porosity Organic: •Toughness •Elasticity •Chromophore •Chemical functionality B: Rule of mixtures Bad Achieve properties not found in either organic or inorganic phase

Different ways to put hybrids together Class 1: No covalent bonds between inorganic and organic phases Example: particle filled polymer Class 2: Covalent bonds between inorganic and organic phases Close-up of hybrid particle Monomers in solvent Gel or dry gel (xerogel)

Key concepts polysilsesquioxanes are made by polymerizing organotrialkoxysilanes the polymerization occurs through the hydrolysis and condensation of the organotrialkoxysilane Silsesquioxane means there is one organic group and 3 siloxane bonds or 1.5 oxygen atoms possible per silicon. Polymerization of organotrialkoxysilanes lead formation of many siloxane rings, with eight membered rings being the most stable. In extreme cases, polyhedral oligosilsesquioxanes are formed. At high concentrations of monomer and with small organic groups, network polymers can form as gels or precipitates. Lower monomer concentrations give soluble polysilsesquioxanes Organotrialkoxysilanes are widely used as coupling agents to modify inorganic filler materials in composites.

Some definitions: silsesquioxanes Trifunctional monomer silsesquioxane If fully condensed, 1.5 oxygens per repeat unit = H, alkyl, aryl, alkenyl alkynyl, and functionalized versions of the latter. sil-sesqui-oxane silicon 1.5 Bonds to oxygen

But polymerization of RSi(OR)3 does not always lead to gels. High monomer concentration, small or reactive R groups Low monomer concentration, bulky R groups High monomer concentration, most R groups POSS Gel Liquid or waxy solid Insoluble May get mixture of products. Rarely get gels

Sol-gel polymerization or organotrialkoxysilanes No Gel No Gel • Phase separation of liquid from solvent prevents further reaction and gelation • Phase separation of particles can lead to precipitate or gels • POSS can also form in any of these cases.

Sol-gel polymerization chemistry. General recipe catalyst Solvent 2 Mole/Liter 3 Moles/Liter Catalyst: Acid catalysts: HCl, H2SO4 (< 0.2 M/Liter) Basic catalysts: NH3, NaOH or KOH Nucleophilic catalyst: Bu4NF Solvent: Alcohol. R’OH – same alcohol formed by monomer hydrolysis EtOH for RSi(OEt)3. Tetrahydrofuran (THF) – phase separates with base. Acetone - not commonly used.

Condensation reactions during organotrialkoxysilane polymerization Soluble products

Polymerization of RSi(OR’)3 at concentrations > 1 M. At higher concentration, intermolecular reactions are faster And compete better with cyclizations. Therefore, more network and less cyclic T8. Distill off solvent during reaction to further concentrate. If R is too bulky, never get gels – POSS or soluble polysesquioxanes

Organotrialkoxysilane Monomers: Aliphatic Substituents Transparent gel opaque gel * * * Transparent gel opaque gel * * Forms gels Only small R groups and very long alkyl groups form gels Otherwise polysilsesquioxane solution

Organotrialkoxysilane Monomers: Sterically hindered Substituents Forms cyclic structures; no gels are formed from any of these monomers Otherwise polysilsesquioxane solution

Organotrialkoxysilane Monomers: Alkenyl and halogenated Substituents * translucent gel transparent gel * * Forms gels Otherwise polysilsesquioxane solution

Organotrialkoxysilane Monomers: Aryl Substituents * * Forms opaque gels Otherwise soluble polysilsesquioxane solution

Organotrialkoxysilane Monomers: Electrophilic Substituents *Gels with just monomer and water Organic groups react under sol-gel conditions Otherwise polysilsesquioxane solution

Isocyanate Functionalized Organotrialkoxysilanes Gels form from neat monomer at acidic, neutral and basic conds. Gel from 1 M Monomer with tetrabutylammonium hydroxide

Epoxide Functionalized Organotrialkoxysilanes Only neat Si(OMe)3 monomers gelled (with NaOH catalyst) Epoxide Group ring opens slower than SiOR polymerization Ring opening occurs under acidic and basic conditions Otherwise soluble polysilsesquioxane solution

Acrylate Functionalized Organotrialkoxysilanes Most cases-sol-gel polym. with retention of vinyl. No vinyl polymerization detected by NMR Trimethoxysilane monomer-also exhibited ester hydrolysis Methacrylic acid detected by NMR, odor neat monomer conc 1.5 equiv H2O/basic-only gel obtained

Amine & Thiol Functionalized trialkoxysilanes *Gels will revert to solutions with heating, solvent or with time

Amine Functionalized trialkoxysilanes No point in adding acid it will just protonate amine group Just add water. No catalyst is needed

Summation of Gelation for Organotrialkoxysilanes Most sol-gel reactions with shown organotrialkoxysilanes do not give gels. Gelation generally does occur when: -the electrophilic functional group reacts under sol-gel conditions. -neat monomer is used. None of the nucleophilic functionalized monomers formed irreversible gels. Insoluble Gels-Usually neat monomer Soluble Thermally Reversible Gels -Usually neat monomer No Gels-Under any circumstances

Ladder polymers: A hypothesis proposed to explain solubility of polysilsesquioxanes Rigid rod polymer Researchers have clung to the ladder polymer hypothesis even after a number of viscosity studies, & NMR experiments have shown it is false

Why don’t most simple pendant silsesquioxanes form gels? Because cyclization to form rings does not allow solid particles to form that can percolate into gels.

Formation of gels Gels are percolating solid particles dispersed in liquid High molecular weight polymer = lower solubility Binodal separation of polymer as particles Higher the functionality of monomer the higher the Molecular Weight.

Polymer molecular weight and functionality number of monomers Dimers and oligomers often liquids May be solid but soluble High molecular weight = particle formation

The problem with silsesquioxanes: formation of rings prevents high molecular weight Liquid oligomers

Polysilsesquioxane Gels: • Don’t form when R is big or bulky pendant group • Gels with R = H, Me, Vinyl, ClCH2-, small or reactive R • Mild Conditions • Concentrations usually > 1M nanoporous • After drying, often get high surface area, porous “xerogel” with nanoscale pores • Gels are insoluble and intractable. • Stable to > 300 °C • Glassy, brittle, hard gels. • Stronger & more hydrophobic than silica

So what can you do with polysilsesquioxane xerogels Most applications are for thin films, rather than bulk: Optical coatings Corrosion protection coatings Water repellant coatings Waveguide materials for optoelectronics Encapsulant material for enzymes and cells Sensor coatings Particles for chromatographic supports Bulk adsorbents for volatile organic contaminants

Other applications of Silsesquioxanes: Silane Coupling Agents Oils or waxy solid in bulk Soluble oligomers & polymers Couple between polymer & silica or other mineral filler Can double or triple strength of composite

Surface modification of particles Not a monolayer – probably 3-4 monomers deep Surface OH’s not close enough for bonds at every silicon

Surface modification sterically slows aggregation & precipitation Better wetting of particle surface with polymer Better particle dispersion Less aggregation

Matching coupling agent to polymer

Silane Coupling Agents Figures courtesy of Geleste

• Increased abrasion resistance • Reduced rolling resistance and improved fuel economy of tires • Better grip on wet and snow/ice surfaces

Hydrophobing mineral fillers PhSi(OMe)3

Recipe for silylating a surface 1) A 95% ethanol – 5% water solution is adjusted to pH 4.5 – 5.5 with acetic acid. 2) Silane is added with stirring to yield a 2% final concentration. Five minutes should be allowed for hydrolysis and silanol formation. 3) Large objects, e.g. glass plates, are dipped into the solution, agitated gently, and removed after 1 – 2 minutes. They are rinsed free of excess materials by dipping briefly in ethanol. Particles, e.g. fillers and supports, are silylated by stirring them in solution for 2 – 3 minutes andcthen decanting the solution. The particles are usually rinsed twice briefly with ethanol. 4) Cure of the silane layer is for 5 – 10 minutes at 110¢XC or for 24 hours at room temperature (<60% relative humidity). For aminofunctional silanes such as A0700 and A0750 this procedure is modified by omitting the additional acetic acid. The procedure is not acceptable for chlorosilanes as bulk polymerization often occurs. Silane concentration of 2% is a starting point. It usually results in deposition of trialkoxysilanes as 3 – 8 molecular layers.

What about other metals with C-M bonds? RGe(OR’)3 R-Sn(OR’)3 These are known, but not R-B(OR’)2 commonly used Most C-M bonds are too reactive with water with the bond polarized with the electron density on carbon. Any other main group IVB (or 14) elements with sesquioxane monomers and polymers. Yes, germanium and tin both can be prepared as the alkoxides above and polymerized. They are more reactive than silicon analog, and the M-C bonds are weaker. Care must be taken with organotin compounds as they are very toxic, though dialkyltin are worse than monoalkyltin (shown). Boron in group IIIB or 13 is trivalent can be prepared or purchased as the dialkoxide. The thermodynamics for condensation for all three metals is less favorable than for silicon. Other metals are too reactive and the carbon-metal bond is simply broken with water resulting in a metal hydroxide and a C-H bond.

Where do you get organotrialkoxysilanes: Commercial sources Sigma Aldrich Chemical company Gelest Dow Corning* Dow Chemical company* Sibond Inc (Dalian, China)* Sigmasil (Wuhan, China)* Power Chemical Corporation (Jiangsu, China)* *Contact company about free research samples

Synthesis of organotrilalkoxysilanes

Synthesis of organotrilalkoxysilanes

Summary polysilsesquioxanes are made by polymerizing organotrialkoxysilanes the polymerization occurs through the hydrolysis and condensation of the organotrialkoxysilane Silsesquioxane means there is one organic group and 3 siloxane bonds or 1.5 oxygen atoms possible per silicon. Polymerization of organotrialkoxysilanes lead formation of many siloxane rings, with eight membered rings being the most stable. In extreme cases, polyhedral oligosilsesquioxanes are formed. At high concentrations of monomer and with small organic groups, network polymers can form as gels or precipitates. Lower monomer concentrations give soluble polysilsesquioxanes Organotrialkoxysilanes are widely used as coupling agents to modify inorganic filler materials in composites.