Physics and Chemistry of Hybrid Organic-Inorganic Materials Lecture 2: Properties of Materials

Physics and Chemistry of Hybrid Organic-Inorganic MaterialsLecture 8: Polysilsesquioxanes

Best
B: Rule of mixtures
Bad
Why make hybrid materials?
Achieve properties not found in either organic or inorganic phase
Inorganic:Thermal stabilityModulusStrengthPorosity
Organic:ToughnessElasticityChromophoreChemical functionality

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

Key concepts
polysilsesquioxanes are made by polymerizing organotrialkoxysilanesthe 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 polysilsesquioxanesOrganotrialkoxysilanes are widely used as coupling agents to modify inorganic filler materials in composites.

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

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

Sol-gel polymerization or organotrialkoxysilanes
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.
No Gel
No Gel
Gel

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

Condensation reactions during organotrialkoxysilane polymerization
Soluble products

Polymerization of RSi(OR)3 at concentrations > 1 M.
At higher concentration, intermolecular reactions are fasterAnd 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
* Forms gels
*
*
*
*
Only small R groups and very long alkyl groups form gelsOtherwise polysilsesquioxane solution
Transparent gel
Transparent gel
opaque gel
opaque gel

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

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

Organotrialkoxysilane Monomers: Aryl Substituents
* Forms opaque gelsOtherwise soluble polysilsesquioxane solution
*

Organotrialkoxysilane Monomers: Electrophilic Substituents
*Gels with just monomer and waterOrganic groups react under sol-gel conditionsOtherwise polysilsesquioxane solution

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

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

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

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

No point in adding acid it will just protonate amine group

Just add water. No catalyst is needed
Amine Functionalized trialkoxysilanes

Summation of Gelation for Organotrialkoxysilanes
Insoluble Gels-Usually neat monomer
Soluble Thermally Reversible Gels-Usually neat monomer
No Gels-Under any circumstances
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.

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

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

Polysilsesquioxane Gels:
Dont 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 coatingsCorrosion protection coatingsWater repellant coatingsWaveguide materials for optoelectronicsEncapsulant material for enzymes and cellsSensor coatingsParticles for chromatographic supportsBulk adsorbents for volatile organic contaminants

Other applications of Silsesquioxanes: Silane Coupling Agents
Soluble oligomers &polymers
Oils or waxy solid in bulk
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 deepSurface OHs not close enough for bonds at every silicon

Better wetting of particle surface with polymerBetter particle dispersionLess 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

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 110XC or for 24 hours at room temperature (