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

2. 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

3. 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)

4. 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.

5. 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

6. 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

7. 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.

8. 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.

9. Condensation reactions during organotrialkoxysilane polymerization
Soluble products

10. 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

11. 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

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

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

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

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

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

17. 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

18. 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

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

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

21. 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

22. 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

23. 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.

24. 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.

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

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

27. 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

29. 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

30. 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

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

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

33. Matching coupling agent to polymer

35. Silane Coupling Agents
Figures courtesy of Geleste

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

38. Hydrophobing mineral fillers
PhSi(OMe)3

39. 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.

40. 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.

41. 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

42. Synthesis of organotrilalkoxysilanes

43. Synthesis of organotrilalkoxysilanes

44. 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.