Transcript Lecture 9 Hybrid POSS

Lecture 9 Hybrid POSS
Class 2A Covalent links at molecular level
Making Hybrid Materials: Class 2A
(Covalent links at molecular level)
• Organic group is attached to network
at molecular level
•Hypercrosslinking is possible
•Pendant or bridging monomers
•Bridging groups can be small or
macromolecule
•This class also includes the
organometallic polymers
Chromatographic
Materials
Photoresists for
Lithography
Low K Dielectrics
There are many hybrid or
organometallic polymers: A quick
survey
Some have been used in making hybrid materials
Many have not.
Hybrid Polymers:
Polysilanes
Polyphosphazenes
Coordination polymers
Polysiloxanes
Purely inorganic:
Poly(sulfur nitride)
Fullerenes
Carbon nanotubes
Graphene
Poly(sulfur nitride)
or Polythiazyl
•First known conducting
inorganic polymer
•Superconducting below 1K
•LED’s and solar cells
Labes, M. M.; Love, P.; Nichols, L. F. (1979). "Polysulfur Nitride - a
Metallic, Superconducting Polymer". Chemical Reviews 79 (1): 1–15.
Poly(sulfur nitride) by CVD
Polysilanes
Wurtz Coupling Reaction
Dehydrocoupling
Polysilanes exhibit σ-delocalization.
UV absorbing/degrading
Semiconductor (4.5 eV)
Ceramic (Si-C) fiber precursor
heat resistant, almost up to 300 oC
Chem. Rev. 1989, 89, 1359-1410
Polyphosphazenes
Over 600 known polymers
Glass transition temp < -60 °C
Thermal stability
Tailored solubility
Can be bioerodible
Polymer electrolytes for fuel cells
Allcock, Harry R. (2003). Chemistry and Applications of Polyphosphazenes.
Wiley-Interscience.
Coordination Polymers
•Many are anisotropic
•Includes metal oxide
framework materials
• catalysts
• gas adsorbents
• electrical conductors &
semiconductors
• Solar cells
If bonding between metal and ligand is not reversible, then small oligomers
If bond formation is reversible, large 3-C crystals can form.
Angew. Chemie 1996, 35, 1602
& Chem. Soc. Rev., 2012,41, 115-147
Polysiloxanes (silicone)
Thermally & chemically stable
Glass transition temp < -123 °C
Melts at -23 °C (liquid at room temperature)
With crosslinking – elastomer
Not flammable
Silsesquioxanes
What about other metals with C-M
bonds?
• RGe(OR’)3
• R-Sn(OR’)3
• R-B(OR’)2
These are known, but not
commonly used
• Most C-M bonds are too reactive with water
with the bond polarized with the electron
density on carbon.
Basic Polysilsesquioxane precursors
Class 2C
Sol-gel polymerization
chemistry. A 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.
Gel
Making Polysilsesquioxane gels as Class
2A
Materials: Sol-Gel Process
•Sol is a dispersion of particles in solvent
•A gel forms when those particles percolate through the solvent
•Aging is the relaxation of the network with time
•Drying removes the solvent leaving the network behind.
But polymerization of RSi(OR)3 does not
always lead to gels.
Low monomer
concentration,
bulky R groups
High monomer
concentration, small or
reactive R groups
High monomer
concentration,
most R groups
POSS
Gel
Liquid or waxy solid
Insoluble
May get mixture of products. Rarely get gels
Why don’t most simple pendant silsesquioxanes form
gels? To answer we must look at formation of gels
Gel
No Gel
No Gel
• Must have solid and liquid phase
• Solid phase (usually particles) must be continuous through liquid (percolation)
• Phase separation of liquid prevents further reaction and gelation
What determines if phase separation occurs? How
to make solid particles?
• very large polymers.
• cross-link polymers (this is easiest)
Functionality = 2, linear siloxane polymers.
Because linear (functionality = 2) siloxanes are generally liquids,
so gels don’t form
When RSi(OR)3 polymerizes and makes rings, its functionality nears 2
Condensation reactions during
organotrialkoxysilane polymerization
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
Organotrialkoxysilane Monomers:
Aliphatic Substituents
*
*
*
*
* Forms gels
Organotrialkoxysilane Monomers:
Sterically hindered Substituents
Forms cyclic
structures; no
gels
Organotrialkoxysilane Monomers:
Alkenyl and halogenated Substituents
*
*
* Forms gels
Organotrialkoxysilane Monomers: Aryl
Substituents
*
* Forms gels
Organotrialkoxysilane Monomers:
Electrophilic Substituents
*Gels with just monomer and water
Organic groups react under sol-gel conditions
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
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.
Insoluble Gels-Usually neat monomer •None of the nucleophilic functionalized
monomers formed irreversible gels.
Soluble Thermally Reversible Gels
-Usually neat monomer
No Gels-Under any circumstances
But polymerization of RSi(OR)3 does not
always lead to gels.
Low monomer
concentration,
bulky R groups
High monomer
concentration, small or
reactive R groups
High monomer
concentration,
most R groups
POSS
Gel
Liquid or waxy solid
Insoluble
May get mixture of products. Rarely get gels
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
If Ladder polymers existed: soluble
polysilsesquioxanes would be thermoplastics
with higher Tg’s and some crystallinity
In reality:
•Most tg < 50 °C
•Soluble polysilsesquioxanes
are weak
Ladder polymers should be stronger
Pack better and have greater non-bonding interactions
Do not expect liquids or low tg solids as with soluble polysilsesquioxanes
Ladder polymers: How to test hypothesis?
Dilute solution viscosity studies
Mark Houwink Sakurada equation
Inherent viscosity
M = molecular weight of polymer
K and a are Mark Houwink Sakurada parameters
For theta solvent and random coil polymer, a = 0.5
For flexible polymers 0.5 < a < 0.8
For semiflexible polymers 0.8 <a < 1.0
For rigid polymers a > 1.0
And for rigid rod polymers, like a ladder polymer, a = 2.0
Ladder polymers(No!!): Dilute solution
viscosity studies
In Chinese Journal of Polymer Science 1987, 5, 335, Fang
showed that a for polyphenylsilsequioxanes was between 0.60.86 (These are not ladder polymers!!!!!)
For theta solvent and random coil polymer, a = 0.5
For flexible polymers 0.5 < a < 0.8
For semiflexible polymers 0.8 <a < 1.0
For rigid polymers a > 1.0
And for rigid rod polymers, like a ladder polymer, a = 2.0
There no ladder polymers, but still researchers
claim to have made them without proof!!! And
with impossible stereochemistry
Syn-isotactic
PolyhedralOligoSilSesquioxane
POSS
Zhang, R. et al. Angew. Chemie. 2006, 45, 3112
•Impossible to make high molecular weight polymer!!!
with cis isotactic stereochemistry.
•Need cis syndiotactic for it to work
Ladder polysilsesquioxanes do not form
through polymerizations, however, they
can be made step-by step
Back to the real world
Gels form with small R
R = H, CH3, Vinyl, ClCH2-, ClCH2Ph-
No ladder polymers from sol-gel polymerizations!!
Polysilsesquioxane Gels:
Class 2A Hybrid
• 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
• 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
nanoporous
So what can you do with
polysilsesquioxane xerogels and aerogels
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