Transcript Physics and Chemistry of Hybrid Organic

Physics and Chemistry of Hybrid
Organic-Inorganic Materials
Lecture 8: Polysilsesquioxanes
Why make hybrid materials?
Best
Inorganic:
•Thermal
stability
•Modulus
•Strength
•Porosity
B: Rule of
mixtures
Organic:
•Toughness
•Elasticity
•Chromophore
•Chemical
functionality
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
Inorganic particles
LInear soluble polymer
Physical Mixing
Solid
solid
with molten
polymer
or
in solvent
followed by
drying
10-10000 nm
Class 2: Covalent bonds between inorganic and organic phases
CH3
HO Si
O
O
CH3
H3C O Si O
O Si
H3C
O
O Si
Si
H3C
H
C
3
O
Si
Si CH3
O
O
Si
O
Si O O
O
H3C
Si O
CHSi
3
O
O
O O
CH3
H3C
Si C
O
Si(OMe)3
H3C
CH3
Si(OMe)3
Si(OMe)3
H3C H3C
H3C Si(OMe)3
H3C Si(OMe)3
(MeO)3Si CH3
H3C
H3C
Si(OMe)3
Si(OMe)3H3C
CH3
(MeO)3Si
(MeO)3Si CH3
Si(OMe)3
Si(OMe)3
H3C Si(OMe)3
H3C
Si(OMe)3
Monomers in solvent
1) Polymerization
H2O + catalyst
2) Phase
separation of
particles
3) Percolation &
gelation
Gel or dry gel (xerogel)
Close-up of hybrid
particle
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
= H, alkyl, aryl, alkenyl
alkynyl, and functionalized
versions of the latter.
If fully condensed, 1.5 oxygens
per repeat unit
sil-sesqui-oxane
silicon
1.5
Bonds to oxygen
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
Sol-gel polymerization or organotrialkoxysilanes
Gel
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.
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
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?
functionality of three
OR'
R Si
OR'
OR'
R
R
O
Si
Si
R O
O
O
O
Si O Si R
R Si O Si R
O
O
O
O
Si O Si
R
R
POSS
crystallizes too
easily into dense
crystals
R
R
R
HO
Si
O
Si
Si O
HO
R
O
R
O
O
O
Si
Si O
R
R Si
HO
O
R
amorhous oligomers:
usually liquids or waxy
solids
O Si
Si
HO
O
Si
R
O
R
OH
R
Si O
Si
O
R
O
O
O
O
OH
R
Si
Si R
O
R
Si
Si
O
Si O Si
O Si
O
O
O
O Si
R R
Si
O
R
R
O
O
R
R
O
R O
Si O
R O
Si
Si
Si
Si
O
O
R
O Si
O
R O
O Si
O
O
Si R
Si
O
O
Si
O
Si O
R
O Si O
R
R
R
O
O
R
HO
O
R
Si O
HO Si
O
HO
Si
R
O
O Si
O
high enough
molecular weight to
form solid particles
Because cyclization to form rings does not allow solid
particles to form that can percolate into gels.
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
• 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
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
Soluble
oligomers &
polymers
Oligosilsesquioxane
HO
Couple
between
polymer &
silica or other
mineral filler
EtO
EtO Si
EtO
Y
> 1.5 n H2 O
Si
O
Y Y
O
O
Y
O
O
Si
OH
Y
Y
Si
Si
OH
Y
Y
O
Si
O
n
Y
Y
Y HO
O
O
O
Si HO
O
Si
Si
Si O
Y
Y
Y
Y
O
Si OH
Y
YHO
O
Si
Si
Y O
Si O
HO Si
O
Y Si O O Y O
Si
O
O
O
Si HO
O
Si
Y
Si
Si
Si O
O
O
O
O
O
Can double or triple strength of composite
OH
Si
Y
Y
Surface
solid in bulk
Surface modification of particles
X
X
X
X
X
O
O Si O Si O
Si O
O Si
Si O
O
Si
HO
O HO Si
HO
X
X
OH O
M
OH
M
X
O HO
O Si O M
M
M O Si
X
Si
M
O
O OH
HO Si
X
X
Si O
O
M
O O
M O Si
Si
O
X
O
X
Si
Si O M
O
MO O
O
Si
X
Si OH
X
HO Si O
O
M
Si O M
X
M
OH O
M O Si O
X
O HO
M
HO Si OH M
X
X
OH
O
O OH Si
O Si
O
O Si O O Si OSi
Si
X
O
X
X
X
X
X
X
X
X
Si(OH)3
Water
X
Si(OR')3
pH 5
HO
M
HO
M
OH
M
OH
M
Not a monolayer – probably 3-4 monomers deep
Surface OH’s not close enough for bonds at every silicon
X
X
X
X
X
X
X
X
O
O Si O Si O
O
Si
Si
O
Si O
O
O HO Si
HO Si HO
X
X
OH O
M
OH
M
X
O HO
O Si O M
M
M O Si
X
Si
M
O
O OH
HO Si
X
X
Si O
O
M
O O
M O Si
Si
O
X
O
X
Si
Si O M
O
O
MO
O
Si
X
Si OH
X
HO Si O
O
M
Si O M
X
M
OH O
M O Si O
X
O HO
M
HO Si OH M
X
X
OH
O
O OH Si
O Si
O
O Si O O Si OSi
Si
X
O
X
X
X
X
X
X
X
X
O
Si
O Si O O
Si O
O Si
Si O
O
O HO Si
HO Si HO
X
X
OH O
M
OH
M
X
O HO
O Si O M
M
M O Si
X
Si
M
O
O OH
HO Si
X
X
Si O
O
M
O O
M O Si
Si
O
X
O
X
Si
Si O M
O
O
M
O
O Si
X
Si OH
X
HO Si O
O
M
Si O M
X
M
OH O
M O Si O
X
O HO
M
HO Si OH M
X
X
OH
O
O OH Si
O Si
O
O Si O O Si OSi
Si
X
O
X
X
X
X
X
+
X
X
X
X
X
Better wetting of particle surface with polyme
Better particle dispersion
Less aggregation
Matching coupling agent to polymer
O
O
N
H
N
H
N
H
n
O
N
H
mn
Hydrocarbon elastomers
HS
Si(OR')3
Polyurethane/polyurea/polyamide
(R'O)3Si
OCN
Si(OR')3
H2N
Si(OR')3
Cl
O
Si(OR')3
OH
OH
n
H3C
O
O
O
n
NC
O
CH3
H2N
O
O
Si(OR')3
Si(OR')3
Cl
Si(OR')3
Si(OR')3
N
H
NC
Si(OR')3
O
N
N
N
H
melamine/formaldehyde
Si(OR')3
O
O
O
N
CH2
Phenol-formaldehyde
O
OCN
R
n
CH3
OH
H3C
R
CH2
O
HN
OH
R
n
H 3C
O
Si(OR')3
x
Si(OR')3
O
H
N
(R'O)3Si
S
Si(OR')3
Si(OR')3
O
O
Si(OR')3
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
• 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.
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
Grignard or Organolithium
R'O
Si
R'O OR'OR'
excess
OR'
R-M
THF
Si
OR'
OR'
M = MgBr or Li
Hydrosilation
R'O
R'O
H
Si
R'O OR'
R
H2PtCl6
R
Si
R'O OR'
R
Metathesis
R'O
R'O
Grubbs catalyst
Si
R'O OR'
R
CH2Cl2
Si
R'O OR'
R
Heck
R'O
R'O
Si
R'O OR'
Pd(OAc)2
Br
R
Ph3P
K2CO3
Si
R'O OR'
R
Synthesis of organotrilalkoxysilanes
Thiols
R'O
R'O
Si
R'O OR'
h
Si
R'O OR'
R
Amines
R'O
R'O
O
Si
R'O OR'
HO
DCC
R
Si
R'O OR'
DMAP
NH2
Isocyanates
O
N
H
R
R'O
R'O
Bu2SnAc2
Si
R'O OR'
HO
NCO
Epoxide
R
S
SH
Si
R'O OR'
R
O
R
N
H
O
R'O
R'O
Si
R'O OR'
H2N
O
O
R
OR'
Si
R'O OR'
Si
R'O OR'
O
HO
O
N
R
OH
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.