Lecture 10 Hybrid POSS
Class 2A Covalent links at molecular level
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
MeSi(OMe)3 sol-gel polymerization
MeSi(OMe)3 gels > 1 M in base
MeSi(OMe)3 gels only without solvent under acidic conditions
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
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 Chemistry
Formation of rings
Larger rings are thermodynamically stable but slower to form
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
They are flexible polymers 0.5 < a < 0.8
and 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!!
Other products of sol-polymerization:
polyhedral oligosilsesquioxanes (POSS)
• Silica like-core
with organic
groups on
surface
• Called smallest
silica particle
8 membered rings (as in T8) are commonly formed
Some examples: OctamethylPolyhedraloligosilsesquioxanes: POSS
1,3,5,7,9,11,13,15octamethylpentacyclo[
9.5.1.13,9.15,15.17,13]oct
asiloxane
No melting point
Insoluble in organic solvents
Sublimes above 240 °C
What about POSS with 6–membered rings?
Instead only T8 &
POSS with 8
membered rings
T6 forms under anhydrous conditions only
25% yield with R = octyl
2 six membered rinbgs
& 3 eight membered rings
Synthesis of T12 POSS
Dropwise add of
15.8 g (80 mmol)
14 days
White crystalline
precipitate
Dalton Trans., 2012, 41, 10585-10588
An Atomic Force Microscope (AFM) image of
a single POSS molecule on a silicon surface
Used to make dielectric layers in computer chips
Class 1 Hybrids: Prefab POSS are
dispersed in an organic polymer.
* Each “black dot” represents a
1.5nm POSS cage
Non-covalently mixed into solid
plastic
POSS in polypropylene
Question: Are the POSS dissolved
or a separate phase?
OctaallylPolyhedraloligosilsesquioxanes: POSS
1,3,5,7,9,11,13,15octapropenylpentacyclo[9.5.1.
13,9.15,15.17,13]octasiloxane
Melts at 71 °C
Soluble in organic solvents
Sublimes above 140 °C
Polymer 2005, 46, 2163
Class 2: Networks based on POSS
as polyfunctional monomers
Octa-functional epoxide versus
commercial epoxide
Impossible to react at all epoxide group
Comparable toughness and strength!! (Just 100X as expensive)
Some Improvement in thermal stability
Chemists often believe network polymers
are infinite and homogeneous in structure
They are not. Particulate morphology suggests otherwise.
Monomer functionality and phase
separation
Gel point = 14% of groups reacted
Degree of condensation
at Gel point
Gel point =
14% of groups
What happens as polymer grows?
Entropy cost for
polymerization increases
with extent of reaction
Enthalpy dominates
solubility thermodynamics
Chemistry and physics of gelation
Sol-gel polymerizations create solid particles that eventually percolate and gel
Kinetics lead to amorphous, high free energy structures in gels
Even this thermodynamically
controlled polymerization gives kinetic
structures
Basic Polysilsesquioxane precursors
Bridged polysilsesquioxanes: Class 2
Ease of gelation related to:
Polymerization kinetics
Solubility thermodynamics
Drawing bridged polysilsesquioxane structures:
Fully condensed: 1.5 oxygens per Si.
Methylene-bridged polysilsesquioxane
Bridged polysilsesquioxanes
Made from monomers with two or more trialkoxysilyl groups
More definitions: Bridged systems
Bridged monomer
Bridged polysilsesquioxane
Often described by chemical name:
Bis(trialkoxysilyl)arylene or alkylene
Functionality of each silicon is THREE
Functionality of each bridged monomer (as above) is SIX
Pendant vs. Bridged Polysilsesquioxanes
Most do not gel
Bridged Systems-Gels Form Readily
Preparation of bridged polysilsesquioxanes:
0.4 M Monomer*
NaOH catalyst
Bridged Monomers; Origins of Control
Commercially Available Sulfide and Amine
Bridged Monomers
What happens when you dry the “wet” gel too fast
Shrinkage with cracking
From aerogel.org
Drying gels – networks collapse due to
capillary forces
• Weakly bonded colloidal network
• Capillary force in small
pores
• irregular solvent front
• 2-300 MPa force
• 50-90% shrinkage
Need to reduce surface tension differential
Eliminate drying stress by supercritical
drying
• No liquid-gas interface
• No drying stress
• Alcohols require high temp
-Methanol: 240 °C, 8.1 MPa
-Ethanol: 241 °C, 6.2 MPa
• Carbon dioxide: 31 °C, 7.4 MPa
Exchange alcohol for liquid CO2, then go supercritica
Differences in size between
equivalent mass xerogels and
aerogels
Bridged xerogels
Bridged Aerogels
Effects of Processing on Gels
(2-HCl-EtOH)
Loy, D. A.; Jamison, G. M.; Baugher, B. M.; Russick, E. M.; Assink, R. A.; Prabakar,
S.; Shea, K. J. J. Non-Cryst. Solids 1995, 186, 44.