Post on 12-Jan-2016
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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 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
MeSi(OMe)3 sol-gel polymerization
MeSi(OMe)3 gels > 1 M in baseMeSi(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, 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 Chemistry
Formation of rings
Larger rings are thermodynamically stable but slower to form
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
If Ladder polymers existed: soluble polysilsesquioxanes would be thermoplastics
with higher Tg’s and some crystallinity
Ladder polymers should be strongerPack better and have greater non-bonding interactionsDo not expect liquids or low tg solids as with soluble polysilsesquioxanes
In reality:•Most tg < 50 °C•Soluble polysilsesquioxanes are weak
Ladder polymers: How to test hypothesis? Dilute solution viscosity studies
Mark Houwink Sakurada equation
= Inherent viscosityM = molecular weight of polymerK and a are Mark Houwink Sakurada parameters
For theta solvent and random coil polymer, a = 0.5For flexible polymers 0.5 < a < 0.8For semiflexible polymers 0.8 <a < 1.0For rigid polymers a > 1.0And for rigid rod polymers, like a ladder polymer, a = 2.0
Ladder polymers(No!!): Dilute solution viscosity studies
For theta solvent and random coil polymer, a = 0.5They are flexible polymers 0.5 < a < 0.8and semiflexible polymers 0.8 <a < 1.0For rigid polymers a > 1.0And for rigid rod polymers, like a ladder polymer, a = 2.0
In Chinese Journal of Polymer Science 1987, 5, 335, Fang showed that a for polyphenylsilsequioxanes was between 0.6-0.86 (These are not ladder polymers!!!!!)
There no ladder polymers, but still researchers claim to have made them without proof!!! And
with impossible stereochemistry
Syn-isotactic
•Impossible to make high molecular weight polymer!!!with cis isotactic stereochemistry.•Need cis syndiotactic for it to work
PolyhedralOligoSilSesquioxane
POSSZhang, R. et al. Angew. Chemie. 2006, 45, 3112
Ladder polysilsesquioxanes do not form through polymerizations, however, they
can be made step-by step
Back to the real world
No ladder polymers from sol-gel polymerizations!!
Gels form with small RR = H, CH3, Vinyl, ClCH2-, ClCH2Ph-
Other products of sol-polymerization: polyhedral oligosilsesquioxanes (POSS)
8 membered rings (as in T8) are commonly formed
• Silica like-core with organic groups on surface• Called smallest silica particle
Some examples: Octamethyl-Polyhedraloligosilsesquioxanes: POSS
No melting pointInsoluble in organic solventsSublimes above 240 °C
1,3,5,7,9,11,13,15-octamethylpentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane
What about POSS with 6–membered rings?
T6 forms under anhydrous conditions only
Instead only T8 & POSS with 8 membered rings
25% yield with R = octyl2 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.
POSS in polypropylene
* Each “black dot” represents a 1.5nm POSS cage
Non-covalently mixed into solid plastic
Question: Are the POSS dissolved or a separate phase?
Octaallyl-Polyhedraloligosilsesquioxanes: POSS
Melts at 71 °CSoluble in organic solventsSublimes above 140 °C
1,3,5,7,9,11,13,15-octapropenylpentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane
Polymer 2005, 46, 2163
Class 2: Networks based on POSS as polyfunctional monomers
Octa-functional epoxide versus commercial epoxide
Comparable toughness and strength!! (Just 100X as expensive) Some Improvement in thermal stability
Impossible to react at all epoxide groups
Chemists often believe network polymers are infinite and homogeneous in structure
They are not. Particulate morphology suggests otherwise.
Monomer functionality and phase separation
Degree of condensation at Gel point
Gel point = 14% of groups reacted
Gel point = 14% of groups reacted
What happens as polymer grows?
Entropy cost for polymerization increases with extent of reactionEnthalpy dominates solubility thermodynamics
Chemistry and physics of gelation
Sol-gel polymerizations create solid particles that eventually percolate and gelKinetics 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 kineticsSolubility 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
Bridged polysilsesquioxaneBridged monomer
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
More definitions: Bridged systems
Pendant vs. Bridged Polysilsesquioxanes
Bridged Systems-Gels Form Readily
Most do not gel
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 crackingFrom aerogel.org
Drying gels – networks collapse due to capillary forces
• Capillary force in small pores• irregular solvent front• 2-300 MPa force • 50-90% shrinkage
• Weakly bonded colloidal network
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 supercritical
Bridged Aerogels
Bridged xerogels
Differences in size between equivalent mass xerogels and 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.