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.