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, ClCH 2 -, 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 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, 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 stronger Pack better and have greater non-bonding interactions Do not expect liquids or low t g solids as with soluble polysilsesquioxanes In reality: Most t g < 50 °C Soluble polysilsesquioxanes are weak

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 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 In Chinese Journal of Polymer Science 1987, 5, 335, Fang showed that a for polyphenylsilsequioxanes was between (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 POSS Zhang, 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 R R = H, CH 3, Vinyl, ClCH 2 -, ClCH 2 Ph-

Other products of sol-polymerization: polyhedral oligosilsesquioxanes (POSS) 8 membered rings (as in T 8 ) are commonly formed Silica like-core with organic groups on surface Called smallest silica particle

Some examples: Octamethyl- Polyhedraloligosilsesquioxanes: POSS No melting point Insoluble in organic solvents Sublimes above 240 °C 1,3,5,7,9,11,13,15- octamethylpentacyclo[ ,9.1 5,15.1 7,13 ]oct asiloxane

What about POSS with 6–membered rings? T 6 forms under anhydrous conditions only Instead only T8 & POSS with 8 membered rings 25% yield with R = octyl 2 six membered rinbgs & 3 eight membered rings

Synthesis of T 12 POSS Dropwise add of 15.8 g (80 mmol) 14 days White crystalline precipitate Dalton Trans., 2012, 41,

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 °C Soluble in organic solvents Sublimes above 140 °C 1,3,5,7,9,11,13,15- octapropenylpentacyclo[ ,9.1 5,15.1 7,13 ]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

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

Bridged polysilsesquioxane Bridged 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 cracking From aerogel.org

Drying gels – networks collapse due to capillary forces Capillary force in small pores irregular solvent front 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 CO 2, then go supercritical

Bridged Aerogels Bridged xerogels Differences in size between equivalent mass xerogels and aerogels

Effects of Processing on Gels 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.