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Lecture 10 Hybrid POSS

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.

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Lecture 10 Hybrid POSS

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  1. Lecture 10 Hybrid POSS Class 2A Covalent links at molecular level

  2. 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

  3. MeSi(OMe)3 sol-gel polymerization MeSi(OMe)3 gels > 1 M in base MeSi(OMe)3 gels only without solvent under acidic conditions

  4. 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

  5. But polymerization of RSi(OR)3 does not always lead to gels. High monomer concentration, small or reactive R groups Low monomer concentration, bulky R groups High monomer concentration, most R groups POSS Gel Liquid or waxy solid Insoluble May get mixture of products. Rarely get gels

  6. Sol-gel polymerization Chemistry

  7. Formation of rings Larger rings are thermodynamically stable but slower to form

  8. 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

  9. 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

  10. 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

  11. Ladder polymers(No!!): Dilute solution viscosity studies In Chinese Journal of Polymer Science1987, 5, 335, Fang showed that a for polyphenylsilsequioxanes was between 0.6-0.86 (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

  12. There no ladder polymers, but still researchers claim to have made them without proof!!! And with impossible stereochemistry PolyhedralOligoSilSesquioxane Syn-isotactic 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

  13. Ladder polysilsesquioxanes do not form through polymerizations, however, they can be made step-by step

  14. Back to the real world Gels form with small R R = H, CH3, Vinyl, ClCH2-, ClCH2Ph- No ladder polymers from sol-gel polymerizations!!

  15. 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

  16. Some examples: Octamethyl-Polyhedraloligosilsesquioxanes: POSS 1,3,5,7,9,11,13,15-octamethylpentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane No melting point Insoluble in organic solvents Sublimes above 240 °C

  17. 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

  18. Synthesis of T12 POSS Dropwise add of 15.8 g (80 mmol) 14 days White crystalline precipitate Dalton Trans., 2012, 41, 10585-10588

  19. An Atomic Force Microscope (AFM) image of a single POSS molecule on a silicon surface Used to make dielectric layers in computer chips

  20. 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 Question: Are the POSS dissolved or a separate phase? POSS in polypropylene

  21. Octaallyl-Polyhedraloligosilsesquioxanes: POSS 1,3,5,7,9,11,13,15-octapropenylpentacyclo[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

  22. Class 2: Networks based on POSS as polyfunctional monomers

  23. Octa-functional epoxide versus commercial epoxide Impossible to react at all epoxide groups Comparable toughness and strength!! (Just 100X as expensive) Some Improvement in thermal stability

  24. Chemists often believe network polymers are infinite and homogeneous in structure They are not. Particulate morphology suggests otherwise.

  25. Monomer functionality and phase separation Gel point = 14% of groups reacted Degree of condensation at Gel point Gel point = 14% of groups reacted

  26. What happens as polymer grows? Entropy cost for polymerization increases with extent of reaction Enthalpy dominates solubility thermodynamics

  27. 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

  28. Even this thermodynamically controlled polymerization gives kinetic structures

  29. Basic Polysilsesquioxane precursors

  30. Bridged polysilsesquioxanes: Class 2 Ease of gelation related to: Polymerization kinetics Solubility thermodynamics

  31. Drawing bridged polysilsesquioxane structures: Fully condensed: 1.5 oxygens per Si. Methylene-bridged polysilsesquioxane

  32. Bridged polysilsesquioxanes Made from monomers with two or more trialkoxysilyl groups

  33. More definitions: Bridged systems 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

  34. Pendant vs. Bridged Polysilsesquioxanes Most do not gel Bridged Systems-Gels Form Readily

  35. Preparation of bridged polysilsesquioxanes: 0.4 M Monomer* NaOH catalyst

  36. Bridged Monomers; Origins of Control

  37. Commercially Available Sulfide and Amine Bridged Monomers

  38. What happens when you dry the “wet” gel too fast Shrinkage with cracking From aerogel.org

  39. 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

  40. 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

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

  42. 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. Solids1995, 186, 44.

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