Physics and Chemistry of Hybrid Organic-Inorganic Materials Lecture 2: Properties of Materials

Physics and Chemistry of Hybrid Organic-Inorganic MaterialsLecture 10: Bridged polysilsesquioxanes

Key concepts
bridged polysilsesquioxanes are made from monomers with two or more trialkoxysilyl groups Bridged polysilsesquioxanes have on organic bridging group in the network rather than hanging from it.Bridged polysilsesquioxanes are prepared by sol-gel polymerization of bridged monomersPolymerization of bridged monomers increases molecular weights so quickly that phase separation of particles and gelation nearly always occur unless very low concentrations Bridged polysilesquioxanes are thermally and chemically stable and can have properties tailored by organicBridged polysilsesquioxane gels are often porous when dried with very high surface areasApplications include anticorrosive coatings, and chromatographic supports for HPLC

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

Bridged polysilsesquioxanes:
Ease of gelation related to:

Polymerization kineticsSolubility thermodynamics

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

Bridged polysilsesquioxanes polymerize by hydrolysis and condensation
Made from monomers with two or more trialkoxysilyl groups

Pendant vs. Bridged Polysilsesquioxanes
Bridged Systems-Gels Form Readily
Most organotrilakoxysilanes do not gel with polymerization

Preparation of bridged polysilsesquioxanes:
0.4 M Monomer*NaOH catalyst

Slow gelation at pH 5

Bridged Monomers; Origins of Control

Commercially Available Sulfide and Amine Bridged Monomers
Aldrich, Gelest sell monomers-look on emolecules or in catalog

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 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 tempMethanol: 240 C, 8.1 MPaEthanol: 241 C, 6.2 MPa Carbon dioxide: 31 C, 7.4 MPa
Exchange alcohol for liquid CO2, then go supercritical

aerogel
gel
Supercritical drying. Mapped out on CO2 phase diagram
Time consuming

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.

(2-HCl-EtOH)

Making and drying hybrid gels by sol-gel polymerization
Dry gels are porous.Porous materials have: -surface area (meter2/gram)-Pore size (nm diameter)

Surface area of dry gels
Calculate from SEM or TEM or AFM orGas sorption porosimetry
Geometric surface area = surface area of particles x number of particles

Silica particles (1 nm diameter) surface area = 2730 m2/gSilica particles (10 nm diameter) surface area = 273 m2/gSilica particles (100 nm diameter) surface area = 27.3 m2/g

If particles are porous, then surface area is higher!!!

3-point bend testing of coated aerogel (using cylinders on side)
Strength of bridged polysilsesquioxanes:
Generally stronger and tougher than silica

Modified organic polymers: Trialkoxysilyl side groups

Polybutadiene with side groups

Grafted triethoxysilyl groups on polyethylene for moisture crosslinking
May also be applied with vinyltriethoxysilane and RF plasma
Excellent for moisture curing polyethylene

Macromolecules, 2004, 37 (10), pp 35323535

Anisotropic Micellar Nanoobjects from Reactive Liquid Crystalline RodCoil Diblock Copolymers

Modified organic polymers: Trialkoxysilyl end groups

Macromolecules 1992,25, 5742-5751
Hydrogenated polybutadiene telechelics with triethoxysilyl groups

Triethoxysilyl terminated polysulfone
Macromolecules 1991;24:344950.
Anneal > 200 C
TiO2 sol in THF
Tailorable refractive index1.6 < n < 1.8

Optical coatings

Drug delivery hybrid gel
Polyethylene glycolTreithoxysilyl group on each endUrea linkage

Drug delivery hybrid gel

PEO Bridged polysilsesquioxane hybrids
Bacteriocide Ag-silsesquioxane coatings Biomacromolecules, 2007, 8 (4), pp 12461254Polymer electrolytes Solid State Ionics, 1999, 116, 197209Coatings for steel. Adv. Technology 2008, 27, 117-126Electrochemically deposited coatings for stints New J. Chem., 2009, 33, 1596-1604Luminescent materials J. Non-Crystal Solids 2006, 352, 22922295 & Chem. Mater., 2004, 16 (13), pp 25302543Contolled druge release Chem. Mater., 2009, 21 (3), pp 463467

PEO bridged polysilsesquioxanes for polymer electrolytes
New J. Chem., 2012, 36, 1218-1223

Summary
Bridged polysilsesquioxanes made with organic bridging group in silsesquioxane networkVery easy to prepareForm as porous xerogels or aerogelsTailored porosity, high surface areaHigh degree of functionalization

A really didnt have a proper section on these polymers in class 2A, so I am putting them in here. These are organic polymers with trialkoxysilyl groups as pendant groups that can be used to crosslink the polymers or integrate with a filler. The following slides will cover some of these in more detail.
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Polymerization of butadiene can give varying amounts of 1,3- and 1,2 polymerization regiochemistries (top left). The result is a number of pendant vinyl groups that can be modified with trialkoxysilyl groups by hydrosilation (top) or S-H radical addition. In both case the oligomers (normally a liquid) are used as coupling agents between filler and rubbery polymers.
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Vinytriethoxysilane is not a very reactive monomer. It only homopolymerizes to give oligomers and it does not incorporate well into many copolymers. However, attaching to polyethylene can be done with free radical grafting that adds one monomer unit without continuing to polymerized. Once the radical grabs another hydrogen atom, the triethoxysilyl group is in place. It only requires a few percent of the monomers modified to allow adequate crosslinking. Curing by hydrolysis and condensation generates a siloxane lionkage between macromolecules. This light level of crosslinking around the entanglements leads to considerable strengthening of the polymer without imbrittlement.
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Chemical structure and a schematic drawing of an asymmetric rodcoil diblock copolymer, PIC(23K)/PS(200K), and AFM images of smectic ordered anisotropic features in THF-cast film: (b) height and (c) phse contrast. The Nylon 1 is made by the anionic polymerization of the isocyanate group on isocyanatopropyltriethoxysilane. The Nylon 1 is a tight helical rigid rod that apparently crystallizes in addition to simple phase segreation to give the unusaul structures in the tems/

Anisotropic Micellar Nanoobjects from Reactive Liquid Crystalline RodCoil Diblock CopolymersAbstractFull Text HTMLHi-Res PDF[219 KB]PDF w/ Links[267 KB]Supporting Info ->FiguresCiting ArticlesJi-Woong Park and Edwin L. Thomas *Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 Macromolecules, 2004, 37 (10), pp 35323535DOI: 10.1021/ma035921jPublication Date (Web): April 17, 2004Copyright 2004 American Chemical SocietySection:Plastics Manufacture and ProcessingSelf-assembly of anisotropic particles can provide novel structures with intriguing properties which are not possible to obtain from spherical particles. Because of anisotropy, their self-organization results in liquid crystalline phases1 or non-fcc lattices. Viruses (0 dimensional), or fibril (1d), or sheetlike (2d) mineral colloids are long-known natural particulate systems. Synthetic methods for nonspherical nanocrystals of minerals or metals have become quite sophisticated in recent years.2,3 Their organic analogues with sizes up to several nanometers are synthesized by supramolecular assembly or polymerization of small molecular compounds. For example, nanometer-sized nonspherical objects were obtained from directional hydrogen bonding of organic molecules.4,5 Disklike aromatic nanoparticles were synthesized from polyarylene dendrimers.6,7 These organic particles offer rich chemistry of functional groups and excellent processability in addition to the anisotropic physical properties.
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Now if the trialkoxysilyl groups could only be put on the ends of the polymers (telechelic means groups on the both ends of a macromolecule), you could crosslink while leaving the macromolecule free to reptate. This should make for stronger elastomers. The above polymers are afew of many that have been prepared. They include polyethylene oxide or PEO, also called polyethylene glycol. Hydrogenated polybutadiene, polysulfone (PSF) and polyether ketone (PEK). All of these polymers had telechelic alcohol or amine groups that reacted with the isocyanatopropyltriethoxysilane to give the urea or urethane linkages to the triethoxysilyl groups (or trimethoxysilyl).
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This is the manner in which the hydrogenated polybutadiene is prepared as a telechelic. It is a bit cumbersome to use the bis isocyanate then react with aminopropyltrialkoxysilane, instead of directly using the isocyanato propyltrialkoxysilane, but they also wanted to make polyurethanes from the telechelic diols.
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If the polysulfone is made with the bisphenol groups on each end, then this approach gives telechelic triethoxysilyl groups that can be used to crosslink the polymer and render it insoluble in solvents. The paper mentioned at the bottom uses the polymers as optical coatings and is only interested in incorporating titania at the silsesquioxane to raise the refractive index.
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There have been many papers using polyethylene oxide as a bridging group between triethoxysilanes. In this case they are used as a drug delivery system. Polyethylene oxide is water soluble and the only thing that keeps this polymer from dissolving is the crosslinks at the silsesquioxane. The PEO segments swell with water and if the crosslinking is done in the presence of a druge, it can be encapsulated into the resulting gel phases between crosslinks and diffuse out when placed in vivo.
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These are just pictures of the gels containing different amounts of the drug sodium diclofenac, an antiinflammatory analgesic. They show a POCC as the crosslinking functionality, but do not believe it. They had no evidence for the POSS forming.
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Here are some of the things PEO bridged polysilsesquixoanes have been used for.
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Interestingly this group failed to cite my groups work in making sol-gel based polymer electrolytes by oxidation of disulfide bridged polysilsesquioxanes. Real part of the global conductivity ac recorded at 20 C versus frequency f, for the M-SO3Hy. Black, red and blue signals refer to M-SO3H0, M-SO3H10 and M-SO3H50 responses, respectively; square, circle and triangle symbols correspond to M = 400, 600 and 1000 g mol1.
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