Physics and Chemistry of Hybrid Organic-Inorganic MaterialsLecture 7: Polymerizing monomers to make hybrids
Key concepts • Hybrid monomers have both organic and inorganic parts in a single molecule • The monomer polymerizes to form a hybrid that has both organic and inorganic components homogeneously mixed rather than phase separated • Hybrid monomers (hybrid polymers): organotrialkoxysilanes (polysilsesquioxanes and POSS), Hexachlorophosphazene (polyphosphazenes), dichlorosilanes (polysilanes and polysiloxanes) • Other hybrids: polymeric phthalocyanines, fullerene based polymers, and many more... • Generally greater thermal stability than organic polymers • Flexible and processable like organic polymers.
Making Hybrid Materials: Class 2A(Covalent links at molecular level) • Organic group is attached to network at molecular level • Hypercrosslinking is possible • Pendant or bridging monomers • Bridging groups can be small or macromolecule • This class also includes the organometallic polymers Chromatographic Materials Low K Dielectrics Photoresists for Lithography
There are many hybrid or organometallic polymers: A quick survey Some have been used in making hybrid materials Many have not. Hybrid Polymers: Polysilanes Polyphosphazenes Coordination polymers Polysiloxanes Purely inorganic: Poly(sulfur nitride) Fullerenes Carbon nanotubes Graphene
Poly(sulfur nitride) or Polythiazyl •First known conducting inorganic polymer •Superconducting below 1K •LED’s and solar cells Labes, M. M.; Love, P.; Nichols, L. F. (1979). "Polysulfur Nitride - a Metallic, Superconducting Polymer". Chemical Reviews79 (1): 1–15.
Polysilanes Wurtz Coupling Reaction Dehydrocoupling Polysilanes exhibit σ-delocalization. UV absorbing/degrading Semiconductor (4.5 eV) Ceramic (Si-C) fiber precursor heat resistant, almost up to 300 oC Chem. Rev. 1989, 89, 1359-1410
Polyphosphazenes Over 600 known polymers Glass transition temp < -60 °C Thermal stability Tailored solubility Can be bioerodible Monomer: Hexachlorophosphazene Allcock, Harry R. (2003). Chemistry and Applications of Polyphosphazenes. Wiley-Interscience.
Coordination Polymers •Many are anisotropic •Includes metal oxide framework materials • catalysts • gas adsorbents • electrical conductors & semiconductors • Solar cells If bonding between metal and ligand is not reversible, then small oligomers If bond formation is reversible, large 3-C crystals can form. Angew. Chemie 1996, 35, 1602 & Chem. Soc. Rev., 2012,41, 115-147
Polysiloxanes (silicone) Thermally & chemically stable Glass transition temp < -123 °C Melts at -23 °C (liquid at room temperature) With crosslinking – elastomer Not flammable
Polysilsesquioxanes organotrialkoxysilanes Hydrolyze alkoxide groups to silanols Condense silanols to Si-O-Si linkages Trifunctional monomer can form gels Next lectures are on polysilsesquioxanes POSS
What determines if phase separation occurs? How to make solid particles? • very large polymers. • cross-link polymers (this is easiest) Functionality = 2, linear siloxane polymers. Because linear (functionality = 2) siloxanes are generally liquids, so gels don’t form When RSi(OR)3 polymerizes and makes rings, its functionality nears 2
What are gels? • Two phase systems: solid particles of polymer + solvent • Jello: solid particles of protein + water • Hybrid gels: solid particles of network polymer percolating through liquid.
Fullerenes covalently incorporated into polymers Prog. Polym. Sci. 29 (2004) 1079–1141
First polymer known was Friedel Craft polymerization of C60 in 1991 JACS 1991, G. Olah
Second polymerization was copolymerization with paraxylene Loy, JACS, 1992
Fullerenes will polymerize photochemically Highly crosslinked. Insoluble powders
Fullerenes will polymerize thermally by step addition, free radical polymerization
Tg went from 97 °C to 163 °C Macromolecules 1994,27, 4836-4837
Summary • Hybrid monomers polymers to homogeneous hybrid solids • Hybrid monomers in solvent can polymerize to form solid particles. Solid particles may form gel. • Hybrid monomers include: organotrialkoxysilanes, dichlorosilanes, hexachlorotriphosphazene • Sol-gel polymerizations involve hydrolysis and condensation reactions and often form gels