Final Hybrids Lecture

Advantages of Hybrids • Greater mechanical strength than organic polymers • Greater flexibility and toughness than inorganic materials • Superior thermal and oxidative stability compared with organic polymers • Harder and tougher films than either inorganic or organic component • Superior gas barrier membranes • Lower energy processing than inorganic materials

Applications of Hybrids Protective coatings wire coatings Pearlescent paint Strong composites Very diverse class of materials

Methods for making hybrids • Physical mixing of preformed organic and inorganic phases • Polymerization of one monomer in another preformed phase • Polymerization of hybrid monomers • Chemical modification of second phase with first • Simultaneous polymerization of organic and inorganic phases • Surfactant Templating

Polymerization of hybrid monomers: Organotrialkoxysilanes High monomer concentration, small or reactive R groups Low monomer concentration, bulky R groups High monomer concentration, most R groups

Synthesis of organotrialkoxysilanes

Synthesis of organotrilalkoxysilanes

Sol-gel polymerization of organotrialkoxysilanes Chemistry: Hydrolysis and condensation reactions Physics: Phase separation

Structure & properties of many hybrids are controlled by phase separation Gel No Gel No Gel • Phase separation of liquid from solvent prevents further reaction and gelation • Phase separation of particles can lead to precipitate or gels • POSS can also form in any of these cases.

Characterizing hybrids Physical appearance – color, solubility, texture, transparency electronic structure – UV-Vis, conductivity Photoelectron spectroscopy XRD – crystallinity or order versus amorphous NMR, IR, XRD – atomic and molecular structure Morphology of phases – SEM, TEM, AFM, SAXS Porosity & surface area – SEM, TEM & gas adsorption Composition – Combustion analysis, X-ray analyses Molecular weight – GPC, Mass spectrometry, viscosity Linear polymer structure - viscosity

Holy Grails in Science A Holy Grail refers to a challenge that everyone agrees is extremely difficult The Holy Grail was an artifact lost by the Catholic Church that was the subject of many searches all over Europe and the Middle East. Indiana Jones and the Last Crusade • It also refers to a Grand, scientific challenge Allen J. Bard, George M. Whitesides, Richard Zare, Fred W. McLafferty Holy Grails of Chemistry, Accounts of Chemical Research 1995, 28

Holy Grails in Science that have been attained: - synthetic plastics as replacement for biomaterials - synthesis of zeolites - Haber ammonia synthesis from nitrogen - structure of DNA - See actual chemical reactions noncontact-AFM Science Express, 2013

Holy Grails in Science Holy Grails that have not been achieved include: - sunlight driven water photolysis to Hydrogen - room temperature superconductors - artificial (and origin of) life.

Holy Grails in Hybrids Holy Grails that have been achieved: High yield, selective POSS syntheses Polymers modified with pendant POSS POSS with atom inside Gyroid structured materials Surfactant templating of structures Characterization of intractable, amorphous materials with solid state NMR. Hierarchical materials Hermetic barriers (gas impermeable)

Holy Grails in Hybrids Holy Grails Remaining: Ladder polymers Structure of amorphous siloxane networks POSS >>> T12 Self-replicating hierarchical structures

Holy Grail #1: Ladder polysilsesquioxanes Rigid rod polymer • Ladder polymers are rigid, hard to break, likely crystalline with very high glass transition temperatures • widely proposed by researchers to explain solubility of polysilsesquioxanes • No convincing evidence for polysilsesquioxane ladder polymers

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

Ladder polymers: How to test hypothesis 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 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!!!!!) 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

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

Ladder polysilsesquioxanes do not form through polymerizations, however, they can be made step-by step Holy Grail: Make ladder polysilsesquioxanes by polymerization

Holy Grail 3: Structure of the Si-O-Si network in amorphous hybrids Cartoon is not reality. Structure is not understood.

Structure in amorphous networks using cleavable bridging groups Characterize fragments with NMR & Mass spectrometry

Holy Grail: T60 polyhedron each blue sphere = silicon each black bond = Si-O-Si linkage Problem: 12 membered rings not thermodynamically favorable. Proposal: Make by oxidizing Si-Si bonds in Si60

Holy Grail 4: Self replicating hierarchical hybrid materials Artificial silicon based Life Tholians from Star Trek Horta from Star Trek There is more left to be done

In summary Organic Inorganic Hybrids are superior in many ways to either organic or inorganic materials New architectures and structures are possibles Many practical applications already exist There are challenges (Holy Grails) left to do.

Acknowledgements Thanks to Professor Hu for inviting me to Harbin, China and allowing me the privilege of teaching Thank you for three weeks of your time.