Chemistry and Physics of Hybrid Organic-Inorganic Materials

Today First hour: Introduction to course: Go over syllabus Introduction to Hybrid Organic Inorganic Materials -10 minute break. Second hour: Quiz Go over quiz answers and discuss (selected students will present their answers to class) Finish introductory material

Instructor: Douglas A. Loy Professor of Chemistry and Biochemistry & Materials Science and Engineering Sabbatical from the University of Arizona in Tucson, Arizona in the United States. Extensive experience with polysilsesquioxanes over 20 years.

Syllabus Five quizs (75 %) Attendance (12.5%) 1st quiz today Attendance (12.5%) Class participation (12.5%)

Syllabus Day 1: Hour1: Introduction to course and Hybrid Organic-Inorganic Materials Hour2: Quiz 1. To see what students know at the beginning of course. Day 2: Hour1: Hybrid organic-inorganics in nature Hour2: Types of material interactions Day 3: Hour1: Strategies for making hybrids. Hour2: Quiz 2. Hybrids in nature, material interactions, strategies for preparation of hybrids Day 4: Hour1: Class 1A preassembled inorganic phase (particles, fibers) dispersed in organic phase . Hour2: Class 1B: inorganic phase is made in-situ in organic continuous phase.

Final exam on…… Day 5: Hour1: Quiz 3. Class 1A-C Hour2: Class 1D: Small organic phase dispersed in continuous inorganic phase Day 6: Class 1E: Phases are formed at same time without covalent attachment - interpenetrating networks Hour2: Class 2A: monomer contains both inorganic and organic components-material is assembled at one time Day 7: Hour1: Class 2A continued Polysilsesquioxanes with pendant groups Hour2: Class 2A Bridged Polysilsesquioxanes. Day 8: Hour1: Quiz 4. Class 1D, 1E, & 2A Hour2: Class 2B & 2C Day 9: Hour1 Class 2D & 2E Hour2: Unanswered questions, remaining challenges, and Holy Grails. Final exam on……

Office hours. Science Building, Room 315 M-F 10 AM –noon & 1 PM – 3 PM. Professor Loy is in Arizona, but will be back in his Harbin office on September 24th

Course website For lecture slides and other info. Not at HIT website, at Loy research website:http://www.loyresearchgroup.com/ • Go to loy research group home page and select “courses” on menu at top. • Class website “Harbin Institute of Technology, Hybrid Materials Course” is the first entry. Direct url:http://www.loyresearchgroup.com/harbin-institute-technology---hybrid-materials-course.html

Resources for course: Sol-gel links at Loy group research website. http://www.loyresearchgroup.com/links1.html Sol-Gel Gateway is a website with many tutorials and links. www.solgel.com SciFinder or Google Search Applications of hybrid organic–inorganic nanocomposites J. Mater. Chem., 2005,15, 3559-3592 J. Mater. Chem., 1996,6, 511-525 Gelest Website: http://www.gelest.com/gelest/forms/generalpages/technology_library.aspx

The history of humanity follows the development of materials The rise of technological human civilization is directly tied to the development of more advanced materials. Until the 20th century, this meant making stronger materials, a process that generally meant higher temperature processing. However, with progress in higher temperature processed materials leveling off and with limits to available energy for such processes, we have turned to see how Nature has solved the same challenges, without using high temperatures at all. One result of this new approach is the class of materials called hybrid organic-inorganic materials. Nature uses organic materials derived from biological sources to organize the formation of complex hierarchical structures that are often drammatically stronger than the inorganic material would normally be expected. Over the last 60 years, and in particular the last twenty, we have made significant strides in emulating Nature and even improving on its work. copper bronze stone iron 25°C 600°C 1100°C T°C stone iron

What are Hybrid Materials? Composite materials mixtures of organic and inorganic components Metal oxide network Generally inorganic materials (labeled as ceramics here) are high temperature stable materials. Unfortunately, they are often brittle. Polymers, on the other hand, are very tough, but not very stable to temperatures above 180 °C. Ideally, a hybrid of inorganic and organic materials would have properties somewhere in between. Tougher than a normal ceramic and more thermally stable than an organic polymer. In the case of many such ceramers (hybrid of ceramics and polymers) that is in fact the case. In this course, we will discuss ceramers and many more types of hybrid organic inorganic materials. Improvement on either organic or inorganic components

Why make hybrid materials? Best Inorganic: •Thermal stability •Modulus •Strength •Porosity Organic: •Toughness •Elasticity •Chromophore •Chemical functionality B: Rule of mixtures Bad Hybrids are useful because it allows us to combine the properties of inorganic and organic materials into one composite material. Inorganic materials bring, thermal stability, greater modulus, strength, and porosity to the hybrids. Organics bring toughness, elasticity, optical properties, and chemical functionality to the hybrids. The graph shows the manner in which the properties of the two constituents can add during the formation of the hybrid. Simple rule of mixtures says the properties will be the mean of the two components. Deviations from the rule of mixtures comes when the hybrid has unique structures or morphologies that are different from either organic or inorganic alone. This synergistic improvement is highly desired because it can lead to major changes in technology. Examples would include porous aerogel composites where the one of the phases has a fibral morpology leading to substantially improved strength. Poor integration of the phases can result in degradation of the properties of the hybrid relative to one or both of the constituents. One way to think about this scenario is that one phase acts as a structural defect for the other phase leading to its premature failure. Another example would be increased opacity (Mie scattering) from the mismatch in refractive indices and micron scale interfaces in an a optical material that needs to be transparent to light. Achieve properties not found in either organic or inorganic phase

Applications of hybrid materials Coupling agents For composites Protective coatings Toughened Composites There are many applications for hybrid materials. Many, but not all, are based on organosilicon hybrids. Coupling agents used for tires are low Tg oligomers of organotrialkoxysilanes that are used to interconnect the silica filler now used in tires with the polyisoprene rubber. Protective coatings typically have a metal alkoxide component covalently linked to a polymerizable organic group that cures into a dense impervious coating to prevent water from artwork or construction, gases from passing through containers or OLED packaging, or aqueous corrosives to metal surfaces. Toughned composites are organic polymers that have been modified with inorganic groups that will crosslink the polymers and improve their toughness. Tethers for Bio-molecules Chromatographic Materials (X-Bridge ®, Waters) Photoresists for Lithography Photographs courtesy of Gelest, Inc. and Waters Co.

More applications of hybrid materials Low K Dielectrics Adhesives Sensor coatings POSS have been widely used as low k dielectric materials for microelectronics. Adhesive based on POSS or other inorganic materials as filler and to improve abrassion resistance are widely available. A number of polysilsesquioxane have been used as coatings for surface accoustic wave guide sensors and electrochemcial sensors. A number of porous hybrids, particularly the bridged polysilsesquioxanes, and mesoprous surfactnan templated mateirals have been used as metal ion scavengers to remove toxic and potentially valuable metals from water. Optical coatings includes those designed to correct chromatic aberrations and to provide scratch resistance to optics. A number of hybrid encapsulants have been developed and are availble. Those with low tg components are suitable for wide temperature range fluctuations due to CTE mismatch. Metal Scavenger “resins” Optical coatings encapsulants Plus ceramic precursors (e.g. SiC)

Hybrid Organic-Inorganic materials are common in nature Animals Organic phase is biopolymers Nacre Plants phytolith Argonite (CaCO3) plates as inorganic with protein (polyamide) as organic Animals use proteins to template the formation of complex hierarchical structures with amazing improvements to strength and complex function. Calcium carbonate is the most commonly used material along with calcium phosphate in bone and silica in sponges. Calcium carbonate is generally very weak (chalk). However as a hybrid organic-inorganic composite it can be very strong, as in nacre or mother of pearl. In nacre the thin plate-like bricks of argonite are glued together with elastic protein to give a strong tough material. Cracks oriented perpendicular to the plates into the nacre have to expend energy at all of the interfaces in order to propagate. The small size of the plates and their organization gives rise to the opalescence typical of mother of pearl. In plants, silica is more commonly used than calcium carbonate. In many grasses and leafy plants, silica structures called “phytoliths” are templated by carbohydrates to provide structural materials when located inside the plant and defense against herbivores, when at the surface. The saw tooth structure on the grass blade make eating the plant less appetizing. The complex architectures are templated by proteins or carbohydrates that can provide a complex topological surface and catalysts for the formation of the inorganic structure. We are only just beginning to learn how to use such templates ourselves. In the last twenty years, we have learned to make hierarchical structures that are beginning to look like bio hybrids in their complexity. Teeth, spines in echinderms Mussel shells, sponges, diatoms and corals are utilize hybrid organic-inorganic materials Carbohydrates are the template and organic phase

Ancient Humans also made Hybrid organic-inorganic materials: Maya Blue Indigo + white clay palygorskite (Mg,Al)2Si4O10(OH)·4(H2O) (also called Fullers Earth) Mayans used a hybrid organic-inorganic material made from the organic dye indigo and a clay. The flat dye fits into pores in the clay normally occupied by water molecules. There it is protected from oxidation and leaching, making this the longest lived organic based dye. Another blue dye (Han blue) also survives from ancient times, but is solely based on a transition metal in a clay without any organics. Indigo is found in the “woad” plant and has long been used as a dye for cloth. It is the dye that give blue jeans their color. Blue jeans fade because the dye is leached out and more importantly oxidized to a less colorful form. The clay protects the indigo allowing it to last for thousands of years. Some of the earliest hybrids in modern times are based on dyes encapsulated into a metal oxide matrix. This can be viewed as a step beyond using alumina mordants or fixing agents to prevent the color in dyed cloth from running. L. A. Polette, N. Ugarte, M. José Yacamán and R. Chianelli, Sci. Am. Discovering Archaeology, 2000, July–August, 46

Different ways to put hybrids together Class 1: No covalent bonds between inorganic and organic phases Example: particle filled polymer Class 2: Covalent bonds between inorganic and organic phases The most common method for classifying hybrids is based on how the organic and inorganic are connected. Class 1 has no covalent bonds between the two phases and class 2 has covalent bonds. Further divisions in each class are based on the size and morphology of the phases and how they are introduced. It is possible to have characteristics of class 1 and class 2 in a single material. Close-up of hybrid particle Monomers in solvent Gel or dry gel (xerogel)

Organic phases Small molecules Macromolecules Biocides & surfactants Dyes Surface modifiers Dansylsilane Macromolecules The organic phases can be small molecules include dyes, biocides, odor agents, and surfactants to list a few. At this length scale, the organic really is not a separate chemical phase but is dispersed or dissolved in the continuous inorganic phase. In class 1 the organics are physically mixed with the inorganic phase. In class two they are covalently or chemically attached. The Rhodamine dye would be physically encapsulated. The Dansylsilane dye would be covalently attached (Class 2). Organic phases can also be macromolecules with molecular weights in the thousands or even millionsof grams/mole. The polyethers, vinyl polymers and polyamides shown are just a few of the many polymers used in hybrids. The polyethers are polar polymers that can be used to phase separate and template meso scale inorganic structures. We will talk about templating in detail as it is the way that biomimetic materials are made. The polyphenyl ether at the bottom has silyl groups attached at the ends of the macromolecules to permit covalent attachment to the inorganic phase.

Some hybrid monomers: Polymerize by hydrolysis and condensation (sol-gel polymerization) Monomers 2-4 polymerize to class 2 materials But act like class 1 in many cases. Used for many of the other classes as the inorganic component. These hybrid will be discussed in the course in class 2A, but are mentioned here because of their similarity to metal oxides in their ability to form porous network materials. Thus they will be used in a number of other “classes” despite their formal inclusion into class 2A.

Inorganic Phases Metal Oxide Networks Organically modified Metal Oxide Networks These are sol-gel polymerizations of metal alkoxides or metal salts (MXn) in the top scheme to afford porous metal oxides. The monomers react, particles forma and phase separate then aggregate into fractal structures with porosity formed by the interstices of the particles. Organically modified particles, generally refers to silsesquioxanes (RSiO1.5)n. They can be copolymerized with tetraalkoxysilanes and other metal oxides if the silsesquioxane is not prone to gelation. In such copolymerizaitons, the silsesquioxane acts like a blocking group or surface modifier so that these copolymers are surfaced with the organic functionality.

Inorganic Phases Preformed inorganic clusters Silica Particles POSS Ti12O16(OPri)16 Ti17O24(OPri)20 Ti18O22(OBun)26(Acac)2 Ti(OR)4-x(acac)x Silica Particles POSS Pre-formed inorganic phases are predominantly oligomeric particles of metal oxides or silica or silsesquioxanes. Depending on their size they may actually be soluble. Others have solids. By preforming the particles, then adding them to the organic phase (in solution), relatively large quantities of inorganic phase can be introduced homogeneously and, once the solvent has evaporated, a composite material is obtained. This approach allows better control over size and quantity of particles than in situ techniques. Another advantage of preforming particles Is that their surfaces can be more readily modified than in situ approaches. Most metals are very reactive with water to the point where precipitates will form instead of gels or well defined compounds. Metals like titanium will form oligomeric complexes by forming additional links through alkoxide group lone pairs. Alternative, chelate ligands can be added to slow the chemistry down and allow the formation of discrete oligomeric species. For larger, narrow size dispersity metal oxide particles, microemulsions can be used. Silica particles can also be made with microemulsions, but they can also be made from tetraethoxysilane without surfactants in aqueous rich ammonical ethanol. Silsesquioxane partilcles can be made, albeit with more difficulty than silica. The interesting silsesquioxane species is the POSS which is essentially a molecular sized particle. These have widely been used as fillers for polymers, though they perform better if covalently attached. Overall, the problem with non-covalent integration of inorganic phases is they may very well aggregate or settle during processing, So ensuring homogeniety is important.

Inorganic Phases Isolated metal atoms in polymeric architectures (organometallic polymers) Polysilanes are a type of polymer called a catenate. The backbone atoms are all silicon (can also be all Ge or Sn) with two organic groups per Si. It is impossible to prepare the carbon analog with two organic groups per carbon. Due to the d-orbital contributions to bonding along the chain, polysilanes are semiconducting and absorb UV light. They are also good precursor materials for silicon carbide ceramics. Available commerically in research quantities. Polyphosphazenes are insulating polymers-many of which are elastomers with very low glass transition temperatures making them of interest for rubber used in arctic regions. Backbone will degrade to phosphate and ammonia making them of interest for biodegradible materials. Metal Organic Frameworks (MOF’s) were recently discovered (Omar Yaghi) and are one of the hottest areas of research in materials science. It may very well lead to a Nobel prize. Made by reacting organic carboxylic acids with metal salts. They have the highest known surface areas of any material and are currently of strong interest as hydrogen storage materials as they have reportedly achieved over 10wt% hydrogen without cryogenics and only moderate pressure. Polysiloxanes are the most widely sold hybrid material. The alternating Si-O bonds are very flexible due to the d-orbitals of the silicon, making these materials rubbery down to -123 °C. They are thermally more stable than organic polymers and only burn when a flame is held to them. They give off white smoke when they burn (silica). Hydroxyterminated polysiloxanes are useful in reacting with metal oxide monomers to make terminal crosslinked hybrid composites. Polymetallophthalocyanines are presently regaining a great deal of interest because their photoconducting properties and great stability to UV and heat as a dye make them extremely attractive as components in photovoltaic cells.

Inorganic Phases Carbon Buckeyballs, nanotubes and graphene Fullerenes or buckyballs are polyhedral carbon forms very similar to the somewhat smaller polyhedral oligosilsesquioxanes (POSS). Fullerenes generally come as the C60 and C70 sizes. They are relatively non-polar so integrating into aqueous rich sol-gel doen’t work well without surface modification of the fullerenes. Unforntunately this also modifies the elctronic properties of the fullerene. Carbon nanotubes were discovered ten years after buckyballs by NEC researchers. This linear analog of fullerenes can be prepard in single wall and multiwall forms (think onions) and can be dispersed into sols and formed into gels and aerogels. Graphene is a more recent discovery than continues this expansion of the field of carbon based materials into well defined forms. Graphene is a single sheet of graphite that has been exfoliated from the stack. Fullerenes in hybrids are interesting for optical applications. Nanotubes and graphenes are attractive to strengthen the hybrid. Nature Materials 9, 868–871 (2010)

Making Hybrid Materials: Class 1A (pre-formed particles and fibers) This is probably one of the oldest hybrid approaches as paints are hybrids of pigment in polymer. Preformed particles can be added to a solution of a polymer as a dispersion, then cast and dried to afford the desired composite. The dispersion allows faster mixing of the particles and polymers and prevents irreversible aggregation of the more reactive smallest particles. One consequence of the lack of covalent attachment is sample homogeneity. One must be wary of the particles sinking or floating or otherwise aggregating before the solvent can be removed sufficiently to raise the viscosity to where the particles are going to be moving fast enough to lose homogeneous distribution. Physical mixing or particles

Making Hybrid Materials: Class 1B (in situ particle growth) Growing particles in situ is generally done with solvent free systems. A liquid monomer is diffused into a film made of an organic polymer that already contains a catalyst and possibly permeated water. The sol-gel polymerization will proceed slowly, generally requiring heating and eventually particles will precipitate form the polymer. The approach works best above the glass transition temperature of the polymer and in systems without a lot of crystallinity. It can be done in the melt of the polymer as long as water can be present for siloxane or metal oxide bond formation. Actual final wt% catalyst is difficult to get to levels possible with preformed particles. No Solvent except for monomer(s) Generally uses low tg organic polymers or in polymer melts (< 100 °C).

Making Hybrid Materials: Class 1C (Polymerizing in pores) Non-porous composite material IN this approach a porous metal oxide is soaked in an organic monomer (e.g. styrene) with a free radical initiator (photoacid generators can be used for Lewis acid catalyzed polymerizations). Porous silica or metal oxides will soak up liquid until the pores are filled and still remain free-flowing powders. Once the polymerization is complete, the porosity will be most gone. While most vinyl polymerizations are accompanied by shrinkage, the pores are generally small enough that this does not lead to enough stress to cause damage. Porous metal oxide Liquid monomer (no solvent) UV, heat, radiation

Making Hybrid Materials: Class 1D (encapsulation of small organics) © Asahi-Kirin By placing soluble organic dyes into the polymerization solution for a silica or silsesquioxane, it is possible to physically entrap or encapsulate some of the dye. This especially true if the dye is large and the pores are small. However, there will be a considerable amount of dye that is left in the solvent and on the surface of the metal oxide. And this dye will leach out or will need to be thoroughly rinsed off unless the metal oxide is a strong mordant. Polymerize metal oxide around organic pores must be small or leakage will occur Solid state dye lasers, filters, colored glass

Making Hybrid Materials: Class 1E (Interpenetrating network) Interpenetrating networks have been reported with synergistic properties. That is to say, they perform better than would be expected based on the rule of mixtures. In the figure the interpenetrating network is made by polymerizing two different monomers in the same reactor. The two monomers cannot react with each other. That is the polymerizations are exclusive so that no copolymers form. Sol-gel and vinyl polymerizations are a good exclusive pair of reactions. It is important that the polymerizations proceed at similar rates. If one polymerization proceeds faster than the other there is a real chance of phase separation before the interpenetrating network can be formed. Both organic and inorganic phases grow simultaneously Timing is more difficult Reproducibility is a challenge May need to use crosslinking organic monomers to ensure solid product

Making Hybrid Materials: Class 2A (Covalent links at molecular level) This approach is how most sol-gels are made. A monomer with organic and inorganic groups is made and polymerized by hydrolysis and condensation reactions if a silicon alkoxide system or through ligation if a MOF. This is also the approach used to make the various organometallic polymers discussed earlier. Organic group is attached to network at molecular level Pendant or bridging monomers Bridging groups can be small or macromolecule This class also includes the organometallic polymers

Making Hybrid Materials: Class 2B (Covalent links at polymer level) IN this system, a polymer with carboxylic acid groups is mixed with a metal oxide that has an affinity to carboxylate groups, such as a tin oxide. The two are melted together allowing the carboxylic acid groups to form bonds with the tin. Relatively underutilized strategy. ligands attached to polymer Reaction rates slow unless in sol. or melt

Making Hybrid Materials: Class 2C (Templating) Shown here with block copolymer  Mean-field prediction of the thermodynamic equilibrium phase structures for conformationally symmetric diblock melts. Phases are labeled as: L (lamellar), C (hexagonal cylinders), G (bicontinuous cubic), S (body-centered cubic spheres). fA is the volume fraction. Heat polymer then cool or cast from solvent

Making Hybrid Materials: Class 2C (Templating) Shown here with block copolymer polyisoprene Sol-gel system Block copolymer PEO, Al2O3 and RSiO1.5 When the glycidaloxypropyltrimethoxysilane and triisobutylaluminum were added to the block copolymer and cast, the resulting mixture would phase separate into structures that depended on the block size used in the copolymer. The sol-gel components dissolved into the polar polyether phaseleaving the polyisoprene block as simple organic. The resulting alumina and silsesquioxane and ring opened epoxy provided a solid matrix that is easy to discern and is resist to dissolution. This allows the block copolymer to be extracted from the system leaving the templated hybrid behind. Block copolymer Multiple phases created by varying size of blocks

Templating with surfactants 1. Davis, H. T., Bodet, J. F., Scriven, L. E., Miller, W. G. Physics of Amphiphilic Layers, 1987, Springer-Verlag, New York • First prepare two phase surfactant system • Add monomer. Sol-gel monomers move into aqueous phase with hydrolysis • Filter precipitate • remove surfactant by calcining or extraction Surfactants or other amphiphilc species will phase segregate when placed into water to hide their water insoluble portions from the continuous aqueous phase. At low surfactant concentrations, this results in micelle. With higher and higher concentrations of surfactant, structures are possible that have the hydrophobic phase continuous through the volume of the mixture. These structures include hexagonal closed packed hollow cylinders, lamellae, and a variety of cubic structures. Many different phases can be accessed

Surfactant templating to make hierarchical materials Surfactant templating involves adding a surfactant to a quantity of with acid base or fluoride as catalyst. Once the surfactant/water system has had a chance to organize into whatever phase is accessible to the conditions used, the monomer is introduced (usually dropwise). The monomer hydrolyzes to the hydroxyl susbtituted species that are water soluble and the hybrid polymer forms in the water phase surrounding the surfactant template. The the reaction run for anywhere from a couple of hours to days before, extracting with alcohol to remove the surfactant or calcining if metal oxides are used.

Surfactant templating Surfactant templating silsesquioxanes has resulted in the widely popular “mesoporous” organosilicas. These materials exhibit long range order in mesopores that makes them appealing to the eye. However, it is not clear that there is any advantage to surfactant templating over simple sol-gel polymerization-which generally is easier and faster and cheaper and gives materials with very high surface areas and relatively narrow pore size distributions. Benzene-silica hybrid material with 3.8 nm pore diameter (Inagaki, Nature, 2002).

Classes 2D &E Covalent coupling agents Class 2D: Attaching organic group onto inorganic material Surface modification of silica has been widely used since the 1960’s to prepare glass or silica fillers to be used in plastics. The surface silanols are reacted with organosilanes (often silsesquioxane). While a monolayer does not form, a low tg oligomer may form that sticks to the surface and provides a pliant interphase to help curb cracking of the resulting composite. Less common is the modification of organics with polymerizable silyl groups. Since vinyl triethoxysilane is a relatively poor monomer, triethoxysilyl groups are generally added by post polymerization modification. Usually by radiation generation of radicals along the chin in the presence of a sea of vinyl triethoxysilane. Once in place simple hydrolysis with humidity is enough to convert alkoxysilyl groups to silanols that can react to afford siloxane cross-links –likely during extrusion or calendaring. Class 2E: Attaching inorganic group onto organic polymer For tough electrical wire coating & shrink fit wrap

Summary Outline for course. Step by step practical review of hybrid materials syntheses, structure, properties, and applications Five quizes, no final exam Attendance is required Lecture notes available at www.loyresearchgroup.com under courses.