lecture 6 hybrid poss

Lecture 6 Hybrid POSS

Lecture 6 Hybrid POSS
Class 1B: inorganic phase is made in-situ in organic continuous phase.
in situ means in place

Making Hybrid Materials: Class 1B (in situ particle growth)
Ideally, no Solvent except for monomer(s)No solvent with low tg organic polymers or in polymer melts (< 100 C) or if monomer will is soluble in polymer.

Otherwise solvent may be used to mix before casting.

Preparation by dissolving inorganic monomer in solid organic polymer then polymerizing to form particles
Solid Organic Polymer
Monomer must be soluble in polymer
Polymerization by:1) hydrolysis & condensation of M(OR)nWater diffuses into membrane from air2) Reduction of metal compound. H2 or polymer itself as reducing agent3) sulfidation of cations (H2S)4) photochemical or thermal decomposition

Inorganic Monomer (liquid) & catalyst
Organic Polymer Inorganic Monomer
Monomer polymerizes & forms particles

Preparation by mixing inorganic monomer with liquid organic polymer & hydrolysis & condensation
Physical mixing
Very few liquid polymers, save some elastomers like polydimethylsiloxane and low molecular weight polybutadiene or polyisoprene
Liquid Organic Polymer
1) Water diffuses in from outside2) Monomer polymerizes 3) Particles phase separate
Silica-PMS materials are looked at later in lecture

Preparation by mixing inorganic monomer with solid organic polymer and allowing particles to form
Physical mixing
Solvent must be removed before polymerization

Water for hydrolysis and condensation would be diffused into dry film from air.
Organic Polymer Inorganic Monomerin solvent
Dissolve
Organic Polymer in solvent
Evaporate solvent
Inorganic Monomer (liquid)
Monomer polymerizes & forms particles
Silica-Nafion materialsmade this way are looked at later in lecture

Polymers used in class 1B systems
Elastomers: silicone, polybutadiene, polyisopreneThermoplastics: polyurethanes, polycarbonates, polyvinylalcohol, polyacrylates, polysulfones, polyethylene oxide (PEO), polypropylene oxide (PPO)Thermosets: epoxiesPolyionomers: Nafion

Review of polymerizations
1) hydrolysis & condensation of M(OR)nor RSi(OR)32) Reduction of metal compound3) sulfidation of cations4) photochemical or thermal decomposition

Hydrolysis & condensation of M(OR)n: the monomers
Silicon: Si(OR)4 or RSi(OR)3
Aluminum: Al(OR)3 or AlCl3 6H2O
Transition metals: Mz(OR)n or MzCln hH2O
z = oxidation number for metaln = number of alkoxide or halide in monomerh = number of coordinating waters

Hydrolysis & condensation of Si(OR)4
Catalyzed by acids (HCl,HNO3) or bases (NH3 aq, NaOH) or fluoride.

Particles from hydrolysis & condensation of Si(OR)4
Typically leads to amorphous spherical particles (not quartz) Can template particles with ordered mesopores with surfactants Stober synthesis from TEOS with NH3 and water gives monodisperse particles Emulsion polymerization (water in oil) gives monodisperse particles Other preps give polydisperse particles

Hydrolysis & condensation of RSi(OR)3

Particles from hydrolysis & condensation of RSi(OR)3
Typically leads to amorphous spherical particles Not as easy to prepare particles as with silica Can template particles with ordered mesopores with surfactants Stober synthesis from TEOS with NH3 and water affords polydisperse particles Emulsion polymerization (two step) gives monodisperse particles

Hydrolysis & Condensation of Mz(OR)n to form MOn/2
Hydrolysis: formation of monomeric MOH species
higher charge & higher pHMore reactive, but too high shuts down condensation
Condensation: formation of oxo (neutral) M-O-M
M = Al, B, Ti, Zr, Cr, Fe..

Hydrolysis & Condensation of Mz(OR)n to form MOn/2
Olation: formation of oxo (neutral) M-O-M
olation >> oxolation

Particles from hydrolysis & condensation of M(OR)n
Particles may be amorphous or crystalline Some amorphous particles will crystallize with time. Stober synthesis from TEOS with NH3 and water does not work Emulsion polymerization (water in oil) gives monodisperse particles Many more molecular clusters are available through olation chemistry

Reduction of metals

Metal and Semiconducting Sulfides

Class 1B: in situ Silica-Nafion Nanocomposite
Solid Nafion
Physical mixing
Nafion & TEOSin ethanol
Dissolve
Nafion in Ethanol
Evaporate solvent
Si(OEt)4 (liquid)
Nafion & TEOS
Silica particles form in membrane
NafionTM

5 weight percent ex situ silica in Nafion
Class 1B: in situ Silica-Nafion Nanocomposite
In situ Silica particles

In situ filled Silica in polydimethylsiloxanes
Journal of Polymer Science Part B: Polymer Physics, 2003, 41, 16

Highly transparent

Does not require mechanical blending


Silver particles made in situ polydimethylsiloxanes

Templating with triblock copolymer is formally a Class 1B material
Polymer is template. After removal, silica remains

Class 1B: In situ formation of inorganic phases in polymers
Method for mixing at nanoscale without mechanical blending required-less chance for aggregation and segregation to occur (steric stabilization)Raises modulus and strength of materialsIn situ polymerization of inorganics selectively in blocks of block copolymers-first step to biomimetic mineralization.

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Class 1 B materials are those in which the inorganic phase is grown from a monomer or precursor in the organic polymer as a solvent or template. It is done to reduce the stress of mechanical mixing (probably not as big a problem as indicated) and to prevent aggregation and segregation experienced during mixing. Detractions: more expensive, time consuming, and control over particle size is generally not as good as in Class 1A materials.
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It is often said that the major advantage of filling a network by this in situ approach is the avoidance of the problems associated with mechanical blending. In this case the monomer is dissolved into the solid polymer then reacted to make the inorganic phase. The size if the inorganic phase is going to depend on the polymer free volume (the space between polymer chains) and how much monomer can be soaked into the polymer. This approach limits the processing to thin films or fibers since diffusion in solid polymers is slow. Polymerizations will be described later, but in several cases there is a coreactant that must also be difused into the polymer. Depending on reaction rates, it is possible to get gradient structures.
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This is the case of polymers like polydimethylsiloxane (PDMS) which are liquids unless they are highly filled with inorganic or crosslinked. In this case, all you have to worry about is the misciblity of the monomer with the polymer. These are often hydrolysis and condensation systems, though metals have been reduced in PDMS as well. To get the water into the system, you generally store samples in humidity chambers under 100 percent relative humidity. The thermosets may also be in this category as the silica may be prepared in situ in the epoxy prepolymer before it is cured or crosslinked.
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A lot of the theromplastics arte no going to have very fast diffusion of monomers into them, so many researchers have resorted to premixing everything in a solvent, then evaporate quickly and allow the particle formation to continue. The polymerization reaction kinetics leading to particles needs to be slow for this to be acceptable. For hydrolysis and condensation, the fact that the water has to come from outside, allows this to work. If you added the water at the same time as monomer youd have problems.
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The number of polymers is pretty extensive since people have been studying this Class1B approach since the 80s when Jim Mark and Garth Wilkes pioneered the field with silicones and polyethers, respectively.
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Lets look at little at the polymerization chemistries involved. We have already spoken about hydroysis and condensation of alkoxides so this will be some review and some new material to build on the old. I will briefly introduce reduction sulfidation and decomposition approaches.
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In situ filling has been done with a lot of different silicon and metal oxides, including the silica and silsesquioxanes at the top of the list. Main group metal oxides like alumina and boron oxides have also been looked at along with a number of transition metals. Magnetic metal oxides (magnetite) in nanocomposites are very popular because you can physically remove nanocomposites from mixtures by a magnet.
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A erview of the hydrolysis and condensation chemistry for tetraalkoxysilanes. These monomers are reactive with water but reaction rates are slow without catalysts. Since the water is added to the Class 1B materials in the vapor phase, the catalyst needs to be in with the monomer. That rules out HCl, or ammonia. One commonly used catalyst class of catalysts are based on tin (II), like dibutyl tin dilaurate 9which are generally liquids). Careful around tin compounds because the dialkyl tin is toxic. So as water diffuses into our polymer impregnated with monomer and tin catalyst it reacts to hydrolyze the alkoxide groups which means alcohol is liberated. The water generated by the condensation step will most get used up by more hydrolyses. Now in the polymer, it is going to be slow for molecules to diffuse and as they grow, it will become even harder to move around. So much of the particle growth will be monomer diffusing to growing clusters and eventually particles.
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This is a summary of sol-gel polymerization of tetraalkoxysilanes outside of a class 1B system, but in the class 1B, the particles will likely be spherical, amorphous and polydisperse. In many cases people have reported larger particles (for a weight percent of precursor) than they would see in a regular low viscosity solvent.
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Now with the silsesquioxane monomers, the only differences are small increases in rates of reaction under acidic conditions and slightly lower rates under basic conditions. The organic group, R, acts as a steric blocking group helping make cyclics but hindering development of networks. The only monomers suitable for in situ polymerization in polymers are the methyltrialkoxysilane, some with reactive functionalities on the R group, and bridged monomers with two or more trialkoxysilane groups. Many of the simple alkyl or aryl groups cause formation of an oily phase that plasticizes the polymer making it weaker.
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Again general characterisitics for organotrialkoxysilane polymerizations. Note Stober method does not afford nice monodisperse particles like it does for tetraethoxysilane.
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Formation of metal oxides like TiO2, Fe2O3, Al2O3, etc. Metal oxides can be prepared from metal salt hydrates, like AlCl36H2O, or from metal alkoxides like Al(Obu)3. In all cases oxidized metals are more electrophilic and generally more reactive with water than silicon. In fact metal salts, when added to water, will lower the pH considerably by forming oxonium species like the intermediate shown in the hydrolysis scheme above. Since every monomer gets a positve charge from the coordinated water, the monomers repel each other and wont react until base is added to neutralize the acid. Metal alkoxides have such electrohilic metals that they dont give up the alkoxide group as a base the way sodium alkoxides would in water. Instead, they get lone pairs from other alkoxides on other metals or on solvent molecules to help satisfy the metals electrophilicity. A simple transition metal ethoxide is actually an oligomer through these tris hapto bonds. Now formaly the hydrolysis and condensations see above are mechanistically similar to the reactions on silicon. Just a lot faster. Usually we try and slow the sol-gel chemistry down to prevent precipitation. So being in a polymer with water as the limiting reagent is actually a good thing. Now metals can have multiple oxidation states, generally, the higher the oxidation state the more electrophilic a metal is. However, two high and the metal grabs all of the electron density of the oxygen making unable to participlate in condensation reactions as a nucleophile. One way to think of these highly oxidized metal oxides is that they have a metal oxygen double bond. So lower oxidation states actually work out better for sol-gel. The reason to put these metals into polymers is that the metal oxides have high Z and high refractive indices (for optical applications) and the metals give rise to nanocomposites that are harder and more abrasion resistant than silicon.
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Now this slide shows you the way that metals grab lone pairs off of oxygens with two bonds already in place. The metals use the oxygeas a ligand using eh lone pairs. This gives rise to a higher level, more compact bonding that observed with silicon. Metals with their full use of the d orbitals can easy form four atom rings whereas silicon cannot.
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Now metal oxides have lower barriers to crystallization than silica. To turn amorphous silica into quartz you need high temperatures and pressues in a hydrothermal environment. In metals amorphous materials can crystallize at room temperature. Stober chemistry doesnt work at all in making monodisperse particles, but emulsion chemistry does. One thing silicon cant do is form the very cool molecular clusters that many metal alkoxides form even before hydrolysis. We will talk more about sol-gel chemistry throughout the course.
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Reduction of metals is a topic we havent broached yet. However, it is a form of chemistry humans have been purposefully using since Roman times. Rake a metal salt and chemical reduce it in solution and you get metal nanoparticles. This was done in ceramics by the Romans with gold nanoparticles that made their ceramics red. Red stained glass from the Middle Ages is often red with gold nanoparticles. Metal nanoparticles are electrically conducting, so adding them to a polymer makes the polymer conducting once you reach percolation levels. They can be made by chemical reduction with hydrazine, hydrogen gas, hydroquinone, silyl hudrides, and alcohols. The last reaction of the metal carbonyls is not a reduction since the metal doesnt change oxidation state. It is a cool way to make particles, though the metal carbonyls are extremely toxic and, in some cases, pyrophoric (spontaneously catch fire).
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Metal sulfides are semiconductors and if made small enough show quantum effects like blue shifting of the absorption and emission bands into the visible range of light. So nanocomposites with metal sulfides would potentially have light emitting diode applications. You make them with hydrogen sulfide, but that is very smelly and toxic so most use either sulfur or thioureas to sulfide metals or metal salts.
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Now back to class 1B materials preparation. This is some work my group did. We were making Nafion silica class 1A materials, but made a couple of Class 1B for comparative purposes. Always design control and baseline experiments based on literature so that your work can be compared with others more easily. Anyway, we wanted to have silica particles in Nafion to see if we could reduce water usage in fuel cells and increase ionic conductivity and thermal stability of the sulfonated fluoropolymer membranes.
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The membrane made by Class 1A methods is on the left, the membrane from class 1B is on the right. We found that you could not systematicaly control of the size of the particles with the Class 1B approach.
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Poly(dimethylsiloxane) (PDMS) [Si(CH3)2O] is probably the most important and the most useful high-performance elastomer, but only when its inherent weakness is overcome by reinforcement with some particulate filler, usually silica. Such fillers are typically mechanically blended into the polymers before crosslinking. There are inherent problems, however, with this type of ex situ blending process. Some of the disadvantages of blended systems include the time-consuming and energy-intensive nature of the blending and the lack of control over the morphology or surface characteristics of the aggregated filler.In a typical two-step process, a polymer network is first formed with PDMS, tetraethoxysilane (TEOS), and stannous octoate (STO). This crosslinked network is swollen in TEOS for a few minutes and then placed in a solution containing acid or base and water to precipitate silica. In this work, the unfilled polymer network was swollen in a solution containing TEOS and a tin salt. No water was added except as was absorbed by the sample from water vapor in the air.
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In a typical two-step process, a polymer network is first formed with PDMS, tetraethoxysilane (TEOS), and stannous octoate (STO). This crosslinked network is swollen in TEOS for a few minutes and then placed in a solution containing acid or base and water to precipitate silica. In this work, the unfilled polymer network was swollen in a solution containing TEOS and a tin salt. No water was added except as was absorbed by the sample from water vapor in the air.
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The strength of the silica polydimethylsiloxane polymer nanocomposites with 0-154 wt% silica made by class 1B approach are shown here. PDMS without filler is a very weak material. Remember tehermoplastics have tensile strengths between 10-60 MPa. The tensile strength of PDMS is less than 0.1 MPa. With the silica in it, the strength went up eight fold to almost 1 MPa.
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IN this work PDMS was filled with silver particles made with a silyl hydride as the in situ reducing agent. The stress strain curve shows improvement in strength but this can be misleading since there is also some curing taking place with the reduction. Thre are many other examples of type 1B nanocompoistes, but in the end using the method to provide mechanical reinforcement is not a economically feasible approach. There are other things you can do with a Class 1B materilal however.
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The block copolymers used to template silica, silsesquioxane or metal oxide growth are basically a Class 1B system. They are lumped in class 2 in this course, but in the future I will probably move them to this lecture. The triblock above has hydrophilic and hydrophobic phases. Unhydrolyzed monoemr will dissovle in the hydrophobic phase but as it hydrolyzes, it becomes hydrophilic enough that it migrates into the hydrophilic phase where it condenses into an amorphous structure tempalted by the 3-D structure of the surfactant. Note if you start with the metal salt hydrates they will go into and stay in the hydrophilic phase. We will talk more about these templated materials in the lectures to come, because they are the closet akin to biohybrids that we have to date.
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lecture 6 hybrid poss

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