Hybrid Materials & POSS Lecture 5: : Class 1A preassembled inorganic phase (particles, fibers) dispersed in organic phase This lecture covers the simplest class of hybrids-those based on inorganic phases physically mixed with a polymer with no covalent bonds. The SEM is of silica nanoparticles dispersed in a polymer electrolyte called Nafion.

Class 1A hybrids are a composite material based on an inorganic particle & an organic polymer Silica particle (130 nm in diameter) Heres another picture of the silica particles in Nafion. The silica particles were synthesized by one of my graduate students using what is called the Stober synthesis then the particles were mixed with Nafion dissolved in ethanol. The solution was cast and dried in an oven before characterizing. The continuous phase means the phase that has no phase boundaries save the walls of the container. In the language of composite science, the continuous phase is called the matrix. The dispersed phase (filler in composites) is the phase with boundaries. In this case the spherical edge of the particles is the boundary between the silica and the Nafion. 5 weight percent silica in Nafion Polymer is the continuous phase or matrix The inorganic particles is the dispersed phase or filler

Preparation by melting polymer and mixing So, making a class 1A material. It’s the easiest of any of the hybrids. Just mix the two components physically. If they are liquids, pour them together and stir. If they are miscible they will form a single phase solution. If they are viscous, it may take time or heating to get them mixed homogeneously. If the liquids are immiscible, the they can be blended together to form an emulsion. Low viscosity emulsions often require surfactants to kinetically stabilize them against separating. Higher viscosity liquids, like melted polymers are harder to blend, but slower to phase segregate, especially if you flash cool the emulsion into a glass. Mixing solids into polymers can be done, as shown here, by mixing the filler phase into the melted polymer. Not all polymers melt. You can polymerize the monomer or prepolymer around the particle (this will be discussed in more detail later). One of your main concerns with class 1A materials is the aggregation and sedimentation (or floating if the filler is low density relative to matrix) of the particles. This results in a heterogeneous material that is inferior in properties.

Preparation by dissolving polymer and mixing Solid Inorganic particles Solid Inorganic particles dispersed in same solvent The other way to prepare a class 1a hybrid is to place both the organic and inorganic phases in solvent. The polymer will dissolve to give a liquid solution while the particle will form a solid dispersion in a liquid. Do not call a dispersion a solution. Dispersions are not solutions-they are two phases mixtures. Solutions are one phase mixtures. Be careful not too have too many particles in the liquid because they might percolate and gel or act as either a thixotrope (shear thining) or dilatant (shear thickening) system. Once the solution and dispersion have been mixed, the solvent must be evaporated before aggregation can oocur. Particle dispersion in solid polymer

Reasons for making a particle-filled polymer Fillers (CaCO3, Silica, Talc, wood powder) are cheaper than some plastics-cut cost. Reduce Coefficient of thermal expansion of polymer Reduce shrinkage during thermoset curing Improve abrasion resistance and hardness Increase modulus Make melt more viscous or gel (thixotrope) Make Flame resistant Aesthetics – pearlesence or opalescence There are many reasons for adding particles (filler) to a polymer. The result is called a composite. These are not the superstrong composites used to make racing bicycles or airplanes or windmills. Those use woven fibers mats or clothes or tapes to provide a continuous high modulus, hign strength inorganic phase. In these materials , the inorganic particles will only interact with each other through non-bonding interactions, or perhaps a few bonds. This is a much weaker interaction. So don’t expect the tensile strength (stretching and breaking) to be a lot higher than without the particles. The most common use for adding particles is to reduce cost, followed by reducing CTE (coefficient of tthermal expansion mismatch-inorganic materials change size only a little bit with changes in temperature where organic polymers change size a great deal with each degree. By adding a bunch of filler you reduce the change in size with temperature that could cause your plastic part or adhesive bond to break. Same thing for reducing cure shrinkage. All monomers increase in density when they polymerize. This means that they will weight the same but occupy less volume. That can cause stress if the plastic is made into a part by polymerizing (like a thermoset eg. Epoxy). Particles are high modulus and strong so they make the composite material hard and resistant to abrasion. The modulus increases because the particles hinder flow of the polymer, but more because they have high modulus, and the modulus of the composite is (rule of averages) in between that of the polymer and the filler. Particles make things more viscous because they can form percolating chains that are resistant to flow (definition of viscosity). IF you particle is flame resistant, then adding some to a combustable polymer makes it less combustable. Aesthetics usually means pretty optical effects like opalescence or pearlesence.

Organic polymers that have been used: Thermoplastics: polystyrene, poly(methyl methacrylate), HDPE, polypropylene, Nylon’s, polycarbonate, polyimides, poly(ethylene oxide), polyurethanes, polyesters…. Elastomers: silicones, polyisoprene,… Thermosets: epoxies, Polyelectrolytes: Nafion Pretty self explanatory. People have been making these things for sixty years now. It doesn’t mean that everythings been done however. New polymers and new filler materials are discovered every year. Graphene is a recent example. practically every commercial polymer known.

Class 1 Hybrids: No covalent bonds between organic & inorganic phases Physically dispersed particles in polymer Generally meta-stable: particles will segregate if given the opportunity Ok. Back to calss 1a materials. Physical mixtures of particles or fibers (dispersed phase) and polymer (continuous phase). I put the dragon fruit in this slide because I like dragon fruit and it looks a lot like the TEM of the POSS in polypropylene next to it. Metastability, means that it is not the thermodynamically most stable state. The particle dispersion may be kinetically stabilized by a solid polymer, but those particles don’t like having all of that surface area. Particle aggregation will be a problem during preparation of the materials. POSS in polypropylene

Just as the colloid made from blending oil and water will return back to two continuous phases This is a liquid liquid two phase system. On the left the oil and water phases have segregated back and have the minimum surface area interface possible. Physically mix the oil and water together and you get the emulsion on the right. It is called an oil in water emulsion simply because there is more water to start with so it gets to be the continuous phase. Without a surfactant, the emulsion will break down and the oil and water layers will reform. This process is fast because the viscous is too low to create a barrier. Surfactants raise the barrier and kinetically stabilize the emulsion. With time Which is the lower energy state?

Aggregation of particles in polymer For class 1a materials, aggregation is like the emulsion on the prior slide breaking down into layers. You can reduce aggregation by charge repulsion (make all of the particles negative or all of them positive and they will repell each other) or you can sterically stabilize them with surfactants or you can increase the viscosity as quickly as you can. Particle sedimentation rates are described by Stokes Einsteins law as being inversely proportional to the viscosity. Silica in polydimethylsiloxane (PDMS). Silica and PDMS in toluene were mixed, then cast and dried. Aggregation occurs during drying stage before viscosity gets too high. Polymer 205, 46, 41270

Sedimentation of particles during mixing and drying This came from my groups research. You remember the slides at the beginning of the talk with the SEMs of silica particles in Nafion? They were made by mixing particles with Nafion dissolved in ethanol. The solution was cast and dried in an oven. However, we found that the particles were floating to the top surface of the membrane. We wanted a homogeneous distribution throughout the Nafion to help the polymer electrolyte conduct protons in a fuel cell, but we were getting the heterogeneous membrane shown by (a) above. (b) (a)

Sedimentation of particles during mixing and drying (b) (a) Solution viscosity was too low Particles floated to the top of the membrane as the solvent dried Solved problem by evaporating solvent while mixing until viscosity was 65 cP. Here are SEMs of the top (with lots of particles) and bottom of the Silica-Nafion composite membrane. There were so many particles on the top that they would fall off when brushed gently. This meant we couldn’t use these membranes because we not only needed a homogeneous distribution, we needed to know how much of the particles were in the membrane. My student eventually discovered that she could rotoevaporate off a bunch of solvent while still keeping the mixture stirred and get the viscosity up above 65 centipoise (same as milliPascal-secomnds). Water has a viscosity of 1 centipoise. Honey has a viscosity of 2000-10000 centipoise. Motar oil (SAE 10) has a viscosity of 65 centapoise.

Particles in polymers: thixotropes Particles are used to stop liquids from flowing until subject to shear. Used in “non-running” or “no-drip” liquid adhesives, paints, and lubricants. Silicone sealant with NO silica Silicone sealant with silica Now, talking about particles in liquids. I mentioned earlier that dispersions of particles in liquids (like solvent) can act as thixotropes or dilatants. Thixotropy or shear thinning is what you see with most non-run adhesives, paints and cosmetics. You apply them and they don’t flow with gravity like a liquid should. Yet they flowed when you applied them with a brush. Conversely, ketchup doesn’t come out of the bottle unless you shake the bottle (apply shear). In all of these cases, there are solid particles that have been added that, upon standing, aggregate into networks that need to be broken up before the entrained liquid will flow. Once the the flow stops, the weak non-bonding interactions reaasemble the networks. Silicone grease is silicone oil with 40 wt% silica added to make it a thixotrope. Dilatant materials act like solids under shear and with gravity will flow like a liquid. This is the now famous corn starch in water demonstration used for school kids. You can walk quickly across a pool of corn starch mixed into water, but will sink immediately if you stop walking and stand still. These types of flow effects complicate the preparation of class 1a materials because with solvent or melt, you will have these “non-Newtonian” viscous behaviors.

How to make inorganic particles Sol-gel “wet” synthesis Emulsion polymerizations (sol-gel in oil & water) Aerosols/flame syntheses (will not make silsesquioxanes) Back to the basics. How to make inorganic particles. There are a number of ways to make particles, but I am listing the three you are most likely to come into contact with. Sol-gel wet synthesis is a homogenous polymerization of a monomer [e.g. Si(OMe)4, AlCl3-6H2O, RSi(OEt)3, etc] to form polymers that phase separate as solid particles. Emulsion polymerizations are polymerizations in a droplet that allow you to control the size based on the emulsion thermodynamics (for silica it would be a water in oil emulsion more than likely). And aerosol/flame synthesis is actually how most particles are made industrially. You simple spray the monomer into a flame and collect the resulting particles.

Sol-gel: Stober synthesis All particles round and same size TEOS Concentrations 0.011M (0.03736g) to 0.28M (0.934g) NH4OH Concentrations 0.1M to 1.2M Most sol-gel polymerizations afford particles of a broad distribution of sizes. A type of sol-gel polymerization that leads to a relative monodisperse population of particles is called the Stober (prounounced as “Stay Ber”) synthesis. In this preparation the monomer is mixed with excess water and a huge excess of ammonia in an ethanolic solution. The result in a few hours is a dispersion of particles that are similar enough in size to allow the formation of an opal. One the particles have stopped growing, they are purified of residual base and monomer with centrifugation and washing. These particles can be used in studies where one wants to vary the size of the particle systematically. Reaction Scale = 15mL and 60mL TEOS distilled NH4OH titrated ( 9M) Anhydrous ethanol J. Colloid Interface Sci., 26 (1968), pp. 62–69

Control of particle size by changing the concentration of ammonium hydroxide with 0.28M TEOS This graph shows the mean silica particle size prepared using the stober method as a function of concentration of ammonia in the reaction mixture. So, to vary size, the only variable was ammonia concentration. We did discover that the ammonia needs to be freshly titirated before using to get an accurate concentration. Not that the small particles are showing the blue “opalesence of Rayliegh scattering (Ray LEE), while the larger particles are showing classical Mie scattering. This is essentially why the sky is blue (single dielectric scattering from gas molecules) and why clouds are white, multiple dielectric centers in large water droplets that make up clouds (clouds are not water vapor, they are water droplets). Rayleigh scattering

Light scattering from particle/polymer composites In principle, the transmittance of incident light (with wavelength, λ) through a filled composite of thickness, x (optical path length) is [111]: (5′) where I and I0 are the intensities of transmitted and incident light, respectively. Filler particle (primary or aggregate or agglomerate) of diameter d, volume fraction ϕv, and refractive index nf are dispersed in a polymer of refractive index nm. The smaller the particles, the less scattering (Mie) of visible light occurs. This is why the new sunscreens contain titanium dioxide or zinc oxide without making you appear white.

Other ways to make particles: Synthesis of T8 POSS “particle” Back to synthesis. Polyhedral oligosilsesquioxanes have been described as the smallest silica particle (the Si-O-Si cage is about 1 nm across). Of course if is covered with eight organic groups that change its surface chemistry dramatically. Furthermore, it really isn’t a particle, because most of the organic modified ones are soluble. They dissolve in solution. The octamethyl is not soluble. While some of these are commercial, its actually cheaper and easier to make them. Old synthetic methods used acid or base in some polar aprotic solvent (like above). But the yields were terrible. There have been some advances, using fluoride catalysts, yields in the 70-90% range are common place. Yields are not always so good

Synthesis of Phenyl T8 POSS Here is the recipe for the high yield synthesis of the octaphenyl T8. It can be made from monomer or from the amorphous polymer, shown here as an intermediate. If you start from the monomer as shown above you can react mole scale. It can all be done in a single glass vessel. First acid catalyzed hydrolysis and condnesaiton in toluene. The more toluene is added along with potassium fluoride and 18-crown-6. Be very careful with these last reagents because the fluoride in the acid reaction solution is extremely toxic and the 18-crown-6 can reportedly cause permanent testicular damage. After 48 hours 110 grams of white crystalline product can be filtered and washed without the need for further purification. Just check the melting point. This preparation can be used with most organitrilakoxysilane monomers. Also works from the polymer!!!! Best way to make POSS

“Two-step” method to prepare silsesquioxane particles Typical recipe: 1) PhSi(OEt)3 (2.4 grams) in 12 mL anethol is mixed with aq. HCl (0.0027 M, 3.6 mL) for 7 h. 2) This sol was added to aq. NH3 (1M, 32.4 mL) and stirred for 20 h. 3) Particles isolated and washed with centrifugation. To make larger polysilsesquioxane particles, one can use a two step acid-base sol-gel polymerization that likely goes through an emulsion. The acid catalyed hydrolysis forms an oligomeric oil that is insoluble in the alcohol solvent and that, with vigorous stirring, can be turned into an emulsion. Addition of base converts the oil droplets into hard particles. The particles are soluble in hydrocarbon solvents but will cure into insoluble material with heating above 200 °C. Loy, D. A. Macromole Mater Eng. 2012, in press. A. Matsuda et al. J. Ceram. Soc. Jap. 2007, 115, 131-135.

Flame synthesis of inorganic particles  From fully coalesced to aggregated and agglomerated nanoparticles: Evolution of flame-made primary particle size (dp, solid line) and agglomerate collision diameter (dc, dashed–dotted line). The end of full coalescence is when fractal-like particles start forming (dc/dp = 1.01, open diamond). The onset of agglomerate formation is when sintering stops and dp levels off (dpH, open square). The dc at that moment corresponds to the largest aggregate diameter (filled, blue square, dcH). The ratio of dcH/dpH is a quantitative measure of the extent of aggregation Langmuir 2004, 20, 5933

Other inorganic fillers include Clays (2-D aluminosilicates)* Fullerene, nanotubes, and graphene* other aluminosilicates Main group metal oxides Transition metal oxide particles Alkali earth carbonates and sulfates Quantum dots Metals *included in this lecture There are many other inorganic particles that can be used in these class1A hybrids. They include the above list. The first two will be discussed later in this lecture. The rest are too numerous to include, but you can find information on them readily enough using scifinder or web of science or other literature search engines.

POSS physically dispersed in polypropylene So back to making a class 1A hybrid. They are made from an inorganic particle (including a POSS, which really isn’t inorganic, but okay) and an organic polymer. It could be possible that the organic phase is not a polymer but that is an very odd and rare thing. It could be an organic glass made from supercooling a molten organic molecule. We have talked briefly about the polymers that could be used and the types of the inorganic particles. And how they are mixed and what to be concerned about during the mixing (aggregation, thixotropy).

How do you characterize a hybrid: XRD of POSS XRD of POSS in HDPE Non attached POSS may not stay as isolated molecules in polymer. Instead it may reform crystals containing many POSS molecules. Macromolecules, 2006, 39 (5), pp 1839–1849

Influence of nanoparticles on melt viscosity Micrographs of the PA-05-S composite (left) and the PA-05-L system (right) (MET) The firs class 1A hyrbids we will consider are simple silica particles mixed with melted polymers, then cooled. Nylon 6 melts at 265 °C. So you may want to keep these under a blanket of nitrogen to avoid oxidation. The hybrid could also be made by dissolving the polymer and mixing with the particles dispersed in solvent (as we did with the Nafion earlier in the lecture). But the ones above were made by melting. Different size particles were used to see if there was an effect on the mechanical properties of the resulting composite. Silica particles mixed into Nylon while melted “Nanofillers in polymeric matrix: a study on silica reinforced PA6,” E. Reynaud, Polymer 2001, 42, 8759

Influence of nanoparticles on melt viscosity Here is the effect of nanoparticle size on the melt viscosity. The plot is viscosity versus shear. Note that with higher shear the viscosity drops (dilatant). The size and wt% of particles are given in the small table to the right. Note that the viscosity is highest with the smallest particles Smaller the size particle, the greater the viscosity “Nanofillers in polymeric matrix: a study on silica reinforced PA6,” E. Reynaud, Polymer 2001, 42, 8759

Modulus increases as inorganic content increases Tensile modulus (stiffness) of nylon 6 nanocomposites as a function of SiO2 content Now lets look at the modulus of the cooled silica nylon composite as a function of silica content. We are not concerned about particle size. The particle distribution should be the same for each point on the graph. For each point the authors performed a stress strain analysis and determined the modulus. The modulus, stiffness, gets higher the more silica is in the polymer. This is what you would expect as the silica has a much higher modulus than the polymer. . Modulus increases as inorganic content increases Composites Part B: Engineering Volume 39, Issue 6 2008 933 - 961

Tensile strength of nylon 6 nanocomposites as a function of SiO2 content & surface modification using coupling agent With surface modification Without surface modification This graph shows the tensile strength for a series of composites made with different weight fractions of silica Generally untreated silica added to polymers results in a slight decrease in tensile strength even though the modulus (previous slide) increases. This is due to poor interface adhesion between the particles and the polymer. However, if the particles are treated with a silane coupling agent, the tensile strength goes up significantly, then drops, probably due to aggregation.

Summary for Class 1: particles in organic polymer Made by solvent or melt mixing Particle aggregation will ruin any positive influence from the inorganic particles Nature of non-bonding interactions will affect strength & modulus trends But generally, modulus and strength increases with decreasing particle size Modulus and strength increases with increasing weight percent particle

Polymer-clay composites (Class 1A) montmorillonite Clay is widely used as a ceramic precursor, geo sealant, and even as an additive in food to modify the rheology. Its abundance makes clay attractive as a starting material for high performance materials. Many clays are 2-D sheets of alumino silicate interspaced with alkali metal cations and stacked on top of each other like plates. Recently its been discovered that cationic surfactants will disrupt the stacking freeing the 2-D alumino silicate sheets to be inorganic fillers for polymer nanocomposites. Exfoliated montmorillonite clay

Polymer-clay composites (Class 1A) Clay: 2-D sheets of alumino-silicate with metal cations in between Replace metal cations with cationic surfactants Replace surfactants with polymers (melted or in solution)-intercalation Heat and apply shear – exfoliation Stronger, fire resistant, less permeable Basic process involves replacing metal cations with big organic cations which opens up the stack structure enough for polymers to wiggle in. If the polymers just wiggle in and expand the lattice but leave it in place, its called intercalation. If the polymers force the plates apart far enough that they become isotropically oriented, it is called exfoliated clay. Intercalation is assisted by heating. Exfoliation is assisted by heating and high shear forces.

Process for forming clay polymer composites intercalated This cartoon just shows the surfactant, then the polymer intercalating, then exfoliating the flat plats (seen here edge on). exfloliated

Detecting intercalation and exfoliation X-ray diffraction The process can be followed by TEM but also by XRD, where the diffraction of the clay get shifted to larger structures (1.1 nm is spread to 1.5 nm and the 2,2 nm peak gets shifted to 3. 0 nm as the clay gets macromolecules inbetween the aluminosilicate plates. Finally, when exfoliation occurs, the diffraction pattern is lost. From Giannelis et al., Adv. Polym. Sci., 118 (1999)

Tensile strength of non-covalently integrated clay-polystyrene-co-acrylate nanocomposites + The introduction of clay into polymers results in greater strengthening and higher modulus than the pure polymer. Remember that particles sometimes lowered the tensile strength while increasing the modulus. The 2-dimensionality of the clay is what makes these materials stronger. Mechanics of Composite Materials 2006, 42, 45.

Just some data for polypropylene-clay composites Just some data for polypropylene-clay composites. Remember many polymers have been made into clay composite.s

Carbon Spheres (Buckyballs) & Nanotubes & graphene as inorganic fillers The last group of Class 1A fillers are the carbon fullerene and graphene allotropes. Fullerenes are spherical carbon molecules with no hydrogens or oxygens. The molecules are electron poor. The 0-dimensionality of the spherical particle limits the reinforcement that can be achieved. Carbon nanotunes show metal like conductivity and theirfiber like characterisitics and great strength make them promising filler materials. Their greatest challenge is wetting the surface with polymers. Graphene is exfoliated graphite. Macromolecules, 2006, 39 (16), pp 5194–5205 Nature Materials 9, 868–871 (2010)

Fullerenes as inorganic particles in polymers The curves of uniaxial deformation of the LDPE films with different fullerene content: 0 (1), 1 (2), 3 (3), 5 (4) and 10 wt% (5) Some stress strain curves for fullerene in low density polyethylene. Increase in tensile strength and modulus with increasing fullerenes up to 10wt% afterwhich the fullerenes crystallize out and form defects that lower strength. J. Mater. Chem., 1997,7, 1097-1109

Summary Class 1A materials rely on non-bonding interactions and so it can be difficult finding optimum conditions to match the surfaces of filler and polymer to give good interactions. Spherical particles are not as good as linear or 2-D fillers in improving mechanical strength. Next we look at in-situ formation of particles where we add the precursor to the polymer and the particle forms in place.