Professor Douglas A. Loy Visiting from the University of Arizona, United States

1. Physics and Chemistry of Hybrid Organic-Inorganic MaterialsLecture 4: The physics of phase separation and solutions Professor Douglas A. Loy Visiting from the University of Arizona, United States

2. Key points for phase separation and solutions • Phase separation is thermodynamic • Phase separation is a part of most hybrids formation • Sol-gel systems form supersaturated solutions that phase separate solid particles (nucleation control). • Sol-gel polymerization of hybrid monomers leads can lead to single solid phases, but there is often a liquid or gas phase created by particle percolation and gelation. • Hybrid based on organic polymers undergo enthalpically driven phase separations • There are nucleation and thermodynamic (spinoidal) controlled phase separations. • Particles form by nucleation phase separation • Surfactant templating is the formation of a material in one phase of a phase separated surfactant-solvent system

3. Phase separation in Hybrid systems • Phase separation occurs frequently in the formation and processing of hybrid organic-inorganic materials • You must be able to recognize how many phases are present in order to characterize and understand a hybrid material. It is not always as easy as with oil and water to tell how many phases are present

4. Hybrid Organic-Inorganic Thermodynamics of Mixing/Phase separation • phase separation and mixing are opposite thermodynamic processes • We will describe the thermodynamics of these processes with Helmhotz free energy, ΔF • For either: • ΔF < 0 means phase change is favorable • ΔF > 0 means the existing state is more stable and no change. ΔF = ΔU –TΔS

5. Thermodynamics of single component phase changes

6. Thermodynamics of single component phase changes

7. Thermodynamics of mixing: Two phases going to become one Dissolution Inorganic F organic 2 phases In this case, the change Helmhotz free energy: ΔF (mixing) = ΔU –TΔS < 0 One phase: a solution Thermodynamically favorable mixing of two phases into one ΔS is generally positive for mixing & gets larger with temperature ΔU is often positive (unfavorable) with mixing polymers. Rare, but this could occur with an inorganic monomer dissolving in a polymer

8. Thermodynamics of phase separation: One phase unable to separate into two Inorganic organic In this case, the change Helmhotz free energy: 2 phases F ΔF(phase separation) = ΔU –TΔS > 0 Thermodynamically unfavorable phase separation: “uphill” The kT at this time is insufficient to drive phase separation. One phase: a solution Hybrids with organic and inorganic components bonded together at the monomer level are unable to phase separate

9. Thermodynamics of mixing: Two phases not changing Inorganic Inorganic organic organic 2 phases Insoluble still two phases ΔF(mixing) = ΔU –TΔS > 0 Either temperature is not high enough to dissolve the particles and/or the ΔU (internal energy; like enthalpy) is too positive for the entropy to overcome This is what happens with mixing inorganic particles and organic polymer.

10. An example of a hybrid composed of inorganic (silica) particles mixed in with a fluorinated polymer electrolyte (Nafion) Two solid immiscible phases Must be physically mixed 5 weight percent ex situ silica in Nafion

11. Thermodynamics of mixing: one phase separating into two One phase: a solution F 2 phases ΔF (Phase separation) = ΔU –TΔS < 0 Thermodynamically favorable phase separation of one phase into two phases: This is how particles form in sol-gel and what can happen when a monomer dissolved in another polymer polymerizes. Inorganic organic

12. Phase separation of particles from an inorganic monomer dissolved in a viscous polymer solution The silica monomer forms oligomers and polymers that eventually nucleate out as spherical particles In situ Silica particles

13. Phase diagram of a hybrid organic inorganic material • Two phases at lower temperatures • One phase at higher temperatures • Maximum insolubility is when there are nearly equal quantities in the mixture.

14. Phase diagram of a hybrid organic inorganic material: Effect Molecular weight on phase separation • With increasing molecular weight of one or both of the solutes, the phase boundary (binodal line) increases in temperature • As Mw increases entropy change becomes less positive. • Hybrids often experience phase separation Bold line – highest molecular weight Dashed line –lowest molecular weight

15. More information from phase diagrams: Plots of ΔF at different temperatures (a) stable (a) (b) (b) (c) (d) (c) unstable (d)

16. Overlaying the plots of ΔF on the original phase diagram reveals a metastable region spinodal line spinodal lines stable (a) (b) organic rich phase composition Inorganic rich phase composition (c) (d) spinodal lines unstable binodal line binodal line metastable metastable red = spinodal line blue - binodal

17. Two phase separation processes: • Spinodal decomposition: • spontaneous. • Fingerprint like patterns. • Between spinodal lines • Nucleation- • not spontaneous, requires nucleation • spherical particles-surface energy important • nucleation kinetics important spinodal nucleation nucleation red = spinodal line blue - binodal

18. Spinodal phase separated materials red = spinodal line blue - binodal spinodal nucleation nucleation Freeze fracture TEM Block copolymers

19. Nucleation phase separated materials red = spinodal line blue - binodal Freeze fracture TEM’s spinodal nucleation nucleation •Spinodal decomposition is mostly about bulk ΔF •Nucleation also has to account for the instability of the particles due to their small size.

20. Thermodynamics of silica particles forming in sol-gel by nucleation • Common monomer for silica is Si(OEt)4 • Reactions with water • Alcohol is solvent because monomer & water are immiscible • In solution, the monomer hydrolyzes to Si(OH)4 & then polymerizes •Volume percent silica is in nucleation zone. • Phase separates as particles only when 1-2 nm in diameter. Why do the particles grow this big before separating from the solution?

21. Surface tension & the importance of interfaces Molecules on surface have fewer neighbors and so exert greater force on adjacent molecules = surface tension (in dynes cm-1 or N m-1 Jm-2) Surface tension γ = surface energy (N m-1 = Jm-2) Nature tries to minimize the surface area of interfaces (spheres and the bigger the better) It costs energy to phase separate and make an interface

22. surface area versus diameter for particles Small particles have higher surface area per gram; higher energy

23. Energy reduction through phase separation with growth of the nucleus with volume (4/3)πr3 Energy “cost” of creating a new interface with an area of 4πr2 Nucleation of a Second Phase in the Metastable Region Small: usually a few nanometers Growth of the second phase occurs only when a stable nucleus with radius r has been formed. γis the interfacial energy between the two phases.

24. Nucleation free energy plot: critical nuclei size

25. Surface energy/size driving force for particle Coalescence Same polymer volume before and after coalescence: In 1 L of latex (50% solids), with a particle diameter of 200 nm, N is ~ 1017 particles. Then ΔA = -1.3 x 104 m2 With ϒ = 3 x 10-2 J m-2,ΔF = - 390 J.

26. Hybrid systems: small inorganic particles in an organic polymer Particles will aggregate into clusters to reduce surface area and lower free energy

27. Other hybrid monomer can undergo a variety of different phase separations Gel No Gel No Gel • Must have solid and liquid phase • Solid phase (usually particles) must be continuous through liquid (percolation) • Phase separation of liquid prevents further reaction and gelation

28. Now, lets look at a two phase system that stays 2 phases with mixing (emulsions) Two immiscible liquids minimizing surface area Two immiscible liquids forced into very high surface area interface Meta-stable because of surfactant Two phases With time Two phases The free energy of mixing the oil and water into a (single phase) solution is very, very, very unfavorable (positive)

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

30. Summation • Phase separation is an important part of how hybrids form • Many hybrids have multiple phases • Some start as mixtures and end as multiphase mixtures. • Hybrid monomers will form single phase bulk materials, but will form porous materials where air or solvent is a second phase. • The phase separations lead to recognizable structures and morphology that can tell the researcher how to manipulate the hybrids productively.