Chemistry and Physics of Hybrid Organic-Inorganic Materials

1. Chemistry and Physics of Hybrid Organic-Inorganic Materials Lecture 3: Material Interactions in Hybrids

2. Material Interactions in Hybrids • Non-bonding interactions • Bonding interactions • Surface tension • Free energy • Changes of phase • Phase separation • Crystalline or amorphous

3. Length Scales

4. Proteins – one of the organic phases from Biohybrid Org-Inorganics • Interactions between atoms within the protein chain • Interactions between the protein and the solvent

5. Bonding (& non-bonding)interactions • London forces < 1 kJ/mole • Dipole-dipole 10 kJ/mole • Hydrogen Bonding 20-40 kJ/mole • Charge-charge interactions 0-100 kJ/mole • Covalent bonds 150-600 kJ/mole 1 kJ mol-1 = 0.4 kT per molecule at 300 K

6. Van der Waals (Non-bonding) Interactions • Nonspecific forces between like or unlike atoms • Decrease with r6 • approximately 1 kJ/mol • If r0 is the sum of van der Waals radii for the two atoms. Van der Waals forces are attractive forces when r> r0 and repulsive when r< r0. ~ 10-21 to 10-20 J, corresponding to about 0.2 to 2 kT at room From 3SCMP

7. Charge-charge (Coulombic) interactions = 10-18J Coulomb interaction between two ions (1-15 A) At close range, Coulomb interactions are as strong as covalent bonds (10-18J or 200-300 kT) Their energy decreases with 1/r and fall off to less than kT at about 56 nm separation between charges In practice, charge-charge interactions have been shown to be chemically significant at up to 15 Å in proteins

8. Hydrogen Bonding • In a covalent bond, an electron is shared between two atoms. • Hydrogen possesses only one electron and so it can covalently bond with only ONE other atom. • The proton is unshielded and makes an electropositive end to the bond: ionic character. • Bond energies are usually stronger than v.d.W., typically 25-100 kT. • H-bonding can lead to weak ordering in water. From 3SCMP

9. 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 Jm2) 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 Small particles have higher surface area per gram; higher energy

10. surface area versus diameter for particles

11. 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. From 3SCMP

12. Covalent Bond Dissociation Energies Two electrons per bonding molecular orbital Si-Si 221 kJ/mole Si-C 300 kJ/mole C-C 350 kJ/mole C-O 375 kJ/mole C-H 415 kJ/mole Al-O 480 kJ/mole Si-O 531 kJ/mole Ti-O 675 kJ/mole Zr-O 750 kJ/mole BDE = potential energy, -dU Force (N or kgms-2) to break a bond = -dU/dr Strength of a bond (Nm-2 or Pa) = Force/cross section area

13. Polymers are weaker than predicted Linear Macromolecules under tension causes polymers to disentangle • Entanglements & non-bonding interactions in linear polymers • Covalent bonds only break with short time scale • Cross-linking with covalent bonds makes materials stronger but more brittle

14. Thermodynamics of Mixing and phase separation • Entropically mixing is usually favorable (+) • Enternal energy ΔU often is crucial component Important for mixing of organic and inorganic precursors to hybrids and for phase separation that might occur upon environmental changes or changes in chemical structure

15. Thermodynamics of mixing of mixing A & B Helmholtz Free Energy (Constant Volume) For small molecules, NA = NB = 1 & ΔS is large and positive. ΔS polymer < ΔS molecule Re-write in terms of an interaction parameter Chi time kT times the volume fractions of A and B Now you can just vary Chi and T and explore phase diagrams

19. When moving from the one-phase to the two-phase region of the phase diagram, ALL concentration fluctuations are stable. Spinoidal decompositon into two phases

20. Spinodal decomposition of mixture of liquid crystals

21. Phases grow in size to reduce their interfacial area in a process called “coarsening”.

22. Block copolymers tie the two immiscible phases together Still spinodal decomposition Coarsening is stopped by connected macromolecules Covalent bonds [provide greater metastability of turing structure

23. Small fluctuations in composition are not stable. Nucleation in metastable regions Onlyf1 and f2* are stable phases! The f2* composition must be nucleated and then it will grow.

24. Nucleated structure: islands of one phase in another Spinodal structure: co-continuous phases From G. Strobl, Polymer Physics, Springer

25. 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 of4πr2 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.

26. Formation of bonds: Polymerization • Hydrolysis: • Condensation: • Net Polymerization: Shown here for formation of a silsesquioxane

27. Most hybrids involve phase separation All nucleation. Rare to see spinodal decomposition

28. Amorphous versus crystalline • Amorphous – kinetic, no long range order, no time for crystals to grow from solution or liquid. How can you tell if a material is amorphous? • Crytsalline: thermdynamic structures made with reversiblity to remove defects and correct growth. Long range order. How can you tell if a material is crystalline?

29. Crystalline materials • Long range order: Bragg diffraction of electromagnetic radiation (or electron beams in TEM) by crystalline lattice into sharp peaks. • Solid structures with geometric shapes, straight lines and flat surfaces, and vertices. • Optical affects like bifringence • Direct visuallization of crystal at molecular level with AFM or STEM. • Melting point (not always though)

30. AFM of polyethylene crystallite microcrystals Inorganic crystals XRD from semicrystalline polymer film Rutile titania crystals in amorphous TiO2 Micrograph of polymer crystalline spherulites

31. XRD (wide angle) • Single crystal or microcrystalline powder (crystals with atomic or molecular scale order)

32. X-ray powder diffraction from polybenzylsilsesquioxane “LADDER” Polymer Big picture is amorphous material. Small sharp peaks are due to contaminant from preparation Not a ladder polymer!!!!!!!!!

33. Amorphous materials • No long range order: diffuse peaks may be present, due to average heavy atom distances. • No crystalline geometries, glass like fractures (conchoidal) • Aggregate spherical particles common • Negative evidence for crystal at molecular level with AFM or STEM. • No Melting point

34. XRD amorphous material Al2O3 thin films prepared by spray pyrolysis J. Phys.: Condens. Matter 13 No 50 (17 December 2001) L955-L959

35. 2012 EPL 98 46001

36. Amorphous materials: XRD amorphous amorphous crystalline

37. Conchoidal Fractures in amorphous materials Crystals break along miller planes Unless microcrystalline

38. If crystals are small compared to impact, conchoidal fracture can occur In metal In sandstone 3 meters tall)

39. Summary: Physics of Hybrids • Bonds & non-bonding forces that hold materials together • Surface tension and surface free energy • Thermodynamics of Mixing and phase separation ( of polymers in particular) • Nucleation and Spinodal decomposition • Blends of immiscible polymers and immiscible block copolymers • Nucleation of particles & sol-gel chemistry • Difference between crystalline and amorphous