Physics and Chemistry of Hybrid Organic-Inorganic Materials Lecture 3: Properties of Hybrids

1. Physics and Chemistry of Hybrid Organic-Inorganic MaterialsLecture 3: Properties of Hybrids

2. Key points • Mechanical properties are strength, modulus, toughness, hardness, elasticity. • Thermal properties of interest include onset of degradation, glass transition temperature, and melting point. • Optical properties include transparency, absorption, scattering, refractive index, etc. • Electric properties include conductivity and dielectric.

3. What properties of hybrids are of interest? • strength • modulus • toughness • transparency • conductivity • Stability • special properties Do not forget baseline (control) measurements.

4. Strength of Materials Force vs. extension • Tensile (Shown) is force used to pull a sample apart. • Compressive strength is the force used to crush. • Flexural strength is the force used to bend and break. • Work or energy per cross-sectional area (kJ/m or Pa) or force per distance (kN/m) • theoretical strength = bond strength/cross sectional area • real strength = function of defects

5. Stress-Strain Analysis

6. Properties: Strength

7. Modulus of Materials • Rigidity of material (kJ/m2 or Pa) • Related to Morse potential • Slope of elastic zone of stress strain curve MPa ΔLength/initial Length

8. Modulus of hybrid materials changes less with temperature than organics °°°°°°° •••••• B. K. Coltrain, C. J. T. Landry, J. M. O’Reilly, A. M. Chamberlain, G. A. Rakes, J. S. Sedita, L. W. Kelts, M. R. Landry and V. K. Long, Chem. Mater., 1993, 5, 10, 1445–1455.

9. Toughness • Energy required to break (Pa or kJ/m2). • Integral of stress strain curve MPa ΔLength/initial Length

10. Mechanical characterization of polymers • Stress-strain curves: • Young’s modulus (brittleness) • Tensile strength-pull sample appart • Flexural strength- bend until it breaks • Compressive strength-crush sample • Dynamic mechanical analyses (same info as above but with cyclic application of stress or strain. • Generate modulus temperature curves • Fatigue studies to predict failure under cyclic stress

11. Properties of Hybrids: high specific strength Ashby plot Organics are considerably less dense than inorganics (glasses, ceramics & metals). Hybrids (composites) are also less dense than inorganics because of their organic component

12. Why hybrid organic inorganic materials: They are stronger than the organic by itself Ashby plot Inorganics (glasses, ceramics & metals) are stronger than organics . Hybrids (composites) are also stronger than inorganics because of their organic component

13. What is the origin of mechanical properties? • Theoretically, mechanical properties depend on bond strengths • In practice, mechnical properties are ruled by defects, morphological features, and non-bonding interactions that give rise to ductility, flexibility, viscoelasticity and limit the ultimate strength.

14. 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

15. 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

16. Origin of strength and modulus: Modulus~ curvature at bottom of well (and strength ~ depth of well) The reality: defects in materials, lower strength by more than 10X

17. For example, 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

18. Transparency • No absorptions due to electronic or vibrational transitions • Scattering from interfaces between phases with large differences in refractive index • 784 × 100 Rayleigh Scattering Two phase system with dispersed phase much smaller in dimension than wavelengths of light. Blue is scattered more than red.

19. Mie scattering scattering from non-absorbing interfaces with roughness similar to wavelengths of light

20. Douglas A. Loy, J. Non-Crystal. Solids 2013, 362, 82-94.

21. Conductivity • electrical • ionic • thermal • Flame resistant ThermobloK

22. Stability PEHS • thermal • chemical • radiation • biological Polymer 2010, 51, 2296

23. Conclusions • Properties of hybrid organic-inorganic materials are often better than either organic or inorganic • Addition of Inorganic improves strength, stability, hardness, abrassion resistance compared with organic • Addition of organic polymer, improves flexibility, elasticity, toughness, and transparency compared with theinorganic