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

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.

What properties of hybrids are of interest?

• • • • • • • strength modulus toughness transparency conductivity Stability special properties Do not forget baseline (control) measurements.

Strength of Materials

• • • • • • 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

Force vs. extension

Stress-Strain Analysis

Properties: Strength

Modulus of Materials

• • • Rigidity of material (kJ/m 2 or Pa) Related to Morse potential Slope of elastic zone of stress strain curve MPa ΔLength/initial Length

Modulus of hybrid materials changes less with temperature than organics n OAc SiO 2 °°°°°°° n OAc •••••• 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.

• •

Toughness

Energy required to break (Pa or kJ/m 2 ). Integral of stress strain curve MPa ΔLength/initial Length

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

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

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

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.

Bonding (& non-bonding)interactions • London forces < 1 kJ/mole ( = (

C

 6 • Dipole-dipole 10 kJ/mole • Hydrogen Bonding 20-40 kJ/mole • Charge-charge interactions 0-100 kJ/mole ( ) = 4 1 2  • Covalent bonds 150-600 kJ/mole 1 kJ mol -1 = 0.4 kT per molecule at 300 K

Covalent Bond Dissociation Energies

Si-Si Si-C C-C C-O C-H Al-O Si-O Ti-O Zr-O 221 kJ/mole 300 kJ/mole 350 kJ/mole 375 kJ/mole 415 kJ/mole 480 kJ/mole 531 kJ/mole 675 kJ/mole 750 kJ/mole Two electrons per bonding molecular orbital 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

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

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

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.

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

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

• • • • electrical ionic thermal Flame resistant

Conductivity

ThermobloK

• • • • thermal chemical radiation biological

Stability

PEHS Polymer 2010, 51, 2296

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