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 This shows a stress strain analyzer (photo) with a sample (called a dog bone because of its shape). A tensile experiment is one where the sample is pulled appart. A compressive strength analysis is where the forces are directed in the opposite vectors from those in the tensile experiment. It is usually conducted on a disk or block. A flexural strength measurement is a bending experiment.

Properties: Strength

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

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.

Toughness Energy required to break (Pa or kJ/m2). 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 Mechanical analyses should be done more often by chemists when they report new polymers. The analyses are easy to do and make a paper a lot more valuable. Just making a new polymer is not enough.

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 Dipole-dipole 10 kJ/mole Hydrogen Bonding 20-40 kJ/mole Charge-charge interactions 0-100 kJ/mole Covalent bonds 150-600 kJ/mole There are a whole bunch of weak non-bonding forces like London and dipole-dipole. They are all weaker than hydrogen bonds, but can add up and be important when surface areas between phases are really large (think bugs crawling on ceiling). Ionic interactions are not the same as the strong ionic bonds in NaCl. These are longer range interactions between fewer groups. None of the non-bonding interactions compare to covalent bonds (or metal or ionic bonds-not ionic interactions). Covalent bonds are strong. So why are materials so weak? We will discuss how to calculate theoretical material strength based on bond strength later 1 kJ mol-1 = 0.4 kT per molecule at 300 K

Covalent Bond Dissociation Energies 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 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 Now on to bonding interactions. These are a select list of covalent bond energies. Remember diamond is the worlds highest melting material (3550 °C). Yet its bonds are only half as strong as zirconium-oxygen bonds. That’s because, diamonds have fewer defects are are closer to their theoretical material strength that’s directly derived from the bond strength. Zr-O has more defects in structure.

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 Polymers typically have tensile strengths of 10-100 MPa. Tensile strength means to take a piece of plastic and pull it into two pieces. So, these macromolecules are full of C-C bonds, yet their strength is at least 2000X lower than the 200 GPa we calculated. Why? Because the plastic is composed of macromolecules that are interconnected by non-bonding interactions, not covalent bonds. This is the weak link that makes them much weaker than diamond. Some more material strengths are on the next page. • 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.

Conductivity electrical ionic thermal Flame resistant ThermobloK

Stability thermal chemical radiation biological Polymer 2010, 51, 2296 PEHS thermal chemical radiation biological 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