Physics and Chemistry of Hybrid Organic-Inorganic Materials Lecture 2: Properties of Materials

Physics and Chemistry of Hybrid Organic-Inorganic MaterialsLecture 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?
strengthmodulustoughnesstransparencyconductivityStability 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 areareal strength = function of defects
Force vs. extension

Stress-Strain Analysis

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
Length/initial Length

Modulus of hybrid materials changes less with temperature than organics


B. K. Coltrain, C. J. T. Landry, J. M. OReilly,A. M. Chamberlain, G. A. Rakes, J. S. Sedita, L. W. Kelts,M. R. Landry and V. K. Long, Chem. Mater., 1993, 5, 10,14451455.

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

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

Properties of Hybrids: high specific strength
Organics are considerably less dense than inorganics (glasses, ceramics & metals).

Hybrids (composites) are also less dense than inorganics because of their organic component
Ashby plot

Why hybrid organic inorganic materials: They are stronger than the organic by itself
Inorganics (glasses, ceramics & metals) are stronger than organics .

Hybrids (composites) are also stronger than inorganics because of their organic component
Ashby plot

What is the origin of mechanical properties?
Theoretically, mechanical properties depend on bond strengthsIn 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-dipole10 kJ/mole

Hydrogen Bonding20-40 kJ/mole

Charge-charge interactions0-100 kJ/mole

Covalent bonds150-600 kJ/mole
1 kJ mol-1 = 0.4 kT per molecule at 300 K

Covalent Bond Dissociation Energies
Si-Si221 kJ/moleSi-C300 kJ/moleC-C350 kJ/moleC-O375 kJ/moleC-H415 kJ/moleAl-O480 kJ/moleSi-O531 kJ/moleTi-O675 kJ/moleZr-O750 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

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

For example, Polymers are weaker than predicted
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
Linear Macromolecules under tensioncauses polymers to disentangle

Transparency

No absorptions due to electronic or vibrational transitionsScattering from interfaces between phases with large differences in refractive index784100
Rayleigh ScatteringTwo 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
electricalionic thermalFlame resistant
ThermobloK

Stability
thermal chemicalradiationbiological
Polymer 2010, 51, 2296
PEHS

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

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.
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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.
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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
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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. Thats because, diamonds have fewer defects are are closer to their theoretical material strength thats directly derived from the bond strength. Zr-O has more defects in structure.
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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.
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