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