Chemistry and Physics of Hybrid Organic-Inorganic Materials
description
Transcript of Chemistry and Physics of Hybrid Organic-Inorganic Materials
Chemistry and Physics of Hybrid Organic-Inorganic Materials
Chemistry and Physics of Hybrid Organic-Inorganic Materials
Lecture 3: Material Interactions in Hybrids
Material Interactions in Hybrids
Non-bonding interactionsBonding interactionsSurface tensionFree
energyChanges of phasePhase separationCrystalline or amorphous
Length Scales
Proteins one of the organic phases from Biohybrid
Org-Inorganics
Interactions between atoms within the protein chainInteractions
between the protein and the solvent
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
Nonspecific forces between like or unlike atomsDecrease with
r6approximately 1 kJ/molIf r0 is the sum of van der Waals radii for
the two atoms. Van der Waals forces are attractive forces when
r> r0 and repulsive when r< r0.
Van der Waals (Non-bonding) Interactions
~ 10-21 to 10-20 J, corresponding to about 0.2 to 2 kT at
room
From 3SCMP
Charge-charge (Coulombic) interactions
Coulomb interaction between two ions (1-15 A)
At close range, Coulomb interactions are as strong as covalent bonds (10-18J or 200-300 kT)
Their energy decreases with 1/r and fall off to less than kT at about 56 nm separation between charges
In practice, charge-charge interactions have been shown to be
chemically significant at up to 15 in proteins
= 10-18J
Hydrogen Bonding
In a covalent bond, an electron is shared between two
atoms.Hydrogen possesses only one electron and so it can covalently
bond with only ONE other atom.The proton is unshielded and makes an
electropositive end to the bond: ionic character.Bond energies are
usually stronger than v.d.W., typically 25-100 kT.H-bonding can
lead to weak ordering in water.
From 3SCMP
Surface tension & the importance of interfaces
Molecules on surface have fewer neighbors and so exert greater
force on adjacent molecules = surface tension (in dynes cm-1 or N
m-1 Jm2)
Surface tension = surface energy (N m-1 = Jm-2)
Nature tries to minimize the surface area of interfaces (spheres
and the bigger the better)
It costs energy to phase separate and make an interface
Small particles have higher surface area per gram; higher
energy
surface area versus diameter for particles
Particle Coalescence
Same polymer volume before and after coalescence:
In 1 L of latex (50% solids), with a particle diameter of 200 nm, N
is ~ 1017 particles. Then A = -1.3 x 104 m2
With = 3 x 10-2 J m-2, F = - 390 J.
From 3SCMP
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
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
Thermodynamics of Mixing and phase separation
Entropically mixing is usually favorable (+)Enternal energy U often
is crucial component
Important for mixing of organic and inorganic precursors to
hybrids and for phase separation that might occur upon
environmental changes or changes in chemical structure
Thermodynamics of mixing of mixing A & B
Re-write in terms of an interaction parameter Chi time kT times the
volume fractions of A and B
Now you can just vary Chi and T and explore phase diagrams
Helmholtz Free Energy (Constant Volume)
For small molecules, NA = NB = 1 & S is large and positive.
S polymer < S molecule
Spinoidal decompositon into two phases
Spinodal decomposition of mixture of liquid crystals
Phases grow in size to reduce their interfacial area in a
process called coarsening.
Block copolymers tie the two immiscible phases together
Still spinodal decomposition
Coarsening is stopped by connected macromolecules
Covalent bonds [provide greater metastability of turing
structure
Nucleation in metastable regions
Only f1 and f2* are stable phases! The f2* composition must be
nucleated and then it will grow.
Nucleated structure: islands of one phase in another
Spinodal structure: co-continuous phases
From G. Strobl, Polymer Physics, Springer
Nucleation of a Second Phase in the Metastable Region
Growth of the second phase occurs only when a stable nucleus with
radius r has been formed.
is the interfacial energy between the two phases.
Small: usually a few nanometers
Formation of bonds: Polymerization
Hydrolysis:
Condensation:
Net Polymerization:
Shown here for formation of a silsesquioxane
Most hybrids involve phase separation
All nucleation. Rare to see spinodal decomposition
Amorphous versus crystalline
Amorphous kinetic, no long range order, no time for crystals to
grow from solution or liquid.How can you tell if a material is
amorphous?Crytsalline: thermdynamic structures made with
reversiblity to remove defects and correct growth. Long range
order.How can you tell if a material is crystalline?
Crystalline materials
Long range order: Bragg diffraction of electromagnetic radiation
(or electron beams in TEM) by crystalline lattice into sharp peaks.
Solid structures with geometric shapes, straight lines and flat
surfaces, and vertices.Optical affects like bifringenceDirect
visuallization of crystal at molecular level with AFM or
STEM.Melting point (not always though)
AFM of polyethylene crystallite
Inorganic crystals
XRD from semicrystalline polymer film
microcrystals
Rutile titania crystals in amorphous TiO2
Micrograph of polymer crystalline spherulites
XRD (wide angle)
Single crystal or microcrystalline powder (crystals with atomic or
molecular scale order)
X-ray powder diffraction from polybenzylsilsesquioxane LADDER
Polymer
Big picture is amorphous material.Small sharp peaks are due to
contaminant from preparationNot a ladder polymer!!!!!!!!!
Amorphous materials
No long range order: diffuse peaks may be present, due to average
heavy atom distances. No crystalline geometries, glass like
fractures (conchoidal)Aggregate spherical particles commonNegative
evidence for crystal at molecular level with AFM or STEM.No Melting
point
XRD amorphous material
Al2O3 thin films prepared by spray pyrolysis
J. Phys.: Condens. Matter 13 No 50 (17 December 2001)
L955-L959
2012 EPL 98 46001
Amorphous materials: XRD
amorphous
amorphous
crystalline
Conchoidal Fractures in amorphous materials
Crystals break along miller planesUnless microcrystalline
If crystals are small compared to impact, conchoidal fracture
can occur
In sandstone 3 meters tall)
In metal
Summary: Physics of Hybrids
Bonds & non-bonding forces that hold materials togetherSurface
tension and surface free energyThermodynamics of Mixing and phase
separation ( of polymers in particular)Nucleation and Spinodal
decompositionBlends of immiscible polymers and immiscible block
copolymers Nucleation of particles & sol-gel
chemistryDifference between crystalline and amorphous
Todays lecture will provide background information on the nature
of bonding and non-bonding interactions and how they contribute to
material properties. Introduce you to some very basic sol-gel
hydrolysis and condensation chemistry involved in making the
inorganic part of these hybrids. And talk some about the
themodynamics of phase separation-both of particles and later of
emulsions and polymers.
*
*
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
*
eo is the permittivity of free space and e is the relative
permittivity of the medium between ions (can be vacuum with e = 1
or can be a gas or liquid with e > 1).With Q1 = z1e, where e is
the charge on the electron and z1 is an integer value.
The interaction potential is additive in crystals
*
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.
*
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.
*
The two phases have a characteristic size scale defined by a
compromise.If the sizes of the phases are too small: energy cost of
extra interfaces is too high.If the phases are quite large, it
takes too long for the molecules to travel the distances required
for phase separation.
*
A schematic illustrating a typical time evolution of domain
formation during spinodal decomposition. With a progress of the
decomposition, a sharp interface between two phases develops to
form a periodic bicontinuous structure (process I). After the
relatively early stage, the bicontinuous structure starts to
coarsen to reduce the interfacial area (process II). The coarsening
occurs with dynamical self-similarity (c, d, e); that is, forms of
the structures are statistically identical at various times, while
the characteristic sizes increase with time. The coarsening process
is followed by fragmentation of the minority phase and subsequent
restructuring due to minimization of surface energy, which finally
results in spherical domains and a continuous matrix (f).
*