chemistry and physics of hybrid organic-inorganic materials
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DESCRIPTIONChemistry and Physics of Hybrid Organic-Inorganic Materials. Lecture 3: Material Interactions in Hybrids. Material Interactions in Hybrids. Non-bonding interactions Bonding interactions Surface tension Free energy Changes of phase Phase separation Crystalline or amorphous. Length Scales. - PowerPoint PPT Presentation
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
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
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
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
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.
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
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.
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
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?
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
XRD from semicrystalline polymer film
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!!!!!!!!!
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
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)
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.