NANOTECHNOLOGY OKLAHOMA

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NANOTECHNOLOGY OKLAHOMA
Organic Chemistry and Polymers Warren T.
Ford Oklahoma State University Department of
Chemistry warren.ford_at_okstate.edu

• What does chemistry have to do with
  nanotechnology?
• Chemistry controls the state of matter solid,
  liquid, gas, and states in between such as liquid
• crystals, gels, and supercritical fluids. We are
  concerned mainly with solids in this course.
• Chemical structure determines the surface
  properties of solid materials.
• Chemical structure controls how the molecules
  pack together and determines the mechanical,
• electrical and magnetic properties of materials.
• Most nanoscale materials are products of chemical
  synthesis.

You may need to consult an organic chemistry
textbook to understand some of this material. Any
standard text will be satisfactory. On the
internet the Virtual Text of Organic
Chemistry, http//www.cem.msu.edu/reusch/vtxtinde
x.htm may serve as a quick reference.
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Ionic and covalent bonds
NaCl, sodium chloride, table salt Molecular
weight 58 g/mole melting point 801 oC, soluble in
water Solid contains Na and Cl- ions in a cubic
crystal lattice held in position by ionic bonds.
Electrostatic forces are strong and long
range. Solution contains dissociated Na and Cl-
ions stabilized by attractive ion-dipole forces
with water.
C4H10, butane Molecular weight 58 g/mole Gas at
room temperature, insoluble in water Atoms are
joined by covalent C-C and C-H bonds formed by
shared pairs of electrons. The bonds have
little polarity the atoms have no formal
charge. Butane has no dipolar attraction to
water. The only intermolecular attractions are
weak van der Waals forces butane has low
melting and boiling points.
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How to represent the structure of butane C4H10,
molecular formula CH3CH2CH2CH3, condensed formula
Dash formula Each line represents a two-electron
covalent bond. There are always 4 bonds to C
and 1 bond to H.
Bond-line formula. Each end of a line or
intersection of lines represents a C atom each
line represents a C-C bond all missing atoms
are H atoms, and all missing bonds are to H
atoms. There always 4 bonds to C.
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Periodic table of the elements Find a good
periodic table on the web, such
as http//ull.chemistry.uakron.edu/periodic_table/
Consider only the second row Li Be B C N
O F Ne Electronegativity the tendency of
the atom to attract electrons from other atoms to
form either ions or polar covalent bonds
increases from left to right. Li loses one
electron to form Li. F gains one electron to
form F-. LiF is a crystalline compound composed
of Li and F- ions. Carbon forms covalent bonds
with most other elements by sharing
electrons. Organic chemistry is defined as the
chemistry of carbon compounds. Most organic
compounds are composed of the elements C and H
and often one or more of The elements N, O, F,
Si, P, S, Cl, Br, I. Inorganic materials may
contain any of the elements. The greater the
difference in electronegativities of covalently
bonded atoms, the more polar the bond. The
electrons of a C-Li bond are pulled toward the C
atom. The electrons of a C-F bond are pulled
toward the F atom. We do not know exactly where
the electrons are due to the Heisenberg
uncertainty principle. We do know probable
distributions of electrons from quantum
mechanical calculations.
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Energies of intermolecular attraction - depend on
interatomic distance r. In order of decreasing
strength Coulombic E ? r-1 Ion-dipole E ?
r-2 Dipole-dipole E ? r-3 (a hydrogen bond in
this case) van der Waals E ? r-6
Na Cl-
Na..OH2
H2O..HOH
The stronger the attractive forces between ions
or molecules in a compound, the higher the
boiling point. van der Waals attractions sum
over all atoms and can be very large for large
molecules such as polymers and carbon nanotubes.
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3D structures of molecules
Methane has a tetrahedral structure with the C
atom at the center. All four H atoms in methane
are equivalent. The HCH bond angles are 109.5o.
Ammonia and water are approximately
tetrahedral. The bond angles are107o and 104.5o
respectively. The unshared pairs of electrons
occupy the remaining corners of a tetrahedron.
C, N, and O must have 8 electrons in the valence
shell in stable compounds and ions.
Ethylene has a C-C double bond. All six atoms of
ethylene lie in a plane. All of the bond angles
are approximately 120o. Planar geometry also
applies to CN, CO, and NN double bonds, and
even to atoms bonded with partial double bond
character, such as the amide groups of proteins.
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References for polymers
For an elementary introduction to polymers, see
the Macrogalleria. http//pslc.ws/mactest/maindir.
htm For a textbook covering synthesis,
characterization, and properties at the
advanced undergraduate - beginning graduate
level, see Allcock, H. R. Lampe,F. W. Mark, J.
E., Contemporary Polymer Chemistry, 3rd edition,
Pearson/Prentice Hall, 2003. For a textbook that
focuses on properties of polymers in solution and
in the solid state, see Sperling, L. H.,
Introduction to Physical Polymer Science, 3rd
edition, Wiley-Interscience, 2001. For a nicely
illustrated introduction to the fundamental
physics of macromolecules, see Grosberg, Yu.
Khokhlov, A. R., Giant Molecules, Academic Press,
1997.
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Polymers Polymers are big molecules
(macromolecules). Because of strong
cumulative van der Waals forces, polymers exist
only as solids or liquids at high temperatures
they decompose. Most organic materials
decompose at lt450 oC. It takes macromolecules
to make plastics, fibers, elastomers, and
adhesives. The properties of polymers depend upon
the chemical structures. Properties may also
depend on the molecular weight (shown below from
the book by Sperling for the melting temperature
of polyethylene), but above lower limits, which
differ for the polymer and the property, most
properties such as tensile modulus and electrical
conductivity are approximately constant. Molecular
weights can be from 103 to 107 g/mole.
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Polymers can be synthetic, such as polyethylene,
polystyrene, Nylon-6,6, or silicone rubber, or
natural such as DNA, proteins, and
cellulose. Polymers have regular repeating
structures that are determined by the way they
are synthesized. DNA and some proteins, such as
enzymes, have very specific structural sequences
that are the basis of their biological
functions. DNA carries the genetic code.
Enzymes are catalysts for specific reactions.
Synthetic polymers contain macromolecules of the
same repeating structure but a distribution of
molecular weights. The molecular weights are
measured and reported as averages. Typical
structures from the book by Allcock are shown on
the next page. -structuren- means that the
structure is repeated n times where n is a
statistical distribution of chain
lengths. Synthetic polymers are commonly
processed as melts or as solutions. Polymers are
soluble in fewer solvents and often to lower
concentrations than low molecular weight
materials due to much smaller entropy of mixing.
Some polymers are not soluble in any solvent.
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Relation of macroscopic to molecular dimensions
of materials
There is an Avogadros number (6.023 x 1023) of
molecules in a mole of a pure material. Moles are
usually expressed as gram molecular weights, the
sum of the atomic weights of the constituent
atoms expressed in grams. On the molecular
scale, weights are expressed in atomic mass
units (amu). One mole of a polymer of average
molar mass 1.00 x 105 g/mole is composed of
6.023 x 1023 molecules having average molecular
mass 1.00 x 105 amu. Small molecules typically
have dimensions of lt2 nm. Single macromolecules
are larger but still have nanoscopic dimensions,
regardless of whether they are in the form of
solid particles, expanded random coils (in
solution or in the amorphous solid or liquid), or
wound into double helices like DNA. (For an
attempt to create single molecule solid particles
of poly(methyl methacrylate) (PMMA) in the form
of 15-nm diameter spheres, see Pilcher, S. C.
Ford, W. T., Macromolecules, 1998, 31, 3454-3460.)
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Macromolecular architecture
Many polymers consist of linear long chain
molecules with small pendant groups, such as
benzene rings in the case of polystyrene. If the
pendant groups are polymeric chains in their own
right, the structure is branched. If many long
chain molecules are linked to one another by
covalent bonds, the structure is cross-linked
into a network. See the drawings on the next
page from the book of Grosberg. A structure that
has one kind of regular repeating structural unit
is a homopolymer. A structure containing more
than one kind of regular repeating unit is a
copolymer. If a copolymer consists of two or
more long chain homopolymers linked together, the
structure is a block copolymer. See the
Macrogalleria for examples. http//pslc.ws/mactest
/maindir.htm
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Physical states of polymers
Solid polymers are either semicrystalline or
amorphous, rarely completely crystalline. The
mix of nanoscale crystalline and amorphous
regions of semicrystalline polymers is
responsible for their properties as
thermoplastics and fibers. At low temperature
solid amorphous polymers are glassy. As a linear
polymer is heated, it reaches a narrow range of
temperature, the glass transition temperature Tg,
at which the glass becomes a rubbery liquid. Tg
is below 0 oC for most elastomeric polymers such
as natural rubber and silicone rubber. Some
polymers such as epoxy resins have no Tg because
they decompose before Tg can be
reached. Semicrystalline polymers have both
amorphous and crystalline domains of nanoscopic
dimensions. As the polymer is heated through
Tg, it transforms from a rigid solid to a
thermoplastic. The crystalline domains are solid
and hold the material together. The amorphous
domains are rubbery liquid and can be
stretched. An illustration from the book of
Allcock of the properties of amorphous and
semicrystalline polymers as a function of
temperature is on the next page.
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Relation of polymer architecture to physical
properties
In amorphous solid and molten polymers long chain
macromolecules exist in random coil
conformations that are tangled together (below,
from the book of Sperling). Any one
macromolecule may occupy a volume in space tens
of nm across (illustrated on the next page from
the book of Grosberg), but this same volume
contains many macromolecules. This tangling of
molecules gives them the properties of plastics
and fibers. If the long chains are
cross-linked, the material cannot be deformed too
much without breaking bonds. If the number of
cross-links is small, the material is rubbery, an
elastomer. If the number of cross-links is
large, the material is a rigid solid, known as a
thermoset.
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Block Copolymers
Block copolymers have two or more homopolymer
chains linked together end-to-end. Both
diblocks, AB, and triblocks, ABA (where A and B
are two different linear polymers), are common.
They are made by sequential polymerization of
the blocks using methods known as living
polymerizations. The lengths of each block can be
controlled to narrower distributions (ideally of
constant length) by living polymerizations than
by conventional chain reactions. The general
structure of a block copolymer of styrene and
methyl methacrylate is shown below.
Polystyrene (PS) and poly(methyl methacrylate)
(PMMA) are immiscible, as are most polymers, but
in a block copolymer the blocks are linked by a
covalent bond. As a result, a solid glassy
polymer consists of domains of PS and domains of
PMMA whose sizes are limited by the sizes of the
blocks. Depending on the relative volumes of the
two phases, the morphology of the solid can be
spheres of PMMA in a PS matrix, cylinders of PMMA
in a PS matrix, layers of PMMA and PS, cylinders
of PS in a PMMA matrix, or spheres of PS in a
PMMA matrix. The spheres and cylinders of the
minor component are ordered at approximately
constant distance apart because of the nearly
uniform size of the minor block. The
morphologies are illustrated on the next slide,
taken from the book of Grosberg. If the minor
phase of the solid can be removed by extraction
or pyrolysis, the remaining material is a solid
having uniform spherical or cylindrical holes.
Read Di Ventra, pp 63-71, to learn about how
these nanostructured materials can be applied in
the solid state. Read Di Ventra, pp 533-546, to
learn how amphiphilic block copolymers with both
water-soluble and water-insoluble blocks form
nanostructures in aqueous solutions that often
resemble biological membranes.
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Dendrimers
A new class of polymers, first reported by
Tomalia and by Newkome in 1985, has dendritic
(tree-like) structures that result from
incorporation of structural units by individual
synthetic steps. Most other polymers are made in
one synthetic operation by chain growth or step
growth reactions. In the synthesis of a
dendrimer, each synthetic step doubles the number
of chain ends of the branched macromolecule. A
scheme from the book of Allcock for a dendrimer
with a trifunctional core is shown below. A
specific example of a poly(propylene imine)
(PPI) dendrimer with 8 NHC(O)C11H23 chain ends is
shown on the next page. Dendrimers having 32 or
more chain ends are forced into globular
conformations by the crowding of the branches.
Because of the globular conformations,
dendrimers have many fewer van der Waals contacts
with their neighbors than do linear polymers.
Dendrimers do not entangle like linear
macromolecules, have Tg far below room
temperature, and often behave as single
macromolecules (single nanoparticles). A PPI
dendrimer having 64 end groups is globular with a
diameter of 4 nm. Dendrimers are potential
single molecule building blocks for
nanostructured materials.
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Fullerenes and carbon nanotubes
Sizeable amounts of the all carbon molecule C60
with the football (soccer ball) structure
appear in the soot that forms in high
concentrations of carbon atom vapors. This 1985
discovery earned Kroto, Smalley and Curl the
1996 Nobel Prize in chemistry. C60 is a sphere
of diameter 0.7 nm (atom to atom counting the
van der Waals radius the diameter is 1.0 nm)
with completely alternating,localized double
bonds in its 6-membered rings. In 1991
Iijima found that under similar conditions in the
presence of a metal catalyst (usually Co or Fe)
carbon nanotubes are formed. Carbon nanotubes
can be either single-walled (SWNT) or coaxial
nests of tubes called multi-walled (MWNT).
Typical SWNT have diameters ranging from 0.7 to
2 nm and lengths of gt 1 ?m. A SWNT having a
diameter of 0.7 nm is basically two hemispheres
of C60 joined by an all-carbon cylinder. The
cylinder is like a roll from a sheet of graphite,
which has a planar structure entirely of
6-membered rings. Most samples contain many
different structures of SWNT due to different
diameters and different helical angles of rolling
the graphene sheet. Statistically about 2/3 of
the structures are semiconductors, and 1/3 are
metallic conductors of electricity. A single
tube has the highest tensile strength per unit
mass of any material known - greater than steel
or Kevlar. SWNT also have very high thermal
conductivity. These properties make SWNT
extremely attractive as components of nanoscale
devices and as components to enhance the
mechanical strength and conductivity of
composite materials. However, SWNT as
synthesized in bulk consist of bundles and ropes
of tubes that are very difficult to separate
because of strong intertube van der Waals
attraction. An alternative way to produce SWNT
is to grow a forest of tubes from spots of
catalyst on a smooth surface such as that of a
silicon wafer.
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Oklahoma has expertise in the manufacture of
SWNT. Prof. Daniel Resasco at OU invented a
method using a Co/Mo catalyst supported on
silica that produces narrower average diameters
and simpler mixtures of tube types than any
other process. He also now has a method to grow
forests of 50 ?m-long SWNT from the surface of a
silicon wafer. Commercial SWNT are available
from SouthWest Nanotechnologies in Norman.
Chapter 6, pp 137-180, of Di Ventra covers
structure, properties, and applications of SWNT.