Naturally occurring hybrids include: bone (hydrated calcium phosphate or
apatite and protein), mother of pearl (nacre: aragonite and protein), chiton lobster
shell (chiton and apatite), silica-protein spines in sea urchins or skeleton of sponges,
magnetite-carbohydrate teeth in Chitons or Zinc and Magnesium-chiton fangs in
spiders, silica-carbohydrate in grasses and horsetails, silica-protein in diatoms,
calcite-protein in mollusk shells, apatite-protein in dentin or enamel of teeth.
Many of these naturally occurring hybrids are hierarchical structures, those
with different structures at different length scales (sizes). Especially good exams
include diatoms, enamel, chiton exoskeletons of lobsters, bone, and nacre.
In the lab, we can prepare hybrids physically or chemically mixing the
inorganic and organic components. Physical mixing would entail melting or
dissolving the polymer then mixing with premade inorganic particles. Cooling the
melt or evaporating the solvent returns the polymer to its solid state and finishes
the hybrid. Some examples of physically mixed hybrids include: silica particles in
nafion, silica in practically any elastomer, thermoset or thermoplastic, fullerenes in
many thermoplastics, and clays intercalated and exfoliated with thermoplastics,
such as Nylon-6 or poly(ethylene terephthalate) or polypropylene.
The inorganic particles used in these hybrids can be purchased or made by sol-gel
polymerizations (silica, alumina and titania and other metal oxides, or POSS), flame
or aerosol syntheses (silica or metal oxides), or emulsion polymerizations (silica,
alumina and titania and other metal oxides, or POSS). Alternative, and less
commonly seen approaches include precipitation, electrochemistry, electroless
reductions or oxidations. The sol-gel polymerizations involve two reactions:
Hydrolysis: replacing alkoxide groups with OH groups and Condensation: reaction
of OH groups to form Si-O-Si groups. Gelation occurs when solid particles dispersed
in a liquid percolate. From sol-gel polyemrizations, the chemistry has to lead to
polymers large enough to phase separate as solid particles (polysilsesquioxanes
often phase separate as liquid droplets that coalesce and never gel). Monomers
with greater functionality than 2 can theoretically form very high molecular weight
polymers that will phase separate out as solid particles. Cyclization lowers the
molecular weight and prevents solid particles from forming when the organic group
is large on pendant silsesquioxanes.
When we physically mix clays with polymers, we must fist intercalate
surfactants into the spaces between the two dimensional clay silicate layers, then
mix molten or dissolved polymer with sufficient shear to replace the surfactant with
polymer. When the layers are still in a 3-D stack but spread apart, they are
intercalated. Here the XRD shows the clays crystal structure with the unit cell
elongated in the z-axis. When the stack is broken up the individual clay sheets are
exfoliated and the crystal structure bragg diffraction disappears.
These exfoliated clay-polymer materials are nanocomposites. When we mix
nanoscale inorganic particles, fibers, plates, etc. with organic polymer we have a
nanocomposite. A nanocomposite has at least one phase that is between 1-100 nm
in dimensions.
Alternatively, hybrids can be made by polymerizing monomers that contain both
inorganic and organic components. These include making polysiloxanes or
polysilanes from dichlorosilanes, polysilsesquioxanes from organotrialkoxysilanes,
and polyphosphaazenes from hexachlorocyclotriphosphazene,