|
Silicone Polymers (Siloxanes)
Silicones are macromolecules comprised of a polymer backbone of alternating
silicon and oxygen atoms
(siloxane bonds) with organic side groups, such as methyl, phenyl or vinyl,
attached to silicon. The number of repeating units, “x,” can range from zero to
several thousand.
Polydimethylsiloxane, the most commonly available
silicone:
By adjusting -Si-O- chain lengths, the functionality of the side groups and
the crosslinking between molecular chains, silicones can be synthesized into an
almost infinite variety of materials, each with unique chemical properties and
performance characteristics.
The types of organic groups attached to the silicon atoms can be controlled
to create a hybrid chemistry capable of imparting important organic
characteristics to the silicone, including compatibility and substantivity.
-
Compatibility – Normally, silicones are not compatible with organic
polymers. But if a larger organic group is attached to a silicone, its
properties are altered, and it behaves more like an organic polymer.
-
Substantivity – Nonreactive silicones lack substantivity. But if an
organoreactive group, such as an aminopropyl group, is attached to the
silicone, the silicone becomes reactive and will form stable bonds with other
organic polymers.
By blending the chemistry of silicon with the chemistry of carbon, it is
possible to create polymers with unique
properties and superior performance characteristics.
Organoreactive Polysiloxanes.
By adding a number of organoreactive groups to a polysiloxane, it is
possible to create a silicon-based organoreactive molecule with the chemical
characteristics of a carbon-based reactive molecule and the physical properties
of a polysiloxane.
F = Organoreactive group
n = Generally ≥ 3
x = 0 to 1,000
y = 1 to 25
This organoreactive silicon-based molecule can be made to any size. Any
number of reactive groups can be designed into it. The organic groups are
usually separated from the silicon by at least a three-carbon chain. This chain
separates the organic reactivity from the effects of the silicon.
Organoreactive groups can be attached to polysiloxanes in a number of ways,
including:
1. Pendant to the polysiloxane backbone, forming “rake” or “comb”
structures
2. At the ends of the polysiloxane chain, forming an ABA structure
3. At one end of the polysiloxane chain, forming an AB structure
Pendant (“rake”) functionality:
ABA functionality:
AB functionality:
R = Amine, carboxy, hydroxy, epoxy, other
n = Generally ≥ 3
x = 0 to 1,000
y = 1 to 25
In rake structures, organic functionality is distributed along the
polysiloxane chain. The resulting silicones are used to chemically bond the
polysiloxane polymer to an organic material, or to give unique orientation of
the silicone on a metal or glass surface.
AB or ABA organoreactive silicones are preferred when molecular architecture
or copolymer synthesis is required because there are only one or two reactive
groups per polymer chain, located predictably at the end of the chain. Reactive
end groups are more exposed than the pendant reactive groups found in rake
structures and are more available to form copolymers with organic
intermediates.
AB and ABA functionalities are ideal for tying silicone and organic
structures together to form silicone-organic or silicone-organic-silicone block
copolymers.
Return to the
Silicon Chemistry Tutorial home page.
|
Change
