Crosslinking
Another page about Crosslinking is available on the Bulk Properties floor.
Crosslinked polymers are also called network polymers or thermosets
.
A string analogy can be used to distinguish between linear,
branched, and crosslinked polymers.
Linear polymers are equivalent to long strings which are not tied
together.
Branched polymers are equivalent to tying the same pieces of
string together, taking care not to tie any closed loops.
If you tie closed loops, then you are creating the model of a
crosslinked system. Notice below the pink dashed lines which show
one of the two 'closed loops' in the system.
A crosslinked polymer molecule is three dimensional, and a cartoon is
shown below:
Crosslink density-
- [Elias-53] defines crosslink density as the mole fraction of monomer
units which are crosslink points.
Example: for a system where 1 of every 20 molecules was a trifunctional
isocyanate, 9 of every 20 molecules was a bifunctional isocyanate, and
10 of every 20 molecules was a bifunctional alcohol, we could say the
the crosslink density was 0.05.
- Crosslink density is sometimes expressed as the average mass of polymer
between crosslinks, and this variable is written M subscript c.
As a coating crosslinks, the
glass transition increases.
Ueberreiter and Kanig reported that the change in glass transition
temperature was directly proportional to crosslink density.
K. Ueberreiter, G. Kanig, J. Chem. Phys., 18, 399 (1950)
J. K. Gillham developed a
time-temperature-transformation reaction
diagram that illustrates gelation and vitrification with respect to
the glass transition temperature.
Vulcanized rubber, developed in (year) by Harvey Firestone, and used for
automobile tires, is a well known example of a crosslinked polymer. Phenol
formaldehyde resins, epoxy resins, amino resins, polyurethanes, unsaturated
polyesters are all examples of polymers that can be used for crosslinked
systems.
Last Update- November 5, 1995- wld