Crosslinking
Another page about Crosslinking is available on the Coatings floor.
The crosslinking of a system effects the tensile strength.
(Sperling-450)
Sperling makes a comment about how the above equation relates Young's
modulus to the crosslink density, and this could be construed to imply
that n is the crosslink density. To me it would seem that the units of
n would be reciprocal volume since n might be, say, number of crosslinks
per cubic centimeter.
Take care not to confuse this 'nRT' with PV=nRT!
- E
is Young's Modulus.
- n
is the number of network chain segments per unit volume. (Sperling-389) yes!
- R
is the real gas constant.
- T
is temperature.
We start by considering the crosslinked system, also referred to as a network,
as short strings between junctions. In the graph below we see a crosslink
between between blue, green, and red, and a crosslink between yellow, red,
and purple.
The blue has a "dangling end" not attached to any other chain section.
We could argue then that the blue section is not doing its job of tying
together two polymer chains. A system with many dangling ends should not
have as high of a tensile strength as a system where nearly all of the
chain segments connect two chains together.
Flory states that these dangling ends contribute nothing to elastic
recovery.
If the stress pulls from left to right, the red and orange chains will
pull against each other and one of the two chains would have to break
before the specimen would separate. This contributes to tensile strength,
and is refered to as a physical crosslink.
Last Update- September 13, 1995- wld