Thin layers of polymers - Paints and Brushes
Polymer Brushes
Suppose you have a bunch of micron-size spheres that you want to
suspend in a solvent (e.g. water)
to make a colloid - maybe to make a paint (little
latex spheres), a medicine (small biodegradable plastic spheres
impregnated with a drug), a magnetic fluid (small magnetic particles).
You will certainly have a problem - the little spheres will glom
together thanks to van der Waals attractions, and you will have
a glob of yecch at the bottom of your bottle,
instead of a nice stable colloid.
What you need is some kind of repulsive force field acting between
the particles to push them apart. A way to do this is to end-attach
a bunch of flexible polymers to the surface of the spheres
to make a molecular forest - or a polymer brush.
The polymers stretch out away from the
surface and away from each other in order to make intimate contact with
the solvent, pushing the particles apart, and stabilizing the colloid.
This technology is used in the real world.
As you can see in this
movie
(made in collaboration with Amit Chakrabarti of Kansas State University)
of a computer simulation of a polymer
brush, the chains do some interesting things. The bottom shows
all of the chains, while the top shows a few selected chains so we
can watch their fluctuation; the red square represents the surface
to which the chains are attached.
Note that the thickness of the layer is well defined, although the
shape of an individual chain fluctuates violently.
On average, the chains
are pretty stretched. But a given chain at a given time can be
very unstretched - in fact there are fascinating
static and dynamic stretching fluctuations
in these polymer brushes. A couple of groups are working on
experiments to study these fluctuations.
Last modified: May 23, 1994
John Marko, marko@msc.cornell.edu