What do they look
like? ........








Photo:
L. Falk







Plastic, rubber, plexiglas, gels and textile fibres
such as nylon and polyester are all synthetic
materials made of polymer molecules.


A Frisbee is a mixture of
crystalline (ordered) and amorphous (disordered)
polymer structures.
This makes the material both strong and
flexible.


Each polymer molecule consists of linked basic
units called monomers. The number of monomers in a
chain can be very large, from thousands to several
millions. The chains may be entangled like
spaghetti (as above) or be ordered in crystalline
formations.


The monomer is specific for every polymer and
determines the name of the substance. A Frisbee is
made of polyethylene which consists of linked
ethylene (CH_{2}CH_{2}) monomers.


If the polymer chain is long,
its size can be described by simple proportion ratios
called scaling laws: When the number of
monomers N is
doubled, the size is increased by the scaling factor
2^{v}. The exponent v is
universal in the sense that it is the same for all
polymer chains although it depends on the polymer
concentration.
In the blob model de
Gennes represents the concentration by a sphere
(blob). The diameter of the sphere is given by the
average distance x between
two chains and increases as concentration decreases.
Inside the sphere the chain segment is swollen like
in a good solvent (v= 3/5). For segments
longer than x the blobs
can be taken as the basic units and then the law for
concentrated solutions holds (v= 1/2).
The blob model is used to
describe polymer solutions, crosslinked polymers
(gels), polymer welds, interfaces, polymers at
surfaces etc.

........ and how do they
move?
The snakelike (reptilelike)
motion of an entangled polymer chain is explained
by imaging that it is confined to a "tube" formed
by adjacent chains. The reptation time
t, the time
needed for the chain to completely move out of the
tube, can be obtained from simple scaling
arguments. The reptation model leads to a smaller
exponent (v= 3) than the measured one
(v= 3.3) but it can nevertheless explain a
number of phenomena and is very powerful in its
simplicity.






Are you familiar with "silly putty", the strange
substance which is both solid and liquidlike? If
you pull it slowly in comparison with the reptation
time, the material flows like a very viscous
liquid. If you form it into a ball and strike it
quickly, it bounces like rubber.

Photo: L. Falk








The basis for the modern physics of polymers
was laid by pioneers such as P.
Debye, W. Kuhn, H.A. Kramers, and P.
Flory. A milestone was set in 1949 when Flory
(Nobel Prize 1974) described the size of a
polymer chain. He took into account that a
certain volume occupied by the chain segments
(selfexcluded volume) is not available to the
other segments of the chain.
The next phase depended
on new experimental methods. With neutron
scattering one could determine the chain
dimensions. Using light scattering
techniques, which advanced after the invention of
the laser in 1960, one could measure the
movements of polymers. At the same time
sophisticated theoretical methods
developed (functional integrals, correlation
functions, renormalisation groups, and the
techniques of manybody theory), which were first
applied to polymers by S.F. Edwards. With high
speed computers and numerical methods models of
polymers could be tested using computer
simulations.
With classical physics as
a basis, by using the latest experimental results
and theoretical tools, PierreGilles de Gennes
reshaped and developed the physics of
polymers.

