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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 (CH2CH2) 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 2v. 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 snake-like (reptile-like) 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 liquid-like? 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
 

Photo: L. Falk

       


Polymer history

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 (self-excluded 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, Pierre-Gilles de Gennes reshaped and developed the physics of polymers.

 



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