MECHANICAL PROPERTIES OF POLYMERS 



125 



The most information about the motions of isolated polymer chains 

 can be obtained by investigating the properties of polymer solutions. 

 This follows from the fact that in pure polymer liquids, and in solids, 

 the mechanical properties are mainly determined by interactions be- 

 tween chains on account of the close packing of the chains. If, however, 

 one dissolves the polymer molecules in a solvent, the inter-chain and 

 intra-chain reactions can be separated as the dilution increases. When 

 the polymer is in the order of one percent of the solvent, the chains on 

 the average touch very seldom and the mechanical properties of the 

 solution are determined by the properties of single molecules. As dis- 

 cussed in Section III, three types of chain segment motion have been 

 isolated, (1) a configurational relaxation of the chain as a whole, (2) a 

 position change of the shortest segment and (3) twisting of the shortest 

 chain segment. Above the frequency of relaxation of this chain segment 

 the joints of the polymer molecule become frozen and the chain be- 

 comes very stiff. These shortest chain relaxations occur also in pure 

 polymer liquids, in rubbers and in non rigid solids with linear chain 

 segments such as polyethylene. In pure liquids a lower frequency quasi- 



3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 



LOG,o FREQUENCY 



F'ig. IB — Mechanical impedance loading for a medium with two relaxation 

 frequencies. 



