MECHANICAL PUOI'KUTIKS OF POLYMERS 



145 



For loiiji; chain segments the values become asymptotic to 8 X 10 cycles 

 and 1()(),()()() cycles which are not far from the two highest relaxation 

 frcciuencics obtained from the solution measurements of Section III. 

 Hence it appears likely that these relaxations are due to the "entangle- 

 ment" motion and the twisting motion of the shortest chain segment. 

 The increased activation energy is due to the fact that more energy has 

 to be applied to the chain segment to break it loose from its equilibrium 

 position when it is surrounded by adjacent polyisobutylene molecides 

 than when it is surrounded by cyclohexane molecules. The stiffness of 

 t he chain is due more to the slope of the potential well than to any in- 

 t rinsic chain stiffness as is shown by Fig. 18, which shows the two stiff- 

 nesses as a function of temperature. These values are obtained by fitting 

 the 15°, 35° and 50° data in a similar manner to that used for the 25° 



40 60 80 100 120 140 160 

 NUMBER OF CHAIN SEGMENTS 



Fig. 17 — Solid Unes are mean values of relaxation frequencies for the two 

 mechanisms plotted against molecular weight. Dotted lines indicate limits of 

 regions assumed to obtain a better fit to the measured values. 



