332 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1952 



aiul chain flexibility obtain, as in polybutadiene and the siUcones, dy- 

 namic properties apparently accord with brittle points hi implying small 

 temperature coefficients of relaxation times. In fact, the temperature 

 coefficient for dynamic viscosity of jjolybutadiene is only al)out 1.5 kcal, 

 whereas a comparable figure for polyisobutylene and polypropylene is 

 12 kcal. 



The frequency range in which the structural comparisons above were 

 made, resides, as discussed earlier, ui the zone of configurational visco- 

 elasticity. That is, over-all shape changes, rather than just nearest 

 neighbor interactions, are predominant even at these comparatively 

 short average chain lengths. Now, other recent studies of polyisobutyl- 

 ene liquids, at 5 to 100 cps frequency, exhibit no rigidity at 25°C and 

 above, although they become non-Newtonian rapidly as temperature is 



Table VII. Shear Dynamics of Polyisobutylene A". 

 (Mv = 1660; 77,25° = 39.6 Poises) 



reduced. ' The questions are, where does the configurational elasticity 

 drop out, as frequency is reduced at 25°C; and does it seem reasonable 

 that this dispersion correlates with a shift in frequencies at lower tem- 

 peratures. Partial answers are given by very recent studies of I. L. 

 Hopkins of Bell Telephone Laboratories. He has eciuipped the tuning 

 fork vibrator described earlier with two parallel vanes filled in between 

 wdth a film of polymer liquid. Pure shear properties can be derived from 

 the response of this system. Table VU lists a few typical figures obtained 

 on polyisobutylene polymer A". These indeed show that the kilocycle 

 relaxation zone (some new data by McSkimin's torsional pulse method 

 are given for it) extends smoothly down to where dynamic and steady 

 stress viscosities are equal. Seemingly there are no new "extra long 

 time" relaxation mechanisms; probably the slow relaxation times some- 

 times indicated for high molecular weight rubbers are just displacements 

 of this configurational relaxation to long times because of high molecu- 

 lar weight and internal viscosity. 



By contrast to the conclusions associated with the data of Table VII, 



