308 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1952 



storage in the sheared molecules. The dynamite shear studies will confirm 

 this. Also dispersion of carbon l)lack and othei' i)igments is restrained 

 by elastic qualities of "iiciuid" polymers (i.e., instead of "mixing", com- 

 pounds just micr()sco})ically deform and later re-form.) Likewise, the 

 efficiency of compounding' and extrusion** depend on how c^uickly the 

 molecules relax after straining. 



Impact Strength, Brittleness and Tenacity 



Toughness, mechanical shock resistance, ultimate elongation and 

 strength reflect the facility with which the polymer molecules can be dis- 

 placed without breaking the piece. Thus, they accommodate to the stress 

 by motions presumably similar to those described above. (The situation 

 is complicated when crystallites are also displaced. ) In any case, time 

 sensitivity in the range 10~^ sec upward exists. ^'^' ^^ The discussion by 

 Morey^ is a valuable survey of these ideas, and explicitly notes the 

 significance of multiple relaxation processes on damping of shock waves. 

 Evidence of the relation of simple changes in chemical structure to the 

 principle relaxation times efTective in these physical properties of plastics 

 and rubbers is thus another part of the dynamics studies. The "brittle 

 point", or volume-temperature transition of amorphous polymers, ' 

 apparently reflects du-ectly the correspondence of the time of experiment 

 with dominant relaxation time of the polymer. ' A few measurements 

 on plasticized polymethyl methacrylate (from which, however, no actual 

 rigidities were calculated) indeed indicate abrupt stiffening as a function 

 of frequency at a given temperature. However, the changes measured 

 were too small and indefinite to indicate any particular molecular relax- 

 ation. Other work ' with plasticized polymers is nevertheless concordant 

 with the current findings that molecular relaxations and not long range 

 order determine embrittlement. The converse of this is, of course, that 

 as some "transition" is approached, hysteresis, heat build up, flex crack- 

 ing and fatigue are greatest. 



Creep, Stress Relaxation and Recovery 



Even these "long time" qualities of plastics, such as found in cold 

 flow, apparently result from integrated displacements of rapidly oscil- 

 lating segments of the chains. A most interesting analysis of stress 

 relaxation in rubbers employs Kuhn's suggestion of a particular distribu- 

 tion of relaxation times. The present point is that, again, these relaxa- 

 tion times reflect processes which should appear directly in reaction of 

 the polymer with high frequency shear waives. 



