INTERACTION OF POLYMERS AND MECHANICAL WAVES 319 



of molecular wciglit, chemical structure (degree of branching in poly- 

 ctliylene), crystallinity, etc. These (luantities, when fitted to a given 

 pattern of /i, X, n' and X' at i)rop('r freiiueiicie.s would yield plastics of 

 optinnnn sei-N-iceahility under the multitude^ of stresses encoimtered in 



US(\ 



A similar li(iuid-like structure-even where the (crystalline) rigidity is 

 much higher and mobile chain segments smaller — apparently occurs in 

 l)olyaniides. Presumably the hydrogen l)onding and dipole interactions 

 are \'ery imperfect in the disordered regions, and there the chain inter- 

 action is reminiscent of polyethylene. For instance, in polyhexamethyl- 

 ene adipamide, measurements in the 8 to 30 megacycle range do indicate 

 that the Lam6 elastic constant X is al)out o.t) X lO' dynes/cm", but 

 only about 3 X 10 for polyethylene. This reflects over-all stifTness 

 dominated by crystallites. Nevertheless, the compressional viscosity, X' 

 is 17-6 poises (going from 8 to 30 mc) for the polyamide, but only 5-2 

 poises for polyethylene. Of course, since there is dispersion in both cases, 

 these relative magnitudes might be quite different at some other fre- 

 (juency or temperature (all above are at 25°C). Yet it remains that the 

 nylon, despite its hardness, also has a liquid-like component more vis- 

 cous than that of polyethylene. Similar relations appear in the shear 

 ^•iscosities, m', also determined for these two systems. For the 6-6 poly- 

 amide, fx' goes from 19 to 7 poises over the 8 to 30 mc interval while 

 polyethylene changes from 15 to 5. These quantities indicate again, as 

 with the polyethylene, that "polymer liciuids" rather than just a few 

 small groups of atoms are the important mechanical elements even at 

 frec|uencies of 10 . Now polystyrene, an amorphous polymer, also has 

 rigidities of about 10'° dynes/cm^ but the m' and X' values at room tem- 

 perature are far below 5 to 20 poises, and glass-like brittleness (although 

 not so bad as silica glass) results. 



So far, then, the two characteristic extremes of polymer mechanics 

 have been discussed: (1) the soft rubbers, whose dynamics at low kilo- 

 cycle frequencies imply, at ordinary temperatures, predominantly over- 

 lapping combinations of relaxation processes whose relaxation elements 

 involve many segments per molecular chain; and (2) the hard, micro- 

 crystalline plastics, whose behaviour is predominated by relaxation proc- 

 esses having times of 10~ to 10~ sec because the longer period (slower) 

 displacements have been relaxed out at the temperatures of normal use. 

 (Likewise, interconvertability by temperature between these two ex- 

 tremes is presumed. Also, a certain correspondence between dielectric 

 and dynamic relaxations in these classes is indicated. "") Next, it is in- 



