318 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1952 



phone Laboratories (described in the last issue), have been used to probe 

 for elemental reactions at the upper end of the frecjuencies presently 

 available. Both longitudinal and shear waves were used. In polyethylene, 

 a wavelength for the shear waxes was 0.0074 cm., at / = 8.55 X 10^ 

 cycles, and in polyhexamethylene adipamidc (the usual G-6 textile ny- 

 lon), the shear w^avelength was 0.0125 cm., for/ = 8.67 X 10^ cycles, 

 all at 25°C. 



The important consequence of these experiments so far has been that, 

 despite the small strains involved, the viscosity appears to be a "poly- 

 mer" viscosity, rather than an inner friction involving just a few liquid- 

 like atoms per unit. Thus, polyethylene of "equilibrium" crystallinity 

 and average molecular weight corresponding to an intrinsic viscosity in 

 xylene of [r/] = 0.89 (at 85°C), was measured over the range from to 

 50°C. The results from both longitudinal and shear wave measurements 



Table III 



are given in Table III. These viscosities are expressed in this case for a 

 Kelvin-Voigt model, of rigidity and viscosity in parallel. The rigidities 

 associated with these viscosities are about 3 X 10 dynes/ cm , or not far 

 from the value under steady pull of about 1 X 10^ 



Now this suggests that the rigid plastic polyethylene retains, even 

 under mechanical impulse of microsecond duration, a shock-absorbing 

 capacity reflected in a shear viscosity of 5-15 poises, and a compres- 

 sional viscosity of 3-8 poises. The former, /x', may roughly correspond 

 to the liquid viscosity of a paraffin-like chain of from 50 to 65 c-atoms 

 in length. Thus, the dynamics measurements seem to relate to basic 

 premises of polymer structure. These are that the amorphous regions 

 (whose existence is shown quite independently by x-ray scattering, den- 

 sity, heat-capacity, etc.) indeed take up and dissipate sudden stresses 

 which the microcrystallites, despite their great strength, would be too 

 brittle to sustain. 



These results give hope that further probing of the dynamics of liquid- 

 like elements in rigid plastics will eventually lead to precise adjustment 



