MECHANICAL PROPERTIES OF POLYMERS 



165 



points of Fig. 32, Most of the loss is accounted for by shear mechanisms, 

 but it appears that some compressional mechanisms may also be present. 



The mechanism causing the relaxation in the megacycle range for 

 polyethylene appears to be the same as for polyisobutylene, namely 

 the relaxation of the shortest chain segment that is free to move. The 

 chain segment acting appears to be longer than six chain units for similar 

 measurements of nylon 6-6 show no relaxations in this frequency range. 

 Fig. 35 shows the longitudinal velocity and Fig. 36 the attenuation per 

 wavelength for longitudinal waves. Since the attenuation per wave- 

 length is still increasing for nylon 6-6 at 25 megacycles a still shorter 

 chain segment may be operating for this material. The shear velocity 

 and attenuation per wavelength for nylon 6-6 are shown by Figs. 37 

 and 38. 



Fig. 39 shows the shear stiffness of polyethylene and nylon 6-6 plotted 



10* X 1.3 



1.2 



61-1 — *- 



> -i 



10 12 14 16 18 20 22 24 26 28 

 FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 37 — Velocity of shear waves in nylon 6-6 plotted as a function of temper- 

 ature and frequency. 



0.34 

 0.32 



10 12 14 16 18 20 22 24 26 

 FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 38 — Attenuation per wavelength for shear waves in nylon 6-6 plotted as a 

 function of temperature and frequency. 



