MECHANICAL PROPERTIES OF POLYMERS 



161 



an intrinsic viscosity in xylene of [n] = 0.89 at 85°C. Fig. 31 shows the 

 longitudinal velocity of polyethylene plotted as a function of frequency 

 and temperature. The velocity rises with frequency and a dispersion 

 is indicated. This is confirmed by the attenuation per wavelength curve 

 for two different frequencies plotted as a function of temperature, Fig. 

 32. A definite dispersion is seen to occur with an activation energy of 

 about 12 ± 2 kilocalories per mole. This could occur in either the X 

 constant or the shearing constants ju, but the data of Figs. 33 and 34 

 show definitely that it occurs in the shear component. Fig. 33 shows 

 the shear velocity for four temperatures plotted as a function of fre- 

 quency. This can be fitted for 30°C with a single relaxation mechanism 

 having a relaxation frequency of 8 megacycles. To agree with the meas- 

 ured attenuation and velocity, there has to be a spreading of the single 

 relaxation over a range as also occurs in liquids. The indicated shear 

 stiffness below this relaxation frequency is 2.6 X 10 dynes/cm . Some 



H H^ H^ H2 II H2 H2 Hg H2 

 N CCCCCCCC 



' \ / \/ \/ \/ \/\/\/\/\ / 



C CCNCCCCC 



Hj H^ •'5 <' Hp Ho Ho H 



^2 "2 "2 



2 "2 "2 "2 



/ 





HN \ 



c=o>.../ 

 \ / < /C=o 



^NH / \ / 



/ HN 



"A. 



< A. 



\ NH, 



C 



/ 



y 



>< 



Fig. 29 — Spatial structure of nylon 6-6. 



