MECHANKWL PKOIMORTIES OF POLYMERS 



167 



against \/T where T is tlie absolute temperature. Both are plotted for 

 8 me and 30 mc. The dispersion in both materials is evident. Below 

 30°C^ the shear elasticity of polyethylene varies exponentially with the 

 temperature with an activation energy of 2.72 kilocalories per mole. 

 Above this temperatiu'e a deviation occurs due to the approach to the 

 melting temperature. Nylon has a smaller variation with temperature. 

 Comparing the longitudinal and shear wave measurements one can 

 calculate the Lame X elastic constant and this is shown plotted on Fig. 40 

 for both polyethylene and nylon 6-6 as a function of temperature for 



12 14 16 1S 20 22 24 26 

 FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 41 — Equivalent shear and compressional viscosities for polyethylene and 

 nylon 6-6 plotted as a function of frequency for a temperature of 25°C. 



two frequencies. The dispersion of X for polyethylene is small but is more 

 prominent in nylon 6-6. This correlates with the larger compressional 

 viscosity component present for nylon 6-6 which as shown from Fig. 41 

 is as large as the shear viscosity. According to the structural rearrange- 

 ment theory of compressional viscosity due to Debye, compressional 

 viscosity can enter when some part of the chain can rearrange from one 

 stable state to another stable state as a function of pressure. This re- 

 arrangement occurs across a potential barrier and hence requires a finite 

 amount of time to occur. This lag in the rearrangement results in a 

 compressional viscosity and as the frequency is increased, a frequency 

 is found for which the motion can no longer occur in the time of a single 



'« P. Debye, Z. Elektrochem., 45, 1939, p. 174. 



