168 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1952 



cycle and the X constant increases. It appears from these measurements 

 that the dipole binding present in nylon 6-6 allows a greater structural 

 rearrangement under pressure than can occur for polyethylene which 

 has only linear chains. 



VI. CONCLUSIONS 



Measurements in dilute solutions, in pure polymer liquids and in non 

 rigid solid polymers have all shown the presence of a shortest segment 

 whose relaxation leads to a crystalline type of elasticity. In dilute 

 polymer solutions the presence of a configurational type of relaxation 

 and an entanglement relaxation of the shortest chain segment have 

 been shown. For pure poljTner liquids a quasi-configurational type of 

 relaxation has been found for chain lengths greater than 60 segments, 

 but for chain lengths less than 40 segments this type of relaxation dis- 

 appears. From the difference between the high frequency shear elastici- 

 ties measured for polyethylene and nylon 6-6 and the static measure- 

 ment of Young's modulus, it appears that there may be other relaxations 

 in these materials for lower frequency ranges. 



For pure polyisobutylene and for nylon 6-6 there appear to be struc- 

 tural changes induced by pressure which account for a compressional 

 viscosity and a dispersion in the X elastic constant. This effect is smaller 

 for polyethylene. 



APPENDIX — EFFECT OF LIQLTEDS ON THE PROPAGATION OF SHEAR W^WES 

 IN RODS 



For radially symmetric rods, the tangential particle displacement Ue 

 in the rod is given by 



Ue = Jiikr)e''''-'' (1) 



where 



k' = p^ + e' (lA) 



All other displacements are zero. In this equation waves are considered 

 to be travelling in the +2 direction with a propagation constant 6 = 

 A -\- jB, where A is the attenuation in nepers per cm and B the phase 

 shift in radians per centimeter, ju is the shear stiffness which may be 

 complex to take account of the dissipation within the rod. 



