Mechanical Properties of Polymers 

 at Ultrasonic Frequencies 



BY WARREN. P. MASON AND H. J. McSKIMIN 



(Manuscript received October 25, 1951) 



Since the mechanical properties of solid polymer materials are largely de- 

 pendent on the motions that segments of the polymer chains can undergo, 

 to understand these properties one must use measuring techniques which can 

 determine these motions. One of the most promising methods is to measure 

 the reaction of polymer materials to longitudinal and shear waves over a 

 frequency spectrum wide enough to determine the relaxation frequencies due 

 to thermal motions of the principle elements of the chain. The presence of 

 relaxations is indicated by a dispersion in the velocity and attenuation 

 constants of the material, or a dispersion in the characteristic impedance 

 of the material if the attenuation is too high to allow velocity measurements. 

 A number of different types of measuring methods are described in this 

 paper which make possible propagation and impedance measurements not 

 only in solid polymers, but also in liquid polymers and in solutions of 

 polymer molecules in typical solvents. 



When these techniques are applied to long chain polymers in dilute solu- 

 tions, the three relaxations observed correspond to motions occurring in isolated 

 molecules since as the dilution increases, the molecules seldom touch. The 

 lowest relaxation corresponds to a configurational relaxation of the molecule 

 as a whole, the highest relaxation corresponds to the twisting of the shortest 

 segment — containing about 40 repeating units — while the intermediate re- 

 laxation corresponds to a transient entanglement of chain segments. All 

 three types of relaxations are present in pure polymer liquids but are spread 

 out over a frequency range due to the perturbing effect of near neighbors of 

 adjacent chains. The high frequency shortest chain relaxation can be traced 

 in solid polymers of the linear chain type such as polyethylene and nylon 

 and produces rubber-like response to mechanical shocks of very short duration. 



I. INTRODUCTION 



The mechanical properties of sohcl polymer materials are largely deter- 

 mined b}^ what motions, parts or segments of the polymer chains can 

 undergo. Toughness, mechanical impact strength and ultimate elongation 

 depend on the facility with which the polymer molecule can be displaced. 



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