AIKCIIA.NK'AL I'UOI'KUTIKS OF POLYM KHS 123 



If only a small motion of the polymer chain can occui" within tlu^ time of 

 the measurement, the material has high elastic stiffness coefiicients and 

 acts similar to a rigid solid. On the other hand, if significant segments 

 of the polymer chain can move at the fi'e(iii(»ncy of measurement, the 

 elastic stiffness is much lower and rul)l)er-like behavior results. An inter- 

 mediate case, which oceiu's when the significant motion of the polymer 

 molecule is near the relaxation time at the frequency of measurement, is 

 that of a damping material such as butyl rubloer. Even "long time" 

 ([ualities of plastics such as creep, stress relaxation and reco\'ery depend 

 on the integrated displacements of rapidly oscillating segments of the 

 chain. 



One of the most promising methods for investigating these motions is 

 to determine the reaction of mechanical waves on the polymer materials 

 over a wide spectrum of wavelengths, eventually going to frequencies 

 comparable with those of thermal \dbrations of significant groups or 

 segments in the macromolecules. 



If one wishes to understand the origins of these motions it is necessary 

 to measure the molecules in the form of liquids or solutions since then 

 the segments of the molecule are less restrained by their neighbors and 

 can perform all the possible ^'ibrations. Polymer liquids are also interest- 

 ing in themselves as sources of damping material. To apply these results 

 to rubbers and solitl materials, one then has to measure the mocUfications 

 of the poljoner chain motion caused by the close approach of near neigh- 

 bors, b}'^ measuring the mechanical properties of these materials. 



By using chfferent types of technitiues, these processes can be applied 

 to molecules in solution, to liciuid polymers and to solid polymers. The 

 principal types of methods used for liquids are the torsional crystal, the 

 torsional wave propagation system and the shear wave reflectance method, 

 all of which are described in Section II. For solids an optical method and 

 an ultrasonic method are. described in Section V. All of these methods 

 involve displacements of 10~^ cm or less so that non-linear effects are 

 negligible. 



All of these methods depend on setting up shear or longitudinal waves 

 in the medium and observing either the velocity and attenuation of the 

 wave, or the reaction of the medium back on the properties of the 

 transducer. If the attenuation of a wave in the medium under consid- 

 eration is low enough to permit the wave parameters, i.e., the velocity 

 and attenuation per wavelength to be determined, the relaxation of 

 some significant part of the polymer molecule is determined by the dis- 

 persion of the wave properties which occur, as shown by Fig. lA, in the 

 form of an increase in velocity and a maximum in the attenuation per 



