MECHANICAL PROPERTIES OF POLYMERS 



159 



Fig. 28 — Photographs of interforcncc pattcrius from sound waves in liciuids 

 and phistics. 



ties ill opaque plastics. The accuracy of the method is better than 1 per 

 cent if the attenuation is low enough to give a number of interference 

 lines. For plastics of high internal loss, the method becomes somewhat 

 inaccurate. 



Typical measurements using this system are shown in Table V. Small 

 changes in chemical composition and plasticizer content are shown up 

 as can be seen from the table. Of particular interest is the difference 

 between nylon 6-6 and polyethylene. Chemically as shown by Fig. 29, 

 the two are identical except for the dipoles occurring for every 6 units 

 of the ethylene chain. These dipoles have the effect of bonding adjacent 

 layers together and result in a higher shearing modulus. 



By attaching shear vibrating crystals to a right angled prism, as shown 

 by the lower part of Fig. 28, with the direction of motion of the crystals 

 parallel to the transmitting face, and setting up shear standing waves 

 between the two crystals, the shear properties of the plastic can be 

 measured. Longitudinal waves are generated in the liquid which inter- 

 fere with one another and cause dark bands perpendicular to the plastic 



