INTERACTION OF POLYMERS AND MECHANICAL WAVES 



327 



icaiit for displacements suporficially quite different from those in macro- 

 scopic viscosit}'. 



The compressional viscosity, X' is also plotted, for the same model, in 

 Fig. 7. It is, within experimental erior, zero for polymer A', as deter- 

 mined by shear and compressional wave studies at 8 mc frecjuency." 

 This is a rare case, then, where the attenuation of sound waves through 

 a liquid has been quantitatively accounted for by the shear viscosity. 

 But, as soon as the a\'erage molecular weight rises to 1000 or so, X' 

 comes ui clearly, and the new mechanism for dissipating compressional 

 or dilatational stresses is de\'eloped. As this presumably represents di- 

 rectl}^ free volume or coordination number changes in liquid struc- 

 ture, ' " its detailed study near Tg , and in connection with brittle 

 ])oints of rubbers, may eventually be especially fruitful. 



Another depiction of influence of average molecular weight in these 

 licjuids on dynamic viscosities occurs in Fig. 8. Here, the Xc curve is 



2 4 6 8 _ 10 I2xl03 



MOLECULAR WEIGHT, M;^ 



Fig. 8 — Dynamic viscosities of i)olyisol)Utyloiic liciuid.s as function of average 

 molecular weight (25°C). 



