INTERACTION OF POLYMERS AND MECHANICAL WAVES 



349 



synthetic rubber, they confer uniciue flow properties, causing the excel- 

 lent proccssibility of GR-S (>(). However, dynamic lenacity, such as in 

 flex crack ^lowth, is (Ungraded \)\ their ])rcs('iicc. Now presumably the 

 excellent extrusion ciualities of synthetic rubber composed of from 00 to 

 80 per cent microgel molecules are because of their individual shear 

 stiffness. Thus, if a wire coating, for instance, is extruded at high rates 

 of shear, chain molecules are deformed, and store energy just as dis- 

 cussed in the earlier sections on li(iuids. After emerghig from the extru- 

 sion die, they relax, and cause the gross retraction, shrinkage and rough- 

 ness shown in the wire insulation of the upper photograph of Fig. 27. A 

 polymer with about 70 per cent microgel molecules gi\es the smooth 

 covering shown in the lower specimen of Fig. 27. Here, the shearing 

 stresses of extrusion seem insufficient to distort the tiny networks of 

 the microgel molecule; in any case, the covering does not roughen or 

 relax. Similar effects have been found for microgel plastics. Neverthe- 

 less, unlike gross or macro gelation, the whole melt can flow. 



On this basis, dilute solutions of microgel molecules ought to hidicate 

 high shear rigidity per molecule. The mechanism /i3 of Fig. 19, in which 

 now the junction pomts are not temporary, but are primary valence 

 cross-links, should be predominant. Fig. 28 shows, for a polybutadiene 

 microgel in cyclohexane," that hb has mdeed risen, compared to equal 



ui 0.020 



o 



o 



y 0.012 



5 10 15 20 25 30 35 40 45 50 

 TEMPERATURE IN DEGREES CENTIGRADE 



_Fig. 26 — Temperature variation of ?? i for 1 per cent solution of polyisobutylene 

 {Ml = 3.87 X 10") in cyclohexane and l)enzene. 



