316 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1952 



per cent of compounding ingvedients in the Butyl g.\\m, and possibility 

 of a small fraction of low mcjlecular weight uncured jjolymer in it. 



Also, Avide variations in the degree of cure of Butyl gums were studied 

 without large changes in jjl'. In this regard, the particular sample of 

 Fig. 3 had an e(iuilibrium swelling ratio (= volume swollen polymer in 

 cyclohexane at 25°C/volume insoluble part of dry vulcanizate) of 4.84. 

 This indicates an Mc value (average molecular weight between cross- 

 links) of < 20,000. ■ Actually many of the dynamic properties can prob- 

 ably be fovnul in individual chain units or segments even smaller than 

 this. This is a significant point in engineering applications where plastics 

 may be cured to reduce creep but where it is desired to retain typical 

 "chain" properties to increase impact toughness. That is, usually some 

 optimum condition for this compromise can be found. The later section 

 on lifiuids will suggest that physical properites typically associated with 

 chain polymers can mdeed reside in even shorter chain sections than the 

 ilf c's observed in usual gum rubbers. 



Filled Polymers 



Marked effects of carbon black and other pigments are of course 

 familiar hi both steady and alternating mechanics of rubbers.^ . - . «. 

 Brief comment on their influence on dynamic shear properties and thus 

 relaxation mechanisms involved may be directed toward plastics, also, 

 however. Thus, technologically it would be desirable to load thermo- 

 plastics with considerable volumes of "inert" fillers, just as is done with 

 rubber. But, almost invariably strength and toughness decline, instead 

 of improvmg, as in the rubber case. A reason for this appears in inves- 

 tigations by Hopkins when carbon black (a standard type of reinforcing 

 black) was added to Butyl rubber of the sort described in Table I. It is 

 that stiffness seems to rise more rapidly than internal viscosity — i.e., a 

 given strain results in proportionately higher stress than the accompany- 

 ing internal viscosity provides means for dissipating the stress (as on 

 impact). Hence, the brittleness which fillers normally engender hi ther- 

 moplastics may represent this change in /x vs. m' balance. Table II illus- 



Table II 



