DETERIORATION OF ORGANIC POLYMERS 



1081 



The value of polymers as structural materials is derived entirely from the 

 fact that they are composed of very large molecules. They are generally 

 classified in two broad groups, the essentially linear or chain-like polymers 

 comprising the thermoplastics and rubbers, and the very highly branched 

 three-dimensional networks which are called thermoset materials. The 

 fundamental difference between these groups is shown diagrammatically in 

 Fig. 6 in which, for convenience, the linear polymers are shown as straight 

 lines instead of in their usual randomly kinked shape. 



The linear polymers are made up of molecules of finite average size, from 

 a hundred to a thousand or more times as long as they are wide^ and the 



SCHEMATIC REPRESENTATION OF A LINEAR POLYMER 



SCHEMATIC REPRESENTATION OF A THERMOSET POLYMER 



Fig. 6— Schematic representation of a linear polymer, above, and of a thermoset 

 ])olymer, below. 



Strength of the material is dependent on the size of these molecules much as 

 the strength of a cotton thread is dependent on the length of the individual 

 fibers of which it is composed. The forces holding the aggregate together 

 are the cumulative interchain forces. In thermoplastics these forces may 

 be quite strong. In rubbers they are weak until the rubber is vulcanized. 

 Vulcanization connects the chain-like molecules into a loose three-dimen- 

 sional network, but the number of cross-links is very low compared to 

 typical thermoset polymers being only about one or two for every hundred 

 chain atoms.* Vulcanized rubbers are therefore still largely linear polymers 

 and their deterioration follows the pattern of the thermoplastics. The 



