372 



BELL SYSTEM TECHNICAL JOURNAL 



Ha 



H 



H2 



H2 



H H 

 C=C 



H2 



H2 



I 

 CH 



CH2 



\ 



C 

 H 



CH 



C 



H2 



C 

 Ho 



/ 



C^ 



1 

 CH 



CH2 



(12) 



Obviously many other combinations are possible which are even more 

 involved. 



There is considerable chemical as well as physical evidence to support 

 the presence of all of these possibilities in the GR-S molecule. It is probable 

 that the butadiene and styrene units enter the chain in an irregular manner, 

 although, as we have seen, one molecule may acquiie more total styrene or 

 butadiene than another. The occurrence of cis and trans forms and head-to- 

 tail arrangements is also irregular. The 1,2 and 1,4 butadiene structures 

 likewise may occur randomly although the amount of 1 ,2 structure appears 

 to vary somewhat depending on the type of reaction. It is not possible to 

 review here the detailed evidence for the randomness and for the occurrence 

 of these various features. The fact that x-rays when diffracted from 

 stretched or cooled samples of GR-S fail to show evidence of crystalline or 

 even of imperfectly cr>'stalline material is proof that a disordered chain 

 structure exists. X-rays, however, do not specify the cause of this disorder. 



Work on synthetic linear polymers of known composition has demon- 

 strated that relatively minute structural changes are able to cause marked 

 disorder in polymer systems^*^- ^^. It is not surprising, therefore, to find 

 that GR-S copolymer is disordered. The important question is: what effect 

 has the disorder on the properties and, if it is deleterious, what can be done 

 about improving the chain structure? Without going into detailed argu- 

 ments there is good reason to believe that an ordered chain structure is 

 desirable for the best properties in a rubber. Only then is it possible for 

 the chain molecules to pack together into crystalline-like regions on stretch- 

 ing and thus provide the resistance to tearing and breaking that are required. 

 Natural rubber possesses this characteristic to an outstanding degree and 

 polychloroprene and polyisobutylene when vulcanized also show considerable 

 crystalline behavior on stretching. Other factors, such as the rate at which 

 crystalline regions develop, are likewise important^'^. But the crucial 

 requirement for toughness is the development of the crystalline type of 

 forces on stressing. 



It must be admitted that no great progress in reducing the chain disorder 

 of GR-S has been attained as yet. Obviously, complete order because of 

 the hybrid nature of the polymer is impossible. This was realized at the 



