SYNTHETIC RUBBER RESEARCH 361 



copolymer produced and the vulcanization properties possessed by it. 

 What was needed, however, were more precise and revealing tests, and tests 

 which could be carried out directly on the copolymer itself. No ordinary 

 chemical methods such as are applicable to the usual type of synthetic 

 chemicals apply, for reasons which should be evident from our previous 

 discussion. New methods of characterization designed to insure uniformity 

 and satisfactory' quality in the GR-S copolymer were required. 



The precise and early control of the copolymer was of utmost importance. 

 Non-uniformity in the product may cause serious troubles in fabricating 

 operations such as are employed in tire plants, wire coating factories, ad- 

 hesives manufacture, etc. Furthermore, with a varying product it often 

 cannot be determined whether the trouble, when it occurs, is in the copolymer 

 or in the method of fabrication being used. 



What are the characteristics which must be controlled to insure a satis- 

 factor}' product? To answer this question it was necessar>' to investigate 

 a variety of GR-S copol3'mers and to conduct service tests on them in order 

 to determine their practical performance. Some of these tests, particularly 

 those on tires, have been ver>' extensive. Some of the characteristics of 

 the copolymer which experience has taught should be measured and con- 

 trolled are: 



1. The over-all or average styrene content in the butadiene-styrene 

 copolymer. 



This necessitates (1) a method of separating the pure copoh-mer 

 (which is the rubber-like component) from non-rubber components 

 such as soap, salts, insoluble matter etc., and (2) a suitable method for 

 determining the styrene content of the purified copolymer. 



2. The percentage soap, fatty acids and low molecular butadiene-styrene 

 compounds in the rubber. 



3. The amount of "gel" fraction, if present, and the swelling volume of 

 the gel.'' 



4. The average molecular size of the "sol" or soluble fraction of the 

 copolymer. 



5. The degree of branching of the sol molecules. 



6. The molecular weight distribution of the sol. 



In addition to the above tests on the final copolymer, control tests which 

 can be used during the poh-merization to tell when the reaction has pro- 

 gressed to the proper point were needed. In the following paragraphs we 

 will take up in some detail the problem of characterization and attempt to 

 show the basis on which methods have been evolved to control some of the 

 quantities listed above. 



