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BELL SYSTEM TECHNICAL JOURNAL 



cell and make a reading of the change in refraction compared to the pure 

 benzene. This value, with the help of a curve relating styrene content to 

 refraction, enables the true styrene content to be determined. The curve 

 of refraction as a function of styrene content must be constructed beforehand 

 and is shown in Fig. 4. This curve is obtained by measuring the refraction 

 of pure polystyrene on the one hand and polybutadiene on the other. Checks 

 also were made by independent methods of estimating composition in the 

 range of the usual Buna S-t}^e synthetic rubber. 



20 30 40 50 60 70 



STYRENE RESIDUES IN WEIGHT PER CENT 



Fig. 4. — Refraction as a function of styrene content for solutions in benzene of polymers 

 containing known percentages of st\Tene. 



Through the use of this method it has been possible to control the styrene 

 content of the copolymer to about ± 0.2 weight percent styrene residues, 

 which is amply close for all purposes. Figure 5 shows the apparatus em- 

 ployed in this determination, the interferometer. More recently, it has 

 been possible to emj^loy a simpler procedure where a milling of the copolymer 

 is introduced to remedy difiiculties early encountered in the determination 

 of the refractive index directly on the solid". Although not as precise as 

 the interferometer method, this method is shorter and as a consequence 

 is tlnding application in process control. It is safe to say that today, with 

 these methods, the control of the average composition of GR-S procuced in 

 this country is now entirely adequate for all purposes. 



