. The Bell System technical journal . 40 50 60 70 STYRENE RESIDUES IN WEIGHT PER CENT Fig. 4.—Refraction as a function of styrene content for solutions in benzene of polymerscontaining known percentages of st\Tene. Through the use of this method it has been possible to control the styrenecontent of the copolymer to about ± 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 hasbeen possible to emj^loy a simpler procedure where a milling of the copolymeris introduced to rem
. The Bell System technical journal . 40 50 60 70 STYRENE RESIDUES IN WEIGHT PER CENT Fig. 4.—Refraction as a function of styrene content for solutions in benzene of polymerscontaining known percentages of st\Tene. Through the use of this method it has been possible to control the styrenecontent of the copolymer to about ± 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 hasbeen possible to emj^loy a simpler procedure where a milling of the copolymeris introduced to remedy difiiculties early encountered in the determinationof the refractive index directly on the solid. Although not as precise asthe interferometer method, this method is shorter and as a consequenceis tlnding application in process control. It is safe to say that today, withthese methods, the control of the average composition of GR-S procuced inthis country is now entirely adequate for all purposes. SYNTHETIC RUBBER RESEARCH 367. 368 BELL SYSTEM TECHNICAL JOURNAL Molecular Weight Distribution l\ GR-S Unlike the linear polyesters whose molecular weight distributions can becalculated from simple assumptions (Fig. 1), the distribution of molecularsizes present in polymerization polymers cannot, at the present state of ourknowledge at least, be accurately predicted. With linear polymers ofuniform composition it is possible to determine experimentally the approxi-mate molecular weight distribution by fractional precipitation of the dis-solved polymer from dilute solution. This procedure, to yield good results,must be carried out under very careful control, and requires considerabletime. The usual procedure is to prepare a solution of the polymer to bestudied and add to it portions of a precipitant. The successive fractionsof the whole polymer precipitated are then examined for average molecularweight by some suitable method. This procedure can give only a crudeseparation but oft
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