374 BELL SYSTEM TECHNICAL JOURNAL 



account a copolymer containing as nearly linear molecules as possible and 

 having neither an excess of high or low molecular fraction is probably 

 preferable. 



Gel GR-S, depending on its swelling volume (see below), is a tough 

 material totally lacking in plasticity. Swelling volumes as low as 10 are 

 hardly distinguishable from vulcanized gum GR-S and in fact resemble it 

 structurally because vulcanization is actually a special kind of gel formation. 

 Ordinarily the swelling volumes of gel in GR-S range between 20-150"' ^^. 



Commercial GR-S may contain both sol and gel, although the trend is to 

 eliminate gel altogether. When large amounts of gel of moderate swelling 

 volume are present the product is hard to mix, although it may extrude 

 smoothly, and after processing, particularly if done hot, it is likely to give 

 products which have higher modulus than copolymer free from gel, and to 

 show poor resistance to cutting and crack growth — properties of great sig- 

 nificance in tires and other applications.^* It is therefore important that 

 we should be able to determine sol and gel in the presence of each other. 

 This need is particularly great in the case of characterization of copolymers 

 after they have been subjected to processing and compounding^' — treat- 

 ments which often are responsible for profound changes in its molecular 

 structure. 



Methods of Characterization of Sol and Gel 



Considerable work has been done at the Laboratories on methods for 

 determining the sol-gel properties of polymers and in investigating the 

 effects of various after treatments of the copolymers on their sol-gel charac- 

 teristics®- ^*. Figure 7 shows the type of apparatus employed for effecting 

 the sol-gel separation''. The weighed copolymer sample is thoroughly dried, 

 cut into small pieces and distributed on stainless steel screens contained in 

 the bulb of the apparatus. About the 100 ml. of benzene is added and the 

 parts assembled. After 24 hours or more standing without disturbance, 

 the benzene containing the soluble part of the copolymer is carefully with- 

 drawn by opening the stop-cock very slightly. The weight of the swollen 

 gel left on the screens is obtained from the difference between the weight 

 of the assembly after draining off the solution and its original weight. This 

 divided by the original weight of the unswoUen gel gives the swelUng volume 

 (SV) of the material. The slight density correction can be neglected. 



The dilute solution viscosity is determined directly on 5 cc. of the solution 

 withdrawn from the vessel and is calculated from equation (2). The con- 

 centration c is determined by evaporating a known volume of the solution 

 and weighing the sohd left after evaporation of the benzene. Figure 8 

 shows the viscometers and bath employed for the measurements of the 

 relative viscosity. A variation of the dilute viscosity method adopted for 



