INTERACTION' OF POLYMERS AND MECHANICAL WAVES 351 



Ilci'ca Rubber Solutions 



The compari.soii of etiual weight coiiceutratioiis of natural rubber m 

 cyclohexane ^vith polyisobutylene in cyclohexane is surprising: 



IIe\ea rul)bcr M„ = .23 X 10' nu = 1350 dynes/cm", 1 per cent 

 sohition (eorr.). 



Polyisobutylene M, = 1.2 X lO'' /x^ = 1000 dynes/cm , 1 per cent 

 solution (corr.). 



Both results are at 20 kc. The higher value for natural rubber may be 

 l)ecause of the double bonds causmg stiffening of the cham. On the other 

 hand, maybe easy rotation around single bonds raises the ms part. Cer- 

 tainly the VISCOUS retardation ivithin natural rubber chains is very low, 

 as noted in the section on solids. However, its interaction with, or con- 

 figuration, in cyclohexane may be peculiar. The [j'b] per average mole- 

 cule is, however, low, being 15 X 10~ dyne cm at 25°C. 



Polystyrene Solutions 



Much work, on light scattering and other properties, has mdicated 

 appreciable intra-chain stiffness for polystyrene, "* but still much freedom 

 compared to polyisobutylene. ^ However, this work, as well as AHp^n of 

 17 kcal compared to '^lO kcal calculated for no steric hindrance, sug- 

 gests comparati\-ely small restraints on ideal flexibility. This needs to be 

 checked by a frequency analysis of dilute solution mechanics, but poly- 

 styrene seems to be a reasonable example of "plastic" behaviour at room 

 temperatiu'e because of interaction between the chains. (It is recalled 

 that, earlier, a-methyl styrene polymer was cited as plastic model show- 

 ing both intra- and inter-chain stiffness. Unlike in polystyrene, the 

 intra-chain factor shows up in a AHp^n of 9-10 kcal, a third less than 

 that calculated if there were no steric hindrance.) Thus, no evidence of 

 unusual stiffness appears in Fig. 29, when, indeed, the mb values are 

 considerably lower, for eriual weight concentrations, than those for nat- 

 ural rubber. The highly milled rubber studied had ikf, very nearly that 

 of M, = 0.234 X lO'^ of the polystyrene, so the [Jb] per a\'erage poly- 

 styrene chain, 4.5 X 10~ dyne cm is less than a third that of the rub- 

 l)er. No wonder that at high temperatures, where the phenyl group 

 interaction between chains is much reduced, polystyrene makes a good 

 rubber. Also, hi Fig. 29 are shown data for a polymer of Mr, = 1.2 X 10 , 

 made in emulsion and having [tj] = 4.350 in b(>nzcne at 25°C. 



