IXTKUACTIOX OF I'OLVM KI{S .\ND M i:( 11 A.\ 1< AL WAVKS 



329 



the l)rittle point tends to iiicroa.so as the molcculo.s j>;ot smallci'. Tliis wa.s 

 supposod to !)(> lu'causc the ultiinalc cloiijial ion (doubtless due to \isco- 

 elastie or (|uasi-c'()iifij>;iirational elasticity' and not kinetic theory ela.s- 

 ticity as sonietini(>s said) declined with chain lena;th, so that the speci- 

 men hroki' at lower and lowci' sti'ains, " e\-en though it was not I'eally in 

 the glassy state. Now, the results above"' '' demonstrate that the shear 

 modulus at a given temperature does fall off for a\erage molecular 

 weights of polyisobutylene b(>low '^5000. Hence, the mechanical em- 

 brittlement of the low molecular weight samples is not because they are 

 stiffer, l>ut because they are weaker. 



pc)LY-cv-mi:thyl styrene: a "plastic" liquid 



Most of the li(iuid studies have been on polyisobutylene pol3''mers, 

 made "hard" or "soft" by temperature or frequency, but under use 



Table V 



conditions, considered rubbery. If one of the methyls in polyisobutylene 

 is replaced by a phenyl, poly-a-methyl styrene, a hard plastic is pro- 

 duced. Low molecular weight polymers of this composition are, however, 

 liciuids at room temperature. Hence, it is interesting to compare their 

 reaction to mechanical waves with that of polyisobutylene liquids of 

 similar macroscopic viscosity. Table V lists a few properties at 25°C. 



Polymer I has roughly the steady flow 7/ of polyisobutylene B. Also, 

 the ^'oigt 11' at 14 mc is similar: 15.2 poises compared to the 23.6 poises 

 of the poly-a-methyl st^^'ene. However, the Voigt /x is already 50 times 

 higher for the phenyl substituted chain. (This shows the shift of the 

 second relaxation range, of course, where nearest neighbor interactions 

 rule.) Even more striking, the temperature coefficient of a /x^ , calculated 

 as the second principal shear viscosity in a frequency independent 

 model," as before, is about 24 kcal, compared to about 12 for Hc for 

 polyisobutylene in its similar model. Thus, although there was no ap- 

 iwrent difference between the mechanical properties of these two poly- 

 mer li([uids in their room temperature state, their dynamics diverged 

 remarkabl3\ This was when they were studied with shear waves whose 



