328 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1952 



for, again, a crude model attempting to show compressional viscosity 

 over the whole frequency range, while the other viscosities are Voigt 

 expressions at 8 (or 14) mc. Extremes of molecular weight averages are 

 shown. 



Comparison of the "soft" or quasi-conhgurational rigidities, expressed, 

 like the n of Fig. 5 as relatively frequency independent Hc , w'ith the 

 "hard" or glassy rigidities is given in Fig. 9. The X and /x values are for 

 the Voigt model at 8 mc. The graph does not show the bend-over of the 

 "soft", He , curve with molecular weight, but that happens more grad- 

 ually. The "hard" rigidities X and h quite readily show this inflection. 

 As before, the relaxing segments must be < 100 chain atoms, according 

 to the behaviour of the molecules at room temperature. 



Concerning influence of molecular weight on engineering "brittle 

 points" of such importance in rubber technology, the present studies 

 agree with earlier proposals. Thus, although the Tg or v-T second order 

 transition point always decreases with decreasing molecular weight, 



LLI LU 



< < 



^ <^^ 1 600 



If) >- 



D Z 



_l < 



Q !5 1200 



i o 



^ I 1 



tu 2 



MOLECULAR WEIGHT, M/^ 



12 X 103 



Fig. 9 — Dynamic rigidities of polyisohutylene liquids as function of average 

 molecular weight (25°C). 



