SYNTHETIC RUBBER RESEARCH 



369 



fall nearer to the low molecular weight end of the distribution curve than 

 does the true weight average molecular weight. Only for a homogeneous 

 system does it coincide with the number average value. Hence, the differ- 

 ence between the two can be used as a rough measure of the broadness of 

 the distribution. 



Light scattering from solutions offers possibly an absolute way of getting 

 the true weight average value. If the molecules are small compared to the 



Fig. 6.- Aiipa.aUis for nu■a^uI■in,■; tlu- itUi.n>il.\ .il M.allci\-(1 li'^lil irom solutions of polymers 



wavelength of the light used and solutions of various dilutions are employed, 

 measurements of turbidity r, i.e. the fraction of the totd light scattered per 

 cm. of path, allow the weight average molecular weight3/«, to be calculated 

 according to 



^'^" H{c/t)o 



where H is a constant, c is the concentration and {c/t)o is the value found 

 by extrapolation to zero concentration'^ Further study of this method is 



