Page 401 - Elements of Chemical Reaction Engineering Ebook

372 Nonelementary ReaoUon Kinetics CMp. 7

8. "he weight-average molecular weight is

(7-5 1)

9. The number-average variance is

(7-52)

10. The polydispersity index (D)
is

(7-53)

A polydispersity of 1 means that the polymers are all the same length
and a polydispersity of 3 means that there is a wi& distribution of poly-
mer sizes. The polydispersity of typical polymers ranges form 2 to 10.

Example 7-4 Parameters Distributions of Pokjmers

A polymer was fractionated into the following six fractions:

Fraction Molecular Weight Mole Fraction
1 10,000 0.1
2 15,000 0.2
3 20,000 0.4
4 25,OOo 0.15
5 30,000 0.1
6 35,000 0.05

The molecular weight of the monomer was 25 Daltons.
Calculate NACL, WACL, the number variance, and the polydispersity,

Solution

Mw j Y jY 12y

10.000 400 0.1 40 16.000
15.000 600 0.2 120 72,000
20,000 800 0.4 320 256.000
25,OOo lo00 0.15 150 150.000
30,000 lux) 0.1 120 144.000
35.000 1400 0.05 70 98,000
820 736,001)