1. Polymer Molecular Weight
Exercise 1

2. Avarege molecular weights
Equations for number average and weight average molecular weights
as well as polydispersity index are defined as follows:
where Mi molecular weight of molecules i
ni number of molecules with molecular weight i
wi mass of the molecules with molecular weight i

3. 2
A sample of polystyrene is composed of a series of fractions of different sized molecules.
Calculate the number average and weight average molecular weights of this sample as well as the PDI.
How would adding styrene oligomer change the average molecular weights? Added amount is 5wt.% of polymer mass and M=1000g/mol.
Table 1. PS fractions.
Fraction
weight fraction
Molecular weight
[g/mol] A
0.130
11000 B
0.300
14000 C
0.400
17000 D
0.170
21000

4. 2a)
Determine the number of moles in each fraction. Assume that the sample is 10 g in the beginning. Number of moles of the fraction is ni = wi / Mi.
fraction wi
[g] Mi
[g/mol] ni
[mmol] A
1.30
11000
0.118 B
3.00
14000
0.214 C
4.00
17000
0.235 D
1.70
21000
0.0810
wi = 10.0 g
ni = mmol

5. 2a) Number average molecular weight: Weight average molecular weight:
Polydispersity index:

6. 2b)
When 5.0 wt-% of styrene oligomer (fraction E) is added, the total mass and number of moles increase as follow:
fraction wi
[g] Mi
[g/mol] ni
[mmol] A
1.30
11000
0.118 B
3.00
14000
0.214 C
4.00
17000
0.235 D
1.70
21000
0.0810 E
0.50
1000
wi = 10.5 g
ni = 1.10 mmol

7. 2b) number average molecular weight weight average molecular weight
Polydispersity index