Polymer Basics - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 102

80,000 g/mol. The polymer M n is (40,000 + 80,000)/2 = 60,000 g/mol. The

M w is 66,666 g/mol.

40,000
120,000 =

40,000 × 13,333

80,000
120,000 =

80,000 53,333

×

66,666

Another common term is polydispersity, which is the ratio of M w /M n .
In the above example, the polydispersity is (66,666/60,000)

1.11. If each
polymer chain in a sample had the exact same molecular weight, then the
polydispersity would be 1.0. As the spread of molecular weight among the
various polymer chains increases, the polydispersity increases. A uniform
process with a catalyst that produces polymer chains of similar molecular
weight might have a reasonably low polydispersity such as 2, and another
system, perhaps with a catalyst that has sites of different activities, might
give a broader distribution of molecular weights and have polydispersity
of, for example, 5 or 6. A blend of polymers with remarkably different
molecular weights might have a polydispersity of greater than 15.
Two polymer samples can have the same weighted average but very
different polydispersity. In this case, many properties will be different
because both the molecular weight and the polydispersity have a strong
influence on properties.
The degree of polymerization of a polymer is the number of repeat units
in that polymer. A particular polyethylene chain with a molecular weight of
28,000 g/mol has a degree of polymerization of 1,000. This is calculated by
dividing 28,000 by the repeat unit weight of 28 (C 2 H 4 ). For this example,
n

=

= 1,000 in the structure below.

H H

CC

H H

n

Because molecular weight is so important in determining physical prop-
erties such as impact and strength, it is routinely measured in the polymer
industry. One convenient method to measure molecular weight is by Gel Per-
meation Chromatography (GPC). This method has some similarity to HPLC
in that a solution of the analyte (in this case the polymer) is passed through a
column at high pressures and then as it exits the column detected by various
methods such as UV, refractive index (RI), or light scattering. In HPLC, the
retention time is determined by affinity of the analyte toward the packing
material of the column. In normal phase HPLC, analytes are separated based