Environmental Engineering Reference
In-Depth Information
Figure 3.4
Left
: Schematic drawing of the molecular weight distribution
of a polymer indicating the two averages
Mn
and
Mw
.
Right
: Schematic
diagram of the molecular weight distribution for polymer samples with low
and high PDI.
The readout obtained is a distribution of weight fractions (
Fig. 3.4
,
left
). The
right-handendofthedistribution hasshort-chainmoleculesthatcanhardly
be called polymer, and the left-hand end of it has the longer-chain high
polymermoleculesinthemix.Thetwoaverages,
Mn
and
Mw
,areindicated
with lines; note that always
Mw
>
Mn
and (
Mw
/
Mn
) is the polydispersity
index (PDI) of the chains. The smaller this value, the narrower will be the
distribution of chain lengths. If all the molecules in the polymer had exactly
the same chain length or molecular weight, then,
Mw
=
Mn
and PDI = 1.
Polymers made by chain reaction generally have values of PDI ranging from
2 to 20, while those made by step reaction have narrower distributions with
P.I. ~ 2.0.
AveragemolecularweightandPDIarebothimportantastheydeterminethe
properties of the polymer. The mechanical properties of plastics that make
them so useful depend on the material having a high molecular weight. As
theaveragemolecularweightisincreased,mechanicalpropertiesliketensile
strength, stiffness, and hardness will also increase. As
Mn
increases, the
melting range as well as the viscosity of the melt also similarly increases.
The higher the
Mn
, the better will be the mechanical properties, but more
difficult will be the processing (which involves molten plastic) due to the
high melt viscosity. Depending on the intended application, usually, a
compromise has to be made between high
Mn
of the plastic and the ease
of processing. At very high or infinite molecular weight, a given property, P,
