Chemistry Reference
In-Depth Information
CHAPTER
2
Basic Principles of Polymer
Molecular Weights
Die Wahrheit ist das Alle .... The truth is the whole.
—G. W. F. Hegel
2.1
Importance of Molecular Weight Control
Both the mechanical properties of solid thermoplastics and their processing
behavior at elevated temperatures depend critically on the average size and the
distribution of sizes of macromolecules in the sample. This is one reason why the
plastics market contains different grades of each polymer. All varieties are often
chemically identical, but some of their molecular-weight-dependent properties
may differ enough that the polymers cannot be interchanged economically. As a
rule of thumb, resistance to deformation increases with increasing average molec-
ular weight. Thus, the thermoplastics that are hardest to force into a final shape in
the softened state will usually yield the strongest solid articles on subsequent
freezing. (Some properties, such as refractive index and hardness at ambient tem-
peratures, are not much dependent on molecular weight, provided this property is
in the normal commercial range for the particular polymer type.)
An example of the influence of average molecular weight has been given
in section 1.4, where various grades of thermoplastic polyester were discussed.
Plasticized poly(vinyl chloride) sheeting and coated fabric provide a similar illus-
tration in heat-sealing applications. If the molecular weight of the polymer is too
high, the material will not flow out enough to weld well under normal sealing
conditions. If the molecular weight is too low, on the other hand, the plastic may
suffer excessive thinning, resulting in a weak weld area or show-through of fabric
backing.
The molecular weights of synthetic polymers are much less uniform, within
any sample, than those of conventional chemicals. The growth and termination of
polymer chains are subject to variations during manufacture that result in the pro-
duction of a mixture of chemically identical molecules of different sizes, and it is
important also to be able to control the distribution of such sizes as well as their
average value. A polymer that can crystallize will tend to form brittle articles, for
example, if there is much low-molecular-weight crystallizable material in the
sample. The presence of appreciable high-molecular-weight material, on the other
hand, makes thermoplastic melts more elastic, and this property can be a
