Chemistry Reference
In-Depth Information
disadvantage in applications like high-speed wire covering or an advantage in
other end uses like extrusion coating of paper.
Molecular weights are not often measured directly for control of production of
polymers because other product properties are more convenient experimentally or
are thought to be more directly related to various end uses. Solution and melt
viscosities are examples of the latter properties. Poly(vinyl chloride) (PVC) pro-
duction is controlled according to the viscosity of a solution of arbitrary concen-
tration relative to that of the pure solvent. Polyolefin polymers are made to
specific values of a melt flow parameter called melt index, whereas rubber is
characterized by its Mooney viscosity, which is a different measure related more
or less to melt viscosity. These parameters are obviously of some practical utility,
or they would not be used so extensively. They are unfortunately specific to par-
ticular polymers and are of little or no use in bringing experience with one poly-
mer to bear on problems associated with another.
Many technical problems that may be encountered, say, with a new thermo-
plastic, will already have been met and solved with polymers, like rubber, that
have been in the marketplace for a comparatively long time. It is not often possi-
ble to recognize and use such parallels, however, if the parameters of the molecu-
lar weight distributions in the different cases are not measured in the same units.
This results in much unnecessary rediscovery of “old” answers, and the engineer
or scientist who can interpret both “Mooney” and “melt index” values in terms of
statistical parameters of the molecular weight distributions of the respective rub-
ber and thermoplastic may save considerable time and effort.
2.2
Plan of This Chapter
We first review the fundamentals of small particle statistics as these apply to syn-
thetic polymers. This is mainly concerned with the use of statistical moments to
characterize molecular weight distributions. One of the characteristics of such a
distribution is its central tendency, or average, and the following main topic shows
how it is possible to determine various of these averages from measurements of
properties of polymer solutions without knowing the parent distribution itself.
Chapter 3 reviews the essentials of practical techniques for measuring average
molecular weights and characterizing molecular weight distributions.
2.3
Arithmetic Mean
The distribution of molecular sizes in a polymer sample is usually expressed as
the proportions of the sample with particular molecular weights. The mass of data
contained in the distribution can be understood more readily by condensing the
