Chemistry Reference
In-Depth Information
1
Introduction
Synthetic copolymers are always polydisperse, i.e., they consist of a large number
of chemically similar species with different molar masses and different chemical
compositions. Owing to this polydispersity, characterization of copolymers does
usually not provide the number of individual molecules or their mole fraction, mass
fraction, etc. but requires the use of continuous distribution functions or their
averages. Continuous thermodynamics, developed by R¨tzsch and Kehlen [
1
],
can be directly applied to the calculation of thermodynamic properties, including
phase equilibria, because this theoretical framework is based completely on contin-
uous distribution functions, which include all the information about these functions
and allow an exact mathematical treatment of all related thermodynamic properties.
Continuous thermodynamics have been used for calculation of phase equilibria of
systems containing two-dimensional distributed copolymers [
1 8
]. The purpose of
this contribution is the application of continuous thermodynamics to copolymer
fractionation according to the chemical composition and molecular weight.
Basic research concerning the physical chemical behavior of polymer solutions
is overwhelmingly confined to a few polymers, like polystyrene, that can be
polymerized anionically to yield products of narrow molecular weight distribution.
One of the reasons for this choice lies in the fact that most polymer properties are
not only dependent on the degree of polymerization but are also strongly affected
by the broadness of the molecular weight distribution. It is desirable to produce
nearly monodisperse polymers. One possibility for doing so is the use of polymer
fractionation. Fractionations of polymers are carried out for two different purposes.
One purpose is the analytical determination of the molar weight distribution and the
other is the preparation of fractions large enough in size to permit study of their
properties. In analytical fractionation, the amount of initial polymer is usually
small. The fractions do not need to be separated and are often characterized online
in automated fractional dissolution procedures. Some column techniques are in use
that are based on thermodynamic equilibrium principles and make use of either
liquid/liquid or liquid/solid phase separations. In preparative polymer fractionation,
scaling-up problems are the main issue, because the necessary amount of initial
polymer increases considerably when the purity requirements of the fractions are
raised [
9 11
].
The fractionation of copolymers presents a special problem. For a chemically
homogeneous polymer, solubility only depends on molecular weight distribution.
In the case of chemically inhomogeneous materials, such as copolymers, solubility
is determined by the molecular weight distribution, as well as by chemical compo-
sition. In the case of copolymers, both distributions can change during the course of
fractionation. The efficiency of any given copolymer fractionation can be estimated
from the data on the heterogeneity of fractions in molecular weight (molecular
heterogeneity) and in composition (composition heterogeneity).
One of the long-sought “Holy Grails” of polymer characterization has been the
simultaneous determination of polymer composition as a function of molecular
