Chemistry Reference
In-Depth Information
angle neutron scattering is an important source of thermodynamic information for
polymer blends over the entire composition range.
3.3 Other Methods
In addition to the experiments briefly discussed above, two further equilibrium
methods and two nonequilibrium procedures are sometimes employed to obtain
thermodynamic information.
The most frequently used additional method is the evaluation of data for liquid/
liquid phase separation, i.e., of critical points and of binodal curves [
21
]. This
information is normally obtained by means of cloud point measurements (either
visually or turbidimetrically) and the analysis of the composition of coexisting
phases. Critical data give access to the system-specific parameters via the critical
conditions, as formulated in (
36
) and (
37
) for the present approach or by means of
equivalent expressions of other theories. If the critical data (temperature, pressure,
and composition) are known for a sufficiently large number of polymer samples
with different molar mass, and the number of parameters required for a quantitative
description of
g
(
) is not too high, this method yields reliable information. Similar
consideration also hold true for the evaluation of binodal curves. In both cases it is
very helpful to formulate a theoretically justified temperature dependence of the
system-specific parameters.
Liquid/solid equilibria also offer access to thermodynamic information. In this
case, it is the differential interaction parameter x of the
polymer
that is obtained
according to (
38
) from the known molar mass of the polymer, its melting tempera-
ture in the pure state, and the corresponding heat of melting plus the polymer
concentration in the solution that is in equilibrium with the pure polymer crystals.
Because of the well-known problems in obtaining perfect crystals in the case of
macromolecules, special care must be taken with the evaluation of such data.
Vapor pressure osmometry [
34 36
] constitutes a very helpful nonequilibrium
method for obtaining thermodynamic information for solutions of oligomers and
polymers of low molar mass, for which osmometry and light scattering experiments
do no longer yield reliable data. Such experiments are based on the establishment of
stationary states for the transport of solvent via the gas phase from a drop of pure
solvent fixed on one thermistor to the drop of oligomer solution positioned on
another thermistor. Because of the heats of vaporization and of condensation,
respectively, this transport process causes a time-independent temperature differ-
ence from which the required information is available after calibrating the
equipment.
Inverse gas chromatography (IGC) represents another nonequilibrium method; it
yields valuable information on polymer solvent interactions in the limit of vanish-
ing solvent content [
37
,
38
]. In experiments of this type, a plug of solvent vapor is
transported in a column over a stationary phase consisting of the pure polymer melt.
The more favorable the solvent interaction with the polymer, the longer it takes
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