Chemistry Reference
In-Depth Information
Surface light scattering methods from thermally induced capillary waves at the
interface [
139 141
] or from electric-field-induced surface waves [
142
,
143
] have
appeared. The technique is limited by the viscosities of the two phases; if the
viscosities are too large, then the spatial damping of the surface capillary waves
is too rapid to be detected by the technique. The applicability of this method for
highly viscous polymeric interfaces has not been verified yet.
Two dynamic methods that have attracted the interest of the scientific commu-
nity are the breaking thread method and the imbedded fiber retraction (IFR)
method. Although they are dynamic methods and, thus, suffer from the high
viscosities and viscoelastic character of polymers, they possess an important
advantage in that they can be used to measure the interfacial tension between two
phases of similar densities. The breaking thread method [
144 147
] involves the
observation of the evolution of the shape of a long fluid thread imbedded in another
fluid. Due to Brownian motion, small distortions of arbitrary wavelength are
generated at the surface of the thread; this leads to a pressure difference between
the inside and the outside of the thread, which induces important deformations
caused by the effect of the interfacial tension that tends to reduce the interfacial
area. It is possible to infer interfacial tension between the polymer forming the
thread and the matrix from the study of the time evolution of the disturbances.
However, the breaking thread method suffers from a major drawback related to
residual stresses during the preparation of the threads; these fibers distort faster and
lead to interfacial tension values much higher than the real value. Moreover, the
fiber should be formed with the material that has the lowest viscosity and, at the
same time, the higher softening temperature. Palmer and Demarquette [
148
] pro-
posed a methodology for the improvement of the accuracy of the method by
utilizing simultaneously the original theory of Tomotika [
144
], which evaluates
the growth rate of the sinusoidal instabilities growing exponentially with time, with
that of Tjahjadi et al. [
146
], which consists of fitting the dynamics of amplitude
growth using curve-fitted polynomials, which are calculated from numerically
generated results of the transient shape using boundary integral techniques.
The IFR method is a dynamic technique that has been widely used to measure the
interfacial tension for blends comprising high molecular weight and/or high viscos-
ity polymers, for which it is difficult or impossible to measure the interfacial tension
using direct equilibrium techniques such as the sessile or pendant drop methods. The
IFR method involves the analysis of the microscopic shape change of a fiber of one
polymer embedded in a matrix of a second polymer [
25
,
149 151
]. In general, the
IFR studies are made on matrix polymers that are solid at room temperature and
have high viscosities, which are obtained directly by compression molding or cut
from large compression-molded samples. These systems require a melting and
embedding step at a temperature below the retraction temperature. However, matrix
polymers that are liquid at room temperature have been used as well [
24
].
The most versatile, convenient, and reliable technique for determining the
surface and interfacial tension of polymer melts is the pendant drop method
[
19
,
20
,
45
,
54
,
56
,
122
,
126
,
127
,
152 155
]. The results obtained by the pendant
drop method constitute the bulk of the available data [
10
,
120
]. The method is
