Four different types of polyurethanes were used in
this work: one of them was a commercially available
material, and the other three were synthesized specifically
for this study. Before each type is presented, we
take a more detailed look at the molecular structure of
the polyurethanes. As mentioned previously, their
molecular chain consists of two segments: one is a soft
segment, and the other is a highly polar hard segment.
This causes the material to have a two-phase microscopic
structure. The hard segments control the dielectric
properties of the polymers, whereas the soft segments,
being more “rubbery,” are responsible for the
elastic properties of the polymers. When the material
is synthesized, the hard segments tend to aggregate
together and form distinct regions. To create these
regions, the hard segments pack themselves close to
one another, stretching the soft portions of the molecules.
This results in a two-phase microscopic morphology
where hard, polar regions are embedded in a
soft, stretched matrix. This morphology, represented
schematically in Figure 1(a), is the common one for
these materials and is referred to as a PS structure.
However, it is possible to synthesize the polyurethanes
in such a way that the two segments mix together,
which results in a PM structure, as illustrated
in Figure 1(b). In this configuration, methyl pendant
groups are attached to the hard segments, to render
them more chemically similar to the soft segments,