based on mean-field calculations and the consideration
of probability distribution functions to model an
amorphous microstructure. Unfortunately, all of these
models predict much smaller coefficients than those
experimentally measured.
At a more fundamental level, there exist two microscopic
hypotheses that propose to explain the basic
mechanisms responsible for polyurethane electrostriction.
One hypothesis, put forth by Balizer and colleagues,
10,11 is a mechanism that relies on phase separation
in the typical polyurethane morphology,
which is that of hard (polar) segments embedded in a
soft, stretched matrix. This hypothesis indicates that
electrostriction is due to the relaxation of the matrix
under an electric field by induced crystallization. A
second hypothesis, introduced by Su et al.,12 suggests
that the presence of electrical charges inside the material
is mostly responsible for the large electrostrictive
strains. Neither of these two hypotheses have
been thoroughly tested, although some evidence
points to space charges as responsible for the phenomenon
of bending electrostriction.13–16 To definitely
identify a mechanism, Balizer proposed that two different
morphologies of polyurethanes be compared: a
phase-separated (PS) semicrystalline morphology and
a homogeneous, phase-mixed (PM) morphology that
could not crystallize. It was the goal of this study to
assess the validity of the proposed hypotheses by the
presentation and analysis of electrostrictive coefficient
data obtained on these structurally different polyurethane
materials. These data, which represent a com-
based on mean-field calculations and the considerationof probability distribution functions to model anamorphous microstructure. Unfortunately, all of thesemodels predict much smaller coefficients than thoseexperimentally measured.At a more fundamental level, there exist two microscopichypotheses that propose to explain the basicmechanisms responsible for polyurethane electrostriction.One hypothesis, put forth by Balizer and colleagues,10,11 is a mechanism that relies on phase separationin the typical polyurethane morphology,which is that of hard (polar) segments embedded in asoft, stretched matrix. This hypothesis indicates thatelectrostriction is due to the relaxation of the matrixunder an electric field by induced crystallization. Asecond hypothesis, introduced by Su et al.,12 suggeststhat the presence of electrical charges inside the materialis mostly responsible for the large electrostrictivestrains. Neither of these two hypotheses havebeen thoroughly tested, although some evidencepoints to space charges as responsible for the phenomenonof bending electrostriction.13–16 To definitelyidentify a mechanism, Balizer proposed that two differentmorphologies of polyurethanes be compared: aphase-separated (PS) semicrystalline morphology anda homogeneous, phase-mixed (PM) morphology thatcould not crystallize. It was the goal of this study toassess the validity of the proposed hypotheses by thepresentation and analysis of electrostrictive coefficientdata obtained on these structurally different polyurethanematerials. These data, which represent a com-
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