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polymers. Ionomers are polymers that contain a small but significant content of ionic or ionizable groups. Due to
their unique transport properties, ionomer membranes are commonly used in proton exchange membrane (PEM)
fuel cells, water electrolysis cells, and chlor-alkali cells. Discover Technologies utilizes the transport properties of
ionomer membranes to develop novel devices that exhibit electromechanical coupling.
An ionomer membrane is a solid state electrolyte. Therefore, it contains a large number of ions that can be
displaced from their resting state by an external stimulus. Examples of this stimulus could be an applied electric
field or an imposed mechanical deformation. For this reason, ionomeric polymer transducers are included in a
category of materials known as electroactive polymers (EAPs). EAPs may be thought of as belonging to a larger
group of materials known as "smart" materials. While not really smart, these materials exhibit active coupling
behavior that allows them to be used as solid-state actuators or sensors. Examples of other types of EAPs are
conductive polymers, dielectric elastomers, and piezoelectric polymers (PVDF). Other common types of non-
polymeric smart materials are piezoelectric ceramics (PZT) and shape memory alloys.
If an electric field is applied to an ionomer membrane, rearrangement of the ions within the polymer results. This
will lead to differential swelling of the membrane and cause the film to deform. In this way, an ionomeric polymer
can be made to function as an actuator. Discover Technologies' ionomeric transducers are fabricated by coating
ionomer membranes with conductive electrodes and swelling the polymer with a diluent to facilitate motion of the
ions within the polymer. Ionomeric polymer actuators can offer several advantages over other technologies:
- low voltage operation (less than 5V maximum input)
- large strain generation
- silent operation
- compliant, compatible with flexible or conformal structures
- large amount of non-converted energy is recoverable
- very small leakage current at DC
Ionomer actuators are typically cut into strips and used as benders. However, numerous actuation modes are
possible and the technology is scalable, meaning that devices can be made in sizes ranging from microns to
meters.
ionomeric sensor is fundamentally similar to the operation of a piezoelectric sensor. The voltage generated by an
ionomer transducer can be correlated with its quasi-static displacement. Additionally, the charge differential
across the thickness of the transducer is proportional to strain. The use of charge as a measurement metric is
generally the most desirable sensing mode.
The principal difference between an ionomeric sensor and a piezoelectric sensor is that the voltage generated by
an ionomer is generally much smaller than that generated by a piezoelectric ceramic due to the much lower
modulus of the ionomer. However, the charge output of an ionomer is generally larger than that of a piezoelectric
because the capacitance of the ionomeric sensor is several orders of magnitude larger than the piezoelectric
sensor. For this reason, the strain coefficient of ionomeric sensors (effectively, the d31 coefficient) is about 2
orders of magnitude larger than piezoelectric ceramics, and about 3 orders of magnitude larger than piezoelectric
polymers.
The main advantage of ionomeric sensors as compared to piezoelectric ceramics is their compliance and
resilience. Because they are soft and flexible, ionomeric sensors can be subjected to large mechanical strains
without breaking. In applications where the sensor will be subjected to bending, maximum strains of larger than
10% are possible. As compared to piezoelectric sensors and bonded foil strain gages, ionomeric sensors have
higher sensitivity and thus the ability to resolve smaller signals.
Another advantage is the ability to measure a wide variety of signals. Ionomeric sensors ca be designed to
respond to bending, extension, torsion, and compression. These sensors also have a wide dynamic range and
superior response at low frequencies as compared to piezoelectrics. Furthermore, the effect of temperature on
the sensitivity of the devices can be easily corrected by on-line compensation.