Characterization of the Time-Dependent Strain Behavior of Electroactive NCC-PEO Composite Polymers

The study of electroactive polymers (EAPs) has garnered a lot of interest due to the ability of these polymer systems to change their size and shape under the application of an external field that in turn generates a large electromechanical actuation. This process eliminates the need for any moving parts, external motors or servos to aid in this type of actuation. This biomimetic functionality creates the opportunity for these types of materials to have a wide-range of possible applications, such as in robotics, biotechnology, mechatronics, prosthetics, micro-valves, and the like. EAPs are also of interest due to their light weight, large actuation performance, fracture tolerance, and their ability to be processed into almost any shape. Ionic-polymer metallic composite (IPMC) electroactive polymers, comprised of a polymer matrix infused with an ionic salt, have been widely studied, wherein the electrically induced ionic motion of the cation and anion generates an actuation response. Actuation is believed to be a result of the size disparity between two types of ions resulting in an unequal expansion and contraction between the two sides. The volumetric changes then generate the observed mechanical bending response.

Nafion, manufactured by DuPont, is the most commonly studied and commercially available IPMC. They can achieve electromechanical strains upwards of 2% at ±3 V DC with response times of under 4 s and work energy densities reaching 5.5 kJ/m³. Nafion is an excellent IPMC actuator, however, it is a perfluorinated polymer and is not biodegradable or recyclable, meaning that at the end of its lifecycle or usefulness, it cannot decompose to a form that is friendly to the environment. As an alternative to Nafion, IPMCs were synthesized using poly(ethylene oxide) (PEO). Lithium perchlorate (LP) was chosen as the ionic salt for this system and their mechanical actuation response was monitored over successive iterations. PEO is a biodegradable polymer that has been well-studied as a solid-polymer electrolyte, capable of having both anions and cations diffuse through its matrix, and has also been FDA approved as a drug delivery system, making it a perfect green alternative to Nafion. To counter the softening effects of increased salt concentration in the PEO matrix, nanocrystalline cellulose (NCC) was added, in various concentrations, to the PEO-LP water-based solution before casting. NCC is also a biodegradable material that is renewable and an inexpensive biomass derivative. Improved Young’s modulus, via hydrogen bonding between the hydroxyl groups along the NCC chains and the atomic oxygen along the PEO backbone and generated a 20% increase in stiffness of the actuators with 5.0 wt.% LP. This also lead to a 33% increase in actuation performance. Strain behavior for the PEO-NCC composites reached 1.1% at ±4 V DC with response times of 2.5 min, and a maximum work energy density of 3.5 kJ/m³. Though these results are below those posted for Nafion, they represent a viable alternative IPMC for research and development that has the added benefit of being a green material.

IPMC actuator performance has been qualitatively described as having three parts: a slow initial response, a constant steady response, and finally a saturated response. Attempts to describe IPMC actuation have fallen into three separate areas: 1) white box, where physical and chemical laws are taken into consideration with describing the performance, 2) black box, where the models used to describe performance are purely empirically driven with little to know consideration for the underlying physics, and 3) gray box, which uses some physical laws in conjunction with empirically derived parameters that account for phenomena that are hard to obtain or difficult to understand with our current knowledge. It was found in this research that the IPMCs followed an Arrhenius-like behavior as they progressed through their actuation. This novel approach was conducted wherein the Arrhenius function was used to model the time-dependent characteristics of the actuators. Successful fitting of the PEO-NCC composites along with published results from other IPMCs such as Nafion and conducting polymers like polypyrrole were conducted and show the flexibility and viability of the proposed fitting equation. As currently applied, this approach falls in the black box area for characterizing the actuator performance, but recent developments are driving it towards a gray-box solution which showcases parameters that are otherwise difficult or impossible to obtain.