Ionic Polymer-Metal Composite:


Shady Alaa Mostafa Kamel (1095150)

History:


Research in active polymer materials has existed for over a century. The first experiments were conducted on rubber bands in 1880. However, the work that led to electroactive polymers (EAPs) did not occur until 1949 when chemically stimulated polymers were discovered and studied. The work with these chemomechanical polymers led directly to synthetic polymers that could be used for actuation. Before long electroactive polymers became prominent because of the convenience of the technology. Even with the increased interest in EAPs, the largest amount of work and progress only began in the 1990’s (Bar-Cohen, 2001). Ionic polymer-metal composites (IPMCs), a type of EAP, were originally developed as fuel cell membranes and not until the early 90’s were their actuation and sensing properties discovered. The two properties were discovered concurrently, but by different researchers. Sensing properties were discovered by Sadeghipoor in 1992, who was using them as hydrogen pressure transducers. The initial sensing use was as a vibration sensor. Also in1992, Oguro described the actuation function of the IPMC’s by bending them under applied voltages (Bar-Cohen, 2001). Since those initial discoveries there has been much work trying
to understand, improve, and apply the performance of IPMCs and other EAPs.

Introduction and Manufacturing:


Ionic Polymer-Metal Composites are synthetic composites that display artificial muscle behavior under an applied voltage[1]. Manufacturing of IPMC starts with an Ion Exchange Polymer such as Nafion, or Flemion, and then the Polymer is plated with a conducting electrode in 2 steps[2]:34181.jpg


  1. Initial Compositing Process:
    • Soak in salt solution such as Pt(NH3)4HCL.
    • Platinum diffuses into polymer.
  2. Surface Electroding:
    • Add reducing agent (LiBH4 or NaBH4).
    • A Layer of Platinum is deposited on the surface.

How Does It Work?


  1. Water molecules pair with cations.
  2. In absence of electric potential, the Water/Cation pairs and the Anions are uniformly distributed as seen in the Left Hand Side of the Image below.
  3. When Voltage is applied, the Water/Cation pairs and the Anions redistribute to balance the charge.
  4. This Causes the negative side of the polymer to swell more than the positive side, resulting in bending actuationa.[2]

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Applications:


IPMCs have many potential uses in various fields such as medicine, robotics, energy harvesting, automotive and small industries due to its ability to perform bending motion when stimulated by low electrical potential (1-3V); Yet, Bending actuators have seen limited application due to integration into larger devices or stronger actuators. Other limitations include the requirement to maintain ion mobility through hydration or other fluids. Higher voltages result in larger actuations, but voltages above 1.23V will result in electrolysis of water, limiting the useful voltage range. [1]

Current research is working to find applications for ionic polymers in both sensing and actuation mode. It is believed that applications can be found to take advantage of the inherent properties of polymers and not limited by the low force production. The high strains of the IPMCs make them attractive as mechanical actuators for applications requiring large motion but little force. Also, three-dimensional actuators have been constructed by joining three individual polymers aligned in different actuation directions. Polymers have also been used to mimic the swimming or flapping motion of fins and wings. The use of polymers as fins is intriguing since most fins are used for balance or stability,not propulsion. Therefore, it is the position of the fin that matters, not the force generated, making the high strain polymers well suited to perform the function. Other industrial applications range from pumps and valves to electromechanical relay switches to musical instruments.



References:


[1] http://www.wikipedia.org
[2] M. Shahinpoor, “Ionic Polymer-Metal Composites: I. Fundamentals, Smart Materials and Structures Volume 1”
[3] Environmental Robotics Inc., http://www.environmental-robots.com/