Review of the use of Fibrous Contractile Ionic Polyacrylonitrile (PAN) in Smart Materials and Artificial Muscles

Abstract

Chapter 5 reviews contractile ionic polyacrylonitrile (PAN) fibers as smart materials and artificial muscles. PAN fibers in an active form (PAN or PAN gel modified by annealing/cross-linking and partial hydrolysis) elongate and contract when immersed in pH solutions (caustic and acidic solutions, respectively). Activated PAN fibers can also contract and expand in polyelectrolyte when electrically and ionically activated with cations and anions, respectively. The change in length of these pH-activated fibers is typically greater than 100%. However, more than 900% contraction/expansion of PAN nanofibers (less than 1 micron in diameter) has been observed in the laboratories. PAN muscles present great potential as artificial muscles for linear actuation. The basic unit of commercially available PAN fibers (Mitsubishi Rayon Co., Japan, Orlon or artificial silk) that can be handled properly from an engineering point-of-view is a “single strand”. One PAN strand consists of approximately two thousand filaments. The typical diameter of each filament is approximately 10 μm in the raw state and 30 μm in the fully elongated state (gel). The advantages of PAN fibers among other electrolyte gels are that PAN fiber gels have good mechanical properties, which can be compared to those of mammalian biological muscles. The large volume change of PAN fiber gels also allows the reduction in the size of the gel, which is an important factor in determining the response time. PAN fibers can convert chemical energy directly into mechanical motion. Based on ion diffusion theory, the response time of swelling will be proportional to the square of the gel fiber diameter. Surface/volume ratio also affects the response time. Note that such pH-induced contraction–expansion of modified PAN fibers can also be induced electrically in a chemical cell by electrolysis and production of H+ and OH− ions.