Smarter Actuator Design with Complementary and Synergetic Functions

Abstract

A general synthetic strategy for multifunctional actuators is presented, by confining desired functions in separate domains of interpenetrating polymer network materials. Specifically, complementary ionic actuator and shape-memory functions are demonstrated by simultaneous, orthogonal reaction pathways. Synergistic effects also allow dynamic programming and two-way linear shape-memory actuation. Discovered in the second half of the 20th century,1, 2 smart polymer materials have more recently started to challenge the structural role as the main mechanical function of a polymer material by also displaying an impressive variety of actuation and stimulus-response behavior.3, 4 Mechanical movement can now be controlled by light,5-7 temperature,7-10 chemicals,11 electric fields,11, 12 and magnetic fields,12-14 and other responses for these stimuli can also include color change,15 light emission,16 viscoelastic properties changes,13 and energy production.16-18 Materials exhibiting several of these functions are highly desired for combined sensing and actuation or replacement of whole devices. The smart materials community, however, is facing a challenge: it is difficult to design multiple functions in one material, without the optimization of one property interfering with the performance of another. New synthetic methodologies are needed. Block copolymers are one way to achieve molecular functionality variation with nanoscale structuring,19, 20 yet this approach does not have a widespread application due to the synthetic complexity of making block copolymers with specific functionality. A simpler approach to similar spatial organization is an interpenetrating polymer network (IPN), a thermodynamically stable arrangement of multicomponent crosslinked polymeric materials.21 The IPN approach has been widely used for creating materials with novel static properties,21 and tougher materials since crack propagation is inhibited at the phase boundaries. More recently this strategy has been used to synthesize a monofunctional material (provide ionic conductivity) in addition to structural (elastic) properties.22, 23 In this work we expand this strategy to conceptually separate the optimization of a single material response function from the design of multifunctional smart materials. Using simultaneous or sequential synthetic approaches, we can design physically separate, nanostructured, continuous phases each with already optimized distinct smart material properties. Using this approach, we demonstrate the development of a multifunctional IPN material with complementary ionic actuator and shape-memory functionalities. We incorporate two major classes of smart materials24-26 in both parts of the IPN, an ionic electroactive polymers (i-EAP) and a two-way shape-memory polymer (SMP) in the same material. i-EAPs are an established smart materials technology,27 with well-known synthesis, able to produce large displacements under low voltages, sense their environmental condition (temperature, concentration, mechanical stimuli) or harvest energy.27-31 i-EAPs operate either by selective swelling of a material, mediated by mobile counterions, causing it to bend,29 or by oxido-reduction of electrically conductive polymers (ECPs) causing expansion due to the intercalation of ions in their structure. i-EAP actuatorseeewwww mechanical properties and low voltage operation make them suitable candidates for the future of soft and microrobotics.27, 28, 32-35 i-EAPs have demonstrated their potential in biomedical devices and bioinspired robots.25, 30 Shape-memory polymers (SMPs) have shown their utility in biomedical tools such as suture implants26 and are promising for applications in the automotive, aerospace, security, and robotics fields.31, 32, 35-37 SMPs can be programmed in a temporary shape and recover the original one trough an external stimulus application. Both classes of materials are usually optimized only for their particular stimulus-response pro