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Journal Club for August 2021 - Electromechanics of Polyelectrolytes

Meredith N. Silberstein's picture

Meredith Silberstein and Max Tepermeister

Cornell University, Sibley School of Mechanical and Aerospace Engineering


Polymers are widely used because of their deformability, low density, and extensive potential for tailoring macroscale properties through chemical composition, molecular architecture, and processing. Polyelectrolytes are a particularly exciting subclass of polymers that are ionically charged. If this is sounding a lot like ionomers (as in the much-studied Nafion™), you’re right! Those are a further subclass with < 15% of the monomers charged. Polyelectrolytes have unique mechanical, ionic, and electromechanical properties enabled by the interactions among the backbone charges and free ions contained within the polymer.[1–5] The charges on the polymer influence backbone conformation, can act as crosslinks, and can drive formation of micelles that are vital both for mechanical properties and ionic conductivity. While we will focus on synthetic polyelectrolytes, it must be mentioned that polyelectrolytes are abundant in biology and are, for example, responsible for many of the pH dependent conformational changes in proteins.


Polyelectrolyte’s unique properties make them useful in a variety of applications including actuators, sensors, energy conversion technologies such as fuel cells and batteries, impact protection, drug delivery, and biocompatible electronics.  For example, their high ionic conductivity and mechanical strength make them ideal ion exchange membranes for low temperature (~80°C) fuel cells. Dynamic ion bonds facilitate self-healing. For example, Surlyn™ will self-heal after a bullet passes through simply from the thermal energy provided by the bullet itself.[6] Finally, their elastic deformability and ability to sustain ion gradients means that they are natural electrochemical actuators. In this journal club entry, we will focus on how polyelectrolytes behave mechanically in electric fields. While the ion transport properties of this material class are also quite interesting, we have decided not to discuss them here since there is an excellent recent imechanica blog on the ionotronics of hydrogel polyelectrolytes. Please forgive the rather small number of citations compared to the vast literature in this field, we have tried to select a mix of seminal papers and review papers (and some self-cites of course) so that readers can follow up according to their areas of interest.



Polyelectrolytes with free counter ions perform as large deformation bending actuators and/or sensors when configured with a field across the thickness direction (Figure 1). Early work in this area focused on ionomers such as Nafion™, ~100um thick, with metal salts deposited on opposite faces. These devices were termed IPMCs (ionic polymer matrix composites).[7,8] When an electric field is applied (typically 1-3V), the dissociated cations move towards the cathode, typically dragging solvent molecules with them. The swelling strain associated with the uneven distribution of cations and solvent molecules causes a bending motion. Conversely, if the IPMC is bent in the absence of an applied field, it will drive the formation of an electric field (~mV). If a DC field is applied for a long time, the polymer inside these actuators relax and the bending is lost or even reverses direction. Extensive modeling work was conducted by Nemat-Nasser’s group in the early 2000’s to explain IPMC dynamics.[9] More recently, polyelectrolyte gels have also been shown to actuate in bending mode. They can be configured identically to IPMCs with metal electrodes attached to the faces and bending according to an identical mechanism (although they do tend to dry out and stop working when operated in this manner). More frequently however, in a gel bending actuator, the gel is placed directly (no metallic coating) in an electrolyte bath with electrolyte between each electrode and the gel.[10,11] This bending was originally explained in terms of a surfactant in the electrolyte being selectively directed to one face of the gel, binding to the fixed anions at the gel surface and causing a contraction. However, gel bending has since been observed in simple salt solutions, bringing this mechanism into question, or at the very least suggesting there must be other possible mechanisms. One subsequent explanation was that the fixed charges in the gel result in an increased free ion concentration within the gel compared to the solution that maintains Donnan equilibrium. When the electric field is introduced, mobile counterions migrate towards one electrode (cations towards the cathode in the case of anionic gels) leaving the solution at one face of the gel (the anode side in the case of anionic gels) with lower mobile ion concentration, resulting in higher osmotic pressure of that gel face, which leads to swelling. It is also possible that bending of these gels in some cases is due to a pH gradient that develops in the electrolyte in conjunction with pH sensitive gel swelling. The gradient would arise from electrochemical water splitting generation of protons at the anode combined with preferential transport of alternative cations over protons to “complete the circuit” due to concentration differences. The pH sensitive gel swelling would arise from pH dependent charge dissociation from the polyelectrolyte.


Figure 1. Schematics of ionic polymer metal composite actuators (left) and in electrolyte gel polyelectrolyte bending actuators (right). Only mobile ions are shown.



The ionic interactions between polyelectrolytes can create strong adhesive forces at gel boundaries and junctions. The ion configurations and therefore these forces can be altered by applying voltages to the systems. Two types of systems have been shown to demonstrate this behavior. In the first type of system, positive and negative linear polyelectrolytes are interpenetrated into separate neutral covalently crosslinked gels.[12,13] These gels are then touched to each other, forming a non-adhesive junction. When a voltage is applied on this junction from the positive to the negatively charged gel, the two gels bond strongly together. It is believed that under this electric field the charged chains diffuse across the interface to ionically bond with each other. The adhesion remains when the field is removed. The adhesion can be reversed by applying a voltage in the opposite direction. This is fundamentally a current-based adhesion, using an electrophoresis-like mechanism (see modeling paper by Qiming Wang). The second type of system, recently introduced by Hayward and Suo, consists of polyelectrolytes with one set of ions incorporated within the polymer backbone and one set of free counterions.[14] It is believed that when the pair of oppositely configured polyelectrolytes are brought into contact, the free ions are driven by chemical potential to diffuse across the boundary, leading to adhesion between the oppositely charged surfaces. When a positive voltage is applied from the positive polyelectrolyte side to the negative side, the free charges are driven to the electrodes, leading to highly charged interfaces from the ions fixed to the polymers, and much stronger adhesion than in the no voltage case. Conversely, a negative voltage will decrease the adhesion. This second system is fundamentally a voltage-based adhesion that works with blocking electrodes. Adhesion takes place in less than a minute given the high diffusivity of the free counter ions.



Bulk mechanical properties

Electric fields can also be used to modulate bulk mechanical properties of polyelectrolytes. One easy way to do this is to use electric fields to adjust the solvent volume fraction. Change in solvent volume fraction will inherently change the crosslink density and there by the elastic modulus of the gel. As early as 1982, it was discovered by Tanaka et. al.[15], that partially hydrolyzed polyacrylamide gels will change size by orders of magnitude under application of an electric current. Many similar demonstrations have been made since, and there has been some really nice theoretical work (e.g. by Doi et. al.[16]), however, the volumetric response of a particular polyelectrolyte gel to an electric field remains a challenging event to understand mechanistically. These mechanisms include pH changes driven by water electrolysis coupled with pH responsive gel properties, electroosmosis that moves water out of the gel (relevant when gel is in contact with the electrode), and stress from the electrodes acting on the fixed ions within the matrix (likely a minimal influence). The relative importance of each possible mechanism depends on gel chemistry, solvent chemistry, voltage, and electrode placement relative to the gels.


In our group’s recent work, we used molecular dynamics (MD) simulations to explore an option for stiffness and strength modulation that does not rely on adjusting solvent quantity/composition, but rather on manipulating the charge aggregation within polyelectrolyte complexes.[17] When a sufficiently large electric field is applied to a polyelectrolyte with no mobile counterions (Figure 2), the bulk properties of the material change. The electric field drives chains to align with the field because of the charge dipoles on the chains, causes rearrangement of the charge clusters that serve as dynamic crosslinks, and alters entanglement density by influencing chain configuration.  This reorientation and rearrangement increases the bulk material stiffness and strength in uniaxial tension applied both parallel and perpendicular to the electric field. The enhancement is greater in the parallel direction because of the chain orientation aspect. While the electric fields we used in the MD study are certainly too large to be practical, real world time scales should enable smaller fields to affect the same nanostructure change. Further, this study suggests mechanisms to aim for in a blocking electrode context (i.e. no change in total solvent or ion content).


Figure 2. MD simulated evolution of ion organization in a polyelectrolyte complex subjected to an electric field.


Since everyone seems to finish these with an outlook, here’s a few thoughts: Even though polyelectrolyte electromechanics have been studied in many different application and basic science contexts since the 1960s, there’s still a lot of room for new discoveries. Given our societal trend towards electrical energy and electronic (or even bio-ionic) devices, these potential discoveries are worth pursuing. The opportunity for new discovery arises from the complexity of the governing mechanisms and diversity of synthetic approaches. Mechanical responses of the polyelectrolytes depend on the fixed and mobile ion arrangements as well as neutral solvent concentration. Electric fields will rearrange those ions, solvent transport is often coupled to ion transport, and ion gradients locally alter electric fields. Applied voltages can also drive electrochemical reactions that generate new ions, changing the chemical environment of the polyelectrolyte and potentially changing the charge on the polyelectrolyte itself. We think that carefully aligning synthetic, experimental, and simulation efforts will be key to making advances.



1.        Hong W, Zhao X, Suo Z. Large deformation and electrochemistry of polyelectrolyte gels. J Mech Phys Solids [Internet]. 2010;58(4):558–77. Available from:

2.        Silberstein MN, Boyce MC. Constitutive modeling of the rate, temperature, and hydration dependent deformation response of Nafion to monotonic and cyclic loading. J Power Sources. 2010;195(17).

3.        Luo F, Sun TL, Nakajima T, Kurokawa T, Zhao Y, Sato K, et al. Oppositely Charged Polyelectrolytes Form Tough, Self-Healing, and Rebuildable Hydrogels. Adv Mater. 2015;27:2722–7.

4.        Liu DS, Ashcraft JN, Mannarino MM, Silberstein MN, Argun AA, Rutledge GC, et al. Spray layer-by-layer electrospun composite proton exchange membranes. Adv Funct Mater. 2013;23(24).

5.        Zhang H, Dehghany M, Hu Y. Kinetics of Polyelectrolyte Gels. J Appl Mech. 2020;87(June):061010-1–15.

6.        Varley R. Ionomers as self healing polymers. In: Self healing materials. Springer; 2007. p. 95–114.

7.        Shahinpoor M, Kim KJ. Ionic polymer – metal composites : I . Fundamentals. Smart Mater Struct. 2001;10:819.

8.        Hao M, Wang Y, Zhu Z, He Q, Zhu D, Luo M. A Compact Review of IPMC as Soft Actuator and Sensor : Current Trends , Challenges , and Potential Solutions From Our Recent Work. Front Robot AI. 2019;6(December):1–7.

9.        Nemat-Nasser S. Micromechanics of actuation of ionic polymer-metal composites. J Appl Phys. 2002;92(5):2899–915.

10.      Osada Y, Okuzaki H, Hori H. A polymer gel with electrically driven motility. Nature. 1992;355:242–4.

11.      Morales D, Palleau E, Dickey MD, Velev OD. Electro-actuated hydrogel walkers with dual responsive legs. Soft Matter. 2014;(10):1337–48.

12.      Xin A, Zhang R, Yu K, Wang Q. Mechanics of electrophoresis-induced reversible hydrogel adhesion. J Mech Phys Solids [Internet]. 2019;125:1–21. Available from:

13.      Asoh T, Kikuchi A. Electrophoretic adhesion of stimuli-responsive hydrogels. Chem Commun. 2010;(46):7793–5.

14.      Kim HJ, Paquin L, Barney CW, So S, Chen B, Suo Z, et al. Low-Voltage Reversible Electroadhesion of Ionoelastomer Junctions. Adv Mater. 2020;2000600:1–7.

15.      TANAKA T, NISHIO I, SUN S-T, UENO-NISHIO S. Collapse of Gels in an Electric Field. Science (80- ). 1982 Oct;218(4571):467 LP – 469.

16.      Doi M, Matsumoto M, Hirose Y. Deformation of Ionic Polymer Gels by Electric Fields. Macromolecules. 1992;25:5504–11.

17.      Raiter PD, Vidavsky Y, Silberstein MN. Can Polyelectrolyte Mechanical Properties be Directly Modulated by an Electric Field? A Molecular Dynamics Study. Adv Funct Mater. 2020;2006969:1–10.

Image icon IPMCvsgelbending.png343.78 KB


Rui Huang's picture

Meredith, thank you for this nice and short summary on the electromechanics of polyelectrolytes! Very interesting topic. I am surprised that there has been no discussions yet. Will come back with some soon.

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