Introduction
Mammalian muscles
Mammalian muscles exhibit highly optimized structures that are consistent across various species Their functionality is governed by a complex mechanism involving chemical reactions that facilitate reversible hydrogen bonding between the proteins actin and myosin, allowing them to slide past each other and generate strain Research indicates that mammalian muscle can achieve peak stress levels of 150-300 kPa at approximately 25% strain, with a maximum power output reaching 150.
Natural muscles exhibit an average power output of 50 W/kg and an energy density ranging from 20-70 J/kg at low to intermediate frequencies Their micro fiber structure allows for easy scalability, but the primary challenge lies in the need for a complex system to sustain and nourish them This challenge has prompted researchers to explore alternative materials and structures that can effectively mimic the properties of natural muscles.
Artificial muscles
Artificial muscles, also known as actuating materials, have gained significant attention over the past two decades due to their potential in soft robotics and programmable motion Researchers are exploring various materials and designs to create robots that can mimic natural bending and flexing, allowing for more organic interactions with living organisms These materials can reversibly contract, expand, or rotate in response to external stimuli, offering compact and lightweight solutions for powering a range of applications, from robots and exoskeletons to medical prosthetics and micro-tools Key materials used in artificial muscles include shape memory alloys (SMAs), electroactive polymers (EAPs), and piezoelectric and dielectric materials, all of which enable oscillatory or reciprocating motions similar to those of natural muscles.
Artificial muscles can be categorized into four types based on their actuation mechanisms: pneumatic, thermal, ionic-based, and electric field-driven, with the latter relying on Coulombic forces The relationship between blocking stresses and actuation strains for various artificial muscles is illustrated in Figure 1.
Pneumatic artificial muscles (PAM), also known as McKibben Artificial Muscles or Fluidic Muscles, are innovative actuators that contract in length when pressurized Typically, a PAM consists of a long, hollow cylinder designed to mimic biological muscle movement, making it a key component in biomimetic applications.
Pneumatic Artificial Muscles (PAMs) are constructed from synthetic or natural rubber encased in a braided mesh shell, designed to operate at a specific angle Their functionality is based on the introduction of compressed air into the cylinder, causing it to contract and expand radially When the air is released, the mesh acts as a spring, returning the cylinder to its initial shape The primary advantage of PAMs in control applications is that only the pressure needs to be managed They are lightweight, easy to manufacture, and exhibit load-length characteristics similar to human muscles However, PAMs face limitations, including low accuracy and force output, as well as the necessity for additional components like a pressurized air source, feedback sensors, and flow control solenoids These challenges highlight the need for ongoing research into alternatives to PAM technology.
Shape memory alloys (SMAs) are prominent thermal actuators known for their ability to revert to their original shape when exposed to specific temperature or magnetic conditions These metallic alloys exhibit unique properties such as superelasticity and visco-elasticity, making them valuable in diverse applications, including automotive, aerospace, robotics, and biomedical fields SMAs, particularly the NiTi type, offer remarkable characteristics like biocompatibility, high stress capacity (200 MPa), significant work per volume (100 J/cm³), and impressive work density (15 kJ/kg) However, they face challenges such as low accuracy, limited strain, fatigue, stability issues, and low energy efficiency Consequently, ongoing research is essential to improve the performance of SMAs.
Recent advancements in thermal actuation have introduced innovative hybrid yarn actuators that incorporate guest materials within multi-walled carbon nanotube bundles, enabling torsional rotation and contraction through electrical, chemical, or photonic excitation Additionally, Haines et al have shown that affordable high-strength polymer fibers, commonly used in fishing lines and sewing threads, can function as effective tensile and torsional muscles These artificial muscles boast an impressive stroke of up to 49% and can generate significant stress, allowing them to lift loads over 100 times greater than that of a human, while maintaining the same length and weight.
The relationship between blocking stress and actuation strain is illustrated for various types of artificial muscles, including piezoelectric polymers (PVDF) and ceramics (PZT), thermal expansion actuators (TE 100K), shape memory alloys (SMA), conducting polymers (CP), ionic polymer metal composites (IPMC), piezoelectric single crystals (PZT-PT), relaxor ferroelectric polymers (P(VDF-TrFE)), dielectric elastomers (DEA), pneumatic artificial muscles (PAM), gel actuators, and natural muscles (NM).
Electric field actuation encompasses piezoelectric and dielectric elastomer actuation (DEAs) Piezoelectric materials experience a reversible dimensional change when exposed to an electric field, leading to electric polarization upon strain These materials are utilized in diverse applications, including sound production, detection, microbalances, and precision optical focusing They also play a crucial role in scientific instruments like scanning probe microscopes, cigarette lighter ignition sources, and quartz watch time references The most prevalent piezo ceramic is lead zirconate titanate (PZT), though its brittleness limits its use in high-strain applications.
Researchers have created flexible materials capable of enduring significant strain for applications involving deformable materials Dielectric elastomer actuators (DEAs) consist of a thin film made from electrically insulating elastomeric materials, which are coated on both sides with compliant, electrically conducting electrodes When a voltage is applied to these electrodes, the films respond accordingly.
Dielectric elastomer actuators (DEAs) are an innovative electroactive polymer technology with potential applications in actuators, strain sensors, and energy-harvesting electrical generators Despite their promise, the adoption of DEAs faces challenges such as early breakdown, high electric field requirements, and the need for large support frames to stabilize the membranes Recent research has focused on miniaturizing the DEA fabrication process to the micro scale, resulting in the creation of thin films just 5 micrometers thick, stacked up to 50 layers to enhance system height and achieve usable displacement while reducing the control voltage to 150 V However, this voltage remains relatively high, limiting their suitability for applications involving human interaction.
Fig 2 Dependence of specific work on actuation strain for various artificial muscles
Ionic-based materials such as gels, ionic polymer-metal composites (IPMCs), carbon nanotubes (CNTs), and conducting polymers play a significant role in various applications Gels, commonly used in manufactured foods, cosmetics, and medical creams, exhibit desirable characteristics including softness, biocompatibility, and biodegradability, making them particularly valuable for medical and in vivo applications.
Ionic gels consist of a crosslinked polymer immersed in an electrolyte solution, which can be either water-based or organic These gels are particularly intriguing as actuators due to their ability to achieve significant dimensional changes, exceeding 40% strain, despite having a maximum blocking force of 0.3 MPa Gel actuators can be categorized based on the type of stimulus they respond to, including temperature-responsive, photoresponsive, and chemically driven gels.
Electrically driven gels can generate a flow of positively charged surfactants in response to an electric field, showcasing their potential as sensors However, the practical applications of gel actuators are limited due to their low blocking force, typically under 4 MPa, and the need for complex pumping systems to deliver chemical energy Additional challenges include slow response times attributed to diffusion rates and the tendency of hydrogels to dry out in non-wet environments Recent research by Spinks highlights the use of gels in McKibben actuators, which demonstrate significant strains of 9% and forces of 2 N when heated or immersed in hot water, though they still exhibit slow performance.
Carbon nanotubes (CNTs), discovered by Iijima in 1991, have garnered significant attention due to their remarkable electrical and mechanical properties Each individual CNT exhibits a high Young's modulus of around 1 TPa, a maximum tensile strength of 100 GPa, and impressive conductivity ranging from 10^6 to 10^7 S/m Moreover, CNTs can be combined with various guest materials to create composite materials, enhancing their overall performance However, despite the exceptional mechanical strength of individual CNTs, the effective strength of CNT bundles is relatively low due to weak shear interactions between adjacent shells and tubes.
Carbon nanotubes (CNTs) have been utilized in tensile and torsional actuators, operating on the principle of double layer capacitance that balances charges from an applied potential, which also causes the nanotubes to separate Despite their high stiffness resulting in a minimal strain of approximately 0.07%, CNTs exhibit a notable strain rate of 19% s^-1 due to their high porosity and rapid ion diffusion However, the cost of mass-producing CNTs remains a significant challenge.
64] which has limited their widespread applications in artificial muscles and other applications to date
Motivation and problem statement
The goal of this thesis is to fabricate, characterize electro-chemo-mechanical properties, and to model the ultrathin PEDOT-based trilayer transducers
To create an effective trilayer structure, a precise and reliable fabrication technique is essential The chosen method is a layer-by-layer fabrication process that integrates seamlessly with photolithography, including plasma dry etching Advanced characterization methods have been established to analyze very thin films and gain insights into their transducer mechanisms However, characterizing trilayer transducers presents challenges due to their thinness and susceptibility to damage from mechanical stress, as well as variations in properties influenced by oxidation states.
To maximize the potential of thin trilayer structures, it is essential to model the behavior of micro-actuators and micro-sensors for design optimization prior to mass production and to enhance device operation understanding The actuation mechanism of PEDOT-based actuators is intricate, involving electrical stimulation, redox reactions, and mechanical strain, which complicates the modeling process and necessitates a comprehensive model that incorporates multiple domains.
This thesis addresses the challenges associated with layer-by-layer fabrication techniques, focusing on the characterization and modeling of trilayer transducers The outcomes of this research will provide valuable insights into the effectiveness of these processes.
Thesis structure
The thesis is divided in six chapters The brief content of each chapter is drawn as below:
PEDOT-based trilayer fabrication process
Introduction
Conducting polymer (CP) actuators operate through a reversible redox chemistry mechanism, where electrochemical oxidation or reduction leads to the insertion or expulsion of ions and solvent molecules from the CP, maintaining electroneutrality and causing volume changes The actuation response, known as electro-chemo-mechanical deformation, is influenced by the degree of oxidation of the CPs, which can be controlled by the applied voltage, affecting the density of polarons that carry electronic charge.
Electrochemical reduction in conducting polymers (CPs) leads to two distinct actuation mechanisms based on the size of the doping anions When small anions, such as ClO4 -, are involved, they exit the CPs into the electrolyte, resulting in a reduced and shrunk state known as anion-driven actuation Conversely, when larger cation clusters like 1-Ethyl-3-methylimidazolium (EMI +) bis-trifluoromethan sulfonylimide (TSFI -) are used, they become immobilized within the polymer network, leading to cation-driven actuation In this scenario, CPs are reduced by the insertion of cations from the electrolyte, causing them to swell due to the additional cations.
Fig 1 Actuation mechanism of conducting polymer actuators: a) anion driven mechanism, b) cation driven mechanism [6] (Reprinted with permission from Springer Nature) The ovals represent positive electronic charge on the backbone
The cation driven reaction of PEDOT, one of the most widely studied conjugated polymers, is described in Fig 2 Providing an ionically conducting medium is available, PEDOT can be used
CP actuators utilize conducting polymers, specifically a solid polymer electrolyte (SPE) film sandwiched between two PEDOT electrodes (PEDOT//SPE//PEDOT), to function effectively in air The SPE layer serves as both an ion reservoir and a structural support, crucial for the actuator's mechanical strength When a voltage, usually less than ±2 V, is applied to the two CP electrodes, the actuator operates efficiently.
CP layer will be reduced leading to an increase in volume whereas the other layer will be oxidized and shrunk in volume Finally, the bending movement is induced
Fig 2 Cation driven actuation in a PEDOT-based actuator
The performance of CP actuators, including their expansion and actuation speed, is influenced by the electrochemomechanical characteristics of the polymer, the size and mobility of ions, and the solid polymer electrolyte's (SPE) effectiveness in promoting ion transport These crucial elements will be explored in detail in Chapter 4.
Fig 3 Bending actuation mechanism of PEDOT-based trilayer actuators The anode contracts due to ion expulsion and the cathode expands due to cation incorporation, leading to bending
A proposed model illustrates two electrochemical doping processes: some ions (II) are deeply trapped near polymer chains and are only released at very low potentials, while other ions (I) are shallowly trapped and create a double layer at a certain distance from the chain.
The bending of the actuator is influenced not only by the redox processes occurring in the conducting polymer (CP) electrodes but also by the formation of a double layer between the electrolyte and the nanostructure of the CP A two-step doping process of polypyrrole conducting polymer thin films in a 1M lithium perchlorate solution in propylene carbonate illustrates this phenomenon At low potentials during the redox process, an ionic double capacitance layer forms at the surface of the polymer chains due to the accumulation of weakly trapped ions near these chains As the applied potential reaches the oxidation potential, further changes occur in the actuator's behavior.
CP involves a specific quantity of ions interacting with polymer chains that have altered their oxidation state Nonetheless, the exact percentage of this contribution to the actuation mechanism is still not determined.
Recent advancements in CP actuators have led to innovative applications, such as the development of a micropump by Naka et al., which utilizes PPy-based conducting polymer soft actuators to transport fluids unidirectionally without backflow This micropump employs two bending structures driven by conducting polymers to effectively open and close (Fig 5a) Additionally, Fang et al introduced a diaphragm pump utilizing circular CP actuators, achieving a flow rate of 1260 µL/min at an operating frequency of approximately 0.5 Hz.
The article presents various innovative applications of conducting polymers in microactuators and pumps It features a schematic of a micropump powered by a polypyrrole (PPy)-based conductive polymer actuator, highlighting its design and functionality Additionally, it includes a diagram illustrating the mechanism of flap check valves alongside a bottom view of the assembled micropump The article also showcases a micro autofocus lens actuator that employs bending conducting polymer actuators, demonstrating their versatility in optical applications Lastly, a prototype of a robotic fish is presented, showcasing the potential of these materials in advanced robotics.
A robotic fish, depicted in Fig 5d, operates autonomously with a flexural joint tail fin powered by a polypyrrole (PPy) actuator developed by McGovern et al [15] The researchers showcased real-time wireless control and directional maneuverability of the robotic fish, achieving a maximum speed of approximately 33 mm/s at a flapping frequency of 0.6 to 0.8 Hz.
Micro autofocus lens actuators utilize a trilayer structure consisting of polyvinylidene difluoride (PVDF) membranes sandwiched between two layers of polyethylenedioxythiophene/poly(styrene sulfonic acid) (PEDOT/PSS) These actuators are fabricated through a casting process, enabling the efficient production of bending conducting polymer actuators that exhibit high mechanical strength.
Jager developed innovative designs for medical applications, including a closable microvial for single-cell studies, constructed from two polypyrrole/Au microactuators This microvial has the potential to function as a microrobot, enabling the manipulation of micrometer-sized objects, such as cells.
To individually control microactuators and prevent short circuits from electrical connections, a novel fabrication method was developed This method utilizes microfabrication technology, including photolithography, to pattern conductive polymer (CP) actuators effectively.
The article features a schematic representation of a closable microvial, showcasing its design and functionality (Fig 6a) [16] Additionally, it includes a sequence of images illustrating the process of grasping and lifting a 100 µm glass bead, accompanied by schematic drawings that depict the motion involved (Fig 6b) [17].
Association for the Advancement of Science)
Wolff and Beiski developed a patented controllable drug delivery system utilizing a polypyrrole actuator in various microvalve configurations, designed for insertion into the oral cavity, such as within a dental prosthesis This innovative system offers significant advantages, including a novel buccal mucosa drug delivery route and a compact, controllable mechanism comparable in size to two teeth.
The selection of materials for CP-based trilayer actuators
Polypyrrole (PPy) and PEDOT are the leading materials for crafting conductive polymer (CP)-based actuators While PPy actuators are recognized for their rapid actuation speeds and significant strain capabilities, they face challenges such as high rigidity, low ionic conductivity, and susceptibility to damage from over-oxidation In contrast, PEDOT actuators are noted for their thermal and chemical stability, along with superior electrochemical stability and conductivity Compared to PPy, PEDOT generally offers higher conductivity, which minimizes voltage drop along the actuator and enhances overall actuation efficiency.
PEDOT and its associated devices are versatile and can be tailored to different fabrication methods, such as spin coating This adaptability makes PEDOT the preferred conductive polymer for the electrodes of the actuators in this research.
The performance of CP-based actuators is influenced not only by the conductivity of the conductive polymer (CP) but also by the solid polymer electrolyte (SPE) layer The electrolyte medium plays a crucial role in facilitating the ion transfer necessary for the redox process, significantly impacting the electromechanical response of the actuators A low ionic conductivity within the SPE layer can result in slower response times and increased energy losses, as discussed in section 4.3.2 Additionally, a stiffer SPE layer can diminish the overall actuation magnitude.
Since the CP materials are often brittle, the mechanical toughness and stretchability of the actuators is mainly dependent on the SPE layer
Therefore, a full interpenetrating polymer network (full-IPN) or a semi interpenetrating polymer network (sIPN) is employed to meet the requirement of both ionic conductivity and mechanical properties
Interpenetrating polymer networks (IPNs) were first discovered in the 1960s, when researchers found that combining two immiscible polymers into an IPN resulted in a shift of their glass-transition temperatures towards each other This phenomenon occurs due to increased miscibility caused by cross-linking IPNs can be classified based on their structure into semi-interpenetrating (sIPN) and full-IPN, as well as by their synthesis pathways into sequential IPN and simultaneous IPN.
In a full-IPN, shown in Fig 7a, two polymers are present as cross-linked networks, however there is negligible bonding between these polymers These can be prepared sequentially or simultaneously
Fig 7 Schematic of a) full-IPN and b) semi-IPN where the pink, blue, and empty dot represent for the crosslinking between two polymer chains
In a semi-interpenetrating polymer network (sIPN), a single cross-linked polymer serves as the host matrix, which can improve the miscibility of the linear polymer within it compared to a full interpenetrating polymer network (IPN) However, the increased mobility of the linear polymer may lead to enhanced phase segregation when the two components lack thermodynamic compatibility Notable applications of IPNs include their use as solid polymer electrolytes in fuel cells and as soft mechanical actuators.
Research has explored various partner materials for creating an optimal interpenetrating polymer network (IPN) for actuator applications Poly(ethylene oxide) (PEO) is often the primary choice due to its exceptional ionic conductivity, which can achieve up to 0.1 S/cm at room temperature when combined with the ionic liquid EMITFSI The PEO network is synthesized through the free radical copolymerization of poly(ethylene glycol) methyl ether.
24 methacrylate and poly(ethylene glycol) dimethacrylate However, PEO films show poor mechanical properties and weak dimensional stability [34] Adding a second networkto PEO
[35], namely hydroxytelechelic polybutadiene (HTPB) [36], or polytetrahydrofurane (PTHF)
[37], or nitrile butadiene rubber (NBR) [32], was proposed to improve the mechanical properties
Table 1 highlights key parameters of HTPB/PEO, PTHF/PEO, and NBR/PEO interpenetrating polymer networks (IPNs) Notably, the NBR/PEO IPN exhibits superior ionic conductivity, high strain at break, low Young’s moduli, and the ability to operate at high frequencies in the kHz range Furthermore, NBR/PEO demonstrates compatibility with the reactive ionic etching process, a crucial step in photolithography for microbeam patterning These advantageous properties position NBR/PEO as a promising candidate for lithography processes and microactuator systems.
Table 1 Summary some important properties of three different PEO-based IPNs
HTPB/PEO PTHF/PEO NBR/PEO Ionic conductivity (S/m)
Elongation at break in dry state
Microactuators benefit from various types of electrolytes, particularly lithium perchlorate (LiClO4) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in propylene carbonate (PC), due to their high ionic conductivity and adjustable concentrations However, these electrolytes are prone to volatility, which can significantly diminish actuator performance during prolonged use To ensure long-term functionality, encapsulation of the actuators is essential.
Ionic liquids offer significant advantages, including a broad electrochemical stability window, high thermal stability, and non-volatility, which contributes to their non-flammability However, their high viscosity typically results in lower conductivity compared to solvent-based electrolytes Notable examples of ionic liquids include 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI), 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (AMITFSI), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMITFSI), and 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide (MPPPTFSI).
The most commonly used ionic liquids in the incorporation of solid polymer electrolyte (SPE) layers in microactuators include 25 osulfonyl)imide (EMIFSI), 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (Pyr14FSI), 1-butylpyridinium tetrafluoroborate (BuPyBF4), and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) As illustrated in Fig 8, these ionic liquids exhibit varying properties, influencing the strain differences produced when integrated into PEDOT-NBR/PEO-PEDOT actuators Notably, EMITFSI demonstrates medium ionic conductivity, significant maximum swelling, and the highest strain difference among the tested ionic liquids.
These are the reason why EMITFSI is chosen as the electrolyte to incorporate to the microactuators in our study
The properties of ionic liquids integrated into an NBR/PEO matrix significantly influence the performance of PEDOT-NBR/PEO-PEDOT actuators Additionally, the strain variations observed in these actuators when swollen with different ionic liquids highlight the impact of ionic liquid selection on actuator functionality.
Microscale actuators require a thickness of less than 20 micrometers to be suitable for etching processes, leading to the development of the layer-by-layer (LbL) approach Bending microactuators are created by sequentially stacking layers through the polymerization of conducting polymer electrodes and a solid polymer electrolyte (SPE) Each layer is precisely deposited using spin-coating to control thickness The conducting polymer PEDOT can be produced in-situ via electropolymerization or vapor phase polymerization (VPP), with the first and last actuator layers being formed through VPP of 3,4-ethylenedioxythiophene (EDOT) The intermediate SPE layer is synthesized as a semi-interpenetrating network (semi-IPN), combining a poly(ethylene oxide) (PEO) network for ionic conductivity and nitrile butadiene rubber (NBR) for enhanced mechanical strength.
The LbL process has previously demonstrated proof of concept; however, its performance has been limited, achieving only 0.13% strain and 0.75 µN of generated force, primarily due to the low electroactivity of PEDOT electrodes produced by VPP The process presents challenges, including the need for fine dimensions and low yields, often requiring multiple attempts to obtain a few successful samples For practical applications of microactuators and their integration into advanced microelectromechanical systems, enhancing mechanical response and yield is essential This thesis presents improvements in both electrical and mechanical responses, although further optimization of the chemistry is necessary to increase yield.
Materials
Poly(ethylene glycol) methyl ether methacrylate (PEGM, Mn = 500 g mol -1) and poly(ethylene glycol) dimethacrylate (PEGDM, Mn = 750 g mol -1) were sourced from Sigma Aldrich, alongside cyclohexanone (>99.8%) and 3,4-ethylenedioxythiophene (EDOT), which was distilled under reduced pressure Iron(III) p-toluene sulfonate Clevios™ CB 55 V2 (55 wt% in butanol) was obtained from HERAEUS Additionally, 1-butanol (99%), the initiator dicyclohexyl peroxydicarbonate (DCPD), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI 99.9%), and nitrile butadiene rubber (NBR) were supplied by Alfa Aesar, Groupe Arnaud, Solvionic, and LANXESS, respectively The molecular structures of these materials are illustrated in Fig 9.
Fig 9 Molecular formula of a) the precursor PEGM, b) PEDGM, c) the linear chain NBR, d) the initiator DCPD, and e) the ionic liquid EMITFSI [46] (Reprinted with permission from American
PEDOT synthesis route
The polymerization of PEDOT occurs through a three-step mechanism initiated by Fe(OTs)3, which produces a radical cation Fe 3+ This process involves the radical termination of two activated EDOT monomers, resulting in carbon–carbon coupling The anion (OTs) - serves as a dopant, stabilizing the positive charge on sulfur The synthesis leads to π-conjugation in the EDOT oligomers, characterized by alternating single and double bonds that delocalize electrons and lower the oxidation potential As a result, existing PEDOT chains grow more rapidly than new ones, since oligomers are more easily oxidized than monomers.
In the polymerization of PEDOT, 28 chains undergo oxidation, resulting in a positive charge along the backbone, with a charge ratio of one for every three or four chain segments A dopant anion is then electrostatically attracted to the polymer, balancing the charge The strength of the oxidant is crucial; it must be sufficiently high to initiate the reaction while remaining low enough to prevent undesirable bond splitting in the monomer, which can lead to imperfect bonds, disrupt conjugation, and ultimately reduce conductivity.
Fig 10 Oxidation of EDOT with iron (III) p-toluene sulfonate
The fabrication process of the PEDOT electrodes was performed as described in the work of Maziz [50] and is depicted in Fig 11
Fig 11 Fabrication process of PEDOT electrodes [51]
The PEDOT electrode layers were obtained from EDOT VPP, first described by Winther-Jensen
The VPP of EDOT was performed using a direct chemical oxidation method with a commercial solution of Iron(III) tosylate (Fe(OTs)3) in butanol, incorporating PEO precursors (mPEG) as a monomer and crosslinker A radical initiator, DCPD, was added to the solution, which was then stirred, degassed, and spin-coated onto a substrate at varying speeds from 1000 to 3500 rpm The process involved exposing the coated solution to EDOT vapor under primary vacuum, followed by a heat treatment to facilitate mPEG polymerization and crosslinking, resulting in PEDOT/PEO composite electrodes The electrodes can be either washed with methanol to remove unreacted materials or left intact for further trilayer fabrication Notably, the incorporation of PEO enhances the specific capacitance, cycling stability, and flexibility of the resulting PEDOT electrodes.
[52], which improves the strain and the life time of the actuator
The final properties of PEDOT electrodes, including thickness, electronic conductivity, and volumetric charge density, were optimized by K Rohtlaid in collaboration with our group Key factors influencing optimization include the composition of the oxidant solution (specifically mPEG content), spin-coater rotation speed, and the time and temperature of EDOT VPP To achieve high electronic conductivity and electroactivity, it is essential to incorporate PEO precursors into the oxidant solution without exceeding a 20% concentration The rotation speed should be adjusted based on the desired thickness of the conductive polymer, while polymerization time and temperature must remain below 90 minutes and 50°C, respectively, to preserve optimal electrochemical properties The resulting PEDOT electrodes demonstrated electronic conductivities around 200 S/cm and electroactivity ranging from 2.3 x 10^7 C/m^3 to 1.0 x 10^8 C/m^3, which is notably lower than the previously reported conductivity of 805 S/cm by Maziz, achieved under different conditions.
In a 30-minute process, a PEDOT electrode with a thickness of 0.34 µm was produced The observed decrease in electronic conductivity can be attributed to several factors, including the fabrication of a thicker PEDOT layer via the VPP process Utilizing a Sawatec vacuum hot plate instead of a bell jar allowed for a higher density of EDOT vapor, leading to an increased PEDOT density.
Achieving a conductivity of 30 in the bell jar is possible, although thicker PEDOT layers exhibit lower electronic conductivity compared to thinner layers Additionally, keeping the PEO content in the oxidant solution below 20% is essential for maintaining a high volumetric charge density, albeit at the expense of reduced electronic conductivity.
To create samples for characterization, specific synthesis parameters were established: 10% mPEG content, 50 minutes of EDOT VPP time at 40°C, and a spin-coating acceleration of 1000 rpm for 30 seconds to achieve optimal electronic conductivity and volumetric charge density The relationship between the spin coater's rotational speed and the PEDOT thickness was analyzed, leading to a final choice of 2000 rpm, resulting in PEDOT electrode thicknesses of 2.2 µm, which yielded the highest bending strain in a trilayer configuration.
The relationship between the rotational speed of the spin-coater and the thickness of the PEDOT electrode on a silicon wafer is illustrated, with a specific focus on a 10% mPEG content in the oxidant solution.
PEDOT-based trilayer fabrication process
This section outlines the fabrication process of CP-based trilayer actuators, which builds on the LbL method established by Maziz [43] Key modifications include optimizing the PEDOT electrodes, which extended the voltage pulse polymerization (VPP) time from 30 to 50 minutes, and utilizing a Sawatec HP-200 vacuum hotplate for enhanced temperature and vacuum control, replacing the previous vacuum bell during VPP The process is conducted in a Class 100 clean room to mitigate the impacts of humidity and temperature, thereby ensuring greater reproducibility.
The trilayer fabrication process involves several key steps, beginning with the coating and synthesis of a trilayer structure on a silicon wafer This is followed by a three-hour heat treatment and femtosecond laser patterning to create a cantilever beam shape Finally, the patterned beams undergo swelling in ionic liquid for about one week Each of these steps will be detailed further below.
Fig 13 Microactuator fabrication process showing the multilayer process followed by laser micromachining
The trilayer actuator was synthesized using the layer-by-layer (LbL) method, where each layer is spin-coated onto the previous one, allowing for precise control over thickness by adjusting the rotation speed An oxidant solution was created by mixing PEO precursors (50 wt% PEGM and 50 wt% PEGDM) with a commercial Fe(OTs)3 solution To enhance adhesion between layers, mPEG PEO precursors were incorporated into each layer and polymerized throughout the trilayer structure, maintaining the same ratios The solution was stirred for 10 minutes before being spin-coated onto a two-inch silicon wafer The electropolymerization of EDOT (VPP) was conducted similarly to the PEDOT electrode fabrication, lasting 50 minutes at a temperature of 40 °C.
The SPE layer was prepared using a semi-IPN architecture by mixing a 20wt% NBR solution with 75wt% PEGM and 25wt% PEGDM precursors, followed by the addition of a 3wt% free radical initiator, DCPD After stirring for 15 minutes and degassing, a homogeneous mixture was achieved and spin-coated onto the first PEDOT electrode layer The impact of spin coating speed on the thickness of the NBR/PEO layer was analyzed, with results comparable to previous studies.
32 by Maziz [39] A rotational speed of 2000 rpm, 1000 rpm s -1 , 30 s was chosen to fabricate the SPE layer in this step The pre-polymerization was carried out under a continuous supply of
N2 at 50°C during 45 min to enable the formation of the PEO network and to improve the adhesion between the first two layers
Fig 14 Relationship between rotation speed of spin-coater for deposition of NBR/PEO and its resulting thickness
The second PEDOT electrode layer was synthesized similarly to the first, involving the preparation of an oxidant solution that was spin-coated onto the PEDOT/SPE bilayer Following this, EDOT vapor phase polymerization (VPP) was conducted under vacuum, mirroring the process used for the initial electrode Additionally, DCPD (3 wt% relative to the PEO network) was incorporated into the oxidant solution to facilitate the polymerization of the PEO network.
After the EDOT VPP process for the second electrode layer, the trilayer underwent a final heat treatment at 50°C for 3 hours under a continuous flow of N2 to polymerize the PEO precursors in each layer, enhancing both intralayer cross-linking and interlayer cobonding Notably, the first PEDOT layer did not contain DCPD to prevent premature polymerization of PEO precursors, which could hinder final cross-linking with the SPE layer During the heat treatment, the initiators from the second and third layers are expected to catalyze the polymerization of PEO precursors in the first layer, thereby improving adhesion among all three layers Afterward, the actuator was washed in methanol to eliminate any unreacted oxidant and weakly polymerized PEDOT, with the washing solution's color varying based on the oxidant solution's thickness A thicker oxidant solution results in a denser PEDOT layer, but also leaves behind short dimers and trimers, which are removed during washing, leading to a blue coloration in the methanol.
33 of short dimers, trimers and such are reduced significantly leading to a more transparency of methanol solution
During the washing step in methanol for trilayer structures of PEDOT:NBR/PEO:PEDOT, different rotational coating speeds were tested: a) 1000:2000:1000, b) 1500:2000:1500, c) 2500:2000:2500, and d) 3000:2000:3000 The acceleration was maintained at 1000 rpm/s, with a coating duration of 30 seconds Observations indicated that faster spinning resulted in thinner layers of the oxidant solution, leading to less coloration during washing, which suggests a lower degree of polymerization of EDOT.
In summary, the parameters selected for the fabrication of trilayer actuators via LbL synthesis aimed to create a thick, highly conductive, and electroactive electrode These parameters include a 10 wt% mPEG content, an oxidant solution applied at a coating speed of 1500 rpm with an acceleration of 1000 rpm/s for 30 seconds, and EDOT VPP conducted at 40°C for 50 minutes, resulting in a 2.2 µm thick PEDOT layer on silicon Additionally, a rotational speed of 2000 rpm, with an acceleration of 1000 rpm/s for 30 seconds, was utilized to fabricate the SPE layer, achieving a thickness of 7 µm for the NBR/PEO layer.
PEDOT-based trilayer patterning
Previous research has explored various methods to shape CP-based trilayer membranes into specific beam configurations at the micro and nanoscale One approach involved electropolymerizing CP materials onto patterned metal electrodes, such as gold or platinum, to create bilayer or trilayer structures However, the poor adhesion between the gold layer and the CP, along with the SPE layer, significantly reduced the actuator's lifespan Additionally, the presence of the gold layer increased the overall Young’s modulus of the actuator, leading to a decrease in strain.
Reactive-ion etching (RIE) has traditionally been used for precision actuator fabrication on CP-based membranes, but it requires a lengthy and expensive photolithography process involving a sacrificial layer and photoresist A novel patterning technique utilizing syringe-based or microcontact printing of oxidant solutions, followed by laser ablation, has emerged, offering versatile deposition of CP layers suitable for mass production However, the resolution limitations of printing devices can affect actuator precision Additionally, ultrafast laser micromachining has proven effective for patterning PPy-based actuators, while electrical discharge machining, though effective, tends to be slower.
In our study, we focus on contactless material processing through laser ablation micromachining, which offers a mask-less, top-down, and flexible machining technique This method allows for the micro-patterning of trilayer structures into microbeams using laser cutting, utilizing a THG crystal to achieve a wavelength of 343 nm.
The 300 femtosecond diode-pumped (DPSS) lasers operate at 1030 nm with a pulse length of 400 fs and a spot size of 10 µm, achieving a maximum power of 2 W and a frequency rate of 200 kHz The sample stage measures up to 300 mm x 300 mm with linear accuracy of +/- 0.5 µm and repeatability of +/- 0.2 µm With patterning conditions set at 10% power, a cutting speed of 10 mm/s, and 30 cut passes, femtosecond laser patterning delivers high precision, minimal heat-affected zones, and short patterning times compared to traditional methods This study explores beams of varying geometries, with lengths and widths between 4 mm to 7 mm and 0.8 mm to 1 mm, respectively, maintaining a length-to-thickness ratio of 200 to 350, indicative of thin beams To facilitate manual handling and experimentation, a rectangular contact pad (3 mm x 2 mm) is integrated at one end of the beam, providing a large clamping area for electrical contact and voltage supply.
Figure 16a illustrates a trilayer coated wafer post-drying, while Figure 16b presents a top view of a cut-out trilayer actuator Following the cutting process, microbeams were immersed in the ionic liquid EMITFSI for approximately one week until they reached the swelling saturation point, facilitating the incorporation of ions essential for the redox process, as depicted in Figure 16c SEM images, akin to those in Figure 16c, confirm that the beam does not experience any increase in length or width after swelling.
The dried wafer, as shown in Fig 16a, features a white trace in the top right corner where material has been peeled off for characterization using EDX and SEM Following the laser patterning process, the trilayer actuator is produced with dimensions of 5 mm in length and 1 mm in width, as depicted in Fig 16b Additionally, Fig 16c presents SEM images of the trilayer after it has been swelled in EMITFSI.
2.6.1 Fabrication of samples for the characterization process
The previous section outlined the LbL fabrication process for trilayer actuators To evaluate the electrochemomechanical properties of these devices, various samples were created for testing, as detailed in Table 2.
Table 2 Summary the sample configuration for different characterizations
Top PEDOT on NBR/PEO
A single sIPN of NBR/PEO (50/50) was fabricated for the ionic conductivity, and Young’s modulus measurement
This study explores the electronic and ionic conductivities of PEDOT electrodes, focusing on their asymmetric properties Two bilayers were created: a top bilayer consisting of NBR/PEO coated with PEDOT/PEO and a bottom bilayer of PEDOT coated with NBR/PEO The thickness of the NBR/PEO layer was increased to facilitate a clearer distinction between the ionic conductivities of PEDOT and NBR/PEO.
The fabricated trilayer structure of PEDOT-PEO/NBR-PEDOT was evaluated for key properties, including volumetric capacitance, potential short circuits between PEDOT electrodes, Young's moduli, damping ratio, and linear strain.
During the fabrication of a single PEDOT layer and the bilayer, the initiator PCDP was incorporated into each layer to facilitate the polymerization of PEO In contrast, for the trilayer configuration, the initiator was only included in the membrane and the second PEDOT layer, which may result in variations in their properties.
Analysis of the texture of the trilayer structure
An EDX analysis was conducted on a trilayer sample to verify its three-layer structure, as illustrated in Fig 17a The analysis revealed sulfur, indicated by the green color, exclusively present in the PEDOT layer, while the red color represented carbon, which is predominant in the NBR region.
The sIPN morphology of the trilayer is described in Fig 17b, where the PEO is covalently crosslinked through all the trilayer improving the adhesion between the electrodes and the
The SPE layer features PEDOT chains synthesized through vapor phase polymerization (VPP) on both the top and bottom electrodes, ensuring that they do not crosslink with each other or with PEO Meanwhile, the linear NBR chains are exclusively present in the middle layer.
Fig 17 a) The EDX of the trilayer structure, b) The anticipated morphology of the trilayer structure PEDOT:NBR/PEO:PEDOT
Table 2 Thicknesses of each layer before and after swelling in EMITFSI
Trilayer Thickness before swelling (àm) Thickness after swelling (àm)
Table 2 presents the thickness measurements of each layer before and after immersion in EMITFSI, based on a set of five trilayer samples prepared using a consistent fabrication process The uniformity of the thickness values in the table highlights the reproducibility of this fabrication method.
The article presents a scanning electron microscopy (SEM) image illustrating the trilayer cross section in a dry state It includes a profilometer analysis of the surface roughness for the top surface of the top electrode layer, as well as the top surface of the solid polymer electrolyte (SPE) in a bilayer PEDOT/SPE configuration Additionally, it features the top surface of the bottom electrode layer and the bottom surface of the bottom electrode layer.
After swelling in EMITFSI, the total thickness of the trilayer actuator can be verified by SEM, but direct measurement of each layer's thickness is hindered by the ionic liquid covering the cross-section To address this, a single SPE layer was fabricated under identical parameters and swollen in EMITFSI for one week, resulting in a 43% increase in thickness This increase allows for the estimation of the thicknesses of the two conducting polymer electrode layers within the trilayer structure The measurement error was determined from five different positions along the sample Notably, the swollen percentage of the SPE layer aligns well with findings reported by Maziz for the same type of SPE.
Despite sharing identical fabrication parameters, the bottom and top PEDOT layers exhibited notable differences in roughness measurements taken while dry Profilometer measurements indicated an average roughness of 0.5 nm for the top surface of the bilayer PEDOT/SPE and approximately 0.1 nm for the single bottom PEDOT layer, both assessed on a silicon wafer These findings highlight the distinct surface characteristics of the top electrode as illustrated in the accompanying figures.
The actuator's average measurement of 0.7 µm on the silicon wafer is comparable to the values obtained on PEDOT/SPE After the lift-off process, the average roughness of the bottom electrode's surface, measured once flipped onto a new silicon wafer, is approximately 0.06 µm (refer to Appendix A.2.2 for the measurement method on various samples) Notably, the average roughness of the top PEDOT electrode surface is ten times greater than that of the bottom electrode.
The LbL fabrication method accounts for the observed differences in surface roughness The spin-coated SPE layer on the PEDOT base undergoes pre-heat treatment, leading to slight contraction and regular surface folding, influenced by the SPE layer's thickness and heat treatment method The top PEDOT layer mirrors the SPE surface, resulting in greater average roughness for the top PEDOT electrode exposed to air compared to that in contact with the silicon substrate This contraction of the SPE layer becomes apparent when microbeams are lifted and swelled in EMITFSI, revealing a slight curvature that extends the bottom PEDOT layer while contracting the top layer, indicating the presence of weak internal stress within the microbeam.
The roughness difference in PEDOT may be attributed to its bottom-up forming mechanism during the vapor-phase polymerization (VPP) of EDOT Research by Brooke et al and Evans et al indicates that during this process, the oxidant solution rises to the surface, facilitating the formation of new PEDOT layers Consequently, this mechanism leads to a denser PEDOT distribution at the bottom surface in contact with silicon, while the top surface exhibits lower density.
Conclusion
This chapter details the fabrication of a trilayer structure composed of semi-NBR/PEO sandwiched between two PEDOT electrodes, achieved through layer-by-layer (LbL) stacking The incorporation of PEO in all three layers enhances ionic conductivity and mechanical adhesion, allowing for a thin and controllable thickness Key improvements include optimizing PEDOT electrodes, which extended the VPP time from 30 to 50 minutes, and utilizing a Sawatec HP-200 vacuum hotplate for better temperature and vacuum control The entire fabrication process was conducted in a clean room environment, ensuring higher reproducibility; ten batches of trilayer samples were produced, with successful bending tests on actuators from four batches Additionally, the PEO percentage in the PEDOT layer was adjusted to 10%.
The study indicates that a 40% composition of PEDOT significantly enhances electronic conductivity, achieving approximately 200 S/cm, while maintaining a high volumetric charge density between 2.3 x 10^7 C/m^3 and 1.0 x 10^8 C/m^3 Additionally, we observed a slight asymmetry in surface roughness between the top and bottom PEDOT layers, which may impact the electrical properties of the PEDOT electrodes and subsequently affect the mechanical characteristics, such as displacement and force, of the actuator.
The VPP process discussed in this thesis utilized a conventional oxidant solution with 55% Fe(TOs)3 in butanol However, this high concentration leads to a highly acidic and reactive solution, causing uncontrollable polymerization and structural defects in the deposited film To enhance the electrochemical properties of the PEDOT electrode, future research should focus on optimizing the oxidant concentration and incorporating base inhibitors like pyridine to better regulate the reaction rate.
Future advancements may lead to a broader selection of trilayers, as suggested by the research of K Rotlaid and F Ribeiro K Rotlaid's work involves substituting PEDOT electrodes with PEDOT:PSS incorporated with PEO, enabling the creation of thicker actuators (approximately 30μm) that generate greater force, although this research has yet to be published.
F Ribeiro, the possibility of incorporating the ionic liquid into the SPE at the time of manufacture is a promising innovation (this work is still unpublished) In fact, the swelling phase of our actuators modifies the total thickness by 160%, which potentially can have consequences on the transfer of electrodes in a complete integration step – where the metal electrodes in contacting with the PEDOT layers will be expanded leading to the facture of the electrode after the actuator is swollen The manufacturing characteristics of these future materials will have to be taken into account to have the necessary elements for a good interpretation of the modeling and their experimental mode of operation
To accurately model the three-layer structure, it is essential to experimentally assess the electrical and mechanical properties of each layer This characterization process will be detailed in the following chapter.
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Chapter 3: Electrochemomechanical characterization of the trilayer structure
3.1 Introduction 46 3.2 Electro-chemical properties 48 3.2.1 Ionic conductivity of the SPE and PEDOT layers 49 3.2.2 Electrical conductivity of the PEDOT electrodes 52 3.2.3 Volumetric capacitance of the PEDOT electrodes 55 3.2.4 Possible short circuit between two PEDOT layers 58 3.3 Mechanical properties 59 3.3.1 Young’s moduli of the SPE layer and of the trilayer actuator 59 3.3.2 Damping ratio 62 3.3.3 Blocking force characterization 63 3.4 Empirical strain-to-charge ratio 64 3.4.1 Strain to charge ratio 64 3.4.2 Linear strain 67 3.5 Conclusion 69
This chapter focuses on the characterization of trilayer structures fabricated using a layer-by-layer method and vapor phase polymerization, essential for optimizing actuator performance Key properties such as geometric, electrochemical, mechanical, and electrochemomechanical attributes are analyzed, including the thickness of conducting polymer layers and separators before and after ionic liquid swelling Measurements of stiffness, ionic and electronic resistances, capacitance, and active deflection under applied voltage are conducted Important parameters for modeling, such as Young’s modulus, volumetric capacitance, ionic and electronic conductivities, and strain to charge ratio—often varying with oxidation—are extracted These characteristics are crucial for simulating the responses of polymer actuators and sensors in subsequent chapters.
The characteristics of the actuators fabricated as described in Chapter 2 are analyzed in detail, beginning with a typical device response As illustrated in Fig 1a, the actuator's output current rapidly decreases as the PEDOT electrodes near a fully charged state The equivalent displacement, shown in Fig 1b, occurs quickly within the first few seconds before stabilizing Fig 1c highlights the relationship between the applied voltage polarity and the displacement direction of the trilayer, where a positive potential causes one PEDOT electrode to contract while the other expands, achieving a bending amplitude of 3 mm This output current and displacement data will be instrumental in extracting information regarding charge and strain.
Fig 1 Actuator behavior under a step voltage a) The applied step voltage and the response current, b) Bending response of the actuator, c) A picture of the bending actuator (length x width x thickness =
4 mm x 1 mm x 0.017 mm), the + and – indicate the polarity of the applied voltage
Several factors influence the strain, strain rate, and blocking force of actuators, including the ionic conductivity of conductive polymer (CP) electrodes and the solid polymer electrolyte (SPE) layer, which can limit both strain rate and bending speed The electronic conductivity of CPs impacts the curvature of the beam during actuation and varies with oxidation state Additionally, the volumetric capacitance of CP material, which indicates its ability to accommodate ions under voltage excitation, changes with scanning rate and applied voltage magnitude The thin trilayer structure and the growth of PEDOT from opposing surfaces may lead to potential short circuits between CP electrodes, affecting efficiency, sensor signal decay rate, and self-heating Lastly, the Young’s modulus of CP material is crucial as it determines the actuator's blocking force at a given strain, and this modulus is influenced by the oxidation state.
This chapter examines the ultrathin PEDOT:NBR/PEO:PEDOT structure by considering various influencing factors It focuses on the electrochemical properties of the actuators, specifically the ionic conductivity of the PEDOT electrodes and NBR/PEO, as well as the electronic conductivity and volumetric capacitance of the PEDOT electrodes Additionally, the potential short circuit between the PEDOT electrodes will be analyzed The relationship between electrochemical and mechanical properties, represented by an empirical strain-to-charge ratio, will be measured Furthermore, the study will explore the Young’s moduli of the solid polymer electrolyte (SPE) and PEDOT electrodes as functions of oxidation state, along with the damping ratio and linear strain of the actuator These parameters are crucial for understanding actuator performance and for developing a predictive model of actuator behavior.
Electro-chemical properties
Before beginning the full characterization, different sample configurations were fabricated to adapt for different measurements (refer to section 2.6.1 for list of fabricated samples)
Trilayers of varying thicknesses were fabricated and tested to identify a dimension range that achieves significant bending actuation for effective characterization Ten trilayer membranes were created on separate silicon wafers, with PEDOT electrode thicknesses adjusted from 0.8 µm to 2.3 µm to examine the impact on strain output Bending actuation experiments conducted on four samples with PEDOT thicknesses ranging from 1.2 µm to 2.3 µm demonstrated bending under voltage excitation, with the greatest displacement observed in a sample with a composition of PEDOT:NBR/PEO:PEDOT at 2.2 µm:7 µm:2.2 µm (pre-swelling dimensions) Following swelling in ionic liquid, the thicknesses of the PEDOT electrodes and the NBR/PEO layer increased to approximately 3.5 µm and 10 µm, respectively.
The selected thickness ratio of PEDOT:NBR/PEO:PEDOT, specifically 3.5 µm:10 µm:3.5 µm after swelling in ionic liquid, will be further analyzed to assess its volumetric capacitance, potential short circuits between PEDOT electrodes, Young’s moduli, damping ratio, linear strain, and bending strain.
3.2.1 Ionic conductivity of the SPE and PEDOT layers
Ionic conductivity significantly impacts current limitations and charging speed, influencing the strain rate and bending speed of trilayer actuators The ionic conductivity of PEDOT was assessed using Electrochemical Impedance Spectroscopy (EIS), following established procedures A Solartron 1287A Potentiostat/Galvanostat, paired with a Solartron 1260A frequency response analyzer, facilitated the collection of frequency responses The measurement involved a 4-point setup across the ionic liquid and the actuator membrane, where a current was applied between the working electrode (W.E.) and counter electrode (C.E.), while local potential drops were recorded between two reference electrodes (R.E.s) The W.E and C.E consisted of glassy carbon, and the R.E.s utilized classic Ag/AgCl wires filled with 4M NaCl.
The article discusses the EIS measurement conducted with a 4-point setup, highlighting the configuration of the NBR/PEO layer as a bottom bilayer and top bilayer Additionally, it includes a detailed image of a specific area within the red rectangular A-A, along with its equivalent circuit that illustrates the current direction at both low and high operating frequencies.
It has been shown that there is an asymmetry between the top and the bottom PEDOT electrodes in a trilayer actuator fabricated by layer-by-layer method (chap 2, section 2.7)
The roughness of the top PEDOT electrode is significantly higher—ten times greater—than that of the bottom PEDOT electrode, which may result in asymmetric properties between the two Consequently, it is essential to characterize each electrode separately To investigate the variation in PEDOT ionic conductivity, a top bilayer (PEDOT polymerized on NBR/PEO) and a bottom bilayer (PEDOT polymerized on a silicon wafer with an NBR/PEO layer) were analyzed, as illustrated in Fig 2b Prior to measurement, both the NBR/PEO membrane and the bilayer samples were immersed in EMITFSI for one week to ensure complete ion uptake.
The resistance of each layer is influenced by its thickness, necessitating the separation of PEDOT ionic resistance from other resistance sources like NBR/PEO and ionic liquid resistance This requires careful consideration of the thickness of the specimens, which includes both the NBR/PEO and PEDOT layers.
1) was increased during the fabrication process It is worth of noticing that the thickness of swollen PEDOT layers were not measured directly but it was determined by SEM in its dry state and then multiplied by 1.66, which is the swelling factor in EMITFSI deriving from chapter 2 (section 2.7)
Fig 3 Impedance values obtained from EIS measurements of a pure EMImTFSI, a top bilayer of NBR/PEO-PEDOT, a bottom bilayer of PEDOT-NBR/PEO, and a single NBR/PEO layer
The experimental results from the Electrochemical Impedance Spectroscopy (EIS) are illustrated in Fig 3, revealing that the resistance between the cannulas of the ionic liquid alone is 114 Ohms When a NBR/PEO matrix is introduced without PEDOT, the resistance increases by an additional 8 Ohms The inclusion of PEDOT layers contributes an extra 2 to 3 Ohms, particularly noticeable in the low-frequency impedance However, at high frequencies, the influence of the conducting PEDOT layer diminishes as its parallel electronic conductivity effectively bypasses the ionic resistance This indicates that the overall impedance is primarily governed by the ionic resistances of both the sample and the electrolyte The ionic conductivity (δ) of the PEDOT and NBR/PEO layers is subsequently determined.
The resistance (R) is calculated using the formula R × A, where h represents the thickness of the PEDOT or NBR/PEO layer, as detailed in Table 1 The sample surface area that comes into contact with the electrolyte is 0.78 cm².
R, is calculated by a subtraction between NBR/PEO containing cell resistance and pure
EMImTFSI resistance (Fig 3) to estimate the resistance of the NBR/PEO alone To find the ionic resistances of the PEDOT layers, the difference in resistance between low EIS frequency
(1 mHz – representing the resistance of all layers) and high EIS frequency (resistance at 1 kHz – representing the resistance of NBR/PEO plus EMImTFSI) is calculated, for the same bilayer
Table 1 Thickness in swollen state and ionic conductivity of the NBR/PEO alone, top bilayer, and bottom bilayer
Ionic liquid - - 0.9 (provided by supplier)
Top PEDOT layer of the bilayer
Bottom PEDOT layer of the bilayer
Table 1 presents the resistance of the NBR/PEO layer, calculated by subtracting the impedance value of the “NBR/PEO matrix” from that of the “pure EMITFSI.” The resistance of the PEDOT layers in the top and bottom bilayer is similarly derived from the impedance differences at low and high frequencies, as illustrated in Fig 4 The ionic conductivity values for both layers are comparable to, yet slightly lower than, the 0.04±0.002 S/m reported by Dobashi et al [8] using aqueous LiTFSI as the electrolyte Notably, Table 1 indicates a minor variation in ionic conductivity between the top and bottom PEDOT electrodes, which may reflect the impact of asymmetry This discrepancy can be attributed to three primary factors: measurement uncertainty in thickness, slight fluctuations in impedance at varying frequencies (represented by error bars), and inherent asymmetry in the PEDOT electrodes The ionic conductivity of the NBR/PEO membrane is recorded at 0.038 S/m, which is lower than the values of 0.1 S/m reported by Vidal [12] and 0.13 S/m by Maziz [13], both of which were measured for thicker full IPN membranes (thickness > 200 μm) where NBR is crosslinked, contrasting with the thinner sIPN utilized in this study.
3.2.2 Electrical conductivity of the PEDOT electrodes
Researchers have shown that the electronic conductivity of PEDOT is influenced by the applied potential Feldman et al demonstrated that a 13.9 µm thick PPy film in a 0.1 M Et4NClO4/CH3CN solution maintains a relatively constant conductivity of approximately 0.3 S/cm when the potential is reduced from 0.4 V to 0 V However, as the potential decreases further to -0.6 V, the behavior of the conductivity may change.
V, the conductivity enormously drops to 10 -6 S/cm Warren et al [3] and Farajollahi et al [4,
A study observed that the conductivity of a 25 µm thick polypyrrole (PPy) film significantly increases from 0.64 S/cm to 270 S/cm as the applied potential rises from -0.8 V to 0.4 V This decrease in conductivity at lower voltages can be attributed to the reduction of the PPy electrode, which leads to a decreased doping level, ultimately lowering the concentration and mobility of charge carriers.
53 conductivity In my work, the electrical conductivities of the thin sIPN PEDOT/PEO electrodes will be characterized to explore their dependences on the potential applied
The three-electrode system used for cyclic voltammetry analysis, as depicted in Fig 4, consists of a platinum foil counter electrode with a surface area larger than that of the trilayer actuator working electrode A standard Ag/AgCl reference electrode, filled with a 4M NaCl solution, is employed to ensure accurate potential measurements The potentiostat, connected to both trilayer electrodes via platinum clamps, is fully immersed in EMITFSI, facilitating the reduction or oxidation of specimens at a specific potential relative to the Ag/AgCl reference.
To measure the electrical conductivity as a function of oxidation state, a potential (versus Ag/AgCl reference electrode) was applied to both sides of a trilayer structure in solution (Fig
5) The dimensions of the specimen are length (L) х width (b) х total thickness (h) = 10 mm х
In the swollen state, the thickness of the PEDOT layers measures 3.5 µm, while the NBR/PEO thickness is 10 µm To accommodate the 4-line measurement setup depicted in Fig 5, the length and width of the sample were adjusted accordingly.
Fig 4 Three-electrode system for setting the oxidation state of the PEDOT and for cyclic voltammetry analysis
To achieve the desired potential for the sample as illustrated in Fig 4, a voltage is applied and maintained for 10 minutes until the response current decreases to zero Following this period, the sample's potential is measured to verify its oxidation or reduction state Once the target potential is reached, the sample is removed from the ionic liquid and placed down for further analysis.
Mechanical properties
In this section, the Young’s moduli of the SPE layer alone and of the trilayer actuator as a function of the oxidation state were measured in a swollen state
3.3.1 Young’s moduli of the SPE layer and of the trilayer actuator
A specimen of the SPE layer with the following dimensions of length (L) x width (b) x thickness
The NBR/PEO thin film, measuring 10 mm x 6 mm x 0.07 mm, was evaluated using a Bose Electroforce 3000 dynamic mechanical analyzer The film was secured at both ends, and a longitudinal displacement of 0.1 mm/s was applied to the sample while the force was recorded.
The relationship between the force F and the displacement ΔL gives a Young’s modulus E SPE for the SPE material of 𝐸 𝑆𝑃𝐸 = 𝐹
𝑏ℎ 𝑠 = 329 ± 50 𝑘𝑃𝑎 This value is consistent with the value reported by Festin [22] and Woehling [23] for the same type of material
Bahrami-Samani and Spinks have demonstrated that the shear modulus of polypyrrole thin films varies with different applied potentials Additionally, Farajollahi highlighted the relationship between the Young's modulus of a penetrated PEDOT layer and its conditions.
We conducted measurements to assess how the Young's modulus of trilayer actuators varies with applied voltage The trilayer actuator, measuring 10 mm in length, 6 mm in width, and 0.017 mm in thickness, was tested using a Bose Electroforce system.
3000 dynamic mechanical analyzer and a potentiostat
Fig 10 Young’s moduli of the NBR-PEO layer alone - and of a trilayer structure as a function of oxidation state - were measured using a Bose Electroforce 3000 dynamic mechanical analyzer
The experimental setup illustrated in Fig 10 is designed to measure the Young’s moduli of a trilayer structure under voltage excitation while in a swollen state The trilayer actuator is secured in a Bose Electroforce 3000 dynamic mechanical analyzer, with a platinum clamp on one end and a plastic clamp on the other, both immersed in EMImTFSI ionic liquid Various positive and negative potentials (± 1.5, ± 1, ± 0.5 V) are applied to oxidize or reduce the PEDOT electrodes simultaneously After voltage application, the open circuit potential is measured to ensure the trilayer has reached the target voltage before subjecting it to controlled displacement/load The Young’s modulus is calculated from the slope of the force/displacement curve and the specimen's dimensions This setup allows for immediate Young’s modulus measurement after achieving the oxidized or reduced state, enabling assessments at both low (-1.5 V) and high (1.5 V) potentials without concerns regarding rapid voltage decay.
The Young’s moduli of the trilayer actuators are measured and the Young’s moduli of the PEDOT layer is deduced in Fig 7
Fig 11 Young’s modulus of the trilayer actuator and the PEDOT layer as a function of applied voltage
From the SPE Young’s modulus and the trilayer Young’s modulus, the Young’s modulus of the PEDOT electrodes can be derived from the following equation [26] 𝐸 𝑃 = (𝐸 𝑡 ℎ 𝑡 −𝐸 𝑆 ℎ 𝑆 )
The total thickness of the trilayer, along with the thicknesses of the NBR/PEO and PEDOT electrode, is represented in Fig 11 Notably, the Young's modulus of both the trilayer and the PEDOT layer exhibits a slight decrease of 14%.
10 MPa to 8.6 MPa and from 24.0 MPa to 20.5 MPa, respectively, when they are switched from reduced state (- 1.5 V) to oxidized state (1.5 V)
The reduction in modulus with increasing potential can be attributed to the insertion and extraction of ions from PEDOT electrodes A negative potential causes the polymer to reduce, allowing EMI+ cations to insert into the PEDOT, which helps balance the charge of the immobile TFSI- ions in the matrix This process may enhance the coupling between PEDOT chains, resulting in an increase in Young’s modulus at negative potentials Conversely, a positive potential appears to increase the separation between the PEDOT chains, thereby reducing the force between them and leading to a decrease in Young’s modulus These findings align with the results reported by Zheng and Shoa, and the percentage reduction is consistent with the values noted by Farajollahi.
In the mechanical model, the natural frequency and damping coefficient are crucial for assessing responses at frequencies influenced by inertia, determined through a vibration experiment This experiment involved an actuator fixed at one end and free at the other, deflected by a 3-axis motorized stage equipped with an FT-RS1002 Microrobotic System and an FT-S100 microforce sensing probe, enabling precise measurements with a range of ±100 àN and a resolution of 5 nN The actuator was quickly released to observe free vibration, with oscillations measured directly using a Keyence laser sensor (Keyence LK-G32), which has a measurement range of ±4.5 mm and a resolution of 0.05 μm.
Fig 12 Experimental setup to measure the damping ratio of the trilayer actuator and the blocking force
Damped oscillation is shown in Fig 13
Fig 13 Beam vibrating as function of time
This response suggests that the actuator is well represented as a second-order system [29] After subtracting the damping coefficient was calculated using the formula below:
The overshoot amplitude, denoted as t, represents the distance from the midpoint of the sinusoidal component to its highest peak, while y indicates the total displacement Additionally, there may be a secondary, prolonged time response due to a longer rise time to reach a steady state, which was not accounted for in the initial model.
The force generated at the tips of the actuator of length x width x thickness: 6 mm x 1 mm x
0.017 mm at various magnitudes and frequencies of a square wave applied voltage were conducted using the setup described in Fig 13, where the clamping is connected to a source of voltage
Fig 14 Force generated as a function of the magnitude and the frequency of the applied voltage from a trilayer actuator of length x width x thickness: 6 mm x 1 mm x 0.017 mm
As the applied voltage increases, the force generated at the tip of the beam exhibits an upward trend, while this force diminishes with rising frequency The blocking force can be calculated using the equation: \( F = E P \alpha \rho \).
The charge density, represented by the equation 2𝐿𝑏ℎ 𝑃, assumes all parameters are constant except for the total charge Q, which varies with time A higher frequency results in a lower charge Q, consequently diminishing the blocking force F At a frequency of 10 Hz, the force may reach zero or yield inaccurate readings due to background noise In this experiment, the maximum force recorded at the tip of the trilayer was 11 μN at a frequency of 0.1 Hz and a voltage of 3.3 Vpp Notably, beyond 2.75 Vpp, the generated force approaches saturation, indicating that both PEDOT electrode layers have reached full oxidation and reduction.
Empirical strain-to-charge ratio
The coupling effect between strain and charge, initially proposed by Baughman et al., describes how the length of polyacetylene thin films changes with ion insertion Della Santa and Madden have suggested a strain-to-charge ratio of approximately 10^-10 m³/C for polypyrrole-based CP actuators In our study, we focused on determining the strain-to-charge ratio for a PEDOT-based actuator We conducted experiments by applying a triangular wave voltage to the trilayer actuator, which allowed it to bend at a slower speed while we recorded the output current and the bending response.
From the actuator bending data, the strain difference produced by one PEDOT electrode (𝜀 𝑎𝑐𝑡𝑢𝑎𝑙 ) can be derived via the following equation 𝜀 𝑎𝑐𝑡𝑢𝑎𝑙 (𝑡)% = ℎ 𝑡
The radius of the actuator in its neutral state (r0) and stimulated state (rt) is crucial for understanding its performance, along with the thickness of the trilayer actuator (ht) An example illustrating the determination of r0 and rt through bending measurements is depicted in Fig 15.
The actuator is depicted in two states: a) its neutral state with a bending radius of r0 = 12 mm, and b) its excited state where the bending radius is reduced to rt = 2.8 mm The background features an orange square measuring 1 x 1 mm for reference.
Note that the strain difference equation above is used instead of the equation ɛ = 2ℎ 𝑡 𝑤
The equation L2 + w2 applies specifically when the actuator starts in a straight position and bends symmetrically under voltage However, in our scenario, where the actuator is initially curved, the resulting strain difference may be inaccurately estimated, either over or under, depending on whether the trilayer's initial curvature aligns with or opposes the actuator's curvature.
From this bending strain, the active strain of the PEDOT electrode, which is the linear strain of a single PEDOT layer, is derived as following equation [33]:
𝐸 𝑃 An error bar of 5 % obtained from the bending strain measurements has been added
The volumetric charge density (ΔQ(t)/V PEDOT), which represents the total ion insertion or extraction per unit volume of the PEDOT electrode, is determined by integrating the current over time.
Figure 16 Current and charge response to a triangular input voltage
Fig 17 Charge density and strain of the trilayer actuator as a function of time under a triangular wave voltage excitation
The strain-to-charge ratio is finally obtained by the ratio: 𝛼 = 𝜀 𝑎𝑐𝑡𝑖𝑣𝑒 (𝑡)𝑉
The active strain to charge density ratio remains relatively constant at 3.6 ± 0.5 x 10^-10 (m³/C), as illustrated in Fig 17 This value aligns with similar measurements observed in polypyrrole and PEDOT, as detailed in Table 2 This strain to charge ratio will be incorporated into the model presented in Chapter 4.
Table 2 Summary strain-to-charge ratio for different materials and actuator configuration
Material Structure Dopant Solution Strain-to-charge ratio value (m 3 C -1 ) x 10 -10 PPy linear actuator DBS -
PEDOT a single layer a trilayer TFSI -
DBS = Dodecylbenzene sulfonate, TBA: tetrabutylammonium, PF6: hexafluorophosphate
The relationship between linear strain and strain difference in the actuator is defined in equation (5) This section explores the linear strain of a trilayer actuator by applying a step voltage to a trilayer specimen to validate this relationship The trilayer actuator, measuring 10 mm in length, 6 mm in width, and 0.017 mm in thickness (the length refers to the beam section between clamps), was subjected to a prestress of 1.5 gf using the Bose Electroforce system.
In the dynamic mechanical analyzer, a constant load of 0.15 MPa is applied to one end of the beam to maintain its straightness in an ionic liquid, as illustrated in Fig 10 A step voltage of 2 Vpp versus Ag/AgCl at a frequency of 0.001 Hz is then applied to both electrodes of the trilayer actuator The resulting uniaxial deformation of the beam is recorded, with L2 and L1 representing the beam's length in its expanded and contracted states, respectively This setup provides a rough estimation of the linear strain of the trilayer actuator, approximately 0.1 mm.
The relationship between linear strain and strain difference reveals that 10mm corresponds to 1%, which exceeds the 0.56% strain difference observed between two PEDOT electrodes in the previous section This aligns with the equation ε_active(t) = 1.61 × ε_actual(t), confirming that 1% is approximately equal to 1.61 times 0.56%.
Fig 18 Uniaxial deformation of the trilayer actuator under a step voltage