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Recent Advances in Thiophene Based Molecular Actuators Anquetil, Patrick A.; Yu, Hsiao-Hua; Madden, John D.; Swager, Timothy M.; Hunter, Ian W. 2003

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Recent Advances in Thiophene Based Molecular Actuators  Patrick A. Anquetil∗a, Hsiao-hua Yub, John D. Madden c, Timothy M. Swager b and Ian W. Hunter a aBioInstrumentation Laboratory, Dept. of Mechanical Engineering; Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA, 02139, USA  bDept. of Chemistry; Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA, 02139, USA c  Dept. of Electrical and Computer Engineering, The University of British Columbia, 2356 Main Mall, Vancouver, BC Canada V6T 1Z4   ABSTRACT  A new class of molecular actuators where bulk actuation mechanisms such as ion intercalation are enhanced by controllable single molecule conformational rearrangements offers great promise to exhibit large active strains at moderate stresses.  Initial activation of poly(quarterthiophene)  based molecular muscles, for example, show active strains in the order of 20%.   Molecular rearrangements in these conjugated polymers are believed to be driven by the formation of π-dimers (e.g. the tendency of π orbitals to align due to Pauli’s exclusion principle) upon oxidation of the material creating thermodynamically stable molecular aggregates.  Such thiophene based polymers, however, suffer from being brittle and difficult to handle.  Polymer composites of the active polymer with a sulfated polymeric anion were therefore created and studied to increase the mechanical robustness of the films. This additional polyelectrolyte is a Sulfated Poly-β-Hydroxy Ether (S-PHE) designed to form a supporting elastic matrix for the new contractile compounds.  Co-deposition of the polyanion with the conducting polymer material provides an elastic mechanical support to the relatively stiff conjugated polymer molecules, thus reducing film brittleness.  The active properties of such poly(quarterthiophene)/S-PHE polymer actuator composites based on intrinsic molecular contractile units are presented and discussed.  Keywords: Molecular actuator, conducting polymer, quarterthiophene, π−π stacking, large strain, molecular design, single molecule, sulfated polyanion, molecular design.  1. INTRODUCTION Recent progress in developing conducting polymer actuators has shown tremendous progress towards incorporation of these contractile materials into products1,2,3,4.  These studies have revealed that polymer actuators exhibit very high active stresses (40 MPa peak) at moderate contractile strains (~2  to 5 %) and speeds (3  to 10 %/s), with high power to mass ratios (150 W/kg), cycle life exceeding the million cycles while being activated at low electrical voltages (~1 to 5_V)5.  Such polymeric contractile materials have been developed to a point where they could be used in medical devices, small robots as well as innovative consumables and toys.  However despite all these successes no material exists to date that matches or exceeds mammalian skeletal muscle in all its figures of merit6.  Such figures of merit include active strain, active stress, active strain rate, power to mass and efficiency.  Actuation in traditional conducting polymer actuators such as polypyrroles or polyanilines is driven by a mechanism of ion intercalation, leading to moderate strains7,8. Our group (like Marsella et al.9) has taken a new approach that moves away from a materials survey approach and towards a materials designer strategy where novel materials are created by incorporating property-designed molecular building blocks.  Such molecular building blocks may include shape changing, load bearing, passively deformable or hinge-like molecular elements combined with precise control of the material morphology at the nanometer scale.  Our vision is that unprecedented actuating materials will be created by a bottom-up approach where specific molecular designs are incorporated into a material to achieve specific properties.   ∗  Correspondance: patanq@mit.edu, http://bioinstrumentation.mit.edu Smart Structures and Materials 2003: Electroactive Polymer Actuators and Devices (EAPAD), Yoseph Bar-Cohen, Editor, Proceedings of SPIE Vol. 5051 (2003) © 2003 SPIE · 0277-786X/03/$15.00 42 Downloaded from SPIE Digital Library on 30 May 2011 to Terms of Use:  http://spiedl.org/terms Within the framework of molecular actuation, we present herein chemically engineered units that are believed to change their molecular shape and produce force and displacement upon the application of an electrical stimulus.  These materials utilize a molecular dimerization mechanism known as π−π stacking as the molecular actuation driving force. We will start by briefly presenting this molecular actuating mechanism and show how it can be incorporated into molecular materials designs.  We will then show initial activation results of polymer films that were synthesized in our laboratory, including beam-bending and isometric actuation characterization.  2. ACTUATING MECHANISMS AT THE MOLECULAR LEVEL AND CANDIDATE MOLECULES 2.1. π−π stacking Typical conducting polymers make use of ion intercalation as the actuation driving force7.  We propose to use molecular driving forces to change the shape of the polymer backbone. Such forces can include the formation of hydrogen bonds, the twisting or planarization of a molecule as a function of backbone electron density and finally the formation of reversible chemical bonds. Units incorporating these reversible molecular conformational transitions are then built into polymer systems that are expected to lead to controllable displacement and work at the macroscopic level.  In this paper we present the use of the formation of π-dimers with aromatic units upon oxidation as molecular actuation driving force. This molecular mechanism referred as π-dimerization or π−π stacking makes use of wavefunction overlap in π- conjugated polymers which is attractive in the oxidized state (electron deficient) and repulsive in the reduced state (filled electronic levels).  The π-stacked structure is typical of oxidized thiophene oligomers and has been studied experimentally by means of X- ray diffraction, Scanning Tunneling Microscopy (STM), Electron Paramagnetic Resonance Spectroscopy (EPR),  as well as theoretical calculation10,11,12,13,14.  As shown in Figure 1, oxidation of two adjacent quarterthiophene molecules creates two radical cations with partially occupied Highest Occupied Molecular Orbitals (HOMO).  This state is observable by means of EPR spectroscopy as it exhibits an unpaired electron in the π-system of the oxidized thiophenes (EPR active). With further oxidation, both HOMO orbitals mix to produce a new doubly occupied molecular orbital at lower energy. The formation of this new molecular orbital provides stabilization of the π-dimer as the electronic energy of the charged quarterthiophene groups has been lowered.  Unlike the singly occupied molecular orbitals, the π-stacked structure is EPR silent due to the electron spin pairing. This process is reversible and described by,  ++•+• ⇔+ 22)(QTQTQT  ,      (1)  where QT describes a quarterthiophene molecule.  As stated above, evidence of a reversible transformation between π- stacked and un-stacked conformations as oxidation state is obtained using EPR spectroscopy.  Figure 2A shows an EPR signal recorded during a 100 mV/s swept cyclic potential (0V to 1.5V vs. Ag/Ag+) for the poly(quarterthiophene) polymer (poly(QT)).  Notice the hysteresis obtained during the cathodic sweep (from 1.5V back to 0V) to revert the signal from EPR silent to EPR active.  Such a large hysteresis indicates that more energy is required to switch poly(QT) from its oxidized state to its reduced state and vice versa, giving strong evidence that a more stable structure (possibly the π-dimer) has been formed as a result of the initial anodic potential sweep in which the quarterthiophene groups were oxidized.  In addition, similar hysteresis effects resulting from π-dimerization are observed from in-situ conductivity measurement. Interdigitated microelectrodes (Abtech Scientifica) allow measuring film conductivity in-situ as a function of oxidation state.  Films of poly(QT) were cycled at 10 mV/s at potentials between 0V and 1V vs. Ag/Ag+  and their conductivity measured against a bias of 40mV (Figure 2B). Notice in Figure 2B that switching the relative conductivity of poly(QT) from conducting to insulating requires ~0.5 V of reverse potential, indicating that the π-dimer aggregated state is harder to reduce and thus confirming its higher stability.  Note that such a behavior is unlikely to be attributed to  a   www.abtechsci.com Proc. of SPIE Vol. 5051     43 Downloaded from SPIE Digital Library on 30 May 2011 to Terms of Use:  http://spiedl.org/terms  2+ EPR Silent NEW HOMO EPR+ Silent Energy HOMO* HOMO  +•  +• EPR Active HOMO HOMO  +• HOMO HOMO EPR Active Increase in oxidative potential Neutral state Oxidized state: π dimer + Electron Paramagnetic Resonance (EPR) signature *  HOMO: Highest Occupied Molecular Orbital   Figure 1: Mechanism of π−π dimer formation in quarterthiophene. The corresponding EPR spectroscopic signature is schematically represented below the 3D space filling molecular model.  Each arrow represents the spin of an electron. A double arrow indicates that the Highest Occupied Molecular Orbital (HOMO) is fully occupied (EPR silent). A single arrow represents an unpaired spin or radical cation that is observable by EPR spectroscopy (EPR active). electrochemical kinetic limitations such as electron transport or ion diffusion as the films relative conductivity is high over the entire potential range of the reduction sweep.  Additionally, theoretical and experimental studies on quarterthiophene dimers in solution show that the barrier of dislocation of π-stacks is quite high: 1.3⋅10-19 J (i.e., 0.8eV or 18.5 kcal/mol), classifying the π-dimer as an electro-         Figure 2: A) EPR signal relative intensity as a function of oxidation potential (100 mV/s scan rate). B) Film relative conductivity       as a function of oxidation potential (10 mV/s) scan rate. 44     Proc. of SPIE Vol. 5051 Downloaded from SPIE Digital Library on 30 May 2011 to Terms of Use:  http://spiedl.org/terms   [ox] [red]        Figure 4: Proposed actuation mechanism for poly(Calix[4]arene bis-bithiophene) showing the expanded (left) and contracted     (right) states. reversible chemical bond15,16.  Notice that this energy value is an order of magnitude larger than the Van der Waals bonding energy and thirty one times bigger than kBT  (where kB is Boltzmann’s constant and T is the absolute temperature; at room temperature 1 kBT is 4.1⋅10-21 J)17.  2.2. Poly(quarterthiophene) Our first molecular actuator candidate is poly(quarterthiophene): (poly(QT)).  This system incorporates directly the π−π stacking molecular actuation mechanism presented in the previous Section.  Polymerization of QT into poly(QT) occurs via oxidative electrochemical deposition.  This system is easily synthesized and is appropriate for studying the mechanism of π−π dimerization.  Figure 3 shows a simplified mechanism of polymerization of poly(QT). 2.3. Poly(calix[4]arene bis-bithiophene) Our second candidate system for molecular actuation is poly(calix[4]arene bis-bithiophene): poly(calixBBT).  It features an accordion-like molecule that can be switched from a zigzag open structure to a collapsed structure upon change of its oxidation state.  It employs mechanically passive cone-shaped hinge molecules (calix[4]arene) interconnected by rigid rods (quarterthiophene) which are the active elements in this system.  As mentioned above, the quarterthiophene active rods are designed to attract one another in the oxidized state, while the calix[4]arene units act as passive hinges, directing the contraction of the material into a folded molecular structure10.  The cone conformation of the calix[4]arene scaffold allows generation of an accordion-like molecule upon monomer polymerization under either electrochemical or chemical conditions.  This molecular contraction is driven by the π−π dimerization of thiophene oligomers rods upon oxidation, producing a reversible molecular displacement and has been described in great detail in previous publications18,19,20.  A 3-dimensional space-filling model showing the collapse and extension of a five-mer molecule is presented in Figure 4.  Calix[4]arene passive hinge Quarterthiophene rigid rod S S S S n S S S S Oxidative Polymerization Quarterthiophene Monomer Poly(quarterthiophene)       Figure 3: Oxidative polymerization of quarterthiophene monomer leading to poly(quarterthiophene) (poly(QT)). Proc. of SPIE Vol. 5051     45 Downloaded from SPIE Digital Library on 30 May 2011 to Terms of Use:  http://spiedl.org/terms 2.4. Polymerization with an elastomeric sulfonate Despite the promising molecular design presented above, these thiophene-based polymers suffer from being brittle and difficult to handle if polymerized electrochemically.  In the case of QT it is our belief that the electropolymerization of the oligomeric quarterthiophene leads to low molecular weight polymers as the large size of the monomer may impair polymer growth. On the other hand, associating the active actuator polymer with a polyelectrolyteb thereby creating a polymer composite can significantly improve mechanical properties.  Following the example of Wallace et al. in polypyrrole2 we used a Sulfated Poly-β-Hydroxy Ether (S-PHE) polymer as matrix-enhancing polyelectrolyte18.  The polyelectrolyte is added to the deposition solution during electrochemical polymerization.  Figure 5 shows the structure of the S-PHE molecule.  The Molar Ratio (MR) of sulfate groups (n) to hydroxyl groups (m) is also referred as the sulfonation ratio and is computed as follows: )/( mnnMR += .  3. EXPERIMENTAL 3.1. Reagents Quarterthiophene (QT) and Calix[4]arene-bis-bithiophene monomers were designed and synthesized in our laboratory according to synthesis techniques presented elsewhere19,20 and their structure verified by NMR.  Tetraethylammonium hexafluorophosphate (TEAP), dichloromethane and acetonitrile were obtained from Aldrichc.  Several Sulfated Poly(β- HydroxyEther) (S-PHE) polyanions candidates with different MR sulfation ratio (0.06, 0.09, 1) were synthesized according to the method of Wernet21.  S-PHE samples with another MR ratio (0.33 and 0.5) were graciously provided by Gordon Wallace and Jie Dingd.  Ag/Ag+ reference electrodes (BAS Bioanalytical Systemse) were constructed from 0.1 M TEAP and 0.01 M AgN03 in acetonitrile and referenced versus the Fc/Fc+ (Ferrocene) redox couple (Aldrich1). Miniature calomel reference electrodes (Acumet) were obtained from Fischer Scientificf. 3.2. Preparation of polymers Synthesis was performed by electrodeposition under galvanostatic or swept potential conditions onto a conducting substrate.  The working electrode materials were glassy carbon (Alfa Aesarg) or 200 nm gold coated PET films (Alfa Aesar) and the counter electrode was a copper sheet (Aldrichc).  Conducting polymer films were grown from a solution of 5 mM quarterthiophene (QT) monomer or calix[4]arene-bis-bithiophene (Calix), 0.1 M TEAP and diverse S-PHE concentrations (0.02; 0.2; 0.5; 1; 2 %weight) as well as MR values (1; 0.5 and 0.33) in acetonitrile, dichloromethane or a 30% acetonitrile – 70% dichloromethane solution.  Galvanostatic depositions were conducted at current densities of 1.25 A/m2 for 2.5 hours resulting in film thickness between 30 and 120 µm.  Deposition took place at room temperature (25 °C).  The resulting films of poly(QT)/S-PHE and poly(calixBBT)/S-PHE were then peeled off the working electrode substrate, rinsed in acetonitrile and conserved in a 0.1 M TEAP in acetonitrile solution.  Poly(QT)/S-PHE films had average conductivities about 10-1 S⋅m-1, densities (in dry state) between 550 and 750 kg·m-3, tensile strengths of 19.6 MPa in their dry form and of 1.3 MPa when soaked in acetonitrile.  Films of poly(calixBBT)  were conserved on their  b   A polyelectrolyte is an ionic polymer. In the case of the S-PHE, the polymer is negatively charged (anion) by sulfonate groups. c   www.aldrich.com d   Intelligent Polymer Research Institute, University of Wollongong, Australia e   www.bioanalytical.com f   www.fishersci.com g   www.alfa.com O OH O O OSO3- O m n TBA+   Figure 5: Structure of the S-PHE polyelectrolyte. 46     Proc. of SPIE Vol. 5051 Downloaded from SPIE Digital Library on 30 May 2011 to Terms of Use:  http://spiedl.org/terms electrode substrate but have not been characterized at the time of this publication.  Polymerization was also performed using swept potential methods (0.0 V to +1.5 V vs. Ag/Ag+ at a rate between 10 and 100 mV/s).  Deposition of the polymers onto the electrode led to an increase in the current indicating polymer deposition after each cycle (potential sweep). Electrochemical strengthh of both poly(QT) and poly(calixBBT)  films were later confirmed by performing cyclic voltammetry on the polymer covered substrate electrodes in 0.1 M TEAP in acetonitrile. 3.3. Techniques and instrumentation Electrodepositions and electroactivations were carried out with a potentiostat (Ameli, Model 2049).  Bulk conductivity measurements were conducted on a custom built four point measurement apparatus connected to a multimeter (Keithleyj, model 2001). In-situ conductivity measurements where acquired using interdigitated microelectrodes (Abtech Scientific)22.  In-situ EPR spectroscopic data was acquired using a platinum working electrode onto which poly(QT) was polymerized. Notice that poly(QT)/S-PHE composites have not been studied by EPR spectroscopy at this time.  Passive and active mechanical testing methods and instrumentation are described in detail in Section 4 of this paper.  4. CHARACTERIZATON OF ACTUATOR PROPERTIES 4.1. Synthesis of poly(QT)/S-PHE free standing films The novel monomers presented herein are designed and synthesized from scratch. Synthesis conditions have to be optimized towards highly conductive and mechanically strong materials.  The freestanding mechanical properties of the active films are very important for building actuators as they determine both the active (contractile) and the passive (load bearing) performance.  Material conductivity is important because it affects the speed at which the material can be activated (i.e. expanded or contracted).  The use of an elastomeric polyelectrolyte such as the S-PHE presented in Section 2.4 enables the creation of mechanically robust materials.  This method, however significantly reduces polymer film conductivity as the S-PHE is an insulating polymer (~ 10-7 S/m).  Studies to be published by Anquetil and Zimet as well as studies by Ding et al.23 of polypyrrole/S-PHE show how the addition of S-PHE to a highly conductive polymer such as polypyrrole significantly  h   By electrochemical strength we meant that the current of the cyclic voltammogram did not decrease after each sybsequent cycle, indicating that the polymer film remained attached to the substrate electrode i   www.amelsrl.com j   www.keithley.com 0.00001 0.0001 0.001 0.01 0.1 1 0.0% 0.5% 1.0% 1.5% 2.0% 2.5% SPHE % weight in deposition solution C o n du ct iv ity  (S/ m ) 0 20 40 60 80 100 120 140 Th ic kn es s (µ m ) Film Conductivity Film thickness  Figure 6:  Poly(QT)/S-PHE sample conductivity (diamonds) and sample thickness (circles) as a function S-PHE % weight in the electropolymerization solution for 2.5 hours deposition at 1.25 A/m2. Proc. of SPIE Vol. 5051     47 Downloaded from SPIE Digital Library on 30 May 2011 to Terms of Use:  http://spiedl.org/terms reduces its conductivity. Similarly, the conductivity of poly(QT)/S-PHE films is affected by the addition of the S-PHE polyanion.  The films presented in Figure 6 were synthesized from an acetonitrile solution containing 5 mM QT, 0.1 TEAP and where the concentration of the S-PHE polyanion (MR = 1) was varied from 0.5 % weight to 2% weight.  All films were deposited for the same amount of time (2.5 hours) under constant current conditions (1.25 A/m2).  Conductivity of the films ranged from 4.2·10-5 S/m to 0.16 S/m.  It was found that conductivity increased as a function of S-PHE present in the deposition solution.  We believe that a larger concentration of S-PHE enhances polymer growth.  The S-PHE is a negatively charged polymer and co-deposits with QT at the positive working electrode.  A larger S-PHE concentration in the deposition solution thus translates into more molecules that will assemble on the substrate, forming an elastomeric matrix onto which the deposited polymer can graft.  This hypothesis is supported by the increase of material thickness as a function of S-PHE concentration while deposition time (2.5 hrs) and deposition current density (1.25 A/m2) remained constant (circles in Figure 6).  In addition the charge on the S-PHE is partially responsible for polymer doping along with PF6- ions.  On the other hand thiophenes have been reported in the literature to reach conductivities of 2⋅104 S/m 24. This significant decrease in conductivity could be explained by a combination of low QT polymer molecular weights and impaired interchain charge hopping due to the large size of the S-PHE backbone (~40,000 molecular weight)25. 4.2. Passive Mechanical properties Passive mechanical testing of the poly(QT)/S-PHE was conducted using a Perkin Elmer Dynamic Mechanical Analyzer (DMA 7e)k.  It allows tensile tests to be performed on polymer films in or out of solution and at a controlled temperature.  This apparatus was used to study the response of poly(QT)/S-PHE samples as a function stress. It was also used to study the mechanical properties of S-PHE films alone.   k  instruments.perkinelmer.com 0 2 4 6 8 10 12 14 16 18 20 0 1 2 3 4 5 6 7 8 9 10 St re ss  (M Pa ) Strain (%) Poly(QT)/S-PHE in air σbreak = 19.6 MPa @ 6.3 %; EYoung = 0.7 GPa Poly(QT)/S-PHE in water + 0.1M TEAP σbreak = 11 MPa @ 4.2 %; EYoung = 0.48 GPa Poly(QT)/S-PHE in acetonitrile + 0.1M TEAP σbreak = 1.32 MPa @ 0.93 %; EYoung = 0.48 GPa 0 0.05 0.1 0.15 0.2 0.25 0 50 100 150 200 250 Strain (%) St re ss  (M Pa ) S-PHE in air σbreak = 0.24 MPa @ 215 % EYoung = 0.001 GPa St re ss  (M Pa ) St re ss  (M Pa ) St re ss  (M Pa )        Figure 7: Mechanical response of the poly(QT)/S-PHE composite (dry, in 0.05 M TEAP in distilled water, in 0.1 M TEAP in      acetonitrile and of standalone dry S-PHE).  48     Proc. of SPIE Vol. 5051 Downloaded from SPIE Digital Library on 30 May 2011 to Terms of Use:  http://spiedl.org/terms Based on the conductivity study presented above (Section 4.1), films of poly(QT)/S-PHE were synthesized such that selected synthesis parameters would lead to films with the highest conductivity.  Films were synthesized from a solution of acetonitrile containing 5 mM QT, 0.1 M TEAP and 2 % weight S-PHE (MR = 1) at room temperature. Polymerization at 1.25 A/m2 for 4.5 hours led to a 100 µm thick film.  The polymers were then peeled off the electrode material and their passive as well as active mechanical properties studied.  The active mechanical properties are presented in Section 4.3.  Figure 7 shows the mechanical response of a poly(QT)/S-PHE sample as a function of applied stress.  Note that the mechanical passive response varies depending on the sample environment (dry, in 0.05 M TEAP in distilled water, in 0.1 M TEAP in acetonitrile). The tensile strength of dry poly(QT)/S-PHE composites peaks at 19.6 MPa, while it decreased to 11 MPa when the samples were placed in 0.05 M TEAP in water and further decreased to only 1.32 MPa when placed in 0.1 M TEAP in acetonitrile.  Note on the other-hand that the tensile strength of the S-PHE alone is 0.22 MPa and it is very elastic (elongation to break ~ 215 %). It is speculated that the poor mechanical properties of poly(QT)/S-PHE composites are due in part to the re-dissolution of the elastomeric S-PHE once placed back into acetonitrile.  The passive mechanical properties of poly(QT)/S-PHE range from tensile strength between 19.6 and 1.32 MPa with elastic moduli between 0.7 and 0.48 GPa depending on the environment, which corresponds to strong mechanical properties for a conducting polymer.  Such properties are useful for building actuators. 4.3. Bilayer beam bending testing Following the method of Pei et al.26, initial electromechanical testing was conducted using the bilayer beam-bending method.  The displacement of a 80 µm thick poly(QT)/S-PHE bilayer was observed under a Zeiss Stemi SV-8 binocular microscopel and recorded using a DFW-S300 digital Sony video cameram under IEEE 1394 transfer protocol.  Inspection of the video micrographs reveals that the part of the bilayer that bends has a length of lo = 0.15 mm (total length of the bilayer is 2.25 mm).  The bilayer bending angle θ  is related to strain (∆l/lo) via,  ϑ⋅=∆ oo l d l l 2  ,       (2)  where d is the thickness of the bilayer27.  Potentials of ±5V vs. Ag/Ag+ were applied manually to the polymer bilayer for less than 4 seconds in an electrochemical cell containing 0.1M TEAP in acetonitrile and a stainless steel counter electrode and the bilayer angle recorded via video microscopy.  Figure 8 shows the calculated active strain performed by the poly(QT)/S-PHE bilayer.  Notice that the poly(QT)/S-PHE samples used for this study were from the same batch as the ones characterized for passive mechanical properties (Section 4.2).  Total strains as high as 21.4 % strains were recorded during these experiments and the best result is presented in Figure 8.  Recoverable strain was 17.7 % and maximum strain rate 15.4 %/s.  Note, however, that while 94.1 % of the high strain exhibited by the poly(QT)/S-PHE system is recoverable and repeatable for several samples, we were not able to observe such high strains for more than one cycle.  It is speculated that the high potentials applied to the system caused electrochemical destruction of the polymer. Notice also these high strains are observed close to the charge injection point into the bilayer, bending the polymer at a small bending radius. We believe that low film conductivity prohibits contractions far away for the point of electrical contact.  l   www.zeiss.com m   www.sony.com Proc. of SPIE Vol. 5051     49 Downloaded from SPIE Digital Library on 30 May 2011 to Terms of Use:  http://spiedl.org/terms  While this result does not describe the ability of the system to perform work it serves the purpose of demonstrating that poly(QT)/S-PHE can be successfully electro-chemo-mechanically activated.  In the next Section we will describe isometric experiments showing that the system can produce a force against a load. 4.4. Low frequency isometric actuator testing To demonstrate that poly(QT)/S-PHE can produce a force against a load, isometric (constant length) electroactive mechanical testing at low frequencies was conducted using a custom built isometric electromechanical testing apparatus. Notice that no commercial tensile testing device exists that adequately combines mechanical testing with electrochemical excitation and monitoring.  In this apparatus28 a sample is held at constant length by a copper alligator clip clamped at each end of the polymer film and immersed horizontally into a 138 mm, 56 mm and 30 mm deep Nylon 6 / 6 bath filled with an electrochemical solution. These copper clips also serve as electrical contacts/charge injection points to the polymer.  Note that while these clips are inserted into the electrochemical solution, their outside is electrically insulated by a Mylar tape.  An aluminum rod connects the sample clamp to a linear translation stage powered by a stepper motor (Parker Automation CompuMotorn).  In series with the clamp is a load cell (Entran ELFS-T3E-10N load cello, with Vishay 2311 Signal Conditioning Amplifierp), allowing one to record the applied force.  The whole testing setup is under computer control via a PCI data acquisition board (Allios 16-bit A/D, D/A, 50 kHz samplingq).  A graphical user interface designed in  n   www.compumotor.com o   www.entran.com p   www.vishay.com q   bioinstrumentation.mit.edu -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0 1 2 3 4 5 6 7 8 -6 0 6 0 1 2 3 4 5 6 7 8 Time (s) St ra in  ∆∆ ∆∆ l/l  (m /m ) Po te n tia l (V ) St ra in  ∆∆ ∆∆ l/l  (m /m ) Po te n tia l (V )    Figure 8: Active strain generated by the poly(QT)/S-PHE system in the bilayer beam-bending configuration 50     Proc. of SPIE Vol. 5051 Downloaded from SPIE Digital Library on 30 May 2011 to Terms of Use:  http://spiedl.org/terms Microsoft Visual Basic 6.0r allows the application of a certain displacement to the polymer sample, while acquiring passive and active stress generated by the material as well as electrochemical activity.  A sample of poly(QT)/S-PHE (length = 12 mm, width = 3.5 mm, thickness = 100 µm) was attached between the two copper alligator clips and held at constant length under a 1 MPa initial load.  The electrochemical solution for this study was a 0.05 M TBAP in distilled water. The electrochemical cell circuit was composed of the polymer sample and a stainless steel counter electrode. Potentials of the cell were controlled by an Amel potentiostat via a Calomel reference electrode (sec. 3.2).  In order to assure rapid double-layer charging at the polymer working electrode while not reaching overoxidation a shaped potential following the methods of Madden et al. was programmed in the testing algorithm28. Starting with a polymer equilibrium voltage of – 0.5 V vs. Calomel, the samples were cycled at 0.27 mHz (1 hour period) during 8 hours between – 1.5 V and + 0.5 V vs. Calomel with allowable potential peaks of ± 1 V vs. polymer equilibrium potential for the fast double-layer charging.  Figure 9 shows a characteristic run under the conditions described above.  Notice that peak stress generated attained 80 kPa and went as low as 35 kPa, showing that poly(QT)/S-PHE composites can produce a force against a load.  It is speculated that low film conductivity prohibits stress generation far away from the points of electrical contact, thus only contracting the material close to its clamping edges.  Further active mechanical characterization will involve isotonic (constant force) testing.  r   www.microsoft.com     Figure 9: Isometric active mechanical testing of poly(QT)/S-PHE composites Proc. of SPIE Vol. 5051     51 Downloaded from SPIE Digital Library on 30 May 2011 to Terms of Use:  http://spiedl.org/terms 4.5. Summary of key relevant properties and discussion Electrical, electrochemical and mechanical (passive and active) characterizations were performed on the novel poly(QT)/S-PHE electroactive polymer.  Table 1 below summarizes the properties of the poly(QT)/S-PHE actuator and compares it with mammalian skeletal muscle.  While initial studies based on the beam-bending method showed strain in levels of 20%, these active material properties need to be measured in a linear configuration.  To this effect a custom- made Electrochemical Dynamic Mechanical Analyzer (E-DMA) has been built in our laboratory that allows testing of samples under isotonic conditions29.  Key parameters to be measured as a function of applied load with this instrument include  maximum active strain, power to mass, stored energy density, efficiency and cycle life.  In addition methods typically used in muscle physiology such as the work-loop method or force velocity curves can also be employed to allow comparison of our artificial actuator technology with nature’s muscle under common conditions30.  Property Mammalian Skeletal Muscle 6 Achieved in Poly(quarterthiophene)    Displacement (Strain) 20 % 21.4 % max. (bilayer)  17.7 % recoverable (bilayer)    Active Stress (Load) 350 kPa 80 kPa @ 1 MPa (isometric)    Velocity (Strain Rate) 100 %/s 15.4 %/s (bilayer)    Power to mass 50-100 W/kg -    Efficiency 30 - 35 % -    Stiffness (wet) 0.3 to 80 MPa (contracted) 0.48 GPa    Tensile Strength (wet) 0.3 MPa 1.4 to 11 MPa    Conductivity - 0.16 S/m      Table 1: Comparison of poly(QT)/S-PHE actuator with mammalian skeletal muscle  5. CONCULSION We presented herein novel materials designed with a molecular mechanistic approach.  Incorporation of an elastomeric sulfated polyanion leads to the creation of mechanically strong films of poly(QT)/S-PHE films.  Initial activation studies of poly(QT)/S-PHE composites using the beam-bending method showed strain at levels of 20% while isometric testing demonstrated that these novel materials are able to produce a significant force against a load.  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