Molecular dynamics simulation study of glass transition in hydrated Nafion

The thermal transition of Nafion is studied using a molecular dynamics simulation through a chemically realistic model. Static and dynamic properties of polymer melts with different water contents are investigated over a wide range of temperatures to obtain viscometric and calorimetric glass transition temperatures. The effect of cooling rate of the simulation on the glass transition of the hydrated polymer is also examined within the well‐known Williams–Landel–Ferry (WLF) equation. Variation of relaxation times versus temperature shows a fragile‐to‐strong transition. The hydration level has a significant impact on the static and dynamic properties of the polymer chains and water molecules confined in nanometric spaces between polymer chains. The results of this study are useful to predict the behavior of Nafion for various applications including fuel cells, sensors, actuators, and shape memory devices at different temperatures. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 907–915     

Modeling of hysteretic behavior in ferroelectric polymers

Controlling the polarization state of ferroelectric materials, and more particularly piezoelectric polymers, is critical to ensure good operation of actuators or sensors using such energy conversion mechanisms. More specifically, the modeling and prediction of the hysteretic behavior of such materials is a critical aspect for the fabrication of robust and accurate devices. The purpose of this article is to present a model based on mathematical functions describing hysteretic behavior as a sum of elementary polarizations arising from combined avalanche and saturation physical effects. Predicted responses show good agreement with experimental measurements, and extension of the model for taking into account electric field‐induced crystallization during operations is presented. Finally, the proposed model is simple to implement and does no require heavy computational and memory requirements, as it relies on pure mathematical functions and only requires unidimensional distribution of elementary polarizations. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 499–508     

Pulse width modulation (PWM) control of the bending displacement and force generation of ionomer‐based polymer actuators

This paper discusses results of the pulse width modulation (PWM) control of the bending displacement and force generation of ionomer‐based polymer actuators. The idea of PWM control for polymer actuators is based on the frequency dependence on relaxation processes of the bending motion of the actuator. The actuators were fabricated by electroless plating of platinum on a perfluorinated sulfonated polymer (i.e. Nafion). It was noted that the duty cycles of PWM can smoothly control both the displacement and force generated by the actuator. The performance of the actuator was evaluated under 40%RH and 90%RH conditions because of the moisture sensitivity of the actuators. Copyright © 2013 John Wiley & Sons, Ltd.     

Electro‐active Polymer Actuator Based on Sulfonated Polyimide with Highly Conductive Silver Electrodes Via Self‐metallization

We report here a facile synthesis of high performance electro‐active polymer actuator based on a sulfonated polyimide with well‐defined silver electrodes via self‐metallization. The proposed method greatly reduces fabrication time and cost, and obviates a cation exchange process required in the fabrication of ionic polymer‐metal composite actuators. Also, the self‐metallized silver electrodes exhibit outstanding metal‐polymer adhesion with high conductivity, resulting in substantially larger tip displacements compared with Nafion‐based actuators.

     

Conducting polymers electromechanical actuators and strain sensors

Electromechanical actuators are being investigated for a wide range of applications in medical, electronics and industrial areas. One attractive application is to incorporate conducting polymer fibre actuators into fabrics for use in prosthetic applications. In this paper, the design of polypyrrole fibre actuators for use in a glove to open and close the human hand (for assisting those with paralysis or hand injuries) is described. The key requirements for this application are the simultaneous generation of 16 mm of contractile movement and 2.9 N of force. Although not critical in the first prototypes, eventually it will also be necessary to produce a rate of movement of around 10 mm sec⁻¹. The effect of the geometry of polypyrrole actuators is examined in this paper and it is shown that a tubular geometry is superior to conventional flat films. Another aspect of the practical use of actuator materials is their control. Fabric strain gauges with polymer actuators is a convenient means for providing feedback control to the actuating element. The fabric strain gauges ideally articulate with fibre actuators to give both the actuating and sensing function in the same fabric structure.     

Modeling and simulation of morphology variation during the solidification of polymer melts with amorphous and semi‐crystalline phases

The solidification of polymer melts in practical processing such as extrusion, injection molding and blow molding can significantly influence the inner structure and performance of final products. The investigation of its mechanism has both scientific and industrial interests. In the study, the three‐dimensional mathematical model is developed for the simulation of morphology variation in the solidification of polymer melts with amorphous and semi‐crystalline phases. The amorphous phase is simulated as the finite extensible nonlinear elastic dumbbell with a peterlin closure approximation (FENE‐P) fluid and the semi‐crystalline phase is approximated as rigid rods that grow and oriented in the flow field. The model of amorphous phase and semi‐crystalline phase are coupled through the stress and momentum balance and the feedback of crystallinity to the system relaxation time. The evolution of crystallization kinetics process are described by using a set of Schneider equation that discriminating the relative roles of the thermal and the flow effect on the crystallization behavior. With the standard Galerkin formulation adopted as basic computational framework, the discrete elastic viscous stress splitting algorithm in cooperating with the streamline upwinding approach serves as a relatively robust numerical scheme by using penalty finite element–finite difference simulation with a decoupled solving algorithm. The proposed mathematical model and numerical method have been successfully applied to the investigation of solidification of polymer melts in the extrusion process. The variations of orientation and crystallization morphology during the solidification process are further discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Ionic electro-active polymer actuator based on cobalt-containing nitrogen-doped carbon/conducting polymer soft electrode

Nanoscale cobalt-containing nitrogen-doped porous carbon (CoNC) materials were prepared by thermolysis of a zeolitic imidazolate framework (ZIF), ZIF-67, at different temperatures and their application for ionic electro-active polymer (EAP) actuator was evaluated. CoNC-700, which was obtained from ZIF-67 pyrolysis at 700 °C, exhibits specific surface area of 753.86 m² g⁻¹, pore volume of 0.5768 cm³ g⁻¹, and specific capacitance of 120.7 F∙g⁻¹. CoNC/conducting polymer soft electrode were fabricated by unitizing effective interaction of CoNC with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). High-performance ionic actuators were developed for the first time using this CoNC/PEDOT:PSS soft electrode. The developed ionic EAP actuator exhibited large peak-to-peak displacement of 20.4 mm and high bending strain of 0.28% (3 V and 0.1 Hz). Therefore, ZIFs or metal organic frameworks (MOFs) can be applied to provide significant improvements in EAP actuators, which can play key roles as technological advances toward bioinspired actuating devices required for next-generation soft and wearable electronics.     

Radiation‐Grafted Fluoropolymers Soaked with Imidazolium‐Based Ionic Liquids for High‐Performance Ionic Polymer–Metal Composite Actuators

On purpose to develop a polymer actuator with high stability in air‐operation as well as large bending displacement, a series of ionic polymer–metal composites (IPMC) was constructed with poly(styrene sulfonate)‐grafted fluoropolymers as ionomeric matrix and immidazolium‐based ionic liquids (IL) as inner solvent. The prepared IPMC actuators exhibited greatly enhanced bending displacement compared to Nafion‐based actuators. The actuators were stable in air‐operation, maintaining initial displacement for up to 10⁴ cycles or 24 h. Investigating the material parameters and morphology of the IPMCs, high ion exchange capacity of the ionomers resulted in high ion conductivity and robust electrode of IPMC, which synergistically contributed to the high bending performance.

     

Thermoplastic Dielectric Elastomer of Triblock Copolymer with High Electromechanical Performance

Dielectric elastomer (DE) actuators have been shown to have promising applications as soft electromechanical transducers in many emerging technologies. The DE actuators, which are capable of large actuation strain over a wide range of excitation frequencies, are highly desirable. Here, the first single‐component DE of a triblock copolymer with attractive electromechanical performance is reported. Symmetric poly(styrene‐b‐butyl acrylate‐b‐styrene) (SBAS) is designed and synthesized. The SBAS actuator exhibits about 100% static actuation area strain and excellent dynamic performance, as evidenced by a wide half bandwidth of 300 Hz and a very high specific power of 1.2 W g^–1 within the excitation frequency range of 300–800 Hz.     

Stretchable and transparent hydrogels as soft conductors for dielectric elastomer actuators

A soft ionic conductor can serve as an artificial nerve in an artificial muscle. A polyacrylamide hydrogel is synthesized containing a hygroscopic salt, lithium chloride. Two layers of the hydrogel are used as ionic conductors to sandwich a dielectric elastomer and fabricate a highly stretchable and transparent actuator. When the two layers of the hydrogels are subject to a voltage, the actuator reduces its thickness and expands. An areal strain of 134% is demonstrated. The voltage‐strain curves are calculated by using a model that accounts for the elastic constraint of the hydrogel and the inhomogeneous deformation of the actuator. For actuators fabricated with the hydrogel of various thicknesses and with the dielectric elastomer of various prestretches, excellent agreements are found between experimental data and theoretical predictions. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1055–1060     

Modeling and Simulation of the Crystallization Behavior of Polymer Melts in the Hollow‐Profile Extrusion Process Using the Coexistence Model of Spherulite and Shish‐Kebab

The thermally and flow induced crystallization behavior of polymer melts has been investigated by using penalty finite element‐finite difference simulation with a decoupled solving algorithm. The coexistence model of spherulite and shish‐kebab is proposed to describe the evolution of crystallization kinetics process. The Schneider equation and Eder equation are adopted to discriminate the relative roles of the thermal and the flow effect on the crystallization behavior. The proposed mathematical model and numerical method have been successfully applied to the investigation of crystallization behavior in the hollow‐profile extrusion process. Both the evolution of crystalline size within the extrusion die and the effects of processing conditions on the crystallization kinetics process are discussed.

     

Selective Decrosslinking in Liquid Crystal Polymer Actuators for Optical Reconfiguration of Origami and Light‐Fueled Locomotion

The ability to optically reconfigure an existing actuator of a liquid crystal polymer network (LCN) so that it can display a new actuation behavior or function is highly desired in developing materials for soft robotics applications. Demonstrated here is a powerful approach relying on selective polymer chain decrosslinking in a LCN actuator with uniaxial LC alignment. Using an anthracene‐containing LCN, spatially controlled optical decrosslinking can be realized through photocleavage of anthracene dimers under 254 nm UV light, which alters the distribution of actuation (crosslinked) and non‐actuation (decrosslinked) domains and thus determines the actuation behavior upon order‐disorder phase transitions. Based on this mechanism, a single actuator having a flat shape can be reconfigured in an on‐demand manner to exhibit reversible shape transformation such as self‐folding into origami three‐dimensional structures. Moreover, using a dye‐doped LCN actuator, a light‐fueled microwalker can be optically reconfigured to adopt different locomotion behaviors, changing from moving in the laser scanning direction to moving in the opposite direction.     

Novel Generation of Liquid Crystalline Photo‐Actuators Based on Stretched Porous Polyethylene Films

The preparation of photo‐actuators based on stretched porous polyethylene and an azobenzene‐containing liquid crystalline polymer network is reported for the first time. It is revealed that this kind of photo‐actuator possesses the following advantages: the lack of a need for using aligning coatings and cells preparation, high deformation of the actuator and its complete reversibility, good mechanical properties, and relatively low cost of fabrication. In addition some kinetic and thermodynamic features of the bending and unbending processes have been studied.     

Electric Double‐Layer Capacitor Based on an Ionic Clathrate Hydrate

Herein, we suggest a new approach to an electric double‐layer capacitor (EDLC) that is based on a proton‐conducting ionic clathrate hydrate (ICH). The ice‐like structures of clathrate hydrates, which are comprised of host water molecules and guest ions, make them suitable for applications in EDLC electrolytes, owing to their high proton conductivities and thermal stabilities. The carbon materials in the ICH Me₄NOH ? 5 H₂O show a high specific capacitance, reversible charge–discharge behavior, and a long cycle life. The ionic‐hydrate complex provides the following advantages in comparison with conventional aqueous and polymer electrolytes: 1) The ICH does not cause leakage problems under normal EDLC operating conditions. 2) The hydrate material can be utilized itself, without requiring any pre‐treatments or activation for proton conduction, thus shortening the preparation procedure of the EDLC. 3) The crystallization of the ICH makes it possible to tailor practical EDLC dimensions because of its fluidity as a liquid hydrate. 4) The hydrate solid electrolyte exhibits more‐favorable electrochemical stability than aqueous and polymer electrolytes. Therefore, ICH materials are expected to find practical applications in versatile energy devices that incorporate electrochemical systems.     

Liquid‐Crystalline Soft Actuators with Switchable Thermal Reprogrammability

Thermal reprogrammability is essential for new‐generation large dry soft actuators, but the realization sacrifices the favored actuation performance. The contradiction between thermal reprogrammability and stability hampers efforts to design high‐performance soft actuators to be robust and thermally adaptable. Now, a strategy has been developed that relies on repeatedly switching on/off thermal reprogrammability in liquid‐crystalline elastomer (LCE) actuators to resolve this problem. By post‐synthesis swelling, a latent siloxane exchange reaction can be induced in the common siloxane LCEs (switching on), enabling reprogramming into on‐demand 3D‐shaped actuators; by switching off the dynamic network by heating, actuation stability is guaranteed even at high temperature (180 °C). Using partially black‐ink‐patterned LCEs, selectively switching off reprogrammability allows integration of completely different actuation modes in one monolithic actuator for more delicate and elaborate tasks.     

Bioinspired Actuators Based on Stimuli‐Responsive Polymers

Organisms exhibit strong environmental adaptability by controllably adjusting their morphologies or fast locomotion; thus providing constant inspiration for scientists to develop artificial actuators that not only have diverse and sophisticated shape‐morphing capabilities, but can also further transfer dynamic and reversible shape deformations into macroscopic motion under the following principles: asymmetric friction, the Marangoni effect, and counteracting forces of the surrounding conditions. Among numerous available materials for fabricating bioinspired artificial actuators, stimuli‐responsive polymers are superior in their flexible features and the ability to change their physicochemical properties dynamically under external stimuli, such as temperature, pH, light, and ionic strength. Herein, different mechanisms, working principles, and applications of stimuli‐responsive polymeric actuators are comprehensively introduced. Furthermore, perspectives on existing challenges and future directions of this field are provided.     

Superelastic,Macroporous Polystyrene‐Mediated Graphene Aerogels for Active Pressure Sensing

Three‐dimensional (3D) graphene‐based polymer/graphene aerogels with excellent mechanical properties are crucial for broad applications. The creation of such polymer/graphene aerogels remains challenging because of the poor dispersion and compatibility of polymer within the graphene matrix. By using the freezing‐directed assembly of graphene under the assistance of surfactant, 3D macroporous polystyrene/graphene aerogels (MPS‐GAs) with lightweight, superelastivity (80 % strain), high strength (80 kPa), and good electrical properties have been achieved in this study. The as‐prepared MPS‐GAs shows excellent electromechanical performance with stable cyclic resilient properties and sensitive resistance responses, thus making the MPS‐GAs promising candidates for applications in actuators, elastic conductors, strain/pressure sensors, and wearable devices.     

A Rewritable,Reprogrammable, Dual Light‐Responsive Polymer Actuator

We report on the fabrication of a rewritable and reprogrammable dual‐photoresponsive liquid crystalline‐based actuator containing an azomerocyanine dye that can be locally converted into the hydroxyazopyridinium form by acid treatment. Each dye absorbs at a different wavelength giving access to programmable actuators, the folding of which can be controlled by using different colors of light. The acidic patterning is reversible and allows the erasing and rewriting of patterns in the polymer film, giving access to reusable, adjustable soft actuators.     

A Rewritable,Reprogrammable, Dual Light‐Responsive Polymer Actuator

We report on the fabrication of a rewritable and reprogrammable dual‐photoresponsive liquid crystalline‐based actuator containing an azomerocyanine dye that can be locally converted into the hydroxyazopyridinium form by acid treatment. Each dye absorbs at a different wavelength giving access to programmable actuators, the folding of which can be controlled by using different colors of light. The acidic patterning is reversible and allows the erasing and rewriting of patterns in the polymer film, giving access to reusable, adjustable soft actuators.