Polymer metal nano composite

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Abstract

Ionic polymer membrane is successfully treated with plasma, by which needle-like structures from 1.5µm to 3µm in height are created on the membrane surface. Subsequently, a thin platinum electrode layer is deposited on the membrane surface by electroless chemical metal plating. By these processes, spherical nano platinum particles of about 80nm in diameter are dispersed uniformly in a localized manner near the electrode surface. The concentration of nano particles is significantly increased with the repetition of the electroless chemical metal plating process. Our results indicate that this type of morphology decreases the surface resistance of IPMNC electrode, whereas electrical capacitance is significantly increased. Localized concentration of nano particles and improved electrical properties of electrode significantly enhances the actuation displacement, force, and operational life of plasma-treated IPMNC actuator in comparison with conventional IPMNC prepared under similar conditions.

Introduction

An ion exchange membrane (IEM) forms the bulk of the material of the ionic polymer metal nano-composites. IEMs are permeable to cations but impermeable to anions because of the unique ionic nature of the fixed perfluorinated polymer backbone. There are several commercial ion exchange material manufacturers, and the popular products that have been used as IPMC materials include from DuPont, from Tokuyama, from Asahi Chemical, and or from Asahi Glass. At present all these products perform fairly well when they are properly treated by the IPMNC chemical plating techniques. In this research nafion from DuPont is used and platinum is selected as electrode material due to its good conductivity, high corrosion resistance, and work densities. In nafion the top and bottom electrode surfaces are formed by exchanging H+ ions with metal ions.

The cross section of IPMNC resembles a sandwich with electrode layers outside and a polymer matrix in the center. Ionic polymer consists of a fixed network with negative charges, balanced by mobile cations. The polymer network consists of pockets of solvents and thin boundary layers are generated by the application of an electric field. A layer deficient in cation forms on the anode side, while a layer containing large number of cation forms on the cathode side. Due to the accumulation of cations on the cathode side, water molecules move to this side and cause hydrophilic expansion. The stress in the polymer matrix causes bending toward the anode . With time, the back diffusion of water molecules causes a slow relaxation toward the cathode. The degree of actuation obtained is a function of various parameters such as polymer thickness, type of polymer, type of counter ion, amount of water, quality of metallization, and surface area of the polymer membrane. When a voltage higher than the electrolysis voltage of water is applied, blistering and damage to the electrodes was observed, which degrades performance of the IPMNC.

In this article, we aim to capture the effect of plasma treatment to IPMNC for the improvement of actuation performance. We describe the fabrication process of multilayer IPMNC and discuss implication of plasma treatment to polymer. We take a look at the needle-like micro structures developed due to oxygen-based plasma etching on the polymer surface and configuration of platinum nano particles after electroless chemical metal plating. Finally we discuss the correlation between actuator electrical properties and actuator performance parameters.

Fabrication of multilayer IPMNC

Nafion-117 is used as ionic polymer. Square sheets of 183m standard thickness were cut into 25-square membranes for the convenience of handling. To generate optimum actuation performance, three layers are staked together in the controlled environment of hot mold press. After nafion staking, plasma treatment was performed.

Plasma treatment to polymers

An important parameter of plasma is the degree of ionization, which denotes the fraction of the original neutral species that have become ionized [6]. In fully ionized plasmas, the degree of ionization approaches unity, and neutral particles play a modest role.

Polymeric materials are chosen for a particular application based on their chemical, mechanical and electrical characteristics . Non-local thermal equilibrium (cold) plasmas are being used with increasing frequency for a wide range of applications related to polymer etching and surface modification; several reviews are published in this regard .

Oxygen plasma and chemical dry etching

Oxidative plasma is a major source to alter the polymer performance . plasma is used for anisotropic chemical etching of polymer surfaces, in order to improve wettability or bondability.

Oxygen plasma causes chemical dry etching of ionic polymer. Plasma reacts with the surface and produces a volatile reaction product . Chemical dry etching is inherently sensitive to differences in bonds and the chemical consistency of a polymer substrate. With anisotropic etching, the removal of vertical and horizontal material proceeds at different rates enabling the formation of fine lines .

The etching process consists of four steps: (a) production of active gas species, e.g. electrons, ions, and free radicals; (b) transport of active species from the bulk plasma to the nafion surface and this accrues mainly by diffusion; (c) absorption of reactive species onto the nafion surface and concurrent ion bombardment which create active etching sites; and (d) drive-out of volatile chemical products from plasma chamber.

Surface morphology

After plasma etching, the scanning electron microscopy image clearly reveals the needle-like structures over the entire membrane surface. A close examination supports the concept of anisotropic etching by oxygen. The relative height with respect to the base width of spikes on the nafion surface increases when polymer is etched for a longer duration. This can be seen in cross-section views of 1.5 m and 3 m etched nafion.

When plasma treatment is extended for a prolonged period, the etch rate is decreased (detail in Table 1) because plasma species have to react with material at larger depths in confined areas surrounded by tall needle-like structures, where reactants need to expend more energy to etch the material. Interestingly, by increasing etch duration, the pointed tips of spikes are eroded and needle-shape structures get transformed into flakes.

Electroless plating

Ionic polymer metal nano composite (IPMNC) samples were prepared by plating platinum electrodes on both sides using the electroless chemical plating method. This consisted of the ion exchange of in the membrane with the cationic platinum complex, followed by a reduction process in an aqueous solution. That is, the compression-molded membrane was washed with deionized water and left at room temperature (25 for one day. The swollen membrane was immersed in the aqueous M solution of tetra-ammine platinum (II) chloride hydrate, Pt .X (Aldrich) for one day at 40 to exchange the platinum complex cation. The impregnated cationic complex was reduced with aqueous 5wt solution of sodium borohyride, (Aldrich) at 40 to 60. The ion exchange and reduction processes were repeated for second time to increase the thickness of platinum layers .

The counter cations of carboxylate groups in the IPMNC were converted to Li+ ions by soaking the IPMNC in aqueous 1N solutions of lithium chloride salt (LiCl) for one day at room temperature and then rinsing with deionized water.

After electroless platinum plating, sharpness of the spikes decreases due to prolonged chemical treatment and as a result the overall gap among protrusion is decreased. By repeating the electroless chemical process for the second time, the depth of protruded structures decreases and the peaks get blunted as shown in .

Platinum nano particles

Dispersion of metal particles was an uncontrollable factor therefore the previous studies have adopted numerous strategies such as repetitive chemical plating, electrical plating and physical loading of conductive materials to improve the electrode condition and ultimately enhance the actuation performance . One of the most promising aspects of oxygen plasma treatment to IPMNC is the localized dispersion of platinum particles, which are of varying size from 50 to 80 nm in diameter. Difference in the distribution of nano particles in plasma-treated and conventional IPMNC.

Results and discussion

Electrode Surface Resistance

The four probe method in wet condition was used at room temperature to measure the surface resistance of the IPMNCs, Summarizes the results of the test specimens. As expected, the surface resistance of IPMNC actuator is significantly reduced by repeating electroless metal plating process [25-29]. The chemical deposition of platinum for the second time is attractive because it can produce a thick homogeneous and localized dispersion of platinum nano particles beneath the plasma-etched electrode surface.

Due to the poor surface resistance of reference IPMNC a significant potential is required to maintain the effective voltage along the surface of actuator. In other words, the voltage drops noticeably along the length of reference electrode surface and a typical set of data is shown in.From these statistics we can observe the advantage of plasma treatment and the repetition of electroless plating on voltage drop along the length of IPMNC specimens. Homogeneity and depth of localized platinum nano particles increases with the number of plating and this improvement reduces the surface resistance for all the samples.

Electrode Capacitance

Capacitance is a critical parameter for IPMNC electromechanical behavior, since it is highly correlated with actuation performance . Experimentally measured capacitance per unit nominal surface area for reference IPMNC varies in the range of 15 to 40F/, whereas this amount is drastically increased to the order of mF/ for plasma-treated cases. IPMNC capacitance can be further enhanced by increasing the number of metal plating layers. More specifically, in it is shown that additional plating on a 1.5m plasma-treated IPMNC enhances the capacitance threefold.

The very large capacitance of plasma-treated IPMNCs cannot be predicted by describing it as a parallel-plate capacitor, that is, a dielectric medium plated by two flat plate electrodes.[REF] Due to locally distributed nano particles inside the electrode, the effective plate surface increases, and the effective distance between the plates decrease. And the capacitance of plasma-treated IPMNCs is governed by double layer effects. A voltage applied across the electrodes produces localized enrichment and depletion of mobile counter-ions at the polymer electrode interfaces.

Actuator Performance Parameters

For actuation tests, IPMNC is cut into 40 mm x 5mm strips by considering tradeoff between actuation displacement and force. Long IPMNC specimens produce large displacement but they cannot generate high tip force and short size. On the other hand, short IPMNC specimens produce small displacement and high tip force.

All the measurements of actuation tests are obtained at 5mm inward from the tip. Actuation properties of IPMNC such as displacement, force and operational life are measured by a laser displacement meter and load cell. represents the schematic diagram for the measuring system used in this experiment.

Actuation Displacement

The electrodynamics associated with the IPMNC actuator are complex. The nano-structure of the ionomer membrane and the morphology of the electrodes determine the cation transportation and the electric conductivity of an IPMNC, and thereby strongly affect its actuation displacement.

The plasma-treated IPMNCs can create a large bending motion under the relatively low input voltage. In previous studies where multilayer IPMNCs are used, the actuation force is improved but as a result the actuation displacement is noticeably reduced compared with single-layer cases. By the adoption of multiple electroless chemical plating process to electrode, the surface resistance is reduced and capacitance is increased. Both of these electrical parameters contribute for the improvement of actuation displacement

shows the comparison of the maximum displacements of the IPMNC cantilever beams actuated at AC 3V with 0.5 Hz, and represents the actuation results measured at 3V DC. For each condition, displacements were measured with 8 samples. All the plasma treated IPMNC samples has shown higher average bending displacements than the reference. As illustrated in displacement of double plated 1.5m specimen is larger than 3m etch single plated plasma treated IPMNC specimen. This superior performance is because of much higher capacitance and relative lower surface resistance of 1.5m specimen.

Actuation Force

IPMNC actuation force is directly related with an actuator size, thickness, and surface resistance. Shorter, thicker and lower surface resistance specimens generate high tip force.

The generated force is measured with a load cell at the tip of IPMNC cantilever specimen. shows the tip forces of the IPMNC specimens. For each condition five specimens were randomly chosen to grasp the behavioral characteristics of actuation force. Plasma treated specimens has generated higher tip forces than the reference specimens, whereas double-plated IPMNC in each category has performed much better than single-plated specimens.

Results plotted in suggest that in case of 1.5 m IPMNC specimen, small surface resistance and large interlayer capacitance created by densely populated nano platinum particles has played a favorable role in the generation of high tip force.

Reliability

To evaluate the reliable performance of IPMNC actuator, displacement and force actuation test are performed repeatedly with single specimen. In similar operating conditions, 1.5m and 3m plasma-treated IPMNC actuator are tested for three consecutive times. In actuation tests water evaporates from the IPMNC specimen and if used for longer durations this can degrade the actuator performance. As a remedial action IPMNCs are immersed in the deionized water for minimum one hour between consecutive actuation tests.

clearly demonstrates the repeatable performance of plasma-treated IPMNC. For each plasma treated condition a very small variation is observed in test results.

In previous studies it was found that by increasing actuation force of plasma-treated IPMNC the actuation displacement was reduced. But in this research using plasma treatment, both actuation force and displacement were increased simultaneously when compared with actuation characteristics of reference IPMNC.

Operational Life

In IPMNC, the solvent evaporates through the platinum electrodes and side edges by electrolysis. During this process, the pores in the membrane are closed gradually . These pores act as a transport channel, which was described in the cluster network model . This contraction by drying increases surface conductivity but has a negligible effect on electrical capacitance. However, it blocks the ion channel through which cations and solvent molecules move, resulting in reduction of IPMNC operational life . For the improvement of actuation operational life of IPMNCs, plasma treatment is performed prior to multiple platinum plating and this finding is supported by the data.

Repeated contraction and relaxation during bending actuation of an IPMNC creates more cracks on the electrode surface and this repetitive movement of IPMNC deteriorates the surface conductivity .

If the IPMNC electrode has large surface resistance, then the cations coupled with polar solvent molecules will move towards the side where power supply is connected to the IPMNC electrode. In cases of cantilever beam style samples, cations coupled with water molecules will move away from the tip due to the electric field gradient. Due to high surface resistance of reference IPMNC an electric field gradient is observed in the specimens. When voltage is applied across the electrodes, the movement of water molecules coupled with cations starts from IPMNC tip towards the side where power supply is connected to IPMNC electrode. The one directional movement of water molecules eventually results in early dry-out of polar solvents from the tip of specimen and operation life of the reference IPMNC is decreased.

Plasma-treated IPMNC has much lower surface resistance. When electric voltage is applied to the electrodes, electric field remains uniform over the entire platinum-plated surface and the water molecules will mostly move up and down through the direction of the thickness.

Another advantage of plasma-treated IPMNC is softer electrode layers and localized nano particles in the IPMNC electrode. This morphology plays a vital role in the enhancement of operational life by reducing the cracks propagation and lower down the electrode surface degradation. In case if cracks appear at the top then it will not be able to propagate across the section due to concentrated region of nano particles.

Conclusions

In summary, plasma treatment to IPMNC actuator was shown to be successful. We have demonstrated that the newly developed actuator has resulted in the localized dispersion of platinum nano particles near the electrode surface. By this unique feature electrical resistance was reduced by 30 times and the capacitance was increased more than 20 times in comparison with reference IPMNC. Furthermore, double plating of electrode layer further improves the performance by promoting the growth of nano particles to larger depths. This improvement in morphology is useful in the development of much reliable IPMNC with enhance service life, large actuation displacement and high actuation force.

Acknowledgements

The authors wish to thank and acknowledge the help of the organizations and individuals whose literature has been cited in this article. Research leading to this article was supported by the Higher Education Commission of Pakistan (HEC) and by the Ministry of Education, Science and Technology (KRF-2008-331-D00023).

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