Ultrasonic Welding of Nafion Ionomers and Aluminum Foils for the Manufacture Of

University of Nevada, Reno

Ultrasonic Welding of Nafion Ionomers and Aluminum Foils for the Manufacture of

Ionic Polymer Metal Composites

A thesis submitted in partial fulfillment of the requirements for the degree of Master of

Science in Mechanical Engineering

Gregory Ross

Dr. Matteo Aureli/Thesis Advisor

May, 2016

Abstract

In this thesis, the use of ultrasonic welding as a method for manufacture of

IPMCs is investigated. There are two potential ways that ultrasonic welding can be used to manufacture IPMCs. The first way is through the welding of metalized electrodes to an ionomer membrane. The second way is through the ultrasonic welding of ionomers or even completed IPMCs to each other. Success with these methods could greatly reduce the costs of IPMC manufacture and open new avenues for research. In this thesis, background information on ultrasonic welding and IPMCs will be covered, followed by the results from a introductory round of testing of the ultrasonic welding method between Nafion ionomer membranes and aluminum foils. Initial results are promising, but further work will be necessary to perfect the technique to the point where it can be useful for the production of IPMCs.

Acknowledgements This project was not possible without the contributions of others. This project, and also the cost of my graduate education, was funded mainly through the Department of Mechanical Engineering and my advisor, Dr. Matteo Aureli. Dr. Matteo Aureli has also spent countless hours helping me to understand complex electrochemical and physical phenomena and guiding me through the research process. I am very thankful for all of this support. I would like to thank Dr. Yiliang Liao for the use of the Ultrasonic Welder machine under his care, and Dr. Wanliang Shan and his graduate student Milad Tatari for allowing me to use the tensile testing machine under their care. I also thank the

Stapla Ultrasonics Corporation and its staff, particularly Kevin Gordon for his effort in welding and mailing specimens and for his welding advice, and Rob Saulnier for his advice. Also I am thankful for Marissa Tsugawa for spending time showing me how to use IPMC related equipment. And I thank Dr. Kam Liang and his assistant James Carrico for providing me with instructions and walking me through the chemical process of making an IPMC in the lab, and for generously leaving some expensive chemicals for me to use in my research after their departure. Also, I would like to thank Dr. Henry Fu for giving me guidance and critique on my oral and visual presentations in front of an audience, and many of my fellow graduate students that have had to take time away from their research and studies at odd hours to help me access and get things I needed for my research from various labs across the vast sprawling campus of UNR, helping me transport heavy loads, and giving general advice. And I'd like to thank all members of the thesis committee for taking time to review this thesis.

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Table of Contents Abstract ...... i

Acknowledgements...... ii

Table of Contents ...... iii

List of Tables ...... v

List of Figures ...... vi

1 Introduction ...... 1

1.1 Problem Statement ...... 1

1.2 Background information on Ultrasonic Welding...... 3

1.2.1 Metal Welding Technique versus Plastic Welding Technique ...... 3

1.2.2 Weld Parameters and Variables ...... 6

1.2.3 Application of Ultrasonic Welding to IPMCs ...... 7

1.3 Background Information on IPMCs ...... 10

1.3.1 Function and Use ...... 10

1.3.2 Conventional IPMC Manufacturing Process ...... 12

1.3.3 Materials used for IPMCs ...... 14

1.3.4 Membrane Customization Techniques ...... 15

2 Experiments ...... 19

2.1 Experimental Setup ...... 19

2.2 Nafion to Nafion Welding ...... 21

2.2.1 Nafion to Nafion Weld Parameters ...... 23

2.2.2 Repeated Welds: Nafion to Nafion ...... 23

2.3 Aluminum Foil to Nafion Welding ...... 24

2.3.1 Nafion to Aluminum Foil Weld Parameters and Factors ...... 25

2.3.2 Repeated Welds: Aluminum Foil to Nafion ...... 29

2.4 Damaged Tooling ...... 33

2.5 Stapla Samples with Less Aggressive Knurling ...... 35

2.6 Bond Strength Testing...... 38

2.6.1 Tensile Testing of Nafion to Nafion Welds ...... 39

2.6.2 Aluminum / Nafion Weld Shear Strength Test ...... 42

2.7 Ultrasonic Welding Overview ...... 44

2.8 Electrochemical Tests of Welded Samples ...... 45

3 Microscope Observations ...... 56

4 Discussion...... 59

5 Future Work ...... 61

5.1 Analysis of Welded Nafion IPMC membranes ...... 61

5.2 Improving the Nafion-Metal Bonding ...... 61

5.3 Forming Proper Surface Characteristics for IPMC Capacitance ...... 63

5.4 Ultrasonic Welding of Complete IPMCs ...... 65

6 Conclusion ...... 70

Appendix ...... 71

A1 Micrometer Measurements ...... 71

A2 Nafion to Nafion Weld Tables ...... 71

A3 Welding Wet Nafion to Aluminum Foil ...... 74

A4 Welding Foil to Double Thick Nafion ...... 75

A5 Welding Nafion/Foil to Nafion/Foil ...... 75

A6 Non-Stick Foil ...... 76

A8 Comparison of IPMC Production Procedures ...... 77

Bibliography ...... 83

List of Tables Table 1 - Vernier Micrometer Measurements...... 71

Table 2 - Nafion to Nafion Weld Table ...... 73

Table 3 - Single Foil Welded to Nafion, then Submerged in Water...... 74

Table 4 - Dry Heavy Duty Aluminum Foil welded to Nafion, then Submerged in Water...... 75

Table 5 - Reynold's "Non-Stick" Foil to Nafion Weld Settings ...... 76

Table 6 - Comparison of Chemicals in Palmre and Oguro methods...... 78

Table 7 - Comparisons of Platinum Solutions ...... 78

Table 8 - Comparison of Other Chemical Solution Quantities ...... 79

Table 9 - Surface Preparation Comparison ...... 80

Table 10 - Comparison of Ion Exchange / Primary Plating Stages ...... 81

Table 11 - Comparison of Secondary Plating Processes ...... 82

Table 12 - Comparison of Final Cleaning Stages ...... 82

Table 13 - Comparison of Final Ion Exchange Processes ...... 82

List of Figures Figure 1 - Two materials between horn and anvil. Comparison of the transverse vibration metal welding method versus longitudinal vibration plastic welding method...... 4

Figure 2 - This figure, drawn using microscope pictures from Wagner et al. [5] as a reference, shows the results of bonding using both the plastic welding technique (a), and the metal welding technique (b), between glass reinforced polymer and aluminum. Bonds were achieved with either method but with very different characteristics...... 5

Figure 3 - Two different ultrasonic welding aids: (a) Energy Directors (projections) and (b) Tie- Layers. Figure redrawn from a similar figure in Tateishi et al. [10] ...... 7

Figure 4 - This drawing was made by tracing the outline of an electron backscatter micrograph from Naji et. al [13], showing an IPMC electrode. Note the extremely complex morphology...... 9

Figure 5 - This diagram demonstrates the motion of an IPMC subject to an electrical potential difference. The positive ions move towards the negative electrode, pulling water (or another solvent) with it. Also, the fixed negative ions near the negative electrode cause stretching of the material near the negative electrodes, and contraction near the positive electrodes. These effects cause expansion on one side and contraction on the other, resulting in a bending motion. A similar diagram can be found in De Luca et al. [15]...... 11

Figure 6 - Left: Beaker on hot plate with clear Nafion. Right- Same sample after metallization. . 13

Figure 7 - Left Branson KET 1 Ultrasonic Plastic Welder With KG1 Sonotrode. Right- Condor ST-30 Ultrasonic Metal Welder ...... 19

Figure 8 - (A) Included anvil and sonotrode (B) Aluminum plate for anvil with yellow 3D printed plastic spacer on sonotrode. (C) Wear caused on aluminum plate by horn...... 21

Figure 9 - Top: Curved sample with welds on same side, compared to flat alternate side welded samples. Bottom: Long double Nafion alternate side welded strip. This long sample was subject to a tensile strength test...... 24

Figure 10 - Left: Thin foil with holes going all the way through. Right: Thicker foil not punctured...... 26

Figure 11 - Left: Bottom of welded sample, in contact with the smooth aluminum anvil during the weld. Right: Top of the same sample, showing effect of aggressively knurled horn...... 27

Figure 12 - Repeated welds caused the sample to curve...... 29

Figure 13 - (A) Tendency of sample to curve. (B) 3D printed Spacer. (C) Spacer on Sonotrode (D) Damage caused by spacer...... 31

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Figure 14 - Sample damage caused by ultrasonic welding. First: Low damage. Second: Major tearing of entire sample. Third: Tearing in foil. Fourth: Foil chipping off on top surface...... 32

Figure 15 - The fresh horn on the left, versus the used horn on the right ...... 34

Figure 16 - Improved welding setups, photos and welds courtesy of Kevin Gordon at Stapla Ultrasonics...... 37

Figure 17 - Sample welded by Kevin Gordon at Stapla Ultrasonics Corporation. This sample features a less aggressive knurl. It is cut in a useful shape for testing to determine the shear strength of the welded joint, and for being used between the electrodes of the electrochemical testing machine. Applications # 6629, 10/15/2015 ...... 37

Figure 18 - Instron 5969 Tensile Testing Machine with 50 kN load cell ...... 38

Figure 19 - Top: Tensile test conducted on a specimen of Nafion. Bottom: Tensile testing on a specimen of Nafion welded to Nafion...... 41

Figure 20 - Shear Strength Test Method ...... 43

Figure 21 - Shear Force Test - Nafion welded to aluminum foil...... 43

Figure 22 - Gamry Instruments Interface 1000 Potentiostat/Galvonostat/ZRA 06109 ...... 46

Figure 23 - Cyclic voltammetry for an IPMC prepared in the normal electroless plating procedure...... 48

Figure 24 - Bare Nafion, Cyclic Voltammetry ...... 49

Figure 25 - Cyclic voltammetry results for Nafion stacked on top of Nafion ...... 49

Figure 26 - Cyclic voltammetry results for Nafion welded to Nafion ...... 50

Figure 27 - Cyclic Voltammetry of sample from the Condor ST-30 machine with aggressive knurling. Shorting out of sample is obvious...... 51

Figure 28 - Left: Sample cut so that foil goes to edge. Right: Stacked Nafion and foil, with Nafion large enough to prevent edge contact...... 52

Figure 29 - A large square of Nafion with a small square of foil below and above it, displaying strange inductive behavior...... 53

Figure 30 - Comparison of Cyclic Voltammetry charts for stacked versus welded configuration for samples from Stapla Ultrasonics Corporation, each with one piece of foil...... 55

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Figure 32 - Microscope images at 10-20x. A/B: This hole, with 2 different lighting modes, shows blackened regions which may be signs of burns. C/D - Other holes found in Nafion layer after welding...... 57

Figure 33 - Sample with hole burned through it. Left- Whole Sample. Right- 10x Microscope image. Note blackened regions and orange Nafion bit...... 58

Figure 34 - Small bits of aluminum foil are left in the Nafion after the foil is peeled back...... 58

Figure 36 - This configuration shows a possibility for using ultrasonically welded IPMCs as a linear actuator. The folded shape on the left is not possible for 2 dimensions. However, the basic idea can be applied in 3 dimensions...... 68

Figure 37 - A construction paper model representing 9 welded IPMCs, with the yellow color indicating the positively charged surface and the purple color indicating the negatively charged surface under hypothetical deflection. It transforms from a relatively flat square shape when folded to long curved shape when unfolded...... 69

Figure 38 - Hypothetical linear force actuator from unfolding ultrasonically welded IPMCs...... 69

Figure 39 - Nafion to Nafion: Weld Time vs. Weld Energy ...... 72

Figure 40 - Tray holding welded samples at each setting...... 72

1 Introduction

1.1 Problem Statement IPMCs, or ionic polymer metal composites, are smart materials which exhibit a bending motion when there is a voltage difference between the two metalized surfaces.

Traditionally, these composites are manufactured by means that are time consuming and expensive, and difficult to control carefully enough to make them consistent in their properties. Current manufacturing processes pose challenges for mass production and adoption of these smart materials.

The electroless plating process of creating the metalized surface is particularly burdensome. In electroless plating, metallic ions are adsorbed into the permeable ionomers. In order to form a layer thick enough to achieve a low enough resistance, this involves carefully monitored chemical reactions which take several hours and a fume hood to conduct safely. The chemicals used are expensive, and the metallic surface is usually made of a precious metal.

If the electroless plating process can be avoided by finding another way to form the metal electrodes, great savings in cost and time can be realized. It is simply not possible with conventional welding techniques to weld a thin layer of metal directly to a polymer material as the heat required to melt the metal would destroy the polymer. But ultrasonic welding is a technique that has found success in combining metals to polymers. Ultrasonic welding binds materials nearly instantaneously simply through

2 vibrations. Ultrasonic welding thus may have the potential to bind the ionomer membrane to a metal foil replacing the tedious chemical baths typically used to metalize the surface.

The current electroless plating process creates a complex interface structure in the region between the ionomer and the metalized surface. One of the difficulties in

IPMC production lies in the reproducibility and fine tuning of the characteristics of the interface structure, and these characteristics have significant impact on the capacitance of the IPMC. Welding a piece of metal directly to an ionomer will create a different interface structure, which requires further investigation. But one advantage of welding one piece of material to another is the potential to control the surfaces of each piece prior to welding. Thus a potential exists to gain more control over the interface through surface treatments.

Another problem in the production of IPMCs by researchers is that they often rely on ionomer membranes that come in flat sheets and only in a few thicknesses.

There are a number of different methods researchers have already found to form thicker ionomer layers, but ultrasonic welding potentially promises to be a faster method of doing so and may also allow for interesting geometric arrangements, such as finger-like projections.

In this first attempt to use this technique, aluminum foils were welded to Nafion membrane weakly, and Nafion membranes were welded to other Nafion membranes strongly. Further work with different equipment will be necessary to make the method

3 viable for replacing the current metallization techniques. Important parameters for weld success include vibration amplitude, vibration direction, the physical shape of the horn and anvil of the welding machines, clamping pressure, and the characteristics of the materials to be welded. In this study, two commercial ultrasonic welding machines were tested with aluminum foils and Nafion ionomer membranes.

1.2 Background information on Ultrasonic Welding.

Ultrasonic welding is method of combining materials through ultrasonic vibrations of around 20 kHz. Essentially, the materials that are to be welded are placed on an anvil, and then a horn, also called a sonotrode, is pressed into the materials and vibrated. The horn will either generally use high amplitude low energy waves or high energy low amplitude waves [1]. In order for the horn to vibrate, an input voltage is converted to alternating voltage of 20 kHz and converted to mechanical oscillations by a reversed piezoelectric effect, and the amplitude of vibrations range from 5 to 50 μm [2].

1.2.1 Metal Welding Technique versus Plastic Welding Technique

There are two primary methods for ultrasonic welding. One is the plastic welding technique, which involves the sonotrode vibrating longitudinally into the sample. The other is the metal welding technique involves the sonotrode vibrating in the transverse direction [3]. Figure 1 illustrates the general arrangement between the horn, the anvil, and the materials to be welded.

Figure 1 - Two materials between horn and anvil. Comparison of the transverse vibration metal welding method versus longitudinal vibration plastic welding method.

While the two different ultrasonic welding methods are named after the most common types of materials welded with each method, the most important reason ultrasonic welding is of interest for this study is because of the promise it has shown for welding two dissimilar materials together by past researchers. Conventional welding techniques are often unsuitable for welding dissimilar materials because the melting point for one material would destroy the other. But ultrasonic welding is not the only technique that has been used to combine metals and polymers. In 2011, Amancio et. al did a study where different methods of joining polymers with metals were used, and found that ultrasonic welding created a stronger bond than other methods such as friction spot welding, induction welding, friction stir pot welding, and adhesive bonding

[4].

Wagner et. al, in 2013, showed that aluminum could be welded ultrasonically to a glass fiber reinforced polyamide 66 polymer, and they showed the success of doing so with both the ultrasonic metal welding technique as well as the ultrasonic polymer welding technique. They found that the ultrasonic metal welding method produced a much stronger bond, but also that it caused the metal to displace the polymer, as shown in figure 2 [5]. According to Balle and Eifler, the transverse waves can heat the polymer beyond the melting point and cause it to flow towards the edges of the horn [6].

Overall, the amount of research done involving the ultrasonic welding of metals other than aluminum to polymers is limited, but Ramarathnam did discuss the joining of steel to polymers in a 1992 paper on joining of polymers to metal [7].

1.2.2 Weld Parameters and Variables

To create and optimize an ultrasonic weld there are several parameters involved.

These include surface roughness, temperature of the weld, [8] surface morphology, thickness of the materials to be welded, the welding force, oscillation amplitude, welding energy, welding time, and the properties of the materials [9]. In some cases, in order to join materials better with ultrasonic welding, the parts need to be geometrically designed to take advantage of the ultrasonic welding technique.

Essentially, this is done by designing energy director structures into the pieces that are to be welded together. For a polymer material, these structures are usually cast.

Tateishi et al. explains that an energy director is a projection which helps create the ideal conditions for creating stress on the asperities of the contacting surfaces to generate heat in the bond region.

Another technique for ultrasonic welding described by Tateishi et al. that is especially useful for bonding dissimilar materials is to use a tie layer. A tie layer is a substance inserted between two substrates to be welded. This inserted layer preferentially absorbs ultrasonic energy thus promoting melting in the bond zone, and can potentially bind materials that would be difficult to directly weld ultrasonically.

Figure 3 illustrates how these methods work [10].

Figure 3 - Two different ultrasonic welding aids: (a) Energy Directors (projections) and (b) Tie-Layers. Figure redrawn from a similar figure in Tateishi et al. [10]

1.2.3 Application of Ultrasonic Welding to IPMCs

Due to the success of others in the ultrasonic welding of polymers to aluminum, and the good corrosion resistance and conductivity qualities of aluminum, choosing to research the bonding of aluminum foil to Nafion by the ultrasonic welding technique seems sensible. The ready availability of cheap aluminum foils also makes aluminum foil a good choice for a starting point.

One of the best things about the ultrasonic welding technique is the short weld times. Ultrasonic welding of ionomers in particular is seldom described in literature, but

DuPont's Surlyn Fastening guide discusses that ultrasonic welding of their Surlyn ionomer material produces a "strong homogenous bond with strength approaching that of the original material" and it only takes 8 seconds plus 2 seconds of holding time to ultrasonically weld Surlyn resins chilled to -20° C [11].

IPMC manufacture typically involves a very time consuming process that involves electroless plating a layer of platinum to the outside surface of the ionomer which provides for the electrodes. If a metal foil could be welded to Nafion on the order of 8

8 seconds, this would be a major advantage over the current plating process. The current electroless plating process must be repeated numerous times and takes upwards of 25 hours to complete.

If ultrasonic welding of Nafion to aluminum foil can be achieved without destroying the ionomer in the process, there is one additional hurdle though. Ultrasonic welding of a smooth metal foil to a polymer is unlikely to create an interlayer interface with similar interface structure to that found in IPMCs manufactured through the electroless plating process.

The structure of this interlayer interface is critical to the proper functioning of an

IPMC. For instance, according to Aureli et. al in a 2010 paper on energy harvesting using an IPMC, the capacitance of the IPMC is critical to its utility as an actuator or energy harvester. This paper describes that the electrode roughness and fractal nature of the

IPMC interlayer interface developed during normal IPMC fabrication is an essential contributor in developing the large IPMC capacitance [12].

According to Naji et. al, as platinum is left behind on the surface through repeated plating processes, the layer of platinum becomes increasingly thick and the capacitance also becomes increasingly higher until the plating process has been repeated about 8 times, creating a porous and high surface area electrode. Naji includes excellent back-scatter electron images of the Nafion-Platinum boundary layer demonstrating this, whether the IPMC is made from commercial membranes or cast

Nafion [13]. Figure 4 demonstrates what the structure of the IPMC metal interface

9 between the polymer looks like, and the shape in the figure was formed by tracing the outline on one of his backscatter electron images. Future work will need to be done in order to modify the interface structure of the welded IPMC to provide similar electrochemical characteristics to the ordinary one manufactured by electroless plating.

Figure 4 - This drawing was made by tracing the outline of an electron backscatter micrograph from Naji et. al [13], showing an IPMC electrode. Note the extremely complex morphology.

While ultrasonic welding as a method to replace the electroless plating process would be a very useful potential application of ultrasonic welding to the manufacture of

IPMCs, using ultrasonic welding bond Nafion to Nafion is also an interesting topic. Since welding dissimilar materials is more difficult, ultrasonic welding of Nafion to Nafion is an easier accomplishment than using ultrasonic welding to form the metalized surfaces of an IPMC. However, there are also presently quite a few different alternative methods available to bind the Nafion membranes, and not so many methods to replace the electroless plating process. The other methods currently used to form customized

Nafion membranes will be discussed in the next chapter of this thesis. Many of these methods have drawbacks which could potentially be overcome with new techniques.

One application of ultrasonic welding that has been investigated by Beck et al. that is similar to the application in this study is the ultrasonic welding of Nafion to carbon paper based gas diffusion electrodes for fuel cells [14]. However, his group did not investigate the welding of Nafion layers together nor the welding of Nafion to a metal foil electrode.

1.3 Background Information on IPMCs

1.3.1 Function and Use

IPMCs are electroactive smart materials that consist of a hydrated and ionized ionomer, or ion exchange membrane, surrounded on both sides by an electrode layer.

The ion exchange membrane has immobile ions in the structure, but in the solution that hydrates the membrane, there are mobile ions are free floating. As a voltage difference is applied to the electrode surfaces a bending motion is created, and conversely, an

IPMC that is bent will develop a current. The bending motion occurs in response to an applied voltage, because free floating ions drag their solvent with them to one side, causing a swelling on that side. Additionally, the fixed ions near the electrodes will also interact with the surface, cause stretching on one surface and contraction at the other.

De Luca et. al does an excellent review paper where this is explained and there is a great illustration that demonstrates how the IPMC functions [15]. A similar figure is shown in figure 5.

Because IPMCs bend when there is a voltage difference, they can be used as an actuator, and because they generate a current when bent, they can be used as a sensor or an energy harvester. Their potential for an actuator is particularly high because they create very large displacement bending motions, are soft, and do not generate a lot of noise [16].

1.3.2 Conventional IPMC Manufacturing Process

The normal process for creating an IPMC consists of a few different stages. The exact procedure varies depending on the intended results, but the general outline of the process will be similar. This particular process is from a manual written by Marissa

Tsugawa and is a recipe developed by Viljar Palmre. It is very similar to one described in a 2014 paper written by Palmre et. al, studying nanothorn electrodes in IPMCs [17]. This recipe was also used to form a sample that was tested for comparison purposes in this study.

This recipe starts from a sheet of commercial extruded activated Nafion ionomer membrane, from which a suitable sized piece is cut off. The first stage of converting this Nafion to an IPMC is surface preparation and cleaning. This process involves a light sanding of the surface followed by chemical baths of heated water, hydrogen peroxide, and concentrated sulfuric acid. The 1 M sulfuric acid used typically needs to be prepared by diluting a more concentrated acid. The acid baths must be handled with care and the process must be monitored. This surface preparation stage lasts about 6 hours. Then there is a primary plating stage involving the use of platinum ions, sodium borohydride, acid and water baths. This stage involves an 8 hour procedure, but must be repeated at least twice. Then, there is a final plating stage which involves Pt(NH3)4Cl2, hydroxylamine hydrochloride, and hydrazine hydrate. The hydrazine hydrate is an extremely dangerous chemical to handle and requires the use of a fume hood. This stage takes about 3 hours, but also may need to be repeated until the

13 measured resistance between the most distant points on the surface of the strip show a resistance of less than ten ohms. Once the final plating stage has been completed, the metalized surface for the IPMC is completed. But the process is not finished yet. A final cleaning stage is still needed, so the sample is put into acid and hot water baths again for a couple of hours. Finally, the sample is allowed to sit in a lithium chloride solution for 24 hours to properly ionize it. Figure 6 shows an IPMC being prepared in a lab at the

Advanced Research Facility in the University of Nevada, Reno.

Figure 6 - Left: Beaker on hot plate with clear Nafion. Right- Same sample after metallization.

Altogether this is a minimum of about 25 hours in the lab before being set in the lithium ion solution, and requires handling of very toxic and very caustic substances.

Other recipes, such as the one from Oguro from the Osaka National Research Institute

[18], are very similar in the overall process, but have minor differences in details for the

14 specific chemicals and their concentrations as well as chemical bath durations. The specific details of each recipe are provided in appendix A8.

Switching to an ultrasonic welding method to metalize the surface intends to reduce the time and risk involved with the preparation of IPMCs by eliminating almost all of this process. Allowing the metalized membrane to soak in lithium chloride or another ionic salt to saturate the ionomer with mobile ions will still be necessary, but the most grueling and dangerous parts of the procedure might be eliminated if ultrasonic welding can replace the electroless plating process.

1.3.3 Materials used for IPMCs

In 2007, Park et al. did a study on IPMCs using a number of different materials used for an IPMC electrode surface, comparing those made of platinum, palladium, and gold. Park et al. also tested IPMCs using ionic liquids other than water [19]. A 2012 review paper by De Luca et al. also discusses the use of other metals such as silver, and copper. That review paper states that non-noble metals are prone to oxidation when used as electrodes for IPMCs. Conductive polymer layers were also discussed in lieu of metallic layers.

For the membrane itself, De Luca et al. describes that while there are a number of ionic polymers to choose from, Nafion or Flemion are the most common, with Nafion being the most common of the two [20].

In the study for this thesis, Nafion hydrated in water is used for the ionomer, and aluminum foil is used to form the electrode surface. Nafion was chosen as it is the most common material used for IPMCs, and aluminum foil was chosen mainly because of the successful welding of aluminum to polymers in other research. The low cost and ready availability of the foils is also an important influencing factor in this decision. While aluminum is not a noble metal, it is fairly resistant to corrosion in water and is a good conductor. If aluminum foils can be readily welded to form an IPMC, other metals that may have more desirable properties might be ultrasonically welded in place of it in the future.

1.3.4 Membrane Customization Techniques

The ionomer material used for the manufacture of IPMCs is normally obtained in sheets of a specific thicknesses. For instance, Nafion membrane is commercially available at thicknesses of 127, 183, or 254 microns from a DuPont, and only generally in a rectangular shape [21]. However, there are some other techniques presently in existence that allow one to form polymer ion exchange membranes of different shapes and thicknesses.

In 1992, Moore et. al investigated melt-processable and liquid solution Nafion membranes, but found that they behaved differently than a commercial membrane due to differences in crystallinity and electrostatic cross links [22]. And in 2002, when Kim and Shahinpoor investigated the use of IPMCs for artificial muscles, they found a need

16 for custom three dimensional shaped IPMCs and membranes thicker than commercially available. For Kim and Shahinpoor, the thicker IPMCs were desired to generate greater forces for the same amount of displacement. Kim and Shahinpoor found that an IPMC of twice the thickness could provide a force eight times as great with the same displacement. They used a process called solution recasting to get the thickness they wanted. This process involves pouring one layer of an ion conducting powder mixed with an electroactive polymer solution, then pouring a layer of just electroactive polymer solution, and then pouring a third layer of ion conducting powder mixed with electroactive polymer solution. Then the layers are thermally cured at elevated temperatures. Finally, they electroplate or electrolessplate metal onto the surface.

Compared to commercially extruded membranes, they found that the solution recasting method resulted in membranes that were much more brittle and cracked easily. They had to be treated with additives to reduce these negative qualities [23].

De Luca et al. also described that the solution recast method is difficult to use to create repeatable results, due to the large number of variable factors such as temperature and concentration of solvent [24].

Seong Kim and his group of researchers did a study on another method of forming thicker IPMCs through the hot-pressing method. In this method, multiple commercial extruded membranes are melted together. That study indicates that hot- pressing is a much simpler and more repeatable process than the solution recast method. But they still noticed some changes in the basic characteristics compared to

17 single layers of Nafion. They found that the hot temperatures resulted in a slight increase in Young's modulus in the material in a specific layer, but also found that the

Young's modulus of the composite decreased with increased numbers of layers.

Laminated plate theory was used to model this, considering the interface regions to have different properties. Seong's group also found that the adhesion strength of the hot pressed Nafion was significantly less than the maximum tensile strength of the

Nafion. However, they found that the strength was sufficient for use as an IPMC and that the overall changes in properties were small [25].

In 2015, Carrico et. al did a study on the fused filament 3D printing of Nafion membranes. 3D printing is a process which can easily create diverse geometries. Their results showed that in some ways IPMCs produced with this method surpassed commercially extruded membrane IPMC performance, showing increased tip deflection, for instance. On the other hand, their response time was considerably slower than for an IPMC produced with commercially extruded membranes [26].

These existing methods of creating custom ionomer shapes and thicknesses for the purposes of creating an IPMCs create a lot of options but most of them are not without limitations. Therefore, new methods are worth pursuing. Without further testing, it is not clear what specific effects the ultrasonic welding technique may have on the Nafion membranes or the IPMCs made from them. Ultrasonic welding of Nafion layers may be a potentially prove a useful method to use as an alternative technique or possibly in conjunction with these methods.

The solution recast method allows for interesting geometries to be formed, but with a lot of downsides to the extruded membranes physical properties.

The hot-pressing method seems to be reported favorably, but ultrasonic welding is still a faster process, and it is possible that the adhesion strength of ultrasonically welded might be higher than the adhesion strength of the hot pressed Nafion. For a typical IPMC, the low adhesion strength of hot pressed Nafion and the generally rectangular shapes may not be a major concern, but possibly it could be a concern for future applications that require different geometries or that will be subject to unusual sources of damage from their operating environments. While both the ultrasonic welding method and the hot pressing method will create an interlayer region with different properties than the bulk medium, the two methods will create a significantly different interlayer region. Without testing it is not possible to know which interlayer region is superior. Also, the overall heating effect that changes the young's modulus of the bulk material may not exist if ultrasonic welding is used instead, as heating from ultrasonic welding is not likely to be as intense, nor as uniform.

The 3D printing method appears to be quite promising for creating complex shapes although for some applications the slower response time could be undesirable.

While the response time of ultrasonic welding is not known, it could be fast.

The solution recasting and 3D printing methods could also potentially be used to complement the ultrasonic welding method. Commercially extruded membranes have flat surfaces, but a cast membrane could be cast with projections that can be used as

19 energy directors or energy directors could be 3D printed. These energy directors may be used to improve the weld strength with the ultrasonic welding technique.

2 Experiments

2.1 Experimental Setup

Figure 7 - Left Branson KET 1 Ultrasonic Plastic Welder With KG1 Sonotrode. Right- Condor ST-30 Ultrasonic Metal Welder

In figure 7, the equipment used to attempt to weld the samples is shown. The

Branson KET-1 ultrasonic plastic welding machine has a smooth sonotrode that is pushed by hand against the sample to be welded.

The Condor ST-30 ultrasonic metal welder uses air pressure to clamp the sample between the horn and the anvil. The air supply for the Condor was set at 81 PSI, and then calibrated. This welder came equipped with an anvil, part number 300-A01-811A, with a 0.010"x0.020" diamond knurling. It also came equipped with a horn, part number

300-A08-679A. This horn is a 4mm pad also with 0.010"x0.020" diamond knurl that has

0.001" DP flats. According to the company that made the welder, this setup was not intended for a thin film application; direct contact between the horn and the anvil, both of which are made out of steel, could cause tooling damage. As a result, the original anvil surface was not used directly.

The Nafion specimens used for welding were all Nafion N117, which measures

183 microns in thickness according to specifications. The aluminum foil used was

Reynold's Wrap brand, both the standard thickness and the heavy duty, which measure

.000725" and 0.0011" in thickness respectively. In Appendix A1, a table showing the thicknesses of other substrates and welded structures from this study are provided.

To determine suitable weld settings, a sweep of the three parameters that could be modified was conducted. These three settings are the pressure, amplitude of vibration, and weld time. For the Condor welder, the number of pistons to generate pressure can also be modified, with the choices of using 2, 4, or 6 pistons. According to the manual, the pressure generally varies from 1.5 to 6.5 bar. With this machine, amplitude varies from 1 to 10, where an amplitude of 1 corresponds to 12 microns, an

21 amplitude of 5 corresponds to 18 microns, and an amplitude of 10 corresponds to 24 microns.

Initially the provided anvil, as shown in figure 8, was used. This anvil has very rough surface similar to that found on the sonotrode. However, due to the possibility of tooling damage from the anvil and sonotrode vibrating directly against each other due to the thin sample size, initial welding was conducted on the surface of an aluminum ruler placed on top of the anvil. This produced very unpredictable results as the ruler would often stick to the sample and need to be repositioned. As a result, a smooth piece of aluminum was screwed into position in place of the anvil as shown in figure 8.

Figure 8 - (A) Included anvil and sonotrode (B) Aluminum plate for anvil with yellow 3D printed plastic spacer on sonotrode. (C) Wear caused on aluminum plate by horn.

2.2 Nafion to Nafion Welding

With the Branson welder that utilizes the plastic welding technique, it was not possible to weld Nafion to Nafion. Since the plastic welding technique is intended for binding plastic to other plastics, probably there is a limitation in the hardware or setup that prevented a weld. The Branson welder is an old device, but it was tested for

22 function by welding together some scrap pieces of ABS and PLA plastic from 3D prints.

With these materials it was difficult to accomplish a weld, but welds were achieved with certain surface geometries that likely served as energy directors.

There are a number of likely reasons why the Branson sonotrode did not work to weld the Nafion. The Branson sonotrode used is almost completely smooth, and the

Nafion samples are also smooth, so there may have been too much slipping or insufficient heating at the interfaces. In an attempt to resolve this problem, welding was attempted again with Nafion roughened with sandpaper, but this still proved insufficient to allow for a weld. While the Branson welder was not able to achieve a weld, it is probably possible to weld Nafion to Nafion with the plastic welding technique. To do so may require special geometries for the Nafion, a different Sonotrode head, or a different welder with more controllable parameters such as weld pressure and a greater range of output. The Branson welder allows modification of weld time and output, but the only way to weld with it is to press into the sample by hand, meaning that weld pressure could not be controlled. Perhaps a slightly rougher sonotrode may have also produced better results. One possibility also, is that if Nafion was etched, cast, or 3D printed in such a manner as to leave pointed projections on the surfaces, that these could serve as energy directors which may allow for the plastic welding method to work.

2.2.1 Nafion to Nafion Weld Parameters

Using the metal welding method and the Stapla welder, Nafion to Nafion welding was readily achieved at a wide range of settings. A sweep of possible amplitude and weld times were conducted, and most of these settings produced a bond. The weld table in Appendix A2 shows some of the settings tested and the results. While many settings produced what appeared to be a good weld, some settings resulted in more surface damage than others. Overall, a weld setting with a pressure value of 1.5, a weld amplitude of 7, and a weld time of 0.5 seconds seemed to form a good bond without excessive damage.

2.2.2 Repeated Welds: Nafion to Nafion

One issue with the current setup is the size of the sonotrode tip. Each time a weld is created, it only welds a small region together the size of the sonotrode tip. In order for this method to be useful for IPMCs, a larger welded region will also be necessary. The sonotrode used created a weld region that was a rectangle approximately 4mm by 4mm, but an IPMC is generally several times longer in one direction and two or three times wider in the other direction.

For connecting Nafion to Nafion, it was possible to enlarge the welded region by doing repeated welds. One phenomenon found with doing repeated welds however, was that the sample begins to curve towards the direction of the horn after multiple welds. It was found that this curving could be eliminated simply by alternating between

24 welding on the top and then the bottom of a sample for each weld in a line, or presumably in a checkerboard pattern if made wider than a single weld. This phenomenon and the solution are illustrated in figure 9. The resulting specimens are very flexible and not very fragile. They can be bent through the normal range of motion that an IPMC would undergo without failure.

2.3 Aluminum Foil to Nafion Welding

No bonding between Nafion and aluminum foil was achieved with the Branson ultrasonic welder. Using the Condor ST-30 Ultrasonic welder however, it was possible to

25 bind the aluminum foil and the Nafion ionomer. Welds were achieved both using a single piece of aluminum foil, as well as by sandwiching a piece of Nafion between two pieces of aluminum foil. Appendices A3, A4, A5, and A6 provide some weld tables and experiment results for different configurations of foil and Nafion.

2.3.1 Nafion to Aluminum Foil Weld Parameters and Factors

When welding Nafion to Nafion, a wide range of settings seemed to produce a good result. However, for welding the aluminum foil to the Nafion, it was very tricky to set the parameters properly to get even a decent weld. Tuning of the three configurable settings, the amplitude of vibration, pressure, and weld time, is critical. The ideal weld would result in a strong bond between the aluminum foil and the Nafion with no damage to either, and the bond would withstand hydration in water. However, many weld settings resulted in extreme damage to both. In some cases, with too much energy, the Nafion would combust entirely into a small fireball leaving very little behind.

The thickness of the foil used also plays an important role. Using the thinner standard 0.000725" thick aluminum foil, there was less success in welding the Nafion to it than with the thicker heavy duty 0.0011" thick foil. With the thinner foils, many settings would do nothing, and as soon as bonding occurred, damage to the sample tended to come with it. While not all weld settings caused destruction of the thinner foil, damage to the thinner foil was difficult to avoid. Figure 10 shows how with the thinner foil, holes would often fully penetrate the aluminum, whereas with the thicker

26 foil this did not appear to happen as often. Probably, a sonotrode with less aggressive knurling would have a less damaging effect on the thinner foils.

Starting with a fresh materials for a single spot weld, it seemed like many weld settings produced fairly consistent results. Testing numerous settings, weld parameters capable of binding a layer of Nafion to a layer of aluminum foil both above and below it, and capable of maintaining this bond after being submerged in water, were identified.

Figure 10 - Left: Thin foil with holes going all the way through. Right: Thicker foil not punctured.

One weld setting that seemed to work favorably for bonding aluminum foil to both sides of the Nafion in one weld is a pressure setting of 2.2 (with 4 pistons), an amplitude setting of 7, and a weld setting of 0.25 seconds. Stapla Ultrasonics

Corporation recommended the use of 2 pistons instead of 4 for thin samples, but the bonding seemed less reliable than with the use of 4 pistons. With 4 pistons, decreasing the pressure down to 2.0, decreasing the weld time to 0.20 seconds, and increasing the amplitude from 7 to 10 could be done and still obtain results that stayed together after

27 hydration. Tables in Appendix A3 describe the results of combining the metal to the foil and submerging it in water at various weld settings.

While the weld settings seemed to work pretty well for the machine at first, with extended use of the machine tooling damage seemed to change the results. As a result of tooling damage, long-term weld repeatability was low.

In addition to problems with consistency there were a lot of potential problems with the characteristics of the welded pieces that would cause issues if they were to be used for a functional IPMC actuator.

One result of having a different surface texture on the anvil versus on the horn is that it causes a difference in texture between the top and the bottom of the welded samples, as shown in figure 11. Assuming that the surface characteristics have an impact on the electrical capacitance or structural rigidity, this may have an undesirable effect on a completed IPMC, whereby deflection in one direction may be greater than the deflection in the other direction.

Figure 11 - Left: Bottom of welded sample, in contact with the smooth aluminum anvil during the weld. Right: Top of the same sample, showing effect of aggressively knurled horn.

If there was the same knurling on both the anvil and the horn, a consistent surface could result. Also, if the top layer of aluminum foil was welded onto the Nafion

28 separately from the bottom layer of aluminum foil, this might also result in a consistent surface. One potential solution would be to weld each metal surface onto the Nafion in a separate step, flipping the sample over between welds. However, this did not work when it was attempted. Instead, it caused the previously welded metal to become undone. It seemed that welding in one step was more reliable.

Temperature and surface characteristics of the welded region are also important. Attempting to weld the same region multiple times without enough cool off time increases the probability of the weld energy becoming too high and the Nafion burning up. Generally, results are also only consistent for one weld on the same part of the sample. If a sample is welded once, and then falls apart, and one attempts to reweld the same sample again, changes within the sample are enough to cause the previous weld setting to have unexpected results. Rewelding a sample will sometimes work with the same settings but often will not. When making repeated welds on a strip to create a longer bond region, the tendency for a previous weld to become undone was made exacerbated by the unpredictability in trying to reweld.

2.3.2 Repeated Welds: Aluminum Foil to Nafion

Assuming that the difference in texture between the top and bottom metal surface could still produce an acceptable result for making an IPMC, a larger weld region than the size of the sonotrode tip will also be necessary. Therefore, repeated welds on a strip need to be successfully conducted to form a weld region the size of a typical IPMC.

Repeated spot welds in close proximity were attempted in this study. However, welds that were positioned too close together often caused the previous weld to separate, or would often be destructive to the surface near the welds. Additionally as repeated welds on the same side were used, the material began to take on a significant curve, just as was the case when welding Nafion to Nafion. But the workaround used with the Nafion to Nafion welding was no solution this time. Simply flipping the sample over for each weld would not work, as in this case it would always cause the previous welds to become undone. The curvature is apparent in figure 12.

Figure 12 - Repeated welds caused the sample to curve.

Also, bending the curved welded sample by hand in the opposite direction of the curving direction causes the sample to separate due to inadequate weld strength. It also subjectively seemed that when multiple bonds were done adjacent to each other, that the overall strength of the bonds were weaker and the samples fell apart more easily.

This low weld strength would pose a major problem for an IPMC that would be expected to bend back and forth in both directions. One attempted solution to the curving problem was to use a 3D printed plastic spacer as shown in figure 13 to keep the sample flat during the welding. However, attempting to use plastic spacers introduced new problems. The plastic spacers exhibited melting and wear through use, which lead to sharp protrusions. The sharp protrusions that formed on the surfaces of the plastic spacers caused damage to the surface of the foil.

Figure 13 - (A) Tendency of sample to curve. (B) 3D printed Spacer. (C) Spacer on Sonotrode (D) Damage caused by spacer.

Debonding after a subsequent weld was not limited to occurring when trying to weld on alternate sides of the strip in order to prevent curving of the sample. Even repeated welds on the same side would sometimes cause old welds to come undone, particularly when there was substantial overlap between the previous weld and the new weld. This problem seemed to worsen as the tooling became more worn out. At one point, it seemed that using a good weld setting for repeated welds in a row could be achieved with the main problem being the sample curling. But perhaps because of tooling wear, it quickly got to the point where damage to the sample became most likely

32 unless the welds were separated by a substantial gap. The types of damage that were seen when trying to do repeated welds are shown in figure 14.

Figure 14 - Sample damage caused by ultrasonic welding. First: Low damage. Second: Major tearing of entire sample. Third: Tearing in foil. Fourth: Foil chipping off on top surface.

A specially designed sonotrode with something built in to keep the sample flat would probably work better than the plastic spacer. Possibly also, if the anvil and sonotrode had a similar texture, curving might be reduced. The more aggressive knurling on the sonotrode side may be causing the surface to become stretched wider compared to the smooth anvil side, thus causing the material to curve towards the sonotrode. Probably, one of the best solutions to the curving problem would be to use a larger welding surface on the sonotrode that would be the size of the desired IPMC. This should prevent curving as well as the repeat weld debonding problems. Possibly, it might increase weld strength too if there is one large weld rather than multiple welds that potentially interfere with each other to some degree.

2.4 Damaged Tooling

After producing many welds, damage to the tooling was noticed. The aluminum panel used in place of the provided anvil quickly became pitted from interaction with the sonotrode. Visible wear on the aluminum anvil began immediately with the first weld. This was predictable considering that the panel was soft aluminum and the only thing separating it from the horn was a about 0.2 mm of material that often got destroyed in the process of attempting a weld. Normally, the ultrasonic welder is supposed to be set to maintain a small gap height between the anvil and the horn, but when welding thin foils and sheets it is difficult to maintain sufficient pressure to hold the pieces together without the anvil coming in contact with the horn. This causes wear of the tooling.

When the aluminum panel began to become too damaged, it became apparent because the foil would begin to tear and have holes in it despite previous successful welds without damage using the same settings. The solution to this problem with this setup is that the aluminum panel had to be removed from the system and polished smooth every few dozen welds, or the samples would begin to become damaged. Each time the panel is replaced after polishing, it also needs to be carefully leveled or it causes an uneven pattern on the sample which tends to damage the edge of the sample closer to the horn and causes peeling at the far edge.

Eventually however, it became apparent that more serious tooling damage was occurring. The first indication was when weld settings that had worked well previously with the new machine started to not work any longer or produced surface damage to the foil, even with the aluminum panel used for the anvil freshly polished to be smooth. When this started to happen, it was noticed, as shown in figure 15, that the horn itself had begun to be damaged. Also it was damaging samples due to uneven wear patterns.

Figure 15 - The fresh horn on the left, versus the used horn on the right

Attempts to rectify this issue by smoothing out the pattern on the sonotrode with sandpaper helped alleviate uneven wear patterns but resulted in different weld characteristics. Since the horn knurling effectively changed, old settings no longer performed the same. Why the horn itself suffered damage is unclear. It could simply be wear from the hundreds or thousands of cycles of use. But this wear may have been accelerated through the nearly direct contact of the aluminum panel used in place of

35 the original anvil. Deformation of the plate, slight deviations from level, and high temperatures caused by igniting Nafion may have added to wear by causing higher pressure regions combined with a weakening the metal under high temperatures.

Not long after noticing the wear on the knurling of the horn, the machine suddenly stopped working. During welds the machine would only fire the ultrasonic vibrations for a moment and would shut off without completing the whole duration of the weld. Also, the machine stopped reporting weld energy and weld power. The machine had to be sent back for service, preventing further investigation of ultrasonic welding with this machine in this study. Whether this total stoppage of the machine was a result of this welding application for which the machine was not originally intended, or due to other causes is unclear.

2.5 Stapla Samples with Less Aggressive Knurling

Kevin Gordon, the Applications Engineer Manager at Stapla Ultrasonics

Corporation, generously conducted some welds between Nafion and Aluminum Foil using ultrasonic welding machines that he had access to that were configured more towards the ultrasonic welding of thinner materials than the ultrasonic welding machine we had. He generated some welds using a horn with a wider, flatter profile of 0.030" x

0.015" with flat sections, and using an anvil with an EDM knurl as shown in figure 16.

He obtained a weld with a less aggressive horn knurling but noted that the weld strength was still low and the process was inconsistent. With his machine set for a

36 pressure of 4.8, an amplitude of 10, and a weld time of 1.25 seconds, he generated some welds. His weld energy was 354 J, which was about twice as much energy as our machine could get into the welds without having the Nafion burn up. Nonetheless, his welds still separated with little effort. He also tried another test with a 0.020"x0.010" sharp knurl horn and an anvil with a 0.020"x0.010" Knurl. His results are labeled under

Application # 6629 at Stapla Ultrasonics Corporation. Receiving his samples, shown in figure 17, the weld strength seemed subjectively similar to the ones we had welded.

In addition to welding some samples, he made some recommendations that might be useful for producing better welds for these types of materials in the future.

Kevin recommended manually raising and lowering the horn in steps separate from the firing of the ultrasonic vibrations, as well as deactivating afterbursts. By waiting an extra few seconds and manually releasing the horn after a weld, it allows for the sample to reach thermal equilibrium. Afterbursts, which occur just after a weld, help prevent a sample from sticking to the horn or anvil. But they may also disrupt a sensitive weld.

This advice did seem to improve weld quality.

Kevin also recommended using a coating on the foil, which would presumably work as a tie-layer. The only coated foil that there was time to test before the welding machine stopped working was the Reynolds Non-Stick Foil but better adhesion was not obtained. Some weld data in a table can be found in Appendix A6.

Figure 16 - Improved welding setups, photos and welds courtesy of Kevin Gordon at Stapla Ultrasonics.

2.6 Bond Strength Testing

In order to test the strength of the welds formed by in this study, the Instron 5969

Tensile Testing machine as shown in figure 18 was used.

Figure 18 - Instron 5969 Tensile Testing Machine with 50 kN load cell

2.6.1 Tensile Testing of Nafion to Nafion Welds

It is interesting to see how welding Nafion to Nafion affects the tensile strength.

To determine tensile strength, both ends of the sample are pulled in opposite directions until the sample breaks. Knowing the maximum force applied before failure, and the cross-sectional area of the sample, the ultimate tensile strength can be determined according to Equation 1.

Equation 1 - The ultimate tensile strength is equal to

the tensile force divided by the cross-sectional area.

A single layer of Nafion measured 0.0068 inches wide using a Vernier micrometer and 6.5 mm wide with a digital caliper. It broke with a tensile force of

13.587 N. This corresponds to an ultimate tensile strength of 12.1 MPa. A welded sample consisting of two layers of Nafion measured .0151 inches on Vernier micrometer and 5.1 mm wide with a digital caliper. It broke with a force of 26.8 N, reaching a maximum ultimate strength of 13.1 MPa. The results of these tensile tests are shown in figure 19.

The welded sample was the long repeat welded strip in figure 9. There was a small non-welded region on either side of the welds. These welds were formed using a pressure setting of 1.5, an amplitude setting of 7, and a free weld time of roughly half a second. The weld operation for the welds in this sample were conducted manually in a mode where the machine continually welded for as long as the button was depressed,

40 rather than using a specific weld time for each spot weld. The increased tensile strength over the single piece of Nafion was unexpected. The machine failed to zero after breaking the single Nafion, and started the second tensile test at -2.4 N. The plot was adjusted to only show positive numbers by adjusting all recorded force up by 2.4 N. But even if somehow this data was off by 2.4 N, that would still place the welded strip at a maximum ultimate strength of 11.9 MPa, which is extremely close in strength to the strength of the unwelded Nafion.

Figure 19 - Top: Tensile test conducted on a specimen of Nafion. Bottom: Tensile testing on a specimen of Nafion welded to Nafion. Tensile testing of long sections of Nafion with aluminum foil welded on both sides was not possible. The samples curved too much and flattening them out enough to conduct a tensile test results in debonding.

2.6.2 Aluminum / Nafion Weld Shear Strength Test

In order to determine the shear strength of a weld, a tensile test can be conducted, and knowing the area of the weld region, the stress can be calculated by dividing the maximum tensile force by the bond area. The test method and the equations are shown in figure 20. The sample used to conduct the test was one provided by Stapla as shown in figure 17. The weld area was a square that was 5.4 mm on each side. The Instron 5969 tensile testing machine equipped with a 50 kN load cell was used, and in figure 21, the load chart for this tensile test is shown. The maximum tensile force applied before breaking was 4.275 N. Therefore, the shear strength was determined to be 0.15 MPa.

Shear strength testing of a Nafion to Nafion weld was not conducted. But based on the ease of separating the welds between aluminum and Nafion, which often happens accidentally during handling, and the relatively strong bonds between two pieces of Nafion, the shear strength of the Nafion to Nafion weld is likely at least several times greater. During the tensile testing of the Nafion to Nafion welded strip, delamination was not significant and the two broken pieces remained stuck together.

Figure 20 - Shear Strength Test Method

Figure 21 - Shear Force Test - Nafion welded to aluminum foil.

2.7 Ultrasonic Welding Overview

With the limited equipment available for testing at the time, the result were mixed. The only welds that seemed consistently reliable were welds between two layers of Nafion. Although there was some adhesion between aluminum foil and the Nafion, overall the process with the current set up is fraught with difficulties and the weld quality is not very good. Many of the samples that were welded suffered either from a very weak bond strength or suffered from surface damage to the foil inflicted by the very rough sonotrode surface. Tooling damage also made it difficult for parameter tests to be compared as the damage affected weld consistency. While the failure of the machine prevented further testing, many problems with this particular setup were identified before the machine failed and further testing of the machine in this specific configuration may not have produced extremely useful results.

The welds between the foil and the aluminum were very low strength, and not likely strong enough to withstand the typical bending motions of an IPMC. One possibility is that ultrasonic welding between aluminum and Nafion is simply not easily done. The welds that did occur typically occurred at relatively high pressures. Much of the adhesion may simply be due to a staking effect from the foil pinching into the Nafion that is pushed by the ultrasonic vibration into the gaps left by the aggressive knurling of the sonotrode.

While ultrasonic welding is a favorable technique to combine dissimilar materials, and Wagner had success bonding aluminum to polyamide 66, it seems that the uncoated aluminum foil and the Nafion are not capable of forming a good ultrasonic weld.

2.8 Electrochemical Tests of Welded Samples

The original goal of this study was to improve the manufacture process of the

IPMC. Therefore, electrochemical testing of the material is of critical interest. Due to limited success in maintaining a bond between hydrated Nafion and aluminum foil compared to dry Nafion and aluminum foil, electrochemical testing was initially delayed in search of a better bond. While some weld settings did allow the samples to remain stuck after submersion in water, a really good bond was not found, and so testing was conducted on what we had.

The goal of the electrochemical testing was to determine whether the welded samples could be suitable for IPMC production. Determining the complex impedance was an important goal. One type of electrochemical test of importance is the cyclic voltammetry test, which can be used to determine the capacitance of the sample. There are some other tests that can also be run to determine frequency dependent impedances and other useful metrics. But running a few cyclic voltammetry tests, the results indicated that the current sonotrode produced welds that were destructive to

Nafion membrane to be useful for IPMC production.

For electrochemical testing, a Gamry Instruments Interface 1000

Porentiostat/Galvanostat was used. This device, shown in figure 22, allows one to run a full battery of electrochemical tests to determine complex impedance and other important electrical properties.

Figure 22 - Gamry Instruments Interface 1000 Potentiostat/Galvonostat/ZRA 06109

Using a cyclic voltammetry test, electrodes are connected to the sample, and voltage is increased slowly over time at a certain known constant rate up to a certain voltage, and then reversed until the negative voltage is reached, and then returned to zero. As the test is conducted, the amount of current at every given moment in time is recorded. A plot of voltage versus current is generated. One can then calculate the charge by integrating the current curve with respect to time. Then, knowing the rate of change of the voltage, the found charge can be divided by the voltage change over a span of time, and the result is the capacitance in farads. This result is shown in Equation

Equation 2 - Equation for finding capacitance from a cyclic voltammetry test

With a capacitor, charge is built up, and it resists a change in voltage. Thus, cyclic voltammetry of a capacitor will show a reversal in current at the same voltage, and a delay before the voltage follows it. The previously given equation for determining the capacitance is only valid under this circumstance. When a cyclic voltammetry plot shows a linear relationship between voltage and current that follows the same line back to where it started, that indicates that it is not acting like a capacitor at all.

Normally, hydrated Nafion, and especially an IPMC, will show a strong capacitance. The cyclic voltammetry plot for the IPMC manufactured according to the electroless plating procedure described in the first section of the report is shown in figure 23. Using equation 2, and taking the region between 0.2 volts and 0.3 volts on the second cycle, a value of 14 mF was calculated for the capacitance of this IPMC. 15.3 mF was calculated in the range from -0.2V to -0.3V.

IPMC Cyclic Voltammetry (100 mV/s)

2.50E-03

2.00E-03

1.50E-03

1.00E-03

5.00E-04

0.00E+00

Amps

-5.00E-04

-1.00E-03

-1.50E-03

-2.00E-03

-2.50E-03 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Volts

Figure 23 - Cyclic voltammetry for an IPMC prepared in the normal electroless plating procedure.

Even bare Nafion will show some capacitive character. The cyclic voltammetry chart for a normal piece of hydrated and activated Nafion is shown in figure 24. Based on this plot, a capacitance of 33.2 μF was determined between 0 volts and 0.075 volts.

We can also look at the capacitance of stacked Nafion versus welded Nafion. In figure 25 and figure 26, cyclic voltammetry charts are shown for stacked Nafion layers and ultrasonically welded Nafion layers. Integrating the region between 0 and 0.1 volts on each chart during the second cycle, a capacitance of 177 μF and 261 μF respectively was determined. Also, the peak amperage of the welded sample is nearly double of that of the stacked sample. Ultrasonic welding of two pieces of Nafion together appears to halve the resistance and increase the capacitance by 47%.

Bare Nafion

4.00E-05

3.00E-05

2.00E-05

1.00E-05

0.00E+00

Amperes -1.00E-05

-2.00E-05

-3.00E-05

-4.00E-05 -0.60 -0.40 -0.20 0.00 0.20 0.40 0.60

Volts

Figure 24 - Bare Nafion, Cyclic Voltammetry

Nafion-Nafion Stacked Cyclic Voltammetry

8.00E-05

6.00E-05

4.00E-05

2.00E-05

0.00E+00

-2.00E-05

Amperes

-4.00E-05

-6.00E-05

-8.00E-05

-1.00E-04 -0.60 -0.40 -0.20 0.00 0.20 0.40 0.60 Volts

Figure 25 - Cyclic voltammetry results for Nafion stacked on top of Nafion

Nafion-Nafion Welded Cyclic Voltammetry

1.50E-04

1.00E-04

5.00E-05

0.00E+00

Amperes

-5.00E-05

-1.00E-04

-1.50E-04 -0.60 -0.40 -0.20 0.00 0.20 0.40 0.60 Volts

Figure 26 - Cyclic voltammetry results for Nafion welded to Nafion

It seems that the process of welding the aluminum foil to the Nafion, using the

Condor ST-30 with the horn shown in figure 15, has caused the welded samples not to act like a capacitor at all. It is as welding caused the foil on each side to touch, creating a shorting out effect. The cyclic voltammetry results for Nafion welded to aluminum foil on both sides, then hydrated for a few minutes, is shown in figure 27. There is a linear voltage to current relationship typical of a resistor, but the extremely high current indicates very low resistance, as if shorting completely.

Welded Sample (37 J Weld Energy) Cyclic Voltammetry 1.50E+00

1.00E+00

5.00E-01

0.00E+00

Amperes

-5.00E-01

-1.00E+00

-1.50E+00 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 Volts

Figure 27 - Cyclic Voltammetry of sample from the Condor ST-30 machine with aggressive knurling. Shorting out of sample is obvious.

Initially it was suspected that the cutting of the samples with the scissors could have caused the foil to curl around the edges and thus short it out. To test this hypothesis, an unwelded sample was tested, formed by stacking a layer of foil, a layer of

Nafion, and a layer of foil and cut the same way and tested. Similar results occurred, indicating that indeed shorting was likely to be occurring around the edges of the sample due to cutting. This was confirmed when another stacked sample was tested with a large square of Nafion and small squares of foil on each side, preventing foil from reaching from one side to the other. Figure 28 illustrates how the sample was normally cut compared to how it could be stacked with a large amount of Nafion on the edges.

Figure 28 - Left: Sample cut so that foil goes to edge. Right: Stacked Nafion and foil, with Nafion large enough to prevent edge contact.

In this case, with the large Nafion extending past the edge of the foil, shorting was averted and a cyclic voltammetry plot matching closer to what was expected was found as shown in figure 29. In the region between 0 and .1 volts on the second cycle, a capacitance of -53.6 μF was calculated. It is odd in that it appears inductive. Normally, the current flips directions before the voltage does, but in this case, the voltage flips direction before the current does.

Since it became apparent that the samples were shorting on the edges, attempts were made to cut and scrape away the foil near the edges of the welded samples with a sharp knife. However, even after making sure that no foil was reaching around the edge of the welded sample, it continued to short out. Most likely this shorting was due to the foil pushing holes all the way through the Nafion. It could be that transverse waves, such as those described by Balle and Eifler, pushed the melted Nafion away from the projecting parts of the sonotrode, causing the aluminum foil on one side to reach the aluminum foil on the other side directly through the Nafion.

Stacked Sample with Large Nafion and small Aluminum

8.00E-05

6.00E-05

4.00E-05

2.00E-05

0.00E+00

Amperes

-2.00E-05

-4.00E-05

-6.00E-05

-8.00E-05 -6.00E-01 -4.00E-01 -2.00E-01 0.00E+00 2.00E-01 4.00E-01 6.00E-01

Volts

Figure 29 - A large square of Nafion with a small square of foil below and above it, displaying strange inductive behavior. While samples from the Condor ST-30 welder seemed to short out when using the horn from figure 15, the samples that Kevin Gordon at Stapla Ultrasonics made with his less aggressively knurled horn and anvil showed more promising results.

Electrochemical test results now appeared more interesting. It is likely that the less aggressive knurling on this setup prevented the foil from shorting through to the other side. The samples prepared by Kevin Gordon only involved welding the foil to one side and not both sides of the Nafion, as shown in figure 17.

In figure 30, the cyclic voltammetry results are shown for a single piece of aluminum foil stacked on top of Nafion, and for the welded sample sent from Stapla.

Unlike with the samples tested using the more aggressive knurling, this sample does appear to have some sort of electrochemical effect occurring that causes it to act differently than a pure resistor. The impedance is very high and it definitely did not short out like the samples tested with the original knurling configuration.

It is difficult to characterize exactly what is happening. Essentially, the material accumulates a charge after the first cycle, and this charge prevents the current from reversing. The current stays positive even when the voltage changes direction. But it does not act like an ideal capacitor and does not appear to store charge the same way in each direction. This appears to be an irreversible electron transfer type of event. The welded sample definitely appears to have more resistance than the stacked sample as currents are four times higher on the stacked sample.

Overall, the less aggressively knurled sonotrode shows greater promise towards being useful for the creation of an IPMC. The resistance is higher than for an IPMC, and without a similar cyclic voltammetry shape. But the Nafion has not been set in a lithium chloride solution, and some work is needed still to improve the weld and structure the metal surface.

Figure 30 - Comparison of Cyclic Voltammetry charts for stacked versus welded configuration for samples from Stapla Ultrasonics Corporation, each with one piece of foil.

3 Microscope Observations

Electrochemical test results indicated that with the Condor ST-30 machine equipped with the horn from figure 15, that the samples were shorting out. Originally, it was believed that when the welded samples were cut, that the foil may have been pushed around the thin Nafion N117 membrane and shorted near the edges. Examining the edge of a cut sample under a 20x microscope, it did appear that foil was bent around the edges.

Attempts were made to carefully cut the very edge of the foil leaving bare Nafion towards the extremes. However, the samples continued to short out during electrochemical tests. Peeling back the foil and examining the surface of samples welded under a microscope seems to confirm that the shape of the horn used for the metal-welding technique was unsuitable.

In figure 31, these photos taken under a microscope show signs of damage to the sample. Under the pressure required to generate the weld, the bumpy surface of the sonotrode seems to have pushed the aluminum foil from the top surface all the way through to the bottom surface. This would account for the samples shorting out during electrochemical analyses.

Figure 31 - Nafion welded to aluminum foil on both sides, with the top foil layer peeled back. This microscope image shows holes penetrating due to the bumps on the sonotrode surface. Each square depression is about half a millimeter across. Analyzing the surface under a microscope also reveals some other interesting characteristics of the material after the weld. There are many holes formed in material that are not present in a normal Nafion sheet, and evidence of burning. Some microscope images of holes can be seen in figure 32.

Figure 32 - Microscope images at 10-20x. A/B: This hole, with 2 different lighting modes, shows blackened regions which may be signs of burns. C/D - Other holes found in Nafion layer after welding.

One sample, which had a noticeable macroscopic hole in it caused by welding a relatively hot sample using a relatively worn out aluminum plate for an anvil and a worn horn was also examined. In the microscope picture in figure 33, there is evidence of

58 burning near this larger. It seems that for overweld conditions, the most common result is intense heat that burns away the sample.

Figure 33 - Sample with hole burned through it. Left- Whole Sample. Right- 10x Microscope image. Note blackened regions and orange Nafion bit. One interesting thing noticed when the foil was peeled from the Nafion is that there are some small bits of aluminum foil that remained stuck into the surface as shown in figure 34. This may be a sign of relatively strong bonding forces compared to the weak foil strength.

Figure 34 - Small bits of aluminum foil are left in the Nafion after the foil is peeled back.

4 Discussion

All of these results indicate that there is a lot of work that will be needed to get ultrasonic welding to the point where it will be capable of being used for replacing the electroless plating process on an IPMC by welding a complete electrode surface directly onto the ionomer membrane. The present welds between the Nafion and the Aluminum foils are far too weak to be useful for this purpose. But even if there was a more successful bonding between the Nafion and the aluminum foils, it is unlikely that the resulting composite would have the proper interlayer interface characteristics to function properly.

From the microscope images and the electrochemical data, it is clear that the

Condor ST-30 horn in figure 15 is far too aggressively knurled, causing depressions that are too deep into the Nafion. As a result it shorts out the sample. When thinner foils are used, it the aggressive knurling will often penetrate the foil as well.

Initially, the goal for this study was to create a bond that was strong enough and resisted soaking and to examine the electrochemical properties of the samples. Since the primary focus of this study was the welding of the aluminum foil to the Nafion, and since most weld settings tried was not strong enough to pass a dunking test, this study did not generate a large battery of quantitative trend-based data. The premature failure of the welding machine prevented much of the data that was intended to be gathered from being collected. But since most weld settings did not perform well enough to even hold the samples together when wet, and the knurling was such that most of the

60 samples shorted out anyway, the utility of a large comparison of electrochemical data or specific strength data based on weld energy is of limited use. It is clear that equipment more suited towards the specific weld application will be necessary, and that even subtle changes in the geometries of the sonotrode, anvil, and samples cause large differences in the necessary weld parameters to create a successful bond.

When Nafion is welded to Nafion, the resulting bond is fairly strong, and repeat welds work very well as long as each weld is conducted on the opposite side in order to prevent the sample from taking on a curved shape. The strength of this bond is such that a long strip of welded Nafion, such as the one shown on the bottom in Figure 9, can be bent so far that the two ends touch and it will not break. Thus, it seems that this ultrasonic weld from Nafion to Nafion would probably be useable for an IPMC even with this ultrasonic welding machine setup which was never intended by the manufacturer for the welding of structures that are as thin as these are.

The effects of ultrasonic welding on the ionomer remain to be fully understood.

According to a thesis paper by Haroon Momand of the University of Birmingham, ultrasound can damage Nafion by causing cavitation bubbles in a hydrated ionomer, and can cause polymerization and depolymerization. Cavitation can cause hydrogen peroxide to form which can attack the fluorinated end groups in the ionomer according to the chemical reaction in equation 3. According to Momand, this causes reduced conductivity and mechanical stability, and the membrane will fail once about 10% of the cation exchange sites have been lost [27]. Determination of whether this would have any practical effect on an IPMC could require a study in itself.

Equation 3 - Attack of Fluorinated end groups by Hydrogen Peroxide. Equation from Momand's Thesis.

5 Future Work

5.1 Analysis of Welded Nafion IPMC membranes

In this study, Nafion was welded to another Nafion, but the electroless plating process was not completed on any of these samples. In order to further understand the potential of these ultrasonically welded membranes, ultrasonically welded membranes should be electroless plated following the same recipe as other membranes made from other techniques to determine whether there are superior performance characteristics with this technique.

5.2 Improving the Nafion-Metal Bonding

One direction for improvement for the Nafion-Metal Bonding may lie in finding a superior horn shape for this application. The horn that was used by Kevin Gordon at

Stapla Ultrasonics is a step up from the one in figure 15, at least in that the shallow knurling allows the material to be welded without pushing the foil through from one side to another. But the weld strength remained too low for use as an IPMC. It is possible that the weld strength is simply always going to be too low when directly trying to weld aluminum foil and the Nafion, regardless of horn used.

Attempting to make IPMCs using different ionomers than Nafion, or the use of different metals for the electrode could also provide better results. The use of a tie-layer

62 between the aluminum foil and the Nafion could also improve the weld strength, although there is a possibility that a tie layer on the interface between the ionomer and the metal surface may interfere with the electrochemical properties desired in an IPMC.

Kevin Gordon recommended a coating on the aluminum foil to improve bonding.

This study experimented with using Reynold's Non-Stick foil, but there are a number of other commercial foils coated in various polymer materials that could improve the weld results. Surlyn as a tie layer, either in the form of a separate sheet or in the form of a metal foil coated in it may be a promising tie-layer material to try, but there are a wide range of different coatings available.

Surlyn, like Nafion, is an ionomer. According to the Surlyn Fastening Guide Surlyn can be laminated with aluminum and copper. The guide recommends using stiffer Surlyn resins and chilling them when ultrasonic welding with that material [28]. Since Surlyn is an ionomer itself, possibly using Surlyn as a tie layer this would form a better interface layer for electrochemical activity than using non ionomer tie layers. Attempting ultrasonic welding in a chilled condition for Nafion or finding harder Nafion resins may also be a way to improve welding with Nafion.

Just looking at one company offering coatings for aluminum foils, there are a huge number of different coatings offered, including rubbers, ethyl and nitro celluloses, methacrylates, polyefin films, homopolymer films, copolymer films [29], polyester, epoxy, LDPE [30],silicone polydimethylsiloxaneare [31] coatings. Acrylic, urethane, PVC, and Surlyn [32] are offered as coatings also, and promoted specifically as adhesion

63 promoters. If any one of these polymers, already bonded to foil, can easily be ultrasonically welded to Nafion, it might provide a pathway to forming a bond with aluminum. It should be easier and stronger in general to bond a polymer to another polymer, so the strength would likely be greater.

DuPont also advertises their Bynel family resins, their Nucrel acid copolymers, and Entira resins as bonding agents in a guide on joining polymers. They also note that

Bynel can help bond to metal, and that acid functionality provides adhesion to aluminum foil and metalized films. They also include a chart that shows that a sodium ionomer or a zinc ionomer will bond with an acid copolymer [33]. Based on that guide, it seems that an acid copolymer would be a strong choice to try as a tie layer between aluminum foil and Nafion.

During future testing to improve bond strength, representative samples from each variation need to be tested to failure to determine trends that will lead to the development of progressively stronger bonds.

5.3 Forming Proper Surface Characteristics for IPMC Capacitance

Merely adhering metal to the Nafion, even if it can be done strongly in the future by ultrasonic welding, is not sufficient. Therefore a great deal of future work will be needed to treat the surfaces of Nafion and/or the metal that is welded to the surface in order to try and form a surface that is more comparable to the porous, rough and fractal-like nature of the normal IPMC interface.

Possibly there are chemical or physical processes that can be used to eat way or build up metal on the surface in order to create a more suitable interface structure.

Pitting the surface of the metal or the ionomer that the metal will be welded into with some sort of sand blasting or sanding process, or some process of depositing molten metal onto the surface of the metal to be welded to the ionomer might be a direction to pursue. Perhaps a mold can be prepared for to cast the metal into which has the proper characteristics and can be used to make very reproducible surfaces. Growing metal crystals on the surface of a flat piece of metal foil may also be an idea, although it could prove equally as time consuming to accomplish as the current electroless plating process which may defeat the purpose. Also, precision computer controlled laser cutting might be able to cut a complicated geometry. This last concept is one that is already being investigated by Dr. Yiliang Liao's research group at the University of Nevada.

One challenge is that any alteration to the surface texture is likely to alter the weld parameters needed to form a good weld, and a good weld is also likely to deform a deliberately textured surface. The interconnected nature of these two variables makes this a challenging problem to solve.

Possibly the ultrasonic welding of a metal fiber reinforced polymer matrix to the ionomer, or direct welding of fine metallic shot particles of varying mixed diameters and shapes could be an approach, instead of trying to integrate a smooth metal foil onto the polymer. Or perhaps, ultrasonic welding could be used to weld such particles to a smooth metal foil prior to welding this foil to Nafion or other ionomer suitable for

IPMCs. There has already recently been some success with using metalized fibers and powdered metal particles in IPMCs.

In 2012, Tang et al. also did a study where they used silver-coated carbon fibers to avoid the electroless plating process by inserting these fibers into a sodium sulfonate ionomer called PSBS. They used ultrasonic vibration to disperse these particles in the fluid resin which was then poured and dried. While this process avoided the use of an electroless plating process, it was not conducted using a Nafion ionomer and is limited to a casting process [34].

The use of silver nano-powders in IPMCs also has some precedence. Back in

2006, Chung et al. investigated using silver nano-powders to enhance an IPMC [35], but they did not use them to replace the electroless plating process. Later, in 2015, Khan et al. used silver nano-powder mixed with Kraton in a solution cast IPMC which replaced the electroless plating process [36], but this process also relies on casting an IPMC, whereas ultrasonic welding of a completed electrode might have a wider scope and be applicable to the less brittle extruded membranes.

5.4 Ultrasonic Welding of Complete IPMCs

There are many different techniques to make a custom IPMC membrane such as hot-pressing, casting, and even 3D printing. The electroless plating process of forming electrodes is time consuming and expensive, but it is a process that does work, and there are now alternatives to that as well, such as with Khan's silver nano-powder

66 electrodes. One potential application of ultrasonic welding that might be without comparably good alternatives could be the ability to bond complete IPMCs directly to each other. One interesting aspect of ultrasonic welding is that it is possible to form relatively strong bonds in very specific layers that may be below the surface, in the manner that a tie layer utilizes by preferentially absorbing vibration energy.

If multiple IPMCs are bonded together, interesting new bending motions and situations could be created. In 2005, Pacquette et al. did a study on multi-layered

IPMCs. In this paper they modeled a lot of the electrochemical considerations of stacked

IPMCs and proposed actuation methods and shapes. They constructed some multi- layered IPMCs using the casting method. If two IPMCs were stacked on top of each other, the top surface of one would share the same charge as the bottom surface of the other one, and thus they would bend in opposite directions. Thus, multiple IPMCs stacked on top of each other could create a wavy motion and interact with each other in feedback loops [37]. Perhaps an IPMC actuator capable of complex motions similar to the IPMC actuator as shown in figure 35 could be produced relatively easily with ultrasonic welding.

Other interesting arrangements can be found as well. Dr. Yantao Shen suggested the possibility of IPMC bonding shapes that would increase linear deflection or force output. Figure 36 shows an arrangement of ultrasonically welded IPMCs that would fold down to a flat stack of IPMCs, but fold out to provide a linear elongation perpendicular to the folded direction. In two dimensions, this stacking would not work as it would self

67 intersect. In three dimensions, a stacking arrangement can occur but it would result in a slightly twisted shape at rest. A construction paper model showing how nine IPMCs might be assembled in a three dimensional configuration is shown in figure 37, forming a relatively flat shape similar to a square when folded. When it unfolds, it transforms into a wavy shape perpendicular to the flat shape. This configuration could provide a relatively large linear deflection.

Figure 35 - Paquette et al. came up with possible motions for multi-layered IPMCs [37]. Ultrasonic welding of IPMCs would allow similar motions. In 3 dimensions, the outer surfaces could be connected neatly by crossing along the side of the membrane and insulated from contacting the inner surfaces while crossing. It may also be possible to generate a linear force from a set of welded IPMCs during an unfolding motion, with an increasing force with increasing numbers of IPMCs.

Figure 38 shows an "origami-like" configuration welded IPMCs that can unfold to generate a force. When the linear force is desired, a voltage can be applied to IPMC

68 electrodes, essentially compressing a spring. When the voltage is disabled, the IPMC set will return to its original flat configuration.

While bare metal surfaces that are facing each other or that are directly welded together will share a charge, other interesting motions might be accomplished by insulating the electrodes except at a small insulated wire connection for each IPMC electrode. In this case, the ultrasonic bond would need to be conducted on a region of each IPMC that is insulated. This would allow the possibility of having two layers face each other when welded to be at opposite charges.

Figure 38 - Hypothetical linear force actuator from unfolding ultrasonically welded IPMCs.

6 Conclusion

Overall, this study concludes an array of introductory investigations into ultrasonic welding as a technique to enhance IPMC production.

This study was unable to achieve a useful weld between Nafion and aluminum foil that would be suitable for use as an IPMC, and found numerous difficulties in using this method for this purpose. But this study still does not entirely rule out the possibility of this technique being viable. With a different sonotrode, and different material selection, it is possible that in the future, more successful bonds may be made.

Improving the bond strength will be a challenging goal, and getting the surface to have the proper texture will possibly be even more challenging. Considering the worthwhile goal of replacing the cumbersome electroless plating process, these great challenges may merit further investigation.

Ultrasonic welding of one Nafion membrane to another forms a bond of sufficient strength to be of use for an IPMC. There are many competitive techniques for forming customizable Nafion membranes, including 3D printing, which is a very user friendly process for creating intricate designs. But the ultrasonically welded Nafion membranes likely warrant some additional investigation to get a better idea of what advantages, if any, that the technique may present over the competing techniques.

Appendix

A1 Micrometer Measurements

Measurements of the thickness of different materials used for welding or formed by welding are shown in table 1. The Nafion N117 appears to be very close in thickness to the advertised 183 microns. Welding materials together results in a thickness that is smaller than the independent pieces would be stacked on top of each other.

Table 1 - Vernier Micrometer Measurements.

Micrometer Measurements of Dried Samples Nafion N117 0.00725 inches Thin Foil 0.00075 inches Thicker Foil 0.0011 inches Nafion Welded to thin foil 0.0069 inches Welded Nafion 0.014 inches Stacked Nafion 0.0145 inches Nafion sandwiched between 2 thick layers 0.0075 inches

A2 Nafion to Nafion Weld Tables

Using the Stapla Condor ST-30 welder, a wide range of weld settings produced relatively strong welds between two layers of Nafion. Figure 39 shows the relationship between amplitude of vibration, weld times, and weld energy. Weld energy increases with increasing amplitude and weld times. An amplitude of 1 corresponds to 12 microns, an amplitude of 5 corresponds to 18 microns, and an amplitude of 10 corresponds to 24 microns.

Nafion to Nafion: Weld Time vs. Weld Energy 120

100

80 y = 52.619x + 0.1786 y = 59.6x - 22.1

60 Amplitude 7 Amplitude 9

Weld Energy (J) 40 Linear (Amplitude 7) Linear (Amplitude 9)

0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Weld Time (s)

Figure 39 - Nafion to Nafion: Weld Time vs. Weld Energy

In table 2, the Nafion to Nafion weld table describes the results of a variety of weld settings. A setting with a pressure of 1.5, an amplitude of 7 (corresponding to around 20-21 microns), and a weld time of 0.5 seconds seems to be a good setting that provides a strong bond without causing obvious tearing, warping, or burning of the surface of the Nafion. Some of the samples produced are shown in figure 40.

Figure 40 - Tray holding welded samples at each setting.

Table 2 - Nafion to Nafion Weld Table

A3 Welding Wet Nafion to Aluminum Foil

Since Nafion must be hydrated in order to be useful as an IPMC, and there were many instances where the weld between the dry Nafion and the foil would separate after submerging into water, (quite likely due to the expansion of the Nafion), one potential workaround was to weld the Aluminum foil to hydrated Nafion. No weld settings were found that formed strong bonds. Often, the aluminum foil would be destroyed during the weld. Most welds formed in this condition formed pretty weak bonds. Most of the samples made using hydrated Nafion would stick to the horn and then fall apart trying to remove them from the horn.

Table 3 shows a weld table showing some results for welding wet Nafion.

Table 3 - Single Foil Welded to Nafion, then Submerged in Water.

While it may be possible to weld the samples after the Nafion is already hydrated, welding a dry piece of Nafion to heavy duty aluminum foil seemed to work the best for getting a sample to stay stuck together after welding. Table 4 shows the results for some submerge tests after dry Nafion was welded to Aluminum foil on both sides. A weld setting with a pressure of

2.2, an amplitude of 7, and a weld time of 0.25 seemed to be the best setting.

Table 4 - Dry Heavy Duty Aluminum Foil welded to Nafion, then Submerged in Water.

A4 Welding Foil to Double Thick Nafion

Multiple welds were attempted between two layers of Nafion and then a layer of foil, with various settings for the weld between the double Nafion layer. No bonds were created while attempting this, but with more study it could probably be done. An exhaustive sweep of weld settings was not conducted, and the weld between the Nafion layers may also play a role.

While successful welds of the foil to the Nafion were unsuccessful, one positive result is that the

Nafion layers never separated from each other during any of these tests, indicating a strong weld between Nafion layers.

A5 Welding Nafion/Foil to Nafion/Foil

After successful welds between a single layer of Nafion and a single layer of aluminum foil, attempts were made to bond them to form a layer with double Nafion thickness wedged between foil. An exhaustive sweep of settings and combinations of weld settings for the previous welds was not conducted, but no weld between the

76 previously welded pieces were obtained. With further testing this may be accomplished.

In most of the cases tried, not only was a new bond not formed, but one or both of the original welds also broke. Occasionally, the initial welds would remain intact, but a new weld was not formed.

A6 Non-Stick Foil

Attempts were made to weld Nafion to Reynold's Non-Stick Foil, hoping that the polymer coating on the aluminum would help form a stronger bond. Only a limited number of tests were conducted on non-stick foil before the ultrasonic welding machine stopped working, and thus the testing was not exhaustive. Table 5 is a weld table showing some of the results.

Instead of welding the substrates together, most of the tested weld settings welded the substrates to the horn or the anvil. Only one sample tested remained bonded after separation from the horn, but the bond strength was weak. This sample had a pressure of 1.5, an amplitude of 10, and a weld time of 1 second.

Table 5 - Reynold's "Non-Stick" Foil to Nafion Weld Settings

A8 Comparison of IPMC Production Procedures

The IPMC that was tested in order to compare to the electrochemical properties to the welded samples in this study was created using a procedure developed by Viljar

Palmre and written down in a manual by Marissa Tsugawa. This procedure has been used by the laboratories under Dr. Kwang Kim, Dr. Kam Leang, and Dr. Matteo Aureli at the University of Nevada, Reno and at the University of Utah. It is very close to the recipe used in a paper written by Palmre et al. in 2014 [38]. In this appendix, this procedure is outlined in detail and compared to that used in a published recipe from

Oguro [39].

Overall, the two procedures are pretty similar as far as the basic steps. The biggest difference include the lack of ammonium hydroxide in the Palmre procedure, the lack of Hydrogen Peroxide in the Oguro procedure, the use of lithium chloride in the

Palmre procedure and the use of sodium ions in the Oguro method. Also, during the cleaning stage, the Oguro procedure uses an ultrasonic cleaner whereas the Palmre procedure does not.

Minor differences include the use of different acids and concentrations, different quantities of some reactants, and different durations and temperatures in many of the baths that the Nafion strip is subjected to. The hot baths used in the Oguro procedure were at higher temperatures but shorter in duration. The chemicals used for each method are compared in table 6, where red text indicates a difference and blue text indicates a similarity.

Table 6 - Comparison of Chemicals in Palmre and Oguro methods.

Oguro Palmre

Hydrochloric Acid 2 M HCl prepared from 37% HCl Sulfuric Acid 1 M H2SO4

Twice as many protons per molecule but Ka2 for bisulfate ion is much weaker. Prepared from 18 M H2SO4

Pt(NH3)4]Cl2 Hydrate 98% Pt(NH3)4]Cl2

NaBH4 NaBH4

Ammonium hydroxide NH4OH ?

? Hydrogen Peroxide

Hydrazine hydrate 20% prepared from 50-60% Hydrazine hydrate 20% Hydrazine Hydrate

Sodium Chloride >= 99.5% Lithium Chloride 1 M

Overall, the chemicals are similar. The acids used are different, but with the same normality for protons. However, a 2 M HCl solution would more freely make the protons available than a 1 M H2SO4 solution. The Palmre method uses lithium chloride and the other method uses sodium chloride in the final ion exchange step.

In table 7, the platinum solutions used are compared.

Table 7 - Comparisons of Platinum Solutions

Oguro Palmre

Depth of Pt solution calculated based on 100 mL of solution regardless of sample size. membrane area. Mass of platinum desired is 3 390 mg total. Same mass of Pt only if the mg Pt/cm2, with 15% excess preferred. sample is 130 cm2

 Concentration is 2 mg Pt/mL = .01 M  Concentration is .02 M Pt 1st Time Pt 1st Time  Concentration is 200mg/300mL =.0033  Concentration is .5 mg Pt/mL= M .0025 M

In Oguro et. al, the concentration is given in mg/mL and in the manual describing the

Palmre process the concentration is given as a molarity. These quantities can be interchanged based on the atomic mass of platinum as 195.084 g/Mol:

Table 8 compares some of the other chemical solution used.

Table 8 - Comparison of Other Chemical Solution Quantities

Oguro Referenced Palmre

Based on Surface Area of Sample Given Values

NaBH4 NaBH4

2ml of 5% per 30 cm2 + 10% 2.4-3.6 g

5% Hyroxylamine HCl 5% Hyroxylamine HCl

 6 mL per 30 cm2  12 mL per repeated Secondary Plate Process, as many times as necessary for 10 Ohm result. 20% Hydrazine Hydrate 20% Hydrazine Hydrate

 3 mL per 30 cm2  6 mL per repeated Secondary Plate Process, as many times as necessary for 10 Ohm result. NaCl LiCl

1 Liter? 100 mL

Tables 9, 10, 11, 12, and 13 compare the specific details of each stage of the procedure. They compare the surface preparation, primary plating, secondary plating, final cleaning, and final ion exchange stages of the procedure respectively.

Procedure

Table 9 - Surface Preparation Comparison

Oguro Referenced Palmre

Sample Width/Length 2mm Longer Not specified

Emery Paper to Roughen perpendicular to bend Sand Paper to Roughen perpendicular to bend direction. direction.

Ultrasonic cleaner used to wash membrane for 30 No Ultrasonic Cleaner Used minutes

BATH TREATMENT BATH TREATMENT

1. Boil with acid 2 M HCl – 30 mins 1. Cool Water Bath - 15 Mins 2. Hot Hydrogen Peroxide Bath - 45 mins 2. Boil with Water – 30 mins 3. Cool Water Bath - 15 mins 4. Acid Bath 1 M H2SO4 - 45 mins 5. Hot Water Bath - 45 mins 6. Hot Hydrogen Peroxide Bath - 45 mins 7. Cool Water Bath - 15 Mins 8. Hot Acid Bath 1 M H2SO4 - 45 mins 9. Hot Water Bath - 45 mins

Table 10 - Comparison of Ion Exchange / Primary Plating Stages

Oguro Palmre

Soak in Platinum Solution Soak in Platinum Solution

 (.01 M Pt Solution)  (.02 M Pt Solution)  Calculated Depth based on Surface Area  Barely Deep Enough to Submerge Sample  NH4OH added  No NH4OH added  Soak Overnight  Soak overnight Reduction with NaBH4 Reduction with NaBH4

 Magnetic Stirrer Used  Magnetic Stirrer Used  Heat in Water to 40 degrees C in 5  Place in 50*C Water minutes  Add proper quantity of 5% liquid NaBH4 every 30 minutes 7 times, 7th time is larger  Add 0.2 g NaBH4 every 30 minutes 6 times. quantity.  NaBH4 quantity based on surface area of  NaBH4 quantity is arbitrary sample

 Slowly increase to 60*C  Stir for 1 hour after final NaBH4 addition  Slowly increase to 65*C  Do not stir longer than normal on last step

Baths Baths

1. Water - 5 minutes 1. Hot Acid – 45 minutes (0.5 M H2SO4) 2. Acid - 1 Hour (0.1 M HCl ) [NOTE – Much higher concentration ] 2. Hot Water – 45 minutes 3. Fresh Hot Water 2 – 45 minutes Primary Plating Stage Done Once Primary Plating Stage done 2-3 times.

Table 11 - Comparison of Secondary Plating Processes

Oguro Palmre

.0025 M Platinum Solution .0033 M Platinum Solution, 300 mL deep

Add 5% NH4OH Do Not Add NH4OH

Magnetic Stirrer Magnetic Stirrer

 Stir at 40*C for one Minute  Heat to 50*C,  Gradually Increase Temperature to 60*C  Gradually Increase Temperature to 65*C over a 4 hour period over a 3 hour period  Add Calculated Quantities of Hydrazine  Add 2 mL of Hydroxylamine HCl and 1 mL Hydrate and Hyroxylamine HCl every 30 of Hyrdazine Hydrate every 30 minutes for minutes for 8 total additions 6 total additions Repeat secondary plating process as desired to Repeat secondary plating process until resistance is make platinum layer thicker. less than 10 ohms.

Table 12 - Comparison of Final Cleaning Stages

Oguro Palmre

Baths Baths

1. Water– 5 minutes 1. Acid – 0.5 M H2SO4 65*C (45 min) 2. Boiling Acid– 30 mins (0.1 M HCL) 2. Hot Water - 45 minutes 3. Water- 5 minutes 3. Fresh Hot Water – 45 minutes

Table 13 - Comparison of Final Ion Exchange Processes

Oguro Palmre

Store in 1 M NaCl Solution for 24 hours Store in 1 M LiCl Solution for 24 hours

1 Liter 100 mL

Cut Edges (Undoubtedly cut edges also although unstated)

Store in deinoized water Store in deionized water

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