Electrical Response of Functionally Graded Graphene-Nylon Segregated Composites Under Quasi-Static Loading

This research examines the fabrication and electro-mechanical properties of functionally graded graphene-nylon segregated composites. A novel production technique was expanded upon to produce segregated nylon-pellet and graphene nanocomposites with low percolation threshold for electrical conductivity. This particle templating procedure effectively disperses graphene within the nylon pellet matrix and is hot-press melted into three-inch diameter discs. While ideally structured for electrical transport, these specimens are mechanically weak along the polymer grain boundaries. To enhance the mechanical properties, a double-rotary shearing step was added to increase mechanical strength without significant sacrifice to electrical properties, signifying the shearing method is a viable trade-off fabrication approach. Lastly, a production technique for fabricating graphene-nylon textiles with conductive properties was investigated. Graphene-coated nylon yarn in a polymer matrix proved to have high electrical conductivity. Chapter one is an introduction to graphene and related studies and applications. The nylon-pellet graphene material fabrication and testing are explored in chapters two and three respectively. Chapter four addresses the nylon-yarn and graphene composites. Electrical conductivity was measured using a high resolution four-point probe method. Three-point bend and tensile testing experiments were used to evaluate mechanical properties.


LIST OF TABLES
lattice. This two dimensional material is the strongest material ever reported in nature and its electrical and thermal conductivity is very high [1]. For this reason, graphene research and its many diverse applications has gained much attention since its discovery in 2004 [1], [2]. Innovative manufacturing methods make it possible to procure graphene nanoparticles in a variety of sizes and capacities. Research involving integrating these nanoparticles into efficient structures can develop into broad ranges of use. The potential applications for graphene-polymer nanocomposites include industries such as automotive, aerospace, construction, and electronic [3]. Graphene nano-composites can form a three-dimensional electrical network if effectively dispersed within the matrix material. Polymers used as matrix material in carbonbased nano-composites have been extensively studied [4][5][6][7]. Studies have shown even a low graphene content in polymer composites exhibits outstanding electrical conductivity compared with conventional filled polymers [8]. The electro conductivity of the composite material depends on efficient dispersion of graphene in the filler, and this can be managed with low graphene content when using a segregated composite structure [9][10][11].
The first part of this research investigates segregated graphene-nylon pellet composites, while the second part is discussed later on and examines graphene-nylon textile composites.
Previous work by Heeder et al. investigated the electrical conductivity of polystyrene-graphene segregated composites [12]. The novel production technique demonstrated "is simple, commercially viable, and does not require hazardous chemicals. It provides the means to form highly organized conductive networks throughout insulating polymeric materials" [12]. A further innovative step to the material production process uses shear manipulation to distort the segregation of the material. The result is a functionally graded material that is nearer to ideal conditions for both mechanical and electrical properties [13].
In advancement from the previous study discussed above which created one-inch diameter samples with a single face distorted exclusively, this research uses three-inch diameter samples and distorts both the top and bottom of the sample discs.
Furthermore, the composite will contain nylon as the matrix material instead of polystyrene. Both polymers are among the most commonly used thermoplastics in commercial industry.
A material fabrication procedure was regimented to achieve the ideal conditions for specimen repeatability. Once the nylon-graphene composite material was efficiently mixed, it was placed into a mold and heated to the appropriate melting temperature before being compacted under pressure. If the sample was categorized as "organized" it was then quenched in a bath of cool water. If the sample was categorized as "sheared", it was sheared on the top and bottom surfaces during the 3 compacting step before quenching. The objective of the shear manipulation is to form an area with an integrated structure to improve mechanical properties by allowing the matrix material to form cohesive bonds. Although this disrupts the electrical networks and diminishes electrical properties, the areas of the sample that remain segregated still have highly conductive characteristics. In this way a sample can possess both ideal electrical and mechanical properties.
The specimens were experimentally tested to determine electrical and mechanical properties. The electrical conductivity for each specimen was established using a differential four-point probe method. The material resistance was calculated by passing a constant current through the specimen and measuring the voltage drop across the known width of the specimen. For mechanical testing, an Instron machine was used for quasi-static three-point bend testing and tensile testing of the material at a slow loading rate. The Instron is equipped with a data acquisition system to record load, deflection, and time.
As postulated, a significant increase in electrical conductivity could be achieved with a small amount of graphene when compared with plain nylon. The organized specimens had the highest conductivity, but were considerably weak mechanically. The sheared specimens had slightly lower electrical conductivity but were appreciably stronger, indicating the shearing method is a viable trade-off approach.
The second part of this research examines combining graphene nanoparticles with nylon yarn to form smart textiles. "Wearable electronic devices, such as etextiles, are of great interest due to their potential applications in portable electronic 4 devices, in multifunctional fabrics, including healthcare units and wearable displays, and even in warfare. [14] Graphene-oxide (GO) is commonly used as it absorbs into textiles more easily, however it does not have high electrical conductivity so efforts are typically made to transform it into reduced graphene-oxide (RGO) [14][15][16][17]. Textile industry standard productions, such as wet spinning, are often used [16], [17]. It has been found that dispersing graphene in an acetone bath results in high electrochemical performance because it "prevents the adulteration effects of impurities into the system." [17] A preliminary investigation was done to determine the feasibility of producing nylon smart textiles. A method of combining graphene with nylon yarn was explored.
The graphene used was pure, few layered graphene as opposed to graphene-oxide, and a hand-mixing method was used that involved an acetone-graphene solvent and dry graphene powder. The graphene-nylon yarn was then preserved in a polyurethane matrix. The electro-conductivity of the graphene-nylon yarn composites were electrically evaluated and found to be within the same order of magnitude as the sheared graphene-nylon pellets from the first part of this research.

MATERIAL FABRICATION
Graphene is a two-dimensional atomic matrix of carbon and is the basic structural element of its other allotropes; graphite, carbon nanotubes, and fullerenes (also known as bucky balls). Although graphene refers to a single atom thickness of carbon and graphite is multi-layered carbon, "few-layered graphene" can be considered to be inbetween with a thickness still in nano-scale. In this thesis, "few-layered graphene" will be used interchangeably with "graphene". Few-layered graphene was selected for use in this study to produce a viable electro-conductive pathway through the macrocomposite of nylon pellets.
The graphene selected for use in this research was xGnP Grade M from XG Sciences. Per the manufacturer's data sheet, the non-oxidizing proprietary process makes the graphene ideal for electrical conductivity. Each graphene nanoplatelet is approximately 6nm thick with a surface area unit mass of 120-150 m 2 /g. The electrical conductivity parallel to the nanaplatelet surface is 10 7 S/m, and the conductivity perpendicular to the surface is 10 2 S/m. the steps for specimen production, which is also described below.
To utilize graphene most efficiently, it is advantageous to identify the percolation threshold, or least amount of graphene to nylon ratio for electroconductance. By distributing graphene along the outer surface of the nylon pellets before melting into a sample, a conductive network is created along that pellet's surface. It can then be conductively linked to the adjoining neighboring pellet, and so on. In this way, only a small amount of graphene was needed in relation to nylon.
Several samples of varying percent volume ratios of graphene to nylon were created 7 and electrically tested. The conductivity increased significantly at 0.3% volume ratio of graphene. 0.3% volume graphene was also the percolation threshold of Heeder's research [12].
Once the appropriate ratios of graphene and nylon were measured, both were placed in a glass jar and shaken vigorously. Graphene adhered to the surface of the nylon pellets likely due to electrostatic attraction. A 3-inch diameter by ½ inch disc specimen contains roughly 3,270 nylon pellets. Photos of the nylon pellets before graphene coating and after graphene coating are shown in Figure 1.     Steel material was selected over aluminum for the mold to withstand repetitive heating and quenching in a bath of cold water. Quenching is necessary to cool the nylon before it is allowed to overheat.
The following is a discussion of lessons learned when manufacturing several iterations of the mold. The first hollowed cylinder die was made from stainless steel and had an inner diameter of approximately 3 inches, an outer diameter of 4 inches, and was 3 inches high. The gap between the die inner diameter and the pistons was only a small fraction of an inch, but this was wide enough to allow leakage of nylongraphene material to escape from the mold as shown in Figure 5.  blanket was used to enclose the heated mold and prevent heat from escaping into the room for more efficient heating. During the winter months a space heater was required in the enclosed room or the mold would never reach the desired temperature. A temperature controller system was preemptively selected and programed to maintain the desired temperature range, but was not needed since it was concluded that the heating cartridges needed to be removed from the mold immediately after the sample reached 200°C to prevent the specimen from burning or melting excessively and losing its conductive pathways. If the mold and specimen was immediately quenched in a cool water bath, it would form internal or external bubbles as shown in Figure 6.
Removing the heat source and Mylar blanket and allowing the mold to cool by convection for approximately five minutes reduced the air bubbles. Figure 6 shows the internal voids in a sample, likely caused by trapped air bubbles. Organized sample that was allowed to cool gradually and did not form voids.
The specimens that were radially sheared had smear patterns that varied depending on the degree and direction of piston rotation. All specimens used for testing were sheared on top and bottom in opposite directions approximately 360°. A sliced section view in Figure 8 shows internal smear patterns. Specimen (A) was only sheared approximately 45° and has an interesting smear pattern that varies between being organized on the outer circumference and center, and smeared in the remaining areas. Specimen (B) was sheared approximately 360° and has areas of extreme smearing with moderate smearing in the interior of the sample. In both cases, the interior remains relatively organized because the center has the least travel distance when the piston is rotated. Figure 9 depicts a sketch of the shearing pattern.  Each 3-inch sample disc was cut to different dimensions depending on the method of mechanical testing. Figure 10 shows ½" wide specimens cut for three-point bend testing. Figure 11 shows ¼" wide specimens used for tensile testing.  The samples needed to be cut in such a way that the nylon would not melt across the freshly cut surface and block the electro-conductive pathways. Samples that were roughly cut with a band saw or milling machine did not display consistent 17 electrical results and the data was excluded. Consistent results were achieved by using a thin spiral blade on a milling machine at 340 rpm. Compressed air was used on the sample during cutting, and only a small amount of material was cut at a time. A total of three-passes were used to cut through the ½ inch sample.
Exposure of the graphene electro-conductive pathways on either sliced end is essential. The next chapter will detail how the sliced ends of the samples were prepared for electrical conductivity testing. The chapter also describes mechanical testing.    Compared with plain nylon, the sheared samples are on average nine orders of magnitude more conductive, and organized samples are on average ten orders of magnitude more conductive.
After electrical testing, samples were subjected to mechanical testing by means of three-point bend and tensile testing. Three-point bend testing examines the flexural properties of the graphene-nylon composite. An Instron was configured for three-point bend using two lower anvils spaced 60mm apart, and one upper anvil in the center.
Samples were side loaded (perpendicular to mold pressure) at a rate of 0.1 mm/min.
The figure below shows a loading direction diagram and a specimen flexing while undergoing three-point-bend, and finally a particle split in the center that indicates a "break" of the material.     Figure 19 below shows experimental results of tensile tests for multiple organized specimens compared with plain nylon properties. Plain nylon has approximately six times the tensile strength as the organized graphene-nylon specimens. Data from sheared specimens will be attempted to be collected before the oral defense.   Although the tensile data for the sheared specimens is listed as N/A, it is known they outlived the organized specimen stress before the sample slipped from the test grips. As would be expected, the conductivity of the experimental specimens are values between the plain nylon and raw graphene properties. There was no manufacturer data provided for flexural strength of the graphene, or for tensile strength in the perpendicular-to-surface direction. Both organized and shear specimens have lower mechanical properties than plain nylon and raw graphene, but this is inherent in the segregated macro-compsite design. The mechanical experiments were not testing a single solid sample, but rather the stress needed to break apart the bonds between the segregated raw materials (graphene and nylon).
The next chapter explores a very different fusion of nylon and graphene composites using nylon yarn.

NYLON-GRAPHENE TEXTILES
A preliminary investigation was accomplished to demonstrate the feasibility of fabricating graphene-nylon textiles. Several trial production methods were explored and one resulted in successfully demonstrating electrical conductivity.
Numerous industry techniques for treating and dyeing textiles were researched.
One intriguing method called mercerization is used to dye and strengthen cellulose materials such as cotton. Mercerization is a process involving a chemical reaction that causes swelling of cellulose fibers so they are more receptive to smaller particles and absorption of dyes. Although nylon is non-cellulose, the absorption mechanism of mercerization was noted as inspiration. A solvent-graphene mixture was created and it was decided to utilize a smaller version of the "few-layered graphene" used in Chapter 3 for nylon-pellets composites.
The nylon textile obtained for this research is called "nylon fluffy yarn". The first attempt to create a graphene nylon-yarn composite with electrical conductive properties was unsuccessful. The yarn was wetted with Methyl Ethyl Ketone (MEK) and then rolled in dry graphene powder (xGnp-M) to coat the outside of the yarn. It was immediately assembled into the mold in a horizontal crossing pattern. This was done to explore the potential for complexity in composite design with the ideal result being a sheet of fabric. Before the yarn dried in the mold, epoxy was poured over and allowed to harden. The textile-graphene epoxy sample was removed from the mold and cut to expose the graphene yarn on all sides. The sides were painted with silver conductive paint and a wire attached similar to the method in Chapter 2. However, no reliable electrical data was collected. It is speculated that either a complete conductive network was never formed or that during cutting the sample melted the epoxy and coated the exposed ends of the graphene-yarn. A second method was developed to construct a graphene-nylon yarn composite. Instead of coating the outside of the yarn, traditional industry methods were investigated to explore depositing graphene inside the nylon yarn fibers.
Traditional dyeing techniques use a liquor ratio of 100:1, which is the ratio of liquid to the material being dyed. [17] Though this turned out to be much more liquid solvent 28 than was needed, the industry liquor ratio was still followed. The yarn was cut into pieces approximately 5 inches long and massed to be .7856 g. This equated to 78.56 g of graphene-MEK (Methyl Ethyl Ketone) solution total. It was decided to retain the 0.3% volume ratio of graphene to MEK. The graphene was changed from xGnP-M to xGnP-C-500 as these were physically much smaller particles of few-layered graphene (6nm thick vs. 1-3nm thick). The nylon yarn was allowed to soak in this liquor to absorb the graphene within its fibers. As can be seen from the photo above, the yarn turned grey in the solvent mixture. To assure that an electrical conductive channel was formed in the next mold production, half the yarn was then coated with dry graphene powder (x-GnP-C-500).
Both sets of graphene nylon yarn were placed immediately into the mold before allowed to dry. The soaked and coated graphene-nylon yarn was on one side while the single soaked graphene-nylon yarn was on the other side. The yarn was lain across 29 horizontally without crossing each other and the side panels were bolted on.
Polyurethane was selected as the matrix material for its ability to not adhere to the mold when mold release is applied. Polyurethane was poured on top and allowed to set per manufacturer's instructions. Graphene-nylon yarn with polyurethane matrix in the mold.
After curing was complete, the polyurethane-graphene-nylon samples were cut into long rectangular sections measuring 5 x 1/2 x 1/8 inches. Silver conductive paint was used on the exposed graphene-nylon-yarn ends of the samples, and wires were attached. Figure 24 shows Samples 1 and 2 which were prepared by both soaking and coating the nylon yarn with graphene, while samples A, B, and C only soaked the nylon yarn with graphene. All samples were tested for electrical conductivity but only specimens 1 and 2 produced results. Specimens A, B, and C likely did not have enough graphene to form a conductive pathway. Adding the step to roll the graphene-soaked yarn in more dry powder graphene establish electrical conductivity throughout the full length of the yarn.
The polyurethane in the sample simply provides the structure for the conductive nylon-graphene yarn and does not contribute to the electrical properties.
Therefore the cross-sectional area of the yarn, the diameter, was used in conductivity calculations rather than including the cross sectional area of the polyurethane matrix.
The fabrication method successfully produced graphene nylon-yarn conductive specimens; however, there may be room for improvement to achieve better electrical results. Unlike the graphene-nylon pellet specimens which were compressed in the mold after heating to form tight efficient graphene networks, the graphene nylon-yarn 32 specimens were allowed to remain "fluffy" and spread out within the polyurethane matrix. When a cellulose material such as cotton goes through the mercerization process, the resulting yarn is always shorter in length and stronger when it dries. This would work to stack the graphene nano-platelets in one direction.

APPENDIX C: RECOMMENDATIONS FOR FUTURE WORK
Graphene-polymer composites:  Vary polymer type and shearing method. For example instead of rotary shearing, perhaps a single directional lateral shear.
 Vary percent volume of graphene to polymer ratio.
 Further quantify the gradation of the material in both x and y axis.
 Measure defects of the material using high resolution photos. Graphene-textile composites:  Use graphene-oxide to promote chemical bonding with textile.
 Incorporate graphene using textile industry equipment for spinning or dying  Utilize mercerization or cationic reagent with cellulose fibers (such as cotton) as an efficient way of incorporating graphene into the fibers and improving fiber strength.