Properties of Pristine Graphene Composites Arising from the

Subscriber access provided by UCL Library Services

Article

Properties of Pristine Graphene Composites Arising from the Mechanism of Graphene Stabilized Emulsion Formation Steven Woltornist, and Douglas H. Adamson Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b05016 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on May 25, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Properties of Pristine Graphene Composites Arising from the Mechanism of Graphene Stabilized Emulsion Formation Steven J. Woltornist† and Douglas H. Adamson†, ‡, * †

Department of Chemistry, ‡Institute of Materials Science, Polymer Program, University of

Connecticut, Storrs, Connecticut 06269, United States.

ABSTRACT: Inexpensive strong and electrically conductive pristine graphene composites are synthesized from graphene-stabilized emulsions. These materials do not utilize oxidized graphite (GO) or reduced oxidized graphite (rGO), instead taking advantage of the recently demonstrated surfactant qualities of pristine graphene sheets. Using monomer as the oil phase, water-in-oil emulsions stabilized by graphene are polymerized to form porous composites with a continuous network of hollow graphene lined spheres. The effects of graphite flake size, emulsion settling, graphite concentration, and solvent ratio on composite properties such as electrical conductivity, strength, and density are investigated. We relate changes in the composite morphology to the observed property changes, and discuss the cause and effect of larger sphere sizes in the composite materials. Controlling and understanding properties such as conductivity and strength is a critical requirement for envisioned applications such as batteries, capacitors, and structural materials.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 20

Introduction

Chemically modified graphitic materials such as graphene oxide1–7 (GO) and reduced

graphene oxide8–12 (rGO) have been used as nanofillers for some time. However, due to the cost and environmental impact of chemical modification, additional processing steps, and degraded properties,13–17 the utilization of pristine graphene in composite materials would be advantageous. The main hurdle to its use has been the difficulty dispersing and exfoliating pristine graphene in solvents and melts. We have recently introduced an approach to suspending and exfoliating pristine graphite that takes advantage of its insolubility in both aqueous and organic solvents. We have shown that graphite will spontaneously spread at the high-energy interface between oil and water, and that this spreading is thermodynamically driven, as placing graphene at the interface lowers the overall energy of the system. In this way, graphene acts as a two dimensional surfactant.18–20 Taking advantage of graphene’s surfactant character, emulsions can be formed with water as the dispersed phase and monomer, such as styrene, as the continuous oil phase. Subsequent polymerization of the monomer and removal of the water results in a polymer foam with hollow spheres lined with pristine graphene sheets forming a continuous network. Each sphere is in contact with neighboring spheres, and windows form at the points of contact. This arrangement provides a percolation path for electrical conductivity as well as providing a pathway to remove the dispersed water. The method is able to take advantage of pristine graphene’s superior electrical, thermal, and mechanical properties21–30 without the degradation and cost that results from chemical modification. There are numerous articles in the literature that contain titles suggesting the formation of foams or emulsions with graphene or graphite. In all case we are aware of, these materials are derived from GO that in many cases has been subsequently reduced or further chemically


2

Page 3 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


modified. While GO has been used as a surfactant to stabilize emulsions for some time, GO emulsions consist of oil dispersed phases and aqueous continuous phases, the opposite of what we find with pristine graphene.31,32 In the system explored in this paper, the emulsions have a continuous oil phase, meaning that polymerization results in solid materials rather than coated polymer particles. In the work presented in this article, we investigate the formation of these solid composites, and explore the effect parameters such as graphite flake size, concentration, solvent ratio, and emulsion settling have on the morphology and properties of the composites. For applications such as super capacitors, batteries, structural materials, and electromagnetic shielding, understanding the role of reaction conditions on properties such as electrical conductivity and compressive strength is of vital importance.

Experimental Section Preparation of Composite with Ar Purge and Extended Stirring. A 250 mL Erlenmeyer flask was charged with a stir bar, 0.88 g graphite (Asbury Carbons grade 2299), 140 mL water (DI), 60 mL styrene (Acros Organics, 99.5%), and 14 mL divinylbenzene (DVB) (Sigma Aldrich, 80%). The contents were then mixed for 1 min, and bath sonicated (Branson 80W B2510DTH) for 30 s to break up large aggregates. 0.18 g azobisisobutyronitrile (AIBN) (Aldrich, 98%) was then added. The flask was then covered and mixed while purging with Argon (Fisher) for 15 min. The contents of the flask were then mixed with a Polytron PT 10-35 stand emulsifier or Waring commercial blender (Model 33BL79) for approximately 1 min on the highest setting. The contents were then poured into a glass jar and sealed under Ar. The jar was then placed in an oven (Thermo Electron Corporation, Model 6500) at ~70 °C for 24 h. After the reaction was


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 20

complete, the composite samples were removed from the jars and kept at ~80 °C for ~2 days to remove all water. Standard Composite Preparation Method A 250 mL Erlenmeyer flask was charged with a stir bar, 0.88 g graphite (Asbury Carbons grade 2299), 140 mL water (DI), 60 mL styrene (Acros Organics,

99.5%),

14

mL

divinylbenzene

(Sigma

Aldrich,

80%),

and

0.18

g

azobisisobutyronitrile (AIBN) (Aldrich, 98%). The flask was then covered and mixed for about 1 min. The contents of the flask were then mixed with a Polytron PT 10-35 stand emulsifier or Waring commercial blender (Model 33BL79) for approximately 1 min on the highest setting. The contents were then poured into a glass jar and sealed. The jar was then placed in an oven (Thermo Electron Corporation, Model 6500) at ~70 °C for 24 h. After the reaction was complete, the composite samples were removed from the jars and kept at ~80 °C for ~2 days to remove all water. Composites with Lower Graphene Concentrations. The samples were prepared using the original method with argon sparging, but with the second sample used 1/4 of the initial amount of graphite (0.22 g), and the third sample used 1/8 of the initial amount of graphite (0.11 g). Larger Initial Graphite Flake Size, and Varying Water/Styrene (w/s) Ratios. For the 50/50 ratio, a 250 mL Erlenmeyer flask was charged with a stir bar, 0.88 g graphite (Asbury Carbons grade Micro 890), 100 mL water (DI), 100 mL styrene (Acros Organics, 99.5%), and 24 mL divinylbenzene (Sigma Aldrich, 80%). The contents were then mixed for about 1 min, and bath sonicated (Branson 80W B2510DTH) for about 30 s to break up aggregates. 0.30 g azobisisobutyronitrile (AIBN) (Aldrich, 98%) was then added. The flask was covered and mixed while purging with Argon (Fisher) for 15 min. The contents of the flask were then mixed with a Polytron PT 10-35 stand emulsifier or Waring commercial blender (Model 33BL79) for


4

Page 5 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


approximately 1 min on the highest setting. The contents were poured into a glass jar and sealed under Ar. The jar was placed in an oven (Thermo Electron Corporation, Model 6500) at ~70 °C for 24 h. After the reaction was complete, the composite samples were removed from the jars and kept at ~80 °C for ~2 days to remove all water. The 60/40 and 70/30 w/s ratio samples were prepared in the same manner, but the water/styrene ratios were varied while keeping the DVB and AIBN amounts constant relative to the amount of styrene. Sample Depth Testing. Samples were made using the standard method, except that they were poured into jars of varying heights to react. The samples were then cut into circular slices using a band saw to cut perpendicular to the axis of the cylindrical composite. Salt Infusion. Same as the original method with Ar sparging, except 140 mL NaCl solution was used as the aqueous phase. Mechanical Testing. Circular disks prepared as above were tested using an Instron Model 5869 in compression mode. Electrical Testing. Samples were first cut into rectangles on the scale of a few centimeters in length and measured. The ends were then covered with silver paint (Ted Pella) and allowed to dry. Copper tape (Ted Pella) was then attached to the silver contacts and the resistance measured using a Keithly Model 2420 SourceMeter using a voltage sweep. Conductivity was determined using the resistance and the length measurements taken when first cut. Density Determination. Density of the samples wes determined by first measuring the dimensions using a digital caliper. The samples were then weighed on an analytical balance. The density was calculated using the mass and volume. Thermogravimetric Analysis. 20 mg of the composites was crushed to a fine powder and analyzed in a TA Instruments TGA Q-500 to determine the graphene loading. The samples were


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 20

heated in a platinum pan in a nitrogen filled chamber from 20 °C to 800 °C at 10 °C per min. The mass of the sample left at 700 °C is taken to be graphene, as the degradation temperature of the polymer was found to be less than 400 °C. Electron Microscopy. To prepare composite samples for the electron microscope, the composites were first cut with a razor blade. The slices were mounted on aluminum stubs and coated with Au/Pd in a sputter coater (Polaron Unit E5100). The samples were characterized with a JEOL 6330 field emission scanning electron microscope with a 10 kV accelerating voltage.

Results and Discussion Standard Preparation Method. To understand and simplify our approach to producing emulsion templated graphene composite foams, we examine several key aspects of the recently developed procedure. Initially the reactions included sparging to remove dissolved O2 and stirring times of 15 minutes in an effort to facilitate graphene spreading at the liquid-liquid interfaces. To test the necessity of these steps, three types of composites are made: one with both Ar purging and extended stirring, one with extended stirring but without sparging with Ar, and one with no sparging and a mixing time of 1 minute. Analyzing the compressive breaking strengths and the scanning electron microscope (SEM) images (See SI for details), we find neither Ar purging nor extended mixing are necessary. Moreover, samples made without bath sonication are found to be nearly identical to samples prepared with bath sonication. All of this drastically reduces the total preparation time. The final procedure for typical composite synthesis is found in the experimental section labeled “Standard Composite Preparation Method.” This is the method used in all work unless otherwise stated.


6

Page 7 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Effect of Graphite Concentration The effects of graphite concentration on the compressive breaking strength and morphology of the composites is tested by three sample types. The first sample is synthesized using the standard amount of starting graphite: 0.88 g graphite/140 ml water. The second and third materials are made with concentrations of 0.22 g graphite/140 ml and 0.11 g graphite/140 ml water, one quarter and one eighth of the standard concentrations respectively. The material synthesized with the standard composition is shown in Figure 1A.

Figure 1. A) Standard 70/30 w/s graphene/polymer composite, B) Composite made with 1/4th of the graphene, C) Composite made with 1/8th of the graphene, D) SEM micrograph of the standard graphene/polymer composite, E) SEM micrograph of composite made with 1/4th of the graphene, and F) SEM micrograph of composite made with 1/8th of the graphene. The spheres in the standard composite are very small, appearing only as surface roughness in the optical image. This is in stark contrast to the foams made with lower concentrations of graphite, as seen in Figure 1B and Figure 1C. SEM images taken from the same samples reveal that the standard concentration material is composed of spheres with


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 20

diameters in the order of 100 µm, as seen in Figure 1D. By decreasing the amount of initial graphite in the system to 25% of the standard amount, the spheres increase in size and become visible to the naked eye, as shown in Figure 1B. Figure 1E shows the sphere size of the sample to be on the order of several hundred µm in diameter. Decreasing the concentration of graphene in the system further, to 12.5% of the standard concentration, the spheres are seen to increase in size even more (Figure 1C). Figure 1F shows the sphere size to have grown to several mm in diameter. Composites with larger spheres have less surface area compared to composites with smaller spheres. Thus decreasing the amount of graphene available to stabilize the liquid/liquid interface requires the emulsion to form larger spheres in order to contain the same volume of water with less interfacial area. The effect of sphere size on the mechanical properties of the composites is illustrated in Figure 2. The stress-strain curves of the three composite materials show that composites with larger spheres exhibit significantly lower compressive breaking strengths. The standard composite material is the strongest, with a compressive breaking strength around 5 MPa. The composite with a graphite concentration one quarter (25%) of that used in the standard material displays an order of magnitude drop in compressive strength, to around 0.5 MPa. Finally, the composite made with one eighth (12.5%) of the standard graphene concentration sees further drop in strength, to around 0.01 MPa.


8

Page 9 of 20

1.00E+01 1.00E+00

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.00E-01 1.00E-02 Standard Composite

1.00E-03

Quarter Concentration

1.00E-04 Eighth Concentration

1.00E-05

0

0.05 Strain

Figure 2. Stress-Strain curves of composite materials made with various concentrations of graphene. Effect of Graphite Flake Size. The size of the graphite flakes used in the composites is also seen to have a significant effect on the final material. We use Asbury Carbons Micro 890 grade, which, with an average flake size of ~10 µm, is roughly an order of magnitude larger than the graphite used in the standard formulation. The standard graphite used, Asbury Carbons grade 2299, has an average flake size on the order of several µm (~3 µm). The larger graphite is seen to dramatically impact the average sphere size of the foams, as seen in Figure 3. The reason for this increase appears to be related to the kinetics of graphene spreading at the liquid/liquid interface. Although one would expect small graphene sheets to be as effective as large ones in stabilizing the oil/water interface as long as the entire interface is covered, there is clearly an effect on the morphology of the foams related to the size of the graphite flakes used. The effect of increasing flake size also appears to be similar to the effect of decreasing the concentration of graphite used


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 20

in the material, as shown previously in Figure 1. Both effects arise from a decrease in available graphene to stabilize the oil/water interface. Although the total mass of large and small graphite flakes is identical, the smaller flakes appear to spread at the interface much faster, thus allowing for significantly more interfacial area to be stabilized. The slower spreading larger sheets thus cannot stabilize as much interface as the smaller sheets, resulting in larger spheres in the final material. In addition to investigating the effect of initial graphite flake size, Figure 3 also demonstrates the effect of the water/oil ratio on the morphology of the composites. In all water/oil ratios, the sphere diameter of the composites is seen to increase by nearly an order of magnitude with the change in graphite size. The sphere diameter is also seen to increase as the water to oil ratio is increased. In all cases, the graphite concentration relative to the volume of oil, in this case styrene, and water is kept constant. As the amount of graphene available to stabilize the oil/water interface is constant, increasing the volume of water relative to oil necessitates larger spheres in order to accommodate the increased water volume without increasing the overall interfacial area.


10

Page 11 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. SEM micrographs of composite materials made with larger initial graphite (Asbury Carbons grade Micro 890) and varied water/styrene ratios: A) 50/50 w/s, B) 60/40 w/s, and C) 70/30 w/s. D, E, and F are SEM micrographs of standard composites made with the smaller Asbury Carbons grade 2299 graphite using the same water/styrene ratios as the images directly above: D) 50/50 w/s, E) 60/40 w/s, and F) 70/30 w/s. With the increase in sphere size also comes a decrease in compressive breaking strength. This may be seen in Figure 4, where the stress strain curves of composites containing different sphere sizes are compared. Care must be taken in comparing these curves, as in addition to sphere size, the density of the foams changes with changing water/oil ratios. In order to observe the effect of sphere size, the blue and red lines in Figure 4, representing samples varying only in graphite flake size and thus sphere size, are compared. The smaller sphere size sample demonstrates a compressive strength approximately five times that of the sample with larger spheres, even though both samples are of the same density. To see the effect of density, the red, green, and aqua curves are compared. In these three samples the ratio of water/oil are varied


11


while the total mass and type of graphite remains constant. Changing the volume fraction of water does have an effect on the sphere size of the foam, as shown in Figures 3A, 3B, and 3C, and so the effect of density alone cannot be determined. However, what is clear is that in all cases, the foams fail gracefully, despite the fact that the poly(styrene) comprising these samples is well below its glass transition temperature (Tg).

6 5

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 20

Asbury Carbons Grade 2299, 50/50 w/s ratio Asbury Carbons Grade Micro 890, 50/50 w/s ratio Asbury Carbons Grade Micro 890, 60/40 w/s ratio Asbury Carbons Grade Micro 890, 70/30 w/s ratio

4 3 2 1 0

0

0.05

0.1 Strain

0.15

0.2

Figure 4: Stress-Strain curves of composite materials made with larger initial graphite and varied water/oil ratios. Sample Depth Analysis. As these composites are synthesized by way of an emulsion template, issues such as settling need to be considered. Upon standing, the emulsion droplets, being comprised of water and thus denser than the continuous oil phase, will tend to settle to the bottom of the reaction container. In practice, the bottom of the samples can look different from


12

Page 13 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the top of the samples, especially if the reaction vessel is deep. To investigate this observation, we cut the sample perpendicular to the gravitational axis to form slices of equal thickness from the top to the bottom. The results of compression, density, and electrical conductivity measurements of these slices are shown in Figure 5.

Figure 5. A) Compression and Conductivity as a function of sample depth. The blue line is conductivity in S/m and the red line is compressive breaking strength in MPa. B) TGA of composite samples cut from top to bottom showing wt% graphene and density as functions of the distance the sample was taken from the top. The blue line is the sample density in g/cm3 and the red line is wt % of graphene in the sample. Figure 5A shows the relationship of conductivity and compressive breaking strength to sample depth, or in other words, the distance from the top of the reactor the sample was taken. The closer to the bottom of the sample (or the furthest from the top) the slice is taken, the more electrically conductive it is. The compressive breaking strength shows an inverse relationship: the lower slices in the sample have progressively lower strengths. While changes in sphere size are shown to affect compressive strength, SEM images show the sphere diameter to be nearly constant between all slices (see SI for details). Instead, what we observe to vary is the amount of polymer in the interstitial space of the composite slices. This corresponds with the data presented


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 20

in Figure 5B. Here we see the density of the slices decreasing as one moves towards the lower slices, while the wt % of graphene in the composites increases at greater sample depths. At the top of the sample, there is more polymer between the spheres, meaning the spheres are not as closely packed. This leads to an increase in compressive breaking strength and density, but a decrease in conductivity as the graphene framework making up the foams is not as fully developed. At the bottom of the sample, conductivity is at its highest, as the graphene coated spheres are pressed more tightly together. Density and compressive breaking strength are both at their lowest in slices further from the top as the samples are increasingly composed of a greater fraction of spheres. As the spheres are filled with water, once the polymerization is complete and the water removed, the lower slices of composite are composed of greater fractions of voids leading to a decrease in both the compressive strength and density. Salt Infusion. One aspect seen in nearly all SEM images is the presence of very small (

Properties of Pristine Graphene Composites Arising from the

Recommend Documents