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Infusion of graphene quantum dots to modulate thermal conductivity and dynamic mechanical properties of polymers
Polymer xxx (xxxx) xxx
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Infusion of graphene quantum dots to modulate thermal conductivity and dynamic mechanical properties of polymers € Joel R. Seibert a, Ozgür Keles¸ a, Jun Wang b, Folarin Erogbogbo c, * a
Chemical, and Materials Engineering Department, San Jose State University, One Washington Square, San Jose, CA, 95112, United States PerkinElmer, 75 Nicholson Ln, San Jose, CA, 95134, United States c Biomedical Engineering Department, San Jose State University, One Washington Square, San Jose, CA, 95112, United States b
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene quantum dos Epoxy Thermal conductivity
Graphene quantum dots are small fragments of graphene that may be advantageous to transferring properties to polymers because of their small size, molecular like interactions, surface chemistry and ability to overcome dispersibility challenges in composite formation. Here, we begin the first thermal conductivity study with gra phene quantum dots by revealing the thermal and mechanical properties of composites made by infusing gra phene quantum dots into epoxy. We found that graphene quantum dots that have the capability to improve the toughness of epoxy by 260%, increase the thermal conductivity by 144%, increase the glass transition temper ature by 10%, increase the storage modulus by 9%, and decrease damping by 45%. We anticipate graphene quantum dots infusion to be a starting point for creating more sophisticated polymer composites.
1. Introduction Previous research on graphene quantum dot synthesis [1–10] and graphene reinforced nanocomposites [11–15] have laid the foundation for potentially new and innovative graphene quantum dot (GQD) com posites [16–19]. This has resulted in a very promising outlook for improved polymer performance in applications across various fields. For example, graphene/polymer nanocomposites have shown promise in a wide range of applications that include, but are not limited to, electronic devices [20,21], energy storage [22,23], fuel cells [24] and biomedical [25,26] applications. GQDs are one of the newest forms of graphene; However, the formation of GQD-polymer composites are underexplored in comparison to composite formation with other graphenic materials [27,28]. There are a few issues related to creating graphene/epoxy compos ites and using GQDs may overcome these issues. Researchers Wan et al. state that graphenic material in nanocomposites “tend to form irre versible agglomerates through van der Waals interactions” and “the structure of graphene is atomically smooth and lacks interfacial bonding, which limits load transfer from the polymer matrix to the graphene [37].” Graphene quantum dots, produced by top down methods, are not atomically smooth because they have functional groups on them that enable their interaction with other moieties in a
molecule like manner. This means GQDs may not form agglomerates in polymers as easily as larger graphenic structures tend to. In addition, the GQDs may have a higher propensity for load transfer due to interfacial bonding with external matrices. Even though there are a significant number of demonstrations showing that graphenic materials can improve the mechanical proper ties of polymers, imparting one property from the graphenic material does not imply all other properties are imparted effectively. This drives the need for simultaneously studying properties such as thermal con ductivity and mechanical attributes on the same sample compositions. Few studies have embarked on discovering thermal and mechanical properties of graphenic nanocomposites at the same time. The studies that do evaluate thermal and mechanical properties of graphenic com posites employ dynamic mechanical methods while simultaneously studying thermal conductivity to comprehensively evaluate the com posite materials. Wang et al. studied mechanical attributes and thermal conductivity of graphene nanoplatelet(GnP)/epoxy composites [43]. By using 2 different size distributions of graphene nanoplatelets they discovered the larger GnPs increased thermal conductivity more than the smaller GnPs [43]. They show that the larger GnPs have a greater effect on increasing the storage modulus and both graphene nano platelet formulations follow a similar trend but with different effec tiveness. This study drives the need to explore graphenic structures on a
* Corresponding author. E-mail address: [email protected] (F. Erogbogbo). https://doi.org/10.1016/j.polymer.2019.121988 Received 9 February 2018; Received in revised form 3 November 2019; Accepted 9 November 2019 Available online 11 November 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Joel R. Seibert, Polymer, https://doi.org/10.1016/j.polymer.2019.121988
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length, 5 mm width, and 3 mm thickness. Samples for thermal conduc tivity were cast into aluminum DSC pans which has sample dimensions of approximately 6 mm diameter and 3 mm thickness. High temperature silicone Apiezon H grease for DSC was obtained from Sigma Aldrich. A polystyrene thermal conductivity reference material was obtained from Sigma Aldrich. 2.2. Methods The overall flow of the experiment is shown in the schematic in the supporting information (Fig. S2). The first task is to synthesize the GQDs from bird charcoal. Next the GQDs are combined with epoxy to create a nanocomposite. Once composite samples have been prepared they are then characterized by either Transmission Electron Microscopy (TEM), Dynamic Mechanical Analysis (DMA), or Differential Scanning Calo rimetry. Details on the methodology of each of these steps is outlined in this section. 2.2.1. GQD synthesis The process used in research by Ye et al. to create GQDs from coal was adapted using bird charcoal as the source material [29]. First, 300 mg of bird charcoal was weighed and mixed into a solution of 60 mL sulfuric acid and 20 mL of nitric acid. The solution was then sonicated for 2 h. Next, the solution was placed in a paraffin oil bath for 24 h at 85 C. Once the oil bath was completed, the solution was cooled to room temperature using ice from distilled water. The pH of the mixture was then neutralized using 3 M sodium hydroxide via titration whilst in an ice bath to prevent unwanted heating effects of the exothermic reaction. The mixture was then filtered using Polytetrafluoroethylene (PTFE) syringe filters, and then is dialyzed for 7 days using 1 kDa Cellu Sep H1 dialysis semi-permeable membrane and changing the water every 24 h. Lastly, the solution was concentrated using a rotary evaporator to remove excess water.
Fig. 1. Schematic of GQD synthesis process and process for creating GQD Epoxy nanocomposite samples.
smaller scale because size effects are readily noticed at the micron scale. Nanoscale materials typically result in size dependent effects and nanoscale dynamics may reveal new fundamentals related to modu lating polymer properties in composites. For graphene oxide, it has been reported that highly-dispersed samples show greater mechanical strength and higher modulus than the poorly dispersed samples especially at higher sample loading. These studies motivate the need to explore smaller, well dispersed forms of graphene. The ease of dispersion of graphene quantum dots means that less post processing may be needed for composite creation for me chanical property or thermal conductivity enhancement. This manuscript aims to contribute to the underexplored area of GQD-polymer composites through simultaneous mechanical attribute and thermal conductivity studies that take advantage of GQD small particle size, atomically uneven surfaces and dispersibility in polymers. Here, we focus on GQDs that were synthesized at 85 � C because they have been shown to enhance the toughness of epoxy by 260% [10]. A schematic of the synthesis of graphene quantum dots and creation of the graphene quantum dot/epoxy composite are depicted in Fig. 1.
2.2.2. GQD/epoxy composite fabrication The synthesized GQD solution was then ready to be infused with an epoxy matrix to create a composite material. All samples were made at the same time, as shown in Fig. S3 so that they underwent the exact same degassing and curing cycles, this way they could be compared without influence from inconsistencies in the degassing and curing processes. The solution was combined with tetrahydrofuran as a dispersion agent. Then the solution was mixed with the appropriate amount of epoxy resin to achieve the desired loading by mechanical stirring for 10 min and then ultrasonication for 10 min. The curing agent was then added to the mixture at 10% by weight of the epoxy resin. The resin mixture then underwent a degassing process where it was placed in a vacuum oven at room temperature and cycled to a pressure of approximately 20 inches Hg and back to atmospheric for 5 min, and then it was ultrasonicated for 5 min. This process of vacuum cycling and ultrasonication was repeated 3 more times until no bubbles could be seen in the mixture. Then the mixture was cast into aluminum molds, or DSC pans, and cured at room temperature for 24 h and then post-cured at 95 C for 24 h. Samples for thermal conductivity on DSC were removed from the aluminum DSC pans and then sanded flat with 600 grit sandpaper as specified by Merzlyakov et al. [44]. An example of a sample prepared for direct contact with the heating element in DSC is shown in Fig. S4.
2. Experimental section 2.1. Materials The source material used for graphene quantum dot (GQD) synthesis was bird charcoal obtained from One Pet Products Co. Ltd. Chemicals used in synthesis include: 98% Sulfuric Acid, 70% Nitric Acid, 3 M So dium Hydroxide, and Paraffin Oil, which were all obtained from Thermo Fisher Scientific. Filtration devices for this process include: 0.45 μm Polytetrafluoroethylene (PTFE) membrane filters from Sartorius Inc., 1 kDa Cellu Sep H1 dialysis semi-permeable membrane from Membrane Filtration Products Inc., and Tangential flow filter from Spectrum Lab oratories Inc. The matrix of the GQD/epoxy composite material was created using Epon 828 epoxy resin and Epikure 3234, which were obtained from Sky Geek. The solvent used for dispersion was 99% tetrahydrofuran which was acquired from Sigma Aldrich. The mold release spray Ease Release 200 was acquired from Renolds Advanced Materials. Aluminum molds for casting samples were made in-house and was lined with PTFE release film as shown in Fig. S1. DMA samples were approximately 30 or 35 mm
2.2.3. Transmission Electron Microscopy (TEM) Transmission electron microscopy was used to characterize the GQDs and the bird charcoal source material using Hitachi H-9500 System at 300 kV in a bright field mode at NASA Ames. Drops of GQD solution were on lacey carbon 300 mesh grids, which then were dried at room temperature for 20 min to remove any liquid from the GQDs. Images taken with the TEM were used to verify the structure of GQDs and compare to the bird charcoal source material. 2
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Fig. 3. Scanning electron micrographs of fractured (A) neat epoxy (B) 1 wt% EGQD (C) 2.5 wt% E-GQD, and (D) 5 wt% E-GQD. The scale bars are all the same at 200 μm.
cycle per second and a strain of .0050 mm. Analysis of the mechanical response measured by DMA using PerkinElmer software gave a calcu lated storage modulus and glass transition temperature. Glass transition temperature for DSC and DMA differ in that DSC measures when the endothermic or exothermic phase change occurs, whereas DMA mea sures when the solid behaves like a glass.
Fig. 2. TEM images of bird charcoal at 2500x (A) and at 50000x (B), and GQDs at 50000x (C) and 500000x (D) GQDs are indicated by the red circles. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
2.2.4. Differential Scanning Calorimetry (DSC) Thermal conductivity was measured using DSC with the method previously mentioned research by Merzlyakov et al. in a program received from the University of Rostock available at [44]. Measuring thermal conductivity using DSC requires additional sample preparation and a modulated temperature scan. This technique is considered an estimation which is within 5% of the actual thermal conductivity [44]. A PerkinElmer dual furnace DSC 8500 was used for this experiment. A temperature step profile was used with a starting temperature of 40 C � with 1 � C heating steps at a rate of 75 C/min, with 1 min isothermic holds with 5 repetitions steps to a final temperature of 45 � C. The tem perature profile is shown in Fig. S5. First, the temperature profile was run and measured with an empty furnace and reference furnace to establish a baseline. Then the profile was run with two different sapphire standards one of which has a mass at least double the other standard as specified in the manual for the software obtained from University of Rostock.
3. Results and discussion 3.1. Microscopic visualization of graphene quantum dots and composites The graphene quantum dots (GQDs) were synthesized from bird charcoal. TEM was used to characterized the starting material in com parison to the graphene quantum dots. Fig. 2 shows the images of bird charcoal in (A) and (B) and images of GQDs in (C) and (D). Fig. 2A shows a low magnification image of 2 bird charcoal particles on a mesh grid at 2500x magnification. Fig. 2B shows a high magnification image of the bird charcoal in which we see a random assortment of layers which combine within the particle. Fig. 2C shows a low magnification image of many GQD particles which look like dark smudges in the image. Fig. 2D shows a high magnification image of GQD particles which are high lighted by the red circles. We can see lattice fringes with 0.24 nm spacing that corresponds with GQDs. A larger TEM image of graphene quantum dots that clearly shows the lattice fringes is provided in the inset of Fig. 2D. Within the images of the GQDs we can see a clear orientation of carbon atoms rather than the random assortment observed in the bird charcoal. The epoxy-graphene quantum dots (E-GQDs) composites were syn thesized as depicted in Fig. 1. Scanning Electron Microscopy (SEM) was used to characterized the different compositions of the E-GQDs. Fig. 3 shows the images of (A) neat epoxy (B) 1 wt% E-GQD (C) 2.5 wt% EGQD and (D) 5 wt% E-GQD. The greater contrast in the image for 1 wt% E-GQD indicates the presence of a rougher surface than that of neat epoxy after fracture. This observation is associated with neat epoxy being more brittle than the E-GQD composites. Roughness similar to that observed in Fig. 3B is also seen in Fig. 3C; however, the spacing between the troughs and peaks are closer. A formation of small spherical shapes is also noticed and is ascribed to E-GQD interphase formed within the epoxy matrix. There is reduced amount of contrast in Fig. 3D in com parison to Fig. 3B and C. This indicates 5 wt% E-GQD is more brittle than 1 wt%E-GQD and 2.5 wt%E-GQD. The formation of rougher patches is attributed to the formation of aggregates in the sample.
2.2.5. Temperature profile used for step analysis Samples were cast into DSC pans with diameter approximately 6.5 mm and then sanded and polished to approximately 0.50 mm with 600 grit sandpaper. After samples had been polished, a small amount of high temperature silicone Apiezon H grease was applied for good ther mal contact with the DSC. Once samples had been measured with the step profile, the program from University of Rostock was used to calculate the thermal conductivity and heat capacity using the method described in the literature review of Merzlyakov et al.‘s research [44, 45]. 2.2.6. Dynamic mechanical analysis (DMA) Dynamic Mechanical Analysis was performed at PerkinElmer in 3point bend configuration using the PerkinElmer DMA 8000. Samples for DMA were cast into the previously mentioned mold in Fig. S1. Samples were carefully removed from the mold as seen in Fig. S6. Samples for DMA were tested in the 3-point bend configuration in a PerkinElmer DMA 8000, as shown in Fig. S7 Samples were tested at 1 3
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Fig. 4. Differential Scanning Calorimetry Heat-Cool-Reheat Cycle for (A) epoxy cured at room temperature for 24 h and (B) epoxy cured at room temperature for 24 h then post cured at 95 C for 4 h.
3.2. Differential Scanning Calorimetry
0.206 W/mK. This is within the range of published values for epoxy [51]. Upon addition of graphene quantum dots at different mass load ings to the epoxy matrix, the thermal conductivity of E-GQD composites rose linearly as depicted in Fig. 5A. The thermal conductivity increased as much as 144% for 5 wt % loading of GQDs in epoxy. The heat capacity of the E-GQD composites are higher than that of neat epoxy. The amount of GQDs loaded into epoxy does not have a clear effect on the heat ca pacity of the composite. Even though the heat capacity values vary, the measurements fall within the error range of one another. For example, the heat capacity value for 1 wt% GQD loaded E-GQD composite is 1.41 � 0.08 J/gK and that overlaps with the heat capacity value for 2.5 wt% E-GQD composite is 1.38 � 0.04 J/gK. The thermal diffusivity of the E-GQD samples are related to thermal conductivity by Equation S (9). The thermal diffusivity follows a linear trend as depicted in Fig. 5c. This is expected because of the proportional relationship thermal diffusivity has with thermal conductivity. The thermal diffusivity of the 5 wt% loaded E-GQD increases by 125% in comparison to neat epoxy. Using the trendlines, an estimate for the thermal conductivity of 100% pure GQD can be calculated to be 6.049 W/mK and the thermal
A Differential Scanning Calorimetry (DSC) heat-cool-reheat cycle scan, from 25 � C to 250 � C, was run on epoxy and composite samples to obtain a hysteresis loop. The first heat run, from 25 � C to 250 � C, shows glass transition temperature and possible residual cure of the epoxy matrix as prepared; the cooling run, from 250 � C to 25 � C, ensures a consistent cooling rate and thermal history is imprinted on the epoxy before a second run; the reheat run, from 25 � C to 250 � C, allows you to determine the glass transition of fully cured epoxy with a minimum and consistent thermal history. The more closely the data from the first pass of heating matches the second pass of heating, the more cured the resin is. Fig. 4A shows heat-cool-reheat cycles for a sample that was cured for 24 h and Fig. 4B shows they cycle for a sample that was cured for 24 h and then post cured at 80 � C for 4 h. In Fig. 4A, the exothermic dip in heat flow after 100 � C indicates that the sample is not fully cured. In Fig. 4B, the reheating cycle matches the 1st heating cycle much better. Mechanical characterization of samples that were cured for 24 h, without a post cure, have been reported in literature [46]; however, our analytical step led us to perform a few iterations of this heat-cool-reheat cycle to settle on a curing process suitable for our experiments as out lined in the methods section. DSC was also used to generate thermal property measurements. Samples in this study were tested on DSC using the profile shown in Fig. S5 and with the procedure outlined in the methods section. The resultant saw-tooth heat flow plot was analyzed using analysis software developed by University of Rostock. The software recursively attempts to fit a polar curve to the points in the sample measurements with a theoretical curve. An example of a measured curve and theoretical curve after fitting is shown in Fig. S8. This results in a measured thermal conductivity and heat capacity for each temperature step in the testing profile. 3.3. Thermal conductivity of E-GQD composites Thermal conductivity methods are still being developed for graphene and its composites with varying degrees of accuracy that can have error margins as high as 30% [47]. There are no reports on the thermal conductivity of graphene quantum dots or how to measure them. The dynamic response of DSC has been used to obtain the thermal conduc tivity of epoxy [48]. The accuracy of this method is typically better than 5% and was applied here to obtain the thermal conductivity of the graphene quantum dots/epoxy (E-GQDS) composites. As the thermal conductivity of graphene ranges between 3000 W/mK to 5000 W/mK, and the thermal conductivity of graphenic materials are considered high [48–50]; it is expected that graphene quantum dots may have some thermal conductivity even though there are no reports on its value. The measured thermal conductivity of epoxy is from our experiments is
Fig. 5. Thermal conductivity (A), heat capacity (B), and thermal diffusivity (C) measured by step analysis of GQD Epoxy composite samples average for the temperature range 40–45C, (D) Trendline of E-GQD thermal conductivity compared to two different graphene nanoplatelet epoxy nanocomposites by Wang et al. 4
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properties of graphene quantum dots in polymer matrices. The average thermal conductivity values from multiple runs are depicted in Table 1.
Table 1 The effects of weight loading percentages on the thermal properties of graphene. Mass% GQD Loading into Epoxy (%)
Average Thermal Conductivity [W/ m K]
Average Cp [J/g K]
Density [g/cm3]
Thermal Diffusivity [mm2/s]
0.0 0.5 1.0 1.5 2.5 5
0.206 0.251 0.259 0.353 0.369 0.503
1.26 1.37 1.41 1.33 1.38 1.36
0.981 1.001 0.993 1.001 1.005 0.983
0.167 0.183 0.185 0.264 0.264 0.376
3.4. Dynamic mechanical analysis 3.4.1. Storage modulus Epoxy is a crosslinked thermoset system with a well-defined Tg, but the deflection temperature under load almost always falls on the steeply sloped part of the modulus curve [52]. The temperature dependence of the modulus is related to the crosslink density. The Tg of highly cross-linked thermosetting resins are often only measurable by DMA because other methods may not be sensitive enough [53]. For DMA studies in this paper, a three-point bending configuration is used because it eliminates the combined loading induced by single or double canti lever mode and it produces measurable strains in stiff materials like epoxy [53,54]. The glass transition temperature can be found from three places in a DMA curve, the first inflection point of the storage modulus curve TgA, the peak of the loss modulus curve TgB, and the peak of the tan delta curve TgC [53,54]. The Tg is different for each of these methods for this investigation. The TgA from the first inflection of the storage modulus can be estimated as epoxy is 85 � C from Fig. 6A and B. The modulus curves of E-GQD do not drop as drastically as that of epoxy; this make it harder to estimate their values for TgA. For getting E-GQD values for TgA, we selected temperatures for E-GQD that have the same modulus value as the TgA for neat epoxy. This value can be extracted from the temperature axis line of Fig. 6B. Those values result in TgA for 1 wt% EGQD as 89 � C, for 2.5 wt% E-GQD as 88 � C, for 5 wt% E-GQD as 78 � C. It is particularly difficult to know where the first inflection point of the storage modulus of 5 wt% E-GQD is because there is no rapid drop in modulus. The TgA for 1 wt% E-GQD and 2 wt% E-GQD increase by approximately 5% and 4% respectively, while the TgA of 5 wt% E-GQD decreases by approximately 8%. The TgA is recommended as the most conservative method for estimating Tg and is recommended by ASTM D 4065; however, that value is unclear for the composites studied here because the onset of the drop in modulus becomes less clear with increasing GQD concentration in the E-GQD composites. Tg reported for a material can differ due to differences in methodology for measuring Tg. We mainly focus on the TgA from the tan delta as the focal Tg for this study because it can easily be identified. For epoxy, we have a typical curve that presents a gradually declining storage modulus that eventually drops rapidly at a particular temperature. For E-GQD composites, we have a higher starting storage modulus than epoxy and a steeper drop in the modulus curve before the glass transition TgA. The steepness of the curves for the pre TgA section increases with the amount of graphene quantum dots in the E-GQD composites as depicted in Fig. 6B. The pre TgA slope of the modulus curve for epoxy is 9.0x106, for 1 wt% E-GQD it is 1.1x107, for 2.5 wt % E-GQD it is 1.2x107, for 5 wt% E-GQD it is 1.3x107. The steepness of the curve after the TgA reduces with the amount of graphene quantum dots in the E-GQD composite. The slopes of the curve measured from the start of the steep slope to the end for epoxy is 9.0x107, for 1 wt% EGQD it is 6.7x107, for 2.5 wt% E-GQD it is 5.1x107, for 5 wt% E-GQD it is 3.7x107. These slopes provide more information about the behavior of the composites that a simple comparison of TgAs. They indicate that, before the glass temperature, the elastic response ability of E-GQDs decreases more rapidly with increasing temperature and the inverse occurs at temperature in the transition region (higher than TgA but lower than the onset temperature that indicates a rubbery phase). Before TgA, dE/dT increases in magnitude as the amount of GQDs in crease in the E-GQD composite. Generally, for epoxy, nothing normally happens after Tg until the sample begins to burn and degrade because the cross links prevent the chains from slipping past each other. For EGQDs, graphene layers may be able to slip against one another causing additional motions that enable storage modulus at higher temperature in comparison to neat epoxy. The larger the amount of graphene, the more the graphene layers may be able to slide against one another.
diffusivity is calculated to be 4.398 mm2/s. Fig. 5d shows the compar ison of the trendline for the thermal conductivity of GQDs loaded into epoxy in contrast to graphene nanoplatelets of two different sizes re ported by Wang et al. [43]. The GnP-5 are around 5 μm in diameter and the GnP-750 are for a distribution of graphene nanoplatelets smaller than 1 μm. Fig. 5 shows that graphene quantum dots can be competitive for thermal property enhancement in comparison to some graphene nanoplatelet formulations. Thermal conductivity measurements of nanometer scale objects are challenging because of the size of the object and the uncertainties associated with the knowing exact amount of dissipated power over the sample [47]. In addition, it is difficult to ascertain the thermal con ductivity of the graphene quantum dots for multiple reasons such as 1.) The theory of mixtures used to obtain values of individual constituents from composites are mainly used for fibers and thus cannot be used to accurately estimate the properties of small particles 2.) The carboxyl and oxide groups on graphene quantum dots can form covalent bonds with the epoxy whereas thermal conductivity calculations for mixtures are simpler when there are no reactions involved 3.) The presence of func tional groups on graphene quantum dots leads to a lower ratio sp2 to sp3 bonds (in comparison to graphene and other larger surface area gra phenic materials) and this may reduce the thermal conductivity [50]. 4.) Direct sample contact needs to be made with the heating surface for DSC thermal conductivity measurements used for obtaining thermal con ductivity values. Composites normally have values lower than their theoretical limit. Hence, it can be assumed that the thermal conductivity of graphene quantum dots is higher than 6.049 W/mK and the thermal diffusivity is higher than 4.398 mm2/s. These values can be viewed as a starting point for understanding the thermally conductive and diffusive
Fig. 6. Storage modulus measured by DMA for GQD epoxy composites for temperature � C ranges of (A) 30–200 (B) 30 to 50 (C) 80 to 110 and (D) 100 to 200. 5
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Table 2 Possible interpretations of the changes to the peak of the tan delta curve as parameters of polymer matrix formation change. Peak of the Tan Delta Curve of composite in comparison to neat epoxy
Possible interpretation
Shifts to the right
Higher cure temperature Increasing levels of cure Increase in test frequency Decreased chain mobility Lower cure temperature Decrease in test frequency Decreasing levels of cure Coupled with increased Tg, it may mean thermal degradation of material Decreased chain mobility
Shifts to the left Broader FWHM Magnitude decreases
Simply put, in areas where there is graphene in contact with graphene, there may be some slippage. The areas that are sub Tg are typically analyzed because they relate to mechanical properties. If this were to be correlated with this study, it indicates that the rate of change of me chanical properties increases with an increasing amount of GQDs in the E-GQD composite. The storage modulus generally has a higher value for the composites at higher temperatures after 100 � C. This is mainly in the rubbery region. For epoxy, the cross linked system normally provides an extended region of stability, though at significantly lower storage modulus, beyond the Tg. This region of stability is seen in Fig. 6D. The rubbery phase shows an increase in modulus as the amount of GQDs increase in the E-GQD composite. The increase in modulus is as high as 300%for 1 wt% EGQDs, 450% for 2.5 wt% E-GQDs and 600% for 5 wt% E-GQDs. On heating above Tg in the rubbery region, the stability at a lower storage modulus is seen for 1 wt% E-GQDS and 2 wt% E-GQDS; however, the 5 wt%E-GQ epoxy composite is gaining mobility and is either getting cured or rearranging into crystallites. The E0 increase in the rubbery region for 5% E-GQD is less likely due to crystallization because this is a thermoset system. The existence of two crosslinking mechanisms in the sub Tg region is more likely - one being the reaction of the crosslinking agent with the epoxy groups and the other being the complex reactions that involves the GQDs. When the GQD load is high, the formation of an E-GQD interphase is more dominant. These short-range, highly immo bilized domains around the GQD’s more or less impede the crosslinking reaction in the sub Tg temperature range. When the material reaches the rubbery region, those immobilized the domains gain more mobility that allows the unreacted crosslink agent and epoxy molecules to diffuse and complete the curing reaction. This interpretation is also supported by the apparent lower E’ at room temperature compared to other samples with lower GQD% [55]. For the rubbery region, the viscosity of composite is known to be dependent on the molecular weight between entanglements or cross links. The modulus of the plateau is proportional to either the number of crosslinks or the chain length between entanglements. For E-GQDs we interpret the data to mean we have an increased number of cross links and short chain lengths between the GQDS and epoxy.
Fig. 7. A) Tan delta measured by DMA for GQD epoxy composites, B) loga rithmic plot of Tan delta measured by DMA for GQD epoxy composites, (C) loss factor percentage decrease, and (D) Full Width Half Maximum (FWHM) of Tan delta plots.
normally compared and interpreted in ways documented in Table 2. The Tan delta of neat epoxy, 1 wt% E-GQD, 2.5 wt% E-GQD, and 5 wt % E-GQD are shown in Fig. 7. We see some features similar to trends reported for graphene nanoplatelets by Wang et al. [51] The similar features are a shift of the TgC to a higher temperature and a decrease in the loss factor (magnitude of the height of the tan delta peak). The different features are that the E-GQDs have a larger increase in TgC than the graphene nanoplatelets/epoxy composites; and the magnitude of the loss factor decreases significantly more than it does for the graphene nanoplatelets/epoxy composites. The glass transition temperature ob tained from the tan delta peak are 100 � C for neat epoxy, 110 � C for 1 wt % E-GQD, 2.5 wt% E-GQD, and 5 wt% E-GQD. The magnitude of the tan delta peak of the tan delta peak compared to that of neat epoxy decreased by 14% for 1 wt% E-GQD, 27% for 2.5 wt% E-GQD, and 45% for 5 wt% E-GQD. We did not have a higher cure temperature during the processing of the E-GQD composites, however it would not be unrea sonable to assume that the higher conductivity of the graphene quantum dots in the epoxy could have enabled the epoxy to cure at a lower temperature. Thus curing epoxy the same way as E-GQD composites are cured is equivalent to curing the epoxy at a lower temperature. If there is a small fraction of epoxy that is uncured, there would be an increased level of cure of the epoxy, in the presence of thermally conductive graphenic materials, under the same curing conditions. We do not attribute this shift to the right of the tan delta peak to stem from decreased chain mobility alone, even though decreased chain mobility may be present; We believe the thermal conductivity of graphene quantum dots contributes to the change in tan delta as this shifting effect has been seen with curing temperature changes alone in neat epoxy
3.4.2. Tan delta The ratio of the loss to the storage in a material is reported as tan delta and is called Damping.1 Damping is a measure of how well a material can get rid of energy and how good it is at absorbing energy. The peak position of the tan delta curve is considered the glass transition temperature, TgC. TgC generated from epoxy and its composites are
Table 3 Average mechanical and thermal properties of epoxy and graphene quantum dot/epoxy composites after triplicate runs from dynamic mechanical analysis.
1
Dynamic Mechanical Analysis (DMA), A beginners guide, Www.perkin elmer.com. (n.d.). https://www.perkinelmer.com/CMSResources/Images/ 44-74546GDE_IntroductionToDMA. 6
Mass% GQD Loading into Epoxy (%)
Average storage modulus at 30 C [GPa]
Average Glass Transition (Tg) [� C]
0.0 1.0 2.5 5
1.76 1.92 1.92 1.74
101 110 110 110
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Table 4 Percentage change of thermal and mechanical properties of graphene quantum dot/epoxy (E-GQD) composite and graphene nanoplatelets(GnP)/epoxy composites in comparison to neat epoxy.
This Investigation Chandrasekaran et al. [26] Wang et al. (GnP-5) [14] Wang et al. (GnP-C750) [14]
Storage Modulus
Glass Transition Temperature
Loss Factor
9.1% 14% 23% 2%
9.6%
40.4%
2.1% 1.4%
7.7% 6.4%
studies. The decreased chain mobility may be inferred from the lower tan delta value for E-GQD. Since there is an enhancement in the storage modulus in the rubbery region depicted in Fig. 6D, it is reasonable to see a decreasing ratio of loss to storage in the composite materials as the amount of GQDs increase in the composite increase. The full width half max (FWHM) of the tan delta peak is narrower than that of neat epoxy for 1 wt%E-GQD and 2 wt%E-GQD composites. However, the FWHM for 5 wt% EGQD is larger than that of neat epoxy. As noted from the Tan delta linear and logarithmic plots there is a shoulder gradually appear ing on the tan delta peak that makes the peak boarder at lower Tan delta values. This shoulder increases as the amount of GQDs increase in the composite. At 5 wt% GQD loading, the shoulder has increased to the point that the FWHM of the tan delta plot for the 5 wt%E-GQD is larger than the FWHM of neat epoxy tan delta plot. If broader peaks are associated with thermal degradation, there is a chance that the E-GQD composites have a more compact configuration that neat epoxy. During curing however, there can be phase separation that leads to a shoulder on the tan delta curve. The spectrally unmixed Tan delta plots are depicted in Fig. S9. When the shoulder on the plot is spectrally unmixed from the main peak for 5 wt% EGQD, it results in a tan delta peak around TgC that is narrower than the Tan delta peaks of epoxy, 1 wt%E-GQD and 2.5 wt% E-GQD at the same temperature. This narrow peak indicates the compact configuration of the E-GQD composite. The second peak from spectral unmixing, slightly left to the tan delta peak, responsible for the appearance of a shoulder is attributed to the interfacial constrains and free volume increase mechanisms falling out of equilibrium [55]. There is a higher contribution from the free volume mechanisms that assists with segmental motion in the 5 wt% E-GQD than are present in the 1 wt % and 2 wt% EGQDs samples that are dominated by the interfacial constrains. Basically, the Tan delta peak enables us to know the quality of the graphene quantum dot dispersion. 1 wt%EGQD and 2.5 wt% E-GQD are well dispersed and 5 wt% E-GQD is sufficiently dispersed. The plots for Figs. 6 and 7 are representative of multiple DMA runs and the average storage modulus from multiple runs at 30 and the average glass transition temperature from the tan delta are included in Table 3. There are a few studies that include dynamic mechanical properties with thermal conductivity. Table 4 compares the increase in storage modulus, glass transition temperature, decrease in loss factor and in crease in thermal conductivity. There are many studies about high thermal conductivity of graphenic materials that are not included here because they were not accompanied by the dynamic mechanical anal ysis. From the studies compared, Graphene quantum dots have a po tential for dramatic changes to the properties of epoxy. We noted the higher drop in loss factor, highest increase in thermal conductivity, highest increase in storage modulus after glass transition. The compar isons to these studies indicate that there is room for graphene quantum dots to make a significant impact on composite formation for different applications. In this study, EPON 828 is used as the investigative epoxy. An interphase also forms around the GQD due to inter and supramolecular interactions. Interphase has properties different than the matrix and can occupy a volume larger than the GQD. Molecular dy namics simulations showed that decreasing particle size increases the interphase stiffness in epoxy [56]. Generally, the interphase thickness is 20%–80% of the filler radius for particles smaller than 25 nm [56–58]. In addition, molecular dynamics simulations showed that an interphase
Thermal Conductivity
Notes
144.2% 14% 114% 14%
5 wt% E-GQD 2% wt loading GnP prepared by 3RM technique 5% wt loading GnP-5 using sonication dispersement 5% wt loading GnP-C750 using sonication dispersement
thickness of 1.14 nm exists around a carbon nanotube of 0.7 nm diam eter in an epoxy matrix [59]. If we assume a spherical GQD of 0.7 nm diameter, the volume of a 1.14 nm thick interphase would be ~76 times the GQD volume. Accordingly, we expect significant changes in the molecular arrangement of epoxy at GQD sizes <2.5 nm even for low GQD concentrations, which could result in an interphase-dominated thermo-mechanical behavior in GQD-epoxy composites. 4. Conclusion Infusing graphene quantum dots(GQDs) into polymers provides intriguing prospects for modulation of the polymeric properties. Gra phene quantum dots with terminal groups are small fragments of gra phene that possess a smaller amount of sp2/sp3 configuration ratios than larger graphenic materials because the functional groups on the GQDS contribute to sp3 bonds on the surface. The GQDs that have larger surface area to volume ratios compared to other forms of graphenic materials like graphene nanoplatelets (GnP). This means properties of GnPs that are largely due to the sp2 configuration could be attenuated in GQDs. In this study, we discover that other properties such as size and dispersibility, allow GQDs to impact mechanical and thermal conduc tivity properties more effectively than GnPs in similar studies. This implies that the size effects, surface chemistry, and dispersibility contribute significantly to imparting graphenic properties to polymers. Author contributions Joel R. Seibert designed experiments, executed experiments, and wrote the manuscript. Ozgur Keles contributed to discussion and revised the manuscript, Jun Wang designed experiment, supervised the research work, and reviewed the manuscript. Folarin Erogbogbo designed ex periments, wrote the manuscript, supervised the research work. All authors approved the final manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The work leading to this manuscript resulted from the efforts of Hildegarde Bell, Anesh Tilwani, Jose Alvarez, Belqais Nagshibandi, Navathej Gobi, Darshan Vijayakumar, Sowbaranigha Chinnusamy Jayanthi. The work leading to these results received funding from SJSU Central Research, Scholarship and Creative Activity (RSCA) Grant Pro gram, The second is the National Science Foundation(NSF) I-Corps program through the California Stae University(CSU) Program for Ed ucation and Research in Biotechnology. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymer.2019.121988. 7
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8
Contents lists available at ScienceDirect
Polymer journal homepage: http://www.elsevier.com/locate/polymer
Infusion of graphene quantum dots to modulate thermal conductivity and dynamic mechanical properties of polymers € Joel R. Seibert a, Ozgür Keles¸ a, Jun Wang b, Folarin Erogbogbo c, * a
Chemical, and Materials Engineering Department, San Jose State University, One Washington Square, San Jose, CA, 95112, United States PerkinElmer, 75 Nicholson Ln, San Jose, CA, 95134, United States c Biomedical Engineering Department, San Jose State University, One Washington Square, San Jose, CA, 95112, United States b
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene quantum dos Epoxy Thermal conductivity
Graphene quantum dots are small fragments of graphene that may be advantageous to transferring properties to polymers because of their small size, molecular like interactions, surface chemistry and ability to overcome dispersibility challenges in composite formation. Here, we begin the first thermal conductivity study with gra phene quantum dots by revealing the thermal and mechanical properties of composites made by infusing gra phene quantum dots into epoxy. We found that graphene quantum dots that have the capability to improve the toughness of epoxy by 260%, increase the thermal conductivity by 144%, increase the glass transition temper ature by 10%, increase the storage modulus by 9%, and decrease damping by 45%. We anticipate graphene quantum dots infusion to be a starting point for creating more sophisticated polymer composites.
1. Introduction Previous research on graphene quantum dot synthesis [1–10] and graphene reinforced nanocomposites [11–15] have laid the foundation for potentially new and innovative graphene quantum dot (GQD) com posites [16–19]. This has resulted in a very promising outlook for improved polymer performance in applications across various fields. For example, graphene/polymer nanocomposites have shown promise in a wide range of applications that include, but are not limited to, electronic devices [20,21], energy storage [22,23], fuel cells [24] and biomedical [25,26] applications. GQDs are one of the newest forms of graphene; However, the formation of GQD-polymer composites are underexplored in comparison to composite formation with other graphenic materials [27,28]. There are a few issues related to creating graphene/epoxy compos ites and using GQDs may overcome these issues. Researchers Wan et al. state that graphenic material in nanocomposites “tend to form irre versible agglomerates through van der Waals interactions” and “the structure of graphene is atomically smooth and lacks interfacial bonding, which limits load transfer from the polymer matrix to the graphene [37].” Graphene quantum dots, produced by top down methods, are not atomically smooth because they have functional groups on them that enable their interaction with other moieties in a
molecule like manner. This means GQDs may not form agglomerates in polymers as easily as larger graphenic structures tend to. In addition, the GQDs may have a higher propensity for load transfer due to interfacial bonding with external matrices. Even though there are a significant number of demonstrations showing that graphenic materials can improve the mechanical proper ties of polymers, imparting one property from the graphenic material does not imply all other properties are imparted effectively. This drives the need for simultaneously studying properties such as thermal con ductivity and mechanical attributes on the same sample compositions. Few studies have embarked on discovering thermal and mechanical properties of graphenic nanocomposites at the same time. The studies that do evaluate thermal and mechanical properties of graphenic com posites employ dynamic mechanical methods while simultaneously studying thermal conductivity to comprehensively evaluate the com posite materials. Wang et al. studied mechanical attributes and thermal conductivity of graphene nanoplatelet(GnP)/epoxy composites [43]. By using 2 different size distributions of graphene nanoplatelets they discovered the larger GnPs increased thermal conductivity more than the smaller GnPs [43]. They show that the larger GnPs have a greater effect on increasing the storage modulus and both graphene nano platelet formulations follow a similar trend but with different effec tiveness. This study drives the need to explore graphenic structures on a
* Corresponding author. E-mail address: [email protected] (F. Erogbogbo). https://doi.org/10.1016/j.polymer.2019.121988 Received 9 February 2018; Received in revised form 3 November 2019; Accepted 9 November 2019 Available online 11 November 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Joel R. Seibert, Polymer, https://doi.org/10.1016/j.polymer.2019.121988
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length, 5 mm width, and 3 mm thickness. Samples for thermal conduc tivity were cast into aluminum DSC pans which has sample dimensions of approximately 6 mm diameter and 3 mm thickness. High temperature silicone Apiezon H grease for DSC was obtained from Sigma Aldrich. A polystyrene thermal conductivity reference material was obtained from Sigma Aldrich. 2.2. Methods The overall flow of the experiment is shown in the schematic in the supporting information (Fig. S2). The first task is to synthesize the GQDs from bird charcoal. Next the GQDs are combined with epoxy to create a nanocomposite. Once composite samples have been prepared they are then characterized by either Transmission Electron Microscopy (TEM), Dynamic Mechanical Analysis (DMA), or Differential Scanning Calo rimetry. Details on the methodology of each of these steps is outlined in this section. 2.2.1. GQD synthesis The process used in research by Ye et al. to create GQDs from coal was adapted using bird charcoal as the source material [29]. First, 300 mg of bird charcoal was weighed and mixed into a solution of 60 mL sulfuric acid and 20 mL of nitric acid. The solution was then sonicated for 2 h. Next, the solution was placed in a paraffin oil bath for 24 h at 85 C. Once the oil bath was completed, the solution was cooled to room temperature using ice from distilled water. The pH of the mixture was then neutralized using 3 M sodium hydroxide via titration whilst in an ice bath to prevent unwanted heating effects of the exothermic reaction. The mixture was then filtered using Polytetrafluoroethylene (PTFE) syringe filters, and then is dialyzed for 7 days using 1 kDa Cellu Sep H1 dialysis semi-permeable membrane and changing the water every 24 h. Lastly, the solution was concentrated using a rotary evaporator to remove excess water.
Fig. 1. Schematic of GQD synthesis process and process for creating GQD Epoxy nanocomposite samples.
smaller scale because size effects are readily noticed at the micron scale. Nanoscale materials typically result in size dependent effects and nanoscale dynamics may reveal new fundamentals related to modu lating polymer properties in composites. For graphene oxide, it has been reported that highly-dispersed samples show greater mechanical strength and higher modulus than the poorly dispersed samples especially at higher sample loading. These studies motivate the need to explore smaller, well dispersed forms of graphene. The ease of dispersion of graphene quantum dots means that less post processing may be needed for composite creation for me chanical property or thermal conductivity enhancement. This manuscript aims to contribute to the underexplored area of GQD-polymer composites through simultaneous mechanical attribute and thermal conductivity studies that take advantage of GQD small particle size, atomically uneven surfaces and dispersibility in polymers. Here, we focus on GQDs that were synthesized at 85 � C because they have been shown to enhance the toughness of epoxy by 260% [10]. A schematic of the synthesis of graphene quantum dots and creation of the graphene quantum dot/epoxy composite are depicted in Fig. 1.
2.2.2. GQD/epoxy composite fabrication The synthesized GQD solution was then ready to be infused with an epoxy matrix to create a composite material. All samples were made at the same time, as shown in Fig. S3 so that they underwent the exact same degassing and curing cycles, this way they could be compared without influence from inconsistencies in the degassing and curing processes. The solution was combined with tetrahydrofuran as a dispersion agent. Then the solution was mixed with the appropriate amount of epoxy resin to achieve the desired loading by mechanical stirring for 10 min and then ultrasonication for 10 min. The curing agent was then added to the mixture at 10% by weight of the epoxy resin. The resin mixture then underwent a degassing process where it was placed in a vacuum oven at room temperature and cycled to a pressure of approximately 20 inches Hg and back to atmospheric for 5 min, and then it was ultrasonicated for 5 min. This process of vacuum cycling and ultrasonication was repeated 3 more times until no bubbles could be seen in the mixture. Then the mixture was cast into aluminum molds, or DSC pans, and cured at room temperature for 24 h and then post-cured at 95 C for 24 h. Samples for thermal conductivity on DSC were removed from the aluminum DSC pans and then sanded flat with 600 grit sandpaper as specified by Merzlyakov et al. [44]. An example of a sample prepared for direct contact with the heating element in DSC is shown in Fig. S4.
2. Experimental section 2.1. Materials The source material used for graphene quantum dot (GQD) synthesis was bird charcoal obtained from One Pet Products Co. Ltd. Chemicals used in synthesis include: 98% Sulfuric Acid, 70% Nitric Acid, 3 M So dium Hydroxide, and Paraffin Oil, which were all obtained from Thermo Fisher Scientific. Filtration devices for this process include: 0.45 μm Polytetrafluoroethylene (PTFE) membrane filters from Sartorius Inc., 1 kDa Cellu Sep H1 dialysis semi-permeable membrane from Membrane Filtration Products Inc., and Tangential flow filter from Spectrum Lab oratories Inc. The matrix of the GQD/epoxy composite material was created using Epon 828 epoxy resin and Epikure 3234, which were obtained from Sky Geek. The solvent used for dispersion was 99% tetrahydrofuran which was acquired from Sigma Aldrich. The mold release spray Ease Release 200 was acquired from Renolds Advanced Materials. Aluminum molds for casting samples were made in-house and was lined with PTFE release film as shown in Fig. S1. DMA samples were approximately 30 or 35 mm
2.2.3. Transmission Electron Microscopy (TEM) Transmission electron microscopy was used to characterize the GQDs and the bird charcoal source material using Hitachi H-9500 System at 300 kV in a bright field mode at NASA Ames. Drops of GQD solution were on lacey carbon 300 mesh grids, which then were dried at room temperature for 20 min to remove any liquid from the GQDs. Images taken with the TEM were used to verify the structure of GQDs and compare to the bird charcoal source material. 2
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Fig. 3. Scanning electron micrographs of fractured (A) neat epoxy (B) 1 wt% EGQD (C) 2.5 wt% E-GQD, and (D) 5 wt% E-GQD. The scale bars are all the same at 200 μm.
cycle per second and a strain of .0050 mm. Analysis of the mechanical response measured by DMA using PerkinElmer software gave a calcu lated storage modulus and glass transition temperature. Glass transition temperature for DSC and DMA differ in that DSC measures when the endothermic or exothermic phase change occurs, whereas DMA mea sures when the solid behaves like a glass.
Fig. 2. TEM images of bird charcoal at 2500x (A) and at 50000x (B), and GQDs at 50000x (C) and 500000x (D) GQDs are indicated by the red circles. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
2.2.4. Differential Scanning Calorimetry (DSC) Thermal conductivity was measured using DSC with the method previously mentioned research by Merzlyakov et al. in a program received from the University of Rostock available at [44]. Measuring thermal conductivity using DSC requires additional sample preparation and a modulated temperature scan. This technique is considered an estimation which is within 5% of the actual thermal conductivity [44]. A PerkinElmer dual furnace DSC 8500 was used for this experiment. A temperature step profile was used with a starting temperature of 40 C � with 1 � C heating steps at a rate of 75 C/min, with 1 min isothermic holds with 5 repetitions steps to a final temperature of 45 � C. The tem perature profile is shown in Fig. S5. First, the temperature profile was run and measured with an empty furnace and reference furnace to establish a baseline. Then the profile was run with two different sapphire standards one of which has a mass at least double the other standard as specified in the manual for the software obtained from University of Rostock.
3. Results and discussion 3.1. Microscopic visualization of graphene quantum dots and composites The graphene quantum dots (GQDs) were synthesized from bird charcoal. TEM was used to characterized the starting material in com parison to the graphene quantum dots. Fig. 2 shows the images of bird charcoal in (A) and (B) and images of GQDs in (C) and (D). Fig. 2A shows a low magnification image of 2 bird charcoal particles on a mesh grid at 2500x magnification. Fig. 2B shows a high magnification image of the bird charcoal in which we see a random assortment of layers which combine within the particle. Fig. 2C shows a low magnification image of many GQD particles which look like dark smudges in the image. Fig. 2D shows a high magnification image of GQD particles which are high lighted by the red circles. We can see lattice fringes with 0.24 nm spacing that corresponds with GQDs. A larger TEM image of graphene quantum dots that clearly shows the lattice fringes is provided in the inset of Fig. 2D. Within the images of the GQDs we can see a clear orientation of carbon atoms rather than the random assortment observed in the bird charcoal. The epoxy-graphene quantum dots (E-GQDs) composites were syn thesized as depicted in Fig. 1. Scanning Electron Microscopy (SEM) was used to characterized the different compositions of the E-GQDs. Fig. 3 shows the images of (A) neat epoxy (B) 1 wt% E-GQD (C) 2.5 wt% EGQD and (D) 5 wt% E-GQD. The greater contrast in the image for 1 wt% E-GQD indicates the presence of a rougher surface than that of neat epoxy after fracture. This observation is associated with neat epoxy being more brittle than the E-GQD composites. Roughness similar to that observed in Fig. 3B is also seen in Fig. 3C; however, the spacing between the troughs and peaks are closer. A formation of small spherical shapes is also noticed and is ascribed to E-GQD interphase formed within the epoxy matrix. There is reduced amount of contrast in Fig. 3D in com parison to Fig. 3B and C. This indicates 5 wt% E-GQD is more brittle than 1 wt%E-GQD and 2.5 wt%E-GQD. The formation of rougher patches is attributed to the formation of aggregates in the sample.
2.2.5. Temperature profile used for step analysis Samples were cast into DSC pans with diameter approximately 6.5 mm and then sanded and polished to approximately 0.50 mm with 600 grit sandpaper. After samples had been polished, a small amount of high temperature silicone Apiezon H grease was applied for good ther mal contact with the DSC. Once samples had been measured with the step profile, the program from University of Rostock was used to calculate the thermal conductivity and heat capacity using the method described in the literature review of Merzlyakov et al.‘s research [44, 45]. 2.2.6. Dynamic mechanical analysis (DMA) Dynamic Mechanical Analysis was performed at PerkinElmer in 3point bend configuration using the PerkinElmer DMA 8000. Samples for DMA were cast into the previously mentioned mold in Fig. S1. Samples were carefully removed from the mold as seen in Fig. S6. Samples for DMA were tested in the 3-point bend configuration in a PerkinElmer DMA 8000, as shown in Fig. S7 Samples were tested at 1 3
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Fig. 4. Differential Scanning Calorimetry Heat-Cool-Reheat Cycle for (A) epoxy cured at room temperature for 24 h and (B) epoxy cured at room temperature for 24 h then post cured at 95 C for 4 h.
3.2. Differential Scanning Calorimetry
0.206 W/mK. This is within the range of published values for epoxy [51]. Upon addition of graphene quantum dots at different mass load ings to the epoxy matrix, the thermal conductivity of E-GQD composites rose linearly as depicted in Fig. 5A. The thermal conductivity increased as much as 144% for 5 wt % loading of GQDs in epoxy. The heat capacity of the E-GQD composites are higher than that of neat epoxy. The amount of GQDs loaded into epoxy does not have a clear effect on the heat ca pacity of the composite. Even though the heat capacity values vary, the measurements fall within the error range of one another. For example, the heat capacity value for 1 wt% GQD loaded E-GQD composite is 1.41 � 0.08 J/gK and that overlaps with the heat capacity value for 2.5 wt% E-GQD composite is 1.38 � 0.04 J/gK. The thermal diffusivity of the E-GQD samples are related to thermal conductivity by Equation S (9). The thermal diffusivity follows a linear trend as depicted in Fig. 5c. This is expected because of the proportional relationship thermal diffusivity has with thermal conductivity. The thermal diffusivity of the 5 wt% loaded E-GQD increases by 125% in comparison to neat epoxy. Using the trendlines, an estimate for the thermal conductivity of 100% pure GQD can be calculated to be 6.049 W/mK and the thermal
A Differential Scanning Calorimetry (DSC) heat-cool-reheat cycle scan, from 25 � C to 250 � C, was run on epoxy and composite samples to obtain a hysteresis loop. The first heat run, from 25 � C to 250 � C, shows glass transition temperature and possible residual cure of the epoxy matrix as prepared; the cooling run, from 250 � C to 25 � C, ensures a consistent cooling rate and thermal history is imprinted on the epoxy before a second run; the reheat run, from 25 � C to 250 � C, allows you to determine the glass transition of fully cured epoxy with a minimum and consistent thermal history. The more closely the data from the first pass of heating matches the second pass of heating, the more cured the resin is. Fig. 4A shows heat-cool-reheat cycles for a sample that was cured for 24 h and Fig. 4B shows they cycle for a sample that was cured for 24 h and then post cured at 80 � C for 4 h. In Fig. 4A, the exothermic dip in heat flow after 100 � C indicates that the sample is not fully cured. In Fig. 4B, the reheating cycle matches the 1st heating cycle much better. Mechanical characterization of samples that were cured for 24 h, without a post cure, have been reported in literature [46]; however, our analytical step led us to perform a few iterations of this heat-cool-reheat cycle to settle on a curing process suitable for our experiments as out lined in the methods section. DSC was also used to generate thermal property measurements. Samples in this study were tested on DSC using the profile shown in Fig. S5 and with the procedure outlined in the methods section. The resultant saw-tooth heat flow plot was analyzed using analysis software developed by University of Rostock. The software recursively attempts to fit a polar curve to the points in the sample measurements with a theoretical curve. An example of a measured curve and theoretical curve after fitting is shown in Fig. S8. This results in a measured thermal conductivity and heat capacity for each temperature step in the testing profile. 3.3. Thermal conductivity of E-GQD composites Thermal conductivity methods are still being developed for graphene and its composites with varying degrees of accuracy that can have error margins as high as 30% [47]. There are no reports on the thermal conductivity of graphene quantum dots or how to measure them. The dynamic response of DSC has been used to obtain the thermal conduc tivity of epoxy [48]. The accuracy of this method is typically better than 5% and was applied here to obtain the thermal conductivity of the graphene quantum dots/epoxy (E-GQDS) composites. As the thermal conductivity of graphene ranges between 3000 W/mK to 5000 W/mK, and the thermal conductivity of graphenic materials are considered high [48–50]; it is expected that graphene quantum dots may have some thermal conductivity even though there are no reports on its value. The measured thermal conductivity of epoxy is from our experiments is
Fig. 5. Thermal conductivity (A), heat capacity (B), and thermal diffusivity (C) measured by step analysis of GQD Epoxy composite samples average for the temperature range 40–45C, (D) Trendline of E-GQD thermal conductivity compared to two different graphene nanoplatelet epoxy nanocomposites by Wang et al. 4
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properties of graphene quantum dots in polymer matrices. The average thermal conductivity values from multiple runs are depicted in Table 1.
Table 1 The effects of weight loading percentages on the thermal properties of graphene. Mass% GQD Loading into Epoxy (%)
Average Thermal Conductivity [W/ m K]
Average Cp [J/g K]
Density [g/cm3]
Thermal Diffusivity [mm2/s]
0.0 0.5 1.0 1.5 2.5 5
0.206 0.251 0.259 0.353 0.369 0.503
1.26 1.37 1.41 1.33 1.38 1.36
0.981 1.001 0.993 1.001 1.005 0.983
0.167 0.183 0.185 0.264 0.264 0.376
3.4. Dynamic mechanical analysis 3.4.1. Storage modulus Epoxy is a crosslinked thermoset system with a well-defined Tg, but the deflection temperature under load almost always falls on the steeply sloped part of the modulus curve [52]. The temperature dependence of the modulus is related to the crosslink density. The Tg of highly cross-linked thermosetting resins are often only measurable by DMA because other methods may not be sensitive enough [53]. For DMA studies in this paper, a three-point bending configuration is used because it eliminates the combined loading induced by single or double canti lever mode and it produces measurable strains in stiff materials like epoxy [53,54]. The glass transition temperature can be found from three places in a DMA curve, the first inflection point of the storage modulus curve TgA, the peak of the loss modulus curve TgB, and the peak of the tan delta curve TgC [53,54]. The Tg is different for each of these methods for this investigation. The TgA from the first inflection of the storage modulus can be estimated as epoxy is 85 � C from Fig. 6A and B. The modulus curves of E-GQD do not drop as drastically as that of epoxy; this make it harder to estimate their values for TgA. For getting E-GQD values for TgA, we selected temperatures for E-GQD that have the same modulus value as the TgA for neat epoxy. This value can be extracted from the temperature axis line of Fig. 6B. Those values result in TgA for 1 wt% EGQD as 89 � C, for 2.5 wt% E-GQD as 88 � C, for 5 wt% E-GQD as 78 � C. It is particularly difficult to know where the first inflection point of the storage modulus of 5 wt% E-GQD is because there is no rapid drop in modulus. The TgA for 1 wt% E-GQD and 2 wt% E-GQD increase by approximately 5% and 4% respectively, while the TgA of 5 wt% E-GQD decreases by approximately 8%. The TgA is recommended as the most conservative method for estimating Tg and is recommended by ASTM D 4065; however, that value is unclear for the composites studied here because the onset of the drop in modulus becomes less clear with increasing GQD concentration in the E-GQD composites. Tg reported for a material can differ due to differences in methodology for measuring Tg. We mainly focus on the TgA from the tan delta as the focal Tg for this study because it can easily be identified. For epoxy, we have a typical curve that presents a gradually declining storage modulus that eventually drops rapidly at a particular temperature. For E-GQD composites, we have a higher starting storage modulus than epoxy and a steeper drop in the modulus curve before the glass transition TgA. The steepness of the curves for the pre TgA section increases with the amount of graphene quantum dots in the E-GQD composites as depicted in Fig. 6B. The pre TgA slope of the modulus curve for epoxy is 9.0x106, for 1 wt% E-GQD it is 1.1x107, for 2.5 wt % E-GQD it is 1.2x107, for 5 wt% E-GQD it is 1.3x107. The steepness of the curve after the TgA reduces with the amount of graphene quantum dots in the E-GQD composite. The slopes of the curve measured from the start of the steep slope to the end for epoxy is 9.0x107, for 1 wt% EGQD it is 6.7x107, for 2.5 wt% E-GQD it is 5.1x107, for 5 wt% E-GQD it is 3.7x107. These slopes provide more information about the behavior of the composites that a simple comparison of TgAs. They indicate that, before the glass temperature, the elastic response ability of E-GQDs decreases more rapidly with increasing temperature and the inverse occurs at temperature in the transition region (higher than TgA but lower than the onset temperature that indicates a rubbery phase). Before TgA, dE/dT increases in magnitude as the amount of GQDs in crease in the E-GQD composite. Generally, for epoxy, nothing normally happens after Tg until the sample begins to burn and degrade because the cross links prevent the chains from slipping past each other. For EGQDs, graphene layers may be able to slip against one another causing additional motions that enable storage modulus at higher temperature in comparison to neat epoxy. The larger the amount of graphene, the more the graphene layers may be able to slide against one another.
diffusivity is calculated to be 4.398 mm2/s. Fig. 5d shows the compar ison of the trendline for the thermal conductivity of GQDs loaded into epoxy in contrast to graphene nanoplatelets of two different sizes re ported by Wang et al. [43]. The GnP-5 are around 5 μm in diameter and the GnP-750 are for a distribution of graphene nanoplatelets smaller than 1 μm. Fig. 5 shows that graphene quantum dots can be competitive for thermal property enhancement in comparison to some graphene nanoplatelet formulations. Thermal conductivity measurements of nanometer scale objects are challenging because of the size of the object and the uncertainties associated with the knowing exact amount of dissipated power over the sample [47]. In addition, it is difficult to ascertain the thermal con ductivity of the graphene quantum dots for multiple reasons such as 1.) The theory of mixtures used to obtain values of individual constituents from composites are mainly used for fibers and thus cannot be used to accurately estimate the properties of small particles 2.) The carboxyl and oxide groups on graphene quantum dots can form covalent bonds with the epoxy whereas thermal conductivity calculations for mixtures are simpler when there are no reactions involved 3.) The presence of func tional groups on graphene quantum dots leads to a lower ratio sp2 to sp3 bonds (in comparison to graphene and other larger surface area gra phenic materials) and this may reduce the thermal conductivity [50]. 4.) Direct sample contact needs to be made with the heating surface for DSC thermal conductivity measurements used for obtaining thermal con ductivity values. Composites normally have values lower than their theoretical limit. Hence, it can be assumed that the thermal conductivity of graphene quantum dots is higher than 6.049 W/mK and the thermal diffusivity is higher than 4.398 mm2/s. These values can be viewed as a starting point for understanding the thermally conductive and diffusive
Fig. 6. Storage modulus measured by DMA for GQD epoxy composites for temperature � C ranges of (A) 30–200 (B) 30 to 50 (C) 80 to 110 and (D) 100 to 200. 5
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Table 2 Possible interpretations of the changes to the peak of the tan delta curve as parameters of polymer matrix formation change. Peak of the Tan Delta Curve of composite in comparison to neat epoxy
Possible interpretation
Shifts to the right
Higher cure temperature Increasing levels of cure Increase in test frequency Decreased chain mobility Lower cure temperature Decrease in test frequency Decreasing levels of cure Coupled with increased Tg, it may mean thermal degradation of material Decreased chain mobility
Shifts to the left Broader FWHM Magnitude decreases
Simply put, in areas where there is graphene in contact with graphene, there may be some slippage. The areas that are sub Tg are typically analyzed because they relate to mechanical properties. If this were to be correlated with this study, it indicates that the rate of change of me chanical properties increases with an increasing amount of GQDs in the E-GQD composite. The storage modulus generally has a higher value for the composites at higher temperatures after 100 � C. This is mainly in the rubbery region. For epoxy, the cross linked system normally provides an extended region of stability, though at significantly lower storage modulus, beyond the Tg. This region of stability is seen in Fig. 6D. The rubbery phase shows an increase in modulus as the amount of GQDs increase in the E-GQD composite. The increase in modulus is as high as 300%for 1 wt% EGQDs, 450% for 2.5 wt% E-GQDs and 600% for 5 wt% E-GQDs. On heating above Tg in the rubbery region, the stability at a lower storage modulus is seen for 1 wt% E-GQDS and 2 wt% E-GQDS; however, the 5 wt%E-GQ epoxy composite is gaining mobility and is either getting cured or rearranging into crystallites. The E0 increase in the rubbery region for 5% E-GQD is less likely due to crystallization because this is a thermoset system. The existence of two crosslinking mechanisms in the sub Tg region is more likely - one being the reaction of the crosslinking agent with the epoxy groups and the other being the complex reactions that involves the GQDs. When the GQD load is high, the formation of an E-GQD interphase is more dominant. These short-range, highly immo bilized domains around the GQD’s more or less impede the crosslinking reaction in the sub Tg temperature range. When the material reaches the rubbery region, those immobilized the domains gain more mobility that allows the unreacted crosslink agent and epoxy molecules to diffuse and complete the curing reaction. This interpretation is also supported by the apparent lower E’ at room temperature compared to other samples with lower GQD% [55]. For the rubbery region, the viscosity of composite is known to be dependent on the molecular weight between entanglements or cross links. The modulus of the plateau is proportional to either the number of crosslinks or the chain length between entanglements. For E-GQDs we interpret the data to mean we have an increased number of cross links and short chain lengths between the GQDS and epoxy.
Fig. 7. A) Tan delta measured by DMA for GQD epoxy composites, B) loga rithmic plot of Tan delta measured by DMA for GQD epoxy composites, (C) loss factor percentage decrease, and (D) Full Width Half Maximum (FWHM) of Tan delta plots.
normally compared and interpreted in ways documented in Table 2. The Tan delta of neat epoxy, 1 wt% E-GQD, 2.5 wt% E-GQD, and 5 wt % E-GQD are shown in Fig. 7. We see some features similar to trends reported for graphene nanoplatelets by Wang et al. [51] The similar features are a shift of the TgC to a higher temperature and a decrease in the loss factor (magnitude of the height of the tan delta peak). The different features are that the E-GQDs have a larger increase in TgC than the graphene nanoplatelets/epoxy composites; and the magnitude of the loss factor decreases significantly more than it does for the graphene nanoplatelets/epoxy composites. The glass transition temperature ob tained from the tan delta peak are 100 � C for neat epoxy, 110 � C for 1 wt % E-GQD, 2.5 wt% E-GQD, and 5 wt% E-GQD. The magnitude of the tan delta peak of the tan delta peak compared to that of neat epoxy decreased by 14% for 1 wt% E-GQD, 27% for 2.5 wt% E-GQD, and 45% for 5 wt% E-GQD. We did not have a higher cure temperature during the processing of the E-GQD composites, however it would not be unrea sonable to assume that the higher conductivity of the graphene quantum dots in the epoxy could have enabled the epoxy to cure at a lower temperature. Thus curing epoxy the same way as E-GQD composites are cured is equivalent to curing the epoxy at a lower temperature. If there is a small fraction of epoxy that is uncured, there would be an increased level of cure of the epoxy, in the presence of thermally conductive graphenic materials, under the same curing conditions. We do not attribute this shift to the right of the tan delta peak to stem from decreased chain mobility alone, even though decreased chain mobility may be present; We believe the thermal conductivity of graphene quantum dots contributes to the change in tan delta as this shifting effect has been seen with curing temperature changes alone in neat epoxy
3.4.2. Tan delta The ratio of the loss to the storage in a material is reported as tan delta and is called Damping.1 Damping is a measure of how well a material can get rid of energy and how good it is at absorbing energy. The peak position of the tan delta curve is considered the glass transition temperature, TgC. TgC generated from epoxy and its composites are
Table 3 Average mechanical and thermal properties of epoxy and graphene quantum dot/epoxy composites after triplicate runs from dynamic mechanical analysis.
1
Dynamic Mechanical Analysis (DMA), A beginners guide, Www.perkin elmer.com. (n.d.). https://www.perkinelmer.com/CMSResources/Images/ 44-74546GDE_IntroductionToDMA. 6
Mass% GQD Loading into Epoxy (%)
Average storage modulus at 30 C [GPa]
Average Glass Transition (Tg) [� C]
0.0 1.0 2.5 5
1.76 1.92 1.92 1.74
101 110 110 110
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Table 4 Percentage change of thermal and mechanical properties of graphene quantum dot/epoxy (E-GQD) composite and graphene nanoplatelets(GnP)/epoxy composites in comparison to neat epoxy.
This Investigation Chandrasekaran et al. [26] Wang et al. (GnP-5) [14] Wang et al. (GnP-C750) [14]
Storage Modulus
Glass Transition Temperature
Loss Factor
9.1% 14% 23% 2%
9.6%
40.4%
2.1% 1.4%
7.7% 6.4%
studies. The decreased chain mobility may be inferred from the lower tan delta value for E-GQD. Since there is an enhancement in the storage modulus in the rubbery region depicted in Fig. 6D, it is reasonable to see a decreasing ratio of loss to storage in the composite materials as the amount of GQDs increase in the composite increase. The full width half max (FWHM) of the tan delta peak is narrower than that of neat epoxy for 1 wt%E-GQD and 2 wt%E-GQD composites. However, the FWHM for 5 wt% EGQD is larger than that of neat epoxy. As noted from the Tan delta linear and logarithmic plots there is a shoulder gradually appear ing on the tan delta peak that makes the peak boarder at lower Tan delta values. This shoulder increases as the amount of GQDs increase in the composite. At 5 wt% GQD loading, the shoulder has increased to the point that the FWHM of the tan delta plot for the 5 wt%E-GQD is larger than the FWHM of neat epoxy tan delta plot. If broader peaks are associated with thermal degradation, there is a chance that the E-GQD composites have a more compact configuration that neat epoxy. During curing however, there can be phase separation that leads to a shoulder on the tan delta curve. The spectrally unmixed Tan delta plots are depicted in Fig. S9. When the shoulder on the plot is spectrally unmixed from the main peak for 5 wt% EGQD, it results in a tan delta peak around TgC that is narrower than the Tan delta peaks of epoxy, 1 wt%E-GQD and 2.5 wt% E-GQD at the same temperature. This narrow peak indicates the compact configuration of the E-GQD composite. The second peak from spectral unmixing, slightly left to the tan delta peak, responsible for the appearance of a shoulder is attributed to the interfacial constrains and free volume increase mechanisms falling out of equilibrium [55]. There is a higher contribution from the free volume mechanisms that assists with segmental motion in the 5 wt% E-GQD than are present in the 1 wt % and 2 wt% EGQDs samples that are dominated by the interfacial constrains. Basically, the Tan delta peak enables us to know the quality of the graphene quantum dot dispersion. 1 wt%EGQD and 2.5 wt% E-GQD are well dispersed and 5 wt% E-GQD is sufficiently dispersed. The plots for Figs. 6 and 7 are representative of multiple DMA runs and the average storage modulus from multiple runs at 30 and the average glass transition temperature from the tan delta are included in Table 3. There are a few studies that include dynamic mechanical properties with thermal conductivity. Table 4 compares the increase in storage modulus, glass transition temperature, decrease in loss factor and in crease in thermal conductivity. There are many studies about high thermal conductivity of graphenic materials that are not included here because they were not accompanied by the dynamic mechanical anal ysis. From the studies compared, Graphene quantum dots have a po tential for dramatic changes to the properties of epoxy. We noted the higher drop in loss factor, highest increase in thermal conductivity, highest increase in storage modulus after glass transition. The compar isons to these studies indicate that there is room for graphene quantum dots to make a significant impact on composite formation for different applications. In this study, EPON 828 is used as the investigative epoxy. An interphase also forms around the GQD due to inter and supramolecular interactions. Interphase has properties different than the matrix and can occupy a volume larger than the GQD. Molecular dy namics simulations showed that decreasing particle size increases the interphase stiffness in epoxy [56]. Generally, the interphase thickness is 20%–80% of the filler radius for particles smaller than 25 nm [56–58]. In addition, molecular dynamics simulations showed that an interphase
Thermal Conductivity
Notes
144.2% 14% 114% 14%
5 wt% E-GQD 2% wt loading GnP prepared by 3RM technique 5% wt loading GnP-5 using sonication dispersement 5% wt loading GnP-C750 using sonication dispersement
thickness of 1.14 nm exists around a carbon nanotube of 0.7 nm diam eter in an epoxy matrix [59]. If we assume a spherical GQD of 0.7 nm diameter, the volume of a 1.14 nm thick interphase would be ~76 times the GQD volume. Accordingly, we expect significant changes in the molecular arrangement of epoxy at GQD sizes <2.5 nm even for low GQD concentrations, which could result in an interphase-dominated thermo-mechanical behavior in GQD-epoxy composites. 4. Conclusion Infusing graphene quantum dots(GQDs) into polymers provides intriguing prospects for modulation of the polymeric properties. Gra phene quantum dots with terminal groups are small fragments of gra phene that possess a smaller amount of sp2/sp3 configuration ratios than larger graphenic materials because the functional groups on the GQDS contribute to sp3 bonds on the surface. The GQDs that have larger surface area to volume ratios compared to other forms of graphenic materials like graphene nanoplatelets (GnP). This means properties of GnPs that are largely due to the sp2 configuration could be attenuated in GQDs. In this study, we discover that other properties such as size and dispersibility, allow GQDs to impact mechanical and thermal conduc tivity properties more effectively than GnPs in similar studies. This implies that the size effects, surface chemistry, and dispersibility contribute significantly to imparting graphenic properties to polymers. Author contributions Joel R. Seibert designed experiments, executed experiments, and wrote the manuscript. Ozgur Keles contributed to discussion and revised the manuscript, Jun Wang designed experiment, supervised the research work, and reviewed the manuscript. Folarin Erogbogbo designed ex periments, wrote the manuscript, supervised the research work. All authors approved the final manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The work leading to this manuscript resulted from the efforts of Hildegarde Bell, Anesh Tilwani, Jose Alvarez, Belqais Nagshibandi, Navathej Gobi, Darshan Vijayakumar, Sowbaranigha Chinnusamy Jayanthi. The work leading to these results received funding from SJSU Central Research, Scholarship and Creative Activity (RSCA) Grant Pro gram, The second is the National Science Foundation(NSF) I-Corps program through the California Stae University(CSU) Program for Ed ucation and Research in Biotechnology. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymer.2019.121988. 7
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