Multi-responsive shape-stabilized hexadecyl acrylate-grafted

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Energy, Environmental, and Catalysis Applications

Multi-responsive shape-stabilized hexadecyl acrylate-grafted-graphene as a phase change material with enhanced thermal and electrical conductivity Ruirui Cao, Yuzhou Wang, Sai Chen, Na Han, Haihui Liu, and Xingxiang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18282 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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ACS Applied Materials & Interfaces

Multi-responsive shape-stabilized hexadecyl acrylate-graftedgraphene as a phase change material with enhanced thermal and electrical conductivity Ruirui Cao1,2,3, Yuzhou Wang1,2,3, Sai Chen1,2,3, Na Han1,2,3, Haihui Liu1,2,3, Xingxiang Zhang1,2,3* 1State

Key Laboratory of Separation Membranes and Membrane Processes, Tianjin

300387, China 2Tianjin

Municipal Key Lab of Advanced Fiber and Energy Storage Technology,

Tianjin 300387, China 3School

of Material Science and Engineering, Tianjin Polytechnic University, Tianjin

300387, China Corresponding Author *Tel/Fax: +86-022-83955238. E-mail: [email protected].

ACS Paragon Plus Environment

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Abstract: A phase change material (PCM) making essentially up of hexadecyl acrylategrafted-graphene (HDA-g-GN) was fabricated via a solvent-free Diels-Alder (DA) reaction. The novel material exhibits multi-responsive, enhanced thermal and electrical conductivity, and valid thermal enthalpy. In addition, the optimum DA reaction conditions were explored. A variety of characterization techniques were used to study the thermal, crystalline, and structural properties of HDA-g-GN. The melting and crystallizing enthalpies of HDA-g-GN were as high as 57 and 55 J/g, respectively. Furthermore, the melting and freezing points of HDA-g-GN were 29.5 and 32.7 °C, respectively. The thermal conductivity of HDA-g-GN reached 3.957 W/(m K), which is well above that of HDA itself and the previously reported PCMs. HDA-g-GN exhibited excellent electric conductivity of 219 S/m. Compared with HDA, the crystalline activation energy of HDA-g-GN decreased from 397 kJ/mol to 278 kJ/mol (Kissinger model), and 373 kJ/mol to 259 kJ/mol (Ozawa model), respectively. Moreover, HDA-g-GN exhibited excellent thermal stability, shape stability and thermal reliability. More importantly, HDA-g-GN can be employed to realize high-performance light-to-thermal, electron-to-thermal energy conversion and storage, which provides wide application prospects in energy-saving buildings, battery thermal management system, bio-imaging, bio-medical devices, as well as real-time and time-resolved applications. Keywords: Solid-solid PCM, Functionalized graphene, Diels-Alder reaction, Enhanced thermal conductivity, Excellent electrical conductivity, Multi-responsive


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Recently, phase change materials (PCMs), which are clean, reusable thermal energy storage materials, have attracted growing attention.1 According to their chemical composition, PCMs are categorized as inorganic and organic PCMs.1,2 In order to realize efficient energy storage with fast thermal storing/releasing rates, a PCM should have appropriate phase change temperature, large thermal enthalpy, and excellent thermal conductivity.3 Hexadecyl acrylate (HDA), as an organic solid-liquid PCM, has comparatively high latent heat, negligible supercooling, good thermal reliability, and no phase separation during phase change.4 While, because of low thermal conductivity, extremely low electrical conductivity, and easy leaking in the molten state, neither electrical nor optical methods can be utilized to drive HDA.5,6 It is well known that HDA is an electrical insulator. The thermal conductivities of organic PCMs are in the range of ~0.1–1 W/(m K),7 and that of stearic acid,8 palmitic acid,9,10 capric acidpalmitic acid eutectic,11 and polyethylene glycol (PEG),12 for example, are around 0.20, 0.17, 0.16 and 0.22 W/(m K), respectively. The low thermal conductivity impacts heat transfer in PCMs, and this greatly limits the practical applications of PCMs. Graphene (GN) is a mono-atom thickness, two-dimensional hexagonal aligned carbon atoms sheet, which is in the sp2 hybridization state, and it offers an outstanding combination of chemical, thermal, and electronic properties.13,14 Thus, the intrinsic thermal conductivity and electrical conductivity of GN are up to 5000 W/(m K) and ~108 S/m, respectively.15 In addition, GN has a large specific surface area, up to 2630 m2/g, which provides a convenient condition for functional modification of the GN surface. The excellent properties of GN provide the possibility of achieving thermal



energy storage, driven either optically or electrically for organic PCMs.5,6 Previous studies to enhance the thermal conductivity of PCMs mainly focused on the use of heat transfer fillers, such as GN sheets, aerogels, and foams.9,16-19 Albeit the thermal conductivity of PCMs has enhanced, most of them are still below 1 W/(m K), which is not enough to meet the practical application requirements. Moreover, other studies to enhance the electrical conductivity of PCMs mainly focused on the utilize of boron nitride (BN)/GO hierarchically interconnected porous scaffolds,20 anisotropic GN aerogels,21 and three-dimensional carbon aerogels.22 However, there are some unavoidable shortcomings in these studies. For example, most of the preparation processes are complex, and the electrical conductivity of these fabricated PCMs is still very low. Therefore, more efforts are still needed to further improve the thermal conductivity and electrical conductivity of PCMs. Recently, some researchers have fabricated all sorts of solid-solid PCMs (SSPCMs) by grafting/crosslinking PCMs onto high melting polymer substrates, such as polyvinyl alcohol,23 polystyrene,24 poly(styrene-co-maleic anhydride),25 and polyethylene terephthalate (PET),26 via in situ grafted/crosslinking method,23 graft polymerization technique,24 esterification,25 and copolymerization,26 respectively. These studies have important guiding significance for the subsequent studies on the fabrication of SSPCMs with carbon-based materials as supporting materials. Wang, et al. fabricated hexadecanol-grafted-graphene oxide (GO) shape-stabilized PCMs by an esterification reaction between carbonyl chloride functionalized GO and hexadecanol.27 In our previous study, HDA and diethylene glycol hexadecyl ether were successfully grafted


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onto GO sheets by free radical polymerization,4,28 atom transfer radical polymerization,29 and esterification,30 respectively. The above studies have solved the leakage problem of PCMs in practical applications; however, the fabrication process is complex and uses a lot of organic solvents, which is not in line with the concept of green chemistry. Furthermore, no previous studies have used GN as a raw material. A simple way to functionalize graphene-based structure is the Diels-Alder (DA) reaction, because it does not produce byproducts and can be carried out under gentle conditions.31 Since 2002, the possibility of the DA reaction has been studied theoretically,32 and many experiments have proved that the DA reaction can be carried out on carbon materials, such as GN, GO, and carbon nanotubes, with the assistance of microwaves,33 ultrasound,34 ball-milling,35 or heating-stirring.36 The DA cycloaddition reaction is highly important in synthetic organic chemistry, due to its high efficiency, versatility, and selectivity,37 and it is also an important functionalized method to incorporate organic moieties onto carbon materials by directly attacking the sp2 carbon of carbon materials. Moreover, [4+2] cyclic adducts are fabricated via the DA reaction between carbon materials and dienophiles.38 However, few studies have studied organic phase change micromolecule-functionalized GN through DA reaction. To overcome the shortcomings of HDA, such as leakage during working, low thermal conductivity, extremely low electrical conductivity, and lack of multiple driving strategies, we attempted to fabricate HDA functionalized GN through a solventfree process and explored optimum DA reaction conditions. GN and HDA are regarded



as diene and dienophile, respectively.39 HDA, with electron withdrawing substituents, is highly reactive in DA cycloaddition reactions. Moreover, the solvent-free DA reaction is simple, environmentally friendly and mild without any additional solvent, which conforms to the concept of green chemistry. The structural, thermal, crystalline, shape-stabilized, light-driven, and electro-driven performances of the fabricated material were characterized using spectroscopic and microscopic analyses, differential scanning calorimetry, infrared thermal imaging, and thermal gravimetric analysis. Results and Discussion The fabrication of HDA-g-GN is illustrated in Figure 1a. The conjugate double bonds of GN react with the unsaturated double bond of HDA, which converts some sp2 hybridized carbons of GN sheets to sp3 hybridized carbons.40 Meanwhile, the HDA molecular chain was grafted onto the surface of GN sheets. The DA reaction is an endothermic reaction,41 and the reaction activation energy is very low, which can be regarded as a low-energy-barrier reaction.42 In this study, we explored the optimum conditions for the DA reaction. The results are displayed in Figure S1. The FT-IR spectra of the obtained products at different reaction temperatures, are displayed in Figure S1a, and the spectra of the HDA and HDA/GN composites (at 180 °C) are shown in Figure S1b. In addition, the DSC curves of the obtained products are displayed in Figure S1e and Figure S1f. By comparison, we found that the DA reaction can happen smoothly when the reaction temperature is greater than or equal to 160 °C. Moreover, the higher the temperature of the reaction system, the more HDA is involved in the DA


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reaction. Figure S1d shows the DSC curves of HDA and HDA/GN composites (at 180 °C). Looking at Figure S1b and Figure S1d, we also found that the HDA is still in its original form in the reaction system, and there is no so-called heat-initiating free radical polymerization, even when the reaction temperature was up to 180 °C. This is attributed to the presence of hydroquinone (HQ), an inhibitor, in the reaction system. However, HDA begins to decompose, when the temperature is over 180 °C, as seen in Figure S1c. Therefore, the optimum temperature for the DA reaction between HDA and GN is in the range of 160–180 °C. In the remainder of this article, without special explanation, HDA-g-GN refers to the obtained product when the reaction temperature is 180 °C. Figure 1b shows the FT-IR spectra of HDA and HDA-g-GN. Characteristic HDA bands were identified at 1273 cm-1 and 1190 cm-1 (C-C stretching of the alkyl chain in HDA), and 1628 cm-1 (C=C stretching).4,28 For HDA-g-GN, the FT-IR spectrum shows the characteristic bands of the alkyl chain in HDA. In addition, these characteristic bands show a slight band shift from 1273 cm-1 to 1260 cm-1 and 1190 cm-1 to 1163 cm-1, compared with HDA. The evident band conversions to lower wavenumber indicate the success of the DA cycloaddition reaction. Further evidence of covalent bonds between GN and HDA was observed in the Raman and XPS spectra, which are displayed in Figure 1c and Figure 1d, respectively. Compared with GN, the ID/IG ratio of HDA-gGN changed from 0.092 to 0.300. The increasing ID/IG ratio shows an increase in the amount of the sp3 carbon, which corresponds to the mechanism of the DA cycloaddition reaction.40 As shown in Figure 1d, the O/C ratio of HDA-g-GN changed from 0.035 to



0.073, compared to GN. Moreover, a new peak in the C1s and O1s XPS spectra of HDA-g-GN was detected and attributed to an O-C=O group (Figure 1e, Figure S2c and Figure S2d). In addition, the intensity of sp3 carbon was enhanced in the HDA-g-GN, as presented in Figure S2a and Figure S2b. In addition, the TEM microscopic images of GN and HDA-g-GN are presented in Figure S4. It can be seen that the transparent and layered morphology of GN was remained after functionalization, while the transparency decreased due to the coating of the GN nanosheets by HDA. To summarize, the DA cycloaddition reaction between GN and HDA was successful.


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Figure 1. Illustration of (a) the fabrication of HDA-g-GN via the Diels-Alder (DA) reaction. FT-TR spectra of (b) HDA and HDA-g-GN, Raman spectra of (c) GN and HDA-g-GN, full-scale XPS spectra of (d) GN and HDA-g-GN and C 1s XPS spectra of (e) GN and HDA-g-GN.



Figure 2. XRD spectrum of GN and HDA-g-GN (a), water contact angles of GN and HDA-g-GN (b), the digital photographs of GN (top) and HDA-g-GN (bottom) (c) dispersed in different solvents. As displayed in Figure 2a, the XRD patterns of HDA-g-GN displays a new peak at 21.360° (2θ, D=11.369 nm), which corresponds to the (110) diffraction peak of HDA.43 As is known, the strength of the XRD diffraction peak is proportional to the crystallinity.44 From Figure 2a, the intensity of the (002) diffraction peak of HDA-gGN, which corresponds to the characteristic diffraction peak of GN, is clearly lower than that of GN. This means that the crystalline structure of GN was damaged to a certain extent during the DA cycloaddition reaction, and the lattice defect of GN is increased. Figure 2b gives the water contact angles (WCA) of GN and HDA-g-GN. The WCA of HDA-g-GN increased from 93.3° (GN) to 112.9°. This is mainly because the HDA, which is grafted onto GN sheets, is hydrophobic. Seven solvents were used to measure the solubility of the fabricated HDA-g-GN, and the results are shown in Figure


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2c. After DA reaction, the solubility of the obtained functionalized GN (HDA-g-GN) sheets in solvent changed significantly, compared with GN. Furthermore, the Hildebrand parameter (δ) was introduced to study the solubilities of HDA, GN and the fabricated HDA-g-GN. On the basis of Hansen’s theory, the cohesive energy density is determined by the dispersion (d), polarity (p) and hydrogen-bonding portion (H).45 δ is the total cohesion energy, and it can be regarded as the sum of these three components. Detailed calculation process of δ is referred to the previous study of our research group.45 As seen in Figure 2c, seven solvents were selected as the dispersants for HDA, GN and HDA-g-GN. The Hansen solubility parameters of the chosen solvents are listed in Table S1. The calculated Hansen solubility parameters of HDA, GN and HDA-g-GN are shown in Table 1. The δ values of HDA, GN and HDA-g-GN are 19.18 MPa1/2, 22.79 MPa1/2, and 19.87 MPa1/2, respectively. Compared with GN, the δ value of HDA-g-GN were smaller and closer to that of HDA. Table 1. Hansen solubility parameters of HDA, GN, and HDA-g-GN. Specimen HDA GN

δd (MPa1/2) 16.68 17.80

δp (MPa1/2) 5.75 11.69

δH (MPa1/2) 7.53 8.13

δ (MPa1/2) 19.18 22.79

HDA-g-GN

17.34

7.24

6.45

19.87



Figure 3. DSC curves of (a) HDA-g-GN (at a rate of ± 10 °C /min), TGA curves of (b) GN, HDA and HDA-g-GN, Kissinger plots (c) and Ozawa plots (d) for evaluating nonisothermal crystallization activation energy, FT-IR spectra (e) and DSC curves (f) of HDA-g-GN after 1 and 100 thermal cycling treatments, photographs of (g, h) HDA-g-


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GN before and after thermal treatment in oven (80 °C, 1 h). The DSC curves of HDA-g-GN are presented in Figure 3a. The corresponding data are listed in Table S2. As displayed in Figure 3a, HDA-g-GN has strong endothermic/exothermic peaks, and the melting and freezing points, melting enthalpy and crystallization enthalpy (∆Hm and ∆Hc) of HDA-g-GN are 29.5 and 32.7 °C, 57 and 55 J/g, respectively. The DSC thermal properties of HDA are also shown in Table S2. Compared with HDA, it is exciting that the phase change temperatures of HDA-g-GN are within the range of the human comfortable temperature, which greatly expands its potential applications, such as smart textile, energy-saving buildings, and battery thermal management system. Figure 3b presents TGA curves of HDA, GN and HDAg-GN. The mass losses of HDA, GN, and HDA-g-GN were 100.00 %, 0.80 %, and 57.74 %, respectively. The difference in the mass loss between GN and HDA-g-GN was 56.94 %, which was attributed mostly to the grafted HDA on GN sheets.4,29,46 Based on the previous studies,4,29,46 the presence of HDA molecules on GN sheets was further analyzed using TGA, as shown in Figure 3b. The grafting ratio of HDA molecules on the unit mass of GN was 134.7 %. That is to say, the grafting densities of HDA on GN sheets were estimated to be 1.347 g (4.544 mmol) per gram of GN, or approximately 5.45 HDA molecules per 100 carbon atoms. In addition, the effect of GN on crystalline activation energy of HDA-g-GN has been studied. Based on previous studies,47,48 the Kissinger and Ozawa models have been utilized to estimate the crystalline activation energy of HDA and HDA-g-GN, and



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these two models were applied. We hypothesized the maximum conversion rate occurs at the peak temperature (Tp) in the DSC curves: d[df(α)/dt]/dt = 0. According to the Kissinger theory and Ozawa theory, a phase change can be portrayed by:

ln

β

2=-

Tp

Ec AR + ln RTp Ec

(1)

and ln β = -

1.0516Ec AEc + ln -5.330 RTp R

(2),

respectively. where, β, Tp, Ec, R and A represent the cooling rate, peak temperatures in the DSC crystallization curves, crystalline activation energy, gas constant (8.314 J/(mol K)), and pre-exponential factor, respectively. The DSC curves of HDA and HDA-g-GN during non-isothermal crystallization at different cooling rates are displayed in Figure S5a and Figure S5b, respectively. The relevant data are shown in Table S3. As presented in Figure S5, the crystallization peaks of HDA and HDA-g-GN are both shifted to lower temperatures and become wider with increasing cooling rate. As plotted in Figure 3c and Figure 3d, both ln(β/Tp2) ~ 1/Tp (Kissinger model) and lnβ ~ 1/Tp (Ozawa model) exhibit linear relationships. Through a series of linear-fitting and calculation, we obtained the Ec of HDA and HDA-g-GN. Compared with pure HDA, the Ec value of HDA-g-GN decreased from 397 kJ/mol to 278 kJ/mol (Kissinger model) and 373 kJ/mol to 259 kJ/mol (Ozawa model), respectively, which is mainly because GN, as a nucleating agent, can induce the crystallization of HDA chains in HDA-g-GN.28 In short, crystallization occurs easily in HDA-g-GN, and less energy is needed to achieve


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crystallization. The thermal stability, thermal reliability, and shape-stabilized properties are considered important parameters of thermal energy storage. As displayed in Figure 3b, the thermal stability of HDA-g-GN is much higher than that of HDA under the same experimental conditions, and HDA-g-GN is thermally stable as high as nearly 340 °C (T5%). In this study, the thermal reliability of HDA-g-GN after 1 and 100 thermal cycles was compared. The FT-IR spectra and DSC curves of HDA-g-GN are presented in Figure 3e and Figure 3f, respectively. Furthermore, the corresponding thermal data from DSC analysis are shown in Table S3. As seen in Figure 3e, Figure 3f and Table S2, the FT-IR spectrum obtained after 1 and 100 thermal cycles show the same characteristic band positions. The DSC curve exhibits a slight change, and the phase change properties of HDA-g-GN remain nearly constant before and after thermal cycles. Figure 3g and Figure 3h display the photographs of HDA-g-GN before and after thermal treatment (80 °C, 1 h), respectively. HDA-g-GN shows excellent shapestabilized behavior even when the test temperature is much higher than its phase change temperature (about 30 °C). No leakage and no size change of HDA-g-GN were observed. In summary, the fabricated HDA-g-GN has excellent thermal stability, thermal reliability, and shape-stabilized property, and it shows broad prospects in the thermal energy storage system, for example, energy-saving buildings, solar energy storage, waste heat recycling and smart fibers.



Figure 4. Schematic of (a) the light-to-thermal conversion measurement and (b) the light-to-thermal imaging process. Schematic of (c) the electro-to-thermal imaging process, Heat storage process (d) and heat release process (e) of GN and HDA-g-GN. Comparison of thermal conductivity of (f) HDA-g-GN in this study and previously reported PCMs. Light-to-thermal conversion curves of (g) HDA-g-GN (100 mW/cm2). Figure 4d and Figure 4e give the heat storage and heat release curves of HDA-g-


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GN with increasing temperature from 0 to 75 °C. In contrast, we also gave the curves of GN under the same test conditions. As seen in Figure 4d and Figure 4e, the required time of HDA-g-GN is significantly longer than GN to reach the same temperature during heating or cooling processes. The longer the time required, the better the ability to store and release heat energy. Moreover, during the cooling process, a thermal buffer platform appeared in the exothermic curve of HDA-g-GN. In other words, the fabricated HDA-g-GN exhibited a heat preservation effect. Therefore, HDA-g-GN can be applied to a series of thermal management systems, such as battery thermal management systems. As is known, the thermal conductivity is positively correlated with the rate at which PCMs store and release energy, reflecting the sensitivity of the thermal response of PCMs.49 In this study, the thermal conductivity of HDA-g-GN was as high as 3.957 W/(m K), which is much higher than that of HDA (about 0.20 W/(m K)),50 and that of most reported results,17,18,51-64 as seen in Figure 4f. The relevant data on the heat conduction of HDA-g-GN are listed in Table S4. The above results strongly evidence that the GN, with a lamellar structure, has enhanced the thermal response properties of the organic PCMs. In addition, the results highlight the potential to improve the working efficiency of PCMs. Interestingly, HDA-g-GN demonstrated fascinating light-to-thermal conversion properties under irradiation with an infrared lamp. Upon infrared irradiation, the temperature of HDA-g-GN increased rapidly, as recorded by an infrared thermal imager



(Figure 4a). As displayed in Figure 4g, under 100 mW/cm2 light, the temperature of HDA-g-GN sharply increased and was higher than that of HDA by nearly 6 °C under the same irradiation time. The high-performance light-to-thermal conversion performance of PCMs displays potential applications in light-to-thermal imaging, and it is a non-destructive method compared to optical imaging.65 An illustration of the light-to-thermal imaging is displayed in Figure 4b. Under infrared irradiation, the temperature of light-to-thermal materials significantly increases, and this temperature change can be captured with an infrared thermal camera, as seen in Figure 4b. As shown in Figure 5a, we prepared a patterned letter “Ƶ” with HDA-g-GN, and with increasing irradiation time, the “Ƶ” pattern became increasingly brighter. The temperature changes with increasing irradiation time are displayed in Figure S6, and a temperature increase of over 26 °C was measured in 120 s, which is sufficient for bio-imaging and biomedical applications.66 Moreover, the contrast of temperature map increases with the increase of irradiation time, which indicates that it has potential application in real-time response and time-resolved response.65,66


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Figure 5. FLIR camera images of (a) HDA-g-GN (patterned letter “Ƶ”) under infrared lamp (60 mW/cm2) and (b) the test specimen (HDA-g-GN) under 30 V electric field. Optical photographs of (c) LED bulb luminescence in a test circuit. In order to further study electrical properties, the electric conductivity of HDA-gGN was characterized via a current-voltage (I-V) test, and the fabricated HDA-g-GN exhibited excellent electric conductivity of 219 S/m. Furthermore, the LED bulb gradually brightened with increasing voltage in the test circuit, as seen in Figure 5c and Video 1 in the Supporting Information. These results indicate that HDA-g-GN has excellent conductivity. Figure 4c displays a schematic of the electro-to-thermal imaging process. The FLIR camera images of HDA-g-GN under a 30 V electric field are shown



in Figure 5b, and more details are shown in Figure S7 and Video 2 in the Supporting Information. As seen in Figure 5b and Figure S7, the temperature of HDA-g-GN increased rapidly to 71.1 °C under the 30 V electric field for 1 min, i.e., and the temperature of HDA-g-GN increases about 41 °C, compared with the surrounding environment. Furthermore, from Video 2, we observed that the temperature of HDA-gGN increased rapidly to more than 70 °C under the 30 V electric field, and when the electric field was removed, the temperature of HDA-g-GN decreased rapidly to room temperature. This indicated that the fabricated HDA-g-GN is an excellent electrodriven phase change material and further verified that HDA-g-GN has enhanced thermal conductivity. Conclusions A multi-responsive HDA-g-GN phase change material with enhanced thermal conductivity was fabricated by a solvent-free DA cycloaddition reaction. This method has the advantages of no external solvent, simple procedures, as well as high efficiency, versatility, and selectivity. The melting and crystallization enthalpies of HDA-g-GN are as high as 57 and 55 J/g, respectively. Furthermore, the melting and freezing points of HDA-g-GN are 29.5 and 32.7 °C, respectively. It is exciting that the phase change temperatures are within the range of human comfortable temperature. The thermal conductivity of HDA-g-GN is up to 3.957 W/(m K), which is much higher than that of most reported results. The fabricated HDA-g-GN exhibited excellent electric conductivity of ≈ 219 S/m. Meanwhile, the fabricated HDA-g-GN has excellent thermal


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stability, thermal reliability, shape-stabilized property, high-performance light-tothermal conversion property, and excellent electro-to-thermal conversion performance, which greatly expand its application to fields, such as smart textile, energy-saving buildings, battery thermal management system, bio-imaging, bio-medical devices, as well as real-time and time-resolved applications. Experimental Section Materials: Graphene powders (GN, KNG-G2, Xiamen KNANO Graphene Technology Co., Ltd) were further purified by hydrochloric acid (HCl, 37 %) and deionized water to remove the impurities on the surface of graphene sheets. Hexadecyl acrylate (HDA, Zhejiang Kant Chemical Co., Ltd) , hydroquinone (HQ, AR) and N, Ndimethylformamide (DMF, AR, Guangfu Fine Chemical Research Institute) were used as received. Solvent-free Diels-Alder reaction of HDA-g-GN HDA-g-GN was fabricated by a solvent-free Diels-Alder (DA) reaction. A mixture of GN (1 g) and HDA (100 g, 337 mmol) were placed in a glass beaker and treated by ultrasound for 2 h to form a uniform suspension. After that, the suspension was poured into a three-necked flask. Under N2 flow (0.15 MPa) protection and mechanical stirring (250 rpm), 0.5 g HQ was added to the reaction flask and this suspension was reacted in an oil bath under different temperatures (120, 140, 160, 170, 175 and 180 °C). After reacting for 12 h, the resultant suspension was rinsed extensively with DMF and filtered



by 0.22-μm membrane. Finally, the obtained products were dried in a vacuum oven at 75 °C for 8 h, and black powder materials were obtained. Characterization The structural, thermal, crystalline, shape-stabilized, light-driven, and electrodriven properties of the fabricated HDA-g-GN were characterized using spectroscopic and microscopic analyses, differential scanning calorimetry, infrared thermal imaging, and thermal gravimetric analysis and so on. For details please refer to the Supporting Information. Acknowledgments This work was supported by the New Materials Research Key Program of Tianjin (No. 16ZXCLGX00090) and the National Key Research and Development Program of China (No. 2016YFB0303000). Supporting Information Available: FT-IR spectra of HDA-g-GNs, HDA and HDA/GN composites, TGA curve of HDA, DSC curves of HDA, HDA/GN composites and HDA-g-GNs; C 1s and O 1s of GN and HDA-g-GN; TEM micro-graphs of GN and HDA-g-GN; SEM micro-graph and EDS spectra of GN and HDA-g-GN; solubility parameters of the chosen solvents; DSC data of HDA-g-GN before and after thermal cycles, and HDA; DSC curves and crystallization temperature of HDA and HDA-g-GN during non-isothermal crystallization at the different crystallization rates; thermal conductivity properties; FLIR camera images at different infrared irradiation times;


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Multi-responsive hexadecyl acrylate-grafted-graphene as a phase change material with enhanced thermal and electrical conductivity 89x33mm (300 x 300 DPI)


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Multi-responsive shape-stabilized hexadecyl acrylate-grafted

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