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Highly flexible and stretchable thermally conductive composite film by polyurethane supported 3D networks of boron nitride
Composites Science and Technology 152 (2017) 94e100
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Highly flexible and stretchable thermally conductive composite film by polyurethane supported 3D networks of boron nitride Hye-Jin Hong a, 1, So Mang Kwan b, 1, Dong Su Lee b, Seung Min Kim b, Yun Ho Kim c, Jin Seong Lim b, Jun Yeon Hwang b, *, Hyeon Su Jeong b, ** a
Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 34132, Republic of Korea Institute of Advanced Composite Materials, Korea Institute of Science and Technology, 92 Chudong ro, Bondong-eup, Wanju-gun, Jeonbuk 565-905, Republic of Korea c Division of Advanced Materials, Korea Research Institute of Chemical Technology, Daejeon, 34114, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2017 Received in revised form 11 August 2017 Accepted 17 September 2017 Available online 21 September 2017
Current interest in flexible and stretchable electronics has been amplified because of their remarkable features for numerous applications. Accordingly, compliant efficient thermal management for such electronics is in great demand. However, new materials simultaneously coping with high thermal conductivity and mechanical flexibility have not been sufficiently studied. Here, we report a simple yet highly efficient method to construct three-dimensional (3D) hexagonal boron nitride (h-BN) network in a polymer composite, which is both thermally conductive and mechanically stretchable. 3D h-BN network is easily fabricated by in-situ incorporation of h-BN onto the surface of a water-borne polyurethane (PU) scaffold during polymerization. The 3D h-BN network in the composite film offers high thermal conductivity up to 10 W/m$K while the PU contributes excellent mechanical flexibility such as folding, twisting and stretching. Moreover, the process developed in this study is highly economical and processable to be scaled up without the need of complex equipment or procedure, which will be of great interest to current manufacturing as well as soft and stretchable electronic devices. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Polymer-matrix composites (PMCs) Flexible composites Thermal properties Interface
1. Introduction Recent advances in flexible and stretchable electronics have gained great attention due to their numerous possible applications in wearable devices [1], power sources [2], sensors [3], robots [4] and healthcare equipment [5]. As one of the soft materials to support next generation devices, new compliant thermal management is necessary because it is well known that the elevated temperature of the device negatively affects performance and safety [6,7]. Thus, a new material that is both flexible and stretchable together with high thermal conductivity is in great demand [8,9]. However, thermally flexible and stretchable materials have not been widely studied yet. The most commonly used heat dissipation method has been relied on polymer-based composite materials, which blends * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.Y. Hwang), [email protected] (H.S. Jeong). 1 Co-first authors. https://doi.org/10.1016/j.compscitech.2017.09.020 0266-3538/© 2017 Elsevier Ltd. All rights reserved.
polymer matrix with high thermal conductive fillers [10]. Among various fillers, ceramic fillers such as hexagonal boron nitride (hBN) are preferred due to their electrical insulation as well as high thermal conductivity [11e14]. In case of carbon based fillers such as graphene, the corresponding composite suffer from current leakage and electrical shorts due to their electrical conductivity, thus limiting their applications where electrical insulation is required. To increase the thermal conductivity of composites, many efforts have been devoted to increase the degree of dispersion of fillers in polymer matrix mainly by surface functionalization of fillers, which expects minimization of interfacial thermal resistance [15e17]. Unfortunately, the thermal conductivity of such composites is still insufficient, and there are difficulties in complex functionalization process [18,19]. The most efficient way to realize high thermal conductivity is constructing a three-dimensional (3D) network of fillers in polymer composite. In recent years, fabricating h-BN 3D scaffolds in polymer composite prepared by the ice-templated method has shown to be an effective way to achieve high thermal conductivity [20e22]. However, the sophisticated fabrication process involved the use of a binder, low processing temperature,
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high calcination temperature and inevitable polymer infiltration process is time consuming, limiting its use in real applications [20,22]. Apart from the thermal conductivity, the flexibility and stretchability of the thermal polymer composite has been rarely reported. Hence, it still remains a technical challenge to fabricate thermally a conductive polymer composite having 3D network of filler with flexibility and stretchability via facile process. Here, we report a simple yet efficient method to construct 3D hBN network in polymer composite, which is highly thermally conductive, electrically insulating and mechanically stretchable. 3D h-BN network was easily constructed by in-situ attaching of h-BN to a surface of water-borne polyurethane (PU) scaffolds during polymerization of PU. The 3D h-BN network in the composite film offered high thermal conductivity of 10 W/m$K while the PU contributed excellent mechanical flexibility (bending and twisting) and stretchability. The composite film developed in this study has significant advantages over other methods for flexible electronics, including from not only high thermal conductivity with elastic properties but also easy and rapid fabrication process. 2. Material and methods 2.1. Materials h-BN was purchased from Denka (particle size: 8 mm) and used as received. The water-borne PU prepolymer (Hypol JT 6000) was provided by Dow Chemical (USA). 2.2. Preparation of h-BN/PU composite foam An amount of 10 g of prepolymer was mixed with 30 g of deionized (DI) water to produce neat PU foam. For the h-BN/PU composite foam, a certain amount of h-BN powder (for example, 1 g of h-BN for a 10 wt% h-BN/PU composite) was initially dispersed in aqueous media by bath-sonication for 30 min. Then, the h-BN solution was vigorously mixed with 10 g of prepolymer for 30 s using a mechanical stirrer. The mixed solution was poured into a rectangular tray. As the polymerization of PU progressed, the composite foam expanded to fill the tray and the tray was removed after 10 min drying at room temperature. The composite foam was washed by immersing it in DI water, squeezing and releasing several times to remove any unattached h-BN particles. The h-BN/ PU composite foam was dried in an oven at 60 C for 24 h. 2.3. Preparation of h-BN/PU composite film In order to make the composite film, subsequently, the composite foam was hot-pressed at 150 C under 100 kgf/cm2 for 2 min. 2.4. Characterization of h-BN/PU composite foam and film Fourier-transform infrared (FT-IR, cary 630, Agilent, USA) was used to investigate the chemical structures of the neat PU and hBN/PU composite. Loading amount of h-BN in the composite was investigated by thermogravimetric analysis (TGA, TA Instrument Q50, USA). Scanning electron microscopy (SEM, Verios 460 L, FEI) equipped with energy dispersive X-ray spectroscopy (EDS) was used to characterize the morphology of interface of the composite. A non-destructive high resolution x-ray microscopy (XRM, Xradia 520 versa, Zeiss, Germany) was also used to determine the internal structure with 3D configuration, and the obtained XRM image was further analyzed by 3D software (Avizo Fire 8.1, FEI). Tensile test was carried out by universal testing machin (5567A, Instron, USA). The in-plane thermal conductivity of the composite film was calculated by multiplying density, specific heat capacity and
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thermal diffusivity of the composite film. The density of composite film was calculated from the weight and dimensions of the composite film (Table S1). The specific heat capacity at room temperature was measured by differential scanning calorimeter (DSC, Q20, TA instrument). The thermal diffusivity was measured based on laser flash method (Laser PIT-M2, ULVAC, Japan). Through-plane thermal conductivity of composite film was measured based on the method of ASTM-D5470 (MVH100P, Hantech, Korea). Thermal IR images of composite film were taken by thermal imaging camera (Testo, 875-1i). 3. Results and discussion 3.1. In-situ incorporation of h-BN onto PU The overall procedure for fabricating 3D networks of h-BN/PU composite film is illustrated in Fig. 1a. The procedure consists of four parts as follows. (i) A certain amount of h-BN dispersed in deionized water with the aid of sonication for 30 min is mixed with a water-borne PU prepolymer; (ii) a foam forming reaction occurs as polymerization proceeds; (iii) the h-BN/PU composite foam is dried; and (iv) the composite film is fabricated by a typical hotpressing method. Our strategy mainly relies on the second step. Polymerization immediately occurs by the reaction between isocyanate groups in prepolymer and water (H2O), resulting in amine group and release of CO2 gas as follows: R-NCO (isocyanate) þ H2O / [R-NHCOOH] / R-NH2 (amine) þ CO2
(1)
This CO2 bubble leads to macro-porous foam structure. The hBN initially dispersed in the water also participated PU polymerization and then immobilized onto the surface of PU foam. In other words, the h-BN naturally attaches to the surrounding of PU surface as the water is consumed in the formation of CO2 as shown in Fig. 1a (an enlarged image of the second step). Thus, 3D h-BN network via PU scaffold can be successfully assembled by an in-situ attachment process. This incorporation process takes less than one minute. The other strategy is to transform foam structure of the composite into a film structure by employing a typical hot-pressing method because as-fabricated macro pores act as thermal insulation spaces. Thus, the pore structure that initially serves as a template for constructing 3D h-BN network in the first place should be removed. After hot pressing, we could effectively remove the pores, resulting in highly dense h-BN/PU composite films shown in Fig. 1a. Fig. 1b shows photographs of the neat PU and h-BN(50)/PU composite (50 wt% of h-BN loading) foam. The overall macroscopic features (porous structure and color) were not significantly changed when h-BN was attached. FT-IR was used to investigate the chemical structures of the neat PU and h-BN/PU composite (Fig. 1c). The pristine h-BN exhibited characteristic bands of B-N in-plane stretching at 1370 cm1 and B-N-B out of plane bending observed at 810 cm1. Also, a broad band at 3200 cm1 indicated the existence of hydroxyl group in BN (-OH) band [23]. The neat PU foam also showed various bands. A broad band at 3295 cm1 corresponded to amine (-NH) groups and that at 1724 cm1 corresponded to free eC¼O. Bands at 1530 and 1221 cm1 corresponded to NH and CN, respectively. These bands indicate the formation of a urethaneeurea bond [24]. The FT-IR spectrum of h-BN(50)/PU showed both PU and h-BN characteristic bands. Characteristic bands of h-BN at 1370 and 810 cm1 were observed in the h-BN/PU spectrum. PU characteristic bands were also observed although PU bands located in 1000e1500 cm1 showed a slight decrease in intensity due to overlapping of the high intensity h-BN band at 1370 cm1. It is noted that the hydroxyl band of pristine h-BN at
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Fig. 1. a) Schematic diagram of the process of fabricating 3D h-BN/PU composite. b) Photographs of the neat PU foam and h-BN/PU composite foam. c) FT-IR data of h-BN, the neat PU foam and h-BN/PU composite foam. SEM images of d) neat PU foam and e) h-BN(50)/PU composite foam. f) A magnified SEM image of (e). g) EDS elemental mapping of hBN(50)/PU composite foam.
3200 cm1 disappeared from the h-BN/PU composite, implying a possible chemical interaction between the hydroxyl groups of h-BN and the isocyanate groups of the prepolymer as follows [25]. R-NCO (isocyanate) þ HO-BN / R-NHCOOBN
(2)
R0-NCO(isocyanate) þ R-NHCOOBN / R0 -N(CONHR)COO-BN
(3)
Thus, the h-BN is not only physically attached to PU polymer but also chemically linked to PU foam by participating in polymerization process. To make a conductive path of fillers, it is important to load enough h-BN content in the composite beyond a critical value, which is the percolation threshold. TGA is a good indicator to show the actual loading content because h-BN is known to have remarkable thermal stability up to 1000 C in air. Fig. S1 (Supporting Information) shows the thermal degradation behaviors of neat PU and the h-BN/PU composites with various loading contents of h-BN. While a weight loss of 100% was observed for neat PU above 500 C, a weight of 12, 21, 33, 40 and 53% weight was remained for 10, 20, 30, 40 and 50 wt% h-BN/PU composite, respectively. This remained weight for each composite is consistent with initial loading content of h-BN in each composite. This consistency is attributed to strong physical and chemical interaction between h-BN and PU, which enables to load enough amount to make 3D connected h-BN networks. Direct observation of h-BN grafted onto PU was investigated by SEM equipped with EDS. As shown in Fig. 1d, the neat PU exhibits a smooth surface and pores. In contrast, the h-BN(50)/PU composite shows a much rough surface, indicating the h-BN wrapped all PU
surface while preserving its original porous structure (Fig. 1e). The trend of surface morphology is obviously a function of loading contents of h-BN in the composite (Fig. S2). A magnified image of the composite clearly shows the presence of two-dimensional h-BN fillers covering the PU surface. A lot of h-BN fillers are compactly connected along all directions, resulting in 3D connected networks of h-BN (Fig. 1f). In Fig. 1g, the EDS mapping is shown for each element (B, C, N) with color coded. EDS analysis further confirms homogenous distribution of h-BN in the PU matrix.
3.2. 3D network of h-BN 3D XRM was used to non-destructively resolve the 3D structure of the h-BN/PU composite, and the results are shown in Fig. 2aec. Fig. 2a shows XRM image of 3D porous structure of neat PU. It is confirmed that a 3D connected porous structure with large pore size about several hundred of micrometers was developed. When h-BN was involved in polymerization process, the features of 3D porous structure were notably changed. The size of pore became smaller and denser than that of neat PU as the loading amount of hBN increased from 0 to 10 wt% (Fig. 2b) and finally to 50 wt% (Fig. 2c). The increase of small pore density is directly related to increase of specific surface area. We analyzed the XRM images with 3D software (Avizo Fire 8.1) to calculate the specific area per volume. As shown in Fig. 2def, the specific area increased from 0.06 (mm2/mm3) to 0.15 for h-BN(10)/PU composite and finally to 0.33 for the h-BN(50)/PU composite. Compared with neat PU, 50 wt% h-BN loading in PU composite has 5.5 times of specific area, indicating 5.5 times of more conductive path for thermal transport is generated.
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Fig. 2. Reconstructed 3D XRM images of a) neat PU foam and h-BN/PU composite foams of b) 10 wt% and c) 50 wt% h-BN. d-f) Processed images showing specific area per unit volume of foam structures corresponding to a-c), respectively.
Another notable change from the XRM investigation is the color change from blue to green as the loading amount of h-BN is increased (Fig. 2aec). This color change indicates a change of interfacial material contacting with air. In other words, the interfacial material contacting with air changes from PU to h-BN, which is consistent with the SEM observations of gradual surface coverage by h-BN on PU (Fig. S2). This change of interfacial environment causes the image contrast visualized by XRM. The XRM results further prove the presence of h-BN anchored onto the PU surface in all direction, which constructs 3D network of h-BN. 3.3. 3D h-BN/PU composite film with high thermal conductivity The h-BN/PU composite foam was transformed into an h-BN/PU composite film with a thickness of 500 mm (Fig. 3a) by using a typical hot-pressing method. A cross-sectional SEM image of the neat PU film (Fig. 3b) reveals that the pores were effectively removed by simple hot pressing. The SEM images in Fig. 3b and c shows cross-sectional morphologies of the prepared neat and composite film, respectively. Compared with the morphology of neat PU, the surface of h-BN(50)/PU is very rough. Like a foam structure, the morphology trend of film is also a function of loading contents of h-BN in the composite (Fig. S3). The magnified SEM image (Fig. 3d) clearly shows the presence of h-BN particles that are compactly packed and connected together. It was further confirmed by EDS mapping that the h-BN fillers were homogenously distributed and well-connected in the PU matrix (Fig. S4). The composite film is expected to have high thermal conductivity arising from highly packed and connected h-BN, simultaneously high mechanical performances arising from the PU matrix. The inand through-plane thermal conductivity of the composite film with various h-BN loadings was measured at room temperature. As shown in Fig. 3e, as the weight fraction of h-BN increased, the inplane thermal conductivity of all composites dramatically increased. Compared to that of neat PU film (0.2 W/m$K), the thermal conductivity of composite at h-BN loadings of 5, 10, 20, 30, 40 and 50 wt% increased by factors of 1.8, 9.5, 29.1, 40.0, 49.2 and 50.4, respectively. In case of h-BN(50)/PU composite film, the inplane thermal conductivity reached up to 10.28 W/m$K. To the
best of our knowledge, this is the highest thermal conductivity reported to date based on filler/PU composites. The threshold for percolation seems to place between h-BN loadings of 5 wt% and 10 wt%. After a dramatic jump in thermal conductivity at 10 wt% hBN, the thermal conductivity linearly increased up to h-BN loading of 40 wt% and became almost constant for further loading. This indicates, at 40 wt% loading, the packing of h-BN is almost saturated by direct contact with each other. Compared to in-plane thermal conductivity, meanwhile, the through-plane thermal conductivity of composite films was not dramatically increased as the weight fraction of h-BN increased (Fig. S5). This indicates the film has anisotropic thermal conductivity. This is probably attributed to partial alignment of h-BN during hot-pressing. An evidence can be also found in EDS mapping of h-BN fillers (B, N) that were partially oriented in-plane direction (Fig. S4). While the h-BN in the composite plays a crucial role in thermal transport, the PU in the composite provides high mechanical properties. The elasticity of the neat PU and composites with various loading amounts was evaluated by tensile stress-strain measurement (Fig. S6). The tensile strength of all composite films continuously increased as the hBN content increased in the PU matrix. In comparison with the neat PU, the tensile strength of the h-BN(50)/PU composite was 3.5 times larger. The continuous increase of tensile strength indicates good interfacial adhesion between PU and h-BN as proved in FT-IR and XPS data. The h-BN well anchored on PU can dissipate of fracture energy, preventing fracture propagation [15,26]. The strain also increased up to h-BN loading of 30 wt% but decreased with further loading of h-BN. Due to good adhesion of h-BN on PU, the fillers follow PU deformation, which helps stretchability. Above 40 wt% loading of h-BN, the excessive fillers start to hinder segmental mobility of polymer matrix, decreasing flexibility. The retained thermal conductivity by saturation of fillers from same loading amount of h-BN supports this speculation (Fig. 3e). Nevertheless, the h-BN(50)/PU composite could be stretched over 100% of the initial length which was still higher than the stretchability of neat PU. It should be noted that the usual trade-off between mechanical and thermal properties as well as processability was not found in our composite [27]. To directly observe the heat transfer ability of the composites,
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Fig. 3. a) a photograph of composite film by hot pressing the composite foam. Cross-sectional SEM images of b) neat PU film and c) h-BN(50)/PU composite film. d) A cross-sectional magnified SEM image of h-BN/(50)/PU composite film. e) In-plane thermal conductivity of the composites as a function of h-BN loading. f) Thermal IR images of the composites with various h-BN contents on hot and cool stages at 30s each. To clarify the image, the h-BN(50)/PU composite film on cool stage is marked by dotted lines in white.
the temperature change of each composite with various loading contents of h-BN was monitored using an infrared (IR) thermal imaging camera. Fig. 3f shows IR images of each composite mounted on hot and cool stages for 30 s each. The trend in heat conduction of the composite is obvious. The more amounts of h-BN presences in the composite, the faster heat transfer was observed for both heating and cooling experiments. In addition to heat transfer, the distribution of temperature was more uniform across the composite film when more h-BN was loaded in the composite. Together with high thermal conductivity, the direct observation on heat transfer further confirms that a 3D network of conductive heat path is well developed by facile in-situ incorporation process. 3.4. Flexibility and stretchability To demonstrate high potential for use in wearable devices as thermal management, the in-plane thermal conductivity of the composite films was measured after various mechanical deformations such as bending, twisting and stretching. Fig. 4a shows the variation in the thermal conductivity with respect to the number of bending tests. In the bending tests, the film was completely folded. It is confirmed that the thermal conductivity curves show plateaus for all composites, indicating high stability of heat conduction under mechanical bending. The thermal conductivity after twisting deformation also retained its initial value for all composites, implying that the network of h-BN was not collapsed by mechanical twisting (Fig. 4b). For stretching test, the composite film was stretched to a certain strain and released to its original state. The stretching cycle for each strain was carried out for 100 times, followed by measuring thermal conductivity of the film. For
strains up to 20%, as shown in Fig. 4c, thermal conductivity of the composite decreased small, but for strains above 40% the value dramatically decreased, indicating breaks in the thermal conductive path. After 100 stretching cycles at a strain of 100%, the hBN(50)/PU composite film showed a decrease in thermal conductivity from 10.3 to 3.3 W/m$K. Even though the decreased thermal conductivity (3.3 W/m$K) is a practical value, a further study that incorporates other shape of fillers such as boron nitride nanotube is worth for improving thermal characteristics of the composite film under mechanical deformation. We proved good thermal conductivity of the composite film after various mechanical tests. However, it still remains a question of in-situ heat transfer under mechanical deformation. Thus, we built a homemade jig that can simultaneously apply heat and strain to the composite film, and observed heat conduction behavior in real-time using an IR camera (Fig. 4d). When there was no strain (0%), the h-BN(50)/PU composite film exhibited thermal distribution gradually changed from red to blue between heat source (100 C) and heat sink (25 C), indicating stable thermal conduction of the composite film (Fig. 4e). When a certain strain up to 50% was applied to the film, the stretched film also showed uniform and stable heat gradient along the strain direction from the heat source to the heat sink across the entire surface area. This result proves that the composite film maintains its own thermal conductive property under mechanical stress. 4. Conclusions In the investigation, the facile fabrication of 3D h-BN network in polymer composite and its application for highly flexible thermally
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Fig. 4. The photographs of the composite film with cycled a) bending, b) twisting and c) stretching test and the corresponding thermal conductivities after each mechanical deformations. d) Schematic illustration of experimental set up. A composite film was suspended across distance-adjustable two stages (the heat source and the heat sink). e) IR images of temperature gradient of h-BN(50)/PU composite film under various strain by using experimental set up (d).
conductive film has been successfully demonstrated. With simple processing steps, 3D h-BN network was easily constructed by utilizing in-situ incorporation of h-BN into PU scaffolds during polymerization. The obtained composite film showed not only high thermal conductivity up to 10 W/m$K but also excellent mechanical flexibility such as folding, twisting and stretching, indicating high potential for use in wearable electronics. Furthermore, this technique can be applied to other fillers to construct 3D networks in polymer composite, allowing versatility in the choice of materials. We believe that the highly economical and versatile process developed in this study provides actual insight to realize composite with multi-functional performances for numerous applications ranging from current electronics to soft and stretchable devices. Acknowledgements This research was supported in part by the Korea Institute of Science and Technology (KIST) Institutional Program (No. 2Z05000) and Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF, 2016 M3A7B4905619). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.compscitech.2017.09.020. References [1] J.A. Rogers, T. Someya, Y. Huang, Materials and mechanics for stretchable electronics, Science 327 (5973) (2010) 1603e1607.
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Highly flexible and stretchable thermally conductive composite film by polyurethane supported 3D networks of boron nitride Hye-Jin Hong a, 1, So Mang Kwan b, 1, Dong Su Lee b, Seung Min Kim b, Yun Ho Kim c, Jin Seong Lim b, Jun Yeon Hwang b, *, Hyeon Su Jeong b, ** a
Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 34132, Republic of Korea Institute of Advanced Composite Materials, Korea Institute of Science and Technology, 92 Chudong ro, Bondong-eup, Wanju-gun, Jeonbuk 565-905, Republic of Korea c Division of Advanced Materials, Korea Research Institute of Chemical Technology, Daejeon, 34114, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2017 Received in revised form 11 August 2017 Accepted 17 September 2017 Available online 21 September 2017
Current interest in flexible and stretchable electronics has been amplified because of their remarkable features for numerous applications. Accordingly, compliant efficient thermal management for such electronics is in great demand. However, new materials simultaneously coping with high thermal conductivity and mechanical flexibility have not been sufficiently studied. Here, we report a simple yet highly efficient method to construct three-dimensional (3D) hexagonal boron nitride (h-BN) network in a polymer composite, which is both thermally conductive and mechanically stretchable. 3D h-BN network is easily fabricated by in-situ incorporation of h-BN onto the surface of a water-borne polyurethane (PU) scaffold during polymerization. The 3D h-BN network in the composite film offers high thermal conductivity up to 10 W/m$K while the PU contributes excellent mechanical flexibility such as folding, twisting and stretching. Moreover, the process developed in this study is highly economical and processable to be scaled up without the need of complex equipment or procedure, which will be of great interest to current manufacturing as well as soft and stretchable electronic devices. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Polymer-matrix composites (PMCs) Flexible composites Thermal properties Interface
1. Introduction Recent advances in flexible and stretchable electronics have gained great attention due to their numerous possible applications in wearable devices [1], power sources [2], sensors [3], robots [4] and healthcare equipment [5]. As one of the soft materials to support next generation devices, new compliant thermal management is necessary because it is well known that the elevated temperature of the device negatively affects performance and safety [6,7]. Thus, a new material that is both flexible and stretchable together with high thermal conductivity is in great demand [8,9]. However, thermally flexible and stretchable materials have not been widely studied yet. The most commonly used heat dissipation method has been relied on polymer-based composite materials, which blends * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.Y. Hwang), [email protected] (H.S. Jeong). 1 Co-first authors. https://doi.org/10.1016/j.compscitech.2017.09.020 0266-3538/© 2017 Elsevier Ltd. All rights reserved.
polymer matrix with high thermal conductive fillers [10]. Among various fillers, ceramic fillers such as hexagonal boron nitride (hBN) are preferred due to their electrical insulation as well as high thermal conductivity [11e14]. In case of carbon based fillers such as graphene, the corresponding composite suffer from current leakage and electrical shorts due to their electrical conductivity, thus limiting their applications where electrical insulation is required. To increase the thermal conductivity of composites, many efforts have been devoted to increase the degree of dispersion of fillers in polymer matrix mainly by surface functionalization of fillers, which expects minimization of interfacial thermal resistance [15e17]. Unfortunately, the thermal conductivity of such composites is still insufficient, and there are difficulties in complex functionalization process [18,19]. The most efficient way to realize high thermal conductivity is constructing a three-dimensional (3D) network of fillers in polymer composite. In recent years, fabricating h-BN 3D scaffolds in polymer composite prepared by the ice-templated method has shown to be an effective way to achieve high thermal conductivity [20e22]. However, the sophisticated fabrication process involved the use of a binder, low processing temperature,
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high calcination temperature and inevitable polymer infiltration process is time consuming, limiting its use in real applications [20,22]. Apart from the thermal conductivity, the flexibility and stretchability of the thermal polymer composite has been rarely reported. Hence, it still remains a technical challenge to fabricate thermally a conductive polymer composite having 3D network of filler with flexibility and stretchability via facile process. Here, we report a simple yet efficient method to construct 3D hBN network in polymer composite, which is highly thermally conductive, electrically insulating and mechanically stretchable. 3D h-BN network was easily constructed by in-situ attaching of h-BN to a surface of water-borne polyurethane (PU) scaffolds during polymerization of PU. The 3D h-BN network in the composite film offered high thermal conductivity of 10 W/m$K while the PU contributed excellent mechanical flexibility (bending and twisting) and stretchability. The composite film developed in this study has significant advantages over other methods for flexible electronics, including from not only high thermal conductivity with elastic properties but also easy and rapid fabrication process. 2. Material and methods 2.1. Materials h-BN was purchased from Denka (particle size: 8 mm) and used as received. The water-borne PU prepolymer (Hypol JT 6000) was provided by Dow Chemical (USA). 2.2. Preparation of h-BN/PU composite foam An amount of 10 g of prepolymer was mixed with 30 g of deionized (DI) water to produce neat PU foam. For the h-BN/PU composite foam, a certain amount of h-BN powder (for example, 1 g of h-BN for a 10 wt% h-BN/PU composite) was initially dispersed in aqueous media by bath-sonication for 30 min. Then, the h-BN solution was vigorously mixed with 10 g of prepolymer for 30 s using a mechanical stirrer. The mixed solution was poured into a rectangular tray. As the polymerization of PU progressed, the composite foam expanded to fill the tray and the tray was removed after 10 min drying at room temperature. The composite foam was washed by immersing it in DI water, squeezing and releasing several times to remove any unattached h-BN particles. The h-BN/ PU composite foam was dried in an oven at 60 C for 24 h. 2.3. Preparation of h-BN/PU composite film In order to make the composite film, subsequently, the composite foam was hot-pressed at 150 C under 100 kgf/cm2 for 2 min. 2.4. Characterization of h-BN/PU composite foam and film Fourier-transform infrared (FT-IR, cary 630, Agilent, USA) was used to investigate the chemical structures of the neat PU and hBN/PU composite. Loading amount of h-BN in the composite was investigated by thermogravimetric analysis (TGA, TA Instrument Q50, USA). Scanning electron microscopy (SEM, Verios 460 L, FEI) equipped with energy dispersive X-ray spectroscopy (EDS) was used to characterize the morphology of interface of the composite. A non-destructive high resolution x-ray microscopy (XRM, Xradia 520 versa, Zeiss, Germany) was also used to determine the internal structure with 3D configuration, and the obtained XRM image was further analyzed by 3D software (Avizo Fire 8.1, FEI). Tensile test was carried out by universal testing machin (5567A, Instron, USA). The in-plane thermal conductivity of the composite film was calculated by multiplying density, specific heat capacity and
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thermal diffusivity of the composite film. The density of composite film was calculated from the weight and dimensions of the composite film (Table S1). The specific heat capacity at room temperature was measured by differential scanning calorimeter (DSC, Q20, TA instrument). The thermal diffusivity was measured based on laser flash method (Laser PIT-M2, ULVAC, Japan). Through-plane thermal conductivity of composite film was measured based on the method of ASTM-D5470 (MVH100P, Hantech, Korea). Thermal IR images of composite film were taken by thermal imaging camera (Testo, 875-1i). 3. Results and discussion 3.1. In-situ incorporation of h-BN onto PU The overall procedure for fabricating 3D networks of h-BN/PU composite film is illustrated in Fig. 1a. The procedure consists of four parts as follows. (i) A certain amount of h-BN dispersed in deionized water with the aid of sonication for 30 min is mixed with a water-borne PU prepolymer; (ii) a foam forming reaction occurs as polymerization proceeds; (iii) the h-BN/PU composite foam is dried; and (iv) the composite film is fabricated by a typical hotpressing method. Our strategy mainly relies on the second step. Polymerization immediately occurs by the reaction between isocyanate groups in prepolymer and water (H2O), resulting in amine group and release of CO2 gas as follows: R-NCO (isocyanate) þ H2O / [R-NHCOOH] / R-NH2 (amine) þ CO2
(1)
This CO2 bubble leads to macro-porous foam structure. The hBN initially dispersed in the water also participated PU polymerization and then immobilized onto the surface of PU foam. In other words, the h-BN naturally attaches to the surrounding of PU surface as the water is consumed in the formation of CO2 as shown in Fig. 1a (an enlarged image of the second step). Thus, 3D h-BN network via PU scaffold can be successfully assembled by an in-situ attachment process. This incorporation process takes less than one minute. The other strategy is to transform foam structure of the composite into a film structure by employing a typical hot-pressing method because as-fabricated macro pores act as thermal insulation spaces. Thus, the pore structure that initially serves as a template for constructing 3D h-BN network in the first place should be removed. After hot pressing, we could effectively remove the pores, resulting in highly dense h-BN/PU composite films shown in Fig. 1a. Fig. 1b shows photographs of the neat PU and h-BN(50)/PU composite (50 wt% of h-BN loading) foam. The overall macroscopic features (porous structure and color) were not significantly changed when h-BN was attached. FT-IR was used to investigate the chemical structures of the neat PU and h-BN/PU composite (Fig. 1c). The pristine h-BN exhibited characteristic bands of B-N in-plane stretching at 1370 cm1 and B-N-B out of plane bending observed at 810 cm1. Also, a broad band at 3200 cm1 indicated the existence of hydroxyl group in BN (-OH) band [23]. The neat PU foam also showed various bands. A broad band at 3295 cm1 corresponded to amine (-NH) groups and that at 1724 cm1 corresponded to free eC¼O. Bands at 1530 and 1221 cm1 corresponded to NH and CN, respectively. These bands indicate the formation of a urethaneeurea bond [24]. The FT-IR spectrum of h-BN(50)/PU showed both PU and h-BN characteristic bands. Characteristic bands of h-BN at 1370 and 810 cm1 were observed in the h-BN/PU spectrum. PU characteristic bands were also observed although PU bands located in 1000e1500 cm1 showed a slight decrease in intensity due to overlapping of the high intensity h-BN band at 1370 cm1. It is noted that the hydroxyl band of pristine h-BN at
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Fig. 1. a) Schematic diagram of the process of fabricating 3D h-BN/PU composite. b) Photographs of the neat PU foam and h-BN/PU composite foam. c) FT-IR data of h-BN, the neat PU foam and h-BN/PU composite foam. SEM images of d) neat PU foam and e) h-BN(50)/PU composite foam. f) A magnified SEM image of (e). g) EDS elemental mapping of hBN(50)/PU composite foam.
3200 cm1 disappeared from the h-BN/PU composite, implying a possible chemical interaction between the hydroxyl groups of h-BN and the isocyanate groups of the prepolymer as follows [25]. R-NCO (isocyanate) þ HO-BN / R-NHCOOBN
(2)
R0-NCO(isocyanate) þ R-NHCOOBN / R0 -N(CONHR)COO-BN
(3)
Thus, the h-BN is not only physically attached to PU polymer but also chemically linked to PU foam by participating in polymerization process. To make a conductive path of fillers, it is important to load enough h-BN content in the composite beyond a critical value, which is the percolation threshold. TGA is a good indicator to show the actual loading content because h-BN is known to have remarkable thermal stability up to 1000 C in air. Fig. S1 (Supporting Information) shows the thermal degradation behaviors of neat PU and the h-BN/PU composites with various loading contents of h-BN. While a weight loss of 100% was observed for neat PU above 500 C, a weight of 12, 21, 33, 40 and 53% weight was remained for 10, 20, 30, 40 and 50 wt% h-BN/PU composite, respectively. This remained weight for each composite is consistent with initial loading content of h-BN in each composite. This consistency is attributed to strong physical and chemical interaction between h-BN and PU, which enables to load enough amount to make 3D connected h-BN networks. Direct observation of h-BN grafted onto PU was investigated by SEM equipped with EDS. As shown in Fig. 1d, the neat PU exhibits a smooth surface and pores. In contrast, the h-BN(50)/PU composite shows a much rough surface, indicating the h-BN wrapped all PU
surface while preserving its original porous structure (Fig. 1e). The trend of surface morphology is obviously a function of loading contents of h-BN in the composite (Fig. S2). A magnified image of the composite clearly shows the presence of two-dimensional h-BN fillers covering the PU surface. A lot of h-BN fillers are compactly connected along all directions, resulting in 3D connected networks of h-BN (Fig. 1f). In Fig. 1g, the EDS mapping is shown for each element (B, C, N) with color coded. EDS analysis further confirms homogenous distribution of h-BN in the PU matrix.
3.2. 3D network of h-BN 3D XRM was used to non-destructively resolve the 3D structure of the h-BN/PU composite, and the results are shown in Fig. 2aec. Fig. 2a shows XRM image of 3D porous structure of neat PU. It is confirmed that a 3D connected porous structure with large pore size about several hundred of micrometers was developed. When h-BN was involved in polymerization process, the features of 3D porous structure were notably changed. The size of pore became smaller and denser than that of neat PU as the loading amount of hBN increased from 0 to 10 wt% (Fig. 2b) and finally to 50 wt% (Fig. 2c). The increase of small pore density is directly related to increase of specific surface area. We analyzed the XRM images with 3D software (Avizo Fire 8.1) to calculate the specific area per volume. As shown in Fig. 2def, the specific area increased from 0.06 (mm2/mm3) to 0.15 for h-BN(10)/PU composite and finally to 0.33 for the h-BN(50)/PU composite. Compared with neat PU, 50 wt% h-BN loading in PU composite has 5.5 times of specific area, indicating 5.5 times of more conductive path for thermal transport is generated.
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Fig. 2. Reconstructed 3D XRM images of a) neat PU foam and h-BN/PU composite foams of b) 10 wt% and c) 50 wt% h-BN. d-f) Processed images showing specific area per unit volume of foam structures corresponding to a-c), respectively.
Another notable change from the XRM investigation is the color change from blue to green as the loading amount of h-BN is increased (Fig. 2aec). This color change indicates a change of interfacial material contacting with air. In other words, the interfacial material contacting with air changes from PU to h-BN, which is consistent with the SEM observations of gradual surface coverage by h-BN on PU (Fig. S2). This change of interfacial environment causes the image contrast visualized by XRM. The XRM results further prove the presence of h-BN anchored onto the PU surface in all direction, which constructs 3D network of h-BN. 3.3. 3D h-BN/PU composite film with high thermal conductivity The h-BN/PU composite foam was transformed into an h-BN/PU composite film with a thickness of 500 mm (Fig. 3a) by using a typical hot-pressing method. A cross-sectional SEM image of the neat PU film (Fig. 3b) reveals that the pores were effectively removed by simple hot pressing. The SEM images in Fig. 3b and c shows cross-sectional morphologies of the prepared neat and composite film, respectively. Compared with the morphology of neat PU, the surface of h-BN(50)/PU is very rough. Like a foam structure, the morphology trend of film is also a function of loading contents of h-BN in the composite (Fig. S3). The magnified SEM image (Fig. 3d) clearly shows the presence of h-BN particles that are compactly packed and connected together. It was further confirmed by EDS mapping that the h-BN fillers were homogenously distributed and well-connected in the PU matrix (Fig. S4). The composite film is expected to have high thermal conductivity arising from highly packed and connected h-BN, simultaneously high mechanical performances arising from the PU matrix. The inand through-plane thermal conductivity of the composite film with various h-BN loadings was measured at room temperature. As shown in Fig. 3e, as the weight fraction of h-BN increased, the inplane thermal conductivity of all composites dramatically increased. Compared to that of neat PU film (0.2 W/m$K), the thermal conductivity of composite at h-BN loadings of 5, 10, 20, 30, 40 and 50 wt% increased by factors of 1.8, 9.5, 29.1, 40.0, 49.2 and 50.4, respectively. In case of h-BN(50)/PU composite film, the inplane thermal conductivity reached up to 10.28 W/m$K. To the
best of our knowledge, this is the highest thermal conductivity reported to date based on filler/PU composites. The threshold for percolation seems to place between h-BN loadings of 5 wt% and 10 wt%. After a dramatic jump in thermal conductivity at 10 wt% hBN, the thermal conductivity linearly increased up to h-BN loading of 40 wt% and became almost constant for further loading. This indicates, at 40 wt% loading, the packing of h-BN is almost saturated by direct contact with each other. Compared to in-plane thermal conductivity, meanwhile, the through-plane thermal conductivity of composite films was not dramatically increased as the weight fraction of h-BN increased (Fig. S5). This indicates the film has anisotropic thermal conductivity. This is probably attributed to partial alignment of h-BN during hot-pressing. An evidence can be also found in EDS mapping of h-BN fillers (B, N) that were partially oriented in-plane direction (Fig. S4). While the h-BN in the composite plays a crucial role in thermal transport, the PU in the composite provides high mechanical properties. The elasticity of the neat PU and composites with various loading amounts was evaluated by tensile stress-strain measurement (Fig. S6). The tensile strength of all composite films continuously increased as the hBN content increased in the PU matrix. In comparison with the neat PU, the tensile strength of the h-BN(50)/PU composite was 3.5 times larger. The continuous increase of tensile strength indicates good interfacial adhesion between PU and h-BN as proved in FT-IR and XPS data. The h-BN well anchored on PU can dissipate of fracture energy, preventing fracture propagation [15,26]. The strain also increased up to h-BN loading of 30 wt% but decreased with further loading of h-BN. Due to good adhesion of h-BN on PU, the fillers follow PU deformation, which helps stretchability. Above 40 wt% loading of h-BN, the excessive fillers start to hinder segmental mobility of polymer matrix, decreasing flexibility. The retained thermal conductivity by saturation of fillers from same loading amount of h-BN supports this speculation (Fig. 3e). Nevertheless, the h-BN(50)/PU composite could be stretched over 100% of the initial length which was still higher than the stretchability of neat PU. It should be noted that the usual trade-off between mechanical and thermal properties as well as processability was not found in our composite [27]. To directly observe the heat transfer ability of the composites,
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Fig. 3. a) a photograph of composite film by hot pressing the composite foam. Cross-sectional SEM images of b) neat PU film and c) h-BN(50)/PU composite film. d) A cross-sectional magnified SEM image of h-BN/(50)/PU composite film. e) In-plane thermal conductivity of the composites as a function of h-BN loading. f) Thermal IR images of the composites with various h-BN contents on hot and cool stages at 30s each. To clarify the image, the h-BN(50)/PU composite film on cool stage is marked by dotted lines in white.
the temperature change of each composite with various loading contents of h-BN was monitored using an infrared (IR) thermal imaging camera. Fig. 3f shows IR images of each composite mounted on hot and cool stages for 30 s each. The trend in heat conduction of the composite is obvious. The more amounts of h-BN presences in the composite, the faster heat transfer was observed for both heating and cooling experiments. In addition to heat transfer, the distribution of temperature was more uniform across the composite film when more h-BN was loaded in the composite. Together with high thermal conductivity, the direct observation on heat transfer further confirms that a 3D network of conductive heat path is well developed by facile in-situ incorporation process. 3.4. Flexibility and stretchability To demonstrate high potential for use in wearable devices as thermal management, the in-plane thermal conductivity of the composite films was measured after various mechanical deformations such as bending, twisting and stretching. Fig. 4a shows the variation in the thermal conductivity with respect to the number of bending tests. In the bending tests, the film was completely folded. It is confirmed that the thermal conductivity curves show plateaus for all composites, indicating high stability of heat conduction under mechanical bending. The thermal conductivity after twisting deformation also retained its initial value for all composites, implying that the network of h-BN was not collapsed by mechanical twisting (Fig. 4b). For stretching test, the composite film was stretched to a certain strain and released to its original state. The stretching cycle for each strain was carried out for 100 times, followed by measuring thermal conductivity of the film. For
strains up to 20%, as shown in Fig. 4c, thermal conductivity of the composite decreased small, but for strains above 40% the value dramatically decreased, indicating breaks in the thermal conductive path. After 100 stretching cycles at a strain of 100%, the hBN(50)/PU composite film showed a decrease in thermal conductivity from 10.3 to 3.3 W/m$K. Even though the decreased thermal conductivity (3.3 W/m$K) is a practical value, a further study that incorporates other shape of fillers such as boron nitride nanotube is worth for improving thermal characteristics of the composite film under mechanical deformation. We proved good thermal conductivity of the composite film after various mechanical tests. However, it still remains a question of in-situ heat transfer under mechanical deformation. Thus, we built a homemade jig that can simultaneously apply heat and strain to the composite film, and observed heat conduction behavior in real-time using an IR camera (Fig. 4d). When there was no strain (0%), the h-BN(50)/PU composite film exhibited thermal distribution gradually changed from red to blue between heat source (100 C) and heat sink (25 C), indicating stable thermal conduction of the composite film (Fig. 4e). When a certain strain up to 50% was applied to the film, the stretched film also showed uniform and stable heat gradient along the strain direction from the heat source to the heat sink across the entire surface area. This result proves that the composite film maintains its own thermal conductive property under mechanical stress. 4. Conclusions In the investigation, the facile fabrication of 3D h-BN network in polymer composite and its application for highly flexible thermally
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Fig. 4. The photographs of the composite film with cycled a) bending, b) twisting and c) stretching test and the corresponding thermal conductivities after each mechanical deformations. d) Schematic illustration of experimental set up. A composite film was suspended across distance-adjustable two stages (the heat source and the heat sink). e) IR images of temperature gradient of h-BN(50)/PU composite film under various strain by using experimental set up (d).
conductive film has been successfully demonstrated. With simple processing steps, 3D h-BN network was easily constructed by utilizing in-situ incorporation of h-BN into PU scaffolds during polymerization. The obtained composite film showed not only high thermal conductivity up to 10 W/m$K but also excellent mechanical flexibility such as folding, twisting and stretching, indicating high potential for use in wearable electronics. Furthermore, this technique can be applied to other fillers to construct 3D networks in polymer composite, allowing versatility in the choice of materials. We believe that the highly economical and versatile process developed in this study provides actual insight to realize composite with multi-functional performances for numerous applications ranging from current electronics to soft and stretchable devices. Acknowledgements This research was supported in part by the Korea Institute of Science and Technology (KIST) Institutional Program (No. 2Z05000) and Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF, 2016 M3A7B4905619). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.compscitech.2017.09.020. References [1] J.A. Rogers, T. Someya, Y. Huang, Materials and mechanics for stretchable electronics, Science 327 (5973) (2010) 1603e1607.
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