Novel Flexible Phase Change Materials with Mussel-Inspired

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Novel Flexible Phase Change Materials with Mussel-Inspired Modification of Melamine Foam for Simultaneous Light-Actuated Shape Memory and Light-to-Thermal Energy Storage Capability Hai-yan Wu, Ruo-tian Chen, Yao-wen Shao, Xiao-dong Qi,* Jing-hui Yang, and Yong Wang* Key Laboratory of Advanced Technologies of Materials, Ministry of Education of China, School of Materials Science and Engineering, Southwest Jiaotong University, Erhuan Road, North I, No. 111, Chengdu 610031, P. R. China Downloaded via UNIV OF SOUTHERN INDIANA on July 31, 2019 at 12:55:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Form-stable phase change materials (PCMs) are widely used for thermal management. However, the strong rigidity and the weak photoabsorption ability have hindered their practical applications. Herein, we report a flexible PCM based on paraffin wax (PW) and polydopamine-coated melamine foam (MF@PDA) for the seamlessly combined light-actuated shape memory and light-to-thermal energy storage capability. The MF@PDA foams with different PDA contents, which act as supporting scaffolds to adsorb PW, are successfully obtained by varying the immersion times and the dopamine concentrations. The low density and high porosity of MF@PDA foams allow for high weight fraction of PW to be incorporated, making the PCMs show high latent heat and encapsulation ability. Simultaneously, the elastic MF@PDA foams impart the PCMs with good shape memory property by triggering the phase transition of PW. More importantly, due to the efficient photothermal effect of PDA coating, the PCMs exhibit excellent lightactuated shape memory effect (shape recovery ratio ∼ 100%, shape recovery speed ∼ 100 s) and solar-to-thermal energy storage efficiency (80.8%). The light-actuated shape recovery rates and the solar-to-thermal transfer rates are increased with the augmented amount of PDA coating. Besides, the PCMs exhibit quite stable shape memory cyclic performance and thermal reliability. This study provides a new strategy to design flexible and light-responsive PCMs with the potential to be used in advanced solar energy storage systems. KEYWORDS: Phase change, Shape memory property, Melamine foam, Polydopamine, Light-responsive

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materials,11,17 polymers,18,19 metal foams,20 carbon materials,21 and so on. Nevertheless, although these conventional formstable PCMs show a good encapsulation effect, they inevitably face the installation difficulty that is derived from the strong rigidity of the supporting materials.22,23 The rigidity of PCMs will cause easy brittle failure and high thermal contact resistance with controlled devices, which has a negative effect on the thermal management efficiency.24,25 Therefore, it is of great importance to fabricate form-stable PCMs with high flexibility and shape variation ability, which are beneficial for the installation of PCMs. In this case, the suitable supporting materials should be carefully considered. Recently, melamine foam (MF) has obtained great attention because of its excellent elasticity, three-dimensional (3D) porous structure, lightweight, and high open porosity.26 Inspired by shape memory polymers (SMPs) that are capable of undergoing a large recoverable deformation upon exposure to external stimuli,27−29 physically blending the elastic MF with the paraffin wax (PW) may make the MF/PW PCMs show shape variation property with temperature as stimulus. In

INTRODUCTION As one kind of advanced energy storage materials, phase change materials (PCMs) have received enormous attention because of their high energy storage capability and small temperature variation during the phase change process.1−3 Among various kinds of PCMs, organic solid−liquid PCMs (e.g., paraffin) are widely used due to their high latent heat, suitable melting temperature, chemical stability, and abundance in nature.4,5 Therefore, organic solid−liquid PCMs show tremendous application prospect in energy-saving buildings,6 smart textiles and clothing systems,7 thermal management of electronics,8 solar energy harvesting systems,9 and so on. However, practical applications of organic PCMs are restricted by one substantial problem: the liquid leakage during the phase change process, which leads to the decrease of energy storage density and environmental pollution.10−13 To solve this problem, form-stable PCMs are extensively studied, which are usually composed of an organic PCM and a supporting scaffold.14 There are mainly two methods to prepare form-stable PCMs. One strategy is the microencapsulation of PCMs.15 Typically, a core−shell structure is constructed in which the core is the organic PCM and the shell is the supporting material.16 The other strategy is incorporating PCMs into effective supporting materials, such as porous © XXXX American Chemical Society

Received: June 5, 2019 Revised: June 25, 2019 Published: June 26, 2019 A

DOI: 10.1021/acssuschemeng.9b03169 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic representations showing the preparation process of MF@PDA/PW PCMs: (a) the framework of MF, (b) MF@PDA foam with PDA particles on the skeleton of MF, and (c) MF@PDA/PW PCMs with PW in the pores or along the skeleton of MF@PDA foams.

leakage of melted PW without compromising the latent heat. Meanwhile, the super elasticity of MF imparts the PCMs with shape memory property by triggering the phase transition of PW. More importantly, MF can be used as a template to deposit a thin layer of PDA coating and thus endows the PCMs with light-responsive properties, which are believed to show great potential in solar energy storage systems.

detail, when the temperature is lower than the melting temperature (Tm) of PW, the MF/PW PCMs will present a rigid state. Once the temperature is higher than the Tm of PW, namely PW turning to liquid, the MF/PW PCMs become softened and can be deformed into a certain shape. Therefore, by triggering the phase transition of PW, the transformation of MF/PW PCMs from rigidity to flexibility is reversible, namely the shape memory effect. By now, the study of MF/PW PCMs mainly concentrates on the phase change property,6 but very little research focuses on the shape memory performance used for the installation in thermal management systems.30 In addition to the leakage and rigidity, another problem of PCMs is the poor photoabsorption, which limits their applications in solar energy utilization.31 Various photothermal fillers including carbon nanotubes,32 carbon sponges,33 graphene,34,35 and dyes36 have been incorporated into PCMs to enhance the photoabsorption ability. Among these, musselinspired polydopamine (PDA), a smart coating material first proposed by Messersmith et al. in 2007, has attracted much attention due to its strong adhesion on almost all substrates.37 PDA can be obtained by the self-polymerization of dopamine (DA) at an ambient temperature, which is considered simple, mild, and controllable.38 Furthermore, PDA shows high photothermal conversion efficiency and has been used for photothermal therapy.39−41 Also, some studies report that PDA can generate heat under near-infrared (NIR) light and induce the shape recovery of SMPs.42−45 In order to solve the issues mentioned above, here we fabricated a novel kind of flexible PCM based on PW and PDA coated MF (MF@PDA), which not only shows light-tothermal energy storage capability but also possesses lightactuated shape memory property. The lightweight and porous structure of MF ensures a high loading of PW and prevents the

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MATERIALS AND METHODS

Materials. PW (Tm: 52 °C) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). MF was obtained from Xiamen Share Nano Technology Co., Ltd. (China). Dopamine (DA) was purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Sample Preparation. Preparation of MF@PDA Foams. The MF@PDA foams were prepared by immersing MF (Figure 1a) into a DA/aqueous solution (pH = 8.5, 0.02 mM Tris) at room temperature (25 °C). The PDA content in the MF@PDA foams (Figure 1b) was adjusted by changing the immersion times and the concentrations of DA solution. The obtained MF@PDA foams were repeatedly washed with deionized water and dried in an oven at 60 °C. Herein, the MF@ PDA foams with different immersion times are denoted as MF@ PDAxh, where x represents the immersion time. The MF@PDA foams with different DA concentrations are denoted as MF@PDAx mg, where x represents the concentration of DA solution. Preparation of MF@PDA/PW PCMs. First, PW (18 g) was dissolved in n-hexane solvent (47 mL) at 60 °C to obtain a homogeneous solution. Second, MF@PDA foams were placed in the PW/n-hexane solution for 10 min to absorb PW under a vacuum environment. Finally, the materials were dried at 50 °C to evaporate solvent, and the resulting MF@PDA/PW PCMs were obtained (Figure 1c). For comparison, the MF/PW PCM was also prepared by the same process. For all the samples, the weight ratio of PW to MF@ PDA foam was 20:1. Characterizations and Measurements. The morphologies of the MF@PDA foams and the representative PCMs were investigated using a scanning electron microscope (SEM, Fei Inspect, NetherB


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Figure 2. (a, b) Digital photographs and SEM images of MF@PDA foams for immersion at different times. The concentration of the DA solution is set as 2 mg/mL. (c, d) Digital photographs and SEM images of MF@PDA foams for immersion in different DA concentrations. The immersion time is set as 12 h. lands). The microstructures of MF@PDA/PW PCMs were characterized using a Fourier transform infrared spectroscope (FTIR, Thermo Nicolet, USA) and a wide-angle X-ray diffractometer (WAXD, Panalytical Empyrean, Netherlands). The encapsulation properties of MF@PDA/PW PCMs were measured by a rheometer (TA Instrument, DHR-1, USA). The temperature was ranged from 20 to 100 °C, and the samples were pressed at a constant force of 0.5 N. The melting and crystallization behaviors of MF@PDA/PW PCMs were investigated by a differential scanning calorimeter (DSC, Netzsch STA 449C, Germany). The samples were first heated from 0 to 100 °C and then cooled from 100 to 0 °C. The heating/cooling rates were 5 °C/min, and the DSC measurements were conducted in nitrogen atmosphere. An infrared thermal imaging camera (FLIR T620, USA) was used to record the thermal energy storage and release behaviors of MF@PDA/PW PCMs. The thermal-actuated and light-actuated shape memory properties of MF@PDA/PW PCMs were measured via a four-step method:46 (1) the sample was placed on a hot stage (80 °C) and then bent into half (“U” shape); (2) the deformed sample was frozen to 25 °C with a constant external force; (3) the force was removed, and the temporary angle (θ1) was detected; (4) the sample was placed on the hot stage (80 °C) or irradiated by a NIR lamp (Philips, 150 W) to allow shape recovery, and the final recovery angle (θ2) was detected. The shape fixing ratio (Rf) and shape recovery ratio (Rr) of MF@PDA/PW PCMs were quantitatively calculated according to the two formulas 1 and 2. Rf =

θ1 × 100% 180

(1)

Rr =

θ1 − θ2 × 100% θ1

(2)

solution to the PDA coating on MF, MF was immersed in 2 mg/mL DA solution for different times and immersed in different concentrations of DA solution for 12 h. As shown in Figure 2a, the increased immersion time in DA solution (2 mg/mL) leads to an obvious color change (darker). The morphology of PDA particles is shown in Figure S1. PDA particles are formed through the self-polymerization and crosslinking reaction of dopamine. The PDA particle size is about 500 nm. The SEM images of the MF@PDA foams coated with different PDA contents are shown in Figure 2b. The surface of the MF skeleton is quite smooth (Figure S2), but the skeletons of MF@PDA foams exhibit a rough texture, suggesting that PDA particles are successfully adhered on MF. The more immersion time, the more PDA particles on the skeleton of MF presents. After immersion for 6 h, some discrete PDA particles form on the surface of MF skeleton. This is ascribed to the random stacking of PDA particles on the substrate in air environment.47 Further increasing the immersion time to 24 h, many more PDA particles adhere with each other, and a denser PDA coating covers the whole surface of the MF skeleton. As for the samples immersed in different concentrations of DA solution, the increased concentration also results in a darker surface (Figure 2c). By increasing the concentration of DA solution, a denser and rougher PDA coating is formed on the skeleton of MF (Figure 2d). Based on the above observations, tunable content of PDA coating can be readily obtained through varying the immersion time and the concentration of DA solution. This phenomenon is consistent with previous studies, which suggest that DA self-polymerizes, in situ forming PDA particles, and PDA particles deposit on the substrates.45,47,48 Besides, the PDA particles do not peel from the skeleton of MF when the MF@PDA12h foam is bent after 30 cycles, indicating the strong adhesion between PDA particles and MF (Figure S3). The PDA coating on the MF matrix consists of some PDA particles and aggregates. The spherical PDA particles are first formed and then linked to each other to form PDA

The solar-to-thermal energy storage and release behaviors of MF@ PDA/PW PCMs were measured by using a xenon lamp (HSX-F300, China) as a solar simulator. The surface temperature of samples was detected using a digital thermocouple.

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RESULTS AND DISCUSSION Morphology of MF@PDA Foams. To investigate the impact of the immersion time and the concentration of DA C


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Figure 3. (a−c) SEM images of MF@PDA/PW PCMs based on MF@PDA with different immersion times. (d) Comparison of the dimension retention ratio of PW, MF/PW, and MF@PDA/PW PCMs. (e) Digital photographs showing the variation of the encapsulate effect during the heating process. Samples from left to right are a) PW, b) MF/PW, and c) MF@PDA12h/PW, respectively.

Figure 4. (a) DSC heating and (b) cooling curves of PW, MF/PW, and MF@PDA/PW PCMs based on MF@PDA with different immersion times. (c) Infrared photographs showing the heat storage and release behaviors of MF/PW and MF@PDA12h/PW PCMs. The samples were placed on a hot stage at 80 °C and then placed at 15 °C.

aggregates.47 Although the PDA coating becomes thicker with increasing the polymerization time and the concentration of

dopamine solution, the thickness of PDA layer adhered to MF is only about several micrometers. The weight of the PDA D


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ACS Sustainable Chemistry & Engineering layer in the MF@PDA foam is negligible. FTIR analyis was applied to confirm the PDA formation on MF. However, it is very difficult to find the difference between MF and MF@PDA foams, since these curves are almost the same (Figure S4). Both MF and PDA have some similar functional groups including N−H, CN, C−N, etc. Therefore, it is hard to verify the PDA formation based on the FTIR characterization. Anyhow, the surface morphologies of MF@PDA foams observed by SEM can be used to confirm the PDA coating on MF. Morphology and Shape Stability of MF@PDA/PW PCMs. MF@PDA foams show a three-dimensional porous structure with macropores of about 100 μm. These interconnected pores have a large capillary force, which allows for the easy infiltration of liquid PW. The microscopic structure of MF@PDA/PW PCMs was investigated by SEM. Figure 3a−c shows that PW occupies the void macropores or PW adheres well to the skeleton of MF@PDA foams. Because the thickness of PDA coating is relatively small, there is no significant difference on the overall morphology of the representative composites PCMs. The SEM images reveal that the MF@PDA foam is capable of being used as a porous scaffold to encapsulate PW through vacuum impregnation. FTIR and XRD analysis were performed to verify the structure of MF@PDA/PW PCMs. No obvious change of characteristic peaks is found in the FTIR and XRD patterns, which suggests that PW and MF@PDA foams are physically combined (Figure S5 and Figure S6). Encapsulation property is one of the key factors which determines the long usage life of PCMs. The shape stability of PW, MF/PW, and MF@PDA/PW were evaluated via different methods. The dimension retention ratios were quantitatively measured at the load of 0.5 N, and the results are presented in Figure 3d and Figure S7. The dimension of pure PW decreases seriously when the temperature is higher than 50 °C at which the solid−liquid phase transition of PW occurs. However, the dimensions of MF/PW and MF@PDA/PW PCMs change little during the heating process. Even if the temperature is 100 °C, the dimension retention ratios of MF@PDA/PW PCMs are still nearly 90%. To further directly observe the encapsulation effect, the cubic PW, MF/PW, and MF@ PDA12h/PW samples were placed on a hot stage at 80 °C. As shown in Figure 3e, these three samples retain their shape well at 30 °C, but pure PW melts with an obvious leakage at 80 °C. The MF/PW and MF@PDA12h/PW PCMs still show high shape stability, and no visible leakage of PW happens even at a load of 20 g counterweight. In this case, the 3D interconnected frameworks of MF and MF@PDA foams are ideal supporting scaffolds to completely encapsulate PW and thus overcome the liquid leakage problem. Thermophysical Properties of MF@PDA/PW PCMs. It is important to measure the latent heat which determines the thermal energy storage capability of PCMs. DSC measurement was then conducted to study the thermophysical properties of the MF@PDA/PW PCMs. The DSC heating and cooling curves are shown in Figure 4a,b and Figure S8. The thermal characteristics are summarized in Table 1 and Table S1. MF@ PDA/PW and pure PW show similar endothermic and exothermic behaviors, indicating that the infiltration of PW into MF@PDA foams has little effect on its thermophysical properties. The Tm and crystallization temperature (Tc) of MF@PDA/PW and pure PW are also quite close. Only a slight decrease of melting enthalpy (ΔHm) and crystallization

Table 1. Thermal Characteristics of PW, MF/PW, and MF@ PDA/PW PCMs Based on MF@PDA with Different Immersion Times sample

Tm/(°C)

Tc/(°C)

ΔHm/(J g−1)

ΔHc/(J g−1)

PW MF/PW MF@PDA3h/PW MF@PDA6h/PW MF@PDA12h/PW MF@PDA24h/PW

57.6 57.1 55.8 56.6 56.8 57.8

46.5 45.8 46.4 43.6 43.6 45.1

157.3 142.4 140.7 140.4 142.5 142.7

155.4 144.7 141.5 141.4 144.9 143.9

enthalpy (ΔHc) is observed for MF@PDA/PW, since a part of the working substance is replaced by MF@PDA foams. Nevertheless, due to the low density and high porosity of MF@PW foams, PW can be infiltrated into MF@PDA foams with an extremely high loading of about 95 wt %. Such a high loading allows for the high thermal energy storage capability, and the phase change enthalpies of these MF@PDA/PW PCMs are about 140 J g−1. The thermal energy storage and release behaviors were further investigated. The MF/PW and MF@PDA12h/PW PCMs were placed on a hot stage at 80 °C for 10 min and then removed to allow natural cooling. The infrared thermal imager was used to record the heating and cooling processes. As shown in Figure 4c, the temperature of MF/PW and MF@ PDA12h/PW increases from 20 to 47.6 °C in 4 min and maintains constant at about 48 °C for 6 min. This heating plateau is ascribed to the melting transition of PW, in which PW absorbs heat during phase change and keeps the temperature around the Tm of PW. After removing the hot stage, the temperature of samples achieves a freezing plateau at about 45 °C for 4 min, which corresponds to the crystalline transition of PW. In this condition, the thermal energy stored during the heating process is released. One can see that the temperature of samples can remain constant for 4 min under the natural cooling. Figure S9 displays the temperature−time curve of MF/PW and MF@PDA12h/PW PCMs in a heating and cooling cycle. Two temperature plateaus are observed during the thermal energy storage and retrieval process, indicating that the MF/PW and MF@PDA/PW PCMs possess excellent thermal management capacity. The thermal heating effect of PDA coating on the MF/PW PCM is shown in Figure S10. There is a negligible temperature variation between MF/ PW and MF@PDA/PW PCMs, suggesting that the PDA coating has very little impact on the thermophysical properties of MF/PW PCM. Thermal-Actuated Shape Memory Effect of MF@PDA/ PW PCMs. In addition to the encapsulation effect and phase change enthalpy, the strong rigidity of PCMs gives rise to the easy fracture and installation difficulty, which greatly hinder the practical applications of PCMs.24 Therefore, flexible PCMs with shape variation property are urgently needed to improve the installation in practical engineering. In this work, MF@ PDA foams possess a good elasticity (Figure S11), and the inner pores of MF@PDA foams are filled by PW. The combination of elastic MF@PDA foams and rigid PW may impart MF@PDA/PW PCMs with high feasibility of shape variation. The thermal-actuated shape memory behavior of MF@PDA12h/PW is recorded by the infrared thermal imager, and the photographs are shown in Figure 5a. MF@PDA12h@ PW can be processed into complicated shapes such as a cross shape, indicating an excellent processability. On one hand, the E


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Figure 5. (a) Infrared thermal imaging photographs showing the thermal-actuated shape memory behavior and the thermal energy storage effect of MF@PDA12h/PW. The digital photographs in the upper left corner showing the shape recovery from a compressed shape to a cross shape. The sample was placed on a hot stage at 80 °C. (b) Shape memory mechanism of MF@PDA/PW PCMs.

temporary shape (Step 2). Next, PW crystallizes in the inner pores of MF@PDA foams through rapid cooling to 25 °C (Step 3). The Tc of PW is higher than 25 °C, and thus the modulus of MF@PDA/PW PCMs derived from PW crystals is much higher than the resilience force of MF@PDA foams. Therefore, the temporary shape can be well fixed at 25 °C due to the existence of rigid PW crystals. Finally, when the sample is reheated to 80 °C, PW crystals melt again (Step 4). The stored resilience force of MF@PDA foams is released without the obstruction of PW crystals and drives MF@PDA/PW PCMs to recover their original shape. The thermal-actuated shape fixing and recovery behaviors of all MF@PDA/PW PCMs are shown in Figure S12. All the samples exhibit good shape memory effect, and the shape recovery speed is almost the same (Figure S13), indicating that the PDA coating does not affect the thermal-actuated shape memory property of MF/PW PCM. Light-Actuated Shape Memory Effect of MF@PDA/ PW PCMs. Light-actuated shape memory effect is also attractive because of its noncontact and region-selective controllability.49,50 It is known that PDA exhibits a strong NIR light absorption and energy conversion effect.43 The NIR light-actuated shape memory behaviors of MF@PDA/PW PCMs are shown in Figure 6. For MF/PW, no instant shape

original shape (“cross” shape) can be compressed into a smaller shape, and the temporary shape is well fixed at room temperature (25 °C), suggesting that MF@PDA12h/PW has a good shape fixing property. On the other hand, when the compressed sample is placed on the hot stage (80 °C), it can gradually recover to its original cross shape in 360 s. More interestingly, the temperature of sample keeps constant around the Tm of PW during the further heating process. This corresponds to the solid−liquid phase transition of PW, and the thermal energy is stored in the form of latent heat. Besides, no liquid leakage of PW is seen during the thermal-actuated shape recovery process, which confirms the enhanced encapsulation ability of MF@PDA/PW PCMs. Therefore, the MF@PDA12h/PW shows simultaneous shape memory and thermal energy storage capabilities, which has great potential in the thermal management of microelectronic devices. The shape fixing and recovery mechanism is explained as follows (Figure 5b). For MF@PDA/PW PCMs, the melting and crystallization of PW play the role of shape deformation and fixation, and the elasticity of MF@PDA foams is responsible for shape recovery. Specifically, upon heating above the Tm of PW, the MF@PDA/PW PCMs become flexible (Step 1) and can be easily compressed into a F


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Figure 6. NIR light-actuated shape recovery behaviors of (a) MF@PDAxh/PW PCMs and (b) MF@PDAx mg/PW PCMs. Shape recovery ratio evolutions of (c) MF@PDAxh/PW PCMs and (d) MF@PDAx mg/PW PCMs as a function of time.

Solar-to-Thermal Energy Storage Property of MF@ PDA/PW PCMs. Nowadays, the utilization of sustainable solar energy is attracting great attention.14 The above results clearly demonstrate that the MF@PDA/PW PCMs have a good encapsulation effect and high phase change enthalpy. The PDA coating has an efficient light absorption spanning the ultraviolet, visible, and NIR regions.41 Then the potential applications of the MF@PDA/PW PCMs in solar energy harvesting, conversion, storage, and release processes were comparatively evaluated. As shown in Figure 7a, a xenon lamp with an intensity of 100 mW/cm2 was used as a stimulant light source, and a digital thermocouple was used to record the temperature variation of the samples. Figure 7c and Figure 7d show the temperature−time curves of samples under light or without light irradiation. Compared with MF/PW, the MF@ PDA/PW PCMs exhibit faster rates in increasing the temperature. Besides, at the same irradiation time, the more immersion time and the more DA concentration induce the samples to exhibit higher temperature, which suggests higher efficiency in solar energy harvesting and conversion. A higher amount of PDA will absorb more solar energy in unit time, thus generating more thermal energy and making the temperature of PCMs rise more quickly. Therefore, the

recovery is driven by the NIR irradiation due to its low light absorption. With the addition of PDA coating, the temporary bended shape (“U” shape) completely recovers to the original straight shape under the NIR irradiation, indicating that MF@ PDA/PW PCMs have a good light-actuated shape memory property. The more immersion time (Figure 6a) and the more DA concentration (Figure 6b) result in a faster shape recovery rate. Figure 6c and Figure 6d display the quantitative shape recovery ratio evolutions of MF@PDA/PW PCMs with increasing irradiation time, which clearly indicate that the light-actuated shape recovery rates are significantly influenced by depositing PDA on the surface of MF skeletons. In this case, the PDA coating works as a light absorption and energy conversion medium, which transforms NIR light energy into heat and makes the temperature of MF@PDA/PW PCMs increase. When the temperature is increased above the Tm of PW, the MF@PDA/PW PCMs become flexible, and then the resilience force of MF@PDA foam triggers the shape recovery. Therefore, the MF@PDA/PW PCMs exhibit excellent thermal-actuated and light-actuated shape memory effects, which are ideally suited for the applications with limited space, such as microelectronic devices and batteries. G


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Figure 7. (a) Schematic representation of the light-to-thermal energy conversion, storage, and release measurement system. (b) Schematic representation of the light-to-thermal energy storage and release behavior of MF@PDA/PW PCMs. Temperature−time curves of MF@PDA/PW PCMs based on MF@PDA foams with (c) different immersion times and (d) different DA concentrations.

Figure 8. (a) Infrared thermal images show the temperature variations of the car without a sunshade board. (b) The temperature variations of the car with an automatically deployable sunshade board. The MF@PDA/PW PCM is used as a sunshade board on the roof. The NIR lamp is chosen as the light irradiation source.

temperature−time curves of MF@PDA/PW PCMs under light irradiation are variable due to the difference of PDA content. The solar-to-thermal energy convention efficiency of the MF@ PDA12h/PW PCM is calculated as 80.8%. Specially, a temperature growth plateau at about 48 °C is observed for the MF@PDA/PW PCMs. This plateau is attributed to the solid−liquid phase transition of PW, in which the transferred heat energy is reserved in the form of latent heat. When the

xenon lamp is turned off, the temperature of MF@PDA/PW PCMs shows a freezing plateau between about 48 and 52 °C, since the thermal energy is released through the crystallization of PW. On the contrary, there are no obvious melting and cooling plateaus for MF/PW PCM, indicating that MF/PW has a low light-absorption effect and the phase change does not happen under the light irradiation. Based on the above results, it can be interpreted that the incorporation of PDA coating is H


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Figure 9. (a) The NIR light-actuated shape recovery effect of MF@PDA12h/PW for 1, 3, and 5 cycles. (b) DSC heating and cooling curves of MF@PDA12h/PW during 5 melting-freezing cycles.

five melting-freezing cycles, suggesting the thermal property of MF@PDA12h/PW is quite stable (Figure 9b). The results indicate that the MF@PDA/PW PCMs have good shape memory cyclic property and thermal reliability, which show the advantage of long-term usability.

an effective strategy to improve the solar-to-thermal energy storage properties of MF/PW PCM. The combination of the photothermal MF@PDA foams and the PW is essential for the successful solar-to-thermal energy convention and storage. The mechanism of the solar-tothermal energy storage and release behavior of MF@PDA/PW PCMs is illustrated in Figure 7b. The PDA coating on MF skeleton works as a solar-to-thermal medium and transforms the solar energy into the heat energy.42 Under the light irradiation, once the temperature of MF@PDA/PW PCMs reaches the Tm of PW, the PW crystals melt, and the heat energy is stored. After removing the light source, the crystallization of PW releases the stored heat energy, and the heat energy is diffused across the whole material. Consequently, the temperature of MF@PDA/PW PCMs is kept constant around the Tm of PW for a long time.51 To clarify its application prospect, a light-responsive and temperature-controlled deployable sunshade board is proposed to avoid the overheating of cars under sunlight. As shown in Figure 8a, the maximum temperature of the car (without a sunshade board) increases to 79.6 °C under light irradiation. A MF@PDA/PW sample is placed on the roof of the car, and the temperature variations of the car under the same light irradiation are shown in Figure 8b. On one hand, PDA absorbs light energy and transforms it into heat energy. When the temperature of the board reaches the Tm of PW, the bended shape gradually recovers to the flat shape, and thus the board covers the roof of the car (10 min). On the other hand, the large phase change enthalpy endows the board with energy storage ability, and the board can maintain the roof at a moderate temperature. Finally (at 20 min), the maximum temperature of the car is 51.5 °C, which is lower than that of the car without the sunshade board. In this case, the energy consumption and expense cost can be reduced by using the automatically deployable sunshade board. Shape Memory Reliability and Thermal Stability of MF@PDA/PW PCMs. Reliability and stability are also important for PCMs from the perspective of long service life. The cyclic light-actuated shape memory performance and the thermal stability of MF@PDA12h/PW were studied. As shown in Figure 9a, almost no change of shape recovery effect and speed is observed after five cycles, which ensures for repeated uses of the flexible PCMs. Meanwhile, the phase change temperatures and enthalpies exhibit negligible change during

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CONCLUSIONS In this work, a novel kind of flexible MF@PDA/PW PCM with simultaneous light-actuated shape memory and solar-tothermal energy storage capability has been prepared. Gradient PDA coatings are first adhered on MF, and then PW is incorporated into MF@PDA foam via vacuum impregnation. Herein, PW works as a latent heat storage material, MF serves as a flexible supporting scaffold, and PDA coating acts as a light absorption medium. Due to the high macroporosity and lightweight of MF@PDA foams, a high loading (nearly 95 wt %) of PW can be absorbed into MF@PDA foams, and thus the PCMs show large latent heat (about 140 J g−1) and good encapsulation ability (dimension retention ratio of 90% at 100 °C). Meanwhile, under the resilience force of MF@PDA foams, the PCMs can realize many shape variation behaviors (bend and compression) by triggering the phase transition of PW. More importantly, the PDA coating shows an excellent photothermal effect, which imparts the PCMs with remarkable light-actuated shape memory effect (shape recovery ratio ∼ 100%, shape recovery speed ∼ 100 s) and solar-to-thermal energy storage efficiency (80.8%). The more content of PDA coating, the higher light-actuated shape recovery rates and the solar-to-thermal transfer rates are. This study demonstrates that the flexible and light-responsive MF@PDA/PW PCMs have great potential prospects in various fields, such as energysaving buildings and solar energy storage systems.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b03169. SEM images of PDA, MF, and MF@PDA12h; FTIR spectra of MF and MF@PDA; XRD, FTIR, and DSC patterns of MF@PDA/PW PCMs based on MF@PDA with different DA concentrations; comparison of dimension retention ratios of PW, MF/PW, and MF@ PDA/PW PCMs; thermal heating effect of MF/PW and I


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MF@PDA/PW PCMs; digital photographs showing shape recovery behavior; shape recovery times of MF@PDA/PW PCMs (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.-d.Q.). *E-mail: [email protected] (Y.W.). ORCID

Yong Wang: 0000-0003-0655-7507 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51473137 and 51803172), the Key Research and Development Program of Sichuan Province (Grant Nos. 2017GZ0409 and 2019YFG0241), and the Fundamental Research Funds for the Central Universities of China (Grant Nos. 2682019JQ04, A0920502051820-45, and 2018GF06). SEM characterizations were kindly supported by the Analytic and Testing Center of Southwest Jiaotong University.

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Novel Flexible Phase Change Materials with Mussel-Inspired

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