Concurrent Superhydrophobicity and Thermal Energy Storage of

Research Article pubs.acs.org/journal/ascecg

Concurrent Superhydrophobicity and Thermal Energy Storage of Microcapsule with Superior Thermal Stability and Durability Gang Wu,*,†,‡ Congjin Hu,‡ Junyi Cui,‡ Si-Chong Chen,† and Yu-Zhong Wang*,† †

Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MOE), College of Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, Sichuan University, 29# Wangjiang Road, Chengdu 610064, China ‡ School of Energy Science and Engineering, University of Electronic Science and Technology of China, 2006# Xiyuan Ave., Chengdu 611731, China S Supporting Information *

ABSTRACT: Despite considerable success in design and preparation of superhydrophobic particles, a facile and low-cost approach to develop multifunctional particles, especially microcapsules with the integrated performances of intrinsically long-lasting and highly stable superhydrophobicity and other passive/active functionalities, remains extremely challenging and is still in its infancy. Herein, we report a microcapsule (MC) with a micro/nano-hierarchical shell and a phase change material (PCM) core by a low-cost one-pot method. The resulting microcapsules (MCs) possess concurrent features of superhydrophobicity and thermal energy storage. Against thermal attack up to approximately 240 °C, the microstructure of MCs is nearly intact to avoid an obvious leakage of encapsulated PCM at high temperature, and meanwhile superhydrophobicity of MCs is enhanced unexpectedly to a static contact angle (CA) of 167.4 ± 0.3° and slide angle (SA) of 5 ± 0.5°. After conventional storage of 80 days, MCs still show a good superhydrophobicity with a nearly constant CA and slightly increasing SA. In addition, encapsulated PCM has high enthalpy up to 176 J/g, nearly unchanged Tm, Tom, and Tos, and negligible change (less than 0.1%) of normalized melting and solidified enthalpies over 100 melting/solidification cycles, indicating high latent heat, low effect of shell on thermal diffusion, and excellent durability during phase transition cycling, respectively. An isothermal stage at around 28 and 26 °C being close to human comfort temperature appears separately in heating-up and cooling-down processes of the epoxy matrix with embedded MCs, revealing a good temperature-regulated property of MCs. Accordingly, the MCs as a promising candidate with all-in-one features of superhydrophobicity, temperatureregulated properties, thermal-resistance, and durability would stimulate wide applications in self-cleaning/energy-saving smart buildings and facilities. KEYWORDS: Superhydrophobicity, Microcapsules, Phase change materials, Self-cleaning, Thermal energy storage

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INTRODUCTION Superhydrophobicity, as one of most special wettability properties, has attracted intensive interest for both fundamental research and practical applications in recent years. Generally, materials with superhydrophobicity mainly include three types, i.e., bulks,1,2 films,3−7 and particles.8−10 Among of them, superhydrophobic particles are of great concern due to numerously promising applications in the related fields of energy, environment, and manufacture, such as oil/water separation,11−14 miniature reactor,15,16 self-cleaning,17,18 antifouling,19 and drag reduction properties.20 Therefore, preparing the superhydrophobic particles is highly required and has stimulated active researches. The mimicking of lotus leaf that possesses a low-surface-energy material and specific surfaces with a suitably designed micro/nano-hierarchical structure is deemed as one of the most representative and successful approaches to obtain diversified superhydrophobic materials.18 Although great efforts have been devoted to preparation of © 2017 American Chemical Society

superhydrophobic particles, most of relevant studies reported typically involve harsh treatments (such as templating),21 expensive materials (e.g., perfluorooctanesulfonic acid and nanotubes),10,17 and multistep procedures.9,14,22 In addition, the superhydrophobicity of materials is easy to be compromised by chemical and physical attacks from the environment. Alternatively, some coatings and fabrics with healable superhydrophobicity against decomposition of low surface energy after O 2 plasma treatment have been prepared.23−28 However, the reported methods primarily depend on the migration or rearrangement of expensive lowsurface energy chemicals, which were predeposited/adsorbed on a matrix by using multistep procedures or harsh treatments and essential triggering stimuli (i.e., temperature or moisture) Received: April 19, 2017 Revised: July 18, 2017 Published: August 3, 2017 7759

DOI: 10.1021/acssuschemeng.7b01226 ACS Sustainable Chem. Eng. 2017, 5, 7759−7767

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Formation Mechanism of Superhydrophobic PCM Microcapsules by One-Pot Method

for the processes.29 Thus, endowing materials, especially particles, with intrinsically long-lasting and highly stable superhydrophobicity properties still remains extremely challenging. The microcapsules (MCs) as a kind of special particles being of micrometer size (>1 μm) and having a spherical or irregular shape consist of two parts, i.e., a core containing different active ingredients (e.g., hardener, perfume, drug, biocide, etc.) and various shells (e.g., polymer, metal, or ceramic).30 According to the diversity of the core materials, MCs with a variety of single functions have been separately applied in many fields, typically controlled release,31 self-healing materials,32−36 and thermal energy storage.37−41 However, in the most existing cases, the shell merely affords a permanent or temporary protection for the core from external surroundings, in principle, which restricts the practical applications in more extensive fields. Recently, we have reported a facile, scalable, and low-cost method to prepare MCs with a superhydrophobic shell for the application of self-healing and self-cleaning coatings.42 This superhydrophobic MC provides the possibility of multifunctional and diversified applications by combining superhydrophobic shells with different core materials. Herein, for the first time to our knowledge, we developed PCM polymeric MCs with thermally stable and long-lasting superhydrophobicity by using a technically simple and low-cost one-pot method. The microstructure of this polymeric MC and its superhydrophobicity show a superior thermal stability up to approximately 240 °C. After storage of 80 days in glass vial, nearly constant CA and slightly increasing SA of MCs reveal a good durability of superhydrophobicity. In addition, the encapsulated PCM with MCs had a high enthalpy up to 176 J/g and an nearly unchanged Tm in comparison to pristine PCM. The normalized melting and solidified enthalpies of PCM MCs changed less than 0.1% over 100 melting/ solidification cycles, indicating a quite high stability during phase transition cycling. By using infrared thermography, an excellent and controllable temperature-regulated property of PCM MCs is demonstrated.

situ polymerization in an oil-in-water emulsion. As shown in Scheme 1, on the surface of MCs, the aggregation and deposition of PUF nanoparticles reported in previous work generated hierarchical micro/nano-roughness,43 meanwhile reactive and volatile hexamethylene diisocyanate (HDI) provided low-surface-energy modification, all of which resulted in superhydrophobicity of MCs with a good durability in a natural environment. On the other hand, dense and crosslinked inner shells endowed MCs with superior thermal stability for structure and superhydrophobicity up to 240 °C. Furthermore, the superhydrophobic PCM MCs showed two-inone function, i.e., self-cleaning antifouling and temperature regulation. Accordingly, this MC could serve as a promising candidate in the field of multifunctional advanced composite materials, such as coatings, textiles, etc. As shown in Figure 1, scanning electron microscopy (SEM) images reveal that MCs had uniform spherical shapes, obvious core−shell structures, dense inner shells, and micro/nanohierarchical surface structures similar to a lotus leaf. Diameters of MCs and nanoparticles and shell thickness of MCs obtained

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RESULTS AND DISCUSSION MCs were prepared by encapsulating a commercial PCM, i.e., n-octadecane (n-Oct), as the core into a poly(urea-formaldehyde) (PUF) shell under gentle reaction conditions via in

Figure 1. SEM images of (a) MCs, (b) MC surface, and (c, d) broken MC. 7760


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ACS Sustainable Chemistry & Engineering

maximum weight loss rates at 250 and 377 °C, respectively, while the pure n-Oct had one maximum weight loss rate at 203 °C. Comparing with TGA results of the shell, the first-stage weight loss of MCs rangeing from 200 to 326 °C was attributed to the gradual release of core materials with primary pyrolysis of the shell. For the second-stage weight loss of MCs between 326 and 400 °C, it could reflect the complete consumption of residual core materials in the inner of MCs with a thick shell. Besides, the weight loss rate of encapsulated n-Oct was lower than that of pure n-Oct. These results indicate that the dense and cross-linked PUF shells can prevent the PCM from a quick loss of weight at high temperature. Using a calcination experiment, the good thermal stability of MCs was solidly supported as shown in Figure 3c−h. The calcined MCs were yellow but still free-flowing powders in comparison with the pristine MCs (Figure 3c and d), and the microstructure and size of the calcined MCs characterized by SEM were nearly identical to the pristine MCs (Figure 3e−h and Figure S2). The resulting PCM MCs with micro/nano-hierarchical structures on the surface demonstrated by SEM resembled a lotus leaf, signifying an analogous water wetting behavior. Indeed, spherical droplets of dye-colored water were formed stably on the surface of MCs at room temperature (Figure 4a), and the unwetted status for the surface of MCs after removing droplets shows the Cassie−Baxter state (suspension) (Figure 4b).45 The water static contact angle (CA) and slide angle (SA) of MCs were 165.1 ± 0.8° and 10 ± 0.5° (Figure 4c and d and Figure S3), respectively, further confirming a good superhydrophobicity of MCs and suggesting a potential self-cleaning capacity for dirt (Figure 4j−m and Figure S4). In addition, after conventional storage of 80 days in a glass vial, the superhydrophobicity of MCs was maintained with nearly constant CA and slightly increasing SA (Figure 4e), indicating that the superhydrophobicity of MCs was intrinsically long lasting. Prominently, for the calcined MCs, the water wetting state was the Cassie−Baxter state as well (Figure 4f and g), and CA and SA were 167.4 ± 0.3° and 5 ± 0.5° (Figure 4h and I and Figure S3), respectively, revealing an excellent stability of superhydrophobicity against thermal attack. The enhanced super-

from statistical data of SEM images was 144.3 ± 24.2 μm, 317 ± 58.9 nm, and 266.8 ± 60.7 nm, respectively (Figure S1). Chemical components of MCs were characterized by Fourier transform infrared spectroscopy (FT-IR) (Figure 2). The

Figure 2. FT-IR spectra of pure PCM (n-Oct), shell, and core.

spectrum of the core was completely identical to that of pure nOct, confirming successful encapsulation of PCM. In regard to the spectrum of the shell, characteristic signals of the core were invisible, indicating no residual PCM in the shell; the signals at 3315, 3067, 1628 (overlapped with CO peak), and 1539 cm−1, separately assigned to N−H, phenyl−H, and CO−NH, support the resorcinol-modified PUF molecular structure of shell. Thus, this one-pot approach to preparing superhydrophobic MCs containing PCM is easy, efficient, and economically feasible. To investigate the thermal stability of MCs, thermal gravimetric analysis (TGA) as shown in Figure 3a and b was employed. The initial weight loss temperature (corresponding to −2.5%/min of derivative weight) of MCs was 231 °C and obviously higher than that of pure n-Oct (139 °C) due to evaporation of n-Oct, suggesting the superior thermal stability of MCs.44 The differential thermal gravimetric (DTG) curves showed a two-stage main weight loss for MCs with two

Figure 3. TGA of pure n-Oct, shell, and MCs: (a) weight loss curves and (b) differential thermal gravimetric curves. Photos of (c) pristine MCs and (d) calcined MCs. SEM images of (a) calcined MCs, (b) calcined MC surface, and (c, d) broken calcined MC. 7761


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Figure 4. Photos of dye-colored water droplets on the surface of (a) MCs and (f) calcined MCs. Photos of the surfaces of (b) MCs and (g) calcined MCs after removing dye-colored water droplets. Still images from contact angle (CA) and slide angle (SA) measurements: (c) CA and (d) SA of MCs and (h) CA and (i) SA of calcined MCs. (e) Superhydrophobicity of MCs at different storage times. (j−m) Photos of self-cleaning antifouling experiment of MCs, in which deionized water was used as a liquid and natural dusts were used as dirt.

Figure 5. (a) DSC curves of pristine n-Oct, shell, and MCs. (b) Normalized melting and solidified enthalpies of MCs as a function of the melting/ solidification cycles.

Table 1. Thermal Properties of n-Oct and MCsa Sample

Tos (°C)

Ts (°C)

ΔHs (J/g)

Tom (°C)

Tm (°C)

ΔHm (J/g)

ΔTs (°C)

R (%)

E (%)

η (%)

n-Oct MCs

25.2 24.8

23.2 20.4

217.1 175.0

27.6 27.3

30.5 30.7

217.4 176.0

7.3 10.3

− 81.0

− 80.8

− 99.7

a

Tos and Tom, the onset temperatures of solidification and melting, respectively. Tm and Ts, the peak temperatures of melting and solidification, respectively. ΔHs and ΔHm, the solidified and melting enthalpies, respectively. ΔTs represents the degree of supercooling and can be evaluated by eq 1.

hydrophobicity of MCs after calcination at 240 °C could result from a consumption of hydrophilic groups (e.g., hydroxyl) on the surface of MCs by thermal elimination reaction. On account of absorbing, storing, and releasing latent heat at a constant temperature during a phase transition, e.g., melting/ solidication, PCMs have been attracting great interest for environmentally friendly, renewable, and sustainable energy uses.46−50 However, migration of melted PCMs as a critical issue restricts their practical applications. The encapsulated

PCMs with MCs not only can handle this problem very well, but also possess three advantages, i.e., increasing the heat transfer area of PCMs, enhancing their thermal conductivity performance, and improving their compatibility with the surrounding matrix.51 Indeed, the optical microscope (OM) images of resulting MCs at molten and solidified states of PCM showed that there was no leakage of PCM (Figure S5), revealing a good sealing tightness of the shell of MCs during the phase change. By using differential scanning calorimeter 7762


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Figure 6. Infrared camera images of epoxy matrixes with embedded different amounts of MCs and pure epoxy matrix as control sample: under heating process (a) control, (b) 10 wt %, (c) 20 wt %, and (d) 30 wt % and under cooling process (e) control, (f) 10 wt %, (g) 20 wt %, and (h) 30 wt %.

DSC curves (Table 1). The Tm of MCs was almost identical with pristine n-Oct, while the Ts of MCs decreased by 2.8 °C due to a heterogeneous nucleating effect of shell on the solidification of encapsulated PCM.52 Both MCs and pristine nOct had fairly consistent Tom and Tos, indicating a low barrier effect of the shell for the thermal diffusion. The difference between Tm and Ts for MCs, i.e., the degree of supercooling (ΔTs) revealing a hysteresis during the phase changes of PCM, increased slightly by 3 °C compared to pristine n-Oct, which is

(DSC), the thermal property of MCs was studied in detail. As shown in Figure 5a, comparing with pristine n-Oct, the endothermic and exothermic peaks from the DSC curve of MCs was slightly broad and low, but in the nearly same position. However, no peak can be found for the DSC curve of shell, which laterally supports the cross-linked structure of the shell and indicates that the latent heat of MCs was completely from the encapsulated PCM. The phase-change temperatures and enthalpies of pristine n-Oct and MCs were measured from 7763


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Figure 7. Temperature of control sample (pure epoxy matrix) and epoxy matrix with embedded different amounts of MCs as a function of time under (a) heating and (b) cooling processes.

much lower than the previous reports.53−55 The ΔHm of MCs were around 176 J/g, leading to an encapsulation ratio (R) of nOct as high as 81% versus the ΔHm of pristine n-Oct (217.4 J/ g). The encapsulation efficiency (E) as an indicator to accurately estimate the working efficiency of MCs was calculated to be 80.8%. Besides, the thermal storage capability (η) of MCs was obtained to be 99.7%, showing that nearly all of the encapsulated PCM can efficiently store the latent heat through the phase change. By using DSC (Figure S6), the repetitive melting/solidification cycles of MCs were conducted to prove the sealing tightness and the endurance of the shell. It can be found that the encapsulated PCM with MCs had a quite high stability during the melting/solidification cycles with negligible change (

Concurrent Superhydrophobicity and Thermal Energy Storage of

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