Solar Thermal Storage and Room-Temperature Fast Release Using a

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Solar Thermal Storage and Room-Temperature Fast Release Using a Uniform Flexible Azobenzene-Grafted Polynorborene Film Enhanced by Stretching Linxia Fu,† Jixing Yang,† Liqi Dong,† Huitao Yu,† Qinghai Yan,† Fulai Zhao,† Fei Zhai,† Yunhua Xu,† Yanfeng Dang,§ Wenping Hu,§ Yiyu Feng,*,†,‡ and Wei Feng*,†,‡,∥ School of Materials Science and Engineering, ‡Tianjin Key Laboratory of Composite and Functional Materials, Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, and §School of Science, Tianjin University, Tianjin 300072, P. R. China ∥ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, P. R. China Downloaded by UNIV AUTONOMA DE COAHUILA at 07:34:34:701 on May 24, 2019 from https://pubs.acs.org/doi/10.1021/acs.macromol.9b00384.

†

S Supporting Information *

ABSTRACT: Deformation-controlled solar thermal storage and release are important for thermal management of dynamic systems. However, few researchers have examined cyclic solidstate solar thermal utilization with different deformations. A uniform flexible stretchable solar thermal fuel film is presented using polynorbornene-templated azobenzene (PNB-Azo) with ring-opening metathesis polymerization and covalent grafting. This film has a high degree of isomerization and good storage stability compared to push−pull electronic interaction. At 20% strain rate, the film combines high-degree photocharging (85%), high energy density (49.0 Wh kg−1), and high rate of heat release induced by blue light (475 nm) at room temperature. Greater free volume improves isomerization and the first-order kinetic constant is increased by 1 order of magnitude. Reversible electric-driven dynamic stretching during charging and discharging enables PNB-Azo“fingers” to release heat more rapidly than static stretched film, resulting in a temperature increase of 1.5 °C. The result indicates that PNB-Azofilm can be used as a high-power dynamic solar heat source by controlling the deformation.

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INTRODUCTION Solid-state solar thermal fuels (STF) are attracting increasing attention for aerospace thermal management because of their ability to convert solar light into storable and releasable heat1−4 without phase changes or liquid evaporation. Photochromic polymers5−8 with various photoisomerizable chromophores show great potential in the preparation of self-supporting deformable STF films as their structural flexibility makes them suitable for stretching or bending as well as solution processing. The azobenzene-based polymer (Azo-polymer) is an ideal candidate due to its tunable trans−cis photoisomerization,9,10 which can be optimized by molecular/electronic interaction and through the microstructure. Azo-Polymer films have some problems, including low energy storage density and limited heat release at room temperature, compared to other templated STFs.11−21 This is due to the combined effects of a relatively low amount of Azochromophore (40 °C) limits its

a Heat release was measured by DSC at a heating rate of 10 °C min−1. bInitial temperature for heat release. cPolymers cannot form the film themselves. dThis Azo-polymer film was obtained by electrodeposition. eThe temperature for the half-lives. fThe half-life of the corresponding monomer. gThere are no data in the ref 35.

35 26 31 31 42 42 our paper 52 70 62 60 75 30 25 40 N/A 180−195 226 (heating rate, 2 °C min−1) 258 (heating rate, 5 °C min−1) 105 (heating rate, ≈4.25 °C min−1) 35 (heating rate, ≈2 °C min−1) 202 189 29 24−26 66 43 35 4 49 43 55 ± 1 (25 °C) 75 (25 °C)e 27.8f (25 °C)e 98.4f (25 °C)e 2.5 (110 °C)e 2.5 (90 °C)e 0.25 (25 °C)e 0.33 (25 °C)e

g

polyacrylate azobenzene polymethacrylate azobenzene polydiacetylenes-butyl ester-azobenzene polydiacetylenes-butyl amide-azobenzene polymethacrylate-hexyl ester-azobenzene polymethacrylate-hexyl ester-1,2-bis(2,6-dimeth-oxyphenyl)-diazene PNB-azo-1

Table 2. Polymer-Templated Azo STFs

c

state materials

e

energy density (Wh kg−1) τ1/2 (h)

power density (W kg−1)a

temp. (°C)b

ref

Macromolecules

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Figure 5. Heat-release performance of PNB-Azo film in static state under blue-light irradiation at room temperature. (a) Optical image of PNB-Azo-1 film on the back of the robot; time-evolved IR thermal imaging of (b) the charged PNB-Azo-1 s-film, I-1, (c) the charged PNB-Azo-1 film without stretching, I-2, and (d) uncharged PNB-Azo-1 film without stretching, I-3. (e) ΔTI at different times and (f) its change in 10 cycles. The temperature is obtained by averaging the data 4 times.

Tmax at 85.0 °C (Figure 4e). Similar results were observed at different heating rates (Figures S12 and S13). This indicates that the stretched film is able to release heat at relatively low temperatures. As a result, the energy density is high, up to 49.0 Wh kg−1, which is 12.2% higher than the original film (43.0 Wh kg−1). This result is consistent with high-degree charging and high-rate discharging (Figure 4a,b). The power density of STF film under different conditions (temperature, heating rate, or irradiation) is another important parameter for solar heat utilization. However, high-rate heat release by STF film at room temperature is highly restricted by slow isomerization.26,27,40,41 Generally, increasing the heating rate at high temperatures results in increased power density and decreased energy density. This is because a certain amount of cisAzo does not isomerize to trans-state in a short time due to steric hindrance. Figure 4e shows that stretching reduces the decrease in energy density at high heating rates by favoring the isomerization. Figure 4f illustrates that the energy density of the s-film only decreased by 18.2% between 1 and 10 °C min−1 compared to 23% for the original film. As a result, the PNB-Azo1 s-film had a high power density of 161.0 W kg−1 and a high energy density of 49.0 Wh kg−1. This indicates that the free volume in the film favors high-degree and high-rate isomerization of the Azo for fast heat release. Furthermore, the s-film shows a high-energy (42.7 Wh kg−1) and high-power (63.0 W kg−1) heat release induced by green light (475 nm, 20 W cm−2) irradiation at room temperature. The PNB-Azo-1 film with high energy density outperforms other STF powders (Table 2), which only release heat at a high rate at high temperatures. A flexible and stretchable PNB-Azo-1 film can be developed as a solar heat source based on room-temperature heat release. We, for the first time, examined the heat-release temperature changes of uniform STF films (with transmittance of 48.5% at

Figure 6. (a) Schematic diagram of the dynamic stretching caused by the bending of the robot’s fingers (left) and the motion of θ1 and θ2 along with the bending of fingers (right). (b) Optical image of PNBAzo-1 film placed on four fingers of the robot during the cyclic bending (the image is cut from the Movie S2) and (c) the corresponding IR thermal images.

application as a solar heat source. This problem can be mitigated by stretching the film. As shown in Figure 4d, the stretched PNB-Azo-1 film released heat at a broad temperature range of 25.0−110.0 °C with the maximum peak (Tmax, the temperature when the PNB-Azo film releases half of the storable heat at different rates; Figure 4e) at 69.5 °C compared to the original film, which had a temperature range of 40.0−135.0 °C and a G

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Figure 7. Heat release performance of PNB-Azo film upon dynamic stretching under blue-light irradiation at room temperature. (a) IR thermal images of charged four fingers (two fingers bends reversibly, II-1 and the other stay II-2); (b) IR thermal images of uncharged four fingers (two fingers bend reversibly, II-3 and the other stay, II-4); (c) ΔTII at different times and (d) its change in 10 cycles. The temperature is obtained by averaging the data for 4 times.

550 nm) before and after stretching, to demonstrate the controllability of the heat output at room temperature. This performance was different from previous studies on heat release induced by high temperature, >40 °C.26,27,40,41 The temperature change was tracked using a high-resolution IR thermal imaging camera. As shown in Figures 5a and 6a, two PNB-Azo-1 films were pasted onto an electrically driven robotic hand as the back and finger. We investigated the temperature changes of the PNB-Azo-1 film under two conditions, static and dynamic stretching during charging and discharging. The temperature was obtained by averaging the data for four regions (Tables S2 and S3) and repeating four samples (Figures S14 and S15).

for 15 min to charge them, and then they were induced using blue-light irradiation. For comparison, four uncharged strips under the same conditions (two stretched TII‑3 and two unstretched TII‑4) were also irradiated for discharging. The temperatures of the four strips on the fingers were tracked to illustrate the effect of dynamic stretching on both the charging and discharging processes (Figure 7a,b). Room-temperature blue-light irradiation resulted in different rates of heat release from the charged PNB-Azo-1 films. The temperature difference (ΔTI = TI‑1 − TI‑3 or TI‑2 − TI‑3 and ΔTII = TII‑1 − TII‑3 or TII‑2 − TII‑4) indicates the heat output of the stretched PNB-Azo-1 film. As shown in Figure 5e, for condition I, the two curves show a similar changing trend for ΔTI as well as the irradiation time for discharging. The PNB-Azo-1 film releases heat, which increases ΔTI until the maximum is reached (ΔTI‑max); this indicates that equilibrium is achieved between heat released by the film and the heat dissipated into the environment. Subsequently, the rate of heat release is lower than the rate of heat dissipation; thus, ΔTI gradually decreases. It can be seen that ΔTI increases more rapidly and has a high ΔTI‑max in the stretched film compared to the unstretched film, 1.25 and 1.05 °C, respectively. Furthermore, ΔTI‑max decreases to zero over the next 42 min due to the high rate of heat release (Figure 5b−e) compared to other polymer-templated Azo-based STF (Table 2). The stretched PNB-Azo-1 demonstrated its potential to cyclically utilize solar heat with a ΔTI‑max of 1.10−1.25 °C over 10 cycles (Figure 5f). Similar results were found in condition II using the stretched finger. Unlike condition I, the PNB-Azo-1 strip was charged and discharged during reversible electric-driven stretching. As shown

(1) Condition I with static stretching: the as-prepared PNBAzo-1 film was cut into three pieces (4 cm × 5 cm × 100 μm; Figure 5a) with approximately the same size as the back of the hand. The first piece (with temperature TI‑1) was stretched at a strain of 20% and charged using ultraviolet irradiation at 365 nm for 15 min. The second piece (TI‑2) was irradiated under the same conditions but without stretching. The third piece (TI‑3) was uncharged and unstretched. The films were irradiated with 475 nm blue light at room temperature. We tracked the temperature of the three pieces on the back of the robotic hand to clarify the effect of static stretching on heat release (Figure 5b−d). (2) Condition II with dynamic stretching: the as-prepared PNB-Azo-1 film was cut into four long strips (4 cm × 1 cm × 100 μm; Figure 6) and pasted onto the fingers of the robotic hand. Two fingers (TII‑1) were stretched reversibly, driven by electricity, and the other two (TII‑2) remained stationary. During the dynamic stretching, the strips were irradiated by 365 nm ultraviolet light H

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Macromolecules in Figure 7c, the dynamic stretching achieved a greater temperature change (ΔTII) than the unstretched one, 1.5 °C (ΔTII‑max, the temperature change arrived maximum), compared to 1.1 °C after 6 min. It also had a high rate of increase and discharged faster than the unstretched one (the dynamic stretching achieved a high-rate temperature increase (0.25 °C min−1) and decrease (0.075 °C min−1) in ΔTII, with the unstretched one 0.09 and 0.003 °C min−1, respectively). This rapid temperature change is due to the high rate of heat released by the stretched strip. The dynamic stretching also produces a stable and fast ΔTII‑ max in the range of 1.3−1.5 °C (Figure 7d). By comparing the results in Figure 5e, we found that the dynamic deformed film has great potential as a high-rate heat output to increase the temperature. The results indicate that the STF film under dynamic stretching during charging and discharging achieves a fast and high temperature increase that is induced by high-degree storage and rapid heat release. The heat-release-induced ΔTI and ΔTII indicate that the PNB-Azo-1 STF film can provide controllable temperature variation at room temperature in both the static and dynamic stretching states. The flexible PNB-Azo-1 STF film may be developed as a high-power solar heat source by controlling the stretching deformation.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.F.). *E-mail: [email protected] (Y.F.).

CONCLUSIONS We prepared two uniform flexible deformable films using PNBAzo-1 and PNB-Azo-2 STF to investigate the stretchingenhanced solar thermal utilization. The PNB was prepared using ring-opening metathesis polymerization. Azo-1 and Azo-2 were covalently grafted onto PNB template via esterification with a long flexible spacer reaching peak Mn values of 66 000 and 68 000, respectively. PNB-Azo-1 with a high grafting density of 48% showed a relatively high-degree trans-to-cis isomerization of 72% as well as stable storage due to the intermolecular interaction and steric hindrance. In contrast, PNB-Azo-2 showed a low-degree Di of 19.2% and rapid reversion due to the pull−push electronic interaction. Stretching further increased the degree and rate of isomerization because it increased the free volume, which favors structural transformation. The stretched PNB-Azo film with a strain of 20% combined high-degree photocharging (85%), high energy density (49.0 Wh kg−1), high power density (161.0 W kg−1), and high-rate heat release, which was induced by blue-light (475 nm) irradiation at room temperature. We also demonstrated that dynamic stretching results in a rapid temperature increase and that a high ΔT was induced by the heat release compared to the static deformation. Reversible electric-driven dynamic stretching enabled the PNB-Azo-1 fingers to release heat at a high rate, resulting in a temperature increase of 1.5 °C. This indicates that the PNB-Azo-1STF film can be used to control temperature variation at room temperature in both the static and dynamic stretching states. The flexible stretchable PNB-Azo film may be developed for a high-power dynamic solar heat source by controlling the deformation.

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evolved UV−vis absorption spectra of PNB-Azo-1 solution in DMF (1 mg mL−1) (a) irradiated by UV light (365 nm, 20 mW cm−2) and (b) in the dark after the irradiation; time-evolved UV−vis absorption spectra of PNB-Azo-1 solution (1 mg mL−1) in the dark after UV irradiation, followed by dissolving it in DMF; timeevolved UV−vis absorption spectra of PNB-Azo-2 solution in DMF (1 mg mL−1) (a) irradiated by UV light (365 nm, 20 mW cm−2) and (b) in the dark after the irradiation; time-evolved UV−vis absorption spectra of PNB-Azo-2 solution (1 mg mL−1) in the dark after UV irradiation, followed by dissolving it in DMF; DSC curves of PNB-Azo-1 film at 1, 5, and 10 °C min−1 heating rates; DSC curves of PNB-Azo-1 s-film at 1, 5, and 10 °C min−1 heating rates; chemical element and the corresponding Gd; and heat-release-induced temperature difference of uncharged and charged PNB-Azo-1 film under static and dynamic stretching at different times (PDF) PNB-Azo-1 film placed on four fingers of the robot during the cyclic bending (MP4)

ORCID

Yunhua Xu: 0000-0003-1818-3661 Yanfeng Dang: 0000-0002-9297-9759 Wenping Hu: 0000-0001-5686-2740 Yiyu Feng: 0000-0002-1071-1995 Wei Feng: 0000-0002-5816-7343 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Key R&D Program of China (No. 2016YFA0202302), the National Natural Science Funds for Distinguished Young Scholars (No. 51425306), the State Key Program of National Natural Science Foundation of China (No. 51633007), the National Natural Science Foundation of China (Nos 51573125, 51573147, and 51803151), National Outstanding Youth Talent Program (2019), and Scientific and Technological Commission of China.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00384. Synthetic route of PNB-Azo; GPC, N1 XPS, and TGA curves of PNB, PNB-Azo-1, and PNB-Azo-2; timeI

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DOI: 10.1021/acs.macromol.9b00384 Macromolecules XXXX, XXX, XXX−XXX