Light-to-Thermal Conversion and Thermoregulated Capability of

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Light-to-thermal conversion and thermoregulated capability of coaxial fibers with a combined influence from comblike polymeric phase change material and carbon nanotube Shuqin Li, Haixia Wang, Huiqin Mao, Jing Li, and Haifeng Shi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 30, 2019

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ACS Applied Materials & Interfaces

Light-to-thermal Conversion and Thermoregulated Capability of Coaxial Fibers with a Combined Influence from Comb-like Polymeric Phase Change Material and Carbon Nanotube

Shuqin Li, Haixia Wang, Huiqin Mao, Jing Li, Haifeng Shi* State Key Lab of Separation Membranes and Membrane Processes, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China

* To whom should be corresponded. E-mail: [email protected] (H. Shi)

Abstract: A series of coaxial fibers with polyethylene terephthalate (PET) as sheath and poly(tetradecyl acrylate) (PTA) comb-like polymeric phase change material as core have been prepared via electrospinning technology with carbon nanotube (CNT) dispersed into core component, denoted as PET/PTA-x CNT where x is the mass fraction of CNT. The morphology, structure and thermal performance of coaxial fibers are characterized. Good thermal stability below 300 ℃ is shown due to the sheath-core structure for PET/PTA-x CNT coaxial fibers. Light-to-thermal conversion effect is contributed from the wide UV-vis light absorbance of CNT and phase change of PTA, and PET/PTA-2% CNT reaches to 60 ℃ after 600 s illumination under 100 mW/cm2. Furthermore, a comparable temperature variation is proved for the covered bottle with PET composite membrane containing PET/PTA-2% CNT coaxial fibers, and after 900 s illumination the inner temperature of bottle gets to 38 oC, which is 3 ℃ higher than that of PET-covered one. The investigations of light-to-thermal conversion and thermoregulated ability of fibers guide an approach to thermal management material and greenhouse film application. Key words: Coaxial fiber; Comb-like polymer; Phase change materials; Light-to-thermal conversion; Thermoregulated ability 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1. Introduction As a clean and renewable energy sources, the conversion, storage and application of solar energy has attracted lots of interests because of energy crisis and environmental protection issue1-4. Good thermal energy storage materials require a stable matrix or container, providing a controllable thermal management process such as storage, conversion, and release capability5-6. As one type of thermal energy storage (TES) materials, phase change materials (PCMs) have been widely used in thermo-regulated fiber and fabrics7-8, waste heat recycling9, green buildings10-11 and electrical devices

12.

Therefore, PCMs with stable thermal energy efficiency are

highly desirable for energy exploration and application. To approach the good-stabilized PCMs with controlled structure, different techniques have been used such as microencapsulation13, topological structure14, blends/copolymers, porous adsorption15-16 and coaxial fiber. Clearly, a good thermal energy conversion and storage capability is the prerequisite condition for the long-term duration for PCMs. Different supports such as porous silicon nanopowder (MCM-41)17, active carbon18 and graphene oxide (GO) nanoplates are employed, and the

thermal

performance

of

poly(ethylene

glycol)

alkyl

ether

and

poly(styrene-co-maleic anhydride)-g-octadecanol19 is greatly improved. Note that, after a multiple heating-cooling process, these hybrid PCMs show a thermal energy loss due to the bad interface compatibility. Recently, by grafting C18 alkyl chains onto GO surface, an improved interfacial interaction and compatibility with poly(ethylene-graft-maleic

anhydride)-g-octadecanol

is

shape-stabilized temperature also is increased to 160 oC

realized, 20.

and

the

However, when the

environmental temperature is further increased, the liquid flow phenomenon still is presented, indicating the hybrid PCMs have a limited temperature range. Coaxial fiber materials, as an interesting support, have demonstrated their promising application for shape-stabilization PCMs. Through melt coaxial electrospinning technology, Mccann et al. fabricated phase change nanofibers with long-chain alkanes as core, and a shape-stabilized performance for the encapsulated core is illustrated during the 2 ACS Paragon Plus Environment

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absorbing and releasing process21. Park et al. prepared a good shape-stabilized shell/core nanofibers with PEG as core22, and Chen et al. also demonstrated PEG/cellulose acetate coaxial nanofiber is stable without leakage23. Recently, Lu et al. prepared paraffin-wax loaded sheath-core nanofibers, further indicating that the sheath/core nanostructure is helpful to decrease the liquid leakage and increase the thermal stability24. Besides the good shape stabilization performance of PCMs, light-to-thermal conversion efficiency and thermal storage capability also are very important for TES materials, especially for the solar energy utilization25-29. Yang’s group fabricated a series of composite PCMs with PEG infiltrated into hybrid graphene aerogels (HGA) or GO/boron nitride (BN) hybrid porous scaffolds (HPSs), respectively, and an increased thermal conductivity by 361% or 479% is found30-31. The porous matrix offers light-to-thermal conversion ability from HGA and HPSs, providing an effective way to convert solar energy to thermal energy. PEG/BN composite PCMs with low content graphene nanoplatelets (GNP) also illustrate the enhanced solar energy conversion ability. This is mainly originated from the BN/GNP thermal conductive network and the photon captor role of GNP32. Accordingly, GO aerogels with various oxidation degree are also employed to fabricate the shape-stabilized PCMs with photo-to-thermal energy conversion ability33. Tang et al. prepared a form-stable PCM by introducing single-walled carbon nanotubes (SWNTs) due to its light absorption and solar energy conversion behavior34. In addition, shaped-stabilized PEG/SiO2 composite PCMs have been prepared by incorporating carbon nanotube, titanium black (Ti4O7) and carbon fiber, showing the different light absorbance and light-to-thermal conversion capability35-37. Moreover, Sheng et al. also demonstrated that GO plays both an enhancement in light-thermal conversion ability and the increase in phase change enthalpy for the cross-linked polyurethane-based solid-solid PCMs14. For the bulky porous composite PCMs, a minimum loading amount is required, and the pore diameter also affects the phase change temperature of PCMs. Thus, balancing the thermal storage ability and the shape-stabilized performance is an 3 ACS Paragon Plus Environment


important issue for the porous composite PCMs. At present, some bulky composite PCMs with a light-to-thermal conversion property have been studied, however, to the best of our knowledge, fiber-based composite PCMs have little been reported, especially for a structure-stabilized comb-like polymeric phase change material as energy storage core and carbon nanotube as a light absorber. Herein, in this paper, coaxial fibers with poly(tetradecyl acrylate) (PTA) comb-like polymer as energy core and polyethylene terephthalate (PET) as sheath are prepared via coaxial electrospinning method. Carbon nanotube (CNT) is incorporated into PTA core component for its excellent thermal conductivity and light absorbance. The structure and morphology, phase change property and the thermal performance of coaxial fibers are characterized systematically by a combined characterized method. And, the light-to-thermal conversion capability of bulky coaxial fibers and PET-based composite membrane containing coaxial fibers are compared deeply, and the potential application is also given to guide the exploration of such composite PCMs.

2. Experimental 2.1 Materials Poly(tetradecyl acrylate) (PTA) (Mn=17000) was fabricated in our laboratory. Carbon nanotube (CNT, length 10-30 μm, inner diameter 20-30 nm) was supported by Beijing Boyu Gaoke New Material Technology Co., Ltd. Polyethylene terephthalate (PET) was supplied by Shenzhen Yingbao Fiber Technology Co. Ltd. Chloroform, dichloromethane and toluene were provided by Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd. Trifluoroacetic acid and Fluorescein isothiocyanate (FITC) were purchased from Shanghai Mecklin Biochemical Co., Ltd.

2.2 Electrospinning of PET/PTA-x CNT coaxial fibers Coaxial fibers with PET as sheath, PTA and CNT as core component are fabricated by electrospinning technology. Briefly, 15 wt% PET is dissolved into the mixed trifluoroacetic acid and dichloromethane (mtrifluoroacetic acid : mchloroform =1:2). A varied 4 ACS Paragon Plus Environment

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contentof CNT, x, is dispersed into a 25wt% PTA-chloroform solution via ultrasonication, where x represents the mass fraction of CNT, changing from 0.5, 1 to 2%, respectively. By controlling the feeding ratio of sheath and core solution at 2:1, PET/PTA-x CNT coaxial fibers are electrospun under a constant collecting distance at 15 cm and the voltage at 10 kV. Similarly, PET/PTA coaxial fibers also are prepared with the same process.

2.3 Preparation of composite membrane A homogeneous PET solution (20 wt%) is used and a PET-based composite membrane is prepared as described in the following. First, PET solution is dropped onto a clean glass plate, and then PET/PTA-2% CNT coaxial fibers are pressed into PET membrane substrate before the solvent is fully evaporated. After that, PET-based composite membrane, (PET/PTA-2% CNT)@PET, is completely dried in a vacuum oven. Similarly, (PET/PTA)@PET and PET membrane also are prepared through the same procedure. To standard the analysis process for composite membrane, a size of 4×3 cm is cut, and the weight for three membrane, PET, (PET/PTA)@PET and (PET/PTA-2% CNT)@PET is 0.084 g, 0.093g and 0.093 g, respectively. The membrane thickness is ca. 50 m.

2.4 Characterization The morphology and structure of coaxial fibers are characterized by field-emission scanning electron microscopy (SEM, S4800, Hitachi, Japan) with 10 kV accelerating voltage. Phase change property is characterized by differential scanning calorimetry (DSC, NETZSCH 200 F3, Germany) in the range of -20 to 120 oC at a scanning rate of 10 oC/min, and the second run is recorded. Thermal stability is detected by thermogravimetric analysis (NETZSCH STA 409 PC/PG TG-DTA, Germany) from room temperature to 800 °C at a heating rate of 10 °C/min under nitrogen. An UV-vis spectrophotometer (UV2600, Shimadzu, Japan) is used. Thermal conductivity is

5 ACS Paragon Plus Environment


measured by a hot disk thermal constant analyzer (TPS 2500, Hot Disk Company, Sweden) by a transient plane heat source method.

2.5 Thermal storage capability The experimental apparatus for thermal storage capability has been described in our previous studies19-20. Taking a 40 °C oven as the heating source, a cut 5×4.5 cm PET/PTA-x CNT coaxial fiber with 0.07g weight is folded to compare and analyze the thermal storing behavior. The temperature variation in energy storage process is recorded by a thermocouple (ECNTER-305, ECNTER technology, Taiwan) every 2 s.

2.6 Light-to-thermal conversion test The light-to-thermal conversion experiment is performed under a simulated sunlight realized by a CEL-HXUV300 xenon lamp (CEAULIGHT, China) with an AM 1.5 filter, as illustrated in Fig.1. The light irradiation power is calibrated by a CEL-NP2000 optical power meter (CEAULIGHT, China). A cut 5×4.5 cm coaxial fibers and 4×3 cm composite membrane are tested, and the irradiation distance keeps constant at 30 cm from the light source. The environmental temperature is ca. 21 oC and 35% relative humidity. The air flow between the inner and the outer environment is circulated by a fan. The temperature-variation process of samples is recorded by thermocouple every 2 s.

Fig.1 Schematic diagram of light-to-thermal conversion experiment. 3. Results and discussion 3.1 Morphology and structure of PET/PTA-x CNT coaxial fibers 6 ACS Paragon Plus Environment

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Coaxial electrospinnng fibers are highly influenced by the sheath/core (S/C) ratio besides the other constant conditions such as the collector distance and the voltage. For the electrospinning process, the different ratio of sheath to core is conducted, and PET/PTA coaxial fibers at S/C=2:1 give a smooth surface and the continuous sheath-core structure. So, the following experiments are conducted under this ratio.

Fig.2 SEM picture of PET/PTA-x CNT coaxial fibers (a-PET/PTA-0.5% CNT; b-PET/PTA-1% CNT; c-PET/PTA-2% CNT), cross-section part (d) and confocal image (e) for PET/PTA-2% CNT fibers, and the average fiber diameter (f).

Fig.2a-c illustrates the SEM picture of PET/PTA-x CNT coaxial fibers with x changing from 0.5, 1 to 2%, respectively. The exhibited smooth surface indicates the incorporated CNT into PTA core has no influence on the formation of coaxial fibers. Moreover, the sheath-core structure is demonstrated from Fig.2d and e. The entrapped CNT in PTA core component is clearly seen from the inserted enlargement image as shown in Fig.2d, illustrating the PTA-CNT core is successfully enclosed by PET sheath. Furthermore, the confocal image of PET/PTA-2% CNT shown in Fig.2e proves the formation of sheath-core composite structure, where the red color is originated from the introduced fluorescein isothiocyanate in the core component comprising of PTA and CNT. And the average diameter of PET/PTA-x CNT coaxial fibers is distributed in the range of 1.13-1.23 μm as exhibited in Fig.2f. 3.2 Thermal performance 7 ACS Paragon Plus Environment


To further analyze the phase change character of coaxial fibers, DSC curves of PTA, PET/PTA with S/C=2:1, PET/PTA-x CNT coaxial fibers and the extracted PET/PTA-2% CNT by toluene are shown in Fig.3. Table 1 summarizes the corresponding phase change data such as the onset melting/crystallization temperature (Tmo/Tco),

the

peak

melting/crystallization

temperature

(Tmp/Tcp)

and

the

melting/crystallization enthalpy (ΔHm/ΔHc). PTA exhibits a clear and narrow endothermic and exothermic peak in the heating and cooling process, where Tmp is 28.6 oC (ΔHm=108 J/g) and Tcp is 20.5 oC (ΔHc= -110J/g). For PET/PTA (S/C=2:1) coaxial fibers, an obvious difference against PTA is seen. PET/PTA coaxial fibers present a broad peak in both melting and crystallization process, which is ascribed to the formed sheath-core fibrous structure. ΔHm and ΔHc of coaxial fibers decrease a lot due to the decreased content of PTA inside fiber. Heating

Cooling

a

b c

f

d

e

Endo

Exo

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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e d f

c b a

0

20

o

Temperature/ C

40 -20

0

20

o

40

Temperature/ C

Fig.3 DSC curves of PTA (a), PET/PTA (b), PET/PTA-x CNT (c-0.5%; d-1%; e-2%) and the extracted PET/PTA-2% CNT (f) coaxial fibers.

On the contrary, for PET/PTA-x CNT coaxial fibers, some changes on phase change behavior are shown due to the incorporated CNT. When CNT content is less than 1%, Tcp of PET/PTA-x CNT is higher than that of PET/PTA coaxial fibers. This is possible due to the nucleation role of CNT onto PTA PCMs. However, when CNT content is up to 2%, Tcp shows a decrease, which is attributed to the molecular restriction instead of nucleating role of CNT. As CNT content exceeds the critical concentration, some aggregation and hindrance to PTA from CNT plays a dominant 8 ACS Paragon Plus Environment

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effect 38. Similarly, the enthalpy of PET/PTA-x CNT coaxial fibers also indicates this change. Therefore, the maximum addition of CNT at 2% is considered. In addition, PET/PTA-2% CNT coaxial fibers demonstrate a good stability after 24 h extraction in toluene. A slight change on phase change temperature, ca. 0.1 oC and 2-3% loss on enthalpy are seen. So, this indicates that the sheath/core structure effectively prohibits the leakage of PTA PCMs from the inner fiber.

Table 1 DSC data of PTA, PET/PTA and PET/PTA-x CNT coaxial fibers. Samples

Tmo (℃)

Tmp (℃)

ΔHm (J/g) Tco (℃)

Tcp (℃)

ΔHc (J/g)

PTA

22.3

28.6

108

22.8

20.5

-110

PET/PTA

21.6

28.8

37

23.1

16.2

-40

PET/PTA-0.5% CNT

22.5

30.4

47

23.5

16.7

-50

PET/PTA-1% CNT

22.2

29.0

41

23.1

16.6

-43

PET/PTA-2% CNT

23.8

29.9

38

23.0

15.9

-42

Extracted PET/PTA-2% CNT

23.4

29.8

37

23.2

16.0

-41

100

CNT 100

CNT

80 PET

Mass/%

90

Mass/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

PET/PTA-2%CNT PET/PTA

80 PTA 70

40

60 300

350

PET/PTA

20

o

Temperature/ C

400

PET

PET/PTA-2%CNT PTA

0 100

200

300

400

500

o

Temperature/ C

600

700

800

Fig.4 TG curves of CNT, PET, PTA, PET/PTA and PET/PTA-2% CNT coaxial fibers. Fig.4 compares the thermal stability of CNT, PET, PTA, and PET/PTA and PET/PTA-2% CNT coaxial fibers. For CNT, a very small weight loss before 800 oC appears, which is caused by the decomposition of amorphous carbon and the adsorbed water. Because the initial decomposition temperature of PTA and PET are close to 9 ACS Paragon Plus Environment


each other, 341 ℃ and 382 ℃, respectively, coaxial fibers present one-step thermal degradation process39. Moreover, PET/PTA and PET/PTA-2% CNT demonstrate better thermal stability than pristine PTA based on the protection of PET sheath. In the meantime, the introduced CNT has no influence on the thermal stability of PET/PTA-x CNT coaxial fibers. Such good thermal stability provides a promising high-temperature application as the shape-stabilized PCMs. Thermal storage capacity is an important performance of PCMs. Fig.5a shows a temperature increasing curve for PET/PTA-x CNT during the heating process. All samples with same size and mass (5×4.5 cm, 0.07 g) present a similar temperature tendency regardless of the CNT content. Interestingly, the introduced CNT affects the temperature increasing rate of PET/PTA-x CNT coaxial fibers. The higher the CNT content is, the faster the heating rate. No thermal storage plateau is observed in the heating process, which possible is attributed to the extremely low weight of PTA PCMs spun inside fiber. Similarly, a certain thermal preservation ability also indicated during the cooling process. And, when the sample mass is further increased to 0.45 g, the thermal preservation for PET/PTA-x CNT coaxial fibers is obvious in light of the increased concentration of PTA comb-like PCMs. The relationship between the temperature increasing rate (R), where R defines as the ratio of ΔT and Δt, and the CNT content is illustrated in Fig.5b. The temperature interval from 22.5 ℃ to 37.5 ℃ is assigned to ΔT, and the time in this temperature region is Δt. Obviously, R shows a dependence on the CNT content. The R of PET/PTA-2% CNT is 0.105 ℃/s, which is enhanced by 29.6%, as compared with PET/PTA (S/C=2:1) coaxial fibers (R=0.08 ℃/s). That indicates the CNT in PTA core improves the temperature increasing rate, which is ascribed to the increased thermal conductivity influenced by the introduced CNT. Against the thermal conductivity of PET/PTA (0.029 W/mK), PET/PTA-2% CNT coaxial fibers show an enhancement of 48.3% (0.043 W/mK). Similarly, this enhanced thermal conductivity also reported by Lu et al., whose study demonstrates a 11.5% enhancement on thermal conductivity after loading 5 wt% CNT into the sheath layer 24. Note that, as the loaded contents of 10 ACS Paragon Plus Environment

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CNT are beyond the critical value for the sheath solution, the spinnability of fiber is greatly decreased. The incorporated CNT in the PTA core component demonstrates a good dispersion and spinnability, and thus the mass loss during the solution extraction is avoided. To give a clear indication of CNT on the thermal storage ability, the compared time difference (Δt’, Δt’=t(S/C=2:1)-t(CNT

content))

at a selected temperature interval is

illustrated in Fig.5c. The weight of coaxial fibers is normalized for 1g. Clearly, both CNT content and temperature range have an influence on Δt’. At a certain temperature interval, Δt’ increases as CNT content rising. Meanwhile, Δt’ also goes up as temperature range broadening. When the temperature region extends from 20 ℃ - 25 ℃ to 20 ℃ - 37.5 ℃, Δt’(0.5% CNT), Δt’(1% CNT) and Δt’(2% CNT) increase from 71 s, 100 s and 114 s to 370 s, 528 s and 657 s, respectively. This demonstrates that the introduced CNT obviously can shorten the temperature-increasing time of PET/PTA-x CNT coaxial fibers in heating process. 0.11

(a)

40

800

0.105

(b)

0.100

700

0.10 600

0.093

35

500

R/( C/s)

0.081

400

(c)

Unit Weight 0.5%CNT 1%CNT 2%CNT

t’ = t(S/C=2:1) - t(CNT content)

o

30

t’/s

0.09

o

Temperature/ C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S/C=2:1 0.5%CNT 1%CNT 2%CNT

25 weight: 0.07g size: 5X4.5 cm 100

200

Time/s

300

300

400

0.06 -0.5

200

R=t

0.07

20 0

0.08

o

o

T=37.5 C-22.5 C

100 0

0.0

0.5

1.0

1.5

2.0

2.5

20-25

20-27.5 20-30

CNT content/%

20-32.5

20-35

20-37.5

o

Temperature range/ C

Fig.5 The temperature increasing process (a); The temperature increasing rate (R) (b) and the time difference (Δt’) (c) of PET/PTA-x CNT coaxial fibers.

3.3 Light-to-thermal conversion performance Due to the excellent sunlight-harvesting and light-to-thermal conversion ability of CNT, PET/PTA-x CNT coaxial fibers demonstrate an improved light absorption capacity in the UV-vis region, as shown in Fig.6a. From the UV-vis spectra, the photo absorption ability increases with CNT content, which contributes to a good light-to-thermal conversion behavior for PET/PTA-x CNT. Fig.6b presents the light-to-thermal conversion process of PET/PTA-x CNT coaxial 11 ACS Paragon Plus Environment


fibers under a simulated solar illumination (AM 1.5) at 100 mW/cm2. A fibrous sample with 0.07g weight and 5×4.5 cm size is used. For PET/PTA (S/C=2:1) coaxial fiber, a slow temperature increasing is seen and a maximum temperature at 37.3 oC after 600 s illumination is shown. Interestingly, PET/PTA-x CNT coaxial fibers exhibit a linear temperature increasing process with the content of CNT changing from 0.5 to 2%. The maximum temperature for PET/PTA-0.5% CNT gets to 40.9 ℃, while PET/PTA-1% CNT and PET/PTA-2% CNT increases to 50.1 ℃ and 60.5 ℃, respectively. The incorporated CNT into PTA core effectively improves the light-to-thermal conversion ability, and the temperature for PET/PTA-2% CNT coaxial fibers is increased by 23.2 ℃ as compared with PET/PTA ones. This demonstrates that the incorporated CNT plays an important light harvesting and temperature increasing effect. After the light is off, the temperature decreases rapidly. That means the incorporated CNT does not alter the thermal preservation effect in combination with the PTA core. In fact, this small content of PTA core inside PET sheath contributes a main reason, and the final balanced temperature of cabinet (23 oC), which is higher than that of the freezing temperature of PTA core, also plays an effect. Then, increasing the incorporated amount of PTA core into fiber can realize an effective temperature management. Accordingly, the introduced CNT plays a light-absorber and thermal conversion ability, and then an enhanced light-to-thermal conversion capability for PET/PTA-x CNT coaxial fibers is demonstrated. To further explore the light-to-thermal conversion capability of PET/PTA-x CNT coaxial fibers, the light-to-thermal conversion cycle is conducted, as shown in Fig.6c. PET/PTA-2% CNT coaxial fibers reach to 60.5 ℃ after 600 s illumination, and then decrease as the light turns off. Five cycles are detected for PET/PTA-2% CNT coaxial fibers, confirming a stable light-to-thermal conversion ability. Fig.6d shows the cycled time with temperature reaching to 55.5 ℃ from 24.6 ℃. A time-interval of 95 s is observed, and the cycled time is stable during the 5-time illumination cycle. This further indicates that PET/PTA-x CNT coaxial fibers have a stable light-to-thermal conversion capability. 12 ACS Paragon Plus Environment

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(a)

(b)

CNT

2.0

Light on

65

Light off 60.5oC

Temperature / oC

60

Absorbance

1.5

1.0 PET/PTA

0.5

300

50.1oC

50 PET/PTA-1% CNT

45

37.3oC PET/PTA

25

400

500

600

700

0

800

200

PET/P

(d) 60

CNT TA-2%

55.5 oC

55

Temperature / oC

60

50

40

30

4 20

Time / s

800

1000

PET/PTA-2%CNT

95s

50 45 40 35

25

cle

24.6 oC

20

Cy

2 1

600

600

30

5

3

200 400

400

Time / s

Wavelength / nm

(c)

40.9oC

PET/PTA-0.5% CNT

40

30

PET

200

PET/PTA-2% CNT

55

35

PET/PTA-2%CNT PET/PTA-1%CNT PET/PTA-0.5%CNT

0.0

o Temperature / C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

200

800

400

600

Time / s

800

1000

Fig.6 UV-vis absorption spectra of PET, CNT and PET/PTA-x CNT coaxial fibers (a), Light-to-thermal conversion curves (b), Five-cycle light-to-thermal conversion process (c) and the cycled time with temperature reaching to 55.5 oC from 24.6 oC (d) for PET/PTA-2% CNT coaxial fibers.

Table 2 Compared light-to-thermal conversion performance from the different composite PCMs. Samples rGO/PU GO/GNP aerogel/PEG BN/GNP/PEG PET/PTA-x CNT coaxial fibers CNT/PEG/SiO2 CF/PEG/SiO2 CNT/SA

Illuminated time, s 3300 2100

Tmax, ℃ 72.5 78

ΔT,

100 mW/cm2

1500

72.5

19.5

32

600

60.5

23.2

This work

0.3 W 0.9 W 1.0 W/cm2

4000 2400 1200

55 72.5 60

12 22.5 14

35

Content, wt% 9.1 1.8

Light intensity

1 2 3 5 33.3

℃ 28.5 18

Ref. 14 30

37 40

* Tmax - the maximum temperature after illumination; ΔT - Tmax difference between the sample and the neat one after illumination. 13 ACS Paragon Plus Environment


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Table 2 compares the light-to-thermal conversion performance from the different composite PCMs with various light absorbers. It clearly demonstrates that the incorporated carbon-based nanofillers play an important light-absorber and solar energy conversion ability. Interestingly, the shown maximum temperature (Tmax) after illumination and the temperature difference (ΔT) exhibit a highly dependent upon the incorporated carbon-based nanofillers such as rGO, GO aerogel and CNT, etc. And, the different light absorbers present the varied Tmax and ΔT based on the irradiation time. For the composite matrix, a critical content of nanofillers is required for approaching shape-stabilized PCMs with no liquid leakage during the phase change process. This is important for the organic PCMs, especially for PEG, fatty acid, fatty alcohol and paraffin. Based on these compared results, PET/PTA-2% CNT coaxial fibers show a good advantage over these composites at a shorter irradiation time. And, Tmax and ΔT demonstrate a comparable enhanced result as compared with the others shown in Table 2. Moreover, this present comparison further indicates these carbon-based light absorbers are crucial to realize the solar energy conversion regardless of the existed location. Especially, PET/PTA-2% CNT coaxial fibers have an advantage over the light-to-thermal conversion ability in light of their large surface area and light weight, as compared with the other bulky samples. To further characterize the possible application of PET/PTA-x CNT coaxial fibers, a composite membrane based on PET matrix is fabricated with PET/PTA-2% CNT as the inner substrate. Fig.7a-c shows the cross-section morphology of PET membrane, (PET/PTA)@PET

and

(PET/PTA-2%

CNT)@PET

composite

membrane,

respectively. Pure PET membrane presents a relatively compact structure, while PET-based composite membranes exhibit a good interfacial compatibility because of the same PET matrix with the sheath of coaxial fibers. An intact sheath-core structure for PET/PTA and PET/PTA-2% CNT coaxial fibers retain, as illustrated in Fig.7b and c. This proves that the covered membrane keeps a good shape-stabilization behavior. Thus, this good shape-stabilized structure avoids the potential leakage during the thermal storage/release process. 14 ACS Paragon Plus Environment

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60

55

(d)

55

(PET/PTA)@PET

PET

(PET/PTA-2%CNT)@PET

top

bottom

bottom

40

top top

35

Temperature / oC

43 mW/cm2

50 45

(e)

50

bottom

Temperature / oC

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


82 mW/cm2

100 mW/cm2

45

40

topmax bottommax

35

30

S1: PET membrane S2: (PET/PTA)@PET S3: (PET/PTA@2% CNT)@PET

30 25 on

light

off

on

light off

on

light off

25

20 0

200 400 600 800 0

200 400 600 8000

200 400 600 800

S1

S2

S3

S1

S2

S3

S1

S2

S3

Samples

Time / s

Fig.7 SEM (a-c); Light-to-thermal conversion curves under 100 mW/cm2 (d) and The maximum temperature of the top and the bottom under various light intensities (e) of PET, (PET/PTA)@PET and (PET/PTA-2% CNT)@PET composite membrane.

Fig.7d compares the light-to-thermal conversion behavior for PET membrane, (PET/PTA)@PET and (PET/PTA-2% CNT)@PET composite membrane under the irradiation intensity of 100 mW/cm2 and 600 s illumination. The top surface temperature (Ttop) and the bottom temperature (Tbottom) of membrane are recorded separately. Ttop and Tbottom of PET membrane are 39.3 ℃ and 40.6 ℃, respectively. For (PET/PTA)@PET composite membrane, Ttop and Tbottom are a little higher than that of PET membrane. While for (PET/PTA-2% CNT)@PET composite membrane, Ttop and Tbottom are 46.7 ℃ and 52.1 ℃, respectively, which are much higher than PET membrane and (PET/PTA)@PET. This is attributed to the light absorption and light-to-thermal conversion ability of the incorporated CNT inside fiber. The temperature difference of Ttop and Tbottom between (PET/PTA-2% CNT)@PET and (PET/PTA)@PET is 5.2 ℃ and 9.7 ℃, respectively, which further demonstrates the light-to-thermal conversion capability of CNT in the composite membrane. 15 ACS Paragon Plus Environment


Interestingly, the deposited PET/PTA-2% CNT coaxial fiber could be close to the bottom of membrane, as shown in Fig.7c, and then the bottom temperature is higher than that of the top one. Fig.7e compares the maximum Ttop and Tbottom for the above three membranes under the irradiation intensity at 43 mW/cm2, 82 mW/cm2 and 100 mW/cm2, respectively. With the light intensity increasing, Ttop and Tbottom all increase. And, the temperature of (PET/PTA-2% CNT)@PET is always the highest one among these three samples. This indicates that the light intensity has an influence on the light-to-thermal conversion effect 41. (b)

40

Temperature / oC

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Bottle

35

Bottle+(PET/PTA-2%CNT)@PET

30

Light on

Bottle+PET

off

25 Tair-bottle

20

Tair-PET Tair-(PET/PTA-2CNT)@PET

15 0

200

400

600

800

1000

1200

Time / s

Fig.8 The simulation experiment set-up (a); Temperature variation process for the covered bottle (b); Schematic process for the light-to-thermal conversion of bottle covered by (PET/PTA-2% CNT)@PET composite membrane (c).

It is well known that PET membrane has been widely used as greenhouse film due to its good mechanical property and chemical stability. However, the covered PET membrane hinders the temperature-increasing process of greenhouse and reflects a part of solar radiation. As proved in Fig.6a, (PET/PTA-2% CNT)@PET composite membrane shows a better light absorption ability based on the incorporated CNT. Therefore, a simulation experiment is designed to compare the effect of those two 16 ACS Paragon Plus Environment

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membranes on the greenhouse temperature. Fig.8a illustrates this experimental detail. The light-to-thermal conversion capability for PET and (PET/PTA-2% CNT)@PET composite membrane is compared, and the covered membrane with a size 4×3 cm is used to analyze the solar energy conversion behavior. Under the irradiation intensity 100 mW/cm2 (one sun), the temperature inside the sealed bottle and beneath the covered membrane is recorded. The temperature variation curves of the bottle in different conditions are presented in Fig.8b. The uncovered bottle shows the fastest heating rate and reaches to 40.3 ℃ after 900 s irradiation. However, for the bottle covered by PET membrane, both the heating rate and the ultimate temperature (35 ℃) show a lot decrease. The covered PET membrane blocks the heat transmission and temperature exchanging between the simulated sunlight and the bottle. Interestingly, the (PET/PTA-2% CNT)@PET covered bottle reaches to 38.0 ℃ after 900 s irradiation, showing a 3 ℃ higher than that of PET-covered one. This temperature difference is mainly from the light-to-thermal conversion capability of the incorporated CNT inside the fiber. When the light is off, the temperature of bottle declines quickly. Obviously, the bottle covered by (PET/PTA-2% CNT)@PET composite membrane shows the lowest cooling rate. This phenomenon is originated from the PTA comb-like PCMs inside PET/PTA-2% CNT coaxial fibers, which releases the stored solar thermal energy and postpones the decline for the inner temperature of bottle. Note that, during the light-to-thermal conversion simulation process, the air temperature inside experimental device changes little, and then the influence from the environment is negligible. That is to say, the combined light-to-thermal performance and thermoregulated capacity of (PET/PTA-2% CNT)@PET composite membrane is beneficial to realize the adaptable temperature period of greenhouse, which is favor of the vegetable growth. In view of practical application, the higher loading of PTA comb-like PCMs is suggested, which can effectively contribute the thermal energy storage or release ability in combination with the photothermal conversion materials. In addition, during the light-to-thermal conversion process, the different thermal 17 ACS Paragon Plus Environment


energy stage exists, as shown in Fig.8c. After the solar energy irradiation, a heat conduction process with the covered membrane is firstly shown, and the part of light will be absorbed and converted by the covered membrane. Thus, the incorporated CNT component plays a crucial effect on the light-absorbing efficiency and capability. Subsequently, the converted thermal energy from the solar light will be exchanged with the inner temperature of bottle through the heat convection process. In this process, the thermal energy loss with the environmental exchanging should be decreased to a minimized degree, and thus the inner temperature gets an enhanced one. At the same time, the covered membrane combining the porous fibrous membrane and PET matrix demonstrate a comparable light transparence, and the light energy can penetrate and improve the inner temperature of bottle. In addition, as a greenhouse film application, a smooth and transparent morphology is required. The fiber morphology ensures the compatibility, the good dispersion, the structure and shape stability during the light-to-thermal conversion process. This shows a good advantage over the encapsulated materials that the dispersion is a big challenge. Therefore, for the light-to-thermal conversion application, fiber-based composite energy storage materials are highly desirable both for the light absorbing and conversion, and the temperature regulation during the thermal energy storage and release process.

4. Conclusion Coaxial fibers consisting of PET sheath and PTA core have been successfully prepared via electrospinning method with CNT as photo-absorber inside core component. PET/PTA-x CNT coaxial fibers clearly demonstrate a good thermal stability, 300 ℃, based on the protection of PET sheath. A good energy storage behavior of PET/PTA-x CNT coaxial fibers is proved, which is contributed from the wide UV-vis light absorbance of CNT and phase change of PTA. After 600 s illumination, PET/PTA-2% CNT coaxial fibers reach to 60.5 oC, which is increased by 23.2 ℃ as compared with PET/PTA ones. Moreover, a composite PET membrane 18 ACS Paragon Plus Environment

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containing PET/PTA-2% CNT coaxial fibers gets to 38 oC, which is 3 oC higher than that of PET-covered one after 900 s illumination. A combined light-to-thermal performance and thermoregulated capacity from (PET/PTA-2% CNT)@PET composite membrane demonstrates a future application as temperature-controllable thermal management material and greenhouse film. Conflicts of interest There are no conflicts to declare. Acknowledgements This work was funded by National Natural Science Foundation of China (Grant No. 21875163), National Key R&D Program of China (Grant No. 2017YFB0309100) and Key Project of Tianjin Municipal Natural Science Foundation (Grant No. 16JCZDJC37000). References (1) Kammen, D. M.; Sunter, D. A. City-integrated renewable energy for urban sustainability. Science 2016, 352 (6288), 922-928. (2) Lewis, N. S. Toward Cost-Effective Solar Energy Use. Science 2007, 315 (5813), 798-801. (3) Chang, C.; Yang, C.; Liu, Y. M.; Tao, P.; Song, C. Y.; Shang, W.; Wu, J. B.; Deng, T. Efficient Solar-Thermal Energy Harvest Driven by Interfacial Plasmonic Heating-Assisted Evaporation. ACS Appl. Mater. Interfaces 2016, 8 (35), 23412-23418. (4) Salunkhe, P. B.; Jaya, K. D. Investigations on latent heat storage materials for solar water and space heating applications. J. Energy Storage 2017, 12, 243-260. (5) Chen, L. J.; Zou, R. Q.; Xia, W.; Liu, Z. P.; Shang, Y. Y.; Zhu, J. L.; Wang, Y. X.; Lin, J. H.; Xia, D. G.; Cao, A. Y. Electro- and photodriven phase change composites based on wax-infiltrated carbon nanotube sponges. ACS Nano 2012, 6 (12), 10884-10892. (6) Sarı, A. Thermal energy storage characteristics of bentonite-based composite PCMs with enhanced thermal conductivity as novel thermal storage building materials. Energy Convers.Manage. 2016, 117, 132-141. (7) Chen, C.; Wang, L.; Huang, Y. Electrospun phase change fibers based on polyethylene glycol/cellulose acetate blends. Appl. Energy 2011, 88 (9), 3133-3139. (8) Shi, H. F.; Zhang, X. X.; Wang, X. C.; Niu, J. J. A new photothermal conversion and thermo-regulated fibres. Indian J Fibre Text 2004, 29 (1), 7-11. (9) Kaizawa, A.; Maruoka, N.; Kawai, A.; Kamano, H.; Jozuka, T.; Senda, T.; Akiyama, T. Thermophysical and heat transfer properties of phase change material candidate for waste heat transportation system. Heat Mass Transfer. 2008, 44 (7), 763-769. (10) Memon, S. A. Phase change materials integrated in building walls: A state of the art review.



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Graphical Abstract


Light-to-Thermal Conversion and Thermoregulated Capability of

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