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graphene aerogel as a high thermal stability phase change material
Journal Pre-proof Polyethylene glycol supported by phosphorylated polyvinyl alcohol/graphene aerogel as a high thermal stability phase change material Jia Shen, Ping Zhang, Lixian Song, Jiapeng Li, Bingqiang Ji, Jiajun Li, Lin Chen PII:
S1359-8368(19)34349-5
DOI:
https://doi.org/10.1016/j.compositesb.2019.107545
Reference:
JCOMB 107545
To appear in:
Composites Part B
Received Date: 25 August 2019 Revised Date:
15 October 2019
Accepted Date: 17 October 2019
Please cite this article as: Shen J, Zhang P, Song L, Li J, Ji B, Li J, Chen L, Polyethylene glycol supported by phosphorylated polyvinyl alcohol/graphene aerogel as a high thermal stability phase change material, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107545. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Polyethylene glycol supported by phosphorylated polyvinyl alcohol/graphene aerogel as a high thermal stability phase change material Jia Shen, Ping Zhang∗, Lixian Song, Jiapeng Li, Bingqiang Ji, Jiajun Li, Lin Chen∗
National Engineering Technology Center for Insulation materials & State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and Technology, Mianyang, 621010, PR China
Abstract Polyethylene glycol (PEG) as a phase change material (PCM) is limited in practical applications due to the three major drawbacks of low thermal conductivity, poor thermal stability, and easy leakage. In this study, a new shape-stabilized PPVA/GA/PEG PCM based on Phosphorylated polyvinyl alcohol (PPVA) and graphene aerogels (GA) as a “double-network” support material was obtained using a one-step method, and the three major obstacles of PEG as a PCM were well resolved in this composite material. 15%-PPVA/GA/PEG composite PCMs still exhibit high energy storage capacity while having high thermal stability and high shape-stabilized property. 15%-PPVA/GA/PEG composite PCMs (0.610 W m-1 K-1), with only 1.60 wt% GA, showed an enhanced thermal conductivity than that of PEG (0.493 W m-1 K-1), and it still exhibited an acceptable latent heat of fusion of 119.6 J g-1. Furthermore, the peak heat release rate of 15%-PPVA/GA/PEG composite PCMs ∗ Corresponding author. Fax: +86-816-2419245 E-mail address: [email protected] (Ping Zhang) ∗ Corresponding author. Fax: +86-816-2419245 E-mail address: [email protected] (Lin Chen) 1
decreased by 19.2% compared with PEG. The above experimental results indicate that the prepared PPVA/GA/PEG composite PCMs have application prospects in thermal energy storage field.
Keywords: A. Polyethylene glycol; A. Phase change materials; B. Double-network structure; B. Thermal properties
1. Introduction The rapid consumption of fossil fuels and the continued increase in greenhouse gas emissions have driven the development and effective utilization of various renewable energy [1, 2], and phase change materials (PCMs) have drawn significant attention for the application of thermal energy storage such as electronics thermal management [3, 4], solar energy utilization [5, 6], industrial waste heat recovery [7] and off-peak electricity storage systems [8] because of its high energy storage density, isothermal storing and small temperature variation during the process of phase transition [9]. Polyethylene glycol (PEG) as a kind of PCM has become an important branch of new PCMs in recent years, because PEG has the advantages of molecular designability, suitable phase transition temperature (4-70 oC), high latent heat of phase change, no supercooling and phase separation, non-toxicity, small corrosivity and stable performance [10]. However, as a typical solid-liquid organic PCM, PEG has three major drawbacks in the practical energy storage applications with low thermal conductivity, easy leakage, and low thermal stability [11]. In the past decades, a number of efforts have been performed to overcome the above challenges of PEG in thermal energy storage applications. Porous materials, such as expanded graphite [12, 13], porous SiO2 [14], expanded perlite [15] are often used as support materials for leakage resistance. The research on improving the thermal conductivity of PEG composite PCMs mainly focuses on the addition of materials with high thermal 2
conductivity, such as silver nanowires [16], carbon nanotubes [17], graphene [18-20], etc. And in terms of thermal stability, the main solution is to use flame retardant which could form a physical barrier on the surface of the substrate when heated to prevent heat transfer, thus improving the thermal stability of the composite PCMs [21-23]. For Instance, Yang et al. [24] prepared lightweight cellulose/GNP aerogels as support materials for PEG using graphene nanoplatelets (GNPs) and microcrystalline cellulose, and the composite PCMs exhibit a large latent heat of fusion of 156.1 J g-1 and a high thermal conductivity of 1.35 W m-1 K-1 due to the highly porous and thermal conductivity of cellulose/GNP aerogels. Yang et al. [25] fabricated PEG/HGA composite PCM with only ca. 0.45 wt% GO and ca. 1.8 wt% graphene nanoplatelets (GNP), showed an enhanced improvement of thermal conductivity (1.43 W m-1 K-1) of 361% compared with pure PEG (0.31 W m-1 K-1). Qian et al. [26] prepared PEG/silsesquioxane composite by in situ sol–gel process, using phosphamide containing organosiloxane as precursor. The peak of heat release rate (PHRR) of PEG/silsesquioxane composite was decreased by 38.6% compared with PEG, showing the excellent flame retardant properties. However, PEG PCMs supported by porous materials are prone to leakage after multiple cycles, and it is easy to occur phase separation phenomenon between inorganic thermally conductive filler and organic phase in the long-term use of the process. Moreover, the latent heat value of the composite PCMs will also reduce to some extent due to the presence of the flame retardant. So, for the practical application of PEG as a PCM, solving the three drawbacks of easy leakage, low thermal conductivity and poor thermal stability in one and the same material is of vital importance. Hence, we report a PPVA/GA/PEG composite PCM with high shape-stabilized property and high thermal stability by using graphene aerogel (GA) and phosphorylated polyvinyl alcohol (PPVA) as 3
support materials. GA with a three-dimensional (3D) framework formed by the self-assembly of Graphene oxide (GO) sheets. Polyvinyl alcohol (PVA) is a kind of water-soluble macromolecular polymer with excellent hydrophilicity, good heat and chemical stability[27]. To further improve the thermal stability of PVA, PPVA was synthesized by esterification reaction of PVA and phosphoric acid in the aqueous solution, which could form a second 3D network structure by entanglement of polymer molecular chains and hydrogen bonding (intramolecular hydrogen bond and intermolecular hydrogen bond) [28-30]. At the same time, the hydrogen bonding between PPVA and graphene oxide can also enhance the 3D framework of GA[31]. The “double-network” structure composed of GA and PPVA can be used as an encapsulation structure to prevent leakage of PEG. At the same time, GA with high thermal conductivity can provide a thermal conduction channel and improve the thermal conductivity of composite PCMs. PPVA with high thermal stability can improve the thermal stability of the composite PCMs. The 15%-PPVA/GA/PEG composite PCMs with only 1.60 wt% GA, showed an enhanced thermal conductivity of 0.610 W m-1 K-1 from 0.493 W m-1 K-1 of pure PEG. Furthermore, the peak of heat release rate of 15%-PPVA/GA/PEG composite PCMs decreased by 19.2% compared with PEG. Moreover, 15%-PPVA/GA/PEG composite PCMs still exhibited an acceptable latent heat of fusion of 119.6 J g-1.
2. Experimental 2.1. Materials Polyethylene glycol (PEG, Mn=6000), Polyvinyl alcohol (PVA, the average degree of polymerization is 1788±50), Urea (H2NCONH2, 99.0%) and Phosphoric acid (H3PO4, 85.0%) was purchased from Chengdu Kelong Chemical Reagent 4
Company, China. And PEG was dried under the vacuum before using (100 oC, 3 h). Natural flake graphite (99.99 wt% purity) was purchased from Yingshida Graphite Co. Ltd., (Qingdao, China), and the average particle diameter is 20 µm. Concentrated sulfuric acid (H2SO4, 95%-98%), Hydrogen peroxide (H2O2, 30.0%), Potassium permanganate (KMnO4, 99.5%), Sodium nitrate (NaNO3) and Potassium Phosphate Monobasic (KH2PO4, 99.0%) were purchased from Chengdu Kelong Chemical Reagent Company, China. L-Ascorbic acid (Vitamin C, C6H8O6, 97.0%) was provided by Tianjin Fuchen Chemical Reagents Factory, China. Ammonium metavanadate (NH4VO3) was purchased from Shanghai Yien Chemical Technology Co., Ltd., China. Ammonium molybdate ((NH4)6Mo7O24·4H2O) was provided by Tianjin Kemiou Chemical Reagent Co., Ltd., China.
2.2. Synthesis of PPVA Phosphorylated
polyvinyl
alcohol
(PPVA)
was
prepared
through
the
esterification reaction in water media, in which urea acted as a catalyst for dehydration and phosphoric acid acted as an esterification agent. And the synthetic route of PPVA was presented in Scheme 1. First, PVA (10 g), deionized water (50 mL), phosphoric acid (50 mL) and urea (0.4 g) were placed in a 250 mL three-necked flask for 24 h at room temperature. Then magnetic stirring reaction for 2.5 h at 96 oC, and the product was cooled down to room temperature after the reaction was finished. Finally, cut the product up and washed it repeatedly with anhydrous ethanol until the pH of the washing liquid reached 5-6, and PPVA was obtained after dried at 60 oC for 12 h.
2.3. Synthesis of GA and GA/PEG composites GO was prepared by a modified Hummers method [32-34], and the GA/PEG composite was prepared through the sol-gel method [35]. First, mixed 0.16 g GO and 5
80 mL deionized water, then treatment under ultrasound conditions for 0.5 h. Then hydrothermal reaction for 4 h at 95 oC after 0.16 g Vitamin C and 8 g PEG were added. Finally, the GA/PEG composite was obtained after drying in a 60 oC vacuum oven for 12 h. GA was synthesized by the same method without adding PEG, and obtained by freeze-drying.
2.4. Synthesis of PVA/GA/PEG and PPVA/GA/PEG composites The PVA/GA/PEG and PPVA/GA/PEG composites were prepared through the sol-gel method. First, mixed 0.16 g GO and 80 mL deionized water, then suffered from ultrasonic treatment for half an hour. Then 0.16 g Vitamin C and 8 g PEG were added, at the same time, add 0.44 g, 0.92 g and 1.47 g PVA or PPVA respectively to the GO suspension (The mass fraction of PVA or PPVA accounted for 5%, 10% and 15%, respectively, and the addition amount of PVA and PPVA contents were calculated on the basis of the composite PCMs). Then hydrothermal reaction for 4 h at 95 oC. Finally, the PVA/GA/PEG and PPVA/GA/PEG composite PCMs were obtained after drying in a 60 oC vacuum oven for 12 h.
2.5.
Determination
of
the
phosphorus
content
in
PPVA
by
spectrophotometry Standard curve of phosphorus: Accurately add 0.10, 0.20, 0.40, 0.60 and 0.8 mL standard phosphorus solutions in 5 volumetric flasks (50 mL) with corresponding phosphorus contents of 0.10, 0.20, 0.40, 0.60 and 0.8 mg, respectively. In addition, a 50 ml volumetric flask was added to deionized water as a control group. Then, 2 ml of 29% nitric acid (HNO3) solution, 2 ml of 0.25% Ammonium metavanadate solution (NH₄ VO₄ ) and 2 ml of 5% ammonium molybdate ((NH4)6Mo7O24·4H2O) solution were sequentially added to each volumetric flask. Finally add deionized water to constant volume to 50 mL. The above samples were measured on an 6
Ultraviolet-visible
spectrophotometer
(Ultraviolet-visible
Spectrophotometer,
UV-2600, Shimadzu Instruments (Suzhou) Co., Ltd.) with a wavelength of 400 nm. The standard curve could be used to calculate the phosphorus content of PPVA. Digestion of PPVA: First, 0.5 g (m0) of PPVA was added to a 50 mL (V0) conical flask, and then 10 mL of nitric acid was added to digest until the sample became black, and then concentrated with nitric acid to further digest until the solution became clear. Then the entire solution was transferred to a 50 mL volumetric flask and diluted with deionized
water to constant volume. Accurately add 5 mL (V1) of the above solution
to a 50 mL volumetric flask, and adjust the pH to neutral with 0.5 M NaOH solution. Then, 2 ml of 29% HNO3 solution, 2 ml of 0.25% NH₄ VO₄ and 2 ml of 5% (NH4)6Mo7O24·4H2O solution were sequentially added to volumetric flask. Finally add deionized water to constant volume to 50 mL. The above sample was measured on an Ultraviolet-visible spectrophotometer with a wavelength of 400 nm, and the phosphorus content of PPVA could be calculated as: P(%)=
P(mg) ×V0 ×100 (1) m0 ×1000×V1
2.6. Characterization The structure of PVA and PPVA were characterized by a Fourier Transform infrared spectra (FT-IR, Spectrum one, PerkinElmer Co., USA). The phosphorus content
of
PPVA
is
measured
by
spectrophotometry
(Ultraviolet-visible
Spectrophotometer, UV-2600, Shimadzu Instruments (Suzhou) Co., Ltd.). The microstructure of the composites was observed by Scanning Electron Microscopy (SEM, UItra55, Carl Zeiss NTS GmbH, Germany). The X-ray diffraction (XRD) patterns of the materials were taken on a PANalytical X’Pert PRO X-ray diffractometer equipped with a copper anode (Cu Kα radiation, λ = 1.54187 Å). The 7
Raman spectrum tests were performed at room temperature using Raman Spectroscopy (in Via, Renishaw Co., U.K.). The size and thickness of GO was determined by Atomic Force Microscope (AFM, NSK Ltd. Japan). Differential scanning calorimeter (DSC Q200, TA Instruments, USA) analysis technique was used to investigate the phase change properties of materials. The DSC analysis was carried out in a N2 atmosphere with the weight of the sample being about 5 mg, and the heating or cooling rate was 10 oC min-1 from 20 to 100 oC. The thermal properties of the composites were analyzed by a Thermogravimetric analyzer (TG, Q500, TA Instruments, USA), and the sample was heated from 30 to 600 oC in a N2 atmosphere. The heating rate was set as 20 oC min-1. The thermal conductivity of the composite PCMs was analyzed with a thermal conductivity meter by a transient plane heat source method, three times measurements of thermal conductivity were performed for each sample. An infrared thermal camera (NEC Co., Ltd., USA) was used to record the heat transfer and temperature distribution of composite PCMs on a hot plate with a constant temperature of 80 oC. The flammability characteristics of composite PCMs were determined by Microscale combustion calorimetry (Govmark MCC-2) according to ASTM D7309-07. Thermogravimetric analysis/infrared spectrometry (TG-IR) of the material was performed using a DT-50 (Setaram, France) instrument which was interfaced to an IRAffinity-1 FT-IR spectrometer. About 10 mg sample was put in an alumina crucible and heated from 30 to 800 oC in a N2 atmosphere (flow rate of 60 mL min-1), with a heating rate of 20 oC min-1.
3. Results and discussion 3.1. Structural properties of GO, GA, PVA, PPVA, GA/PEG, PVA/GA/PEG, and PPVA/ GA/PEG composite PCMs 8
Phosphorus content of PPVA is measured by spectrophotometry, and the principle is to convert the phosphorus in the phosphate ester to the orthophosphate ion by oxidation, and then determine by colorimetry[36]. The standard curve of P content is shown in Fig. 1. The linear relationship between absorbance and phosphorus content is very obvious (R2=0.9990), so it can be used as a standard curve for calculating the phosphorus content of the PPVA. The absorbance of the sample was 0.74 through UV-visible absorption spectroscopy testing. According to the regression equation y=2.4575x+0.02825, P(mg)=0.2896 mg. And the phosphorus content in PPVA can be determined as 0.58% according to formula (1). The XRD pattern of natural graphite, graphene oxide (GO) and graphene aerogel (GA) are shown in Fig. 2a. It can be seen that a characteristic diffraction peak with a large intensity and a narrow and sharp peak shape appearing at 2θ=26.5° corresponds to a typical (002) crystal plane of graphite, indicating that the natural graphite has good crystallinity and ordered layered structure [37]. After Natural graphite is oxidized by Hummers method, the typical (002) crystal plane is transferred from 2θ=26.5° to 2θ=8.5°, indicating successful synthesis of GO. And the reason of the 2θ value of the (001) plane was only 8.5° could be the high degree of oxidation and the presence of water molecules between layers of GO [38]. Atomic force microscopy (AFM), as a direct tool, was applied to investigate the microstructure and thickness of the GO [39]. As shown in Fig. 2c, the GO is comprised of thin nanosheets with a lateral size of 0.5–5.0 µm and a thickness of about 1.1 nm. Raman spectroscopy is one of the important technical means used to characterize carbon materials [40, 41]. In the Raman spectroscopic analysis of graphene materials, the intensity ratio of the D band (structural defects of graphene) and G band (In-plane vibrations of sp2 carbon atoms) is often used to measure the regularity of the graphene structure. The intensity ratio of 9
the D band and G band (ID/IG) is smaller, the degree of graphitization is higher, and the graphene structure is more regular. L-Ascorbic acid, which is also known as Vitamin C, is a natural antioxidant in the cells. It is often used as a environment-friendly reducing agent of graphene oxide due to its mild reducibility and non-toxicity [42]. The Raman spectra of GO and GA are shown in Fig. 2b, and the ID/IG of GO and GA were 1.16 and 0.92, respectively, indicating that GO is well reduced by Vitamin C. The FT-IR spectroscopy is an effective way to detect whether PPVA is successfully grafted. The test results of PVA and PPVA were shown in Fig. 3. Two new peaks were observed at 1189 cm-1 and 991 cm-1 in spectrum of PPVA than that of PVA. The stretching peak of P-O-C was located at 1055-950 cm-1. Esters C-O stretching vibration absorption peak was located at 1300-1100 cm-1. With the C-O connected to the different groups, the absorption peak position would change. Therefore, the absorption peak near the 1189 cm-1 is the coincidence of the absorption peak of the C-O stretching vibration and the P-O-C absorption peak. So these results show that the P-O-C bond does exist in PPVA, i.e., PPVA was synthesized successfully. The microscopic structures of the GA and the GA/PEG composite PCMs were revealed by Scanning Electron Microscopy (SEM) (Fig. 4a, b). The numerous of folds and pores of GA provided enough void space for the permeation of PEG (Fig. 4a), and the SEM image revealed that the PEG infiltrated and completely occupied the void space of GA (Fig. 4b). The SEM image of 15%-PPVA/GA/PEG composite PCMs (Fig. 4d) revealed that a network structure could be seen more clearly compared with that of 15%-PVA/GA/PEG composite PCMs (Fig. 4c), and these network pores adhered to a large number of PEG molecules, and no obvious interface 10
was observed between the GA and PEG, which indicated that GA and PEG hold good compatibility and PEG could be absorbed and bounded by the void space of the “double-network” structure. The SEM images of char layer, which were gained by 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs after calcined in a microwave muffle furnace at 550 oC (Fig. 4e, f), revealed that the char layer of 15%-PPVA/GA/PEG composite PCMs was denser than that of 15%-PVA/GA/PEG composite PCMs. This is because the PPVA can form a denser char layer on the surface material when the 15%-PPVA/GA/PEG composite PCMs was pyrolysed, which can protect the matrix more efficient than that of 15%-PVA/GA/PEG composite PCMs.
3.2. Thermal energy storage Phase change temperature and enthalpy of PCMs were measured by DSC. The DSC curves of PEG, PVA/GA/PEG and PPVA/GA/PEG composite PCMs were shown in Fig. 5, and the corresponding data were presented in Table 1. The thermal properties of the samples include melting temperature (Tm), Phase transition enthalpy of melting (∆Hm), crystallizing temperature (Tc) and phase transition enthalpy of crystallization (∆Hc). As shown in Fig. 4 and Table 1, the phase transition enthalpy of pure PEG the were 177.3 J g-1 (∆Hm) and 173.8 J g-1 ( ∆Hc), respectively. When the mass fraction of PPVA was 5%, the ∆Hm and ∆Hc of PPVA/GA/PEG composite PCMs were 145.8 J g-1, 144.1 J g-1, respectively. When the mass fraction of PPVA is gradually increased (10% and 15%), the ∆Hm were 133.4 J g-1, 119.6 J g-1 and the ∆Hc were 133.3 J g-1, 118.2 J g-1, respectively. That is to say, with the increase of the mass ratio of PPVA, the phase transition enthalpy of PPVA/GA/PEG composite PCMs showed a downward trend, and the PVA/GA/PEG composite PCMs showed the same rules. This is because PVA or PPVA forms a network structure due to the 11
entanglement of the molecular chain and hydrogen bonding , when the phase transition of PEG occurs, this network structure, coupled with the 3D network of GA, will directly hinder the chain movement of PEG molecules, thus the processes of melting or crystallization of polyethylene glycol are influenced. Furthermore, PPVA formed a denser network structure than that of PVA because of the more complex side groups of PPVA, so that the PPVA/GA/PEG composite PCMs have a greater loss in phase change enthalpy. Moreover, it can be seen from Table 1 that PVA/GA/PEG and PPVA/GA/PEG composite PCMs show a lower melting temperature and higher crystallizing temperature than PEG, indicating that GA provides an excellent thermal path for composite PCMs, while the effect of different mass fraction of PVA and PPVA on the phase transition temperature of the composites is almost negligible. Importantly, after 20 and 50 melting-crystallizing cycles, the DSC curves of 15%-PPVA/GA/PEG composite PCMs remain almost unchanged compared to the initial (Fig. 8b), indicating that the composite PCMs have excellent thermal reversibility.
3.3. Shape-stabilized property As the solid-liquid organic phase change material is prone to liquid leakage in the phase transition, which affects the practical application, so the shape-stabilized property becomes an important parameter of PCMs. The samples were pressed into a sheet with a uniform size to perform the shape-stabilized test by a flat vulcanizer (LP-S-50), and the testing results were shown in Fig. 6. First, pure PEG material and GA/PEG composite PCMs were placed on the plate at 70 oC. After 10 minutes, pure PEG began to melt and completely melted into liquid after 20 minutes, but GA/PEG composite PCMs did not leak liquid after 40 minutes. This is due to the PEG molecules entered into the network pores of the GA, which makes PEG to flow out 12
difficultly. When GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs were placed on the plate at 110 oC, 130 oC, 150 oC, respectively, the results showed that liquid leakage of the three materials did not occur at 110 oC and 130 oC for 30 min. When these three composites were removed from their original position at 150
o
C for 30 min (Fig. 6a, b), It could be found that GA/PEG and
15%-PVA/GA/PEG composite PCMs exhibited varying degrees of liquid leakage, and 15%-PPVA/GA/PEG composite PCMs was almost no leakage. These results indicate that when the PVA molecules enter the GA/PEG composites, the binding capacity of PEG molecules to a “double-network”
structure consisting of PPVA and
GA is more attractive than that of the "single-network" structure of the GA, so it is more difficult to leak out. Meanwhile, the oxygen containing functional groups on GA, PVA and PPVA can form hydrogen bonds with PEG, further preventing the leakage. Moreover, the PPVA molecular can form a more complex network structure, so the anti-leakage ability of PPVA/GA/PEG composite PCMs is stronger than that of PVA/GA/PEG composite PCMs. Therefore, the addition of PPVA can greatly improve the anti-leaking ability of PEG composite phase change materials, and the PPVA shows more efficiency than that of PVA.
3.4. Thermal conductivity The thermal conductivities of pure PEG, GA/PEG, PVA/GA/PEG and PPVA/GA/PEG composite PCMs were measured by Hot Disk Thermal conductivity analyzer using the transient plane heat source method, and the thermal conductivity results were collected in Table 2. The results showed that the thermal conductivity of pure PEG, PVA and PPVA were 0.493, 0.152 and 0.112 W m-1 K-1, respectively, and the thermal conductivity of GA/PEG (0.687 W m-1 K-1), 15%-PVA/GA/PEG (0.667 W m-1 K-1) and 15%-PPVA/GA/PEG (0.610 W m-1 K-1) composite PCMs could 13
increase 39.4%, 35.3% and 23.7%, respectively, while the mass fraction of GA was only 1.6 wt%. Compared with pure PEG, the thermal conductivity of the composite PCMs have improved. This is due to the fact that the GA provides a thermal conduction channel for PEG phase change heat transfer. Meanwhile, as the heat transfer of organic molecules (such as PEG, PVA and PPVA) mainly rely on chain transfer, with poor heat transfer rate, and the heat transfer rate is low, so PVA and PPVA will form a thermal resistance between the GA and the PEG molecules, leading a
certain
decrease
in
thermal
conductivity
of
15%-PVA/GA/PEG
and
15%-PPVA/GA/PEG composite PCMs. An infrared thermal imager was used to record the temperature response during the heating process, and the temperature distribution images of pure PEG and PPVA/GA/PEG composite PCMs at 0, 60 and 100 s were shown in Fig. 8a. It can be clearly seen that the temperature increase was much faster in 15%-PPVA/GA/PEG composite PCMs, further indicating its higher thermal conductivity and rate of thermal diffusion than those of pure PEG.
3.5. Thermal stability Thermogravimetric analysis is a convenient method to determine the thermal stability of materials, and thermal stability is one of the most significant parameters for PCMs applications. The thermal analysis curves of PVA and PPVA are shown in Fig. 7a. At 280 oC, the weight loss rate of PVA was 8.0%, while the weight loss rate of PPVA was 40%. This is because the phosphorus in PPVA will catalyze the PPVA into carbon, so that PPVA has a greater weight loss rate. When PVA and PPVA were completely pyrolyzed, the weight loss of PVA was 97.0%, but the weight loss of PPVA was 80.0%. This is because PPVA can make a more efficient char layer on the surface of composite PCMs. The thermogravimetric curves of PEG, GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs were shown in Fig. 14
6b. The weight loss of PEG, GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs were 98.0%, 97.0%, 93.5% and 91.5% at 550 oC, respectively, in other words, the corresponding char layer content were 2.0 wt%, 3.0 wt%, 6.5 wt% and 8.5 wt%, respectively. In the GA/PEG composite PCMs system, GA with three-dimensional porous network can not only act as a support material for PEG but also as a mass-transport barrier, thus improving the thermal stability of the composite PCMs. And it can be seen from the curves that the thermal stability of 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs improve significantly compared with GA/PEG composite PCMs. It is attributed to the fact that PPVA can promote the carbon formation of the PEG molecular chain in the composite PCMs due to its high thermal stability (As shown in Fig. 7a) , and the char layer can serve as a thermal insulation layer and a barrier between oxygen and polymeric decomposition gases [43]. Thereby, the synergistic effect between char layer and GA could further enhance the thermal stability of the PPVA/GA/PEG composite PCMs. The property of char layer is one of the important parameters for the thermal stability of the material. The GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composites were calcined at 550 oC and the char layer was characterized by Raman spectroscopy
(Fig.
7).
The
ID/IG
of
GA/PEG,
15%-PVA/GA/PEG
and
15%-PPVA/GA/PEG composite PCMs were 0.97, 0.98 and 1.16, respectively. These results indicate that PPVA could hinder the orderly carbon formation of PEG molecules with the increase of the calcining temperature, because the molecular chains of PPVA dispersed in that of PEG, and PPVA showed higher thermal stabilities than that of PEG, when PEG was pyrolyzed, the molecular chains of PPVA could hinder the orderly carbon formation of PEG, resulting in an increase in the ID/IG value of the char layer of the 15%-PPVA/GA/PEG composite PCMs. In all, PPVA, 15
which holds more thermal stabilities than that of pure PEG, can promote the carbonization of the PEG molecular chain on the surface of the 15%-PPVA/GA/PEG composite PCMs to form a disordered and dense char layer as a protective layer of the system. And it is consistent with the results of the SEM characterization (Fig. 4e, f). Microscale combustion calorimeter is further used to investigate the flame retardant of PPVA/GA/PEG composite PCMs, and the heat release rate curves of PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs were shown in Fig. 7d. The results showed that the peak heat release rate (HRR) of PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs were 689.3 W g-1, 578.6 W g-1 and 557.5 W g-1, respectively. So the HRR of 15%-PPVA/GA/PEG composite PCMs decrease 19.2% compared to pure PEG. The improvement of flame retardant properties of 15%-PPVA/GA/PEG composite PCMs is attributed to that PPVA can promote the carbonization of the PEG molecular chain on the surface of the composite PCMs to form a disordered and dense char layer as a mass-transport barrier, thus blocking the heat transfer during combustion, and finally achieving the flame retardant effect. TG-IR technique was employed to investigate the volatile products in the thermal degradation process of PEG [44-47] and 15%-PPVA/GA/PEG composite PCMs, and the three-dimensional (3D) TG-IR spectrum of pyrolysis gas products of PEG and 15%-PPVA/GA/PEG composite PCMs were shown in Fig. 9a, b. In order to investigate the influence of support material (GA and PPVA) on the thermal degradation of PEG, the FT-IR spectra of the volatile products of pure PEG and 15%-PPVA/GA/PEG composite PCMs evolved from 250 oC to 380 oC were extracted, as shown in Fig. 8c and d, respectively. It could be seen from Fig. 9c that the main decomposition products of PEG were combustible hydrocarbon gas (about 2870 cm-1), 16
aldehydes (about 1744 cm-1) and ether linkage (about 1130 cm-1). For pure PEG, the detection of combustible hydrocarbon gas (about 2870 cm-1) at 250 °C indicates that the initial decomposition of the PEG molecular chain begins at 250 °C and the relative intensity of the main decomposition products peaks at 340 °C reaches the maximum. However, compared with PEG, there is a certain delay in the thermal pyrolysis of 15%-PPVA/GA/PEG composite PCMs. The chemical structural changes during thermal degradation take place at 280 °C as shown in Fig. 9d, and the relative intensity of the flammable gas peak reached the maximum at 380 °C. The enhanced thermal stability of the 15%-PPVA/GA/PEG composite PCMs is attributed to the fact that PPVA can promote the carbon formation of the PEG molecular chain in the composite PCMs, and the char layer can serve as a thermal insulation layer and a barrier between oxygen and polymeric decomposition gases. In summary, the support material and the char layer synergistically improve the thermal stability properties of the 15%-PPVA/GA/PEG composite PCMs.
4. Conclusions In this work, the target composite PCMs with high thermal stability were synthesized by one-step method. In the composite PCMs system, a “double-network” structure was constructed as a support material by PPVA and GA, at the same time, PEG as a PCM was integrated into the “double-network” structure by the capillary force. The “double-network” structure can be used as an encapsulation structure to prevent leakage of PEG. Moreover, GA and PPVA can improve the thermal conductivity
and
thermal
stability
of
the
composite,
respectively.
15%-PPVA/GA/PEG composite PCMs with excellent shape-stabilized property could maintain at 150 oC for 30 minutes. The thermal conductivity of 15%-PPVA/GA/PEG composite PCMs could increase 23.7% compared with that of pure PEG, while the 17
mass fraction of GA was only 1.60 wt%. The 15%-PPVA/GA/PEG composite PCMs maintained outstanding thermal reliability after 50 thermal cycles. In addition, 15%-PPVA/GA/PEG composite PCMs still exhibited an acceptable latent heat of fusion of 119.6 J g-1. Thus the PPVA/GA/PEG composite PCMs with high thermal stability, high thermal conductivity and high anti-leakage ability were prepared, and the three major drawbacks of PEG as a PCM were solved in one and the same material. In all, 15%-PPVA/GA/PEG composite PCMs are highly promising for practical application for energy storage due to these excellent properties.
Acknowledgements We acknowledge support from the National Natural Science Foundation of China (No.51503173), National Engineering Technology Center for Insulation Materials, Southwest University of Science and Technology (No.16kfjc09), Longshan academic talent research supporting program of SWUST (17LZX636, 18LZX629) and
Graphene
Engineering
Technology
Research
Center
of
Sichuan
(2018SCGCZX05).
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Figures Figure captions Scheme 1. Synthetic route of PPVA. Fig. 1. Standard curve of P content. Fig. 2. (a) XRD patterns of Natural graphite, GO and GA; (b) Raman spectra of GO and GA (The inset of (b) shows the ID/IG data for GO and GA); (c) AFM images of GO. Fig. 3. FT-IR spectrum of PVA and PPVA. Fig. 4. SEM images of GA (a) and GA/PEG (b), 15%-PVA/GA/PEG (c) and 15%-PPVA/GA/PEG (d) composite PCMs section. (e) and (f) are the SEM images of char layer correspond to (c) and (d).
24
Fig. 5. DSC curves of PVA/GA/PEG and PPVA/GA/PEG composite PCMs with different ratios of PVA and PPVA. Fig.
6.
Shape-stability
test
of
PEG,
GA/PEG,
15%-PVA/GA/PEG
and
15%-PPVA/GA/PEG composite PCMs with increasing temperature. Fig. 7. (a) TG curves of PVA and PPVA; (b) TG curves of PEG, GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs. Fig. 8. (a) Infrared camera images of PEG and 15%-PPVA/GA/PEG composite PCMs during heating for 0, 60 and 100 s; (b) DSC curves of 15%-PPVA/GA/PEG composite PCMs before and after 20 and 50 thermal cycles; (c) Raman spectra of char layer of GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs; (d) Heat release rate curves of PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs. Fig. 9. 3D and TG-IR spectra of volatilized products at various temperature during the thermal degradation of PEG and 15%-PPVA/GA/PEG composite PCMs.
Scheme 1. Synthetic route of PPVA
25
Fig. 1. Standard curve of P content 26
Fig. 2. (a) XRD patterns of Natural graphite, GO and GA; (b) Raman spectra of GO and GA (The inset of (b) shows the ID/IG data for GO and GA); (c) AFM image of GO.
27
Fig. 3. FT-IR spectrum of PVA and PPVA.
28
Fig. 4. SEM images of GA (a) and GA/PEG (b), 15%-PVA/GA/PEG (c) and 15%-PPVA/GA/PEG (d) composite PCMs section. (e) and (f) are the SEM images of char layer correspond to (c) and (d).
29
Fig. 5. DSC curves of PVA/GA/PEG and PPVA/GA/PEG composite PCMs with different ratios of PVA and PPVA.
30
Fig.
6.
Shape-stability
test
of
PEG,
GA/PEG,
15%-PVA/GA/PEG
15%-PPVA/GA/PEG composite PCMs with increasing temperature.
31
and
Fig. 7. (a) TG curves of PVA and PPVA; (b) TG curves of PEG, GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs.
32
Fig. 8. (a) Infrared camera images of PEG and 15%-PPVA/GA/PEG composite PCMs during heating for 0, 60 and 100 s; (b) DSC curves of 15%-PPVA/GA/PEG composite PCMs before and after 20 and 50 thermal cycles; (c) Raman spectra of char layer of GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs; (d) Heat release rate curves of PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs.
33
Fig. 9. 3D and TG-IR spectra of volatilized products at various temperature during the thermal degradation of PEG and 15%-PPVA/GA/PEG composite PCMs.
34
Tables Table captions Table 1 The effect of PVA and PPVA on the heat storage of PEG. Table 2 The thermal conductivity of PVA, PPVA, PEG, GA/PEG composite PCMs, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs.
35
Table 1 The effect of PVA and PPVA on the heat storage of PEG PEG
PVA (5%)
PVA (10%)
PVA (15%)
PPVA (5%)
PPVA (10%)
PPVA (15%)
∆Hm (J g-1)
177.3
162.7
155.2
141.7
145.8
133.4
119.6
Tm (oC)
63.9
61.2
60.8
62.5
61.2
61.2
60.9
∆Hc (J g-1)
173.8
159.6
153.0
139.3
144.1
133.3
118.2
Tc (oC)
41.3
45.0
43.8
43.7
41.6
43.0
43.6
36
Table 2 The thermal conductivity of PVA, PPVA, PEG, GA/PEG composite PCMs, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs Sample
Thermal conductivity
Ambient T (oC)
PVA
(W m-1 K-1) 0.152
25.05
PPVA
0.112
24.95
PEG
0.493
25.36
GA/PEG
0.687
25.52
15%-PVA/GA/PEG
0.667
25.60
15%-PPVA/GA/PEG
0.610
25.66
37
Highlights A "double-network" structure as support material for Polyethylene glycol was constructed by the use of Phosphorylated polyvinyl alcohol and graphene aerogels. Graphene aerogels and Phosphorylated polyvinyl alcohol as support materials can improve the thermal conductivity and thermal stability of composite phase change materials, respectively. The three major drawbacks of Polyethylene glycol as a phase change material (low thermal conductivity, poor thermal stability and easy leakage) were solved overall in one and the same composites.
1
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S1359-8368(19)34349-5
DOI:
https://doi.org/10.1016/j.compositesb.2019.107545
Reference:
JCOMB 107545
To appear in:
Composites Part B
Received Date: 25 August 2019 Revised Date:
15 October 2019
Accepted Date: 17 October 2019
Please cite this article as: Shen J, Zhang P, Song L, Li J, Ji B, Li J, Chen L, Polyethylene glycol supported by phosphorylated polyvinyl alcohol/graphene aerogel as a high thermal stability phase change material, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107545. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Polyethylene glycol supported by phosphorylated polyvinyl alcohol/graphene aerogel as a high thermal stability phase change material Jia Shen, Ping Zhang∗, Lixian Song, Jiapeng Li, Bingqiang Ji, Jiajun Li, Lin Chen∗
National Engineering Technology Center for Insulation materials & State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and Technology, Mianyang, 621010, PR China
Abstract Polyethylene glycol (PEG) as a phase change material (PCM) is limited in practical applications due to the three major drawbacks of low thermal conductivity, poor thermal stability, and easy leakage. In this study, a new shape-stabilized PPVA/GA/PEG PCM based on Phosphorylated polyvinyl alcohol (PPVA) and graphene aerogels (GA) as a “double-network” support material was obtained using a one-step method, and the three major obstacles of PEG as a PCM were well resolved in this composite material. 15%-PPVA/GA/PEG composite PCMs still exhibit high energy storage capacity while having high thermal stability and high shape-stabilized property. 15%-PPVA/GA/PEG composite PCMs (0.610 W m-1 K-1), with only 1.60 wt% GA, showed an enhanced thermal conductivity than that of PEG (0.493 W m-1 K-1), and it still exhibited an acceptable latent heat of fusion of 119.6 J g-1. Furthermore, the peak heat release rate of 15%-PPVA/GA/PEG composite PCMs ∗ Corresponding author. Fax: +86-816-2419245 E-mail address: [email protected] (Ping Zhang) ∗ Corresponding author. Fax: +86-816-2419245 E-mail address: [email protected] (Lin Chen) 1
decreased by 19.2% compared with PEG. The above experimental results indicate that the prepared PPVA/GA/PEG composite PCMs have application prospects in thermal energy storage field.
Keywords: A. Polyethylene glycol; A. Phase change materials; B. Double-network structure; B. Thermal properties
1. Introduction The rapid consumption of fossil fuels and the continued increase in greenhouse gas emissions have driven the development and effective utilization of various renewable energy [1, 2], and phase change materials (PCMs) have drawn significant attention for the application of thermal energy storage such as electronics thermal management [3, 4], solar energy utilization [5, 6], industrial waste heat recovery [7] and off-peak electricity storage systems [8] because of its high energy storage density, isothermal storing and small temperature variation during the process of phase transition [9]. Polyethylene glycol (PEG) as a kind of PCM has become an important branch of new PCMs in recent years, because PEG has the advantages of molecular designability, suitable phase transition temperature (4-70 oC), high latent heat of phase change, no supercooling and phase separation, non-toxicity, small corrosivity and stable performance [10]. However, as a typical solid-liquid organic PCM, PEG has three major drawbacks in the practical energy storage applications with low thermal conductivity, easy leakage, and low thermal stability [11]. In the past decades, a number of efforts have been performed to overcome the above challenges of PEG in thermal energy storage applications. Porous materials, such as expanded graphite [12, 13], porous SiO2 [14], expanded perlite [15] are often used as support materials for leakage resistance. The research on improving the thermal conductivity of PEG composite PCMs mainly focuses on the addition of materials with high thermal 2
conductivity, such as silver nanowires [16], carbon nanotubes [17], graphene [18-20], etc. And in terms of thermal stability, the main solution is to use flame retardant which could form a physical barrier on the surface of the substrate when heated to prevent heat transfer, thus improving the thermal stability of the composite PCMs [21-23]. For Instance, Yang et al. [24] prepared lightweight cellulose/GNP aerogels as support materials for PEG using graphene nanoplatelets (GNPs) and microcrystalline cellulose, and the composite PCMs exhibit a large latent heat of fusion of 156.1 J g-1 and a high thermal conductivity of 1.35 W m-1 K-1 due to the highly porous and thermal conductivity of cellulose/GNP aerogels. Yang et al. [25] fabricated PEG/HGA composite PCM with only ca. 0.45 wt% GO and ca. 1.8 wt% graphene nanoplatelets (GNP), showed an enhanced improvement of thermal conductivity (1.43 W m-1 K-1) of 361% compared with pure PEG (0.31 W m-1 K-1). Qian et al. [26] prepared PEG/silsesquioxane composite by in situ sol–gel process, using phosphamide containing organosiloxane as precursor. The peak of heat release rate (PHRR) of PEG/silsesquioxane composite was decreased by 38.6% compared with PEG, showing the excellent flame retardant properties. However, PEG PCMs supported by porous materials are prone to leakage after multiple cycles, and it is easy to occur phase separation phenomenon between inorganic thermally conductive filler and organic phase in the long-term use of the process. Moreover, the latent heat value of the composite PCMs will also reduce to some extent due to the presence of the flame retardant. So, for the practical application of PEG as a PCM, solving the three drawbacks of easy leakage, low thermal conductivity and poor thermal stability in one and the same material is of vital importance. Hence, we report a PPVA/GA/PEG composite PCM with high shape-stabilized property and high thermal stability by using graphene aerogel (GA) and phosphorylated polyvinyl alcohol (PPVA) as 3
support materials. GA with a three-dimensional (3D) framework formed by the self-assembly of Graphene oxide (GO) sheets. Polyvinyl alcohol (PVA) is a kind of water-soluble macromolecular polymer with excellent hydrophilicity, good heat and chemical stability[27]. To further improve the thermal stability of PVA, PPVA was synthesized by esterification reaction of PVA and phosphoric acid in the aqueous solution, which could form a second 3D network structure by entanglement of polymer molecular chains and hydrogen bonding (intramolecular hydrogen bond and intermolecular hydrogen bond) [28-30]. At the same time, the hydrogen bonding between PPVA and graphene oxide can also enhance the 3D framework of GA[31]. The “double-network” structure composed of GA and PPVA can be used as an encapsulation structure to prevent leakage of PEG. At the same time, GA with high thermal conductivity can provide a thermal conduction channel and improve the thermal conductivity of composite PCMs. PPVA with high thermal stability can improve the thermal stability of the composite PCMs. The 15%-PPVA/GA/PEG composite PCMs with only 1.60 wt% GA, showed an enhanced thermal conductivity of 0.610 W m-1 K-1 from 0.493 W m-1 K-1 of pure PEG. Furthermore, the peak of heat release rate of 15%-PPVA/GA/PEG composite PCMs decreased by 19.2% compared with PEG. Moreover, 15%-PPVA/GA/PEG composite PCMs still exhibited an acceptable latent heat of fusion of 119.6 J g-1.
2. Experimental 2.1. Materials Polyethylene glycol (PEG, Mn=6000), Polyvinyl alcohol (PVA, the average degree of polymerization is 1788±50), Urea (H2NCONH2, 99.0%) and Phosphoric acid (H3PO4, 85.0%) was purchased from Chengdu Kelong Chemical Reagent 4
Company, China. And PEG was dried under the vacuum before using (100 oC, 3 h). Natural flake graphite (99.99 wt% purity) was purchased from Yingshida Graphite Co. Ltd., (Qingdao, China), and the average particle diameter is 20 µm. Concentrated sulfuric acid (H2SO4, 95%-98%), Hydrogen peroxide (H2O2, 30.0%), Potassium permanganate (KMnO4, 99.5%), Sodium nitrate (NaNO3) and Potassium Phosphate Monobasic (KH2PO4, 99.0%) were purchased from Chengdu Kelong Chemical Reagent Company, China. L-Ascorbic acid (Vitamin C, C6H8O6, 97.0%) was provided by Tianjin Fuchen Chemical Reagents Factory, China. Ammonium metavanadate (NH4VO3) was purchased from Shanghai Yien Chemical Technology Co., Ltd., China. Ammonium molybdate ((NH4)6Mo7O24·4H2O) was provided by Tianjin Kemiou Chemical Reagent Co., Ltd., China.
2.2. Synthesis of PPVA Phosphorylated
polyvinyl
alcohol
(PPVA)
was
prepared
through
the
esterification reaction in water media, in which urea acted as a catalyst for dehydration and phosphoric acid acted as an esterification agent. And the synthetic route of PPVA was presented in Scheme 1. First, PVA (10 g), deionized water (50 mL), phosphoric acid (50 mL) and urea (0.4 g) were placed in a 250 mL three-necked flask for 24 h at room temperature. Then magnetic stirring reaction for 2.5 h at 96 oC, and the product was cooled down to room temperature after the reaction was finished. Finally, cut the product up and washed it repeatedly with anhydrous ethanol until the pH of the washing liquid reached 5-6, and PPVA was obtained after dried at 60 oC for 12 h.
2.3. Synthesis of GA and GA/PEG composites GO was prepared by a modified Hummers method [32-34], and the GA/PEG composite was prepared through the sol-gel method [35]. First, mixed 0.16 g GO and 5
80 mL deionized water, then treatment under ultrasound conditions for 0.5 h. Then hydrothermal reaction for 4 h at 95 oC after 0.16 g Vitamin C and 8 g PEG were added. Finally, the GA/PEG composite was obtained after drying in a 60 oC vacuum oven for 12 h. GA was synthesized by the same method without adding PEG, and obtained by freeze-drying.
2.4. Synthesis of PVA/GA/PEG and PPVA/GA/PEG composites The PVA/GA/PEG and PPVA/GA/PEG composites were prepared through the sol-gel method. First, mixed 0.16 g GO and 80 mL deionized water, then suffered from ultrasonic treatment for half an hour. Then 0.16 g Vitamin C and 8 g PEG were added, at the same time, add 0.44 g, 0.92 g and 1.47 g PVA or PPVA respectively to the GO suspension (The mass fraction of PVA or PPVA accounted for 5%, 10% and 15%, respectively, and the addition amount of PVA and PPVA contents were calculated on the basis of the composite PCMs). Then hydrothermal reaction for 4 h at 95 oC. Finally, the PVA/GA/PEG and PPVA/GA/PEG composite PCMs were obtained after drying in a 60 oC vacuum oven for 12 h.
2.5.
Determination
of
the
phosphorus
content
in
PPVA
by
spectrophotometry Standard curve of phosphorus: Accurately add 0.10, 0.20, 0.40, 0.60 and 0.8 mL standard phosphorus solutions in 5 volumetric flasks (50 mL) with corresponding phosphorus contents of 0.10, 0.20, 0.40, 0.60 and 0.8 mg, respectively. In addition, a 50 ml volumetric flask was added to deionized water as a control group. Then, 2 ml of 29% nitric acid (HNO3) solution, 2 ml of 0.25% Ammonium metavanadate solution (NH₄ VO₄ ) and 2 ml of 5% ammonium molybdate ((NH4)6Mo7O24·4H2O) solution were sequentially added to each volumetric flask. Finally add deionized water to constant volume to 50 mL. The above samples were measured on an 6
Ultraviolet-visible
spectrophotometer
(Ultraviolet-visible
Spectrophotometer,
UV-2600, Shimadzu Instruments (Suzhou) Co., Ltd.) with a wavelength of 400 nm. The standard curve could be used to calculate the phosphorus content of PPVA. Digestion of PPVA: First, 0.5 g (m0) of PPVA was added to a 50 mL (V0) conical flask, and then 10 mL of nitric acid was added to digest until the sample became black, and then concentrated with nitric acid to further digest until the solution became clear. Then the entire solution was transferred to a 50 mL volumetric flask and diluted with deionized
water to constant volume. Accurately add 5 mL (V1) of the above solution
to a 50 mL volumetric flask, and adjust the pH to neutral with 0.5 M NaOH solution. Then, 2 ml of 29% HNO3 solution, 2 ml of 0.25% NH₄ VO₄ and 2 ml of 5% (NH4)6Mo7O24·4H2O solution were sequentially added to volumetric flask. Finally add deionized water to constant volume to 50 mL. The above sample was measured on an Ultraviolet-visible spectrophotometer with a wavelength of 400 nm, and the phosphorus content of PPVA could be calculated as: P(%)=
P(mg) ×V0 ×100 (1) m0 ×1000×V1
2.6. Characterization The structure of PVA and PPVA were characterized by a Fourier Transform infrared spectra (FT-IR, Spectrum one, PerkinElmer Co., USA). The phosphorus content
of
PPVA
is
measured
by
spectrophotometry
(Ultraviolet-visible
Spectrophotometer, UV-2600, Shimadzu Instruments (Suzhou) Co., Ltd.). The microstructure of the composites was observed by Scanning Electron Microscopy (SEM, UItra55, Carl Zeiss NTS GmbH, Germany). The X-ray diffraction (XRD) patterns of the materials were taken on a PANalytical X’Pert PRO X-ray diffractometer equipped with a copper anode (Cu Kα radiation, λ = 1.54187 Å). The 7
Raman spectrum tests were performed at room temperature using Raman Spectroscopy (in Via, Renishaw Co., U.K.). The size and thickness of GO was determined by Atomic Force Microscope (AFM, NSK Ltd. Japan). Differential scanning calorimeter (DSC Q200, TA Instruments, USA) analysis technique was used to investigate the phase change properties of materials. The DSC analysis was carried out in a N2 atmosphere with the weight of the sample being about 5 mg, and the heating or cooling rate was 10 oC min-1 from 20 to 100 oC. The thermal properties of the composites were analyzed by a Thermogravimetric analyzer (TG, Q500, TA Instruments, USA), and the sample was heated from 30 to 600 oC in a N2 atmosphere. The heating rate was set as 20 oC min-1. The thermal conductivity of the composite PCMs was analyzed with a thermal conductivity meter by a transient plane heat source method, three times measurements of thermal conductivity were performed for each sample. An infrared thermal camera (NEC Co., Ltd., USA) was used to record the heat transfer and temperature distribution of composite PCMs on a hot plate with a constant temperature of 80 oC. The flammability characteristics of composite PCMs were determined by Microscale combustion calorimetry (Govmark MCC-2) according to ASTM D7309-07. Thermogravimetric analysis/infrared spectrometry (TG-IR) of the material was performed using a DT-50 (Setaram, France) instrument which was interfaced to an IRAffinity-1 FT-IR spectrometer. About 10 mg sample was put in an alumina crucible and heated from 30 to 800 oC in a N2 atmosphere (flow rate of 60 mL min-1), with a heating rate of 20 oC min-1.
3. Results and discussion 3.1. Structural properties of GO, GA, PVA, PPVA, GA/PEG, PVA/GA/PEG, and PPVA/ GA/PEG composite PCMs 8
Phosphorus content of PPVA is measured by spectrophotometry, and the principle is to convert the phosphorus in the phosphate ester to the orthophosphate ion by oxidation, and then determine by colorimetry[36]. The standard curve of P content is shown in Fig. 1. The linear relationship between absorbance and phosphorus content is very obvious (R2=0.9990), so it can be used as a standard curve for calculating the phosphorus content of the PPVA. The absorbance of the sample was 0.74 through UV-visible absorption spectroscopy testing. According to the regression equation y=2.4575x+0.02825, P(mg)=0.2896 mg. And the phosphorus content in PPVA can be determined as 0.58% according to formula (1). The XRD pattern of natural graphite, graphene oxide (GO) and graphene aerogel (GA) are shown in Fig. 2a. It can be seen that a characteristic diffraction peak with a large intensity and a narrow and sharp peak shape appearing at 2θ=26.5° corresponds to a typical (002) crystal plane of graphite, indicating that the natural graphite has good crystallinity and ordered layered structure [37]. After Natural graphite is oxidized by Hummers method, the typical (002) crystal plane is transferred from 2θ=26.5° to 2θ=8.5°, indicating successful synthesis of GO. And the reason of the 2θ value of the (001) plane was only 8.5° could be the high degree of oxidation and the presence of water molecules between layers of GO [38]. Atomic force microscopy (AFM), as a direct tool, was applied to investigate the microstructure and thickness of the GO [39]. As shown in Fig. 2c, the GO is comprised of thin nanosheets with a lateral size of 0.5–5.0 µm and a thickness of about 1.1 nm. Raman spectroscopy is one of the important technical means used to characterize carbon materials [40, 41]. In the Raman spectroscopic analysis of graphene materials, the intensity ratio of the D band (structural defects of graphene) and G band (In-plane vibrations of sp2 carbon atoms) is often used to measure the regularity of the graphene structure. The intensity ratio of 9
the D band and G band (ID/IG) is smaller, the degree of graphitization is higher, and the graphene structure is more regular. L-Ascorbic acid, which is also known as Vitamin C, is a natural antioxidant in the cells. It is often used as a environment-friendly reducing agent of graphene oxide due to its mild reducibility and non-toxicity [42]. The Raman spectra of GO and GA are shown in Fig. 2b, and the ID/IG of GO and GA were 1.16 and 0.92, respectively, indicating that GO is well reduced by Vitamin C. The FT-IR spectroscopy is an effective way to detect whether PPVA is successfully grafted. The test results of PVA and PPVA were shown in Fig. 3. Two new peaks were observed at 1189 cm-1 and 991 cm-1 in spectrum of PPVA than that of PVA. The stretching peak of P-O-C was located at 1055-950 cm-1. Esters C-O stretching vibration absorption peak was located at 1300-1100 cm-1. With the C-O connected to the different groups, the absorption peak position would change. Therefore, the absorption peak near the 1189 cm-1 is the coincidence of the absorption peak of the C-O stretching vibration and the P-O-C absorption peak. So these results show that the P-O-C bond does exist in PPVA, i.e., PPVA was synthesized successfully. The microscopic structures of the GA and the GA/PEG composite PCMs were revealed by Scanning Electron Microscopy (SEM) (Fig. 4a, b). The numerous of folds and pores of GA provided enough void space for the permeation of PEG (Fig. 4a), and the SEM image revealed that the PEG infiltrated and completely occupied the void space of GA (Fig. 4b). The SEM image of 15%-PPVA/GA/PEG composite PCMs (Fig. 4d) revealed that a network structure could be seen more clearly compared with that of 15%-PVA/GA/PEG composite PCMs (Fig. 4c), and these network pores adhered to a large number of PEG molecules, and no obvious interface 10
was observed between the GA and PEG, which indicated that GA and PEG hold good compatibility and PEG could be absorbed and bounded by the void space of the “double-network” structure. The SEM images of char layer, which were gained by 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs after calcined in a microwave muffle furnace at 550 oC (Fig. 4e, f), revealed that the char layer of 15%-PPVA/GA/PEG composite PCMs was denser than that of 15%-PVA/GA/PEG composite PCMs. This is because the PPVA can form a denser char layer on the surface material when the 15%-PPVA/GA/PEG composite PCMs was pyrolysed, which can protect the matrix more efficient than that of 15%-PVA/GA/PEG composite PCMs.
3.2. Thermal energy storage Phase change temperature and enthalpy of PCMs were measured by DSC. The DSC curves of PEG, PVA/GA/PEG and PPVA/GA/PEG composite PCMs were shown in Fig. 5, and the corresponding data were presented in Table 1. The thermal properties of the samples include melting temperature (Tm), Phase transition enthalpy of melting (∆Hm), crystallizing temperature (Tc) and phase transition enthalpy of crystallization (∆Hc). As shown in Fig. 4 and Table 1, the phase transition enthalpy of pure PEG the were 177.3 J g-1 (∆Hm) and 173.8 J g-1 ( ∆Hc), respectively. When the mass fraction of PPVA was 5%, the ∆Hm and ∆Hc of PPVA/GA/PEG composite PCMs were 145.8 J g-1, 144.1 J g-1, respectively. When the mass fraction of PPVA is gradually increased (10% and 15%), the ∆Hm were 133.4 J g-1, 119.6 J g-1 and the ∆Hc were 133.3 J g-1, 118.2 J g-1, respectively. That is to say, with the increase of the mass ratio of PPVA, the phase transition enthalpy of PPVA/GA/PEG composite PCMs showed a downward trend, and the PVA/GA/PEG composite PCMs showed the same rules. This is because PVA or PPVA forms a network structure due to the 11
entanglement of the molecular chain and hydrogen bonding , when the phase transition of PEG occurs, this network structure, coupled with the 3D network of GA, will directly hinder the chain movement of PEG molecules, thus the processes of melting or crystallization of polyethylene glycol are influenced. Furthermore, PPVA formed a denser network structure than that of PVA because of the more complex side groups of PPVA, so that the PPVA/GA/PEG composite PCMs have a greater loss in phase change enthalpy. Moreover, it can be seen from Table 1 that PVA/GA/PEG and PPVA/GA/PEG composite PCMs show a lower melting temperature and higher crystallizing temperature than PEG, indicating that GA provides an excellent thermal path for composite PCMs, while the effect of different mass fraction of PVA and PPVA on the phase transition temperature of the composites is almost negligible. Importantly, after 20 and 50 melting-crystallizing cycles, the DSC curves of 15%-PPVA/GA/PEG composite PCMs remain almost unchanged compared to the initial (Fig. 8b), indicating that the composite PCMs have excellent thermal reversibility.
3.3. Shape-stabilized property As the solid-liquid organic phase change material is prone to liquid leakage in the phase transition, which affects the practical application, so the shape-stabilized property becomes an important parameter of PCMs. The samples were pressed into a sheet with a uniform size to perform the shape-stabilized test by a flat vulcanizer (LP-S-50), and the testing results were shown in Fig. 6. First, pure PEG material and GA/PEG composite PCMs were placed on the plate at 70 oC. After 10 minutes, pure PEG began to melt and completely melted into liquid after 20 minutes, but GA/PEG composite PCMs did not leak liquid after 40 minutes. This is due to the PEG molecules entered into the network pores of the GA, which makes PEG to flow out 12
difficultly. When GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs were placed on the plate at 110 oC, 130 oC, 150 oC, respectively, the results showed that liquid leakage of the three materials did not occur at 110 oC and 130 oC for 30 min. When these three composites were removed from their original position at 150
o
C for 30 min (Fig. 6a, b), It could be found that GA/PEG and
15%-PVA/GA/PEG composite PCMs exhibited varying degrees of liquid leakage, and 15%-PPVA/GA/PEG composite PCMs was almost no leakage. These results indicate that when the PVA molecules enter the GA/PEG composites, the binding capacity of PEG molecules to a “double-network”
structure consisting of PPVA and
GA is more attractive than that of the "single-network" structure of the GA, so it is more difficult to leak out. Meanwhile, the oxygen containing functional groups on GA, PVA and PPVA can form hydrogen bonds with PEG, further preventing the leakage. Moreover, the PPVA molecular can form a more complex network structure, so the anti-leakage ability of PPVA/GA/PEG composite PCMs is stronger than that of PVA/GA/PEG composite PCMs. Therefore, the addition of PPVA can greatly improve the anti-leaking ability of PEG composite phase change materials, and the PPVA shows more efficiency than that of PVA.
3.4. Thermal conductivity The thermal conductivities of pure PEG, GA/PEG, PVA/GA/PEG and PPVA/GA/PEG composite PCMs were measured by Hot Disk Thermal conductivity analyzer using the transient plane heat source method, and the thermal conductivity results were collected in Table 2. The results showed that the thermal conductivity of pure PEG, PVA and PPVA were 0.493, 0.152 and 0.112 W m-1 K-1, respectively, and the thermal conductivity of GA/PEG (0.687 W m-1 K-1), 15%-PVA/GA/PEG (0.667 W m-1 K-1) and 15%-PPVA/GA/PEG (0.610 W m-1 K-1) composite PCMs could 13
increase 39.4%, 35.3% and 23.7%, respectively, while the mass fraction of GA was only 1.6 wt%. Compared with pure PEG, the thermal conductivity of the composite PCMs have improved. This is due to the fact that the GA provides a thermal conduction channel for PEG phase change heat transfer. Meanwhile, as the heat transfer of organic molecules (such as PEG, PVA and PPVA) mainly rely on chain transfer, with poor heat transfer rate, and the heat transfer rate is low, so PVA and PPVA will form a thermal resistance between the GA and the PEG molecules, leading a
certain
decrease
in
thermal
conductivity
of
15%-PVA/GA/PEG
and
15%-PPVA/GA/PEG composite PCMs. An infrared thermal imager was used to record the temperature response during the heating process, and the temperature distribution images of pure PEG and PPVA/GA/PEG composite PCMs at 0, 60 and 100 s were shown in Fig. 8a. It can be clearly seen that the temperature increase was much faster in 15%-PPVA/GA/PEG composite PCMs, further indicating its higher thermal conductivity and rate of thermal diffusion than those of pure PEG.
3.5. Thermal stability Thermogravimetric analysis is a convenient method to determine the thermal stability of materials, and thermal stability is one of the most significant parameters for PCMs applications. The thermal analysis curves of PVA and PPVA are shown in Fig. 7a. At 280 oC, the weight loss rate of PVA was 8.0%, while the weight loss rate of PPVA was 40%. This is because the phosphorus in PPVA will catalyze the PPVA into carbon, so that PPVA has a greater weight loss rate. When PVA and PPVA were completely pyrolyzed, the weight loss of PVA was 97.0%, but the weight loss of PPVA was 80.0%. This is because PPVA can make a more efficient char layer on the surface of composite PCMs. The thermogravimetric curves of PEG, GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs were shown in Fig. 14
6b. The weight loss of PEG, GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs were 98.0%, 97.0%, 93.5% and 91.5% at 550 oC, respectively, in other words, the corresponding char layer content were 2.0 wt%, 3.0 wt%, 6.5 wt% and 8.5 wt%, respectively. In the GA/PEG composite PCMs system, GA with three-dimensional porous network can not only act as a support material for PEG but also as a mass-transport barrier, thus improving the thermal stability of the composite PCMs. And it can be seen from the curves that the thermal stability of 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs improve significantly compared with GA/PEG composite PCMs. It is attributed to the fact that PPVA can promote the carbon formation of the PEG molecular chain in the composite PCMs due to its high thermal stability (As shown in Fig. 7a) , and the char layer can serve as a thermal insulation layer and a barrier between oxygen and polymeric decomposition gases [43]. Thereby, the synergistic effect between char layer and GA could further enhance the thermal stability of the PPVA/GA/PEG composite PCMs. The property of char layer is one of the important parameters for the thermal stability of the material. The GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composites were calcined at 550 oC and the char layer was characterized by Raman spectroscopy
(Fig.
7).
The
ID/IG
of
GA/PEG,
15%-PVA/GA/PEG
and
15%-PPVA/GA/PEG composite PCMs were 0.97, 0.98 and 1.16, respectively. These results indicate that PPVA could hinder the orderly carbon formation of PEG molecules with the increase of the calcining temperature, because the molecular chains of PPVA dispersed in that of PEG, and PPVA showed higher thermal stabilities than that of PEG, when PEG was pyrolyzed, the molecular chains of PPVA could hinder the orderly carbon formation of PEG, resulting in an increase in the ID/IG value of the char layer of the 15%-PPVA/GA/PEG composite PCMs. In all, PPVA, 15
which holds more thermal stabilities than that of pure PEG, can promote the carbonization of the PEG molecular chain on the surface of the 15%-PPVA/GA/PEG composite PCMs to form a disordered and dense char layer as a protective layer of the system. And it is consistent with the results of the SEM characterization (Fig. 4e, f). Microscale combustion calorimeter is further used to investigate the flame retardant of PPVA/GA/PEG composite PCMs, and the heat release rate curves of PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs were shown in Fig. 7d. The results showed that the peak heat release rate (HRR) of PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs were 689.3 W g-1, 578.6 W g-1 and 557.5 W g-1, respectively. So the HRR of 15%-PPVA/GA/PEG composite PCMs decrease 19.2% compared to pure PEG. The improvement of flame retardant properties of 15%-PPVA/GA/PEG composite PCMs is attributed to that PPVA can promote the carbonization of the PEG molecular chain on the surface of the composite PCMs to form a disordered and dense char layer as a mass-transport barrier, thus blocking the heat transfer during combustion, and finally achieving the flame retardant effect. TG-IR technique was employed to investigate the volatile products in the thermal degradation process of PEG [44-47] and 15%-PPVA/GA/PEG composite PCMs, and the three-dimensional (3D) TG-IR spectrum of pyrolysis gas products of PEG and 15%-PPVA/GA/PEG composite PCMs were shown in Fig. 9a, b. In order to investigate the influence of support material (GA and PPVA) on the thermal degradation of PEG, the FT-IR spectra of the volatile products of pure PEG and 15%-PPVA/GA/PEG composite PCMs evolved from 250 oC to 380 oC were extracted, as shown in Fig. 8c and d, respectively. It could be seen from Fig. 9c that the main decomposition products of PEG were combustible hydrocarbon gas (about 2870 cm-1), 16
aldehydes (about 1744 cm-1) and ether linkage (about 1130 cm-1). For pure PEG, the detection of combustible hydrocarbon gas (about 2870 cm-1) at 250 °C indicates that the initial decomposition of the PEG molecular chain begins at 250 °C and the relative intensity of the main decomposition products peaks at 340 °C reaches the maximum. However, compared with PEG, there is a certain delay in the thermal pyrolysis of 15%-PPVA/GA/PEG composite PCMs. The chemical structural changes during thermal degradation take place at 280 °C as shown in Fig. 9d, and the relative intensity of the flammable gas peak reached the maximum at 380 °C. The enhanced thermal stability of the 15%-PPVA/GA/PEG composite PCMs is attributed to the fact that PPVA can promote the carbon formation of the PEG molecular chain in the composite PCMs, and the char layer can serve as a thermal insulation layer and a barrier between oxygen and polymeric decomposition gases. In summary, the support material and the char layer synergistically improve the thermal stability properties of the 15%-PPVA/GA/PEG composite PCMs.
4. Conclusions In this work, the target composite PCMs with high thermal stability were synthesized by one-step method. In the composite PCMs system, a “double-network” structure was constructed as a support material by PPVA and GA, at the same time, PEG as a PCM was integrated into the “double-network” structure by the capillary force. The “double-network” structure can be used as an encapsulation structure to prevent leakage of PEG. Moreover, GA and PPVA can improve the thermal conductivity
and
thermal
stability
of
the
composite,
respectively.
15%-PPVA/GA/PEG composite PCMs with excellent shape-stabilized property could maintain at 150 oC for 30 minutes. The thermal conductivity of 15%-PPVA/GA/PEG composite PCMs could increase 23.7% compared with that of pure PEG, while the 17
mass fraction of GA was only 1.60 wt%. The 15%-PPVA/GA/PEG composite PCMs maintained outstanding thermal reliability after 50 thermal cycles. In addition, 15%-PPVA/GA/PEG composite PCMs still exhibited an acceptable latent heat of fusion of 119.6 J g-1. Thus the PPVA/GA/PEG composite PCMs with high thermal stability, high thermal conductivity and high anti-leakage ability were prepared, and the three major drawbacks of PEG as a PCM were solved in one and the same material. In all, 15%-PPVA/GA/PEG composite PCMs are highly promising for practical application for energy storage due to these excellent properties.
Acknowledgements We acknowledge support from the National Natural Science Foundation of China (No.51503173), National Engineering Technology Center for Insulation Materials, Southwest University of Science and Technology (No.16kfjc09), Longshan academic talent research supporting program of SWUST (17LZX636, 18LZX629) and
Graphene
Engineering
Technology
Research
Center
of
Sichuan
(2018SCGCZX05).
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Figures Figure captions Scheme 1. Synthetic route of PPVA. Fig. 1. Standard curve of P content. Fig. 2. (a) XRD patterns of Natural graphite, GO and GA; (b) Raman spectra of GO and GA (The inset of (b) shows the ID/IG data for GO and GA); (c) AFM images of GO. Fig. 3. FT-IR spectrum of PVA and PPVA. Fig. 4. SEM images of GA (a) and GA/PEG (b), 15%-PVA/GA/PEG (c) and 15%-PPVA/GA/PEG (d) composite PCMs section. (e) and (f) are the SEM images of char layer correspond to (c) and (d).
24
Fig. 5. DSC curves of PVA/GA/PEG and PPVA/GA/PEG composite PCMs with different ratios of PVA and PPVA. Fig.
6.
Shape-stability
test
of
PEG,
GA/PEG,
15%-PVA/GA/PEG
and
15%-PPVA/GA/PEG composite PCMs with increasing temperature. Fig. 7. (a) TG curves of PVA and PPVA; (b) TG curves of PEG, GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs. Fig. 8. (a) Infrared camera images of PEG and 15%-PPVA/GA/PEG composite PCMs during heating for 0, 60 and 100 s; (b) DSC curves of 15%-PPVA/GA/PEG composite PCMs before and after 20 and 50 thermal cycles; (c) Raman spectra of char layer of GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs; (d) Heat release rate curves of PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs. Fig. 9. 3D and TG-IR spectra of volatilized products at various temperature during the thermal degradation of PEG and 15%-PPVA/GA/PEG composite PCMs.
Scheme 1. Synthetic route of PPVA
25
Fig. 1. Standard curve of P content 26
Fig. 2. (a) XRD patterns of Natural graphite, GO and GA; (b) Raman spectra of GO and GA (The inset of (b) shows the ID/IG data for GO and GA); (c) AFM image of GO.
27
Fig. 3. FT-IR spectrum of PVA and PPVA.
28
Fig. 4. SEM images of GA (a) and GA/PEG (b), 15%-PVA/GA/PEG (c) and 15%-PPVA/GA/PEG (d) composite PCMs section. (e) and (f) are the SEM images of char layer correspond to (c) and (d).
29
Fig. 5. DSC curves of PVA/GA/PEG and PPVA/GA/PEG composite PCMs with different ratios of PVA and PPVA.
30
Fig.
6.
Shape-stability
test
of
PEG,
GA/PEG,
15%-PVA/GA/PEG
15%-PPVA/GA/PEG composite PCMs with increasing temperature.
31
and
Fig. 7. (a) TG curves of PVA and PPVA; (b) TG curves of PEG, GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs.
32
Fig. 8. (a) Infrared camera images of PEG and 15%-PPVA/GA/PEG composite PCMs during heating for 0, 60 and 100 s; (b) DSC curves of 15%-PPVA/GA/PEG composite PCMs before and after 20 and 50 thermal cycles; (c) Raman spectra of char layer of GA/PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs; (d) Heat release rate curves of PEG, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs.
33
Fig. 9. 3D and TG-IR spectra of volatilized products at various temperature during the thermal degradation of PEG and 15%-PPVA/GA/PEG composite PCMs.
34
Tables Table captions Table 1 The effect of PVA and PPVA on the heat storage of PEG. Table 2 The thermal conductivity of PVA, PPVA, PEG, GA/PEG composite PCMs, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs.
35
Table 1 The effect of PVA and PPVA on the heat storage of PEG PEG
PVA (5%)
PVA (10%)
PVA (15%)
PPVA (5%)
PPVA (10%)
PPVA (15%)
∆Hm (J g-1)
177.3
162.7
155.2
141.7
145.8
133.4
119.6
Tm (oC)
63.9
61.2
60.8
62.5
61.2
61.2
60.9
∆Hc (J g-1)
173.8
159.6
153.0
139.3
144.1
133.3
118.2
Tc (oC)
41.3
45.0
43.8
43.7
41.6
43.0
43.6
36
Table 2 The thermal conductivity of PVA, PPVA, PEG, GA/PEG composite PCMs, 15%-PVA/GA/PEG and 15%-PPVA/GA/PEG composite PCMs Sample
Thermal conductivity
Ambient T (oC)
PVA
(W m-1 K-1) 0.152
25.05
PPVA
0.112
24.95
PEG
0.493
25.36
GA/PEG
0.687
25.52
15%-PVA/GA/PEG
0.667
25.60
15%-PPVA/GA/PEG
0.610
25.66
37
Highlights A "double-network" structure as support material for Polyethylene glycol was constructed by the use of Phosphorylated polyvinyl alcohol and graphene aerogels. Graphene aerogels and Phosphorylated polyvinyl alcohol as support materials can improve the thermal conductivity and thermal stability of composite phase change materials, respectively. The three major drawbacks of Polyethylene glycol as a phase change material (low thermal conductivity, poor thermal stability and easy leakage) were solved overall in one and the same composites.
1
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: