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Synthesis and thermal properties of a novel nanoencapsulated phase change material with PMMA and SiO2 as hybrid shell materials
Accepted Manuscript Title: Synthesis and thermal properties of a novel nanoencapsulated phase change material with PMMA and SiO2 as hybrid shell materials Author: Jian Shi Xiaolin Wu Xianzhu Fu Rong Sun PII: DOI: Reference:
S0040-6031(15)00336-6 http://dx.doi.org/doi:10.1016/j.tca.2015.08.022 TCA 77321
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
20-4-2015 5-5-2015 17-8-2015
Please cite this article as: J. Shi, X. Wu, X. Fu, R. Sun, Synthesis and thermal properties of a novel nanoencapsulated phase change material with PMMA and SiO2 as hybrid shell materials, Thermochimica Acta (2015), http://dx.doi.org/10.1016/j.tca.2015.08.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Synthesis and thermal properties of a novel nanoencapsulated phase
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change material with PMMA and SiO2 as hybrid shell materials
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Jian Shi1, 2, Xiaolin Wu1,*, Xianzhu Fu1, Rong Sun1, * 1
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Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China
Nano Science and Technology Institute, University of Science and Technology of
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China, Suzhou 215123, China
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Abstract:
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A novel composite PCMs nanocapsules (nanoPCMs) with PMMA and SiO2 as hybrid
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shell material and paraffin as core were successfully synthesized by concerted
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reaction of interfacial polymerization, sol-gel and self-assembly methods. SEM
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images showed that the structure of nanocapsules was affected by the mass ratio of
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TEOS and MMA. When the mass ratios of TEOS/MMA were 1:3.5 and 1:5.75, the
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nanocapsules were spherical and homogeneous with the diameter around 120nm.
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However, the nanocapsules were inhomogeneous and irregular when the mass ratio of
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TEOS/MMA was 1:12.5. Based on the results of DSC, moderate mass ratio of
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TEOS/MMA can improve thermal properties of nanocapsules. When the mass ratio of
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TEOS/MMA was 1: 3.5, the thermal properties of [email protected]
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nanocapsules was improved with the average enthalpy about 71 J/g and encapsulated
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efficiency of paraffin was 57.4%, which was higher than [email protected]
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nanocapsules. Besides paraffin which was encapsulated in nanocapsules has a better
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Corresponding author: Wu Xiaolin,+86-0755-86392196,xl[email protected]; Sun Rong, +86-0755-86392195, [email protected]
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thermal stability than that of unencapsulated, and the nanocapsules prepared with
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different mass ratio of MMA/TEOS can provide almost the same thermal stability.
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Key words: interfacial polymerization; PMMA-SiO2 hybrid shell; sol-gel and
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self-assembly methods，nanocapsules; thermal energy storage.
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1. Introduction:
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With the development of the industry and the consumption of non-renewable energy,
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energy issue is threatening the survival of mankind[1]. Sustainable development of
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energy is becoming the target of the world, and energy conservation has become one
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of the crucial problems for humanity. Therefore, the effective utilization of energy
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became the focus of many scientific researches. As one of the most effective way,
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thermal energy storage[2],[3] has attracted more and more attention over the past
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decades. Phase change materials[4] (PCMs) have a great potential for thermal energy
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storage because of their large latent heat density and chemical stability. So far,
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many researches on PCMs had been reported including paraffin[5], fatty acids[6],[7],
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stearyl alcohol[8] and salt hydrates[9], etc. In particular, paraffin possesses many
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irreplaceable advantages[10] such as non-corrosive, nontoxic, good thermal durability
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properties and low cost, which make it become a promising industrial candidate for
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thermal energy storage. However, the inherent low thermal conductivity of paraffin
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has greatly restricted its application on a large scale.
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At present, many methods[11],[12] have been developed to improve the thermal
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performance. Typically, the micro-encapsulation[13] of PCMs not only can
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effectively improve the thermal conductivity by increasing the heat-transfer area[14],
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but also can protect the PCMs against the environment with PCMs as core and
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inorganic or organic polymer as shell. Up to date, the shell materials mainly include
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the organic polymer, inorganic and composite materials. Wu Dezhen[15] had
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successfully synthesized a new phase change material (PCM) microcapsules with
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n-octadecane as core and calcium carbonate (CaCO3) as shell to enhance the thermal
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conductivity and serving durability. Li Hui and his co-workers[16] had prepared
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microencapsulated paraffin composites with SiO2 shell as thermal energy storage
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materials. The results showed that the SiO2 shells can improve the thermal stability of
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the microencapsulated paraffin composites. However, organic polymer materials as
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the most popular shell materials are extensively studied and applied because of their
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moderate properties, corrosion resistant, acceptable cost and non-toxic, such as
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poly(methyl methacrylate) (PMMA)[17], polystyrene[2] , polyurea resin[18] and so
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on[13],[19]. In recent years, PMMA[20] as shell material attracts more attention
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because of its hydrophilic property and excellent mechanical properties. In fact, Cemil
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Alkan and Ali Karaipekli[21] had already prepared the microencapsulated n-eicosane
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PCMs with PMMA as shell, which had good thermal properties. Wang Yi[22] had
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synthesized nanocapsules with PMMA as shell and eicosanoic-stearic acid (EA-SA)
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eutectic as core using ultraviolet photo-initiated emulsion polymerization. Over the
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past few years, more and more researchers shift their focus on the composite shell
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materials in combination with a variety of methods for microencapsulation. Tang
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Guoyi[23]
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(3-(trimethoxysilyl) propyl methacrylate) and VTMS (vinyltrimethoxysilane) as raw
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successfully
synthesized
n-octadecane
microPCMs
using
MPS
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materials of hybrid shells. Yin Dezhong[24] investigated a novel template of
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Pickering emulsion for microPCMs with covalently bonded polymer-silica hybrid
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shell. Xu Peng[25] had synthesised PMMA-SiO2 nanocomposite particles through
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micro-emulsion polymerization from MMA and silica particles coated with
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3-(trimethoxysilyl) propyl methacrylate (MSMA). Although a few composite
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core-shell structures of microPCMs have been prepared by these methods, the
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research of novel composite microPCMs is needed to improve the preparation
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methods, thermal stability and structural characteristics of microPCMs.
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There are a lot of methods for preparing microcapsules; however, the emulsion
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polymerization is one of the main methods, which can be further divided
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into interfacial polymerization[26], in-situ polymerization[27], sol-gel[28] and
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self-assembled[29] methods, etc. Fang He and his co-workers[30] prepare
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[email protected] by sol-gel method using sodium silicate as a silica precursor. Wu
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Dezhen[31] designed magnetic microcapsules based on an n-eicosane core and
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Fe3O4/SiO2 hybrid shell. Firstly, Fe3O4 nanoparticles were self-assembled on the
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interface of n-eicosane droplets in a Pickering emulsion system, and then the silica
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matrix was synthesized through interfacial poly-condensation of TEOS during the
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sol-gel process.
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In this paper, a novel composite nanocapsules with PMMA and SiO2 as hybrid shell
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and paraffin as core were synthesized by concerted reaction of interfacial
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polymerization, sol-gel method and self-assembly method. The preparation process
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was reviewed in detail and the experimental parameter influence of TEOS/MMA
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mass ratio on thermal properties and thermal stability of composite nanocapsules was
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discussed. The DSC and TGA were employed to characterize the thermal properties
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and thermal stability of nanocapsules. The structure and composition of the
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nanocapsules were confirmed by FT-IR. The morphology was observed by SEM. The
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results
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synthesized by concerted reaction with good thermal stability and thermal
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characteristics, which will have a huge potential application for thermal energy
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storage.
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2. Experiment
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2. 1 Materials and methods
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MMA (AR) and TEOS (AR) were supplied by Tianjin Da Mao chemical reagent
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company. Paraffin (industrial-grade, with Hm=121.0J/g, Hc=124.5J/g, Tm=26.6 ºC,
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Tc=21.6ºC and carbon number is about 13-18) was used as core materials. Sodium
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dodecyl benzene sulfonate (SDBS, AR) as emulsifier and ammonium persulfate as
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initiator were purchased from Shanghai Ling Feng chemical reagent company.
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Deionized water was used in the whole experiment process. All reagent are used
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directly without further purification.
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2.2 Synthesis of nanocapsules:
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Specific experimental process is as follows: at first, 1 g SDBS and 42.5 mL deionized
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water were mixed in flask and emulsified 10 min under mechanical agitation to
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prepare uniform water phase at 60 ºC. Meanwhile, paraffin, TEOS and MMA with
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different weight proportions were mixed by ultrasonic for 10 min to prepare oil phase.
that
[email protected]
nanocapsules
are
successfully
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The 13.5 g total mass of precursors remained the same. Then, the oil phase was
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gradually added into the water phase to prepare O/W emulsion, which was emulsified
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30 min at 80 ºC. After emulsification, aqueous initiator solution of ammonium
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persulfate (0.13 g/mL, pH=4) was dropwise added into the emulsion and the
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temperature was adjusted to 85 °C. After 30 min, the temperature was adjusted back
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to 80 °C. The polymerization reaction and hydrolysis reaction would occur for 2 h. At
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last, the nanocapsules were collected by filtration and washed with distilled water for
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3-5 times and the white powder obtained after natural drying.
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2.3 Characterization
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The morphology of nanocapsules was observed through scanning electron microscope
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(SEM, FEI Nova NanoSEM450). The molecular structure was analyzed by Fourier
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transform-infrared spectrum (FT-IR, BRUKER VERTEX 70). The thermal properties
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and thermal stability were investigated by differential scanning calorimeter (DSC, TA
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Q20) and thermo gravimetric analysis (TGA, TA SDT-Q600). The thermal
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characteristics were tested by DSC with a heating rate of 5 ºC/min under nitrogen
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atmosphere. TGA was employed to investigate the thermal stability from room
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temperature to500 ºC with the heating rate of 10ºC/min under nitrogen atmosphere.
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2.4 Performance test
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The actual encapsulation efficiency (ϕ) of nanocapsules was calculated by the
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following equation.
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Micro (Hm+Hc) in the equation represented the plus of melting and crystallizing
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enthalpy from microcapsules. PCM (Hm+Hc) was the plus of melting and
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crystallizing enthalpy from PCM.
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3. Results and discussion:
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3.1 Synthesis mechanism of nanoPCMs
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The paraffin@PMMA-SiO2 nanocapsules had been successfully synthesized by the
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concerted reaction method of interfacial polymerization, sol-gel and self-assembly on
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the
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[email protected] nanocapsules is illustrated in Fig.1. The oil/water (O/W)
140
emulsion was prepared with MMA, TEOS and paraffin as uniform mixed oil phase
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and SDBS as emulsifier. The acidic aqueous solution of ammonium persulfate was
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used as the initiator of polymerization and the catalyst of hydrolysis reaction.
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Therefore, the reactions of polymerization and hydrolysis occur simultaneously,
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which should limit the rate of the two reactions and improve the structural uniformity
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of the composite nanocapsules. According to the principle of concentration gradient,
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the polymerization and hydrolysis reactions lead to the decrease of interfacial
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concentration of MMA and TEOS, which will cause the spontaneous migration of
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MMA and TEOS from the internal to the interface of the oil phase. And by the
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self-assembly from Si(OH)4 and PMMA, PMMA-SiO2 composite shell was
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successfully synthesized on the surface of the paraffin droplet.
of
O/W
emulsion.
The
chemical
reaction
schematic
of
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Fig.1 The synthesis schematic of [email protected] nanocapsules
3.2 Microstructure of nanocapsules
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The SEM images of [email protected] nanocapsules prepared under different
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mass ratio of MMA and TEOS are presented in Fig.2. The results indicated that the
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microstructure of composite nanocapsules was affected by the mass ratio of MMA
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and TEOS. When the mass ratios of TEOS/MMA were 1:3.5 and 1:5.75, the prepared
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nanocapsules were spherical and homogeneous with the diameter around 120nm.
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Compared with p[email protected] nanocapsules in Fig2. (d), in fact, the presence of
160
SiO2 had no obvious influence on the microstructure of [email protected]
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nanocapsules. However, the structure of nanocapsules was unhomogeneous and
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irregular when the mass ratio of TEOS/MMA was 1:12.5.
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(a)
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(b)
(d)
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Fig.2 SEM images of paraff[email protected] nanocapsules prepared under different mass ratio of
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TEOS/MMA: (a) 3:10.5, (b) 2:11.5, (c) 1:12.5 and (d) 0:13.5 ([email protected]).
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3.3 FT-IR analysis
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FT-IR spectra of paraffin, PMMA, SiO2, [email protected] nanocapsules and
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[email protected] nanocapsules were presented in Fig.3, respectively. FT-IR
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spectra of paraffin showed that the main characteristic peaks near 2950 cm-1 and 1560
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cm-1 were C-H stretching peaks. The characteristic peaks of PMMA were observed at
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1440 cm-1 and 1200 cm-1, which can be assigned to C-H bending peak and C-O
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stretching peak of the ester group[32], respectively. Compared with the FT-IR curve
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of para[email protected], the characteristic peak of SiO2 in [email protected]
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nanocapsules was observed near 1080cm-1 without frequency biases, which indicated
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that SiO2 and PMMA were physically blended and compatible with each other. And
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the characteristic peaks of paraffin were found in the FT-IR spectra of
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[email protected] nanocapsules, which could prove the existence of paraffin in
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paraffin@ PMMA-SiO2 nanocapsules.
d
SiO2
M
[email protected]
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Transmittance%
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PMMA
0
180 181
prarffin 1000
2000
3000
4000
-1
Wavenumbers(cm )
Fig.3 FT-IR spectra of the pure paraffin, PMMA, SiO2 and [email protected] nanocapsules
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3.4 Thermal performance
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The latent heat and phase change temperature of paraffin, [email protected] and
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paraf[email protected] nanocapsules prepared under TEOS/MMA mass ratio of
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1:12.5, 2:11.5, and 3:10.5 were tested by DSC in Fig.4 and the detail results were
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listed in Table 1. The DSC heating and cooling curves show two clearly resolved
187
peaks, which revealed that the industrial-grade paraffin is a mixture which is 10
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composed of alkanes with two different range of carbon atoms number. Meanwhile,
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the lower temperature peak should be the phase transition peak of paraffin wax with
190
small carbon atoms number in melting and solidification process. These peaks can be
191
interpreted due to the melting and solidification of different paraffin composition.
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However, the total mass of precursors and the mass of paraffin remained the same; the
193
results indicated that phase change temperature, enthalpy and encapsulation efficiency
194
increased with the mass ratio increase of TEOS/MMA. Therefore, the adding of SiO2
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can improve the encapsulation efficiency and the phase change characteristics of
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[email protected] nanocapsules.
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0: 13.5 1: 12.5 2: 11.5 3: 10.5 paraffin
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HeatFlow(mW)
M
exo
endo
-10
0
10
20
30
40
50
60
Temperature(°C)
197 198
Fig.4 DSC curves of paraffin and [email protected] nanocapsules prepared under different mass
199
ratio of TEOS/MMA: 0:13.5, 1:12.5, 2:11.5, and 3:10.5 for sample 4, 3, 2 and 1, respectively.
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Compared with [email protected] nanocapsules, when the mass ratio of TEOS/MMA
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was 3:10.5, the thermal performance of [email protected] nanocapsules was
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significantly improved with the average enthalpy about 71 J/g. And the PCM
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encapsulated efficiency of [email protected] nanocapsules was 57.4%, which
204
was higher than [email protected] nanocapsules. Table 1. The thermal characteristics of [email protected] nanocapsules prepared under different
206
mass ratio of MMA and TEOS T (ºC) TEOS(g)
Hm (J/g) Tm
Tc
10.5
3
26.8
19.8
69.9
Sample 2
11.5
2
26.6
19.6
57.5
Sample 3
12.5
1
27.1
19.5
53.5
Sample 4
13.5
0
28.0
19.8
paraffin
0
0
26.6
62.1
M 21.6
71.0
an
Sample 1
Hc (J/g)
121
Φ (%)
cr
MMA(g)
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Samples
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57.4
57.5
46.9
53.5
43.6
62.4
50.7
124.5
100
Besides, the thermal stability of nanocapsules was investigated by TGA in Fig.5.
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As shown in Fig. 5, one-step degradation of paraffin began between 110ºC and 130 ºC
209
and ended at the range of 200-220ºC. There were obvious multi-step weight losses of
210
nanocapsules in the TGA curves. The first step weight loss of nanocapsules was the
211
decomposing process of paraffin in the temperature region 140ºC-220ºC; therefore it
212
can calculate the mass fraction of encapsulated paraffin by the loss which is in line
213
with the observations from the DSC results. A larger mass loss for the first
214
degradation step indicates more encapsulated paraffin. Although there is little
215
difference between the onsets of degradation for the different nanocapsules, which
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means there was nearly no effect on thermal stability, It is clear that the initial
217
decomposition temperature of paraffin increases with encapsulation. Based on the
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results of TG, we can conclude that the paraffin which was encapsulated in
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nanocapsules has a better thermal stability than that of unencapsulated, and the
220
nanocapsules prepared with different mass ratio of MMA/TEOS can provide almost
221
the same thermal stability.
0: 13.5 1: 12.5 2: 11.5 3: 10.5 Paraffin
cr
100
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60 40
an
Weight(%)
80
20
-20 50
150
200
250
300
350
400
450
500
Temperature(°C)
d
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Fig.5 The TGA curves of paraffin, [email protected] nanocapsules and [email protected]
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100
M
0
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nanocapsules prepared under different mass ratio of TEOS/MMA: 1:12.5, 2:11.5, 3:10.5, respectively
225
4. Conclusion:
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Using concerted reaction of interfacial polymerization, sol-gel and self-assembly
227
methods, PC[email protected] nanocapsules prepared under different mass ratios of
228
TEOS/ MMA were successfully synthesized with PMMA-SiO2 as hybrid shell and
229
paraffin as core. The optimal [email protected] nanocapsules was prepared under
230
3:10.5 mass ratio of TEOS/MMA, which were spherical and homogeneous with the
231
diameter around 120nm. Meanwhile, the thermal properties were improved with the
232
average enthalpy about 71 J/g and the PCM encapsulated efficiency around 57.4% 13
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compared with [email protected] nanocapsules. Besides, paraffin encapsulated in
234
nanocapsules has a better thermal stability than that of unencapsulated, and the
235
nanocapsules prepared with different mass ratio of MMA/TEOS can provide almost
236
the same thermal stability. In summary, the concerted reaction of interfacial
237
polymerization, sol-gel and self-assembly methods is a suitable and effective method
238
for the synthesis of nanocapsules with PMMA-SiO2 hybrid shells as well as other
239
similar configurations, which will have potential applications for thermal energy
240
storage.
241
5. Acknowledgement:
242
This work was supported by Shenzhen Science and Technology Program
243
(JCYJ20120831180118531), Guangdong and Shenzhen Innovative Research Team
244
Program
245
Packaging Materials Engineering Laboratory (The Development and Reform
246
Commission of Shenzhen 【 2012 】 372) and National S&T Major Project
247
(2011ZX02709).
an
M
2011D052,
KYPT20121228160843692),
Shenzhen
Electronic
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an
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[24] D.Z. Yin, L. Ma, J.J. Liu, Q.Y. Zhang, Pickering emulsion: A novel template for microencapsulated phase change materials with polymer-silica hybrid shell, Energy, 64 (2014) 575-581.
M
[25] P. Xu, H. Wang, R. Tong, Q. Du, W. Zhong, Preparation and morphology of SiO2/PMMA nanohybrids by microemulsion polymerization, Colloid Polym Sci, 284 (2006) 755-762. [26] J. Wei, Z. Li, L. Liu, X. Liu, Preparation and Characterization of Novel Polyamide Paraffin
Ac ce pt e
4588-4593.
d
MEPCM by Interfacial Polymerization Technique, Journal of Applied Polymer Science (2013) [27] H.S. Park, H.S. Park, M.S. Gong, Preparation of silver/poly(methyl methacrylate) nanocomposites by in-situ radical polymerization using silver carbamate complex, Macromol Res, 18 (2010) 897-903. [28] S.T. Latibari, M. Mehrali, M. Mehrali, T.M.I. Mahlia, H.S.C. Metselaar, Synthesis, characterization and thermal properties of nanoencapsulated phase change materials via sol-gel method, Energy, 61 (2013) 664-672.
[29] N. Vitorino, J.C.C. Abrantes, J.R. Frade, Highly conducting core-shell phase change materials for thermal regulation, Appl Therm Eng, 66 (2014) 131-139. [30] F. He, X.D. Wang, D.Z. Wu, New approach for sol-gel synthesis of microencapsulated n-octadecane phase change material with silica wall using sodium silicate precursor, Energy, 67 (2014) 223-233.
[31] F. Jiang, X. Wang, D. Wu, Design and synthesis of magnetic microcapsules based on n-eicosane core and Fe3O4/SiO2 hybrid shell for dual-functional phase change materials, Appl Energ, 134 (2014) 456-468. [32] L. Zhang, J.Q. Zhu, W.B. Zhou, J. Wang, Y. Wang, Characterization of polymethyl methacrylate/polyethylene glycol/aluminum nitride composite as form-stable phase change material prepared by in situ polymerization method, Thermochimica Acta, 524 (2011) 128-134.
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Page 16 of 17
Highlights We introduced a fairly novel method for the nanoencapsulation of paraffin into a PMMA shell, in the absence and presence of in situ prepared silica.
ip t
The mass ratios of TEOS/MMA had a significant effect on the morphology and particle size of nanocapsules.
cr
Nanocapsules prepared with different mass ratio of MMA/TEOS can provide
us
almost the same thermal stability which is better than that paraffin.
The average enthalpy of [email protected] nanocapsules about 71.0 J/g and
an
encapsulated efficiency of paraffin was 57.4% when the mass ratio of
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d
M
TEOS/MMA was 1: 3.5.
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Page 17 of 17
S0040-6031(15)00336-6 http://dx.doi.org/doi:10.1016/j.tca.2015.08.022 TCA 77321
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
20-4-2015 5-5-2015 17-8-2015
Please cite this article as: J. Shi, X. Wu, X. Fu, R. Sun, Synthesis and thermal properties of a novel nanoencapsulated phase change material with PMMA and SiO2 as hybrid shell materials, Thermochimica Acta (2015), http://dx.doi.org/10.1016/j.tca.2015.08.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Synthesis and thermal properties of a novel nanoencapsulated phase
2
change material with PMMA and SiO2 as hybrid shell materials
3
Jian Shi1, 2, Xiaolin Wu1,*, Xianzhu Fu1, Rong Sun1, * 1
4
Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China
Nano Science and Technology Institute, University of Science and Technology of
us
2
cr
5 6
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1
China, Suzhou 215123, China
7
Abstract:
9
A novel composite PCMs nanocapsules (nanoPCMs) with PMMA and SiO2 as hybrid
10
shell material and paraffin as core were successfully synthesized by concerted
11
reaction of interfacial polymerization, sol-gel and self-assembly methods. SEM
12
images showed that the structure of nanocapsules was affected by the mass ratio of
13
TEOS and MMA. When the mass ratios of TEOS/MMA were 1:3.5 and 1:5.75, the
14
nanocapsules were spherical and homogeneous with the diameter around 120nm.
15
However, the nanocapsules were inhomogeneous and irregular when the mass ratio of
16
TEOS/MMA was 1:12.5. Based on the results of DSC, moderate mass ratio of
17
TEOS/MMA can improve thermal properties of nanocapsules. When the mass ratio of
18
TEOS/MMA was 1: 3.5, the thermal properties of [email protected]
19
nanocapsules was improved with the average enthalpy about 71 J/g and encapsulated
20
efficiency of paraffin was 57.4%, which was higher than [email protected]
21
nanocapsules. Besides paraffin which was encapsulated in nanocapsules has a better
Ac ce pt e
d
M
an
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Corresponding author: Wu Xiaolin,+86-0755-86392196,xl[email protected]; Sun Rong, +86-0755-86392195, [email protected]
Page 1 of 17
thermal stability than that of unencapsulated, and the nanocapsules prepared with
23
different mass ratio of MMA/TEOS can provide almost the same thermal stability.
24
Key words: interfacial polymerization; PMMA-SiO2 hybrid shell; sol-gel and
25
self-assembly methods，nanocapsules; thermal energy storage.
26
1. Introduction:
ip t
22
With the development of the industry and the consumption of non-renewable energy,
28
energy issue is threatening the survival of mankind[1]. Sustainable development of
29
energy is becoming the target of the world, and energy conservation has become one
30
of the crucial problems for humanity. Therefore, the effective utilization of energy
31
became the focus of many scientific researches. As one of the most effective way,
32
thermal energy storage[2],[3] has attracted more and more attention over the past
33
decades. Phase change materials[4] (PCMs) have a great potential for thermal energy
34
storage because of their large latent heat density and chemical stability. So far,
35
many researches on PCMs had been reported including paraffin[5], fatty acids[6],[7],
36
stearyl alcohol[8] and salt hydrates[9], etc. In particular, paraffin possesses many
37
irreplaceable advantages[10] such as non-corrosive, nontoxic, good thermal durability
38
properties and low cost, which make it become a promising industrial candidate for
39
thermal energy storage. However, the inherent low thermal conductivity of paraffin
40
has greatly restricted its application on a large scale.
41
At present, many methods[11],[12] have been developed to improve the thermal
42
performance. Typically, the micro-encapsulation[13] of PCMs not only can
43
effectively improve the thermal conductivity by increasing the heat-transfer area[14],
Ac ce pt e
d
M
an
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cr
27
2
Page 2 of 17
but also can protect the PCMs against the environment with PCMs as core and
45
inorganic or organic polymer as shell. Up to date, the shell materials mainly include
46
the organic polymer, inorganic and composite materials. Wu Dezhen[15] had
47
successfully synthesized a new phase change material (PCM) microcapsules with
48
n-octadecane as core and calcium carbonate (CaCO3) as shell to enhance the thermal
49
conductivity and serving durability. Li Hui and his co-workers[16] had prepared
50
microencapsulated paraffin composites with SiO2 shell as thermal energy storage
51
materials. The results showed that the SiO2 shells can improve the thermal stability of
52
the microencapsulated paraffin composites. However, organic polymer materials as
53
the most popular shell materials are extensively studied and applied because of their
54
moderate properties, corrosion resistant, acceptable cost and non-toxic, such as
55
poly(methyl methacrylate) (PMMA)[17], polystyrene[2] , polyurea resin[18] and so
56
on[13],[19]. In recent years, PMMA[20] as shell material attracts more attention
57
because of its hydrophilic property and excellent mechanical properties. In fact, Cemil
58
Alkan and Ali Karaipekli[21] had already prepared the microencapsulated n-eicosane
59
PCMs with PMMA as shell, which had good thermal properties. Wang Yi[22] had
60
synthesized nanocapsules with PMMA as shell and eicosanoic-stearic acid (EA-SA)
61
eutectic as core using ultraviolet photo-initiated emulsion polymerization. Over the
62
past few years, more and more researchers shift their focus on the composite shell
63
materials in combination with a variety of methods for microencapsulation. Tang
64
Guoyi[23]
65
(3-(trimethoxysilyl) propyl methacrylate) and VTMS (vinyltrimethoxysilane) as raw
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d
M
an
us
cr
ip t
44
successfully
synthesized
n-octadecane
microPCMs
using
MPS
3
Page 3 of 17
materials of hybrid shells. Yin Dezhong[24] investigated a novel template of
67
Pickering emulsion for microPCMs with covalently bonded polymer-silica hybrid
68
shell. Xu Peng[25] had synthesised PMMA-SiO2 nanocomposite particles through
69
micro-emulsion polymerization from MMA and silica particles coated with
70
3-(trimethoxysilyl) propyl methacrylate (MSMA). Although a few composite
71
core-shell structures of microPCMs have been prepared by these methods, the
72
research of novel composite microPCMs is needed to improve the preparation
73
methods, thermal stability and structural characteristics of microPCMs.
us
cr
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66
There are a lot of methods for preparing microcapsules; however, the emulsion
75
polymerization is one of the main methods, which can be further divided
76
into interfacial polymerization[26], in-situ polymerization[27], sol-gel[28] and
77
self-assembled[29] methods, etc. Fang He and his co-workers[30] prepare
78
[email protected] by sol-gel method using sodium silicate as a silica precursor. Wu
79
Dezhen[31] designed magnetic microcapsules based on an n-eicosane core and
80
Fe3O4/SiO2 hybrid shell. Firstly, Fe3O4 nanoparticles were self-assembled on the
81
interface of n-eicosane droplets in a Pickering emulsion system, and then the silica
82
matrix was synthesized through interfacial poly-condensation of TEOS during the
83
sol-gel process.
84
In this paper, a novel composite nanocapsules with PMMA and SiO2 as hybrid shell
85
and paraffin as core were synthesized by concerted reaction of interfacial
86
polymerization, sol-gel method and self-assembly method. The preparation process
87
was reviewed in detail and the experimental parameter influence of TEOS/MMA
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Page 4 of 17
mass ratio on thermal properties and thermal stability of composite nanocapsules was
89
discussed. The DSC and TGA were employed to characterize the thermal properties
90
and thermal stability of nanocapsules. The structure and composition of the
91
nanocapsules were confirmed by FT-IR. The morphology was observed by SEM. The
92
results
93
synthesized by concerted reaction with good thermal stability and thermal
94
characteristics, which will have a huge potential application for thermal energy
95
storage.
96
2. Experiment
97
2. 1 Materials and methods
98
MMA (AR) and TEOS (AR) were supplied by Tianjin Da Mao chemical reagent
99
company. Paraffin (industrial-grade, with Hm=121.0J/g, Hc=124.5J/g, Tm=26.6 ºC,
100
Tc=21.6ºC and carbon number is about 13-18) was used as core materials. Sodium
101
dodecyl benzene sulfonate (SDBS, AR) as emulsifier and ammonium persulfate as
102
initiator were purchased from Shanghai Ling Feng chemical reagent company.
103
Deionized water was used in the whole experiment process. All reagent are used
104
directly without further purification.
105
2.2 Synthesis of nanocapsules:
106
Specific experimental process is as follows: at first, 1 g SDBS and 42.5 mL deionized
107
water were mixed in flask and emulsified 10 min under mechanical agitation to
108
prepare uniform water phase at 60 ºC. Meanwhile, paraffin, TEOS and MMA with
109
different weight proportions were mixed by ultrasonic for 10 min to prepare oil phase.
that
[email protected]
nanocapsules
are
successfully
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d
M
an
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cr
revealed
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Page 5 of 17
The 13.5 g total mass of precursors remained the same. Then, the oil phase was
111
gradually added into the water phase to prepare O/W emulsion, which was emulsified
112
30 min at 80 ºC. After emulsification, aqueous initiator solution of ammonium
113
persulfate (0.13 g/mL, pH=4) was dropwise added into the emulsion and the
114
temperature was adjusted to 85 °C. After 30 min, the temperature was adjusted back
115
to 80 °C. The polymerization reaction and hydrolysis reaction would occur for 2 h. At
116
last, the nanocapsules were collected by filtration and washed with distilled water for
117
3-5 times and the white powder obtained after natural drying.
118
2.3 Characterization
119
The morphology of nanocapsules was observed through scanning electron microscope
120
(SEM, FEI Nova NanoSEM450). The molecular structure was analyzed by Fourier
121
transform-infrared spectrum (FT-IR, BRUKER VERTEX 70). The thermal properties
122
and thermal stability were investigated by differential scanning calorimeter (DSC, TA
123
Q20) and thermo gravimetric analysis (TGA, TA SDT-Q600). The thermal
124
characteristics were tested by DSC with a heating rate of 5 ºC/min under nitrogen
125
atmosphere. TGA was employed to investigate the thermal stability from room
126
temperature to500 ºC with the heating rate of 10ºC/min under nitrogen atmosphere.
127
2.4 Performance test
128
The actual encapsulation efficiency (ϕ) of nanocapsules was calculated by the
129
following equation.
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d
M
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cr
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Page 6 of 17
130
Micro (Hm+Hc) in the equation represented the plus of melting and crystallizing
132
enthalpy from microcapsules. PCM (Hm+Hc) was the plus of melting and
133
crystallizing enthalpy from PCM.
134
3. Results and discussion:
135
3.1 Synthesis mechanism of nanoPCMs
136
The paraffin@PMMA-SiO2 nanocapsules had been successfully synthesized by the
137
concerted reaction method of interfacial polymerization, sol-gel and self-assembly on
138
the
139
[email protected] nanocapsules is illustrated in Fig.1. The oil/water (O/W)
140
emulsion was prepared with MMA, TEOS and paraffin as uniform mixed oil phase
141
and SDBS as emulsifier. The acidic aqueous solution of ammonium persulfate was
142
used as the initiator of polymerization and the catalyst of hydrolysis reaction.
143
Therefore, the reactions of polymerization and hydrolysis occur simultaneously,
144
which should limit the rate of the two reactions and improve the structural uniformity
145
of the composite nanocapsules. According to the principle of concentration gradient,
146
the polymerization and hydrolysis reactions lead to the decrease of interfacial
147
concentration of MMA and TEOS, which will cause the spontaneous migration of
148
MMA and TEOS from the internal to the interface of the oil phase. And by the
149
self-assembly from Si(OH)4 and PMMA, PMMA-SiO2 composite shell was
150
successfully synthesized on the surface of the paraffin droplet.
of
O/W
emulsion.
The
chemical
reaction
schematic
of
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M
interface
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Page 7 of 17
ip t cr an
us 152
M
151
Fig.1 The synthesis schematic of [email protected] nanocapsules
3.2 Microstructure of nanocapsules
154
The SEM images of [email protected] nanocapsules prepared under different
155
mass ratio of MMA and TEOS are presented in Fig.2. The results indicated that the
156
microstructure of composite nanocapsules was affected by the mass ratio of MMA
157
and TEOS. When the mass ratios of TEOS/MMA were 1:3.5 and 1:5.75, the prepared
158
nanocapsules were spherical and homogeneous with the diameter around 120nm.
159
Compared with p[email protected] nanocapsules in Fig2. (d), in fact, the presence of
160
SiO2 had no obvious influence on the microstructure of [email protected]
161
nanocapsules. However, the structure of nanocapsules was unhomogeneous and
162
irregular when the mass ratio of TEOS/MMA was 1:12.5.
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d
153
8
Page 8 of 17
(a)
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(b)
(d)
M
an
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cr
163
d
164
Fig.2 SEM images of paraff[email protected] nanocapsules prepared under different mass ratio of
166
TEOS/MMA: (a) 3:10.5, (b) 2:11.5, (c) 1:12.5 and (d) 0:13.5 ([email protected]).
167
3.3 FT-IR analysis
168
FT-IR spectra of paraffin, PMMA, SiO2, [email protected] nanocapsules and
169
[email protected] nanocapsules were presented in Fig.3, respectively. FT-IR
170
spectra of paraffin showed that the main characteristic peaks near 2950 cm-1 and 1560
171
cm-1 were C-H stretching peaks. The characteristic peaks of PMMA were observed at
172
1440 cm-1 and 1200 cm-1, which can be assigned to C-H bending peak and C-O
173
stretching peak of the ester group[32], respectively. Compared with the FT-IR curve
174
of para[email protected], the characteristic peak of SiO2 in [email protected]
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165
9
Page 9 of 17
nanocapsules was observed near 1080cm-1 without frequency biases, which indicated
176
that SiO2 and PMMA were physically blended and compatible with each other. And
177
the characteristic peaks of paraffin were found in the FT-IR spectra of
178
[email protected] nanocapsules, which could prove the existence of paraffin in
179
paraffin@ PMMA-SiO2 nanocapsules.
d
SiO2
M
[email protected]
an
Transmittance%
[email protected]
us
cr
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175
Ac ce pt e
PMMA
0
180 181
prarffin 1000
2000
3000
4000
-1
Wavenumbers(cm )
Fig.3 FT-IR spectra of the pure paraffin, PMMA, SiO2 and [email protected] nanocapsules
182
3.4 Thermal performance
183
The latent heat and phase change temperature of paraffin, [email protected] and
184
paraf[email protected] nanocapsules prepared under TEOS/MMA mass ratio of
185
1:12.5, 2:11.5, and 3:10.5 were tested by DSC in Fig.4 and the detail results were
186
listed in Table 1. The DSC heating and cooling curves show two clearly resolved
187
peaks, which revealed that the industrial-grade paraffin is a mixture which is 10
Page 10 of 17
composed of alkanes with two different range of carbon atoms number. Meanwhile,
189
the lower temperature peak should be the phase transition peak of paraffin wax with
190
small carbon atoms number in melting and solidification process. These peaks can be
191
interpreted due to the melting and solidification of different paraffin composition.
192
However, the total mass of precursors and the mass of paraffin remained the same; the
193
results indicated that phase change temperature, enthalpy and encapsulation efficiency
194
increased with the mass ratio increase of TEOS/MMA. Therefore, the adding of SiO2
195
can improve the encapsulation efficiency and the phase change characteristics of
196
[email protected] nanocapsules.
an
us
cr
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188
0: 13.5 1: 12.5 2: 11.5 3: 10.5 paraffin
Ac ce pt e
d
HeatFlow(mW)
M
exo
endo
-10
0
10
20
30
40
50
60
Temperature(°C)
197 198
Fig.4 DSC curves of paraffin and [email protected] nanocapsules prepared under different mass
199
ratio of TEOS/MMA: 0:13.5, 1:12.5, 2:11.5, and 3:10.5 for sample 4, 3, 2 and 1, respectively.
200
Compared with [email protected] nanocapsules, when the mass ratio of TEOS/MMA
201
was 3:10.5, the thermal performance of [email protected] nanocapsules was
202
significantly improved with the average enthalpy about 71 J/g. And the PCM
11
Page 11 of 17
203
encapsulated efficiency of [email protected] nanocapsules was 57.4%, which
204
was higher than [email protected] nanocapsules. Table 1. The thermal characteristics of [email protected] nanocapsules prepared under different
206
mass ratio of MMA and TEOS T (ºC) TEOS(g)
Hm (J/g) Tm
Tc
10.5
3
26.8
19.8
69.9
Sample 2
11.5
2
26.6
19.6
57.5
Sample 3
12.5
1
27.1
19.5
53.5
Sample 4
13.5
0
28.0
19.8
paraffin
0
0
26.6
62.1
M 21.6
71.0
an
Sample 1
Hc (J/g)
121
Φ (%)
cr
MMA(g)
us
Samples
ip t
205
57.4
57.5
46.9
53.5
43.6
62.4
50.7
124.5
100
Besides, the thermal stability of nanocapsules was investigated by TGA in Fig.5.
208
As shown in Fig. 5, one-step degradation of paraffin began between 110ºC and 130 ºC
209
and ended at the range of 200-220ºC. There were obvious multi-step weight losses of
210
nanocapsules in the TGA curves. The first step weight loss of nanocapsules was the
211
decomposing process of paraffin in the temperature region 140ºC-220ºC; therefore it
212
can calculate the mass fraction of encapsulated paraffin by the loss which is in line
213
with the observations from the DSC results. A larger mass loss for the first
214
degradation step indicates more encapsulated paraffin. Although there is little
215
difference between the onsets of degradation for the different nanocapsules, which
216
means there was nearly no effect on thermal stability, It is clear that the initial
217
decomposition temperature of paraffin increases with encapsulation. Based on the
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d
207
12
Page 12 of 17
results of TG, we can conclude that the paraffin which was encapsulated in
219
nanocapsules has a better thermal stability than that of unencapsulated, and the
220
nanocapsules prepared with different mass ratio of MMA/TEOS can provide almost
221
the same thermal stability.
0: 13.5 1: 12.5 2: 11.5 3: 10.5 Paraffin
cr
100
us
60 40
an
Weight(%)
80
20
-20 50
150
200
250
300
350
400
450
500
Temperature(°C)
d
222
Fig.5 The TGA curves of paraffin, [email protected] nanocapsules and [email protected]
Ac ce pt e
224
100
M
0
223
ip t
218
nanocapsules prepared under different mass ratio of TEOS/MMA: 1:12.5, 2:11.5, 3:10.5, respectively
225
4. Conclusion:
226
Using concerted reaction of interfacial polymerization, sol-gel and self-assembly
227
methods, PC[email protected] nanocapsules prepared under different mass ratios of
228
TEOS/ MMA were successfully synthesized with PMMA-SiO2 as hybrid shell and
229
paraffin as core. The optimal [email protected] nanocapsules was prepared under
230
3:10.5 mass ratio of TEOS/MMA, which were spherical and homogeneous with the
231
diameter around 120nm. Meanwhile, the thermal properties were improved with the
232
average enthalpy about 71 J/g and the PCM encapsulated efficiency around 57.4% 13
Page 13 of 17
compared with [email protected] nanocapsules. Besides, paraffin encapsulated in
234
nanocapsules has a better thermal stability than that of unencapsulated, and the
235
nanocapsules prepared with different mass ratio of MMA/TEOS can provide almost
236
the same thermal stability. In summary, the concerted reaction of interfacial
237
polymerization, sol-gel and self-assembly methods is a suitable and effective method
238
for the synthesis of nanocapsules with PMMA-SiO2 hybrid shells as well as other
239
similar configurations, which will have potential applications for thermal energy
240
storage.
241
5. Acknowledgement:
242
This work was supported by Shenzhen Science and Technology Program
243
(JCYJ20120831180118531), Guangdong and Shenzhen Innovative Research Team
244
Program
245
Packaging Materials Engineering Laboratory (The Development and Reform
246
Commission of Shenzhen 【 2012 】 372) and National S&T Major Project
247
(2011ZX02709).
an
M
2011D052,
KYPT20121228160843692),
Shenzhen
Electronic
Ac ce pt e
d
(No.
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6. References: [1] J.M. Khodadadi, L.W. Fan, H. Babaei, Thermal conductivity enhancement of nanostructure-based colloidal suspensions utilized as phase change materials for thermal energy storage: A review, Renew Sust Energ Rev, 24 (2013) 418-444. [2] Y. Fang, X. Liu, X. Liang, H. Liu, X. Gao, Z. Zhang, Ultrasonic synthesis and characterization of polystyrene/n-dotriacontane composite nanoencapsulated phase change material for thermal energy storage, Appl Energ, 132 (2014) 551-556.
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[3] M. Iten, S. Liu, A work procedure of utilising PCMs as thermal storage systems based on air-TES systems, Energ Convers Manage, 77 (2014) 608-627.
[4] S.M. Shalaby, M.A. Bek, A.A. El-Sebaii, Solar dryers with PCM as energy storage medium: A
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review, Renew Sust Energ Rev, 33 (2014) 110-116.
[5] J.L. Song, Q.G. Guo, Y.J. Zhong, X.Q. Gao, Z.H. Feng, Z. Fan, J.L. Shi, L. Liu, Thermophysical
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properties of high-density graphite foams and their paraffin composites, New Carbon Mater, 27 (2012) 27-34.
[6] A. Sari, K. Kaygusuz, Thermal energy storage system using some fatty acids as latent heat energy
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storage materials, Energ Source, 23 (2001) 275-285.
[7] Y. Konuklu, H.O. Paksoy, M. Unal, S. Konuklu, Microencapsulation of a fatty acid with Poly (melamine-urea-formaldehyde), Energ Convers Manage, 80 (2014) 382-390. [8] M. Li, M. Chen, Z. Wu, Enhancement in thermal property and mechanical property of phase change
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microcapsule with modified carbon nanotube, Appl Energ, 127 (2014) 166-171. [9] H.W. Ryu, S.W. Woo, B.C. Shin, S.D. Kim, Prevention of Supercooling and Stabilization of Inorganic Salt Hydrates as Latent-Heat Storage Materials, Sol Energ Mat Sol C, 27 (1992) 161-172.
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[10] X. Fang, L.W. Fan, Q. Ding, X.L. Yao, Y.Y. Wu, J.F. Hou, X. Wang, Z.T. Yu, G.H. Cheng, Y.C.
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Hu, Thermal energy storage performance of paraffin-based composite phase change materials filled with hexagonal boron nitride nanosheets, Energ Convers Manage, 80 (2014) 103-109. [11] D.M. Zhao, Z.W. Li, L.D. Liu, Y.H. Zhang, D.C. Ren, J. Li, Progress of Preparation and Application of Graphene/Carbon Nanotube Composite Materials, Acta Chim Sinica, 72 (2014) 185-200.
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cr
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Highlights We introduced a fairly novel method for the nanoencapsulation of paraffin into a PMMA shell, in the absence and presence of in situ prepared silica.
ip t
The mass ratios of TEOS/MMA had a significant effect on the morphology and particle size of nanocapsules.
cr
Nanocapsules prepared with different mass ratio of MMA/TEOS can provide
us
almost the same thermal stability which is better than that paraffin.
The average enthalpy of [email protected] nanocapsules about 71.0 J/g and
an
encapsulated efficiency of paraffin was 57.4% when the mass ratio of
Ac ce pt e
d
M
TEOS/MMA was 1: 3.5.
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