In this study, novel nanoencapsulated phase change materials (PCMs) with n-eicosane as a core and PMMA (Poly (methyl methacrylate)) as a shell with a weight ratio of 50/50 were synthesized by the mini-emulsion polymerization method. The size of these particles was evaluated by Dynamic Light Scattering (DLS) and ranged from 71nm to 566nm but the average size of the particles was 242nm. These particles were almost spherical and soft as can be seen in Optical Microscope (OP), Transmission Electron Microscopy (TEM) and the Scanning Electron Microscope (SEM) tests figures. The chemical stability of sample was tested by Fourier transform infrared spectroscopy (FT-IR) and all the peaks of n-eicosane and nanocapsules were checked and the result of that, confirms the synthesis high accuracy. The thermal characteristics and thermal stability of the sample were measured by Differential Scanning Calorimetry (DSC) and Thermo-gravimetric analysis (TGA) tests. DSC indicates that melting and freezing latent heats and temperatures of n-eicosane/PMMA nanocapsules are 124.7 J/g and 34.66°C, 119.55 J/g and 32.92°C respectively. TGA test also represents a good thermal stability of the sample. With suitable attributes of PMMA nanocapsules obtained from aforementioned tests, this study presents a novel nano-encapsulated PCM with a good capacity for thermal energy storage applications.
In recent years, Phase Change Materials (PCMs) have been used to store thermal energy in order to optimize energy consumption and prevent the spread of environmental pollutants problems. The use of these materials is because PCMs can absorb or release a large amount of heat, while their temperature stays almost constant from solid to liquid, or vice versa. PCMs are usually divided into the following three categories: organic PCMs, inorganic PCMs and eutectic PCMs. Although inorganic PCMs have high latent heat, some of the inherent problems in these PCMs, such as the need for a special container due to corrosion, excessive super cooling, decomposition and etc. are still remained. Some of the major problems of organic PCMs are the low conductivity and sometimes very low levels, the low degree of stability of these materials under different conditions, and liquid leakage when the phase changes from solid to liquid. Therefore, encapsulating PCMs is an effective solution to the inherent problems in various types of PCM. The benefits of encapsulating are: (I) protecting the PCM from external environmental influences, (II) increasing the heat transfer surface, and (III) The core materials, due to the shell around it, can withstand the change in the volume of PCM that occurs due to the phase shift. (IV) allowing the small and portable TES systems to emerge. Phase change material (PCMs) can be used as energy storage material in TES applications such as solar energy, energy efficiency in buildings, medical applications and thermal regulation textile materials.
Recent studies have shown that different types of microencapsulated phase change materials (MPCMs) are made in a variety of ways. Three of the most important of these are as follows. The first method is in-situ polymerization. Among those who have used this method, Li et al. have prepared microcapsules with n-octadecane as a core and melamine formaldehyde (MF) as a shell. The microcapsules made by them have a soft spherical surface and a high thermal stability. Also, Yu et al. produced n-dodecanol microcapsules and melamine-formaldehyde resin as a shell. Their experiment showed that an increase in the amount of n-dodecanol mass could reduce the microcapsule thermal stability.
The second method is the interfacial polymerization which Liang et al. prepared the microcapsules from the Butyl Stearate as a core and the shell of Toluene-2,4-diisocyanate (TDI) and Ethylenediamine (EDA) with periodic thermal properties and suitable encapsulation rates. Also Pascu et al., have conducted an experiment to produce microcapsules with epoxy resins and carboxylic acids with this form of polymerization. Their investigations led to an inverse relationship between the agitator speed and the size of the produced microcapsules.
The third method for producing PCM microcapsules is the suspension polymerization method that Chang et al. made n-octadecane and poly (methyl methacrylate)-silica hybrid microcapsules. In their conclusion, they suggested that the most effective solution for the production of microcapsules is the pre-polymer solution method. The same way, You and his colleagues prepared a kind of microcapsules with n-octadecane as a core and the styrene (St)-divinylbenzene (DVB) co-polymer shell. Following their experiments, they injected the microcapsules into PU foam, which resulted in the improvement of most of the thermal properties of the foam after the addition of the microcapsules.
The applications of microcapsules and nanocapsules are used in building materials, fabrics and thermal fluids. In the case of thermal fluids, (Microencapsulated Phase Change Materials) MPCMs do not perform well during repeated cycles because large MPCM particles not only increase liquid viscosity but also often break down during pumping and thus block the pipe system. For these kinds of reasons, the process of developing nanoencapsulated PCMs began. Compared to microcapsules, nanocapsules have a larger surface to volume ratio, which provides a stronger stimulant for accelerating the thermodynamic processes. In recent years, PCM nanocapsules have been made using mini-emulsion polymerization method.
Fang et al. have produced a kind of polystyrene nanocapsule as shell and n-octadecane as a core. Sari et al. investigated the nanoencapsulation of n-octacosane with the poly (methyl methacrylate) shell (PMMA). Their tests confirm the appropriateness of n-octacosane as a phase change substance. Black and his colleagues used nanocapsules from n-hexadecane as the nucleus and poly (alkyl methacrylate) as a shell. Their results showed that the overall size of the capsules is determined by the total hexadecane fraction in the capsules, which means that a larger amount of hexadecane produces larger particles. It is anticipated that the change in the viscosity of the oil, plays an important role in the final particle size. Alay et al. prepared nanocapsule PMMA/n-hexadecane that can be used as an additive to the fiber in textile applications and improves the thermal properties of nanofibers.
By reviewing various articles in this field, it can be seen that a new method for the production of nanocapsules with the eicosane paraffin core and the PMMA polymer shell is used. Eicosane is a substance of the family of paraffin chains, which has been selected as the nucleus due to its suitable melting point and latent heat. On the other hand, PMMA is also selected as a shell for its smooth flexibility, as well as mechanical strength and high thermal stability, so that nanocapsules with a soft and spherical surface can be obtained. These spherical soft capsules have a variety of applications that are based on the temperature range and the heat transfer rate, they can be used in nanofibers and textiles, as well as building materials and thermal fluids, and as extensions to solar and electronic systems for controlling temperature. The main goal of this paper is to produce nanocapsules in a simpler way, with the thermal energy storage by the latent heat and its use in the mentioned applications. DLS, FT-IR, SEM, TEM, DSC and TGA from nanocapsules have been tested to confirm the morphology and thermal properties of nanocapsules, as can be seen in later sections.
The accuracy of the synthesized nanocapsules was confirmed by a Fourier Transform Infrared Spectra (FTIR) test (Bruker Tensor27) with a sample container of KBr. This device operates at ambient temperatures and in the wavelength range of 600 cm-1 to 4000 cm-1.
The particle size distribution (PSD) of the particles was obtained by Dynamic Light Scattering (DLS) method and by the Zeta Potential Analyzer (ZetaPlus Brookhaven).
The morphology of the nanocapsules was first examined by the light microscope (Leitz Wetzlar Metallux 2). Subsequently, they tested by Scanning Electron Microscopy (SEM) with a ZEISS FOS-UT model with a 30 kV accelerator voltage. The specimen was covered with gold before the test. The Transmission Electron Microscope Test (TEM) was used to examine the shape and size of the particles. The Philips CM-30 model with 150 kV accelerator voltage was used for this purpose.
Since thermal properties are one of the important properties in TES nanocapsules, in order to study thermal properties such as temperature determination of melting and freezing (crystallization) as well as the latent heat of melting and freezing (crystallization), DSC and TGA tests were taken from nanocapsules. The Differential Scanning Calorimeter (DSC) test was performed with the METTLER TOLEDO TGA / DSC 1 and the N2 atmosphere with a flow rate of 10ml/min, and the heating rate of 10°C/min, as well as the cooling rate of 10°C/min. The test was carried out at a temperature of 10 to 100 °C as a reciprocal period. At the last step, the Thermal Gravimetric Analysis (TGA) was taken from the sample. The METTLER TOLEDO TGA / DSC 1 was the model. The test was performed at a temperature of 25 to 400 °C at a heating rate of 10°C /min and under the N2 atmosphere at 50 ml/min flow rate.
Materials and Method
All the materials used in the experiment include n-eicosane and AIBN (Azobisisobutyronitrile) with a purity of 98% and MMA (methyl methacrylate) with a purity of 99% were purchased from Merck. Also, SLS (Sodium Lauryl Sulfate) is also used as the surfactant. The deionized water used is commercial and has not been distilled again.
The method used for the production of eicosane / PMMA nanocapsules is the mini-emulsion polymerization. Another important point is that for the production of nanocapsules, the blending ratio of MMA monomer and eicosane is equal. Initially, in a beaker, the amount of 1gr of SLS was solved in 100cc distilled water. In another beaker, 6g of eicosane and 6g of MMA monomer with 4g of AIBN as an initiator of the reaction were combined. Then, the aforementioned solutions were mixed together for 30 minutes at 50 ° C and 400 rpm on a magnetic stirrer (IKA RET basic). After completing the mixing phase, the beaker was placed in the ultrasonic water bath (Bandelin SONOREX Digital 10P). The temperature of the water inside the sonicator was 50° C and the duration of beaker exposure within it was 6 minutes. In the next step, the beaker was placed on a stirrer for 24 hours at 75 ° C and 600 rpm. At this stage, polymerization will be improved and completed. In the final stage and after 24 hours, the nanocapsules were purified from coarse impurities by the filter paper onto the Buchner funnel and using a vacuum pump. After the abovementioned steps, nanocapsules are obtained as a suspension in water and distributed uniformly.
Results and Discussion
1) Chemical Analysis (FT-IR)
n-eicosane and eicosane/PMMA nanocapsules FT-IR (Fourier-transform infrared spectroscopy) spectrometers are shown in Fig. X. By evaluating the spectrum of each of them and the location of the peaks of the graphs, it can be seen that the nanocapsule production reaction has been done correctly. The graph shows that the peak of 717 cm-1 is clearly related to the spectrum of eicosane and the in-plane rocking vibration of the methylene group of this compound. On the other hand, the peaks of 2847 cm-1, 2912 cm-1, and 2957 cm-1 are related to stretching vibration of the C-H bond in the methyl and methylene group in eicosane. Similarly, the bending vibration of the C-H bond shows itself at the 1470 cm-1 peak. The important thing is that these peaks are overlapping in the spectrum of eicosane, PMMA and eicosane/PMMA nanocapsules. In the following, by observing the produced nanocapsule spectrum, the presence of PMMA was checked to confirm the correctness of the reaction. The two peaks of 1207 cm-1 and 1729 cm-1 are the major peaks related to the stretching vibration of the C-O and C=O bonds. Both of these bonds represent the PMMA ester group and its presence in the nanocapsule. As a result of the overall review of the nanocapsule spectrum, the accuracy of the test is confirmed.
2) Morphology Analysis (DLS, Light Microscope, SEM, TEM)
The size and shape of the particles can significantly affect the physical and functional properties of a material. To check these effects, a thorough examination of the size and shape of the nanocapsule particles was performed by the relevant tests. At the beginning of the experiment, the DLS (Dynamic Light Scattering) test was conducted to measure the size of nanocapsule particles. The particle size distribution (PSD) of nanocapsules is shown in figure Q. As shown in Fig. Q, the variation in the intensity of the particle size is from 71 to 566 nm, with an average particle diameter of about 242 nm. Another point is that the number of the nanocapsule particle size distribution is between 117 and 160 nm, and in this case, the average diameter in is also 135 nm that is clear in Fig. Q. According to other similar studies, these sizes are good values for nanocapsules, which can improve their performance in various areas, including textiles. After DLS test, the shape and structure of the nanocapsules were investigated. This was initially done by optical microscopy. Pictures of W are photos of dispersed nanocapsules in a soluble phase, in which nanocapsules are well seen as small and smooth spheres in this phase. The size of these particles corresponds to the values reported by the DLS test. In the next step, the SEM test was taken from the particles. The images of this test are shown in figure E. These images were taken after evaporation of the sample and converting the nanocapsule solution to the solid phase. According to the images, they have a smooth spherical surface that is located in a cluster structure. The dimensions of this structure and the nanocapsules in it are also consistent with the reported particle size. Finally, the TEM test examined the apparent structure of nanocapsules. TEM test images of nanocapsules are shown in figure R. These images show that nanocapsules have a relatively spherical structure and their size also confirms the results of the DLS. The heat transfer surface of nanocapsules and, in general, their thermal properties play a significant role in the performance of a TES (Thermal Energy Storage) substance. For this reason, the study of the structure and morphology of nanocapsules is very important.
3) Thermal Analysis (DSC, TGA)
DSC (Differential Scanning Calorimetry) and TGA (Thermogravimetric Analysis) have been used for micro/nanocapsule thermal analysis. The DSC test was used to determine the melting temperature, latent heat of fusion and material stability during the phase shift, and the result is shown in Fig. Y. The pure eicosane melting and freezing temperature is 38.63°C and 29.57 °C, respectively. The latent heat of fusion of this substance is 208.56 J/g and latent heat of crystallization is 198.22 J/g, which is a suitable number for a specific application. Also prepared nanocapsules with eicosane core and PMMA shell have melting temperature, freezing temperature, latent heat of fusion and latent heat of crystallization of 34.66 °C, 32.92 °C, 124.7 J/g and 119.55 J/g, respectively. It is clear that this decrease in the latent heat of melting and crystallization in the produced nanocapsules is due to the shell. Due to the appropriate melting temperature and melting heat of the nanocapsules, they can be used in a variety of applications, such as heating of buildings by solar radiation or other heating devices, battery-powered systems, or in cases where temperature control is needed, such as the textile industry. The formula for calculating the core encapsulation ratio in the nanocapsule is as follows:
In the formula above, the numerator denotes the nanocapsules latent heat of fusion and the denominator indicating the pure eicosane melting heat. It should be noted that the ratio of encapsulation changes with the stirring rate, type of core, type of shell and type of synthesis. According to the above formula, the ratio of the encapsulation of the core in this study was approximately 62%, and the rest of the nanocapsule is formed by shell. In the study 34, the amount of heptadecane encapsulated in heptadecane/PMMA microcapsules was 38%, in the study 33, the rate of formation of octacosane core in microcapsules with a PMMA shell was 43%, in the study 67, the rate of formation of nonadecane in micro/nanocapsules prepared with The PMMA shell was 60.3% and in the research 81 the rate of tetradecane encapsulation in the microcapsules with Polystyrene shell was 39.1%. The results obtained in this study clearly show the proper core selection, behavioral stability in phase shift, considering the optimum stirring speed and proper maintenance temperature.
The TGA test was used to measure the chemical stability of the materials at high temperatures and heat. The results can be seen in fig. U. As shown in Fig. U, the initial degradation temperature in pure eicosane is approximately 50 °C, which reaches about 100 °C in nanocapsules. Since the application of micro/nanocapsules produced is in the moderate temperature, non-degradation up to 100 °C is a desirable result that can be used in some cases, such as textiles, photovoltaic panels, buildings, and so on. The slope of degradation of nanocapsules with increasing temperature is so low that at about 200 °C the nanocapsules only lost 5% of their weight. The degradation gradient of the nanocapsules is sharpened after approximately 230 °C, which only lost 10 percent of their weight, and completely degraded at 400 °C. The results of the TGA test clearly demonstrate that nanocapsules have excellent chemical stability and can be used even at operating temperatures above 100 °C.
Due to the focus of the paper that includes synthesis, preparation, and defines a series of features and potentials for introducing a suitable product for use in the thermal energy storage, several tests have been performed to measure the properties of the micro/nanocapsules produced.
In the first step, the FT-IR test showed the correct formation of nanocapsules and polymerization of the materials. The eicosane/PMMA nanocapsules with a mean particle size of 240 nm, measured by DLS, have a soft and spherical surface that is well visible in optical microscopy, SEM, and TEM images. These micro/nanocapsules, which were prepared by the method of mini-emulsion with the optimum mixing speed, also had good thermal analysis results. The DSC test showed the suitable melting temperature and latent heat of the nanocapsules and the close proximity of pure eicosane melting temperature and prepared nanocapsules. Also, the TGA test proved a good chemical stability of these materials at high temperatures (100-200 °C).
From all the above tests and their desirable results, reducing super-cooling due to nanoencapsulation and a very favorable core encapsulation rate in nanocapsules, it can be concluded that the micro/nanocapsules prepared in this study have a good potential for use in thermal energy storage applications.