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Mater Renew Sustain Energy (2017) 6:14 DOI 10.1007/s40243-017-0098-0 ORIGINAL PAPER Preparation and characterization of 100% bio-based polylactic acid/palmitic acid microcapsules for thermal energy storage 1 1 Maryam Fashandi Siu N. Leung Received: 2 April 2017 / Accepted: 24 June 2017 The Author(s) 2017. This article is an open access publication Abstract Phase change materials (PCM) have gained Keywords Bio-based microPCM Encapsulation extensive attention in thermal energy storage applications. Palmitic acid Polylactic acid Solvent evaporation In this work, microencapsulation of vegetable-derived palmitic acid (PA) in bio-based polylactic acid (PLA) shell by solvent evaporation and oil-in-water emulsification was Introduction investigated. Fourier transform infrared spectroscopy and scanning electron microscopy were conducted to confirm Around 40% of the world’s energy consumption, and one- the successful encapsulation of PA in PLA shells. Differ- third of the global greenhouse gas emissions are related to ential scanning calorimetry was performed to evaluate the the building sector [1]. Greenhouse gases are known to be thermal properties, thermal reliability, and core content of the main reason of global warming and scientists have the fabricated PCM microcapsules (microPCM). Through a predicted that by the end of twenty-first century, the series of parametric studies, the effects of PCM and solvent average temperature of the Earth can rise by up to 7 C[2]. content, oil phase-to-aqueous phase ratio, as well as sur- The reduction in energy consumption and an improved factant type and content on the morphology, particle size, energy preservation system represent reasonable ways on and thermal properties of the PCM microcapsules were suppressing the greenhouse gases emission, and thereby investigated. Experimental results showed that PVA was a reducing the rate of global warming. In this context, phase superior emulsifier to SDS in the emulsion systems being change materials (PCM) seem to be a possible solution that studied. There also existed an optimal PVA concentration can help reduce the need to non-renewable fossil fuels. to reduce the average size of microPCM. When the PVA During the phase transition (i.e., solid–liquid, solid–gas or concentration was above this optimal level, the emulsifier liquid–gas), the energy will be stored in or released from molecules tend to form micelles among themselves. This PCM [3]. When PCM undergo isothermal melting, subli- led to the adhesion of tiny microspheres on the surface of mation or vaporization, energy is absorbed from the sur- microPCM as well as larger microPCM. In short, this work rounding and saved in the material. In contrast, when PCM has demonstrated the possibility of using the solvent solidify or condense, energy is released. Due to their wide evaporation method to fabricate 100% bio-based PCM- range of melting points, different PCM are suitable for polymer microcapsules for thermal energy storage thermal energy storage applications under diverse envi- applications. ronments. Although PCM undergoing liquid–gas transition have higher heat storage capacity than those undergoing solid–liquid transition, the latter have the higher practi- cality because of their smaller volume changes in com- & Siu N. Leung parison to PCM that undergo liquid–gas transition [4, 5]. sunny.leung@lassonde.yorku.ca PCM can be classified as organic, inorganic. and eutectics. The amount of thermal energy that can be stored Department of Mechanical Engineering, Lassonde School of in inorganic PCM, like salts, salt hydrates, metals and Engineering, York University, Toronto, ON M3J 1P3, alloys, are up to two times the amount that can be stored in Canada 123 14 Page 2 of 10 Mater Renew Sustain Energy (2017) 6:14 organic ones. They are also nonflammable, cheaper and enhance the heat transfer performance, improved durabil- possess a better thermal conductivity in comparison to ity, and suppressed supercooling [14]. Extensive researches organic PCM. However, their corrosiveness as well as sub- were conducted to encapsulate various organic and inor- cooling and super-cooling behaviors have limited their ganic PCMs by a wide range of polymers, such as poly- applications [6, 7]. Organic PCM can be subdivided into styrene [15], poly(methyl methacrylate-co-divinylbenzene) paraffinic and non-paraffinic materials. Paraffinic petro- [16], phenolic resin [17], vinyl trimethoxysilane [18], and leum-based PCM are the most commonly used PCM. They urea–formaldehyde resin [19]. Furthermore, researchers possess respectable amount of latent heat. They also have investigated different fabrication techniques to encapsulate limited super-cooling, low reactivity, good thermal and PCM. These include in situ polymerization [16, 20] inter- mechanical stability, low vapor pressure, as well as self- facial polymerization [14], emulsion polymerization [21], nucleating ability. However, their drawbacks include low condensation polymerization [22], and solvent evaporation thermal conductivity, flammability, and being more [23]. Depending on their melting points, encapsulated PCM expensive than inorganic PCM [4, 8, 9]. Non-paraffinic can be used in many different areas, including food and PCM are mostly bio-derived from vegetable oil (e.g., pharmaceutical preservation, blood transport, solar power soybean oil, coconut oil, palm oil, etc). Furthermore, fatty plants, electronic devices, space equipment’s, textile, acids, esters, alcohols and glycols are some well-known sportswear, hot and cold therapies, building materials, etc. examples of this subcategory of PCM [10]. By covering a [24, 25]. wide range of melting temperatures, these bio-based PCM While microPCM represent a unique means to promote are appropriate candidates for thermal energy storage substantial energy conservation through their passive applications in different environments. Similar to paraffinic thermal regulating and thermal energy storage capabilities, PCM, they have high latent heat, low vapor pressure, good it is possible to promote further their environmental sus- chemical and thermal stability, fast self-nucleating behav- tainability if both the microscopic shells and PCM cores ior, and high abundance. Due to their fully hydrogenated are bio-based. To the best knowledge of the authors, the structure, they can undergo thousands of thermal cycles design and fabrication of 100% bio-based microPCM have without oxidation. With lower flammability and cheaper yet been reported. As a result, the goals of this study are to price than paraffinic PCM while possessing all the positive develop a fabrication strategy to produce 100% bio-based properties of them [11, 12], bio-based PCM represent a microPCM as well as to investigate the effects of different great choice for thermal energy storage purposes. processing parameters on their morphological and thermal When PCM undergo phase transition from their solid properties. In this context, palmitic acid (PA) and PLA state to liquid state, they start to flow. In this context, shell were used herein as case examples of the PCM core encapsulation is a widely used technique to prevent PCM and the protective shell, respectively. PA is a fatty acid from migrating and reacting with their environment. If the with negligible supercooling and volume change during encapsulation results in PCM capsules in micron-scale phase transition. It also has high latent heat of fusion, [i.e., denoted as microencapsulated phase change material superior thermal stability, and no toxicity. In order to (microPCM)], it would also increase their surface-to- enhance its thermal conductivity, researchers have dis- volume ratio, thereby promoting heat transfer into and persed graphite [26] and carbon nanotubes [27] in them. from their cores. It must be noted that a proper encap- Furthermore, PA has also been added as a filler to SiO sulating material should meet the criteria under which the [28],TiO [29] and polypyrrole [30], as well as an eutectic microPCM operates, such as compatibility, strength and PCM with stearic acid [31], capric acid [32], myristic acid, flexibility, needed in specific applications. On the one and n-octadecane [33]. PLA is a biodegradable and bio- hand, the encapsulating shells should protect PCM from based polymer with its physical and mechanical properties any damage or leakage during operation. On the other comparable to conventional commodity polymers such as hand, they should be compatible with the operating polystyrene. This has made PLA an attractive option to be environment while they are in direct contact with food, used as the shells of microPCM to promote their environ- medicine, people, etc [13]. mental sustainability [34]. Although synthesizing this bio- Two main methods for encapsulating PCM are based polymer through various methods, such as ring macroencapsulation and microencapsulation. They are opening polymerization, polycondensation, enzymatic different in terms of the final capsule sizes. Microencap- polymerization, etc., is quite complicated, their production sulation is defined as wrapping any material in a capsule needs 25–55% less energy in comparison to petroleum- with its diameter between 1 and 1000 lm. Beyond this based polymers. Moreover, this FDA-approved polymer range, the capsules are called macrocapsules [4]. Com- can be degraded into non-toxic materials, which makes it a paring the two encapsulation approaches, microencapsu- great alternative to conventional petroleum-based polymers lation results in greater surface area-to-volume ratio to in different applications [35, 36]. 123 Mater Renew Sustain Energy (2017) 6:14 Page 3 of 10 14 solution to 60 g of aqueous solution. The oil–water system Experimental was first stirred by a magnetic stirrer at 200 rpm for three Materials hours at room temperature. The emulsion was then soni- cated at an amplitude of 50% level for 5 min using a PLA (Ingeo 8052D NatureWorks LLC) was used as the sonicator probe (QSonica Q700). In the next step, DCM was evaporated by elevating the emulsion temperature to polymeric shell of the microPCM. PA (Acros organics) was used as the PCM core. 87–89% hydrolyzed polyvinyl 70 C while the emulsion was continuously stirred at 200 rpm for 1 h. The remaining solution was kept at room alcohol (PVA, Sigma Aldrich) with an average molecular -1 weight of 146,000–186,000 g mol and sodium dodecyl temperature for 48 h to precipitate the fabricated microPCM. Eventually, microPCM were repeatedly sulfate (SDS, Sigma Aldrich, ReagentPlus grade) were washed with deionized water at 50 C and filtrated to used as emulsifiers. Dichloromethane (DCM, Caledon -3 remove the PVA residues. The washed microPCM were Laboratory Chemical), with the density of 1320 kg m , dried in a vacuum oven at 50 C for 12 h. Table 1 sum- was the organic solvent for the preparation of the oil phase. Deionized water was the aqueous medium. All materials marizes the material compositions used to fabricate the PLA–PA microPCM in this work. PCM0.6 was denoted as and chemicals were used as received. the base case for comparison purpose. Preparation of PLA–PA microPCM Characterization of PLA–PA microPCM Microencapsulation of PA core in PLA shell was con- The chemical structures of microPCM and each of its ducted by the solvent evaporation method accompanied by components (i.e., PLA and PA) were analyzed using oil-in-water emulsification [37]. This method has been Fourier transform infrared (FTIR) spectroscopy (Bruker widely used in the pharmaceutical industry to encapsulate Alpha-P FT-IR Spectrophotometer). The spectra were different medicines with bio-based and biodegradable collected by averaging signals from 32 scans at a resolution -1 -1 polymers such as PLA and poly(lactic-co-glycolic acid) of 4 cm in the range of 400–4000 cm . (PLGA) due to its simplicity and ability to produce Scanning electron microscopy (SEM) (FEI Company, repeatable results [38]. In this method, pre-synthesized Quanta 3D FEG) was used to observe the morphologies PLA is used, and this eliminates the complicated PLA (i.e., surface features and sphericity) and sizes of polymerization step. Unlike emulsion polymerization, no microPCM. The fabricated microPCM were sputter coated monomer, initiator or catalyst is used. Hence, it allows the with gold (Denton Vacuum, Desk V Sputter Coater) before fabrication of microspheres with high purity [39]. In gen- the observation. The particle sizes were obtained by ana- eral, the fabrication process involves four main steps: (1) lyzing the SEM micrographs using ImageJ (NIH Image). dissolution of PA and PLA in DCM; (2) emulsification of The interior morphology of microPCM was exposed by this organic phase (i.e., dispersed phase), in a continuous microtoming microcapsules using a diamond knife. aqueous phase of deionized water containing PVA; (3) The enthalpy of fusion and the melting point of extraction and evaporation of solvent from the dispersed microPCM were determined by a differential scanning phase, which transform the dispersed phase into solid calorimetry (DSC) (TA Instrument, DSC Q20). These microspheres (i.e., microPCM); and (4) recovery and dry- measurements were performed in the temperature range -1 ing of microPCM to eliminate residual solvent and from 40 to 90 C at a heating rate of 10 C min . In order emulsifier. to determine the thermal stability of the microPCM, the For the oil phase, 1.2 g of PLA and different amounts of enthalpy of fusion was analyzed after samples [i.e., the PA (i.e. 0.4, 0.6, or 0.8 g) were added to DCM. The mix- base case (PCM0.6)] were subjected to 50 thermal cycles at ture was stirred for 2 h at 36 C, which was lower than the the same temperature range and heating rate. boiling point of DCM (i.e., 40 C), to obtain a uniform PLA–PA solution. For the aqueous phase with PVA as the emulsifier, a uniform PVA solution was prepared by dis- Results and discussion solving PVA in deionized water. The solution was cured for 30 min at room temperature to swell PVA. After that, it Chemical structures of PLA–PA microPCM was heated to 80 C for 3 h to ensure complete dissolution of PVA. For the aqueous phase with SDS as the emulsifier, The FTIR spectra of PLA, PA, and PLA–PA microPCM a desired amount of SDS was dissolved in deionized water (i.e., PCM0.4) were illustrated in Fig. 1. From curve (a), it at room temperature. Consequently, the oil-in-water can be observed that PLA had obvious absorption peaks at -1 emulsion was prepared by adding 5 or 10 g of oil phase 2996 and 2946 cm , which corresponded to stretching 123 14 Page 4 of 10 Mater Renew Sustain Energy (2017) 6:14 Table 1 Conditions for the preparation of PLA–PA microPCM Sample Oil phase Aqueous phase Oil-in-water ratio PLA (g) PA (g) DCM (mL) PVA (g) SDS (g) DI water (g) PCM0.4 1.2 0.4 29 5 – 95 1:12 PCM0.6 1.2 0.6 29 5 – 95 1:12 PCM0.8 1.2 0.8 29 5 – 95 1:12 PCM0.6 1.2 0.6 14.5 5 – 95 1:12 DCM0.5 PCM0.6 1.2 0.6 14.5 2 – 98 1:12 PVA2 PCM0.6 1.2 0.6 14.5 3 – 97 1:12 PVA3 PCM0.6 1.2 0.6 14.5 4 – 96 1:12 PVA4 PCM0.6 1.2 0.6 14.5 2 – 98 1:6 O/W92 PCM0.6 1.2 0.6 14.5 2 2 96 1:12 PVA2-SDS2 PCM0.6 1.2 0.6 14.5 – 0.5 99.5 1:12 SDS0.5 PCM0.6 1.2 0.6 14.5 – 1 99 1:12 SDS1 PCM0.6 1.2 0.6 14.5 – 2 98 1:12 SDS2 PCM0.6 1.2 0.6 14.5 – 3 97 1:12 SDS3 PCM0.6 1.2 0.6 14.5 – 4 96 1:12 SDS4 of C=O and the out of plane bending of –OH, respectively. [28, 40]. The FTIR spectrum of the PLA–PA microPCM is shown -1 in curve (c). The characteristic peak at 2846 cm , attrib- uted to CH group, belonged to the PA. The absorption -1 peak at 2919 cm was related to CH bond, which was related to both the PLA shell and the PA core. The sharp -1 peak at 1750 cm was attributed to C=O group, which could also be found in both materials. However, the sig- nificantly lower transmittance in the peak related to the CH group comparing to the FTIR spectrum of pure PA revealed that both PLA and PA existed in the microPCM. It can be seen that the absorption peaks of PLA–PA microPCM were consistent with those of pure PA and PLA. Therefore, it can be concluded that PA was well encapsulated by PLA resin and no chemical interaction occurred between the core and the shell materials. Thermal properties and thermal reliability of PLA– Fig. 1 FTIR spectra of: a PLA; b PA; and c PLA–PA microcapsules PA microPCM vibration of CH bond in the molecular structure. The sharp PLA–PA microPCM fabricated by using different material -1 peaks around 1747 and 1180 cm were related to the compositions were analyzed by DSC to compare their stretching vibration of C=O and C–O–C bond. performances and operating temperatures for thermal Curve (b) shows the FTIR spectrum of PA. The sharp energy storage. Neat PA’s melting point and latent heat of -1 peak at 2847 cm was resulted from the stretching fusion were 62.8 C and 167.3 J/g, respectively. The latent -1 vibration of CH groups and the peak at 2915 cm was heat of fusion of microPCM can be determined by inte- caused by the stretching vibration of CH group in the PA 3 grating the area under the endothermic peak in a DSC -1 structure. The absorption band from 2500 to 3300 cm thermogram. By comparing this with the latent heat of belongs to the stretching vibration of –OH group and the fusion of pure PA using Eq. (1), the core content of PLA– -1 peaks at 1694 and 940 cm were related to the stretching PA microPCM can be calculated. 123 Mater Renew Sustain Energy (2017) 6:14 Page 5 of 10 14 Table 2 Thermal properties of PLA–PA microPCM Sample Melting point (C) Enthalpy of fusion (J/g) Core content (%) PCM0.4 61.9 40.7 24.3 PCM0.6 62.3 59.9 35.8 PCM0.8 62.1 70.1 41.9 PCM0.6 62.4 55.1 32.9 DCM0.5 PCM0.6 62.5 52.8 31.5 PVA2 PCM0.6 62.2 54.3 32.4 PVA3 PCM0.6 62.3 51.9 31.0 PVA4 PCM0.6 62.0 62.2 37.1 O/W92 PCM0.6 –– – PVA2-SDS2 PCM0.6 61.9 33.2 19.8 SDS0.5 PCM0.6 62.4 12.0 7.2 SDS1 PCM0.6 –– – SDS2 PCM0.6 –– – SDS3 PCM0.6 –– – SDS4 filtration and washing steps. Although lowering the SDS DH Core Content ¼ 100%; ð1Þ content to 1.0 wt% or below helped to sustain some PA DH PCM cores in the microPCM, the resultant core contents were where DH and DH are the latent heat of fusion of PCM significantly lower than those of microPCM prepared by microPCM and that of PA, respectively. using PVA as the emulsifier. As a result, SDS was an The melting temperatures, the enthalpies of fusion, and inappropriate surfactant for this oil-in-water emulsion the core contents of different microPCM samples are system [42] (Table 2). summarized in Table 2. The results show that the melting The microPCM must maintain their performances in temperatures for all microPCM samples were virtually practice after long-term uses. Therefore, it is important that unchanged, with a mean temperature and standard devia- they have insignificant change in thermal properties after tion of 62.2 and 0.2 C, respectively. In contrast, the core repeated thermal cycles. In this context, a thermal cycling content was strongly dependent on the PA content as well test was conducted on the microPCM prepared under the as the type of emulsifier used. For microPCM prepared by base case conditions (i.e., PCM0.6) to determine the ther- using PVA as the emulsifier, as the PA loading increased mal reliability of PLA–PA microPCM. After undergoing from 0.4 to 0.8 g, the core content increased from 24.3 to 50 thermal cycles over a temperature range of 40–90 C, 41.9%. Nevertheless, changing the DCM content, the PVA the enthalpy of fusion decreased by only 1.0 to 58.9 J/g. content, or the oil-to-water ratio only had minor effects on This result indicates that there was no chemical degrada- the core contents of PLA–PA microPCM. tion in the fabricated microPCM during thermal cycling When comparing the two emulsifiers (i.e., PVA and and provides evidence to confirm the microPCM were SDS), experimental results reveal that PVA was signifi- stable chemically after repeated thermal cycling. cantly more effective than SDS for the microencapsulation of PA cores by PLA shells. While all microPCM prepared Morphology and size distribution of PLA–PA by using 0.6 g of PA and various amounts of PVA resulted MicroPCM in similar core contents, the endothermic peak of PA was either absent or suppressed in the DSC thermograms of Figure 2a and b show the SEM micrographs of a batch of microPCM prepared by using SDS as the lone emulsifier or the base case PLA–PA microPCM (i.e., PCM0.6) and the together with PVA. The hydrophile-lyophile balance cross-sections of individual microcapsules, respectively. (HLB) of SDS is higher than that of PVA [41]. While PVA Figure 2a reveals that the fabricated microPCM were had a good solubilizing ability to prepare micro-emulsion generally spherical but with a wide range of size distribu- of oil phase in the aqueous phase, the excessively high tion and relatively uniform exterior characteristics. This HLB value of SDS, together with its high PA solubility, suggests successful microencapsulation of PA cores by had made it act as a perfect solubilizing agent to dissolve PLA shells. The interior features of microPCM were PA completely in the aqueous phase. As a result, the PA revealed by performing SEM on the microtomed samples. core materials were removed with the aqueous phase in the Figure 2b illustrates that individual microPCM possessed 123 14 Page 6 of 10 Mater Renew Sustain Energy (2017) 6:14 Fig. 2 SEM micrographs of PLA–PA microPCM (i.e., PCM0.6): a a batch of microPCM; and b cross-sections of individual microPCM expressed as an empirical model in Eq. (2)[37]. When preparing PLA–PA microPCM, increasing the PA content would increase the viscosity of the dispersed phase, and thereby resulted in larger microPCM sizes. 0:25 d ¼ A 100%: ð2Þ where d is the average diameter of the microspheres, A is a coefficient dependent on process conditions, l is the vis- cosity of the dispersed (oil) phase, and l is the viscosity of the continuous (aqueous) phase. Figure 4a–c illustrates SEM micrographs of microPCM consisting of 0.4, 0.6, and 0.8 g of PA while keeping a fixed PLA content (i.e., 1.2 g). It can be observed that the microPCM’s shapes and surface morphologies were vir- tually unchanged as the PA content increased. Further- more, while the microPCM demonstrated their sphericity, Fig. 3 Effect of PA content on microPCM’s sizes some uneven surface morphologies with the presence of tiny microspheres were observed. multi-core morphologies, which was consistent with the The effects of oil and aqueous media on the microPCM general expectation for a microencapsulation process morphologies and sizes were investigated by varying either involving an emulsification step [43]. the amount of DCM or the oil phase-to-aqueous phase In this section, the effects of (i) PCM core content; (ii) ratio. Figure 5 reveals that the average size of microPCM oil and aqueous media; and (iii) emulsifier type and content decreased as the oil phase-to-water phase ratio doubled. on microPCM’s morphology and size distribution are dis- This can be attributed to the reduced amount of emulsifier cussed. The average sizes of microPCM prepared by dif- (i.e., PVA) in emulsion system, which enhances the ability ferent material compositions were analyzed using SEM and for the PVA molecules to adsorb on the surface of the plotted, with the error bars representing the standard dispersed oil phase instead of forming PVA micelles. The deviations of the microPCM’s sizes. governing mechanism of this observation can be found in Figure 3 plots the average microPCM sizes and the the later part of this manuscript when discussing the effects standard deviations for samples consisted of different PA of emulsifier content on the microPCM’s sizes and mor- contents. All samples were fabricated at the same pro- phologies. Figure 6a shows the SEM micrograph of cessing conditions as the base case. The results reveal that microPCM prepared by using half the DCM volume of the increasing PA contents would slightly increase base case when preparing the oil phase. It can be observed microPCM’s average sizes as well as the degrees of size that reducing the DCM content resulted in a higher degree variation. Research related to the encapsulation of drug by of microPCM agglomerations as well as rougher surfaces. the solvent evaporation approach reported that increasing Reducing the amount of organic solvent used would the viscosity of the dispersed phase in the emulsion would increase the viscosity of the dispersed phase, and thereby increase the size of the microcapsule. This relationship was 123 Mater Renew Sustain Energy (2017) 6:14 Page 7 of 10 14 Fig. 4 SEM micrographs of PLA–PA microPCM that consist of different core contents: a PCM0.4; b PCM0.6; and c PCM0.8 Different loadings of two different emulsifiers (i.e., SDS and PVA) were used to prepare microPCM. Comparing the two types of emulsifier as shown in Fig. 7a–c, the uses of SDS eased the filtration and washing steps during the fabrication process. Unlike the case of using PVA as the emulsifier, no residues of surfactant were seen in the samples prepared by SDS. It is believed that this can be attributed to SDS’s higher HLB and its higher solubility in water. The effects of emulsifier contents on the microPCM’s surface morphologies and sizes were studied. Figure 8a and b show that, regardless of the type of emulsifier, there existed a U-shaped relationship between the emulsifier content and the average microPCM size. By increasing the amount of emulsifier from a very low content, more sur- factant molecules were available to adsorb to the oil–water interface and reduce the surface energy. This would allow Fig. 5 Effects of oil and aqueous media on microPCM’s sizes the dispersed oil phase to achieve smaller droplet sizes, lead to larger microPCM sizes as shown in Fig. 6.This which also yielded larger total surface area. However, trend was again consistent with the prediction based on when the emulsifier content continued to increase beyond a Eq. (2). By comparing Fig. 6b to Fig. 4b, it can be threshold concentration, the fabricated microPCM became observed that the sphericity and surface morphology of larger. This threshold concentration is called the critical microPCM were not affected by changing the oil phase-to- micelle concentration (CMC) in the oil-in-water emulsion. aqueous phase ratio from 1:12 to 1:6 in the emulsion. Over the CMC, excess PVA molecules would more likely Fig. 6 SEM micrographs of PLA–PA microPCM fabricated by different material compositions: a PCM0.6 and DCM90.5 b PCM0.6 O/W92 123 14 Page 8 of 10 Mater Renew Sustain Energy (2017) 6:14 Fig. 7 SEM micrographs of PLA–PA microPCM fabricated by different material compositions: a PCM0.6 ; b PCM0.6 ; and PVA3 SDS3 c PCM0.6 SDS0.5 Fig. 8 Effects of emulsifier type and content on microPCM’s sizes: a PVA and b SDS form micelles among themselves, rather than adsorbed on SDS content was investigated in an attempt to overcome the surfaces of the dispersed oil phase [44, 45]. This the loss of PA to the aqueous phase when higher loading of reduced affinity of the surfactant molecules at the oil–water SDS was used. interfaces would promote the aggregation of dispersed oil phase, and thereby led to the formation of larger microPCM. This was evidenced by the presence of small Conclusions microspheres (i.e., PVA micelles) on the surfaces of microPCM, as shown in Fig. 4a–c, in all samples prepared Solvent evaporation method, which was commonly used in by using 5 wt% of PVA solution as the aqueous phase. In the pharmaceutical industry for drug encapsulation, was contrast, PVA residues were virtually invisible on the applied to microencapsulate a bio-based PCM [i.e., pal- surfaces of the microcapsules and there were negligible mitic acid (PA)] with a bio-based polymeric shell [i.e., agglomerates in Fig. 4b and 6a. This could be attributed to polylactic acid (PLA)]. Successful encapsulation of PA the lower number of PVA molecules caused by the reduced core by PLA shell was confirmed by Fourier transform concentration of PVA or decreased amount of aqueous infrared analyses. Parametric studies were conducted to phase. Furthermore, it is also interesting to note that, for investigate the effects of core content, organic solvent microPCM prepared using SDS, reducing the emulsifier content, emulsifier type and content, as well as oil phase- content to 0.5 wt% would yielded very small number of to-aqueous phase ratio on the characteristics of PLA–PA microPCM while a large amount of un-encapsulated microPCM. Experimental data indicated that higher PA materials were observed. In other words, the extremely low content would yield higher core content. Moreover, emulsifier content caused a low encapsulation efficiency of increasing the PA content or decreasing the DCM content the PA core. As discussed in an earlier section, this low would slightly increase the microPCM’s size due to the 123 Mater Renew Sustain Energy (2017) 6:14 Page 9 of 10 14 7. Cabeza, L.F., Castell, A., Barreneche, C., de Gracia, A., Fer- higher viscosity of the oil phase. 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Materials for Renewable and Sustainable Energy – Springer Journals
Published: Jun 30, 2017
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