Roles of Polymerization Temperature and Initiator Type on Thermal Properties of Rubitherm^® 21 PCM Microcapsules

Abstract

Thermal energy storage offers a viable solution for managing intermediate energy availability challenges. Phase change materials (PCMs) have been extensively studied for their capacity to store thermal energy when available and release it when needed, maintaining a narrow temperature range. However, effective utilization of PCMs requires its proper encapsulation in most applications. In this study, microcapsules containing Rubitherm^®(RT) 21 PCM (Tpeak = 21 °C, ΔH = 140 kJ/kg), which is suitable for buildings, were synthesized using a suspension polymerization technique at different operating temperatures (45–75 °C). Two different water-insoluble thermal initiators were evaluated: 2,2-Azobis (2,4-dimethyl valeronitrile) (Azo-65) and benzoyl peroxide (BPO). The prepared microcapsules were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), particle size distribution (PSD), scanning electron microscope (SEM), and optical microscopy (OM). Additionally, the microcapsules were subjected to multiple melting and freezing cycles to assess their thermal reliability and performance stability. DSC results revealed that the microcapsules using BPO exhibited a latent heat of melting comparable to those produced with Azo-65 at an operating temperature of 75 °C. However, the onset crystallization temperature for the BPO-encapsulated PCMs was approximately 2 °C lower than that of the Azo-65-encapsulated PCMs. The greatest latent heat of melting, 107.76 J/g, was exhibited by microcapsules produced at 45 °C, representing a PCM content of 82 wt. %. On the other hand, microcapsules synthesized at 55 °C and 75 °C showed latent heats of 96.02 J/g and 95.66 J/g, respectively. The degree of supercooling for PCM microcapsules was reduced by decreasing the polymerization temperature, with the lowest supercooling observed for microcapsules synthesized at 45 °C. All microcapsules exhibited a monodisperse and narrow PSD of ~10 µm, indicating uniformity in microcapsule size and demonstrating that temperature variations had no significant impact on the particle size distribution. Future research should focus on low-temperature polymerization with extended polymerization times.

1. Introduction

The buildings sector is the largest contributor to energy consumption, making up to ~30–40% of global energy consumption [1,2]. As such, a major share of the energy is used in heating and cooling applications, with an estimated 10–20% of energy used for heating, ventilation, and air conditioning (HVAC) [3,4,5]. Phase change materials (PCMs) are organic or inorganic materials that can store and release large amounts of thermal energy during a change in phase, such as the process of melting from solid to liquid, with only a small change in temperature [6]. PCMs have been researched extensively in HVAC applications to cut energy consumption in the building sector [7,8], in conjunction with improving thermal comfort by reducing temperature fluctuations [9,10]. However, PCMs require containment during their incorporation into building materials to prevent them from interacting with the building structure and altering the properties of the building materials, along with reducing the risk of PCM leakage over the lifetime of the building and in case of building destruction [11].

Recent advances in UV photo-induced polymerization for the microencapsulation of PCMs have successfully cut back the polymerization time to as low as a few minutes, which is a remarkable improvement over the traditional thermal method. Moreover, because it could be performed at low temperatures, this technology could be used for encapsulating a broader range of PCMs, particularly those with high volatilities that were previously challenging to encapsulate [12,13,14,15]. As a result, it has boosted encapsulation to an entirely new level via a faster and more energy-efficient process. Ikutegbe et al. (2022) [16] employed a photo-induced suspension polymerization technique to encapsulate low-melting-point PCMs (T_peak = 6.2 °C) using an ultraviolet perfluoroalkoxy (UV PFA) coiled tube reactor. The produced microencapsulated PCMs (microPCMs) had a peak melting temperature of 8.2 °C and an energy storage capacity of 131.1 J/g, equivalent to approximately 87.4 wt. % PCM content. MicroPCMs encapsulated with UV irradiation-initiated MMA polymerization were prepared [17]. DSC measurements showed an encapsulation efficiency of 61.2% and melting temperatures ranging from 24 to 33 °C. Iron (III) chloride, as a photo initiator, and MMA monomer were used to encapsulate stearic acid via an emulsion polymerization method [18]. UV light was used to initiate the polymerization at a temperature higher than the melting temperature of the PCM, which was 60 °C. The results showed that the PCM content of the prepared microPCMs reached a maximum of 52.20 wt. %.

UV photo-induced polymerization presents several challenges that can affect the efficiency and safety of the process [19,20,21]. UV photo-induced polymerization relies on UV light to initiate the polymerization reaction that forms the microcapsule shell around the PCM. The effectiveness of this process depends highly on the penetration depth of UV light into the encapsulating medium. UV light, especially at a wavelength of 360 nm (which is the activation wavelength for photopolymerization), suffers from limited penetration in materials that are opaque or have high absorbance. This can lead to incomplete polymerization or uneven shell formation and result in microcapsules with weak, incomplete, or non-uniform shells, thus affecting the overall stability and performance of the encapsulated PCM. Also, UV light, particularly at wavelengths around 360 nm, poses significant health and safety risks. Prolonged exposure can be harmful to the skin and eyes, potentially causing burns or other injuries. Additionally, this wavelength falls within the UV-A range, which, although less energetic than UV-B or UV-C, still requires careful handling and protective measures to avoid accidental exposure. Moreover, the generation of UV light at this wavelength may require specific equipment and safety protocols to mitigate these risks, increasing the complexity and cost of the encapsulation process.

The use of a free radical thermal oil-soluble azo initiator in the field of the microencapsulation of PCM presents the potential to carry out polymerization at around room temperature, which presents an alternative to photo reaction as a radical generator [22]. This is due to its relatively low activation energy and half-life at low temperatures, which makes it a promising radical generator for a wide range of applications from versatile polymers and cosmetics-related use to the microencapsulation of PCMs. Zhao et al. (2023) [23] prepared microPCMs containing an n-octadecane core shelled by styrene–divinylbenzene copolymer using a free radical suspension polymerization method. The azo-iso-butyronitrile (AIBN), with a 10 h half-life decomposition temperature of 65 °C (in toluene), was used as an initiator. The polymerization reaction was carried out at 85 °C for 3 h. The prepared microPCMs exhibited a melting enthalpy and an encapsulation efficiency of 111.5 J/g and 51.4%, respectively. Carreira et al. (2017) [24] synthesized acrylic-based microcapsules containing octadecane via suspension polymerization using different types of initiators, including benzoyl peroxide (BPO), azobisisobutyronitrile (AIBN) and Trigonox 23 (TRIG). The reaction temperature was set at 80 °C and carried out for 5.5 h under a nitrogen atmosphere and continuous stirring. The microcapsules exhibited mean particle sizes of ~12 μm and melting enthalpies of ~175 J/g, corresponding to a PCM content of ~70%. Qiu et al. (2012) [25] fabricated microPCMs by combining 2,2′-azobisisobutyronitrile (AIBN) and redox initiators at 45 °C and compared these with microPCMs produced with AIBN or benzoyl peroxide (BPO) at 85 °C. The results revealed that the microPCMs prepared by combining 2,2′-azobisisobutyronitrile (AIBN) and redox initiators at 45 °C show lower heat capacities and thermal stabilities than those of microPCMs prepared with AIBN or benzoyl peroxide (BPO) at 85 °C. Su et al. (2019) [26] synthesized polymethyl methacrylate microPCMs via a suspension polymerization method. Azo-AIBN with weight percentages of 0.5%, 1.0%, 2.0%, and 3.0% was used as an oil-soluble thermal initiator. The microencapsulation reaction was then carried out at 80 °C for 5 h. The results revealed that the morphologies of the microPCMs were particularly influenced by the type of surfactant and the dosage of the thermal initiator, where the optimum capsules’ morphology was achieved with a surfactant of S-1DS and a 1 wt. % thermal initiator.

The proper selection of polymerization initiator is a key factor in the polymerization process. One aspect involved in indicating the stability of the initiator is its 10 h half-life temperature, which is the temperature at which the concentration of the initiator decreases to half its original amount over a period of 10 h. This concept is crucial in understanding how initiators work in chemical reactions, particularly in polymerization processes. A higher half-life temperature often indicates that the initiator is more stable at elevated temperatures, which can be beneficial in processes that operate at higher temperatures. It allows for more-controlled reactions, as the initiator will decompose more slowly, providing a longer working time before the reaction starts. On the other hand, a lower half-life temperature means the initiator will decompose more quickly, which can be advantageous for fast reactions or when immediate initiation is required. This can be beneficial in processes that operate at lower temperatures, reducing energy costs and the risks associated with high-temperature operations.

To our knowledge, one relatively unexplored free radical thermal oil-soluble azo initiator in the field of the microencapsulation of PCMs, known as 2,2-Azobis (2,4- dimethyl valeronitrile) (ADVN-Azo-65), presents the potential to carry out polymerization at around room temperature, which indicates it as an alternative to photo reaction as a radical generator. This is due to its relatively low activation energy of 117.8 kJ mol⁻¹ and 10 h half-life temperature of 51 °C in toluene [27]. When compared with AIBN, the 10 h half-life temperature of Azo-65 polymerization initiator is 10 °C or lower than AIBN, which makes Azo-65 a promising radical generator for a wide range of applications, from versatile polymers to cosmetics-related use. The primary objective of this study was to investigate the influence of polymerization temperature and thermal initiator type on the efficiency and stability of PCM microencapsulation using a suspension polymerization method. Additionally, the study aimed to assess the performance of this approach in comparison with existing methods. The hypothesis of this study was based on optimizing the polymerization temperature and selecting the appropriate thermal initiator, which was expected to result in improved microcapsule characteristics, including higher encapsulation efficiency and enhanced thermal stability. This hypothesis guided the design and execution of the experiments, providing a clear scientific framework for the study.

The issue of supercooling in PCM microencapsulation is a common challenge; the desire is to reduce or eliminate supercooling in microPCMs. The small size of microcapsules reduces the availability of nucleation sites that are critical for the phase change process. When the radius of the PCM core is below a critical threshold, nucleation can be entirely suppressed, making it difficult for the material to solidify at its bulk PCM freezing point, and this causes freezing at lower temperatures [28,29,30]. Supercooling is calculated as the difference between melting and freezing temperatures and is typically avoided by incorporating nucleating agents that allow nucleation points to form during crystallization [31]. Despite the fact that adding nucleation agents minimizes the supercooling degree, it also results in a reduction in the amount of PCM encapsulated and hence reduces a microPCM’s latent heat of fusion. Thus, the influence of the polymerization temperatures (75, 65, 55, and 45 °C) involved in PCM supercooling was also studied.

2. Materials and Methods

2.1. Materials

Commercial Rubitherm^® 21 (RT-21) (Rubitherm Technologies GmbH, Berlin, Germany was used as a PCM. RT-21 is a common paraffinic organic PCM with a melting point of 21 °C and a bulk latent heat of fusion of 135 J/g and is often used in thermal energy storage for building applications. Methyl methacrylate (MMA) (99%, contains ≤30 ppm mono methyl ether hydroquinone (MEHQ) as inhibitor) and ethylene glycol dimethacryalte (EGDM) (98%, contains 90–110 ppm MEHQ) were obtained from Sigma Aldrich (Auckland, New Zealand) and used as shell materials. Polyvinyl alcohol (PVA) and sodium dodecyl sulfate (SDS) were obtained from Sigma Aldrich (Auckland, New Zealand), and were used as non-ionic and ionic surfactants, respectively. Benzoyl Peroxide (Luperox^® A75-contains 25% water) and 2,2-Azobis (2,4- dimethyl valeronitrile) (ADVN), known as Azo-65, were obtained from Sigma Aldrich (Auckland, New Zealand) and FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan)respectively, and were used as thermal initiators. All chemicals were used as received without any further purification. Deionized (DI) water was used to form the initial PCM–water emulsion.

2.2. Preparation of MicroPCMs

2.2.1. Emulsification

An oil–water emulsion is typically prepared by mixing two immiscible phases: an organic phase (oil) and an aqueous phase (water) using an emulsifying technique. The typical procedure for the preparation of an oil–water emulsion used for encapsulating PCMs is described in our previous publication [32]. The procedure starts by preparing an organic phase solution and an aqueous phase solution as outlined below. An organic phase solution containing the monomers MMA (18 g) and EGDM (2.5 g) and the thermal initiator (0.5 g) is thoroughly mixed by stirring at 400 rpm using a magnetic stirrer. This initial mixing ensures the uniform distribution of all components in the organic phase. After thorough mixing of the monomer, cross-linker, and initiator, RT-21 PCM (33 g) is added to the organic mixture. The mixture is stirred vigorously until the color of the solution transforms from yellowish to transparent. This color change indicates the homogenous dispersion of the PCM in the organic phase and a proper interaction between the PCM and the monomers. The aqueous phase solution starts with 300 g of DI water to avoid impurities or ionic interference that could affect the stability of the emulsion. PVA (1 g) and SDS (0.2 g) are added to the aqueous phase. PVA acts as a stabilizer and emulsifier, providing a protective layer around the oil droplets and preventing coalescence. Meanwhile SDS is an anionic surfactant that helps reduce the surface tension between the oil and water phases, ensuring the formation of a stable emulsion. The combination of PVA and SDS allows for the successful formation of a stable oil–water emulsion [32]. The final step in the emulsification process involves creating the oil–water emulsion by combining the organic and aqueous phase solutions under high shear mixing of 3000 rpm for 10 min using a Silverson (L5M-A Laboratory Mixer Silverson Machines Ltd., Chesham, UK). The Silverson L5M-A Laboratory Mixer is a standard high-shear mixer commonly used in laboratory-scale emulsification processes. It features a high-speed rotor–stator system that applies intense shear to the mixture, facilitating the breakdown of immiscible phases into fine emulsions. The mixer has several interchangeable heads and screens that allow for customizing the mixing action, ensuring the right balance between shear force and droplet size, as displayed in Figure 1. The PCM-to-monomer mass ratio was kept constant for all samples in this study (1.5:1), and only the effects of different polymerization temperatures and types of thermal initiators were investigated. However, in our previous publication [32], the effects of the PCM-to-monomer mass ratio, as well as the impact of different surfactant types and their mixing ratios, were thoroughly examined.

2.2.2. Polymerization

A 2 L three-neck glass reactor (LR-2.ST laboratory reactor-IKA-Werke GmbH & Co. KG, Staufen, Germany) was immersed in a thermostat water bath. The reactor was purged with nitrogen prior to the addition of the emulsion and also during polymerization, as shown in Figure 2. Agitation was performed by a mechanical stirrer set at ~250 rpm (IKA ^® RW 20 Digital Stirrer, GmbH & Co. KG, Staufen, Germany), during the polymerization process, which was carried out over a period of 6 hours. The polymerization temperature was sustained by the water bath at 75 °C when using BPO as a thermal initiator. Alternatively, when using Azo-65 as a thermal initiator, the polymerization was carried out at 75, 65, 55, and 45 °C. Following polymerization, the system was allowed to cool naturally to room temperature before being washed with hexane 3 times to remove unreacted monomers and unencapsulated PCM. The washed microPCMs were filtered then spread on a tray and placed in an oven at 50 °C for 24 h to obtain a dried microPCM powder. Figure 3 outlines the microPCM formation steps.

2.3. Characterizations of MicroPCMs

2.3.1. Fourier Transformed Infrared Spectroscopy (FT-IR)

FTIR spectra of bulk RT21, the polymer shell, and the microPCMs were obtained using a Fourier Transform Infrared Spectrometer (PerkinElmer Spectrum 400, PerkinElmer Inc., Waltham, MA, USA) equipped with a universal attenuated total reflection (ATR) diamond prism to enhance the reflected signal.

2.3.2. Optical Microscopy (OP) and Scanning Electron Microscopy (SEM)

The morphology of the microPCMs was visualized using a Nikon Eclipse 80i Optical Microscope (Nikon Corporation, Tokyo, Japan), offering a broader view of the microcapsules’ overall structure and size distribution. A small droplet of the microcapsule suspension solution (before filtration) was carefully spread onto a 1 cm × 3 cm glass slide. The sample was then covered with a glass cover slip, ensuring an even distribution of the microcapsules and minimizing air bubbles, which could interfere with image clarity. A 50 µm scale was later drawn onto the images to provide a reference for size measurement and facilitate accurate comparisons between microcapsules in terms of size and uniformity.

To investigate the surface morphology of the microcapsules more thoroughly, some selected samples were analyzed using a FEI QUANTA 200F scanning electron microscope (FEI Company, Hillsboro, OR, USA). N₂O (nitrous oxide) was used as imaging gas. N₂O provides the necessary environmental conditions for low-vacuum SEM imaging, which reduces charging on non-conductive materials like microPCMs. The SEM chamber was maintained at a pressure of 0.58 Torr, a low-vacuum environment suitable for observing non-conductive samples without the need for conductive coatings, which could alter the surface features of the microcapsules. The temperature inside the SEM chamber was set to −5 °C, to ensure that the PCM core remained in its solid phase during the imaging process and prevent any melting phase change from occurring, which could have distorted the microcapsule structure. In addition, an accelerating voltage of 10 kV was applied to provide adequate imaging resolution without damaging the microcapsule surface, offering a balance between penetration depth and image clarity. The resulting SEM images provide detailed information about the surface morphology, encapsulation structure, and potential defects of the microcapsules.

2.3.3. Particle Size Distribution (PSD)

A Malvern Mastersizer 2000 (Malvern Panalytical Ltd., Malvern, UK) was used to measure the PSD of the microcapsules. This device employs laser diffraction to analyze the scattering patterns produced by particles suspended in a liquid medium, enabling accurate determination of the PSD. The refractive index of the PMMA (polymethyl methacrylate) shell, which encapsulates the PCM, was set to 1.490. The microcapsules were dispersed in water to create a suspension suitable for analysis. Ensuring uniform dispersion prevents particle agglomeration and provides an accurate representation of individual microcapsule sizes. Three measurements were performed for each sample of freshly prepared microcapsules and the average was taken, which ensures reproducibility and provides a reliable estimation of average particle sizes. The calculated standard deviation was within ±3 of the mean value.

2.3.4. Differential Scanning Calorimetry (DSC)

For calorimetric analysis of both pure PCM and microPCMs, a DSC-60 SHIMADZU (Shimadzu Corporation, Kyoto, Japan) differential scanning calorimeter was utilized. The DSC technique allows for the evaluation of key thermal properties, such as phase change temperatures and latent heats, which are critical for assessing the performance of PCMs in thermal energy storage applications. The measurements were performed in an inert nitrogen atmosphere, where a continuous flow of nitrogen gas was maintained at 100 mL·min⁻¹ during the DSC analysis to prevent the oxidation or degradation of the PCM or microPCM during the analysis. The heating and cooling rates were set to 3 °C.min⁻¹. Slower heating/cooling rates and a smaller sample size (approximately 5 mg) allow for clearer identification of the phase change onset, peak, and endset points, along with more precision [33].

The percentage PCM core content can be estimated by calculating the ratio of the average latent heat of pure PCM to the average latent heat of the microencapsulated PCMs (microPCMs) using the following equation:

$$\mathrm{\%} \mathrm{P}\mathrm{C}\mathrm{M} \mathrm{c}\mathrm{o}\mathrm{r}\mathrm{e} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}={\displaystyle \frac{{∆\mathrm{H}}_{\mathrm{m}\mathrm{i}\mathrm{c}\mathrm{r}\mathrm{o}\mathrm{P}\mathrm{C}\mathrm{M}\mathrm{s}}}{{∆\mathrm{H}}_{\mathrm{p}\mathrm{u}\mathrm{r}\mathrm{e} \mathrm{P}\mathrm{C}\mathrm{M}}}}\times 100\mathrm{\%}$$

ΔH_purePCM and ΔH_microPCMs are obtained from DSC measurements and represent the average latent heats of fusion (J/g) for pure PCM (non-encapsulated) and microPCMs, respectively. Three measurements were performed for each sample and the average was taken. The calculated standard deviation was within ±2 of the mean value.

2.3.5. Thermal Gravimetric Analyzer (TGA)

The thermal stability of microcapsules was analyzed using a SHIMADZU TGA-50 (Shimadzu Corporation, Kyoto, Japan). Analysis was carried out from room temperature up to 500 °C, at a heating rate of 10 °C. min⁻¹ in an argon atmosphere, with argon flowing at a flow rate of 75 mL min⁻¹. Assuming the components of the microcapsules decomposed separately, the mass loss recorded using TGA can be used to calculate the PCM’s core content in microcapsules through analysis of the mass loss plot [31]. From the thermogravimetric curve, the extrapolated onset temperature of PCM degradation (which represents the temperature at which PCM mass loss begins) can also be assessed [34]. Three measurements were performed for each sample and the average was taken. The calculated standard deviation was within ±3 of the mean value.

2.3.6. Thermal Cycling

For microcapsules containing PCM to be used effectively in practical applications, such as incorporation into building material, they are required to undergo multiple melting and freezing cycles without any noticeable changes in their properties. MicroPCMs and pure RT-21 were put through 50 accelerated thermal cycles between 0 and 30 °C. MicroPCMs (8 g) and pure RT-21 (6 g) were placed in 10 mL sealed cylindrical containers with K-type thermocouples positioned in the center of the sample. The thermocouples were connected to a data logger to measure and record the temperatures every 5 s during heating–cooling cycling.

3. Results

3.1. MicroPCMs Formation

FTIR provides characteristic absorption peaks corresponding to the functional groups of both the core PCM and the shell material. Therefore, the presence of peaks corresponding to both components in the FTIR spectrum of the microcapsules confirms the presence of both materials. The infrared absorption peaks of the functional groups present in bulk RT21, polymethyl methacrylate (PMMA) solid polymer microspheres, and the microPCMs are presented in Figure 4. The FTIR absorbance peaks of bulk RT21 at 2960, 2920, and 2850 cm⁻¹ can be attributed to the asymmetric stretching vibrations of simple alkane (C–H) bonds. The peak at 720 cm⁻¹ corresponds to the in-plane rocking vibration of the CH₂ group, while the peaks at 1466 and 1377 cm⁻¹ are characteristic of RT21. The intense peaks of the polymer shell at 1725 and 1145 cm⁻¹ are due to the presence of the ester carbonyl group (C=O) and the C–O (ester bond) stretching vibration, respectively. The FTIR spectrum of the microPCMs shows peaks matching those observed in both the bulk RT21 and the polymer shell, indicating that RT21 has been successfully encapsulated.

3.2. Morphology and Size of the MicroPCMs

The surface morphology of microPCMs determines how well the core material (PCM) is protected by the encasing shell. The presence of a smooth shell surface and proper formation implies a closed container for the PCM, where no amount of the subject is exposed, thus preventing any factors of degradation. Meanwhile, a rough surface morphology may suggest a non-uniform thickness of the encapsulation shell layer over the PCM core, which can lead to a situation where the PCM core does not receive adequate protection all the time. This may reduce the lifespan of operation for these microcapsules in the long run. However, although a rough surface could suggest weaknesses or defects within the encapsulating shell layer, which would promote PCM leakage or the degradation of thermal properties over time, it is useful in improving the adherence of the microcapsules to the composite matrix. Since increased surface area and texture enhance the interfacial bonding of the microcapsules, this should enhance the overall mechanical properties of the composite material [35].

Figure 5 highlights the role of both the initiator type and the polymerization temperature in influencing the surface morphology of the microPCMs, with temperature playing a more significant role in determining surface texture and smoothness. MicroPCMs produced at higher temperatures (75 °C) using both Azo-65 and PBO initiators display a rough surface texture, as evidenced by the dimples and surface irregularities shown in Figure 5a,b. This roughness is more visible under higher magnification in the SEM images (right side), which offer a more detailed look at the microstructure. This surface roughness is likely caused by higher reaction rates at higher temperatures, leading to faster polymer growth and potentially uneven surface formation. Higher temperatures might also affect the diffusion of reactants to the surface of the microcapsules, contributing to the formation of surface defects like dimples. In contrast, microPCMs produced at a lower temperature (45 °C) with the Azo-65 initiator exhibit a smooth surface with no visible dimples or irregularities (Figure 5c). This is due to the slower reaction rate at this lower temperature, where the oligomers (small polymer chains) have more time to diffuse and migrate uniformly to the surface of the microcapsule. This controlled migration allows the oligomers to form a more ordered shell, resulting in a more spherical and smoother morphology. Also, the slow growth of the polymer chains enables better packing and alignment, which minimizes surface defects like dimples or roughness. However, it is noted that far fewer microcapsules are observed in Figure 5c (using the Azo-65 initiator at 45 °C), indicating that large amounts of PCM were left unencapsulated. When comparing the effects of different initiators on the microPCMs’ surface morphology, no notable difference in surface morphology was found for microPCMs produced using either PBO or Azo-65 at 75 °C (Figure 5a and Figure 5b, respectively). Both initiators resulted in microcapsules with rough surfaces. This indicates that, at elevated temperatures, the choice of initiator does not significantly influence the surface morphology, as the temperature itself plays a more dominant role in determining the surface characteristics.

In PCM microcapsules, a uniform particle size distribution (PSD) is crucial, since uniform particle sizes ensure more-consistent heat transfer rates. Since, in such cases, each microcapsule contains the same amount of PCM, they undergo phase transitions (melting/freezing) at similar times, thus improving the efficiency of energy storage and release. In addition, microcapsules with a narrow, uniform PSD tend to have more-consistent mechanical properties [36]. The uniform microcapsule size ensures an even distribution of stress across the capsules, reducing the likelihood of weak points where failure could occur. A consistent microcapsule size also means that the wall thickness is more uniform, contributing to the overall mechanical integrity. Several studies provide detailed analyses of how microcapsule surface characteristics and size distribution impact their mechanical strength, durability, and performance in various applications. The findings confirm that optimized morphology and uniform size distribution enhance the structural integrity and mechanical stability of microcapsules [37,38,39]. From a commercial point of view, a consistent PSD simplifies the manufacturing process, making it easier to scale up production while ensuring that the quality and performance of the microcapsules remain uniform. Figure 6 presents the particle size distribution (PSD) for all samples produced with varying reaction temperatures and thermal initiator types. As seen in Figure 6, all microcapsules exhibited a monodisperse and narrow PSD, with an average particle size of 10 µm. This indicates that the emulsions remained stable throughout the process; instability in the emulsions could have led to coalescence of PCM droplets, resulting in a broader PSD. The narrow PSD also confirms that the emulsification process was sufficiently long, allowing a uniform droplet size across the samples. These results agree with a previous work using the mixed surfactants of SDS and PVA [32].

3.3. Thermal Properties of the MicroPCMs

The thermal properties of pure RT-21 and the synthesized microPCMs, including the PCM melting/freezing temperatures and latent heat of melting/freezing, are shown in Figure 7. In the heating DSC analysis (Figure 7a), the onset, peak, and endset melting temperatures of the microPCMs show slight variations. Notably, the peak melting temperature of the microPCMs is shifted forward by ~2–4 °C above that of the pure PCM (RT-21).

Table 1. Thermal properties for pure and microencapsulated RT-21 (PCM) from DSC analysis.

Initiator	Tom (°C)	Tpm (°C)	Tem (°C)	ΔHm (J.g−1)	Toc (°C)	Tpc (°C)	Tec (°C)	ΔHc (J.g−1)	Core Content (%)
RT21 (PCM)	16.53	22.10	23.85	132.53	21.38	19.39	14.50	134.48	-
PBO (75 °C)	19.55	24.49	27.92	99.69	10.42	6.51	2.56	108.33	77.9
Azo-65 (75 °C)	19.98	24.92	28.68	96.02	12.24	7.08	4.41	89.75	69.7
Azo-65 (65 °C)	19.05	25.58	30.36	94.21	11.51	7.83	2.99	95.34	71.0
Azo-65 (55 °C)	18.66	24.31	28.19	95.66	11.36	8.44	3.51	97.44	72.3
Azo-65 (45 °C)	17.59	25.63	29.05	107.76	21.29	15.66	4.67	111.18	82.0

Note: T_om and T_oc are the onset temperatures of the DSC heating and cooling curves, respectively. T_pm and T_pc are the peak temperatures of the DSC heating and cooling curves, respectively. T_em and T_ec are the endset temperatures of the DSC heating and cooling curves, respectively. ∆H_m and ∆H_m are the enthalpies of the DSC heating and cooling curves.

summarizes the thermal properties of the pure PCM and the microPCMs synthesized at different conditions. The table shows that the highest latent heat of melting (107.76 J/g) is observed in microcapsules produced at 45 °C using the Azo-65 initiator, corresponding to a PCM content of 82 wt. %. Comparable latent heat values of 96.02 J/g and 95.66 J/g were obtained at 75 °C and 55 °C, respectively. This suggests that the Azo-65 initiator facilitates the synthesis of microcapsules at lower polymerization temperatures without compromising the latent heat while enhancing the PCM content. Table 1 also shows that there is no notable change in the latent heat of melting between the microPCMs utilizing the BPO and Azo-65 initiators at 75 °C. This suggests that at higher polymerization temperatures, both initiators perform similarly in encapsulating the PCM, resulting in comparable latent heat values. It is possible that at 75 °C, the polymerization reaction reaches a state where the thermal properties of the microcapsules, particularly the latent heat, are no longer significantly influenced by the choice of initiator. This could be due to more-complete polymerization or improved compatibility between the PCM and the polymer shell at higher temperatures, minimizing any differences between the initiators’ effects.

Supercooling occurs when the phase change material (PCM) does not solidify at its expected freezing point after being microencapsulated due to the absence of nucleating sites required to initiate the crystallization process. In the case of microcapsules synthesized via suspension polymerization, the process involves the diffusion of oligomers (small polymer chains) from the organic phase to the PCM–water interface, where they crosslink to form the shell of the microcapsules. However, the rate of oligomers’ formation and diffusion is influenced heavily by the polymerization temperature. At higher polymerization temperatures, the reaction rate is much faster, meaning oligomers are produced more quickly. While this may accelerate shell formation, it also causes a problem of slow oligomer diffusion to the PCM–water interface to keep up with the rate of oligomer production. As a result, oligomers begin to accumulate in the core of the microcapsule, leading to a mixture of PCM and oligomers inside the capsule. The congestion of oligomers inside the microcapsule core prevents the PCM from forming a pure, crystalline structure during freezing. This creates a barrier to the crystallization process, and thus, the PCM remains in a supercooled liquid state until the temperature drops significantly lower than its normal freezing point. This explains the large degree of supercooling observed at higher polymerization temperatures, as shown in Figure 7b and Table 1. On the other hand, the reaction rate is significantly slower at lower polymerization temperatures (e.g., at 45 °C), as reported elsewhere [40]. This allows for the formation of oligomers at a more controlled pace. As a result, oligomers have sufficient time to diffuse to the PCM–water interface without becoming congested in the core of the microcapsule. With fewer oligomers in the core, the formation of nucleating sites becomes more likely, allowing the PCM to freeze at a temperature closer to its normal freezing point. This may explain why the microcapsules produced at 45 °C using the Azo-65 initiator exhibit a significantly reduced degree of supercooling, with a peak freezing temperature of 15.66 °C, compared to the much lower peak freezing temperature of 7.08 °C for microcapsules produced at 75 °C. In the pure RT-21 PCM sample, as there are no monomers or oligomers to interfere with the PCM’s crystallization; as such, nucleation occurs readily, resulting in minimal supercooling. This is why the pure RT-21 PCM sample shows little to no supercooling, as shown in Figure 7b.

It can also be seen in Figure 7b that microcapsules produced using Azo-65 at 45 °C exhibit a broad freezing peak. This is attributed to the low thermal conductivity of the cross-linked polymethacrylate shell. The low thermal conductivity slows the transfer of heat within the microcapsule, causing the PCM to freeze unevenly across a wider range of temperatures [36]. Additionally, the reduced nucleation rate results in fewer sites for crystallization, which further broadens the range of freezing temperatures. This broad range of freezing temperature limits the effectiveness of the microcapsules as they will be unable to release heat over a narrow range of temperatures, which is critical for applications such as thermal energy storage and temperature regulation.

3.4. Thermal Stability and Core Content

Thermal stability refers to the ability of microPCMs to maintain their functionality at specific working temperatures without degradation. One of the most common methods to evaluate thermal stability is TGA, which analyses the weight loss of a material as a function of temperature. TGA degradation curves offer crucial data about thermal decomposition temperatures for the polymer shells and the PCM itself. Therefore, they offer evidence that neither the polymer shell nor the PCM decomposes if the encapsulation process is performed under the onset degradation temperatures of both the shell and PCM. Also, TGA results suggest safe temperature operation limits, which would assure long-term stability, performance, and durability for those microcapsules.

The two-step degradation process observed in the TGA curves of microPCMs, as shown in Figure 8, is a typical thermal behavior for microencapsulated systems where both the PCM core and the polymer shell have distinct decomposition temperatures. The first degradation plateau in the TGA curve corresponds to the decomposition of the PCM inside the microcapsules. As the temperature increases, the PCM reaches its thermal stability limit. Beyond this limit, it begins to decompose or vaporize. This leads to a noticeable weight loss in the TGA curve. When the PCM decomposes, the microcapsules lose their primary latent heat storage capability. This step marks a point where the microcapsule no longer functions as intended for thermal energy storage, as the core material responsible for the phase change process is destroyed. The second degradation step is associated with the decomposition of the polymer shell surrounding the PCM. This step marks the thermal breakdown of the microcapsule’s structural framework. As the polymer shell degrades, the microcapsule collapses, thus leading to further weight loss in the TGA curve. The degradation of the polymer shell is typically more gradual than that of the PCM due to the more robust nature of polymeric materials. At this stage, the polymer shell material (cross-linked PMMA) degrades into smaller molecules, resulting in the total loss of the microcapsule’s structural integrity. This leaves no shell structure behind, and the weight loss approaches the final level, as seen in the second degradation plateau (Figure 8).

The TGA results in Figure 8 show a clear correlation between the polymerization temperature and the thermal stability of the microPCMs. As the polymerization temperature decreases, the thermal degradation of the microPCMs occurs at lower temperatures (

Table 2. Onset thermal degradation temperature and PCM core content (%) of the prepared microPCMs at different polymerization temperatures, measured via TGA.

Initiator	Teo (°C)	${\mathit{C}\mathit{o}\mathit{r}\mathit{e}}_{\mathit{P}\mathit{C}\mathit{M}}$ (%)
MicroPCMs-PBO (75 °C)	181.40	85
MicroPCMs-Azo-65 (75 °C)	187.72	84
MicroPCMs-Azo-65 (65 °C)	182.62	80
MicroPCMs-Azo (55 °C)	173.03	85
MicroPCMs-Azo (45 °C)	164.57	93

Note: T_eo: Extrapolated onset temperature (°C), Core_PCM (%): % PCM core content.

). This trend can be attributed to a thinner polymer shell formed at lower polymerization temperatures. The thinner shell fails to offer sufficient protection to the PCM core, which leads to lower thermal stability and earlier degradation. Interestingly, Table 2 also shows that microPCMs produced at 65 °C using the Azo-65 initiator have an onset degradation temperature of 182.62 °C, which is almost identical to the 181.40 °C observed for microPCMs produced using the BPO initiator at 75 °C. This suggests that even though Azo-65 allows for a reduction in polymerization temperature to 65 °C, it does not compromise the thermal stability of the microPCMs. The similarity in the onset degradation temperatures for both BPO at 75 °C and Azo-65 at 65 °C implies that Azo-65 is an effective initiator for producing microPCMs with comparable thermal properties to those produced with BPO, despite the lower polymerization temperature.

TGA can also be used as a tool to roughly estimate the percentage of core content, as described in a previous publication [13,31]. Based on the TGA measurements, the microPCMs produced using the Azo-65 initiator at 45 °C exhibit the highest core content of 93%, which is significantly greater than the core contents of 75.3% achieved by Qiu et al. [25]. While this high core content is advantageous for maximizing the latent heat (thermal storage capacity) of the microcapsules, it also comes with a trade-off, where such microcapsules have thinner polymer shells due to the high PCM-to-shell ratios. The comparison with Qiu et al.’s results suggests that the use of PETRA or a mixture of other cross-linkers could be an effective strategy to enhance the thermal stability of the microPCMs. Cross-linking agents strengthen the polymer network, making it more heat-resistant and slowing down their thermal degradation process. Incorporating such cross-linkers into microPCMs could help prevent early thermal degradation, even when the core content is high, thereby improving the overall performance and stability of the microcapsules.

3.5. Thermal Reliability

The thermal reliability of microencapsulated PCM was evaluated by exposing PCMs to 50 cooling–heating cycles by forcing changes in the surrounding’s temperature and monitoring the temperature responses of the encapsulated PCM over repeated cycles. The observations from the first and fiftieth cooling–heating thermal cycles shown in Figure 9 highlight the impact of the microcapsule structure on heat transfer rates. Pure PCM (RT-21) responds faster to temperature changes, as demonstrated by the greater delay in temperature recorded between the water bath and RT-21 sample compared to the microPCM samples. This is because there are no barriers impeding the transfer of heat in pure PCM. On the other hand, heat transfers through microPCMs more slowly due to the higher heat transfer resistance caused by the polymer shell, which has low thermal conductivity. In addition, the microPCMs are not tightly packed and may contain air gaps between them, which act as thermal insulators, adding to the delay in temperature response. The results in Figure 9, consistent with the DSC analysis (Figure 7), show significant supercooling in all microPCMs except for the Azo-65 microcapsules synthesized at 45 °C. The phase change temperature of microPCMs produced at 45 °C is visible on the graph as a brief plateau, where the temperature remains stable even as that of the water bath continues to drop. This plateau is an indicator of latent heat release during the phase transition, showing that the PCM is absorbing energy to change its state, thus “resisting” the temperature change. This behavior demonstrates that this microPCM is effectively performing its intended function: storing thermal energy when melting and providing thermal regulation at the desired phase transition temperature of around 21 °C when freezing.

Thermal reliability ensures that the material will not degrade or lose efficiency after repeated phase transitions. For the encapsulated PCM, the microPCMs need to maintain their thermal properties, including melting points and latent heat storage capacities through many heating and cooling cycles since this is crucial for the successful deployment of microPCMs in various industrial and residential applications, ensuring both long-term functionality and economic viability. Consistent thermal repeatability ensures predictable behavior, making it easier for engineers to design energy systems with precise thermal management plans based on known thermal characteristics. Further, systems using reliable microPCMs reduce maintenance and replacement costs, since frequent failures or the degradation of encapsulated PCMs can lead to costly repairs or inefficient energy storage solutions. The data presented in Figure 9b, compared to the initial thermal cycle in Figure 9a, display an altered response to heating and cooling after 50 thermal cycles, particularly for the Azo-synthesized microcapsules at 45 °C. The result indicates poor thermal stability over repeated use for the microPCM sample synthesized at 45 °C. This is a critical drawback because microPCMs are expected to maintain consistent thermal behavior over many cycles. The altered response is corroborated by the TGA results, where these microcapsules showed the lowest onset degradation temperature of 164.57 °C (Table 2). This lower degradation temperature signals more thermal vulnerability compared to other samples, leading to loss of structural integrity and diminished performance. In contrast, the Azo microcapsules produced at 75 °C, as well as most other microcapsule samples, exhibit consistent thermal responses even after 50 thermal cycles, as shown in Figure 9b. This suggests they have good thermal stability and reliability over time, thus meeting the expectations for repeated thermal cycling without significant degradation or performance loss. The improved stability of the microPCMs synthesized at 75 °C could be due to more-robust encapsulation, better shell formation, or enhanced crosslinking during synthesis, which provides greater resistance to thermal stress.

3.6. Comparing the Results of This Study with Related Publications

The results of this study were also compared with related studies using other polymerization techniques, as summarized in Table 3. Many studies, as well as using commercially available PCM microcapsule from manufacturers, have employed in situ polymerization (interfacial) techniques for synthesizing PCM microcapsules. UF or MF resins are some of the commonly reported shell materials utilized for the fabrication of microcapsules using the in situ polymerization technique. However, due to their toxic nature, the production of formaldehyde-based microcapsules needs to adopt a significant preparation route coupled with stringent safety precaution measures. In addition, some of the residues of these formaldehyde resins in shells can cause environmental and health problems. Other researchers used coacervation techniques for the microencapsulation of PCM using natural shell materials (Arabic gum, natural chitosan) [41,42]. Although the coacervation technique was judged to be simple and eco-friendly, the materials used to encapsulate PCM via the complex coacervation method are hosts for bacteria growth; hence, they are not commonly used in building and medical applications. The microPCMs in this study were fabricated by means of suspension polymerization and using polymethyl methacrylate polymer shells that possess high mechanical strength and good chemical stability and are nontoxic, and easy to use [11,43]. In terms of PCM content, the highest reported in the literature using UV-assisted microencapsulation at a low temperature by means of suspension polymerization was 87.4 [16]. However, UV light at specific wavelengths around 360 nm poses significant health and safety risks. This study presents a high PCM content of 82%, representing a significant improvement in the microencapsulation of heat-sensitive PCM.

4. Applications of MicroPCMs

The wide-ranging applications of PCM microcapsules highlight their versatility and growing importance in improving energy efficiency, temperature regulation, and environmental sustainability. Extensive research has focused on their incorporation into various sectors, with promising results [44,45,46]. For instance, studies have assessed the thermal behavior of cement mortar incorporating microencapsulated bio-based phase change materials (PCM) at both wall and building scales, demonstrating energy savings of up to 33% for heating and 31% for cooling, contingent on climate conditions [47]. Further research has examined the use of microencapsulated PCM slurries for cooling in micro-channel heat exchangers, revealing that a 10% microPCM slurry can increase the heat transfer coefficient by approximately 3.5 times compared to pure water. The enhancement was attributed to an increase in fluid thermal capacity caused by PCM melting, with additional improvements from micro-mixing within the capsules.

Table 3. Summary of PCM microencapsulation techniques and key findings reported in the literature [15,16,48,49,50,51,52,53,54,55].

Microencapsulation Technique	Core Material	Shell Material	Polymerization Temperature	Peak Melting Temperature (Tamp) °C	Latent Heat of Melting (J/g)	PCM Content (%)
in situ polymerization	eutectic mixture (75% SA + 25% CA)	melamine formaldehyde (MF)	70	34.5	103.9	85.3
in situ polymerization	C18 paraffin	melamine formaldehyde (MF)	70	20.5	164.8	84.3
interfacial polymerization and the sol–gel process	SCD-DHPD eutectic PCM	Inorganic Silica shell	40	53.1 °C.	97.7	45.8
coacervation	poly (octadecyl acrylate)	natural chitosan	85	32.9–47.3	92.9–131.4	49.8–69.0
interfacial polymerization	butyl stearate	chitosan-based polyurethane (c-PU)	50–90	21	106.3	71.4
emulsion polymerization technique	paraffin	titanium dioxide (TiO2)-modified chitosan (CS)	40	125.5	125.5	81.3
suspension-like polymerization	n-octadecane	poly (methyl-methacrylate) doped with titanium dioxide nanoparticles	80	4.5	89.0–153.8	43.10–73.20
suspension-like polymerization	pure temp (PT) 6	cross-linked polymethyl methacrylate	30	8.2	131.10	87.40
suspension-like polymerization	butyl stearate	comb-like acrylic co-polymer	85	23.5	102.3	86.6
in situ polymerization (commercially available microcapsules)	paraffin and bio-base methyl ester	melamine based capsules	70	−10, 6, 18, 24, 28, 29, 32, 37	~150–205	-
suspension-like polymerization (This work)	paraffin (RT21)	cross-linked polymethyl methacrylate	45	25.6	107.8	82.0

Table 3. Summary of PCM microencapsulation techniques and key findings reported in the literature [15,16,48,49,50,51,52,53,54,55].

Microencapsulation Technique	Core Material	Shell Material	Polymerization Temperature	Peak Melting Temperature (Tamp) °C	Latent Heat of Melting (J/g)	PCM Content (%)
in situ polymerization	eutectic mixture (75% SA + 25% CA)	melamine formaldehyde (MF)	70	34.5	103.9	85.3
in situ polymerization	C18 paraffin	melamine formaldehyde (MF)	70	20.5	164.8	84.3
interfacial polymerization and the sol–gel process	SCD-DHPD eutectic PCM	Inorganic Silica shell	40	53.1 °C.	97.7	45.8
coacervation	poly (octadecyl acrylate)	natural chitosan	85	32.9–47.3	92.9–131.4	49.8–69.0
interfacial polymerization	butyl stearate	chitosan-based polyurethane (c-PU)	50–90	21	106.3	71.4
emulsion polymerization technique	paraffin	titanium dioxide (TiO2)-modified chitosan (CS)	40	125.5	125.5	81.3
suspension-like polymerization	n-octadecane	poly (methyl-methacrylate) doped with titanium dioxide nanoparticles	80	4.5	89.0–153.8	43.10–73.20
suspension-like polymerization	pure temp (PT) 6	cross-linked polymethyl methacrylate	30	8.2	131.10	87.40
suspension-like polymerization	butyl stearate	comb-like acrylic co-polymer	85	23.5	102.3	86.6
in situ polymerization (commercially available microcapsules)	paraffin and bio-base methyl ester	melamine based capsules	70	−10, 6, 18, 24, 28, 29, 32, 37	~150–205	-
suspension-like polymerization (This work)	paraffin (RT21)	cross-linked polymethyl methacrylate	45	25.6	107.8	82.0

Although the slurry increased the pressure drop, the heat transfer enhancement outweighed this drawback [56]. In another study, the use of microencapsulated PCM slurry flow within a porous double-pipe heat exchanger was evaluated and optimized. The research indicated that PCM microcapsule slurry can improve cost-effectiveness when meeting lower performance evaluation criteria (PEC), with negative impacts on PEC at high Reynolds numbers, leading to a 13% reduction in PEC when the mass concentration was increased from 5% to 15% [57]. In the field of textiles, the incorporation of 1-tetradecanol microcapsules on cotton fabric for enhanced thermoregulation has been studied. The microcapsule coating, using a 75:25 microcapsule–binder ratio, achieved the highest add on (55%) and showed good durability after 25 home washes. The thermal insulation R-value of the fabric was significantly enhanced at 40 °C, demonstrating improved thermoregulation properties. Thermal imaging revealed a 2.8 °C temperature difference and lower emissivity for the coated fabric [58]. Cold-chain logistics applications have benefitted from PCM integration in insulated containers used for temperature-controlled transportation. By incorporating PCM with a melting/freezing point of 5 °C, the internal temperature of the container was maintained at 4–5 °C for over 80 h, compared to just 1 h without PCM. This significantly extended the temperature-controlled time, showcasing the potential of PCM microcapsules in improving the efficiency and sustainability of temperature-sensitive supply chains [59]. The promising contribution of microPCMs to energy savings, thermal regulation, and environmental sustainability underscores their growing significance across various industries and highlights the need for continued development in PCM microcapsule synthesis.

5. Conclusions

Cross-linked polyMMA microcapsules containing commercial PCM (RT-21) were successfully synthesized using a suspension polymerization method. The influence of polymerization temperatures and type of initiator on the microPCMs’ thermal properties (including PCM content, thermal stability, PCM supercooling, and reliability against thermal cycling) were investigated. The choice of initiator (Azo-65 or BPO) impacted the thermal properties of the microcapsules, with the Azo-65 initiator leading to reduced thermal stability at lower polymerization temperatures due to thinner polymer shells.

The synthesized microPCMs exhibited a monodisperse and narrow size distribution, with an average particle size of 10 µm, indicating that the emulsification process was well controlled and thus preventing instability and coalescence. Rough surface textures and dimples were seen in most of the microPCMs produced at the higher temperature (75 °C), regardless of the type of initiator used, BPO or Azo-65. However, microencapsulated PCMs synthesized at 45 °C using the Azo-65 initiator showed spherical particles with smooth surfaces due to the slower reaction rate at this lower temperature.

Based on the TGA measurements, the PCMs microencapsulated at lower temperatures, specifically at 45 °C using the Azo-65 initiator, displayed the highest core content (93.66%) but also had the thinnest polymer shell, leading to compromised thermal stability. The onset of supercooling was observed across all microcapsules, with the exception of those produced at 45 °C, which experienced phase change temperatures closer to that of the pure PCM (non-encapsulated).

TGA results showed that the highest extrapolated onset temperature was obtained for microcapsules polymerized at 75 °C using the Azo-65 initiator (Tonset = 182 °C), whereas the microcapsules synthesized at 45 °C exhibited the lowest extrapolated onset temperature of 164.54 °C. As a result, those synthesized at low temperatures have lower thermal stabilities. This is due to the lower polymerization reaction rate, which reduces the shell thickness and thus results in a weaker shell. The microcapsules synthesized at lower temperatures, especially 45 °C using Azo-65, also demonstrated a poor thermal reliability after multiple thermal cycles, showing altered temperature responses and reduced stability. However, the microcapsules with thicker shells (e.g., those produced at higher polymerization temperatures), showed better thermal reliability and were thus more suitable for long-term applications.

Overall, this work underscores the trade-off between polymerization temperature and thermal stability, suggesting that, while lower temperatures can enhance certain thermal properties, they may also compromise the durability or thermal stability of the microcapsules. Nonetheless, low-temperature polymerization presents clear advantages, such as reduced energy costs and the ability to microencapsulate temperature-sensitive and volatile PCMs that have previously been challenging to encapsulate.

It is recommended that future research focus on exploring low-temperature polymerization (e.g., at 45 °C) with extended polymerization times of 12, 18, and 24 h. This approach is anticipated to yield microcapsules with enhanced thermal stability and PCM content and diminished supercooling effects, thereby improving the overall efficiency and applicability of PCMs in building applications.