## 4 Microencapsulation Techniques
### Chemical Process
#### 4.1.1 Microencapsulation by Heterophase Polymerization
The resulting microcapsules exhibit relative thermal performances and can be employed for textile, building, or food packaging materials, in thermal fluids. [PERSON] et al. obtained a higher \(n\)-hexadecane encapsulation ratio, smaller size, and enhanced morphology by using glycolidyl methacrylate or ethylene glycol dimethacrylate as a crosslinker agent [91]. The ultraviolet (UV) light initiating polymerization is improved technology for preparing PCMs/PMMA microcapsules due to its significant advantages compared to the traditional thermal initiated polymerization, such as the temperature independence of the radical formation higher reaction rate. The light-induced polymerization limits the colloidal particle destabilization as the reaction is conducted at ambient temperature. In addition, the reaction rate of polymer can be maintained by varying intensity of light and time of irradiation [92]. The efficiency of this approach has been established by [PERSON] et al. to prepare paraffin/PMMA microcapsules [93]. They obtained a loading content of 61% wt with the use of Photocure(r) 2959 as a water-soluble photoinitiator. [PERSON] et al. prepared mPCMs using  
Figure 5: Schematic representation of different stages of emulsion polymerization. 2-hydroxyl-2-methyl-1-phenyl acetone [94], or iron (III) chloride as photoinitiator [95]. They demonstrated that the types of surfactants affect thermal properties and particle size distribution, even if cationic and nonionic emulsifiers are suitable for preparing nanocapsules. In addition, the particle size distribution narrowed with the addition of crosslinking agent and the reduction in initiator and monomer concentration. The latent heat of nanocapsules decreased with the increase in monomer and initiator concentration. [PERSON] et al. developed paraffin wax/polystyrene microcapsules. The average size of obtained microcapsules was about 18 \(\upmu\)m [96]. The particle size decreased by increasing the amount of surfactant and the stirring rate, enhancing the microencapsulation efficiency. Four parameters of this microencapsulation process were optimized to encapsulate lauric acid (LA), i.e., LA/polystyrene, and surfactant/polystyrene weight ratios, stirring rate, and temperature; to obtain an encapsulation efficiency about of 92% [97]. The miniemulsion polymerization method is employed to form PCMs nanocapsules (50-500 nm) following the same process of emulsion polymerization. This technique is a simple method for the preparation of PMMA nanocapsules. The core/monomer mass ratio and crosslinking agents such as ethylene glycol dimethacrylate are the principal factors that influence the size, morphology, thermal stability, and thermal storage behavior of nanocapsules. Encapsulation efficiency and heat transfer rate can be improved by the application of ultrasonic radiation [98]. This method was used for the encapsulation of _n_-eicosane/PMMA microcapsules [99]. Traditional low molecular weight emulsifiers are physically adsorbed on the polymer and influence the stability of particles. [PERSON] et al. demonstrated that the employment of reactive emulsifiers such as alkyl vinyl sulfonate stabilizes the emulsion, participates in polymerization with monomers, and is integrated into the polyacrylate shell [100]. In addition, the morphology of nanocapsules could be enhanced using a comonomer. [PERSON] et al. reported the development of _n_-dodecanol/PMMA nanocapsules using butyl acrylate, acrylamide, and acrylic acid as comonomers. They concluded that the comonomer's hydrophilicity increased the polymer chain's hydrophilicity, which affected the encapsulation of oil-soluble PCM _n_-dodecanol efficiency. They also noticed that the type and amount of comonomers had a significant influence on the thermal properties and morphology of the nanocapsules [101]. The dispersion polymerization involves a single step, and all reactants are dissolved in the continuous phase to form a homogeneous solution.