## 4 Microencapsulation Techniques
### Chemical Process
#### 4.1.1 Microencapsulation by Heterophase Polymerization  
* Suspension polymerization  
The suspension polymerization includes two phases, i.e., (i) the dispersed phase containing the active agent, water-insoluble monomers, and initiator; (ii) the continuous phase involving solvent and reactant of shell materials (Figure 4) [37]. PCM droplets are obtained by vigorous agitation of the mixture and dissolution of stabilizers in aqueous phases such as poly(vinyl alcohol), polyvinyl pyrrolidone, or cellulosic derivative. This method is mainly used to encapsulate with polystyrene, poly(methyl methacrylate), and poly(vinyl chloride) shells [80]. The advantages of this method are the formation of uniform spherical particles, excellent heat control of the reaction, and its cost-effectiveness. The main drawback is the coalescence of particles during polymerization [81]. The complex interaction between the polymer system and the continuous phase on the surface of the droplets defines the efficiency of encapsulation and particles properties and depends on several factors. It was reported that the thermal energy storage capacity depends strongly on surfactant concentration and core/polymer ratios. In addition, the polymerization temperature has a significant effect on particle morphology without impacting the particle size [82]. Moreover, the loading content and particle size depend on the nature of core materials [83]. To improve polystyrene shell properties, [PERSON] et al. studied the development of paraffin microcapsules by using copolymerization of styrene (St) and methyl methacrylate (MMA) [84]. They observed that the MMA/St mass ratio affects the polymerization rate and that the particles size decreased with the increase in MMA amount. In addition, the weight ratios MMA/St of 4 and monomers/paraffin of 4 and 3 were established as suitable conditions to favorite paraffin microencapsulation. Furthermore, [PERSON] et al. employed this method to entrap caprylic acid in a PMMA shell with a mean particle diameter of 4 \(\upmu\)m [85]. The microcapsules exhibited enhanced thermal properties for thermo-responsive textile fabrics, PCMs slurries, and thermo-responsive functional coatings. Figure 4: Schematic representation of the microencapsulation by suspension polymerization. * Emulsion/miniemulsion polymerization  
The emulsion polymerization involves emulsifying monomers and generating a cross-linked system in chemical, thermal, or enzymatic ways. The mean diameter of the resulting particles is between 50 nm and 1 \(\upmu\)m and is influenced by the mass ratio of monomer/aqueous phase, surfactant and initiator concentration, and polymerization temperature [86]. In emulsion polymerization, unlike suspension polymerization, the initiator is soluble in the aqueous phase. The first step consists of the formation of monomers micelles with the aid of a surfactant. Monomers are insoluble or scarcely soluble in the polymerization medium. The polymerization starts by introducing water-soluble initiators leading to free radical formation and diffusion through the swollen monomer micelles [87] (Figure 5). The advantage of this method is the possibility of employment of high molecular weight polymer. The main drawback is the necessity of an additional step to remove polymerization adjuvants and surfactants that remain in the polymer [37]. Polymethyl methacrylate (PMMA), polystyrene, polyvinyl acetate, poly(ethyl methacrylate) are widely used for PCMs encapsulation because of their excellent chemical resistance, non-toxic and easy handling. [PERSON] et al. worked on the preparation of \(n\)-alkanes/PMMA microcapsules using \(n\)-octacosane [88], docosane [89], \(n\)-nonadecane [90] as core materials. PCMs microcapsules with a loading content of 43, 28, and 60 wt% were obtained, respectively. They indicated that the type of \(n\)-alkanes, the weight ratio of monomer/PCMs, and the crosslinker agent significantly influence the morphology, particle size, and encapsulation efficiency.