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- RESULTS AND DISCUSSION
PRS® paraffin wax was encapsulated by means of suspension-like copolymerization of methyl methacrylate (MMA) with butyl acrylate (BA). The effects of the polymeric shell dry glass transition temperature (Tg) and the reaction temperature (Tr) were then studied. Additionally, the evolution of particle diameter, molecular weight, conversion, and Tg during polymerization was also researched. The chemical properties of the shell material (acrylic polymer), together with those found in the core material (PRS® paraffin wax), for instance: polarity and interfacial tensions, largely determine whether the morphology of the microcapsules will be thermodynamically favored or not. The high polarity of MMA (γ0 = 18 mN m−1) and BA (γ0 = 24 mN m−1) should provide a thermodynamic driving force to cover the paraffin wax droplet which would result in a core/shell thermodynamically favored structure. However, most systems are defined by kinetics rather than thermodynamics such as the monomers dry Tg and Tr. It was observed that penetration of polymer radical chains was severely limited when the dry Tg was ≥10°C above the reaction temperature, resulting in irregular and undifferentiated particles. However, penetration did occur when the copolymeric shell dry Tg was ∼10°C below the reaction temperature which led to uniform and spherical particles being synthesized. POLYM. ENG. SCI., 54:208–214, 2014. © 2013 Society of Plastics Engineers
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- RESULTS AND DISCUSSION
Nowadays, the rise in world energy demand calls for new energy solutions. Furthermore, conventional fossil energy sources are limited and environmentally unsustainable due to harmful greenhouse gases. In this context, thermal energy storage has proved to be important technology for use in renewable energy systems and has received growing attention in recent years [1, 2].
Phase change materials (PCM) are a series of functional materials capable of storing and releasing energy when undergoing a phase transition. PCMs absorb heat when they melt (phase change from solid to liquid) and release it when they solidify (phase change from liquid to solid). Some applications they have, described elsewhere, are: thermal storage of solar energy , cooling , thermal protection of food , for use in the textile industry  and in energy conservation in buildings with thermal comfort [7, 8]. Some of the more commonly used PCMs are linear chain hydrocarbons known as paraffin waxes (or n-alkanes), hydrated salts, polyethylene glycols (PEGs), fatty acids and mixtures or eutectics of organic and nonorganic compounds. Commercial paraffin waxes compared to others PCMs are widely available, have a moderate thermal storage density of (200 kJ kg−1), a wide range of melting temperatures and are chemically inert and inexpensive .
Moreover, they can be encapsulated with a plastic or crosslinked polymer shell to prevent liquids from leaking during their use. The most common methods for paraffin microencapsulation are interfacial polymerization [10, 11], emulsion polymerization [12, 13], in situ polymerization [14, 15], layer by layer deposition of polyelectrolyte [16, 17], coacervation , spray drying  and suspension polymerization [20, 21].
The choice of shell material is one of the most important aspects of the process for controlling the mechanical strength, thermal properties and the microcapsules morphology and the most widely used materials to encapsulate paraffin waxes are gelatine and arabic gum [22, 23], melamine-formaldehyde prepolymer [24, 25], silica  and methyl methacrylate-based polymer [27, 28].
In previous research [21, 29], PCMs were successfully encapsulated in a polystyrene shell using a suspension-like polymerization technique. However, cross-section micrographs of the poly(methyl methacrylate-co-styrene) revealed a composite salami-like internal morphology (as opposed to a more traditional core/shell) due to the lack of a strong driving force for phase separation of the polystyrene formed within the paraffin wax droplet. The complex interaction between the polymer system (initiators, monomers, and polymers) and the continuous phase (usually water and the suspension agent) on the surface of the droplets is responsible for the success in the encapsulation. Thus, the chemical property differences between the encapsulating “shell” material, and the “core” material, such as polarity and interfacial tensions, determined to a large extent whether the morphology of the microcapsules would be thermodynamically favored or not. Here, the polystyrene polarity value was quite similar to that of the paraffin wax, and thus the core/shell morphology was not thermodynamically favored during polymerization .
The paraffin wax was also microencapsulated by means of a suspension-like polymerization using a shell based on a methyl methacrylate and styrene copolymer . The average energy storage capacity (87.5 J g−1) of the microcapsules obtained using this copolymer was higher than the values reported in previous papers for styrene alone (41.7 J g−1). Hence, the higher polymerization rate and polarity of methyl methacrylate compared to styrene favored the development of a core/shell morphology. Moreover, a higher latent heat value was achieved.
Additionally, the microcapsules containing PCMs were synthesized using acrylic-based polymer shells to improve the paraffin encapsulation efficiency. The acrylic shell composition was modified so as to achieve greater encapsulation efficiency (93.5%) and a higher latent heat value (94.8 J g−1), which was more than double that of the value obtained with a polystyrene shell .
Copolymerization of methyl methacrylate with butyl acrylate (P(MMA-co-BA)) as shell material should form a thermodynamically favored core/shell morphology. The greater combined polarity of MMA (γ0 = 18 mN m−1) and BA (γ0 = 24 mN m−1) in comparison with styrene (γ0 = 32 mN m−1)  should provide a thermodynamic driving force to cover the paraffin wax droplet.
However, Sundberg et al.  stated that the majority of systems are defined by kinetic factors such as the monomer dry glass transition temperature (Tg) and the reaction temperature (Tr) and with these considerations, PMMA-based paraffin encapsulation should have produced microcapsules with completely separated phases. However ESEM micrographs showed there to be a uniform mixture of both paraffin wax and PMMA polymer domains . This was probably due to the fact that the wet polymer glass transition temperature (94°C) was markedly higher than the reaction temperature (70°C) and, as a result, mobility of the polymeric radicals was low due to their glassy state. Thus, copolymerization of MMA with BA, whose Tg is −40°C , would decrease the copolymeric shell Tg. According to Sundberg et al. , the penetration of polymer radical chains fully occurs when the copolymeric shell dry Tg is about 10–15°C below the reaction temperature.
In this article the effect of Tg on the microencapsulation process was evaluated by varying the copolymeric shell dry Tg. Alay et al.  synthesized microcapsules by using emulsion polymerization of poly(butyl acrylate) (PBA) for a shell and n-hexadecane for a core. Zhang et al.  microencapsulated paraffin wax with poly (methyl methacrylate-co-acrylic acid) for a shell by means of emulsion polymerization.
The evolution of particle diameter, molecular weight, conversion, and dry glass transition temperature during the microencapsulation process was then studied so as to achieve a better understanding of the microencapsulation process.