obtained composite capsules exhibit a relatively high encapsulation ratio (89.5%), which is the ratio of the fusion enthalpy of encapsulated phase change composites to that of bulk PCMs.[25] Docosane‐loaded microcapsules with PU shell were prepared via miniemulsion interfacial polymerization. The size of the microcapsules plays a vital role in the thermal properties of the final product.[26]
Figure 2.3 Illustrations of (a) preparation routes and (b) various architectures of micro/nanoencapsulated PCMs.
Source: Aftab et al. [20]. Reproduced with the permission from the Royal Society of Chemistry.
Aside from supporting effect, these polymer shell materials are required to impart some important characteristics such as biocompatibility, thermal stability, and flame retardancy to micro/nanoencapsulated phase change composites. For example, the introduction of phosphorus–nitrogen containing diamine into MF/n‐octadecane phase change composites significantly increased the limiting oxygen index (LOI) and effectively inhibited the release of heat and smoke. This flame‐retardant nanoencapsulated phase change product exhibits considerable potential in TES applications such as energy saving construction and thermoregulated textile.[27]
Although polymers have many excellent properties, their applications are limited by several key problems, such as poor thermal response and flammability. To solve the dilemmas, inorganic materials (e.g. SiO2, [28, 29] TiO2,[30] CaCO3,[31] and hybrids [32]) with excellent thermal conductivity, thermal stability, flame retardancy, and mechanical strength have been used to prepare encapsulated phase change composites with organic PCMs, especially for low‐molecular compounds. Among them, SiO2 becomes the most widely studied inorganic shell material for composite phase change micro/nanocapsules, which is attributed to its unique superiorities such as high melting point and low thermal expansion coefficient. However, they suffer from brittleness and poor compatibility. Therefore, the high thermal conductivity of inorganic shell and the good compatibility of polymer shell are integrated to develop emerging encapsulated phase change composites with organic–inorganic hybrid shell, which are expected to become the next‐generation star products. The typical organic–inorganic hybrid encapsulation method includes microemulsion polymerization, pickering emulsion polymerization, seeded emulsion polymerization, and soap‐free emulsion polymerization.
Yang et al.[33] fabricated a polyurea/TiO2 hybrid shell to encapsulate n‐octadecane by a two‐step liquid phase deposition method. The n‐octadecane/polyurea pre‐microcapsules were first synthesized through interfacial polymerization, followed by the deposition of TiO2 on the surface of pre‐microcapsules. Tang et al.[34] synthesized a PMMA/SiO2 hybrid shell to encapsulate n‐octadecane PCMs using a novel photocurable pickering emulsion method, and the introduction of ultraviolet (UV) irradiation effectively shortened the polymerization time and decreased the processing temperature. Furthermore, TiC was introduced into the PMMA‐SiO2 hybrid shell to improve the thermal conductivity and thermal stability of the polymeric phase change composites.[35] Similarly, a PMMA/TiO2/boron nitride (BN) hybrid shell with high thermal conductivity has been developed to encapsulate PW through a pickering emulsion method (Figure 2.4a).[36] MF, graphene oxide (GO), and PW acted as shell, extra protective screen, and core, respectively, to prepare microencapsulated phase change composites with high encapsulation ratio (up to 93.9 wt%) through in‐situ polymerization (Figure 2.4b).[37] Along this line, carbon nanotube (CNT) as thermally conductive enhancement component was added into MF/GO shell system to facilitate thermal charging/discharging of microencapsulated phase change composites.[38]
Figure 2.4 Schematic diagrams of the fabrication route of composite phase change microcapsules with paraffin core and (a) PMMA/BN/TiO2 hybrid shell.
Source: Sun and Xiao [36]. Reproduced with the permission from the American Chemical Society
or (b) MF/GO hybrid shell.
Source: Zhang et al. [37]. Reproduced with the permission from Elsevier Ltd.
2.2.2 Physical Blending
It is quite appealing that supporting materials are directly introduced into the phase change matrices via physical blending to produce shape‐stabilized phase change composites. Compounding organic PCMs, especially PW, with robust or flexible polymers featuring high melting temperature or network structure is an effective strategy to achieve this goal. Accordingly, a facile melt blending has been conducted to fabricate polymeric phase change composites, including polyethylene (PE)/PW,[39] ethylene propylene diene monomer (EPDM)/PW,[40] styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene (SEBS)/PW,[41] olefin block copolymer (OBC)/PW,[42] OBC/hexadecane,[43] epoxy/PW,[44] polyolefin elastomer (POE)/lauric acid,[45] POE/stearic acid,[46] PMMA/stearic acid,[47] etc. For example, Chen et al.[48] introduced high‐density PE (HDPE), low‐density PE (LDPE), and linear LDPE (LLDPE) into PW through melt blending to fabricate three kinds of polymeric PCMs. The results indicated that the leakage behavior could be controlled by the co‐continuous structure, and the PE phase had a minor influence on the crystallinity structure and thermal transition temperatures of PW phase. For these polymeric PCMs, the organic PCMs content is maintained in the range of 30–60 wt%, and most of them can be used as shape memory devices. To impart high thermal conductivity to these polymeric PCMs, functional fillers can be added to prepare thermally conductive polymeric phase change composites,[49, 50] and more discussion will be provided in the next section.
Figure 2.5 Microstructures of (a) EVM.
Source: Deng et al. [51].
(b) EP.
Source: Zhang et al. [52].
(c) diatomite.
Source: Qian et al. [53].
(d) EG.
Source: Wang et al. [54].
(e) graphene.
Source: Shi et al. [18].
and (f) GO.
Source: Yang et al. [55].
The innovations of nanotechnology open up new frontiers in energy conversion and storage materials and systems, and thus multifarious functional nanomaterials can act as supporting materials to prepare shape‐stabilized polymeric phase change composites. Effective supporting nanomaterials shown in Figure 2.5 possess the following three main characteristics:
1 Micro/nanoporous structure can provide capillary force to accommodate PCMs. Typical candidates include expanded vermiculite (EVM),[51] expanded perlite (EP),[52] diatomite,[53] expanded graphite (EG),[54] and activated carbon[56].
2 Large specific surface area can yield strong surface tension to absorb PCMs. The typical representatives are two‐dimensional (2D) graphene with a specific surface area of 2630 m2 g−1[57] and MXene[58].
3 Hydrogen