or negligible supercoolingGood compatibilityNontoxicity and noncorrosionDesirable thermal and chemical stability for long‐term useAbundant natural resources
Polymeric phase change composites, as TES units, can be roughly divided into two categories: phase change polymers presented in Figure 2.2 as working substance and common polymers as supporting component. In this chapter, we put our emphases on the innovations of shape‐stabilized and thermally conductive polymeric phase change composites, covering processing method, structural design, and materials selection. Also, energy conversion routes and potential applications associated with polymeric phase change composites are highlighted. Finally, a brief perspective on future opportunities and challenges for high‐performance and multifunctional polymeric phase change composites is proposed.
2.2 Shape‐stabilized Polymeric Phase Change Composites
Organic solid–liquid PCMs suffer from liquid phase leakage in the process of phase transition, resulting in decrease of energy storage density, pollution of related devices, and security risks. Shape‐stabilized PCMs are composed of working substance and supporting material, and the latter predominately includes polymers with good compatibility and thermal stability, nanomaterials with desirable structure, and porous scaffolds with excellent mechanical properties.[13] Micro/nanoencapsulated techniques, physical blending with supporting materials, incorporating porous scaffolds, and crosslinking‐solidity strategy have been adopted to improve the shape stability of PCMs, which are magnified in this chapter.
To the best of our knowledge, the methods used to evaluate the shape stability of PCMs have not been unified and standardized. Thermal mechanical analyzer (TMA) and dynamic rheometer are usually used to quantitatively measure the size change of materials to evaluate their shape stability. With the increase of temperature, the size or dimension of samples will automatically adjust when subjected to constant normal force, and thus the shape stability can be determined by the change degree of the size or dimension. Generally speaking, the smaller the change of the sample size, the better the shape‐stabilizing effect of the material.[14] In addition to monitoring change of sample size in rheological test, dynamic temperature scanning can be used to compare the difference of storage modulus between pure sample and composites, further to judge the shape‐stabilizing effect.[15] Moreover, facile and qualitative leakage experiments are conducted to intuitively reflect the shape‐stabilizing effect in most cases. The samples are directly placed on a hot stage or in an oven and then heated to different temperatures for varying durations, followed by tracking real‐time photography with a digital camera to record the change of shape and size and leakage status of samples. It is more reasonable to calculate the mass change of samples before and after leakage test.[16, 17] Additionally, Shi et al.[18] measured the dropping point to determine the shape‐stabilizing effect of samples.
2.2.1 Micro/Nanoencapsulated Method
Micro/nanoencapsulation is one of the common techniques to fabricate shape‐stabilized PCMs with core‐shell structure, in which the core and the shell are, respectively, the working medium performing energy storage and functional coating layer providing complete and stable structure. In general, PCMs with phase change temperature in the range of −10–80 °C can be encapsulated.[19] Micro/nanostructures of phase change capsules can be tuned by using different preparation methods and controlling synthesis conditions, mainly forming single core‐shell, multi‐shell and polynuclear structures (Figure 2.3).[20] Microencapsulated phase change products present different appearances, such as sphere, tubular, and other irregular shapes. The coating materials mainly include organic polymers, inorganics (e.g. silica, metal oxides, and hydroxides), and their hybrids.[21] The choice of coating material depends on the compatibility with the core materials and the synthesis methods. Usually, monomer polymerization (e.g. emulsion polymerization, interfacial polymerization, and in‐situ polymerization) and sol–gel method are adopted for preparing organic and inorganic shell layers, respectively.[21, 22]
Owing to their low cost, light weight, outstanding flexibility, good processability, and desirable compatibility, polymers, including polystyrene (PS), polymethylmethacrylate (PMMA), polyurethane (PU), urea–formaldehyde resin (UF), melamine–formaldehyde resin (MF), and many others, are widely used in the preparation of core‐shell phase change composites.[19, 23, 24] For example, a direct miniemulsion method has been adopted to prepare spherical poly(ethyl methacrylate) (PEMA)/n‐octadecane nanocapsules with the average size of 140 nm and PMMA/n‐octadecane nanocapsules with the average size of 119 nm, and the 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
