The addition of functional PCMs can significantly improve their performance. For example, the electrochemical performance and cycling life of the batteries depend heavily on the operating temperature, especially in high‐power scenarios, and thus the thermal management of the battery is particularly important. Conventional heat dissipation methods such as forced air cooling and liquid cooling have been widely developed. Although these thermal management systems can ensure heat safety, the whole system is too bulky, sophisticated, and costly in terms of fans, pumps, pipelines, and other accessories, resulting in an increase in the weight of the whole system and the dependence on external energy input. It has been proved that the thermal management system associated with PCMs for the batteries can overcome these shortcomings to a great extent, prolonging their service life.[128]
Polymeric phase change composites have been employed as thermal management component to protect the electronic systems, including batteries,[129] chips,[61] mainboards,[130] etc. Yang et al.[61] fabricated leakage‐proof phase change composites through impregnating low‐temperature PEG into hierarchically porous PVA aerogel. The obtained composites can work as a temperature control component to prevent the chip from overheating (Figure 2.8a). Flexible melamine foam‐supported shape‐stabilized PCMs can be packed in the electronic systems to work as temperature‐controlled deployable panel for the mainboard.[130] Furthermore, graphene and BN nanosheets have been deposited on the surface of melamine foam framework to improve the thermal response performance of the composites as a thermal protection component in electronic systems.[131] A solid–solid polymeric PCMs/EG composite with high cooling efficiency has been designed for the battery modules.[129] Apart from thermal management performance of polymeric phase change composites, other capabilities such as electromagnetic interference shielding and heat dissipation are crucial for the next‐generation electronic devices.
2.5.2 Smart Textiles
Apart from the thermal management of electronics, PCMs can be utilized to tune the on‐body temperature, named as smart clothing, which exhibits great potentials in the clothing for extreme conditions such as spacesuits and fire protective garments. Comfort related with temperature is an important function of clothing, and one of the basic functions of smart clothing is to maintain relatively stable temperature for human body. Integrating PCMs featuring large heat storage capacity and almost constant phase change temperature with personal thermal management textiles can greatly improve their heat preservation performance and temperature regulation function. At present, the main approach to make smart clothing with PCMs is to infiltrate microencapsulated phase change products into fibers, fabrics, and foams. In addition, PCMs with the working temperature range of 18–35 °C are most suitable for the production of smart clothing.[125]
Phase change component can be directly integrated into the fabrics through printing, coating, and padding routes.[132, 133] Another plausible alternative is to construct core‐shell structure, in which phase change matrix and polymer fiber serve as core and shell, respectively. Apart from typical coaxial electrospinning,[134] melt spinning as a clean and high‐efficiency spinning strategy is available for the preparation of polymeric phase change fibers.[135] Very recently, a novel freeze‐spinning strategy integrating freeze‐casting with spinning has been proposed to yield polymeric phase change composite fibers for personal thermoregulation textiles (Figure 2.8b).[127, 136] Porous aerogel fibers are likely to be competitive candidates for the carriers of phase change clothing. Wu et al.[137] developed PU/graphite foam solid–solid phase change composites with the ability of electro‐to‐thermal energy conversion. The obtained composites are able to be assembled into the fabric as an active protective layer (or a thermal buffering layer) to fight the cold.
2.5.3 Shape Memory Devices
Shape memory polymers (SMPs), as a class of intelligent materials, are capable of fixing the deformed (temporary) shape and recovering to the original (permanent) shape upon exposure to external stimuli, such as heat, light, electricity, moisture, chemicals, etc.[138] Common SMPs consist of two structural components, namely, the reversible switching phase determining the shape memory transition and the permanent phase responsible for memorizing the original state. Enlightened by this dual‐phase structure, the polymeric phase change composites prepared by compounding PCMs with elastic supporting materials are able to exhibit shape memory function. With this in mind, the supporting component plays a significant role in the macroscopic physical properties of shape memory polymeric phase change composites. Elastomers represented by SEBS and OBC as well as melamine foam with 3D porous structure have been proved to be good supporting candidates for PCMs‐based shape memory composites.[41, 42, 130]
PCMs‐based shape memory composites present a rigid state when the temperature is below the phase change temperature (melting point) of PCMs. Once the temperature is higher than their melting temperature, PCMs melt into a liquid. Meanwhile, the composites soften and can be deformed into a targeted shape. Therefore, accompanying the phase transition of PCMs, the transformation of the composites from rigidity to flexibility is reversible. PCMs and elastic supporting materials can be facilely blended to produce binary thermally induced phase change composites.[130] Additional functional component such as EG can facilitate the performance owing to the improvement of thermal conductivity.[139] For light‐actuated shape memory composites (Figure 2.8c), photothermal absorbers such as PDA,[140] CNT,[141] and graphene[126] have been added to improve their photoadsorption ability.
2.6 Conclusions and Outlook
Great advances in PCMs with excellent shape‐stability and high thermal conductivity have been witnessed in past few decades, largely extending their applications. In this chapter, polymeric phase change composites with enhanced comprehensive performance have been highlighted. Micro/nanoencapsulation, facile blending, porous structural scaffolds and solid–solid transition routes have been adopted to fabricate leakage‐proof phase change composites. Further to improve the thermal conductivity, conductive metals and carbon materials as well as insulating ceramics have been employed as thermally conductive components. Polymeric phase change composites exhibit immense potential applications in energy conversion, thermal management, smart clothing, and shape memory device. High‐performance and multifunctional phase change composites as an advanced TES technique are bound to play an increasingly important role in energy storage‐related applications.
Although high‐performance polymeric phase change composites have been greatly developed, many technologies are not mature enough to achieve mass production. There is a long way to go from the laboratory to the factory, in particular high‐efficiency 3D porous macroarchitectures. Of note, the introduction of functionality is at the expense of energy storage density, but high energy storage density is the intrinsic essence of PCMs. It is unwise to ignore the original intention when developing new polymeric phase change composites. Therefore, it is of significance to optimize the structural design and processing technology to improve the utilization efficiency of functional materials. Moreover, the effect of nanomaterials and nano/microstructures on the phase change behaviors needs to be further clarified. Accordingly, green, safety, efficiency, and cost, these timeless pursuits, are still the concerns. More attention is well‐advised to be paid to sustainable, flexible, and intelligent polymeric phase change composites in the future. (i) For the perspective of polymeric supporting component, biodegradable materials are considered as plausible alternatives, including nanomaterials represented by nanocellulose and biomasses like porous wood. (ii) With the emergence of Internet of Things (IoT), flexible devices for wearable applications are becoming research priorities, and flexible phase change energy storage composites are an indispensable part. (iii) Breaking through existing application scenarios, polymeric phase change composites are likely to be more intelligent in the future, such as multiresponsive materials, self‐adaptive devices, infrared stealth
