Elsevier

Energy Storage Materials

Volume 34, January 2021, Pages 508-514
Energy Storage Materials

An intrinsically flexible phase change film for wearable thermal managements

https://doi.org/10.1016/j.ensm.2020.10.014Get rights and content

Abstract

Phase change materials (PCMs) involving significant amounts of latent heat absorbing and releasing at a constant transition temperature have been extensively utilized for thermal management of electronic devices. However, it is still a great challenge to apply thermal management for wearable devices using PCMs due to their solid rigidity and liquid leakage. Although considerable efforts have been dedicated to constructing flexible composite PCMs by physically blending bulk PCMs with different flexible polymers, design and fabrication of intrinsically flexible PCM films still remain unexplored. Herein, we report an intrinsically flexible PCM film with apparent solid-solid phase transition performance, outstanding self-support and shape-conformable property for wearable thermal management. Remarkably, the as-obtained PCM film behaves adjustable phase transition enthalpy and temperature in the region from about (5 to 60)°C, long-term cycling stability up to 1000 cycles, highly mechanical flexibility and remarkable shape tailorability and foldability. Notably, such flexible PCM films are easily integrated into wearable devices with a flexible graphene film as thermal source, revealing superior temperature control behaviors, together with unprecedented electro-thermal and photo-thermal energy conversion performance. The intrinsically flexible PCM film developed in this work is holding great potential for next-generation flexible thermal management electronics.

Introduction

Thermal energy has always been the indispensable energy resources for the development of human society. Most of renewable or non-renewable energy, such as fossil fuel, electric power, solar energy and nuclear energy, are generally utilized directly or indirectly by means of thermal energy [1]. Meanwhile, a significant amount of waste heats can be generated in the process of energy consumption, which greatly reduces the efficiency of energy utilization and increases the probability of thermal safety accidents [2], [3]. Consequently, the thermal management is of particular significance to utilize and convert thermal energy resources in an efficient and safe way. Phase change materials (PCMs) have attracted much attention recently due to their unique capability of absorbing or releasing large amounts of latent heat at almost constant temperatures during phase transition process [4]. Therefore, PCMs are extensively acknowledged as an ideal carrier of thermal energy storage and temperature control techniques, being widely used in the field of thermal management including industrial waste heat recovery, solar thermal energy utilization, energy saving building, intelligent temperature regulating clothing, temperature control of functional electronics, and so on [5], [6], [7].

On the other hand, wearable electronics or devices have aroused great interest rapidly in design and construction of smart wearable human activity and health monitoring or supporting systems [8], which extremely need an advanced thermal management technique to efficiently utilize the limited energy sources due to the portable weight and space limitations. However, the currently used PCMs for thermal management such as paraffin waxes, fatty acids, polyols and inorganic salt hydrates [9], [10], [11], [12], [13], are mainly rigid solid or leaking liquid materials without any flexibility, making it unsuitable for application in this wearable thermal management scenario. Although considerable efforts have been dedicated to constructing flexible composite PCMs by physically blending or chemically modifying bulk PCMs with different flexible polymers, such as polyethylene, polymethyl methacrylate, poly(vinyl chloride), polyurethane and olefin block copolymer [14], [15], [16], [17], [18], it is still a great challenge to design and fabrication of intrinsically flexible PCM films with a stable phase transition property, shape-conformability, tailorability and foldability for wearable thermal managements.

To this goal, we have developed a general polymerization strategy of synthesizing large-area intrinsic PCM films (MTPEG) with remarkable self-support, ultra-flexible and shape-conformable properties by polymerically chemical grafting of melamine and toluene-2,4-diisocyanate (TDI) with polyethylene glycol (PEG) materials, which has been frequently employed as PCMs due to their low cost, high phase change enthalpy, biocompatibility, stable chemical structure and high molecular weight with long folded chain [19]. The most attractive feature of PEG is their easily chemical modification of the included hydroxyl groups, which can be cross-linked by chemical reaction to construct polymeric PCMs with stable chemical structures and excellent solid-solid phase transition performance [20], [21]. The resultant flexible PCM film exhibits tunable phase change temperature from about (5 to 60)°C with varying PEG molecular weights, relatively high latent heat (118.7 Jg−1), highly stable apparent solid-solid transition in 1000 heating-cooling cycles, and remarkably shape tailorability and foldability. More importantly, a wearable thermal management device, constructed using a flexible ultrathin graphene film (GF) as thermal sources and the flexible PCM film as management carrier, is demonstrated for high-performance thermal management. The resulting wearable device exhibits remarkable capability of efficiently thermal management and photo-thermal or electro-thermal energy conversion.

Section snippets

Preparation of MTPEG

Polyethylene glycol (PEG, Mn = 4000, 6000, 8000, 10000, 12000) samples were commercially provided by J&K Scientific Ltd, and all PEG samples were dried at 80 °C in a vacuum oven for 48 h to remove the water before use. Toluene diisocynate (TDI), dibutyltin dilaurate (DBT) and N, N-dimethylformamide (DMF) were purchased from Aladdin Industrial Corporation. The synthesis was conducted through two steps. Step 1, PEG, TDI, dibutyltin dilaurate and N,N-dimethylformamide were added into three necks

Results and discussion

The synthesis of MTPEG involves two step polymerization of PEG with melamine and TDI (Fig. 1a and S1), and MTPEG film can be consequently obtained by casting MTPEG solution in a round or rectangle glass mold. The thickness of MTPEG film is about 1 mm, and less thickness can result in more flexible film. This synthetic procedure and casting method can be easily scaled up to fabricate large-area flexible PCM films (Fig. 1c, d). It should be pointed out that most of flexible PCMs reported

Conclusion

In conclusion, an intrinsically flexible PCM film is designed using a chemical polymerization strategy and developed for wearable thermal management applications. This PCM film behaves adjustable phase transition enthalpy and temperature in the region from about (5 to 60)°C, long-term cycling stability up to 1000 cycles, and high thermal and mechanical stability. Compared with previously reported flexible PCM composites synthesized using physical blending method, our PCM film exhibits

CRediT authorship contribution statement

Yan Kou: Conceptualization, Data curation, Formal analysis, Funding acquisition, Writing - original draft. Keyan Sun: Data curation, Formal analysis, Writing - original draft. Jipeng Luo: Data curation, Formal analysis. Feng Zhou: Data curation, Formal analysis. Haibo Huang: Data curation, Formal analysis. Zhong-Shuai Wu: Conceptualization, Funding acquisition, Writing - review & editing. Quan Shi: Conceptualization, Funding acquisition, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by Dalian Institute of Chemical Physics (Grants DICP I202036), National Natural Science Foundation of China (Grants 21903082, 51572259, and 51872283), Science and Technology Major Project of Liaoning Province (2019) under grant 2019JH1/10300002, Scientific Instrument Developing Project of the Chinese Academy of Sciences under grant No. YJKYYQ20190046, DICP ZZBS201608, DICP ZZBS201708, DICP ZZBS201802, and National Key R&D Program of China (Grants

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