Thermosetting solid–solid phase change materials composed of poly(ethylene glycol)-based two components: Flexible application for thermal energy storage

Highlights

• The thermosetting PCMs composed of PEG-based two components were synthesized.

• The thermosetting PCMs were endowed with the flexible application capability.

• The maximum enthalpy in heating (cooling) process is 98.2 J/g (102.0 J/g).

• The thermal energy storage mechanism and crystalline properties were studied.

Abstract

The thermosetting solid–solid phase change materials (SSPCMs) composed of poly(ethylene glycol)-based two components, isocyanate-terminated prepolymer and tetrahydroxy prepolymer, were synthesized via solvent-free bulk polymerization, endowing the thermosetting PCMs with flexible application capability. The chemical structures, crystalline properties, phase change properties, thermal energy storage mechanism, thermal reliability and stability of the synthesized PCMs were respectively characterized by Fourier transform infrared spectroscopy, X-ray diffraction, differential scanning calorimetry, polarizing optical microscopy, accelerated thermal cycling testing and thermogravimetric analysis. The intrinsic crystalline structure in SSPCMs is not influenced by the crosslinking reaction between two components. However, compared with the poly(ethylene glycol) (PEG), the lamellae and spherulite size in SSPCMs decrease to some entente and dramatically, respectively. The SSPCMs have proper phase change temperature of about 45 °C and high phase change enthalpy of about 100 J/g much less than the PEG, resulted from the small spherulite and lamellae size working together. The SSPCM-based thermal energy storage relies on the phase change from the crystalline state to the amorphous state and vice versa. The SSPCMs have good thermal reliability and stability with the degradation temperature higher than 300 °C.

Introduction

Phase change materials (PCMs) have drawn an increasing attention for thermal energy storage (TES) in the last two decades [1], [2], [3]. They can absorb or release abundant amounts of heat with small temperature fluctuations when transforming one physical state (e.g. crystalline state) to another (e.g. amorphous state), thus contributing to thermal inertia at phase change temperature (T_pc) [4], [5], [6]. Among PCMs, the polymeric solid–solid PCMs based on polyethylene glycol (PEG) as a phase change ingredient are of much importance in consideration of no leakage when used and needless encapsulation [7]. In addition, PEG has many advantages of good biodegradability [8], excellent biocompatibility [9], high phase enthalpy and proper T_pc [10], tunable molecular weight [11], [12] and low cost [13], which will endow the PEG-based PCMs with great application prospect.

At present, solid–solid PCMs based on PEG have been developed as thermoplastic and thermosetting polymers for latent heat TES [14], [15]. Meng et al. [16] synthesized thermoplastic PEG-based block polyurethane by strictly controlling the mole ratio of isocyanate and hydroxy from isophorone diisocyanate as well as PEG-3400 and 1,4-butanediol. Xi et al. [17] reported the preparation of polyurethane PCMs via a two-step condensation reaction of 4,4′-diphenylmethane diisocyanate, PEG and a novel tetrahydroxy compound (terephthalic acid bis-(2-hydroxy-1-hydroxymethyl-ethyl) ester). The thermoplastic products were synthesized based on the intrinsic reactivity derived from the steric hindrance. Sari et al. [18] studied the synthesis method and thermal properties of thermoplastic polystyrene-graft-PEG copolymers as new kinds of solid–solid PCMs for TES. The PEG percentage in the whole materials altered from 25 wt.% to 50 wt.%. Chen et al. [19] prepared three kinds of thermosetting polyurethane with three hexahydroxy components (i.e. sorbitol, dipentaerythritol and inositol) by the thermal postcuring of hexahydroxy components. Chen et al. [20] also investigated synergistic polyurethane PCMs with PEG as the phase change functional chain and glucose as the crosslinking agent.

Generally, thermoplastic solid–solid PCMs have intrinsic advantage in good processability due to the linear molecular structure [21]. Unfortunately, the PEG content, positively related to the phase change enthalpy in thermoplastic PCMs, was restricted to a great extent. This is because that the thermoplastic solid–solid PCMs can get flowed and have a very weak mechanical strength once the PEG content surpass a critical value, when the temperature reaches or exceeds the T_pc of thermoplastic PCMs [7], [18], [22]. The higher PEG content, the weaker mechanical strength above T_pc. Different from thermoplastic solid–solid PCMs, the thermosetting solid–solid PCMs have both the high PEG content and the good mechanical strength higher than T_pc [23]. The reason is that the crosslinking structure in PCMs strongly contributes to the nonflowing state during phase change and high mechanical strength over T_pc. For example, our group has reported PEG-based solid–solid PCMs with biodegradable Span 80 and Tween 80 as the crosslinking agent [24]. However, the loss of processability in thermosetting solid–solid PCMs is the big disadvantage, seriously limiting the practical application. Therefore, the synthesis of thermosetting solid–solid PCMs based on biodegradable PEG with a flexible application ability will be a potential subject for latent heat TES.

In this work, a novel synthesis method based on solvent-free two components has been developed to solve the flexible application problem of thermosetting solid–solid PCMs. Meanwhile, there are no effects on the high PEG content and the good mechanical strength over T_pc, due to that the change occurs not in the ultimate product composition but in the synthesis process. The PEG-based two components including isocyanate-terminated prepolymer (component A) and tetrahydroxy prepolymer (component B) were synthesized respectively by the solvent-free bulk polymerization. In the component A, the diphenylmethane diisocyanate (MDI) was considered as the blocking agent and the high molecular weight PEG was utilized as phase change functional chains. In the component B, the diethanol amine was selected as the blocking agent to synthesize tetrahydroxy prepolymer, MDI was used as the skeleton, and the high molecular weight PEG is also chosen as phase change functional chains. Followed, the thermosetting solid–solid PCMs were fabricated by the curing reaction of the component A and B, when applied in a given fields such as coating [25], adhesive [26], building materials [27], solar energy storage [28], and textile products [29]. The chemical composition, crystalline properties, phase change properties, thermal energy storage mechanism, and thermal stability of two-component solid–solid PCMs were investigated via Fourier transform infrared spectroscopy, X-ray diffraction, differential scanning calorimetry, polarizing optical microscopy and thermogravimetric analysis, respectively. An accelerated thermal cycling testing was also performed to reveal the thermal reliability of two-component solid–solid PCMs.