Binary shape-stabilized phase change materials based on poly(ethylene glycol)/polyurethane composite with dual-phase transition
Given the acute shortage of fossil fuels and ravenous demand of clean energy globally, latent heat storage (LHS) has recently attracted increasing attention. Phase change materials (PCMs) are extensively used in LHS systems due to their ability to store and release significant amounts of thermal energy repeatedly. PCMs have several advantages such as high heat storage density and small volume change on melting/freezing, so they are expected to play significant roles in many applications including solar energy harnessing, energy efficient buildings, central air-conditioning systems, and thermoregulating textiles. Owing to high heat of fusion, abundant categories and low cost, solid–liquid PCMs have been recognized as the most favorable candidates in LHS systems.
The common method to solve LHS PCMs’ fluidity and leakage problems is to prepare shape-stabilized PCMs (also called form-stable PCMs) in which solid–liquid PCMs are loaded within supporting framework material. Owing to various noncovalent interactions (e.g., hydrogen bonds, dipole–dipole interactions, Van der waals forces and capillary forces), shape-stabilized PCMs can protect PCMs from flowing away when heated over melting point. The traditional supporting materials in shape-stabilized PCMs are generally substances that don’t phase change. They not only make zero contribution to the latent heat storage of shape-stabilized PCMs, but also have a negative effect on the crystallization of the solid–liquid PCM.
In order to solve the problem, authors firstly reported the novel shape-stabilized composite PCMs using synthesized polymeric glucose/MDI/PEG solid–solid PCMs to load PCMs poly (ethylene glycol) (PEG). They combine solid–solid PCMs and solid–liquid PCMs for co-thermoregulation at overlapped phase change transition range to achieve higher heat of fusion. As shown in Fig 1, The PU copolymer containing PEG segment exhibits a sharp endothermic peak and exothermic peak near that of pristine PEG with ΔH_c=99.1 J/g and ΔH_m=96.2 J/g. The decrease in phase change temperature and enthalpy is caused by the lower crystallinity in the PU copolymer.
Fig. 1. The phase change enthalpies and temperatures of PEG, PU copolymer and CPCMs for a melting process and b crystallization process.
It is clear that there are two types of phase transition occurring in the CPCMs during the heating/cooling processes (as shown in Fig. 2). The ‘free’ PEG molecules and PEG segments in PU undergo transformation between an ordered arrangement and disordered entangled ‘melt’ during heating/cooling. From the perspective of thermal properties, the shape-stabilized PCMs based on solid–liquid PCM/solid–solid PCM composite with single endothermic/exothermic peak have better synergetic and cumulative effect by the two components.
Fig. 2. Schematic of phase transition process of CPCMs
In general, a crosslinked polyurethane copolymer comprising Hp-β-CD and PEG linkages with a solid–solid phase transition was synthesized and used as a supporting material for loading additional ‘free’ PEG (which exhibits a solid–liquid phase transition). The two components have good compatibility in the CPCM. Since the solid–liquid phase transition of ‘free’ PEG and solid–solid phase transition of PU copolymer occurred simultaneously in the CPCMs, the CPCMs have far higher heat storage density than that of traditional shape-stabilized PCMs with the same ‘free’ PEG loading percent. The strategy of preparing the novel CPCMs in this work could overcome the common problem of enthalpy decline of shape-stabilized PCMs for future use in thermoregulation or energy storage. In addition, this work further deepens understanding toward synergetic phase transition behavior of binary shape-stabilized PCMs based on solid–liquid PCM/solid–solid PCM composites.
This result has been published in Journal of Materials Science, 2018, 53, 16539-16556. The corresponding authors are Dr Changzhong Cheng (Xiangnan University), and Prof. Linge WANG (AISMT, South China University of Technology).
Source from South China Advanced Institute for Soft Matter Science and Technology