With the upgrade growth of population and the increment of energy consumption, it is necessary to develop various thermal energy storage (TES) measures to alleviate the mismatch between energy demand and supply[1, 2]. Attribute to the capacity of storing latent heat during the phase change process, the energy storage medium termed phase change materials (PCMs) is widely used to utilize the renewable energy and provide support to energy conservation. [3, 4]. Many materials applied for TES had been studied including inorganic materials (such as salts[5], hydroxides[6] and salt hydrates[7]) and organic materials (such as paraffin[8], polyethylene glycol(PEG)[9] and fatty acids[10]). Among the different types, organic phase change materials had attracted great attention in thermal management based on electronic equipment[11], smart fibers[12] or textiles[13] due to their large energy storage capacity and low supercooling degree. In recent years, PEG and various organic structures based on it have been employed as PCMs due to their large energy storage density, excellent thermochemical stability, suitable phase transition temperature and non-toxicity, [14, 15]. In order to promote the development and application of PCMs, it is still a key challenge to obtain overall excellent properties for PCMs, such as thermal conductivity[16], shape stability[17], and energy storage density[18], etc. Polyurethane phase change energy materials (PUPCMs) are favored for thermal energy storage because of their advantages of no leakage, good stability, high phase change enthalpy and wide phase change temperature range[19–21]. There are mainly three types of PUPCMs based on their molecular structure: linear structure[22], crosslinking structure[23] and composite structure[24], etc.
A kind of shape-stable and super elastic flexible linear PCMs with classical polymerization strategy were prepared by Liao et al. [25]. The elongation at break and latent heat of the prepared PCMs was 420% and 61.48 J/g by adjusting the molecular weight of PEG. And crosslinking solid-solid PCMs were synthesized through solvent-free bulk polymerization with xylitol as curing agent [26]. The obtained PCMs could be used as promising TES materials with energy storage density exceeding 60 J/g and phase change temperature between 20–45°C. Since the properties of PUPCMs are strongly relative to the structure of soft segment, blending and modification of chemical structure are employed in previous studies to improve thermal energy storing properties. In general, PU with cross-linking structure based on phase change polyols as PU soft segment, polyisocyanate and chain extender as PU hard segment shows better shape and thermal stability than linear PU [27]. The type and ratio of soft and hard segments, chemical structure, molecular weight are designed to adjust the properties of PU. However, there were few researches focus on the synthesis process, wettability and mechanical properties of PU based on mixed soft segments and its.[28–30] Liu et al. [31] reported the thermoplastic polyurethane elastomer (TPU) composing of hydroxylterminated polybutadiene (HTPB) and polypropylene glycol (PPG) as segments. The soft structure of TPU was suppressed by randomly alternated bi-soft segments, and the phase change temperature range of bi-soft segments TPU was more broad than that of single soft segments. However, there was few reports on the preparation of PUPCMs based on mixed soft segment.
In order to obtain stable phase change materials with good thermal conductivity and heat storage capacity, a new type of cross-linked hybrid soft segment PUPCMs were designed. Firstly, PEG/PCDL-based PUPCM was prepared by using PEG and polycarbonate(PCDL) as composite double soft segments. PCDL contains carbonate repeating unit, which has strong polarity and rigidity, so it is an excellent raw material for preparing PUPCM, which can improve the mechanical properties of the PUPCMs. Because the phase transition enthalpy of PEG is better than that of PCDL, the increase of PCDL content is accompanied by the decrease of transition enthalpy and phase transition temperature and the decrease of thermal stability, but the mechanical properties of PEG/PCDL-based PUPCM are improved. When the content of PCDL is less than 33.33%, PEG/PCDL-based PUPCM can maintain relatively good comprehensive properties. Based on the study of PEG/PCDL-based PUPCM, polytetramethylene ether glycol(PTMG) was introduced into the composite soft segment. PTMG is a homopolyether with regular methylene, and the molecular segment is compliant[29]. Compared with PEG, it has lower phase transition temperature, suitable enthalpy and better water resistance. It is confirmed that the addition of PTMG reduces the phase transition temperature of macromolecules and improves the thermal stability. At the same time, a series of linear PUPCMs were prepared, and the effects of composite three soft segments and cross-linked structure on the properties of PUPCMs were studied. The chemical structure, crystal state, phase transition properties and thermal stability of PUPCMs were studied. The lowest phase transition temperature is 30.23 ℃ and the phase transition enthalpy is 54.01J/g, which is expected to be used in the fields of textile and smart wearable device.