Fabrication And Performance Of Composite Phase Change Material Through Self-Assembly Technology

Phase change materials (PCM) can absorb, store, and release large amounts of latent heat over a defined temperature range while undergoing phase changes, thus, they have gained a great scientific interest in modern technology due to the impending shortage and increasing cost of energy resources. Composite PCM have attracted more and more attention in recent years, since they can prevent PCM from leaking and harmful interaction with the environment, and can supply a large heat capacity to control the volume changes when phase change occurs. However, there are still some limitations for traditional composite PCMs, such as poor thermal and physical stability, and quite low thermal conductivity. Therefore, in this thesis, we synthesized a series of composite PCMs with toughening-reagent modified polymers as wall material, which supply a new method to improve the thermal and physical stability of traditional PCM. We also develop a novel inorganic encapsulation technique to enhance thermal conductivity of PCM, and a series of organic PCM/inorganic hybrids with novel regular structure were prepared. The influence of synthetic conditions on the morphology and performance of composite PCM was also studied. First, a series of micro-PCMs based on n-octadecane core and resorcinol-modified melamine-formaldehyde shell were synthesized by in-situ polymerization method. The influence of various self-assembly templates (SMA, SDS, and PVA) and core/shell weight ratio on the performance of micro-PCMs was studied. Results suggested that using SMA as self-assembly template is optimal for the fabrication of the microcapsules. The micro-PCMs fabricated with a core/shell weight ratio of 75/25 by using SMA, have a compact spherical surface with a mean particle size of about 16μm. This sample has a much better phase change properties and a higher efficiency of encapsulation (about 92%) than the others, while it also exhibits a better stability through the anti-osmosis measurement. These micro-PCMs can be used in the manufacture of thermal-regulated fibers, fabrics and building materials. Then, the micro-PCMs based on n-octadecane core and polyurea shells containing different soft segments in the molecular chain were successfully synthesized through interfacial polycondensation. Various amines (i.e. EDA, DETA, and Jeffamine) were used as water-soluble monomers to investigate the changes of morphology and chemical structure for the wall materials of micro-PCMs. The microcapsules synthesized by using Jeffamine as the amine monomer have a smoother and more compact surface than those using EDA and DETA. It is also found that the microcapsules synthesized by using Jeffamine have a large mean particle size with a centralized size distribution. These microcapsules under this condition also exhibit much better phase change properties, higher encapsulation efficiency, and better anti-osmosis property than the other two samples, although they have poorer thermal stabilities. Furthermore, the core/shell weight ratio of 70/30 is optimal to synthesize micro-PCMs with good pergormance. Furthermore, a novel microencapsulated PCM based on an n-octadecane core and an inorganic silica shell was designed to enhance thermal conductivity and phase-change performance. These silica microcapsules were synthesized by using TEOS as an inorganic source through a sol-gel process. They exhibit a spherical morphology with a well-defined core-shell microstructure. Furthermore, the silica microcapsules synthesized at 2.45 display a smooth and compact surface and present a large particle size range of 7-16μm, thus n-Octadecane inside the silica microcapsules still retains a good crystallinity. These silica microcapsules have good thermal stability, and the encapsulated n-octadecane can achieve good phase change behavior, high encapsulation efficiency, and good antiosmosis property by controlling the loading of core material and acidity of the reaction solution during the sol-gel process. Especially, the thermal conductivity of these silica microcapsules is significantly enhanced due to the presence of the high thermal conductive silica wall. In addition, the silica microcapsules were also synthesized through interfacial condensation. The aim of this study is to discuss the influence of acidity in a large rage (pH=0.93-4.07) on the micro-structure of silica microcapsules. Although the silica microcapsules obtained in different conditions present a spherical morphology with well-defined core-shell microstructure, the samples formed at pH 2.89 achieve a compact silica wall with fairly smooth surface as well as a large mean particle size of about 17.0μm, suggesting an optimal acidity for the preparation of silica microcapsules. Compared to the samples synthesized in chapter 4, the thermal conductivity of these silica microcapsules is more highly improved, which indicates that the silica wall is more compact than that synthesized by sol-gel process. Therefore, encapsulation of n-octadecane with the silica wall material through interfacial condensation can be a perspective way to prepare micro-PCMs with enhanced thermal transfer and phase-change performance for potential applications to thermal-regulating textiles and fibers. Interestingly, the lamellar-mesostructured PEGDS/silica hybrids have been synthesized by using amphiphilic PEGDS as template through self-assembly in sol-gel process. A delicate balance between the hydrophilic interactions at the surfactant head group-silanol interface and the hydrophobic interactions between the stearate segments in the surfactant can be maintained in a severe range of TEOS/PEGDS weight ratio, resulting in the final lamellar mesostructure of the hybrids only containing 20.73-31.25 wt% PEGDS. The molecular chains of PEGDS with 2D degree of freedom could still crystallize, though they were stranded one dimensionally between the silica interlayers. However, the confinement effects of the lamellar mesostructure caused a significant decrease in both the melting and crystallization temperatures of the PEGDS. This work not only presents a novel synthetic route to polymer/silica hybrids with the lamellar mesostructure, but also creates opportunities to study the confined crystallization within a new long-range-ordered space. Besides, a series of novel n-octadecane/calcium hybrids were synthesized by using sodium lauryl benzenesulfate (DBS) as template through self-assembly process. Since inorganic crystals of defined shape and orientation are produced by DBS, the synthesized hybrids have two crystalline forms, calcite and vaterite. As the hybrids containing 20 wt.% n-octadecane synthesized with 0.4 wt.% DBS, vaterite crystals were mainly formed since self-assembly template stereoselectively bind with Ca2+, initiate targeted crystal nuclei and inhibit its growth, tending to induce the nucleation of vaterite. While with high weight percent of n-octadecane and DBS, the interplay of nucleation and growth becomes more complicated, and the viscosity of solution increases, whereas the influence of DBS on the CaCO3 crystal growth was decreased, thus the hybrids tend to form calcite crystals. In addition, the hybrids with vaterite crystals can effectively encapsulate n-octadecane, however, the 3D structure of vaterite seriously inhibits the motion of n-octadecane molecular chains which induce a significant decrease in phase chase properties of encapsulated n-octadecane.