Preparation And Crystallization Behavior Of Octadecane Phase Transition Composite Membrane And Microcapsule

Phase change materials (PCMs) can storage or release a large number of latent heat and the temperature keep constant when the phase is changing, so it is an environmentally friendly material. In recent years, it has been widely used in thermostat, thermo-regulated fibers, energy-saving building materials and solar energy utilization. However, when the particle size is decreased, the onset crystallization temperature of the PCMs will change, such as the nanoPCMs or microPCMs cannot crystallize in the region of the required temperature, and this phenomenon limits their applications. This thesis followed by the international forefront of PCMs, draw on the experience of the preparation and crystallization of the PCMs, in order to study the crystallization of phase change composite membrane and the supercoiling of the microencapsulated phase change materials.In this study, synthesis of Poly (methyl methacrylate)-b-polystyrene (PMMA-b-PS) was carried out in emulsion polymerization, using a novel chain transfer agent DPE (1,1-diphenylethylene). Through controlling the self-assembly of block copolymer, adsorbing n-octadecane, the phase change composite membrane was prepared. The microcapsules containing n-octadecane with polyethylene wax was synthesized by emulsion polymerization using Poly (methyl methacrylate-co-allyl methacrylate) as shell. Then, the structure of the block copolymer, the surface morphologies of the membrane and microcapsules, thermal stabilities and crystallization process of membrane and microcapsules were investigated using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis(TG), Hydrogen nuclear magnetic resonance spectrometer(’HNMR), respectively. The conclusions are presented as follows:1. The block copolymer that the molecular weight is 6.2x 104 has been successfully prepared, the ratio of the PS segment and the PMMA segment is about 1:1. The morphology of the copolymer membrane presents nano-dots. After etching the PMMA segment, the morphology of the copolymer presents nano-porous with PS as substratum. The diameter of the nano-porous is about 80 nm and its pitch is about 200 nm. The effect of solvent or high temperature annealing on block copolymer membrane self-assembly was studied by the scanning electron microscope (SEM). Solvent annealing is a better method to form small and uniform nano-porous, because of the copolymer in a supersaturated vapor pressure condition.2. The n-Oct composite membrane has a lower initial melting temperature (18.0 ℃) and onset crystallization temperature (20℃).3. Through the comparison of ZnO nanoparticles nucleating agent and polyethylene wax nucleation agent, a more efficient nucleating agent has been selected. The supercooling crystallization temperature of MicroPCMs with nano ZnO is 6.9℃, it is lower only 1.0℃ than the supercooling crystallization temperature of MicroPCMs without ZnO nanoparticles. Furthermore, the ZnO nanoparticles has caused some damages to the wall materials of MicroPCMs. So it is not an ideal nucleating agent. However, polyethylene wax which is not only a nucleating agent, but also a phase change material, has little effect on the enthalpy of core material. The supercooling crystallization temperature of MicroPCMs with polyethylene wax could be reduced to 1.2℃, and the stability of MicroPCMs has not been destroyed, either. So polyethylene wax is a better nucleating agent.4. Microcapsules are all spherical particles, the surfaces has some wrinkles, and the average particle size is 2.8μm, the addition of polyethylene wax has little impact on their particle sizes and surfaces. The heat-resistance temperature of microcapsules is higher than the bulk about 90℃ and the heat resisting properties of the microcapsules are constant after adding a bit of polyethylene wax.5. The addition of polyethylene wax has a great effect on the thermal properties. When the amount of polyethylene wax is 0.06 g, it can effectively reduce the supercoiling crystallization behavior of the microcapsules. The polyethylene wax does not affect the crystallographic system of n-octdecane, but it can induce heterogeneous nucleation.