Researches On PEG-macromolecule Solid-solid Phase Changed Energy Storage Materials

As demand for energy resources increasing, the researches of energy storage phase changed materials (PCM) has become a hotspot in recent years, especially the solid-solid phase changed energy storage materials with some unique advantages has now become the most potential application materials. However, some defects do exist in the solid-solid phase changed energy storage materials. For example, the applications of the polyhydric alcohols solid-solid phase changed energy storage materials have their limitations with plastic crystals existing; inorganic salt phase changed materials in low temperatures are not enough; the varieties of macromolecules is limited. Their phase change enthalpy and thermal conductivity properties are small and poor. So, researches on development of some new solid-solid phase changed materials are urgent -needed.In this paper, the researches were made on polyethylene glycol phase changed materials with outstanding phase change characteristics and heat storage properties. We started our work from molecule design, used untraditional grafting methods, made achievements on solid-solid phase change from traditional PEG solid-liquid phase change, and prepared high-performance, multi-functional solid - solid phase changed materials. Four kinds of Polyethylene glycol phase changed materials were prepared in this work. The analysis results show that the novel polymers prepared possess of higher phase transition enthalpy value and stable thermal performance with excellent prospect of application.The main results of this study are as follow:1. The MPEG with same hydroxyl reactive activity of the PEG was used as the basic phase change unit. MPEG macromolecule phase changed monomer MPEG-TDI-NMA (PD) containing double bonds was synthesized by introducing NMA with double bonds, and the PD was copolymerized with Vinyl acetate (VAc). In that way, a new solid-solid phase changed heat storage material (MGVM) with side chains containing MPEG was prepared. The composition and structure of the MGVG were characterized with Fourier transform infrared spectroscopy (FT-IR) and hydrogen-nuclear magnetic resonance spectrum (~1H-NMR). Properties of the MGVM were tested through differential scanning calorimetry (DSC), thermal gravimetric analysis (TG), and partial Optical microscopy (POM).The results show that the MGVG has been successfully prepared. Furthermore, MGVM has no crystal peak until the ratio of PD: VAc was more than 1:50. When the ratio of PD: VAc is 1:2, its melting enthalpy value reaches 80.66 J/g and melting phase change temperature is 72℃. When the ratio was more than 1:50, crystal peak appeared and the crystal properties were better, but the crystallization enthalpy value became lower, and the decomposition temperature was 415℃. By analyzing the heat storage behavior of the MGVM after 12 months and its recyclable property, we found that the ultimate heat storage intensity only decreased 10%, whose durability satisfied the demand of the heat storage materials.2. In order to improve the crystal properties and thermal stability of the MGVG, and enhance its phase transition enthalpy value, we used styrene (St) containing rigid benzene ring as the polymerization unit instead of the VAc, synthesized the novel solid-solid phase change materials (MGPM) which had side chains containing MPEG. The composition and structure of the MGPM were characterized by FT-IR and ~1H-NMR; DSC was used to analyze the transformation properties of the MGPM; its thermal properties were analyzed via TG and DTG; heat storage and crystal properties of the MGPM were analyzed via polarizing optical microscopy (POM) and wide-angle X-ray diffraction (WAXD).The results showed that thermal stability of the MGPM gradually improved with the MPEG decreasing, and its effects on the behavior of heat storage and crystallization of the MGPM were also analysis. Researches on their non-isothermal crystallization kinetics of the MGPM and MGPM/Opal were made; the characteristics, essence and main factors of the crystallization were reveals. The transformation of the MGPM was a kind of solid - solid phase change, and the heat storage happened when the soft part of the side chain MPEG transformed from crystal phase to amorphous phase. The phase change enthalpy value of the MGPM enhanced with the copolymerization proportion of PD and St increasing. If necessary, we can produce a series of solid-solid PCM with different phase change enthalpy value and different phase change temperature by changing the copolymerization proportion. When the ratio of PD to St was 1:2, and the percentage of the MPEG was 71%, the phase change enthalpy value of the MGPM reached the highest, with the melting enthalpy value of 98.5 J/g and the crystallization enthalpy value of 71.6 J/g. Compared with the enthalpy value of the MGPM synthesized with the ratio of PD to VAc of 1:2, high crystallization enthalpy value of the MGPM appeared. The decomposition temperature of the MGPM with St as the polymerization unit was apparently higher than the MGVM’s with aliphatic vinyl acetate as the polymerization unit with the same ratio. In that way, a excellent solid-solid changed material was produced by introducing the styrene monomer instead of the vinyl acetate monomer for polymerization.By adding Opal to MGPM materials to produce a kind of MGPM/opal composite material, its properties of crystallization and transformation was improved. Both the crystal structures of the MGPM and MGPM/Opal were characterized with WAXD diffraction, and their crystal properties and nucleation mechanisms were also analyzed. The results showed that the crystal type has been not changed after adding Opal with proper quantity, but the crystal grain was smaller and the crystallinity was increased. The crystallinity of the MGPM/opal increased 3.12% by adding 0.7% opal, with higher transitional enthalpy value of 109.07J/g, and crystallization rate is apparently increased. Comparing the MGPM with the MGVM, the time shortened is 99-second. Generally speaking, the thermal stability of the MGPM/Opal was improved, which would not decompose below 420°C, and also the crystallization properties.The non-isothermal kinetics of the MGPM showed that: crystal peaks of the MGPM moved to lower temperature zone with the rate of temperature decreasing, and the exothermal crystallization increased, for a new balance of the MGPM formed by molecular movement after some time. With lower rate of temperature decreasing, crystallization happened earlier, and the temperature of the crystal peak was higher. When the whole molecular chain, chain segment and chain element moved, internal friction should be overcome, which made instant movement impossible. So the higher rate of temperature decreasing, the later the crystallization happened, and the crystallization peak are moved to low temperature district. Otherwise, the higher rate of temperature decreasing, the shorter the time of the movement of molecular chain arrives at the new equilibrium state, the bigger the energy of overcoming internal friction, and the more the heat produced, so the crystallization peak is stronger.The results of the analysis of the different models show that the non-isothermal dynamics process of MGPM don’t match Jeziprny model, but the Ozawa model is suitable to describe its non-isothermal crystallization behaviors. For the MGPM, the Avrami exponent obtained based on the Ozawa model lies in the range of 3 to 4. The results show that the nucleating type of MGPM is sporadic nucleation during the crystallization initial stage. As the crystalline time increasing, the nucleating type of MGPM is changed into instantaneous nucleation. The Avrami exponent of MGPM / Opal based on the Ozawa model is closer to 3. This showed that after the accession of opal, there is influence to the nucleating type of MGPM/Opal. The nucleating type of MGPM / Opal mainly is instantaneous nucleation. In other words, the introducing Opal increases the nucleating rate of MGPM/Opal, and endows with the excellent phase change properties for the MGPM/Opal.3. In order to research novel polymeric ways for synthesizing unique phase change materials, we synthesized a new phase change monomer with double-hydroxyl and PEG unit. The new phase change material (MGEM) was prepared with the monomer via step copolymerization. The composition and structure of MGEM were characterized by FTIR and 1 H-NMR. The phase change performance and thermal properties were analyzed by DSC and TG. The results show that the composition and structure of MGEM is the same as that of the anticipated polymer. At same time, the copolymer possesses of excellent phase transition behaviors and thermal stability, and the phase transition enthalpy value of MGEM is 75.6 J / g.4. In the last section of the paper, we synthesized a hyperbranched solid - solid phase change materials (PUPCM). In the PUPCM, the higher of the degree of branching, the higher the phase change unit. So the PUPCM has higher phase change behaviors, and the highest phase change enthalpy value could reach 105.35 J/g. By using the PEG with different molecular weigh, different PUPCM were synthesized. Furthermore, the crystalline type, crystallization, and the phase change enthalpy value of different PUPCM were researched. The results showed that the crystallization and the phase change enthalpy value of PUPCM were enhanced with the increase of molecular weight of PEG. The structure of PUPCM is regular, the crystalline type is perfect ball-crystal, and thermal stability is good.By adding inorganic nanomaterial-Opal to PUPCM materials to produce a kind of PUPCM/Opal composite material, its properties of crystallization and phase change behaviors was further improved. The analysis of DSC and POM of PUPCM/Opal show that the PUPCM/Opal has higher phase change enthalpy value and appropriate phase change temperature.