| The phase behavior and phase transition of materials directly determine their properties and applications. Therefore, it is the subject of natural science to understand the nature of phase and phase transition. At present, the polymer composites modified by nanoparticles are rich and colorful. The performance of the materials can be improved by only a small amount of nano particles filled in the polymers, in which it has achieved great success in both experiments and applications.However, there is a lack of a unified understanding of the basic principles about the effect of nanoparticles on the properties of composites and it is difficult to give a systematic interpretation of the experimental results, which restricts the effective design and application for polymer nanocomposites. The most notable feature of the addition of nanoparticles in polymers is the formation of polymer/filler interfaces,which will cause the change of the dynamic behavior of the polymer segments and will affect the glass transition or crystallization behavior of polymer system. It is very important to find out the principle of glass transition and crystallization behavior of the polymers limited by interfaces. In this paper, the effect of interface behavior on the glass transition and crystallization behavior of polymer materials was studied by using dynamic Monte Carlo simulation. The main research contents and conclusions are as follows:1. Study on the interfacial segment dynamics and glass transition of polymer system filled with nanoparticles.Without interfacial interaction, the segments in interfacial region is more movable and the glass temperature is lower than that in the bulk,and it is the density of chain segments that controls the glass transition temperature distribution. Under the condition of weak interfacial interaction, the free energy reduction of segments will make up for the loss of conformational entropy, resulting in the dynamics of the chain segments and the glass transition temperature near the interface are the same as those of the bulk. When the interface interaction is strong, a lot of confined polymers with various mobility or glass transition temperature are formed near the interface and the glass transition temperature distribution is controlled by the interfacial interaction strength.2. Study on the dynamics and glass transition of ultrathin films of polymer confined by interfaces.When there is no polymer-substrate interaction, the density of the interface layer is lower and the average movement probability of the segments is higher than that of the middle layer. It is the geometric constraints which lead to heterogeneous dynamics. Accordingly, the glass transition temperature of the interface layer is lower than that of the interlayer and the segment density is the main factor to determine the local glass transition temperature distribution. In the presence of polymer-substrate interaction, the interfacial layer density is higher, and the glass transition temperature increases linearly with the increase of the interfacial interaction. A glass layer is formed on the surface and the glass transition temperature of the middle layer is reduced under the strong attractive interaction. The interfacial interaction is the main factor control the glass transition temperature of the interface layers, while the density of the segment is the main factor to control the glass transition temperature of the middle layers.3. Study on the effect of one-dimensional filler network on the crystallization behavior of polymers.The addition of filler network significantly accelerated the nucleation rate of the polymer and improved the final crystallinity, due to the nano fillers can provide effective nucleation sites for the crystallization of polymers, so that more segments can participate in crystallization. The density of the segments in the polymer-filler interface region is significantly higher than that of the external region, and the internal molecular chain conformation is restricted by the filler network. The orientation of the chain along the direction of the long axis of the filler is enhanced,which leads to the molecular chain in the interface region of the filler network to participate in the crystallization process first. A new type of crystal structure of nanohybrid Shish-Kebabs is formed and the spacing of the filler network directly affects its nucleation mechanism. The nucleus appears in the vicinity of the interface of the filler network when the filler spacing is small, and with the increase of the filler spacing, the confinement effect on the segments of the filler network is weakened, and the nucleus is no longer formed in the filler network.4. Study on the influence of anisotropic filler network on the relaxation and crystallization behavior of oriented polymer melt.Compared with the pure polymer system without filler, the nucleation induction period of the oriented polymer melt with filler network is shorter. The comparison for the effects of the three kinds of polymer-fillers interactions indicates that the appropriate mutual attraction interaction can improve the nucleation and crystallization of the oriented polymer melt. The proper amount of nano fillers can improve the crystallization ability of the oriented polymer melt, and the crystallization ability will be reduced by too low or too high content of fillers. The maximum nucleation and crystallization ability can be obtained when the filler spacing is five lattices. When the packing space is large, the restriction of chain conformation is weak and the crystallization ability is weak, as well. When the filler spacing is small, the effect of confinement is stronger, but the number of confined chains is limited, which results in the weak crystallization ability.5. Study on the crystallization behavior of polymer system grafted on two dimensional filler.Compared with the polymer/filler blend system, the nucleation induction period of the grafted polymer system is shorter and the final crystallinity is slightly lower.The higher the grafting density, the shorter the nucleation induction period and the higher the final crystallinity. With the increase of the grafting density, the density of the segments in the vicinity of the grafting point also increases,the chains become more extended, and the conformational entropy of the chains and the critical nucleation free energy barrier decrease, while the melting point and the supercooling degree increase. For the polymer system with low grafting density, the crowding effect of the molecular chain is relatively weak, which leads to the formation of crystal nucleus in the mode of intramolecular chain-folding nucleation. For the polymer system with high grafting density,the crowding effect of polymer chains is very strong, and the degree of conformational extension is large, which results in the form of intermolecular fringed-micelle nucleation. The crystal orientation also changes with the change of grafting density. The crystal prefers to parallel to the surface of the filler when the grafting density is low,while the crystal is perpendicular to the surfac, of the filler for the system with high grafting density.6. Effect of grafting density and polymer-filler interaction on crystallization process.For the system with low grafting density, with the increase of the interaction between polymer and filler, the nucleation induction period of the system gets shorter and the heterogeneous nucleation is more and more obvious, and there is an amorphous layer on the surface of the filler for the system without polymer-filler interaction. For the moderate grafting density system, the nucleation induction period becomes shorter and shorter, and the supercooling degree is lower, with the increase of the interaction between polymer and filler; The relatively strong crowding effect hinders the direct contact between the nucleus and the filler surface, and there is no heterogeneous nucleation; The enhancement of nucleation ability of polymer chains is caused by the increase of the density of segments and the confinement effect in the region near the filler; The nucleation mode changed from the intermolecular nucleation mode to the intramolecular nucleation mode, and the crystal orientation gradually changed from the orientation perpendicular to the filler surface to the that along the filler surface with increasing polymer-filler interaction. For the system with high grafting density, the excessive crowding effect controls the crystallization process, which is hardly affected by the interaction.In this paper, we understand the complex effect relationship of the interface on the glass transition and crystallization behavior by the microscopic information of the mobility of polymer chains. The influence of the interfaces is related to its type,quantity, spacing and interfacial interaction. The various and even inconsistent findings in the experiment should be attributed to the differences in these factors, for which we can get a unified interpretation through the present simulation research.The results of this work have important theoretical significance for the design,preparation and application of nano polymer composites. |
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