Interface Design Investigate Of Polymer Nanocomposite By Molecular Dynamics Simulation

Polymer nanocomposites have been received significant attention from academia due to their excellent mechanical properties and functionality.Several types of interfaces are formed in polymer nanocomposites due to the introduction of fillers or changes in the type of matrix polymer.These interfaces with large specific surface areas give the composites a number of excellent properties,but they also bring a greater challenge to the preparation of the materials and the adjustment of the material properties.With the development of productivity,human requirements for the performance of polymer nanocomposites are increasing,therefore,understanding the mechanisms by which interfaces in materials affect material properties is essential for the preparation of high-performance polymer nanocomposites.Interfaces in polymer nanocomposites usually involve multiple scales and substances.Thus,it is difficult to characterize them in more detail and depth with the current traditional experimental techniques.Molecular dynamics(MD)simulation methods have the advantage of controllable variables and detailed characterization,which can effectively investigate such complex objects.In this paper,a variety of interfacial systems in polymer nanocomposites are constructed using molecular dynamics simulation,and the molecular mechanisms of their interfacial effects are investigated through deep characterization and analysis.The main research content of each chapter is as follows:(1)The system that polymer grafted nanoparticles are filled in a lamellar matrix is constructed based on the coarse-grained model.By quantitatively characterizing a series of microscopic parameters such as the distance between the nanoparticles and the lamellar phase interface,the percentage and orientation of different type grafted chains,the radial distribution function between the parts of the nanocomposite,and the density distribution of grafted chains along the normal direction of lamellar interface,the effects of the filling number of nanoparticles,the grafting density,and the compatibility between the grafted chains and the A-block on the localization of nanoparticles near the lamellar phase interface are systematically investigated.The results show that increasing the grafting density facilitates the grafted chains to separate the nanoparticles from the lamellar phase interface,resulting in the nanoparticles moving away from the interface.Increasing the compatibility between the grafting chain and the A-block results in nanoparticles being dragged from the lamellar phase interface into the A-block by the grafting chains.However,when the compatibility is too strong,the grafting chains near the interface will orient along the lamellar phase interface dominated by the enthalpy,leading to a decrease in the segregation effect of the grafting chains and the nanoparticles close to the interface.When the number of nanoparticles is large,the nanoparticles are distributed in the vicinity of the interface in a layer.Moreover,as the number of nanoparticles increases,the nanoparticles gradually migrate towards the lamellar interface under the spatial repulsion effect between the grafted chains of different layers.(2)Polymer nanocomposite systems filled with two-dimensional fillers are constructed based on the coarse-grained model,in which the surface of the twodimensional filler is chemically heterogeneous.The effect of surface heterogeneity of two-dimensional fillers on the mechanical properties of the composite is investigated,and it is found that there is an optimal surface heterogeneity that results in the highest mechanical strength of the nanocomposite.By analyzing the entanglement between polymers,the overlap between polymers and fillers,and the conformation and adsorption of polymers on the filler surface,it is found that the mechanical strength of the composites is mainly affected by the combination of entanglement between polymers and bridging between polymers and fillers.And the filler surface heterogeneity can influence the above two factors by affecting the adsorption between the fillers and the polymers.For this purpose,the relationships among the fillers dispersion,the filler surface property,and filler adsorption capacity under different heterogeneities are further investigated.The results show that these two factors synergistically influence the adsorption of the filler on the polymer chain.Among them,the non-monotonic variation of filler dispersion with the filler heterogeneity is the main reason for the non-monotonic variation of the adsorption of the fillers on the polymers and ultimately the non-monotonic variation of the tensile stress of the nanocomposite.(3)Loaded polymer ultrathin film systems is constructed based on the coarse-grained model.In this system,the loop chains are adsorbed on a solid substrate and the linear chains are covered on the loop chains,which together form the polymer film.The effect of chain length and rigidity of the loop adsorption chains on the mobility of the linear chains that compose the polymer ultrathin films is investigated by analyzing the diffusion coefficient,spatial distribution,and entanglement of different polymer chains.It is found that there exists an optimal loop chain rigidity that can maximize the confinement of linear chain motion,which is caused by the competition between the existence range of the loop chain and the entanglement of different polymer chains.Meanwhile,it is found that increasing the length of the loop chain is more effective in confining the motion of the linear chain than increasing the rigidity of the loop chain.The reason is that increasing the length of loop chain is more effective in increasing the entanglement between the loop chain and linear chain than increasing the rigidity of loop chain.Besides,expanding the mismatch in length and rigidity between the loop and linear chains amplifies the dynamic asymmetry between them because increasing the mismatch reduces the interpenetration between the loop and linear chains,which in turn hinders the synergistic motion of the loop and linear chains.Finally,by using Gaussian Process Regression model,the prediction model of diffusion coefficient on the segmental and chain length scales of the linear chains for different conditions of length and rigidity of loop chains in constructed,which is further validated by our simulation.