Study Of Crystallization Behaviors In Polymer Nanocomposites By Dynamic Density Functional Theory

The thermodynamics,mechanics and optoelectronic properties of the polymer nanocomposites mainly depend on the formation and regulation of the mesostructures in the composite systems.Under suitable particle-particle,polymer-polymer and particle-polymer interactions,nanoparticles and polymer chains will self-assemble into some specific conformations and form ordered lattice structures,which will bring the materials some unique properties.Based on the mentioned microscopic interactions,analyzing the different factors that affect the mesoscopic structures and then determine the macroscopic properties of the materials is of great significance to design and develop the new classes of materials.In this paper,the disorder-order transitions and self-assembly processes of nanoparticles and polymers are studied by analyzing the formation and growth processes of two-dimensional(2D)and three-dimensional lattices.The effects of different media environments on ordered structures and morphologies are studied to explain the microscopic mechanisms of the ordered self-assembly.Specific research contents include the following aspects:(1)Based on the framework of the classical dynamic density functional theory(DDFT),a new time-evolving free energy functional theory is constructed considering the contributions of lattice tensor,oriented direct correlation function,polymer chain connectivity,bond bending and chain conformation.This time-evolving theory combines the mentioned thermodynamic properties and the dynamical equations of motion,which can be used to quantitatively characterize the density and energy evolution of 2D or three-dimensional lattices and the morphology changes during the formation and growth of the ordered structures in polymer nanocomposites.In order to test the reliability of the theoretical model,the lattice structure parameters are determined by optimizing the local and global free energies,the equilibrium densities of different crystal-liquid(melt)phases are predicted,and the free energy barrier changes during the formation processes of ordered structures are analyzed.The prediction results are in good agreement with the corresponding experimental or molecular simulation data.This part lays the theoretical foundation for the study of disorder-order transitions and self-assembly dynamics behaviors.(2)Using the tested DDFT method,the polyacrylic acid(PAA)-induced 2D self-assembly of cadmium selenide(CdSe)quantum dots on the carbon film surface is studied.It is found that the concentration of PAA,the local supersaturation of CdSe,the ratio of the CdSe diameter to PAA segment size and the surface chemical properties of CdSe all affect the disorder-order transitions and self-assembly behaviors of CdSe.By calculating the energy barriers of CdSe self-assembly processes at different supersaturations,it is found that the nucleation energy barrier is close to zero when the supersaturation of the CdSe density reaches 1.025.Under this condition,disorder-order transition can occur spontaneously.Corresponding to the theoretical study,an experimental study of CdSe/PAA self-assembly is performed.By comparing the self-assembly morphologies of CdSe in CdSe/PAA monolayer films,it is found that when the mass ratio of PAA to CdSe is 0.056 g:0.022 g,the supersaturation of CdSe formed on the surface of carbon film reaches 1.05,which extends the spontaneous conditions predicted by the theoretical method.Therefore,CdSe forms an ordered morphology of 2D hexagonal-close-packed lattice.Comparison of theoretical and experimental results shows that the theoretically predicted 2D ordered array is basically consistent with the experimental structure.The incomplete structure observed in experiment is mainly caused by the nonideal and uncontrollable external factors such as gravity.These external interference factors,however,are not considered in the theoretical study.(3)Taking face-centered cubic lattice as an example,the formation and growth of three-dimensional simple crystal under homogeneous and heterogeneous conditions are investigated using the DDFT method.Studies have shown that the impurity changes the growth process of crystal.The impurity affects the ordered arrangement of the lattices,resulting in lattice defects.As the crystal grows,the effect of the impurity declines and the lattice defects heals automatically.When the impurity size is consistent with the particle size,the lattice defects mainly occur in three repetition periods.When the impurity size is not consistent with the particle size,the lattice defects will extend to 4?5 repetition periods.Exceeding the mentioned repetition periods,the lattice will be repaired,that is to say,the ordered structures have self-healing ability.The whole self-assembly process around an impurity can be divided into three typical stages:density aggregation,self-healing and stable growth,in which the self-healing is the rate-determining step.(4)Focusing on the polymer crystallization in polymer nanocomposites,polyethylene(PE)crystal is taken to study the ordered self-assembly behaviors of the polymer.The crystallization processes of PE from a homogenous phase,on a solid surface and on an amorphous PE surface are performed and analyzed.Under homogeneous condition,the PE lattice shows different growth rates in different directions of crystal planes.When the PE crystallizes on the solid surface,due to surface confinement,crystal growing in the interfacial region have lattice defects.As the influence of the solid surface declines,the PE lattice also shows the self-healing ability.When PE crystallizes on the surface of amorphous PE,due to the roughness of the surface,multiple nucleation sites exist in the interfacial region,resulting in the nucleation and growth of multiple lattices.At the same time,the lattice defects and morphology defects are caused.During the continuous growth of crystal,the lattice defects can be healed automatically,but the morphology defects are always present.