Dissipative Particle Dynamics Simulaiotn Study On Phase Behavior Of Polymeric Ternary Blends

Blending is an effective approach to improve the properties of polymeric materials. However, polymer blends tend to macroscopically phase separate because of the low blending entropy, leading to poor quality materials. To circumvent this problem, one typically introduces compatibilizers into the system in order to modify the interfacial quality. There are plenty of factors impacting on the phase behavior of polymer blends, though one generally only studied ternary blends which are relatively simple in experiments and theoretical calculations. However, there are few reports relating to studies on the phase behavior of ternary blends by dissipative particle dynamics (DPD) method. In this thesis, the DPD method was used to study the phase behavior of symmetric and asymmetric polymeric ternary blends, and various characterization means which have clear physical meaning were used to characterize the corresponding phase structures. The relatively detailed summaries were as follows.1. The coarse-grained model was constructed based on the experimental symmetric polymeric ternary blend poly(ethylene)/poly(ethylene oxide)/poly(ethylene)-poly(ethylene oxide) (PE/PEO/PE-PEO) used in Bates's study. The phase and phase transition are well described by order parameter, structure factor, radial distribution function and density distribution function. LAM spacing, macroscopic interfacial tension and microscopic molecular orientation were studied through changing the volume fraction ratio between homopolymers and diblock copolymer in order to establish the relationship between molecular order degree, ordered phase domain size and interfacial tension and obtain the phase diagram at last. The results show that (1) The equilibrium phase will change from lamellar (LAM) to bicontinuous microemulsion (PBME) and then to macroscopic phase separation (PS) as the volume fraction of homopolymersΦH is increased. The sequences and their locations are in accordance with the experimental results, and the PBME region is near the theoretical Lifshitz point, which is in agreement with theoretical results; (2) The LAM spacing increases with the increasing ofΦH in LAM phase region; (3) The interfacial tension decreases with increasing volume fraction of diblock copolymer (1-ΦH) in PS phase region and the formation of stable PBME requires the interfacial tension reduce to zero; (4) In PBME region, the value of order parameter is very small, which indicates that the interfaces formed by diblock copolymer is tortuous; (5) The calculated structure factor of PBME phase is in accordance with the Teubner-Strey static scattering model of micro-emulsion. In addition, the effect of temperature impacting on the phase behavior of symmetric ternary blend also studied by DPD method. The result shows that the phase behavior of symmetric ternary blend is not sensitive to temperature. Finally, the structure factor was used to characterize the LAM structures, and the results are in agreement with the conclusion obtained by radial distribution function and density distribution.2. Breaking the molecular symmetry of diblock copolymer, A3B7 was chosen as coarse-grained model of diblock copolymer to construct the DPD coarse-grained model of asymmetric ternary blend. Phase and phase transition were studied by changing the volume fraction ratio between homopolymers and diblock copolymer. The phases were preliminary determined and the rough phase diagram is also established. The results show that the equilibrium phase change from hexagonal packing cylinder (HPC) to "bicontinuous microemulsion (PBME)" then to droplet-microemulsion (DLME) and at last to macroscopic phase separation (PS) as the volume fraction of homopolymersΦH is increased.