Mechanical Properties Of Heterogeneous Polymer Materials By Computer Simulation

Macroscopic mechanical characteristics are the basis of physical properties of polymer materials and a topic of fundamental importance with innumerable industrial applications. Therefore, an effective prediction method for the mechanical properties with wide applicability is very necessary. In this work, we proposed a sequential mesoscopic simulation method that utilized MesoDyn for morphologies and lattice spring model (LSM) for mechanical properties, and then we applied this combined method to several kinds of polymer materials. Simulation results agreed well with the experimental or theoretical findings. The lattice spring model (LSM) was optimized or improved to fit for different systems, and mechanical models were proposed during the simulation. Besides, molecular dynamics method was adopted in the simulation, which is related to the chain movement. This work is divided into the following five parts:(1) The mechanical behavior of binary polymer blends was studied by a sequential mesoscopic simulation method. The dynamic density functional theory approach embodied in MesoDyn method was adopted to obtain the meso-structures of polymer blends. The output of MesoDyn served as the input of a micromechanical LSM. Mechanical properties, such as young’s modulus, stress distribution and fracture position was obtained through applying strain in systems with different blend ratios. An interfacial stiffness model for immiscible polymer blends was proposed and incorporated in LSM to simulate the Young’s modulus along the tensile direction. Simulation results agreed well with experiments in polystyrene (PS)/polyproplene (PP) system. As for fracture position prediction, a stress-related probabilistic method was integrated in LSM (PLSM) to study the fracturing process of materials. Fracture positions were shown in detail, which had close relationship with the meso-structures in PS/PP blends. Subsequently, we varied the minimum fracture stress (related to toughness) of different components and the interface to study the influence of toughness on the fracture position. This work yields a guideline for the study of mechanical properties of immiscible polymer blends.(2) The structural evolution and mechanical properties of "hard-soft" block copolymers and its nanocomposites were determined by a hybrid simulation technique. The result showed that selective distribution of nanoparticles in hard or soft phase would narrow the phase domain size in bicontinuous structures. Then the output from our morphological studies served as the input to the LSM, which was used to simulate the stiffness, critical fracture strain and stress relaxation behavior. The pure elastic model was replaced by an anelastic solid model (Zener model) to capture the viscoelastic behavior. The result revealed the stress transfer mechanism between the polymer matrix and nanoparticles in different composites, which was critical to determine the stiffness enhancement. In dispersed structures, adding nanoparticles in hard phase could increase the elastic modulus and maintain its high extensibility and viscosity, while the viscosity was impaired dramatically by adding nanoparticles in bicontinuous structures. This work correlate meso-structures of polymer nanocomposites (especially the distribution of nanoparticles) and macroscopic mechanical responses via viscoelastic LSM.(3) Multiblock copolymers containing a large number of blocks have distinct microstructures and mechanical responses that are different from that of conventional diblock and triblock copolymers. The sequential mesoscopic simulation method was adopted in this work to study the mechanical behavior. The loops and bridges of polymer chains were incorporated into interface strength in the probabilistic LSM calculation. Simulation results showed that tensile strength increases dramatically with the increase in the number of blocks within "hard-soft" multiblock copolymers. One-dimensional lamellae were built to provide an ideal morphology for studying the influence of lamellar orientation on multiblock copolymer mechanical properties. Stiffness anisotropy was found in this model system and the simulation result had a good agreement with the theoretical (Voigt and Reuss models) and experimental findings. The interfacial strength model for multiblock copolymers was proposed to simulate the difference in chain structures. During tensile tests different failure processes were observed with two kinds of interface strength that corresponded to brittle and ductile fracture in real polymers.(4) The compatibilizing effect of block copolymer addition on mechanical properties of polymer blends was studied by the sequential mesoscopic simulation method. In this work, the original two-component LSM was extended to the application in multi-component systems, the stiffness, strength and fracture models were also modified to fit for new systems. The mechanical responses such as the Young’s modulus, tensile strength and fracture position were analyzed as a function of the concentration of the additive. Simulation results showed the Young’s modulus varied slightly with the increase of the concentration of the additive. The tensile strength increased dramatically with the addition of compatibilizer. The fracture position moved from the interface to matrix with the increase of the volume fraction of compatibilizer, it also led to an enhancement of tensile strength. The compatibilizing effect of block copolymer addition was examined by the analysis of fracture position in LSM.(5) Uniaxial tensile test was adopted to study the deformation of layered crystalline/amorphous structures by molecular dynamic method. The single crystal structure was created from a coarse-graining poly (vinyl alcohol)(CG-PVA) model via a heterogeneous nucleation process, and then the lamellar structure was obtained by melting and quenching processes. In the mechanical test, the stress-strain curve and the variation of crystallinity were examined at different strain rates. The increase of strain rates would accelerate the destruction of crystalline domain. The density distribution, chain orientation and crystallinity distribution were studied during the tensile test at slow strain rate. The result showed that the yield point was corresponding to the rotation of polymer chains along the tensile direction, polymer chains in crystalline domain had memory effect and the stress-induced crystallization was also observed in this work.