Shengwei Deng

Simulation of Morphologies and Mechanical Properties of A/B Polymer Blend Film

Abstract The effects of blend composition and micro-phase structure on the mechanical behavior of A/B polymer blend film are studied by coupling the Monte Carlo (MC) simulation of morphology with the lattice spring model (LSM) of micro mechanics of materials. The MC method with bond length fluctuation and cavity diffusion algorithm on cubic lattice is adopted to simulate the micro-phase structure of A/B polymer blend. The information of morphology and structure is then inputted to the LSM composed of a three-dimensional network of springs to obtain the mechanical properties of polymer blend film. Simulated results show that the mechanical response is mainly affected by the density and the composition of polymer blend film through the morphology transition. When a force is applied on the outer boundary of polymer blend film, the vicinity of the inner cavities experiences higher stresses and strains responsible for the onset of crack propagation and the premature failure of the entire system.

Mechanical properties of high-performance elastomeric nanocomposites: a sequential mesoscale simulation approach

The incorporation of nanoparticles into elastomeric block copolymers affords engineers an opportunity to obtain polymer nanocomposites that potentially rival the most advanced materials in nature. A computationally efficient simulation method that utilized MesoDyn for the morphologies and the lattice spring model (LSM) for the mechanical properties was adopted in this work. The simulation results show that the selective distribution of nanoparticles in hard or soft segment microdomains of block copolymers will narrow the phase domain size in bicontinuous structures. The Zener model was incorporated into pure elastic LSM to capture the stress relaxation behavior. Mechanical tests reveal that the stress transfer between the polymer matrix and nanoparticles in different composites is critical to the stiffness enhancement. In dispersed structures, adding nanoparticles in a hard microdomain can increase the elastic modulus and maintain high extensibility without impairing its viscosity dramatically. The methods developed in this work yield guidelines for formulating elastomeric nanocomposites with desired macroscopic mechanical responses.

Multiscale simulation of shear-induced mechanical anisotropy of binary polymer blends

Mechanical properties of polymer blends are not only determined by characteristics of individual polymer but also depend significantly on processing such as shear fields. A sequential mesoscopic simulation method was adopted to study the influence of shear processing on morphology orientation and mechanical responses. This method utilizes mesoscopic dynamic simulation (MesoDyn) for structural evolution and lattice spring model (LSM) for correlating the structure and mechanical behaviour. The dispersed phase in meso-structures moves from spherical to elliptical and then to columnar structure with the increase of shear rates. The morphology orientation leads to the anisotropy of elastic modulus. During the tensile test, different fracture processes were observed with two kinds of toughness relationship in blends which correspond to brittle phases dispersed in a ductile matrix and in reverse ductile phases dispersed in a brittle matrix. The tensile strength along shear processing direction increases with the increase of shear rates when the dispersed phase is ductile, while the strength decreases when the dispersed phase is brittle. The strength perpendicular to shear processing direction is mainly related to the soft matrix and interfacial strength. The morphologies of polymer blends at different shear rates and their corresponding mechanical behavior are well correlated by the mesoscopic simulation. The simulation results also yield guidelines to manufacture desired polymer blends with shearing process, e.g. extrusion or injection molding.

Controlled Assembly of Nanocellulose-Stabilized Emulsions with Periodic Liquid Crystal-in-Liquid Crystal Organization

Colloidal particles combined with a polymer can be used to stabilize an oil-water interface forming stable emulsions. Here, we described a novel liquid crystal (LC)-in-LC emulsion composed of a nematic oil phase and a cholesteric or nematic aqueous cellulose nanocrystal (CNC) continuous phase. The guest oil droplets were stabilized and suspended in liquid-crystalline CNCs, inducing distortions and topological defects inside the host LC phase. These emulsions exhibited anisotropic interactions between the two LCs that depended on the diameter-to-pitch ratio of suspended guest droplets and the host CNC cholesteric phase. When the ratio was high, oil droplets were embedded into a cholesteric shell with a concentric packing of CNC layers and took on a radial orientation of the helical axis. Otherwise, discrete surface-trapped LC droplet assemblies with long-range ordering were obtained, mimicking the fingerprint configuration of the cholesteric phase. Thus, the LC-in-LC emulsions presented here define a new class of ordered soft matter in which both nematic and cholesteric LC ordering can be well-manipulated.

Tensile deformation of semi-crystalline polymers by molecular dynamics simulation

In this work, the microstructure evolution of semi-crystalline polymers during tensile deformation is analyzed by molecular dynamics simulation. A perfect semi-crystalline lamellar structure with crystalline/amorphous interface perpendicular to tensile direction is created with the help of coarse-grained (CG) model of poly(vinyl alcohol) (PVA). During the tensile test, two kinds of strain rates are applied to the lamellar stack to determine the stress–strain curves, yield stresses, and crystallinities. Consistent with experimental findings, two yield points were observed in the semi-crystalline sample which was corresponded to the fine and coarse crystallographic slips in the lamellar structure, where the crystal stems gradually rotated into the direction of applied stress during chain slips. After the second yielding point when the crystal stems had been rotated fully into the direction of applied stress, the lamellar structure was destroyed and it resulted in a decrease of crystallinity. In addition, the increase of the strain rate led to the acceleration of destruction of crystal structures. It is worth noting that the stress induced crystallization was observed in the interfacial region, and newly crystallized beads were belonged to the same microcrystalline domain as crystalline region due to memory effects. This work provides direct comparison of structure evolution between crystalline and amorphous region in semi-crystalline polymers during tensile deformation, and it is helpful for the design and mechanical property analysis of semi-crystalline polymers.