Design Novel High-performance Elastomeric Materials By Molecular Dynamics Simulation Assistance

With the rapid development of industry,elastomer materials have become an extremely important research field in materials science and engineering.To prepare elastomer materials with excellent performance,it is the key point to establish the relationship between the microstructure of the composite material and the macro dynamic and static mechanical properties.Since elastomer materials,especially elastomer-based nanocomposites,have complex interactions at multiple levels and scales,which is difficult to accurately characterize their microstructures using traditional experimental methods,meaning it can not establish the quantitative relationship microstructures and macroscopic performance.With the rapid development of computer technology,it is possible to use molecular dynamics simulation methods to characterize the microstructure of elastomer-based nanocomposites and to prepare new elastomeric materials,which provides guidance for experiments.In this paper,molecular dynamics simulation methods are used to carry out in-depth and detailed research around the microstructure design and macroscopic properties of elastomer materials,and several important results are obtained.The details are as follows:(1)Using molecular dynamics simulation methods and establishing coarse-grained models,six kinds of molecular chain architecture were designed and the effects of different molecular chain architectures on the dispersion of nanoparticles and the mechanical properties of their corresponding nanocomposite were analyzed.First,we investigated the influence of the interfacial interaction between polymer molecular chains and nanoparticles on the dispersion of nanoparticles,and found that for the nanocomposites with various molecular chain architectures,both high and low interfacial strength caused the aggregation of nanoparticles,indicating that there was an appropriate interfacial strength that the nanoparticles obtained the best thermodynamically stable dispersion.Moreover,among the six molecular chain architectures,the multi-arm star structure and the multi-branched branch structure were more benefitial for the dispersion of nanoparticles.Second,we investigated the effect of molecular chain structure on the chain orientation and strain hardening behavior of nanocomposites under uniaxial deformation.We found that the nanocomposites corresponding to the multi-arm star-shaped molecular chains with the best dispersion of nanoparticles exhibited the best static mechanical properties and more complete crosslinking network structure.Third,we studied the effects of end-functionalization of molecular chains with different chain architectures on nanoparticle dispersion,chain orientation and strain hardening behavior.The results showed that nanoparticles were dispersed well in a branched matrix with multiple branches.Moreover,when the tensile strain was small,the branched structure with fewer branches showed relatively better mechanical properties.When the tensile strain was large,the nanocomposite with multi-arm star structure and the multi-branched structure exhibited better mechanical properties than others.(2)By constructing a coarse-grained model,a new type of single-component polymer-grafted nanoparticles was designed,and the self-assembly structure of the nanoparticle was studied by changing the length,density,and stiffness of the grafted chain.First,the effect of the length and density of the grafted polymer chain on the dispersion of the nanoparticles was studied by characterizing the radial distribution function between the nanoparticles and observing the visualization results.The results showed that when the grafted chain was long,the arrangement regularity of the nanoparticles was very low,and as the density of the grafted polymer chain increased,the arrangement regularity of the nanoparticles increased monotonously,signaling that the shorter grafted chain and higher grafted density contributed to the regularity of the nanoparticle arrangement.Second,we studied the effect of the stiffness of the grafted polymer chain on the regularity of the nanoparticles arrangement.The results showed that when the grafted density was high,as the flexibility of the grafted chain decreased,the regularity of the self-assembled structure would improve,then gradually tended to a fixed value and oscillated near a certain horizon.Third,we studied the effect of different grafted chain stiffness on the static mechanical properties of the system.The research results showed that with a certain stiffness of the grafted chain,the regularity of the nanoparticle arrangement was improved and the static mechanical properties of the system became worse.For the flexible chain with a high grafted density,the system possessed a high degree of regularity and excellent mechanical properties simultaneously.Fourth,in order to systematically summarize the effects of grafted chain density and grafted chain stiffness on the regularity of self-assembled structures,we drew a phase diagram and divided the structure of nanoparticles into three types:amorphous structure,ordered structure and superlattice structure.(3)Using molecular design and full-atom molecular dynamics simulation,a new type of silicone elastomer with high cold-resistance and crystalline-free was designed by using three different siloxane structural units(dimethylsiloxane,diethylsiloxane and methylepoxysiloxane),through random copolymerization.We employed three different methods(specific volume-temperature,non-bonded potential energy-temperature and conformational transition-temperature)to calculate the glass transition temperature of E-MEVQ by molecular dynamics simulations,and compared it with the glass transition temperature that determined by differential scanning calorimetry.The glass transition temperature obtained by simulation and experiment was very consistent,and its value was lower than the glass transition temperature of polydimethylsiloxane(PDMS).Furthermore,E-MEVQ was crystalline-free at extremely low temperature.(4)Using full-atom molecular dynamics simulation a kind of siloxane with a long-side chain(LSC)composed of silicon bond was designed,and then it was copolymerized randomly with dimethylsiloxane.This new silicone elastomer obtained a wide temperature service range.Its glass transition temperature was unprecedentedly low(-150?),and had a relatively high decomposed temperature(higher than 400?).The experiment results proved the simulated results were correct.In addition,we proposed a new characterization method based on the change of the mean square displacement changing rate of the molecular chain with temperature,which could accurately characterize the decomposed temperature of the elastomer.