Computer Simulation Study Of The Interfacial Properties And Diffusion Mechanism Of C₆₀ In Polyolefin/C₆₀ Composites

It is well known that adding nanoparticles?NPs?into polymer material can often dramatically enhanced mechanical,optical,electrical and processing properties.Nanoparticles generally have a relatively large specific surface area,and changes in the properties of the composite material are directly related to the formation of the interfacial layer.In the study of composite interface regions,the thickness of the interface layer is also a major concern.The thickness of the interface layer often depends on the specific properties of the NP and the polymer chains,such as the size,shape and rigidity of the NPs,and the flexibility of the polymer chains,as well as the molecular weight?MW?.The thickness of the interface layer also varies for the different properties to study.In addition,extensive theoretical works have reported on the diffusion dynamics of NPs in polymer melts and solutions.Recently,some theoretical work elaborated the mechanism of hopping diffusion dynamics of nanoparticles from the perspective of free energy in long-term regions in a dense medium system.These theories point out that in polymer melt the hopping diffusion mechanism is relevant only for particles with size2R/_Td^.15-.20?R is the radius of the NP andd_T is the tube diameter?,while smaller particles are considered to slip through the entanglement mesh.However,very recently,Volgin et al.found that in polyimide?PI?matrix small C₆₀0 NP also adopted hopping diffusion.This phenomenon is obviously beyond the above theoretical predictions.The hopping diffusion of small C₆₀0 in PI matrix can be attributed to the attractive interactions between C₆₀0 and the polymerchainbackbone.Indeed,suchparticularfeaturesofthe“polymer–nanoparticle”interactions can significantly affect the properties of nanocomposites materials.However,due to rich variety of polymers,how the interactions between NPs and polymer segments especially the side groups of polymer influence the NP dynamics remain largely unexplored.To better understand the mechanism of nanoparticles on the properties of materials,we need to understand and study the effects of nanoparticles on the structure and dynamic properties of different polymer chains,as well as the diffusion mechanism of nanoparticles in the system.So that we can explain the underlying mechanism at the molecular level to better design materials.In this article,by performing all-atom molecular dynamics simulations,we investigate the diffusion dynamics of C₆₀0 in two linear polyvinyl polymers,namely PS and PP melts.The interfacial structure and dynamics properties of polymer segments around C₆₀0 surface in both composite systems are also investigated.In the case of PS,there are attractive interactions between C₆₀0 and phenyl rings on side group of PS chain,while such interactions are absent in the system of PP/C₆₀.In particular,we find a hopping diffusion process of C₆₀0 in PS melt due to attractive interactions between C₆₀0 and phenyl rings.We also find a dynamic coupling between C₆₀0 and a PS tetramer which has a similar size with NP.Phenyl rings on the PS chain backbone are found to be parallel to the surface of C₆₀0 in its vicinity.At the same time,rotational dynamics of phenyl rings and translational diffusion of styrene monomers are found to be slowed down in this interface area,while we do not observe any hopping motion in PP system.