Computer Simulation Study Of Creep Mechanism Of Polymer Nanocomposites

With the continuous development of advanced materials,the dimensional stability of polymer nanocomposites(PNCs)has been put under higher demands,especially in some high-tech applications such as flexible sensors,foldable display devices and precision instruments.Under the long-term stress environment,PNCs need to follow better design rules to obtain the resistance to creep deformation.The creep properties of PNCs are closely related to the microscopic network structure formed by filled nanoparticles(NRs)in the matrix.Although many studies on filler networks have gradually revealed the dispersion mechanism of NPs in the filler networks,the contribution of the NPs network structure to the creep resistance of polymer materials has been rarely mentioned.In view of the development of computer simulation technology and the success of some previous simulation works on the simulation of the structure and mechanical properties of PNCs,we adopt the coarse-grained molecular dynamics(CGMD)simulation method in this paper to investigate the effects of the filling fraction of NPs and the strength of the interaction between polymer chains and NPs on the creep mechanism of PNCs.Here we use the prevalence of the three structures of NPs in the radial distribution function in the PNCs system to quantitatively characterize the binding structure of NPs to the matrix chains and the strain after a certain time under constant force to quantitatively characterize the creep behavior.Our specific work was carried out as follows:I.The differences in creep behavior of cross-linked and non-cross-linked polymer systems;We first investigated the effect of temperature on the creep behavior of the pure polymer system and found that the shifts of creep strain at higher crosslinked densities lagged slightly behind that of the non-cross-linked system with temperature when changing the temperature conditions.In addition,we also found that crosslinking at low stresses did not significantly affect the creep behavior of the system,but at high stresses,the crosslinking was effective in suppressing the creep strain growth of the system due to the crosslinking network.Ⅱ.The effect of the filling fraction of NPs on the creep behavior of PNCs was investigated;We set up a series of systems containing different filling fractions of NPs and simulated the creep behavior of these systems and found that the addition of NPs significantly reduced the creep compliance of PNCs.In addition,we found that the change in creep strain with increasing filler volume fraction can be roughly divided into three different phases,which are strongly related to the structure of PNCs:It is concluded that the single-chain bridge structure(BB=1)in the system can effectively limit the slip of the matrix chains,thus improving the creep resistance of PNCs.Ⅲ.The effect of the interaction between polymer chains and NPs(εnp)on the creep properties of PNCs;In this section,we first characterized the creep resistance of PNCs at the same filling fraction under four εnp.The creep strain is smaller for larger εnp systems at the same filler fraction,which is due to the fact that the larger εnp is,the tighter the bond between the molecular chains and the NPs.Then we increased the filler volume fraction,at which time we found that the addition of NPs is beneficial to the creep resistance of the material only when is strong enough,and the creep strain increases with the increase of the filler volume fraction for smaller εnp systems.This is because the slip of polymer matrix chains is determined by the attraction between polymer chains and NPs together with the repulsion between NPs.Only when the attractive forces between the molecular chains and the NPs dominate,the slip between the matrix chains is limited by the effect of the NPs.