A Computer Simulation Study Of Theproperties Of Linear Polystyrene/Cross-linked Polystyrene Nanoparticle Composite

Adding nanoparticles(NPs)into polymers has becomea widely used method to optimize the properties of materials,such as mechanical,optical and electrical properties.However,the structure-property relationship and the mechanism of performance improvement in the polymer/NP composite(PNCs)systems are still remaining largely unresolved.Due to the identical chemical composition of melt polymers and nanoparticles,the composites of linear polystyrene and cross-linked polystyrene nanoparticle can be served as an ideal system for the study of the performance improvement mechanism of PNC systems.Due to the identical composition of the nanoparticle and polymer chains,the enthalpic interactions can be reduced to a minimum,whileonly the entropic effect is dominant in the system.In this thesis,by performing coarse-grained molecular dynamics simulation based on a coarse-grained(CG)polystyrene(PS)model obtained by iterative Boltzmann inversion methoed,we investigate the structur and dynamical properties in the aforementioned PNC system.By using this CG-PS model,the structural properties,such as the density,radial distribution function,bond-length distribution,angle distribution and radius of gyration of PS chains in the melt state can be well captured.Moreover,by a simulated intrachain cross-linking reaction process in dilute solution.The size of nanoparticlewe obtained from our simulation is in good agreement with that obtained in experimental measurements.For the composite system composed of linear PS chaind and cross-linked single-chain NP,we firstly investigate the interface properties at the NP surface area.Since the extremely large interface area is a main characteristic in nanocomposite system,it is widely believed that interface properties play a keyrole.Results of the density profile at the interface of melt chain and soft nanoparticles demonstrate that melt polymer chain and NP are mutually penetrated with each other.Their density profiles are smooth,which is in a strong contrast to the density distortions usually often observed in the interface region of inorganic nanoparticles.Besides,short segments of melt chains in the interface can be trapped and therefore be compressed in rugged nanoparticle surface structures,and therefore radius of gyration of these small segments reduces.On the contrary,the relative long segments are elongated along the tangential direction of the nanoparticle surface,which leads to swellon of melt chains.While in the radial direction,the chain dimension are reduced.We also investigate the dynamics in the interface by the analyses such as the self-scattering function of monomers and the mean-square displacement of segments.The results show that due to the influence of nanoparticles the dynamics of short segments at the interface regionare decelerated,while the dynamics of long segments areaccelerated.Recently,the diffusion of nanoparticles in polymer melts has also drawn a wideattention.In an early theoreticalwork by de Gennes and Brochard-Wyart,they predicted that when the radius of NP is smaller than the tube diameterof melt polymers and chain dimension(radius of gyration),the diffusion coefficient of nanoparticles is independent of the melt chain length(N);on the contrary,the motion of nanoparticles is determined by the melt polymer chain segments which havesimilar size with NP.These predictions have also been confirmed in recently reported molecular dynamics simulations of hard-core NPs diffusion in polymer melts.Here we verify that changing the nanoparticle surface from softness to hardness does not affect the aforementioned N-independence of NP diffusivity,in the composites composed of linear PS chainsand cross-linked PSnanoparticles.In addition,we find that the motion of nanoparticles is non-Gaussian,which is unexpected in theories and undetected in previous simulations.Afteranalysing the mean-square displacement and displacement possiblility distribution of polymer chain segments at different time and lenth scales,wefind that the non-Gaussianity of nanoparticle diffusion originates from the friction applied by the segments whose diffusion is also non-Gaussian and which has a similarsize with nanoparticle.Based on the understanding of interface properties and diffusion dynamics of nanoparticles,we further investigate the viscosity reduction effect caused by the addition of nanoparticles.Previous experiments and simulations have demonstrated that the addition of nanoparticles,whose radiusis smaller than the radius of gyration of melt chains,will result in a significant reduction in viscosity.There is a consensus that the disentanglement effect might be themain reasonfor theviscosity reduction.However,our simulations demonstrate that the extend of the viscosity reduction is much larger than that from the disentanglement effect.Besides,we find that the visocisty reduction has a melt polymer chain length dependence,namely for a given loading of NP,systems with longer melt chains are found to have larger viscosity reduction by adding NPs.By a comprehensive analysis,we find thatthe mechanism of the significant reduction in viscosity is as follows:(i)the relaxation of melt chain segments atthe NP interfaceregion can be accelerated by the fast deformation of the NP surface;(ii)NPs are found to have a constant diffusion constant in entangled systems,NP diffusivity does not slow down when the melt chain length and therefore matrix viscosity increase;(iii)In entangled systems,due to the fast diffusion of NPs and its N-independent diffusivity,the probability for the melt polymer chains to collide with NPs will increase cubically with N.As a consequence,melt chain relaxation will be largely accelerated due to a collective effect induced by multiple collosions with NPs in the system.Base on this mechanism,three phenomena can be well explained: significant reduction in viscosity,chain length dependence of viscosity reduction and invariance of plateau shear modulus after addition of NPs.