Simulation Study On Conformation And Dynamics Of Semi-flexible Polymer Chains In The Presence Of Nanoparticles

It is a hot research topic in polymer nanocomposites to improve the physical and chemical properties of polymer materials by adding nanoparticles to a polymer matrix.In this thesis,the dynamics and conformation of semi-flexible polymer chains in the presence of periodically distributed nanoparticles are simulated by using off-lattice Monte Carlo simulations.It is divided into the following two parts:(1)Dynamics of a semi-flexible polymer in the presence of nanoparticles.Our simulations find that for the repulsive or weak attractive nanoparticles,the dynamics is slowed down monotonically by increasing the chain stiffness ko or decreasing the inter-particle distance d.However,for the strong attractive nanoparticles,the dynamics shows non-monotonic behaviors with k o and d.The non-monotonic behavior of polymer's dynamics with k? is explained by the competition between the weakening effect of chain stiffness on the nanoparticle's attraction and the intrinsic effect of chain stiffness which reduces the dynamics of polymer.In addition,the non-monotonic behavior of polymer's dynamics with d is explained by the competition between the nanoparticle-exchange motion of polymer dominated at small d and the desorption-and-adsorption motion at large d.An interesting result is that a stiff polymer may move faster than a flexible one.The underlying mechanism is that the nanoparticle's attraction is weakened by the chain stiffness,and the excluded volume effect of nanoparticles plays more important roles for stiffer polymer.(2)Based on annealing simulations,the effects of nanoparticles on the glass transition temperature Tg and self-diffusion coefficient D of polymer melts have been studied by changing the concentration ?NP and size DNP of nanoparticles.It is found that as the nanoparticle concentration ?NP increases,Tg gradually increases,while D gradually decreases.Moreover,the effect of small nanoparticles is stronger than that of big nanoparticles.After importing the dynamical confinement parameter ?d/2Rg,we find that all the normalized glass transition temperature Tg/Tg0 and diffusion coefficients D/D0 collapse onto a master curve of Tg/Tg0=1/[1-exp(-?d/2Rg)]and D= D0[1-exp(-?d/2Rg)],respectively.Here Tg0,Do and Rg are glass transition temperature,the diffusion coefficient and gyration of pure polymer melt,respectively.(?d = ?·ID,ID = d—DNP is the spacing between inter-particle surface.a is related to the nanoparticle size DNP and system temperature T.At last,we study the effect of chain stiffness.For the weak attractive nanoparticles,the dynamics is slowed down monotonically by increasing the chain stiffness k? similar with pure polymer melt.However,D first decreases and then increases with the increase for ke when nanoparticles are strong attractive.