Simulation Study On The Conformation And Dynamics Of Semiflexible Ring Polymers

Semiflexible ring polymer is an issue of importance in a variety of fields including polymer materials,biochemistry,and biotechnology.The research on the spatial configuration and dynamic properties of semiflexible ring polymer chains will provide a new method for fabricating polymer nanocomposites with novel mechanical,thermal and photonic properties.And semiflexible ring polymer chains provide an appropriate theoretical research model for cyclic biomacromolecules,such as:circular chromosomes of Escherichia coli,mitochondrial DN A of eukaryotes,circular plasmids and circular monosaccharides.In this dissertation,based on molecular dynamics simulation method and coarse-grained model,we have extensively studied the conformation and dynamics of semiflexible ring polymers including:semiflexible ring polymer nanocomposite melts,semiflexible ring polymers in ring-linear blends,semiflexible ring polymers confined in two parallel surfaces,binary semiflexible ring polymer mixtures confined in hard sphere.These results contribute to understand the structure of semiflexible ring polymers and their distribution under restricted conditions.In Chapter 2,we explore the dynamics of nanoparticles(NPs)in semiflexible ring polymer nanocomposite melts.A novel glass transition is observed for NPs in semiflexible ring polymer melts as the bending energy(Kb)of ring polymers increases.For NPs in flexible ring polymer melts(Kb=0),NPs move in the classic diffusive behavior.However,for NPs in semiflexible ring polymer melts with large bending energy,NPs diffuse very slowly and exhibit the glassy state in which the NPs are all irreversibly caged be the neighbouring semiflexible ring polymers.This glass transition occurs well above the classical glass transition temperature at which microscopic mobility is lost,and the topological interactions of semiflexible ring polymers play an important role in this non-classical glass transition.This investigation can help us understand the nature of the glass transition in polymer systems.In Chapter 3,we investigate different size-dependent dynamics of NPs in semiflexible linear and ring polymer nanocomposites.Large NPs diffuse faster slightly in flexible ring polymer nanocomposites than in flexible linear polymer ones.However,the motions of large NPs in semiflexible ring polymer nanocomposites with large bending energies are very slow and NPs exhibit the glassy state in which the NPs are all caged by the neighbouring semiflexible ring polymers,while large NPs in semiflexible linear polymer nanocomposites transport much faster than those in semiflexible ring polymer ones because rod-like structures of semiflexible linear polymers can favour the motions of NPs.Meanwhile,the diffusion coefficients Dpolymer of semiflexible ring polymers decrease monotonously with increasing bending energy Kb,while the minimum diffusion coefficient for semiflexible linear polymers occurs at the moderate bending energy such as Kb=6,in which the semiflexible linear polymers hold the maximum entanglement density.NP size,polymer topological constraint as well as chain stiffness affect the dynamics of NPs in polymer nanocomposites seriously,and this investigation can help us understand the complicated dynamical behaviours for NPs in polymer nanocomposites.In Chapter 4,conformations and dynamical properties of semiflexible ring polymers(SRPs)in semiflexible ring-linear blends are investigated by using molecular dynamic simulation.The conformations of SRPs rely mainly on bending energy(Kbjinear)and chain length(Nlinear)of semiflexible linear polymers as well as the fraction of SRPs(?ring)in semiflexible ring-linear blends.SRPs are randomly immersed in flexible or short semiflexible linear polymer melts,however,ordered aggregation structures of SRPs are formed in the matrix of semiflexible linear polymers,especially for long semiflexible linear polymers.These ordered aggregation structures are induced by entropy of SRPs,and long semiflexible linear polymers can improve the overlap of SRPs in semiflexible ring-linear blends.In addition,the center-of-mass diffusion coefficient(D)of SRPs first decreases abruptly and then gradually with the bending energy of linear polymers increasing in semiflexible ring-linear blends,and SRPs diffuse very slowly in the blends of long semiflexible linear polymers due to the strong threading by long semiflexible linear polymers.Our results may help us understand special topological conformations and dynamical behaviors of SRPs in semiflexible ring-linear blends.In Chapter 5,we explore the conformations of semiflexible ring polymers(SRPs)confined in two parallel surfaces.The conformations of confined SRPs rely on the distance between two parallel surfaces D and the chain stiffness as well as the interaction between SRPs and the surface.For strong confinements with the repulsive surface,SRPs are almost parallel to the surface,however,for weak confinements with the attractive surface,some SRPs are perpendicular to the surface owing to the competition between the attractive interactions and the topological constrains of SRPs.For the attractive surface,adsorption ability of SRPs decreases with the increase of chain stiffness Kb because disk-like conformations hinder more monomers to be adsorbed on the attractive surfaces,which is different completely from semiflexible linear polymers.Meanwhile,the shape of SRPs near the repulsive surface is oblate,while most of SRPs are prolate near the attractive surface.Moreover,confinements hinder large stack structures to be formed for SRPs,especially for the attractive surface.Our results can help us understand topological constraints for confined SRPs.In Chapter 6,coarse-grained molecular dynamics simulations are used to investigate the conformations of binary semiflexible ring polymers(SRPs)of two different lengths confined in a hard sphere.Segregated structures of SRPs in binary mixtures are strongly dependent upon the number density of system(p),the bending energy of long SRPs(Kbjong),and the chain length ratio of long to short SRPs(?).With a low p or a weak Kb,long at a small ratio ?,long SRPs are immersed randomly in the matrix of short SRPs.As p and bending energy of long SRPs(Kb,long)are increased up to a certain value for a large ratio ?,a nearly complete segregation between long and short SRPs is observed,which can be further characterized by the ratio of tangential and radial components of long SRPs velocity.These explicit segregated structures of the two components in spherical confinement are induced by a delicate competition between the entropic excluded volume(depletion)effects and bending contributions.In Chapter 7,coarse-grained molecular dynamics simulations are used to investigate the conformations of semiflexible ring-linear blends in ellipsoidal confinement.Ordered structures of semiflexible ring polymers(SRPs)in ring-linear blends rely strongly on the number density of blends(p),the chain length(Nlinear)and the bending energy(Kb,linear)of semiflexible linear polymers,as well as the curvature of confinement.For a low number density p,SRPs are immersed randomly in the matrix of linear polymers in ellipsoid confinement.However,for a high number density p,a complete separation of SRPs from flexible linear polymers occurs,and a highly ordered aggregation structure of SRPs at the equatorial plane is formed in the mixture of semiflexible linear polymers with large Kb,linear.These explicit ordered aggregations of SRPs in ellipsoidal confinement are induced by a delicate competition between the entropic excluded volume(depletion)effects and bending contributions.This investigation can help us understand the complicated conformations of ring-linear blends in ellipsoidal confinement.