Entropic Interaction In Semiflexible Polymer Melts And The Condensation-decondensation Transition Induced By Ionic Interaction

With the development of science and technology and the intercross and penetration among disciplines, polymer science has close relations with physics,chemistry, biology, medicine, material and other related disciplines, so polymer science attracts many researchers' attention. In the binary or multibody system related with polymer, there exist the entropic packing constraints associated with mismatch between species of different size and geometry, the enthalpic interaction of van der Waals attraction, Coulomb electrostatic interaction, and other specific polymer-particle attractive interactions. The competition between the interactions mentioned above results in abundant physical images of polymer system. Understanding the factors which affects the depletion interaction between nanoparticles in the polymer is meaningful to the efficiency self-assembly nanoparticles and the formation of hierarchical structure and provide a reference for the development self-healing materials, environment response materials and other novel functional materials.DNA-cationic agent plays an irreplaceable role in gene therapy. Knowing the mechanism of how the ion interaction influence the complex has potential applicationsfor gene repair, gene silence, drug delivery.In recent years, the booming of computer technology provides great help for the investigation of polymer system at the molecular level. On the basis of Molecular Dynamics and Coarse-Grained model introduced in the chapter two, computational simulation methods are employed to investigate polymer nanocomposites and DNA-cationic agent complexation and to obtain phase behaviors of the two complexes.The analysis of the data can figure out the contribution and influence from specific interactions, respectively. After that relationship between properties and conformations is established.In the chapter three, molecular dynamics simulations are used to explore the effective depletion zone for nanoparticles (NP) immersed in semiflexible polymer melts and calculate the entropic depletion interactions between a pair of NPs in semiflexible polymer nanocomposite melts. The average depletion zone volumes rely mainly on polymer chain stiffness and increase with chain stiffness increasing. In the semiflexible polymer nanocomposite melt, the entropic depletion interactions changes from anisotropic attraction to stronger isotropic attraction with chain stiffness increasing. Meanwhile, the attractive interactions between NPs and polymers can also affect strongly the entropic depletion interactions. For NPs in the rodlike polymer melts, a mixture structure of contact/"bridging" aggregations for NPs is formed at a strong attractive NP/polymer interaction. Our calculations can provide an effective framework to predict the morphology of NPs immersed in semiflexible polymer melts.In the chapter four, molecular dynamics simulations and atomic force microscopy(AFM) are combined to study the decondensation process of DNA chains induced by multivalent cations at high salt concentrations in the presence of short cationic chains in solutions. The valence and concentration of anions are taken into consideration as influence factors. The typical simulation conformations of DNA chains with varying salt concentrations for multivalent cations imply that the concentration of salt cations and the valence of multivalent cations have a strong influence on the process of DNA decondensation. The DNA chains are condensed in the absence of salt or at low salt concentrations, and the compacted conformations of DNA chains become loose when a number of cations and anions are added into the solution. It is explicitly demonstrated that cations can overcompensate the bare charge of the DNA chains and weaken the attraction interactions between the DNA chains and short cationic chains at high salt concentrations. The condensation-decondensation transitions of DNA are also experimentally observed in mixing spermidine with lambda-phage DNA at different concentrations of NaCl / MgCl2 solutions.In the chapter five,the adsorption-desorption transition of DNA in DNA-dendrimer solutions is observed when high-valence anions, such as hexavalent anions, are added to the DNA-dendrimer solutions. The effect of the valence and concentration of salt cations is the focus of the research. In the DNA-dendrimer solutions with low-valence anions, dendrimers bind tightly with the V-shaped double-stranded DNA. When high-valence anions, such as pentavalent or hexavalent anions, are added to the DNA-dendrimer solutions, the double-stranded DNA chains can be stretched straightly and the dendrimers are released from the double-stranded DNA chains. In fact, adding high-valence anions to the solutions can change the charge spatial distribution in the DNA-dendrimer solutions, and weaken the electrostatic interactions between the positively charged dendrimers and the oppositely charged DNA chains. Adsorption-desorption transition of DNA is induced by the overcharging of dendrimers. This investigation is capable of helping us understand how to control effectively the release of DNA in gene/drug delivery because an effective gene delivery for dendrimers includes non-covalent DNA-dendrimer binding and the effective release of DNA in gene therapy.In the chapter six,the self-assembly behaviors and phase transitions of binary nanoparticles (NPs) adsorbed on a soft elastic shell are investigated through molecular dynamics simulation. The conformations of adsorbed binary NPs depend on the bending energy Kb of elastic shell and the binding energy Do between the NPs and the elastic shell. The ordered structures of binary NPs are observed at the moderate adhesive strength and bending energy, in which the small NPs are located near the vertices of regular pentagons as well as the large NPs are distributed along the sides of the regular pentagons. The shape of soft elastic shell can be adjusted by adding the adsorbed binary NPs, and this investigation can provide an effective way to regulate and reshape surfaces or membranes with the sizes in the micrometer range or smaller.