Biomedical subjects

Hongdong Zhang

Publications and source records attributed to Hongdong Zhang.

7 recordsLinked to original sources

PubMed2006

Shape of fluid vesicles anchored by rigid rod.

A method combined the self-consistent field theory (SCFT) for the rigid rod with the Helfrich curvature elasticity theory for the vesicle has been developed for studying the shape of vesicles anchored by rigid rod. Both the deformation of the vesicle and the density distribution of rod segments can be obtained. Because of the vesicle's impenetrability for the rod segments and the decrease of the available space for the rod orientational configurations, the anchored rod segments exert the inhomogeneous entropic pressure on the vesicle and induce the change of vesicle shape. The interaction between the rod segments and the vesicle membrane exerts an extra tension to the membrane. Thus the interaction between the vesicle membrane and the rod segments, the rod length, and the bending rigidity of vesicle are investigated as the important factors to the shape transformation of the vesicle and the density distribution of rod segments. This method can be extended to more complicated and real biological systems, such as polymers with different topological architectures/vesicle, multiple chains/vesicle, protein inclusions, etc.

Algorithms↗

PubMed2006

Aggregates in solution of binary mixtures of amphiphilic diblock copolymers with different chain length.

The polydispersity effect of amphiphilic AB diblock copolymers on the self-assembled morphologies in solution has been investigated by the real-space implementation of self-consistent field theory (SCFT) in two dimensions (2D). The polydispersity is artificially obtained by mixing binary diblock copolymers where the hydrophilic or hydrophobic blocks are composed of two different lengths while the other block length is kept the same. The main advantage is that this simple polydispersity can easily distinguish the difference of aggregates in the density distribution of long and short block length intuitionally and quantitatively. The morphology transition from vesicles to micelles is observed with increasing polydispersity of copolymers due to the length segregation of copolymers. For polydisperse hydrophilic or hydrophobic blocks, the short blocks tend to distribute at the interfaces between hydrophilic and hydrophobic blocks while the long blocks stretch to the outer space. More specifically, by quantitatively taking the sum of all the concentration distribution of long and short chains over the inside and outside surface areas of the vesicle, it is found that long blocks prefer to locate on the outside surface of the vesicle while short ones prefer the inside. Such length segregation leads to large curvature of the aggregate, thus resulting in the decrease of the aggregate size.

Journal Article↗

PubMed2005

Rigid rod anchored to infinite membrane.

We investigate the shape deformation of an infinite membrane anchored by a rigid rod. The density profile of the rod is calculated by the self-consistent-field theory and the shape of the membrane is predicted by the Helfrich membrane elasticity theory [W. Helfrich, Z. Naturforsch. 28c, 693 (1973)]. It is found that the membrane bends away from the rigid rod when the interaction between the rod and the membrane is repulsive or weakly attractive (adsorption). However, the pulled height of the membrane at first increases and then decreases with the increase of the adsorption strength. Compared to a Gaussian chain with the same length, the rigid rod covers much larger area of the membrane, whereas exerts less local entropic pressure on the membrane. An evident gap is found between the membrane and the rigid rod because the membrane's curvature has to be continuous. These behaviors are compared with that of the flexible-polymer-anchored membranes studied by previous Monte Carlo simulations and theoretical analysis. It is straightforward to extend this method to more complicated and real biological systems, such as infinite membrane/multiple chains, protein inclusion, or systems with phase separation.

Adsorption↗

PubMed2005

Phase separation patterns for diblock copolymers on spherical surfaces: a finite volume method.

We explore phase separation on spherical surfaces by solving the Cahn-Hilliard equation modified for diblock copolymers using a finite volume method. The spherical surface is discretized into almost uniform triangles by employing successive dyadic refinements of the spherical icosahedron, a methodology that avoids potential mathematical and numerical problems related to the poles in spherical coordinates. The finite volume method is based on averaging Voronoi cells built from triangular meshes to calculate the Laplace-Beltrami operator on the curved surface, which greatly improves both the accuracy and speed of calculation as compared to the conventional finite difference method. By using this method we simulate the phase separation of diblock copolymers on a spherical surface. It is found that stable and intrinsic defects, which would not occur in a flat space after sufficient annealing, appear in the periodic arrangement of the domains on the curved surface due to the distinct Euler characteristic of the surface.

Journal Article↗

PubMed2005

Predicting shapes of polymer-chain-anchored fluid vesicles.

We propose a general method for predicting shapes of fluid vesicles with anchored polymers. The method combines the Helfrich curvature elasticity theory for fluid membranes and the self-consistent field theory for polymers, to determine stable and metastable shapes of the vesicles as well as the segment distributions of the anchored chains. We illustrate the method by investigating the shape change of a fluid vesicle induced by a single anchoring polymer chain. Extension to more complicated systems, such as vesicles with multiple anchored chains, block copolymers, or semiflexible chains, is straightforward.

Biopolymers↗

PubMed2005

Threading-unthreading equilibrium in solution of molecular nanotubes and linear flexible polymer chains.

Using the Flory-Huggins lattice model, we investigate the threading-unthreading equilibrium in a solution of linear flexible polymer chains and molecular nanotubes formed by covalently bonding the ringlike molecules, such as cyclodextrins (CDs). It is found that the threading-unthreading equilibrium depends on the temperature and the molar concentrations of the ringlike molecules and polymer chain segments but is independent of the polymer chain length, which agrees with the experimental observations. By fitting the experimental data of alpha-CD and poly(ethylene glycol) (PEG), the inclusion energy between alpha-CD and PEG, which includes the conformation energy loss of PEG resulted from the inclusion, is calculated to be approximately -20.45 kJ/mol per PEG unit.

Journal Article↗

PubMed2004

Morphology and phase diagram of complex block copolymers: ABC linear triblock copolymers.

Using a real space implementation of the self-consistent field theory for the polymeric system, we explore microphases of ABC linear triblock copolymers. For the sake of numerical tractability, the calculation is carried out in a two-dimensional (2D) space. Seven microphases are found to be stable for the ABC triblock copolymer in 2D, which include lamellae, hexagonal lattice, core-shell hexagonal lattice, tetragonal lattice, lamellae with beads inside, lamellae with beads at the interface, and hexagonal phase with beads at the interface. By systematically varying the composition, triangle phase diagrams are constructed for four classes of typical triblock polymers in terms of the relative strengths of the interaction energies between different species. In general, when both volume fractions and interaction energies of the three species are comparable, lamellar phases are found to be the most stable. While one of the volume fractions is large, core-shell hexagonal or tetragonal phases can be formed, depending on which of the blocks dominates. Furthermore, more complex morphologies, such as lamellae with beads inside, lamellae with beads at the interface, and hexagonal phases with beads at the interface compete for stability with lamellae structures, as the interaction energies between distinct blocks become asymmetric. Our study provides guidance for the design of microstructures in complex block copolymers.

Journal Article↗