Xuehao He

Self-assembly of the symmetric diblock copolymer in a confined state: Monte Carlo simulation

Self-assembly of symmetric diblock copolymers in confined state has been investigated by means of Monte Carlo simulation method. The symmetric diblock copolymers were confined in two- (parallel walls or circle) or in three-dimensional (spherical or cylindrical) space. There are interactions between these boundaries and the symmetric diblock polymers. These interactions and boundary shape resulted in the formation of novel self-assemble structures, e.g., strip, circle, core-multishell, and multibarrel-layer structures. Simulation results predicated that it is possible to design different phase structures for block copolymers by adjusting boundary shape and boundary-block copolymer interactions.

Effect of surface field on the morphology of a symmetric diblock copolymer under cylindrical confinement

We have used lattice Monte Carlo simulations to investigate the molecular assembly of symmetric diblock copolymer melts within cylindrical nanochannels. We studied the effect that the surface field has on the copolymer morphology in three cylinders having different diameters. Upon varying the strength of the surface field, we observed a variety of morphologies, including stacked-disk, single-helix, catenoid-cylinder, gyroidal, stacked-circle, and concentric cylindrical barrel structures. The results of these simulations should be helpful when designing polymeric nanomaterials confined in cylindrical nanochannels.

Monte Carlo simulation of hyperbranched copolymerizations in the presence of a multifunctional initiator

The Monte Carlo method is applied to simulate the copolymerization of a self-condensing vinyl monomer and a conventional vinyl monomer in the presence of a multifunctional initiator at equal rate constanits. With the increase of the molar ratio of self-condensing vinyl monomer in the monomer composition, the number average degree of polymerization decreases and the weight average degree of polymerization decreases at a lower conversion of double bonds but increases at a higher conversion of double bonds. The molecular weight distribution, PI, changes from 1.01 to 12.0 and the average degrec of branching, DB, increases from 0 to 0.456, We discovered that the addition of conventional vinyl monomer can modify the ratio of multi-arm hyperoranched macromolecules in the product, i.e., the content of macromolecules with core moiety will increase with the increase of the ratio of conventional vinyl monomer. The possibility of controlling PI and DB by changing the conversion and monomer compositions is demonstrated.

Localizations of junction points of ABC 3-miktoarm star terpolymers

Microdomain structures of ABC 3-miktoarm star terpolymers in three-dimensional space were investigated by dynamic density functional theory (DDFT). Analysis of DDFT demonstrates that the junction points are restricted to lie on the periodically spaced parallel lines formed by the mutual intersections of different domains at the strong segregation between distinct blocks. With the improvement of compatibility of one component with the two other components, the localization of junction points is changed to the tendency of distribution over intermaterial dividing surface between the domains.

Self-assembly of star block copolymers by dynamic density functional theory

The dynamic mean-field density functional method, driven from the generalized time-dependent Ginzburg–Landau equation, was applied to the mesoscopic dynamics of the multi-arms star block copolymer melts in two-dimensional lattice model. The implicit Gaussian density functional expression of a multi-arms star block copolymer chain for the intrinsic chemical potentials was constructed for the first time. Extension of this calculation strategy to more complex systems, such as hyperbranched copolymer or dendrimer, should be straightforward. The original application of this method to 3-arms block copolymer melts in our present works led to some novel ordered microphase patterns, such as hexagonal (HEX) honeycomb lattice, core–shell HEX lattice, knitting pattern, etc. The observed core–shell HEX lattice ordered structure is qualitatively in agreement with the experiment of Thomas et al. [Macromolecules 31, 5272 (1998)].

Dynamics of vesicle formation from lipid droplets: Mechanism and controllability

A coarse-grained model developed by Marrink et al. [J. Phys. Chem. B 111, 7812 (2007)] is applied to investigate vesiculation of lipid [dipalmitoylphosphatidylcholine (DPPC)] droplets in water. Three kinds of morphologies of micelles are found with increasing lipid droplet size. When the initial lipid droplet is smaller, the equilibrium structure of the droplet is a spherical micelle. When the initial lipid droplet is larger, the lipid ball starts to transform into a disk micelle or vesicle. The mechanism of vesicle formation from a lipid ball is analyzed from the self-assembly of DPPC on the molecular level, and the morphological transition from disk to vesicle with increasing droplet size is demonstrated. Importantly, we discover that the transition point is not very sharp, and for a fixed-size lipid ball, the disk and vesicle appear with certain probabilities. The splitting phenomenon, i.e., the formation of a disk/vesicle structure from a lipid droplet, is explained by applying a hybrid model of the Helfrich membrane theory. The elastic module of the DPPC bilayer and the smallest size of a lipid droplet for certain formation of a vesicle are successfully predicted.

Kinetics of multicompartment micelle formation by self-assembly of ABC miktoarm star terpolymer in dilute solution

Multicompartment micelles, as novel nanoscopic structures, have great potentialities in the fields of multifunctional nanoreactors and carriers. In this work, multicompartment micelle formation in an initially homogeneous dilute solution of ABC miktoarm star terpolymer is investigated with polymeric external potential dynamics. Apart from the hamburger micelle, toroidal micelle, raspberry micelle, worm micelle and laterally structured vesicle, which have been reported before, three novel morphologies are observed, including laterally structured vesicle with a core, spotted vesicle with a core and segmented cage-like micelle, by varying the concentration of copolymers, the volume fractions of three blocks, the solvophobicity of the solvophobic A and C blocks, and the solvophilicity of the solvophilic B block. The structural stability of the prolate vesicle with alternating ring-shape AC strips is demonstrated using Helfrichs membrane model combining with the strong segregation theory of block copolymers. In the dynamics of vesicle formation, two formation pathways of multicompartment vesicles are found: when the volume fraction of B block is smaller, the formation pathway of vesicles includes nucleation, coalescence and growth; when the volume fraction of B block is larger, the formation process of vesicles only includes nucleation and growth. The formation mechanisms of toroidal and cage-like micelles are also studied in this work. Our simulation results enrich the knowledge of the morphologies of multicompartment micelles and reveal the formation mechanism of complex multicompartment micelles of miktoarm star terpolymer in solution.

Monte Carlo simulation for the modification of polymer via grafting

Abstract Monte Carlo method has been applied to investigate the kinetic of grafting reaction in free radical copolymerization. The simulation is quite in agreement with that of theoretical and experimental results. It proves that the Monte Carlo simulation is an effective method for investigating the grafting reaction of free radical copolymerization.

Diffusion-limited hyperbranched polymers with substitution effect.

Highly branched structure has the essential influence on macromolecular property and functionality in physics and chemistry. In this work, we proposed a diffusion-limited reaction model with the consideration of macromolecular unit relaxations and substitution effect of monomers to study the structure of hyperbranched polymers prepared by slow monomer addition to a core molecule. The exponential relationship (R(g) ∼ N(λ)) between the radius of gyration R(g) and the degree of polymerization N, was systematically analyzed at various branching degrees. It is shown that the effective exponent λ(eff) decreases at lower N and but increases toward that of diffusion-limited aggregation (DLA) clusters (λ(DLA) = 0.4) with the degree of polymerization increasing. The substitution effect of monomers in reaction strongly influences the evolution pathway of λ(eff). With the static light scattering technique, the fractal property of internal chains was further calculated. A general law about the radial distribution of the units of diffusion-limited hyperbranched polymers was found that, at smaller reactivity ratio k(12), the radial density of all monomer units D(A) declines from the center region to the peripheral layer revealing the dense core structure; however, at larger k(12), the density distribution shows a loose-dense-loose structure. These structural characteristics are helpful to deeply understand the property of hyperbranched polymers.

Phase transition of a single star polymer: A Wang-Landau sampling study

Star polymers, as an important class of nonlinear macromolecules, process special thermodynamic properties for the existence of a common connecting point. The thermodynamic transitions of a single star polymer are systematically studied with the bond fluctuation model using Wang-Landau sampling techniques. A new analysis method employing the shape factor is proposed to locate the coil-globule (CG) and liquid-crystal (LC) transitions, which shows a higher efficiency and accuracy than the canonical specific heat function. The LC transition temperature is found to obey the identical scaling law as the linear polymers. However, the CG transition temperature shifts towards the LC transition with the increasing of the arm number. The reason is that for the star polymer a lower temperature is needed for the attractive force to overcome the excluded volume effect of the polymer chain because of its high arm density. This work clearly proves the structural distinction of the linear and star polymers can only affect the CG transition while has no influence on the LC transition.