Yu-qiang Ma

Ordering stripe structures of nanoscale rods in diblock copolymer scaffolds

We report a simulation on the formation of ordered stripe structures of nanoscale rods driven by symmetric diblock copolymer melts. Due to the preferential adsorption of one species of the diblock onto the mobile rods, the phase ordering process will couple with the movement of rods. We find that the self-assembly of rods on the copolymer scaffold produces the highly ordered nanowires of rods, and copolymer blends in turn form the well-oriented lamellar structure. This is due to the interplay among the micro-phase separating dynamics in the diblock copolymer, the wetting interaction between rods and diblock copolymer, and the nematic ordering dynamics of rods. We examine the influence of the domain size, the wetting strength, and the rod number density on the formation of such a nanoscale structure. Additionally, we indicate that the orientation of the pattern can be well controlled by external fields acting on the rods. The results suggest that our model system may provide a novel and simple way to contr...

Controlling structural organization of binary phase-separating fluids through mobile particles

The structural organization in binary immiscible fluids is studied in the presence of mobile particles. We show that the particles, which are included in the favorable phase due to a selective affinity and assemble therein, can drive the favorable phase to self-organize into cluster, interconnected, and network structures, depending on the particle concentration. The detailed studies enable us to confirm the physical origin of the morphology transitions: At low particle concentration, the assembly of particles would amplify the spatial heterogeneity by depleting the favorable phase in the particle-free region, where the hydrodynamic flows further accelerate the dropletlike formation of favorable phase; with increasing the particle concentration, the assembly of particles becomes energetically unfavorable due to the considerably excluded volume effect of particles and they tend to stabilize the percolated structure of the favorable phase. Furthermore, the respective crossover behaviors of the growth dynamics are also observed, corresponding to different structure formations.

Confined crystallization of cylindrical diblock copolymers studied by dynamic Monte Carlo simulations

We report dynamic Monte Carlo simulations of polymer crystallization confined in the cylindrical microdomains of diblock copolymers. The microdomains were prepared via spontaneous microphase separation from homogeneous melt, and the major component was then frozen in a vitreous amorphous state to make a hard confinement to the crystallization of the minor component. We found that during the isothermal crystallization at high temperatures, crystal orientations are dominantly perpendicular to the cylinder axis at the early stage of crystal nucleation and remain to the final state; while if the block junctions are broken before crystallization, crystal orientations are dominantly parallel at the early stage of crystal nucleation, and eventually other orientations take the place of parallel preferences. Analysis of bond orientations in the heterogeneous melts demonstrates the microscopic origin of oriented crystal nucleation.

Ordering mechanism of asymmetric diblock copolymers confined between polymer-grafted surfaces.

Using self-consistent-field calculation, we study the equilibrium morphology of asymmetric block copolymer melts between two brush-covered surfaces. We discuss systematically the morphologies formed as a function of the grafting density of brushes and show a series of reentrant structural transformations between the lamellar and hexagonal phases. Such a selection of ordered microdomains has been attributed to the interplay among entropy effects of stretched brushes, the preferential interaction of the blocks with the grafted substrate, and the bulk microphase-separated behavior of asymmetric copolymers. Interestingly for the affinitive brush-copolymer interfaces, the bulk hexagonal phase may still be remained due to entropy elasticity of brushes, and the frustration can be relieved by deforming the shape of soft polymer-brush surface, in contrast to the hard-wall cases. The result demonstrates a simple way to realize molecular self-assembly for confined copolymer films with well-controlled thickness and interfacial environment.

Effective attraction interactions between like-charge macroions bound to binary fluid lipid membranes

Using integral equation theory of liquids to a binary mixed fluid lipid membrane, the authors study the membrane-mediated interactions between binding macroions and the redistribution of neutral and charged lipids due to the macroions. The authors find that when the concentration of binding macroions is infinitely dilute, the main contribution to the attractive potential between macroions is the line tension between neutral and charged lipids of the membrane. As the relative concentration of charged lipids is increased, the authors observe a repulsive-attractive-repulsive potential transition due to the competition between the line tension of mixed lipids and screened electrostatic macroion-macroion interactions. For the finite concentration of macroions, the main feature of the attraction is similar to the infinite-diluted case. However, the corresponding line tension of binary lipids under single macroion is lowered with the formation of multicomplexes by the charged lipids and the macroions, and the maximum of attractive potential will shift toward the higher values of charged lipid concentration.

Structure formation in a phase-separating polymer blend with randomly driven particles

We investigate the phase separation of a binary polymer blend–particle system under fluctuating fields. Our results show that the disordered stochastic field would drive the formation of ordered stripe structures, and the domain growth dynamics is intensively affected by both the blend–particle coupling effect and stochastic motion of driven particles. Particularly, for large fluctuating force and enough particle number, the domain morphology shows the better orientational stripe patterns in two-dimensional case, and the cylindrical structure along the fluctuating force direction in three-dimensional case. In addition, the particle mobility is also one of the crucial factors dominating the domain growth at the late stage.

Orientational structures of a phase-separating system under oscillatory particles

We investigate the ordering mechanism of the striped pattern of a binary mixture in the presence of periodic oscillatory particles with a special addictive to one of the two immiscible phases. Due to the presence of oscillatory particles, the phase separation of the system will be significantly perturbed, and composition fluctuations are suppressed anisotropically. By changing the oscillatory frequency ω and amplitude γ, we observe the striped domain structures either parallel or perpendicular to the oscillatory direction, and obtain a γ−ω phase diagram describing the orientational ordering of the phase-separating structures. The results suggest that our model system may provide a novel and simple way to control the ordering morphology of soft materials.

Interaction between a rodlike inclusion and a supported bilayer membrane

The interactions between a rodlike inclusion and a supported copolymer bilayer membrane are investigated by using the self-consistent field theory. For different system parameters, physical observables, such as the interaction free energy, entropy, and translocation energy barrier, are obtained. Particular emphasis is put on the closely energetic and entropic analyses of the interaction. It shows that the interfacial energy provides a qualitative trend and dominates the basic shape of the interaction free energy curve; the combination of chemical potential energy and total entropy contribution is responsible for the translocation energy barrier and the weak attraction in the vicinity of upper monolayer surface. We also specify the nature, height, and shape of the energy barrier to translocation. Particularly, the height is roughly proportional to the rod radius.

Stripe patterns in frustrated spin systems

We investigate the effects of the long-range dipolar interaction on the formation of the stripe patterns in two-dimensional (2D) spin systems with competing short- and long-range interactions by using the continuous time Monte Carlo technique. We find that there exists an optimal temperature region at which the best stripe patterns are prone to be formed, and the stability and the orientational ordering of striped structures mainly depend on the temperature T, while the width of the striped domains is determined by the strength of the long-range dipolar interaction g. Furthermore, a complete T-g phase diagram is obtained. The results provide a simple and universal picture to account for striking and substantial physics revealed in the prevalent striped morphologies of 2D spin systems.

Domain patterns in ternary mixtures with different interfacial properties

Numerical simulation of the phase separation kinetics in ternary mixtures with different interfacial properties is performed by means of the cell dynamics approach. We discuss the effects of the competition between the phase separation and the wetting phenomena on the system morphology and growth dynamics. Morphological changes and domain growth due to different interfacial properties including wetting and interface tension difference between two interfaces are observed in the late stage of spinodal decomposition.