Qinghua Jin

Self-assembly of diblock copolymers confined in cylindrical nanopores

Self-assembly of AB diblock copolymers confined in cylindrical nanopores is studied using a simulated annealing technique. The pore diameter and surface preference are systematically varied to examine their effects on the self-assembled morphologies and the chain conformations. For bulk lamella-forming and cylinder-forming diblock copolymers, novel structures such as helices and concentric (perforated) lamellae spontaneously form when the copolymers are confined in cylindrical pores. The observed equilibrium morphologies are compared with that obtained from experiments, theory, and other simulations. A simple model is proposed for symmetric diblock copolymers, which gives a reasonable description of the layer thickness for the concentric lamellae. It is found that chains near the pore surfaces are compressed relative to the bulk chains, which can be attributed to the existence of the surfaces. The dependence of the chain conformation on the degree of confinement and strength of the surface preference are reasonably explained. The energetics is discussed qualitatively and used to account for the appearance of the complex phase behavior observed for certain intermediate conditions.

Hierarchically helical mesostructured silica nanofibers templated by achiral cationic surfactant

Recently, ordered chiral mesoporous silica with a twisted hexagonal rod-like morphology and hexagonally ordered chiral channels has been synthesized by using chiral anionic surfactants as a liquid crystal template (S. Che, Z. Liu, T. Ohsuna, K. Sakamoto, O. Terasaki and T. Tatsumi, Nature, 2004, 429, 281). In this work, we report an observation of hierarchically helical mesoporous silica nanofibers organized by the achiral cationic surfactant cetyltrimethylammonium bromide (CTAB). These nanofibers (diameter ranging around 100–300 nm) grew from a two-phase system (H2O, CTAB, HCl for the aqueous phase and tetraethylsiloxane (TEOS) in hexane for the oil phase). SEM and TEM characterizations were performed and the results indicate that these nanofibers possess rope-like twisted hexagonal morphology and helical (chiral) mesoporous channels running inside winding around the fiber axis. These twisted hexagonal nanofibers could further curve spirally to form a second-level helical morphology (hierarchically helical morphology). As no chiral molecules are used in the synthesis, the hierarchically helical morphology of nanofibers could be explained by the different kinds of topological defects existing in the silicate liquid crystal seeds formed in a diffusion-controlled kinetic process, and these defects could initiate and direct the growth of particular forms of mesostructured silica. Formation of the ordered chiral mesorporous silica would be expected to be a general phenomenon in the cooperative assembly between amphiphilic organic molecules (templates) and inorganic species, no matter whether the templates are chiral or achiral.

Confined self-assembly of cylinder-forming diblock copolymers: effects of confining geometries

The effects of confining geometries on the self-assembly of cylinder-forming asymmetric diblock copolymers are studied using a simulated annealing technique. Morphological transitions of block copolymers confined inside two parallel flat walls, cylindrical channels, as well as spherical and ellipsoidal cavities are systematically investigated. Depending on the copolymer composition, confining geometry and degree of structural frustration, a very rich array of confinement-induced morphologies is predicted by the simulations. The results reveal that the dimensionality of the confinement can affect the structure, symmetry and degeneracy of the self-assembled structures. In particular, the effect of spherical confinement is much stronger than that of thin film or cylindrical confinement.

Simulated annealing study of morphological transitions of diblock copolymers in solution

The simulated annealing method was applied to study the self-assembling process of diblock copolymers in selective solvents for one block. The simulation results illustrated that the morphologies of the copolymer aggregates strongly depend on the interactions between the core-forming blocks and the solvents and on the length of the corona-forming blocks. Multiple morphological transitions were observed in one system. The transition sequence (disordered state-spherical micelles-short rodlike micelles-long rodlike micelles-onionlike aggregates) was observed for copolymers with increasing core-solvent interaction. Similar transitions were observed with the decrease of the length of the corona-forming blocks. The mechanisms of these transitions are investigated. The simulation results are compared with experiments and other simulations.

Cylinder-gyroid-lamella transitions in diblock copolymer solutions: A simulated annealing study

The morphological transition of an asymmetric diblock copolymer [A3-b-B9] in A-selective solvents is investigated using a simulated annealing technique. The study was carried out at high copolymer concentrations. Phase-transitions among hexagonally packed cylinders (C), gyroid (G), and lamellae (L) are observed. The phase transition sequence, C-->G-->L, was obtained with decreasing copolymer concentration and/or increasing B-solvent interaction. The predicted phase-transition sequence is consistent with experiments of diblock copolymers with similar volume fractions in selective solvents of different selectivity. The morphological transitions were further analyzed in terms of the average contact numbers for A or B monomers with other molecules and the total surface area of the core or matrix in each structure. It is found that these quantities correlate with the structures, providing an understanding of the phase-transition mechanisms.

Simulated annealing study of asymmetric diblock copolymer thin films

We report a simulated annealing study of the morphology of asymmetric diblock copolymer thin films confined between two homogeneous and identical surfaces. We have focused on copolymers that form a gyroidal morphology in the bulk. The morphological dependence of the confined films on the film thickness and the surface-polymer interaction has been systematically investigated. From the simulations it is found that much richer morphologies can form for the gyroid-forming asymmetric diblock copolymer thin films, in contrast to the lamella-forming symmetric and cylinder-forming asymmetric diblock copolymer films. Multiple morphological transitions induced by changing the film thickness and polymer-surface interactions are observed.

Phase Behavior of Binary Blends of Diblock Copolymers

The phase behavior of binary blends of a long symmetric AB diblock copolymer and a short asymmetric AB diblock copolymer is studied using the self-consistent mean-field theory. The investigation focuses on blends with different short diblocks by constructing phase diagrams over the whole blending compositions and a large segregation regime. The influences of the chain length ratio (R) of the long and short diblock copolymers on their miscibility and on the stability of various ordered structures are explored. The theoretical results reveal that the blends have a much more complex phase behavior than each constituent copolymer. With the increase of the volume fraction of the short diblocks in the blends, multiple transitions from a long-period lamellar phase to phases with nonzero interfacial curvatures including cylindrical and spherical phases, and finally to a short-period lamellar phase or disordered phase, are predicted. In particular, consistent with experiments, the theory predicts that the cylindrical phase is stabilized over a wide blending compositions region in the strong segregation region, even though the two constituent diblock copolymers are both lamella-forming. When the ratio R is large enough, macrophase separation occurs over a wide range of blending compositions in a relatively strong segregation regime. Various coexisting phases, including those of lamellar and disorder, lamellar and cylindrical, cylindrical and cylindrical, cylindrical and disorder, spherical and disorder, and cylindrical and spherical, are predicted. In addition, the density profiles of the typical ordered structures are presented in order to understand the self-organization of the different copolymer chains.

Self-assembly of grafted Y-shaped ABC triblock copolymers in solutions

Self-assembled morphologies of grafted Y-shaped ABC triblock copolymers are investigated using a simulated annealing method. The block copolymers are composed of two incompatible arms (A and B) and a short stem (C), with the C-stems grafted onto a flat surface. A rich array of novel morphologies is discovered. The formation of these morphologies is controlled by polymer grafting density, the incompatibility between the A-B-blocks, as well as the quality and selectivity of the solvents. In particular, it is observed that solvent selectivity drives lateral and/or perpendicular microphase separation. A phase diagram for systems with low grafting density is constructed. It is predicted that multiple morphological transitions, such as these from mixed or core-shell micelles to internally segregated micelles, to hamburger-like micelles, to segmented wormlike micelles, to connected micelles, and to split micelles, can be induced by varying either the incompatibility between the two arms or the quality of the solvents. These results are consistent with previous experiments and theories.

A simulated annealing study of Si, Al distribution in the omega framework

Abstract The Si,Al framework distribution of zeolite omega is studied by three different models, in which the model 3 provides numerical results manifesting the fact that site T B , one of the two crystallographically unequivalent tetrahedral sites, is preferentially occupied by Al atoms in parent zeolite omega. The dependence of the partitioning ratio of Al atoms in the two crystallographically unequivalent tetrahedral sites, on the Si/Al is predicted correctly. The agreement between the calculated Si building units, {Si k ( n Al); k=A or B, n =0–4}, and that obtained from 29 Si MAS NMR is excellent.

A simulated annealing study of Si,Al distribution in the faujasite framework

Abstract Several important properties of faujasite are concerned with the Si,Al distribution in its framework. In the present study, a starting configuration of the distribution is generated with randomly positioned Si,Al. After the simulated annealing procedure, some orderings appear in the optimized configurations of the system and lead to a series of consequences, some of which have already been obtained by pioneer studies concerning ordering schemes. In optimized configurations, rations, a certain degree of disorder reasonably is maintained because of the use of standard Monte Carlo finite temperature techniques, which bring about a realistic employ of the numerical simulation. The calculated Al distribution on the first shell of the Si site is assessed by29SiMAS n.m.r. experiments to demonstrate the rationality of the present theory. The calculated Al distribution on the second shell of the Al site is related to the acidity, which leads to the proposition that the acid amount of faujasite is in proportion to the number of single Al four rings. The electrostatic energy calculation provides possible explanations of the observed discontinuity in the plot of unit cell parameter against Al content.