Datong Ding

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.

Facile preparation of Pd/organoclay catalysts with high performance in solvent-free aerobic selective oxidation of benzyl alcohol

A facile method to prepare Pd/organoclay catalysts was reported. At room temperature, in H2PdCl4 solution the quaternary ammonium surfactant modifiers intercalated in the organoclay adsorbed PdCl42− specifically and quickly. After reduction at room temperature, Pd nanoparticles were formed and well-dispersed in the organoclay matrix. The Pd/organoclay catalysts were used in aerobic oxidation of benzyl alcohol to benzaldehyde and exhibited high and stable activity. Particularly, the 0.2 wt% Pd/organoclay gave a remarkably high turnover frequency (up to 6813 h−1) under base- and solvent-free conditions. Our method can be generally applied for the preparation of Ru, Au and Pt/organoclay catalysts.

Anionic surfactant-templated mesoporous silica (AMS) nano-spheres with radially oriented mesopores.

Mesoporous silica nano-spheres with pore size larger than 3 nm were synthesized using an anionic surfactant as the template. These nano-spheres possess centrosymmetric radial mesopores (emanating from the spherical center to the exterior surface) and form stable suspension. The spherical size and mesostructure can be finely tuned by changing the pH value of the synthetic system in the range of 8.8 to 6.4. In addition, when the pH value was decreased to 5.8, instead of spheres, anisotropic morphologies such as elliptical, peanutlike and trifurcate particles were obtained, exhibiting core/shell structure due to the different orientations of the mesopores in the core and the shell of the particles. It is proposed that the evolution of the morphologies and mesostructures of the products templated by anionic surfactants strongly depend on the pH value of the synthetic system.

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.

Reversible Cross-Linking, Microdomain Structure, and Heterogeneous Dynamics in Thermally Reversible Cross-Linked Polyurethane as Revealed by Solid-State NMR

Polyurethane material is widely utilized in industry and daily life due to its versatile chemistry and relatively easy handling. Here, we focused on a novel thermally reversible cross-linked polyurethane with comprehensive remarkable mechanical properties as reported in our recent work (Adv. Mater. 2013, 25, 4912). The microphase-separated structure and heterogeneous segmental dynamics were well revealed by T2 relaxometry experiments, which was also first utilized to in situ monitor the reversible cross-linking associated with Diels-Alder (DA) and retro-Diels-Alder (RDA) reactions. On the basis of T2 relaxometry results, we determined the actual temperature of the (R)DA reaction as well as the corresponding activation energies of the motion of soft segments. Besides, the roles of the temperature and cross-linker contents on the microdomain structure and dynamics are discussed in detail. It is found that the microphase separation is enhanced by the increase of temperature as well as the incorporation of cross-linkers. Also, the polyurethane samples are still thermal-stable even at a high temperature beyond the disassociation of the cross-linkages. Furthermore, Baum-Pines and three-pulse multiple-quantum NMR experiments are utilized to investigate the heterogeneous structures and dynamics of the mobile and rigid segments, respectively. Both the results obtained from the T2 relaxometry and multiple-quantum NMR experiments are in good agreement with the macroscopic mechanical properties of the polyurethane. Finally, it is also well demonstrated that proton T2 relaxometry combined with multiple-quantum NMR is a powerful method to study the heterogeneous structures and dynamics of a multiphase polymer system.

Lewis acid sites on dehydroxylated zeolite HZSM-5 studied by NMR and EPR

Abstract Zeolite HZSM-5 calcined at different temperatures were studied by 29 Si MAS NMR and EPR methods. It is proposed that over a certain temperature range, dealumination did not necessarily occur simultaneously with framework dehydroxylation and thus framework Lewis acid sites may emerge. A two-step mechanism of dehydroxylation may be present, and it dominates the dehydroxylation and dealumination processes at different temperatures.