Morphology of Symmetric ABC Triblock Copolymers in the Strong Segregation Limit

Thermodynamic Picture of Phase Segregation during the Formation of Bicontinuous Concentric Lamellar (bcl) Silica

The thermodynamic picture describing the formation mechanism of bicontinuous concentric lamellar (bcl) nanostructured silica particles, bcl silica, was investigated thoroughly. A series of classical kinetics of bcl silica by varying the synthesis time were employed to observe the morphological evolution of bcl silica. The formation mechanism of bcl silica is proposed as the hydrolysis and condensation reactions in the reverse micelle, followed by the phase segregation process. The images of the whole part and the cross-section of bcl silica reveal that bcl silica can be obtained just 30 min after the synthesis starts. The particle morphology evolves from bicontinuous lamellar (bl) morphology, with the absence of the dense part in the center of the particle, to bicontinuous concentric lamellar (bcl) morphology. The theoretical part of this study is focused on the phase segregation process of the mixture. This process is divided thermodynamically into several reversible processes based on the reduced Helmholtz free energy state function. The type of the lamellar orientation (i.e., parallel or perpendicular orientation) changed as the stacked lamellae changed in thickness and was followed by the decrease in the free energy. It was merely shown that the segregation of the thin slab of the lamellar polysiloxane stack favors the perpendicular orientation. In contrast, the thick slab of the lamellar polysiloxane stack yields a complex lamellar structure consisting of perpendicular and parallel orientations. A lamellar polymer confined between two planar substrates can experience a topological transformation into a sphere due to an unfavorable environment, i.e., high surface tension. After the topological transformation, lamellae with a perpendicular orientation form bicontinuous lamellae, whereas the complex lamellar structure transforms into a bicontinuous concentric lamellar morphology.

Self-Assembly of Single-Diamond-Surface Networks

Biological scaffolds with hyperbolic surfaces, especially single gyroid and single-diamond structures, have sparked immense interest for creating novel materials due to their extraordinary physical properties. However, the ability of nature to create these unbalanced surfaces has not been achieved in either lyotropic liquid crystals or block copolymer phases due to their thermodynamical instability in these systems. Here, we report the synthesis of a porous silica scaffold with a single-diamond-surface structure fabricated by self-assembly of the poly(ethylene oxide)-b-polystyrene-b-poly(L-lactide) and silica precursors in a mixed solvent of tetrahydrofuran and water. The single-diamond structure with tetrahedral interconnected frameworks was revealed by the electron crystallographic reconstruction. We assume that the formation of single networks is induced by the structural transition and related to the energetic change due to the fluctuations of the Gaussian curvature. This work may provide new insights into these biologically relevant surfaces and related self-assembly systems.

Discovery of single gyroid structure in self-assembly of block copolymer with inorganic precursors

Triply periodic hyperbolic surfaces have attracted great attention due to their unique geometries and physical properties. Among them, the single gyroid (SG) is of significant interest due to its inherent chirality as well as the potential applications in energy and environmental science. However, the formation of the thermodynamically unstable structure is still unclear. In this work, we show the formation of SG structure in the structural transformation from the cylindrical to shifted double diamond (SDD) scaffold in a self-assembly system of diblock copolymer and silica precursors in solution. It has been found that the cylindrical tubes with zero Gaussian curvature were split and curved into hyperbolic surfaces and extruded to form SG structures and further evolved into the SDD networks. This growth or extrusion process suggests the SG structure is an intermediate phase of the cylindrical and SDD, and this transformation is found similar to the formation of butterfly wing scales (Thecla opisena), which has not been observed in neither the theoretical calculation nor the experimental self-assembly of amphiphilic molecules. We hope the structural relationship may bring new insights in understanding the formation of single networks in the biological system and the creation of new functional materials.

Phase behavior of ABC-type triple-hydrophilic block copolymers in aqueous solutions

The phase behavior of symmetric ABC triple-hydrophilic triblock copolymers in concentrated aqueous solutions is investigated using a simulated annealing technique. Two typical cases, in which the hydrophilicity of the middle B-block is either stronger or weaker than that of the end A- and C-blocks, are studied. In these two cases, a variety of phase diagrams are constructed as a function of the volume fraction of the B-block and the copolymer concentration ([Formula: see text] for both non-frustrated and frustrated copolymers. Structures, such as two-color alternatingly packed cylinders or gyroid, and lamellae-in-lamellae etc. that do not occur in the melt system, are obtained in solutions. Rich phase transition sequences, especially re-entrant phase transitions involving complex continuous networks of alternating gyroid and alternating diamond are observed for a given copolymer with decreasing [Formula: see text] . The difference in hydrophilicity among different blocks can result in inhomogeneous distribution of solvent molecules in the morphology, and with the decrease of [Formula: see text] , the distribution of solvent molecules presents a non-monotonic variation. This results in a non-monotonic variation of the effective volume fraction of each domain with the decrease of [Formula: see text] , which induces the re-entrant phase transitions. The presence of a good solvent for all the blocks can cause changes in the effective segregation strengths between different blocks and also in chain conformations, hence can alter the bulk phases and results in the occurrence of new structures and phase transitions. Especially, structures having A-C interfaces or A-C mixed domains can be obtained even in the non-frustrated copolymer systems, and structures obtained in the frustrated systems may be similar to those obtained in the non-frustrated systems. The window of the alternating gyroid structures may occupy a large part of the phase diagram for non-frustrated copolymers with stronger B-hydrophilicity. This behavior can be used to tune the self-assembled structures of block copolymers.

Complex crystal structures formed by the self-assembly of ditethered nanospheres

We report the results from a computational study of the self-assembly of amphiphilic ditethered nanospheres using molecular simulation. As a function of the interaction strength and directionality of the tether-tether interactions, we predict the formation of four highly ordered phases not previously reported for nanoparticle systems. We find a double diamond structure comprised of a zinc blende (binary diamond) arrangement of spherical micelles with a complementary diamond network of nanoparticles (ZnS/D), a phase of alternating spherical micelles in a NaCl structure with a complementary simple cubic network of nanoparticles to form an overall crystal structure identical to that of AlCu2Mn (NaCl/SC), an alternating tetragonal ordered cylinder phase with a tetragonal mesh of nanoparticles described by the [8,8,4] Archimedean tiling (TC/T), and an alternating diamond phase in which both diamond networks are formed by the tethers (AD) within a nanoparticle matrix. We compare these structures with those observed in linear and star triblock copolymer systems.

Computer simulation of structure and microphase separation in model A-B-A triblock copolymers

A set of computer simulations for three symmetric A-B-A triblock copolymer microarchitectures at varying temperatures is reported. By using the cooperative motion algorithm we obtain energy, specific heat, end-to-end distance, and bridging fraction as a function of the reduced temperature. The order-disorder transition temperatures are determined, an outline of a symmetric A-B-A triblock copolymer phase diagram is presented, and the visualization of different microstructures is given. A bicontinuous microstructure is reported at 67% fraction of A component.

Supramolecular organometallic polymer chemistry: multiple morphologies and superstructures from the solution self-assembly of polyferrocene-block-polysiloxane-block-polyferrocene triblock copolymers

The solution self-assembly of an organometallic-inorganic triblock copolymer, poly(ferrocenyldimethylsilane)-block-poly(dimethylsiloxane)-block-poly-(ferrocenyldimethylsilane) (PFDMS-b-PDMS-b-PFDMS, 3b; block ratio 1:13:1; Mn = 2.88 x 10(4) gmol(-1), polydispersity (PDI) 1.43 (gel permeation chromatography, GPC)) was studied in n-hexane, a PDMS block selective solvent. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and TEM with negative staining analysis of these micellar solutions after solvent evaporation revealed the presence of multiple micellar morphologies including spheres, cylinders, and novel flower-like supramolecular aggregates. TEM analysis of samples fractionated by ultracentrifugation and preparative size-exclusion chromatography suggest that the formation of multiple morphologies is a consequence of compositional variations. When micellar solutions were prepared at 50 degrees C (above the glass transition of the PFDMS core-forming block) flower-like micellar aggregates similar to those present in micellar solutions prepared at room temperature also formed. However, after solvent evaporation, TEM analysis of micellar solutions prepared in decane at about 150 degrees C, above the melt temperature of the PFDMS core (ca. 120-145 degrees C), revealed the presence of spherical micelles (when decane solutions at 150 degrees C were rapidly cooled to room temperature) and rod-like cylindrical micelles (when decane solutions at 150 degrees C were slowly cooled to room temperature). In contrast, poly(ferrocenylmethylethylsilane)block-poly(dimethylsiloxane)-block-poly(ferrocenylmethylethylsilane) (PFMES-b-PDMS-b-PFMES, 4; block ratio 1:16:1; Mn=2.90x10(4)g mol(-1), PDI= 1.42 (GPC)) and poly(ferrocenylmethylphenylsilane)-block-poly(dimethylsiloxane)-block-poly(ferrocenylmethylphenylsilane) (PFMPS-b-PDMS-b-PFMPS, 5; block ratio 1:15:1; Mn=3.00 x 10(4) gmol(-1), PDI = 1.38 (GPC)), which possess completely amorphous organometallic core-forming blocks, formed only spherical micelles in hexane at room temperature. These observations indicate that crystallinity of the insoluble polyferrocenylsilane block is a critical factor in the formation of the nonspherical micelle morphologies.