Lamellar-in-Lamellar Structure of A-b-(B-b-C)m-b-B-b-A Multiblock Copolymers

Self-Assembly of Lamellae-in-Lamellae by Double-Tail Cationic Surfactants

The molecular structures of surfactants play a pivotal role in influencing their self-assembly behaviors. In this work, using simulations and experiments, an unconventional hierarchically layered structure in the didodecyldimethylammonium bromide (DDAB)/water binary system: lamellae-in-lamellae is revealed, a new self-assembly structure in surfactant system. This self-assembly structure refers to a lamellar structure with a shorter periodic length (inner lamellae) embedded in a lamellar phase with a longer periodic length (outer lamellae). The normal vectors of these two lamellar regions orient perpendicularly. In addition, it is observed that this lamellar-in-lamellar phase disappears when the two tails of the cationic surfactants become longer. The formation of the lamellar-in-lamellar architecture arises from multiple interacting factors. The key element is that the short tails of the DDAB surfactants enhance hydrophilicity and rigidity, which facilitates the formation of the inner lamellae. Moreover, the lateral monolayer of the inner lamellae provides shielding from the water and prompts the formation of the outer lamellae. These findings indicate that molecular structures and flexibility can profoundly redirect the hierarchical self-assembly behaviors in amphiphilic systems. More broadly, this work presents a new strategy to deliberately program hierarchical nanomaterials by designing specific surfactant molecules to act as tunable scaffolds, reactors, and carriers.

Formation of Perpendicular Three-Dimensional Network Nanostructures in ABC-Star Copolymers

Perpendicular arrangements in hierarchical nanostructures show superior mechanical properties and provide great opportunities for the development of advanced membranes because different channels are connected by the perpendicular blocks. To obtain these perpendicular hierarchical nanostructures, we use a simple ABC-star terpolymer because of the existence of a conjunction point by using the A block as a polymer network template, which guides the BC phase separation accordingly. When χ_BC is 10, the formed phase and the corresponding phase diagram of ABC-star are similar to those of the AB₂ triblock because of the mixture between the B and C blocks. Interestingly, at increased χ_BC, the B and C blocks phase separate, leading to the formation of a series of perpendicular nanostructures, including perpendicular lamellae-in-lamellae (L_⊥), perpendicular lamellae-in-cylinder (C_⊥), and even perpendicular three-dimensional polymer networks (G_⊥). The corresponding stability regime of each phase is identified through the dedicated comparison of free energy, which can well explain the missing phases in Monte Carlo simulations. Our proposed design route according to the target structures and the calculated phase diagram can provide useful guidance for the experimental observation of these perpendicular nanostructures.

Distinctive Optical Properties of Hierarchically Ordered Nanostructures Self-Assembled from Multiblock Copolymer/Nanoparticle Mixtures

Hybrid materials with hierarchical nanostructures are of great interest for their advanced functions. However, the effect of the formation of hierarchical nanostructures on properties is not well understood. Here, through combining dissipative particle dynamics simulation and the finite-difference time-domain method, the optical properties of hierarchically ordered nanostructures formed by mixtures of A(BC)_n multiblock copolymers and nanoparticles (NPs) are investigated. A series of hierarchically ordered nanostructures with multiple small-length-scale hybrid domains are obtained from the self-assembly of A(BC)_n /NP. An increase and blueshift in optical absorption are observed when the number of small-length-scale hybrid domains increases. The small-length-scale hybrid domains enhance light scattering, which consequently contributes to the improved optical performance. These findings can yield guidelines for designing hierarchically ordered functional nanocomposites with light-harvesting characteristics.

Olefin metathesis in multiblock copolymer synthesis

Multiblock copolymers constitute a basis for an emerging class of nanomaterials that combine various functional properties with durability and enhanced mechanical characteristics. Our mini-review addresses synthetic approaches to the design of multiblock copolymers from unsaturated monomers and polymers using olefin metathesis reactions and other ways of chemical modification across double C=C bonds. The main techniques, actively developed during the last decade and discussed here, are the coupling of end-functionalized blocks, sequential ring-opening metathesis polymerization, and cross metathesis between unsaturated polymers, or macromolecular cross metathesis. The last topic attracts special interest due to its relative simplicity and broad opportunities to tailor the structure and hence the properties of the copolymer products. Whenever possible, we analyze the structure-property relations for multiblock copolymers and point to their possible practical applications.

Monte Carlo simulations of the phase separation of a copolymer blend in a thin film

Monte Carlo simulations were carried out to study the phase separation of a copolymer blend comprising an alternating copolymer and/or block copolymer in a thin film, and a phase diagram was constructed with a series of composed recipes. The effects of composition and segregation strength on phase separation were discussed in detail. The chain conformation of the block copolymer and alternating copolymer were investigated with changes of the segregation strength. Our simulations revealed that the segment distribution along the copolymer chain and the segregation strength between coarse-grained beads are two important parameters controlling phase separation and chain conformation in thin films of a copolymer blend. A well-controlled phase separation in the copolymer blend can be used to fabricate novel nanostructures.

Perpendicular lamellar-in-lamellar and other planar morphologies in A-b-(B-b-A)2-b-C and (B-b-A)2-b-C ternary multiblock copolymer melts

Ordered planar morphologies in A-b-(B-b-A)2-b-C and (B-b-A)2-b-C terpolymer melts are studied within the framework of the self-consistent field theory for volume fractions of components A, B, and C in the ratio 1:1:2 and the Flory-Huggins interaction parameters satisfying χ(AB) = 2χ(AC). The stable phases turn out to be the disordered, hexagonal, parallel lamellar-in-lamellar L∥ (including the simple lamellar) as well as non-shifted and shifted (L⊥ and SL⊥) perpendicular lamellar-in-lamellar morphologies. Depending on the value of the ratio r = Θ(AB)/Θ(BC), where Θ is a characteristic temperature of the units involved, different sequences of phase transitions are shown to occur. The hexagonal phase is characteristic for r ≅ 1. The L⊥ and SL⊥ morphologies occur at weak and intermediate segregations whereas the L∥ morphology appears for stronger degrees of segregation. For (B-b-A)2-b-C a reduction in r favors the shifted SL⊥ phase over the non-shifted L⊥ one, whereas for A-b-(B-b-A)2-b-C we find re-entrant phase transitions SL⊥ - L⊥. The physics determining the particular phase behavior is discussed.

Hierarchical self-assembly of two-length-scale multiblock copolymers

The self-assembly in diblock copolymer-based supramolecules, obtained by hydrogen bonding short side chains to one of the blocks, as well as in two-length-scale linear terpolymers results in hierarchical structure formation. The orientation of the different domains, e.g. layers in the case of a lamellar-in-lamellar structure, is determined by the molecular architecture, graft-like versus linear, and the relative magnitude of the interactions involved. In both cases parallel and perpendicular arrangements have been observed. The comb-shaped supramolecules approach is ideally suited for the preparation of nanoporous structures. A bicontinuous morphology with the supramolecular comb block forming the channels was finally achieved by extending the original approach to suitable triblock copolymer-based supramolecules.

Hierarchically ordered microstructures self-assembled from comb-coil block copolymers

Using the real-space implemented self-consistent field theory, we undertook an investigation on the hierarchical self-assembly behaviors of comb-coil block copolymers. The comb-coil block copolymers can self-assemble into hierarchical microstructures with two different length scales. Various structure-within-structure morphologies, such as parallel and perpendicular lamella-within-lamella, cylinder-within-lamella, lamella-within-cylinder, and cylinder-within-cylinder, were observed. In the hierarchical structures, the large-length-scale structures are produced by segregation between the coil blocks and comb blocks, and the small-length-scale structures are formed by microphase separation within the comb blocks. Effects of interaction strength and coil block length on the hierarchical phase behaviors were studied, and the phase diagram was mapped out accordingly. Furthermore, the large-length-scale lamellar period as a function of interaction strength was examined. It was found the lamellar periods are greatly dependent on interaction strengths.

Nonconventional morphologies in two-length scale block copolymer systems beyond the weak segregation theory

The order-disorder and order-order transitions (ODT and OOT) in the linear multiblock copolymers with two-length scale architecture A(fmN)(B(N2)A(N2))(n)B((1-f)mN) are studied under intermediate cooling below the ODT critical point where a nonconventional sequence of the OOTs was predicted previously [Smirnova et al., J. Chem. Phys. 124, 054907 (2006)] within the weak segregation theory (WST). To describe the ordered morphologies appearing in block copolymers (BCs) under cooling, we use the pseudospectral version of the self-consistent field theory (SCFT) with some modifications providing a good convergence speed and a high precision of the solution due to using the Ng iterations [J. Chem. Phys. 61, 2680 (1974)] and a reasonable choice of the predefined symmetries of the computation cell as well as initial guess for the iterations. The WST predicted sequence of the phase transitions is found to hold if the tails of the BCs under consideration are symmetric enough (mid R:0.5-fmid R:</=0.05); the quantitative agreement between the WST and SCFT phase diagrams is reasonable in a narrow (both in f and chi=chiN) region close to the critical point, though. For mid R:0.5-fmid R:>0.05, a large region of the face-centered cubic phase stability is found (up to our knowledge, first within the SCFT framework) inside of the body-centered cubic phase stability region. Occurrence of the two-dimensional and three-dimensional phases with the micelles formed, unlike the conventional diblock copolymers, by the longer (rather than shorter) tails, and its relationship to the BC architecture is first described in detail. The calculated spectra of the ordered phases show that nonmonotonous temperature dependence of the secondary peak scattering intensities accompanied by their vanishing and reappearance is rather a rule than exception.