Study of self-assembly of symmetric coil-rod-coil ABA-type triblock copolymers by self-consistent field lattice method

The self-assembly of symmetric coil-rod-coil ABA-type triblock copolymer melts is studied by applying self-consistent field lattice techniques in a three-dimensional space. The self-assembled ordered structures differ significantly with the variation of the volume fraction of the rod component, which include lamellar, wave lamellar, gyroid, perforated lamellar, cylindrical, and spherical-like phases. To understand the physical essence of these phases and the regimes of occurrence, we construct the phase diagram, which matches qualitatively with the existing experimental results. Compared with the coil-rod AB diblock copolymer, our results revealed that the interfacial grafting density of the separating rod and coil segments shows important influence on the self-assembly behaviors of symmetric coil-rod-coil ABA triblock copolymer melts. We found that the order-disorder transition point changes from f(rod)=0.5 for AB diblock copolymers to f(rod)=0.6 for ABA triblock copolymers. Our results also show that the spherical-like and cylindrical phases occupy most of the region in the phase diagram, and the lamellar phase is found stable only at the high volume fraction of the rod.     

Microphase separation of weakly charged block polyelectrolyte solutions: Donnan theory for dynamic polymer morphologies

A mean-field dynamic density functional theory for the phase behavior of concentrated weakly charged block polyelectrolyte solutions is developed, using the Donnan membrane equilibrium approach to account for electrostatic interactions. In this limit all long-range electrostatic interactions are canceled and the net charge density in any region on a coarse-grained scale is zero. The phase diagram of a model triblock polyelectrolyte in solution as a function of the charge of the solvophilic block and the solvent concentration is established. Different mesoscopic structures (lamellar, bicontinuous, hexagonal, micellar, and dispersed coexisting phases) are formed depending on the copolymer charge asymmetry. It is found that upon changing the charge of the solvophilic copolymer block the polyelectrolyte solution does not follow the lyotropic sequence of phases of this polymer. Upon increase in the charge of the solvophilic blocks, changes in copolymer morphology take place by means of change in curvature of polymeric domains.     

Morphology and Conformation Analysis of Self‐Assembled Triblock Copolymer Melts

Summary: We used the dissipative particle dynamics method to simulate the self‐assembly of symmetric triblock copolymers of the type ABA. Depending on the volume fraction of the end blocks f_A, several mesophases including lamellar, perforated lamellar, gyroid, hexagonal cylinders and bcc spherical micelles were obtained. The order‐disorder transition (ODT) at f_A = 0.5 was found to be about χN = 19.8. The ODT for the cylindrical mesophase at symmetrical points on the phase diagram had different values, indicating asymmetry in the phase diagram. We were also able to estimate the bridge fraction in the different mesophases. They range from about 0.44 for the lamellar mesophase to about 0.75 for the spherical micelles. Our simulation results are in good agreement with previously reported theoretical calculations and experimental observations.

The hexagonal cylinders generated with the A₆B₄A₆ copolymer.     

Self-assembly of rod-coil-rod ABA-type triblock copolymers

Self-assembled behavior of symmetric ABA rod-coil-rod triblock copolymer melts is studied by applying self-consistent-field lattice techniques in three-dimensional space. The phase diagram is constructed to understand the effects of the chain architecture on the self-assembled behavior. Four stable structures are observed for the ABA rod-coil-rod triblock, i.e., spherelike, lamellar, gyroidlike, and cylindrical structures. Different from AB rod-coil diblock and BAB coil-rod-coil triblock copolymers, the lamellar structure observed in ABA rod-coil-rod triblock copolymer melts is not stable for high volume fraction of the rod component (f(rod)=0.8), which is attributed to the intramolecular interactions between the two rod blocks of the polymer chain. When 0.3相似文献   

Effect of molecular architecture on phase behavior of graft copolymers

Influence of molecular architecture on phase behavior of graft copolymer melts was studied by using a reciprocal-space self-consistent filed theory (SCFT). The phase diagrams were examined as functions of the architectural parameters describing the graft copolymers (i.e., the number of grafts and the position of first junction). In comparison with the well-known phase diagram of diblock copolymers, the phase diagrams of the graft copolymers are asymmetric. When the number of grafts or the position of first junction varies, the boundaries of order-order transitions have shifts due to the variation in the chain stretching energy. The change in molecular architecture also significantly alters the domain spacing of ordered structures but has weak impact on the density distributions of graft copolymers. For comparison of the theoretical predictions with the existing experimental results, the phase diagrams of graft copolymers were also calculated at strong segregation. The SCFT calculations can accurately capture the characteristics of the phase behavior of graft copolymer melts.     

Structure formation in films of weakly charged block polyelectrolyte solutions

A mean-field dynamic density functional theory is used to describe a phase diagram of concentrated solutions of weakly charged flexible block polyelectrolytes in a film. Electrostatics is taken into account by applying the local electroneutrality constraint (the Donnan membrane equilibrium approach). In the Donnan limit it is assumed that a salt added to the solution perfectly screens long-range electrostatic interactions. The phase diagram of a solution of a triblock polyelectrolyte in a film as a function of the solvent concentration and the charge of the polyelectrolyte (solvophilic) block is calculated for a given film thickness. The phase behavior of the block polyelectrolyte film arises from the interplay between surface-induced alignment and the electrostatically-driven structure formation. The observed mesoscopic structures (lamellar, perforated lamellar, cylindrical, micellar, and mixed phases) are oriented parallel to the surfaces for the considered case of morphologies unfrustrated by the film thickness. Structures with connections between parallel layers (bicontinuous, etc.) are not formed. As a result of surface-induced ordering, the region of ordered phases in a film is wider than in bulk and the phase boundary between ordered and disordered phases is more diffuse. As in the case of unconfined block polyelectrolyte solution, the solution in a film does not follow the lyotropic sequence of phases of such a block copolymer upon increase in the charge of the polyelectrolyte block. Upon changing the charge of the solvophilic copolymer block, transformations of copolymer morphology take place via change in curvature of polymeric domains. Due to confinement of a polyelectrolyte film, no swelling of solvophilic domains is observed.     

A self-consistent field theory study on the morphologies of linear ABCBA and H-shaped (AB)(2)C(BA)(2) block copolymers

By using a combinatorial screening method based on the self-consistent field theory, we investigate the equilibrium morphologies of linear ABCBA and H-shaped (AB)(2)C(BA)(2) block copolymers in two dimensions. The triangle phase diagrams of both block copolymers are constructed by systematically varying the volume fractions of blocks A, B, and C. In this study, the interaction energies between species A, B, and C are set to be equal. Four different equilibrium morphologies are identified, i.e., the lamellar phase (LAM), the hexagonal lattice phase (HEX), the core-shell hexagonal lattice phase (CSH), and the two interpenetrating tetragonal lattice phase (TET2). For the linear ABCBA block copolymer, the reflection symmetry is observed in the phase diagram except for some special grid points, and most of grid points are occupied by LAM morphology. However, for the H-shaped (AB)(2)C(BA)(2) block copolymer, most of the grid points in the triangle phase diagram are occupied by CSH morphology, which is ascribed to the different chain architectures of the two block copolymers. These results may help in the design of block copolymers with different microstructures.     

Comparing the morphology and phase diagram of H-shaped ABC block copolymers and linear ABC block copolymers

By using a combinatorial screening method based on the self-consistent field theory (SCFT) for polymers, we have investigated the morphology of H-shaped ABC block copolymers (A2BC2) and compared them with those of the linear ABC block copolymers. By changing the ratios of the volume fractions of two A arms and two C arms, one can obtain block copolymers with different architectures ranging from linear block copolymer to H-shaped block copolymer. By systematically varying the volume fractions of block A, B, and C, the triangle phase diagrams of the H-shaped ABC block copolymer with equal interactions among the three species are constructed. In this study, we find four different morphologies (lamellar phase (LAM), hexagonal lattice phase (HEX), core-shell hexagonal lattice phase (CSH), and two interpenetrating tetragonal lattice (TET2)). Furthermore, the order-order transitions driven by architectural change are discussed.     

Self-assembly of amphiphilic ABC star triblock copolymers and their blends with AB diblock copolymers in solution: self-consistent field theory simulations

The self-assembled morphologies of amphiphilic ABC star triblock copolymers consisting of hydrophilic A blocks and hydrophobic B and C blocks and the blends with their counterpart linear AB diblock copolymers in solution are investigated by 2D real-space implementation of self-consistent field theory (SCFT) simulation. The star triblock copolymers self-assemble in solution to form various micellar structures from hamburger, to segmented wormlike, to toroidal segmented micelles, and finally to vesicles with simultaneously increasing hydrophobic lengths of blocks B and C. When the length of hydrophobic blocks B and C is asymmetric, specific bead-on-string worm micelles are found. Particularly, when the star ABC triblock copolymer is in a strong segregation regime and both B and C blocks are strongly hydrophobic, quite long segmented wormlike micelles are obtained, which had not been found in previously investigated diblock and linear ABC triblock copolymers solution. Additionally, raspberry micelles with beads dispersed on the core also occur in the strong segregation regime of bulk star ABC triblock copolymers. Furthermore, the aggregate morphology of ABC star triblock copolymers is strongly influenced by the addition of linear AB diblock copolymers. The most significant feature is that the long segmented worms will become shorter, to form hamburger micelles with the addition of AB diblock copolymers. These simulations are in good agreement with the experimental findings by Lodge's group.     

Predicting the Morphology of Metallo‐Supramolecular Block Copolymers with Bulky Counter Ions

Summary: The phase behavior of metallo‐supramolecular block copolymers with bulky counter ions is theoretically studied within the framework of a mean‐field dynamic density functional theory and compared with recent experiments on a polystyrene–poly(ethylene oxide) metallo‐supramolecular diblock copolymer, PS₂₀‐[Ru]‐PEO₇₀, with tetraphenylborate counter ions. The copolymer is modeled as a triblock polyelectrolyte, in which the metal complex is treated as the polyelectrolyte block. The topology and kinetics of the formation of the observed three‐domain lamellar morphology in which the polyelectrolyte blocks and bulky counter ions are located together to form electroneutral complexes, are in good agreement with experimental results. In addition, the model predicts the existence of core–shell morphologies. The agreement with and variations from the experimental phase diagram are discussed in detail.

Morphological transformations in a metallo‐supramolecular block copolymer with bulky counter ions upon increasing the temperature.     

Unexpected consequences of block polydispersity on the self-assembly of ABA triblock copolymers

Controlled/"living" polymerizations and tandem polymerization methodologies offer enticing opportunities to enchain a wide variety of monomers into new, functional block copolymer materials with unusual physical properties. However, the use of these synthetic methods often introduces nontrivial molecular weight polydispersities, a type of chain length heterogeneity, into one or more of the copolymer blocks. While the self-assembly behavior of monodisperse AB diblock and ABA triblock copolymers is both experimentally and theoretically well understood, the effects of broadening the copolymer molecular weight distribution on block copolymer phase behavior are less well-explored. We report the melt-phase self-assembly behavior of SBS triblock copolymers (S = poly(styrene) and B = poly(1,4-butadiene)) comprised of a broad polydispersity B block (M(w)/M(n) = 1.73-2.00) flanked by relatively narrow dispersity S blocks (M(w)/M(n) = 1.09-1.36), in order to identify the effects of chain length heterogeneity on block copolymer self-assembly. Based on synchrotron small-angle X-ray scattering and transmission electron microscopy analyses of seventeen SBS triblock copolymers with poly(1,4-butadiene) volume fractions 0.27 ≤ f(B) ≤ 0.82, we demonstrate that polydisperse SBS triblock copolymers self-assemble into periodic structures with unexpectedly enhanced stabilities that greatly exceed those of equivalent monodisperse copolymers. The unprecedented stabilities of these polydisperse microphase separated melts are discussed in the context of a complete morphology diagram for this system, which demonstrates that narrow dispersity copolymers are not required for periodic nanoscale assembly.     

Morphology re-entry in asymmetric PS-PI-PS' triblock copolymer and PS homopolymer blends

Here, we report the morphology variation in a series of PS-b-PI-b-PS' asymmetric triblock copolymer and PS homopolymer (hPS) blends, where PS' and PS are polystyrene blocks with a molecular weight ratio of approximately 0.11 and PI is poly(isoprene). We find that adding a small amount of hPS results in significant order–order transition (OOT) boundary deflection toward higher PS volume fractions f_PS, which is accompanied by morphology re-entry. For example, the neat triblock copolymer with a PS + PS' volume fraction of f_PS = 0.38 exhibits a lamellar microphase; adding a small amount of hPS reverts the morphology into a hexagonal phase with PS cylinders, while further increasing the hPS fraction leads to normal OOTs from PS cylinders to lamellae, to PI cylinders and finally to spheres. The morphology variation reported here is significantly different from that reported in binary blends of diblock or symmetric triblock copolymer with homopolymer. While the domain features of the LAM structure can be correctly reproduced by self-consistent field theory (SCFT), the observed morphology re-entry is absent in the theoretical SCFT phase diagram. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 169–179     

Mechanism of the transition between lamellar and gyroid phases formed by a diblock copolymer in aqueous solution

The mechanism of the transition from a lamellar phase to a gyroid phase in an aqueous solution of a diblock copolymer has been studied by time-resolved synchrotron small-angle X-ray scattering. The transition occurs via a metastable perforated lamellar structure. The perforations initially have liquidlike ordering before developing hexagonal packing. The transient phase of irregularly perforated layers is revealed by the development of diffuse scattering peaks, just below the Bragg peaks of the lamellar structure. The diffuse scattering is modeled by Monte Carlo simulations of perforated layers. Following the formation of perforations, Bragg peaks characteristic of a hexagonal structure signal an ordering into a hexagonal lattice (with the concomitant loss of diffuse scattering). Computer simulations based on a dynamic density functional model reproduce these features. The hexagonal perforated lamellar phase is rapidly replaced by the gyroid phase. The domain spacing of the gyroid phase is larger than that of the perforated lamellar structure. The perforated lamellar and gyroid phases coexist for a defined period. The reverse transition from gyroid to lamellae occurs directly, with no transient or metastable intermediates.     

Self-assembled ordered polymer nanocomposites directed by attractive particles

We theoretically investigate general conditions under which an inorganic phase can direct the self-assembly of an ordered polymer nanocomposite. For this purpose, we consider a solution of triblock copolymers forming a hexagonal phase of micelles and investigate the effect of adding attractive particles. We show that if the triblock is functionalized at its ends by attaching groups with specific affinity for the particles, thus effectively becoming a pentablock, the particles direct the self-assembly of the system into phases where both the polymers and the particles exhibit mesoscopic order. Different lamellar and gyroid phases (both with Ia3d and I4(1)32 space symmetries) are presented in detail. Our results show that functionalization is a very powerful route for directing self-assembly of polymer nanocomposites. We briefly discuss the connections with recent theoretical and experimental results in diblock melts with nanoparticles as well as for problems where polymers are used to template the growth of an inorganic phase in solution.     

Structural Changes in Block Copolymer Solutions under Shear Flow as Determined by Non‐Equilibrium Molecular Dynamics

Summary: A non‐equilibrium molecular dynamics computer simulation on microsegregated solutions of symmetrical diblock copolymers is reported. As the polymer concentration increases, the system undergoes phase transitions in the following order: body centered cubic (BCC) micelles, hexagonal (HEX) cylinders, gyroid (GYR) bicontinuous networks and lamellae (L), which are the same morphology reported for block copolymer melts. Structural classification is based on the patterns of the anisotropic static structure factor and characteristic 3‐dimensional images. The systems in the BCC micellar (ρσ³ = 0.3) and HEX cylindrical (ρσ³ = 0.4) phases were then subjected to a steady planar shear flow. In weak shear flow, the segregated domains in both systems tend to rearrange into sliding parallel close‐packed layers with their normal in the direction of the shear gradient. At higher shear rates, both systems adopt a perpendicular lamellar structure with the normal along the neutral direction. A further increase in the shear rate results in a decrease in lamellar spacing without any further structural transitions. Two critical shear rate values that correspond to the demarcation of different structural behaviors were found.

Shear‐induced BCC‐LAM phase transition.     

Self-consistent field theory for lipid-based liquid crystals: hydrogen bonding effect

A model to describe the self-assembly properties of aqueous blends of nonionic lipids is developed in the framework of self-consistent field theory (SCFT). Thermally reversible hydrogen bonding between lipid heads and water turns out to be a key factor in describing the lyotropic and thermotropic phase behavior of such systems. Our model includes reversible hydrogen bonding imposed in the context of the grand canonical ensemble and exact conditions of binding equilibrium. The lipid molecules are modeled as a rigid head and a flexible Gaussian tail, and the water molecules are treated explicitly. Here, we focus on systems where the lipid molecule has a relatively small hydrophilic head compared to the hydrophobic tail, such as monoolein in water. Experimentally, this system has both normal phase sequences (inverted hexagonal to inverted double gyroid cubic phase) and reverse phase sequences (lamellar to inverted double gyroid cubic phase) as the water volume fraction increases. From SCFT simulations of the model, two phase diagrams corresponding to temperature independent or dependent interaction parameters chi are constructed, which qualitatively capture the phase behavior of the monoolein-water mixture. The lattice parameters of the simulated mesophases are compared with the experimental values and are found to be in semiquantitative agreement. The role of various structural and solution parameters on the phase diagrams is also discussed.     

Solid-supported block copolymer membranes through interfacial adsorption of charged block copolymer vesicles

The properties of amphiphilic block copolymer membranes can be tailored within a wide range of physical parameters. This makes them promising candidates for the development of new (bio)sensors based on solid-supported biomimetic membranes. Here we investigated the interfacial adsorption of polyelectrolyte vesicles on three different model substrates to find the optimum conditions for formation of planar membranes. The polymer vesicles were made from amphiphilic ABA triblock copolymers with short, positively charged poly(2,2-dimethylaminoethyl methacrylate) (PDMAEMA) end blocks and a hydrophobic poly( n-butyl methacrylate) (PBMA) middle block. We observed reorganization of the amphiphilic copolymer chains from vesicular structures into a 1.5+/-0.04 nm thick layer on the hydrophobic HOPG surface. However, this film starts disrupting and dewetting upon drying. In contrast, adsorption of the vesicles on the negatively charged SiO2 and mica substrates induced vesicle fusion and formation of planar, supported block copolymer films. This process seems to be controlled by the surface charge density of the substrate and concentration of the block copolymers in solution. The thickness of the copolymer membrane on mica was comparable to the thickness of phospholipid bilayers.     

Self-assembly of ternary cubic, hexagonal, and lamellar mesophases using the lattice-Boltzmann kinetic method

We use a kinetic lattice-Boltzmann method to simulate the self-assembly of the cubic primitive (P), diamond (D), and gyroid (G) mesophases from an initial quench composed of oil, water, and amphiphilic particles. Here, we also report the self-assembly of the noncubic hexagonal phase and two lamellar phases, one with periodic convolutions. The periodic mesophase structures are emergent from the underlying conservation laws and quasi-molecular interactions of the lattice-Boltzmann model. We locate regions of the model's parameter space where the sequence of appearance of mesophases lamellar --> primitive --> hexagonal is in agreement with pressure jump experiments and the sequence cubic --> lamellar is in agreement with compositional variations reported in the literature. The ability of our lattice-Boltzmann model to simulate self-assembly of cubic and noncubic phases in a unified and consistent manner opens the way for further investigations into the transition pathways and kinetics of the phase transitions between these states as well as of the rheology of these phases.     

Mesoscopic Simulation on Phase Behavior of Pluronic P104 Aqueous Solution at Moderate Concentrations

The mesoscopic dynamics is used to simulate the microphase separation of a block copolymer P104 in water. With simple models, the hexagonal and lamellar phases are produced at moderate concentrations. For comparison, the phase behavior of charged P104 is also investigated. Simulated results indicate a more ordered hexagonal phase with increasing charge of EO blocks. For lamellar phase, however, its structure becomes ordered first and then turns to disordered when the charge arrives at a certain value. Obtained results might be used to forecast new phase behavior of charged P104.     

Monte Carlo phase diagram for diblock copolymer melts

A partial phase diagram is constructed for diblock copolymer melts using lattice-based Monte Carlo simulations. This is done by locating the order-disorder transition (ODT) with the aid of a recently proposed order parameter and identifying the ordered phase over a wide range of copolymer compositions (0.2相似文献