Morphology Of Diblock Copolymers Research Articles

Monomer sequence of partially hydrolyzed poly(4-tert-butoxystyrene) and morphology of diblock copolymers composing this polymer sequence as one block

The molecular characteristics of poly(4-tert-butoxystyrene) (O) upon hydrolysis reaction were investigated. It is known that O can be converted into poly(4-hydroxystyrene) (H) through hydrolysis reaction using strong acid. In this study, a set of poly(4-tert-butoxystyrene-co-4-hydroxystyrene)s (O/H copolymers) having various conversion rates, fHs, were prepared. Hydrolysis reaction is found to occur uniformly by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) where the average fH obtained was consistent with that from 1H NMR though a certain distribution in the number of hydrolyzed units was conceived. Monomer sequence of O/H copolymers was determined by 13C NMR and the changes in triad concentration were obtained by spectra subtraction method. As a result, 13C NMR reveals that O and H are statistically distributed. To evaluate the effect of hydrolysis on microphase-separated structure, we observed the morphology of partially hydrolyzed poly(4-tert-butylstyrene-block-4-tert-butoxystyrene) (BO) by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Samples with fH from 0.21 to 0.67 form both lamellar (major component) and cylindrical (minor component) structures reflecting both the statistical manner of hydrolysis reaction and the possible localized distribution of H sequence.

Solvent-Driven Evolution of Block Copolymer Morphology under 3D Confinement

Nanoparticles with concentric layered structures were generated from a lamellae-forming poly(styrene-b-isoprene) diblock copolymer using controlled precipitation from a tetrahydrofuran/water mixture. Chloroform, a good solvent for both blocks, was used to swell and anneal the nanoparticles suspended in aqueous media. The three-dimensional morphologies of particles were reconstructed by transmission electron microtomography throughout the process of solvent annealing. A transition from concentric lamellae to PI cylinders in a PS matrix occurred upon annealing, presumably due to a slight selectivity of chloroform for PS. These cylindrical microdomains were further divided into PS-core-PI-shell spherical structures in a PS matrix upon extended annealing, a structure that is unique among reported microphase separated morphologies of diblock copolymers.

Mesoscale simulation on patterned nanotube model for amphiphilic block copolymer

Self-assembly of AB diblock copolymer confined in concentric-cylindrical nanopores was studied using MesoDyn simulation. Our calculation shows that in this confined geometry a zoo of exotic structures can be formed. These structures include bicontinuous phases like carbon nanotube, imperfect single helixes and double helixes. Moreover, the dependence of the chain conformation on the volume fraction, concentration, the interactions between blocks and the diameter of the cylindrical pore are investigated. The results of these simulations can be used to predict the diblock copolymer morphologies confined in concentric-cylindrical nanopores and should be helpful in designing polymeric nanomaterials in the future.

Controllable stacked disk morphologies of charged diblock copolymers

Monte Carlo simulations are used to demonstrate the controlled stacking of charged block copolymer disk morphologies that can be obtained under certain thermodynamic conditions. We examine a partially charged block copolymer where 75% of the blocks are neutral and 25% of the blocks are charged. The presence of strong electrostatic interactions promotes charge agglomeration thereby changing morphologies in these systems. This study relates different thermodynamic quantities for which disk-like stackings can be obtained. The long-range order can be sustained even if hydrophobicity is increased albeit with lower dimensional structures. Our simulation results agree very well with recent experiments and are consistent with theoretical observations of counterion adsorption on flexible polyelectrolytes.

Diblock copolymers in a cylindrical pore

We show that a simple Ginzburg-Landau type theory can predict a tremendous rich "zoo" of diblock copolymer morphologies in cylindrical nanopores. Using the cell dynamics simulation we study in detail lamellar-, cylinder-, and eventually sphere-forming diblock copolymers melts in cylindrical nanopores. A very fast simulation method is proposed to be used as a research precursor for more elaborate computational techniques.

Sphere-forming diblock copolymers in slit confinement: A dynamic density functional theory study

With mean-field dynamic density functional theory, we study the morphologies of sphere-forming diblock copolymers confined between two homogeneous surfaces. The effects of the film thickness and the surface field strength on the phase behavior of sphere-forming copolymer film are investigated. The morphologies deviating from the bulk sphere-forming structure are revealed, including cylinders oriented perpendicular to the surface, cylinders oriented parallel to the surface, perforated lamellae and lamellae by varying the film thickness, and surface field strength. We also construct the phase diagram of surface reconstruction, in which some interesting phase transitions are presented. Besides, we compare the present phase diagram with the relevant phase diagram of cylinder-forming block copolymer film.

Effect of confinement on the ordering of symmetric diblock copolymers: application of interfacial statistical associating fluid theory

Diblock copolymers consist of two chemically distinct polymer chains (blocks) covalently bonded together. Due to the differences in the physiochemical properties of these blocks, the copolymers can undergo order–disorder transitions (ODT) at the molecular level, resulting in microphase-separated states. Control of these microstructures has significant impact on the technological applications of diblock copolymers. Diblock copolymers exhibit a variety of microstructures, which depend on the relative compositions of the two distinct blocks. In the case of symmetric diblock copolymers, the equilibrium structure is the lamellar structure where each lamellar layer is an A–B or B–A repeat unit. We use interfacial statistical associating fluid theory (iSAFT) density functional theory to analyse the morphologies of symmetric diblock copolymers in bulk melts and ultra-thin films confined between two planar surfaces. For bulk melts, the effect of the length of the copolymer and incompatibility between the blocks of the copolymer on the lamellar structure is discussed. For copolymer thin films, the effect of the surface interactions and the film thickness on the microstructure is investigated.

Surface-induced morphologies of lamella-forming diblock copolymers confined in nanorod arrays

Using the real-space self-consistent field theory, we investigate the effects of geometry confinement and surface field on the morphologies of lamella-forming diblock copolymers confined in the periodic nanorod arrays. The array confinement provides a fractional confinement dimension where the diblock copolymers are incompletely confined at each spatial direction and the novel morphologies may be surface induced. In the arrays with neutral surfaces, we observe several confinement-induced morphologies, such as perpendicular lamellae and double-cylinder network structures, by varying the degree of confinement. For the arrays with preferential surfaces, a rich variety of morphologies including undulated lamellae, perforated lamellae, and cylindrical lamellae are investigated as a function of the strength of surface field. The predicted morphologies and the corresponding ordered transitions can be reasonably comprehended based on the commensurability between the array period and the bulk lamella period, as well as the cylindrical symmetry about the surface field.

Microphase separation of diblock copolymers with amphiphilic segment

We present a statistical mechanical approach for predicting the self-assembled morphologies of amphiphilic diblock copolymers in the melt. We introduce two conformationally asymmetric linear copolymer models with a local structural asymmetry, one of a “comb-tail” type and another that we call “continuous jackknife model.” The copolymers consist of amphiphilic and “monophilic” (non-amphiphilic) blocks, which have different segmental volume and tend to segregate into subphases. Using a self-consistent field theory (SCFT) framework, we explore the phase diagrams for these copolymers and compare them with that known for conventional, conformationally symmetric diblock copolymers. To determine the impact of structural effects on the self-assembly of copolymer melts, copolymers with a variation in both molecular architecture and chemical composition, f, are studied for different values of the Flory–Huggins parameter, χ. The composition dependence of the phase diagrams is shown to be basically determined by the conformational asymmetry. Remarkably, the stable lamellar structures exist even in the very compositionally asymmetric case, f < ¼. An interesting geometric distinction of the “direct” and “inverse” morphologies is introduced. The presence of an internal structure is found to influence the high χ behavior, where a stable two-scale (structure-in-structure) hexagonal morphology is found to be formed for some compositions. Therefore, the local chemical structure of monomer units can dictate the global morphology of copolymer melts.

Ring pattern solutions of a free boundary problem in diblock copolymer morphology

The cross section of a diblock copolymer in the cylindrical phase is made up of a large number of microdomains of small discs with high concentration of the minority monomers. Often several ring like microdomains appear among the discs. We show that a ring like structure may exist as a stable solution of a free boundary problem derived from the Ohta–Kawasaki theory of diblock copolymers. The existence of such a stable, single ring structure explains why rings exist for a long period of time before they eventually disappear or become discs in a diblock copolymer. A variant of Lyapunov–Schmidt reduction process is carried out that rigorously reduces the free boundary problem to a finite-dimensional problem. The finite-dimensional problem is solved numerically. A stability criterion on the parameters determines whether the ring solution is stable.

The complex influence of the oscillatory shear on the melt of linear diblock copolymers

The phase morphologies of symmetric linear diblock copolymers subjected to the oscillatory shear are investigated with the aid of dissipative particle dynamics simulations. The frequency dependent reorientations of the lamellar phase (LAM) have been identified. We find that the parallel orientation of LAM (i.e., the lamellar normal is parallel to the velocity gradient) appears at high shear frequency, whereas the perpendicular orientation of LAM (the lamellar normal being perpendicular to the velocity gradient) takes place at low shear frequency. In both of the cases, the reorientations undergo similar processes: the original LAM phase prepared in equilibrium breaks down rapidly, and it takes a very long time for the perfectly oriented LAM being reformed. Moreover, the shear-induced isotropic to lamellar phase transitions are observed when the oscillatory shear amplitude is large enough. It indicates that the shear amplitude plays a dominant role in the order-disorder transition. The viscosity and the modulus of the melt are found to be dependent on the shear amplitude and the shear frequency in a complex way.

Flow‐Induced Morphologies of Diblock Copolymers in a Nanotube Studied by Dissipative Particle Dynamics Simulation

AbstractMorphologies of diblock copolymers confined in a cylindrical nanotube under a Poiseuille flow were studied by dissipative particle dynamics simulation. Varying with flow intensity, a symmetric diblock copolymer in a neutral tube exhibits parallel lamellae or stacked disks, stacked bowls, or cylinders; the morphologies in a non‐neutral tube are relatively less influenced by flow. An asymmetric diblock copolymer is helical in a neutral tube, but becomes hexagonal under flow; the morphologies in a non‐neutral tube are parallel to the tube axis and vary in a less pronounced manner than in a neutral tube. The simulation results reveal that confinement and external flow significantly affect the phase transitions among a variety of morphologies, particularly for copolymers with domains not parallel to the tube axis.magnified image

Confinement Effect on the Body‐Centered‐Cubic Phase of Diblock Copolymers in Film

AbstractIn this paper, we study the morphology of diblock copolymers with the body‐centered‐cubic (bcc) phase confined between two flat surfaces. Employing the Landau–Brazovskii mean field theory and the single mode approximation, we obtain three amplitude parameters as functions of temperature, surface field strength, and film thickness. Because of the effect of confinement size and the surface inducement, the morphology of confined diblock copolymers is different from the bulk structure. By controlling confinement size and surface field strength, lamella, undulated lamella, cylinder, and distorted cylinder can be observed in the bcc bulk phase of diblock copolymers. Also, we construct a “phase diagram” of confinement‐induced structures at different surface field strengths. We compare the present theoretical results with the other relevant theoretical results. The predictions about these interesting confinement‐induced structures should be observable in the experiments under suitable conditions.magnified image

Spherical Solutions to a Nonlocal Free Boundary Problem from Diblock Copolymer Morphology

The $\Gamma$-limit of the Ohta–Kawasaki density functional theory of diblock copolymers is a nonlocal free boundary problem. For some values of block composition and the nonlocal interaction, an equilibrium pattern of many spheres exists in a three-dimensional domain. A subrange of the parameters is found where the multiple sphere pattern is stable. This stable pattern models the spherical phase in the diblock copolymer morphology. The spheres are approximately round. They satisfy an equation that involves their mean curvature and a quantity that depends nonlocally on the whole pattern. The locations of the spheres are determined via a Green's function of the domain.

Bipolar Copolymers as Host for Electroluminescent Devices: Effects of Molecular Structure on Film Morphology and Device Performance

Bipolar transport polymers have been developed as host materials for electroluminescent devices by incorporating both electron-transporting and hole-transporting functionalities into copolymers. Two different copolymers having the same molecular weight (Mn ∼ 30 kg/mol) and the same fraction of electron-transporting monomers (fOXA = 0.50) have been synthesized in the form of random and diblock copolymers, respectively. The effect of molecular structure and film morphology of these bipolar polymers on device performance has been studied. For the diblock copolymers, pronounced phase segregation forming different nanomorphologies has been observed by modern microscopic techniques, which is not observed for the random counterparts under the same thin film preparation conditions. The results of single-layer polymer light emitting diodes (PLEDs) show that the nanophase separation morphology of diblock copolymers has a significant effect on device performance: lowering charge transport and facilitating the hole−...

MANY DROPLET PATTERN IN THE CYLINDRICAL PHASE OF DIBLOCK COPOLYMER MORPHOLOGY

The Ohta–Kawasaki density functional theory of diblock copolymers gives rise to a nonlocal free boundary problem. In a proper range of the block composition parameter and the nonlocal interaction parameter, an equilibrium pattern of many droplets exists in a general planar domain. A sub-range of the parameters is identified where the multiple droplet pattern is stable. This stable droplet pattern models the cylindrical phase in the diblock copolymer morphology. Each droplet is close to a round disc. The boundaries of the droplets satisfy an equation that involves the curvature of the boundaries and a quantity that depends nonlocally on the whole pattern. The locations of the droplets are determined via a Green's function of the domain. In constructing the droplet pattern we overcome three obstacles: interface oscillation, droplet coarsening, and droplet translation.

Single Droplet Pattern in the Cylindrical Phase of Diblock Copolymer Morphology

The Ohta–Kawasaki density functional theory of diblock copolymers gives rise to a nonlocal free boundary problem. Under a proper condition between the block composition fraction and the nonlocal interaction parameter, a pattern of a single droplet is proved to exist in a general planar domain. A smaller parameter range is identified where the droplet solution is stable. The droplet is a set that is close to a round disc. The boundary of the droplet satisfies an equation that involves the curvature of the boundary and a quantity that depends nonlocally on the whole pattern. The location of the droplet is determined by the regular part of a Green’s function of the domain. This droplet pattern describes one cylinder in space in the cylindrical phase of diblock copolymer morphology.

Micro-phase separation of diblock copolymer in a nanosphere: Dissipative particle dynamics approach

Micro-phase separation and morphologies of diblock copolymer in a nanosphere with different size and interfacial energy were studied by dissipative particle dynamics approach. The surface of a nanosphere is modeled by a layer of frozen soft particles. For symmetric diblock copolymer A 5B 5, if the surface is neutral, i.e., with the same interactions toward different blocks, the morphology consists of bended distorted lamellae connected by channels. They might be mushroom-like, helical or bi-continuous depending on the sphere size. When the surface is non-neutral, i.e., with the different interactions toward different blocks, the meso-structure is onion like. The outmost layer is the block with weaker repulsion toward the surface. The number of the alternative inner layers is determined by the size. For asymmetric diblock, the morphology is strong associated with the composition. For A 3B 7, owing to the strong restriction of the nanosphere, the hexagonal phase originally in the bulk does not occur. When the surface is neutral, the minority block forms bended sticks touched the surface. With the increase of the sphere size, more and more cylindrical or star like joints link these sticks. When the surface is non-neutral and less repulsive toward the minority block, more bended sticks near the nanosphere surface than those in the neutral nanosphere. Increasing the sphere size causes spherical and cylindrical domain formed in the center of the sphere. For A 2B 8, the minority A-block forms spheres or peanut like domains consisting concentric spheres; the number of them is related to the sphere size. However, for A 1B 9, no micro-phase separation was found. The polymer surfactant in aqueous solution was also studied. The results indicate that the polymer in a nanosphere can be used to mimic the polymer surfactant in some cases.

Morphologies of diblock copolymer confined in a slit with patterned surfaces studied by dissipative particle dynamics

Diblock copolymers with ordered mesophase structures have been used as templates for nano-fabrication. Unfortunately, the ordered structure only exists at micromete rscale areas, which precludes its use in many advanced applications. To overcome this disadvantage, the diblock copolymer confined in a restricted system with a patterned surface is proved to be an effective means to prohibit the formation of defects and obtain perfect ordered domains. In this work, the morphologies of a thin film of diblock copolymer confined between patterned and neutral surfaces were studied by dissipative particle dynamics. It is shown that the morphology of the symmetric diblock copolymer is affected by the ratio of the pattern period on the surface to the lamellar period of the symmetric diblock copolymer and by the repulsion parameters between blocks and wall particles. To eliminate the defects in the lamellar phase, the pattern period on the surface must match the lamellar period. The difference in the interface energy of different compartments of the pattern should increase with increasing film thickness. The pattern period on the surface has a scaling relationship with the chain length, which is the same as that between the lamellar period and the chain length. The lamellar period is also affected by the polydispersity of the symmetric diblock copolymer. The total period is the average of the period of each component multiplied by the weight of its volume ratio. The morphologies of asymmetric diblock copolymers are also affected by the pattern on the surface, especially when the matching period of the asymmetric diblock copolymer is equal to the pattern period, which is approximately equal to the lamellar period of a symmetric diblock copolymer with the same chain length.

Micellar morphologies of self-associated diblock copolymers in acetone solution

We describe the synthesis and solution morphologies of poly(vinyl phenol- b-styrene) (PVPh- b-PS) micelles and the effects that changing the copolymer composition and concentration have on self-assembly structures of PVPh- b-PS in acetone (a good solvent for PVPh). These PVPh- b-PS copolymers aggregated into spherical, rod-like, and vesicular morphologies. The transformations of the PVPh- b-PS block copolymer micelles in acetone depended on a number of parameters, including the relative block lengths, their concentrations, and the degree of self-association through hydrogen bonding of the coronal PVPh chains. We also investigated the morphologies of the micelles formed from acetone solutions of poly(4- tert-butoxystyrene- b-styrene) (P tBOS- b-PS) copolymers having the same degree of polymerization as the precursor of PVPh- b-PS copolymer before hydrolysis reaction. Our results indicate that the micelles formed from PVPh- b-PS copolymers in acetone were more complicated than those prepared from P tBOS- b-PS copolymers in acetone because hydrogen bonding occurs in the micelle corona of the PVPh block. Finally, we also discussed the morphology transition when the self-association hydrogen bonding of PVPh block was destroyed by adding proton acceptor poly(4-vinylpyridine) (P4VP).