Asymmetric Diblock Copolymers Research Articles

Nanoparticle assembly under block copolymer confinement: The effect of nanoparticle size and confinement strength

We investigate the self-assembly of cylinder-forming polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers (BCP) mixed with metal nanoparticles (NP) coated with short-chain polystyrene (PS) ligands. The NP formed hierarchical superstructures under confinement of cylindrical PS domains of PS-b-P4VP BCP. The complexity of NP superstructures was found to depend on the ratio between PS cylindrical domain size and NP size (DC/DNP). As the DC/DNP ratio increased, the number of NP layers normal to the cylinder axis also increased. However, the packing density of the NP decreased at higher DC/DNP. Furthermore, the morphology of the structures obtained during different solvent casting conditions revealed that the initial clustering of NP and micellization around these clusters act as a precursor for the subsequent formation of closely packed structures of NP in cylinders. The experimental results were further supported by modeling results obtained from molecular dynamics (MD) simulation. Based on MD simulations, we constructed structural phase diagram of nanoparticle assemblies in the presence of asymmetric diblock copolymers comprising short NP-attractive blocks. The MD simulation results indicate that NP undergo transition from spherical to cylindrical assemblies depending on the NP size, the overall concentration of components and the degree of affinity of the minor block to NP.

A15, σ, and a Quasicrystal: Access to Complex Particle Packings via Bidisperse Diblock Copolymer Blends.

A renewed focus on the phase behavior of nominally single-component, compositionally asymmetric diblock copolymers has revealed a host of previously unanticipated Frank-Kasper (FK) and quasicrystalline phases. However, these periodic and aperiodic particle packings have thus far only been reported in low molecular weight, highly conformationally asymmetric diblock copolymers, leaving researchers with a relatively small library of polymers in which these phases can be studied. In this work, we report on a simple approach to access these morphologies: blending two diblock copolymers with the same corona block length and varied core block lengths. Compositionally symmetric and asymmetric polystyrene-b-1,4-polybutadiene (SB) diblock copolymers with constant corona block lengths were blended together and shown via small-angle X-ray scattering and transmission electron microscopy to order into the FK A15 and σ phases, as well as a dodecagonal quasicrystal, providing a route to various particle packings in high molecular weight diblock copolymer melts.

Enhanced sampling of cylindrical microphase separation via a shell-averaged bond-orientational order parameter.

The formation of a hexagonal phase from disordered phase is one of the typical order-disorder transitions (ODTs) observed in asymmetric diblock copolymer systems. In order to drive this transition in a particle-based simulation, we introduce a shell-based bond-orientational order parameter that selectively responds to the mesoscopic order of the hexagonal cylinder phase. From metadynamics simulations in a bond-free particle model system, the characteristic pathway involved with the underlying free energy surface is deduced for the disordered-to-hexagonal transition. It is shown consecutively that the transition pathway and the metastable state are reproduced in dissipative particle dynamics simulations for the corresponding transition in a bulk asymmetric block copolymer melt system. These agreements suggest that efficient strategies for enhanced sampling with particle-based simulations of block copolymer systems can be devised using coarse-grained pictures of the mesoscopic order.

Correlations in Disordered Melts of Asymmetric Diblock Copolymers

Simulations of several different coarse-grained models are used to characterize how the structure factor S(q) in melts of compositionally asymmetric diblock copolymers varies with changes in the vo...

Origins of low-symmetry phases in asymmetric diblock copolymer melts

Cooling disordered compositionally asymmetric diblock copolymers leads to the formation of nearly spherical particles, each containing hundreds of molecules, which crystallize upon cooling below the order-disorder transition temperature (TODT). Self-consistent field theory (SCFT) reveals that dispersity in the block degrees of polymerization stabilizes various Frank-Kasper phases, including the C14 and C15 Laves phases, which have been accessed experimentally in low-molar-mass poly(isoprene)-b-poly(lactide) (PI-PLA) diblock copolymers using thermal processing strategies. Heating and cooling a specimen containing 15% PLA above and below the TODT from the body-centered cubic (BCC) or C14 states regenerates the same crystalline order established at lower temperatures. This memory effect is also demonstrated with a specimen containing 20% PLA, which recrystallizes to either C15 or hexagonally ordered cylinders (HEXC) upon heating and cooling. The process-path-dependent formation of crystalline order shapes the number of particles per unit volume, n/V, which is retained in the highly structured disordered liquid as revealed by small-angle X-ray scattering (SAXS) experiments. We hypothesize that symmetry breaking during crystallization is governed by the particle number density imprinted in the liquid during ordering at lower temperature, and this metastable liquid is kinetically constrained from equilibrating due to prohibitively large free energy barriers for micelle fusion and fission. Ordering at fixed n/V is enabled by facile chain exchange, which redistributes mass as required to meet the multiple particle sizes and packing associated with specific low-symmetry Frank-Kasper phases. This discovery exposes universal concepts related to order and disorder in self-assembled soft materials.

Designing an ordered template of cylindrical arrays based on a simple flat plate confinement of block copolymers: a coarse-grained molecular dynamics study.

In this paper we study the morphology formed by asymmetric di-block copolymers (di-BCPs) under various confinements using a large-scale coarse-grained molecular dynamics (CGMD) framework. We start with a simple flat plate confinement with the bottom and the top substrate attractive to the minor phase. Studies at a lower confinement length of 17σ have shown that there exists a critical chain length above which a transition from a three-domain morphology to a two-domain morphology is observed. Increasing the confinement length to 42σ, where the chains experience considerably lower confinement effects, also revealed the existence of a critical chain length - a transition from a multi-domain morphology (>3) to a three-domain morphology. The results obtained from the flat plate study with two confinement dimensions were used to design a topography of silica pillars with and without a bottom substrate to form ordered cylindrical BCP arrays. The least and highest radial separation lengths between adjacent pillars are kept at 17σ and 42σ, respectively. A direct correlation was observed in the number of continuous micro-domains of the maximum and minimum confinement dimensions with the 17σ and 42σ flat plate trials. With the optimum chain length employed, the surfaces with affinity to the minor phase can direct the BCP self-assembly to form ordered arrays of minor phase cylinders. The current study thus elucidates a useful tool to predict the morphology formed in an intricate nano-lithographic template by using simple length scale arguments derived from a flat plate confinement study.

Stimuli-responsive properties of special micellar nanocarriers and their application for delivery of vitamin E and its analogues

Stimuli-responsive micelles were formed by asymmetric diblock and triblock copolymers, which comprised chemically complementary (methoxy)poly(ethylene oxide) and poly(acrylic acid) blocks of different length. They were used as nanocarriers to encapsulate and deliver in vivo a poorly soluble homolog of α-tocopheryl acetate (α-TOCAh), commonly known as an analogue of vitamin E. Self-assembly of the copolymer macromolecules into special micellar structures in aqueous solutions depended on the solution pH. The micellar structures arising at pH < 5 due to hydrogen bonds between the blocks and the subsequent segregation of the hydrophobic bound segments in the aqueous medium were stable in the temperature range 20–60 °C and with dilution of the solution. The micelle formation/destruction in the copolymer solutions at pH alteration was shown to be an equilibrium process. Various morphological forms of micelles depending on the copolymer structure and the blocks’ asymmetry were revealed by the TEM method. All types of micelles showed high binding degrees (about 100%) of α-TOCAh, which did not depend on the encapsulation pathway (“in situ” or “ex situ”). Encapsulation of α-TOCAh resulted in hydrophobization of the micelle surface, thereby enhancing the aggregation of micelles in aqueous solution. Encapsulated α-tocopheryl acetate was in vivo fully digested by the animals (white mice) and showed increased biological activity, which allowed reducing its therapeutic dose by 25 times.

Chemoepitaxial guiding underlayers for density asymmetric and energetically asymmetric diblock copolymers

Currently, high χ block copolymers (BCPs) are being investigated as a method to extend optical lithography due to their ability to microphase separate on small size scales. Typically, BCPs with larger Flory–Huggin’s χ parameters are composed of more dissimilar homopolymers. However, having dissimilar blocks changes how BCPs interact with their guiding underlayers. Several BCPs are simulated annealing on chemoepitaxial guiding underlayers using a coarse-grained molecular dynamics model to explore the effect that either energetic asymmetry or density asymmetry in the BCP have on the pattern registration. It is found that in varying the background region composition four regimes can be found. Minor variations in pinning stripe width are shown to have little effect on the window where well-aligned vertical lamellae form. For BCPs without an energetic mismatch, incommensurate films have the largest window for well-aligned vertical lamellae. However, with an energetic mismatch, the defectivity has a more complicated dependence on film thickness. Two different mixed lamellae (ML) morphologies can form depending on the film volume fraction and the relative compressibilities of the two blocks. It is found that more preferential background regions can be used when the BCP is transitioning between the two ML morphologies. This transition volume fraction shifts for a density asymmetric BCP, likely due to a difference in compressibilities of the two blocks.

Asymmetric supramolecular double-comb diblock copolymers: From plasticization, to confined crystallization, to breakout

The combination of asymmetric P4VP-b-PAPI diblock copolymers (i.e. fP4VP≠fPAPI) and 3-NDP surfactants in hydrogen-bonded [poly(4-vinylpyridine)-block-poly(N-acryloylpiperidine)](3-nonadecylphenol)x (P4PA(3-NDP)x) supramolecular double-comb diblock copolymers could potentially result in rather interesting morphologies. However, plasticization of the copolymer and the ability of 3-NDP to crystallize were found to affect their self-assembly significantly. In general, for high comb densities x, the complex's tendency to crystallize and its preference to form a flat interface dominated microphase separation. Lower values of x on the other hand gave uneven distribution of 3-NDP, resulting in a higher glass transition temperature of the P4VP block. Crystallization of 3-NDP's aliphatic tails was therefore in most cases restricted to the preferential PAPI microdomains, thereby maintaining the large length scale block copolymer morphology that was already present in the melt. Such behavior is identical to self-assembly of linear semicrystalline diblock copolymers, as in these type of systems structure formation depends on the segregation strength and relative magnitude of the order-disorder transition (TODT), Tg and Tc, leading to mechanisms like confined crystallization or breakout.

Compositions of Anticancer Drug with Micellar Nanocarriers and Their Cytotoxicity

Asymmetric diblock (DBC) and triblock (TBC) copolymers contained biocompatible chemically complementary polyacrylamide and poly(ethylene oxide) (PAAm-b-PEO-b-PAAm) or its monomethyl ether (MEPEO-b-PAAm), and also partially hydrolyzed triblock copolymer derivative (TBChydr) were used to create micelles of a special type. The micelles obtained are characterized by small CMCs and large values of the Gibbs micellization energy, thus indicating a high stability of DBC, TBC and TBChydr micelles in aqueous solutions and the capabilities of their use to encapsulate and deliver poorly soluble and/or toxic drugs in living organism. Morphological features and size of DBC and TBC micelles were determined by TEM. The electron images demonstrated spherical micelles of a polymolecular type, monomolecular type and separate micelle aggregates. TBC and TBChydr micelles were used to examine in vitro anticancer activity of their compositions with doxorubicin (Dox). The created micelle systems showed the enhanced cytotoxicity as compared to individual Dox against murine leukemia cells of L1210 line, murine transformed fibroblasts of L929 line and human T-leukemia cells of Jurkat line and allow to achieve a high efficacy at low Dox concentrations (0,1÷3 µg·cm-3) that opens the great prospects for essential decrease in drug dose at chemotherapy.

Micellar nanocarriers for anticancer drug melanin

ABSTRACTThe effective biocompatible and partially biodegradable micellar nanocarriers for encapsulation of the anticancer drug eumelanin were created. Such nanocarries are the products of self-assembly of asymmetric diblock and triblock copolymers, which comprised chemically complementary (methoxy)poly(ethylene oxide) and poly(acrylic acid) blocks of a variable length. A chemical structure and solubility of the eumelanin, the vital activity product of black yeast “Nadsoniella nigra sp. X-1, was characterized. Peculiarities of the micellization process, morphology, size and stability of the developed micellar nanocarries were highlighted. The eumelanin encapsulation into micellar nanocarries with different morphology and a state of the obtained drug/micelle systems were studied and discussed.

Nucleation of the BCC phase from disorder in a diblock copolymer melt: Testing approximate theories through simulation.

We examine nucleation of the stable body-centred-cubic (BCC) phase from the metastable uniform disordered phase in an asymmetric diblock copolymer melt. Our comprehensive, large-scale simulations of the time-dependent, mean-field Landau-Brazovskii model find that spherical droplets of the BCC phase nucleate directly from disorder. Near the order-disorder transition, the critical nucleus is large and has a classical profile, attaining the bulk BCC phase in an interior that is separated from disorder by a sharp interface. At greater undercooling, the amplitude of BCC order in the interior decreases and the nucleus interface broadens, leading to a diffuse critical nucleus. This diffuse nucleus becomes large as the simulation approaches the disordered phase spinodal. We show that our simulation follows the same nucleation pathway that Cahn and Hilliard found for an incompressible two-component fluid, across the entire metastable region. In contrast, a classical nucleation theory calculation based on the free energy of a planar interface between coexisting BCC and disordered phases agrees with simulation only in the limit of very small undercooling; we can expand this region of validity somewhat by accounting for the curvature of the droplet interface. A nucleation pathway involving a classical droplet persists, however, to deep undercooling in our simulation, but this pathway is energetically unfavourable. As a droplet grows in the simulation, its interface moves with a constant speed, and this speed is approximately proportional to the undercooling.

INTERRELATIONS BETWEEN MORPHOLOGY AND SOFTENING BEHAVIOR IN SELF-ASSEMBLED POLY (BUTADIENE-BLOCK-(STYRENE-STAT-BUTADIENE)) COPOLYMERS

ABSTRACTTwo series of poly(butadiene-block-(styrene-stat-butadiene) (PB-SBR) diblock copolymers were synthesized to study interrelations between microphase-separated state and relaxation dynamics. Series I consists of six symmetric PB-SBR samples (ΦSBR ∼ 0.50) with different styrene contents in the random SBR block (21 mol% ≤ xS,SBR ≤ 52 mol%). Series II contains six asymmetric PB-SBR diblock copolymers with almost constant styrene content in the SBR block (xS,SBR = 32 ± 4 mol%) and SBR volume fractions in the range of 0.20 ≤ ΦSBR ≤ 0.69. All PB-SBR diblock copolymers have a total molecular weight of about 200 kg/mol with 1,2-vinyl content of about 8.5 and 15.8 mol% in the soft PB and in the hard SBR block, respectively. A novel synthesis route has been developed so as to maintain low 1,2-vinyl contents in both blocks and a random distribution of the styrene units in the SBR block. The softening behavior of non–cross-linked PB-SBR diblocks was investigated by differential scanning calorimetry, providing first information about miscibility. The morphology of cross-linked PB-SBR diblock copolymers, which have been vulcanized with sulfur at 150 °C using standard procedures, was studied by atomic force microscopy. Dynamic shear measurements aimed at understanding the influence of morphological changes on the segmental dynamics of both phases, αPB and αSBR, were performed on vulcanized samples. Atomic force microscopy results and shear data indicate that symmetric diblock copolymers with styrene content higher than 27 mol% in the SBR block are well microphase separated. Strongly segregated states are also observed for asymmetric diblock copolymers with volume fractions 0.28 ≤ ΦSBR ≤ 0.60 showing cylindrical and lamellar morphologies. The origin of changes in the αPB and αSBR dynamics depending on morphology and segregation strengths is considered. Implications for the use of rubbery PB-SBR block copolymers in composites for tire treads are discussed.

Topology and Shape Control for Assemblies of Block Copolymer Blends in Solution

We study binary blends of asymmetric diblock copolymers (AB/AC) in selective solvents with a mesoscale model. We investigate the morphological transitions induced by the concentration of the AC block copolymer and the difference in molecular weight between the AB and AC copolymers, when segments B and C exhibit hydrogen-bonding interactions. To the best of our knowledge, this is the first work modeling mixtures of block copolymers with large differences in molecular weight. The coassembly mechanism localizes the AC molecules at the interface of A and B domains and induces the swelling of the B-rich domains. The coil size of the large molecular weight block copolymer depends only on the concentration of the short block copolymer (AC or AB), regardless of the B–C interactions. However, the B–C interactions control the morphological transitions that occur in these blends.

Arrangement of Maghemite Nanoparticles via Wet Chemical Self-Assembly in PS-b-PNIPAM Diblock Copolymer Films.

The structure and magnetic behavior of hybrid films composed of maghemite (γ-Fe2O3) nanoparticles (NPs) and an asymmetric diblock copolymer (DBC) polystyrene61-block-polyN-isopropylacrylamide115 are investigated. The NPs are coated with PS chains, which allow for a selective incorporation inside the PS domains at different NP concentrations. Upon incorporation of low amounts of NPs into the DBC thin films, the initial parallel (to film surface) cylinder morphology changes to a well ordered, perpendicularly oriented one. The characteristic domain distance of the DBC is increased due to the swelling of the PS domains with NPs. At higher NP concentrations, the excess NPs which can no longer be embedded in the PS domains, are accumulated at the film surface, and NP aggregates form. Irrespective of NP concentration, a superparamagnetic behavior of the metal oxide-DBC hybrid films is found. Such superparamagnetic properties make the established hybrid films interesting for high density magnetic storage media and thermoresponsive magnetic sensors.

Fiber‐Like Micelles from the Crystallization‐Driven Self‐Assembly of Poly(3‐heptylselenophene)‐block‐Polystyrene

The crystallization‐driven self‐assembly (CDSA) of crystalline‐coil polyselenophene diblock copolymers represents a facile approach to nanofibers with distinct optoelectronic properties relative to those of their polythiophene analogs. The synthesis of an asymmetric diblock copolymer with a crystallizable, π‐conjugated poly(3‐heptylselenophene) (P3C7Se) block and an amorphous polystyrene (PS) coblock is described. CDSA was performed in solvents selective for the PS block. Based on transmission electron microscopy (TEM) analysis, P3C7Se18‐b‐PS125 formed very long (up to 5 μm), highly aggregated nanofibers in n‐butyl acetate (nBuOAc) whereas shorter (ca. 500 nm) micelles of low polydispersity were obtained in cyclohexane. The micelle core widths in both solvents determined from TEM analysis (≈ 8 nm) were commensurate with fully‐extended P3C7Se18 chains (estimated length = 7.1 nm). Atomic force microscopy (AFM) analysis provided characterization of the micelle cross‐section including the PS corona (overall micelle width ≈ 60 nm). The crystallinity of the micelle cores was probed by UV–vis and photoluminescence (PL) spectroscopy and wide‐angle X‐ray scattering (WAXS).

Coarse-graining in simulations of multicomponent polymer systems.

We investigate the mapping required between the interaction parameters of two different coarse-grained simulation models to ensure a match of the long-range structural characteristics of multicomponent polymeric system. The basis for our studies is the recent work of Morse and workers, which demonstrated the existence of a mapping between the interaction parameters of different coarse-grained simulation models which allow for a matching of the peak of the disordered state structure factor in symmetric diblock copolymers. We investigate the extensibility of their results to other polymeric systems by studying a variety of systems, including, asymmetric diblock copolymers, symmetric triblock copolymers, and diblock copolymer-solvent mixtures. By using the mapping deduced in the context of symmetric diblock copolymers, we observe excellent agreement for peak in the inverse structure between both two popular coarse grained models for all sets of polymeric melt systems investigated, thus showing that the mapping function proposed for diblock copolymer melts is transferable to other polymer melts irrespective of the blockiness or overall composition. Interestingly, for the limited parameter range of polymer-solvent systems investigated in this article, the mapping functions developed for polymer melts are shown to be equally effective in mapping the structure factor of the coarse-grained simulation models. We use our findings to propose a methodology to create ordered morphologies in simulations involving hard repulsive potentials in a computationally efficient manner. We demonstrate the outcomes of methodology by creating lamellar and cylindrical phases of diblock copolymers of long chains in the popularly used Kremer-Grest simulation model.

Asymmetric block copolymers for supramolecular templating of inorganic nanospace materials.

This review focuses on polymeric micelles consisting of asymmetric block copolymers as designed templates for several inorganic nanospace materials with a wide variety of compositions. The presence of chemically distinct domains of asymmetric triblock and diblock copolymers provide self-assemblies with more diverse morphological and functional features than those constructed by EOn POm EOn type symmetric triblock copolymers, thereby affording well-designed nanospace materials. This strategy can produce unprecedented nanospace materials, which are very difficult to prepare through other conventional organic templating approaches. Here, the recent development on the synthesis of inorganic nanospace materials are mainly focused on, such as hollow spheres, tubes, and porous oxides, using asymmetric triblock copolymers.

High aspect ratio PS-b-PMMA block copolymer masks for lithographic applications.

The control of the self-assembly (SA) process and nanostructure orientation in diblock copolymer (DBC) thick films is a crucial technological issue. Perpendicular orientation of the nanostructures in symmetric and asymmetric poly(styrene)-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymer films obtained by means of simple thermal treatments was demonstrated to occur in well-defined thickness windows featuring modest maximum values, thus resulting in low aspect ratio (h/d < 2) of the final lithographic mask. In this manuscript, the thickness window corresponding to the perpendicular orientation of the cylindrical structures in asymmetric DBC is investigated at high temperatures (190 °C ≤ T ≤ 310 °C) using a rapid thermal processing machine. A systematic study of the annealing conditions (temperature and time) of asymmetric PS-b-PMMA (Mn = 67.1, polydispersity index = 1.09) films, with thicknesses ranging from 10 to 400 nm, allowed ordered patterns, with a maximum value of orientational correlation length of 350 nm, to be obtained for film thicknesses up to 200 nm. The complete propagation of the cylindrical structures through the whole film thickness in a high aspect ratio PS template (h/d ≈ 7) is probed by lift-off process. Si nanopillars are obtained having the same lateral ordering and characteristic dimensions of the DBC lithographic mask as further confirmed by grazing-incidence small-angle X-ray scattering experiments.

σ Phase Formed in Conformationally Asymmetric AB-Type Block Copolymers.

The stability of various spherical phases formed in conformationally asymmetric AB diblock and architecture asymmetric ABm miktoarm block copolymers is investigated using self-consistent field theory. Both the conformational and architecture asymmetries are unified into a parameter of conformationally asymmetric degree, ε. We find that a complex spherical phase, the σ phase, becomes stable and its phase region expands between bcc and hexagonal phases as increasing ε. Only for large conformational asymmetry, for example, ε = 9 (or m = 3), the A15 phase becomes stable in the region between the σ phase and the hexagonal phase and its phase region terminates at the intermediate segregation region. Compared with the σ phase, the A15 phase has more favorable interfacial energy by enabling the formation of larger spherical domains, and therefore, it becomes more stable in the region of more symmetric volume fraction and stronger segregation.