Mixtures Of Diblock Copolymers Research Articles

Effect of nanorods on the mesophase structure of diblock copolymers

Mixtures of symmetric diblock copolymers and rigid nanorods (NRs) that are preferentially wetted by one of the blocks (A) are systematically investigated via dissipative particle dynamics simulations. The morphology of such composites depends not only on the characteristics of the copolymers, but also on the physical or chemical features of the NRs, such as NR volume fraction, size, and polymer-NR interaction. We find that the presence of NRs influences the phase behavior of copolymers and the phase-separated domains of copolymers in turn confine the NRs spatial distribution and positional orientation. The morphologies and phase transitions of hybrids and the corresponding NRs distributed and oriented regularities can be rationalized on the basis of the relative enthalpic and entropic effects involving all of the species, A and B blocks, and NRs. These results suggest that by choosing appropriate parameters, our model systems may provide a prediction to control and design the high-performance composites.

Hydrogen bond mediated supramolecular micellization of diblock copolymer mixture in common solvents

We report a new approach toward preparing self-assembled hydrogen-bonded complexes having vesicle and patched spherical structures from two species of block copolymers in nonselective solvents. Two diblock copolymers, poly(styrene- b-vinyl phenol) (PS- b-PVPh) and poly(methyl methacrylate- b-4-vinylpyridine) (PMMA- b-P4VP), were synthesized through anionic polymerization. The assembly of vesicles from the intermolecular complex formed after mixing PS- b-PVPH with PMMA- b-P4VP in THF was driven by strong hydrogen bonding between the complementary binding sites on the PVPH and P4VP blocks. In contrast, well-defined patched spherical micelles formed after blending PS- b-PVPh with PMMA- b-P4VP in DMF: the weaker hydrogen bonds formed between the PVPh and P4VP blocks in DMF, relative to those in THF, resulted in the formation of spherical micelles having compartmentalized coronas consisting of PS and PMMA blocks.

Self-Assembly Behavior of AB/AC Diblock Copolymer Mixtures in Dilute Solution

The self-assembly behavior of AB/AC amphiphilic diblock copolymer mixtures in dilute solution was studied by a real-space-implemented self-consistent field theory in three dimensions. The AB and AC copolymers have a common hydrophobic block A but different hydrophilic blocks B and C. Two cases were studied: one in which copolymers AB and AC have same hydrophobic and hydrophilic block lengths and one in which copolymers AB and AC have different hydrophobic and hydrophilic block lengths. It was found that the two copolymers can cooperatively self-assemble into hybrid aggregates. The morphologies of the formed aggregates were found to be dependent on the mixture ratio and the interaction between the B and C blocks. For the AB/AC copolymers with different hydrophobic and hydrophilic lengths, chain segregation was found in the formed hybrid aggregates. Based on the obtained calculation results, phase diagrams as functions of the mixture ratio and interaction between the B and C blocks were constructed.

Hydrogen bond‐mediated self‐assembly and supramolecular structures of diblock copolymer mixtures

AbstractThis review summarizes recent advances in the preparation of hydrogen bonding block copolymer mixtures and the supramolecular structures they form through multiple hydrogen bonding interactions. Hydrogen bonding in block copolymer mixtures that form nanostructures and have unusual electronic, photonic and magnetic properties is a topic of great interest in polymer science. Combining the self‐assembly of block copolymers with supramolecular structures offers unique possibilities to create new materials with tunable and responsive properties. The self‐assembly of structures from diblock copolymer mixtures in the bulk state is readily controlled by varying the weight fraction of the block copolymer mixture and the copolymer composition; in solution, the morphologies are dependent on the copolymer composition, the copolymer concentration, the nature of the common solvent, the amount of the selective solvent and, most importantly, the hydrogen bonding strength. Copyright © 2008 Society of Chemical Industry

Microfluidic Fabrication of Monodisperse Biocompatible and Biodegradable Polymersomes with Controlled Permeability

We describe a versatile technique for fabricating monodisperse polymersomes with biocompatible and biodegradable diblock copolymers for efficient encapsulation of actives. We use double emulsion as a template for the assembly of amphiphilic diblock copolymers into vesicle structures. These polymersomes can be used to encapsulate small hydrophilic solutes. When triggered by an osmotic shock, the polymersomes break and release the solutes, providing a simple and effective release mechanism. The technique can also be applied to diblock copolymers with different hydrophilic-to-hydrophobic block ratios, or mixtures of diblock copolymers and hydrophobic homopolymers. The ability to make polymer vesicles with copolymers of different block ratios and to incorporate different homopolymers into the polymersomes will allow the tuning of polymersome properties for specific technological applications.

Morphological and Scattering Studies of Binary Mixtures of Block Copolymers: Cosurfactant Effects Observed in the Parameter Space of Temperature, Blend Composition, and Molecular Weight Ratio

Thermoreversible order-order transitions or “order−distorted order” transitions (defined “OOT” for simplicity) between various morphologies occurring on binary mixtures of diblock copolymers were investigated by using small-angle X-ray scattering. The mixtures were composed of a long asymmetric polystyrene-block-polyisoprene (SI) having a spherical morphology and a short symmetric SI, which are hereafter denoted as as and s3, respectively. The symmetric diblock, s3, is short enough to be in a disordered state at room temperature. Upon raising temperature from ambient temperature toward order−disorder or “distorted order-disorder” transition temperature, a unique re-entrant bicontinuous-cylinder- bicontinuous OOT was observed for as/s3 = 85/15 and 82/18 (w/w). Other types of OOT, such as cylinder-sphere and lamella-bicontinuous, were observed on the as/s3 mixtures of different compositions. These OOTs were characterized by monitoring the evolution of the inverse peak scattered intensity, the square of the half-width at half-maximum of the peak, and the characteristic period of the microdomains, as a function of temperature, T. The complex morphological behaviors observed here can be understood on the basis of two key features of the binary blends: the cosurfactant effect and the possibility for the chemical junctions of the shorter diblock to migrate away from the interface as segregation power decreases. As a summary of this series of studies, a three-dimensional phase diagram was constructed in the parameter space of overall volume fraction of PS block in the mixture, the ratio of the degree of polymerization of as to that of s3 and T. Such a phase diagram illustrates that the cosurfactant effect opens perspectives in tailoring morphologies of self-assembled, nanostructured materials.

Supramolecular Micellization of Diblock Copolymer Mixtures Mediated by Hydrogen Bonding for the Observation of Separated Coil and Chain Aggregation in Common Solvents

AbstractThis paper describes a new approach towards preparing self‐assembled hydrogen‐bonded complexes that have vesicle and patched spherical structures from two species of block copolymer in non‐selective solvents. The assembly of vesicles from the intermolecular complex formed after mixing polystyrene‐block‐poly(4‐vinyl phenol) (PS‐b‐PVPh) with poly(methyl methacrylate)‐block‐poly(4‐vinylpyridine) (PMMA‐b‐P4VP) in tetrahydrofuran (THF) is driven by strong hydrogen bonding between the complementary binding sites on the PVPh and P4VP blocks. In contrast, well‐defined patched spherical micelles form after blending PS‐b‐PVPh with PMMA‐b‐P4VP in N,N‐dimethylformamide (DMF): weaker hydrogen bonds form between the PVPh and P4VP blocks in DMF, relative to those in THF, which results in the formation of spherical micelles that have compartmentalized coronas that consist of PS and PMMA blocks.magnified image

WITHDRAWN: Double-responsive core-shell-corona micelles from mixtures of binary diblock copolymers

WITHDRAWN: Double-responsive core-shell-corona micelles from mixtures of binary diblock copolymers

Self-Assembly of Diblock Copolymer Mixtures in Confined States: A Monte Carlo Study

The self-assembly of diblock copolymer mixtures (A-b-B/A-b-C or A-b-B/B-b-C mixtures) subjected to cylindrical confinement (two-dimensional confinement) was investigated using a Monte Carlo method. In this study, the boundary surfaces were configured to attract blocks A but repel blocks B and C. Relative to the structures of the individual components, the self-assembled structures of mixtures of the diblock copolymers were more complex and interesting. Under cylindrical confinement, with varying cylinder diameters and interaction energies between the boundary surfaces and the blocks, we observed a variety of interesting morphologies. Upon decreasing the cylinder's diameter, the self-assembled structures of the A(15)B(15)/A(15)C(15) mixtures changed from double-helix/cylinder structures (blocks B and C formed double helices, whereas blocks A formed the outer barrel and inner core) to stacked disk/cylinder structures (blocks B and C formed the stacked disk core, blocks A formed the outer cylindrical barrel), whereas the self-assembled structures of the A(15)B(7)/B7C15 mixtures changed from concentric cylindrical barrel structures to screw/cylinder structures (blocks C formed an inside core winding with helical stripes, whereas blocks A and B formed the outer cylindrical barrels) and then finally to the stacked disk/cylinder structures.

Morphological Studies of Binary Mixtures of Block Copolymers: Temperature Dependence of Cosurfactant Effects

A unique phase behavior was investigated for binary mixtures of diblock copolymers in order to explore the cosurfactant effect as a function of temperature. The mixtures are composed of a long asymmetric polystyrene-block-polyisoprene (SI) copolymer and a short symmetric SI copolymer denoted, respectively, as as and s3. The neat as copolymer forms polystyrene (PS) spherical microdomains on a body-centered-cubic lattice in polyisoprene (PI) matrix from ambient temperature to 180 °C, while neat s3 is always in the disordered state in the same temperature range. The mixtures of as/s3 = 45/55 and 75/25 (w/w) are mainly focused in this study. Both of them show lamellar morphology at low temperatures, then undergo an “order−order transition (OOT)” to a “bicontinuous structure” with a considerable distortion in the long-range order, and finally reach the truly disordered state through the “order−disorder transition (ODT)” with increasing temperature. The reason why quotation marks were put in “OOT” and “ODT” is explained in the text. We found that the invariants (the integrated scattered intensity) do not change across the “OOT” for these two mixtures. It is intriguing to note that these two mixtures exhibit a quite opposite temperature dependence of the characteristic length D for the ordered structures, which seems to be related to the “cosurfactant effect” as discussed in the text. The cosurfactant effect was shown to decrease with increasing temperature and to promote the distorted bicontinuous phase.

An ESR Spin-Label Study on Molecular Mobility in the Interface between Microphases of a Diblock Copolymer: Effects of Admixture of Homopolymers That Are Miscible with One of the Blocks

Molecular mobility in the interfacial region of a microphase-separated structure was studied in binary mixtures of AB-type diblock copolymers and homopolymers (miscible with only A) by the spin-label technique. In this study, we prepared (a) binary blends of polystyrene-block-poly(methyl acrylate) (PS-block-PMA) and homopolymer poly(cyclohexyl acrylate)'s having the number-averaged molecular weight (Mn) of 1000 (PCHA-S) and (b) having the Mn of 17300 (PCHA-L), and (c) binary mixtures of the PS-block-PMA and homopolymer PS with the Mn of 900 (PS-1). Emphasis was placed on effects of the molecular weight and the miscibility of added homopolymers on the mobility in the interfacial region of the microphase separation. Selective incorporation of the added PCHA-S, PCHA-L, and PS-1 into the PS phase of the PS-block-PMA was confirmed by modulated-temperature differential scanning calorimetry (MDSC) measurement as a decrease in the glass transition temperature of the PS phase. Moreover, the MDSC and small-angle X-ray scattering (SAXS) measurements suggested that the spatial distributions of the PCHA-S and PS-1 in the PS phase were relatively uniform because of their small Mn. On the other hand, the distribution of the PCHA-L in the PS phase was somewhat heterogeneous because of the large Mn of the PCHA-L. The spin-label at the junction point of the PS-block-PMA allowed us to estimate the mobility in the interfacial region of the microphase separation. Influence of the PCHA-S and PCHA-L on the mobility in the interfacial region was negligible even though the relatively uniform distribution of the PCHA-S in the PS phase was suggested by the SAXS and MDSC. More uniform distribution of the PS-1 than that of the PCHA-S in the PS phase was suggested by the SAXS, and the mobility in the interfacial region was slightly enhanced by the addition of the PS-1. However, the mobility was almost constant against an increase in the PS-1. The PS-1 was considered to be penetrated into the interfacial region and activated the mobility, but the fraction of the PS-1 in the interfacial region was constant irrespective of the blended amount of the PS-1. These results suggest that effects of homopolymers on the mobility in the interface are significantly related to their spatial distribution in the host phase.

Aggregates in Solution of Binary Mixtures of Amphiphilic Diblock Copolymers with Different Chain Length

The polydispersity effect of amphiphilic AB diblock copolymers on the self-assembled morphologies in solution has been investigated by the real-space implementation of self-consistent field theory (SCFT) in two dimensions (2D). The polydispersity is artificially obtained by mixing binary diblock copolymers where the hydrophilic or hydrophobic blocks are composed of two different lengths while the other block length is kept the same. The main advantage is that this simple polydispersity can easily distinguish the difference of aggregates in the density distribution of long and short block length intuitionally and quantitatively. The morphology transition from vesicles to micelles is observed with increasing polydispersity of copolymers due to the length segregation of copolymers. For polydisperse hydrophilic or hydrophobic blocks, the short blocks tend to distribute at the interfaces between hydrophilic and hydrophobic blocks while the long blocks stretch to the outer space. More specifically, by quantitatively taking the sum of all the concentration distribution of long and short chains over the inside and outside surface areas of the vesicle, it is found that long blocks prefer to locate on the outside surface of the vesicle while short ones prefer the inside. Such length segregation leads to large curvature of the aggregate, thus resulting in the decrease of the aggregate size.

Epidermal Growth Factor-Conjugated Poly(ethylene glycol)-block- Poly(δ-valerolactone) Copolymer Micelles for Targeted Delivery of Chemotherapeutics

Epidermal growth factor (EGF)-conjugated copolymer micelles were prepared from a mixture of diblock copolymers of methoxy poly(ethylene glycol)-block-poly(delta-valerolactone) (MePEG-b-PVL) and EGF-PEG-b-PVL for targeted delivery to EGF receptor (EGFR)-overexpressing cancers. The block copolymers and functionalized block copolymers were synthesized using PEG as the macroinitiator and HCl-diethyl ether as the catalyst. The MePEG-b-PVL and the carboxyl-terminated PEG-b-PVL (HOOC-PEG-b-PVL) copolymers were found to have molecular weights of 5940 and 5900, respectively, as determined by gel permeation chromatography (GPC) analyses. The HOOC-PEG-b-PVL copolymers were then activated by N-hydroxysuccinimide and subsequently reacted with EGF to form the EGF-PEG-b-PVL copolymers. The efficiency for the conjugation of EGF to the copolymer was found to be 60.9%. A hydrophobic fluorescent probe, CM-DiI, was loaded into both the nontargeted MePEG-b-PVL micelles and the targeted EGF-conjugated PEG-b-PVL micelles. The effective mean diameters of the CMDiI-loaded nontargeted and the CMDiI-loaded targeted micelles were found to be 32 +/- 1 nm and 45 +/- 2 nm, respectively, as determined by dynamic light scattering (DLS). The zeta potentials for the nontargeted micelles (no CM-DiI-loaded), CM-DiI-loaded nontargeted micelles, and CM-DiI-loaded targeted micelles were found to be -6.5, -8.7, and - 13.5 mV, respectively. Evaluation of the in vitro release of CM-DiI from the MePEG-b-PVL micelles in phosphate buffer saline (0.01 M, pH = 7.4) containing 10% (v/v) fetal bovine serum at 37 degrees C revealed that approximately 20% of the probe was released within the first 2 h. Confocal laser scanning microscopy (CLSM) analysis revealed that the targeted micelles containing CM-DiI accumulated intracellularly in EGFR-overexpressing MDA-MB-468 breast cancer cells following a 2 h incubation period, while no detectable cell uptake was observed for the nontargeted micelles. Results obtained from the confocal images were confirmed in an independent study by measuring the intracellular CM-DiI fluorescence in cell lysate. In addition, the presence of free EGF was found to decrease the extent of uptake of the targeted micelles. Nuclear staining of the cells with Hoechst 33258 indicated that the targeted micelles mainly localized in the perinuclear region and some of the micelles were localized in the nucleus. These results demonstrate that the EGF-conjugated copolymer micelles developed in this study have potential as vehicles for targeting hydrophobic drugs to EGFR-overexpressing cancers.

Phase separations in a copolymer–copolymer mixture

We propose a three-order-parameter model to study the phase separations in a diblockcopolymer–diblock copolymer mixture. The cell dynamical simulations provide richinformation about the phase evolution and structural formation, especially the appearanceof onion-rings. The parametric dependence and physical reason for the domain growth ofonion-rings are discussed.

Phase behavior of a blend of polymer-tethered nanoparticles with diblock copolymers

Using the self-consistent field theory (SCFT), we investigate the phase behavior of a mixture of diblock copolymers and nanoparticles with monodisperse polymer chains tethered to their surfaces. We assume the size of the nanoparticles to be much smaller than that of the attached polymer chains and therefore model the particles with their grafted polymer "shell" as star polymers. The polymer chains attached to the particles are of the same species as one of the blocks of the symmetric diblock copolymer. Of primary interest is how to tune the shell of the particle by changing both the length and number of tethered polymers in order to achieve higher loading of nanoparticles within an ordered structure without macrophase separation occurring. We find that the phase behavior of the system is very sensitive to the size of the particle including its tethered shell. The region of microphase separation is increased upon decreasing the star polymer size, which may be achieved by shortening and/or removing tethered polymer chains. To explore the possible structures in these systems we employ SCFT simulations that provide insight into the arrangement of the different species in these complex composites.

Self-directed self-assembly of nanoparticle/copolymer mixtures

The organization of inorganic nanostructures within self-assembled organic or biological templates is receiving the attention of scientists interested in developing functional hybrid materials. Previous efforts have concentrated on using such scaffolds to spatially arrange nanoscopic elements as a strategy for tailoring the electrical, magnetic or photonic properties of the material. Recent theoretical arguments have suggested that synergistic interactions between self-organizing particles and a self-assembling matrix material can lead to hierarchically ordered structures. Here we show that mixtures of diblock copolymers and either cadmium selenide- or ferritin-based nanoparticles exhibit cooperative, coupled self-assembly on the nanoscale. In thin films, the copolymers assemble into cylindrical domains, which dictate the spatial distribution of the nanoparticles; segregation of the particles to the interfaces mediates interfacial interactions and orients the copolymer domains normal to the surface, even when one of the blocks is strongly attracted to the substrate. Organization of both the polymeric and particulate entities is thus achieved without the use of external fields, opening a simple and general route for fabrication of nanostructured materials with hierarchical order.

Triblock terpolymer micelles: A personal outlooka

This outlook paper focuses on micelles formed by ABC triblock copolymers (triblock terpolymers) and related systems resulting from mixtures of diblock copolymers. Micelles with different internal structure such as micelles with a heterogeneous core and a homogeneous corona or micelles with a homogeneous core and a mixed corona are presented. More complex nanoobjects such as vesicles and Janus particles are also reviewed. Finally, potential applications of these objects are discussed.

Fabrication of a Gradient Heterogeneous Surface Using Homopolymers and Diblock Copolymers

The strength of the interfacial interactions and the length scale over which these interactions occur are key factors in understanding the thin film behavior of polymer blends and diblock copolymers, adhesion, wettability, and recognition processes of cells and random heteropolymers on surfaces. Here, gradient heterogeneous surface topographies were prepared using thin films of mixtures of homopolymers and diblock copolymers to vary the lateral size scale of heterogeneities from the microscopic to nanoscopic. Dewetting, phase separation, and cell adhesion were used to demonstrate the utility of these surfaces having gradient heterogeneous topographies. By tuning the lateral size scale of the heterogeneities, surface patterns can be engineered to meet a specific function. Gradient surfaces offer a straightforward method to optimize various length scales of heterogeneity.

Computer Simulation of Morphologies and Optical Properties of Filled Diblock Copolymers

We integrate two computational techniques in order to determine the optical properties of self-assembled mixtures of diblock copolymers and nanoparticles. To determine the morphology of the system, we first use an approach that combines a self-consistent-field theory (SCFT) for the diblocks with a density functional theory (DFT) for the particles. Using this SCF/DFT model, we focus on the lamellar phase of AB diblocks and calculate volume fraction profiles for systems containing selective or nonselective nanoparticles. We use the volume fraction profiles from the SCF/DFT and assign typical dielectric constants for the different polymer domains and the particles to characterize the dielectric properties of the composite. A finite difference time domain (FDTD) technique is then used to simulate the propagation of light through this heterogeneous material. The results of this study allow us to determine how the polymer−particle interactions affect the spatial distribution of fillers within the polymer matrix...

Simulating the morphology and mechanical properties of filled diblock copolymers.

We couple a morphological study of a mixture of diblock copolymers and spherical nanoparticles with a micromechanical simulation to determine how the spatial distribution of the particles affects the mechanical behavior of the composite. The morphological studies are conducted through a hybrid technique, which combines a Cahn-Hilliard (CH) theory for the diblocks and a Brownian dynamics (BD) for the particles. Through these "CH-BD" calculations, we obtain the late-stage morphology of the diblock-particle mixtures. The output of this CH-BD model serves as the input to the lattice spring model (LSM), which consists of a three-dimensional network of springs. In particular, the location of the different phases is mapped onto the LSM lattice and the appropriate force constants are assigned to the LSM bonds. A stress is applied to the LSM lattice, and we calculate the local strain fields and overall elastic response of the material. We find that the confinement of nanoparticles within a given domain of a bicontinous diblock mesophase causes the particles to percolate and form essentially a rigid backbone throughout the material. This continuous distribution of fillers significantly increases the reinforcement efficiency of the nanoparticles and dramatically increases the Young's modulus of the material. By integrating the morphological and mechanical models, we can isolate how modifications in physical characteristics of the particles and diblocks affect both the structure of the mixture and the macroscopic behavior of the composite. Thus, we can establish how choices made in the components affect the ultimate performance of the material.