Self-assembly of block copolymers

Block copolymer (BCP) self-assembly has attracted considerable attention for many decades because it can yield ordered structures in a wide range of morphologies, including spheres, cylinders, bicontinuous structures, lamellae, vesicles, and many other complex or hierarchical assemblies. These aggregates provide potential or practical applications in many fields. The present tutorial review introduces the primary principles of BCP self-assembly in bulk and in solution, by describing experiments, theories, accessible morphologies and morphological transitions, factors affecting the morphology, thermodynamics and kinetics, among others. As one specific example at a more advanced level, BCP vesicles (polymersomes) and their potential applications are discussed in some detail.     

Self-assembly of rod-coil block copolymers

We present a self-consistent field theory model for the self-assembly behavior of rod-coil block copolymers. The orientational interactions between the rods were modeled through a Maier-Saupe interaction, while the enthalpic interactions between rods and coils were modeled through a standard Flory-Huggins approach. We outline a "real-space" numerical approach to solve the self-consistent field equations for such rod-coil block copolymers. A major focus of our work is upon the nonlamellar phases observed in the experiments on such polymers. To develop a physical understanding of these phases and their regimes of occurrence, we compute the two-dimensional phase diagram for our model. The latter shows significant departures from the one-dimensional phase diagram, but matches qualitatively with the existing experimental results. We also present scaling arguments that rationalize the numerical results for the self-assembly behavior.     

Self-assembly of polypeptide-based copolymers into diverse aggregates

Recently, increasing attention has been given to the self-assembly behavior of polypeptide-based copolymers. Polypeptides can serve as either shell-forming or core-forming blocks in the formation of various aggregates. The solubility and rigidity of polypeptide blocks have been found to have a profound effect on the self-assembly behavior of polypeptide-based copolymers. Polypeptide graft copolymers combine the advantages of a grafting strategy and the characteristics of polypeptide chains and their self-assembly behavior can be easily adjusted by choosing different polymer chains and copolymer architectures. Fabricating hierarchical structures is one of the attractive topics of self-assembly research of polypeptide copolymers. These hierarchical structures are promising for use in preparing functional materials and, thus, attract increasing attention. Computer simulations have emerged as powerful tools to investigate the self-assembly behavior of polymers, such as polypeptides. These simulations not only support the experimental results, but also provide information that cannot be directly obtained from experiments. In this feature article, recent advances in both experimental and simulation studies for the self-assembly behavior of polypeptide-based copolymers are reviewed.     

Self-assembly of polypeptide-based block copolymer amphiphiles

The past decade has seen growing interest in the investigation of self-assembling nanostructures, particularly in aqueous solution. In this context, polypeptide-based copolymers show considerable promise as building blocks that allow enhanced control over intra- and intermolecular interactions, in concert with stable, yet modifiable, secondary and tertiary structures. We will focus here on the most recent advances in the formation of micelles and vesicles from peptide–polymer conjugates or from copolypeptide systems, and on the capacity of these structures to manifest stimuli-driven variation in size and shape. We will also discuss a new generation of materials based on protein-like copolymers that offer precise control over molecular composition and structure along with predetermined biological functionality.     

双刚性共轭嵌段共聚物体系的自组装

棒杆-棒杆(rod-rod)共轭嵌段共聚物体系是近几年发展起来的一类新型共轭聚合物材料,由于其特有的电学活性以及通过自组装实现纳米尺度结构可控等特性正逐渐成为人们研究的热点.构筑单元的刚性棒状结构使得rod-rod共轭嵌段共聚物体系倾向于自组装形成囊泡或层状结构等低曲率聚集体.本文总结了近年来关于rod-rod共轭嵌段共聚物体系自组装行为的研究,分别介绍了溶液中以及薄膜状态下双刚性共轭嵌段共聚物体系的自组装行为,在此基础上进一步讨论了rod-rod共轭嵌段共聚物薄膜结构与性能的关系.     

Self-assembly of dendritic-linear block copolymers with fixed molecular weight and block ratio

Block copolymers built on a dendritic block having fixed molecular weight but different number of peripheral PEG chains were synthesised. At a fixed block ratio, these block copolymers exhibited topological transition of cylindrical micelles depending on the number of PEG chains.     

Biohybrid block copolymers: towards functional micelles and vesicles

This critical review covers the elaboration of micelles and vesicles made from block copolymers containing peptide or oligonucleotide blocks with a focus on recent developments toward responsive and functional assemblies (166 references).     

Self-assembly of block copolymer micelles in an ionic liquid

Four amphiphilic poly((1,2-butadiene)-block-ethylene oxide) (PB-PEO) diblock copolymers were shown to aggregate strongly and form micelles in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)]). The universal micellar structures (spherical micelle, wormlike micelle, and bilayered vesicle) were all accessed by varying the length of the corona block while holding the core block constant. The nanostructures of the PB-PEO micelles formed in an ionic liquid were directly visualized by cryogenic transmission electron microscopy (cryo-TEM). Detailed micelle structural information was extracted from both cryo-TEM and dynamic light scattering measurements, with excellent agreement between the two techniques. Compared to aqueous solutions of the same copolymers, [BMIM][PF(6)] solutions exhibit some distinct features, such as temperature-independent micellar morphologies between 25 and 100 degrees C. As in aqueous solutions, significant nonergodicity effects were also observed. This work demonstrates the flexibility of amphiphilic block copolymers for controlling nanostructure in an ionic liquid, with potential applications in many arenas.     

Cross-linked block copolymer micelles: functional nanostructures of great potential and versatility

Supramolecular self assembly techniques have provided a versatile means by which to selectively assemble polymer molecules into well-defined three dimensional core-shell nanostructures. The covalent stabilisation and tailoring of these dynamic nanostructures can be achieved using a range of chemistries within the assembly to afford robust functional nanoparticles. Many examples of the stabilisation, functionalisation and decoration of these nanoparticles have been reported in the literature and this tutorial review will focus on these recent developments and highlight their potential applications.     

Self-assembly behavior of carbon nanotubes modified by amphiphilic block copolymer

Block copolymers of poly(tert-butyl methyacrylate) (PtBMA) and polystyrene (PSt) were grafted onto multi-walled carbon nanotubes (MWNTs) by the reaction of azide groups at the copolymer chain end with the surface of MWNTs. After hydrolysis, PtBMA block was transformed to polymethyacrylic acid (PMAA) block, and amphiphilic diblock copolymer-modified MWNTs were finally obtained. The modified MWNTs were characterized by XPS, TGA, FTIR, and Raman, and the results showed that the amphiphilic diblock copolymers were grafted onto MWNTs by the covalent bond. The TEM and SEM observation showed that PMAA-b-PSt copolymer modified MWNTs (S2) formed self-assembly tube bundles with the size up to 20 μm in both ethanol and chloroform. However, PtBMA-b-PSt copolymer modified MWNTs (S1) only formed small-size aggregates or dispersed as single-modified MWNTs. The dispersion stability tests showed that S1 had good dispersion stability in several solvents (water, ethanol, acetone, and chloroform) even after 20 days. Due to the big-size tube bundles formed by self-assemble S2, the dispersion stability of S2 in above all solvents decreased, but it was still much better than that of pristine MWNTs.     

Self-assembly in mixed aqueous solutions of amphiphilic block copolymers and vesicle-forming surfactant

The self-assembly behavior of mixed solutions consisting of poly(isoprene-b-ethylene oxide) (IEO) copolymer micelles and vesicle-forming didodecyldimethylammonium bromide (DDAB) was investigated. Dynamic light scattering indicated the presence of two populations of nanoassemblies in the solutions. By aid of atomic force microscopy, the larger ones were identified as block copolymer modified surfactant vesicles (BCMSVs) and the smaller ones as surfactant-modified block copolymer micelles (SMBCMs). This identification is based on the amphiphilic character of the low and high molecular weight molecules and the notion that exchange of unimers of both types can take place between the initial nanoassemblies in aqueous solution. Electrophoretic light scattering experiments showed that the nanostructures carry positive charges originating from the surfactant. The sizes of the nanoassemblies depend on the relative concentrations of both components. The behavior of the mixed systems was also found to depend on block copolymer composition and temperature. Nanoassemblies of smaller sizes were formed at higher temperatures. BCMSVs and SMBCMs are thermosensitive, in contrast to the temperature stability of pure block copolymer micelles. On the other hand, BCMSVs showed lesser sensitivity to temperature increase compared to the pure DDAB vesicles. This indicates that incorporation of macromolecules into the DDAB bilayer increases the stability of the vesicles.     

Preparation of reactive block copolymers and their transformation to hollowed nanostructures

A novel polymeric hollow nanostructure was generated using micellar template method through a three‐step procedure. First, the block copolymers were synthesized via ring‐opening metathesis polymerization by sequentially adding monomers 7‐oxanorborn‐5‐ene‐exo‐exo‐2,3‐dicarboxylic acid dimethyl ester and the mixture of norbornene and 2,3‐bis(2‐bromoisobutyryloxymethyl)‐5‐norbornene in chloroform, and also atom transfer radical polymerization of 4‐(3‐butenyl)styrene was carried out by using the as‐obtained block copolymer poly(7‐oxanorborn‐5‐ene‐exo,exo‐2,3‐dicarboxylic acid dimethylester)‐block‐poly(norbornene‐co‐2,3‐bis(2‐bromoisobutyryloxymethyl)‐5‐norbornene as macroinitiator to afford a graft copolymer bearing poly(4‐(3‐butenyl)styrene) branch poly(7‐oxanorborn‐5‐ene‐exo,exo‐2,3‐dicarboxylic acid dimethylester)‐block‐poly(norbornene‐co‐2,3‐bis(2‐bromoisobutyryloxymethyl)‐5‐norbornene)‐graft‐poly(4‐(3‐butenyl)styrene). Second, the shell‐crosslinked micelles were prepared by ruthenium‐mediated ring‐closing metathesis of poly(4‐(3‐butenyl)styrene) branches in intramicelle formed from the copolymers self‐assembly spontaneously in toluene. Finally, the hollowed spherical nanoparticles were presented by removing the micellar copolymer backbone through the cleavage of the ester bonds away from the crosslinked network of branches. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Composite block copolymer stabilized nanoparticles: simultaneous encapsulation of organic actives and inorganic nanostructures

We describe the preparation and characterization of hybrid block copolymer nanoparticles (NPs) for use as multimodal carriers for drugs and imaging agents. Stable, water-soluble, biocompatible poly(ethylene glycol)-block-poly(epsilon-caprolactone) NPs simultaneously co-encapsulating hydrophobic organic actives (beta-carotene) and inorganic imaging nanostructures (Au) are prepared using the flash nanoprecipitation process in a multi-inlet vortex mixer. These composite nanoparticles (CNPs) are produced with tunable sizes between 75 nm and 275 nm, narrow particle size distributions, high encapsulation efficiencies, specified component compositions, and long-term stability. The process is tunable and flexible because it relies on the control of mixing and aggregation timescales. It is anticipated that the technique can be applied to a variety of hydrophobic active compounds, fluorescent dyes, and inorganic nanostructures, yielding CNPs for combined therapy and multimodal imaging applications.     

Covalent stabilization of nanostructures: Robust block copolymer templates from novel thermoreactive systems

Structurally robust block copolymer templates with feature sizes of approximately 10 nm were prepared from functionalized poly(methyl methacrylate)‐b‐polystyrene block copolymers. By the inclusion of benzocyclobutene crosslinking groups in the polystyrene block, the covalent stabilization of thin films to both thermal treatment and solvent exposure became possible. In addition, the crosslinking of the poly(styrene‐benzocyclobutene) domains at 220 °C, followed by the removal of poly(methyl methacrylate), provided a robust, crosslinked nanostructure with greater processing and fabrication potential. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1028–1037, 2005     

Self-assembly of stimuli-responsive water-soluble [60]fullerene end-capped ampholytic block copolymer

A well-defined, water-soluble, pH and temperature stimuli-responsive [60]fullerene (C(60)) containing ampholytic block copolymer of poly((methacrylic acid)-block-(2-(dimethylamino)ethyl methacrylate))-block-C(60) (P(MAA-b-DMAEMA)-b-C(60)) was synthesized by the atom transfer radical polymerization (ATRP) technique. The self-assembly behavior of the C(60) containing polyampholyte in aqueous solution was characterized by potentiometric and conductometric titration, dynamic light scattering (DLS), and transmission electron microscopy. This amphiphilic mono-C(60) end-capped block copolymer shows enhanced solubility in aqueous medium at room and elevated temperatures and at low and high pH but phase separates at intermediate pH between 5.4 and 8.8. The self-assembly of the copolymer is different from that of P(MAA-b-DMAEMA). Examination of the association behavior using DLS revealed the coexistence of unimers and aggregates at low pH at all temperatures studied, with the association being driven by the balance of hydrophobic and electrostatic interactions. Unimers and aggregates of different microstructures are also observed at high pH and at temperatures below the lower critical solution temperature (LCST) of PDMAEMA. At high pH and at temperatures above the LCST of PDMAEMA, the formation of micelles and aggregates coexisting in solution is driven by the combination of hydrophobic, electrostatic, and charge-transfer interactions.     

Self-assembly of inorganic nanoparticle vesicles and tubules driven by tethered linear block copolymers

Controllable self-assembly of nanoscale building blocks into larger specific structures provides an effective route for the fabrication of new materials with unique optical, electronic, and magnetic properties. The ability of nanoparticles (NPs) to self-assemble like molecules is opening new research frontiers in nanoscience and nanotechnology. We present a new class of amphiphilic "colloidal molecules" (ACMs) composed of inorganic NPs tethered with amphiphilic linear block copolymers (BCPs). Driven by the conformational changes of tethered BCP chains, such ACMs can self-assemble into well-defined vesicular and tubular nanostructures comprising a monolayer shell of hexagonally packed NPs in selective solvents. The morphologies and geometries of these assemblies can be controlled by the size of NPs and molecular weight of BCPs. Our approach also allows us to control the interparticle distance, thus fine-tuning the plasmonic properties of the assemblies of metal NPs. This strategy provides a general means to design new building blocks for assembling novel functional materials and devices.     

Self-assembly morphology effects on the crystallization of semicrystalline block copolymer thin film

Self-assembly morphology effects on the crystalline behavior of asymmetric semicrystalline block copolymer polystyrene-block-poly(L-lactic acid) thin film were investigated. Firstly, a series of distinctive self-assembly aggregates, from spherical to ellipsoid and rhombic lamellar micelles (two different kinds of rhombic micelles, defined as rhomb 1 and rhomb 2) was prepared by means of promoting the solvent selectivity. Then, the effects of these self-assembly aggregates on crystallization at the early stage of film evolution were investigated by in situ hot stage atomic force microscopy. Heterogeneous nucleation initiated from the spherical micelles and dendrites with flat on crystals appeared with increasing temperature. At high temperature, protruding structures were observed due to the thickening of the flat-on crystals and finally more thermodynamically stable crystallization formed. Annealing the rhombic lamellar micelles resulted in different phenomena. Turtle-shell-like crystalline structure initiated from the periphery of the rhombic micelle 1 and spread over the whole film surface in the presence of mostly noncrystalline domain interior. Erosion and small hole appeared at the surface of the rhombic lamellar micelle 2; no crystallization like that in rhomb 1 occurred. It indicated that the chain-folding degree was different in these two micelles, which resulted in different annealing behaviors.     

Induction of periodic nanostructures in polythiophene films through block copolymer templating

A new class of periodically nanostructured polythiophene materials with high regularity and numerous morphologies is prepared through the cooperative self‐assembly of polythiophene derivatives with a templating block copolymer (BCP) and poly(1,4‐isoprene)‐block‐poly(methacrylic acid) (PMA). The selection of the hydrophilic and aprotic triethylene glycol (TEG) group as side chains on polythiophene and the use of hydrophilic and protic PMA are crucial to producing well‐ordered nanostructures in polythiophene films, as it enables selective coassembly within the hydrophilic domain through hydrogen bonding. The composite films are shown to have formed hexagonally packed cylinders with 28 nm periodicities based on small‐angle X‐ray scattering and transmission electron microscopy. The formation of hydrogen bonding is revealed by a shift in the carbonyl peak of PMA in the Fourier transform infrared spectra of the composite film relative to the neat film. This suggests that the TEG‐functionalized polythiophene selectively incorporates into PMA. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 1105–1112     

Temperature‐dependent location of a weakly segregated block copolymer in binary blends of block copolymers

The phase behaviors of binary blends of poly(styrene‐b‐butadiene) block copolymers were investigated by a small‐angle X‐ray scattering technique. The blends were composed of weakly segregated one in a random micellar phase and the other in a cylindrical phase with similar molecular weights and complementary volume fractions. Morphologies, domain spacings, and order–disorder transition temperatures of the blends indicated that the junctions of the constituent block copolymers share the interface at low temperatures. The domain spacing decreased as temperature increased in a blend with a small amount of the weakly segregated block copolymer. In the cases of the blends with a large amount of the weakly segregated constituent, domain spacing increased with increasing temperature. These results implied that some of the weakly segregated block copolymer moved from the interface to one microdomain at higher temperatures. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 470–476