Self-assembly of block copolymers

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The spontaneous formation of nanostructured materials by molecular self-assembly of block copolymers is an active area of research, driven both by its inherent beauty and by a wealth of potential technological applications. The so-called “supramolecular” structures can be used to build functional materials with nanoscopic dimensions, such as sensors for biochips or smart drug delivery vehicles. Block copolymer vesicles have attracted increasing interest, particularly in view of possible applications in drug delivery and in protein reconstitution. Conventional methods utilizing synthetic lipid membranes for protein functionality assays have yielded much information with respect to the membrane protein behavior. Furthermore, amphiphilic block copolymer membrane, allowing proper protein refolding while preserving protein function, have been developed to improve the efficiency of these proteins in robust devices. Even if spherical structures are still the most common supramolecular structures generated by self-assembly of block copolymers, a remarkable variety of other morphologies have now been demonstrated, such as rod-like micelles and nanotubes. Soft nanotubes made from biocompatible organic molecules and polymers could find applications in biotechnology and medicine. However, polymer hollow tubes are rare and have so far only been described in organic solvents where their fabrication often requires elaborate procedures. Recently, a series of ABA triblock copolymer composed of poly(dimethylsiloxane)-block- poly(2-methyloxazoline)-block- poly(dimethylsiloxane) (PMOXA-b-PDMS-b-PMOXA) able to mimic biomembranes has been introduced. The PMOXA blocks have hydroxyl end groups that allow functionalization with methacrylic acid. In aqueous solution, the triblock macromonomers form supramolecular assemblies that can be chemically cross-linked by polymerization of the methacrylic acid groups. Established and new preparation methods have been used to prepare superstructures in water with various hydrophobic-hydrophilic ratios of the PMOXA-b-PDMS-b- PMOXA ABA-triblock copolymers. Suitable choice of the block lengths and preparation method allowed controlling the shape of the self-assemblies. We have particularly introduced a preparation method for nanovesicles using detergents and bio-beads that is a suitable alternative devoid of organic solvents leading to improved reconstitution of functional membrane proteins. We have also developed a simple method for the preparation of soft, water-filled nanotubes via self-assembly of PMOXA-b-PDMS-b-PMOXA in aqueous media. Polymer nanotubes have been loaded with water-soluble substances and used as highly specific templates for inorganic synthesis.

In the past several decades, the self-assembly of block copolymers in selective solvents has attracted extensive interest due to the formation of various aggregates including spherical and cylindrical micelles, vesicles, tubes, helices, toroids, and other complex forms. Research has been focused on aggregates with spherical, flowerlike, tubular or sheet-like superstructures, obtained through the hierarchical self-assembly of Janus micelles or Janus nanoparticles, owning to their formation mechanisms and potential applications in biomedical materials and new nanodevices. The self-assembled aggregates can also be constructed from amphiphilic comb-like graft copolymers in water or organic solvents. Despite that many interesting aggregates including petal-like micelles, spindle-like micelles, wormlike micelles, chiral helices, and so on, are obtained by the change of structural and environmental parameters, in most cases conventional spherical micelles and vesicles are observed. In comparison with block copolymers, self-assembled aggregates of amphiphilic comb-like graft copolymers reveal the morphological characteristics of obviously low diversity and complexity. Considering the shape plays a crucial role in determining physical and chemical properties of aggregates, the controlled fabrication and switch of morphologies have been paid a great attention, which has been implemented by regulating the conditions, such as solvent, pH, redox, and so on. Typically, crystallization is an important factor utilized to control the self-assembly of semicrystalline block copolymers consisting of the crystallizable block and amorphous block, which has been actively researched recently. Inspired by this specific class of copolymer, we have designed an amphiphilic comb-like graft copolymer consisting of poly(p-dioxanone) (PPDO) as a crystallizable hydrophobic side chain and poly(vinyl alcohol) (PVA) as a hydrophilic main chain, and found that this copolymer presented an unique self-assembled behavior. In fact, recently we reported a “star anise”-like nanoaggregate from a PPDO-based branched alternating multi-block copolymer. Herein, by directly dispersing this PPDO-based amphiphilic comb-like graft copolymer into water, more complex and regular aggregates with a well-defined snowflake-like superstructure was obtained, and their origin involving a dynamically disorder–order change from nanoto submicroscale was visualized. Moreover, the aggregates showed thermally induced multimorphological evolution from a snowflake-like to cluster-like structure. As an original and significant work, this superstructure enriches the self-assembled morphology of amphiphiles, especially comb-like macromolecules. Furthermore, the direct water phase self-assembled strategy of PPDO-based amphiphilic copolymers and temperature-adjusted morphological change could provide a new idea to construct complex multimorphological and multiscale objects. The precursors of the copolymers were synthesized through ring-opening polymerization and the acylation, respectively. The copolymers were prepared in DMSO by a coupling reaction between carboxyl groups presenting on PPDO chain-end and hydroxyl groups of PVA (Scheme 1). The information of the molecular structure of copolymers is listed in the Supporting Information, Table S1. The self-assembly was achieved by adding PVA500-g5.6%PPDO15 to water at room temperature, heating the system to homogenous phase (to erase the thermal history), and aging it at 25 8C. Tests of dynamic light scattering (DLS) indicated the presence of monodispersed and stable particles with 791 nm of intensity-averaged hydrodynamic diameter ( ) and a polydispersity index (PDI) of 0.053 (Figure 1a, their and PDI after 7 days was shown in the Supporting Information, Figure S1). Transmission electron microscopy (TEM) images revealed well-defined snowflakelike aggregates having about 260 nm of average thickness, 530 nm of average width, and 620 nm of average length, respectively (marked by short arrows in Figure 1band 1c). For monodisperse spherical aggregates, they have same diffusion coefficient in every direction, and their size from CONTIN analysis is independent of scattering angles. Conversely, for the anisotropic or polydisperse aggregates, the results of DLS are dependent of scattering angles. By multi-angle DLS experiment, we found that the diameter and apparent [a] G. Wu, Dr. S.-C. Chen, Dr. X.-L. Wang, Dr. K.-K. Yang, Prof. Y.-Z. Wang National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan) State Key Laboratory of Polymer Materials Engineering College of Chemistry, Sichuan University 29 Wangjiang Road, Chengdu 610064 (P.R. China) Fax: (+86)28-85410259 E-mail : chensichong@scu.edu.cn yzwang@scu.edu.cn Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201103961.

The self-assembly of block copolymers has attracted attention for many decades because it can yield polymeric nanoobjects with a wide range of morphologies. Membrane emulsification is a fairly novel technique for preparation of various types of emulsions, which relies on the dispersed phase passing through a membrane in order to effect droplet formation. In this study, we have prepared polymeric nanoparticles of different morphologies using self-assembly of asymmetric block copolymers in connection with membrane emulsification. Shirasu Porous Glass (SPG) membranes has been employed as the membrane emulsification equipment, and poly (oligoethylene glycol acrylate)-block-poly (styrene) (POEGA-b-PSt) copolymers prepared via RAFT polymerization. It has been found that a number of different morphologies can be achieved using this novel technique, including spheres, rods, and vesicles. Interestingly, the results have shown that the morphology can be controlled not only by adjusting experimental parameters specific to the membrane emulsification step such as membrane pore size and pressure, but also by changing the nature of organic solvent. As such, this method provides a novel route to these interesting nanoobjects, with interesting prospects in terms of exercising morphology control without altering the nature of the block copolymer itself.

Self-assembly of block copolymers has been identified as a potential candidate for high density fabrication of nanostructures. However, the factors affecting its reliability and reproducibility as a patterning technique on various kinds of surfaces are not well-established. Studies pertaining to block copolymer self-assembly have been confined to ultra-flat substrates without taking into consideration the effect of surface roughness. Here, we show that a slight change in the angstrom-scale roughness arising from the surface of a material creates a profound effect on the self-assembly of polystyrene-polydimethylsiloxane block copolymer. Its self-assembly was found to be dependent on both the root mean square roughness (Rrms) of the surface and the type of solvent annealing system used. It was observed that surface with Rrms< 5.0 Å showed self-assembly. Above this value, the kinetic hindrance posed by the surface roughness on the block copolymer leads to its conforming to the surface without observable phase separation.

Mesogen-jacketed liquid crystal like block copolymers: synthesis and self-assembly to prepare 1D PC thin films

A diblock copolymer system constituting both achiral and chiral blocks, polystyrene-block-poly(L-lactide) (PS-PLLA), was designed for the examination of chiral effects on the self-assembly of block copolymers (BCPs). A unique phase with three-dimensional hexagonally packed PLLA helices in PS matrix, a helical phase (H*), can be obtained from the self-assembly of PS-rich PS-PLLA with volume fraction of PLLA f PLLAv = 0.34, whereas no such phase was found in racemic polystyrene-block-poly(D.L-lactide) (PS-PLA) BCPs. Moreover, various interesting crystalline PS-PLLA nanostructures can be obtained by controlling the crystallization temperature of PLLA (T(c,PLLA) ), leading to the formation of crystalline helices (PLLA crystallization directed by helical confined microdomain) and crystalline cylinders (phase transformation of helical nanostructure dictated by crystallization) when T(c,PLLA) < T(g,PS) (the glass transition temperature of PS) and T(c,PLLA) ≧ T(g,PS) , respectively. As a result, a spring-like behavior of the helical nanostructure can be driven by crystallization so as to dictate the transformation (i.e., stretching) of helices and to result in crystalline cylinders. For PS-PLLA with PLLA-rich fraction (f PLLAv = 0.65), another unique phase, a hexagonally packed core-shell cylinder phase with helical sense (CS*), in which the PS microdomains appear as shells and PLLA microdomains appear as matrix and cores, can be found in the self-assembly of PLLA-rich PS-PLLA BCPs. The formation of those novel phases: helix and core-shell cylinder is attributed to the chiral effect on the self-assembly of BCPs, so we named this PS-PLLA BCP as chiral BCP (BCP*). For potential applications of those materials, the spring-like behavior with thermal reversibility might provide a method for the design of switchable nanodevices, such as nanoscale actuators. In addition, the PLLA blocks can be hydrolyzed. After hydrolysis, helical nanoporous PS bulk and PS tubular texture can be obtained and used as templates for the formation of nanocomposites.

Understanding self-assembly behavior and resulting morphologies in block co-polymer films is an essential aspect of chemistry and materials science. Although the self-assembly of amorphous coil-coil block co-polymers is relatively well understood, that of semicrystalline block co-polymers where each block has distinct crystallization properties remains unclear. Here, we report a detailed study to elucidate the rich self-assembly behavior of conjugated thiophene-selenophene (P3AT- b-P3AS) block co-polymers. Using a combination of microscopy and synchrotron-based X-ray techniques, we show that three different film morphologies, denoted as lamellae, co-crystallized fibers, and patchy fibers, arise from the self-assembly of these block co-polymers over a relatively narrow range of overall degrees of polymerization (30 < N < 90). Crystallization-driven phase separation occurs at a very low N (<35), and lamellar films are formed. Conversely, at medium N (50-60) and high N (>80), the thiophene and selenophene blocks co-crystallize into nanofibers, where medium N leads to much more mixing than high N. The overall tendency for phase separation in these systems follows rather different trends than phase separation in amorphous polymers in that we observe the greatest degree of phase separation at the lowest N. Finally, we demonstrate how each morphology influences transport properties in organic thin-film transistors comprised of these conjugated polymers.

Solution self-assembly of amphiphilic block copolymers into inverse bicontinuous cubic mesophases is an emerging strategy for directly creating highly ordered triply periodic porous polymer nanostructures with large pore networks and desired surface functionalities. Although there have been recent reports on the formation of highly ordered triply periodic minimal surfaces of self-assembled block copolymer bilayers, the structural requirements for block copolymers in order to facilitate the preferential formation of such inverse mesophases in solution have not been fully investigated. In this study, we synthesized a series of model block copolymers, namely, branched poly(ethylene glycol)-block-polystyrene (bPEG-PS), to investigate the effect of the architecture of the block copolymers on their solution self-assembly into inverse mesophases consisting of the block copolymer bilayer. On the basis of the results, we suggest that the branched architecture of the hydrophilic block is a crucial structural requirement for the preferential self-assembly of the resulting block copolymers into inverse bicontinuous cubic phases. The internal crystalline lattice of the inverse bicontinuous cubic structure can be controlled via coassembly of branched and linear block copolymers. The results presented here provide design criteria for amphiphilic block copolymers to allow the formation of inverse bicontinuous cubic mesophases in solution. This may contribute to the direct synthesis of well-defined porous polymers with desired crystalline order in the porous networks and surface functionalities.

The solution-based self-assembly of block copolymers (BCPs) into nanoparticulate or microparticulate inverse cubic mesophases, or polymer cubosomes, is of growing interest. This phenomenon could yield new polymeric mesoporous materials with three-dimensionally organized mazes of large water channels. In addition to the ratio of its hydrophilic and hydrophobic blocks, the architecture of a BCP critically influences self-assembly. BCP bilayers having triply periodic minimal surfaces with cubic lattice structures are formed. Here we report the synthesis and self-assembly of BCPs with T8 polyhedral oligomeric silsesquioxane (POSS), an inorganic cage molecule. POSS can be asymmetrically functionalized to host structural modules as branching units of the hydrophilic block that contain various spatial and chemical environments. BCPs, each containing seven poly(ethylene glycol) chains and the hydrophobic polystyrene block PEG3507-POSS-b-PS, were prepared to investigate the effect of the highly branched hydrophilic block architecture on self-assembly. We found that the BCPs self-assembled into polymer cubosomes. Regardless of the block ratio, only the primitive cubic phase (Schwarz P surface, Im3̄m space group) was observed in the cubosomes. This is in contrast to the self-assembly of conventional BCPs into inverse mesophases, in which the internal lattice preferentially transforms into double-diamond lattices to yield a Schwarz D surface as a consequence of block ratio changes. Our results suggest that BCP architecture may be related to the symmetry of the lattice formed via self-assembly in solution.

The effects of chain topology on the self-assembly of block copolymers are examined using an ABAT block copolymer, composed of an AB diblock copolymer with an extra A block tethered onto the B block, as a model system. The topology of the ABAT block copolymer is regulated by the tethering point, such that the block copolymer changes continuously from linear ABA triblock copolymer to A2B miktoarm star copolymer as the tethering position moves from the B end to the AB junction. The phase diagrams of ABAT copolymers of different tethering positions are constructed using the self-consistent field theory. The theoretical results reveal that the phase behavior of the system depends sensitively on the topology of the ABAT copolymers. In particular, a considerably wide stable region of the perforated lamellar (PL) phase is predicted for ABAT with proper tethering positions. The PL phase could even completely replaces the gyroid phase at relatively strong segregation. Furthermore, a large window of the hexagonally...

In this work, we present the synthesis of uniform PMAs, where the number of repeat units and their stereochemical arrangement are precisely defined. Utilizing an iterative convergent approach with orthogonally protected dimandelic acid building blocks, we achieved high molecular weight PMAs with the desired number of repeat units, extending up to 144 mandelic acids. Additionally, stereochemically defined poly(l-mandelic acid)s with up to 32 repeat units were successfully synthesized. These uniform PMAs were subsequently coupled with uniform branched poly(ethylene glycol) blocks to create uniform dendritic-linear block copolymers. The self-assembly of these block copolymers in solution was systematically investigated. In solution self-assembly, the synthesized block copolymers showed multiple phases from cylinder to inverse cubic as the molecular weight of PMA increased. In the case of solvent diffusion-evaporation-mediated self-assembly, the block copolymers underwent a phase transition as the rate of water addition decreased.

Poly(furfuryl isocyanate) (PFIC), which includes the reactive furan group, was synthesized by anionic polymerization using a sodium benzhydroxide (Na-BH), self-assembly initiator. We determined the optimum polymerization conditions by varying both the reaction time and the molar ratio of the monomer to the initiator. Block copolymer, poly(furfuryl isocyanate)-b-poly(n-hexyl isocyanate), was synthesized under optimized polymerization conditions. The PFIC was modified by Diels-Alder reactions with C60 for functionalization. Transmission electron microscopy (TEM) was used to study the self-assembly of block copolymers and modified block copolymer with C60. C60 formed highly ordered aggregates on the PFIC domains via self-assembly of the block copolymer.

New advances into the chirality effect in the self-assembly of block copolymers (BCPs) have been achieved by tuning the helicity of the chiral-core-forming blocks. The chiral BCPs {[N=P(R)-O2C20H12](200-x)[N=P(OC5H4N)2](x)}-b-[N=PMePh]50 ((R)-O2C20H12 = (R)-1,1'-binaphthyl-2,2'-dioxy, OC5H4N = 4-pyridinoxy (OPy); x = 10, 30, 60, 100 for 3 a-d, respectively), in which the [N=P(OPy)2] units are randomly distributed within the chiral block, have been synthesised. The chiroptical properties of the BCPs ([α]D vs. T and CD) demonstrated that the helicity of the BCP chains may be simply controlled by the relative proportion of the chiral and achiral (i.e., [N=P(R)-O2C20H12] and [N=P(OPy)2], respectively) units. Thus, although 3 a only contained only 5% [N=P(OPy)2] units and exhibited a preferential helical sense, 3 d with 50% of this unit adopted non-preferred helical conformations. This gradual variation of the helicity allowed us to examine the chirality effect on the self-assembly of chiral and helical BCPs (i.e., 3 a-c) and chiral but non-helical BCPs (i.e., 3 d). The very significant influence of the helicity on the self-assembly of these materials resulted in a variety of morphologies that extend from helical nanostructures to pearl-necklace aggregates and nanospheres (i.e., 3 b and 3 d, respectively). We also demonstrate that the presence of pyridine moieties in BCPs 3 a-d allows specific decoration with gold nanoparticles.

To construct a smart artificial antioxidative enzyme on a nano-scaffold, a novel method for designing glutathione peroxidase (GPx) active sites on block copolymer vesicles was developed by simple blending predesigned temperature-sensitive block copolymers with the main catalytic units of GPx. A series of functional block copolymers, poly(N-isopropylacrylamide)-b-polyacrylamides loaded with recognition and catalytic sites, were synthesized viaATRP and click chemistry. Through altering the molar ratio of the functional copolymers, the optimum GPx mimic based on copolymer vesicles was obtained by self-assembly of temperature-sensitive block copolymers through a blending process. Significantly, the catalytic activity of the optimum GPx mimic can be well modulated by changing the temperature. It was proved that the change in self-assembly structure of the block copolymer played an important role in the modulation of the catalytic activity. This method not only bodes well for designing smart antioxidative enzyme mimics that could be used in cosmetics as antioxidative additives but also highlights the construction of other regulatory biologically related functional biomaterials.