Alpha-cyclodextrin-induced self-assembly of a double-hydrophilic block copolymer in aqueous solution

Double-hydrophilic poly(ethylene oxide)-b-poly(acrylic acid) (PEO-b-PAA) self-assembled into nanostructures in basic solution upon the addition of alpha-cyclodextrin (alpha-CD) as a result of the complexation between alpha-CD and PEO. The nanostructures produced were spherical in shape as observed by transmission electron microscopy (TEM) and possessed radii that were much larger than that of a single stretched polymeric chain. The ratio of Rg/Rh (where Rg is the z-average radius of gyration and Rh is the hydrodynamic radius) obtained from laser light scattering (LLS) was approximately approximately 1.0, and the aggregation number was approximately 4100. The zeta-potential of complex particle was -45 mV, suggesting that the particle possessed a stable negatively charged surface, attributed to ionized PAA segments. The above results suggested that the nanostructures formed in the PEO-b-PAA/alpha-CD solution at high pH were likely to be spherical vesicles.     

Pegylated thermally responsive block copolymer micelles and nanogels via in situ RAFT aqueous dispersion polymerization

A very straightforward approach was developed to synthesize pegylated thermoresponsive core‐shell nanoparticles in a minimum of steps, directly in water. It is based on RAFT‐controlled radical crosslinking copolymerization of N,N‐diethylacrylamide (DEAAm) and N,N′‐methylene bisacrylamide (MBA) in aqueous dispersion polymerization. Because DEAAm is water‐soluble and poly(N,N‐diethylacrylamide) (PDEAAm) exhibits a lower critical solution temperature at 32 °C, the initial medium was homogeneous, whereas the polymer formed a separate phase at the reaction temperature. The first macroRAFT agent was a surface‐active trithiocarbonate based on a hydrophilic poly(ethylene oxide) block and a hydrophobic dodecyl chain. It was further extented with N,N‐dimethylacrylamide (DMAAm) to target macroRAFT agents with increasing chain length. All macroRAFT agents provided excellent control over the aqueous dispersion homopolymerization of DEAAm. When they were used in the radical crosslinking copolymerization of DEAAm and MBA, the stability and size of the resulting gel particles were found to depend strongly on the chain length of the macroRAFT agent, on the concentrations of both the monomer and the crosslinker, and on the process (one step or two steps). The best‐suited experimental conditions to reach thermosensitive hydrogels with nanometric size and well‐defined surface properties were determined. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2373–2390, 2009     

Perhydroxycucurbit[6]uril-induced self-assembly of a double-hydrophilic block copolymer in aqueous solution

We reported the preparation of the poly(N-vinyl pyrrolidone)-b-polyethylene glycol (PVP-b-PEG) double-hydrophilic block copolymer by atom transfer radical polymerization (ATRP). The PVP-b-PEG samples were characterized by nuclear magnetic resonance spectroscopy and Fourier translation infrared spectrum. By taking advantage of its exceptional binding affinity, the perhydroxycucurbit[6]uril ((HO)₁₂CB[6]) was used to interact with PVP-b-PEG by hydrogen bond and constructed micelle-like core–shell aggregates with the physical cross-link PVP segments as the core and the extending PEG chains as the outer palisade. The size of aggregates was about 50–300 nm. The particle size of this type of assemblies could be controlled by the weight content of (HO)₁₂CB[6]. The size distribution was broad at high concentrations. The broad size distribution at high (HO)₁₂CB[6] concentration was due to the coexistence of isolated particles and clusters. These novel micelle-like core–shell aggregates could be controlled by pH value. At pH lower than 6 or pH larger than 13, hydrogen bond interactions were destroyed and the formed (HO)₁₂CB[6]-induced PVP-b-PEG assemblies could be destructed.     

In situ SAXS studies of a prototypical RAFT aqueous dispersion polymerization formulation: monitoring the evolution in copolymer morphology during polymerization-induced self-assembly

Small-angle X-ray scattering (SAXS) is used to characterize the in situ formation of diblock copolymer spheres, worms and vesicles during reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate at 70 °C using a poly(glycerol monomethacrylate) steric stabilizer. ¹H NMR spectroscopy indicates more than 99% HPMA conversion within 80 min, while transmission electron microscopy and dynamic light scattering studies are consistent with the final morphology being pure vesicles. Analysis of time-resolved SAXS patterns for this prototypical polymerization-induced self-assembly (PISA) formulation enables the evolution in copolymer morphology, particle diameter, mean aggregation number, solvent volume fraction, surface density of copolymer chains and their mean inter-chain separation distance at the nanoparticle surface to be monitored. Furthermore, the change in vesicle diameter and membrane thickness during the final stages of polymerization supports an ‘inward growth’ mechanism.

In situ small-angle X-ray scattering is used to monitor the formation of diblock copolymer spheres, worms and vesicles during reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate.     

Synthesis and self-assembly of a triarm star-shaped rod-rod block copolymer

We designed and synthesized a triarm star-shaped rod-rod block copolymer(BCP),(poly{2,5-bis[(4-methoxyphenyl)-oxycarbonyl]styrene}-block-poly(γ-benzyl-L-glutamate))3,(PMPCS-b-PBLG)3. The triarm core with three PMPCS-N3 segments was prepared by copper-mediated atom transfer radical polymerization of 2,5-bis[(4-methoxyphenyl)-oxycarbonyl]styrene initiated by a trifunctional initiator and a subsequent azide reaction. And the PBLG block with alkyne functionality was synthesized through ring-opening polymerization of γ-benzyl-L-glutamate N-carboxyanhydride initiated by propargylamine. Finally, Huisgen's 1,3-dipolar cycloaddition was employed to combine the triarm(PMPCS-N3)3 and PBLG segments. The chemical structure of the BCP was confirmed by 1H-NMR spectroscopy, Fourier-transform infrared spectroscopy, and gel permeation chromatographic analysis. Results from differential scanning calorimetry, polarized light microscopy, one-dimensional and two-dimensional wide-angle X-ray diffraction, and transmission electron microscopy techniques demonstrate that the triarm star-shaped rod-rod BCP self-assembles into a hexagon-in-lamella morphology, with the PMPCS block in the columnar nematic phase and the PBLG block in the hexagonal columnar arrangement packed in bilayers due to the rigid nature of the two blocks and the covalent connections in the star-shaped BCP.     

Miniemulsion polymerization of styrene with a block copolymer surfactant

A series of miniemulsion systems based on styrene/azobisisobutyronitrile in the presence of poly(methyl methacrylate‐b‐2‐(dimethylamino)ethyl methacrylate) as a surfactant and hexadecane (HD) as a cosurfactant were developed. For comparison, a series of pseudoconventional emulsions also were carried out with the same procedure used for the aforementioned series but without the cosurfactant (HD). Both the droplet size and shelf life were also measured. Experimental results indicate that it is possible to slow the effect of Ostwald ripening and thereby produce a stable miniemulsion with the block copolymer as the surfactant and HD as the cosurfactant. In addition, the extent to which varying the surfactant concentration and copolymer composition could affect both the polymer particle size during the polymerization and the polymerization rate was examined. Variation in the polymer particle sizes during polymerization indicates that droplet and aqueous (micellar or both homogeneous) nucleation occurs in the miniemulsion polymerization. With the same concentration of the surfactant used in the miniemulsion polymerization, the polymerization rates of systems with M₁₂B₃₆ are faster than those of systems with M₁₂B₁₂. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1818–1827, 2000     

Fabrication of cage-like particles via unstable seeded dispersion polymerization: A new concept in the polymerization-induced self-assembly

The formation mechanism of hollow micron-sized polystyrene (PS) particles having numerous dents on the surface, so-called cage-like particles, obtained from seeded dispersion polymerization (SDP) of 2-ethylhexyl methacrylate (EHMA) with low molecular weight (MW) PS particles stabilized by poly(vinyl alcohol) (PVA) in the presence of hexadecane droplets was investigated. It was found that association of poly(2-ethylhexyl methacrylate) (PEHMA)/hexadecane phases which occurs due to the instability of the obtained composite particles followed by a diffusion of PS ellipsoidal particles into each other is the main process responsible for the production of such unique morphology. Time course monitoring of the SDP showed that diffusion of hexadecane and/or PS and/or PEHMA phase into PS/PEHMA/hexadecane composite particles through PS shell which happens based on Ostwald ripening is the main phenomenon which results in the formation of the dents on the surface of final particles. Moreover, the experimental results revealed that in this reaction system, the polymerization develops in a faster manner rather than the SDP employing seed particles having higher MWs. Furthermore, it was observed that particles with different surface morphologies can be produced by using different hydrocarbons. The elimination of small particles which are produced in addition to the cage-like ones via decreasing the concentration of the stabilizer was another interesting finding of this research. The acquired results showed that unstable SDP is expected to be a new concept in polymerization-induced self-assembly (PISA) which employs instability of a dispersion for self-assembly of polymeric particles, and therefore, production of polymeric unique objects.     

In situ synthesis of nano‐assemblies of the high molecular weight ferrocene‐containing block copolymer via dispersion RAFT polymerization

The in situ synthesis of the nano‐assemblies of the high molecular weight ferrocene‐containing block copolymer of poly(ethylene glycol)‐block‐poly(4‐vinylbenzyl ferrocenecarboxylate) (PEG‐b‐PVFC) via dispersion reversible addition‐fragmentation chain transfer (RAFT) polymerization was discussed. Taking the advantage of the accelerated polymerization rate of the dispersion RAFT polymerization, the nano‐objects of the well‐defined PEG‐b‐PVFC diblock copolymer with the polymerization degree (DP) of the ferrocene‐containing PVFC block up to 300 were prepared. It was found that the morphology of the PEG‐b‐PVFC diblock copolymer nano‐assemblies was dependent on the DP of the PEG and PVFC blocks, and nanospheres favorably formed in the case of the long PEG block and vesicles containing a thick and porous membrane were formed in the case of the short PEG block and long PVFC block, respectively. Our results demonstrate that the dispersion RAFT polymerization is an effective way to prepare the high molecular weight ferrocene‐containing block copolymer with interesting morphologies. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 900–909     

The role of cooling rate in crystallization-driven block copolymer self-assembly

Self-assembly of crystalline-coil block copolymers (BCPs) in selective solvents is often carried out by heating the mixture until the sample appears to dissolve and then allowing the solution to cool back to room temperature. In self-seeding experiments, some crystallites persist during sample annealing and nucleate the growth of core-crystalline micelles upon cooling. There is evidence in the literature that the nature of the self-assembled structures formed is independent of the annealing time at a particular temperature. There are, however, no systematic studies of how the rate of cooling affects self-assembly. We examine three systems based upon poly(ferrocenyldimethylsilane) BCPs that generated uniform micelles under typical conditions where cooling took pace on the 1–2 h time scale. For example, several of the systems generated elongated 1D micelles of uniform length under these slow cooling conditions. When subjected to rapid cooling (on the time scale of a few minutes or faster), branched structures were obtained. Variation of the cooling rate led to a variation in the size and degree of branching of some of the structures examined. These changes can be explained in terms of the high degree of supersaturation that occurs when unimer solutions at high temperature are suddenly cooled. Enhanced nucleation, seed aggregation, and selective growth of the species of lowest solubility contribute to branching. Cooling rate becomes another tool for manipulating crystallization-driven self-assembly and controlling micelle morphologies.

In the self-assembly of crystalline-coil block copolymers in solution, heating followed by different cooling rates can lead to different structures.     

Macromolecular self-assembly in dilute sequence-controlled block copolymer/homopolymer blends

Conventional block copolymers consist of two long contiguous monomer sequences (‘blocks’) that can, in the same fashion as low-molar-mass surfactants, self-assemble into various microstructural elements (e.g., micelles at low copolymer concentrations) to minimize repulsive contacts in the presence of a parent homopolymer. In this work, we explore the existence of segment-specific interactions, as well as the possibility of tailoring these blend morphologies (and producing altogether new ones), with novel sequence-controlled block copolymers. These copolymers are comprised of at least one block that is a random segment composed of both constituent monomer species. Transmission electron microscopy is employed here to examine the bilayered membranes and channel structures that form in two different series of such copolymers in dilute copolymer/homopolymer blends.     

A new paradigm for protein design and biological self-assembly

Very few molecules with biological origins contain the element fluorine. Nature's inability to incorporate fluorine into biomolecules is related to the low concentration of free fluoride in sea and surface water. However, judicious introduction of fluorine into proteins, nucleic acids, lipids and carbohydrates has allowed mechanistic scrutiny of enzyme catalysis, control of protein oligomerization in membranes, clustered display of ligands on surfaces of living cells, and in increasing the protease stability of protein and peptide therapeutics.     

Preparation of block copolymer vesicles in solution

Block copolymer vesicles can be prepared in solution from a variety of different amphiphilic systems. Polystyrene‐block‐poly(acrylic acid), polystyrene‐block‐poly(ethylene oxide), and many other block copolymer systems can produce vesicles of a wide range of sizes; those in the range of 100–1000 nm have been explored extensively. Different factors, such as the absolute and relative block lengths, the presence of additives (ions, homopolymers, and surfactants), the water content in the solvent mixture, the nature and composition of the solvent, the temperature, and the polydispersity of the hydrophilic block, provide control over the types of vesicles produced. Their high stability, resistance to many external stimuli, and ability to package both hydrophilic and hydrophobic compounds make them excellent candidates for use in the medical, pharmaceutical, and environmental fields. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 923–938, 2004     

Kinetically constrained block copolymer self-assembly a simple method to control domain size

In this work asymmetric polystyrene-block-polyethylene oxide (PS-PEO) diblock copolymers were blended with high and low molecular polystyrene (PS) homopolymer and spin cast, resulting in the rapid self-assembly of vertically oriented PEO cylinders in a matrix of PS. Due to the kinetically constrained phase separation of the system, increasing addition of homopolymer is shown to reduce the diameter of the PEO domains, even when the homopolymer was of significantly higher molecular weight than the PS block in the PS-PEO diblock copolymer and would be predicted to macro-phase separate from the copolymer. The outcomes of this study provide a novel method that requires the adjustment of a single variable to tune the size of vertically oriented PEO domains between 10 and 100 nm, with potential applications in a number of areas including membrane technologies.     

Nanopatterning of substrates by self-assembly in supramolecular block copolymer monolayer films

Many technological applications require templates with nanoscale patterns.Block copolymer self-assembly is a method of choice for obtaining a large variety of such patterns,with greatest flexibility achieved when combined with a supramolecular approach.One of the ways to fabricate block copolymer templates is the Langmuir-Blodgett (LB) technique.Here,we briefly summarize recent work with LB films of polystyrene-poly(4-vinyl pyridine) (PS-P4VP) mixed with 3-n-pentade cylphenol (PDP),illustrating the different types of patterns possible and the principles governing them.One interesting pattern that can be easily achieved with this system is the so-called nanostrand network,which,when used as a template for gold deposition,can produce double striped lines of gold.Here,we show how this pattern can be modified by acetone swelling to give rise to gold monolayer ribbons with internal structure.The results also suggest new insights into the early stages of morphology formation at the air/water interface.     

Aqueous reversible addition‐fragmentation chain transfer dispersion polymerization of thermoresponsive diblock copolymer assemblies: Temperature directed morphology transformations

In this work, we demonstrate the formation of various 3D structures formed by a structural reorganization; a process not governed by self‐assembly. First, we packaged well‐defined diblocks, thermoresponsive poly(N‐isopropylacrylamide‐b‐styrene) or P(NIPAM‐b‐STY), into spherical particles made in situ using a reversible addition‐fragmentation chain transfer (RAFT) nanoreactor technique in water to obtain high polymer solids. The resultant spheres reorganized through a temperature stimulus to form equilibrium and kinetically trapped structures; we denote this process as temperature directed morphology transformation (TDMT). Cylinder and vesicle structures, other more unusual loop, buckled sphere and cauliflower (via ultrasound) structures were observed below the lower critical solution temperature (LCST) of PNIPAM. These structures were produced efficiently, rapidly, reproducibly at high polymer solids and can be stored for years in solution or be freeze‐dried and rehydrated without a change in structure, which becomes important in biological applications. We generated the first phase diagram for the TDMT changes of thermoresponsive diblock micelles in concentrated solutions (> 7% polymer solids) produced from narrow molecular weight diblock copolymers (PDI ～ 1.1). The sphere/cylinder and cylinder/vesicle transition in the phase diagram were surprisingly found to be predominantly dictated by the length of the hydrophilic block, poly(N‐isopropylacrylamide). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Polymerization‐induced self‐assembly of block copolymer through dispersion RAFT polymerization in ionic liquid

Polymerization‐induced self‐assembly of block copolymer through dispersion RAFT polymerization has been demonstrated to be a valid method to prepare block copolymer nano‐objects. However, volatile solvents are generally involved in this preparation. Herein, the in situ synthesis of block copolymer nano‐objects of poly(ethylene glycol)‐block‐polystyrene (PEG‐b‐PS) in the ionic liquid of 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([BMIN][PF₆]) through the macro‐RAFT agent mediated dispersion polymerization is investigated. It is found that the dispersion RAFT polymerization of styrene in the ionic liquid of [BMIN][PF₆] runs faster than that in the alcoholic solvent, and the dispersion RAFT polymerization in the ionic liquid affords good control over the molecular weight and the molecular weight distribution of the PEG‐b‐PS diblock copolymer. The morphology of the in situ synthesized PEG‐b‐PS diblock copolymer nano‐objects, e.g., nanospheres and vesicles, in the ionic liquid is dependent on the polymerization degree of the solvophobic block and the concentration of the fed monomer, which is somewhat similar to those in alcoholic solvent. It is anticipated that the dispersion RAFT polymerization in ionic liquid broads a new way to prepare block copolymer nano‐objects. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1517–1525     

Synthesis,self-assembly and pH sensitivity of a novel fluorinated triphilic block copolymer

A novel fluorinated triblock copolymer incorporating 2-ethylhexyl methacrylate (EHMA),tert-butyl methacrylate (tBMA) and 1H,1H,2H,2H-perfluorodecyl acrylate (FA) (PEHMA-b-PtBMA-b-PFA) was first synthesized using three successive reversible addition fragmentation chain transfer (RAFT) polymerization and the subsequent hydrolyzing at acidic condition.The as-fabricated triblock copolymer exhibited an interesting morphology evolution from the multi-compartment rod-like structure to spherical structure along with the addition of a selective solution.At the same time,a visible phase separation domain could be seen in the core area due to the existence of fluorocarbon segments.Furthermore,the selfassembly behavior of the triphilic copolymer at different pH was also verified by transmission electron microscopy,as well as the dynamic light scattering.These stimuli-responsive multi-compartment nanostructures may have potential applications in drug delivery.     

Oil-induced aggregation of block copolymer in aqueous solution

The oil-induced aggregation behavior of PEO-PPO-PEO Pluronic P84 [(EO)19(PO)39(EO)19] in aqueous solutions has been systematically investigated by 1H NMR spectroscopy, freeze-fracture transmission electron microscopy (FF-TEM), and dynamic light scattering (DLS). The critical micellization temperature (CMT) for P84 in the presence of oils decreases with increasing oil concentration. The effectiveness of various oils in decreasing the CMT of block copolymer follows the order m-xylene (C(8)H(10)) > toluene (C(7)H(8)) > benzene (C(6)H(6)) > n-octane (C(8)H(18)) > n-hexane (C(6)H(14)) approximately cyclohexane (C(6)H(12)). It was found that the amount of anhydrous PO methyl groups increases whereas the amount of hydrated PO methyl groups decreases upon the addition of oils. At low oil concentration, the oil molecules are entrapped by the micellar core, but as the oil concentration increases above a certain value, the micellar core swells significantly as a result of the penetrated oil molecules, and much larger aggregates are formed. Intermolecular rotating-frame nuclear Overhauser effect (ROE) measurements between P84 and benzene were performed at 10 and 40 degrees C. The specific interaction between benzene and the methyl groups of PPO was determined, and it was observed that the interaction site remained unchanged as the temperature was increased.