One‐Step Approach for the Preparation of Organic‐Inorganic Janus‐Like Particles by Alkalization of Polystyrene‐block‐Poly(2‐vinylpyridine)/FeCl3 Complex Micelles

A novel approach to organic‐inorganic Janus‐like particles based on the alkalization process of polystyrene‐block‐poly(2‐vinylpyridine) (PS‐b‐P2VP) micelles containing FeCl₃ precursors in the P2VP cores in toluene is presented. It is found that by addition of a small amount of NaOH solution to a solution of the spherical PS‐b‐P2VP/FeCl₃ micelles, organic‐inorganic Janus‐like particles with a produced α‐FeOOH domain on one side and PS‐b‐P2VP block copolymers on the other can be prepared. The Janus‐like nanoparticles obtained by this facile approach may have potential application in biomedical areas.     

Folate‐Conjugated Poly(N‐(2‐hydroxypropyl) methacrylamide)‐block‐Poly(benzyl methacrylate): Synthesis,Self‐Assembly,and Drug Release

Well‐defined amphiphilic diblock copolymers of poly(N‐(2‐hydroxypropyl)methacrylamide)‐block‐poly(benzyl methacrylate) (PHPMA‐b‐PBnMA) are synthesized using reversible addition–fragmentation chain transfer polymerization. The terminal dithiobenzoate groups are converted into carboxylic acids. The copolymers self‐assemble into micelles with a PBnMA core and PHPMA shell. Their mean size is <30 nm, and can be regulated by the length of the hydrophilic chain. The compatibility between the hydrophobic segment and the drug doxorubicin (DOX) affords more interaction of the cores with DOX. Fluorescence spectra are used to determine the critical micelle concentration of the folate‐conjugated amphiphilic block copolymer. Dynamic light scattering measurements reveal the stability of the micelles with or without DOX. Drug release experiments show that the DOX‐loaded micelles are stable under simulated circulation conditions and the DOX can be quickly released under acidic endosome pH.     

Block Copolymer Micelle Nanotubes by the Solvent‐Annealing‐Induced Nanowetting in Anodic Aluminum Oxide Templates

Block copolymer micelles are generally formed by the self‐assembly of amphiphilic copolymer molecules in aqueous medium. Although different types of block copolymer micelles have been studied, the self‐assembly behavior of block copolymer micelles in confined geometries have been rarely studied. In this work, the fabrication of polystyrene‐block‐poly(4‐vinylpyridine) (PS‐b‐P4VP) micelle nanotubes in the cylindrical nanopores of anodic aluminum oxide templates using a solvent‐annealing‐assisted wetting method is presented. The PS‐b‐P4VP chains wet the pore walls in the form of micelles when the sample is annealed in the vapor of toluene, a good solvent for PS and a nonsolvent for P4VP. The formation of the PS‐b‐P4VP micelle nanotubes instead of nanorods implies that the micelles wet the nanopores in the complete wetting regime. This study not only contributes to a deeper understanding of the self‐assembly behavior of block polymer micelles in confined geometries, but also provides more possible variations and design freedoms in the application of block copolymer micelles.

     

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

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

Solution Self‐Assembly of Poly(ferrocenylphenylphosphine)‐block‐polydimethylsiloxane: Formation of Spherical Micelles with an Organometallic Poly(ferrocenylphenylphosphine) Core

The novel organometallic‐inorganic diblock copolymer, poly(ferrocenylphenylphosphine)‐block‐polydimethylsiloxane (PFP‐b‐PDMS), with narrow molecular weight distribution has been synthesized by living anionic polymerization through sequential monomer addition. These block copolymers self‐assemble into “star‐like” spherical micelles in hexane with a dense organometallic PFP core surrounded by a swollen corona of the PDMS chains. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) were used to characterize these micellar aggregates. It was found that the block copolymer micelles have a relatively narrow core size distribution, but an overall broader distribution of hydrodynamic size in hexane. Significantly, the preparation method of the micelle solution was also found to have an influence on the size and size distribution of the resulting micellar structures.     

The Aggregation Behavior of Poly(ethylene oxide)‐Poly(methyl methacrylate) Diblock Copolymers in Organic Solvents

Poly(ethylene oxide)‐poly(methyl methacrylate) and poly(ethylene oxide)‐poly(deuteromethyl methacrylate) block copolymers have been prepared by group transfer polymerization of methyl methacrylate (MMA) and deuteromethyl methacrylate (MMA‐d₈), respectively, using macroinitiators containing poly(ethylene oxide) (PEO). Static and dynamic light scattering and surface tension measurements were used to study the aggregation behavior of PEO‐PMMA diblock copolymers in the solvents tetrahydrofuran (THF), acetone, chloroform, N,N‐dimethylformamide (DMF), 1,4‐dioxane and 2,2,2‐trifluoroethanol. The polymer chains are monomolecularly dissolved in 1,4‐dioxane, but in the other solvents, they form large aggregates. Solutions of partially deuterated and undeuterated PEO‐PMMA block copolymers in THF have been studied by small‐angle neutron scattering (SANS). Generally, large structures were found, which cannot be considered as micelles, but rather fluctuating structures. However, ¹H NMR measurements have shown that the block copolymers form polymolecular micelles in THF solution, but only when large amounts of water are present. The micelles consist of a PMMA core and a PEO shell.     

Tertiary Amine Methacrylate‐Based ABC Triblock Copolymers: Synthesis,Characterization, and Self‐Assembly in both Aqueous and Nonaqueous Media

Water soluble poly[2‐(diisopropylamino)ethyl methacrylate]‐block‐poly[2‐(dimethylamino) ethyl methacrylate]‐block‐poly[2‐(N‐morpholino)ethyl methacrylate] triblock copolymers are synthesized via group transfer polymerization. They are molecularly soluble in acidic solution but give PDPA‐core three‐layer “onion‐like” micelles in alkaline solution. They also give two types of micelles in hexane depending on whether a cosolvent is used: i) PMEMA‐core “onion‐like” reverse micelles are formed with a cosolvent, or, ii) [PDMA‐b‐PMEMA]‐core core–shell micelles without. In addition, novel shell cross‐linked micelles and reverse SCL micelles are also synthesized by cross‐linking the inner PDMA shell of both PDPA‐core micelles in water and PMEMA‐core reverse micelles in hexane.

     

SANS Study of Coated Block Copolymer Micelles

Summary: Multishell particles were prepared by γ‐radiation‐induced polymerization of methyl methacrylate (MMA) in polystyrene‐block‐poly(methacrylic acid) (PS‐b‐PMA) aqueous micellar solution and their structure was studied by small‐angle neutron scattering (SANS). Before polymerization, almost all MMA molecules are distributed in aqueous phase and only 1% of MMA is accumulated inside the micelles. The newly formed polymer (PMMA) is deposited on the surface of PS cores of the original micelles. The effect of the MMA concentration, micelle concentration, absorbed radiation dose, and absorbed dose rate on the characteristics of the resulting particles was examined. The thickness of the PMMA shell (20–218 Å for the presented series of samples) can be easily controlled by variation of monomer and/or micelle concentration. Universal plots of the core volume (PS+PMMA) and SANS curve were presented. These plots facilitate choosing proper monomer and micelle concentrations and detecting possible irregularities in the parameters of the resulting particles.

     

Nanoporous Gold Film Prepared by the Epoxidation of Poly(styrene‐b‐butadiene) Diblock Copolymer Templated Micelles

A new approach is developed for the preparation of nanoporous gold (Au) films using diblock copolymer micelles as templates. Stable Au nanoparticles (NPs) with a narrow distribution are prepared by modifying NPs functionalized with 4‐(dimethylamino)pyridine ligands (DMAP Au NPs) and a spherical micelle formed through the epoxidation of poly(styrene‐b‐butadiene) diblock copolymer to produce poly(styrene‐b‐vinyl oxirane) (PS‐b‐PBO) in tetrahydrofuran–acetonitrile solution. The exchange reaction of 4‐aminothiophenol of PS‐b‐PBO diblock copolymer micelles with DMAP Au NPs can produce block copolymer–Au NPs composite films. After the pyrolysis of the diblock copolymer templates at a specific temperature to avoid the collapse of the Au NPs, a nanoporous Au film is prepared.     

Disassembly of Crystalline Platelets of an Amphiphilic Triblock Copolymer Mediated by Varying pH and Organic Diacids

Regulation of crystalline micelles is difficult to achieve because of the strong solidification of crystallization. In this present work, an amphiphilic triblock copolymer poly(ethylene oxide)‐b‐poly(ε‐caprolactone)‐b‐poly(4‐vinylpyridine) (PEO‐b‐PCL‐b‐P4VP) is prepared, in which the P4VP block serves as an H‐bonding acceptor. It is originally self‐assembled into crystalline lamellar micelles in aqueous solution. Subsequently, the effect of varying pH and organic diacids on morphological transition is investigated in detail. Lamellae‐to‐cylinder‐to‐sphere transitions are observed after decreasing the pH or with the addition of organic diacids with different chain spacers. The decreasing pH causes increasing hydrophilicity of the P4VP block, while adding organic diacids results in an increasing corona swelling of the P4VP segment, both of which lead to a decreasing crystallinity of the poly(ε‐caprolactone) core. Consequently, morphological variety changing from lamellar to worm‐like to spherical micelles can be achieved.     

Controlled Anionic Block Copolymerization with N,N‐Dialkylacrylamide as a Second Block

Summary: Novel well‐defined block copolymers composed of polystyrene, poly(2‐vinylpyridine), poly(ethylene oxide), or poly(tert‐butyl methacrylate) as the first block and poly(N,N‐dialkylacrylamide) (PDAlAAm) as the second block were synthesized by ligated anionic polymerization. The latter was carried out in tetrahydrofuran (THF) initiated by 1,1‐diphenyloligostyryllithium in the presence of ZnEt₂ and LiCl. At first the role of the additives LiCl and ZnEt₂ on the mode of the anionic homopolymerization of N,N‐dialkylacrylamide was investigated. Polymerization in the presence of ZnEt₂ resulted in syndiotactic polymers with narrow molecular weight distribution only. In the presence of both additives, the reaction mixture became heterogeneous with a high degree of isotacticity of the polymers. Despite the fact that the polymerizations were performed in heterogeneous phase, the DAlAAm monomers were polymerized in a quantitative yield. The efficiency of the first block of active sites was always higher than 0.71. Preliminary studies using dynamic light scattering of aqueous hydrochloric acid solutions of poly[(2‐vinylpyridine)‐block‐(N,N‐diethylacrylamide)] block copolymers at different temperatures and at pH 2 showed that above 45 °C, micelle‐like aggregates were formed. The heating and cooling cycles were reversible but showed hysteresis, which was obviously due to the isotactic structure of the poly(N,N‐diethylacrylamide) block.

Temperature dependence of the scattering intensity of various poly[(2‐vinylpyridine)‐block‐(N,N‐diethylacrylamide)] block copolymers.     

Cover Picture: Macromol. Chem. Phys. 23/2005

Summary: The complexation between polystyrene‐block‐poly(acrylic acid) (PS‐b‐PAA) micelles and poly(ethylene glycol)‐block‐poly(4‐vinyl pyridine) (PEG‐b‐P4VP) is studied and a facile strategy is proposed to prepare three‐layered core–shell–corona micellar complexes. Micellization of PS‐b‐PAA in ethanol gives rise to spherical core–shell micelles with the PS block as the core and the PAA block as the shell. When PEG‐b‐P4VP is added into the core–shell micellar solution, the P4VP block penetrates into the PAA shell and is absorbed into the core–shell micelles to form spherical core–shell–corona micellar complexes with the PS block as the core, the bonded PAA/P4VP blocks as the shell, and the PEG block as the corona. The core radius, R_c, of the core–shell–corona micellar complexes, 15.1 nm, is equal to that of the core–shell micelles, and the thickness, D_s, of the PEG corona is about 1.5 nm, while the shell thickness, D_s, ranges from 12.2 to 15.7 nm depending on the weight ratio of PEG‐b‐P4VP to PS‐b‐PAA.

Schematic formation of core–shell–corona micellar complexes.     

Polystyrene‐block‐Polyisoprene Diblock‐Copolymer Micelles: Coupled Pressure and Temperature Effects

The structural changes of diblock‐copolymer micelles under pressures from 200 to 16 000 psi are investigated using small‐angle neutron scattering (SANS). Asymmetric polystyrene‐block‐polyisoprene (PS–PI) diblock copolymers are dissolved in decane, a selective solvent for PI, to form spherical micelles with a core of PS and a corona of PI. The micellar solutions are put under pressure at temperatures of 25 to 60 °C. At room temperature, elevating the pressure from 200 to 16 000 psi has no effect on the size of the micelles. While the micellar solutions remain stable, instantaneous association of micelles is detected. In contrast to micelles at atmospheric pressure, increasing the temperature at elevated pressures does not lead to dissociation of micelles; instead, the micelles aggregate and evolve into sheet‐like structures, reminiscent of a macroscopic phase separation. Furthermore, higher pressures lead to a smaller temperature range in which shape transitions take place.

     

Spherical Compound Micelles with Lamellar Stripes Self‐Assembled from Star Liquid Crystalline Diblock Copolymers in Solution

Detailed investigations on the self‐assembly of amphiphilic star block copolymers composed of three‐arm poly(ethylene oxide) (PEO) and poly(methacrylate) (PMAAz) with an azobenzene side chain (denoted as 3PEO‐b‐PMAAz) into stable spherical aggregates with clear lamellar stripes in solution are demonstrated. Four block copolymers, 3PEO₁₂‐b‐PMA(Az)₃₃, 3PEO₂₂‐b‐PMA(Az)₃₁, 3PEO₂₂‐b‐PMA(Az)₆₂, and linear PEO₆₈‐b‐PMA(Az)₃₁, are synthesized. The liquid crystalline properties of the block copolymers are studied by differential scanning calorimetry, polarized optical microscopy techniques, and wide‐angle X‐ray diffraction. The morphologies of the compound micelles self‐assembled in tetrahydrofuran (THF)/water mixtures are observed by means of transmission electron microscopy and scanning electron microscopy. The size of the spherical micelles is influenced by the self‐assembly conditions and the lengths of two blocks. The well‐defined three‐arm architecture of the hydrophilic blocks is a key structural element to the formation of stable spherical compound micelles. The micelle surface integrity is affected by the lengths of PEO blocks. The lamellar stripes are clearly observed on these micelles. This work provides a promising strategy to prepare functional stable spherical compound micelles self‐assembled by amphiphilic block copolymers in solution.     

Crystallization in ABC Triblock Copolymers with Two Different Crystalline End Blocks: Influence of Confinement on Self‐Nucleation Behavior

The influence of different confinements active during crystallization within polybutadiene‐block‐polyisoprene‐block‐poly(ethylene oxide) (PB‐b‐PI‐b‐PEO) and the corresponding hydrogenated polyethylene‐block‐poly(ethylene‐alt‐propylene)‐block‐poly(ethylene oxide) (PE‐b‐PEP‐b‐PEO) triblock copolymers on the self‐nucleation behavior of the crystallizable PEO and PE blocks is investigated by means of differential scanning calorimetry (DSC). In triblock copolymers with PEO contents ≤ 20 wt.‐% crystallization of PEO is confined within small isolated microdomains (spheres or cylinders), and PEO crystallization takes place exclusively at high supercoolings. Self‐nucleation experiments reveal an anomalous behavior in comparison to the classical self‐nucleation behavior found in semicrystalline homopolymers. In these systems, domain II (exclusive self‐nucleation domain) vanishes, and self‐nucleation can only take place at lower temperatures in domain III_SA, when annealing is already active. The self‐nucleation behavior of the PE blocks is significantly different compared with that of the PEO blocks. Regardless of the low PE content (10–25 wt.‐%) in the investigated PE‐b‐PEP‐b‐PEO triblock copolymers a classical self‐nucleation behavior is observed, i.e., all three self‐nucleation domains, usually present in crystallizable homopolymers, can be located. This is a direct result of the small segmental interaction parameter of the PEP and PE segments in the melt. As a consequence, crystallization of PE occurs without confinement from a homogeneous mixture of PE and PEP segments.

Self‐nucleation regimes of a block copolymer showing confined crystallization by means of DSC.     

Micro‐ and Macrophase Separation in Phenolic Resol Resin/PEO‐PPO‐PEO Block Copolymer Blends: Effect of Hydrogen‐Bonded PEO Length

We demonstrate microphase‐separated thermosets based on blends of phenolic resol resin and poly(ethylene oxide)‐block‐poly(propylene oxide)‐block‐poly(ethylene oxide) (PEO‐PPO‐PEO), i. e., so‐called Pluronics. Three triblock copolymers are used (PE 9200, PE 10300 and PE 9400) where the molecular weights of the PPO blocks are nearly equal and the weight fractions of the PEO blocks f_PEO are 0.20, 0.30 and, 0.40, respectively. The blends are prepared in a particularly straightforward way using aqueous solutions and thermal crosslinking. Structure formation is characterized using transmission electron microscopy and small‐angle X‐ray scattering. PPO turns out to be sufficiently repulsive to allow microphase separation in the bulk crosslinked phase and the tendency for macrophase separation upon curing is reduced due to the hydrogen bonding between the PEO and resol. The weight fraction of PEO‐PPO‐PEO in the present blends has been limited to a relatively small value, i. e., 20 wt.‐%, and spherical microphase‐separated structure is observed for f_PEO = 0.40 with a long period of the order 120 Å. Macrophase separation manifests upon curing if the weight fraction of the PEO blocks is smaller, i. e., f_PEO = 0.30 or f_PEO = 0.20. In addition, in order to prevent macrophase separation, the molecular weights of PEO blocks and resol resin before curing are of the same order. In that respect, the system behaves qualitatively similar to the corresponding thermoplastic homopolymer/block copolymer blends.     

A Novel Route for the Preparation of Discrete Nanostructured Carbons from Block Copolymers with Polystyrene Segments

Well‐defined nanostructured carbon was prepared by pyrolysis of core cross‐linked micelles formed from block copolymers containing polystyrene segments. These micelles were obtained by self‐assembly of poly(ethylene oxide)‐block‐polystyrene (PEO₁₁₃‐b‐PS₅₂) diblock copolymer or brush macromolecules containing polymethacrylate backbone and side chains with PS and poly(acrylic acid) block segments (PBPEM₃₃₀‐g‐PS₄₀‐b‐PAA₁₁₁) in selective solvents. UV irradiation was used to induce cross‐linking of the PS core in the micelles. After pyrolysis, the cross‐linked PS cores were converted to a partially graphitic carbon, while the shells were sacrificed, resulting in the formation of discrete carbon nano‐objects. The formation of carbon material was confirmed by Raman scattering spectroscopy while the morphology of the precursor and the resulting pyrolyzed product was studied by atomic force microscope (AFM).

     

Tunable Morphologies From Bulk Self‐Assemblies of Poly(acryloxypropyl triethoxysilane)‐b‐poly(styrene)‐b‐poly(acryloxypropyl triethoxysilane) Triblock Copolymers

Reactive poly(acryloxypropyl triethoxysilane)‐b‐poly(styrene)‐b‐poly(acryloxypropyl triethoxysilane) (PAPTES‐b‐PS‐b‐PAPTES) triblock copolymers are prepared through nitroxide‐mediated polymerization (NMP). The bulk morphologies formed by this class of copolymers cast into films are examined by small‐angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM). The films morphology can be tuned from spherical structures to lamellar structures by increasing the volume fraction of PS in the copolymer. Thermal annealing at temperatures above 100 °C provides sufficient PS mobility to improve ordering.     

Functionalized Nanoporous Thin Films From Blends of Block Copolymers and Homopolymers Interacting via Hydrogen Bonding

Blends of poly(acrylic acid)‐block‐polystyrene (PAA‐b‐PS) copolymers and poly(ethylene oxide) (PEO) homopolymers in which PAA and PEO interact via hydrogen bonds were used as precursors of nanoporous PS thin films with cavities decorated by PAA blocks. The presence of free carboxylic acid groups inside the pores was evidenced by fluorescence spectroscopy after reacting them with a diazomethane functionalized fluorescent dye. These nanoporous thin films were then used as templates for the preparation of dense arrays of silica nanodots.     

Self‐Assembly of a Hydrophobic Polypeptide Containing a Short Hydrophilic Middle Segment: Vesicles to Large Compound Micelles

This report describes a facile route to prepare the vesicles and large compound micelles (LCMs) from a series of poly(ε‐benzyloxycarbonyl L ‐lysine)‐block‐poly[diethylene glycol bis(3‐amino propyl) ether]‐block‐poly(ε‐benzyloxycarbonyl L ‐lysine) (PZLL‐DGBE‐PZLL) in their water solution, depending on molecular weight of the polypeptides. A pyrene probe is used to demonstrate the aggregate formation of PZLL‐DGBE‐PZLL in solution, and also to measure their critical micelle concentration as a function of molecular weight of the polymer. Transmission electron microscopy, atomic force microscopy, dynamic light scattering and confocal laser scanning microscopy are used to observe their aggregate morphologies. Rhodamine B is used as a fluorescent probe to confirm the structure of large compound micelles composed of many reverse micelles with aqueous cores. These polypeptides are prepared by ring‐opening polymerization of α‐amino acid N‐carboxyanhydrides with a small molecule as the initiator. Their structures are confirmed by NMR and SEC‐MALLS. These vesicles and large compound micelles are extremely expected to be used in drug delivery.