Expulsion of Unimers from Polystyrene‐block‐poly(acrylic acid) Micelles

Summary: The pH response of the micelles of polystyrene‐block‐poly(acrylic acid) (PS₂₀₀‐b‐PAA₇₈) in water is studied using a combination of techniques: static light scattering (SLS), dynamic light scattering (DLS), and transmission electron microscopy (TEM). The structure of the micelles in dilute aqueous solution is dependent on pH. At pH values <2.5, the micelles precipitate. At pH values from 2.5 to 3.5, the micelles associate to form micellar clusters. At pH values ranging from 3.5 to 8.0, the micelles are dynamically frozen. At pH > 8.1, some PS₂₀₀‐b‐PAA₇₈ unimers gradually escape from the micelles and subsequently re‐associate to form smaller micelles.

The pH‐responsive behavior of the PS₂₀₀‐b‐PAA₇₈ micelles in solution.     

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.     

Amphiphilic PEG‐b‐PMCL‐b‐PDMAEMA Triblock Copolymers: From Synthesis to Physico‐Chemistry of Self‐Assembled Structures

A synthetic route toward a new family of amphiphilic mPEG‐b‐PMCL‐b‐PDMAEMA triblock copolymers is reported. Chemical structures and compositions are confirmed by ¹H NMR and SEC. Polydispersity indices are typically <1.4, indicating good control of the reactions. The physicochemical parameters associated with mPEG‐b‐PMCL‐b‐PDMAEMA self‐assembled structures are investigated. Nanoparticles are prepared via a co‐solvent method, and parameters such as nanoparticle $\overline {M} _{{\rm w}} $ , N_agg, A₂, and R_h are calculated based on static and dynamic light scattering data. Critical aggregation concentrations for the polymers are determined by measuring surface tensions of polymer solutions. TEM is employed to visualize the morphology of the assemblies.

     

pH‐Sensitive Micelles Formed by Interchain Hydrogen Bonding of Poly(methacrylic acid)‐block‐Poly(ethylene oxide) Copolymers

pH‐sensitive micelles formed by interchain hydrogen bonding of poly(methacrylic acid)‐block‐poly(ethylene oxide) copolymers were prepared and investigated at pH < 5. Both and R_h of the micelles increase with decreasing pH of the solution, displaying an asymptotic tendency at low pH values. The observed micelles are well‐defined nanoparticles with narrow size distributions (polydispersity ΔR_h/R_h ≤ 0.05) comparable with regular diblock copolymer micelles. The CMCs occur slightly below c = 1 × 10^?4 g · mL^?1. The micelles are negatively charged and their time stability is lower than that of regular copolymer micelles based purely on hydrophobic interactions.

     

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).     

Random Poly(methyl methacrylate‐co‐styrene) Brushes by ATRP to Create Neutral Surfaces for Block Copolymer Self‐Assembly

Random copolymer brushes of styrene and methyl methacrylate (MMA) on silicon wafers by atom transfer radical polymerization (ATRP) are synthesized using CuCl/CuCl₂/HMTETA. It is found that with increasing amount of styrene the thickness of the brush layer could no longer be well controlled by the amount of free (sacrificial) initiator in the reaction. At constant concentration of free initiator a constant thickness is obtained for various ratios of MMA to styrene. Within 30–70% MMA in the monomer feed the composition of the free polymer corresponds well to the monomer feed ratio, displaying a water contact angle in agreement with the theoretical value for a random copolymer. These copolymers are shown to create a neutral surface directing spin‐coated poly(styrene‐b‐MMA) into a perpendicular lamellae orientation.     

Poly(2‐vinylnaphthalene)‐block‐poly(acrylic acid) Block Copolymer: Self‐Assembled Pattern Formation,Alignment, and Transfer into Silicon via Plasma Etching

P2VN‐b‐PAA is a novel diblock copolymer which has potential as a self‐assembled nanoscale patterning material. Thin spin cast P2VN‐b‐PAA films rapidly reorganize to vertical lamellar with exposure to acetone vapor. P2VN‐b‐PAA lamellar morphology was aligned by electric field under acetone vapor at a significantly faster rate and at lower electric field strengths than other polymer systems. Observed dry etching selectivity for P2VN to PAA were comparatively high for a variety of etch gases, consistent with estimations from Ohnishi and ring parameters. Block copolymer self assembled patterns were transferred to silicon via two‐step CF₄ and SF₆ etching.

     

Self‐Assembled Structures in Block Copolymer Blends: SAXS,SANS and TEM Study

Phenomena associated with the order‐disorder transition, microdomain morphology and phase behavior of a deuterated block copolymer (BCP) blend I / II (where I is dPS‐block‐dPMMA and II is dPS‐block‐PI) were studied by SAXS, SANS and TEM. The studied, almost symmetric, copolymers differ essentially in microdomain morphology. One of them ( I ) is in disordered microdomain state, while the other ( II ) displays lamellar morphology at ordinary temperatures. Self‐assembled structures in blends were investigated as a function of concentration of the added microphase‐separated copolymer and temperature. The ODT positions were located in all the blends, the position of ODT depending only slightly on the concentration of the ordered copolymer. A systematic increase in long period D of the lamellar phase is observed with the growing content of the disordered copolymer. The evaluation of TEM shows the gradual diminishing of macrophase separated regions of disordered copolymer I with growing content of the lamellar copolymer II .

     

Self‐Assembly of Carbon Nanotubes Modified by Amphiphilic Block Polymers in Selective Solvent

Block copolymers of polystyrene and poly(tert‐butyl methyacrylate) were prepared by ATRP. Halogen atoms at the chain ends were transformed into azide groups to obtain  N₃ terminated block copolymers, which were connected to the surface of multi‐walled carbon nanotubes (MWNTs) by reacting  N₃ with MWNT's surface. Amphiphilic diblock copolymer modified MWNTs were obtained after PtBMA blocks were hydrolyzed to polymethyacrylic acid (PMAA). Results showed that the amphiphilic diblock copolymer was grafted onto MWNTs by covalent bonds. TEM showed that they formed self‐assembly structures by hydrophilic/hydrophobic interaction in good solvents. As the block length of PMAA increased, the self‐assembly structures of amphiphilic MWNTs became increasingly ordered and uniform.

     

Solvent‐Vapor‐Induced Rapid Assembly of Block‐Copolymer Film via Prevacuumizing

The undesired long time for the self‐assembly of block‐copolymer (BCP) thin films restricts their application as a template in lithography and other technologies. To shorten the assembly time, a facile but versatile strategy of solvent‐vapor‐induced rapid assembly into a uniform ordered morphology of polystyrene‐block‐poly(methyl methacrylate) (PS‐b‐PMMA) thin film via prevacuumizing is reported. Factors such as the prevacuum pressure and the temperature during the solvent‐vapor‐annealing process are investigated for their effects on the assembly time. The morphologies are observed by transmission electronic microscopy (TEM) and the results indicate that the time for the assembly of PS‐b‐PMMA with a PS‐cylinder‐forming composition into a morphology of hexagonally arranged PS spheres in the film, induced by the solvent vapor at a prevacuum pressure 0.02 atm, is only 4 min at 20 °C and shortens more to 1 min at 60 °C.

     

Poly(ethylene oxide)‐ and Poly(perfluorohexylethyl methacrylate)‐Containing Amphiphilic Block Copolymers: Association Properties in Aqueous Solution

Amphiphilic di‐ and triblock copolymers containing poly(ethylene oxide) (PEO) as the hydrophilic block and poly(perfluorohexylethyl methacrylate) (PFMA) as the hydrophobic block were synthesized by atom‐transfer radical polymerization using hydroxy‐terminated PEO as the macroinitiator. The copolymers were characterized by size exclusion chromatography and ¹H NMR spectroscopy. Self‐association in aqueous solution has been investigated using surface tension measurements, dynamic light scattering (DLS), and transmission electron microscopy (TEM). From surface tension measurements in water, a characteristic concentration (c*) can be detected, which is interpreted as the critical micelle concentration (cmc). The cmc decreases with an increase in fluoro content in the triblock copolymer up to 11 wt.‐% PFMA (solubility limit). DLS studies have been carried out for different samples above the cmc, showing small aggregates (micelles) and single chains for diblock copolymer solutions. In the case of triblock copolymers large clusters were the dominant aggregates in addition to the micelles and single chains. The effect of temperature and concentration on the micelle and cluster formation has been investigated by DLS. Micelle size was found to be resistant to any change by temperature, however, a slight but significant increase in apparent hydrodynamic radius was observed with an increase in concentration, while both temperature and concentration affected the formation of large clusters, especially in concentrated solutions. TEM has been carried out to visualize the morphology of the aggregates after transferring the solution to carbon film. The initial concentration for the preparation of TEM samples was found to have a strong influence on the morphology of the aggregates. By adding colloidal gold particles to the solutions, the typical covering by the polymer was observed by TEM.

Decay‐rate distributions for PEO₁₀F5 (4.0 g · L^?1); obtained from the time correlation functions.     

Mesh‐Like Vesicles Formed From Blends of Poly(4‐vinyl pyridine)‐b‐polystyrene‐b‐poly(4‐vinyl pyridine) Block Copolymers via Gradual Blending Method

In this study, we developed a novel blending strategy, namely, the gradual blending method, to tune the micellar structure. Different from the most commonly used premixing blending method, which different block copolymers are premixed in a common solvent before their individual self‐assembly, the gradual blending method involves gradually adding one type of block copolymer into the pre‐generated micellar solution formed from another type of block copolymer. Moreover, we obtained a novel mesh‐like vesicle from the self‐assembly of the mixtures of P4VP₄₃‐b‐PS₂₆₀‐b‐P4VP₄₃ and P4VP₄₃‐b‐PS₃₆₆‐b‐P4VP₄₃ in 1,4‐dioxane/water solution using the gradual blending method.     

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.     

Synthesis and Self‐Assembly of Functionalized Cyclooctene Block Copolymers via ROMP

A novel block copolymer consisted of MACIT and HCO segments was synthesized via ROMP in the ionic liquid [bmim][PF₆] with good control over the polymerization process. The molecular weight of the block copolymer was estimated by ¹H NMR, and the molar composition ratio of repeating units in the MACIT block to those in the HCO block was 100:120 (120:120 in feed). The micellar characterization was carried out by DLS, AFM, and TEM. The hydrodynamic diameter of the micelles, measured by DLS, was 129 ± 0.09 nm with a narrow distribution (PDI = 0.034). The TEM image showed spherical micelles.

     

Block Copolymer Micelles with an Intermediate Star‐/Flower‐Like Structure Studied by 1H NMR Relaxometry

¹H NMR relaxation is used to study the self‐assembly of a double thermoresponsive diblock copolymer in dilute aqueous solution. Above the first transition temperature, at which aggregation into micellar structures is observed, the trimethylsilyl (TMS)‐labeled end group attached to the shell‐forming block shows a biphasic T₂ relaxation. The slow contribution reflects the TMS groups located at the periphery of the hydrophilic shell, in agreement with a star‐like micelle. The fast T₂ contribution corresponds to the TMS groups, which fold back toward the hydrophobic core, reflecting a flower‐like micelle. These results confirm the formation of block copolymer micelles of an intermediate nature (i.e., of partial flower‐like and star‐like character), in which a part of the TMS end groups folds back to the core due to hydrophobic interactions.

     

Biocompatible Poly(D,L‐lactide)‐block‐Poly(2‐methacryloyloxyethylphosphorylcholine) Micelles for Drug Delivery

Well‐defined amphiphilic PLA‐b‐PMPC diblock copolymers were synthesized. Bimimetic micelles were prepared and applied for release of anti‐cancer drugs (DOX). TEM and DLS analysis revealed a regular spherical shape with small diameter (less than 50 nm) of the micelle. The biocompatibility of PLA‐b‐PMPC micelles was studied, and it was found that the micelles possessed excellent cytocompatibility due to the zwitterionic phosphorylcholine group. DOX could be efficiently loaded into the micelles with a loading efficiency of 44–67%. The DOX‐loaded micelles showed lower cytotoxicity than free drugs and efficiently delivered and released the drug into cancer cells. With these properties, the PLA‐b‐PMPC polymer micelles are attractive as drug carriers for pharmaceutical application.

     

Multivalent (Nitrilotriacetic Acid)‐End‐Functionalized Polystyrenes by ATRP and Their Self‐Assembly

The synthesis of tri‐(nitrilotriacetic acid) (NTA)‐end‐functionalized polystyrenes using an initiator containing tert‐butyl protected NTA moieties, by atom transfer radical polymerization (ATRP) of styrene is described. First, a suitable ATRP initiator is prepared and subsequently characterized by ¹H and ¹³C NMR spectroscopy, gel‐permeation chromatography (GPC), and matrix‐assisted laser desorption ionization time‐of‐flight (MALDI‐TOF) mass spectro­scopy. The structures of the tri‐NTA‐end‐functionalized polystyrenes (tri‐NTA‐PS) are confirmed by ¹H and ¹³C NMR spectroscopy. Tri‐NTA‐PS itself produces self‐assembled spherical aggregates with ≈40–60 nm diameters in water/THF, whereas nickel‐complexed tri‐NTA‐PS produces spherical core–shell hybrid aggregates with ≈90–115 nm diameters with His‐tagged GFP in water/DMF (DMF 4 vol%), as confirmed by transmission electron microscopy and dynamic light scattering measurements.

     

Effect of the End‐Positioning of a Lithium Sulfonate Group on the Aggregation and Micellization Behavior of ω‐Lithium Sulfonate Polystyrene‐block‐polyisoprenes

Summary: ω‐Lithium sulfonate polystyrene‐block‐polyisoprenes were synthesized by anionic polymerization high vacuum techniques and a post‐polymerization reaction with 1,1‐diphenylethylene and 1,3‐propane sultone. The solution properties of ω‐lithium sulfonate block copolymers were studied in a low polarity solvent (CCl₄) which is a good solvent for both blocks. The study revealed the formation of aggregates due to the association of the lithium sulfonate groups (SuLi). The mass, size and shape of the aggregates were studied by static and dynamic light scattering and viscometry. The aggregation number decreased as the length of the chain increased and the aggregates showed a behavior similar to that of star polymers. The micellization of ω‐functionalized block copolymers was also studied in a polar selective solvent for polystyrene, N,N‐dimethylacetamide (DMA). Micelles formed from block copolymers containing the sulfonate group at the polyisoprene end exhibited an association number which was about 80% of that of the neutral block copolymer and had a higher critical micelle concentration. The samples having the polar group at the PS end showed a similar behavior.

The structures of the end‐functionalized block copolymers studied.     

Self‐Assembly of Amphiphilic Block Copolymers in Ternary Solvent Mixtures: Lyotropic Liquid Crystalline Phase Behavior and Structure

The effects of the simultaneous addition of two organic solvents (glycerol and ethanol) on the lyotropic liquid crystalline (LLC) phase behavior and structure formed by PEO–PPO–PEO amphiphilic block copolymers in water are examined. Glycerol and ethanol have been selected because of their opposite effect: structure promoting and structure dissolving, respectively. Lattice parameters of Pluronic P105 (EO₃₇PO₅₈EO₃₇) and F127 (EO₁₀₀PO₇₀EO₁₀₀) LLC samples in the presence of any type and amount of E+G solvents decrease with increasing volume fraction of the PPO‐rich domains. Lattice parameters for the two block copolymers fall on the same line when normalized with the square root of polymer molecular weight, suggesting weak block segregation.     

Synthesis and Self‐Assembly of Novel Amphiphilic Six‐Armed Star Copolymers TP[PDMAEMA‐b‐PSt]6

A triphenylene (TP)‐based hexafunctional initiator was prepared and used in successive ATRP of DMAEMA and St. Well‐defined six‐armed star block copolymers TP[PDMAEMA‐b‐PSt]₆ bearing hydrophilic backbones inside and hydrophobic blocks outside were successfully synthesized. The self‐assembly behaviors of the novel amphiphilic copolymer were further investigated. Co‐existing spherical and bowl‐shaped aggregates were observed from their neutral aqueous solution, while large spherical structures with different dimensions were obtained from their diluted HCl and CF₃COOH aqueous solution, respectively. Dynamic light scattering in different aqueous solutions were conducted to give further confirmation. The possible mechanism of the morphology formation was proposed.