Polystyrene-block-polyglycidol micelles cross-linked with titanium tetraisopropoxide. laser light and small-angle X-ray scattering studies on their formation in solution

Hybrid micelles from polystyrene-block-polyglycidol (PS-b-PG) copolymers with chemically cross-linked cores by titanium tetraisopropoxide (Ti(OC(3)H(7))(4)) were prepared in toluene solution. Additionally, micellization of PS-b-PG copolymers with different mass fractions of polyglycidol (x(PG)), was studied by static and dynamic light scattering as well as small-angle X-ray scattering. It was observed that copolymers with x(PG) smaller than 0.5 self-assembled in toluene into spherical core-shell micelles with hydrodynamic radii R(h) between 12 and 23 nm. On the other hand, copolymers with larger PG content formed particles with R(h) = 50-70 nm and aggregation numbers of several thousands. The presence of these aggregates in solution was attributed to the nonequilibrated form of block copolymers upon dissolving, most probably due to hydrogen bonding. In the following, spherical PS-b-PG micelles were loaded in toluene with hydrochloric acid and titanium tetraisopropoxide. Confined hydrolysis of Ti(OC(3)H(7))(4) induced by HCl in the micellar core was confirmed by small-angle X-ray scattering experiments. The subsequent condensation of the precursor with hydroxyl groups of polyglycidol chains led to covalently stabilized hybrid organic-inorganic particles. The presence of cross-linked PS-b-PG micelles was proven in two ways. First, micelles with "frozen" core showed stable hydrodynamic size in time upon dilution below critical micellization concentration while non-cross-linked PS-b-PG micelles underwent disintegration under the same conditions within several hours. Second, light scattering experiments revealed the presence of stable, swollen particles in N,N-dimethylformamide, which is a good solvent for both blocks.     

Cadmium sulphide quantum dots in morphologically tunable triblock copolymer aggregates

Cadmium sulfide (CdS) quantum dots (QDs) are formed within poly(ethylene oxide)-block-polystyrene-block-poly (acrylic acid) (PEO-b-PS-b-PAA) triblock copolymer aggregates of different architectures. These structures are obtained starting with the same ionically cross-linked primary micelles consisting of a cadmium acrylate core, a PS shell, and a PEO corona. One morphology is a worm-shaped micelle prepared in tetrahydrofuran (THF) in which the CdS QDs are surrounded by the PAA and aligned as a loose necklace in the PS matrix. The PEO serves as a corona around the PS rod. Another structure is a multicore spherical (ca. 50 nm) water soluble PS micelle, surrounded by PEO chains. The CdS particles within these two latter structures are formed by the reaction of cadmium ions present in the acrylate cores with hydrogen sulfide. In a third structure, the CdS QDs are located on the surface of PS micelles. A fourth spherical single-core micelle structure is postulated to exist in dilute THF solutions. The dimensions in all the aggregates can be controlled by the block length.     

由可控聚合反应直接制备不同形貌的聚集体

由可控聚合,包括活性阴离子和自由基聚合直接制备不同形貌纳米材料,是近几年来合成化学领域的一个重要研究成果.与两亲性嵌段共聚物在选择性溶剂中自组装方法不同,在选择性溶剂中进行的分散聚合,首先生成两亲性嵌段共聚物,并逐渐增加第二段聚合物的链长,以实现相分离,形成球形胶束;聚合物链继续增长,实现形貌转变,从而制备预期的聚合物形貌,包括球形胶束、纳米棒、纳米线、囊泡和复合囊泡等.本文综述了乳液聚合法制备球形胶束等形貌;描述了不同聚合体系形成的形貌以及它们的性质和应用,讨论了形貌的形成机理和控制方法,同时指出了存在的问题.     

双亲半柔性两嵌段共聚物在溶液中自组装形态的模拟退火研究

针对一系列疏溶剂嵌段与溶剂间的相互作用,用模拟退火方法研究了双亲半柔性两嵌段共聚物在溶液中的自组装形态.模拟结果显示共聚物在溶液中均形成核-壳聚集体,其中疏溶剂嵌段形成聚集体的核,亲溶剂嵌段形成聚集体的壳.当上述相互作用较小时,核呈球形,而壳如同长在核上的刺.随着上述相互作用的增大,核逐渐增大.在较大的相互作用时,核呈柱形;而随着相互作用的加大,长在核上的刺逐渐伏贴于核表面.在更大的相互作用时,核又呈球状,壳伏贴于核表面.     

Self-organization of double-C60 end-capped poly(ethylene oxide) in chloronaphthalene and benzene solvent mixtures

Well-defined and narrowly distributed double-C60 end-functionalized poly(ethylene oxide) (C60-PEO-C60) was prepared by reacting azido-terminated PEO with C60. The self-organization behavior of such C60-modified copolymers in different mixtures of chloronaphthalene (CN) and benzene (BN) was studied by a combination of static and dynamic laser light scattering. For C60-PEO-C60 in pure CN, a selective solvent only for C60, self-organization occurred to form a large micelle-like core-shell aggregate, probably with some C60 interlocking with each other with the collapsed PEO chains as the core. The addition of BN, a second selective solvent for core-forming PEO chains, has a significant effect on the structures and compactness of the resultant aggregates because the introduction of BN increases the stretching of the PEO chains inside the core and modifies the interfacial energy of the core-corona interface. On the other hand, for C60-PEO-C60 in pure BN, a non-solvent of C60, several smaller flower-like micelles may self-organize to form a large spherical-like aggregation complex.     

Effect of block length, polydispersity, and salt concentration on PEO-PDEAMA block copolymer structures in dilute solution

A series of poly(ethylene oxide)-block-poly(N,N-diethylaminoethyl methacrylate) (PEO-PDEAMA) block copolymers with relatively high polydispersity (1.36 < PDI < 1.96) have been prepared to determine the effect that polydispersity has on the self-assembly of amphiphilic block copolymers in dilute solution. Because monodisperse macroinitiators were used for the ATRP reactions, the polydispersity resides within the hydrophobic block. By adjusting the relative block lengths, spherical micelles, wormlike micelles, vesicles, or a precipitate is formed. Here, we show that relatively high polydispersity in the block copolymer does not preclude efficient self-assembly. We also discuss the effect of increasing the concentration of NaCl in the systems and show that this can result in a shift from one morphology to another. These shifts are reversible in some cases, but for PEO12-PDEAMA39, this method allows access to giant vesicles of between 500 nm and 1 microm in diameter.     

Synthesis and micelle formation of triblock copolymers of poly(methyl methacrylate)-b-poly(ethylene oxide)-b-poly(methyl methacrylate) in aqueous solution

Amphiphilic triblock copolymers of poly(methyl methacrylate)-b-poly(ethylene oxide)-b-poly(methyl methacrylate) (PMMA-b-PEO-b-PMMA) with well-defined structure were synthesized via atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) initiated by the PEO macroinitiator. The macroinitiator and triblock copolymer with different PMMA and/or PEO block lengths were characterized with ¹H and ¹³C NMR and gel permeation chromatography (GPC). The micelle formed by these triblock copolymers in aqueous solutions was detected by fluorescence excitation and emission spectra of pyrene probe. The critical micelle concentration (CMC) ranged from 0.0019 to 0.016 mg/mL and increased with increasing PMMA block length, while the PEO block length had less effect on the CMC. The partition constant K_v for pyrene in the micelle and in aqueous solution was about 10⁵. The triblock copolymer appeared to form the micelles with hydrophobic PMMA core and hydrophilic PEO loop chain corona. The hydrodynamic radius R_h,app of the micelle measured with dynamic light scattering (DLS) ranged from 17.3 to 24.0 nm and increased with increasing PEO block length to form thicker corona. The spherical shape of the micelle of the triblock copolymers was observed with an atomic force microscope (AFM). Increasing hydrophobic PMMA block length effectively promoted the micelle formation in aqueous solutions, but the micelles were stable even only with short PMMA blocks.     

The Role of inclusion size in toughening of epoxy resins by spherical micelles

We report our finding of an optimal length scale for toughening of epoxies using spherical micelles formed by block copolymers. The amphiphilic diblock copolymer poly(hexylene oxide)‐poly(ethylene oxide) (PHO‐PEO) with 30 wt % PEO self‐assembled to form spherical micelles in a bisphenol A epoxy resin with a phenol novolac hardener. We systematically increased the size of the spherical micelles from 20–30 nm to 0.5–10 μm by swelling their PHO core using PHO homopolymer. Although all the blends were tougher than the unmodified epoxy, the largest enhancement of fracture resistance was measured in blends containing 0.1–1 μm spherical inclusions. This enhanced toughness was correlated with plastic deformation by shear banding in tensile test and greater roughness of the fracture surface. Smaller micelles neither induced plastic deformation nor contributed to surface roughness significantly whereas larger micelles acted as local defects resulting in early failure. These findings provide a framework in assessing the toughening effects of blended block copolymers on epoxy resins. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1125–1129, 2009     

Micellar aggregation in blends of linear and cyclic poly(styrene-b-isoprene) diblock copolymers

The morphology of micelles formed from blends of linear and cyclic poly(styrene-b-isoprene) (PS-b-PI) block copolymers has been investigated in solution using dynamic light scattering (DLS) and in thin solid deposits by atomic force microscopy (AFM) and transmission electron microscopy under cryogenic conditions (cryo-TEM). Micelles of the pure cyclic PS(290)-b-PI(110) copolymers are wormlike cylindrical objects built by unidirectional aggregation of 33 nm wide sunflower micelles, while the linear block copolymer having the same volume fraction and molar mass forms spherical micelles 40 nm in diameter. The DLS, AFM, and cryo-TEM results consistently show that the addition of the linear copolymer (even for amounts as low as 5% w/w) to the cyclic copolymer rather favors the formation of spherical micelles at the expense of the cylindrical aggregates. Those results clearly show that the linear block copolymer chains can be used to stabilize the thermodynamically unstable elementary sunflower micelle. The thermal stability of the micelles (from the pure copolymers and from the blends) has been examined in solid deposits with in situ AFM measurements. Coalescence starts at about 70 degrees C, and the surface roughness shows a two-step decrease toward a fully homogeneous and flat structure.     

水溶液中Pluronic嵌段共聚物聚集行为的介观模拟

通过介观动力学方法(MesoDyn)研究了低浓度下的三嵌段共聚物PEO27PPO61PEO27 (P104)水溶液的聚集行为, 讨论了聚合物浓度、模拟时间对P104水溶液相行为的影响. 在聚合物浓度较低(φ<35%)的情况下, 可以形成三种不同的胶束聚集体：球形胶束(spherical micelle)、胶束簇(micellar cluster)和盘状胶束(disk-like micelle). (1) 球形胶束(5%-10%, φ), 模拟的胶束结构表明疏水的PPO嵌段形成球形内核(micellar core), 而亲水的PEO嵌段形成核壳(micellar corona), 并有水分子存在内核和核壳之中；(2) 胶束簇(11%-15%, φ), 由于球形胶束之间的缔合, 形成直径明显高于球形胶束的聚集体, 其半径比球形胶束大1 nm左右；(3) 盘状胶束(16%-25%, φ), 胶束簇核壳PEO嵌段之间的相互缠绕, 形成了成串的类似盘状的胶束. 模拟中有序参数随浓度的变化证明了这种结构划分的合理性.     

Evaluation of Isoprene Chain Extension from PEO Macromolecular Chain Transfer Agents for the Preparation of Dual, Invertible Block Copolymer Nanoassemblies

Two RAFT-capable PEO macro-CTAs, 2 and 5 kDa, were prepared and used for the polymerization of isoprene which yielded well-defined block copolymers of varied lengths and compositions. GPC analysis of the PEO macro-CTAs and block copolymers showed remaining unreacted PEO macro-CTA. Mathematical deconvolution of the GPC chromatograms allowed for the estimation of the blocking efficiency, about 50% for the 5 kDa PEO macro-CTA and 64% for the 2 kDa CTA. Self assembly of the block copolymers in both water and decane was investigated and the resulting regular and inverse assemblies, respectively, were analyzed with DLS, AFM, and TEM to ascertain their dimensions and properties. Assembly of PEO-b-PIp block copolymers in aqueous solution resulted in well-defined micelles of varying sizes while the assembly in hydrophobic, organic solvent resulted in the formation of different morphologies including large aggregates and well-defined cylindrical and spherical structures.     

Mixed self-assembly of poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers and sodium dodecyl sulfate in aqueous solution

Interaction of amphiphilic poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers with anionic sodium dodecyl sulfate (SDS) has been investigated in aqueous solution. Formation of mixed micelles has been confirmed by surface tension measurements, whereas the influence of the surfactant on the copolymer self-assembling has been studied by measurement of the 1H NMR self-diffusion coefficients and by small-angle neutron scattering. As a rule, the surfactant decreases the heterogeneity of the micellar structures formed by the copolymer in water. Moreover, increasing the content of SDS results in the increasingly more important extension of the poly(ethylene oxide) (PEO) corona chains and the copolymer micelle deaggregation. The stability of the micelles against SDS increases with the length of the hydrophobic block. Preliminary two-dimensional NMR measurements with nuclear Overhauser enhancement have confirmed the spatial vicinity between SDS and the constitutive blocks of the copolymer.     

酶促开环聚合合成双亲性H型嵌段共聚物及其自组装

通过固定化酶Novozyme435（NV435）催化聚乙二醇（PEG）开环聚合己内酯（CL）得到端基带有羟基的ABA型三嵌段聚合物,用2,2-二氯代乙酰氯将聚合物的端羟基功能化形成H型大分子引发剂,在CuCl／HMTETA体系中引发4-乙烯基吡啶（4VP）进行原子转移自由基聚合反应（ATRP）,得到了具有两亲性的H型五嵌段聚合物（PVP）2-b—PCL-b．PEG-PCL-（PVP）2,用红外光谱（FT IR）,核磁共振（^1H NMR）,凝胶渗透色谱（GPC）对其结构与分子量及其分子量分布进行了表征,结果表明：H型五嵌段聚合物分子量46121g／mol,分子量分布1．30．并利用动态光散射（DLS）和原子力显微镜（AFM）对聚合物在水溶液中的自组装行为进行了研究,H型嵌段聚合物的胶束呈球形结构,平均直径为70nm左右．     

Micellization phenomena of biodegradable amphiphilic triblock copolymers consisting of poly(beta-hydroxyalkanoic acid) and poly(ethylene oxide)

This paper reports the studies on micelle formation of new biodegradable amphiphilic poly(ethylene oxide)-poly[(R)-3-hydroxybutyrate]-poly(ethylene oxide) (PEO-PHB-PEO) triblock copolymer with various PHB and PEO block lengths in aqueous solution. Transmission electron microscopy showed that the micelles took an approximately spherical shape with the surrounding diffuse outer shell formed by hydrophilic PEO blocks. The size distribution of the micelles formed by one triblock copolymer was demonstrated by dynamic light scattering technique. The critical micellization phenomena of the copolymers were extensively studied using the pyrene fluorescence dye absorption technique, and the (0,0) band changes of pyrene excitation spectra were used as a probe for the studies. For the copolymers studied in this report, the critical micelle concentrations ranged from 1.3 x 10(-5) to 1.1 x 10(-3) g/mL. For the same PEO block length of 5000, the critical micelle concentrations decreased with an increase in PHB block length, and the change was more significant in the short PHB range. It was found that the micelle formation of the biodegradable amphiphilic triblock copolymers consisting of poly(beta-hydroxyalkanoic acid) and PEO was relatively temperature-insensitive, which is quite different from their counterparts consisting of poly(alpha-hydroxyalkanoic acid) and PEO.     

用SAXS/WAXS研究聚氧乙烯/聚氧丁烯嵌段共聚物在n-己烷中的结晶性球形胶束结构

嵌段共聚物在选择性溶剂中能够自组装形成胶束,胶束的不同形状与嵌段共聚物的结构、溶剂和浓度有关．无定形嵌段共聚物通常形成球形胶束,在某些情况下也可以形成其它形状的胶束,关于结晶性嵌段共聚物在无定形链段选择性溶剂中的胶束结构和形状的报道非常少．由于结晶和相似相溶两种作用力的竞争,使得这类胶束的形状丰富多变．通常结晶作用较强时,结晶性嵌段共聚物形成片状的胶束,当结晶组分比较少时,可形成棒状胶束,尽管理论上已经指出存在球形胶束,但尚无关于这方面的报道。     

Chain-length dependence of diblock copolymer micellization kinetics studied by stopped-flow pH-jump

A series of well-defined poly(ethylene oxide)- b-poly(2-(diethylamino)ethyl methacrylate) (PEO- b-PDEA) diblock copolymers containing PEO block of identical chain length and PDEA block with varying degrees of polymerization (DP, in the range of 32-154) were prepared via atom transfer radical polymerization (ATRP) employing a PEO-based macroinitiator (DP = 113). Upon a pH-jump from 3 to 12 under highly efficient stopped-flow mixing conditions, PEO- b-PDEA copolymers spontaneously form spherical micelles of increasing sizes and aggregation numbers ( N agg) with increasing PDEA chain lengths. Stopped-flow light scattering technique was used to probe the pH-induced micellization kinetics of PEO- b-PDEA copolymers, aiming to elucidate the PDEA chain-length effects on the unimer-to-micelle transition process. Upon a stopped-flow pH-jump from 3 to 12, the obtained dynamic traces can be well-fitted with double exponential functions. The calculated fast and slow characteristic relaxation times (tau 1 and tau 2) can be ascribed to the formation of quasi-equilibrium micelles (fast process) and subsequent relaxation into final equilibrium micelles (slow process), respectively. For PEO 113- b-PDEA 32 and PEO 113- b-PDEA 61, tau 2 is almost independent of polymer concentrations, suggesting that the relaxation from quasi-equilibrium micelles into final equilibrium micelles mainly proceeds via insertion/expulsion of unimer chains. Upon increasing the DP of pH-responsive PDEA block to 89, 117, and 154, the obtained slow relaxation time, tau 2, tends to decrease with increasing polymer concentrations, suggesting that the slow process is dominated by the micelle fusion/fission mechanism. The apparent activation energy ( E a) associated with tau 2 has also been determined from temperature-dependent micellization kinetics for five PEO- b-PDEA copolymers. It was found that during micellization, copolymers with longer PDEA blocks exhibit much lower E a compared to those with shorter blocks. Thus, we observed experimentally for the first time that increasing the hydrophobic block length in double hydrophilic block copolymers (DHBCs) can transform the mechanism of the slow process from unimer insertion/expulsion to micelle fusion/fission.     

Morphological transitions in aggregates of thermosensitive poly(ethylene oxide)‐b‐poly(N‐isopropylacrylamide) block copolymers prepared via RAFT polymerization

Double hydrophilic poly(ethylene oxide)‐b‐poly(N‐isopropylacrylamide) (PEO‐b‐PNIPAM) block copolymers were synthesized via reversible addition‐fragmentation chain transfer (RAFT) polymerization, using a PEO‐based chain transfer agent (PEO‐CTA). The molecular structures of the copolymers were designed to be asymmetric with a short PEO block and long PNIPAM blocks. Temperature‐induced aggregation behavior of the block copolymers in dilute aqueous solutions was systematically investigated by a combination of static and dynamic light scattering. The effects of copolymer composition, concentration (C_p), and heating rate on the size, aggregation number, and morphology of the aggregates formed at temperatures above the LCST were studied. In slow heating processes, the aggregates formed by the copolymer having the longest PNIPAM block, were found to have the same morphology (spherical “crew‐cut” micelles) within the full range of C_p. Nevertheless, for the copolymer having the shortest PNIPAM block, the morphology of the aggregates showed a great dependence on C_p. Elongation of the aggregates from spherical to ellipsoidal or even cylindrical was observed. Moreover, vesicles were observed at the highest C_p investigated. Fast heating leads to different characteristics of the aggregates, including lower sizes and aggregation numbers, higher densities, and different morphologies. Thermodynamic and kinetic mechanisms were proposed to interpret these observations, including the competition between PNIPAM intrachain collapse and interchain aggregation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4099–4110, 2009     

Rupturing polymeric micelles with cyclodextrins

Small-angle neutron scattering has been used to investigate the associative structures formed by triblock copolymers of poly(ethylene oxide) (PEO)-polypropylene oxide (PPO)-poly(ethylene oxide) (PEO) (also known as Pluronics) and to monitor the structural changes occurring upon complexation with heptakis(2,6-di-O-methyl)-beta-cyclodextrin (hbeta-CD) over the temperature range from 5 to 70 degrees C. At low temperature, the Pluronics are dispersed as unimers. Close to ambient temperature, the hydrophobicity of PPO causes the aggregation of the polymers into spherical micelles with core sizes between 40 and 50 A and a high inclusion of solvent. The aggregation number increases with temperature as the hydrophobicity of the core is gradually enhanced. hbeta-CD spontaneously forms pseudopolyrotaxanes with the triblock copolymers either when in their unimer form or micellized. The complexation results in an increase in the effective critical micellar concentration. It is suggested that the cyclodextrins thread onto the polymer backbone to localize preferentially on the central PPO block, therefore improving its water solubility. At temperatures where the polymers exist in micellar form, complexation with hbeta-CD gives rise to a complete disruption of the aggregates. These processes are highly temperature-dependent. Above 50 degrees C, the break-up of the aggregates is inhibited, and large-scale aggregation is observed.     

Structures and properties of block copolymers in solution

Structures and properties of block copolymers in solution depend on the total block length, the block length ratio, the block sequence and the solvent quality. The supramolecular structures formed by core/shell or flower-like micelles in the concentrated solution region are examined by dynamic light scattering to ascertain some of the conditions which can enhance the formation of open associated structures using a triblock copolymer in a selective solvent for the middle block.