Mechanism of gold metal ion reduction, nanoparticle growth and size control in aqueous amphiphilic block copolymer solutions at ambient conditions

Spontaneous formation and efficient stabilization of gold nanoparticles with an average diameter of 7 approximately 20 nm from hydrogen tetrachloroaureate(III) hydrate (HAuCl4.3H2O) were achieved in air-saturated aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer solutions at ambient temperature in the absence of any other reducing agent. The particle formation mechanism is considered here on the basis of the block copolymer concentration dependence of absorption spectra, the time dependence (kinetics) of AuCl4- reduction, and the block copolymer concentration dependence of particle size. The effects of block copolymer characteristics such as molecular weight (MW), PEO block length, PPO block length, and critical micelle concentration (cmc) are explored by examining several PEO-PPO-PEO block copolymers. Our observations suggest that the formation of gold nanoparticles from AuCl4- comprises three main steps: (1) reduction of metal ions by block copolymer in solution, (2) absorption of block copolymer on gold clusters and reduction of metal ions on the surface of these gold clusters, and (3) growth of metal particles stabilized by block copolymers. While both PEO and PPO blocks contribute to the AuCl4- reduction (step 1), the PEO contribution appears to be dominant. In step 2, the adsorption of block copolymers on the surface of gold clusters takes place because of the amphiphilic character of the block copolymer (hydrophobicity of PPO). The much higher efficiency of particle formation attained in the PEO-PPO-PEO block copolymer systems as compared to PEO homopolymer systems can be attributed to the adsorption and growth processes (steps 2 and 3) facilitated by the block copolymers. The size of the gold nanoparticles produced is dictated by the above mechanism; the size increases with increasing reaction activity induced by the block copolymer overall molecular weight and is limited by adsorption due to the amphiphilic character of the block copolymers.     

PEO-PPO-PEO在生物医用材料方面的研究进展

聚氧乙烯-聚氧丙烯-聚氧乙烯三嵌段共聚物因其具有良好的生物相容性和蛋白抗性,近年来在生物医用材料中的应用越来越广泛.聚氧乙烯-聚氧丙烯-聚氧乙烯水溶液具有温度敏感的胶束化和热可逆凝胶化特点,被认为是一种具有许多优点的药物传输载体,药物与胶束的核心结合增加了药物的溶解性、代谢稳定性和体内循环时间.本文对聚氧乙烯-聚氧丙烯-聚氧乙烯在生物医用方面的研究进展进行了综述,并重点介绍了其在药物传输载体,组织工程等方面的研究进展.     

Effect of the structure of ethylene oxide-propylene oxide block copolymers on their interaction with biological membranes

Partition coefficients of ethylene oxide-propylene oxide block copolymers between the lipid phase and water have been estimated via equilibrium dialysis. It has been shown that for the triblock copolymer (Pluronic L61), the partition coefficient is 45 ± 9, while for the diblock copolymer (REP), this parameter is as high as 78 ± 17. The effect of the copolymers on the permeation of the charged organic ion carboxyfluorescein across the lecithin bilayer membrane changes in the same direction. Even though the triblock copolymer binding is weaker, it shows a stronger effect on the rate of transbilayer migration of lipids and on the permeation of the uncharged substance (doxorubicin). The incorporation of cholesterol into the membrane decreases its sensitivity to the action of copolymers; however, the character of changes induced by both copolymers remains invariable. The experimental data of this study indicate that the triblock structure of amphiphilic macromolecules is responsible for their higher ability to disturb lipid bilayer membranes.     

双亲三嵌段共聚物PEO-PPO-PEO在水溶液中相行为的理论研究

用自洽平均场理论的谱方法研究了双亲三嵌段共聚物PEO-PPO-PEO的分子量对其在水溶液相行为的影响. 理论预测了各种溶致液晶相的稳定区域. 通过系统地改变聚合物的分子量, 我们给出了各种PEO-PPO-PEO三嵌段共聚物在水溶液中的相图. 此外, 也研究了分子量对自组装结构各组分浓度分布的影响. 发现在给定的温度下, 聚合物的分子量是体系发生相分离的一个重要驱动力. 我们的理论结果与相关的实验很好地符合.     

Rheology of aqueous solutions of polyglycidol-based analogues to pluronic block copolymers

The aqueous solution properties of a series of polyglycidol-poly(propylene oxide)-polyglycidol (PG-PPO-PG) block copolymers were investigated by means of rheology. The copolymers are considered as analogues to the commercially available Pluronic, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), block copolymers in which the flanking PEO blocks are substituted by blocks of structurally similar PG bearing a hydroxyl group in each repeating monomer unit. In the dilute regime, the samples normally behave as Newtonian fluids. Shear thinning was observed only for the solutions of LGP65 (the copolymer of 50 wt % PG content) as well as at concentrations well above the critical micellization concentration for the rest of the copolymers. The zero shear viscosities exhibited pronounced maxima at PG content of 50 wt % and were found to decrease with increasing temperature. The concentrated solutions were investigated using oscillatory measurements. Large hystereses were observed during the temperature sweeps 15-70-15 degrees C. The evolutions of the loss and storage moduli with frequency, PG content, and temperature displayed transitions from a non-elastic to elastic behavior of the solutions. A phase diagram showing areas of predominant elasticity or fluidity was constructed.     

Solubility behavior of amphiphilic block and random copolymers based on 2‐ethyl‐2‐oxazoline and 2‐nonyl‐2‐oxazoline in binary water–ethanol mixtures

The solution properties of random and block copolymers based on 2‐ethyl‐2‐oxazoline (EtOx) and 2‐nonyl‐2‐oxazoline (NonOx) were investigated in binary solvent mixtures ranging from pure water to pure ethanol. The solubility phase diagrams for the random and block copolymers revealed solubility (after heating), insolubility, dispersions, micellization as well as lower critical solution temperature (LCST) and upper critical solution temperature behavior. The random and block copolymers containing over 60 mol % pNonOx were found to be solubilized in ethanol upon heating, whereas the dissolution temperature of the block copolymers was found to be much higher than for the random copolymers due to the higher extent of crystallinity. Furthermore, the block copolymer containing 10 mol % pNonOx exhibited a LCST in aqueous solution at 68.7 °C, whereas the LCST for the random copolymer was found to be only 20.8 °C based on the formation of hydrophobic microdomains in the block copolymer. The random copolymer displayed a small increase in LCST up to a solvent mixture of 9 wt % EtOH, whereas further increase of ethanol led to a decrease in LCST, which is probably due to the “water‐breaking” effect causing an increased attraction between ethanol and the hydrophobic part of the copolymer. In addition, the EtOx‐NonOx block copolymers revealed the formation of micelles and dynamic light scattering demonstrated that the micellar size is increasing with increasing the ethanol content due to the enhanced solubility of EtOx. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 515–522, 2009     

Interaction of urea with pluronic block copolymers by 1H NMR spectroscopy

Solution 1H NMR techniques were used to characterize the interaction of urea with poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers. The urea was established to interact selectively with the PEO blocks of the block copolymer, and the interaction sites were found not to change with increasing temperature. Such interactions influence the self-assembly properties of the block copolymer in solution by increasing the hydration of the block copolymers and stabilizing the gauche conformation of the PPO chain. Therefore, urea increases the critical micellization temperature (CMT) values of PEO-PPO-PEO copolymers, and the effect of urea on the CMT is more pronounced for copolymers with higher PEO contents and lower for those with increased contents of PPO segments.     

Bicomponent Block Copolymers Derived from One or More Random Copolymers as an Alternative Route to Controllable Phase Behavior

Block copolymers have been extensively studied due to their ability to spontaneously self‐organize into a wide variety of morphologies that are valuable in energy‐, medical‐, and conservation‐related (nano)technologies. While the phase behavior of bicomponent diblock and triblock copolymers is conventionally governed by temperature and individual block masses, it is demonstrated here that their phase behavior can alternatively be controlled through the use of blocks with random monomer sequencing. Block random copolymers (BRCs), i.e., diblock copolymers wherein one or both blocks are a random copolymer comprised of A and B repeat units, have been synthesized, and their phase behavior, expressed in terms of the order–disorder transition (ODT), has been investigated. The results establish that, depending on the block composition contrast and molecular weight, BRCs can microphase‐separate. We also report that large variation in incompatibility can be generated at relatively constant molecular weight and temperature with these new soft materials. This sequence‐controlled synthetic strategy is extended to thermoplastic elastomeric triblock copolymers differing in chemistry and possessing a random‐copolymer midblock.     

聚醚树枝体线性聚(N-异丙基丙烯酰胺)不对称嵌段共聚物的 合成及自组装

采用原子转移聚合方法合成了聚N-异丙基丙烯酰胺和聚醚树技体的不对嵌段共聚物Dendr.PE-PNI-PAM。实验结果表明Dendr.PE-PNIPAM分子在水中能通过疏水缔合作用形成具有双分子膜结构的超分子聚集体。临界缔合浓度(CAC)、聚集体的大小及形貌对树枝体的代数具有明显的依赖关系。该聚集体对温度刺激具有响应性,并在人体体温温度(37.5℃)发生相转变。在高于相转变温度时,Dendr.PE-PNIPAM分子形成管状、带状等多重形态的超级结构的聚集体。     

Kinetics and Morphology of an Epoxy Resin Modified with PEO-PPO-PEO Block Copolymers

Block copolymers are a special class of polymers having the ability to self-assemble into nanoscale ordered structures which depend on molecular composition of the blocks. With the aim of studying the influence of copolymer composition, the kinetics of a 4,4′-diaminodiphenylmethane-cured diglycidyl ether of bisphenol-A (DGEBA) epoxy system modified with a PEO-PPO-PEO block copolymers has been investigated by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR), taking into account the relation between blocks in the copolymer as well as different copolymer contents. DSC results show that the rate of cure reaction decreases when the copolymer is added, which can be attributed to the interaction between the hidroxyl groups of the growing epoxy thermoset and the ether groups of the block copolymer observed by FTIR. The experimental results obtained have been related to the morphologies observed by atomic force microscopy (AFM).     

两亲嵌段共聚物溶液内胶束形成的温度效应

合成了一系列具有两亲嵌段结构的聚（乙二醇）（PEO）一聚（丙二醇）（PPO）共聚物．利用荧光探针及示差量热法测定了共聚物水溶液的临界胶束形成温度（CMT）值．发现二嵌段共聚物（PEO－PPO）和三嵌段共聚物（PEO－PPO－PEO）有着类似的变化规律，即随共聚物分子中疏水链（PPO）长度的增大，其CMT值降低．但三嵌段共聚（PPO－PEO－PPO）则因疏水链段处于共聚物分子的两端，因而在溶液中有可能形成立体网状交联结构．此外，利用探针分子在不同极性溶剂中荧光峰值波长发生位移的现象可以对形成胶束内核的组织程度、极性大小进行估测．     

beta-casein and symmetrical triblock copolymer (PEO-PPO-PEO and PPO-PEO-PPO) surface properties at the air-water interface

A comparison of beta-casein and symmetrical triblock copolymer (PEO-PPO-PEO and PPO-PEO-PPO) adsorption layer properties at the air-water interface has been carried out by bubble tensiometry and ellipsometry. It has been verified that the equation of state parameters (pi approximately gamma(y)) obtained from surface pressure (pi) and ellipticity in Brewster conditions (rhoB), which is proportional to the surface concentration (gamma) data, are the same as those obtained from dilational modulus epsilon and pi data. These two consistent approaches give further support to the theoretical model of block copolymers which has been previously developed for protein adsorption at fluid interfaces. It is shown that the interfacial behavior of the copolymer adsorption layer changes strongly as a function of the length of the hydrophilic and hydrophobic block sequences. The theoretical model may be used for the interpretation of the adsorption properties of the synthetic copolymers only when the size of the blocks is large enough. In the case of block copolymers, the coil is in a self-avoiding walk conformation (y = 3) whatever the temperature, while in the case of beta-casein, the polypeptide chain is partly collapsed at room temperature due to thermolabile noncovalent bonds. At the end of the first semidilute regime, there is clear evidence for a crossover toward a second semidilute regime for synthetic copolymers as well as for beta-casein but it is presently only partially characterized.     

New side chain cholesterol-functionalised aliphatic polycarbonate copolymer: synthesis and phase behaviour

A new side cholesterol-functionalised liquid crystal (LC) copolymer based on aliphatic polycarbonate backbone was synthesised. The chemical structures of the block copolymers obtained in this study were characterised with Fourier Transform Infrared Spectroscopy (FT-IR) and Proton Nuclear Magnetic Resonance (¹H NMR) spectra. Their thermal stability and phase behaviours were investigated with thermogravimetric analysis (TGA) measurements, differential scanning calorimetry, and polarising optical microscopy. The molecular organisation in the mesophase was studied by temperature-dependent X-ray diffraction (XRD). As a result, the block copolymer bearing side cholesteryl groups showed a glass transition at 15.8°C and a smectic A (SmA) to isotropic phase transition at 151.3°C on heating cycle. XRD indicated that the block LC copolymer showed an interdigitated molecular arrangement of the mesogenic units within the smectic layers. This partial bilayer structure was similar to the SmA phase formed by polar mesogens.     

Small-angle-neutron-scattering from giant water-in-oil microemulsion droplets. II. Polymer-decorated droplets in a quaternary system

Amphiphilic block copolymers of the type poly(ethylenepropylene)-co-poly(ethyleneoxide) dramatically enhance the solubilisation efficiency of non-ionic surfactants in microemulsions that contain equal volumes of water in oil. Consequently, the length scale of the microstructure of such bicontinuous microemulsions is dramatically increased up to the order of a few 100 nm. In this paper, we show that this so-called efficiency boosting effect can also be applied to water-in-oil microemulsions with droplet microstructure. Such giant water-in-oil microemulsions would provide confined compartments in which chemical reactions of biological macromolecules can be performed on a single molecule level. With this motivation we investigated the phase behavior and the microstructure of oil-rich microemulsions containing D(2)O, n-decane(d22), C(10)E(4) and the amphiphilic block copolymer PEP5-PEO5 [poly(ethylenepropylene)-co-poly(ethyleneoxide), weight per block of 5000 g/ mol]. We found that 15 wt % of water can be solubilised by 5 wt % of surfactant and block copolymer when about 6 wt % of surfactant is replaced by the block copolymer. Small-angle-neutron-scattering experiments were performed to determine the length scales and microstructure topologies of the oil-rich microemulsions. To analyze the scattering data, we derived a novel form factor that also takes into account the scattering contribution of the hydrophobic part of the block copolymer molecules that reside in the surfactant shell. The quantitative analysis of the scattering data with this form factor shows that the radius of the largest droplets amounts up to 30 nm. The novel form factor also yielded qualitative information on the stretching of the polymer chains in dependence on the polymer surface density and the droplet radius.     

A Morphological Transition of Inverse Mesophases of a Branched‐Linear Block Copolymer Guided by Using Cosolvents

We report here a strategy for influencing the phase and lattice of the inverse mesophases of a single branched‐linear block copolymer (BCP) in solution which does not require changing the structure of the BCP. The phase of the self‐assembled structures of the block copolymer can be controlled ranging from bilayer structures of positive curvature (polymersomes) to inverse mesophases (triply periodic minimal surfaces and inverse hexagonal structures) by adjusting the solvent used for self‐assembly. By using solvent mixtures to dissolve the block copolymer we were able to systematically change the affinity of the solvent toward the polystyrene block, which resulted in the formation of inverse mesophases with the desired lattice by self‐assembly of a single branched‐linear block copolymer. Our method was also applied to a new solution self‐assembly method for a branched‐linear block copolymer on a stationary substrate under humidity, which resulted in the formation of large mesoporous films. Our results constitute the first controlled transition of the inverse mesophases of block copolymers by adjusting the solvent composition.     

Langmuir monolayer and Langmuir–Blodgett films of amphiphilic triblock copolymers with water-soluble middle block

Langmuir monolayers and Langmuir–Blodgett (LB) film morphology of amphiphilic triblock copolymers are studied using surface pressure-area measurements and atomic force microscopy (AFM), respectively. The triblock copolymers are composed of long water-soluble poly(ethylene oxide) (PEO) chains as middle block with very short poly(perfluorohexylethyl methacrylate) (PFMA) end blocks. The surface pressure-area isotherms show phase transitions in the brush regime. This phase transition is due to a rearrangement of PFMA block at the air–water interface. It becomes more significant with increasing PFMA content in the copolymer. LB films transferred at low surface pressures from the air–water interface to hydrophilic silicon substrates show surface micelles in the size range of 50–100 nm. A typical crystalline morphology of the corresponding PEO homopolymer is observed in LB films of copolymers with very short PFMA blocks, transferred in the brush region at high surface pressure. This crystallization is hindered with increasing PFMA content in the copolymer.     

Gauging the effect of impurities on lipid bilayer phase transition temperature

We report on the gel-to-fluid phase transition behavior of unilamellar vesicles formed with 1,2-dimyristoyl-sn-phosphatidylcholine (14:0 DMPC). We have interrogated the gel-to-fluid transition temperature of these bilayer structures using the chromophore perylene incorporated in their nonpolar region. We observe a discontinuous change in the reorientation time of perylene sequestered within the bilayer at the known melting transition temperature of 14:0 DMPC, 24 degrees C. The perylene reorientation data reveal a local viscosity of 14.5 +/- 2.5 cP in the gel phase, and 8.5 +/- 1.5 cP in the fluid phase. We have also incorporated small amounts of 1,2-dimyristoleoyl-sn-glycero-3-phosphocholine (14:1 DMPC) into these unilamellar vesicles and find that the melting transition temperature for these bilayers varies in a regular manner with the amount of 14:1 DMPC present. These data demonstrate that very little "contaminant" is required to cause a substantial change in the gel-to-fluid transition temperature, even though these contaminants do not alter the viscosity of the bilayer sensed by perylene, either above or below the melting transition.     

Novel pH sensitive block copolymer micelles for solvent free drug loading

Novel pH sensitive biodegradable block copolymers (MPEG-PDLLA-OSM) composed of mono-methoxy poly(ethylene glycol) (MPEG), poly (D,L-lactide) (PDLLA) and sulfamethazine oligomer (OSM) were synthesized via ring-opening polymerization and a dicyclohexyl carboimide (DCC) coupling reaction. These copolymers had a relatively low critical micelle concentration (CMC) due to the strong hydrophobic properties of non-ionized OSM at pH 7.0. Also, the pH sensitive block copolymers showed the micelle-unimer transition due to the ionization-non-ionization of OSM in the pH range (pH 7.2-8.4) above the CMC. Due to the pH sensitive properties of the block copolymer, the hydrophobic drug paclitaxel (PTX) was incorporated into a pH sensitive block copolymer micelle by the pH induced micellization method, without using an organic solvent. The block copolymer micelle prepared by pH induced micellization showed a relatively high PTX loading efficiency, and good stability for 2 d at 37 degrees C. Furthermore, the PTX loaded micelle showed a sustained release of PTX with a small burst in vitro over 2 d. The present results suggest that the pH induced micellization method due to the micelle-unimer transition of the pH sensitive block copolymer would be a novel and valuable drug incorporation tool for hydrophobic and protein drugs, since no organic solvent is involved in the formulation.     

Self-assembly of Asymmetric Dimer Particles in Supported Copolymer Bilayer

Using self-consistent field and density functional theories, we investigate the self-assembly behavior of asymmetric dimer particles in a supported AB block copolymer bilayer. Asym-metric dimer particles are amphiphilic molecules composed by two different spheres. One prefers to A block of copolymers and the other likes B block when they are introduced into the copolymer bilayer. The two layer structure of the dimer particles is formed within the bilayer. Due to the presence of the substrate surface, the symmetry of the two leaflets of the bilayer is broken, which may lead to two different layer structures of dimer particles within each leaflet of the bilayer. With the increasing concentration of the asymmetric dimer particles, in-plane structure of the dimer particles undergoes sparse square, hexagonal, dense square, and cylindrical structures. In a further condensed packing, a bending cylindrical structurecomes into being. Here we verify that the entropic effect of copolymers, the enthalpy of the system and the steric repulsion of the dimer particles are three important factors determing the self-assembly of dimer particles within the supported copolymer bilayer.     

Thermoresponsive hydrogels from symmetrical triblock copolymers poly(styrene-block-(methoxy diethylene glycol acrylate)-block-styrene)

A series of symmetrical, thermo-responsive triblock copolymers was prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization, and studied in aqueous solution with respect to their ability to form hydrogels. Triblock copolymers were composed of two identical, permanently hydrophobic outer blocks, made of low molar mass polystyrene, and of a hydrophilic inner block of variable length, consisting of poly(methoxy diethylene glycol acrylate) PMDEGA. The polymers exhibited a LCST-type phase transition in the range of 20-40 °C, which markedly depended on molar mass and concentration. Accordingly, the triblock copolymers behaved as amphiphiles at low temperatures, but became water-insoluble at high temperatures. The temperature dependent self-assembly of the amphiphilic block copolymers in aqueous solution was studied by turbidimetry and rheology at concentrations up to 30 wt %, to elucidate the impact of the inner thermoresponsive block on the gel properties. Additionally, small-angle X-ray scattering (SAXS) was performed to access the structural changes in the gel with temperature. For all polymers a gel phase was obtained at low temperatures, which underwent a gel-sol transition at intermediate temperatures, well below the cloud point where phase separation occurred. With increasing length of the PMDEGA inner block, the gel-sol transition shifts to markedly lower concentrations, as well as to higher transition temperatures. For the longest PMDEGA block studied (DP(n) about 450), gels had already formed at 3.5 wt % at low temperatures. The gel-sol transition of the hydrogels and the LCST-type phase transition of the hydrophilic inner block were found to be independent of each other.