Synthesis and photoluminescence of titania nanoparticle arrays templated by block-copolymer thin films.

High-density arrays of titania nanoparticles were prepared using a polystyrene-b-poly(ethylene oxide) block copolymer (PS-b-PEO) as a template and a titanium tetraisopropoxide based sol-gel precursor as titania source via a spin-coating method. The hydrophilic titania sol-gel precursor was selectively incorporated into hydrophilic PEO domains of PS-b-PEO and form titania nanoparticle arrays, due to a microphase separation between the PS block and the sol-gel/PEO phase. Field emission scanning electron microscopy (FESEM) and scanning probe microscopy (SPM) images showed that the uniformity and long-range order of the titania/PEO domains improved with increasing sol-gel precursor amount. Grazing incidence small-angle X-ray scattering (GISAXS) results indicate that the ordered structures exist over large length scales. Titania nanocrystal arrays of anatase modification were obtained by calcination at 600 degrees C for 4 h. After calcination, separated particles were observed for low and medium amounts of sol-gel precursors. Films with higher precursor amounts showed wormlike structures due to the aggregation between neighboring particles. Removal of the polymer matrix via UV treatment leads to highly ordered arrays of amorphous titania while retaining the domain size and interparticle distance initially present in the hybrid films. Photoluminescence (PL) properties were investigated for samples before and after calcination. The PL intensity increases with the increasing amount of sol-gel precursor. Bands at 412 nm were ascribed to self-trapped exitons and bands at 461 and 502 nm to oxygen vacancies, respectively. Uncalcined or UV-treated samples also showed PL properties similar to calcined samples, indicating that the local environment of the titanium atoms is similar to the environment of the crystalline anatase modification.     

Size-selective organization of enthalpic compatibilized nanocrystals in ternary block copolymer/particle mixtures

Dependent on the relative particle core size, two distinct types of particle topologies in block copolymer/nanocrystal blends have been identified, that is, the localization of particles along the intermaterial dividing surface or at the center of the respective polymer domain. In ternary systems consisting of block copolymer and two different-sized nanocrystal species, the distinct morphological types are conserved, resulting in autonomous size-selective separation and organization of the respective nanocrystals within alternating arrays and sheets.     

Melting temperature: from nanocrystalline to amorphous phase

By extrapolating the mean grain size of nanocrystal to an infinitesimal value, an amorphous phase has been obtained from the Voronoi construction. The molecular dynamics simulations indicated that for nanocrystal, the grain size variation of melting temperature exhibits two characteristic regions. As mean grain size above about 4 nm for Ag, the melting temperatures decrease with decreasing grain size. However, with grain size further shrinking, the melting temperatures almost keep a constant. This is because the dominant factor on the melting temperature of nanocrystal shifts from grain phase to grain boundary. As a result of fundamental difference in structure, the amorphous phase has a much lower solid-to-liquid transformation temperature than that of nanocrystal.     

Determinants of thermal conductivity and diffusivity in nanostructural semiconductors

The origin of size effects in the thermal conductivity and diffusivity of nanostructural semiconductors was investigated through the establishment of a unified nanothermodynamic model. The contributions of size-dependent heat capacity and cohesive energy as well as the interface scattering effects were considered during the modeling. The results indicate the following: (1) both the thermal conductivity and diffusivity decrease with decreasing nanocrystal sizes (x) of Si and Si/SiGe nanowires, Si thin films and Si/Ge(SiGe) superlattices, and GaAs/AlAs superlattices when x > 20 nm; (2) the heat transport in semiconductor nanocrystals is determined largely by the increase of the surface (interface)/volume ratio; (3) the interface scattering effect predominates in the reduction of thermal conductivity and diffusivity while the intrinsic size effects on average phonon velocity and phonon mean free path are also critical; (4) the quantum size effect plays a crucial role in the enhancement of the thermal conductivity with a decreasing x (<20 nm). These findings provide new insights into the fundamental understanding of high-performance nanostructural semiconductors toward application in optoelectronic and thermoelectric devices.     

Self-assembly and morphology control of new L-glutamic acid-based amphiphilic random copolymers: giant vesicles, vesicles, spheres, and honeycomb film

New amphiphilic random copolymers containing hydrophobic dodecyl (C12) chain and hydrophilic L-glutamic acid were synthesized, and their self-assembly in solution as well as on the solid surfaces was investigated. The self-assembly behavior of these polymers are largely dependent on their hydrophilic and hydrophobic balances. The copolymer with a more hydrophobic alkyl chain (～90%) self-assembled into giant vesicles with a diameter of several micrometers in a mixed solvent of ethanol and water. When the hydrophobic ratio decreased to ca. 76%, the polymer self-assembled into conventional vesicles with several hundred nanometers. The giant vesicles could be fused in certain conditions, while the conventional vesicles were stable. When the content of the hydrophilic part was further increased, no organized structures were formed. On the other hand, when the copolymer solutions were directly cast on solid substrates such as silicon plates, films with organized nanostructures could also be obtained, the morphology of which depended on solvent selection. When ethanol or methanol was used, spheres were obtained. When dichloromethane was used as the solvent, honeycomb-like morphologies were obtained. These results showed that through appropriate molecular design, random copolymer could self-assemble into various organized structures, which could be regulated through the hydrophobic/hydrophilic balance and the solvents.     

Converting self-assembled gold nanoparticle/dendrimer nanodroplets into horseshoe-like nanostructures by thermal annealing

A simple and effective nonlithographic method to produce a novel organization of noble metal nanoparticles into horseshoe-like nanostructures via self-assembly is described. The adsorption of Au nanoparticles stabilized with the dendrimer 1,2,3,4,5,6-hexakis[(3',5'-bis(benzyloxy)benzyl)sulfanylmethyl]benzene (S(6)G(1)) on hydrophilic surfaces (native oxide-terminated Si(111)) resulted in the formation of spatially correlated droplet aggregates. Annealing of Au/S(6)G(1) in thin films caused amalgamated droplets to form arrays of horseshoe-like nanostructures with an average size of approximately 250 nm and an average height of 13 nm. The mobility and the manner in which the semicapped Au nanoparticles are distributed on the hydrophilic substrate are believed to be the promoters that control the growth of the nucleation to create the horseshoe-like structures. Atomic force microscopy (AFM) measurements demonstrated the changes in height and size of the nanoparticles before and after the annealing process. Oxygen plasma etching was used to remove the S(6)G(1) dendrimer to reveal the orientation of the Au nanocrystals in the nanostructure matrix.     

环氧树脂水基分散体系的相反转乳化

以聚乙二醇－邻苯二甲酸酐－环氧树脂Ｅ－４４多元嵌段共聚体为乳化剂,将环氧树脂Ｅ－４乳化成水包油的稳定水基乳液。用乳液体系电导率和粘度的变化表征了相反转乳化过程。研究了乳化剂浓度、三元多嵌段共聚体中亲水嵌段分子量和乳化温度对相反转乳化过程的影响。实验结果表明,体系在较高乳化剂浓度（９．１％）下为完全相反转,在低乳化剂浓度（４．１％）下为不完全相反转。相反转时水与环氧树脂Ｅ－４４的重量比值随乳化剂浓度     

The synthesis of biodegradable graft copolymer cellulose-graft-poly(L-lactide) and the study of its controlled drug release

According to the concept of green chemistry and sustainable development, a new biodegradable copolymer comprised of hydrophobic poly(l-lactide) (PLLA) segments and hydrophilic cellulose segment (cellulose-g-PLLA) was designed and synthesized. The structure of the copolymer was characterized by (1)H NMR, FT-IR, (13)C NMR, DSC and WAXD. The cytotoxicity study shows that cellulose-g-PLLA exhibits good biocompatibility. The copolymer can self-assemble into micelles in water with the hydrophobic PLLA segments at the cores of micelles and the hydrophilic cellulose segments as the outer shells. Transmission electron microscopy (TEM) shows that the micelles exhibit nanospheric morphology within a size range of 30-80nm. The drug loaded micelles formed by the copolymer in aqueous media show sustained drug release which indicates their potential applicability in drug carrier.     

Influence of prepolymers molecular weight on the viscoelastic properties of aqueous HEUR solutions

A series of hydrophobically modified ethoxylated urethanes (HEURs) were synthesized by a step growth polymerization of polyethylene glycol with dicyclohexylmethane diisocyanate (H12MDI). The thickeners were produced with different sizes of the hydrophilic section by changing the molar ratios of reactants. The size of the hydrophobic ends was constant for all prepared samples. The changes in hydrophilic lengths were correlated with the rheological properties of HEURs aqueous solutions. The intrinsic viscosity measurements showed that associates are present even at very low concentration. The response of these HEUR systems in aqueous solution to both steady shear and oscillatory shear was determined as a function of hydrophilic chain length and polymer concentration. Dramatic increases in viscosity are observed with decreasing molecular weight of the prepolymer (with a decrease of the hydrophilic components' size and at the same time an increased ratio between hydrophobic and hydrophilic sections of HEURs). Also, a steep increase in viscosity with increasing thickener concentration is obtained. The rheological properties of aqueous solutions of HEUR polymers can be described using a simple Maxwell model with a single relaxation. The dynamic measurements verified the results obtained from the steady state measurements about the hydrophilic section size and its effect on the association phenomenon.     

单分散CdS纳米晶/PNIPAM复合水凝胶的制备及其温敏荧光特性

采用液体-固体-溶液法(LSS)制备单分散CdS纳米晶;通过自由基聚合制备单分散CdS纳米晶/聚N-异丙基丙烯酰胺(CdS/PNIPAM)复合温敏水凝胶.采用HRTEM、XRD、FTIR、DSC、PL等对CdS纳米晶、CdS/PNIPAM温敏复合凝胶的微观结构与性能进行了表征,变温荧光光谱研究了温度对凝胶荧光性能的影响.结果表明,CdS纳米晶粒径约为2.8 nm,单分散性良好;复合凝胶的荧光发射强度与环境温度存在一定的关联性,且呈可逆性.     

ROD-LIKE AGGREGATES FROM POLYSTYRENE-b-POLY(4-VINYLPYRIDINE)-b-POLYSTYRENE TRIBLOCK COPOLYMER IN AQUEOUS MEDIA

INTRODUCTIONRecently, solution-state assembly of block copolymers has attracted much interest. On the one hand, theversatility of morphology control can be used for the preparation of unique nanostructured materials with variousarchitectures[1-8]. On the other hand, some self-assembled structures are biomimetic[9,10]. The balance betweenthree major forces acting on the system affects block copolymer morphologies in solutions[11,12]. These threeforces include the stretching of the core-for…     

Efficient polymer passivation of ligand‐stripped nanocrystal surfaces

Water‐dispersible, polymer‐wrapped nanocrystals are highly sought after for use in biology and chemistry, from nanomedicine to catalysis. The hydrophobicity of their native ligand shell, however, is a significant barrier to their aqueous transfer as single particles. Ligand exchange with hydrophilic small molecules or, alternatively, wrapping over native ligands with amphiphilic polymers is widely employed for aqueous transfer; however, purification can be quite cumbersome. We report here a general two‐step method whereby reactive stripping of native ligands is first carried out using trialkyloxonium salts to reveal a bare nanocrystal surface. This is followed by chemically directed immobilization of a hydrophilic polymer coating. Polyacrylic acids, with side‐chain grafts or functional end groups, were found to be extremely versatile in this regard. The resulting polymer‐wrapped nanocrystal dispersions retained much of the compact size of their bare nanocrystal precursors, highlighting the unique role of monomer side‐chain functionality to serve as effective, conformal ligation motifs. As such, they are well poised for applications where tailored chemical functionality at the nanocrystal's periphery or improved access to their surfaces is desirable. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Block copolymer-mediated synthesis of size-tunable gold nanospheres and nanoplates

We have successfully controlled the size and shape of isotropic and anisotropic gold nanocrystals through a one-step reaction by using amphiphilic polyethylene oxide-polystyrene oxide block copolymers as both reductant and stabilizing agents in water solution. Spherical or quasispherical nanoparticles were obtained at room temperature with tunable mean sizes and polydispersities depending on reaction conditions, that is, on copolymer block length, and copolymer and gold salt concentrations. By moderate increases of reaction temperature up to 65 degrees C, progressive formation of single-crystalline gold nanoplates in good yields takes place (up to 70%) without the necessity of additional reactants or growing solutions. These nanoplates are characterized by lateral mean sizes between 0.1-1.2 microm depending on copolymer concentration and reaction temperature, with mainly truncated or rounded triangular shapes with {111} planes as two basal surfaces. This allows us to tune the surface plasmon band of the nanoplates from ca. 850 nm to more than 1100 nm, well inside the near-infrared region (NIR), which enables the use of these type of nanostructures as a very promsing materials in applications such as optical coatings, SERS, and cancer cell hyperthermia. We proposed that the growth of these nanostructures can stem from a decrease in the reaction rate as temperature increases due to an enhanced copolymer hydrophobicity, which gives rise to a structure of interacting micelles formed from the fluid via a percolation transition (known as "soft gel") at elevated temperatures. In this way, reduction becomes slow enough to allow kinetic control of the reaction, and preferential adsorption of the copolymer molecules/micelles on certain crystallographic planes can favor the growth of certain nanocrystal facets to give the final structure. This alternative water-based system provides a more convenient and environmentally benign route to the synthesis of shape-controlled noble-metal nanocrystals in high yield because it does not involve toxic organic solvents or reagents and serves as a bridge between two frontline discipline: the block copolymeric science and anisotropic nanoparticles.     

PELGE nanoparticles as new carriers for the delivery of plasmid DNA

Biodegradable monomethoxy(polyethyleneglycol)-poly(lactide-co-glycolide)-monomethoxy(poly-ethyleneglycol) (PELGE) copolymers were synthesized by ring-opening polymerization to formulate plasmid DNA loaded nanoparticles. A double emulsion method with polyvinyl alcohol as the emulsifier in the external aqueous phase was employed to prepare nanoparticles. The effects of monomethoxypoly(ethyleneglycol) (mPEG) segments in the polymer on particle size, zeta potential, encapsulation efficiency and in vitro release were investigated. It was found that the introduction of a certain amount of hydrophilic mPEG segments in the copolymer chains could improve the affinity of copolymer with plasmid DNA and enhance the emulsification ability of the copolymer. Thus DNA loaded nanoparticles with smaller particle sizes and higher encapsulation efficiencies were obtained by using PELGE copolymer as the matrix.     

Branched surface-active polylactides

A series of new branched block copolymers was synthesized and investigated, and their physicochemical and surface properties were determined. It was shown that all of the prepared block copolymers demonstrate well-defined surface activity dependent on the ratio of hydrophilic and lipophilic fragments of macromolecule as well as on the copolymer structure. The resulting copolymers were used as surfactants to obtain microemulsions and microcapsules. It was found that the size of emulsion drops stabilized by branched copolymers increased more slowly than that of drops stabilized by linear copolymers. During the preparation of microcapsules through the use of branched copolymers as surfactants, unlike that through the use of linear copolymer surfactants, a high effectiveness of encapsulation and a smaller particle size were observed.     

Temperature-responsive magnetite/PEO-PPO-PEO block copolymer nanoparticles for controlled drug targeting delivery

In this study, temperature-responsive magnetite/polymer nanoparticles were developed from iron oxide nanoparticles and poly(ethyleneimine)-modified poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer. The particles were characterized by TEM, XRD, DLS, VSM, FTIR, and TGA. A typical product has an approximately 20 nm magnetite core and an approximately 40 nm hydrodynamic diameter with a narrow size distribution and is superparamagnetic with large saturation magnetization (51.34 emu/g) at room temperature. The most attractive feature of the nanoparticles is their temperature-responsive volume-transition property. DLS results indicated that their average hydrodynamic diameter underwent a sharp decrease from 45 to 25 nm while evaluating the temperature from 20 to 35 degrees C. The temperature-dependent evolution of the C-O stretching band in the FTIR spectra of the aqueous nanoparticles solution revealed that thermo-induced self-assembly of the immobilized block copolymers occurred on the magnetite solid surfaces, which is accompanied by a conformational change from a fully extended state to a highly coiled state of the copolymer. Consequently, the copolymer shell could act as a temperature-controlled "gate" for the transit of guest substance. The uptake and release of both hydrophobic and hydrophilic model drugs were well controlled by switching the transient opening and closing of the polymer shell at different temperatures. A sustained release of about 3 days was achieved in simulated human body conditions. In primary mouse experiments, drug-entrapped magnetic nanoparticles showed good biocompatibility and effective therapy for spinal cord damage. Such intelligent magnetic nanoparticles are attractive candidates for widespread biomedical applications, particularly in controlled drug-targeting delivery.     

Polystyrene-b-poly(acrylic acid) vesicle size control using solution properties and hydrophilic block length

Polymeric vesicles have attracted considerable attention in recent years, since they are a model for biological membranes and have versatile structures with several practical applications. In this study, we prepare vesicles from polystyrene-b-poly(acrylic acid) block copolymer in dioxane/water and dioxane/THF/water mixtures. We then examine the ability of additives (such as NaCl, HCl, or NaOH), solvent composition, and hydrophilic block length to control vesicle size. Using turbidity measurements and transmission electron microscopy (TEM) we show that larger vesicles can be prepared from a given copolymer by adding NaCl or HCl, while adding NaOH yields smaller vesicles. The solvent composition (ratio of dioxane to THF, as well as the water content) can also determine the vesicle size. From a given copolymer, smaller vesicles can be prepared by increasing the THF content in the THF/dioxane solvent mixture. In a given solvent mixture, vesicle size increases with water content, but such an increase is most pronounced when dioxane is used as the solvent. In THF-rich solutions, on the other hand, vesicle size changes only slightly with the water concentration. As to the effect of the acrylic acid block length, the results show that block copolymers with shorter hydrophilic blocks assemble into larger vesicles. The effect of additives and solvent composition on vesicle size is related to their influence on chain repulsion and aggregation number, whereas the effect of acrylic acid block length occurs because of the relationship among the block length, the width of the molecular weight distribution, and the stabilization of the vesicle curvature.     

Formation of gold nanoparticles in microreactor composed of helical peptide assembly in water

A novel microreactor was prepared by self-assembly of an amphiphilic block copolymer composed of a hydrophobic helical peptide unit with a naphthyl group at the C terminal and a hydrophilic poly(ethylene glycol) unit. The copolymer formed a self-assembly in water, taking a vesicular structure. Noticeably, when the copolymer was dispersed in an Au(3+) aqueous solution, gold nanoparticles were formed without addition of any reducing reagent. The naphthyl groups, which are located at the inner surface of the vesicular assembly, promoted the reduction of Au(3+) ions with accompanying pH decrease.     

Photochemical instability of CdSe nanocrystals coated by hydrophilic thiols

The photochemical instability of CdSe nanocrystals coated by hydrophilic thiols was studied nondestructively and systematically in water. The results revealed that the photochemical instability of the nanocrystals actually included three distinguishable processes, namely the photocatalytic oxidation of the thiol ligands on the surface of nanocrystals, the photooxidation of the nanocrystals, and the precipitation of the nanocrystals. At first, the thiol ligands on the surface of a nanocrystal were gradually photocatalytically oxidized using the CdSe nanocrystal core as the photocatalyst. This photocatalytic oxidation process was observed as a zero-order reaction in terms of the concentration of the free thiols in the solution. The photogenerated holes in a nanocrystal were trapped onto the thiol ligands bound on the surface of the nanocrystal, which initiated the photooxidation of the ligands and protected the nanocrystal from any photooxidation. After nearly all of the thiol ligands on the surface of the nanocrystals were converted into disulfides, the system underwent several different pathways. If the disulfides were soluble in water, then all of the disulfides fell into the solution at the end of this initial process, and the nanocrystals precipitated out of the solution without much variation over their size and size distribution. When the disulfides were insoluble in water, they likely formed a micelle-like structure around the nanocrystal core and kept it soluble in the solution. In this case, the nanocrystals only precipitated after severe oxidation, which took a long period of time. If the system contained excess free thiol ligands, they replaced the photochemically generated disulfides and maintained the stability and solubility of the nanocrystals. The initiation stage of the photooxidation of CdSe nanocrystals themselves increased as the thickness and packing density of the ligand shell increased. This was explained by considering the ligand shell on the surface of a nanocrystal as the diffusion barrier of the oxygen species from the bulk solution into the interface between the nanocrystal and the surface ligands. Experimental results clearly indicated that the initiation stage of the photooxidation was not caused by the chemical oxidation of the system kept in air under dark conditions or the hydrolysis of the cadmium-thiol bonds on the surface of the nanocrystals, both of which were magnitudes slower than the photocatalytic oxidation of the surface ligands if they occurred at all. The results described in this contribution have already been applied for designing new types of thiol ligands which dramatically improved the photochemical stability of CdSe nanocrystals with a ligand shell that is as thin as approximately 1 nm.     

Aggregation of poly(ethylene oxide)-poly(propylene oxide) block copolymers in aqueous solution: DPD simulation study

The dissipative particle dynamics (DPD) simulation method was applied to simulate the aggregation behavior of three block copolymers, (EO)16(PO)18, (EO)8(PO)18(EO)8, and (PO)9(EO)16(PO)9, in aqueous solutions. The results showed that the size of the micelle increased with increasing concentration. The diblock copolymer (EO)16(PO)18 would form an intercluster micelle at a certain concentration range, besides the traditional aggregates (spherical micelle, cylindrical micelle, and lamellar phase); while the triblock copolymer (EO)8(PO)18(EO)8 would form a spherical micelle, cylindrical micelle, and lamellar phase with increasing concentration, and (PO)9(EO)16(PO)9 would form intercluster aggregates, as well as a spherical micelle and gel. New mechanisms were given to explain the two kinds of intercluster micelle formed by the different copolymers. It is deduced from the end-to-end distance that the morphologies of the diblock copolymer and triblock copolymer with hydrophilic ends were more extendible than the triblock copolymer with hydrophobic ends.