Controlled Phase Separation Research Articles

Control of Gel Swelling and Phase Separation of Weakly Charged Thermoreversible Gels by Salt Addition

Doping of thermoreversible polymer gels with charged monomers provides a way to control phase separation and gelation conditions by coupling the properties of the gel with a tunable ionic environment. We analyze the dependence of the gelation and phase separation conditions on the amount of salt present using a mean field model of weakly charged associative polymers. The ions and co-ions present are explicitly considered at the mean field level, and we determine their concentrations in the different equilibrium phases when the system undergoes phase separation. For weak polymer charge, the entropic contributions of the ions to the free energy of the system play a central role in the determination of the location of phase equilibrium. In the simplest case, when the associative interaction responsible for gel formation is independent of the electrostatic interaction, the addition of salt changes the polymer equilibrium concentrations and indirectly changes the measurable swelling of the gel. We construct phase diagrams of these systems showing the location of the coexistence region, the gel-sol boundary and the location of the tie-lines. We determine the swelling of the gel within the co-existence region. Our main result is that the description of the effect of the salt on the properties of the weakly charged gel can be described through an extra contribution to the effective immiscibility parameter χ proportional to the square of the doping degree f(2) and to the inverse square of the added salt concentration s(-2).

Preparation of Polymer Microspheres by Phase-Separation/Aggregation (PSA) Technique for Color Toner Applications

A particle formation method related to controlled phase separation of organic and aqueous phases, and limited aggregation/coalescence is reported. The method can be applied effectively to form microspherical polymer particles for use as electrophotographic toners. An organic solution containing a polymer resin dissolved in a solvent and an aqueous phase containing an anti-aggregation agent are prepared. The aqueous solution is added to the organic solution during vigorous stirring to facilitate aggregation of gel colloids and diffusion of solvent into the aqueous phase due to the partial aqueous solubility of the solvent. As opposed to the case when the organic phase is added to the aqueous phase under an extremely high speed of agitation (>10,000 rpm), this phase-separation/aggregation process only requires an agitation speed of 1,000∼1,500 rpm. Furthermore, removal of solvents due to diffusion into the aqueous phase is in process at the same time as the formation of gel particles. The depletion of solvents in the organic phase results in solid particle formation. Fine control of particle size distribution can be achieved by careful adjustments of the viscosity values of both the organic and the aqueous solutions. This phase-separation/aggregation method offers numerous advantages for preparation of toners including fast production run and extremely sharp particle size distribution, with mean volume-averaged diameter to mean number-averaged diameter ratios dv/dn smaller than 1.1. The particle shape can also be tuned by the selection of different aqueous solubility of the solvents. Both styrene/acrylate based and polyester based materials can be applied for formation of microspherical particles by using the phase-separation/aggregation technique.

From a two-dimensional chemical pattern to a three-dimensional topology through selective inversion of a liquid–liquid bilayer

Soft organic surfaces with more and more complex topologies are required daily to engineer appropriate microstructures for many different applications such as DNA array technology, biological optics for advanced photonic systems and microfluidics. Complementarily to conventional lithographic processes, several pioneering methods have been developed recently, by controlling phase separation of polymer blends, spinodal decomposition of homopolymers or by using the action of additional external forces driving diverse instabilities. Here we present a method that not only provides original concepts towards the three-dimensional (3D) structuring of liquids, on the basis of the synergistic effects of molecular diffusion and confined nucleation, but also suggests original solutions for the transport, mixing and filtering of small volumes of liquid. Through the intrinsic destabilization of a liquid-liquid bilayer, the 2D pattern of a chemically structured surface with 'hydrophilic' and 'hydrophobic' domains is transferred to a solid/liquid interface as a 3D topography with either 'positive' or 'negative' replication. This easy-to-use process has potential applications in various technological realms requiring a specific topography at interfaces such as microfluidics or biosensors.

Molecular Interface Effect of Heterogeneous Molecular Junctions Consisting of Nonfluorinated and Fluorinated Phthalocyanines

We report the tunneling behavior of homogeneous and heterogeneous molecular junctions using p-type molecules of iron phthalocyanine (FePc), phthalocyanine (H(2)Pc), and copper(II) octaalkoxyl substituted phthalocyanine (CuPcOC8) and n-type molecule of copper hexadecafluorophthalocyanine (F(16)CuPc). The molecular films formed on the electrode surfaces were inspected by X-ray photoelectron spectroscopy (XPS). The measured characteristic tunneling curves of single-component phthalocyanines revealed comparable energy gaps for homogeneous tunneling junctions using the photoemission method. In contrast, for the heterogeneous tunnel junctions of mixed phthalocyanines including fluorinated phthalocyanine a distinctive offset of the energy gaps to the positive bias voltage direction can be clearly identified. It is suggested that the substitution of phthalocyanines and surface affinity of phthalocyanines could contribute to the controlled phase separation within the heterogeneous tunneling junctions. The apparent shift of the tunneling spectra is attributed to the existence of an internal electric field originated with the phase separation of the binary mixture of p-type and n-type phthalocyanines within the tunneling junction.

New Photovoltaic Concept: Liquid‐Crystal Solar Cells Using a Nematic Gel Template

Liquid-crystal (LC) gels are normally used as insulating, scattering media in LC displays. Organic solar cells generate electrical power by the separation of photocreated electrons and holes at the interface between electron-donating and electron-accepting organic semiconductors. We combine these two technologies to create a new type of plastic solar cell. The efficiency of solar cells can be improved by enlarging the interface area to maximize charge separation and by vertically separating the two materials to provide different paths for electrons and holes to the electrodes. We use a new electron-donating, visible-light absorbing, LC gel to form a surface with nanometer-sized grooves with a large interface area to an overlying electron-accepting layer. This forms a photovoltaic cell, which exhibits a monochromatic power conversion efficiency of 0.6 % at an incident intensity of 45 mW cm. Improvements will provide low-cost, robust solar cells compatible with plastic electronics and roll-to-roll processing. Most organic photovoltaics use either main-chain conjugated polymers, often in conjunction with fullerenes, or low-molecular-weight glasses, achieving power-conversion efficiencies up to 4.2 %. Charge separation is achieved in solar cells by ionization of the photogenerated exciton at a heterointerface between electron-donating material with a low ionization potential (IP) and electron-accepting species with a high electron affinity (EA). The diffusion length of the exciton before recombination is of the order of 100–200 A, so that distributed, rather than planar, heterointerfaces have been used to maximize charge separation. These have been formed by phase-separated blends of electron-donating and electronaccepting organic materials. The bulk-distributed heterojunctions are formed randomly and do not guarantee a continuous pathway for charge carriers to the electrodes. The required vertical segregation of the blend materials can be obtained by surface treatment or in diffuse bilayer photovoltaics. A blend of an electron-donating columnar LC, hexabenzocoronene, and an electron-accepting perylene dye vertically segregates spontaneously to give an efficient photovoltaic effect. Unfortunately, the spatial scale of the phase separation cannot be controlled reproducibly in these devices. Another problem for phase-separated configurations is poor stability of the domain size lowering the device efficiency. To date the photovoltaic effect has only been demonstrated with calamitic LCs via an ionic mechanism, although electronic photoconductivity has been well researched. Polymerizable LCs, with two polymerizable groups attached through flexible aliphatic spacers to an aromatic core, are attracting interest as organic semiconductors for electroluminescence and organic transistors. Their advantages include spontaneous self-assembly, relatively high charge mobility (1 × 10 cm V s), easy deposition by spin-coating or ink-jet printing and lithographic photopatterning to form multilayer polymer networks. We now report the use of reactive mesogens in a novel way to demonstrate a new approach to photovoltaics. Liquid-crystalline gels are formed from a homogeneous mixture of a reactive mesogen and a non-polymerizable LC. Polymerization results in controlled phase separation of the two components and the formation of a polymer network matrix around the nematic droplets. To date, transparent LC gels have typically been used in direct-view reflective displays, switchable polarizers, directional reflectors, and so forth. Polymerization-induced phase separation of a homogeneous mixture of a LC and a polymer-forming material brings the concept of a paintable display closer. A key step in our new approach is the use of mixtures of hole-transporting reactive mesogens with a low IP and analogous LCs with the same aromatic core to form an electron-donating LC gel. An ideal LC composite photovoltaic is illustrated in Figure 1. It has a vertically separated, large-area interface between electron-donating and electron-accepting layers providing complete pathways for transport of holes and electrons. An electron-blocking polymer network of the electron-donating reactive mesogen is deposited first on an electrode surface by spin-coating from solution and then crosslinking in order to provide a high open circuit. A mixture of the same electron-donating reactive mesogen and the corresponding non-polymerizable analogue are then deposited in a similar C O M M U N IC A TI O N S

Ordered Silica Microspheres Unsymmetrically Coated with Ag Nanoparticles, and Ag‐Nanoparticle‐Doped Polymer Voids Fabricated by Microcontact Printing and Chemical Reduction

Colloid particles usually have charges or nanoparticles uniformly distributed over their surface. The design and preparation of unsymmetrically coated colloid particles have been a long-standing challenge in surface and colloid science. These particles would remedy some limitations of their spherical counterparts for potential applications in modeling the behaviors of highly irregular colloids that are more commonly found in industrial products, in the fields that require lattices with lower symmetries and high complexities, and as potential building blocks in generating three-dimensional (3D) photonic crystals with complete bandgaps. However, due to the thermodynamic limitations of the reaction, there have been only a few reports about the preparation of microspheres unsymmetrically coated with nanoparticles, which typically involve Langmuir–Blodgett techniques, the evaporation of metals on colloidal particles, controlled phase separation, and using the gas/liquid and liquid/solid interface action. Although some of these methods are effective to fabricate unsymmetrically coated particles, challenges in this field still exist. For example, the assembly of these particles into ordered arrays, enhancement of the unsymmetrical coating density, and precise control of the tropism of these particles have not been well developed. Two-dimensional (2D) structured arrays and patterns are also important due to their potential applications in engineering microelectronic and optoelectronic devices, the fabrication of biological and chemical sensors, and for controlled crystallization. Existing, elegant approaches that involve lithography, imprinting, and soft lithography techniques have been successfully applied to create patterned surfaces in microelectronic and plastic electronics. Among them, soft lithography encompasses a set of flexible methods for patterning materials. As a branch of soft lithography, microcontact printing (lcp) has also been used to modify solid surfaces with different properties, such as charge nature and wettability, to direct the deposition of colloidal microspheres on special regions of substrates. Recently, lift-up soft lithography and modified microcontact printing methods have been developed to pattern colloidal crystals. In this communication, the fabrication of ordered silica microspheres unsymmetrically coated with Ag nanoparticles using lift-up soft lithography and chemical reduction is reported. Taking advantage of the flexibility of lcp, these microspheres are easily transferred onto polymer-coated solid substrates and precisely realize a tropism conversion. By etching away the silica microspheres, ordered Ag-nanoparticle-doped polymer voids are obtained. These silica microspheres unsymmetrically coated with Ag nanoparticles and Ag-nanoparticle-doped polymer voids can also be used as templates to fabricate ordered Ag-nanoparticle-doped polymer and gold composite voids with different morphologies. Compared with previous methods, it is believed that some progress in preparing ordered microspheres unsymmetrically coated with nanoparticles and nanoparticle-doped composite voids has been made, in that: a) the unsymmetrical coating density on the microspheres is increased, b) the ordered array of these unsymmetrically coated microspheres has been realized, c) the tropism of these ordered unsymmetrically coated microspheres can be well controlled, and d) ordered nanoparticle-doped polymer or polymer and metal composite voids with different morphologies can be easily obtained. Due to this progress, this method will provide a powerful platform for fabricating ordered, versatile, colloidal microspheres unsymmetrically coated with nanoparticles, and hybrid patterns. Figure 1 outlines the procedure for preparing ordered silica microspheres unsymmetrically coated with Ag nanoparticles and Ag-nanoparticle-doped polymer voids. First, monodisperse silica microspheres are assembled into colloidal crystals on a silicon wafer. Using the lift-up soft lithography technique, a single layer of close-packed silica microspheres are then transferred onto the surface of a poly(dimethylsiloxane) (PDMS) stamp. After depositing Ag nanoparticles on these microspheres by chemical reduction and spin-coating a thin C O M M U N IC A TI O N S

Processing of Macroporous Titania Thin Films: From Multiscale Functional Porosity to Nanocrystalline Macroporous TiO2

Titania thin films with hierarchical pore structure (macropores and nested mesopores) were obtained in one step by a combination of sol−gel synthesis with controlled phase separation. Poly(ethylene...

Hierarchically structured transparent hybrid membranes by in situ growth of mesostructured organosilica in host polymer

The elaborate performances characterizing natural materials result from functional hierarchical constructions at scales ranging from nanometres to millimetres, each construction allowing the material to fit the physical or chemical demands occurring at these different levels. Hierarchically structured materials start to demonstrate a high input in numerous promising applied domains such as sensors, catalysis, optics, fuel cells, smart biologic and cosmetic vectors. In particular, hierarchical hybrid materials permit the accommodation of a maximum of elementary functions in a small volume, thereby optimizing complementary possibilities and properties between inorganic and organic components. The reported strategies combine sol-gel chemistry, self-assembly routes using templates that tune the material's architecture and texture with the use of larger inorganic, organic or biological templates such as latex, organogelator-derived fibres, nanolithographic techniques or controlled phase separation. We propose an approach to forming transparent hierarchical hybrid functionalized membranes using in situ generation of mesostructured hybrid phases inside a non-porogenic hydrophobic polymeric host matrix. We demonstrate that the control of the multiple affinities existing between organic and inorganic components allows us to design the length-scale partitioning of hybrid nanomaterials with tuned functionalities and desirable size organization from ångström to centimetre. After functionalization of the mesoporous hybrid silica component, the resulting membranes have good ionic conductivity offering interesting perspectives for the design of solid electrolytes, fuel cells and other ion-transport microdevices.

Insulin Particle Formation in Supersaturated Aqueous Solutions of Poly(Ethylene Glycol)

Protein microspheres are of particular utility in the field of drug delivery. A novel, completely aqueous, process of microsphere fabrication has been devised based on controlled phase separation of protein from water-soluble polymers such as polyethylene glycols. The fabrication process results in the formation of spherical microparticles with narrow particle size distributions. Cooling of preheated human insulin-poly(ethylene glycol)-water solutions results in the facile formation of insulin particles. To map out the supersaturation conditions conducive to particle nucleation and growth, we determined the temperature- and concentration-dependent boundaries of an equilibrium liquid-solid phase separation. The kinetics of formation of microspheres were followed by dynamic and continuous-angle static light scattering techniques. The presence of PEG at a pH that was close to the protein’s isoelectric point resulted in rapid nucleation and growth. The time elapsed from the moment of creation of a supersaturated solution and the detection of a solid phase in the system (the induction period, tind) ranged from tens to several hundreds of seconds. The dependence of tind on supersaturation could be described within the framework of classical nucleation theory, with the time needed for the formation of a critical nucleus (size <10nm) being much longer than the time of the onset of particle growth. The growth was limited by cluster diffusion kinetics. The interfacial energies of the insulin particles were determined to be 3.2–3.4 and 2.2mJ/m2 at equilibrium temperatures of 25 and 37°C, respectively. The insulin particles formed as a result of the process were monodisperse and uniformly spherical, in clear distinction to previously reported processes of microcrystalline insulin particle formation.

Controlled Phase Separation of Polyfluorene Blends via Inkjet Printing

We report the use of inkjet printing (IJP) to produce thin polymer films with controlled phase separation of binary polymer blends. Photovoltaic devices comprising a blend of poly(9,9‘-dioctyfluorene-co-bis-N,N‘-(4-butylphenyl)-bis-N,N‘-phenyl-1,4-phenylenediamine) (PFB) with poly(9,9‘-dioctylfluorene-co-benzothiadiazole) (F8BT) and light-emitting diodes (LEDs) constituting a blend of F8BT with poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) were fabricated. The phase separation in these polymer blends was analyzed using atomic force microscopy and photoluminescent optical microscopy. The lateral phase separation in both blend films spin-coated from p-xylene solution is typically a few microns; however, by inkjet printing a solution of an identical composition, the feature size can be reduced to ∼300 nm when combined with an elevated substrate temperature during the printing process. This results from the rapid solvent evaporation of the inkjet-printed droplets. LEDs made by IJP show wel...

Phase separation of water–alcohol binary mixtures induced by the microheterogeneity

The relationship between liquid-liquid phase separation and microheterogeneity in water-primary alcohol mixtures was examined by analysing the mass spectra of clusters generated through the fragmentation of liquid droplets. By comparing the cluster structures of water-ethanol, -1-propanol, and -1-butanol binary mixtures at various alcohol concentrations, we discovered differences in the molecular clusters that control phase separation. We also studied the role of water in alcohol self-association. Alcohol self-association is promoted in the presence of a small amount of water (ca. 10 approximately 20 wt%), in which the water-water hydrogen-bonding network is weak and does not contribute to alcohol self-association. We have demonstrated that alcohol self-association is also promoted by non-ideal mixing with other alcohols. The self-association of alcohol molecules complements the loss of stabilization energy caused by the relatively weak coexisting interactions. This complementary relationship among intermolecular interactions is an inherent property of solutions, and plays a key role in the phase separation process.

Oil Core−Polymer Shell Microcapsules Prepared by Internal Phase Separation from Emulsion Droplets. I. Characterization and Release Rates for Microcapsules with Polystyrene Shells

Microcapsules with an oil core surrounded by a polymeric shell have been prepared by the controlled phase separation of polymer dissolved within the oil droplets of an oil-in-water emulsion. The dispersed oil phase consists of the shell polymer (polystyrene), a good solvent for the polymer (dichloromethane), and a poor solvent for the polymer (typically hexadecane). Removal of the good solvent results in phase separation of the polymer within the oil droplets. If the three interfacial tensions between the core oil, the shell-forming polymer, and the continuous phase are of the required relative magnitudes, a polymer shell forms surrounding the poor solvent. A UV-responsive organic molecule was added to the oil phase, prior to emulsification, to investigate the release of a model active ingredient from the microcapsules. This molecule should be soluble in the organic core but also have some water solubility to provide a driving force for release into the continuous aqueous phase. As the release rate of the active ingredient is a function of the thickness of the polymeric shell, for controlled release applications, it is necessary to control this parameter. For the preparative method described here, the thickness of the shell formed is directly related to the mass of polymer dissolved in the oil phase. The rate of volatile solvent removal influences the porosity of the polymer shell. Rapid evaporation leads to cracks in the shell and a relatively fast release rate of the active ingredient. If a more gentle evaporation method is employed, the porosity of the polymer shell is decreased, resulting in a reduction in release rate. Cross-linking the polymer shell after capsule formation was also found to decrease both the release rate and the yield of the active ingredient. The nature of the oil core also affected the release yield.

Ordered Block-Copolymer Assembly Using Nanoimprint Lithography

Nanoimprint lithography and self-assembly of poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer are combined to induce order in the phase-separated domains. Tailored periodic arrays of poly(methyl methacrylate) (PMMA) cylinders normal or parallel to neutralized silicon surfaces can be formed inside the gap of imprint molds. This method opens up a new route to the controlled phase separation of block copolymers with precise placement of the phase-separated domains.

Two-dimensional supramolecular organization of copper octaethylporphyrin and cobalt phthalocyanine on Au(111): molecular assembly control at an electrochemical interface.

Mixed adlayers of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine copper(II) (CuOEP) and cobalt(II) phthalocyanine (CoPc) were prepared by immersing Au(111) substrate in a benzene solution containing CuOEP and CoPc molecules, and they were investigated in 0.1 M HClO(4) by cyclic voltammetry (CV) and in-situ scanning tunneling microscopy (STM). The composition of the mixed adlayer consisting of CuOEP and CoPc molecules was found to vary depending on the immersion time. CoPc molecules displaced CuOEP molecules during the modification process with increasing immersion time, and the CuOEP molecules were completely replaced with CoPc molecules in the mixed solution after a long modification time. The two-component adlayer consisting of CuOEP and CoPc, which has a structure with the constituent molecules arranged alternately, was found to form either a p(9 x 3(square root)7R - 40.9 degrees) or a p(9 x 3(square root)7R - 19.1 degrees) structure, each involving two molecules on the Au(111) surface. The surface mobility and the molecular reorganization of CuOEP and CoPc were accelerated by modulation of the electrode potential. Different surface structures were produced at different electrode potentials, and hence potential modulation should allow a precisely controllable phase separation to take place in aqueous HClO(4).

Reactive core/shell type hyperbranched blockcopolyethers as new liquid rubbers for epoxy toughening

Novel reactive core/shell-type hyperbranched blockcopolyethers were tailored as new class of liquid rubbers useful as flexibilizers and toughening agents of anhydride-cured epoxy resins. Anionic ring-opening polymerization of glycidol onto a six-arm star poly(propylene oxide-block-ethylene oxide) afforded hyperbranched polyether cores as macroinitiators for propylene oxide graft copolymerization. The hydroxy end groups of the resulting polyether–polyols have been modified in order to prepare stearate, hydroxy benzoate and epoxy derivatives. The modification afforded reactivity and polarity design which has been the key to improved blend performance with epoxy resins. In comparison to conventional hyperbranched epoxy-terminated polyesters, the influence of molecular architectures on thermal, mechanical and morphological properties of hexahydrophthalic acid anhydride-cured bisphenol-A diglycidylether was examined. As a function of polarity and reactivity design, it was possible to control phase separation and to vary mechanical properties from highly flexible to stiff and tough.

Phase separation in the solution of water glass and poly(vinyl alcohol)

Silica gel samples with macropores were prepared from solutions of silicate and poly(vinyl alcohol) (PVA), where macropores were formed by fixing a transitional structure of phase separation. Among the silica sources tested, tetraethoxysilane (TEOS), colloidal silica and water glass, only the system with water glass shows phase separation and forms macroporous silica gel. In the system with TEOS, ethanol formed during hydrolysis of TEOS becomes good solution and stabilizes the system not to induce phase separation. In the system with colloidal silica, dense structure of silica is probably not suitable for controlling phase separation and gelation. In the system with water glass, driving force of phase separation is considered to be a repulsive interaction between solvent molecules and PVA interacting with silica surface and the solution separates into a phase rich in solvent and that rich in silica and PVA. One of the features in the water glass–PVA system is insensitivity of macropore size against compositional change in the solution, i.e. macroporous morphology in the resultant silica gel hardly changes by changing the composition ratio in the solution. This would be an advantage in the preparation of well-defined macroporous silica from water glass, whose composition varies among the product lot number, because reproducibility in macroporous morphology is ensured regardless of the lot number of the water glass.

Photocatalytic Fiber with Gradient Surface Structure Produced from a Polycarbosilane and Its Applications

We developed a new in‐situ process for preparing functional ceramic fibers with a gradient surface layer by means of precursor methods using a polycarbosilane. After incorporation of selected low‐molecular‐mass additives into the precursor polymer from which the ceramic forms, thermal treatment of the resulting bodies leads to controlled phase separation (“bleed out”) of the additives. Subsequent calcination stabilizes the compositionally changed surface region, generating a functional surface layer. Using this technology, we developed a strong photocatalytic fiber (TiO2‐covered SiO2 fiber), which effectively decomposed many types of organic chemicals and bacteria into CO2and water by irradiation with UV light. Furthermore, we performed some field tests using a circulation purifier with a module composed of the cone‐shaped felt material of our new fiber.

Versatile Double-Templating Synthesis Route to Silica Monoliths Exhibiting a Multimodal Hierarchical Porosity

Monolithic silica exhibiting a trimodal, hierarchical pore structure has successfully been prepared via sol−gel-processing. Monolithic bodies with interconnected macropores in the micrometer range are a result of controlled phase separation and gelation kinetics, whereas textural mesopores in the 10−20 nm range originate from voids between particles. Furthermore, the particles exhibit internal mesoporosity with pore diameters in the 2−4 nm range originating from supramolecular templating by the surfactant. Poly(ethylene glycol) has been used together with alkylammonium surfactants to control the particle aggregation and internal structure, respectively. The present work sheds light on the mechanism of formation of the complex framework architectures and the application potential of the material.

Factors Controlling Phase Separation in Water−Salt Solutions of DNA and Polycations

Factors affecting phase separation in water-salt solutions of polyelectrolyte complexes (PECs), formed by DNA and integral or pendant polycations with a quaternary amino group in every monomer unit, have been studied. When no salt was added, quantitative DNA precipitation occurred at a stoichiometric charge ratio, = [+]/[-] 1. In DNA mixtures with poly(N,N'-dimethyldiallylammonium chloride) (PDMDAAC, a pendant polycation), insoluble PECs formed in the range 0.7 2. For different aliphatic ionene bromides (integral polycations), the range of corresponding to insoluble PECs was significantly broader, mainly due to the poor ability of the ionenes to form soluble, positively charged PECs. The range was also relatively broad for poly(N-ethyl-4-vinylpyridinium bromide) (a pendant polycation) and became broader with decreasing degree of polymerization of the polycation. The formation of insoluble PECs was favored by the addition of salt (NaCl), and the effect was more pronounced when decreasing the relative content of the solubilizing component, i.e., the nucleic acid at 1. At moderate ionic strength, 0.12 M < [NaCl] < 0.6 M, quantitative precipitation of DNA was attained by addition of PDMAAC in the whole region studied: 1 < < 4.5. The data obtained strongly suggest that phase separation in solutions of DNA-containing PECs follows general rules revealed by studying PECs formed by flexible vinyl polyanions. However, the high rigidity of the DNA double helix appears to be responsible for the key feature revealed in the phase diagrams, i.e., significant broadening of the region for insoluble PECs at the expense of the region in which soluble DNA-containing PECs are formed. This feature may severely limit the application of DNA-containing PECs in medicine and biology but could be beneficial in the development of simple and effective procedures for DNA separation in biotechnology.

La2O3‐Al2O3‐SiO2Glasses for High‐Power, Yb3+‐Doped, 980‐nm Fiber Lasers

Single‐mode semiconductor pumps have failed to keep pace with the increasing power requirements of Er‐doped fiber amplifiers (EDFAs), so there is a need for high‐powered 980‐nm sources. Yb3+‐doped tapered fiber lasers can provide high‐power output by conversion of a low‐brightness, high‐powered, 920‐nm, multimode broad stripe laser diode to a high‐brightness, 980‐nm, single‐mode output. The tapered fiber laser requires a fiber with high numerical aperture (NA) (&gt;0.4), a rectangular core, and good Yb3+spectroscopy for efficient operation. CVD‐based fiber fabrication methods are incapable of delivering fibers with an NA &gt; ∼0.3 or with good efficiency at 980 nm so a new method of high‐NA fiber fabrication was developed. The core glass composition is critical for maintaining a high‐NA fiber with good power extraction while avoiding phase separation, loss, and clustering. The SiO2level controlled the NA and interdiffusion between core and clad, while the Al2O3/La2O3ratio controlled phase separation. A La2O3‐Al2O3‐SiO2glass was developed that is compatible with a pure SiO2cladding glass and has a laser slope efficiency of 70% at 980 nm. The optimized fiber composition yielded 450 mW of 980‐nm power in a single‐mode fiber.