Hierarchical Inorganic–Organic Nanocomposites Possessing Amphiphilic and Morphological Complexities: Influence of Nanofiller Dispersion on Mechanical Performance

Novel nanocomposites possessing ternary compositions and complex morphologies have been prepared from amphiphilic crosslinked hyperbranched fluoropolymer–poly(ethylene glycol) (HBFP–PEG) in the presence of pristine and chemically functionalized nanoscopic fillers, single‐walled carbon nanotubes (SWNTs) and silica nanoparticles (SiO₂). Both SWNTs and SiO₂ were engineered specifically to become phase‐designated reinforcing functional materials, SWNT‐g‐PEG and SiO₂‐g‐HBFP, which (1) improved the dispersion of fillers, nanotubes, or spherical nanoparticles in the amphiphilic matrices, (2) enhanced the non‐covalent interactions between nanofillers and polymers, and more importantly, (3) maintained reactive functionalities to be further covalently integrated into the complex networks. Tensile moduli (E_dry) for these as‐prepared SWNT‐containing composites increased by up to 430% relative to the unfilled material, while those incorporated with SiO₂ had a 420% increase of E_dry. After swelling in water, the water absorption within the micro‐ and nanochannels of PEG‐rich domains rigidified or softened the entire crosslinked network, as determined by the amount of PEG.     

Nanocomposite Electrolytes with Fumed Silica and Hectorite Clay Networks: Passive versus Active Fillers

The use of nanocomposites constitutes a versatile and robust approach in the development of novel electrolytes with tailored electrochemical and mechanical characteristics. In this study, we examine the morphology, rheology, and ion‐transport properties of two types of nanocomposite electrolyte gels, one consisting of branched silica nanoparticles and the other composed of hectorite clay. In the first system with hydrophobic (fumed) silica, oligomers of poly(ethylene oxide) (PEO), and lithium salt, the silica acts as a passive filler and does not participate in ion transport. The electrochemical properties are controlled by the salt–PEO electrolyte, allowing for ionic conductivities greater than 10^–3 S cm^–1 at ambient temperature. At sufficiently high concentrations, the silica forms an elastic gel possessing a large open network structure that provides for unimpeded ion mobility. In the second system composed of lithium‐exchanged hectorite filler, the nanoscale platelets serve as the anion. This active filler yields ionic conductivities in excess of 10^–4 S cm^–1 and lithium transference numbers approaching unity. Similar to fumed silica, the hectorite clay also forms an elastic gel network. However, the morphologies of the two systems are distinctively different both in terms of network structure and characteristic length scale. These morphological differences manifest themselves in different rheological responses with regard to gel modulus and yield stress.     

Evolution‐Based Design of an Injectable Hydrogel

A new class of simple, linear, amphiphilic peptides are developed that have the ability to undergo triggered self‐assembly into self‐supporting hydrogels. Under non‐gelling aqueous conditions, these peptides exist in a random coil conformation and peptide solutions have the viscosity of water. On the addition of a buffered saline solution, the peptides assemble into a β‐sheet rich network of fibrils, ultimately leading to hydrogelation. A family of nine peptides is prepared to study the influence of peptide length and amino acid composition on the rate of self‐assembly and hydrogel material properties. The amino acid composition is modulated by varying residue hydrophobicity and hydrophilicity on the two opposing faces of the amphiphile. The conformation of peptides in their soluble and gel state is studied by circular dichroism (CD), while the resultant material properties of their gels is investigated using oscillatory sheer rheology. One weight percent gels formed under physiological conditions have storage modulus (G′) values that vary from ≈20 to ≈800 Pa, with sequence length and hydrophobic character playing a dominant roll in defining hydrogel rigidity. Based on the structural and functional data provided by the nine‐peptide family members, an optimal sequence, namely LK13, is evolved. LK13 (LKLKLKLKLKLKL‐NH₂) undergoes triggered self‐assembly, affording the most rigid gel of those studied (G′=797 ± 105). It displays shear thin‐recovery behavior, allowing its delivery by syringe and is cytocompatibile as assessed with murine C3H10t1/2 mesenchymal stem cells.     

Biopolymer Networks for the Solid‐State Production of Porous Magnetic Beads and Wires

Aqueous solutions of sodium carboxymethyl cellulose are used for the morphosynthesis of spherical and wire‐shaped biopolymer networks, in which Fe³⁺ cations serve as a crosslinking and hardening agent. Their morphology remains intact upon drying, resulting in monolithic beads (1 mm) and wires (ca. 80 μm), which are exploited as reaction vessels to pre‐encapsulate poly(ethylene glycol) 400 (PEG 400) and cobalt cations. A solid‐state reaction in an inert atmosphere at 600 °C affords porous carbonaceous xerogels, macroscopically shaped as beads or wires and decorated with nanocrystalline magnetic iron oxide, metallic iron, or iron–cobalt alloy particles, thus imparting magnetic properties to the products. As such the reduction of Fe³⁺ species to α‐Fe nanoparticles can be achieved without H₂ treatment, since poly(ethylene glycol) serves as a reducing agent and the encapsulated Co²⁺ aids in the subsequent growth of the metallic iron particles. Particularly interesting are the magnetic properties of the carbon–α‐Fe composite, in which the size of the magnetic particles, estimated near the boundaries of the single magnetic domain, gives rise to increased coercivity compared with that of bulk iron.     

Amyloid Fibrils form Hybrid Colloidal Gels and Aerogels with Dispersed CaCO3 Nanoparticles

New hybrid colloidal gels are reported formed by amyloid fibrils and CaCO₃ nanoparticles (CaNPs), with Ca²⁺ as charge screening ions and CaNPs as physical crosslinking agents to establish and stabilize the network. The gel is characterized by rheological measurements and diffusing wave spectroscopy, complemented by microscopic observations using transmission and scanning electron microscopy. The hybrid colloidal gels show a two orders of magnitude improved gel strength at significantly shorter gelation times compared to previous calcium ion‐induced amyloid fibril gels. Supercritical CO₂‐dried colloidal aerogels allow demonstrating that amyloid fibrils, combined with smaller (higher specific surface area) CaNPs, constitute a denser fibrils network, resulting in stronger gels. By varying the amyloid fibril concentration and the CaNPs size and concentration, the complete phase diagram is mapped out. This enables identifying the sol–gel phase transition and a window for gel formation, which widens with increasing CaNPs size. Finally pH responsiveness and self‐healing properties of this hybrid colloidal gel are also demonstrated, making these systems a suitable candidate for biological applications.     

Solid‐State NMR Investigations of the Unusual Effects Resulting from the Nanoconfinement of Water within Amphiphilic Crosslinked Polymer Networks

Two types of solid‐state ¹⁹F NMR spectroscopy experiments are used to characterize phase‐separated hyperbranched fluoropolymer–poly(ethylene glycol) (HBFP–PEG) crosslinked networks. Mobile (soft) domains are detected in the HBFP phase by a rotor‐synchronized Hahn echo under magic‐angle spinning conditions, and rigid (hard) domains by a solid echo with no magic‐angle spinning. The mobility of chains is detected in the PEG phase by ¹H → ¹³C cross‐polarization transfers with ¹H spin‐lock filters with and without magic‐angle spinning. The interface between HBFP and PEG phases is detected by a third experiment, which utilized a ¹⁹F → ¹H–(spin diffusion)–¹H → ¹³C double transfer with ¹³C solid‐echo detection. The results of these experiments show that composition‐dependent PEG inclusions in the HBFP glass rigidify on hydration, consistent with an increase in macroscopic tensile strength.     

Biofunctional Polyelectrolyte Multilayers and Microcapsules: Control of Non‐Specific and Bio‐Specific Protein Adsorption

Layers of the polyelectrolytes poly(allylamine hydrochloride) (PAH, polycationic) and poly(styrene sulfonate) (PSS, polyanionic) are consecutively adsorbed on flat silicon oxide surfaces, forming stable, ultrathin multilayer films. Subsequently, a final monolayer of the polycationic copolymer poly(L ‐lysine)‐graft‐poly(ethylene glycol) (PLL‐g‐PEG) is adsorbed onto the PSS‐terminated multilayer in order to impart protein resistance to the surface. The growth of each of the polyelectrolyte layers and the protein resistance of the resulting [PAH/PPS]_n(PLL‐g‐PEG) multilayer (n = 1–4) are followed quantitatively ex situ using X‐ray photoelectron spectroscopy and in situ using real‐time optical‐waveguide lightmode spectroscopy. In a second approach, the same type of [PAH/PSS]_n(PLL‐g‐PEG) multilayer coatings are successfully formed on the surface of colloidal particles in order to produce surface‐functionalized, hollow microcapsules after dissolution of the core materials (melamine formaldehyde (MF) and poly(lactic acid) (PLA; colloid diameters: 1.2–20 μm). Microelectrophoresis and confocal laser scanning microscopy are used to study multilayer formation on the colloids and protein resistance of the final capsule. The quality of the PLL‐g‐PEG layer on the microcapsules depends on both the type of core material and the dissolution protocols used. The greatest protein resistance is achieved using PLA cores and coating the polyelectrolyte microcapsules with PLL‐g‐PEG after dissolution of the cores. Protein adsorption from full serum on [PAH/PPS]_n(PLL‐g‐PEG) multilayers (on both flat substrates and microcapsules) decreases by three orders of magnitude in comparison to the standard [PAH/PPS]_n layer. Finally, biofunctional capsules of the type [PAH/PPS]_n(PLL‐g‐PEG/PEG‐biotin) (top copolymer layer with a fraction of the PEG chains end‐functionalized with biotin) are produced which allow for specific recognition and immobilization of controlled amounts of streptavidin at the surface of the capsules. Biofunctional multilayer films and capsules are believed to have a potential for future applications as novel platforms for biotechnological applications such as biosensors and carriers for targeted drug delivery.     

Protein‐Release Behavior of Self‐Assembled PEG–β‐Cyclodextrin/PEG–Cholesterol Hydrogels

This paper reports on the degradation and protein release behavior of a self‐assembled hydrogel system composed of β‐cyclodextrin‐ (βCD) and cholesterol‐derivatized 8‐arm star‐shaped poly(ethylene glycol) (PEG₈). By mixing βCD‐ and cholesterol‐derivatized PEG₈ (molecular weights 10, 20 and 40 kDa) in aqueous solution, hydrogels with different rheological properties are formed. It is shown that hydrogel degradation is mainly the result of surface erosion, which depends on the network swelling stresses and initial crosslink density of the gels. This degradation mechanism, which is hardly observed for other water‐absorbing polymer networks, leads to a quantitative and nearly zero‐order release of entrapped proteins. This system therefore offers great potential for protein delivery.     

Substantial Recoverable Energy Storage in Percolative Metallic Aluminum‐Polypropylene Nanocomposites

Chemisorption of the activated metallocene polymerization catalyst derived from [rac‐ethylenebisindenyl]zirconium dichlororide (EBIZrCl₂) on the native Al₂O₃ surfaces of metallic aluminum nanoparticles, followed by exposure to propylene, affords 0–3 metal‐isotactic polypropylene nanocomposites. The microstructures of these nanocomposites are characterized by X‐ray diffraction, transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. Electrical measurements show that increasing the concentration of the filler nanoparticles increases the effective permittivity of the nanocomposites to ?_r values as high as 15.4. Because of the high contrast in the complex permittivities and conductivities between the metallic aluminum nanoparticles and the polymeric polypropylene matrix, these composites obey the percolation law for two‐phase composites, reaching maximum permittivities just before the percolation threshold volume fraction, v_f ≈ 0.16. This unique method of in situ polymerization from the surface of metallic Al particles produces a new class of materials that perform as superior pulse‐power capacitors, with low leakage current densities of ≈10^?7–10^?9 A/cm² at an applied field of 10⁵ V/cm, low dielectric loss in the 100 Hz–1 MHz frequency range, and recoverable energy storage as high as 14.4 J/cm³.     

Preparation of Uniform,Water‐Soluble,and Multifunctional Nanocomposites with Tunable Sizes

Novel, thiol‐functionalized, and superparamagnetic, silica composite nanospheres (SH‐SSCNs) with diameters smaller than 100 nm are successfully fabricated through the self‐assembly of Fe₃O₄ nanoparticles and polystyrene₁₀₀‐block‐poly(acrylic acid)₁₆ and a subsequent sol‐gel process. The size and magnetic properties of the SH‐SSCNs can be easily tuned by simply varying the initial concentrations of the magnetite nanoparticles in the oil phase. By incorporating fluorescent dye molecules into the silica network, the composite nanospheres can be further fluorescent‐functionalized. The toxicity of the SH‐SSCNs is evaluated by choosing three typical cell lines (HUVEC, RAW264.7, and A549) as model cells, and no toxic effects are observed. It is also demonstrated that SH‐SSCNs can be used as a new class of magnetic resonance imaging (MRI) probes, having a remarkably high spin–spin (T₂) relaxivity (r₂* = 176.1 mM ^?1 S^?1). The combination of the sub‐100‐nm particle size, monodispersity in aqueous solution, superparamagnetism, and fluorescent properties of the SH‐SSCNs, as well as the non‐cytotoxicity in vitro, provides a novel and potential candidate for an earlier MRI diagnostic method of cancer.     

Polymeric Multilayers that Contain Silver Nanoparticles can be Stamped onto Biological Tissues to Provide Antibacterial Activity

The design of polyelectrolyte multilayers (PEMs) that can be prefabricated on an elastomeric stamp and mechanically transferred onto biomedically‐relevant soft materials, including medical‐grade silicone elastomers (E’～450–1500 kPa; E’‐elastic modulus) and the dermis of cadaver skin (E’～200–600 kPa), is reported. Whereas initial attempts to stamp PEMs formed from poly(allylamine hydrochloride) and poly(acrylic acid) resulted in minimal transfer onto soft materials, we report that integration of micrometer‐sized beads into the PEMs (thicknesses of 6–160 nm) led to their quantitative transfer within 30 seconds of contact at a pressure of ～196 kPa. To demonstrate the utility of this approach, PEMs were impregnated with a range of loadings of silver‐nanoparticles and stamped onto the dermis of human cadaver skin (a wound‐simulant) that was subsequently incubated with bacterial cultures. Skin dermis stamped with PEMs that released 0.25 ± 0.01 μg cm^?2 of silver ions caused a 6 log₁₀ reduction in colony forming units of Staphylococcus epidermidis and Pseudomonas aeruginosa within 12 h. Significantly, this level of silver release is below that which is cytotoxic to NIH 3T3 mouse fibroblast cells. Overall, this study describes a general and facile approach for the functionalization of biomaterial surfaces without subjecting them to potentially deleterious processing conditions.     

Mechanically Robust,Highly Ionic Conductive Gels Based on Random Copolymers for Bending Durable Electrochemical Devices

Mechanically robust, highly ionic conductive gels based on a random copolymer of poly[styrene‐ran‐1‐(4‐vinylbenzyl)‐3‐methylimidazolium hexafluorophosphate] (P[S‐r‐VBMI][PF₆]) and the ionic liquid 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) are successfully prepared. The gels with either homo P[VBMI][PF₆] or conventional PS‐block‐poly(methyl methacrylate)‐block‐PS (SMS) show significant trade‐off between ionic conductivity and mechanical resilience. In contrast, the P[S‐r‐VBMI][PF₆]‐based gels exhibit both large elastic modulus (≈0.105 MPa) and ionic conductivity (≈1.15 mS cm⁻¹) at room temperature. To demonstrate that these materials can be used as solid‐state electrolytes, the ion gels are functionalized by incorporating electrochromic (EC) chromophores (ethyl viologen, EV²⁺) and are applied to EC devices (ECDs). The devices show low‐voltage operation, large optical transmittance variation, and good cyclic coloration/bleaching stability. In addition, flexible ECDs are fabricated to take advantage of the mechanical properties of the gels. The ECDs have excellent bending durability under both compressive and tensile strains. The versatile P[S‐r‐VBMI][PF₆]‐based gel is anticipated to be of advantage in flexible electrochemical applications, such as batteries and electrochemical displays.     

Modulation of Viscoelasticity and HIV Transport as a Function of pH in a Reversibly Crosslinked Hydrogel

Materials that respond to physiological stimuli are important in developing advanced biomaterials for modern therapies. The reversibility of covalent crosslinks formed by phenylboronate (PBA) and salicylhydroxamate (SHA) has been exploited to provide a pH‐responsive gel for application to the vaginal tract. Dynamic rheology reveals that the gel frequency‐dependent viscoelastic properties are modulated by pH. At pH 4.8 the viscous component dominates throughout most of the frequency range. As the pH increases, the characteristic relaxation time continues to increase while the G′_Plateau levels off above pH 6. At pH 7.5, the elastic component dominates throughout the frequency sweep and is predominately independent of frequency. Particle tracking assesses the transport of both fluorescently labeled HIV‐1 and 100‐nm latex particles in the PBA–SHA crosslinked gel as a function of pH. At pH 4.8 the ensemble‐averaged mean squared displacement at lag times greater than three seconds reveals that transport of the HIV‐1 and 100‐nm particles becomes significantly impeded by the matrix, exhibiting diffusion coefficients less than 0.0002 µm² s^?1. This pH‐responsive gel thus displays properties that have the potential to significantly reduce the transport of HIV‐1 to susceptible tissues and thus prevent the first stage of male‐to‐female transmission of HIV‐1.     

Supercritical‐Fluid‐Assisted One‐Pot Synthesis of Biocompatible Core(γ‐Fe2O3)/Shell(SiO2) Nanoparticles as High Relaxivity T2‐Contrast Agents for Magnetic Resonance Imaging

Monodisperse iron oxide/microporous silica core/shell composite nanoparticles, core(γ‐Fe₂O₃)/shell(SiO₂), with a diameter of approximately 100 nm and a high magnetization are synthesized by combining sol–gel chemistry and supercritical fluid technology. This one‐step processing method, which is easily scalable, allows quick fabrication of materials with controlled properties and in high yield. The particles have a specific magnetic moment (per kg of iron) comparable to that of the bulk maghemite and show superparamagnetic behavior at room temperature. The nanocomposites are proven to be useful as T₂ MRI imaging agent. They also have potential to be used in NMR proximity sensing, theranostic drug delivery, and bioseparation.     

Functionalized Nanostructures with Liquid‐Like Behavior: Expanding the Gallery of Available Nanostructures

Recently, we have developed a novel family of functionalized nanostructures that exhibit liquid‐like behavior in the absence of solvents and preserve their nanostructure in the liquid state. The gallery of nanostructures developed so far includes functionalized silica and magnetic iron oxide nanoparticles, layer‐like organosilicate nanoparticles, polyoxometalate clusters, and organic–inorganic hybrid networks. In an effort to demonstrate the wider applicability of this concept and to provide a deeper insight into this class of materials, the present work cites additional paradigms of functionalized nanostructures with similar behavior as above. In one case, surface functionalization of anatase nanoparticles (TiO₂, an inorganic nanostructure) with a quaternary ammonium organosilane leads to ionically modified nanoparticles that, when electrostatically combined with a poly(ethylene glycol) (PEG)‐tailed sulfonate anion, exhibit liquid‐like behavior in the absence of solvents. In a different but quite interesting case of a bionanostructure, ion‐exchange functionalization of a DNA oligonucleotide with a PEG‐tailed quaternary ammonium cation leads to an easily separable liquid derivative with attractive features. These examples show the versatility of this concept over a range of nanostructures.     

Hybrid Titanium Oxide/Polymer Amphiphilic Nanomaterials with Controlled Size for Drug Encapsulation and Delivery

This work describes for the first time the synthesis of hybrid drug‐loaded TiO₂/polymer amphiphilic nanoparticles of finely controlled size utilizing a simple and reproducible sol–gel process that comprises the formation of a titanium(IV)/acetone oxo‐organo complex followed by its blending with an amphiphilic poly(ethylene oxide)‐b‐poly(propylene oxide) block copolymer in acetone and its aqueous‐phase nanoprecipitation. The size of the hybrid nanoparticles is governed by the complex aging, e.g., hybrid nanoparticles display diameter between 228 and 53 nm for aging of 1 and 36 days, respectively (dynamic light scattering). In addition, they show excellent physical stability in water owing to the coating of the surface with poly(ethylene oxide) blocks of the copolymer. Conversely, polymer‐free TiO₂ particles are large and precipitate fast. Incorporation of a model hydrophobic drug, nitazoxanide, to the precursor solution results in hybrid nanoparticles containing 12.9% w/w cargo that is released following a bimodal profile. High‐resolution transmission electron microscopy (Titan Cubed Themis G2 300) analysis reveals the porous amorphous nanostructure of these novel hybrids and colocalization of drug and copolymer in the nanoparticle bulk. Finally, upon ultrasonication, our hybrid nanoparticles produce reactive oxygen species in vitro which paves the way for their use in sonodynamic and drug release therapies.     

Hybrids of Magnetic Nanoparticles with Double‐Hydrophilic Core/Shell Cylindrical Polymer Brushes and Their Alignment in a Magnetic Field

The synthesis of double‐hydrophilic core/shell cylindrical polymer brushes (CPBs), their hybrids with magnetite nanoparticles, and the directed alignment of these magnetic hybrid cylinders by a magnetic field are demonstrated. Consecutive grafting from a polyinitiator poly(2‐(2‐bromoisobutyryloxy)ethyl methacrylate) (PBIEM) of tert‐butyl methacrylate (tBMA) and oligo(ethylene glycol) methacrylate (OEGMA) using atom‐transfer radical polymerization (ATRP) and further de‐protection yields core/shell CPBs with poly(methacrylic acid) (PMAA) as the core and POEGMA as the shell, which is evidenced by ¹H NMR, gel permeation chromatography (GPC), and dynamic and static light scattering (DLS and SLS). The resulting core/shell brush is well soluble in water and shows a pH responsiveness because of its weak polyelectrolyte core. Pearl‐necklace structures are observed by cryogenic transmission electron microscopy (cryo‐TEM) at pH 4, while at pH 7, these structures disappear owing to the ionization of the core. A similar morphology is also found for the polychelate of the core/shell CPBs with Fe³⁺ ions. Superparamagnetic magnetite nanoparticles have also been prepared and introduced into the core of the brushes. The hybrid material retains the superparamagnetic property of the magnetite nanoparticles, which is verified by superconducting quantum interference device (SQUID) magnetization measurements. Large‐scale alignment of the hybrid cylinders in relatively low magnetic fields (40–300 mT) can easily be performed when deposited on a surface. which is clearly revealed by the atomic force microscopy (AFM) and TEM measurements.     

Pegylated Composite Nanoparticles Containing Upconverting Phosphors and meso‐Tetraphenyl porphine (TPP) for Photodynamic Therapy

The utilization of upconverting nanophosphors (UCNP) for photodynamic therapy (PDT) has gained significant interests due to its ability to convert deep‐penetrating near‐infra red (NIR) light (i.e., 978 nm) to visible light. Previous attempts to co‐localize UCNPs with photosensitizers suffer from low photo­sensitizer loading and problems with nanoparticle aggregation. Here, the preparation of a novel composite nanoparticle formulation comprising 100 nm β?NaYF₄:Yb³⁺,Er³⁺ UCNPs, and meso‐tetraphenyl porphine (TPP) photo­sensitizer, stabilized by biocompatible poly(ethylene glycol‐block‐(dl )lactic acid) block copolymers (PEG‐b‐PLA) is presented. A photosensitizer loading of 10 wt% with respect to UCNP crystal was achieved via the Flash NanoPrecipitation (FNP) process. A sterically stabilizing PEG layer on the composite nanoparticle surface prevents nanoparticle aggregation and ensures nanoparticle stability in water, PBS buffer, and culture medium containing serum proteins, resulting in nanoparticle suitable for in vivo applications. Based on in vitro studies utilizing HeLa cervical cancer cell lines, the composite nanoparticles are shown to exhibit low dark toxicity and efficient cancer cell‐killing activity upon NIR excitation. Exposure with 134 W cm^?2 of 978 nm light for 45 min resulted in 75% HeLa cell death. This is the first quantification of the cell‐killing capabilities of the UCNP/TPP composite nanoparticles formulated for photodynamic therapy.     

Poled Sol–Gel Materials With Heterocycle Push–Pull Chromophores that Confer Enhanced Second‐Order Optical Nonlinearity

Hybrid organic–inorganic materials doped with zwitterionic push–pull chromophores with high hyperpolarizability have been synthesized by a sol–gel procedure. A large chromophore concentration was reached by using N‐(hydroxyethyl)carbazole as a physical spacer (preventing the dye aggregation). Spin‐coated doped films were electrically poled and second harmonic generation measurements performed in situ. During the thermally assisted poling under a N₂ atmosphere, only the carbazole molecules degraded. Second‐harmonic generation measurements gave an estimation of the nonlinear coefficient, r₃₃, of 38 pm V^–1 at 1064 nm.     

Polymer Light‐Emitting Electrochemical Cells: Doping Concentration,Emission‐Zone Position,and Turn‐On Time

Direct optical probing of the doping progression and simultaneous recording of the current–time behavior allows the establishment of the position of the light‐emitting p–n junction, the doping concentrations in the p‐ and n‐type regions, and the turn‐on time for a number of planar light‐emitting electrochemical cells (LECs) with a 1 mm interelectrode gap. The position of the p–n junction in such LECs with Au electrodes contacting an active material mixture of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV), poly(ethylene oxide), and a XCF₃SO₃ salt (X = Li, K, Rb) is dependent on the salt selection: for X = Li the p–n junction is positioned very close to the negative electrode, while for X = K, Rb it is significantly more centered in the interelectrode gap. Its is demonstrated that this results from that the p‐type doping concentration is independent of salt selection at ca. 2 × 10²⁰ cm^–3 (ca. 0.1 dopants/MEH‐PPV repeat unit), while the n‐type doping concentration exhibits a strong dependence: for X = K it is ca. 5 × 10²⁰ cm^–3 (ca. 0.2 dopants/repeat unit), for X = Rb it is ca. 9 × 10²⁰ cm^–3 (ca. 0.4 dopants/repeat unit), and for X = Li it is ca. 3 × 10²¹ cm^–3 (ca. 1 dopants/repeat unit). Finally, it is shown that X = K, Rb devices exhibit significantly faster turn‐on times than X = Li devices, which is a consequence of a higher ionic conductivity in the former devices.