Enhancing light extraction in top-emitting organic light-emitting devices using molded transparent polymer microlens arrays.

The light extraction efficiency in organic light-emitting devices (OLEDs) is enhanced by up to 2.6 times when a close-packed, hemispherical transparent polymer microlens array (MLA) is molded on the light-emitting surface of a top-emitting device. The microlens array helps to extract the waveguided optical emission in the organic layers and the transparent top electrode, and can be manufactured in large area with low cost.

Biomimetic mineralization of woven bone-like nanocomposites: role of collagen cross-links.

Ideal biomaterials for bone grafts must be biocompatible, osteoconductive, osteoinductive and have appropriate mechanical properties. For this, the development of synthetic bone substitutes mimicking natural bone is desirable, but this requires controllable mineralization of the collagen matrix. In this study, densified collagen films (up to 100 µm thick) were fabricated by a plastic compression technique and cross-linked using carbodiimide. Then, collagen-hydroxyapatite composites were prepared by using a polymer-induced liquid-precursor (PILP) mineralization process. Compared to traditional methods that produce only extrafibrillar hydroxyapatite (HA) clusters on the surface of collagen scaffolds, by using the PILP mineralization process, homogeneous intra- and extrafibrillar minerals were achieved on densified collagen films, leading to a similar nanostructure as bone, and a woven microstructure analogous to woven bone. The role of collagen cross-links on mineralization was examined and it was found that the cross-linked collagen films stimulated the mineralization reaction, which in turn enhanced the mechanical properties (hardness and modulus). The highest value of hardness and elastic modulus was 0.7 ± 0.1 and 9.1 ± 1.4 GPa in the dry state, respectively, which is comparable to that of woven bone. In the wet state, the values were much lower (177 ± 31 and 8 ± 3 MPa) due to inherent microporosity in the films, but still comparable to those of woven bone in the same conditions. Mineralization of collagen films with controllable mineral content and good mechanical properties provide a biomimetic route toward the development of bone substitutes for the next generation of biomaterials. This work also provides insight into understanding the role of collagen fibrils on mineralization.

Two-photon absorption and degenerate four-wave mixing studies of sulfide semiconductor nanoparticles in polymeric solutions.

The two-photon absorption coefficients (beta) and the third-order nonlinear susceptibilities (chi(3)) of several semiconductor nanoparticles (CdS, Cd(x)Ag(1-x)S, and core-shell CdS/Ag2S) that are confined and stabilized by random and block ionomers have been measured by nonlinear transmission and degenerate four-wave mixing techniques using 21 picosecond laser pulses at near-infrared spectral region. The imaginary part of the third-order nonlinear susceptibility that is related to the two-photon absorption coefficient was then calculated. The absorptive nonlinearity of the nanoparticles (2 approximately 9 nm) was found to be dependent on the particle size, composition and wavelength, i.e., larger CdS particles exhibit higher two-photon absorption coefficients and the presence of Ag improves two-photon absorption of CdS nanoparticles. The obtained two-photon absorption coefficients of nanoparticles corrected for their volume fraction in solution are significantly greater that those of corresponding bulk semiconductors.

Effects of various salts on structural polymorphism of reconstituted type I collagen fibrils.

Even though the behavior of collagen monomers self-assembling into fibrils is commonly understood in terms of hydrophobic and electrostatic interactions, the mechanisms that drive their ordered, longitudinal alignment to form a characteristic periodicity are still unclear. By introducing various salts into the collagen fibrillogenesis system, the intermolecular interactions of fibril formation were studied. We found that the pH and ion species play a critical role in forming native fibrils. Turbidity and electron microscopy revealed that collagen self-assembled into fibrils with a native banding pattern in the presence of multivalent ions. The isoelectric point of collagen in 12mM of NaCl is pH 8.9; it shifted to pH 9.4 and pH 7.0 after adding 10mM CaCl2 and Na2SO4, respectively. Native fibrils were reconstituted at pH 7.4 in salts with divalent anions and at pH 9.0 in salts with divalent cations. Circular dichroism spectroscopy showed a loss of helicity in the conditions where fibrillogenesis was unable to be achieved. The multivalent ions not only change the surface charge of collagen, but also facilitate the formation of fibrils with the native D-periodic banding pattern. It is likely that the binding multivalent ions induce the like-charge attraction and facilitate monomers' longitudinal registration to form fibrils with the native banding.

Surface modification of cured cement pastes by silane coupling agents.

X-ray photoelectron spectroscopy (XPS) and static contact angle measurements were used to study the interaction between silane coupling agents and cured cement paste. Three different silane coupling agents were investigated: aminopropyltriethoxy silane (APTES), 3-glycidyloxypropyltrimethoxy silane (GPTMS), and methoxy-terminated polydimethxyl siloxane (PDMS). These silanes have different end groups, so the change in surface energy after undergoing a successful reaction between the silane and hydroxyls on the surface of the cement paste was demonstrated by a change in contact angle. Relative to untreated samples, APTES samples decreased the contact angle, PDMS samples increased the contact angle, and GPTMS did not show a significant change in contact angle. Samples with a water-to-cement ratio (w/c) of 0.5 showed a larger change in contact angle than 0.4 w/c ratio samples, because of a greater number of hydroxyl groups at the surface. Deconvolution of the O 1s and Si 2p XPS peaks were performed to determine contributions from bridging and nonbridging atoms. An increase in bridging silicon and oxygen atoms relative to untreated samples indicated successful silane condensation and that a covalent bond was formed between the cement paste and silanes.

Complex hygrothermal effects on the glass transition of an epoxy-amine thermoset.

Anomalous hygrothermal behavior in an epoxy-amine thermoset was observed in terms of increases in glass-transition temperatures (T(g)) after immersion in water at different temperatures. Fourier transform near-infrared (FT-NIR) spectroscopic measurements showed that an increase in conversion was responsible for the increase in glass transition, while plasticization occurred simultaneously, rendering the hygrothermal behavior to be complex. To consider other factors affecting this complex hygrothermal behavior, a relationship between T(g) and conversion was constructed for the unexposed system and compared to the corresponding T(g) values for the exposed system at the same point of each conversion value. With this method, the conversion was successfully excluded to compare the T(g) values directly between the unexposed and exposed systems. This indicates the effects of other factors are negligible compared with the large plasticization effect. The plasticization effect was also evaluated quantitatively by observing T(g) differences between the unexposed and the exposed system as a function of absorbed water amount estimated by a characteristic water peak in the NIR spectra. The result indicates that there is additional anomalous behavior in which DeltaT(g) changes independently of the water amount at higher exposure temperatures.

Patterning inorganic (CaCO3) thin films via a polymer-induced liquid-precursor process.

The biomimetic synthesis of patterned mineral thin films, based on a combination of the microcontact printing technique and a novel crystallization process called the polymer-induced liquid-precursor (PILP) process, is demonstrated. The PILP process enables the deposition of smooth and continuous calcitic mineral films (up to 1500 nm in thickness) under low-temperature and aqueous-based processing conditions. The films are formed by deposition of colloidal droplets composed of a liquid-phase mineral precursor that is induced by a polymeric process-directing agent (polyaspartate or polyacrylate salts). The droplets can be preferentially deposited onto patterned substrates templated with self-assembled monolayers (SAMs) of alkanethiolate on gold. The droplets coalesce to form an amorphous mineral film, which then transforms (solidifies and crystallizes) while retaining the shape of the patterned template, providing a means for patterning the location and morphology of two-dimensional calcite crystals. A vertical substrate experiment supports the premise that the calcite films are created by adsorption of colloidal droplets from solution, rather than heterogeneous nucleation and growth of an amorphous phase on the SAMs. Large single-crystalline domains, on the order of 50-100 microm, can be "molded" into nonequilibrium morphologies by constraining the mineral precursor to a chemically defined "compartment". Biominerals are well recognized for their elaborate nonequilibrium molded crystal morphologies, and increasing evidence suggests that many biominerals are formed from an amorphous precursor that is stabilized by polyanionic proteins. The biomimetic system examined here, which consists of a polyanionic process-directing agent in combination with a functionalized organic template, offers a practical tool for generating complex inorganic structures such as those found in biominerals.

Instabilities during the formation of electroactive polymer thin films.

The solvent-induced film structure of poly(n-vinyl carbazole) (PVK) thin films on indium tin oxide (ITO)-coated glass was examined. PVK thin films were prepared via spin-coating using five different solvents. We investigated the relationship between the solvent characteristics and film properties, including surface roughness and structure, film thickness, and density. The spin-coated polymer thin films are not in thermodynamic equilibrium; rather, the film properties are affected by the dynamics of the spin-coating process. We found that water present in tetrahydrofuran (THF) induces dewetting of PVK films during the spin-coating process. Solvents with a high evaporation rate lead to high surface roughness due to Marangoni convection. The results show that the surface roughness and structure of the films are dominated by the dynamics of the film formation process, rather than thermodynamic interactions between the polymer and solvents.

A new paradigm for biomineral formation: mineralization via an amorphous liquid-phase precursor.

Biologically mineralized tissues are well recognized for their unusual crystal morphologies and hierarchically organized composite structures. The soluble acidic macromolecules associated with biominerals are thought to play an important role in modulating the mineral morphology. Our in vitro studies, which use acidic polypeptide additives to modify crystal growth of calcium-based minerals, have demonstrated a crystallization mechanism that proceeds via a liquid-phase mineral precursor. Various features of the crystals produced via this mechanism, such as "extruded" mineral fibers and mineralized collagen composites, have led us to propose the hypothesis that an amorphous, liquid-phase precursor could play a fundamental role in the morphogenesis of calcium-based biominerals. Although in vivo evidence of this process remains to be determined, we demonstrate crystallization features that mimic bone and dental enamel and suggest that this process could be relevant to biomineralization in both vertebrates and invertebrates.