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1. Field of the Invention
This invention pertains generally to preparing mesostructured materials, and more particularly to controlling the nucleation rate and growth of block-copolymer-templated silica domains to yield highly macroscopically aligned mesostructured materials.
2. Background Discussion
Surfactant-templated mesoporous materials have been a field of interest since their discovery in the early 1990s. In the past decade, extensive research has focused on the development of mesostructured materials from a wide range of surfactants with applications as catalyst supports, membranes, and as hosts for optical devices. Emphasis has been placed on syntheses of diverse compositions (e.g., inorganic oxides), phases (e.g., cubic, hexagonal, and lamellar), and morphologies (e.g., powders, films, fibers, and monoliths). Recently, there has been substantial interest in producing and controlling macroscopic orientational ordering in such mesostructured materials. Photovoltaic cells, light emitting diodes (LEDs), waveguides, fuel cells, etc. all may benefit from the anisotropic properties of orientationally ordered channels and/or guest molecules in aligned mesostructured composite materials. In addition, aligned mesoporous materials may be useful for promoting anisotropic growth of single crystals.
Mesoscale materials have also received substantial attention for their potential to become an inexpensive and efficient new technology in the fields of sensors, membranes, catalysis, and optics. Many potential applications require or would benefit from macroscopic alignment of anisotropic mesostructures with uniform arrays of mesochannels that are accessible to different guest species.
Furthermore, incorporation of photo- (or synonymously, optically) responsive molecules into aligned mesostructured composite materials has potential technological benefits in anisotropic absorption or emission in optical devices, as well as providing evidence of the anisotropic nature of the oriented mesochannels.
In prior work described in U.S. Patent Application Publication No. US-2007-0248760-A1 (Mesostructured Inorganic Materials Prepared with Controllable Orientational Ordering), the entire disclosure of which is incorporated herein by reference, we showed that mesostructured inorganic-organic materials, in the form of patterned films, monoliths, and fibers, can be prepared with controllable orientational ordering over macroscopic length scales. The materials were synthesized by controlling solvent removal rates across material interfaces, in conjunction with the rates of surfactant self-assembly and inorganic cross-linking and surface interactions. In that work, we described a method for controlling the rates and directions of solvent removal from a heterogeneous material synthesis mixture that allows the nucleation and directional alignment of self-assembling mesostructures to be controlled during synthesis. The aligned mesostructured inorganic-organic materials and mesoporous inorganic or carbon materials can be prepared in the form of patterned films, monoliths, and fibers with controllable orientational ordering. Such materials possess anisotropic structural, mechanical, optical, reaction, or transport properties that can be exploited for numerous applications in opto-electronics, separations, fuel cells, catalysis, MEMS/microfluidics, for example.
It has now been found that, with careful control over the rates and directions of the solvent and co-solvent removal, one can control the nucleation rate and growth of block-copolymer-templated silica and titania domains to yield highly macroscopically aligned mesostructured materials.
Accordingly, an aspect of the present invention described herein is control over solvent and cosolvent removal, one embodiment of which is the use of soft-lithographic patterning materials with specific solubility properties for the volatile species in the block copolymer sol-gel precursor solution. The PDMS stamping protocol allows for this control, as well as for the simultaneous patterning of the mesostructured composite material, establishing where domain nucleation occurs, and the direction(s) that they grow, thereby directing the ultimate alignment of a resulting hexagonal mesostructure. Product films result with macroscopically anisotropic properties that can be exploited in membrane and optical applications. One example is the incorporation of photo-responsive supra- or macromolecular guest species, the resulting nanocomposite materials of which may exhibit anisotropic optical properties.
Another aspect of this invention is to develop and control macroscopic orientational ordering of new patterned mesostructured silica or titania films. The hexagonal and lamellar phases of block-copolymer/silica mesostructured materials are of particular interest, due to their intrinsic anisotropy, compared to the cubic phase. Alignment of the mesostructured domains perpendicular to the substrate is of interest because of the importance of creating mesochannel contacts between the substrate and the external surfaces of films in applications for sensors, membranes, and opto-electronic devices. Other orientations of the mesostructured domains parallel to the substrate also have applications in anisotropic optical materials, fuel cell devices, or field effect transistors. Materials with the combination of a high degree of mesoscopic ordering, with controllable alignments, are expected to yield anisotropic properties with significant technological advantages for polarized absorption or emission of light, oriented crystal growth, semipermeable membranes, catalysts, or device assembly.
Another aspect of the invention is a method of forming patterned mesostructured silica or titania films with control over the direction of alignment of a block-copolymer-directed hexagonal mesostructure across macroscopic lengths scales. By way of example, and not of limitation, this can be achieved by using a patterned poly(dimethylsiloxane) soft-lithography stamp to control the rates and directions of the solvent removal, e.g. water, and/or cosolvents, such as ethanol or tetrahydrofuran, from the block-copolymer/silica (or titania) sol-gel solution. In addition to sol-gel composition and block-copolymer architecture, key variables are solution acidity, solvent selection(s), the solvent concentrations in the PDMS stamp, PDMS surfaces in contact with the precursor solution, and surrounding atmosphere, and temperature. These variables collectively influence the relative rates of solvent diffusion and/or evaporation, block-copolymer self-assembly, domain growth, and silica cross-linking. By controlling the direction(s) of solvent and cosolvent fluxes out of the drying film (patterned or otherwise), control can be exerted over the interfaces where the self-assembling domains first nucleate and the direction that they grow.
Vertical, longitudinal, or lateral orientational ordering of patterned, hexagonally mesostructured silica-P132 films have been demonstrated. In particular, mesostructures with high degrees of alignment perpendicular to the substrate can be produced with radially integrated (100) diffraction peak widths as narrow as 3 degrees FWHM observed. The high degree of alignment was also shown to be present over large macroscopic regions of the entire film area (2.25 cm2). Cross-sectional TEM imaging corroborated the vertical alignment of the mesochannels and established that they form a continuous contact between the film surface and the substrate. X-ray diffraction studies have shown that in some regions of the patterned films, vertically and laterally aligned mesostructures may coexist, though it is not yet clear how such mixed domains form.
The principles by which the direction and flux of solvent species out of the patterned block-copolymer/sol-gel films were controlled during synthesis was extended to produce hexagonal mesostructures orientationally ordered parallel to the substrate with their cylinders aligned along the longitudinal axis of the microchannels. Radial integration of the (100) diffraction peaks also shows high degrees of alignment with widths of 10 degrees FWHM. The results indicate that the formation of longitudinally aligned and hexagonally mesostructured silica-P123 is favored when thick (˜7 mm) PDMS stamps, saturated with ethanol, are used to pattern and direct the nucleation of self-assembling domains from ethanolic solutions. It has been shown that the longitudinal alignment is extended over macroscopic length scales.
The anisotropic properties of orientationally ordered mesostructured silica/block-copolymer films are expected to enable new applications in separations, catalysis, and optoelectronics. Removal of the structure-directing block copolymer species by calcination or solvent extraction results in porous films that can be functionalized to introduce desirable interior surface properties for selective adsorption or permeability. Alternatively, functional guest molecules can be introduced during syntheses of orientationally ordered mesostructured host films, provided that the guest species can be solubilized and co-assembled during the patterning process. For example, photo-responsive guest molecules were included in one-pot syntheses to co-assemble and thereby incorporate the guest-molecules (including semiconducting polymers and J-aggregated porphyrin dyes in patterned, hexagonally mesostructured and vertically aligned silica-P123 films. It has also been shown that inclusion of guest molecules by backfilling hydrophobically functionalized mesopores, following removal of the block-copolymer species, is feasible. The alignment of photo-responsive guest molecules in orientationally ordered mesostructured hosts matrices is expected to induce anisotropic optical properties, with potential device applications in light-emitting diodes, photovoltaics, and optoelectronics.
Another aspect of the invention is a method of controlling orientational ordering in self-assembled materials. One embodiment of this aspect comprises controlling solvent removal from a precursor solution.
Another embodiment comprises preparing a patterned stamp/mold for use as a mold for directing the patterning of the self-assembled material as it forms from a precursor solution; producing the precursor solution; drying the precursor solution in the presence of the patterned stamp/mold; and controlling the rate and direction of solvent/co-solvent species removal from the drying precursor solution.
In another embodiment, a said self-assembled material contains a surfactant or block-copolymer species. In a further embodiment, the said self-assembled material includes a network-forming component. In still another embodiment, the network-forming component comprises an organic or inorganic component. In one embodiment, the organic component comprises a resin. In one embodiment, the inorganic comprises silica or titania.
In a still further embodiment a said self-assembled material contains a functional guest species. In one embodiment, the guest species comprises a photo-responsive molecule or nanoparticle. In another embodiment, the orientational ordering occurs for hexagonal, lamellar, cubic or other phases, including crystalline phases.
In one mode, the orientational ordering occurs in a patterned film. In another mode, the orientational ordering occurs in a monolith or fiber.
In another embodiment, removal of solvent/co-solvent species is controlled with respect to the rate of removal. In a further embodiment, removal of solvent/co-solvent is controlled with respect to the direction of removal. In a still further embodiment, the surfactant or block-copolymer species are chosen, along with solvent/co-solvent and stamp/mold surface properties, so that said self-assembled material nucleates and grows at a surface. In another embodiment, removal of solvent/co-solvent is combined with the use of other externally applied fields, such as an electric field, a magnetic field, light, or fluid flow. In still another embodiment, solvent/co-solvent removal or externally applied fields are varied transiently.
Another aspect of the invention is the formation of self-assembled materials formed according to one or more of the methods described above.
Another aspect of the invention is an assembled structure comprising a substrate and a mesostructured material supported by the substrate; said mesostructured material having a perpendicular axis, a longitudinal axis, a lateral axis, or a radial axis, relative to the substrate; said mesostructured material comprising a plurality of mesochannels orientationally ordered along the same axial direction in relation to a said axis of the substrate.
A further aspect of the invention is an assembled structure comprising a substrate, a microchannel supported by the substrate where the microchannel has a perpendicular axis, a longitudinal axis, and a lateral axis, wherein the microchannel comprises a mesostructure and wherein the mesostructure comprises a plurality of mesochannels orientationally ordered along the same axial direction in relation to a said axis of the microchannel. In one embodiment, the substrate comprises a metalized or oxide substrate. In further embodiments, the substrate comprises titanium or aluminum. In various modes, the orientational ordering occurs for hexagonal, lamellar, cubic or other phases, including crystalline phases.
Another aspect of the invention is an orientationally ordered mesostructure that exhibits anisotropic properties. In various embodiments, the anisotropic properties are selected from the group consisting of anisotropic ion-transport, diffusion, reaction, photoluminescent properties, light-emission and light-absorption.
Another aspect of the invention is an orientationally ordered mesostructure that includes a photo-responsive molecule or nanoparticle.
Another aspect of the invention is a mesostructure that exhibits orientational order>100 nm from a surface and >100 μm in one or more dimensions.
Another aspect of the invention is a mesostructure that contains organic, inorganic, or a mixture of such species in a covalently bonded network. In one embodiment, the covalently bonded network contains species that aid the incorporation and/or influence the location or interactions of guest species within said mesostructure.
Another aspect of the invention is a self-assembled structure in the form of a film. In one embodiment, the film is a patterned film.
Another aspect of the invention is a self-assembled structure in the form of a monolith.
Another aspect of the invention is a self-assembled structure in the form of a fiber.
In one embodiment, the mesostructure comprises a hexagonal mesostructure that is orientationally ordered predominantly perpendicular to the microchannel.
In another embodiment, the mesostructure comprises a hexagonal mesostructure that is orientationally ordered predominantly lateral to the microchannel.
In a further embodiment, the mesostructure comprises a hexagonal mesostructure that is orientationally ordered predominantly longitudinal to the microchannel.
In another embodiment, the mesostructure contains guest species that are also orientationally ordered.
Further aspects of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.
The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only:
If one is to make an efficient material, and ultimately a device that takes advantage of the intrinsically anisotropic properties of a mesophase host matrix, the mesostructure should possess a high degree of alignment, as well as include guest molecules that adopt aligned configurations within the host structures. Furthermore, the film should be patternable for fabrication and integration into devices. These properties are provided by block-copolymer-templated silica composites, which can yield aligned, hexagonal inorganic-organic mesostructures that can serve as anisotropic host matrices for guest molecules. For example, for photo-responsive guests, anisotropic optical properties are expected, provided that the guest species are orientationally ordered in the aligned channels of a mesostructured host. Furthermore, it is desirable to control the specific orientation of any resulting alignment (relative to the substrate or pattern features), which would impart significant versatility for a wide range of different applications.
Self-assembled materials rely on amphiphilic surfactants (ionic, or in the present case, non-ionic triblock copolymers), which form micelles and eventually liquid-crystal-like phases, as the surfactant concentration increases due to solvent evaporation. In the case of amphiphilic triblock copolymers, the surfactant is composed of at least two types of different monomer units that self-assemble to minimize interactions with one another during micellization or mesostructure formation. The phase obtained depends on the composition (e.g., type of block copolymer, water, cosolvent, inorganic precursor, and guest/solute contents) and conditions (e.g., temperature and pressure) of the mixture, according to the balance of entropic and enthalpic interactions among the different species present. These interactions, and resulting phase behaviors, have been well studied for different non-ionic block copolymer water-alcohol mixtures under equilibrium conditions.
Block-copolymer/silica mesostructured materials, however, are much more complicated multi-component, non-equilibrium, and heterogeneous systems. Nevertheless, their phase behaviors both in precursor solutions and in the final products can be estimated and manipulated using guidance from equilibrium phase diagrams and from general predictive methods. Ternary block-copolymer-water-cosolvent (e.g. ethanol, butanol, etc.) phase diagrams can be used to guide the selection of the compositions required for the formation of specific phases. This includes the mesostructures of different bulk macroscopic morphologies, such as films, fibers, monoliths and powders, where non-equilibrium drying, domain nucleation and growth, and silica cross-linking processes can exist.
Mesostructured composite materials have been prepared as patterned films that permit independent control of structural ordering on multiple, discrete length scales, including those relevant to micro- and opto-electronic devices. This has been achieved by using soft lithography, which addresses similar length scales as conventional photolithography methods. However, by exploiting favorable thermodynamics of self-assembly from solution, soft lithographic processing may be much less expensive. In soft lithography, a patterned mesostructured film can be prepared directly by use of a pre-patterned mold (herein referred to as a “stamp” or “micromold”) that directs the shape and form of the self-assembling mesostructured (2-50 nm) composite on a substrate over microscopic (1-10 μm) and macroscopic (>100 μm) length scales.
Without control over the drying and thus nucleation processes, an unaligned mesostructured material typically forms with self-assembled mesophase domains oriented isotropically. In addition, the pH of the solution relative to the isoelectric point of the network-forming inorganic species (e.g., silica: pH 1.7-2.5) controls the relative rates of silica hydrolysis and condensation, along with electrostatic interactions with the block copolymer species. Yet, by selecting processing conditions that allow the rates or directions of solvent and cosolvent (e.g., water, ethanol, or THF) removal to be controlled, nucleation and alignment of the mesostructured domains can also be controlled. Other methods for drying have also yielded highly aligned mesostructures in monoliths and in capillaries. In dip-coating, the initial nucleation has been reported to occur at the triple interface between the block-copolymer/sol-gel solution, the surrounding vapor, and the substrate. It is the combined thermodynamic and kinetic properties of the self-assembling components and their interactions with the surfaces across which the solvent and cosolvent species are removed, that account for the nucleation, growth, and alignment of the first and all subsequent hexagonal domains.
When the substrate is hydrophilic, the hexagonal mesostructure tends to align parallel to the substrate with hydrophilic components at the substrate interface, as shown in
Thus, controlling only the relative hydrophilicity/hydrophobicity of the substrate is generally not enough to induce the hexagonal mesostructure to adopt an orientation perpendicular to the substrate. If perpendicular alignment is to occur, the substrate should be energetically favorable, and the nucleation rate should be slow enough to control the location of nucleation (as opposed to simultaneous nucleation throughout the microchannels, which would lead to an isotropic distribution of mesostructured domain orientations).
By controlling nucleation, we can control the formation of a mesostructured aggregate at a point where the hexagonal mesochannel axes will grow perpendicular to the substrate. This is accomplished by controlling the direction of solvent removal from the system, as well as controlling the relative concentrations of the solvent species present in the block copolymer/sol-gel solution. Moreover, as indicated in
Control over the directions and rates of solvent removal in the patterned films can be accomplished by using a soft lithographic micromold stamp formed from a material with appropriate solubility and diffusion properties for the solvent species. The rates and directions of solvent and cosolvent removal from the self-assembling precursor solution depend on whether the solvent species diffuse preferentially into the stamp material versus evaporating at the air interfaces at the ends of the microchannels. The micromolding process therefore allows one to control the drying of the self-assembling block-copolymer/sol-gel solution confined within the patterned channels of a pre-made soft lithography mold and the substrate.
Of the numerous choices available for a soft lithographic stamping material, highly cross-linked poly(dimethylsiloxane) (PDMS) is a material that has several benefits. The absorption of water in PDMS can be partially controlled by varying the degree of cross-linking of the elastomer precursor. Additionally, PDMS is available commercially and can easily be formed into patterned stamps by polymerizing on a hard-silicon master pattern formed using standard photolithography techniques. PDMS is also sufficiently rigid so as to maintain the three-dimensional shape of micrometer-scale pattern features (with tolerable mechanical deformation) when removed from the master or applied to a surface. Cross-linked PDMS is clear, flexible, and easy to mechanically manipulate.
The capacity of the PDMS to absorb solvent is also an important factor for obtaining directionally oriented nucleation of an aligned hexagonal mesostructure, with higher capacity allowing for better control. Control over the rates and directions of absorption and diffusion of solvent (e.g., ethanol) and cosolvent (e.g., water) is particularly important for directing mesostructure alignment. Control can be achieved by controlling the atmosphere (i.e., partial pressures of solvent and cosolvent species) surrounding the PDMS stamp, as the solvent and cosolvent absorb into the stamp from the patterned precursor solution.
Referring also to
Referring also to
Alignment of Hexagonal Mesostructured Block-Copolymer-Templated Silica Films
Here, we describe in detail a reliable and reproducible method for obtaining hexagonal inorganic-organic mesostructures with high degrees of macroscopic orientational order. By simultaneously controlling the directions and rates of solvent removal and interface hydrophobicity/hydrophilicity, it is possible to control the location at which nucleation of mesostructural domains occur and influence their direction of growth. This can be achieved by using soft-lithography to prepare patterned, hexagonally mesostructured block-copolymer/silica films with controlled alignment and pores/channels that can accommodate orientationally ordered photo-responsive guest molecules.
The general method for preparing aligned mesostructural composites involves the creation of a patterned PDMS stamp to be used as a micromold for directing the patterning of the mesostructured silica/P123 as it forms from a block-copolymer sol-gel precursor solution on a metalized substrate. The drying period extends over a period of 6-7 days under fixed environmental conditions that control the rate(s) of solvent/co-solvent species removal from the precursor solution. After the drying period, the PDMS stamp is removed, leaving the patterned mesostructured material on the substrate for characterization by SAXS, and cross-sectional TEM.
Four-inch silicon [100] wafers (Wafer World Inc., West Palm Beach, Fla.), were patterned by photolithography and subsequently used as a master replica from which patterned micromold PDMS stamps were prepared. The master pattern was formed by spin-coating photoresist AZ5214, developed according to a desired pattern, followed by 6 s etch cycles for a total of 30-36 s. After coating the silicon wafer with 1H,1H,2H,2H-perfluoro-decyltricholorosilane to prevent significant adhesion of the PDMS to the silicon surface, a mixture of Sylgard® 184 elastomer and a dimethyl-methylhydrogen siloxane curing agent in a 10:1 ratio was poured on top of the patterned silicon master and cured overnight at 65° C. under vacuum. The pattern imprinted onto the PDMS stamp was comprised of long microchannels 1.5 cm in length, 1 μm in height, and 5, 7, or 12 μm in width. The thickness of the stamps above the channels was controlled by adjusting the amount of elastomer poured on top of the patterned silicon master.
Thin metalized Kapton® was used as a substrate for the films, providing a smooth surface for film deposition. The Kapton® support is transparent to X-rays and allows for efficient characterization of the mesostructured silica by transmission-mode SAXS. Substrates for the films were prepared by depositing titanium metal via physical vapor deposition methods using an electron beam evaporator and a 99.999% titanium source. Titanium metal was chosen because of its excellent corrosion resistance under the acidic conditions of the synthesis. The titanium was deposited onto a 0.05 inch thick Kapton® support (DE350—Dunmore Corporation, Bristol, Pa.) or a thin borosilicate glass slide. The glass slide was used when calcination was performed to remove the structure-directing triblock copolymer surfactant species at temperatures at which the Kapton® would not withstand.
Glass desiccators having a volume of 2.4 L were used as an environmental chamber to achieve a 97% relative humidity. The relative humidity was controlled by placing a saturated salt solution of K2SO4, while at a constant temperature of 25° C. Other relative humidity environments (percents shown) were created using different saturated salt solutions: KCl (84%), Kl (69%), Mg(NO3)2 (54%), MgCl2 (33%).
The mesostructured silica films were synthesized by solution precipitation in the presence of amphiphilic triblock copolymer species. Soluble silica precursor species were prepared by hydrolyzing tetraethoxysilane, (TEOS, Aldrich Chemicals) in an acidic, ethanol-based solution at room temperature for one hour. A second solution containing the amphiphilic triblock copolymer species poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EO20PO70EO20, Pluronic® P123, BASF, Mount Olive, N.J.) was separately prepared by dissolution in ethanol at room temperature, after which the two solutions were combined and mixed under stirring for one hour. In a typical synthesis, the molar ratio of materials used was 1 TEOS:0.0172 P123:22.15 EtOH:0.02HCl:5.00H2O. 11 μL of this triblock-copolymer/silica sol was then placed on a metalized substrate (typically titanium-coated Kapton® or glass), after which the patterned PDMS stamp (thickness ˜1 mm) was placed down on top of the solution and pressure applied in such a manner that the entire stamp area was wetted. The solution was allowed to dry over a period of several days to 1 week in a fixed volume chamber maintained at 97% (or other) relative humidity. After drying, the PDMS was carefully removed, leaving the patterned, mesostructured silica-P123 film adhering to the substrate surface.
Similar mesostructured silica-P123 films were prepared using the more hydrophobic solvent tetrahydrofuran (THF). In this case tetraethoxysilane, (TEOS, Aldrich Chemicals) was hydrolyzed in an acidic, tetrahydrofuran-based solution for one hour and then mixed with a solution of EO20PO70EO20, (Pluronic® P123) triblock copolymer species also dissolved in tetrahydrofuran. In a typical synthesis, 1.17 mL of THF, 0.23 mL of TEOS, and 0.09 mL of 0.07 M HCl were mixed at room temperature in a small vial, then added to 0.09 g of Pluronic® P123 to dissolve the surfactant, followed by the addition of another 2.2 mL of THF. As above, the mixed precursor solution was then placed on a metalized substrate (typically titanium-coated Kapton® or glass), after which the patterned PDMS stamp (thickness ˜1 mm) was placed on top of the solution and pressure applied in such a manner that the entire stamp area was wetted. The solution was allowed to dry over a period of 2 days in a fixed volume chamber maintained at 53% relative humidity through a saturated salt solution of NaBr, after which the stamp was removed.
After removing the stamp and drying in room temperature air overnight, removal of the structure-directing triblock copolymer species was achieved by calcining the mesostructured silica-P123 films in air. The temperature of the oven was ramped from 25° C. to 500° C. at 1° C. per min, then held for eight hours and allowed to cool. After calcination, 2D SAXS measurements were conducted to confirm that the mesostructural order and alignment were still present, and to assess any changes in d-spacing.
Small-angle X-ray scattering (SAXS) measurements were made using an ultra-SAXS diffractometer with a copper anode (λ=1.54 Å) and a two-dimensional (2D) image plate with a sample-to-detector distance of 1.725 m. An intermediate-SAXS (i-SAXS) diffractometer with similar features, but a sample-to-detector distance of 0.758 m, was also used.
Cross-sectional TEM micrographs were obtained using a FEI Tecnai T20 electron microscope operating at 200 keV. Samples were prepared using a FEI DB235 Dual-Beam Focus Ion Beam System to cut 150-nm-thick samples out of individual microchannels with a Magnum ion column operating at 300 pA.
(a) Spectrographic Analysis
The mesostructural and orientational ordering of patterned silica/block-copolymer films can be characterized by small-angle X-ray scattering (SAXS) and focused ion-beam (FIB) transmission electron microscopy (TEM). SAXS scattering measurements are routinely used in the study of mesoscale materials, because they provide information on the mesostructural ordering of the silica/block-copolymer films. Analysis of the azimuthal distribution, that is, variance in scattering intensity at differing distances from the center of the diffraction pattern, allows one to infer the presences of different lattice planes in the mesostructured material. The presences of particular lattice planes are associated with different phases with different mesostructural ordering. For example, it is known that the diffraction pattern of a cubic mesostructure will contain principally the (100), (200), and (211) diffraction planes, whereas a hexagonal mesostructural will contain principally the (100), (200) and (300) planes (and several others possibly present depending on the orientation of the mesostructure). The number of diffraction planes present is also a measure of the degree of mesostructural ordering, with more diffraction planes corresponding to higher long-range ordering. Likewise, analysis of the radial distribution (i.e., in a circle at a fixed distance from the center of the diffraction pattern) provides information on the orientation of particular lattice planes relative to the X-ray beam. A circular (ring) diffraction pattern is characteristic of an isotropically oriented sample. Diffraction “spots” are characteristic of alignment in a particular diffraction plane, where the width of the spot (i.e., how narrow the distribution of intensity) allows for the quantification of the degree of alignment. Through analysis of a 2D SAXS diffraction pattern, one can therefore characterize the degrees of alignment and mesostructural ordering.
The expected diffraction pattern from a patterned silica/block-copolymer film varies greatly depending on the direction of alignment of the hexagonal mesostructure and the orientation of the sample relative to the incident X-ray beam.
In GI-SAXS, the sample substrate is almost parallel to the X-ray beam 102 (approximately 2-3° off the beam path), as illustrated in
In addition to SAXS, FIB TEM is a desirable tool for the characterization of silica/block-copolymer films to ascertain details of the mesostructural ordering on a nanometer scale that are not provided by SAXS. TEM samples are prepared by using a focused ion beam (FIB) to cut thin (approximately 125-200 nm) cross-sectional slices of the patterned film. The example shown in
(b) Substrates
The acidic conditions of the silica and block copolymer precursor species are necessary for the co-assembly of the organic and inorganic species, but create additional considerations for choosing the substrate material. The addition of acid into the silica precursor solution, as described above, is required for the hydrolysis of the TEOS silica source. The TEOS hydrolyzes into Si(OH)4 and later cross-links to form the inorganic matrix of the silica-P123 mesostructured film. However, the low pH (˜1.75) of the silica and block copolymer precursor solution also serves to slow the silica cross-linking reaction. If the mesostructured silica-P123 film is to form, the cross-linking of the silica should occur after self-assembly of the P123 polymer species. After self-assembly, the silica can then polymerize, fixing the organic. However, the acidic conditions also can cause corrosion of the metalized substrate used to provide a smooth surface for film growth. This corrosion is evident in the TEM image showing the aluminum-coated Kapton® substrate 202 in
Therefore, subsequent patterned and mesostructured P123/silica films were prepared on a substrate of titanium-coated Kapton® or glass, as described above. The titanium-coated substrate allows a low enough pH in the silica-P123 precursor solution to sufficiently slow the polymerization of the silica, while not corroding the metalized substrate.
(c) Vertical Alignment
Such characterization methods were applied to assess the degree and direction of orientational ordering of mesostructured films prepared under the conditions described above. When the block copolymer sol-gel precursor solution was allowed to dry under the micro-patterned PDMS stamp in an atmosphere with high relative humidity, the mesostructured composite film aligned with a large fraction of the hexagonal mesochannels perpendicular to the substrate. The alignment of the mesostructure was first characterized through SAXS to confirm the vertical alignment before using FIB to obtain a cross-sectional TEM image.
The 2D transmission-mode SAXS diffraction pattern 206 shown in
To prepare a TEM sample, a focus ion been was used to cut a cross-sectional slice into a single 1 μm high by 7 μm wide microchannel, shown in
We believe that the vertical alignment of the mesostructure results principally from absorption and diffusion of the solvent and cosolvent species into the PDMS itself, rather than evaporation-induced self-assembly. Here, the aim is to test and generalize this hypothesis by selecting and controlling whether the solvent species are removed by diffusion or evaporation and the interfaces and directions where these processes occur. For example, if the stamp is not saturated with solvent, continuous absorption, diffusion, and removal of the solvent species will occur into the stamp. This results in a concentration gradient that eventually results in the solvent concentration within the microchannels diminishing to the point where self-assembly of the structure-directing block copolymer species can take place. The point(s) in the microchannels where this first occurs is expected to be nearest the points where the solvent is being most rapidly removed. These will be at either at the PDMS or air interfaces along or at the ends of the microchannels, where nucleation and growth of the mesostructure composite is expected to occur. Provided such mesophases nucleate, grow, and fill the microchannels before the silica cross-links and solidifies, formation of thermodynamically favored mesostructure domains with high degrees of mesoscopic and orientational order are expected to result.
To control the alignment of cylinders in hexagonal mesostructured domains, a system is preferably selected in which a hexagonal phase is thermodynamically favored and takes into account the relative surface properties of the self-assembling block-copolymer components and the interface at which mesophase nucleation occurs. Phase selection can be achieved, based on guidance from available (often ternary) phase diagrams and the expected compositions of the final block-copolymer silica composite (assuming complete removal of solvent species and that the silica can be classified as a hydrophilic component). For Pluronic®-type triblock-copolymer species, the silica and PEO components form continuous hydrophilic regions, while the PPO blocks are relatively hydrophobic. For the case of EO20PO70EO20 (P123) in mixed water/alcohol solutions, a relatively large region exists at low alcohol concentrations over which the hexagonal (H1) phase forms. As the water and ethanol solvent species are removed by diffusion into the PDMS stamp or evaporation from an air/sol interface, the system follows a trajectory through a complicated multi-component phase diagram. Nucleation of liquid-crystal-like mesophases occurs when and where the solvent composition drops to the point where dense aggregates first form. As the solvent concentration continues to drop, a mesophase (e.g., hexagonal) develops and grows. Because solvent is being depleted from the microchannels into the PDMS stamp or across an air interface at the channel ends, nucleation will invariably occur at whichever of these interfaces the flux of solvent is the greatest.
The orientations of the intrinsically anisotropic hexagonal domains at their nucleation sites depend on whether either of the hydrophilic PEO/silica or hydrophobic PPO moieties preferentially interact with the interface at which nucleation occurs. Because PDMS is relatively hydrophobic, the relatively hydrophobic PPO cylinders of hexagonal P123-silica domains will tend to maximize their contact with the PDMS surface, according to the balance of surface and bulk phase energies. For example, orientational ordering of hexagonally mesostructured domains can occur, such that the PPO cylinders are oriented perpendicular to the PDMS interface and the metalized substrate. This appears to occur because the solvent species (here, ethanol, water, and/or THF) are removed approximately unidirectionally from the patterned microchannels into the PDMS stamp, fixing the nucleation interface for the mesostructure at the top microchannel surfaces.
As shown in
We believe that nucleation occurs at the corners of the microchannel/PDMS interfaces, where the solvent flux is expected to be highest. The hydrophobicity of the PDMS surface yields preferential contact with the hydrophobic PPO regions/moieties of the structure-directing triblock copolymer species (in this case, Pluronic® P123), promoting perpendicular alignment of the mesostructure, as domains grow downward toward the titanium-coated Kapton® substrate. We have also observed that the locations of the six diffraction spots do not vary between samples or along the longitudinal axes of the microchannels. This indicates that there is a preferential orientation for the hexagonal mesostructure as each domain nucleates, supporting the hypothesis that the alignment of the mesostructure is a result of solvent removal into the PDMS stamp and giving the mesostructure crystal-like ordering over macroscopic (˜1 cm) length scales. These results indicate that nucleation appears to first occur at the microchannel corners of the PDMS stamp pattern, where solvent flux is expected to be the highest, and then progress inward toward the center of the microchannel and downward to the lower substrate. Along the two corners of each microchannel, the side walls of the PDMS stamps may influence mesostructure growth and thus alignment. The net effect is that the hexagonal mesostructure aligns with the same orientation at all points along the microchannel and with few grain boundaries in the final film (presumably because they self-anneal prior to silica cross-linking).
We achieved similar mesostructural ordering and alignment using a more hydrophobic solvent, specifically THF. PDMS-patterned silica-P123 films were prepared from THF precursor solutions using similar methods as described above, although the most reproducible alignment occurred when drying occurred at 53% relative humidity. When THF is used as a solvent, the drying rate increases dramatically, with the patterned silica-P123 mesostructure formed within 48 h, as opposed to the six to seven days required when using ethanol as the principal solvent. THF is more volatile than ethanol, yet the difference in their vapor pressures at room temperature does not account for the dramatic difference in drying rates. Instead, it is better explained by the increased solubility of THF in the PDMS stamp, leading to an increased diffusive flux of THF out of the microchannel. It should be noted that PDMS swells to a much higher extent in THF, which can cause difficulties in the stamping process. In the presence of high concentrations of THF, the PDMS stamp can curl away and delaminate from the substrate, disrupting confinement of the block-copolymer/silica sol and control of the solvent removal direction. The problem of PDMS swelling was found to be diminished as the PDMS thickness increased, presumably because of a small concentration gradient and lower swelling stresses within the stamp. Mesostructured silica films prepared from THF solvents were therefore achieved using PDMS stamps with an average thickness of 8 mm.
The 2D SAXS pattern,
It is our belief that alignment of the hexagonal silica-P123 mesostructure can achieved by controlling the flux of the solvents/co-solvents into the PDMS and that the degree of alignment should be consistent along the entire longitudinal axis of the microchannel, if end effects are not present. To support this belief, we synthesized patterned, hexagonally mesostructured silica-P123 films using PDMS stamps with the ends of the microchannels either closed off by the PDMS (where end effects along the microchannel axes will be minimized) as illustrated by the structure 300 with closed ends 302 shown in
In both cases, the 2D transmission-mode diffraction pattern showed alignment of the hexagonal phase perpendicular to the substrate. In cases where the microchannel ends were exposed to the atmosphere, the degree of perpendicular alignment decreased closer to the microchannel ends. By comparison, for cases where the ends of the microchannels in the PDMS stamp were closed to the atmosphere, the degree of vertical alignment of the hexagonal mesostructured silica-P123 appeared not to vary along the 15 mm lengths of the ensemble of microchannels examined within the 1 mm2 X-ray beam.
We believe that, when the microchannel ends are left exposed to the atmosphere, a portion of the solvent is able to evaporate into the surroundings, instead of diffusing into and through the PDMS stamp, resulting in multiple directions of solvent removal and thus, lower extents of orientational ordering in these regions. These observations correlate with our belief that solvent removal from the block-copolymer/silica sol in the center of the stamped region occurs principally via diffusion into the stamp, primarily perpendicular to the underlying substrate, leading to high extents of vertically aligned hexagonal mesostructured domains that appear to persist over macroscopic (˜1 cm) length scales. Near the open microchannels ends at the stamp edges, solvent evaporation at the air interfaces can also contribute to solvent removal, disrupting mesostructural alignment.
These beliefs were validated by small-angle X-ray scattering results that characterize mesostructural order and alignment at different regions across the macroscopic dimensions of patterned silica-/P123 films. For example,
The film examined was prepared by using a PDMS stamp with closed microchannel ends and allowing the block-copolymer/silica precursor sol to dry in a controlled atmosphere at 97% relative humidity, as depicted in
The 2D radial integration of a representative SAXS pattern (pattern iii in
When the ends of the longitudinal axes of the microchannels are open, an interface exists between the block-copolymer/silica precursor sol and the atmosphere, which allows evaporation of volatile solvent and cosolvent species. According to the proposed hypothesis on the mechanism for alignment of the hexagonal silica-P123 mesostructure described above, the evaporative end-effects are expected to contribute to the flux of solvent and cosolvent species and potentially disrupt the mesostructural alignment near the open microchannel ends.
Further validation of our hypotheses on alignment of the hexagonal silica-P123 mesostructure were provided through X-ray scattering results that characterize mesostructural order and alignment at different regions across the macroscopic dimensions of the patterned silica-P123 film, focusing near the ends of the microchannel axes. It is in this region that effects from evaporation at the block copolymer/silica sol and air interface would be most prevalent.
A mesostructured silica film examined was prepared by using a PDMS stamp with its microchannel ends exposed to the surrounding environment and allowing the block-copolymer/silica precursor sol to dry in a controlled atmosphere at 97% relative humidity, as depicted in
The observations concerning the differing degrees of alignment, depending on whether the microchannel PDMS stamp ends are open or closed, illustrates the importance of controlling the rate and direction of mass transfer of the solvent/cosolvent species as the block-copolymer/silica precursor solution dries. To explore the effects of varying the concentration of water in the film as the mesostructure self-assembles, ethanolic block-copolymer/silica sol-gel precursor solutions were prepared and dried using a thin (˜1 mm) patterned PDMS stamp with open microchannel ends. The stamped films were allowed to dry in environments with differing fixed relative humidities, as depicted in
Refer, for example, to
Characterization of the mesostructural ordering and alignment was accomplished by 2D SAXS, as shown in
These SAXS patterns reveal a similar and consistent effect on the mesostructural ordering and alignment as observed for the variation of the water content in the atmosphere surrounding the PDMS stamp (
The water and cosolvent profiles in the PDMS stamp are expected to be complicated and transient, but approaching steady-state over several (˜24) hours. For a fresh, dry PDMS stamp, the solubilities of water and ethanol are estimated to be 5.2·10−4 moles/g PDMS and 1.6·10−3 moles/g PDMS, respectively, as measured by the differences in the mass of a PDMS sample before and after saturation with the respective species. The diffusion coefficients of the water and ethanol can be approximated by molecular dynamical modeling, while their respective diffusivities are 1.5·10−5 cm2/s and 2.0·10−6 cm2/s, respectively. For a humid atmosphere without ethanol, water will absorb into the PDMS stamp both from the humidified atmosphere and from water in the block-copolymer/silica solution filling the microchannels. When a sufficiently thin PDMS stamp (˜1 mm) is used for a given high humidity, diffusion through the top surface of the stamp may be significant enough to affect the rate of water absorption from the microchannels, diminishing the rate of water removal, and thus slowing the rate of silica cross-linking. There are many variables affecting the rate of solvent and cosolvent removal from the block-copolymer/silica solution in the microchannels, and modeling efforts are underway to shed further investigate the timescales of the drying process.
Having shown the ability to direct the alignment of the hexagonal mesostructure perpendicular to the substrate over a large length scale, it is desirable for the mesostructured composite matrix to remain intact when the surfactant is removed. While it is advantageous to incorporate photo-responsive molecules through a one-pot synthesis during mesostructural alignment, another possible route of guest molecule incorporation may be through backfilling of the mesopore void spaces that result after surfactant removal.
Before calcination, the 2D SAXS pattern and its radial integration in
(d) Lateral Alignment
For transmission-mode SAXS patterns showing no reflected intensity, such as in
For example, refer to
As can be seen,
The FIB TEM image,
Transmission-mode diffraction results for mesostructured films that show evidence of vertical alignments also occasionally show co-existing regions of perpendicularly and laterally oriented cylinders within the 1-mm2 X-ray beam.
For example, refer to
(e) Longitudinal Alignment
Similar to the methods used to produce and characterize vertically and laterally aligned patterned, hexagonal mesostructured silica films, it may also be desirable to achieve longitudinal alignment of the mesochannels across the macroscopic length scales of the microchannel pattern. Previous results have confirmed the hypothesis that a high degree of orientational order and alignment of the hexagonal silica-P123 mesochannels can be obtained by influencing the solvent and cosolvent fluxes out of the silica-P123 triblock-copolymer precursor solution. These controlled fluxes result in control over where the hexagonal silica-P123 mesostructure nucleates and the direction that they propagate. It is thought that by changing the location of nucleation, it will be possible to change the thermodynamic effects that govern the direction of orientational ordering of the hexagonal mesostructure to induce longitudinal alignment. This alignment is desired for possible applications in membranes, as well as to illustrate how control over the mesostructure nucleation location can control the direction of alignment.
It is thought that these principles can also be used to form well ordered mesochannels that are oriented longitudinally down the microchannel axes by directing the solvent flux out the ends of the microchannels where they are exposed to the atmosphere. By promoting evaporation at the ends of the microchannels rather than by absorption into the PDMS stamp, the nucleation of the hexagonal mesostructure may occur at the interface between the atmosphere and the silica/P123 block-copolymer precursor sol. At this interface, the hydrophobic PPO moieties of the triblock-copolymer are expected to be preferentially distributed at the relatively hydrophobic air interface, from which the hexagonal mesochannels grow longitudinally along a microchannel axis.
The hypothesis that longitudinally aligned hexagonal mesophases can be controllably obtained was tested by enhancing the rate of solvent removal from open microchannel ends vis-à-vis diffusion into the PDMS stamp. This was achieved by saturating the PDMS with ethanol prior to preparing the stamped patterned film (
For example, refer to
It is hypothesized, based on the low intensity of the diffraction spots, that the saturation of the PDMS stamp inhibits the rate of solvent and cosolvent removal from the silica/P123 block-copolymer precursor solution enough to disrupt mesostructural ordering. Without such ordering, of course, mesoscale alignment does not develop. The low intensity of the diffraction spots could also indicate the presence of a mix of longitudinal and lateral alignment with forbidden X-ray reflections (as previously discussed). TEM samples showing the presence of no mesostructural ordering in the microchannel make this the less likely of the two hypotheses. The TEM micrographs in
Referring to
The diffraction patterns shown in
The longitudinal alignment is explained by and consistent with the solvent being removed from the block-copolymer/silica precursor sol by evaporation out the open ends 306 of the stamp microchannels across a boundary perpendicular to the stamp, shown in
According to this drying protocol, one would expect that as the distance from the microchannel ends toward the PDMS stamp center increases, a greater fraction of the solvent may be removed perpendicularly by diffusion into and through the stamp. Farther away from the stamp edges, the rate of solvent/cosolvent removal by evaporation out the ends of the microchannels may become comparable to the rates of solvent absorption and diffusion into the PDMS stamp. In such a case, mesostructure nucleation and growth may occur at interfaces and directions that lead to a distribution of domain orientations, resulting in a ring diffraction pattern.
Interestingly, in mesostructured silica-P123 films prepared at 33% relative humidity using thin (˜1 mm) unsaturated PDMS stamps, 2D SAXS measurements in
By controlling the rates and directions of the removal of solvent species during drying of the block-copolymer/silica precursor sol, micropatterned silica films have been synthesized with hexagonal mesostructures having different relative alignments. These include hexagonal silica-P123 mesophases where the hydrophobic PPO cylinders in the microchannels are aligned perpendicular to the metalized lower substrate, or parallel to the substrate and laterally or longitudinally oriented with respect to the axes of patterned microchannels.
The hexagonal mesostructure is formed during drying of the block-copolymer/silica precursor sol. As the relative concentration of the block-copolymer species increases, micelles begin to form. Eventually, these micelles self-assemble and the hexagonal (or other liquid-crystal-like) mesostructure first nucleates. By controlling the location of this nucleation, it is possible to affect the direction of propagation of the hexagonal cylinders (or other anisotropic liquid-crystal-like structures), leading to an aligned mesostructure. After self-assembly of the triblock-copolymer occurs, the silica then cross-links, forming an inorganic-organic mesostructured film. By selecting conditions so that the self-assembly takes place while silica polymerization kinetics are slow (low pH (e.g., ˜1.75) and/or low temperature), the mesostructure can form before extensive silica cross-linking occurs. The low pH, however, can corrode the metalized substrates used to provide a smooth surface for film growth. Some metals are resistant to the acidic conditions present during the self-assembly process, as shown in Pourbaix diagrams,
Vertical and lateral directions of alignment occur when a dry PDMS stamp is placed over the block-copolymer/silica precursor sol. Drying of the sol occurs as the solvent and cosolvent species absorb and diffuse through the stamp, resulting in initial nucleation of mesostructure domains at the corners of the microchannels. If the relatively hydrophobic PPO blocks preferentially interact with the relatively hydrophobic top PDMS surface of the stamp microchannel, the mesostructure will tend to propagate perpendicular to the substrate, resulting in vertically aligned silica/P123 hexagonal mesostructure domains. If the PPO blocks preferentially interact with the PDMS side wall of the stamp microchannel, the mesostructure will tend to propagate parallel to the substrate across the microchannel width. Among the variables that govern such nucleation and anisotropic growth processes, the rates and directions of solvent/cosolvent removal into the PDMS stamp or evaporating from sol-air interfaces appear to be valuable for establishing the direction of mesostructure alignment.
Longitudinal alignment occurs through a similar process of solvent removal, mesostructure nucleation and growth, followed by silica polymerization. The principal difference is that the solvent and cosolvent species are removed predominantly by evaporation at the ends of the exposed microchannels, where there are air interfaces with the silica/block-copolymer precursor sol. The solvent is inhibited from absorption and diffusion through the PDMS by saturating the stamp in ethanol before its use to micromold ca. 11 μL of the precursor sol (
The development of anisotropic growth and alignment from the original nucleation sites or interfaces is useful for producing uniform orientational order in 3D monoliths. The principles of directed solvent removal may applied to thick free-standing films, monoliths, and fibers to develop long-range mesostructurally and orientationally ordered solids with anisotropic bulk properties that would be useful in separations, catalysis, sensor, or optics applications. In an attempt to form such a P123/silica monolith with a highly aligned hexagonal mesostructure, a THF-based silica/block-copolymer precursor sol was prepared and poured into a Teflon® mold and covered with a patterned-PDMS stamp illustrated schematically in
Other materials may be suitable to use as stamps for soft-lithographic patterning. For example, fluorosilicone resins could potentially be cured into a similar stamp as that of the PDMS. Fluorosilicone resins often exhibit low solubilities for absorbing solvent species and so may be good candidates for promoting solvent removal out the ends of the patterned microchannels, instead of through the stamp itself.
A desirable stamp criterion is that the silica mesostructure should not adhere strongly to the stamp material, so that it remains on the lower substrate when the stamp is removed. Also, the material should be sufficiently flexible that when the stamp is pressed down over the block-copolymer/silica solution it promotes even wetting of the substrate, while simultaneously sealing tightly to the substrate to confine the solution within the microchannels. Lastly, the material should be sufficiently rigid to be patternable and permit microchannel arrays to be stamped/molded without significant mechanical deformation across or along the microchannel dimensions.
By simulation of various PDMS stamps (or stamps from other materials) with different macroscopic and/or microchannel configurations, different solvent solubilities and diffusivities and in different controlled atmospheres, it is anticipated that optimum compositions and processing conditions can be estimated for generating macroscopic alignment of diverse inorganic organic mesostructured materials. The combination of close feedback among synthesis, processing, characterization and modeling results are expected to improve material properties, broaden their ranges of properties, and assist with their integration into new processes and devices.
The general method for preparing aligned mesostructural composites involves the creation of a patterned PDMS stamp to be used as a mold for directing the patterning or form of mesostructured silica/P123 as it forms from a block-copolymer sol-gel precursor solution on a substrate. The drying period extends over a period of 6-7 days under fixed environmental conditions that control the rate(s) of solvent/co-solvent species removal from the precursor solution. After the drying period, the PDMS stamp is removed, leaving the patterned mesostructured material on the substrate for characterization by SAXS, and cross-sectional TEM.
Four-inch silicon [100] wafers (Wafer World Inc., West Palm Beach, Fla.), were patterned by photolithography and subsequently used as a master replica from which patterned micromold PDMS stamps were prepared. The master pattern was formed by spin-coating photoresist AZ5214, developed according to a desired pattern, followed by 6 s etch cycles for a total of 30-36 s. After coating the silicon wafer with 1H,1H,2H,2H-perfluoro-decyltricholorosilane to prevent significant adhesion of the PDMS to the silicon surface, a mixture of Sylgard® 184 elastomer and a dimethyl-methylhydrogen siloxane curing agent in a 10:1 ratio was poured on top of the patterned silicon master and cured overnight at 65° C. under vacuum. The pattern imprinted onto the PDMS stamp was comprised of long microchannels 1.5 cm in length, 1 μm in height, and 5, 7, or 12 μm in width. The thickness of the stamps above the channels was controlled by adjusting the amount of elastomer poured on top of the patterned silicon master.
Thin metalized Kapton® was used as a substrate for the films, providing a smooth surface for film deposition. The Kapton® support is transparent to X-rays and allows for efficient characterization of the mesostructured silica by transmission-mode SAXS. Substrates for the films were prepared by depositing titanium metal via physical vapor deposition methods using an electron beam evaporator and a 99.999% titanium source. Titanium metal was chosen because of its excellent corrosion resistance under the acidic conditions of the synthesis. The titanium was deposited onto a 0.05 inch thick Kapton® support (DE350—Dunmore Corporation, Bristol, Pa.) or a thin borosilicate glass slide. The glass slide was used when calcination was performed to remove the structure-directing triblock copolymer surfactant species at temperatures at which the Kapton® would not withstand.
Amphiphilic surfactant species were used to direct the formation of mesostructured silica. Soluble hydrophilic silica precursor species were prepared by first hydrolyzing tetraethoxysilane, (TEOS, Aldrich Chemicals) in an acidic, ethanol-based solution for one hour at room temperature. A second solution was prepared by dissolving poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EO20PO70EO20, Pluronic P123, BASF, Mount Olive, N.J.) triblock copolymer species in ethanol, which was stirred at room temperature for one hour. This solution was then added to a small mass of tetrakis (p-sulfonatophenyl) porphyrin dye (TPPS4). The two solutions were mixed under stirring for an additional hour at room temperature, yielding an overall mixture for a typical synthesis with a composition (molar ratios) of 1.0 TEOS:0.017 P123:22.15 EtOH:0.02HCl:5.00H2O:0.019 TPPS4. This solution was then placed on a metalized (typically titanium, due to the metal's stability under acidic conditions) substrate, after which the patterned PDMS stamp (thickness ˜1 mm) was placed on top of the precursor solution and pressure applied in such a manner that the entire stamp area was wetted. The solution was allowed to dry over a period of several days to 1 week in a fixed volume chamber (2.4 L in volume) maintained at 25° C. at 97% relative humidity through a saturated salt solution of K2SO4. After drying, the PDMS stamp was carefully removed by scoring at the film edge with a razor blade at one edge of the stamp and slowly peeling the PDMS away from the substrate, leaving the patterned, mesostructured silica/P123/TPPS4 composite adhering to the substrate surface.
Similar patterned mesostructured silica/P123 films containing conjugated polymer guest species were prepared by using the more hydrophobic solvent tetrahydrofuran (THF). In this case, tetraethoxysilane, (TEOS, Aldrich Chemicals) was hydrolyzed in an acidic, tetrahydrofuran-based solution for one hour and then mixed with a solution of EO20PO70EO20 (Pluronic P123, BASF, Mount Olive, N.J.) triblock copolymer species also dissolved in tetrahydrofuran. In a typical synthesis, 1.17 mL of THF, 0.23 mL of TEOS, and 0.09 mL of 0.07 M HCl were mixed at room temperature in a small vial, then added to 0.09 g of Pluronic® P123 to dissolve the surfactant, followed by the addition of another 2.2 mL of THF containing 0.20-3.8 mg/mL of the semiconducting polymer poly(9,9-dioctylfluorine) (PF8). As above, the precursor solution was placed on a metalized (typically titanium) substrate, the patterned PDMS stamp (thickness ˜8 mm) was placed on top of the precursor sol, and pressure was applied so the entire stamp area was wetted. The solution was allowed to dry over a period of 2 days in a fixed volume chamber (2.4 L in volume) maintained at 53% relative humidity through a saturated salt solution of NaBr, after which the stamp was removed as described above.
Small-angle X-ray scattering (SAXS) measurements were conducted using an Ultra-SAXS diffractometer with a copper anode (λ=1.54 Å) and a two-dimensional (2D) image plate with a sample-to-detector distance of 1.725 m. An intermediate-SAXS (i-SAXS) diffractometer was also used with similar features, but a sample-to-detector distance of 0.758 m.
Fluorescence measurements were made using a Perkin Elmer LS 55 Luminescence Spectrometer with excitation at 380 nm for the PF8 semiconducting polymer and 275-325 nm for the conjugated oligomers. Slit widths of 5 nm were used along with a 1% attenuation filter, and multiple-scan (typically 5 scans) averaging to improve the signal-to-noise ratio. Fluorescent micrographs were acquired using an Olympus BX41 optical microscope with a LUCPLFLN 20× objective with an ultra-violet excitation mirror unit providing excitation in the range of 330-385 nm with a 420 nm emission filter.
Polarized optical light microscopy images were obtained using a Nikon Optiphot-2 optical microscope with cross-polarizing accessories. The films were mounted to a glass slide and fixed upright on a movable stage in such a manner that the microchannels of the patterned film were parallel to that of the incident light path. To show birefringent properties, the sample stage was rotated in a plane perpendicular to the light path.
In some cases, the structure-directing surfactant species were removed from the mesostructured material through solvent extraction by refluxing in ethanol for 2 days at 100° C. This was followed by washing in deionized water over night at 80° C., and then drying in an oven for 8 h at 100° C. The films were then re-characterized by SAXS to show the preservation of mesostructural order and alignment and determine any changes d-spacing.
In some studies, cubic SBA-16 mesostructured silica was used to test grafting of n-butyltrichlorosilane onto the mesopore surfaces after surfactant removal. The cubic SBA-16 mesostructured silica was synthesized hydrothermally by dissolving 4.0 g of Pluronic® F127 (EO106PO70EO106, BASF, Mount Olive, N.J.) in 30 g of deionized water with 120 g of 2 M HCl and stirring at room temperature for 20 min. 8.5 g of TEOS were added followed by an additional 20 min of stirring. The solution was then sealed and aged at 80° C. for 2 days.
To promote the backfilling of hydrophobic optical guest species into the mesopores, the pore walls were hydrophobically functionalized with n-butyltrichlorosilane after removal of the surfactant species. Typically, the mesoporous material was placed in a sealed 1 L HDPE chamber (under an Ar environment) containing 200 μL of the functionalizing molecule (in liquid form). The chamber was then sealed and heated to 65° C. for 24 h. To characterize the efficacy of n-butyltrichlorosilane grafting onto the pore walls, NMR measurements were conducted to analyze the fraction of Q2, Q3, and Q4 silica species before and after the grafting procedure. Solid-state NMR measurements were made using a Bruker AVANCE-500 wide bore spectrometer (11.7 T) operating at 99.3 MHz for 29Si. The sample was loaded into a 4 mm rotor by packing approximately 100 mg of the SBA-16 powder on either side of a 3 mg piece of cross-linked PDMS, which served as an internal chemical shift and spin-counting standard. 29Si NMR spectra were acquired under magical-angle-spinning conditions of 10 kHz at room temperature.
The incorporation of co-self-assembled and aligned guest species in orientationally ordered mesostructured silica films is expected to be general, provided that the aspect ratios and solubilities of the guests, e.g., supramolecular aggregates, macromolecules, nanoparticles, etc., are compatible with and conform to the mesochannel dimensions and components. One example is the inclusion of supramolecular porphyrin J-aggregates in aligned, hexagonally mesostructured silica, both of which form under mutually compatible, strongly acidic conditions. Based on separate recent results, the porphyrin species were hypothesized to become incorporated into the surfactant during the formation of the mesostructure, resulting in J-aggregated TPPS4 molecules that will yield desired anisotropic optical properties. Specifically, TPPS4 porphyrin dye species were introduced into orientationally ordered mesostructured silica films at low dye weight loading (˜1 wt %) to ensure solubility in the hydrophobic (PPO) channels and minimal disruption of the hexagonal mesostructural order. Higher TPPS4 loadings or longer drying periods resulted in the porphyrin molecules phase separating and disrupting mesostructural alignment. Due to the small scale of the anisotropic dimension of such films, the characterization of anisotropic optical properties is difficult. Polarized Optical Microscopy (POM) can be a useful tool for distinguishing between isotropic and anisotropic materials, including aligned mesostructured silica films containing J-aggregated porphyrins, based on the observance of birefringence behavior.
In POM, one expects extinction (no light transmission) when the anisotropic axis of a birefringent material is aligned parallel to one of the cross-polarizers, and maximum light transmission when the anisotropic axis is at 45 degrees between the two polarizers. When the anisotropic axis of the aligned mesostructured silica composite film (as established through 2D SAXS diffraction measurements to determine the mean direction of alignment) is parallel with one of the two crossed polarizers,
The small amount of residual light that is visible in the background of
A second example of co-assembly and alignment of guest species in hexagonally and orientationally ordered mesostructured silica is the incorporation of conjugated polymer species in patterned films. In such systems, care should especially be taken to select synthesis mixture compositions and conditions to maintain the mutual solubilities of the highly hydrophobic conjugated polymer guest and mesostructure-directing block copolymer species. In the case of Pluronic®-type block copolymers, tetrahydrofuran is an excellent solvent for both EOx and POy blocks, as well as many conjugated polymers. In addition to promoting the solubilities of hydrophobic guest species, THF-based sols dry much faster, providing less time for the guest molecules to macroscopically phase-separate, as the mesostructure-directing Pluronic® triblock copolymers self-assemble and the silica cross-links and solidifies. Previous syntheses that sought to include photo-responsive guest molecules, specifically J-aggregated porphyrin dyes, in polar, ethanol-based silica sols showed such phase separation to be a major challenge.
More specifically,
Because not all guest molecules are compatible with the synthesis conditions required to incorporate them by co-assembly in a “one-pot” method, it is desirable to show that backfilling of oriented mesopores is also possible. To do so, following otherwise identical PDMS patterning, mesostructure self-assembly, alignment, and silica cross-linking (as described in the previous section above), the P123 surfactant species were removed prior to introducing the photo-responsive guest species. Solvent extraction at 100° C. in ethanol was used to remove the soluble triblock copolymer species, while preventing additional cross-linking of the silica network and thereby retaining silanol sites for grafting functionalizing agents onto the pore walls. For example, grafting of alkylsiloxanes can subsequently be used to impart favorable hydrophobicity to the pore wall surfaces, which interact favorably with hydrophobic organic guest molecules, including many photo-responsive species.
To examine the efficacy of surface grafting strategies, a powder sample of cubic SBA-16 mesoporous silica was synthesized, functionalized, and characterized by NMR. Following solvent extraction of the structure-directing triblock copolymer species and subsequent drying of the SBA-16 powder, n-butyltrichlorosilane was grafted onto the interior silica mesopore surfaces by using a vapor deposition method as described in experimental methods.
The SAXS diffraction patterns, shown in
In the solvent-extracted silica, only the dominant (110) reflection (d-spacing of 11.2 nm) and a poorly resolved (200) reflection (d-spacing of 7.9 nm) are present. The low resolution of the higher order reflections indicates relatively poor long-range mesostructural ordering. The solvent-extracted powder was subjected to a vapor-grafting procedure by which n-butyltrichlorosilane species reacted with surface silanol species to become covalently bonded to the mesopore surfaces. Single-pulse 1D 29Si MAS NMR measurements were conducted to confirm the formation of T2 and T3 sites, indicating the presence of grafted organosiloxane moieties onto the silica framework. The very narrow and intense 29Si peak at −22 ppm is from PDMS added as an internal chemical shift and spin-counting standard, which was used to quantify the 29Si peak intensities and associated species populations before and after functionalization. The weak and broader peak at 14 ppm in both spectra is from the PDMS standard as well, likely from sites of incomplete cross-linking. The 29Si MAS spectrum shown in
With the cubic mesostructured silica pores functionalized to provide hydrophobic interior surfaces, conjugated organic oligomers could then be introduced to incorporate photo-responsive guest species by further post-synthesis modifications. In particular, the use of oligomers, as opposed to much larger molecular-weight polymers, was expected to provide lower resistances to mass-transfer into the mesopores during loading, and thus better and more uniform penetration.
For example, refer to
This broadening is consistent with decreased inhomogeneous aggregation of the oligomer species, now confined inside the mesopores, indicated by increased emission from the 0-1 band relative to the 0-0 band, which has been observed previously for semiconducting polymers incorporated into a silica mesoporous host.
Characterization of co-assembled mesostructured P123-silica nanocomposite systems containing porphyrin dyes or semiconducting polymers indicates that a vertically aligned mesostructure can be formed, while simultaneously incorporating photo-responsive guest molecules with different supramolecular architectures into the patterned films. Solid-state 2D NMR (under ultrafast MAS conditions) and anisotropic optical measurements could be used to establish unambiguously whether the guest molecules can be incorporated and aligned into the mesostructured silica host films as opposed to being isotropically aggregated among different domains or on the external surface of the patterned mesostructured films.
By incorporating a semiconducting polymer into the mesostructured host matrix, mesostructured films can be prepared on conducting ITO substrates. This would allow integration into an electroluminescent device, in which the total emission from electronic excitation of the polymer could be used to assess the degree of connectivity between both electrical contacts. When compared against a standard sample of the same weight loading of the semiconducting polymer in an unaligned mesostructure, co-alignment of the mesostructured host film and semiconducting polymer guest species are expected to yield improved performance of an LED device. Once such an electroluminescent device is formed, polarized electroluminescence measurements can be undertaken to quantify the emission anisotropy from the semiconducting polymer.
Orientationally ordered mesostructured titania with controllable directional alignments can be prepared by using the same strategies as for silica, but with precursor solution compositions and processing conditions adapted for the different chemistries of titania and silica. Titania has a number of interesting optical and electronic properties that are not shared by silica and that make titania suited for diverse applications in solar cells, catalysis, and semiconductor devices. Among the differences between titania and silica in the syntheses of orientationally ordered mesostructured films are that titania precursor species often cross-link more rapidly than silica, making self-assembly difficult. This problem may be managed by using acetylacetone as a chelating agent to slow the rate of titania cross-linking.
To impart interesting opto-electronic, catalytic, or semiconducting properties, guest species, such as conjugated polymers, organic dye molecules, or inorganic nanoparticles can be incorporated into the orientationally ordered mesostructured titania material, which serves as a host matrix. This, however, presents a number of additional challenges, with respect balancing mutual solubilities, processabilities, and other compatibilities of the various components under synthesis and conditions that promote high extents of mesostructural ordering and controllable orientational ordering.
For example, while mesostructured silica and titania can be synthesized in polar solvents (e.g., water and ethanol), finding a suitable solvent system for the inorganic precursor species, the structure-directing agents (SDA) (e.g., non-ionic poly(ethyleneoxide)-poly(polypropyleneoxide)-poly(ethyleneoxide) Pluronic® P123 and F127 triblock copolymers, low-molecular-weight surfactants (e.g., ethyleneoxide-alkyl Brij®-56), and one or more guest species is often challenging. This is especially true for relatively hydrophilic mesostructured titania and highly hydrophobic guest molecules, such as conjugated polymers (e.g., MEH-PPV), for which mutually compatible solvents are few. The solvent tetrahydrofuran (THF) balances many of the compatibility issues and is suitable for the alignment procedures described here. Furthermore, by judicious selection of the composition and processing conditions for the different solvent, inorganic, structure-directing, and guest species, one can exert significant control on how and where the species self-assemble, at what surfaces they nucleate, and how the resulting mesophase domains grow.
One way to implement this embodiment of the invention is to deposit 11 μL of a precursor solution onto a desired substrate using a pipette. Glass, titania-coated Kapton® (a polymer), or silicon are commonly used as substrates, but many other substrates are suitable for use, including other inorganic substrates or organic substrates (e.g., polymers or organic surface-coatings), based on their adhesion, device application, or other properties. Once the precursor solution has been deposited on the substrate, it is subsequently covered by a stamp or mold that can be patterned arbitrarily, according to device needs and tolerances. For the present applications, a ca. 8-mm thick poly(dimethylsiloxane) (PDMS) stamp/mold was used with micropatterned channels 1 μm deep, 7 μm wide, and several millimeters long (
A suitable titania precursor solution can be prepared by mixing 1 mL tetraethoxy-titanium (TEOT, Ti(OC2H5)4) with 0.35 mL of concentrated aqueous hydrochloric acid. This causes a precipitate to form, which dissolves upon stirring after several minutes. 10 min after the addition of the acid, 0.35 mL acetylacetone (acac) is added, which causes the solution to turn yellow. This titania precursor solution is then added to a solution containing the structure-directing agent (SDA), e.g., low-molecular-weight surfactant species such as Brij®-56, or block-copolymer species such as Pluronic® P123 or F127. For Brij®-56, the SDA precursor solution contains 0.47 g of the Brij®-56 dissolved in 4 mL of THF. For Pluronic® P123, 0.53 g of the P123 is dissolved in 11.47 g of THF.
If functionalization of the titania network is desired, then species, such as trimethoxycyclopentadienyl titanium (TMCPT), can be added before casting or patterning the film. For example, 10 μL of TMCPT can be added to introduce hydrophobic character and/or phenyl groups into the resultant titania network, which can be achieved without disrupting mesostructural ordering of the final material. Finally, if guest molecules are to be incorporated, a guest-molecule precursor solution is mixed with the SDA precursor solution before casting or patterning. For example for the conjugated polymer MEH-PPV, the guest-molecule precursor solution consists of 1.2 to 12 mg of MEH-PPV dissolved in 4 mL of THF. This solution is heated to 55° C. for approximately 1 h and then filtered with 5.0 and 0.45 μm Teflon® filters prior to being combined with the SDA and titania precursor solutions and then used for casting or patterning a film.
Controlling the direction of solvent flux provides way to control the alignment of mesostructured inorganic materials. The same or similar precursor solutions as described above can be used to prepare mesostructured titania films with orientational ordering that can be controlled according to the material composition and processing conditions. Important considerations that influence the formation of aligned mesostructured materials are the rates and direction(s) of solvent removal, the type and anisotropic character of the mesostructure(s) formed, and the interactions between the self-assembling SDA species and the surface(s) from which the solvent species leave the precursor solution (e.g., within the PDMS-patterned microchannels), temperature, etc. By adjusting these, mesostructured titania can be prepared with orientational ordering, for example, as hexagonal phases without or with guest species, such as MEH-PPV, and/or without or with functionalized titania networks, such as TMCPT, and with cylinder alignments predominantly perpendicular to the substrate (i.e., ‘vertically’), with alignments predominantly in the plane of the substrate oriented perpendicular to the long microchannel axes (e.g. ‘laterally’), or with alignments predominantly in the plane of the substrate oriented parallel to the long microchannel axes (e.g. ‘longitudinally’).
Under the conditions used here, the use of Brij®-56 SDA in THF with a 8-mm PDMS stamp/mold (the latter of which is predominantly devoid of dissolved solvent species) tends to form hexagonal mesostructured titania with orientationally ordered cylinders perpendicular (vertical) to the substrate. By comparison, using similar precursor solutions and procedures as described above, but with a thinner PDMS stamp ca. 1 mm thick and covered by a glass or metal plate, most of the solvent species are removed (by diffusion) from the microchannels laterally via the sides of the stamp, rather than being removed in a direction perpendicular to the substrate. This causes the hexagonal mesostructured titania to self-assemble and grow in domains that are aligned in the plane of the substrate, with alignments that can be controlled to be lateral or longitudinal with respect to the microchannel axes, according to the predominant direction(s) of solvent removal.
By using a rectangular stamp and placing solvent selectively, such as along the shorter ends, the direction of solvent removal can be restricted predominantly to a single axis that results in preferential orientational ordering of the resulting mesostructure along that axis. A patterned film (or monolith) can be oriented at an arbitrary angle relative to such an axis or axes, so as to produce a film (or monolith) with laterally, longitudinally, or other uniaxially aligned mesostructural order. More complicated orientational ordering may be achieved by controlling the time-dependent removal of one or more solvent species, optionally in different directions.
Another way to control the orientational ordering of mesostructured titania is by the selection of the structure-directing agent (SDA), according to the relative hydrophobicity-hydrophilicity of its substituent groups, compared to the hydrophobicity or hydrophilicity of the solvent(s) and mold or substrate surfaces at which the mesostructured phases nucleate and grow. For example, for hexagonally mesostructured titania films, vertical alignment of the cylindrical-aggregates normal to the substrate can be achieved in THF and at relatively hydrophobic PDMS surfaces by using a structure-directing agents with more hydrophobic non-polar (e.g., alkyl) chains, such as Brij®-56. By comparison, laterally or longitudinally aligned cylindrical-aggregates of mesostructured titania can be controllably achieved in THF by using structure-directing agents with more polar cores, such as the propyleneoxide chains present in block copolymers like Pluronic® P123.
The properties of mesostructured titania films, and specifically their anisotropic orientational ordering, can be characterized by a variety of methods, including Small Angle X-ray Scattering (SAXS) and transmission electron microscopy (TEM).
Two-dimensional (2D) SAXS provides insight into the type and extent of mesostructural ordering of the materials and into the degrees to which they are orientationally ordered with respect to the incident 1-mm2 X-ray beam, as illustrated in
Transmission electron microscopy (TEM) is a powerful technique that allows direct visualization of the mesostructure over relatively small regions of a sample. TEM images can be obtained for films in cross-section by using a focused-ion-beam (FIB) milling to cut a trench into or through a microchannel, which can then be imaged from the side in the plane of the substrate. An example of a cross-sectional FIB-TEM image acquired from a micropatterned, hexagonal mesostructured titania-Brij®-56 film is shown in
The left portion of
The image shows a high degree of vertical orientational ordering of the cylinders relative to the free microchannel surface that is representative of other such images acquired at different locations within the same film and other films prepared under similar conditions. These results are complementary to and consistent with the results obtained by SAXS in
The incorporation of photo-responsive guest molecules into mesostructured titania films can be studied by using fluorescence confocal microscopy, in combination with SAXS and TEM. SAXS and TEM measurements establish that the mesostructure ordering and alignment of the material appears to be undisturbed by the introduction of guest molecules. Confocal microscopy is useful to assess whether macroscopic aggregation and phase-separation of guest species may have occurred, by being able to detect aggregates on the scale of 100 nm to 1000 μm in size.
For example,
Mesostructured titania films can be prepared with controllable orientational ordering by judicious selection of precursor solution compositions, the compositions, structures and/or surface properties of patterning stamps/molds, the directions and rates of solvent removal, temperature, surface substrate properties, surrounding atmosphere, pressure, etc. Furthermore, a wide variety of functional guest species can be incorporated during or after film syntheses, such as MEH-PPV or other photo-responsive organic molecules, inorganic species, such as semiconducting, conducting, or catalytic nanoparticles or clusters, organic species, organometallic groups, acidic or other ionic moieties, adsorption- or transport-selective species, or mixtures thereof. These materials and associated methods of preparation are novel and have a number of promising applications, particularly in opto-electronic devices, such as solar cells, as semipermeable membranes, as sensors, or as catalysts. The methods described can be combined with the use of other externally applied fields (e.g., electric, magnetic, light, flow, etc.), which can be furthermore applied transiently to allow more complicated patterning or alignments to be achieved. In addition, the methods described are not limited to films, but can be used for monoliths with different shapes, fibers, or other objects.
Although the description above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.”
This application claims priority from U.S. provisional application Ser. No. 61/013,919 filed on Dec. 14, 2007, incorporated herein by reference in its entirety. This application is also a continuation-in-part of U.S. patent application Ser. No. 11/735,252 filed on Apr. 13, 2007, incorporated herein by reference in its entirety, which claims priority from U.S. provisional application No. 60/792,050, filed on Apr. 13, 2006, incorporated herein by reference in its entirety.
This invention was made with Government support under Grant No. DMR-02-33728 awarded by the National Science Foundation. The Government has certain rights in this invention.
Number | Date | Country | |
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61013919 | Dec 2007 | US | |
60792050 | Apr 2006 | US |
Number | Date | Country | |
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Parent | 11735252 | Apr 2007 | US |
Child | 12335225 | US |