Patent Application
20190030490
A long-standing goal in materials science is to provide highly ordered or periodic nanostructures with useful properties over large length scales or technologically relevant dimensions. Bottom-up approaches involve the self-assembly of atomic, molecular and colloidal building blocks as a promising way to achieve this goal at potentially lower cost or higher throughput than top-down strategies (Whitesides et al., 2002, Proc. Nat. Acad. Sci. 99:4769-4774). The translation of these ideas to polymeric materials enables highly compelling applications, such as precisely tailored nanoporous membranes (Jackson and Hillmyer, 2010, ACS Nano 4:3548-3553; Seo and Hillmyer, 2012, Science 336:1422-1425), photonic band gap materials (Yoon et al., 2005, MRS Bull. 30:721-726), and high resolution lithography using self-assembled structures as pattern transfer masks (Bita et al., 2008, Science 321:939-943). In these three cases, as in others, reliable control over the orientation of the self-assembled structures is needed to enable the applications as envisioned.
In designing nanoporous films for use as membranes, and also for pattern transfer applications (e.g. block copolymer lithography), the pores should all ideally possess the same diameter and be aligned parallel to the macroscopic transport direction, i.e. in the “thickness”, “through-plane”, or “vertical” direction (Hu et al., 2014, Soft Matter 10:3867-3889). The current state of the art departs considerably from this ideal, however. Membranes produced by conventional techniques, such as phase inversion, have highly tortuous, interconnected pore networks and a large variation in pore diameters. The broad distribution of pore diameters severely limits the selectivity of such membranes as evident from their molecular-weight cutoff characteristics (Gin and Noble, 2011, Science 332:674-676; Merkel et al., 2002, Science 296:519-522; Baker, 2012, Membrane Technology and Applications, Wiley). Additionally, the highly tortuous pore networks negatively impact membrane permeability, as the distance of molecular transport will be greater than the thickness of the membrane (Baker, 2012, Membrane Technology and Applications, Wiley, Phillip et al., 2010, ACS Applied Materials and Interfaces 2:847-853).
In principle, the aforementioned permeability and selectivity issues can be circumvented using self-assembled materials such as block copolymers (BCPs) or small-molecule liquid crystals (LCs) that feature nanostructures with thermodynamically defined characteristic dimensions which are therefore narrowly distributed (Jackson and Hillmyer, 2010, ACS Nano 4:3548-3553; Dorin et al., 2014, Polymer 55:347-353; Gin et al., 2006, Adv. Funct. Mater. 16:865-867; Gin et al., 2001, Acc. Chem. Res. 34:973-980). Leveraging self-assembled materials to fabricate ideal membranes requires both physical continuity and vertical alignment of nanostructures over large areas in thin films. As one might expect, such morphologies in general do not result spontaneously during self-assembly of these systems (Askay et al., 1996, Science 273:892-898). Considerable effort must often be expended, for example through the use of interfacial engineering (Bates et al., 2012, Science 338:775-779) or external fields (Majewski et al., J. Polym. Sci., Part B: Polym. Phys. 50:2-8; Firouzi et al., 1997, J. Am. Chem. Soc. 119:9466-9477), to control alignment while physical continuity is determined principally by the propensity to form defects.
One area of interest is in the development of membrane morphologies in systems where self-assembly provides access to pore sizes in the 1 nm regime. This length scale is of considerable interest in water purification as it permits nanofiltration of multivalent salts and small molecule solutes, including boron, which is a particularly challenging contaminant (Shannon et al., 2008, Nature 452:301-310). Recently, the use of magnetic fields to generate uniformly oriented, 1 nm pores in mechanically robust polymer films by field-induced alignment of a self-assembled cross-linkable LC mesophase was reported (Feng et al., 2014, ACS Nano 8:11977-11986). While the method is extremely effective, field alignment is best suited to morphological control in the bulk, i.e. where surface forces do not play a prominent and potentially confounding role. In the reported work the systems considered were generally 10 μm or larger in thickness. Practical membranes are often considerably thinner, however—transmembrane flux is maximized through thickness minimization for a given pressure differential. For example, the dense permselective layer in thin-film composite membranes used for reverse osmosis is only ca. 100-200 nm thick (Phillip et al., 2010, ACS Applied Materials and Interfaces 2:847-853; Geise et al., 2014, Prog. Polym. Sci. 39:1-42; Ghosh et al., 2008, J. Membrane Science 311:34-45). The need for mechanical integrity of the membrane, on the other hand, imparts a lower threshold on film thickness. A highly useful membrane therefore requires generating ideal pore morphology—physical continuity and vertical alignment—in a suitably thin yet mechanically robust film, ca. 1 μm in thickness or less. As a practical consideration, the morphological control should be amenable to large area films. Taken together, these requirements represent an extremely compelling yet unfulfilled goal.
The production of useful polymers from renewable or sustainably-derived materials represents an increasingly important societal concern. Synthetic routes have been developed for the polymerization of a broad range of sustainably-derived monomers, including vegetable oils and fatty acids, terpenes, lactic acid, and saccharides (Wilbon, et al., 2013, Macromol. Rapid Comm. 34:8; Gandini et al., 2016, Chem. Rev., 116:1637; de Espinosa, et al., 2011, Eur. Polym. J., 47:837; Xia and Larock, 2010, Green Chem., 12:1893; Yao and Tang, 2013, Macromolecules, 46:1689; Mikami, et al., 2013, J. Am. Chem. Soc., 135:6826; Meier, et al., 2007, Chem. Soc. Rev. 36:1788). The appeal of sustainable polymers from environmental and economic perspectives has traditionally been tempered by inferior properties, particularly mechanical properties, relative to petroleum-derived materials.
There is a need in the art for thin, nanoporous membranes with vertically aligned nanostructures. There is also need in the art for useful nanoporous polymers made from renewable or sustainably-derived materials. The present invention addresses these unmet needs.
In one aspect of the invention, a method of aligning nanopores in a polymeric film is described. In one embodiment, the method includes the steps of: depositing a solution of at least one monomer in a solvent onto a surface of a first substrate to form a mesophase comprised of nanopores; applying a second substrate onto a surface of the mesophase, such that the mesophase is in contact with both the first substrate and the second substrate, and wherein the nanopores at least partially align in response to the second substrate; and polymerizing the mesophase to form a polymeric film containing the at least partially aligned nanopores. In one embodiment, the method further includes the steps of: raising the temperature of the mesophase such that the mesophase is in a disordered state; and controlling the rate of cooling of the mesophase as it returns to an ordered state. In another aspect of the invention, a polymeric film formed by this method is described.
In one embodiment, the monomer is sodium 3,4,5-tris(11′-acryloyloxyundecyloxy)benzoate (Na-GA3C11). In one embodiment, the solvent further comprises a photoinitiator. In one embodiment, the photoinitiator is 2,4,6-trimethylbenzoyl-diphenylphosphine oxide. In one embodiment, the amount of photoinitiator is about 0.5%. In one embodiment, the monomer is polymerized by exposing the mesophase to UV light.
In one embodiment, the first substrate is a polydimethylsiloxane (PDMS) substrate. In one embodiment, the second substrate is a polydimethylsiloxane (PDMS) substrate. In one embodiment, the first substrate is a glass substrate. In one embodiment, the second substrate is a glass substrate. In one embodiment, the solvent is removed prior to the application of the second substrate.
In one embodiment, the polymeric film has a pore diameter of about 1 nm. In one embodiment, the polymeric film has a thickness ranging from about 200 nm to 40 μm. In one embodiment, the polymeric film has a hexagonal pore arrangement. In one embodiment, the nanopores are vertically aligned.
In another aspect of the invention, a composite material is described. In one embodiment, the composite material includes a first substrate, a second substrate, and a layer between the first and second substrate which comprises at least one monomer, at least one photoinitiator, and a plurality of nanopores, wherein the plurality of nanopores are at least partially aligned in the layer. In one embodiment, the at least one monomer is sodium 3,4,5-tris(11′-acryloyloxyundecyloxy)benzoate (Na-GA3C11).
In another aspect of the invention, a method of fabricating a polymeric film is described. In one embodiment, the method includes the steps of: depositing a solution of at least one monomer in a solvent onto a surface of a first substrate to form a mesophase comprised of nanopores; removing the solvent; applying a second substrate onto a surface of the mesophase, wherein the mesophase is in contact with both the first substrate and the second substrate, and wherein the nanopores at least partially align in response to the second substrate; raising the temperature of the mesophase such that the mesophase is in a disordered state; controlling the rate of cooling of the mesophase as it returns to an ordered state; and polymerizing the mesophase to form a polymeric film containing the at least partially aligned nanopores. In one embodiment, the method further includes the step of removing at least one of the substrates from the composite material after the polymerizing step. In one embodiment, the monomer is sodium 3,4,5-tris(11′-acryloyloxyundecyloxy)benzoate (Na-GA3C11). In one embodiment, the photoinitiator is 2,4,6-trimethylbenzoyl-diphenylphosphine oxide.
In one embodiment, the first substrate is a polydimethylsiloxane (PDMS) substrate. In another embodiment, the second substrate is a polydimethylsiloxane (PDMS) substrate. In one embodiment, the first substrate is a glass substrate. In another embodiment, the second substrate is a glass substrate. In one embodiment, the solvent is removed prior to the application of the second substrate.
In another aspect of the invention, a method of fabricating a polymeric film of aligned nanopores is described. In one embodiment, the method includes the steps of: depositing a mixture of at least one monomer and at least one template compound onto a surface of a first substrate to form a mesophase; polymerizing the mesophase to form a polymeric film; rinsing the polymeric film with NaOH in DMSO to remove the template compound; and wetting the polymeric film with water to form aligned nanopores. In one embodiment, the method further includes the steps of applying a second substrate prior to polymerization. In one embodiment, the method further includes the steps of: applying a magnetic field to the mesophase; rotating the mesophase about the normal of the first substrate; heating the mesophase; and gradually cooling the mesophase to room temperature. Also described is a polymeric film formed by this method.
In one embodiment, the first and second substrates are glass substrates. In one embodiment, the first and second substrates are coated with poly(sodium styrene sulfonate). In one embodiment, the first substrate and second substrate are coated with octadecyltrimethoxysilane.
In one embodiment, the mixture further includes at least one crosslinker and at least one photoinitiator. In one embodiment, the photoinitiator is benzoin methyl ether. In one embodiment, the crosslinker is selected from the group containing divinylbenzene, butyl acrylate, and 1,6-hexanediol diacrylate.
In one embodiment, the monomer is an unsaturated fatty acid. In one embodiment, the monomer is an epoxidized fatty acid. In one embodiment, the template compound is 1,3,5-tris(1H-benzo[d]imidazol-2-yl)benzene. In one embodiment, the monomer and template compound are in the mixture in a ratio of about 3:1. In one embodiment, the template compound can be recycled for later use.
The following detailed description of preferred embodiments of the invention will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities of the embodiments shown in the drawings.
It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for the purpose of clarity, many other elements found in composite materials and polymeric thin films. Those of ordinary skill in the art may recognize that other elements and/or steps are desirable and/or required in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. The disclosure herein is directed to all such variations and modifications to such elements and methods known to those skilled in the art.
As used herein, each of the following terms has the meaning associated with it in this section. Unless defined otherwise, all technical and scientific terms used herein generally have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
The articles “a” and “an” are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element.
As used herein, the term “about” will be understood by persons of ordinary skill in the art and will vary to some extent depending on the context in which it is used. As used herein when referring to a measurable value such as an amount, a temporal duration, and the like, the term “about” is meant to encompass variations of ±20% or ±10%, more preferably ±5%, even more preferably ±1%, and still more preferably ±0.1% from the specified value, as such variations are appropriate to perform the disclosed methods.
As used herein, the term “mesophase” refers to the ordered phases of matter formed by anisotropic molecular or colloidal species as a function of temperature, concentration, pressure, ionic strength (salt content) or combinations thereof.
As used herein, the term “mesogen” refers to the constituents of mesophases.
As used herein, the term “liquid crystal” refers to a thermodynamic stable phase characterized by anisotropy of properties without the existence of a three-dimensional crystal lattice, generally lying in the temperature range between the solid and isotropic liquid phase.
As used herein, the term “lyotropic” refers to molecules that form phases with orientational and/or positional order in a solvent. Lyotropic liquid crystals can be formed using amphiphilic molecules (e.g., dodecyltrimethylammonium bromide, sodium laurate, phosphatidylethanolamine, lecithin). The solvent can be water.
As used herein, the term “nanopore” refers to a pore, channel or passage formed or otherwise provided in a membrane. A nanopore may have a characteristic width or diameter in a range of 0.1 nanometers to about 1000 nm.
As used herein, the term “monomer,” refers to any molecule that can be polymerized, that is, linked together via a chemical reaction to form a higher molecular weight species.
As used herein, the term “polymer” denotes a covalently bonded chain of monomer units, and is intended to include both homopolymers and copolymers.
As used herein, the term “initiator,” in accordance with the definition adopted by the IUPAC, refers to a substance introduced into a reaction system in order to bring about reaction or process generating free radicals or some other reactive reaction intermediates which then induce a chain reaction.
As used herein, the term “photoinitiator,” in accordance with the definition adopted by the IUPAC, refers to a substance capable of inducing the polymerization of a monomer by a free radical or ionic chain reaction initiated by photoexcitation.
The term “crosslinker” refers to compounds that are able to react with the functional group or groups on the polymer chains to lengthen them and/or connect them, e.g., to form a crosslinked network like that of a cured elastomer.
As used herein, the term “Na-GA3C11” refers to 3,4,5-tris(11′-acryloyloxyundecyloxy)benzoate.
As used herein, the term “Darocur TPO” refers to 2,4,6-trimethylbenzoyl-diphenylphosphine oxide.
Throughout this disclosure, various aspects of the invention can be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 2.7, 3, 4, 5, 5.3, and 6. This applies regardless of the breadth of the range.
As described herein, the present invention relates in part to the facile and scalable synthesis of thin polymer films containing vertically oriented nanopores. For example, as illustrated in
As contemplated herein, the resulting polymeric thin films may be used in membranes, such as small molecule, size-based separation membranes, or other functional composite films, where large pore size distributions and tortuosity hinder performance and have proven challenging to overcome.
As contemplated herein, a LC mesophase is formed from the self-assembly of one or more mesogens. In one embodiment, the mesogen is a monomer. The LC mesophase may be composed of one or more photoinitiators or other chemical constituents. In certain embodiments, the mesophase may be a single-component material. In other embodiments, the mesophase may be multicomponent, having tunable physicochemical properties. In other embodiments, the mesophase may be anisotropic, lyotropic, thermotropic and/or metallotropic.
The mesophase contains at least one monomer. In one embodiment, the mesogen is itself a monomer, capable of being reacted to form a polymer. In one embodiment, the monomer is amphiphilic. As used herein, the term “amphiphilic” refers to a compound which has at least one hydrophilic moiety and at least one hydrophobic moiety. The one or more amphiphilic monomer induce a structural ordering of the LC system. In a non-limiting example, an amphiphilic monomer may comprise at least one polar group such as hydroxyl, carboxyl, sulfonate, phosphate, amino or any salts thereof, and at least one non-polar group such as n-alkyl, branched alkyl, alkenyl, or alkynyl. In another embodiment, the monomer contains one or more polymerizable constituents. For example, the mesophase may include one or more types or category of monomer suitable for forming a polymeric structure. The monomer may be synthetic, organic, or any other type of polymerizable monomeric molecule. The monomer may contain a type of polymerizable group. A polymerizable group is a chemical moiety that polymerizes under certain chemical conditions. In general, the type of polymerizable group is not critical, so long as the polymerizable group is capable of polymerization with a monomer of the instant invention. Examples of polymerizable groups include double-bond containing moieties which are polymerized by photopolymerization or free radical polymerization. In some embodiments, the polymerizable group is a vinyl group, acryl group, alkylacryl group (i.e. acryl group having an alkyl substituent, such as methacryl). As used herein, acryl (alkylacryl, methacryl, etc) includes acryl esters as well as acryl amides. In another embodiment, the monomer may be any alkyl methacrylate. In another embodiment, the monomer may be styrene, vinyl acetate, vinyl pyridine, n-isopropylacrylamide or a vinyl ether. In one embodiment, the monomer is sodium 3,4,5-tris(11′-acryloyloxyundecyloxy)benzoate (Na-GA3C11). Na-GA3C11 is a wedge-shaped amphiphilic molecule possessing a large hydrophobic body and a small hydrophilic head, forming supramolecular columnar LC mesophases with closely-packed, ordered hydrophilic nanochannels. Moreover, the introduction of triple reactive acrylate groups at the periphery of Na-GA3C11 enables structural lock-in of the hexagonal columnar (Colh) order by photo-crosslinking into a mechanically and chemically robust polymer. In another embodiment, the monomer is the acid form, 3,4,5-tris(11′-acryloyloxyundecyloxy)benzoic acid (GA3C11). In another embodiment, the monomer is a metallic salt of 3,4,5-tris(11′-acryloyloxyundecyloxy)benzoic acid (GA3C11). In another embodiment, the monomer is sodium 2,3,4-tris(11′-acryloylundecyl-1′-oxy)benzenesulfonate. In one embodiment, the monomer self-assembles into a hexagonal columnar LC mesophase at room temperature. Non-limiting examples of other LC mesophases include cubic, reverse hexagonal, lamellar, and reverse micellar. Upon vertical alignment, these columns form nanochannels within the mesophase.
In one embodiment, the monomer is a natural or unnatural unsaturated fatty acid. Unsaturated fatty acids possess a carboxylic acid head group and a long aliphatic chain with one or more alkene groups. Examples of unsaturated fatty acids include, but are not limited to, 3-hexenoic acid, trans-2-heptenoic acid, 2-octenoic acid, 2-nonenoic acid, cis- and trans-4-decenoic acid, 9-decenoic acid, 10-undecenoic acid, trans-3-dodecenoic acid, tridecenoic acid, cis-9-tetradeceonic acid, pentadecenoic acid, cis-9-hexadecenoic acid, trans-9-hexadecenoic acid, 9-heptadecenoic acid, cis-6-octadecenoic acid, trans-6-octadecenoic acid, cis-9-octadecenoic acid, trans-9-octadecenoic acid, cis-11-octadecenoic acid, trans-11-octadecenoic acid, cis-5-eicosenoic acid, cis-9-eicosenoic acid, cis-11-docosenoic acid, cis-13-docosenoic acid, trans-13-docosenoic acid, cis-15-tetracosenoic acid, cis-17-hexacosenoic acid, and cis-21-triacontenoic acids, as well as 2,4-hexadienoic acid, cis-9-cis-12-octadecadienoic acid, cis-9-cis-12-cis-15-octadecatrienoic acid, eleostearic acid, 12-hydroxy-cis-9-octadecenoic acid, and the like. In one embodiment, the monomer is conjugated linoleic acid (CLA).
In one embodiment, the monomer is an epoxidized fatty acid. Epoxidized fatty acids are fatty acids that have been treated with a chemical agent to convert internal alkenes into epoxide moieties. Fatty acids suitable for epoxidation include, but are not limited to, all fatty acids discussed herein. In one embodiment, the epoxidized fatty acids are polymerized with reagents known to those of skill in the art. Epoxidzed fatty acid polymerizing agents include, but are not limited to, aliphatic amines, aromatic amines, polyamide resins, tertiary and secondary amines, imidazoles, polymercaptans, polysulfide resins, anhydrides, boron trifluoride-amine complexes, dicyandiamide, organic acid hydrazides, and photo- and ultraviolet
In one aspect of the invention, a template compound is employed to control the orientation of the monomers. In one embodiment, the monomer and template compound form a stable mesophase when mixed in a 3:1 ratio. In one embodiment, the template compound is 1,3,5-tris(1H-benzo[d]imidazol-2-yl)benzene. Other examples of template compounds include, but are not limited to, 2-[3,5-bis(1H-benzimidazol-2-yl)cyclohexyl]-1H-benzimidazole, 2,2′,2″-(1α,3α,5α-Cyclohexanetriyl)tris(1-azonia-1H-benzoimidazole), 2-[3,5-bis(6-methyl-1H-benzimidazol-2-yl)phenyl]-6-methyl-1H-benzimidazole, 2,2′-(2-((1H-benzo[d]imidazol-2-yl)methyl)propane-1,3-diyl)bis(1H-benzo[d]imidazole), N,N-bis(1H-benzimidazol-2-ylmethyl)-3-phenylpropan-1-amine, 2-[2-(1H-benzimidazol-2-ylmethyl)phenyl]-1H-benzimidazole, 2-[2,4,5-tris(1H-benzimidazol-2-yl)phenyl]-1H-benzimidazole, N,N-bis(1H-benzimidazol-2-ylmethyl)-1-phenylmethanamine, N,N-bis(1H-benzimidazol-2-ylmethyl)-2-phenylethanamine, 2-[3-[2-amino-1,3-bis(1H-benzimidazol-2-yl)propan-2-yl]phenyl]-1,3-bis(1H-benzimidazol-2-yl)propan-2-amine, 4-phenyl-2-(4-phenyl-1H-benzimidazol-2-yl)-1H-benzimidazole, 2-[[3-(1H-benzimidazol-2-ylmethyl)-1-adamantyl]methyl]-1H-benzimidazole, 6-methyl-N,N-bis(6-methyl-1H-benzimidazol-2-yl)-1H-benzimidazol-2-amine, 2-[2,4,5-tris(1H-benzimidazol-2-yl)phenyl]-1H-benzimidazole, N,N,N′,N′-tetrakis(1H-benzimidazol-2-ylmethyl)propane-1,3-diamine, 2-[3-(1H-benzimidazol-2-yl)-5-[3,5-bis(1H-benzimidazol-2-yl)phenyl]phenyl]-1H-benzimidazole, N,N,N′,N′-tetrakis(1H-benzimidazol-2-ylmethyl)butane-1,4-diamine, 2-[1,2-bis(1H-benzimidazol-2-yl)-2-(1,3-dihydrobenzimidazol-2-ylidene)ethylidene]benzimidazole, N,N,N′,N′-tetrakis(1H-benzimidazol-2-yl)ethane-1,2-diamine, N, N,N′,N′-tetrakis(1H-benzimidazol-2-yl)ethane-1,2-diamine, 2-[2-(1H-benzimidazol-2-yl)propan-2-yl]-1H-benzimidazole, 2-[3-(1H-benzimidazol-2-yl)-5-[3,5-bis(1H-benzimidazol-2-yl)phenyl]phenyl]-1H-benzimidazole, benzene-1,3,5-tricarboxylic acid, and the like.
In some embodiments of the invention, it is desirable to remove the template compound from the polymerized film. In one embodiment, the template compound is removed by immersing the polymer film in 0.1% NaOH in DMSO, followed by rinsing with water. In one embodiment of the invention, the polymer film is then soaked in water to restore the nanotubes. In one embodiment of the invention, the template molecule and monomer co-system creates a hexagonal distribution of nanopores in the polymer film. In one embodiment, the template molecule can be recycled following its removal via NaOH in DMSO and can be re-used in later polymer-forming processes.
The mesophase may also include one or more crosslinkers and/or initiators, depending on the mechanism and the amount of polymerization and crosslinking desired. As contemplated herein, any type of crosslinker and/or initiator may be used as would be understood by those skilled in the art. Examples of crosslinkers include, without limitation, polycarboxylic acids, polyamines, polyisocyanates, polyepoxides, and polyhydroxyl containing species. Other crosslinkers include bi- and multifunctional vinyl ethers, acrylamides and acrylates. Exemplary crosslinkers include 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (also referred to as triallylisocyanurate), triallyl cyanurate, diallyl bisphenol A, diallylether bisphenol A, triethyleneglycol divinyl ether, 1,4-bis(4-vinylphenoxy)butane, cyclohexanedimethanol divinyl ether, multi-functional norbornene monomers prepared by reaction of multifunctional acrylates with cyclopentadiene, norbornadiene, 1,2,4-benzenetricarboxylic acid tris[4-(ethenyloxy)butyl]ester, vinylcyclohexene, 1,2,4-trivinylcyclohexane, diallyl malate, diallyl monoglycol citrate, allyl vinyl malate, glycol vinyl allyl citrate, monoglycol monoallyl citrate, monoglycol monoallyl fumarate, ethylene glycol dimethacrylate, N,N-methylene-bismethacrylamide, diethylene glycol dimethacrylate, glycerine trimethacrylate, and the like. In one embodiment, the crosslinker is divinylbenzene. In one embodiment, the crosslinker is butyl acrylate (BA). In one embodiment, the crosslinker is 1,6-hexanediol diacrylate (HDA). In one embodiment, the crosslinker is poly(ethylene glycol)-400 dimethacrylate. In some embodiments, one or more crosslinkers can be used in combination.
Examples of initiators include, but are not limited to, thermal initiators, photoinitiators, redox reaction initiators, persulfates, ionizing radiation initiators, and ternary initiators. Other photoinitiators and thermal initiators include those based on benzophenones as well as those based on peroxides. In a preferred example, the initiator is a photoinitiator.
In one embodiment, the initiator is an organic photoinitiatior. In another embodiment, the photoinitiator is acetophenone. Non-limiting examples of photoinitiators include 2-benzyl-2-(dimethylamino)-4′-morpholinobutyrophenone, 4′-tert-butyl-2′,6′-dimethylacetophenone, 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide/2-hydroxy-2-methylpropiophenone blend, 4′-ethoxyacetophenone, 3′-hydroxyacetophenone, 4′-hydroxyacetophenone, 1-hydroxycyclohexyl phenyl ketone, 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone, 2-hydroxy-2-methylpropiophenone, 2-methyl-4′-(methylthio)-2-morpholinopropiophenone, 4′-phenoxyacetophenone, benzoin, benzoin ethyl ether, benzoin methyl ether, 4,4′-dimethoxybenzoin, 4,4′-dimethylbenzil, benzophenone, benzophenone-3,3′,4,4′-tetracarboxylic dianhydride, 4-benzoylbiphenyl, 4,4′-bis(diethylamino)benzophenone, 4,4′-bis[2-(1-propenyl)phenoxy]benzophenone, 4-(diethylamino)benzophenone, 4,4′-dihydroxybenzophenone, 4-(dimethylamino)benzophenone, 3,4-dimethylbenzophenone, 3-hydroxybenzophenone, 4-hydroxybenzophenone, 2-methylbenzophenone, 3-methylbenzophenone, 4-methylbenzophenone, methyl benzoylformate, Michler's ketone (4,4′-bis(dimethylamino)benzophenone), bis(4-tert-butylphenyl)iodonium perfluoro-1-butanesulfonate, bis(4-tert-butylphenyl)iodonium p-toluenesulfonate, bis(4-tert-butylphenyl)iodonium triflate, boc-methoxyphenyldiphenylsulfonium triflate, (tert-butoxycarbonylmethoxynaphthyl)-diphenylsulfonium triflate, (4-tert-butylphenyl)diphenylsulfonium triflate, diphenyliodonium hexafluorophosphate, diphenyliodonium nitrate, diphenyliodonium perfluoro-1-butanesulfonate, diphenyliodonium p-toluenesulfonate, diphenyliodonium triflate, (4-fluorophenyl)diphenylsulfonium triflate, n-hydroxynaphthalimide triflate, n-hydroxy-5-norbornene-2,3-dicarboximide perfluoro-1-butanesulfonate, (4-iodophenyl)diphenyl sulfonium triflate, (4-methoxyphenyl)diphenylsulfonium triflate, 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine, (4-methylphenyl)diphenyl sulfonium triflate, (4-methylthiophenyl)methyl phenyl sulfonium triflate, 1-naphthyl diphenylsulfonium triflate, (4-phenoxyphenyl)diphenylsulfonium triflate, (4-phenylthiophenyl)diphenylsulfonium triflate, triarylsulfonium hexafluoroantimonate salts, triarylsulfonium hexafluorophosphate salts, triphenylsulfonium perfluoro-1-butanesufonate, triphenylsulfonium triflate, tris(4-tert-butylphenyl)sulfonium perfluoro-1-butanesulfonate, tris(4-tert-butylphenyl)sulfonium triflate, anthraquinone-2-sulfonic acid, 2-tert-butylanthraquinone, camphorquinone, diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, 9,10-phenanthrenequinone, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, 1-chloro-4-propoxy-9h-thioxanthen-9-one, 2-chlorothioxanthen-9-one, 2,4-diethyl-9h-thioxanthen-9-one, isopropyl-9h-thioxanthen-9-one, 10-methylphenothiazine, and thioxanthen-9-one. In one embodiment, the photoinitiator is 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (Darocur TPO). In one embodiment, the photoinitiator is benzoin methyl ether.
In some embodiments, the initiator is a thermal initiator. In a non-limiting example, the thermal initiator polymerizes the monomer upon exposure to heat, as would be understood by one of ordinary skill in the art. Examples of thermal initiators include, but are not limited to, azo compounds, peroxides and persulfates. Suitable persulfates include, but are not limited to, sodium persulfate and ammonium persulfate.
Suitable azo compounds include, but are not limited to, non-water-soluble azo compounds, such as 1-1′-azobiscyclohexanecarbonitrile, 2-2′-azobisisobutyronitrile, 2-2′-azobis (2-methylbutyronitrile), 2-2′azobis (propionitrile), 2-2′-azobis (2, 4-dimethylvaleronitrile), 2-2′ azobis (valeronitrile), 2-(carbamoylazo)-isobutyronitrile and mixtures thereof and water-soluble azo compounds, such as azobis tertiary alkyl compounds, including 4-4′-azobis (4-cyanovaleric acid), 2-2′-azobis (2-methylpropionamidine) dihydrochloride, 2, 2′-azobis [2-methyl-N-(2-hydroxyethyl) propionamide], 4,4′-azobis (4-cyanopentanoic acid), 2,2′-azobis (N, N′-dimethyleneisobutyramidine), 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis (N, N′-dimethyleneisobutyramidine) dihydrochloride and mixtures thereof.
Suitable peroxides include, but are not limited to, hydrogen peroxide, methyl ethyl ketone peroxides, benzoyl peroxides, di-t-butyl peroxides, di-t-amyl peroxides, dicumyl peroxides, diacyl peroxides, decanol peroxide, lauroyl peroxide, peroxydicarbonates, peroxyesters, dialkyl peroxides, hydroperoxides, peroxyketals and mixtures thereof.
LC mesophases of the invention may be formed by first depositing a solution comprising at least one monomer, and optionally at least one initiator, in a solvent on a surface of a first substrate. In one embodiment, the solvent is an organic solvent. Non-limiting examples of organic solvents include tetrahydrofuran, (THF), xylene (ortho, meta, or para), methanol, ethanol, isopropanol, acetone, ethyl acetate, acetonitrile, hexane, hexene, octane, pentane, cyclohexane, iso-octane, and 1-hexene. In one embodiment, the solvent is tetrahydrofuran. In another embodiment, the solvent is water. The thickness of the mesophase may be adjusted by varying the concentration of the monomer in the solution and by the amount of solution deposited on the substrate. In other embodiments, the mesophase may include one or more other solvents, such as water, as would be understood by those skilled in the art. Upon deposition of the solution onto the surface of the first substrate, the monomer can self-assemble into supramolecular columns, forming pores within the mesophase.
The substrate may be formed from any material that would permit the formation of a LC mesophase upon its surface. In some embodiments, the substrate is a silicone elastomer substrate. In one embodiment, the silicone elastomer is polydimethylsiloxane (PDMS). Non-limiting examples of other polysiloxanes polydiethylsiloxane, polydiphenylsiloxane, polymethylvinylsiloxane, polyethylvinylsiloxane, polyphenylvinylsiloxane, polyethylmethylsiloxane, polymethylphenylsiloxane, and polyethylmethylsiloxane. In one embodiment, the first substrate is a PDMS substrate. In another embodiment, the first substrate is a glass substrate. In another embodiment the substrate is Kapton. In another embodiment the substrate is a silicon wafer. In another embodiment the substrate is composed of a polymeric material such as polyacrylic acid or polyvinylalcohol.
In one embodiment, the substrate is treated with a chemical agent to modify its surface properties. In one embodiment, the surface agent is poly(sodium 4-styrenesulfonate (PSS). In one embodiment, the chemical agent is applied using spin-coating. In another embodiment, the surface agent is octadecyltrimethoxysilane (OTMS). In one embodiment, the chemical agent is applied by exposing the surface to a vapor of the chemical agent.
In one embodiment, the solvent is removed from the mesophase prior to application of the first substrate. This may be accomplished, for example, by passing a stream of a gas or air over the mesophase surface, or by allowing the mesophase to remain exposed to air such that the solvent evaporates over time.
The mesophase may be of any volume, and is not limited to any particular geometry. Thus, the mesophase may take the shape and size of any substrate, mold, or container in order to produce, upon polymerization, a polymerized structure of desired geometry. For example, in one embodiment, the mesophase is shaped to form a thin-film polymer.
In certain embodiments, the mesophase may include an amount of monomer equal to about 10%, about 15%, about 16%, about 17%, about 18%, about 19%, about 20%, about 25%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, about 90%, about 95% and about 99%. In some embodiments, the monomer is the mesogen, and the mesophase is comprised almost exclusively of the monomer with a small amount of initiator.
In certain embodiments, the mesophase may not include any photoinitiator. In other embodiments, the mesophase may include an amount of photoinitiator equal to about 0.1%, about 0.2%, about 0.3%, about 0.4%, about 0.5%, about 0.6%, about 0.7%, about 0.8%, about 0.9%, about 1%, about 2%, about 3%, about 4% or about 5%. In one embodiment, the mesophase includes about 0.5% of photoinitiator by weight. In one embodiment, the mesophase includes about 1.0% of photoinitiator by weight.
In certain embodiments, the mesophase may not include any crosslinker. In other embodiments, the mesophase may include an amount of crosslinker equal to about 0.1%, about 0.5%, about 0.6%, about 0.7%, about 0.8%, about 0.9%, about 1%, about 1.1%, about 1.2%, about 1.3%, about 1.4%, about 1.5%, about 2%, about 3%, about 4% or about 5%.
The mesophase is preferably in the form of a thin film of substantially constant thickness. In some embodiments, the thin film has a thickness ranging from about 10 nm to 500 μm. In another embodiment, the thin film has a thickness ranging from about 200 nm to 40 μm. In one embodiment, the thickness of the thin film is about 350 nm. The influence of the thickness on the alignment can be readily understood in the context of the finite elasticity of the medium, which specifies the distance over which the memory of a given orientation will decay under thermal forces. That is, the length scale where the elastic energy due to a non-uniform orientation of the LC director is less than or equal to kBT. This length scale can be considered in a dimensionally consistent manner, as a persistence length λ defined by the ratio of an effective bending rigidity of the mesophase Keff and kBT, where the effective bending rigidity originates from the elasticity of the mesophase. The quality of the alignment then decays exponentially with distance z from the interface, as captured by the tilt away from the boundary condition, cos θ=e−z/λ.
Preferably, the thin film has a pore diameter ranging from about 0.1 nm to 10 nm. In one embodiment, the pore diameter is about 1 nm. In another embodiment, the pore diameter is between 1 and 2 nm. In one embodiment, the pore diameter is between 1.2 and 1.5 nm.
The thin films have pores of substantially uniform size. By “substantially uniform” it is meant that at least 75%, for example 80% to 95%, of pores have pore diameters to within 30%, within 10%, or within 5%, of average pore diameter. More preferably, at least 85%, for example 90% to 95%, of pores have pore diameters to within 30%, within 10%, and within 5%, of average pore diameter.
The pores are preferably cylindrical in cross-section, and preferably are vertically aligned, or present or extend through the thickness of the thin film, such that the pores are aligned parallel to the macroscopic transport direction.
The structure of the thin films of the present invention has a periodic arrangement of pores having a defined, recognizable topology or architecture, for example cubic, lamellar, oblique, centred rectangular, body-centred orthorhombic, body-centred tetragonal, rhombohedral, or hexagonal. In one embodiment, the thin film has a pore arrangement that is hexagonal, in which the film is perforated by a hexagonally oriented array of pores that are of uniform diameter and continuous through the thickness of the film.
In some embodiments, at least one of the substrates is removed from the thin film. In another embodiment, both the first and second substrates are removed from the thin film. In one embodiment, at least one substrate is comprised of a dissolvable material which can be selectively removed from the film by dissolution in a solvent. Non-limiting examples of dissolvable materials include polyacrylic acid and polyvinyl alcohol. In one embodiment, the solvent is water. In another embodiment, the solvent is an organic solvent. In another embodiment, both substrates are comprised of a dissolvable material.
The vertical alignment of nanopores in a thin film may be driven by subjecting the mesophase to soft confinement prior to annealing and optional cross-linking. As contemplated herein, soft confinement may include, without limitation, applying a second substrate onto the surface of the formed LC mesophase, thereby sandwiching the mesophase between the first and second substrates. In this conformation, the mesophase is in contact with both the first substrate and the second substrate, which induces homeotropic anchoring of the columnar nanopores at the interfaces between the mesophase and the first and second substrates, resulting in vertical alignment of the nanopores. Moreover, the presence of the second substrate prevents exposure of the mesophase to air, which can induce distortion of the thin film structure. In one embodiment, the second substrate is a PDMS substrate. A PDMS substrate is particularly useful, as it is both easily fabricated and can be scaled to cover large areas. In another embodiment, the second substrate is a glass substrate. In some embodiments, the first substrate and the second substrate are formed from the same material. In other embodiments, the first substrate and the second substrate are formed from different materials. Any size substrate may be used for the soft confinement step, provided that the area of the first substrate and the area of the second substrate are each equal to, or greater than, the area of the mesophase.
The temperature of the mesophase system may be manipulated during all or any portion of time that the second substrate is applied to the surface of the mesophase. In certain embodiments, control of temperature may be automated through a programmable temperature controller (Omega, Stamford, Conn.) that provides temperature control within 0.1° C. of set points, for example. Initially, the mesophase system may be heated above a threshold or temperature suitable for transitioning the mesophase monomers and other constituents from an ordered state to a disordered state. For example, the mesophase is heated above its order-disorder transition temperature, TODT, which facilitates rapid alignment. For example, if the threshold temperature is about 65° C., the system can be raised to a first temperature such as between 70-90° C., and held at that temperature for a period of time before cooling through Tour to a second temperature of between about 20-30° C. at a rate between 0.1 and 30° C./minute. In a preferred embodiment, the mesophase system may be heated to a threshold or melting temperature of about 75° C., and held at that temperature for about 1 minute before cooling through Tour to a second temperature of about 25° C. at a rate of about 0.1° C./minute. In another embodiment, the cooling rate is 0.1° C./minute. In some embodiments, the threshold temperature is the temperature at which the mesophase exhibits an isotropic phase. As used herein, the term “isotropic” indicates a single continuous phase, such as a liquid. The temperature at which a mesophase exhibits an isotropic phase can be determined using any method known in the art, such as differential scanning calorimetry, temperature resolved polarized optical microscopy or temperature resolved x-ray scattering.
In one embodiment, a magnetic field can be applied to the polymer film with concurrent rotation about the normal of the plane of the film. In one embodiment, this magnetic field and rotation is maintained while the system is heated to the isotropic point and cooled to room temperature. In one embodiment, the magnetic field strength is between 0.001 and 10 T. In one embodiment, the magnetic field strength is between 1 and 10 T. In one embodiment, the magnetic field strength applied is 6 T. In some embodiments, the magnetic field can be necessary if the film thickness is greater than 5 μm.
Polymerization
After alignment of the mesophase system via application of the second substrate, the system may be polymerized to form a polymer film having aligned or oriented nanopores therein. Polymerization may be performed via, thermal polymerization, free radical polymerization, catalyst induced polymerization, or any other polymerization technique as would be understood by those skilled in the art. In certain embodiments, the polymerization is free radical polymerization. In certain embodiments, the film may include a well maintained alignment of structures substantially perpendicular to the film surface.
The invention is now described with reference to the following Examples. These Examples are provided for the purpose of illustration only, and the invention is not limited to these Examples, but rather encompasses all variations that are evident as a result of the teachings provided herein.
The materials and methods employed in these experiments are now described.
The synthesis and characterization of the amphiphilic monomer Na-GA3C11 is referenced in a previous report (Gin et al., J. Am. Chem. Soc., 1997, 119 (17), pp 4092-4093). Silicon elastomer kit was obtained from Dow Corning. All other chemicals were purchased from Aldrich and used as received. For UV-induced cross-linking, Na-GA3C11 was doped with a small amount of a commercially available radical photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (0.5 wt %). Glass slides and silicon wafers were cleaned by acetone and water and then dried by nitrogen gas before use. Polydimethylsiloxane (PDMS) elastomer pads were prepared by following the standard procedure. Briefly, a homogeneous fluid mixture of a silicon monomer and a curing agent with a weight ratio of 9:1 was poured into a Petri dish in which a clean silicon wafer was placed at the bottom. Subsequently, bubbles trapped in the fluid mixture were eliminated by vacuum. Thermal curing at 70° C. resulted in a transparent PDMS elastomer pad (5 mm) with a smooth surface.
LC films with two thicknesses (i.e., 80 and 28 μm) were prepared by sandwiching the mesophase by two substrates with the corresponding 80 or 28 μm spacers, respectively. LC films with a thickness ranging from 200 nm to 10 μm were obtained by casting of an LC/THF solution onto a glass or silicon substrate followed by solvent evaporation. The thickness of the film was generally controlled by the concentration and the amount of the solution cast on the substrate. Polymer films were prepared by photo-cross-linking of LC films (with 0.5 wt % initiator) obtained by following the above described procedure.
A PDMS elastomer pad (1.5 cm×1.5 cm×0.5 cm) was pressed onto a LC film (doped with photo-initiator) lying on a substrate. The LC film was then thermally annealed by heating up to the isotropic phase (75° C.) and allowing it to cool back to room temperature at a rate of 0.1° C./min. Subsequently, a polymer film was obtained by cross-linking the LC film immediately through exposure to 365 nm UV light (8W UVL-18 EL lamp at a distance of ca. 10 cm) for 24 h at room temperature.
POM images were obtained by a Zeiss Axiovert 200 M inverted microscope. Conoscopy studies were performed by using a Zeiss Axio Imager M2m microscope. The conoscopic images were obtained with a 40× objective and a Bertrand lens introduced between the analyzer and the ocular.
To obtain a thin specimen for cross-sectional TEM imaging, a polymer film was embedded in an epoxy and then the epoxy along with the sample was cured at 60° C. overnight to enhance the rigidity for microtoming. The cured epoxy block was then sectioned at room temperature by a diamond knife mounted on a Leica EM UC7 ultramicrotome. Thin sections (ca. 60 nm thick) on top of water were picked up onto a TEM grid and stained in vapor of a 0.5 wt % aqueous solution of RuO4 for 10 min. Specimens were then visualized by an FEI Tecnai Osiris TEM with an accelerating voltage of 200 kV. To visualize a polymer film with a thickness of 350 nm along the through-plane direction, the film was detached from the substrate by sonication in a water/ethanol (9/1, volume ratio) bath. Specimens were then obtained by placing several drops containing suspension of tiny pieces of films onto a TEM grid.
SAXS measurements on the Colh mesophase and cross-linked polymer films were performed by using a Rigaku 007 HF+ instrument with a rotating anode Cu Kα X-ray source (λ=1.542 Å) and a 2-D Saturn 994+ CCD detector. A silver behenate standard (d-spacing of 58.38 Å) was employed for the calibrations of the resultant 2-D SAXS data. All the 2-D scattering patterns were integrated into 1-D plots of scattering intensity (I) versus q, where q=4π sin(θ)/λ and the scattering angle is 20. Particularly for measurements on a polymer film, as schematically illustrated by
The results of the experiments are now described.
The results described herein demonstrate the fabrication of sub-μm polymer films possessing vertically oriented 1 nm pores using simple and readily accessible tools. These films were produced by subjecting sub-μm films of a cross-linkable hexagonal columnar (Colh) LC to soft confinement using an elastomeric pad of polydimethylsiloxane or PDMS (
As described herein, the orientation of the nanoporous structure in polymer thin films in the absence of any external field was investigated. Here the orientation of the columnar nanopores is determined only by surface confinement effects. As schematically illustrated by
A thin LC monomer film was prepared by casting a dilute THF solution of Na-GA3C11 containing a small amount of photo-initiator (0.5 wt %) on a silicon substrate and allowing the solvent to evaporate. The film was thermally treated with heating to the isotropic phase (75° C.) and cooling back to room temperature at a rate of 0.1° C./min. A polymer film (ca. 350 nm thick) was obtained by subsequent photo-cross-linking.
When a soft PDMS elastomer pad was pressed onto the LC film before thermal annealing and cross-linking, the columnar nanopores adopted a strikingly different orientation.
The different orientations of columnar nanopores found in open versus confined films originate from different surface anchoring of the columnar structures at the free air surface relative to the PDMS interface. To elucidate the influence of surface anchoring on the alignment of columnar nanopores over more macroscopic dimensions, polarized optical microscopy (POM) and conoscopy studies were performed on LC films (in the absence of any photo-initiator) under different surface confinement conditions. All the investigated LC films were thermally treated following the same protocol described above.
In contrast to the sandwiched sample, an open film of the same thickness (28 μm) with one surface exposed to the air possessed randomly oriented columnar nanopores, as evidenced by a birefringent POM texture and poorly defined conoscopic image (
SAXS studies on the polymer films were performed to provide complementary information regarding the orientation of the columnar nanopores.
The discernable differences between the 14 and 4.5 μm half-thickness films indicates that the persistence length is similar in magnitude to these dimensions. Therefore, it can be hypothesized that when the film thickness is reduced to the sub-μm range, highly persistent alignment of columnar nanopores should be achieved. The TEM observations in
Conductivity measurements on cross-linked polymer films prepared in sandwiched and open-to-air geometries respectively provide strong evidence of their different transport properties (
TEM imaging of the polymer/air interface shows homogeneous anchoring as distinguished by the occurrence of several areas in which circular hexagonally packed features are clearly visible, consistent with end-on views of the hexagonally packed columnar structures (
The data obtained from POM, SAXS, and TEM studies demonstrates that both the glass and PDMS surfaces induce homeotropic anchoring of the columnar nanopores, but the free air interface results in degenerate planar anchoring. Film exposed to air were therefore subjected to antagonistic boundary conditions which induce distortion of the structure. In sufficiently thin films one expects uniform orientation of the nanopores throughout the film because distortion of the LC director on sub-thickness length scales in such a film would be precluded by the very large associated elastic energies. Assuming equilibrium conditions, the particular orientation observed would be determined by the surface which has stronger anchoring strength. Although not wishing to be bound by any particular theory, the TEM image of the open sub-μm thin film (
In conclusion, the results described herein demonstrate a simple strategy to produce polymeric thin films with physically continuous and vertically aligned 1 nm pores. This method relies on confinement and subsequent photo-cross-linking of thin films of a Colh mesophase formed by a wedge-shaped amphiphile. TEM, SAXS and POM studies provide clear experimental verification of the role of different anchoring conditions in producing the observed morphologies, and of the physical continuity of nanopores through the film thickness. Thin films of any desired area can be easily processed by this technique to achieve vertical nanopore orientation, as the only requirement is for the area of the substrate and the confining material, glass or PDMS, to be matched with that of the film.
The materials and methods employed in these experiments are now described.
TBIB was synthesized using a single-step reaction adopted from literature.28 The product was purified twice by sublimation prior to use. NMR spectra (
CLA and TBIB with a molar ratio of 3:1, respectively, were dissolved in chloroform. A small amount of methanol (˜5 wt %) was then added to the solution to assist dissolution of TBIB and formation of the supramolecular complex. The resulting solution was then allowed to stand for 30 min under ambient conditions before solvent evaporation at room temperature under nitrogen atmosphere. The obtained supramolecular discotic complex, TBIB/(CLA)3, was then dried in vacuum overnight.
NMR spectra of the TBIB/(CLA)3 complex are displayed in
Homogeneous mesophases were prepared by addition of various quantities of acrylate or vinyl co-monomers to the supramolecular discotic TBIB/(CLA)3 complex. The co-monomers aided crosslinking and also served as a means to modify the phase behavior of the system. A mixture of butyl acrylate (BA) and 1,6-hexanediol diacrylate (HDA) with a fixed weight ratio of 4:1 was chosen for forming acrylate co-monomer-containing mesophases, and divinylbenzene (DVB) for vinyl co-monomer-containing mesophases. To ensure homogenous mixing, after the addition of a desired amount of co-monomer/cross-linker the mesophases were vortexed and centrifuged for a minimum of 10 cycles.
Mesophases containing a small amount of radical photo-initiator (˜1 wt %) 2-methoxy-2 phenylacetophenone were polymerized by exposure to 365 nm UV light using a focused spot UV beam (100 Watt Sunspot SM Spot Curing System at a distance of ˜2 cm) for 1 h followed by a benchtop lamp (8 Watt UVL-18 EL lamp at a distance of ˜2 cm) for 24 h.
Polymerized mesophase samples were immersed into a DMSO solution of NaOH (0.1 wt %) for 24 hours at room temperature (˜21° C.). The polymer samples were then rinsed with de-ionized water to eliminate any residual NaOH/DMSO solution.
DSC measurements were performed using a Q200 DSC (TA Instruments) with a heating/cooling rate of 10/min.
A Zeiss Axiovert 200 M inverted microscope was employed to obtain POM images.
Polymer films obtained after polymerization were embedded in epoxy, which was then cured at 60° C. overnight to ensure enough rigidity for microtoming. Epoxy blocks containing samples were sectioned at room temperature by a diamond knife mounted on a Leica EM UC7 ultramicrotome. The thickness of thin sections was controlled to be around 60 nm. To improve the contrast, the thin sections were stained by vapor of a 0.5% aqueous solution of RuO4 for 10 min. An FEI Tecnai Osiris TEM was employed to visualize the sample at an accelerating voltage of 200 kV.
X-ray scattering measurements on mesophases and polymerized/cross-linked polymer films were performed using a Rigaku 007 HF+ instrument equipped with a rotating anode Cu Kα X-ray source (A=1.542 Å) and a 2-D Saturn 994+ CCD detector. A silver behenate standard (d-spacing of 58.38 Å) was employed for the calibrations of the resultant 2-D SAXS data. All the 2-D scattering patterns were integrated into 1-D plots of scattering intensity (I) versus q, where q=4π sin(θ)/λ and the scattering angle is 20.
UV-Vis spectra were recorded in transmission mode using a dual beam configuration on a Cary 300 spectrometer.
FT-IR spectra of the samples were measured using FTIR/Raman Thermo Nicolet 6700 in the attenuated total reflection (ATR) method.
Surface alignment of the mesophases was achieved by sandwiching their thin films (ca. 15 μm thick) with two surface-modified glass slides of a thickness of 100 μm. The glass slides were cleaned by a piranha solution and UV-ozone for 20 min (UV Ozone Cleaner—ProCleaner, Bioforce Nanosciene) before further chemical treatment. The surfaces of the cleaned glass slides were then modified by either spin coating a thin layer of polyelectrolyte for face-on (homeotropic) anchoring or silanization for edge-on (planar) anchoring, respectively. The spin-coating was carried out with a 1 wt % aqueous solution of PSS [poly(sodium 4-styrenesulfonate), Mw ˜70,000] and a spin-speed of 3000 rpm for 1 min. For silanization, the cleaned glass slides were placed inside a container with a 15 μl drop of octadecyltrimethoxysilane (OTMS) on an aluminum foil. The container was heated to 100° C. to evaporate OTMS, and the glass slides were exposed to silane vapor for 3 h. After removed from the chamber, the glass slides were rinsed by isopropanol and dried by nitrogen prior to use.
A magnetic field with a field strength of 6 T provided by a superconducting magnet (American Magnetics, Inc.) was utilized to align the mesophases containing acrylates or DVB in films with a thickness above 5 μm that were not able to align uniformly through the whole thickness by surface effects. Vertical orientation of the supramolecular columns was achieved by continuous rotation of the mesophase film sandwiched by PSS coated glass slides about the normal of the film that was positioned perpendicular to the magnetic field direction. Under continuous rotation, the sample was heated to the isotropic phase, followed by slow cooling back to room temperature with a cooling rate of 0.2° C./min.
Three water soluble dyes including cationic methylene blue (MB) and Rhodamine 6G (R6G) and anionic methyl orange (MO) were chosen for testing. For in situ UV-vis spectroscopy, the weight factions of MB, R6G and MO in water solutions (2.8 g) were 3.6×10−6, 9×10−6, and 12×10−6, respectively, which corresponded to a relative absorbance of approximately 1. Experiments were performed using deionized, neutral pH water. For the demonstration of selective adsorption of dye molecules through visualizing color changes of water solutions containing two dyes by eyes, the weight faction of MB, R6G and MO in MB/R6G and MB/MO solutions (2.8 g) was fixed to 5×10−5. For both UV-vis spectroscopy and eye visualization, 2 mg polymer films possessing aligned nanopores with a thickness of 15 μm were added into the dye solutions.
For TBIB/(CLA)3, the weight fraction of TBIB is 0.34 (MW of TBIB is 426 g/mol and MW of CLA is 280 g/mol). In a binary system the volume fraction of a species ϕ1 is related to its mass fraction f1 as ϕ1=f1/[f1+(1−f1)α] where α=ρ1/ρ2 is the ratio of the mass densities of the two species. While the exact densities of CLA and TBIB in the mesophase are not known, the ratio of their densities can reasonably be bracketed between 1.4 and 2.2 based on mass densities of CLA relative to analogous aromatic species in disordered liquids (low) and π-π stacked ordered mesophases (high). On this basis, the TBIB volume fraction is between 0.19 and 0.27 and correspondingly the column diameter in the mesophase can be estimated using Equation 1 to be between 1.19 and 1.42 nm. This calculation assumes effectively circular cross-sections and that there is a sharp boundary between TBIB and the CLA matrix. While these assumptions are not quantitatively correct, the calculation provides a useful estimate for the effective size of the columns.
In the presence of 20 wt. % co-monomer, the TBIB volume fraction lies between 0.152 and 0.216. For the DVB system with a d-spacing of 2.69 nm, the corresponding cylinder diameter lies between 1.27 and 1.52 nm. For the acrylate containing system, with a d-spacing of 2.52 nm, the cylinder diameters lie between 1.18 and 1.41 nm.
The specific surface area Sv (area per mass of non-porous material) is given by Equation 2 below where ε, porosity, is TBIB volume fraction.
For the DVB containing sample (used in adsorption experiments), Sv varies from a minimum of 4×0.152/(1.52E-9 m×1000 kg·m−3×1000 g·kg−1×0.848) to a maximum of 4×0.216/(1.27E-9×1000 kg·m−3×1000 g·kg−1×0.784), i.e. 470 to 870 m2·g−1, where a density of 1000 kg·m−3 is assumed for the polymeric matrix (liquid monomer densities are 900 and 914 kg·m−3 for CLA and DVB respectively).
For the acrylate containing sample, the specific surface area ranges from 4×0.152/(1.41E-9 m×1000 kg·m−3×1000 g·kg−1×0.848) to 4×0.216/(1.18E-9×1000 kg·m−3×1000 g·kg−1×0.784), i.e. 510 to 930 m2·g−1, where a density of 1000 kg·m−3 is assumed for the polymeric matrix (liquid monomer densities are 900 and 911 kg·m−3 for CLA and acrylate mixture (890 and 1010 kg·m−3 for BA and HDA respectively, present at 4:1 mass ratio).
The number of functional groups per unit area is calculated by mass balance based on the distance a between pores as shown in Equation 3 below where φ is the volume fraction of CLA in the system and M is the molar mass of CLA. Alternatively, the same expression can be used for the volume fraction of CLA+comonomer, but the relevant molar mass is now MCLA+(fCLA/fco)Mco where f is the weight fraction of the species.
For the DVB system, σ varies between 3.9 and 4.2 nm−2.
For the acrylate system, σ varies between 3.6 and 3.9 nm−2.
The results of the experiments are now described.
A core-templated strategy for the synthesis of vertically aligned nanopores in polymer films is realized here using 1,3,5-tris(1H-benzo[d]imidazol-2-yl)benzene (TBIB) as the templating core species and conjugated linoleic acid (CLA) as the renewable monomer. There are two key features. The carboxylic acid headgroup of CLA can form hydrogen bonds with the basic benzoimidazole ring on TBIB to yield a supramolecular discotic complex composed of one TBIB and three CLA molecules (i.e. TBIB/(CLA)3) that then undergoes LC self-assembly. Additionally, the use of a conjugated form of linoleic acid aids free-radical initiated crosslinking as the reactivity of conjugate dienes is significantly higher than their non-conjugated counterparts. The TBIB/(CLA)3 self-assembles into a thermotropic Colh mesophase that can be vertically aligned with high fidelity using a simple surface-confinement method. This alignment method can be optionally coupled with magnetic fields if desired. Cross-linking of the aligned TBIB/(CLA)3 mesophase followed by chemical removal of TBIB results in thin films with vertically aligned nanopores of ˜1.0-1.5 nm diameter. These polymer films display sharp selectivity for molecular solutes with different sizes and charges, as demonstrated by the size and charge selective adsorption of model penetrant molecules in aqueous solutions.
The C3 symmetric TBIB/(CLA)3 complex (
The diameter, D, and volume fraction, φ, of columns in the system are related to the d-spacing for hexagonal packing as shown in Equation 1. For TBIB/(CLA)3, the weight fraction of TBIB is 0.34. The column diameter in the mesophase is estimated to be between 1.19 and 1.42 nm, based on estimates for the volume fraction of packed CLA in the system using the relevant mass densities of CLA and TBIB domains.
CLA derived from plant oils is composed of an ill-specified mixture of various C18 fatty acids. The LC formation behavior of TBIB with single component C18 fatty acids such as linolenic acid and oleic acid, as well as their mixtures, was investigated to determine whether mesophase formation was sensitive to the feedstock composition. Stable Colh mesophases can also be formed at the 3:1 molar ratio of fatty acid to TBIB for both of these species and mixtures thereof (
Drying oils contain mixtures of unsaturated fatty acids and are used to create tough thin coatings or varnishes upon exposure to air. The ‘drying’ or hardening of the material is due to oxygen-induced oxidative crosslinking of the fatty acids rather than any evaporation of volatile species. Likewise, exposure of the TBIB/(CLA)3 Colh mesophase to air under ambient conditions resulted in the formation of a dense crosslinked sample from an initially gel-like LC (
Photo-initiated free radical polymerization and crosslinking have been employed effectively to lock in the structure of polymerizable LC assemblies (Gin, et al., 2006, Adv. Funct. Mater. 16:865; Yoshio, et al., 2006, J. Am. Chem. Soc., 128:5570.). CLA is amenable to polymerization and cross-linking using a range of chemistries including diene metathesis by Grubbs catalysts, controlled radical polymerizations and thiol-ene click chemistry (de Espinosa and Meier, 2011, Eur. Polym. J. 47:837; Yao and Tang, 2013, Macromolecules, 46:1689) A simple UV-initiated free-radical polymerization route was pursued in this example. This reaction necessitated the use of a more reactive co-monomer to crosslink the system due to the limited free-radical reactivity of CLA (
Stable, homogeneous Colh mesophases with addition of small quantities of either a mixture of BA and HDA, or DVB, were formed (
Co-monomer containing mesophases of TBIB/(CLA)3 with added photo-initiator were photo-polymerized by exposure to 365 nm UV light at room temperature for 24 h, resulting in the formation of rigid polymer films (
The participation of the CLAs in the copolymerization was verified by FT-IR analysis of thin films of the Colh mesophases before and after polymerization.
Effective utilization of the crosslinked nanostructured materials derived here requires alignment of the columnar structures and, in particular, vertical alignment in thin films, which may be mediated using surface anchoring and film confinement of the Colh mesophases. The hydrogen-bonded TBIB/(CLA)3 is effectively an amphiphilic supramolecular species consisting of a rigid core with partial ionic character due to strong hydrogen bonding by acid-base proton sharing, and an oleophilic periphery due to the aliphatic CLA and comonomer chains. Thus, a surface exhibiting strong affinity for the cores favors face-on (i.e. homeotropic, vertical) alignment of the supramolecular columns, and a surface with ionic character can provide the required affinity. Conversely, an oleophilic or hydrophobic surface favors edge-on orientation of the columnar structure, or a planar anchoring.
The surfaces of glass slides were modified by depositing a layer of polyelectrolyte poly(sodium styrene sulfonate) (PSS) via spin coating. POM was used to characterize optical textures in samples sandwiched between PSS-coated glass slides during slow cooling (0.2° C./min) from the isotropic phase. The growth of dendritic morphologies (inset of
Surface-induced alignment using treated glass slides was also successful in the case of co-monomer containing samples, albeit with a greater sensitivity to the film thickness. Data are shown here for DVB-containing samples. An increase in the population of rectilinear and other LC defects on increasing co-monomer content at a fixed film thickness of 15 μm (
For films more than 5 μm thick, a 6 T magnetic field was employed to assist the alignment of the mesophases and annihilate LC defects. In analogy to Coln mesophases formed by discotic mesogens bearing aromatic cores, the TBIB/(CLA)3 mesophases possess negative magnetic anisotropy and therefore the columnar axes degenerately align perpendicular to the field direction (Lee, et al., 2006, Mater. Chem., 16:2785). That is, the easy axis for magnetic alignment is in the plane of the TBIB, and the hard axis is along the columns. Degeneracy due to negative magnetic anisotropy can be broken by rotation of the sample around an axis perpendicular to the field, resulting in alignment of the hard axis parallel to the axis of rotation (Majewski and Osuji, 2009, Soft Matter, 5:3417; Majewski and Osuji, 2010, Langmuir, 26:8737). Uniform and non-degenerate vertical alignment was successfully achieved in this manner, by continuous rotation of the sample around an axis normal to the field, and normal to the film surface (
Removal of the TBIB core molecule in the aligned polymer to create ordered nanopores was carried out by immersion of the magnetically aligned, crosslinked films (30 μm thick) into a 0.1 wt % NaOH solution in DMSO for 48 h, followed by rinsing in deionized water. The efficacy of TBIB removal was verified by FT-IR spectroscopy (
2-D SAXS characterization was utilized to verify the retention of both the columnar nanopores and their alignment in the samples after TBIB removal. Scattering was performed with X-rays incident perpendicular to the film thickness, providing a cross-sectional view. The equatorial scattering in the 2-D SAXS pattern of the pristine crosslinked polymer confirms the vertical alignment of the supramolecular columns (top panel of
The absence of scattered intensity in the dried samples is a clear indication that the pores collapsed during drying, likely due to the large Laplace pressures associated with nm-scale pores (Gopinadhan, et al., 2014, Adv. Mater., 26:5148; Cavicchi, et al., 2004, Macromol. Rapid Comm., 25:704). Such collapse is not uncommon in nanoporous polymers and it is predicted that its occurrence can be preempted if required by increasing the bulk modulus of the polymer by increasing the crosslink density of the material. The enhanced scattering intensity in the non-dried films is consistent with the expected increase in electron density contrast on replacing TBIB by water and confirms that well-aligned solvent-accessible pores were successfully produced by TBIB removal. An increase of 0.14 nm in the d100 spacing of the water impregnated nanoporous material was observed after TBIB removal, relative to the pristine sample (2.83 vs 2.69 nm). The marginal nature of the change in d-spacing indicates that the material does not swell appreciably in water. This result suggests that the dimensions of the pores as set by TBIB are well preserved in the final nanoporous material.
The selectivity of the nanopores was investigated via characterization of the adsorptive uptake of molecular species in water with different sizes and charges.
The ability of the polymer film to completely adsorb MB from solution is due to its large specific surface area and, presumably, the accessibility of that area. The specific surface area and areal density of sodium carboxylate groups at the pore wall can be estimated based on the structural data of the nanoporous polymers. The specific surface area Sv is between roughly 470 and 870 m2/g. The areal density of sodium carboxylate groups on the pore wall is ˜4 nm−2.
Using Sv=670 m2·g−1 as a representative value for specific surface area, for the 2 mg of nanoporous polymer utilized, the available area is 1.34 m2, neglecting the contribution from the external film surface area (3×10−4 m2). The test solution contained 4.4×10−7 moles of MB. Assuming a projected molecular area for MB of 1.35 nm2 and a maximum packing fraction of 0.547 for random sequential adsorption, complete MB uptake would require approximately 0.65 m2. Complete uptake would produce an MB areal density of 0.4 nm−2, well away from the ˜4 nm−2 that would be required for charge inversion of the nanopore surface based on the estimate of the areal density of sodium carboxylate groups. Notwithstanding any uncertainties in the relevant experimental measurements and the projected molecular area, the proximity of the required area (0.65 m2) and estimated available area (1.34 m2) suggests that a large fraction of the pore surface is in fact solvent accessible.
The issue of accessibility was further considered in experiments which examined the relative adsorption kinetics of aligned and non-aligned material. These experiments also highlight the critical role of pore alignment in determining the transport properties and performance of nanoporous membranes. Both polymer films possessing the same thickness of 15 μm and weight of 2 mg were immersed into two 2.8 g MB water solutions with a MB weight fraction of 3.6×10−6. The time-dependent UV-vis absorbance of the MB solutions was measured in the presence of nanoporous polymer films (
In conclusion, a novel approach for fabricating polymer films with highly aligned, vertically oriented nanopores using sustainably-derived materials has been developed. This approach relies on the use of a molecular template to guide the self-assembly of polymerizable fatty acids into a hexagonally packed columnar mesophase, that yields nanopores upon removal of the templating species. The template species can be recovered from solution by crystallization and reused for subsequent fabrications. The alignment methods are highly scalable and facile production of large area thin films for membrane applications may be possible using film confinement alone, or of thicker materials by combining confinement with magnetic field alignment.
The nanoporous materials produced here demonstrate remarkable size and charge selectivity in adsorption experiments, and accessibility of the pore surfaces. The existence of highly ordered and aligned nanostructures with well-defined dimensions allows robust quantification of parameters of interest for applications of these materials, including functional group density and accessible area. These aligned nanoporous polymers will be useful in a wide range of applications from analytical chemistry to nanofiltration and lithographic pattern transfer. Work is ongoing to quantitatively determine the accessible pore surface area as well as the permeability and solute rejection characteristics of these materials as nanofiltration membranes.
The disclosures of each and every patent, patent application, and publication cited herein are hereby incorporated herein by reference in their entirety. While this invention has been disclosed with reference to specific embodiments, it is apparent that other embodiments and variations of this invention may be devised by others skilled in the art without departing from the true spirit and scope of the invention. The appended claims are intended to be construed to include all such embodiments and equivalent variations.
This application claims priority from U.S. Provisional Patent Application Ser. No. 62/305,695, filed Mar. 9, 2016, the entire contents of which is incorporated herein by reference.
This invention was made with government support under CMMI-1246804 awarded by the National Science Foundation. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US17/21617 | 3/9/2017 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62305695 | Mar 2016 | US |
