Patent Application
20160369076
This section provides background information related to the present disclosure which is not necessarily prior art.
Layer-by-layer (LbL) assembled PEMs have been considered as a versatile platform for surface modification. In 1990s, Gero Decher pioneered the LbL technique to build multilayers by dipping a positive-charged substrate into polyanion and polycation solutions alternately, resulting in PEMs with precise structure control in nanometer scale. (Decher G. Science, 1997, 277(5330): 1232-1237.) For conventional LbL process, the dipping time for polyanion and polycation solutions is around 10 minutes or more. Sufficient washing steps are also required. And, to achieve a proper thickness, 10 bilayers or more are always needed. Thus, slow processing becomes one of the major issues for the industrializing PEM products. The short-time LbL technique originally pioneered by Grunlan et al., (Hagen D A, Foster B, Stevens B, et al. ACS Macro Letters, 2014, 3(7): 663-666.) is an effective tool for fabricating porous PEM structures.
The conventional LbL process is extremely time consuming and it takes several hours to fabricate a PEM film of desired thickness. Hence, although the process has been developed more than a decade ago, and extensive research, both in terms of fundamentals as well as applications have been carried out, this LbL process has not seen industrial acceptance.
Multi-scale porous structures have been successfully built up either with a micro-sized porous structure on top of a nano-sized porous structure or the other way around. According to the previous studies about porous PEM films, either only one porous structure was developed in one sample (Hiller J A, et al., Nature Materials, 2002, 1(1): 59-63; Berg M C, et al., Biomacromolecules, 2006, 7(1): 357-364; Cho C, Zacharia N S. Langmuir, 2011, 28(1): 841-848), or with micro- and nano-structures on top of the surface randomly (Fu J, Ji J, Shen L, et al. Langmuir, 2008, 25(2): 672-675.).
This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
Porous polyelectrolyte multilayer (PEM) films have been created with precise pore size control ranging from nano- to micro-scale. Layer-by-layer (LbL) technique has been applied for fabricating PEMs, and the porous treatment has been carried out under acidic condition.
The primary purpose of our work aims at reducing the processing time of the process to fabricate porous PEM films without compromising on the quality of the prepared films. This will enable the tremendously versatile LbL coating process to be economically fabricated. Besides shortening the processing time, we also tried polyelectrolytes with high molecular weight. This enables a broader control of pore size. Multi-scale porous structures have been first developed in this work, with either micro-sized porous structure on top of nano-sized porous structure or the reverse.
Porous polyelectrolyte multilayer (PEM) films have been fabricated via fast layer-by-layer (LbL) technique, followed by acidic treatment with pH varying from 1.8 to 2.4. In our approach, the dipping time has been shortened significantly. The dipping time can be as short as 10 seconds or can be extended to about 15 minutes. The film thickness can be tuned by manipulating dipping time, molecular weight, number of bilayers, etc.
In this work, we use Poly(acrylic acid) (PAA) as the polyanion, and Poly(allylamine hydrochloride)(PAH) as polycation. In order to achieve a broader control of pore size, PAA with high molecular weight (Mw=225,000 g/mol) has been tried. This high molecular weight PAA can form special microfibrous structure on the surface via LbL assembly with dipping time longer than 5 minutes. However, with 10 second dipping, the surface is flat and smooth, and after porous treatment with pH of 2.0, pore size of 20-50 nm can be obtained, which is much smaller than what has been reported in literatures. (Cho C, Zacharia N S. Langmuir, 2011, 28(1): 841-848. Berg M C, et al., Biomacromolecules, 2006, 7(1): 357-364.)
In this invention, we can control the micro-sized and nano-sized porous regions. To fabricate multi-scale porous structure, we first make the bottom porous structure via LbL assembly followed by acid treatment and crosslink the structure. Then the top porous structure can be further built up through the same way. If the bottom porous structure is with a nano-sized structure, the top porous structure can be built up with no need to consider the molecular weight of polyelectrolytes. However, if the bottom has micro-sized porous structure, higher molecular weight of polyelectrolytes is required for the top porous structure since the polymer chain needs to be large enough to avoid filling into bottom pores.
This invention has potential for various applications, such as 1) membranes, 2) drug delivery, and 3) super hydrophobic coatings (i.e., self-cleaning). For drug delivery, the porous structure can be considered as a drug reservoir. This invention allows the design of certain porous structures to fulfill the release requirements, such as initial burst release, sustained release, or a combination of both. The release kinetics can be precisely controlled by tuning the porous structure. For membrane applications, these porous PEM structures can also be used to replace the porous polysulfone and polyamide layers of reverse osmosis (RO) membranes. In addition, micro- and nano-structured surface can be achieved with our approach, which can be further modified by fluorinated silane molecules to obtain super hydrophobic (self-cleaning) surfaces.
The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.
In
In
Example embodiments will now be described more fully with reference to the accompanying drawings.
Composites containing a PEM film on a substrate are prepared layer-by-layer by sequentially applying layers of polycation and polyanion on a substrate, such as a silicon, glass, plastic slide, or—as disclosed below—a non-woven web or non-woven fabric. A wide range of negatively charged and positively charged polymers is suitable for making the layered materials. Suitable polymers are water soluble and sufficiently charged (by virtue of the chemical structure and/or the pH state of the solutions) to form a stable electrostatic assembly of electrically charged polymers. Sulfonated polymers such as sulfonated polystyrene are commonly used as the negatively charged polyelectrolyte. Quaternary nitrogen-containing polymers such as poly (diallyldimethylammonium chloride) (PDAC) are commonly used as the positively charged electrolyte.
Polyelectrolytes include positively and negatively charged polymers, and are also divided among “strong” and “weak” polyelectrolytes depending on whether the charged groups do or do not maintain their charge over a wide pH range. For example, a sulfonated polymer is considered a strong polyelectrolyte because it is negatively charged over a wide pH range; an acrylic acid polymer is considered a weak polyelectrolyte because it is protonated below a pH of about 4 but contains a negative charge at higher pH. Strong polyelectrolytes include sulfonated polystyrene (SPS) and poly (diallyldimethyl ammonium chloride) (PDAC). Weak polyelectrolytes include polyacrylics such as polyacrylic acid, as well as positively charged polyelectrolytes such as poly (allyl amine) and branched and linear polyethyleneimines as their respective ammonium salts.
In various embodiments, polyelectrolyte multilayers are prepared by applying a first charged polyelectrolyte to a substrate surface by electrostatic interaction. The nature of the first polyelectrolyte applied (polyanion or polycation) depends on the charge state of the substrate surface. Thereafter, additional layers of polyelectrolyte are deposited in alteration between positive and negative. If a substrate surface is hydrophobic and not capable of electrostatic interactions with a polyelectrolyte (an example is an un-plasma treated silicone surface), it is possible to apply a first polyelectrolyte that interacts with the hydrophobic surface by hydrophobic interactions, but that is capable of interacting with a subsequent polyelectrolyte layer. For example, layers of PDAC/SPS cannot be assembled on a hydrophobic (non-plasma treated) surface of PDMS. However by starting with one layer of PAH, at a pH of 7.5, SPS/PDAC can be assembled on PDMS, where PAH interacts with PDMS by hydrophobic interactions and SPS/PDAC can be built on the PAH by electrostatic interactions. This is further explained and illustrated in Park et al., Advanced Materials 16, 520-525 (2004), the disclosure of which provides background information and is hereby incorporated by reference.
Applying the polycation and polyanion and building up the alternating layers of polyelectrolyte on the substrate are accomplished with any suitable method. In a first method, the substrate or a substrate containing built-up layers is dipped or immersed in a solution of polycation or polyanion. After each application of polyelectrolyte, the substrate is removed and is preferably rinsed. Following the rinse step, the substrate is dipped or immersed again in a solution of the oppositely charged polyelectrolyte. Following a rinse step, the process is repeated as desired to build up a number of layers. This layer by layer assembly method is well known and is described for example in Decher, Science 277, 1232 (1997), the disclosure of which is helpful for background information and is hereby incorporated by reference.
In other embodiments, the polyelectrolytes are applied by 1) spin casting, 2) solution casting, or 3) spray assembly. After application of one layer, the applied layer is preferably rinsed before the next layer is applied. In this way, alternating layers of polycation and polyanion are applied to the surface until the desired number of bilayers is achieved.
Methods of assembling the PEMs are well known. The methods can be conveniently automated with robots. Polycation and polyanion is alternately applied layer-by-layer to a substrate. When the substrate surface is capable of electrostatic interactions with a positively charged material (that is, when it is negatively charged), a polycation is first applied to the substrate, preferably followed by a rinse step. The polycation is followed with application of a polyanion. The procedure is repeated as desired until a number of layers are built up. A bilayer consists of a layer of polycation and a layer of polyanion. Thus for example, 10 bilayers contain 20 layers, while 10.5 bilayers contain 21 layers. With an integer number of bilayers, the top surface of the PEM has the same charge as the substrate. With a half bi-layer (e.g. 10.5 illustrated) the top surface of the PEM is oppositely charged to the substrate.
Multilayer films are abbreviated as (x/y)z where x is the first polyion deposited, y is the second polyelectrolyte deposited and z is the number of bilayers. Half a bilayer means that x was the last polyelectrolyte deposited.
Pores in the PEM thin films described herein are classified as nanopores or micropores depending on their size. Nanopores are characterized by dimensions on the order of nanometers, and in any event less than 1 micron (which equals 1000 nm). Micropores where used indicates a pore with a dimension of 1 micron or greater. In one embodiment described herein, a film contains nanopores having a dimension of 20 nm to 50 nm. For pores that are nearly circular, such as many of the nanopores illustrated in the Figures, the word diameter can be used interchangeably for the size of the pore. But the use of “diameter” here or in the claims is not to be taken as an indication that the description is limited to round or perfectly round pores. Rather, it is a short hand way to describe the minimum dimension of a pore; if that minimum dimension is less than a micron, it is a nanopore.
An important parameter in LbL assembly is the dipping time, or the time for which the growing film is exposed to solutions of polyanion or polycation. As detailed herein, dipping varies from 1 second, to 10 seconds, up to 15 minutes. Dipping is at room temperature unless otherwise stated. It has been discovered that the morphology the PEM film can be designed and altered by selecting suitable values for dipping times and the nature and molecular weight of the polyelectrolytes.
In one embodiment, a polyelectrolyte multilayer (PEM) thin film is made having pores in the film, and wherein at least some of the pores have a diameter in a range of 20-50 nm. In various embodiments, the film contains alternating layers of polycation and polyacrylic acid, with the polyacrylic acid preferably having a higher weight average (molecular) than conventional polyacrylics used in the LbL technique. In a non-limiting embodiment, the weight average molecular weight of polyacrylic acid is over 100,000 g/mol. Methods of making the PEM thin film are also provided.
In another embodiment, a PEM thin film has a hydrophobic surface characterized by a contact angle with water of greater than 150°, and in some embodiments, 160° or greater. In various embodiments, the film contains alternating layers of polycation and polyacrylic acid. In an exemplary embodiment, the weight average molecular weight of the polyacrylic acid is greater than 100,000, for example about 225,000 g/mol. In various embodiments, the thin film is porous treated to introduce nanoporous features into a microfibrous morphology of the thin film. Optionally, the surface of the PEM thin film comprises a fluorine compound that contributes to hydrophobicity or super hydrophobicity of the surface. In exemplary embodiments, the fluorine compound is bound to the surface by a covalent attachment of a silane functional group of the fluorine compound to a functional group on the surface of the thin film.
In another embodiment, a PEM thin film is made of built-up alternating layers of polycation and polyanion, wherein the polyanion comprises polyacrylic acid having a weight average molecular weight above that conventionally used for PEM thin films. In various embodiments, the polyacrylic acid has a weight average molecular weight greater than 50,000, greater than 60,000, greater than 70,000, greater than 80,000, or greater than 100,000 g/mol. In an exemplary embodiment, a polyacrylic acid with a weight average molecular weight of about 225,000 g/mol is used. Optionally the surface is covered or partially covered with a fluorine compound, as in the preceding paragraph.
In another embodiment, a method of making a PEM thin film involves layer by layer assembly of alternating polycation and polyanion. In the method, a first layer of polycation is applied to a negatively charged substrate, and alternating layers of polycation and polyanion are built up to make the thin film. The method involves alternately dipping the coated substrate in a solution containing polyanion and then in a solution containing polycation. The dipping time in each solution is 1 minute or less, in order to provide a smooth morphology that can be subsequently porous treated. As in some other embodiments, the polyanion comprises a polyacrylic acid that has weight average molecular weight higher than the weight of polyacrylic acid normally used in LbL procedures. In various embodiments, the molecular weight (weight average) of an acrylic polymer used in the method is above 100,000 g/mol.
In another embodiment, the LbL assembly method described in the preceding paragraph is used, but the dipping time in each solution (of polyanion and of polycation) is greater than 1 minute. The use of a polyacrylic acid having an above normal molecular weight (e.g. above 100,000 g/mol), together with the dipping time greater than 1 minute, leads to microfibrous morphology, which can be subsequently porous treated to provide microporous structure.
In the various methods of making PEM thin films by LbL assembly of polycation and polyanion wherein the polyanion has a high molecular weight (such as above 100,000 or about 225,000), the film is post treated and then exposed to a fluorine containing molecule in the gas phase to produce a super hydrophobic surface. Post treating includes the steps of porous treatment (or equivalently porous induction) at a high pH or at a low pH, followed by rinsing and drying the porous-treated film, followed by crosslinking. After crosslinking, the surface of the crosslinked film is exposed to a vapor of a fluorine compound in a simple chemical vapor deposition process (CVD). The fluorine applied by CVD gives hydrophobic or super hydrophobic properties to the surface of the thin film.
The PEM thin films made in the methods disclosed herein are industrially useful in a variety of applications, owing to their unique structures. In one embodiment, a method of filtering water during water treatment to remove impurities involves passing water through a filter, wherein the filter comprises any of the polyelectrolyte multilayer thin films described herein, or any film made by any of the methods described herein.
In other embodiments, a drug delivery system is provided that comprises a PEM thin film described herein or made by any of the methods described herein. The thin film comprises a pore and there is an active pharmaceutical agent disposed in the pore.
The structure of the PEM thin films and methods for making them are now described with reference to the figures.
In one embodiment, porous treatment involves exposing the thin film to a low pH solution, such as by dipping or immersion. The solution has a pH below the pKa of the polyanion used in making the film. For polyacrylic acid and other polymers containing carboxylic groups, the pKa is on the order of 4.2. in various embodiments, porous treatment is carried out at a pH that is one unit below or about two units below the pKa. In illustrative embodiments, porous treatment is carried out at a pH of 1.6, 1.7. 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, or 2.4. In some embodiments, porous treatment is carried out within a pH range of about 1.8 to about 2.6.
In another embodiment, porous treatment involves exposing the thin film to a high pH solution. The pH is above the pKb of the polycation used to make the thin film. In non-limiting fashion, the pH of the high pH solution is at least one unit higher or at least two units higher than the pKb. For example, the pH is in the range of 10 to 12, 10-11, 11-12, and so on.
A porous structure is formed after the post treatment, which includes which includes porous treatment as described, rinsing in DI water, drying and cross-linking. During post treatment, the porous-treated PEM thin film is cross-linked, for example by dipping in a glutaraldehyde solution or by heating at an elevated temperature such as 180° C. for about 2 hours.
A key variable of the process is the so called dipping time, measuring the amount of time that the substrate is dipped in respectively the polyanion and polycation in order to build up multiple bilayers. As illustrated in the following Figures, experimentally the dipping time was set as 10 seconds, 1 minute, 5 minutes, 10 minutes, or 15 minutes.
In addition to a difference in thickness illustrated in
The micrographs shown in the bottom of
The result of porous treatment at different pHs is illustrated in the micrographs shown in
Similarly,
Further morphologies of films produced with different dipping times and porous treatment at various pHs are shown in
As illustrated above, after alternately applying polyanion and polycation to build up a film of the desired thickness, the thin film is optionally porous treated, after which the thin film can be further crosslinked.
The polyanion used in the films and composites described herein is selected from one having carboxylate groups and is exemplified by polyacrylic acid. It is possible to use other polyanions, so long as their use enables the microfibrous structure observed with long dipping times and also the nanoporous structure resulting from making films with low dipping times and high molecular weight polyanion. The molecular weight in these embodiments is higher than the polyacrylic conventionally used in layer by layer construction of PEM thin films. The cutoff as to molecular weight can be determined empirically for any particular polyanion under a given set of synthetic conditions. For polyacrylic acid, it has been observed that a molecular weight above 100,000 g/mole suffices to achieve the desired morphology illustrated in the Figures. In an exemplary embodiment, the molecular weight is about 225,000 g/mole. A polyanion molecular weight at or above the value at which the microfibrous structure is formed (generally at the higher dipping times) is classified as “high molecular weight.”
Pores in the PEM thin films described herein are classified as nanopores or micropores depending on their size. Nanopores are charactized by dimensions on the order of nanometers, and in any event less than 1 micron (which equals 1000 nm). Micropores where used indicates a pore with a dimension of 1 micron or greater. In one embodiment described herein, a film contains nanopores having a dimension of 20 nm to 50 nm. For pores that are nearly circular, such as many of the nanopores illustrated in the Figures, the word diameter can be used interchangeably for the size of the pore. But the use of “diameter” here or in the claims is not to be taken as an indication that the description is limited to round or perfectly round pores. Rather, it is a short hand way to describe the minimum dimension of a pore; if that minimum dimension is less than a micron, it is a nanopore.
The discoveries described herein have led to development of so-called multi-scale porous structures. These structures combine thin films with pores on the order of microns in size (microsized) and thin films with pores on the order of less than a micron (nanosized), for example in the range of 20-50 nm, 50-100 nm, 10-100 nm, 10-500 nm, 20-500 nm 50-500 nm, 100-500 nm, and so on. In various embodiments, a microsized porous structure is provided on top of nanosized porous structure or, conversely, a nanosized porous structure is provided on top of a microsized porous structure.
In various embodiments, a microsized porous structure as used herein is formed when a PEM thin film is built up of alternating polycation and polyanion with dipping times of greater than a minute (such as 5 minutes, 10 minutes, or 15 minutes) using the higher molecular weight polyanion. This forms the microfibrous morphology shown, for illustration, in
Together, the microsized porous structure and the nanosized porous structure form a composite that can be applied to a variety of substrates using standard LbL technology. In a non-limiting example, a composite can be applied to a substrate comprising a non-woven fabric or non-woven web to provide membranes for various industrial applications. In non-limiting examples, they can be applied to reduce the COD level of wastewater samples. They tend to foul less than commercially available membranes. Further, the solution fluxes of the membranes are higher than commercial reverse osmosis membranes, making them less energy-demanding.
Illustrative multi-scale composites are given in
PEM films have been widely applied for surface modification of membranes used for water treatment applications. Commercial Ultrafiltration (UF) and Nanofiltration (NF) membranes have been modified by LbL to yield higher rejection and sometimes even higher fluxes than commercial RO membranes. However the underlying porous structures of these commercially available membranes impose certain limitations on the PEMs in terms of property enhancement. With the help of the above fabricated porous PEM structures we seek to overcome this limitation.
A commercially available NF membrane has several structural components. The bottommost support layer is usually made from non-woven PET fabrics followed by a microporous polysulfone layer. This in turn is followed by the membrane skin layer made from polyamide which usually has pores in the range of 1-5 nm. We adopt a simple bottom-up approach for mimicking the above mentioned structure using the multi-scale porous PEM structures. Both the microporous as well as the polyamide layers can be replaced by PEMs with equivalent pore sizes. The higher molecular weight of PAA can be used for fabricating the nanoporous layer with very minute pore diameters similar to what is usually observed for NF membranes. For RO applications the membrane has to be made suitable for rejecting even small monovalent ions.
This might necessitate the deposition of a few bilayers of non-porous PEMs on the nanoporous layers which would serve as a barrier to the passage of unwanted ions. Overall it can be claimed that a truly hierarchical porous structure with layers that are microporous, nanoporous and even non-porous, can be built using the simple yet versatile LbL process.
PEMs are known to be hydrophilic and by virtue of the LbL process there is control over the thickness and the pore-sizes of the fabricated membrane. This is ideal for developing a highly perm-selective membrane which should potentially eliminate the need for high pressure demands, as is presently the case. For actual desalination purposes even the best commercially available RO membranes require a transmembrane pressure of around 50-60 bar. This high pressure accounts for the lion's share of the electricity cost involved in running a desalination plant. The hierarchical porous structure provided by a multi-scale composite as described herein disposed on a porous substrate can help reduce the energy demands of the present RO membranes.
A thin-film LbL deposition technique is used to make RO membranes. In the prior art, the individual components of the membranes have to be fabricated using different processing techniques. For example, a polysulfone layer is prepared by solvent casting and a polyamide layer from interfacial polymerization. The new method described herein enables the membrane to be fabricated in a more synergistic way, whereby all the components can be synthesized using the same LbL process. This simplifies the process and saves time and money. The short-time LbL would make sure that the manufacturing of the membranes would not take long as usually expected from conventional LbL. It should also be noted that these porous structures are thermally cross-linked following the porous treatment in order to retain their structures. The cross-linking step gives the membranes mechanical strength sufficient to sustain the high pressure requirements of an RO process.
In conclusion, it can be claimed that a highly permeable RO membrane with high rejection can be made from porous LbL films. In our work we have focused on overcoming the limitation of the LbL process being time-consuming, by reducing the time of each individual step. The currently available RO membranes can be made using one simple approach without having to change the processing technique for every individual component of the membrane. Our fabrication process will not only bring down the manufacturing cost of the membranes but also the electricity cost usually required to operate these membranes in a desalination plant. Lastly, cross-linking of the multilayers would provide high mechanical strength to the membranes to be used for actual applications.
Controlled drug release from the surface has drawn more and more attention in the biomedical field. It can facilitate local delivery and increase the drug efficiency. For controlled release, the release rate should be different according to the change of drug needed for different release stage. Although LbL technique has already been applied for fabricating drug loaded PEMs (Wood K C, Boedicker J Q, Lynn D M, et al. Langmuir, 2005, 21(4): 1603-1609.), the release profile is linear for most cases, which means the release rate remains constant all the time. This is because drug release is controlled by dissociation or degradation of polyelectrolytes. In addition, this approach requires the drug to be hydrophilic, which can be alternatively deposited onto the surface with the other polyelectrolyte. For hydrophobic drug, a special amphiphilic drug carrier is required (Kim B S, Park S W, Hammond P T. Acs Nano, 2008, 2(2): 386-392.).
Dr. Rubner first developed the porous PEM films and applied for controlled drug release (Berg M C, Zhai L, Cohen R E, et al. Biomacromolecules, 2006, 7(1): 357-364.). With porous PEM films, hydrophobic drugs can be easily incorporated. The average surface pore size of their porous films ranged from 100 nm to 1 μm. In this invention, our porous structure can be precisely controlled from 20 nm to 10 μm by fast LbL assembly. What's more important, multi-scale porous structures have been successfully built up to achieve different release rate for different stage. The release kinetics can be precisely controlled by tuning the porous structure. This will help maximize the drug efficiency. The multi-scale porous structure allows us more possibility to fulfill certain needs.
A super hydrophobic (i.e., self-cleaning) surface requires advancing contact angle of at least 150°. Surface roughness and topography influence the surface hydrophobicity profoundly. Micro- and nano-structure are both desired on the surface to achieve super hydrophobicity. For example, the surface of lotus leaf contains 3-10 micron-sized hills and valleys that are decorated with nano-sized hydrophobic particles. Current studies (Zhai L, Cebeci F C, Cohen R E, et al. Nano Letters, 2004, 4(7): 1349-1353.) generated super hydrophobic surface always by fabricating micro-sized structure first and then introducing nano-sized structure. With our approach, micro and nano-structured surface can be achieved at the same time by tuning the dipping time and molecular weight of polyelectrolytes. The surface can be further modified with fluorinated silane molecules to obtain a super hydrophobic surface.
Suitable fluorinated silane molecules (also called fluoroalkyl silanes) contain a perfluorinated or partially perfluorinated hydrocarbon chain attached to a silane group functionalized to bond to functional groups like hydroxyls found on the surface. A non-limiting example is trichloro-(1H, 1H, 2H, 2H-perfluorooctyl)-silane, which is commercially available from Aldrich. After post treatment, the porous surface is exposed to a fluorinated silane or other suitable molecule in the gas phase using a simple chemical vapor deposition process. For example, several droplets of the silane are placed in an open vial sitting next to the samples to be coated. The vial and samples are placed in a beaker and the beaker was sealed and placed in an oven. Heating is carried at 130° C. for 2 hours and then at 180° C. for 2 hours. The result is a hydrophilic surface covered with sufficient silane to increase the observed contact angle of water with the surface. Coverage of the fluorinated silane on the surface of the sample is inferred from the observed increase in the contact angle, and can be confirmed with an elemental analysis.
The invention has been described with exemplary embodiments based on a combination of a variety of features, each of which can take various values. It is to be understood that the various values of the features can be substituted to provide other embodiments. A non-limiting set of embodiments includes:
1. A polyelectrolyte multilayer thin film having pores in the film, wherein at least some of the pores have a diameter of 20 to 50 nm.
2. The film of embodiment 1, wherein the film comprises alternating layers of polycation and polyacrylic acid.
3. The film of embodiment 2, wherein the polyacrylic acid has a weight average molecular weight over 100,000 g/mole.
4. The film of embodiment 2, wherein the polyacrylic acid has a weight average molecular weight of about 225,000 g/mole.
5. The film of embodiment 2, wherein the polycation comprises poly(allyamine hydrochloride).
6. The film of embodiment 1, wherein the film has a thickness of 250-500 nm.
7. The film of embodiment 1, comprising high molecular weight polyanion.
8. A polyelectrolyte multilayer thin film, having a hydrophobic surface characterized by a contact angle with water of greater than 150°.
9. The film of embodiment 8, wherein the contact angle is 160° or greater.
10. The film of embodiment 8, wherein the film comprises alternating layers of polycation and of polyacrylic acid.
11. The film of embodiment 10, wherein the polycation comprises polyethleneimine, poly(allylamine hydrochloride), or DADMAC.
12. The film of embodiment 10 wherein the polyacrylic acid has a weight average molecular weight greater than 100,000.
13. The film of embodiment 10 wherein the polyacrylic acid has a weight average molecular weight of about 225,000 g/mole.
14. The film of claim 8, comprising high molecular weight polyanion.
15. A polyelectrolyte multilayer thin film comprising built up alternating layers of polycation and polyanion, wherein the polyanion comprises poly acrylic acid having a weight average molecular weight of greater than 100,000 g/mole.
16. The polyelectrolyte multilayer thin film according to embodiment 15, wherein the polyacrylic acid has a weight average molecular weight of about 225,000 g/mole.
17. The polyelectrolyte multilayer thin film according to embodiment 15, wherein the film has a smooth morphology.
18. The polyelectrolyte multilayer thin film according to embodiment 15, wherein the film has a microporous morphology.
19. The film of embodiment 15, having nanopores with a diameter in the range of 20 to 50 nm.
20. The film of embodiment 20, comprising high molecular weight polyanion.
30. A method of making a polyelectrolyte multilayer thin film using layer by layer assembly of alternating polycation and polyanion, the method comprising applying a first layer of polycation to a negatively charged substrate, and building up alternating layers to make the thin film by alternately dipping the coated substrate in a solution containing polyanion and a solution containing polycation, wherein the dipping time in each solution is one minute or less, and wherein the polyanion comprises high molecular weight polyanion or polyacrylic acid having a weight average molecular weight above 100,000 g/mole.
31. The method according to embodiment 30, comprising building up at least 10 bilayers comprising polyanion and polycation.
31. The method according to embodiment 30 or 31, further comprising porosity treating the thin film by exposing it to a solution having a pH of 1.8-2.4.
32. The method according to embodiment 31, wherein the solution has a pH of 1.8-2.2.
33. The method according to embodiment 30 through 32, wherein the dipping time is 30 seconds or less.
34. The method according to embodiment 33, wherein the dipping time is about 10 seconds.
35. The method according to embodiment 30 to 34, wherein the polyacrylic acid has a weight average molecular weight of about 225,000 g/mole.
36. The method of any of embodiments 30-35, further comprising crosslinking the thin film.
40. A method of making a polyelectrolyte multilayer thin film using layer by layer assembly of alternating polycation and polyanion, the method comprising applying a first layer of polycation to a negatively charged substrate, and building up alternating layers to make the thin film by alternately dipping the coated substrate in a solution containing polyanion and a solution containing polycation, wherein the dipping time in each solution is greater than one minute, and wherein the polyanion comprises polyacrylic acid having a weight average molecular weight above 100,000 g/mole.
41. The method according to embodiment 40, wherein the molecular weight is about 225,000 g/mole
42. The method according to claim 40, further comprising porous treating the thin film by exposing it to a solution of pH 1.8-2.4.
43. A method of filtering watering during water treatment to remove impurities, the method comprising passing water through a filter, wherein the filter comprises a polyelectrolyte thin film according to any of embodiments 1-20.
44. A method of filtering watering during water treatment to remove impurities, the method comprising passing water through a filter, wherein the filter comprises a polyelectrolyte thin film made according to the method of any of claims 30-42.
45. A method according to embodiment 43 or claim 44, wherein the method comprises reverse osmosis.
46. A drug delivery system comprising a polyelectrolyte multilayer thin film of any of claims 1-29, wherein the thin film comprises a pore and an active pharmaceutical agent disposed in the pore.
47. A composite comprising a first PEM thin film having a microsized porous structure disposed on a second PEM thin film having a nanosized porous structure.
48. The composite of embodiment 47, wherein one or both of the first and second PEM thin films comprise polyacrylic acid.
49. A membrane comprising a composite according to embodiment 47 disposed on a porous substrate.
50. The membrane of embodiment 49, wherein the porous substrate is a non-woven web.
51. The composite of embodiment 48, comprising polyacrylic acid having a weight average molecular weight of greater than 100,000 g/mole.
52. A method according any of embodiments 30 through 42, further comprising applying a fluoroalkyl silane to the surface of the thin film following post treatment, wherein the post treatment comprises porous treatment, rinsing, drying, and crosslinking.
53. The method of embodiment 52, wherein the fluoroalkyl silane comprises trichloro-(1H, 1H, 2H, 2H-perfluorooctyl)silane.
In this work, we focused on the design of porous polymeric films with nano and micro sized pores existing in distinct zones. The porous thin films were fabricated by the post treatment of layer-by-layer (LbL) assembled poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) multilayers. In order to improve the processing efficiency, the dipping time was shortened to −10 s. It was found that fine porous structures could be created even by significantly reducing the processing time. The effect of using polyelectrolytes with widely different molecular weights was also studied. The pore size was increased by using the high molecular weight PAH, while high molecular weight of PAA minimized the pore size to nanometer scale. Having gained a precise control over the pore size, layered multi-scale porous thin films were further built up with either micro-sized porous zone on top of nano-sized porous zone or vice versa.
Porous polymeric films are in demand for a wide range of applications including foams[1], insulators[2], membranes[3], catalytic supports[4], anti-reflection coatings[5], superhydrophobic coatings[6] and drug delivery systems[7]. For many applications, sophisticated porous structures with a precise control on the pore sizes ranging from nano to micro are desired. For example, hierarchical (i.e., micro and nano sized) porous surfaces also help achieve superhydrophobicity[8]. For controlled drug release, the release rate is highly dependent on the pore size[9]. A well-controlled porous structure enables a tunable drug release over time[10]. In addition, commercial nanofiltration/reverse osmosis membranes have an asymmetric structure with two distinct types of porous zones; the bottom one consists of micro-sized pores while the upper zone consists of nano-sized pores. Motivated by these prospects, the multi-scale porous thin films with well-defined micro and nano-sized porous regions were developed in this work.
Layer-by-layer (LbL) assembly is considered as a highly versatile deposition technique for fabricating functional thin films and coatings[11]. LbL assembled polyelectrolyte multilayers (PEMs) followed by simple post-treatment steps provides one of the most promising methods to generate porous polymeric frameworks. Rubner and coworkers first demonstrated the formation of porous networks using poly (allylamine hydrochloride) (PAH)/poly (acrylic acid) (PAA) multilayers. The PEMs were fabricated with the PAH solution at a pH of 7.5 or 8.5 and the PAA solution at a pH of 3.5. The porous structure was formed after the post treatment, which includes acidic immersion within the pH range of 1.8-2.6, rinsing in DI water, drying and cross-linking[5-6, 9, 12]. Both nano and micro sized porous films were able to be achieved by tuning the post treatment conditions. In addition, free standing porous PAH/PAA films can be obtained through an ion-triggered exfoliation method[12d]. Porous thin films can also be fabricated by salt-induced structural changes in PAH/PAA multilayers[13]. The porous structures were formed by exposing the PAA/PAH multilayers fabricated in the presence of salt to pure water. However, the pore size was limited to the nanometer scale. Another way to make porous thin films via LbL assembly is through the treatment of hydrogen-bonded poly(4-vinylpyridine) (PVP)/PAA multilayers in aqueous solution at pH of 12.5 when PAA was dissolved followed by the reconfirmation of PVP chains[14]. Only micro-sized pores were obtained, and the stability of the hydrogen bonded LbL films over a broad range of pH is always an issue. Thus, in this work, we applied acidic treatment to induce the porous formation in PAH/PAA multilayer films.
Immersion of PAH/PAA films in a low-pH aqueous solution causes rearrangement of the polymer chains[5, 12b, 12c]. This rearrangement is induced by the breakage of the ionic cross-links of PAA due to protonation of the carboxylate groups and charge repulsion among the free, positively charged amine groups of PAH. The rinsing step with DI water allows ion pairs to reform and form small water pocket by rejecting water from the film. By drying water out from the water pockets and cross-linking the polymer chains, stable porous films were obtained.
In order to create distinct zones with different scales of the pore size, the pre-requisite was to have a very precise control on each of the zones independently and to understand the factors that affect the formation of those zones. Once those factors were identified, their combination could lead us to form multi-scale porous frameworks. Previous studies mainly investigated the effect of the number of layers[12a, 12d], pH[5, 9, 12a, 12c, 15] and time[12a, 12d, 15a] of the post treatment on the morphology of the porous PAH/PAA films. However, one major obstacle in commercializing any of these films is the long processing time that goes into fabricating the PAH/PAA films using LbL technique. Recently, several studies initiated using short dipping time to address this issue and apply for gas barrier films[16]. The dipping time was shortened from conventional 15-20 min to less than 1 min. It has been found that different dipping time leads to varied film compositions and structures[16a]. Since the formation of porous PAH/PAA films is mainly dependent on the interaction between PAA and PAH and the reorganization of polymer chains, the changes in film composition and polymer distribution may further alter the porous structure. However, no research has been focused on studying the effect of dipping time on the porous structure, or how efficiently the porous thin films can be built up. In addition, the mobility of the individual polymer chains also plays a crucial role. During the post treatment, the reorganization of polymer chains is highly influenced by the chain mobility and the interaction among functional groups. In this regard, the molecular weight of polyelectrolytes could be one of the critical and intrinsic parameters to tune the porous structure since it highly affects the chain mobility and the intramolecular and intermolcular interactions. It has been reported that molecular weight of polyelectrolytes plays an important role during LbL assembly[17]. However, few studies focused on the molecular weight effect of polyelectrolytes on the porous structure[15b]. In order to study the molecular weight effect thoroughly, we fabricated PAH/PAA multilayers using PAA with molecular weight of 15,000 g/mol (PAAL) and 225,000 g/mol (PAAH) and PAH with molecular weight of 15,000 g/mol (PAHL) and 900,000 g/mol (PAHH). In this study, we focused on the effect of dipping time and molecular weight of polyelectrolytes on the porous morphology in order to shorten the fabrication time and obtain a wider and more precise control on the porous structure at the same time.
In this work, the PAH/PAA multilayers were constructed by the alternate deposition of PAH at pH=8.5 and PAA at pH=3.5. According to the literature, the degree of ionization of PAA in aqueous solution with pH=3.5 is less than 10%[18], while the degree of ionization of PAH in aqueous solution with pH=8.5 is around 50% [18a, 19]. Under this pH condition, a high level of interlayer diffusion occurs in order for charge compensation to take place, leading to an exponential growth in the thickness of the multilayer films[18b, 20]. The variations in the thickness of (PAHL/PAAL)20.5 films as a function of the dipping time are shown in
Considering the efficiency of fabricating porous films, 10 s dipping was further applied to different molecular weight systems in order to study the molecular weight effect on the porous morphology. The acid treatment was still carried out by immersing PAH/PAA multilayers in pH=2.0 aqueous solution for 5 min followed by 5 min of washing with DI water.
Based on the porous structures described above, multi-scale porous films were fabricated which constituted of a macro-sized porous zone on top of nano-sized porous zone (
As shown in
In the above mentioned scenario, the underlying porous portion had very small surface pore size, therefore the molecular weight of the polyelectrolytes used to build up the top porous portion, is not a matter of serious concern. However, if the bottom portion is made with relatively larger surface pore size, PAA with high molecular weight is required for the top porous portion. This is because the polymer chain size needs to be large enough for not diffusing into the porous bottom. In addition, after the bottom portion was thermally cross-linked, the surface became more hydrophobic, which helped trap air inside the porous structure and block the polyelectrolytes outside. As shown in
In summary, multi-scale porous thin films have been developed for the first time with either micro-sized porous structure on top of nano-sized porous structure or vice versa. In order to build up the porous thin films more efficiently, the effect of dipping time on the morphology of porous films was investigated for the first time in this work. Compared to conventional 15 or 20 min dipping, we were able to shorten the dipping time to 10 s but still maintain fine porous structures. The molecular weight effect of both PAH and PAA were also studied. While an increase in the molecular weight of PAH led to an increase in the pore size, a decrease in the pore size was observed for a high molecular weight of PAA. The layered multi-scale porous thin films were further fabricated by tuning the tipping time and molecular weight of polyelectrolytes. The porous thin films developed in the present work may broaden the applications of porous thin films for membrane filtration, drug delivery, etc.
Poly (acrylic acid, sodium salt) solutions with different molecular weight (PAAL, Mw=15,000, 35% aqueous solution, and PAAH, Mw=225,000, 20% aqueous solution) were purchase from Sigma Aldrich and Polyscience, respectively. Both poly(allylamine hydrochloride) (PAHL, Mw=15,000 g/mol and PAHH, Mw=900,000 g/mol) were purchased from Sigma-Aldrich. All aqueous solutions were prepared using 18.2 MΩ Millipore water at a concentration of 10 mM with respect to the repeat unit and adjusted to the required pH using 0.1M HCl or NaOH solutions. Glass slides from Globe Scientific Inc. were cleaned by sonication for 20 min each in ethanol and DI water and then exposed to oxygen plasma generated by a Harrick plasma cleaner (Harrick Scientific Corporation, Broading Ossining, NY) for 20 min, producing hydrophilic moieties and negative charges on the surface.
All LbL films were assembled with a programmable Carl-Zeiss slide-stainer. After the oxygen plasma treatment, the glass substrates were immediately dipped into PAH solution (without adjusting the pH) for 20 min to form the precursor layer, followed by three washing steps. Then, the substrates were introduced in the aqueous solution of PAA (pH=3.5) for required dipping time, followed by three washing steps with DI water (pH=3.5) for sufficient time. Subsequently, the substrates were immersed in the PAH (pH=8.5) aqueous solution with the same dipping time as PAA, and washed again three times with DI water (pH=8.5). The dipping process was repeated 20 times. In total, 20.5 bilayers were deposited on the substrate, including the first PAH precursor layer. Dipping time of 10 s, 1 min, 5 min, 10 min and 15 min were applied in this work.
The assembled polyelectrolyte multilayer films were immersed in the water solution with pH of 2.0 for 5 min followed by washing with DI water (pH=5.5) for 5 min. After the porosity induction, the films were dried and then heated at 180° C. for 2 hours to cross-link the films and prevent the porous structure from being distorted. This post treatment helped create porous films as described by other researchers.[5, 9, 12c, 15a]
For fabricating the layered multi-scale porous thin films, the bottom porous portion was first built up by repeating the previous steps and considered as the substrate for the following LbL assembly. The bottom porous portion was further introduced in the aqueous solution of PAA (pH=3.5) for required dipping time and then PAH (pH=8.5) aqueous solution with three washing steps in between. After 20 bilayers of PAA/PAH were built up, the entire thin film went through the post treatment again for making the top portion porous. Eventually, the entire thin film was further thermally cross-linked at 180° C. for 2 hours.
The thickness of the thin films before and after the post-treatment was measured in the dry state using a Dektak surface profiler. A JEOL 6610LV Scanning Electron Microscopy (SEM) was used to observe the surface and fractured cross-section morphology of the porous thin films. All specimens were coated with gold before examination under the SEM.
Porous induction was carried out after the LbL assembly of PAA (pH 3.5) and PAH (pH 8.5). The assembled polyelectrolyte multilayer films were porous treated by immersing in a water solution at pH=2.0 for 5 min followed by washing with deionized water (pH=5.5) for 5 min. After the porous induction, the films were dried and then heated at 180° C. for 2 hours to cross-link the films and prevent the porous structures from being distorted. In the end, a chemical vapor deposition process using trichloro-(1H, 1H, 2H, 2H-perfluorooctyl)silane was carried out at 130° C. for 2 hours, followed by heating at 180° C. for 2 hours to remove free fluoroalkyl silane molecules. The effect of molecular weight and dipping time on the roughness and wettability of the porous surface was investigated.
A droplet of water-soluble ink solution was placed on the surfaces of the (PAHH/PAAL)20.5 samples with 1 min dipping and pH=2.0 for porous induction before and after CVD process, respectively. The result is shown in
In summary, by tuning the dipping time, molecular weight of polyelectrolytes and pH of the porous induction, we successfully fabricated a surface with hierarchical structure by depositing porous (PAHH/PAAL)20.5. A switch from super hydrophilicity to super hydrophobicity was achieved via a simple chemical vapor deposition of fluoroalkylsilane molecules. And it was possible to shorten the dipping time from conventional 15 or 20 min to only 1 min.
Wettability is a fundamental property of a solid surface and plays a key role in addressing the problems related to fouling[1], oil/water separation[2], corrosion[3], fogging[4], water collection[5], etc. In order to achieve superwettability, surface chemistry and topography are the two key factors. Comparing to other surface modification methods, Layer-by-Layer (LbL) assembly can be carried out under much milder conditions and provides highly tunable surface properties. LbL assembly always provides hydrophilic surface due to the nature of polyelectrolytes. Fluoroalkylsilane molecules can be grafted onto the surface by chemical vapor deposition (CVD) and change the surface wettability to hydrophobic. However, without proper surface topography, it is hard to achieve superwettability.
LbL assembly is able to generate smooth surface with roughness in nanometer level. Rough surface is always hard to achieve via LbL assembly of polyelectrolytes. Special LbL conditions were required to achieve a rough surface. Shen et al.[6] prepared a superhydrophobic surface via fluorinating polyelectrolyte multilayer with exponential-growth behavior. It was found that the exponential growth behavior could facilitate the formation of micro/nano hierarchical structures. The resultant surfaces exhibited superhydrophobicity after the CVD of (tridecafluoroctyl)-triethoxysilane. However, in order to achieve exponential growth, the LbL assembly was carried out while two polyelectrolytes were mostly not charged, leading to a possible issue with film stability. In addition, long processing time was also required for the adsorption of polyelectrolytes. In this work, the PAH/PAA multilayers were fabricated by the alternate deposition of PAH at pH 8.5 and PAA at pH 3.5. Then, a CVD process of Trichloro(1H, 1H,2H, 2H-perfluoro-octyl)silane was done at 130° C. for 2 hours, followed by heating at 180° C. for 2 hours to remove free fluoroalkylsilane molecules. We investigated the effect of molecular weight and dipping time on the surface roughness. The surface morphologies for (PAHL/PAAL)20.5 with different dipping time are shown in
In order to increase surface roughness and achieve superhydrophobicity, the LbL technique has to be combined with other techniques together to control surface structures. Rubner et al.[7] fabricated a porous PAH/PAA multilayer film via LbL assembly followed with a simple acidic treatment. The surface was then coated with silica nanoparticles and modified with semi-fluorinated silane via CVD process to achieved superhydrophobicity. Zhang et al.[8] prepared the polyelectrolyte multilayers covered by gold cluster via a combination of LbL technique and electrochemical deposition. A stable superhydrophobic surface was achieved after the further modification by n-dodecanethiol. However, the combination of LbL assembly with other techniques increased the complexity of fabrication by requiring more materials and equipments and increasing the processing time. Therefore, in this work, only porous PAA/PAH multilayers were applied to achieve superwettability. The porous induction was carried out after the LbL assembly of PAA (pH 3.5) and PAH (pH 8.5). The assembled polyelectrolyte multilayer films were immersed in the water solution at a certain pH for 5 min followed by washing with DI water (pH=5.5) for 5 min. After the porous induction, the films were dried and then heated at 180° C. for 2 hours to cross-link the films and prevent the porous structures from being distorted. In the end, the same CVD process of Trichloro(1H, 1H,2H, 2H-perfluoro-octyl)silane was applied to make the surface hydrophobic.
Another parameter which affects the surface topography significantly is the pH for porous induction. The (PAHH/PAAL)20.5 samples with 1 min dipping were treated at pH from 1.8 to 2.4. The surface SEM images are shown in
Further, we put a droplet of water-soluble ink solution on the surfaces of the (PAHH/PAAL)20.5 samples with 1 min dipping and pH=2.0 for porous induction before and after CVD process, respectively. The image is shown in
In summary, by tuning the dipping time, molecular weight of polyelectrolytes and pH of the porous induction, we successfully fabricated a surface with hierarchical structure by depositing porous (PAHH/PAAL)20.5. A switch from superhydrophilicity to superhydrophobicity was achieved via a simple chemical vapor deposition of fluoroalkylsilane molecules. We were able to shorten the dipping time from conventional 15 or 20 min to only 1 min.
This application claims the benefit of U.S. Provisional Application No. 62/254,371 filed on Nov. 12, 2015, and is a continuation-in-part of U.S. patent application Ser. No. 14/937,955 filed on Nov. 11, 2015, which claims the benefit of U.S. Provisional Applications No. 62/180,982 filed on Jun. 17, 2015; 62/080,296 filed on Nov. 15, 2014; and 62/080,010 filed on Nov. 14, 2014. The entire disclosures of the above applications are incorporated herein by reference.
This invention was made with Government support under W912HQ-12-C-0020 awarded by the Department of Defense, Strategic Environmental Research and Development Program (SERDP). The U. S. Government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 62254371 | Nov 2015 | US | |
| 62180982 | Jun 2015 | US | |
| 62080296 | Nov 2014 | US | |
| 62080010 | Nov 2014 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14937955 | Nov 2015 | US |
| Child | 15157526 | US |
