Patent Application
20140213136
Materials to form devices that can be switched between, for example, a superhydrophilic surface or a superhydrophobic surface, have garnered great attention for fundamental considerations and for applications as self-healing surfaces, selective molecular separation, controlled drug delivery, and smart textiles. In general, these switchable materials are engineered to respond to an external stimulus, such as light irradiation, pH changes, solvent exposure, electrical potential, magnetic field, mechanical force, or temperature. Thermally responsive materials are of particular interest due to the narrow temperature response and the predictable properties the materials can display upon undergoing the transition. The switchable responses result from structural changes to the material as the properties change. Although generally a single property is the focus in a study of a material, other properties of the material will simultaneously change, for example, a temperature change that switches a surface property of a material, can simultaneously switch a bulk property of the material. The rate of switching of these properties and the response profile will depend on the shape and size of the article in addition to the chemical composition of the article.
Poly(N-isopropylacrylamide) (PNIPA) has been explored extensively with respect to its temperature responsive properties. PNIPA displays a lower critical solution temperature (LCST) of 32-33° C. in water. The polymer's repeating units display a reversible hydrogen bonding preference for water molecules or other repeating units of the polymer due to enthalpic and entropic contribution to the free energy of PNIPA chains. Below the LCST, enthalpic contributions dominate over entropic contributions and the polar groups (C═O and N—H) of PNIPA form intermolecular hydrogen bonds with water molecules, which places the PNIPA polymer chains in “extended conformations”, when the polymer is dissolve in water. Above the LCST, entropic contributions dominate the enthalpic contributions of bonding with water, and hydrogen bonding occurs between repeating units of the polymer chains rather than with water molecules, which causes the PNIPA chains to exist in collapsed “globular conformations” and to precipitate from solution.
To fabricate surfaces that display reversible extreme wettability (REW) characteristics, PNIPA has been processed by layer-by-layer, hydrothermal, surface entrapment, phase separation, self-assembled monolayers, electrochemical deposition, and other methods. These techniques are complicated, typically requiring multiple process steps to produce a responsive surface for the observation of reversible wettability. Wang et al., Macromol. Rapid Comm.,2008, 29, 485-89, teaches the electrospinning of a poly(N-isopropylacrylamide)/polystyrene composite film. The films were spun from tetrahydrofuran solutions, leaving a combination of microparticles and nanofibers. The use of a composite solution high in poly(N-isopropylacrylamide) (6:10:90 PNIPA/PS/THF) resulted in poor spinibility dominated by microfibers with a size distribution of 0.5 to 2 μm with connected microparticles with diameters in excess of 10 μm, whereas a composite high in polystyrene (1:10:90 PNIPA/PS/THF) did not display switchable wettability and little nanofiber content with microparticles having diameters of 5 to 25 μm. At an intermediate composition (2:10:90 PNIPA/PS/THF), a switchable wettability was achieved although spinning still resulted in a combination of microparticles and nanofibers, where microparticles dominated the structure and were 3.5 to 30 μm in diameter.
To exploit reversible wetting properties (REW) or other surface properties, and/or bulk properties for practical applications on a large scale, a simple fabrication technique is required that can achieve a structural consistency, where the structure is superior to that presently achieved for these materials, for example, a structure that is very small and controllable so that the switching rate can be designed and rapid.
Embodiments of the invention are directed to a fibrous properties-switching article whose properties rapidly and reversibly switch over a range of temperatures. The article comprises a mat consisting of fibers of at least one polymer, copolymer, polymer blend, and/or polymer network with narrow fiber size distribution, having a diameter of about 2 μm or less, where the material undergoes a structural change over the range of temperatures, which causes the surface and/or bulk property of the mat to change over the range of temperatures. In exemplary embodiments of the invention, the structural change is a conformational change in N-isopropyl acrylamide units of poly((N-isopropylacrylamide), where the hydrogen bonding of the amide switches from bonding with water at low temperatures to intramolecular bonding between repeating units of the polymer at higher temperatures. Fiber mats of a polymer blend of polystyrene and poly((N-isopropylacrylamide) (b1-PS/PNIPA) and of crosslinked poly(N-isopropylacrylamide-co-methacylicacid) (x1-PNIPAMAA) display dramatic changes in their hydrophilicity over a relatively narrow temperature range. The switching speed that can be achieved depends on the diameter of the fibers in the mat, with very high switching speeds possible for very small diameter fibers.
Embodiments of the invention are directed to the preparation of mats of very small fibers by electrospinning. By the proper choice of parameters, including the polymer concentration and the solvent, a mat is formed that displays exclusively fibers by SEM. The b1-PS/PNIPA fiber mat is spun from dimethylformamide (DMF), which, suprisingly, gives exclusively fibers with no particles being observed, as is the case when a THF solvent is employed. A PNIPAMAA fiber mat is spun from DMF and subsequently heated to crosslink the fibers.
Embodiments of the invention are directed to preparing and using articles having surface and/or bulk properties that change upon a change of temperature. In an embodiment of the invention, a fiberous surface that displays reversible extreme wettability (REW) is formed by electrospinning, as shown in
In exemplary embodiments of the invention, properties switching articles are prepared from polystyrene/poly(N-isopropylacrylamide) blends (b1-PS/PNIPA), and poly(N-isopropyl acrylamide-co-methacylicacid) (PNIPAMAA) are used to produce REW articles by electrospinning. The fiber mats of blended PS/PNIPA (b1-PS/PNIPA) and crosslinked PNIPAMAA (x1-PNIPAMAA) display REW properties while maintaining integrity at temperature ranges that are broad. In one embodiment of the invention a fiber mat was produced from b1-PS/PNIPA that exhibited exclusively fibers, as shown in
Response times of hydrogels have been shown to be directly proportional to the square of the gel dimension and inversely proportional to the network diffusion coefficient:
τ=r2/Dcoop Equation 1
where, τ is the characteristic swelling time for diffusion to achieve equilibrium, r represents the smallest gel dimension, and Dcoop is the cooperative diffusion coefficient of the network, Tanaka et al. J. Phys. Rev. Lett. 1985, 55, (22), 2455-8. The value of Dcoop for PNIPA varies between 10−12 and 10−10 m2 s−1 depending upon the crosslinking density, polymer concentration, and temperature, with an inverse correlation between Dcoop and temperature, and with an order of difference in diffusion coefficients, 5×10−12 and 2×10−11 m2 s−1, for de-swollen gel at temperatures greater than 32° C. and water swollen gel at temperatures less than 32° C., respectively. As it is difficult to increase the value of Dcoop by a factor of 102 or more, the actual response time of a hydrogel largely depends upon gel thickness, or the fiber diameter for the fiber mats, according to an embodiment of the invention.
Because the fiber diameter can be controlled, the response time can be controlled. As calculated in Table 1, below, the rate at which the fiber can switch depends on heat transfer and/or water (or other chemical) diffusion through the fiber, which are processes whose rates vary with the cross-sectional area of the fiber. As can be seen in Table 1, the rate of switching can be dramatically decreased as the fiber's diameter decreases, allowing switching times that can occur in milliseconds or less when fiber diameters drop below a micrometer (μm). As surface properties, such as wettability, and bulk properties, such as elastic modulus, can vary dramatically, such materials can be useful in applications where an article comprising a polymeric material must respond to the environment it experiences, for example, a tire. The property change results from a structural change of the material comprising the fiber; for example, the fiber can be constructed of a polymer that has functionality that undergoes a conformational change, a change in association, or a change in solvation. The fibers can be those of a homopolymer, a copolymer, a polymer blend, or a polymer network. In embodiments of the invention, the polymer changes structure from a polymer hydrogen bonded to water at low temperatures to a self hydrogen bonding polymer at higher temperatures, such that water is released when the polymer adapts a conformation for intramolecular hydrogen bonding.
Contact angle (CA) measurements were undertaken for the 870 nm fiber bl-PS/PNIPA and the 1950 nm fiber xl-PNIPAMAA electrospun fiber mats, according to embodiments of the invention. At 15° C., the bl-PS/PNIPA fiber mat shows a response consistent with a superhydrophilic surface, which is defined as a surface with a CA value of ≦5°. At 65° C., a 138.0°±4.5 CA is observed, which is near the value, ≧150°, considered to indicate a superhydrophobic surface. The CA values for the b1-PS/PNIPA and x1-PNIPAMAA fiber mats at various temperatures are given in Table 2, below, and shown as a plot in
CA measurements carried out on electrospun PS/PNIPA fiber mats, shown in
cos θCB=fs(1+cos θY)−1 Equation 2
where θCB is the apparent contact angle on a rough surface, θY is the equilibrium (Young's) contact angle on a smooth surface, and fs is the fraction of the wet solid contact area. The electrospun fiber mats are a nonwoven fiber network with three dimensionally interconnected pores and grooves between the fibers. The CA values at 65° C. given in Table 4 agree with values calculated using the CB model where a reduction in the fiber's diameter reduces the fraction of fiber area in contact with the water droplet such that a fiber mat constructed of sufficiently fine diameter fibers displays superhydrophobicity, with water CA values ≧150°, as given in Table 4. Electrospun PS/PNIPA fiber mats, according to an embodiment of the invention, show superhydrophobic to superhydrophilic switching.
aAt 24° C. the contact angle (CA) was 0° for all the fiber mats;
bMetal bar temperature was maintained at −30 ± 3° C. using liquid N2; stage temperature was maintained at 24 ± 1° C. using cold water bath-circulator;
cfrozen.
The response time for a change from the maximum CA to the minimum CA for the PS/PNIPA blended fiber mats from different diameter fibers given in Table 4, above, corresponds to the change of a water droplet as illustrated in
The REW properties of the x1-PNIPAMAA fiber mats is indicated by a transition temperature that can be observed in a DSC measurement and occurs at the temperature where the fiber mat's surface changes from hydrophilic to hydrophobic during heating and from hydrophobic to hydrophilic properties during cooling. The x1-PNIPAMAA fiber mat exhibits an upward shift in transition temperature, observed as a strong broad peak at 82.7° C. in a DSC plot, which differs from that of ˜32° C. for the transition displayed by PNIPAMAA in water, indicating that transition temperature for the material increases upon crosslinking, as indicated in
The fiber mats, according to embodiments of the invention, are those where the solubility of the fiber mat in water is inhibited. In one embodiment, the solubility is inhibited by blending a polymer that is water soluble below the LCST with a polymer that is insoluble in water at all temperatures. In another embodiment, a water soluble polymer is crosslinked to a water swellable, yet insoluble material. Leaching experiments were carried out where vacuum dried fiber mats were water washed with stirring at 10° C. for 24 hours and agian vacuum dried. Result of the experiments, as inidicated in Table 5, below, suggest that other factors than just the dissolving of water soluble portions of the mats affected the results. For example, the 83.3% weight loss of the bl-PS/PNIPA indicated a retention of only 16.7% of the mass, even though approximately 70% of the mat's mass was blended polystyrene, which is water insoluble. In contrast, the x1-PNIPAMAA fiber mat lost 53.3% of its mass upon washing, although it was, in principle, a crosslinked mass that should swell but not dissolve in water.
The integrity of the b1-PS/PNIPA and x1-PNIPAMAA fiber mats in areas where the fiber mats had experienced the heating and cooling cycles to determine REW by CA measurements was determined by SEM analysis. As can be seen in
The crosslinking reactions that occur in PNIPAMAA can include: anhydride formation between carboxylic acid groups of PNIPAMAA; esterification between carboxylic acid groups of PNIPAMAA and alcohol groups of poly(vinyl alcohol) (PVA); and/or imidization between carboxylic acid groups and amide groups of PNIPAMAA. It is reasonable that all three of the reactions with the carboxylic acid groups contribute to crosslinking the electrospun PNIPAMAA fibers during heat treatment at 160° C. in a vacuum oven.
Polystyrene (PS) (Mn 170,000 g/mol and Mw 350,000 g/mol), poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAMAA) (Mn 60,000 g/mol, 90 mol % PNIPA and 10 mol % MAA), disodium hydrogen phosphate (DSHP), and dimethylformamide (DMF) were used as received from Sigma-Aldrich. Poly(N-isopropylacrylamide) (PNIPA) (Mv ˜40,000 g/mo) was used as received from Polyscience Incorporation. Poly(vinyl alcohol) (PVA) (75% hydrolyzed and MW 2,000) was used as received from Acros Organics. Glacial acetic acid (99.9% HOAc) was used as received from Fisher.
A 15% wt blend solution of PS and PNIPA (PNIPA/PS 30/70 wt/wt) was prepared by dissolving the polymers in DMF. The blend solution was placed in a 3 mL syringe, fitted with an 18-gauge stainless steel needle (inner diameter of 0.965 mm). The syringe was fixed horizontally on a syringe pump (Model: BSP-99M, Braintee Scientific Inc.), and an electrode connected to a high voltage power supply (Model: ES30N-5W, Gamma High Voltage Research) was attached to the tip of the metallic needle. A grounded stationary square collector (10 cm×10 cm) covered by a piece of clean aluminum foil was used for fiber collection. Electrospinning, to produce bl-PS/PNIPA with 870 nm fibers, was carried out using the blend solution under the following operating conditions: a flow rate (FR) of 0.90 mL/h; an electric field (EF) of 0.8 kV/cm; and a distance between the needle and the collecting plate (DCP) of 11 cm. Electrospinning was performed for about 30 mins.
PS/PNIPA blended fiber mats with diameter of the fiber 380, 990, 1.5K and 16K nm were fabricated by varying the blend solution concentration, flow rate, distance between the needle tip and collector surface (DCP) or gap distance, electric field, and needle gauge in electrospinning given in Table 6, below. Attempts to produce higher diameter fibers yielded fibers that were fused together.
Water stock solutions of 15 wt NIPAMAA/HOAc, 15% wt PVA/DI water, and 10% wt DSHP/DI were prepared. A formulation was generated by mixing 0.68 g of the PNIPAMAA/HOAc solution with 15 wt PVA/DI water to yield 5% wt PVA relative to PNIPAMAA and 30% wt DSHP relative to PVA. Electrospinning was carried out using the formulation under the following operating conditions: FR of 0.43 mL/h; EF of 1 kV/cm; and DCP of 20 cm. Electrospun fibers were collected on a 1 mm thick glass slide (size 7.6 cm×2.5 cm) for 3 mins. The bottom of the glass slide was fixed to aluminum foil using a double-sided copper tape. The collected electrospun fiber mats were kept in a vacuum oven at room temperature (RT) overnight, followed by a heat treatment at 160° C. for 30 minutes in a vacuum oven. Subsequently, samples were washed in cold water (10° C.) followed by hot water (100° C.) and this washing cycle was repeated two additional times. Contact angle (CA) measurements were carried out on the fiber mats collected on a glass slide.
The surface morphology of b1-PS/PNIPA and x1-PNIPAMAA fiber mats were examined using a field emission gun SEM (Model: 6335F, Jeol), where a small portion of electrospun fiber mat was cut and fixed to a SEM stub using a double-sided adhesive carbon tape. Sample was sputter coated with a thin film of gold-palladium to aid in SEM analysis, and analyzed in SEM with an accelerating voltage of 10 kV. Additionally, the blended structure of b1-PS/PNIPA fiber mat was examined using TEM (Philips CM30), where a thin web of electrospun sample was collected on a copper grid and directly examined in TEM at an accelerating voltage of 300 kV.
CA measurements were carried out on electrospun samples using a Goniometer (Model: VCA Optima, AST Products, Inc.) instrument equipped with an automated dispensing system and a 30 gauge flat-tipped stainless steel needle. The probe fluid, water, having resistivity >18 MΩ-cm was collected using a nanopure Milli-Q purification system (Millipore Inc.). Sessile drop images were captured, by placing 2 μL or 4 μL water droplets onto the fiber mat at 5 different places. The CA data were then obtained by Drop-Snake analysis, a plug-in for Image J software.
About 10 mg of the b1-PS/PNIPA or x1-PNIPAMAA vacuum dried fiber mat was added to a vial containing de-ionized water (25 mL at 10° C.) and stirred at 50 rpm for 24 h. Subsequently, the fiber mat was removed from the vial and dried overnight in a vacuum oven at RT.
Mean fiber diameters of electrospun fiber mats were analyzed using Image J, a general purpose image processing software. Ten SEM images were obtained at different sites on each fiber mat. All fibers present in an image were measured for determining mean fiber diameter, where at least 150 individual fibers were measured for the analysis of each fiber mat.
Hydrated x1-PNIPAMAA fiber mats weighing ˜20 mg were used for DSC analysis. Temperature scans were performed between 5° C. and 100° C. to analyze sample transition temperatures. A 7.5% wt PNIPAMAA in DI water was analyzed by DSC to determine its transition temperature.
For temperature dependent CA measurements, heating was performed using a thin-flexible Kapton® heater (Model: KH-203/10, Omega Engineering, Inc.) and cooling was performed by a cryostage (Product Number: 39467506, Subzero™ Freezing Microtome Stage, Leica) attached to a cooler maintained at 24±1° C. using a water bath-circulator. Fiber mats, either collected on aluminum foil or a glass slide, were attached to a silicon wafer by a double-sided carbon adhesive tape. The silicon wafer was fixed to a flexible heater and a cryostage using scotch tape and the entire setup was placed on Goniometer stage. A thin thermocouple (Model: SA1-K-SRTC, Omega Engineering, Inc.) connected with a temperature meter (Model: BS5001k2, Omega Electronics, Inc.) was adhered to the fiber mat to read its surface temperature. The flexible Kapton® heater was powered by a DC power supply (Model: 6218A, Agilent HP), the temperature on the fiber mat was adjusted by controlling the voltage current. The DC power supply was switched on during heating cycles and the cryostage was switched on during cooling cycles. The temperature was measured with ±1° C. accuracy.
To impose a very quick step-function temperature reduction and, therefore, a rapid response, the 24° C. cryostage was replaced with a metal bar at a temperature of −30±3° C., that was maintained using liquid N2 as the coolant. Droplets placed over 16K nm diameter fiber mats froze after 25-30 seconds, whereas response time measurements using a stage at 24±1° C., was found to be 47.4±1.9 s. A 380 nm fiber mat's response time using the stage at 24±1° C., was 13.3±0.9 s, which is much slower than when the metal bar was used. Due to the limitations encountered in the experimental setup, response time for nanofibers may be shorter than that measured in this study.
The fraction of the wet solid contact area of electrospun fiber mats was obtained using Image J software. The image was first converted 32-bit type: image>type>32-bit. The threshold level was determined by adjusting and measuring to obtain the fraction of the wet solid contact area: image>adjust>threshold.
PS/PNIPA Blended fiber mats response time was investigated by capturing and analyzing the video for the transition from a maximum to minimum CA. The camera captures 60 frames per second. Fiber mat collected on aluminum (Al)-foil was glued to silicon (Si)-wafer using double-sided adhesive carbon tape to ensure a flat fiber mat surface that facilitated the response time studies. A thermocouple (Model: SA1-K-SRTC, Omega Engineering, Inc.) was glued on top of the fiber mat and was connected to a temperature meter (Model: BS5001k2, Omega Electronics, Inc.) to monitor the fiber mat's surface temperature. A fiber mat was heated to 65±1° C. by resistive heating, using a thin and flexible Kapton® heater (Model: KH-203/10, Omega Engineering, Inc.) with help of a DC power supply (Model: 6218A, Agilent HP), where upon reaching 65° C., a 4 μL volume dye solution (50 ppm concentration Procion red dye prepared in water) was placed above the fiber mat using a pipette. The fiber mat was transferred to the top of a metal bar maintained at −30±3° C. using liquid N2. The start time was when the Si-wafer with the fiber mat assembly fully contacted the metal bar and the end time was noted when the dye solution reached a minimum CA value. The response time was determined as an average of 5 values from 5 different spots. The measurements were conducted in an enviroment with relative humidity and temperature of 45% and 25° C., respectively.
All patents, patent applications, provisional applications, and publications referred to or cited herein, supra or infra, are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification.
It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application.
The present application claims the benefit of U.S. Provisional Application Ser. No. 61/528,040, filed Aug. 26, 2011, which is hereby incorporated by reference herein in its entirety, including any figures, tables, or drawings.
This invention was made with government support under Contract No. DE-AC04-94AL85000 awarded by the Department of Energy National Nuclear Security Administration. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2012/052070 | 8/23/2012 | WO | 00 | 2/25/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61528040 | Aug 2011 | US |
