U.S. patent application number 14/240963 was filed with the patent office on 2014-07-31 for fast response nanofiber articles with tunable wettability and bulk properties.
This patent application is currently assigned to SANDIA CORPORATION. The applicant listed for this patent is Tim Boyle, Palanikkumaran Muthiah, Wolfgang M. Sigmund. Invention is credited to Tim Boyle, Palanikkumaran Muthiah, Wolfgang M. Sigmund.
Application Number | 20140213136 14/240963 |
Document ID | / |
Family ID | 47756744 |
Filed Date | 2014-07-31 |
United States Patent
Application |
20140213136 |
Kind Code |
A1 |
Sigmund; Wolfgang M. ; et
al. |
July 31, 2014 |
FAST RESPONSE NANOFIBER ARTICLES WITH TUNABLE WETTABILITY AND BULK
PROPERTIES
Abstract
A fibrous properties-switching article comprises a mat
consisting of fibers having a fiber diameter of 2 microns or less.
The fibers comprise a polymer, copolymer, polymer blend, or polymer
network, wherein the fibers have a diameter of 2 gm or less. The
surface and/or bulk property of the mat changes over a range of
temperatures, wherein the polymer, copolymer, polymer blend, or
polymer network undergoes a structural change over the range of
temperatures. The fiber mat is formed by electrospinning. In an
exemplary embodiment, a blend of polystyrene and poly((N-isopropyl
acrylamide) (b1-PS/PNIPA) in dimethylformamide (DMF) is electrospun
to form a mat consisting of fibers with a diameter less than 2
.mu.m that shows a transition from a superhydrophilic surface to a
nearly superhydrophobic surface over a temperature range from
30.degree. C. to 45.degree. C. A fiber mat formed by
electrospinning a DMF solution comprising poly(N-isopropyl
acrylamide-co-methacylicacid) (PNIPAMAA), comprises fibers having a
diameter less than 2 .mu.m and are cross linked after spinning. The
crosslinked PNIPAMAA, (x1-PNIPAMAA) fiber mat displays a transition
from a hydrophilic surface to a nearly hydrophobic surface over a
temperature range from 30.degree. C. to 45.degree. C.
Inventors: |
Sigmund; Wolfgang M.;
(Gainesville, FL) ; Muthiah; Palanikkumaran;
(Tamillnadu, IN) ; Boyle; Tim; (Albuquerque,
NM) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sigmund; Wolfgang M.
Muthiah; Palanikkumaran
Boyle; Tim |
Gainesville
Tamillnadu
Albuquerque |
FL
NM |
US
IN
US |
|
|
Assignee: |
SANDIA CORPORATION
ALBUQUERQUE
NM
UNIVERSITY OF FLORIDA RESEARCH FOUNDATION, INC.
GAINESVILLE
FL
|
Family ID: |
47756744 |
Appl. No.: |
14/240963 |
Filed: |
August 23, 2012 |
PCT Filed: |
August 23, 2012 |
PCT NO: |
PCT/US2012/052070 |
371 Date: |
February 25, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61528040 |
Aug 26, 2011 |
|
|
|
Current U.S.
Class: |
442/341 ; 264/6;
442/351 |
Current CPC
Class: |
Y10T 442/615 20150401;
D04H 1/544 20130101; D04H 1/728 20130101; Y10T 442/626 20150401;
D01F 6/56 20130101; D01D 5/0007 20130101; D01F 6/36 20130101 |
Class at
Publication: |
442/341 ;
442/351; 264/6 |
International
Class: |
D04H 1/728 20060101
D04H001/728 |
Goverment Interests
[0002] 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.
Claims
1. A fibrous properties-switching article, comprising a mat
consisting of fibers, wherein the fibers comprise a polymer,
copolymer, polymer blend, or polymer network, wherein the fibers
have a diameter of 2 .mu.m or less, wherein the polymer, copolymer,
polymer blend, or polymer network undergoes a structural change
over a range of temperatures, and wherein a surface and/or bulk
property of the mat changes over the range of temperatures.
2. The fibrous properties-switching article of claim 1, wherein the
structural change is a conformational change and/or a change in
solvation.
3. The fibrous properties-switching article of claim 1, wherein the
fiber mat comprises a polymer blend of polystyrene and
poly((N-isopropyl acrylamide) (b1-PS/PNIPA).
4. The fibrous properties-switching article of claim 3, wherein the
polystyrene and the poly((N-isopropyl acrylamide) are in a weight
ratio of 7 PS to 3 PNIPA.
5. The fibrous properties-switching article of claim 1, wherein the
fiber mat comprises a polymer network of crosslinked
poly(N-isopropyl acrylamide-co-methacylicacid) (x1-PNIPAMAA).
6. The fibrous properties-switching article of claim 1, wherein the
surface property change comprises a change from hydrophobicity to
hydrophilicity.
7. The fibrous properties-switching article of claim 6, wherein a
response rate for the change from hydrophobicity to hydrophilicity
is less than one minute.
8. The fibrous properties-switching article of claim 6, wherein a
response rate for the change from hydrophobicity to hydrophilicity
is less than 5 seconds.
9. A method for preparing a fiber mat of a property switching
material according to claim 1 comprising: providing a solution
comprising at least one polymer or copolymer; optionally including
a catalyst and/or a reagent for crosslinking; and electrospinning
the solution onto a target substrate to form a mat consisting of
fibers, wherein the fibers are 2 .mu.m or less in diameter.
10. The method of claim 9, wherein the solution comprising at least
one polymer is a blend of PS and PNIPA in dimethylformamide
(DMF).
11. The method of claim 9, wherein the solution comprising a
copolymer is PNIPAMAA in DMF.
12. The method of claim 11, wherein the catalyst is disodium
hydrogen phosphate (DSHP).
13. The method of claim 11, wherein the reagent is polyvinyl
alcohol (PVA).
14. The method of claim 9, further comprising heating the mat,
wherein crosslinking of the fibers occurs.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] 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.
BACKGROUND OF THE INVENTION
[0003] 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.
[0004] 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.degree. 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.dbd.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.
[0005] 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 .mu.m with connected microparticles with diameters in
excess of 10 .mu.m, 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 .mu.m. 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 .mu.m in
diameter.
[0006] 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.
BRIEF SUMMARY OF THE INVENTION
[0007] 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 .mu.m 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.
[0008] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 shows digital photographic images of dye solutions
placed on a) a blended polystyrene/poly((N-isopropylacrylamide)
(b1-PS/PNIPA) fiber mat according to an embodiment of the invention
and b) a crosslinked poly(N-isopropylacrylamide-co-methacylicacid)
(x1-PNIPAMAA) fiber mat according to an embodiment of the invention
showing reversible, extreme wettability (REW) properties.
[0010] FIG. 2 shows scanning electron microscopy (SEM) images of a)
a b1-PS/PNIPA fiber mat according to an embodiment of the invention
and b) x1-PNIPAMAA fiber mat according to an embodiment of the
ivention, and c) a transmission electron microscopy (TEM) image of
the b1-PS/PNIPA fiber mat where the PS and PNIPA blending at the
nanoscale dimensions are apparent.
[0011] FIG. 3 shows a plot of the contact angle (CA) over a range
of temperatures for b1-PS/PNIPA and x1-PNIPAMAA fiber mats
according to embodiments of the invention.
[0012] FIG. 4 shows a plot of CA against temperature during cycling
of the temperature for b1-PS/PNIPA and x1-PNIPAMAA fiber mats
according to embodiments of the invention, where the b1-PS/PNIPA
fiber mats were cycled between 15.degree. C. and 65.degree. C. and
the x1-PNIPAMAA fiber mats were cycled between 20.degree. C. and
95.degree. C.
[0013] FIG. 5 shows SEM images of b1-PS/PNIPA fiber mat according
to an embodiment of the invention, with fibers of diameter: a) 380;
b) 990; c) 1500; and d) 16000 nm.
[0014] FIG. 6 shows photographs of water droplets on a 380 nm
diameter fiber bl-PS/PNIPA fiber mat, according to an embodiment of
the invention at 65.degree. C. and 25.degree. C., where the mat is
superhydrophobic and superhydrophilic, respectively.
[0015] FIG. 7 shows selected photographic images taken at the
indicated time in seconds after placing a fiber mat, according to
an embodiment of the invention, supporting a water droplet on top a
metal bar maintained at -30.+-.3, where a) a 380 nm fiber
b1-PS/PNIPA mat transformed from superhydrophobic to
superhydrophilic within 5 seconds, and where b) a 16000 nm fiber
b1-PS/PNIPA mat resulted in freezing of the water droplet before
wetting of the surface to an appreciable degree.
[0016] FIG. 8 shows DSC plots of PNIPAMAA dissolved in water and
hydrated x1-PNIPAMAA fibers.
[0017] FIG. 9 shows SEM images of the sections of fiber mats in the
presence of water after the heating and cooling cycles that are
illustrated in FIG. 4 were carried out, for mats of a) b1-PS/PNIPA
and b) x1-PNIPAMAA according to embodiments of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] 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 FIG. 1 for exemplary
embodiments. The electrospinning readily produces a mat of fibers,
where the rate of response and the switching rate of the resulting
fiber mat is controlled by the diameter of the fiber. In exemplary
embodiments of the invention, the fibers have diameters that range
from about 100 nm to about 2 .mu.m in diameter depending on
predetermined conditions employed. This process involves the
imposition of a high electrical field, for example, 1-5 kV/cm, to a
polymer droplet as it exits an orifice, for example, the end of a
needle. The applied high electrical force overcomes the surface
energy of the droplet and forms a Taylor cone where a stream of
liquid erupts from the droplet. Subsequently, as the liquid stream
dries in flight, the mode of current flow changes from ohmic to
convective as the charge migrates to the surface of the fiber. The
stream is elongated by a whipping process caused by electrostatic
repulsion initiated at small bends in the fiber, until the fibers
are deposited on a grounded collector. The elongation and thinning
of the fiber, resulting from bending instability, leads to
formation of uniform fibers, often displaying nanometer-scale
diameters, and results in a mat of fibers on the collector that
possess a high surface area to mass ratio.
[0019] 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 FIG. 2a. This contrasts with the
mat observed by Wang et al., Macromol. Rapid Comm., 2008, 29,
485-89 where it was not possible to achieve exclusively fiber
structures. In an embodiment of the invention, the fiber mat was
produced by electrospinning the blend from a dimethylformamide
(DMF) solution of the b1-PS/PNIPA, indicating that the nature of
the solvent is critical to the electrospinning of fibers. Analysis
by TEM of the b1-PS/PNIPA fiber mat displays a blended structure
where two distinct polymer phases with nanometer dimentions are
apparent, as can be seen in FIG. 2c. For exemplary embodiments of
the invention, the surface morphologies of the b1-PS/PNIPA and
x1-PNIPAMAA fiber mats, as shown by the SEM images in FIG. 2,
indicate that the fiber mats are exclusively fibers that have a
uniform size distribution and a mean fiber diameter of 1950 nm for
the mat from b1-PS/PNIPA and 870 nm for the mat from
x1-PNIPAMAA.
[0020] 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:
.tau.=r.sup.2/D.sub.coop Equation 1
where, .tau. is the characteristic swelling time for diffusion to
achieve equilibrium, r represents the smallest gel dimension, and
D.sub.coop is the cooperative diffusion coefficient of the network,
Tanaka et al. J. Phys. Rev. Lett. 1985, 55, (22), 2455-8. The value
of D.sub.coop for PNIPA varies between 10.sup.-12 and 10.sup.-10
m.sup.2 s.sup.-1 depending upon the crosslinking density, polymer
concentration, and temperature, with an inverse correlation between
D.sub.coop and temperature, and with an order of difference in
diffusion coefficients, 5.times.10.sup.-12 and 2.times.10.sup.-11
m.sup.2 s.sup.-1, for de-swollen gel at temperatures greater than
32.degree. C. and water swollen gel at temperatures less than
32.degree. C., respectively. As it is difficult to increase the
value of D.sub.coop by a factor of 10.sup.2 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.
[0021] 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 (.mu.m). 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.
TABLE-US-00001 TABLE 1 Calculated switching response times for
fibers of different diameters. Fiber Diameter (nm) r.sup.2
(.mu.m.sup.2) Calc Response time (msec) 10,000 2.5 833 1,000 0.25
8.3 100 0.025 0.083
[0022] 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.degree. 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 .ltoreq.5.degree.. At 65.degree. C., a
138.0.degree..+-.4.5 CA is observed, which is near the value,
.gtoreq.150.degree., 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 FIG. 3. The switching occurs between 30.degree. C. and
45.degree. C. for the fiber mat of b1-PS/PNIPA. At 45.degree. C.,
the b1-PS/PNIPA sample displays a CA that is 90% of the steady
state CA value for higher temperatures. This REW of b1-PS/PNIPA and
x1-PNIPAMAA fiber mats are shown in FIG. 4 for the data of Table 3,
below, for wetted fiber mats, where the temperature is cycled 5
times between temperatures displaying hydrophilic or
superhydrophilic behavior and hydrophobic or superhydrophobic
behavior.
TABLE-US-00002 TABLE 2 CA values of bl-PS/PNIPA and xl-PNIPAMAA
fiber mats at various temperatures. 870 nm bl-PS/PNIPA Fiber Mat
1950 nm xl-PNIPAMAA Fiber Mat Temperature (.degree. C.) CA .+-. SD
(.degree.) Temperature (.degree. C.) CA .+-. SD (.degree.) 15 0.0
.+-. 0.0 20 20.5 .+-. 6.3 25 0.0 .+-. 0.0 40 16.6 .+-. 3.8 30 0.0
.+-. 0.0 60 63.2 .+-. 4.4 35 65.4 .+-. 6.1 70 82.2 .+-. 3.6 40
101.8 .+-. 4.3 75 87.8 .+-. 3.1 45 124.2 .+-. 7.8 80 87.7 .+-. 6.7
55 134.5 .+-. 3.6 85 87.4 .+-. 4.2 65 138.1 .+-. 4.4 90 89.5 .+-.
2.7 -- -- 95 87.3 .+-. 2.7
TABLE-US-00003 TABLE 3 Temperature responsive reversibility CA
values of bl-PS/PNIPA and xl-PNIPAMAA fiber mats where each cycle
was between 15.degree. C. and 65.degree. C. for bl-PS/PNIPA fiber
mats and 20.degree. C. and 95.degree. C. for the xl-PNIPAMAA fiber
mats. Number of bl-PS/PNIPA fiber mat xl-PNIPAMAA fiber mat cycles
CA .+-. SD (.degree.) CA .+-. SD (.degree.) 0 0.0 .+-. 0.0 19.7
.+-. 1.0 0.5 144.0 .+-. 6.3 90.7 .+-. 2.6 1 0.0 .+-. 0.0 17.2 .+-.
2.3 1.5 144.7 .+-. 9.4 88.1 .+-. 1.6 2 0.0 .+-. 0.0 22.3 .+-. 1.7
2.5 140.0 .+-. 9.1 88.5 .+-. 4.8 3 0.0 .+-. 0.0 12.2 .+-. 1.9 3.5
133.4 .+-. 4.4 87.3 .+-. 3.6 4 0.0 .+-. 0.0 17.1 .+-. 5.1 4.5 136.8
.+-. 4.5 90.9 .+-. 1.6 5 0.0 .+-. 0.0 15.3 .+-. 1.6
[0023] CA measurements carried out on electrospun PS/PNIPA fiber
mats, shown in FIG. 5, with different diameter fibers, according to
an embodiment of the invention, are reported in Table 4, below. The
interaction between a liquid droplet and a porous structure
conforms to the Cassie-Baxter (CB) model, where the relationship
between CA and the porosity of a material is:
cos .theta..sub.CB=f.sub.s(1+cos .theta..sub.Y)-1 Equation 2
where .theta..sub.CB is the apparent contact angle on a rough
surface, .theta..sub.Y is the equilibrium (Young's) contact angle
on a smooth surface, and f.sub.s 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.degree. 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 .gtoreq.150.degree., as given in Table 4.
Electrospun PS/PNIPA fiber mats, according to an embodiment of the
invention, show superhydrophobic to superhydrophilic switching.
TABLE-US-00004 TABLE 4 Response time on PS/PNIPA blended fiber mats
with different diameter fibers Fraction of wet Fiber dia Mat
thickness solid contact .theta..sub.c at 65.degree. C. Response
time (nm) w/Al-foil (.mu.m) area (f.sub.s) (.degree.).sup.a Cold
source.sup.b (sec) 380 .+-. 100 44 .+-. 6 0.39 .+-. 0.03 144 .+-. 5
metal bar 4.2 .+-. 0.9 990 .+-. 300 55 .+-. 10 0.39 .+-. 0.02 146
.+-. 4 metal bar 4.3 .+-. 0.6 1.5K .+-. 900 53 .+-. 10 0.40 .+-.
0.01 140 .+-. 4 metal bar 4.8 .+-. 0.7 16K .+-. 1.2K 70 .+-. 10
0.48 .+-. 0.03 135 .+-. 6 metal bar >25-30.sup.c 16K .+-. 1.2K
70 .+-. 10 -- 135 .+-. 6 stage 47.4 .+-. 1.9 380 .+-. 100 44 .+-. 6
-- 144 .+-. 5 stage 13.3 .+-. 0.9 .sup.aAt 24.degree. C. the
contact angle (CA) was 0.degree. for all the fiber mats;
.theta..sub.c: contact angle, .sup.bMetal bar temperature was
maintained at -30 .+-. 3.degree. C. using liquid N.sub.2; stage
temperature was maintained at 24 .+-. 1.degree. C. using cold water
bath-circulator; .sup.cfrozen.
[0024] 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 FIG. 6. FIG. 7 graphically
displays this transformation for a mat of a) 380 nm diameter fibers
and of b) 16 .mu.m diameter fibers. Table 4 gives the fiber
diameter, fiber mat thickness, CA value measured at 65.degree. C.,
and the cold source used to study response time. All fiber mats are
superhydrophilic with a 0.degree. CA value at 25.degree. C. All
mats displayed response times below one minute, and mats with fiber
diameters of 380, 990 and 1.5K nm display a fast response time, of
<4-5 seconds, while the mats with fiber diameter of 16K nm
displayed a significantly slower response time, where the droplet
froze within 30 s on the low temperature cold bar before wetting of
the fibers could proceed, where movement of the ice drop at 35
seconds is shown in FIG. 7b.
[0025] 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.degree. C. in
a DSC plot, which differs from that of .about.32.degree. C. for the
transition displayed by PNIPAMAA in water, indicating that
transition temperature for the material increases upon
crosslinking, as indicated in FIG. 8. Additionally, the presence of
PVA in x1-PNIPAMAA causes the fiber mat to be more hydrophilic than
in the absence of PVA, shifting the transition point to a higher
temperature. In contrast to b1-PS/PNIPA fiber mats, the CA
measurements of the x1-PNIPAMAA fiber mat reveal a response between
hydrophilic and nearly hydrophobic, where hydrophobicity is
indicated by CA value that is .gtoreq.90.degree.. The CA values
vary from 20.5.degree..+-.6.5 to 87.0.degree..+-.3.0 for the
xl-PNIPAMAA fiber mat as the temperature varies from 20.degree. C.
to 95.degree. C., as indicated in Table 2, above, and plotted in
FIG. 3. The transition point observed for the x1-PNIPAMAA fiber
mat, as determined by DSC, was .about.83.degree. C., although the
CA measurements for the x1-PNIPAMAA fiber mat attained 94% of the
steady state CA value at 70.degree. C. over a relatively broad
switching temperature range of 40 to 70.degree. C., as indicated in
Table 3, above. FIG. 4 shows that the temperature cycling of the
x1-PNIPAMAA results in a switching between hydrophilic and nearly
hydrophobic behavior of the mat's surface.
[0026] 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.degree. 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.
TABLE-US-00005 TABLE 5 Weight loss by water washing of bl-PS/PNIPA
and xl-PNIPAMAA mats at 10.degree. C. Fiber mat sample % Mass loss
% Retained Mass Bl-PS/PNIPA 83.3 .+-. 4.2 16.7 .+-. 4.2 xl-PNIPAMAA
53.3 .+-. 0.5 46.7 .+-. 0.5
[0027] 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 FIG. 9a, the b1-PS/PNIPA fiber
mat displays damage to the fibrous structure. In contrast, the
x1-PNIPAMAA fiber mat retains its structure after experiencing
heating and cooling cycles, as can be seen in FIG. 9b.
Interestingly for the b1-PS/PNIPA, even though fiber damage
occurred, the fiber mat continued to demonstrate consistent REW
properties over all tested cycles, as indicated in FIG. 4.
[0028] 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.degree. C. in a vacuum oven.
Materials and Methods
Materials
[0029] Polystyrene (PS) (M.sub.n 170,000 g/mol and M.sub.w 350,000
g/mol), poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAMAA)
(M.sub.n 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)
(M.sub.v .about.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.
Preparation of PS/PNIPA Blended Fiber Mat
[0030] 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.times.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 (D.sub.CP) of
11 cm. Electrospinning was performed for about 30 mins.
[0031] 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 (D.sub.CP) 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.
TABLE-US-00006 TABLE 6 PS/PNIPA 70/30 w/w preparation conditions,
and the fibers and mats produced. Wt % Flow rate D.sub.CP EF in
Needle Time in Fiber dia. Mat thickness blend in .mu.L/min. in cm
kV/cm Gauge min. in nm in .mu.m 15 3 20 0.43 24 180 380 .+-. 100 44
.+-. 6 15 3 20 0.43 18 150 990 .+-. 300 55 .+-. 10 15 15 11 0.77 18
45 1500 .+-. 900 53 .+-. 10 30 150 11 0.77 18 5 16000 .+-. 1200 70
.+-. 10 15 3 20 0.43 18 60 1500 .+-. 400 83 .+-. 14 15 3 20 0.43 18
180 600 .+-. 100 163 .+-. 16 15 3 20 0.43 18 600 800 .+-. 200 258
.+-. 32
Preparation of Crosslinked PNIPAMAA Fiber Mat
[0032] 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 D.sub.CP of 20 cm. Electrospun fibers were
collected on a 1 mm thick glass slide (size 7.6 cm.times.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.degree. C. for 30
minutes in a vacuum oven. Subsequently, samples were washed in cold
water (10.degree. C.) followed by hot water (100.degree. 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.
Surface Morphology and Structure Analysis
[0033] 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.
Contact Angle Measurements
[0034] 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 M.OMEGA.-cm was collected using a
nanopure Milli-Q purification system (Millipore Inc.). Sessile drop
images were captured, by placing 2 .mu.L or 4 .mu.L 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.
Water Resistance of Mats
[0035] 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.degree. 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 Size Analysis
[0036] 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.
Determination of Transition Temperature
[0037] Hydrated x1-PNIPAMAA fiber mats weighing .about.20 mg were
used for DSC analysis. Temperature scans were performed between
5.degree. C. and 100.degree. C. to analyze sample transition
temperatures. A 7.5% wt PNIPAMAA in DI water was analyzed by DSC to
determine its transition temperature.
Temperature Dependent CA Measurements
[0038] For temperature dependent CA measurements, heating was
performed using a thin-flexible Kapton.RTM. heater (Model:
KH-203/10, Omega Engineering, Inc.) and cooling was performed by a
cryostage (Product Number: 39467506, Subzero.TM. Freezing Microtome
Stage, Leica) attached to a cooler maintained at 24.+-.1.degree. 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.RTM.
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.degree. C. accuracy.
[0039] To impose a very quick step-function temperature reduction
and, therefore, a rapid response, the 24.degree. C. cryostage was
replaced with a metal bar at a temperature of -30.+-.3.degree. C.,
that was maintained using liquid N.sub.2 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.degree.
C., was found to be 47.4.+-.1.9 s. A 380 nm fiber mat's response
time using the stage at 24.+-.1.degree. 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.
Determination of the Wet Solid Contact Area
[0040] 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.
Determination of Response Time on PS/PNIPA Blended Fiber Mats
[0041] 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.degree. C. by resistive heating, using a thin and flexible
Kapton.RTM. heater (Model: KH-203/10, Omega Engineering, Inc.) with
help of a DC power supply (Model: 6218A, Agilent HP), where upon
reaching 65.degree. C., a 4 .mu.L 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.degree. C. using liquid
N.sub.2. 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.degree. C.,
respectively.
[0042] 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.
[0043] 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.
* * * * *