U.S. patent application number 11/017546 was filed with the patent office on 2006-03-16 for electrospun electroactive polymers.
This patent application is currently assigned to U.S.A.as represented by the Administrator of the National Aeronautics and Space Administration. Invention is credited to Kristin J. Burney, Joycelyn S. Harrison, Zoubeida Ounaies, Cheol Park, Emilie J. Siochi.
Application Number | 20060057377 11/017546 |
Document ID | / |
Family ID | 36034368 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060057377 |
Kind Code |
A1 |
Harrison; Joycelyn S. ; et
al. |
March 16, 2006 |
Electrospun electroactive polymers
Abstract
Electroactive polymers are produced via electrospinning. The
induction of electroactivity via electrospinning can be utilized
with one or more soluble polymers with polarizable moieties.
Suitable polymer classes include but are not limited to polyimides,
polyamides, vinyl polymers, polyurethanes, polyureas,
polythioureas, polyacrylates, polyesters, and biopolymers. Any one
or more solvents sufficient to dissolve the one or more polymers of
interest and make a spinnable solution can be utilized. The polymer
can be electrospun into fiber and fibrous nonwoven mat. The
electroactive polymer can be doped with inclusions, such as
nanotubes, nanofibers, and piezoceramic powders for dielectric
enhancement The availability of electroactive polymer fibers and
fibrous nonwoven mat will enable many new applications for
electroactive polymers.
Inventors: |
Harrison; Joycelyn S.;
(Hampton, VA) ; Burney; Kristin J.; (Newport,
RI) ; Ounaies; Zoubeida; (Richmond, VA) ;
Park; Cheol; (Yorktown, VA) ; Siochi; Emilie J.;
(Newport News, VA) |
Correspondence
Address: |
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION;LANGLEY RESEARCH CENTER
MAIL STOP 141
HAMPTON
VA
23681-2199
US
|
Assignee: |
U.S.A.as represented by the
Administrator of the National Aeronautics and Space
Administration
Washington
DC
|
Family ID: |
36034368 |
Appl. No.: |
11/017546 |
Filed: |
December 18, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60530637 |
Dec 19, 2003 |
|
|
|
Current U.S.
Class: |
428/364 |
Current CPC
Class: |
H01L 41/193 20130101;
D04H 3/02 20130101; D01F 1/10 20130101; H01L 41/257 20130101; B82Y
30/00 20130101; D04H 3/16 20130101; D01D 5/0038 20130101; H01L
41/45 20130101; Y10T 428/2913 20150115 |
Class at
Publication: |
428/364 |
International
Class: |
D02G 3/00 20060101
D02G003/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OF DEVELOPMENT
[0002] The invention described herein was made in part by employees
of the United States Government and may be manufactured and used by
and for the Government of the United States for governmental
purposes without the payment of any royalties thereon or therefore.
Claims
1. An electrospun electroactive polymer.
2. The electroactive polymer of claim 1, wherein said polymer is a
fiber.
3. The electroactive polymer of claim 2, wherein said fiber has a
diameter between approximately 10 nm and approximately 10
.mu.m.
4. The electroactive polymer of claim 1, wherein said polymer is a
fibrous nonwoven mat.
5. The electroactive polymer of claim 1, wherein said polymer is
electrospun from a solution comprising one or more polymers having
polarizable moieties dissolved in one or more solvents, wherein
said one or more solvents dissolves the one or more polymers of
interest to make a spinnable solution.
6. The electroactive polymer of claim 5, wherein said one or more
polymers having polarizable moieties is selected from the group
consisting of polymides, polyamides, vinyl polymers, polyurethanes,
polyureas, polythioureas, polyacrylates, polyesters and
biopolymers.
7. The electroactive polymer of claim 6, wherein said one or more
polyimides is selected from the group consisting of
2,6-bis(3-aminophenoxy)benzonitrile
((.beta.-CN)APB)/4,4'oxydiphthalic anhydride (ODPA)
((.beta.-CN)APB-ODPA) and amorphous polyimide.
8. The electroactive polymer of claim 7, wherein said amorphous
polyimide is amorphous polyetherimide.
9. The electroactive polymer of claim 6, wherein said one or more
polyamides is an odd-numbered nylon.
10. The electroactive polymer of claim 6, wherein said one or more
vinyl polymers are selected from the group consisting of
polyvinylidene fluoride (PVDF), copolymer of vinylidene fluoride
and trifluoroethylene (PVDF/TrFE), poly(vinyl alcohol) (PVA), graft
elastomer, and vinyl copolymer.
11. The electroactive polymer of claim 6, wherein said one or more
polyacrylates is poly(methyl methacrylate) (PMMA).
12. The electroactive polymer of claim 6, wherein said one or more
biopolmers is selected from the group consisting of polypeptide and
keratin.
13. The electroactive polymer of claim 5, wherein said one or more
solvents is selected from the group consisting of
N,N-Dimethylformamide (DMF), N,N-Dimethylacetamide (DMAc),
N-methylpyrrolidinone (NMP), toluene, and cosolvent.
14. The electroactive polymer of claim 13, wherein said cosolvent
is DMF/acetone.
15. The polymer of claim 1, further comprising inclusions for
dielectric enhancement.
16. The polymer of claim 15, wherein said inclusions are selected
from the group consisting of nanotubes, nanofibers, and
piezoceramic powders.
17. The polymer of claim 16, wherein said nanotubes are selected
from the group consisting of single-walled carbon nanotubes and
multi-walled carbon nanotubes.
18. A process for producing polymeric materials for electroactive
applications comprising electrospinning of a solution comprising
one or more polymers having polarizable moieties dissolved in one
or more solvents, wherein said one or more solvents dissolves the
one or more polymers of interest to make a spinnable solution.
19. The process of claim 18, wherein said one or more polymers
having polarizable moieties is selected from the group consisting
of polyimides, polyamides, vinyl polymers, polyurethanes,
polyureas, polythioureas, polyacrylates, polyesters and
biopolymers.
20. The process of claim 19, wherein said one or more polyimides is
selected from the group consisting of
2,6-bis(3-aminophenoxy)benzonitrile
((.beta.-CN)APB)/4,4'oxydiphthalic anhydride (ODPA)
((.beta.-CN)APB-ODPA) and amorphous polyimide.
21. The process of claim 20, wherein said amorphous polyimide is
amorphous polyetherimide.
22. The process of claim 19, wherein said one or more polyamides is
an odd-numbered nylon.
23. The process of claim 19, wherein said one or more vinyl
polymers are selected from the group consisting of polyvinylidene
fluoride (PVDF), copolymer of vinylidene fluoride and
trifluoroethylene (PVDF-/TrFE), poly(vinyl alcohol) (PVA), graft
elastomer, and vinyl copolymer.
24. The process of claim 19, wherein said one or more polyacrylates
is poly(methyl methacrylate) (PMMA).
25. The process of claim 19, wherein said one or more biopolmers is
selected from the group consisting of polypeptide and keratin.
26. The process of claim 18, wherein said one or more solvents is
selected from the group consisting of N,N-Dimethylformamide (DMF),
N,N-Dimethylacetamide (DMAc), N-methylpyrrolidinone (NMP), toluene,
and cosolvent.
27. The process of claim 26, wherein said cosolvent is
DMF/acetone.
28. The process of claim 18 wherein said solution further comprises
inclusions for dielectric enhancement.
29. The process of claim 28, wherein said inclusions are selected
from the group consisting of nanotubes, nanofibers, and
piezoceramic powders.
30. The process of claim 18, wherein said polymeric material is a
fiber.
31. The process of claim 30, wherein said fiber has a diameter
between approximately 10 nm and approximately 10 .mu.m.
32. The process of claim 18, wherein said polymeric material is a
fibrous nonwoven mat.
33. A process for producing polymeric materials for electroactive
applications, comprising the in situ induction of polar phase and
spontaneous dipolar orientation of electroactive polymers via
electrospinning a polymer solution, wherein said solution comprises
one or more polymers having polarizable moieties dissolved in one
or more solvents, wherein said one or more solvents dissolves the
one or more polymers of interest to make a spinnable solution.
Description
CLAIM OF BENEFIT OF PROVISIONAL APPLICATION
[0001] Pursuant to 35 U.S.C. .sctn. 119, the benefit of priority
from provisional application having U.S. Ser. No. 60/530,637, filed
on Dec. 19, 2003, is claimed for this nonprovisional
application.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates generally to the production of
electroactive polymers via electrospinning. It relates in
particular to the induction of the polar phase in electroactive
polymers, and the spontaneous orientation of dipoles in a single
step process. These materials have widespread application in
numerous fields, including aerospace, biomedical, clothing, and
defense.
[0005] 2. Description of the Related Art
[0006] Due to their exceptional thermal, mechanical, and dielectric
properties, polyimides are widely utilized, such as for matrix
materials in composite aircraft components and as dielectric
materials in microelectronic devices. Further, electroactive
polymers have properties that are advantageous in numerous fields.
Unfortunately, current methodology for producing electroactive
polymers entails melt-pressing of the polymer at high temperatures,
followed by stretching and corona-poling at high electric field
strengths in order to produce the polar phase and electroactive
behavior. This current process is labor-intensive and
time-consuming, and requires large equipment that is complex to
maintain.
[0007] Currently, polyvinylidene fluoride (PVDF) is the only
commercially available piezoelectric polymer. PVDF is
ferroelectric, having a polar axis that can be reoriented when an
electric field is applied, and piezoelectric, when subjected to
mechanical stress, changing its electrical polarization, or vice
versa, with a change in electrical polarization resulting in
mechanical movement. PVDF exhibits piezoelectricity after being
subjected to a poling process which applies a high electric field
to force molecular dipole alignment. PVDF can exist in several
solid state phases, .alpha., .beta., .delta., and .gamma.. The
.beta.-phase is of most importance because it is this phase that
shows the largest electroactive response. The different phases can
be attained by application of mechanical, thermal, or electrical
stress depending on the initial and desired state of the polymer.
The process that leads to the .beta.-phase formation is labor
intensive and time consuming, requiring initial melt processing and
drawing into a film, followed by further stretching of the film at
an elevated temperature either uniaxially or biaxially to induce
the polar phase. Finally, it is passed under corona wires at high
voltage to cause the induced dipoles to orient. PVDF has the
potential to be used in many applications due not only to its
electroactive properties but also because it is lightweight,
flexible, tough and conformable. In addition, it possesses
significant resistance to fatigue, abrasion, deformation,
chemicals, and solar radiation. These properties make it an
attractive material for aerospace as well as medical
applications.
[0008] In the fiber-spinning process known as electrospinning, a
high voltage is applied to a polymer in solution to create
nanofibers and nonwoven mats. The polymer solution is loaded into a
syringe, and high voltage is applied to the needle of the syringe.
Charge builds up on a droplet of solution that is suspended at the
tip of the syringe needle. Gradually, as this charge overcomes the
surface tension of the solution, this droplet elongates and forms a
Taylor cone. Finally, the solution exits out of the tip of the
Taylor cone as a jet, which travels through the air to its target
medium. While traveling, the solvent evaporates, leaving fibers.
Overall, electrospinning is advantageous for many reasons. It is
simple, and the technique is fast and easy to run. Only a small
amount of material is required, and there is very little waste. The
products of this process also have advantages over currently
available materials; the fibers are very thin and have a high
length to diameter ratio, which provides a very large surface area
per unit mass. Finally, the process is versatile. Fibers can be
spun onto any shape using a wide range of polymers. While
electrospinning is an advantageous processing method to apply to
polymers, it has not been applied for the purpose of producing
electroactive polymers. An additional advantage of electrospinning
is the ability to produce fibers and fibrous nonwoven mats; current
methods for the production of electroactive polymers generally
produce films.
[0009] While electroactive polymers themselves embody many useful
properties, doping electroactive polymers with inclusions, such as
nanotubes, nanofibers, and piezoceramic powders for dielectric
enhancement is advantageous. Carbon nanotubes have become
increasingly interesting due to their very unique properties: high
tensile strength and modulus, high electrical conductivity, and
high thermal conductivity.
SUMMARY OF THE INVENTION
[0010] It is accordingly a primary object of the present invention
to overcome the difficulties and avoid the inadequacies presented
by existing processes for the production of electroactive polymers.
The present invention produces electroactive polymers via
electrospinning. The induction of electroactivity via
electrospinning can be utilized with one or more soluble polymers
with polarizable moieties. Suitable polymer classes include but are
not limited to polyimides, polyamides, vinyl polymers,
polyurethanes, polyureas, polythioureas, polyacrylates, polyesters,
and biopolymers. The polyimides include but are not limited to
2,6-bis(3-aminophenoxy)benzonitrile
((.beta.-CN)APB)/4,4'oxydiphthalic anhydride (ODPA)
((.beta.-CN)APB-ODPA) and an amorphous polyimide such as amorphous
polyetherimide (such as the commercially available Ultem.RTM.). The
polyamides include but are not limited to odd-numbered nylons. The
vinyl polymers include but are not limited to PVDF, PVDF/TrFE
(copolymer of vinylidene fluoride and trifluoroethylene),
poly(vinyl alcohol) (PVA), a graft elastomer such as that claimed
in U.S. Pat. No. 6,515,077, and vinyl copolymers. The polyacrylates
include but are not limited to poly(methyl methacrylate) (PMMA).
The biopolmers include but are not limited to polypeptides and
keratin. Any one or more solvents sufficient to dissolve the one or
more polymers of interest and make a spinnable solution can be
utilized. Suitable solvents include but are not limited to
N,N-Dimethylformamide (DMF), N,N-Dimethylacetamide (DMAc),
N-methylpyrrolidinone (NMP), toluene, and cosolvents including but
not limited to DMF/acetone. The polymer can be electrospun into
fiber and fibrous nonwoven mat forms. The availability of
electroactive polymer fibers and fibrous nonwoven mat will enable
many new applications for electroactive polymers.
[0011] The present invention allows for the production of
electroactive polymers using a simple set-up and a processing
method that is fast and easy to run. Additionally, electroactive
fibers can be created that are only nanometers in diameter. In situ
induction of polar phase and spontaneous dipolar orientation of
electroactive polymers by a single step electrospinning process
produces electroactive polymers from a polymer solution. The need
for direct contact or corona filed poling is eliminated, resulting
in arc-free processing. Further, nanofibers and fibrous nonwoven
mats are produced with minimal pre- and post-processing. Enabling
materials are provided for a wide expanse of applications in such
fields as aerospace, biomedical, military and environmental.
[0012] Additional objects and advantages of the present invention
are apparent from the drawings and specification which follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic of one embodiment of an
electrospinning apparatus.
[0014] FIG. 2 is a schematic of one embodiment of an
electrospinning apparatus.
[0015] FIG. 3 illustrates Differential Scanning Calorimetry (DSC)
measurements for PVDF dissolved in DMF.
[0016] FIG. 4 illustrates X-ray Diffraction (XRD) results for PVDF
dissolved in DMF.
[0017] FIG. 5 illustrates Infrared Spectroscopy (IR) results for
PVDF dissolved in DMF.
[0018] FIG. 6 illustrates Thermally Stimulated Current (TSC)
results for PVDF dissolved in DMF.
[0019] FIG. 7 illustrates the size and proportion of fibers versus
droplets produced for PVDF dissolved in DMF, for varying PVDF
concentrations.
[0020] FIG. 8 illustrates the results for voltage variation for
PVDF dissolved in DMF.
[0021] FIG. 9 illustrates results for PVDF dissolved in DMF, for
distance variation between needle and collector.
[0022] FIG. 10 illustrates results for PVDF dissolved in DMF, for
varying infusion rates.
DETAILED DESCRIPTION OF THE INVENTION
[0023] According to the process of the present invention, a
solution is provided which comprises one or more polymers having
polarizable moieties dissolved in a solution. Polymers with
polarizable moieties have asymmetric strong dipoles. Suitable
polymer classes include but are not limited to polyimides,
polyamides, vinyl polymers, polyurethanes, polyureas,
polythioureas, polyacrylates, polyesters, and biopolymers. The
polyimides include but are not limited to
2,6-bis(3-aminophenoxy)benzonitrile
((.beta.-CN)APB)/4,4'oxydiphthalic anhydride (ODPA)
((.beta.-CN)APB-ODPA) and amorphous polyimides such as amorphous
polyetherimide (such as the commercially available Ultem.RTM.). The
polyamides include but are not limited to odd-numbered nylons. The
vinyl polymers include but are not limited to PVDF, PVDF/TrFE
(copolymer of vinylidene fluoride and trifluoroethylene),
poly(vinyl alcohol) (PVA), a graft elastomer such as that claimed
in U.S. Pat. No. 6,515,077, and vinyl copolymers. The polyacrylates
include but are not limited to poly(methyl methacrylate) (PMMA).
The biopolmers include but are not limited to polypeptides and
keratin. Any one or more solvents sufficient to dissolve the one or
more polymers of interest and make a spinnable solution can be
utilized. Suitable solvents include but are not limited to
N,N-Dimethylformamide (DMF), N,N-Dimethylacetamide (DMAc),
N-methylpyrrolidinone (NMP), toluene, and cosolvents including but
not limited to DMF/acetone.
[0024] Doping the one or more polymers with inclusions, such as
nanotubes, nanofibers, and piezoceramic powders for dielectric
enhancement would be advantageous. The inclusions can be induced
prior to electrospinning using processes known in the art.
[0025] Referring now to the drawings, and more particularly to FIG.
1, an electrospinning apparatus, generally known in the art, is
referenced generally by numeral 10. A housing 110, such as a
benchtop fume hood having a ventilated shell, encloses the entire
electrospinning process, and ensures protection from hazardous
solvent fumes as well as electric fields. A high voltage supply 120
(such as that manufactured by Spellman High Voltage Electronics
Corp.) charges the polymer solution contained in a syringe (such as
that manufactured by Becton Dickinson) with a voltage in the range
of approximately 0 to 30 kilovolts (kV). The voltage can be applied
to the syringe needle 130 via an alligator clip 140 or other
suitable connection. At a predetermined distance from the needle
130 tip (generally approximately 3-10 inches), a grounded collector
150 is suspended so that the collector 150 is generally
approximately perpendicular to the needle 130. The collector can be
customized in size and shape depending on the particular morphology
and pattern desired. Grounding is required, although the collector
150 may be positioned between the needle 130 tip and ground. Any
material for the collector is suitable, including both conductive
and nonconductive materials. Examples of suitable materials include
but are not limited to glass and metals, wherein the metal may be
coated, such as with Teflon.RTM., to make material removal easier.
The glass may also be coated, such as with Indium-Tin Oxide (ITO),
for enhanced conductivity. Any material and associated coating is
suitable, as long as a potential can be provided between the needle
130 tip and the collector 150 substrate. The charge on the solution
eventually overwhelms the surface tension of the solution, and a
jet is ejected from the needle 130 tip 140 in the direction of the
collector 150. During jet travel, the solvent evaporates and the
remaining solid polymer fiber is deposited on the collector 150.
Fibers accumulate and spread on the collector 150, as long as the
needle 130 tip is continually supplied with polymer solution, such
as via a syringe infusion pump 160 (such as that manufactured by
Fisher Scientific). It is advantageous for the solution to be
infused at a selected rate automatically.
[0026] The spun product is allowed to dry until solvent-free.
Drying time should be sufficient to allow solvent evaporation to
occur. Drying may occur in a vacuum oven or at room
temperature.
[0027] In an alternate embodiment, referenced generally in FIG. 2
by the numeral 200, the electrospinning apparatus comprises a
rotating collector 210. The collector 210 is attached to base 220
and rotates about its longitudinal axis, such as via a motor 230
and shaft 240 coupling. Alternatively, the collector 210 can move
laterally along the base, such as by a lead screw 250 attached to a
second motor 260 to allow for full coverage of the collector 210.
Again, various collector 210 geometries can be utilized as needed
for particular morphology and pattern applications. Examples
include but are not limited to a rotating plate, cylinder, or
patterned substrate. Rotation can occur at steady or variable
speed, and can be utilized to induce preferential alignment of the
fibers.
[0028] An additional embodiment of the electrospinning apparatus
includes the incorporation of a heater to regulate temperature of
the electrospinning environment.
[0029] The polymer solution is electrospun to produce fibers of
approximately 10 nm to approximately 10 .mu.m in diameter, as well
as fibrous nonwoven mats of customizable size, shape, fiber
orientation, and thickness. Depending on the spun product desired,
the collector 160 is designed accordingly. Parameters in the
processing include infusion rate, applied voltage, collector's
material and rotation and/or translation speed, distance between
the needle 140 tip and collector 160, and drying time.
[0030] Parameters sufficient to form dry, uniform diameter solid
fibers are preferred, and are customized based on the polymer of
interest. The concentration of polymer in solution generally
correlates with fiber size, with lower concentrations producing
smaller diameter fibers. Additionally, the size and proportion of
fibers versus droplets generally increase with increase in
concentration. Voltage will generally be in the approximate range
of 5 to 30 kV, and is adjusted to achieve the volume and diameter
of fibers desired. Distance between the needle 140 tip and
collector 160 will generally be 3-10 inches. It is desirable to
have an infusion rate that delivers the appropriate amount of
solution, with balancing of the infusion rate and applied voltage
to maintain a pendant drop of solution at the tip of the nozzle.
Infusion rates will generally be approximately between 3 and 10
mL/hr.
[0031] The following examples are illustrative of the present
invention, and are not intended to limit the ambit thereof.
EXAMPLE 1
(PVDF/DMF)
[0032] PVDF pellets (M.sub.W 530,000, Aldrich Chemical Company,
Inc.) were dissolved into solvent DMF at a concentration of 30
weight percent (wt %) PVDF. The solutions were electrospun into
fibers using the apparatus illustrated in FIG. 2. The polymer
resins were delivered to the system using a plastic syringe (Becton
Dickinson) equipped with an 18-gauge blunt needle tip. Metered
infusion of the solution into the system was accomplished with a
digitally-controlled syringe pump (KD Scientific, model 100).
Voltage was applied to this supply system via an alligator clip
connected to the needle and to a high voltage power supply unit
(Spellman High Voltage Electronics Corporation, model CZE 1000R).
The grounded target was a rotating drum, which imparted some degree
of fiber orientation to the collected mat. For these experiments,
the infusion pump was set to deliver polymer resin at a rate of 6.0
milliliters per hour (mL/hr). Distance between the needle tip and
the grounded target was held constant at approximately 23
centimeters (cm). The applied voltage was varied at 10 kV, 15 kV,
and 20 kV, resulting in applied electric fields of 0.44 kV/cm, 0.66
kV/cm, and 0.87 kV/cm, respectively. Fiber morphology and size were
assessed by optical microscopy (OM) and scanning electron
microscope (SEM) (JEOL, model 6400).
Differential Scanning Calorimetry Analysis (DSC)
[0033] Differential scanning calorimetry (DSC) measurements were
completed using a Perkin Elmer Pyris 1 calorimeter. Film and
electrospun mat samples ranged in mass from 3.0 to 6.0 milligrams
(mg); PVDF pellet and commercial film samples were 5.0 to 11.4 mg.
The thermal program consisted of heating from -65.degree. C. to
250.degree. C. at a rate of 10.degree. C./min. Heat flow was
recorded for all samples. Melt temperature (T.sub.m), defined as
the temperature at the maximum of the endotherm, and heat of fusion
(.DELTA.H), defined as the area of the melt peak, were determined
from the first heat.
[0034] DSC analysis was used to indicate degree of crystallinity
and specific crystal form present in the PVDF samples. Table 1
summarizes the DSC results for .beta.-phase PVDF
(commercially-purchased stretched and poled film), .alpha.-phase
PVDF (commercially purchased pellets), and electrospun fibers. It
was observed that the value of the main melting point T.sub.m for
the 15 kV and 20 kV electrospun fibers was higher than that of the
pellets and closer to that of the commercial PVDF films. T.sub.m of
the 10 kV electrospun fibers was similar to that of the
.alpha.-phase pellets. This indicates that the 15 kV and 20 kV
electrospun fibers exhibit .beta.-phase. The same was true for the
.DELTA.H, where all three electrospun cases showed values
comparable to the stretched and poled .beta.-phase commercial
films. DSC results for the three spun cases are summarized in FIG.
3, where samples are identified by applied voltage during
electrospin processing. TABLE-US-00001 TABLE 1 DSC results. First
heat Material Form T.sub.m (.degree. C.) .DELTA.H (J/g) MSI Poled
.beta.-form 165.763 53.954 melt-cast, Film poled MSI Unpoled
.beta.- 168.270 59.687 melt-cast, form Film unpoled Aldrich
.alpha.-form Pellets 158.950 26.14 Pellets 30 wt % electrospun
154.260* 51.900 Aldrich fibers (10 kV) 158.123 in DMF 30 wt %
electrospun 156.099* 49.294 Aldrich fibers (15 kV) 166.084 in DMF
30 wt % electrospun 157.083* 53.360 Aldrich fibers (20 kV) 165.100
in DMF *Shoulder on lower side of T.sub.m.
[0035] As illustrated in FIG. 3, the DSC of the electrospun fibers
exhibits a shoulder on the melt peak. This shoulder present on the
low side of the melting temperature was more pronounced for the 15
kV and 20 kV cases and may indicate the co-existence of both
.alpha.-phase and .beta.-phase in varying proportion in the
electrospun fibers. Since both processing conditions and thermal
history influence melting temperature and heat of fusion of a
polymer sample, other characterization techniques (such as FTIR,
XRD and TSC below) were used to make more definitive
conclusions.
X-Ray Diffraction Analysis (XRD)
[0036] X-ray diffraction (XRD) was performed using a Philips
Analytical X'Pert Pro X-ray Diffraction System with a step size of
0.0080.degree. 2.theta. and scan step time of 120 seconds. The
range of interest for the measurements was for 2.theta. between 10
and 40.degree.. The phases present in the films were determined by
examining the characteristic absorption bands of the respective
crystalline phases.
[0037] An assessment of the degree and type of crystallinity of the
three electrospun samples using XRD data supports the findings of
the DSC data. FIG. 4 illustrates that the peaks in all four cases
(pellets, 10 kV, 15 kV, and 20 kV) suggest that both a-phase and
.beta.-phase are present in the fibers. The data also show that the
2.theta. peak location and intensity depend on polymer processing
conditions. Characteristic .alpha.-phase peaks are seen at 2.theta.
values of 18.degree., 20.degree., and 27.degree.. Characteristic
.beta.-phase peaks are located at 2.theta. values of 20.4.degree.
and 37.degree.. All three electrospun cases show a decrease in
.alpha.-phase character in favor of .beta.-phase. In the 10 kV
case, the 18.degree. peak disappeared. The peak that was at
20.degree. shifted to 20.4.degree., indicating a transformation
from .alpha. to .beta.. In comparison, the 15 kV case shows a much
smaller .beta.-phase peak at 20.4.degree., still displays a large
.alpha.-phase peak at 18.degree., and exhibits no hint of a
.beta.-peak at 37.degree.. Furthermore, none of the electrospun
cases exhibit the characteristic .alpha.-phase peak 27.degree..
[0038] XRD data clearly confirm the DSC results. The presence of a
shoulder on the low end of the melt peak seen in DSC corresponds to
the finding by XRD that a second crystalline form is present in the
electrospun fibers. This shoulder is attributed to .beta.-form. In
mixed systems, .beta. form has smaller reflections than
.alpha.-form, which are dominant, so detecting small amounts of
.beta.-form becomes difficult.
Fourier Transform Infrared Spectroscopic Analysis (FTIR)
[0039] Infrared spectroscopy (IR) was performed on the samples
using a ThermoNicolet IR 300 Spectrometer. Data analysis was
performed with Omnic version 6.2 software. The measurements were
taken from 600-1500 wavenumbers (cm.sup.-1) with 128 scans
performed per sample. Data analysis consisted of qualitative visual
comparison of intensities of characteristic transmittance peaks for
PVDF crystalline phases.
[0040] FIG. 5 further confirms that electrospinning induces
formation of the .beta.-phase in PVDF and suggests orientation of
the .beta.-phase. Characteristic .alpha.-phase peaks at 614, 762,
795, and 975 cm.sup.-1 are evident in only the .alpha.-phase and
unpoled .beta.-phase film samples. For clarity, peaks at 762, 795,
and 975 cm.sup.-1 are detailed in the insets. .alpha.-phase peaks
are absent in all three electrospun cases. Strong peaks at 840 and
1280 cm.sup.-1, which indicate .beta.-phase PVDF, are present in
all samples except the .alpha.-phase film sample. These peaks are
nearly as strong for the electrospun cases as they are for the
poled .beta.-phase film sample. The similarity between the poled
.beta.-phase film and all three fiber cases suggests that
electrospin processing causes orientation of the poled
.beta.-phase. Note that evidence of some .alpha.-phase in the
unpoled .beta.-phase film sample is unexpected but indicates
incomplete conversion from .alpha. to .beta. phase during
stretching.
Thermally Stimulated Current Analysis (TSC)
[0041] A SETARAM TSC 3000, automated Thermally Stimulated Current
(TSC) equipment, was used to track the relaxation processes in the
polymeric films and fibers. Heating of the sample at a constant
rate accelerates the real charge decay, which can be observed as a
current release. The current as a function of temperature was
measured by a sensitive electrometer connected to the electrodes.
No additional poling was performed on any of the samples between
initial electrospin processing and TSC analysis. All samples were
subjected to the same cycle. The samples were heated from
25.degree. C. to 150.degree. C. at a rate of 2.degree. C./min.
[0042] Thermally Stimulated Current (TSC) measures the release of
stored dielectric polarization in the form of charge or current.
Because piezoelectricity in PVDF arises from orientation
polarization of the --CF.sub.2-- dipoles in the polar phase, TSC
analysis can be used to reveal the presence of this orientation
polarization current peak. The peak was shown to shift from
90.degree. C. to 130.degree. C. as more perfect and thermally
stable crystallites are formed [13]. A second peak, due to space
charge release, typically occurs at higher temperatures.
[0043] FIG. 6 shows the depolarization current spectrum of the
electrospun 15 kV fibers next to the current spectrum of a
stretched, poled PVDF film (MSI). The depolarization current
spectrum of the 15 kV sample is consistent with poled and stretched
PVDF films. The peak centered at 120.degree. C. is consistent with
orientation polarization in .beta.-phase PVDF, and is due to
relaxation of dipoles in the crystalline regions. A second peak
centered around 145.degree. C. is most likely due to space charge,
coupled with onset of crystallite melting.
[0044] To assess the potential piezoelectricity of the 15 kV
fibers, the pyroelectric coefficient, p, was calculated using the
following equation: p = I A .times. d T d t ##EQU1## where I is the
depolarization current, A is the area of the electrode, and dT/dt
is the heating rate during TSC. The pyroelectric coefficient was
measured with respect to current released up to 140.degree. C.,
since the peak of interest is the orientation polarization peak
located below this temperature. The measured pyroelectric
coefficient was 1.5.times.10.sup.-5 C./.degree. C.-m.sup.2 for the
15 kV fibers. Values for stretched, poled PVDF are typically in the
range of 2.0-3.5.times.10.sup.-5 C./.degree. C.-m.sup.2.
EXAMPLE 2
[0045] PVDF solutions in the solvent DMF with and without single
wall nanotubes (SWNTs) were electrospun at various concentrations,
as outlined in Table 2. PVDF pellets (M.sub.W 530,000, Aldrich
Chemical Company, Inc.) were dissolved into DMF at a concentration
of 30 weight percent (wt %) PVDF. A SWNT stock solution (1% w/w) in
DMF was prepared to mix with PVDF/DMF solution to prepare various
compositions of SWNT/PVDF/DMF solutions. The solutions were
electrospun into fibers using the apparatus illustrated in FIG. 2.
The polymer resins were delivered to the system using a plastic
syringe (Becton Dickinson) equipped with an 18-gauge blunt needle
tip. Metered infusion of the solution into the system was
accomplished with a digitally-controlled syringe pump (KD
Scientific, model 100). Voltage was applied to this supply system
via an alligator clip connected to the needle and to a high voltage
power supply unit (Spellman High Voltage Electronics Corporation,
model CZE 1000R). The grounded target was a rotating drum, which
imparted some degree of fiber orientation to the collected mat. For
these experiments, the infusion pump was set to deliver polymer
resin at a rate of 6.0 milliliters per hour. Distance between the
needle tip and the grounded target was held constant at
approximately 23 centimeters (cm). The applied voltages were 10 kV,
15 kV, and 20 kV, resulting in applied electric fields of 0.44
kV/cm, 0.66 kV/cm, and 0.87 kV/cm, respectively. Fiber morphology
and size were assessed by optical microscopy (OM) and scanning
electron microscope (SEM) (JEOL, model 6400). TABLE-US-00002 TABLE
2 Concentrations in wt % of solutions in DMF PVDF SWNT Wt % WT % 15
0 20 0 25 0 30 0 35 0 15 0.1 15 1.0 25 0.1 25 0.2
[0046] The varying concentrations of PVDF and PVDF with SWNT shown
in Table 2 were spun onto Indium-Tin Oxide (ITO) coated glass
plates and standard glass microscope slides utilizing the
electrospinning apparatus illustrated in FIG. 1. Numerous trials
were competed with varying concentrations of PVDF and SWNTs.
Voltage, distance between nozzle and grounding plate, infusion
rate, and collection medium type were varied.
[0047] The 30 wt % PVDF was also spun onto a rotating roller to
produce an oriented fibrous mat, using the apparatus of FIG. 2. The
roller was a hollow metal cylinder fixed on an axis that rotated at
varying speeds. The cylinder was moved from side to side as well,
to ensure complete coverage of the length of the roller.
[0048] Once spun, on either plates or the roller, optical
microscopy was completed at 100.times. and 200.times. magnification
using an Olympus BH-2 optical microscope in conjunction with Scion
Image, v. 1.62, software. These images were utilized to compare and
observe trends as a function of electrospinning parameters.
[0049] Mats spun on a roller were dried in a vacuum oven at
60.degree. C. overnight or over a weekend. The vacuum oven removed
any remaining DMF solvent from the fibers. Solvent vapors were
allows to flow over dry ice, were condensed into liquid form, and
were then contained in a liquid trap. Once dried, the PVDF mats
were prepared for Differential Scanning Calorimetry (DSC),
Thermogravimetric Analysis (TGA), and Dynamic Mechanical Analysis
(DMTA) by cutting to appropriate size.
[0050] The concentration of PVDF in DMF solution correlated with
fiber size. The lowest concentration, 15 wt %, produced wet fibers
with a small diameter. The size and proportion of fibers versus
droplets produced increased as concentration was increased, as
shown in FIG. 7. It was determined that a concentration of 30 wt %
PVDF produced the best quality fibers as determined by desirable
morphological features such as the fiber dryness and the number of
fibers present as compared to solution droplets. FIG. 7 also
indicates that the substrate material had little effect on fiber
formation.
[0051] Various electrospinning parameters were examined for the 30
wt % PVDF in DMF solution to determine the optimal conditions.
Voltage was varied from 5 kilovolts (kV) to 30 kV. As is
illustrated in FIG. 8, voltage affected the amount of fibers spun,
the amount of solvent that was splattered on the slide, and the
diameter of the fibers. Optimum fibers were produced at 10 kV.
[0052] FIG. 9 illustrates results for distance variation. Distance
was varied from 5 to 13 inches. When the collecting plate was close
to the syringe, the slides and the fibers growing off of the edges
of the slides were wet. Solution splattering was also evident with
short distances. As the distance was increased, the plate and
fibers were less wet and splattering was minimized. In addition,
fibers were thick and prominent. As a maximum distance of 13 inches
between collecting plate and nozzle was exceeded, fewer fibers
attached to the collecting plate; instead attaching to some other
surface in the hood.
[0053] Infusion rate was not a dramatic factor in determining fiber
morphology. However, in choosing an infusion rate, it was desirable
to have a rate that delivered the appropriate amount of solution.
The infusion rate must be balanced with applied voltage to maintain
a pendant drop of solution at the tip of the nozzle. In addition,
the appropriate infusion rate varied with solution viscosity. The
infusion rate was varied from 3 to 10 mL/hr for 30 wt % PVDF. As
can be seen in FIG. 10, an infusion rate between 6 mL/hr and 9
mL/hr produced fibers that did not exhibit excessive amounts of wet
solution droplets. If infusion rate exceeded 9 mL/hr, the solution
dripped out of the needle. Furthermore, if the infusion rate
exceeded 6 mL/hr, the fibers produced were wet. For these reasons,
6 mL/hr was an optimal infusion rate.
[0054] Optimal conditions for 30 wt % PVDF were found to be a
voltage of 10 kV, an infusion rate of 6 mL/hr, and a distance of 9
inches from the needle point to the collecting plate.
EXAMPLE 3
[0055] PVDF combined with carbon nanotubes was tested beginning
with 0.1% SWNT in 15 wt % PVDF and 1.0% SWNT in 15 wt % PVDF. Small
and few fibers formed. When the 1.0% SWNT was electrospun, black
drops fell out of the solution as a result of the excessive
concentration of SWNTs. A 0.1 % SMNT in 25 wt % PVDF spun well and
was able to be spun onto a rotating roller, producing a nonwoven
mat which was vacuum-dried at 60.degree. C. A nonwoven mat was also
produced from 0.2% SWNT in 25 wt % PVDF. Optimal conditions for the
PVDF solutions with carbon nanotubes were slightly different
depending on amount of carbon nanotubes and concentration of
PVDF.
[0056] Although the present invention has been described relative
to specific embodiments thereof, there are numerous variations and
modifications that will be readily apparent to those skilled in the
art in light of the above teachings. It is therefore to be
understood that, within the scope of the appended claims, the
invention may be practiced other than as specifically
described.
* * * * *