U.S. patent application number 11/141205 was filed with the patent office on 2006-01-26 for field-responsive superparamagnetic composite nanofibers and methods of use thereof.
Invention is credited to T. Alan Hatton, Gregory C. Rutledge, Harpreet Singh, Mao Wang.
Application Number | 20060019096 11/141205 |
Document ID | / |
Family ID | 35463487 |
Filed Date | 2006-01-26 |
United States Patent
Application |
20060019096 |
Kind Code |
A1 |
Hatton; T. Alan ; et
al. |
January 26, 2006 |
Field-responsive superparamagnetic composite nanofibers and methods
of use thereof
Abstract
The present invention relates to magnetic field-responsive
fibers, which comprise magnetite particles and a polymeric matrix.
The invention also provides methods of producing the same, in
particular via electrospinning of a stably dispersed or
monodispersed polymer solution, either aqueous or organic,
comprising the magnetite particles, and applications thereof.
Inventors: |
Hatton; T. Alan; (Sudbury,
MA) ; Rutledge; Gregory C.; (W. Newton, MA) ;
Singh; Harpreet; (Cambridge, MA) ; Wang; Mao;
(Malden, MA) |
Correspondence
Address: |
PEARL COHEN ZEDEK, LLP
10 ROCKEFELLER PLAZA
SUITE 1001
NEW YORK
NY
10020
US
|
Family ID: |
35463487 |
Appl. No.: |
11/141205 |
Filed: |
June 1, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60575423 |
Jun 1, 2004 |
|
|
|
Current U.S.
Class: |
428/364 |
Current CPC
Class: |
D01F 1/10 20130101; Y10T
428/2913 20150115; D01F 6/16 20130101; D01D 5/0038 20130101; D01F
6/14 20130101; D01D 5/0007 20130101 |
Class at
Publication: |
428/364 |
International
Class: |
D02G 3/00 20060101
D02G003/00 |
Goverment Interests
GOVERNMENT INTEREST STATEMENT
[0002] This invention was made in whole or in part with government
support under Contract DAAD-19-02-D0002 awarded by the United
States Army through the Institute for Soldier Nanotechnologies, The
government may have certain rights in the invention.
Claims
1. A superparamagnetic fiber comprising magnetite particles and a
polymeric matrix.
2. The superparamagnetic fiber of claim 1, wherein said fiber is a
nanofiber.
3. The superparamagnetic fiber of claim 2, wherein said nanofiber
is less than 500 nm in diameter.
4. The superparamagnetic fiber of claim 2, wherein said nanofiber
diameter ranges -from 10 nm-1 .mu.m.
5. The superparamagnetic fiber of claim 1, wherein said matrix
comprises polyethylene oxide, polyvinyl alcohol or a combination
thereof.
6. The superparamagnetic fiber of claim 1, wherein said polymeric
matrix comprises a polysaccharide, an oligosaccharide, a
surfactant, a polyethylene glycol, a lignosulfonate, a
polyacrylamide, a polypropylene oxide, a cellulose derivative a
polyacrylic acid or a combination thereof.
7. The superparamagnetic fiber of claim 1, wherein said polymeric
matrix comprises any polymer that can be electrospun from
solution.
8. The superparamagnetic fiber of claim 1, wherein said
superparamagnetic fiber is magnetic field-responsive.
9. The superparamagnetic fiber of claim 1, wherein said magnetite
nanoparticles are stably dispersed within said polymeric
matrix.
10. A device or apparatus comprising the superparamagnetic fiber of
claim 1.
11. The device or apparatus of claim 10, wherein said device or
apparatus is used as a filter or a sensor.
12. The device or apparatus of claim 10, wherein said device or
apparatus is used for information storage.
13. The device or apparatus of claim 10, wherein said device or
apparatus is used for magnetic imaging.
14. The device or apparatus of claim 10, wherein said device or
apparatus is used for magnetic shielding.
15. The device or apparatus of claim 10, wherein said device or
apparatus is used as a tunable mechanical reinforcement component
in a composite
16. The device or apparatus of claim 10, wherein said device or
apparatus is used as a piezomagnetic transducer
17. A fabric comprising the superparamagnetic fiber of claim 1,
wherein said fabric may be woven or nonwoven.
18. A field-responsive fiber comprising ferromagnetic nanoparticles
and an organic polymeric matrix.
19. The fiber of claim 18, wherein said fiber is a nanofiber.
20. The fiber of claim 19, wherein said nanofiber has a diameter
ranging from 10-500 nm.
21. The fiber of claim 18, wherein said matrix comprises polymethyl
methacrylate.
22. The fiber of claim 18, wherein said nanoparticles are
monodispersed within said polymeric matrix.
23. The fiber of claim 18, wherein said fiber has a high saturation
magnetization, ranging from 250 kA/m to 2000 kA/m.
24. The fiber of claim 18, wherein said fiber has a tunable Neel
relaxation time which ranges from 2 milliseconds to 4 seconds.
25. The fiber of claim 24, wherein said tunable Neel relaxation
time is a function of nanoparticle size.
26. A device, apparatus or fabric comprising the fiber of claim
18.
27. A method of producing a field-responsive fiber comprising
magnetite particles and a polymeric matrix, the method comprising
the step of electrospinning a polymer solution comprising magnetic
nanoarticles.
28. The method of claim 27, wherein said field-responsive fiber is
a nanofiber.
29. The method of claim 27, wherein said nanofiber is less than 500
nm in diameter.
30. The method of claim 29, wherein said nanofiber diameter ranges
from 10 nm-1 .mu.m.
31. The method of claim 27, wherein said field-responsive fiber is
superparamagnetic.
32. The method of claim 31, wherein said polymer solution comprises
polyethylene oxide.
33. The method of claim 32, wherein said polyethylene oxide is at a
concentration of between 1% and 3% by weight.
34. The method of claim 33, wherein said polymer solution has a
conductivity of between 0.1 and 10000 .mu.S/cm.
35. The method of claim 31, wherein said polymer solution comprises
polyvinyl alcohol.
36. The method of claim 35, wherein said polyvinyl alcohol is at a
concentration of between 6.5% and 15% by weight.
37. The method of claim 31, wherein said polymer solution comprises
SDS.
38. The method of claim 31, wherein said polymer solution comprises
a polysaccharide, an oligosaccharide, a surfactant, a polyethylene
glycol, a lignosulfonate, a polyacrylamide, a polypropylene oxide,
a cellulose derivative a polyacrylic acid or a combination
thereof.
39. The method of claim 31, wherein said polymer solution is an
aqueous solution.
40. The method of claim 27, wherein said field-responsive fiber is
ferromagnetic.
41. The method of claim 40, wherein said matrix comprises
polymethyl methacrylate.
42. The method of claim 40, wherein said nanoparticles are
monodispersed within said polymeric matrix.
43. The method of claim 40, wherein said fiber has a high
saturation magnetization, ranging from 250 kA/m to 2000 kA/m.
44. The method of claim 40, wherein said fiber has a tunable Neel
relaxation time which ranges from 2 milliseconds to 4 seconds.
45. The method of claim 44, wherein said tunable Neel relaxation
time is a function of nanoparticle size.
46. The method of claim 40, wherein said polymer solution is an
organic solution.
47. The method of claim 27, wherein said polymer solution comprises
any polymer that can be electrospun from solution.
48. A superparamagnetic fiber produced by the method of claim 27.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This Application claims the benefit of U.S. Provisional
Application No. 60/575,423, filed Jun. 1, 2004, which is hereby
incorporated it its entirety.
FIELD OF THE INVENTION
[0003] The present invention relates to field-responsive, composite
nanofibers, methods of producing the same, and applications
thereof. The present invention has wide application in such fields
as magnetic filters, sensors, information storage, magnetic
shielding, tunable composites, magnetic separation, SMART fabrics
and piezomagnetic transducers.
BACKGROUND OF THE INVENTION
[0004] Magnetic composite fibers, in which magnetic nanoparticles
are embedded into a polymeric fiber matrix, can be expected to
exhibit interesting magnetic field-dependent mechanical behavior
with potential applications in a range of areas. Magnetic composite
fibers, with particles of ferromagnetic materials such as iron
oxide below approximately 100 nm in diameter would, in theory, no
longer exhibit the cooperative phenomenon of ferromagnetism found
in the bulk, due to thermal fluctuations sufficient to reorient the
magnetization direction of entire particles and instead, might be
superparamagnetic, exhibiting strong paramagnetic properties with
large susceptibility. The relative magnitudes of the stiffness
enhancement and fiber deformation by such fibers are expected to
increase as the diameter of the embedding polymer fiber is reduced,
and therefore, to date, production of magnetic composite nanofibers
(i.e. with diameters on the order of 100 nm or less) with
superparamagnetic properties, with defined mechanical properties,
has not been achieved.
[0005] Electrospinning is an effective method for the production of
polymeric nanofibers with diameters ranging from a few nanometers
to a few micrometers. This technique has attracted interest over
the last decade due to potential applications for nanofibers
numerous applications. In a typical electrospinning process, a
polymer solution or melt is extruded through a capillary and, in
the presence of a strong electric field, deforms, resulting in
ejection of a charged jet from the apex of the cone, which is
accelerated toward a grounded collecting device, traveling first as
a steady jet for a certain distance, and then in some instances
undergoing an electrostatically driven whipping instability that
bends and stretches the jet. The result of the whipping instability
is a dramatic reduction in the diameter of the jet, typically by
about 2 orders of magnitude, which allows for rapid solidification
of the jet through solvent evaporation (for solution) or cooling
(for melts). The solid fibers are deposited on an electrically
grounded collecting device in the form of threads or as a non-woven
fabric.
[0006] The incorporation of nanoparticles into nanofibers by
electrospinning a nanoparticle-filled polymer solution, however,
necessitates dispersion stability of the nanoparticles in the
polymer solution.
SUMMARY OF INVENTION
[0007] In one embodiment, this invention provides a
superparamagnetic fiber comprising magnetite particles and a
polymeric matrix. In one embodiment, the fiber is a nanofiber,
which in another embodiment is less than 500 nm in diameter, or in
another embodiment, the nanofiber has a diameter that ranges from
10 nm-1 .mu.m.
[0008] In one embodiment, the matrix comprises polyethylene oxide,
polyvinyl alcohol or a combination threof. In another embodiment,
the superparamagnetic fiber is magnetic field-responsive.
[0009] In another embodiment, this invention provides a
field-responsive fiber comprising ferromagnetic nanoparticles and
an organic polymeric matrix. According to this aspect, and in one
embodiment, the fiber is a nanofiber, and in another embodiment,
has a diameter ranging from 10-500 nm. In another embodiment, the
organic polymeric matrix comprises polymethyl methacrylate.
[0010] In another embodiment, according to this aspect of the
invention, the nanoparticles are monodispersed within said
polymeric matrix. In another embodiment, the fiber has a high
saturation magnetization, ranging from 250 kA/m to 2000 kA/m, In
another embodiment, the fiber has a tunable Neel relaxation time
which ranges from 2 milliseconds to 4 seconds, or in another
embodiment, the tunable Neel relaxation time is a function of
nanoparticle size.
[0011] In another embodiment, this invention provides a device or
apparatus comprising a field-responsive or superparamagnetic fiber
of this invention. In one embodiment, the device or apparatus is
used as a filter or a sensor, or in another embodiment, is used for
information storage, or in another embodiment, is used for magnetic
imaging, or in another embodiment, is used for magnetic shielding.
In another embodiment, this invention provides a fabric comprising
a fiber of this invention, which in another embodiment, is a SMART
fabric.
[0012] In another embodiment, this invention provides a method of
producing a field-responsive fiber comprising magnetite particles
and a polymeric matrix, the method comprising the step of
electrospinning a polymer solution comprising magnetic
nanoparticles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 demonstrates a DLS curve for a 2.5 wt % magnetite
nanoparticle solution.
[0014] FIG. 2 is a micrograph obtained by transmission electron
microscopy of magnetite fluid.
[0015] FIG. 3 is a plot of magnetization (M) versus magnetic field
(H) for 2.5 wt % as-synthesized magnetite fluid.
[0016] FIG. 4 demonstrates some representative SEM images of PEO
and PEO/magnetite nanofibers: (a) PEO (1%), (b) PEO
(1%)+Fe.sub.3O.sub.4 (3.52%), (c) PEO (2%), PEO
(2%)+Fe.sub.3O.sub.4 (0.75%).
[0017] FIG. 5 demonstrates some representative SEM images of PVA
and PVA/magnetite nanofibers: (a) PVA (7.5%), (b) PVA
(7.5%)+Fe.sub.3O.sub.4 (0.75%), (c) PVA (7.5%)+SDS (1%), (d) PVA
(7.5%)+SDS (1%)+Fe.sub.3O.sub.4 (0.75%).
[0018] FIG. 6 demonstrates TEM images of superparamagnetic
nanofibers. (a) PEO nanofiber with 28 wt % magnetite nanoparticles.
(b) PVA nanofiber with 8 wt % magnetite nanoparticles.
[0019] FIG. 7 demonstrates magnetization curves of
superparamagnetic nanofibers: (a) PEO nanofiber with (28 wt %)
magnetite nanoparticles, (b) PVA nanofiber with (8 wt %) magnetite
nanoparticles.
[0020] FIG. 8 schematically depicts a tip-sample interaction during
an indentation test.
[0021] FIG. 9 demonstrates indentation curves for PVA/magnetite (8
wt %) nanofiber: (a) calibration on hard surface (mica), cantilever
bending without indentation; (b) indentation curve on PVA/magnetite
nanofiber, cantilever bending and indent; (c) indentation curve on
PVA/magnetite after subtracting the cantilever bending.
[0022] FIG. 10 demonstrates field responsive behaviors of
PVA/magnetite fabric (a) without magnetic field, (b) within a low
gradient of magnetic field, (c) within a high gradient of magnetic
field.
[0023] FIG. 11 demonstrates TEM images of magnetite nanoparticles.
The nanoparticles are 8, 14 and 16 nm in size, respectively (left
to right).
[0024] FIG. 12 demonstrates a representative SEM image of PMMA
fiber containing 37 wt % of 16 nm magnetite nanoparticles.
[0025] FIG. 13 demonstrates magnetization curves of PMMA fiber
containing 37 wt % of 16 nm magnetite nanoparticles.
DETAILED DESCRIPTION OF THE INVENTION
[0026] In one embodiment, this invention provides a
field-responsive fiber, comprising nanoparticles and a polymeric
matrix.
[0027] In one embodiment, the nanoparticles range in size from
about 4 nm to about 100 nm. In one embodiment, the nanoparticles
are magnetic and comprise iron, oxides of iron, cobalt, oxides of
cobalt, alloys of iron and cobalt, platinum, alloys of iron and
platinum, alloys of cobalt and platinum, manganese oxide, alloys of
manganese and iron or alloys of nickel and iron or nickel and
cobalt.
[0028] In one embodiment, the nanoparticles which comprise the
fibers of this invention can be synthesized by an organic route or,
in another embodiment, by an aqueous route, as exemplified herein,
and as will be appreciated by one skilled in the art.
[0029] In one embodiment, synthesis of the nanoparticles via an
aqueous route may comprise the steps of co-precipitating metal
salts at high pH (14) in the presence of a stabilizing polymer. The
stabilizing polymer has carboxylic moieties, in some
embodiments.
[0030] The present invention provides, in one embodiment a
field-responsive, superparamagnetic fiber comprising magnetite
particles and a polymer matrix. As exemplified herein, and
representing some embodiments of the invention, the
polymer/magnetite nanofibers exhibited superparamagnetic behavior
at room temperature, and deflected in the presence of an applied
magnetic field.
[0031] In one embodiment, this invention provides a
superparamagnetic fiber comprising magnetite particles, which in
one embodiment, refers to an iron ore that is strongly attracted by
a magnet. In one embodiment "magnetite" refers to a molecule with a
general formula of Fe.sub.3O.sub.4, which in another embodiment,
possesses a Fe.sup.2+ to Fe.sub.3+ ratio of about 1:1.5 to about
1:2.5, or in another embodiment, about 1:2.
[0032] In another embodiment, the superparamagnetic nanofibers may
comprise particles which are chemical equivalents of magnetite,
such as, for example, and in one embodiment, (Fe,M)OFe.sub.20.sub.3
where M may be, in one embodiment, Zn, Co, Ni, Mn, or Cr. In
another embodiment, the Fe.sup.2+ to Fe.sup.3+ ratio includes any
ratio that permits the formation of the superparamagnetic fibers of
the present invention.
[0033] In another embodiment, the concentration of magnetite in
suspsension is 2.5 wt %. In another embodiment, the concentration
of magnetite in suspension is 0.75 wt %. In another embodiment, the
concentration of magnetite in suspsension is 0.75-50 wt %, or in
another embodiment, the concentration of magnetite in suspsension
is 0.75-2.5 wt %, or in another embodiment, the concentration of
magnetite in suspsension is 0.75-5.0 wt %, or in another
embodiment, the concentration of magnetite in suspsension is
0.75-10 wt %, or in another embodiment, the concentration of
magnetite in suspsension is 0.75-15 wt %, or in another embodiment,
the concentration of magnetite in suspsension is 0.75-20 wt %, or
in another embodiment, the concentration of magnetite in
suspsension is 0.75-25 wt %, or in another embodiment, the
concentration of magnetite in suspsension is 0.75-30 wt %, or in
another embodiment, the concentration of magnetite in suspsension
is 0.75-35 wt %, or in another embodiment, the concentration of
magnetite in suspsension is 0.75-40 wt %, or in another embodiment,
the concentration of magnetite in suspsension is 0.75-50 wt %, or
in another embodiment, the concentration of magnetite in
suspsension is 2.5-10 wt %, or in another embodiment, the
concentration of magnetite in suspsension is 2.5-20 wt %, or in
another embodiment, the concentration of magnetite in suspsension
is 2.5-25 wt %, or in another embodiment, the concentration of
magnetite in suspsension is 2.5-30 wt %, or in another embodiment,
the concentration of magnetite in suspsension is 2.5-40 wt %, or in
another embodiment, the concentration of magnetite in suspsension
is 2.5-50 wt %, or in another embodiment, the concentration of
magnetite in suspsension is 10-20 wt %, or in another embodiment,
the concentration of magnetite in suspsension is 10-30 wt %, or in
another embodiment, the concentration of magnetite in suspsension
is 10-40 wt %, or in another embodiment, the concentration of
magnetite in suspsension is 10-50 wt
[0034] The superparamagnetic fibers of this invention comprise, in
one embodiment, magnetite particles and a polymer matrix. In one
embodiment, the polymers comprising the polymer matrix of this
invention may be copolymers. In another embodiment, the polymers
may be homo- or, in another embodiment heteropolymers. In another
embodiment, the polymers may be synthetic, or, in another
embodiment, natural polymers. In another embodiment, the polymers
may be water-soluble. In another embodiment, the polymers
comprising the polymer matrix of this invention may be free radical
random copolymers, or, in another embodiment, graft copolymers. In
one embodiment, the polymers may comprise polysaccharides,
oligosaccharides, proteins, peptides or nucleic acids. It is to be
understood that any polymers, which may be utilized to produce a
superparamagnetic fiber of this invention, such as, any material
that may be electrospun into a fiber, including, in other
embodiments, any natural or synthetic polymer, are to be considered
as part of this invention.
[0035] In one embodiment, the choice of polymer utilized may be a
function of the particles employed. In one embodiment, the polymer
may comprise polyacrylic acid, polystyrene sulfonic acid, polyvinyl
sulfonic acid, polyethylene oxide polypropylene oxide, polyvinyl
alcohol, or a combination thereof.
[0036] In another embodiment, the polymer comprises a surfactant, a
polyethylene glycol, a lignosulfonate, a polyacrylamide or a
biopolymer. In another embodiment, the biopolymer may comprise
polypeptides, cellulose and its derivatives such as hydroxyethyl
cellulose and carboxymethyl cellulose, alginate, chitosan, lipid,
dextan, starch, gellan gum or other polysaccharides, or a
combination thereof.
[0037] In another embodiment, the polymer comprises polyethylene
oxide at a concentration of 2-4 wt %. In another embodiment, the
polymer comprises polyacrylic acid, at a concentration of 6.5-15 wt
%. In another embodiment, the polymeric matrix may comprise
poyacrylic acid and SDS. In another embodiment, the SDS or other
similar ionic surfactant may be at a concentration of 0.5-10 wt
%
[0038] According to this aspect of the invention, and in another
embodiment, the molecular weight of the polyacrylic acid may be
5,000 Da, or in another embodiment, 5,000-20,000 Da. In one
embodiment, the PAA and Jeffamine are used during nanoparticle
synthesis to form a "corona" on the magnetite particles that allows
them to be suspended in solution and stabilizes them against
aggregation. In another embodiment, magnetite particles may be
similarly prepared, via methods known to one in the art, to form
stable suspensions in solution.
[0039] In another embodiment, this invention provides a
field-responsive fiber comprising ferromagnetic nanoparticles and
an organic polymeric matrix. In another embodiment, according to
this aspect of the invention, the nanoparticles are monodispersed
within said polymeric matrix.
[0040] The ferromagnetic nanoparticles can exhibit a spontaneous
magnetization, and may comprise Fe, Co, Ni, Gd, Dy, MnAs, MnBi,
MnSb, CrO.sub.2, MnOFe.sub.2O.sub.3, FeOFe.sub.2O.sub.3,
NiOFe.sub.2O.sub.3, CuOFe.sub.2O.sub.3, MgOFe.sub.2O.sub.3, EuO,
Y.sub.3Fe.sub.5O.sub.12.
[0041] In one embodiment, the organic route synthesis uses
organometallic precursors decomposed at a high temperature, which
in one embodiment, is at a range of between about 200-400.degree.
C., in a high boiling point organic liquid. In one embodiment, the
organic liquide is a benzyl ether, or in another embodiment, a
phenyl ether, or in another embodiment, octanol, or others, as will
be appreciated by one skilled in the art. In one embodiment, the
process is conducted in the presence of stabilizers like oleic acid
and olylamine. According to this aspect of the invention, and in
one embodiment, the method for preparing the nanoparticles via an
organic route produces monodisperse magnetic nanoparticles.
[0042] In one embodiment, the nanoparticles thus prepared result in
particles relatively small in size. In one embodiment, seed
mediated growth can be used to synthesize larger nanoparticles, in
which the smaller nanoparticles synthesized can be used as seeds in
a subsequent synthesis of larger-sized nanoparticles.
[0043] According to this aspect, and in one embodiment, the
functionalization of the nanoparticles with an organic surface
coating provides nanoparticles compatible with organic solvents. In
one embodiment, the organic route synthesis permits a wide range of
particle compositions, including those with larger intrinsic
magnetic moments, which, in another embodiment, provide a longer
Neel relaxation time. This was exemplified herein, in Example 6 via
SQUID test, where the remnant magnetization at zero field for the
16 nm particles indicated ferromagnetic behavior, rather than the
superparamagnetic behavior exhibited by the smaller particles
synthesized via aqueous route.
[0044] In one embodiment, a longer Neel relaxation time allows for
changes in mechanical properties under a uniform applied field at
conventional rates of deformation. Due to coupling of the particle
magnetic moment with the applied field, deformation of the magnetic
fibers requires additional work, resulting in increased stiffness
and lower strain, compared to the equivalent nonmagnetic fibers at
equal deformation energy.
[0045] According to this aspect of the invention, and in one
embodiment, the fiber has a high saturation magnetization, ranging
from 250 kA/m to 2000 kA/m. In another embodiment, the fiber has a
tunable Neel relaxation time which ranges from 2 milliseconds to 4
seconds, or in another embodiment, the tunable Neel relaxation time
is a function of nanoparticle size.
[0046] In one embodiment, the fibers of this invention are formed
from a solution whose concentration of polymer may range from
0.5-40 wt %, or in another embodiment, the concentration ranges
from 2-20 wt %, or in another embodiment, the concentration ranges
from 5-15 wt %, or in another embodiment, the concentration ranges
from 6.5-15 wt %. In one embodiment, the polymer concentration will
be a function of the chemistry, molecular weight, or combination
thereof of the polymer and/or solvent used.
[0047] In one embodiment, the fibers of this invention are
nanofibers. In one embodiment, the nanofiber is less than 500 nm in
diameter. In another embodiment, the nanofiber diameter ranges from
10nm-1 .mu.m. In one embodiment, the nanofiber diameter ranges from
65-100 nm, or, in another embodiment, the nanofiber diameter ranges
from 65-200 nm, or, in another embodiment, the nanofiber diameter
ranges from 65-300 nm, or, in another embodiment, the nanofiber
diameter ranges from 65-400 nm, or, in another embodiment, the
nanofiber diameter ranges from 65-200 nn, or, in another
embodiment, the nanofiber diameter ranges from 100-200 nm, or, in
another embodiment, the nanofiber diameter ranges from 150-250 nm,
or, in another embodiment, the nanofiber diameter ranges from
100-300 nm, or, in another embodiment, the nanofiber diameter
ranges from 100-400 nm, or, in another embodiment, the nanofiber
diameter ranges from 100-250 nm, or, in another embodiment, the
nanofiber diameter ranges from 200-300 nm, or, in another
embodiment, the nanofiber diameter ranges from 200-350 nm, or, in
another embodiment, the nanofiber diameter ranges from 200-450 nm,
or, in another embodiment, the nanofiber diameter ranges from
10-200 nm, or, in another embodiment, the nanofiber diameter ranges
from 75-500 nm. In another embodiment, the nanofiber diameter
ranges from 10 nm-10 .mu.m. In another embodiment, the nanofiber
has a diameter that is less than 10 nm.
[0048] In one embodiment, the magnetite nanoparticles produced via
aqueous route are stably dispersed within the polymeric matrix. In
one embodiment, the term "stably dispersed" or "stabilized" or
"stabilization" refers to the stability of the resulting polymer
solution or matrix, following their production. In one embodiment,
the terms "stably dispersed" or "stabilized" or "stabilization"
refer to the fact that the magnetite particles do not aggregate or
"settle out" in solution, or in another embodiment, are readily
dispersed, such as, via vortexing in solution. In another
embodiment, the magnetite nanoparticles dispersed within the
polymeric matrix are "colloidally-stable". In one embodiment, the
term "colloidally-stable" refers to refer to the fact that the
magnetite particles do not aggregate or "settle out" in solution,
or in another embodiment, form a homogeneous solution, wherein the
particles, in another embodiment, cannot be separated by ordinary
filtration or centrifugation.
[0049] In one embodiment, the magnetite nanoparticles produced via
organic route are monodispersed within the polymeric matrix. In one
embodiment, the term "monodispersed" refers to a relative average
particle size, with a coefficient of variance of less than 10.
[0050] In another embodiment, the magnetite particles do not change
in their chemical composition over a particular period of time.
[0051] In another embodiment, the fibers of this invention may
further comprise a targeting moiety. The term "targeting moiety",
in one embodiment, refers to a specificity conferred to the moiety,
which results in attachment of the moiety to a cognate partner, or,
in another embodiment, an ability to specifically "target" the
moiety to a desired cognate partner molecule. The targeting moiety
may, in one embodiment, facilitate attachment of the the fibers,
through the targeting moiety, to a molecule of interest, such as a
protein or glycoprotein, in one embodiment, or, in another
embodiment, to a nucleic acid of interest, or in another
embodiment, to a cellular fraction of interest. Such a property may
be additionally useful, in another embodiment, in application of
the superparamagnetic fibers of this invention in filtration or
magnetic separation.
[0052] In one embodiment, the targeting moiety enhances attachment
to a molecule in low abundance, which is of interest. In another
embodiment, the targeting moiety enhances attachment following
supply of an energy source, such as a UV light source. In one
embodiment, the targeting moiety is chemically attached to the
polymers via a chemical cross-linking group, or in another
embodiment, forms a stable association with a polymer, or, in
another embodiment, forms an association with the polymer, yet
readily dissociates following changes in solution conditions, such
as, for example, salt concentration or pH.
[0053] In one embodiment, the targeting moiety may be an antibody,
which specifically recognizes a molecule of interest, such as a
protein or nucleic acid. In another embodiment, the antibody may
specifically recognize a reporter molecule attached to a molecule
of interest. In another embodiment, the targeting moiety may be an
antibody fragment, Protein A, Protein G, biotin, avidin,
streptavidin, a metal ion chelate, an enzyme cofactor, or a nucleic
acid. In another embodiment, the targeting moiety may be a
receptor, which binds to a cognate ligand of interest, or
associated with a cell or molecule of interest, or in another
embodiment, the targeting moiety may be a ligand which is used to
"fish out" a cell via interaction with its cognate receptor.
[0054] It is to be understood that any component of interest, such
as a cell, or component thereof, wherein its separation from other
materials is desired, which is amenable to the present technology
is to be considered as part of this invention.
[0055] The fibers of this invention are magnetic field-responsive,
such as was demonstrated, for example with superparamagnetic fibers
of this invention, in Example 5. In one embodiment, the
superparamagnetic fiber property, or ferromagnetic property of
magnetic field-responsiveness is exploited in a variety of
applications, as is discussed further hereinbelow.
[0056] In one embodiment, the term "magnetic field-responsive"
refers to the property of the fibers of this invention to exhibit a
structural modification, in response to the application of an
external magnetic field. In one embodiment, such responsiveness is
completely reversible, or, in another embodiment, mostly, or in
another embodiment, partly reversible. In another embodiment,
magnetic field responsiveness results in a high stiffness exhibited
in the fibers of this invention. In another embodiment, it results
in deformation or change of shape, such as, for example, exhibited
in the superparamagnetic fibers. Such changes in stiffness and/or
deformation may be rate-sensitive, in another embodiment.
[0057] In one embodiment of this invention, the fibers of this
invention are produced via an electrospinning technique.
Preparation of superparamagnetic or ferromagnetic polymeric
nanofibers via electrospinning is exemplified hereinbelow. In one
embodiment, for preparation of the former, electrospinning is
conducted as described herein, on colloidally-stable suspensions of
magnetite nanoparticles in polyethylene oxide and polyvinyl alcohol
solutions. In some instances, the magnetite nanoparticles were
aligned in columns parallel to the fiber axis direction within the
fiber by the electrospinning process. In another embodiment, for
the preparation of the ferromagnetic nanofibers, the
electrospinning is conducted on monodispersed magnetite
nanoparticles of size up to 16 nm, in THF.
[0058] In one embodiment, this invention provides a method of
producing a field-responsive fiber comprising magnetite particles
and a polymeric matrix, the method comprising the step of
electrospinning a polymer solution comprising stably dispersed
superparamagnetic magnetite particles, or monodispersed magnetite
particles.
[0059] In one embodiment, the method of producing a
field-responsive fiber of this invention via electrospinning
comprises the step of preparing desired concentrations of
polymer/magnetite nanoparticle solutions, in which the
nanoparticles are dispersed.
[0060] For preparation of superparamagnetic fibers of this
invention, in one embodiment such dispersions are prepared by
adding the desired amount of polymer solution directly to a
magnetite nanoparticle aqueous solution, which may be accompanied
by, in another embodiment, vigorous stirring, which may be
accomplished, in another embodiment, for a period of time of at
least 24 hours at room temperature.
[0061] In one embodiment, aqueous solutions of magnetite
nanoparticles may be prepared as follows: aqueous solutions
containing iron (III) chloride hexahydrate, iron (II) chloride
tetrahydrate, and graft copolymer may be dissolved in deoxygenated
water, where the graft copolymer may comprise Jeffamine and
polyacrylic acid.
[0062] In one embodiment, the particles are co-precipitated in the
presence of a stabilizing polymer as described hereinbabove, which,
in another embodiment, attaches to the particle surfaces and
confers steric stabilization to the particle dispersion in the
polymer solution. In another embodiment, the magnetic fluid thus
formed, comprising an aqueous solution of stably dispersed
magnetite particles in polymer, may be subjected to centrifugation
and/or filtration, which, in another embodiment, may serve remove
excess polymer and/or salts.
[0063] For preparation of ferromagnetic fibers of this invention,
in one embodiment such dispersions are prepared by by adding the
desired amount of polymer solution directly to a magnetite
nanoparticle organic solution, which may be accompanied by, in
another embodiment, vigorous stirring, which may be accomplished,
in another embodiment, for a period of time of at least 24 hours at
room temperature. In one embodiment, the solution will comprise THF
and DMF.
[0064] In one embodiment, electrospinning may be conducted with the
aid of any suitable apparatus as will be known to one skilled in
the art. In one embodiment, a parallel-plate electrospinning
apparatus may be used, such as that described by Shin et al [Shin
M., Hohman M. M., Brenner M. P., and Rutledge G. C., Appl. Phys.
Lett. 2001; 78:1149-1151] and/or Fridrikh et al [Fridrikh S. V., Yu
J. H., Brenner M. P., and Rutledge G. C. Phys. Rev. Lett. 2003;
90:144502].
[0065] In one embodiment, electrospinning is conducted with two 10
cm in diameter aluminum disks, arranged parallel to each other, at
a distance of up to 30 cm. In one embodiment, the electrical
voltage, solution flow rate and distance between the two parallel
plates are adjusted to obtain a stable jet.
[0066] In one embodiment, the methods of this invention produce
superparamagnetic fibers in which magnetite particles line up
within the fibers in parallel to the fiber axis direction.
[0067] In one embodiment, the method employs magnetite
nanoparticles dispersed as stable suspensions in PEO solutions, for
producing superparamagnetic fibers. Such solutions comprising the
nanoparticles will, in another embodiment, exhibit increased
conductivity, such as, for example, that shown in Table 1.
[0068] In one embodiment, the methods of this invention will employ
electrospinning wherein the parameters comprise a flow rate of
between 0.005 to 0.5 ml/minute, or in another embodiment, 0.01 to
0.03 ml/minute. In another embodiment, the electrospinning
parameters may comprise a polymer/particle solution viscosity of
0.1 to 20 (Pas), or more.
[0069] In one embodiment, the polymer is PVA, at a concentration
ranging from 6.5-15 wt %, and the viscosity ranges from 0.05-20
(Pas), or more. In another embodiment, the solution further
comprises SDS, which increases the viscosity. According to this
aspect of the invention, and in one embodiment, a solution
comprising 1 wt % SDS, exhibits a viscosity of 0.6-21 (Pas). In one
embodiment, the addition of SDS increases viscosity and
conductivity of the solution, however does not affect surface
tension properties. In another embodiment, the inclusion of SDS in
the polymer solution decreases the diameter of the fiber
formed.
[0070] In another embodiment, flow rate, viscosity, concentration,
or combination thereof may vary, and be any value which enables
electrospinning of the solution to produce a superparamagnetic
fiber, or ferromagnetic fiber of this invention.
[0071] It is to be understood that any embodiment listed herein in
reference to the fibers of this invention, may characterize the
fibers as obtained by the methods provided herein, and is to be
considered as part of this invention.
[0072] In one embodiment, in regard to the methods of this
invention producing superparamagnetic fibers of this invention,
which comprise a PEO polymer, the PEO polymer used is at a
concentration of between 1% and 3% by weight, and in another
embodiment, solutions comprising the same have a conductivity of at
least 1000 .mu.S/cm. In another embodiment the solution may have a
conductivity ranging <1 microSeimen/cm up to 1300
microSeimens/cm, or greater.
[0073] In another embodiment, according to this aspect of the
invention, the methods of this invention emloy the use of a polymer
solution comprising polyvinyl alcohol, which in another embodiment,
is at a concentration of between 6.5% and 15% by weight, or in
another embodiment, comprises SDS, which in another embodiment,
exhibits a conductivity of at least 1000 .mu.S/cm.
[0074] In another embodiment, this invention provides a device or
apparatus comprising the field-responsive fibers of this invention.
It is to be understood that such a device or apparatus may comprise
any embodiment of any fiber of this invention.
[0075] In one embodiment, the device or apparatus is used as a
filter. In one embodiment, the superparamagnetic fibers of this
invention are so arranged in a matrix as to form an impermeable
barrier, when not under the influence of an external magnetic
field. According to this aspect of the invention, and in one
embodiment, upon exposure to an external magnetic field the fibers
deform in a given orientation, such that gaps are introduced within
the matrix, thereby introducing permeability. In another
embodiment, the filter is suited for use in magnetoseparation.
According to this aspect, and in another embodiment, magnetic
particles suspended in a liquid are adsorbed and separated off with
use of a layer of filter medium comprising the superparamagnetic
fibers of this invention. In one embodiment, application of an
external magnetic field may result in deformation of the
superparamagnetic fibers comprising the filter, facilitating
passage of debris, and other non-desired components, while
materials adhered to magnetic particles remain attached to the
filter. These are some, non-limiting applications of the
superparamagnetic fibers as filters, however many other
applications will be appreciated by one skilled in the art, and are
to be considered as embodiments of this invention.
[0076] In one embodiment, the device or apparatus is used as a
sensor. In one embodiment, the sensor will take advantage of the
deformation of the superparamagnetic fibers in response to an
externally applied magnetic field. In one embodiment, such sensors
may be utilized for remotely sensing the alternating currants (AC)
in a set of substantially parallel conductors, from the magnetic
fields generated by these currents in the vicinity of the
conductors. It is to be understood that any application wherein
detection of the presence of a magnetic field is desired, and a
sensor comprising the superparamagnetic fibers of this invention
may be utilized is to be considered as part of this invention. In
another embodiment, as a tunable reinforcement material, the
superparamagnetic fibers of this invention, or a fabric comprising
the same, could be included in a composite or multilayer
construction such that the composite on multilayer is made stiffer
when a magnetic field is turned on, such stiffness increase being
higher when the field is higher and/or when the rate of deformation
is faster. In another embodiment, as a piezomagnetic transducer,
deformation of the fabric may result in a measureable magnetic
field to sense or actuate other components. In another embodiment,
introduction of a magnetic field may result in deflection or
deformation of the piezomagnetic fiber, fabric or multilayer
material.
[0077] In another embodiment, the superparamagnetic fibers of this
invention may be used for information storage. In one embodiment,
the superparamagnetic fibers of this invention are used in a
magnetic storage medium containing a magnetic material. The
magnetic material may be any magnetic material, which can store
data by the alignment of the directions of the spins in the
material. In one aspect, the each magnetite particle within the
superparamagnetic fibers of this invention may be adapted to store
one bit of data.
[0078] In another embodiment, the superparamagnetic fibers of this
invention may be used for magnetic imaging. In one embodiment, an
image may be formed by applying an external magnetic field to
selected regions of a composite medium comprising the
superparamagnetic fibers of this invention. In response to the
applied field, deformation of the fibers occurs, producing a latent
image, which may, in another embodiment, be developed by exposure
to magnetic fluid or powders. The image may be erased, in another
embodiment, by removal of the magnetic field, or in another
embodiment, by exposure to an AC demagnetizing field or a DC sweep
magnet.
[0079] In one embodiment, the superparamagnetic fibers are used in
magnetic shielding. In another embodiment, this invention provides
a fabric-comprising the superparamagnetic fibers.
[0080] In many situations, such as, for example, in military
shelters, protection against electro-magnetic interference signals
is desired. In one embodiment, shielding of electronic equipment is
desired, as the equipment would malfunction if subjected to
electro-magnetic waves. In another embodiment, when certain types
of electronic equipment are used, it is desirable to have
shielding, which prevents detection of the location from which the
signals are generated. In one embodiment, such shielding may be
provided by the superparamagnetic fibers of this invention.
[0081] In one embodiment, a fabric comprising the superparamagnetic
fibers, or ferromagnetic fibers of this invention may be thus
utilized.
[0082] In another embodiment, a fabric comprising the
superparamagnetic or ferromagnetic fibers of this invention, may
further comprise additional materials which do not materially
affect their properties, such as, for example, pigments,
antioxidants, stabilizers, surfactants, and others as will be
appreciated by one skilled in the art. In one embodiment, a fabric
of this invention may be a SMART fabric, in which stiffness can be
controlled by use of an external magnetic field, and which absorb
impact at pre-determined rates. These properties are in addition to
the field-dependent deflection behavior demonstrated
previously.
[0083] In one embodiment, the surface coating of nanoparticles will
be compatible (for example, hydrophilic or hydrophobic) with the
polymer solution in which they are electrospun (magnetic
nanoparticles should disperse uniformly in the polymeric
solution).
[0084] In one embodiment, the magnetic nanoparticles will have a
high saturation magnetization and tunable Neel relaxation time
(with size). The magnetization of the material may vary, in some
embodiments, from 250 kA/m to 2000 kA/m. The Neel relaxation time
may vary, in some embodiments, from milliseconds to seconds,
depending on the size of nanoparticles.
[0085] The following examples are presented in order to more fully
illustrate some embodiments of the invention. They should, in no
way be construed, however, as limiting the scope of the
invention.
EXAMPLES
Materials and Methods
Materials:
[0086] Poly(ethylene oxide) (PEO, Mv 2,000,000), poly(vinyl
alcohol) (PVA, 87%-89% hydrolyzed, Mw: 85 k-146 k)and dodecyl
sulfate, sodium salt (98%)(SDS) were obtained from Aldrich and used
for making electrospinnable solution. Poly(acrylic acid) (PAA; 50
wt % in water, Mw=5000), iron(III) chloride hexahydrate (97%),
iron(II) chloride tetrahydrate (99%), ammonium hydroxide (28 wt %
in water), dimethyl formamide (DMF) and dicyclohexylcarbodiimide
(CDI) were obtained from Aldrich (Milwaukee, Wis.) and used for
synthesizing nanoparticles. Jeffamine XTJ-234 (PEO/PPO-NH2,
EO:PO=6.1:1, Mw=3000) is an amine-terminated random copolymer of
ethylene oxide (EO) and propylene oxide (PO) repeat units with 6.1
EO units per PO unit. It was donated by Huntsman Corp. (Houston,
Tex.) and has characteristics similar to that of pure PEO.
Preparation of Nanoparticles:
[0087] The graft copolymer was prepared by reacting the Jeffamine
with the carboxyls on PAA via amidation chemistry as described
[Moeser G. D., K. A. R., Green W. H., Laibinis P. E., and Hatton T.
A. Ind. Eng. Chem. Res. 2002; 41:4739-4749]. Only a small
percentage (16%) of carboxyl groups were grafted with Jeffamine
since free carboxyl groups are required for chelation with surface
iron atoms and stabilization of the magnetite nanoparticles.
[0088] In a typical procedure for the synthesis of the magnetite
nanoparticles [Moeser, supra], an aqueous solution containing 2.35
g of iron (III) chloride hexahydrate, 0.86 g of iron (II) chloride
tetrahydrate, and 1 g of graft copolymer was prepared by dissolving
the reagents in 40 mL of deoxygenated water. Deoxygenation was
achieved by bubbling with nitrogen under vigorous stirring for 30
min before reaction. The aqueous solution was heated to 80.degree.
C., and 5 ml of 28 wt % of ammonium hydroxide was added to
precipitate iron oxide in the form of magnetite. The growth of
spherical nanoparticles was arrested by the polymer in the
solution, which caps the magnetite nanoparticles as soon as they
form and stabilizes them sterically against aggregation. The
resulting mixture was then aged for 30 min at 80.degree. C. This
procedure produces 1 g of magnetite in 40 mL of water, which is
equivalent to a 2.5 wt % suspension of magnetite. The final
magnetic fluid was washed in a centrifuge with an ultrafilter
(Millipore, Centricon Plus 80, MWCO 100,000) to remove excess
polymer and salts.
Preparations of Spinning Solutions:
[0089] PEO solutions ranging from 1% to 3% by weight were prepared
by directly adding the PEO polymer to distilled water. The
solutions were stirred vigorously for at least 24 h at room
temperature in order to obtain homogeneous solutions. PVA solutions
ranging from 6.5% to 15% by weight were prepared by directly adding
the polymer into distilled water, with vigorous stirring for at
least 3-4 hours at 70.degree. C.
[0090] Various concentrations of PEO/magnetite nanoparticle
dispersions were prepared by adding the desired amount of PEO
solution directly to the nanoparticle aqueous solution prepared as
described above, with vigorous stirring for at least 24 h at room
temperature. A range of PVA/magnetite nanoparticle suspensions was
prepared similarly, and then mixed using a vortex mixer (VWR
Scientific Products) for at least ten minutes before spinning, as
the particle suspension was not particularly stable against
aggregation and settling.
Electrospinning:
[0091] The parallel-plate electrospinning apparatus used was
similar to that described by Shin et al [Appl. Phys. Lett. 2001;
78:1149-1151] and Fridrikh et al [Phys. Rev. Lett. 2003;
90:144502]. Briefly, two aluminum disks with diameters of 10 cm
were arranged parallel at a distance of up to 30 cm apart. The
fluid was pumped at a constant flow rate by a syringe pump (Harvard
Apparatus PHD 2000) to a stainless steel capillary with inner
diameter 1 mm located in the center of the upper disk. An
electrical potential was applied to the upper disk by a high
voltage power supply (Gamma High Voltage Research ES-30P). Current
was measured by a Digital multimeter (Fluke85 III) as the voltage
drop across a 1.0 MW resistor between the lower disk and ground.
The electrical voltage, solution flow rate and distance between the
two parallel plates were adjusted to obtain a stable jet.
Measurement and Characterization of Composite Nanofibers:
[0092] Viscosity was measured on an AR-7000 Rheometer (TA
Instruments) at 25.degree. C. A. Kruss 10 tensiometer was used to
determine surface tensions, while conductivity was measured using a
Cole Parmer 19820 conductivity meter.
Dynamic Light Scattering (DLS):
[0093] Dynamic Light Scattering (DLS) was performed to determine
the hydrodynamic diameters of the coated nanoparticles using a
Brookhaven BI 200-SM system at a fixed angle of 900. The
autocorrelation function was fitted with an exponential curve to
obtain the diffusion coefficient, which was then used to calculate
the hydrodynamic diameter via the Stokes-Einstein equation.
Scanning Electron Microscopy (SEM):
[0094] Specimens for Scanning Electron Microscopy (SEM) were
prepared by direct deposition of the electrospun nanofibers on an
aluminum foil and sputter-coating with gold using a Desk II cold
sputter/etch unit (Denton Vacuum LLC, NJ). The images of the
electrospun fiber were obtained using a JEOL-6060SEM (JEOL Ltd,
Japan), and the fiber diameters were determined using AnalySIS
image processing software (Soft Imaging System Corp., Lakewood,
USA) by measuring 20 randomly selected fibers for each sample.
Transmission Electron Microscopy (TEM):
[0095] For Transmission Electron Microscopy (TEM), a dilute
magnetite nanoparticle solution was dried on a carbon grid and
visualized under the JEOL JEM200 CX TEM microscope (JEOL Ltd,
Japan) to estimate the core sizes of the particles. The electrospun
nanofibers were directly deposited onto a copper grid for TEM
analysis.
Super Conducting Quantum Interference Device (SQUID) Test:
[0096] The SQUID test was conducted using an MPMS XL magnetometer
(Quantum Design Inc., San Diego) for both PVA/magnetite and
PEO/magnetite nanofibers. The sample was scanned in 40-50 equal
increments with an applied magnetic field ranging from
approximately -0.6 Tesla to 0.6 Tesla.
Nanoindentation:
[0097] Nanoindentation experiments were performed using a Nanoscope
IV, Dimension.TM. 3100 AFM (Digital Instrument, Santa Barbara) with
a RTESP single-beam silicon probe (Digital Instrument) (fR=280-361
kHz, k=30-40 N/m). All the nanofibers were conditioned in a vacuum
oven for at least two days before experiments, at room temperature
for PEO and PEO/magnetite nanofibers and at 60.degree. C. for PVA
and PVA/magnetite nanofibers, respectively. During these AFM
indentation tests, PVA and PVA/magnetite nanofibers were treated as
a group, as were the PEO and PEO/magnetite nanofibers. Within each
group, the maximum indentation force, Pmax, was the same. Pmax for
the PVA group was twice that for the PEO group. Within each group,
mica and a flat reference sample of epoxy were indented using the
same probe and parameters. The mica has an elastic modulus of
.about.171 GPa and Poisson ratio of .about.0.3 [27-29]. The elastic
modulus of the reference epoxy sample was determined independently
using a Triboindenter with Berkovich-type indentation tip (Hysitron
Inc., Minneapolis). For each sample, at least 20 individual force
curves were obtained.
Field Responsive Testing:
[0098] A rectangular strip
(length.times.width.times.thickness=1.8.times.0.555.times.0.004 cm)
of electrospun nonwoven mat was placed on the surface of a table,
with one end fixed by tape onto the table surface, A permanent
laboratory magnet with a rectangular cross-section (1.8.times.0.6
cm) was suspended some distance away above the mat, and the
response behavior of the nonwoven mat to the laboratory magnet was
recorded by a digital camera.
Example 1
Synthesis of Composite Nanofibers Containing Magnetite
Nanoparticles
[0099] The size distribution of the magnetite nanoparticles was
determined by DLS (FIG. 1), and corresponded to an average
hydrodynamic diameter of 25 nm. An analysis of TEM images of the
as-synthesized magnetite nanoparticles (FIG. 2) indicated an
average core size, assuming a log normal distribution, of
7.5.+-.2.9 nm. Only the magnetite cores were visible in TEM
measurements, as the polymer coatings were of low contrast, and
could not be discerned in these images. The difference between the
average hydrodynamic diameter and core size yielded a thickness of
about 9 nm for the polymer shell.
[0100] The dependence of the magnetization, M, of the magnetite
fluid on the applied magnetic field in the SQUID tests is shown in
FIG. 3. The magnetite nanoparticle suspension exhibited
superparamagnetic behavior in that there was zero remnant
magnetization at zero applied field. The saturation magnetization
was approximately 0.5-0.7 Tesla.
[0101] The magnetite nanoparticles were readily dispersed as stable
suspensions in PEO solutions, increasing their conductivity
dramatically, as shown in Table 1. TABLE-US-00001 TABLE 1 Solution
properties and electrospinning processing parameters of some
representative nanofibers Flow Fiber Conductivity Viscosity Voltage
Rate Distance Current Diameter Composite (.mu.S/cm) (Pa s) (kV)
(ml/min) (cm) (nA) (nm) PEO 107.9 1.565 9.0 0.010 25 83 390 .+-. 40
(2 wt %) Fe.sub.3O.sub.4 (0.75%) 1277 1.506 9.0 0.020 25 353 400
.+-. 80 PVA 351 0.2905 29.0 0.010 25 412 170 .+-. 40 (7.5%)
Fe.sub.3O.sub.4 (0.75%) 1372 0.3926 29.0 0.010 25 1534 320 .+-. 40
Fe.sub.3O.sub.4 (0.75%) + SDS (1%) 2740 1.941 28.5 0.016 25 1050
140 .+-. 30
[0102] The preferred electrospinning parameters (Table 1) were
almost identical for PEO and PEO/magnetite solutions. Some
representative SEM pictures of electrospun nanofibers of PEO and
PEO/magnetite are shown in FIG. 4. At low PEO concentration (1 wt
%) in the absence of magnetite nanoparticles, the fibers adopted a
bead-on-string morphology (FIG. 4 (a)), while fibers with uniform
diameters were obtained when 3.52 wt % magnetite nanoparticles were
added to the spin solution (FIG. 4(b)). At higher concentrations
(2-3 wt %) of PEO, uniform fiber morphologies were obtained for
both PEO and PEO/magnetite solutions with little change in fiber
diameters on the addition of magnetite nanoparticles, as shown in
FIGS. 4 (c) and (d).
[0103] The magnetite nanoparticles were easily dispersed in PVA
solutions, as well, but these solutions were not as stable as in
the case of PEO, and the magnetite nanoparticles settled overnight.
The settling of the suspension on standing may have been due to
PEO-based shells around the nanoparticles not being as compatible
with the PVA in solution as they were with PEO solutions. The PVA
suspensions were easily homogenized, however, using a Vortex mixer
for 10 minutes immediately before electrospinning. The presence of
the magnetite nanoparticles in the PVA solutions increased the
conductivity of these solutions (Table 1). Again, there was little
change in the preferred electrospinning processing parameters for
PVA solutions when magnetite nanoparticles were added (Table 1).
SEM pictures of electrospun nanofibers using PVA and PVA/magnetite
solutions (FIG. 5) show that the magnetite nanoparticles lead to
increased fiber diameters, but that addition of SDS to the solution
counteracted this effect, as SDS generally reduces the fiber
diameters (Table 1). The results of a detailed study of the effect
of SDS on the PVA fiber morphology are shown in Table 2.
TABLE-US-00002 TABLE 2 Effect of SDS On PVA Solution Properties and
PVA Fiber Morphology Conductivity Viscosity Surface Tension Fiber
Diameter (S/cm) (Pa s) (mN/m) (nm) PVA SDS SDS SDS SDS (wt %) No
SDS (1 wt %) No SDS (1 wt %) No SDS (1 wt %) No SDS (1 wt %) 6.5%
329 1223 0.1187 0.6456 41.58 42.04 Beaded 96.04 .+-. 17.13 8% 315
1187 0.2905 1.375 39.5 42.42 146.67 .+-. 10.90 73.72 .+-. 14.51 10%
449 1156 0.8931 2.954 39.23 39.5 201.01 .+-. 27.85 158.19 .+-.
22.43 12% 496 1194 2.019 6.822 37.24 34.1 356.96 .+-. 15.96 193.98
.+-. 48.14 15% 515 1230 8.372 20.64 34.61 29.71 478.9 .+-. 18.37
297.03 .+-. 14.74
[0104] While SDS increased both the conductivity and the viscosity
of the solutions significantly, it had surprisingly litte effect on
the surface tension. The diameters of the nanofibers were decreased
by adding 1 wt % SDS to various concentrations of PVA. At the
lowest PVA concentration (6.5 wt %), the fiber morphology changed
from bead-on-string (with no SDS) to uniform fibers on the addition
of 1 wt % SDS.
Example 3
Electrospinning Effects on Nanofiber Characteristics
[0105] In order to further characterize PEO/magnetite and
PVA/magnetite nanofibers, transmission electron microscropy was
utilized to visualize the fibers (FIG. 6). The weight percentages
of magnetite nanoparticles within the fibers were 28%, and 8% for
PEO/magnetite and PVA/magnetite nanofibers, respectively. The
relatively large size of the PEO fiber and high content of
nanoparticles within the fiber made it difficult to focus the TEM
pictures, but the contour of the alignment of the nanoparticles
into columns along the fiber axis direction was readily visible.
For the PVA/magnetite fiber, the images were clearer, and
demonstrated magnetite nanoparticle alignment in columns parallel
to the fiber axis direction within the fiber.
[0106] Magnetite nanoparticles can form chains in solution owing to
magnetic coupling effects between particles. The number of
nanoparticles, n.sub.0. in a chain in the fluid, at zero external
field, can be estimated using the following formula: n 0 = [ 1 - 2
3 .times. ( .PHI. .lamda. 3 ) .times. e 2 .times. .lamda. ] - 1 ( 1
) ##EQU1## where, .phi. is the volume fraction of particles in the
fluid, and .lamda. is the coupling coefficient, which measures the
strength of particle-particle interactions. .lamda. is given by
.lamda. = .mu. 0 .times. M 2 .times. V 14 .times. kT ( 2 ) ##EQU2##
where, .mu..sub.0 is the permeability of free space, M is intensity
of magnetization of the magnetic particles, V is the volume of the
magnetic particles, k is Boltzmann's constant, and T is the
absolute temperature in degrees Kelvin.
[0107] For magnetite particles 7.5 nm in diameter, .lamda.<1
according to equation (2), which means the particle-particle
interaction energy is less than the thermal energy and no chain
forms in solution prior to electrospinning for any concentrations
of magnetite particles Thus the column alignment of magnetite
nanoparticles within the fiber observed in FIG. 6(b) was a result
of the electrospinning process itself. Possible causes for this
alignment may have been hydrodynamics in the capillary, steady jet,
or whipping jet regions, or induction by the local electric
field.
[0108] Super Conducting Quantum Interference Device (SQUID)
magnetization curves for both 28% PEO/magnetite and 8%
PVA/magnetite nanofibers demonstrated the superparamagnetic
behavior of the fibers at room temperature (FIG. 7). At low
temperature (5K), both systems were characterized by a narrow
hysteresis and a small remnant magnetization at zero field. These
can be explained by considering the magnetic relaxation of the
nanoparticles. For 7.5 nm diameter particles, Neel relaxation
dominates the Brownian rotation mechanism. The Neel relaxation time
varies exponentially with inverse temperature. For example, at 300
K the Neel relaxation time for magnetite particles 8 nm in diameter
in kerosene carrier is approximately 10.sup.-9 s, and increases to
approximately 13 s at 5K. At low temperature, when the applied
field reached zero, the dipole moments of some nanoparticles were
still polarized, and therefore a small remnant magnetization was
observed. The superparamagnetic behavior of the nanofibers at room
temperature may be useful, in some embodiments of this invention,
for applications in which alternating nonuniform fields are needed,
as this would reduce the dissipative energy in a device comprising
the nanofibers.
Example 4
Structural Characterization of the Composite Nanofibers Comprising
Magnetite Nanoparticles
[0109] The elastic modulus of the fibers was evaluated using an AFM
indentation technique according to the following formula
[Vanlandingham M. R., et al., J. Adhesion 1997; 64: 31-57; Sneddon
J. N., Int. J. Engng. Sci. 1965; 3: 47-56; Pharr G. M., et al., J.
Mater. Res. 1992; 7: 613-617; Vanlandingham M. R., et al.,
Composites Part A1999; 30:75-83; Drechsler D., et al., Appl. Phys.
A 1998; 66: S825-S829]: S = d P d .DELTA. .times. .times. Z i
.times. P max = 2 .times. E * .function. ( A .pi. ) 1 2 ( 3 )
##EQU3##
[0110] Here, S was the slope of the unloading curve at P.sub.max, P
was the applied load, A was the contact area, .DELTA.Z.sub.i is the
indentation depth, and E* was the effective Young's modulus of the
contact as defined by 1 E * = 1 - v s 2 E s + 1 - v t 2 E t ( 4 )
##EQU4##
[0111] In equation (4), E.sub.s and E.sub.t were the elastic
moduli, and v.sub..sigma. and v.sub.t the Poisson ratios of the
sample and the tip, respectively. A diamond tip was used, with
asymmetric pyramidal geometry; indent size was characterized by the
lateral distance from the apex to the base of the triangular
impression [Vanlandingham, supra]. E.sub.t and v.sub.t were assumed
to be 130 GPa and 0.2, respectively, corresponding to the bulk
values of diamond [Kracke B., Damaschke B. Appl. Phys. Lett. 2000;
77:361-363; Vanlandingham M. R., et al., J. Mater. Sci. Lett. 1997;
16:117-119; Klapperich C., et al. J. Tribology 2001; 123: 624-631].
The nanofibers were indented in the radial direction. A schematic
depiction of the tip-sample interaction during the indentation test
is shown in FIG. 8. The method was applicable here, in view of the
fact that the diameters of the fibers (>150 nm) were much larger
than the diameters of the contact area (<10 nm).
[0112] The results of the indentation tests for PVA/magnetite
nanofiber are shown in FIG. 9. Mica was also indented to evaluate
the bending of the AFM cantilever, which was then subtracted from
the raw PVA/magnetite data to determine fiber properties. In these
indentation tests, the slopes of the top portions of the unloading
curves were used to evaluate the modulus of each sample.
[0113] Assuming the tip geometry is the same for all the
indentations, the relative changes in indent size was sufficient to
relate contact areas; here, the apex to base distance was equated
to a contact radius, r. The Poisson ratio, v, was assumed to be the
same for all fibers within each group. The ratio of modulus of
different samples within each group was then evaluated using the
formula: ( d P / d .DELTA. .times. .times. Z i ) .times. P max 1 (
d P / d .DELTA. .times. .times. Z i ) .times. P max 2 = r 1 .times.
E 1 r 2 .times. E 2 ( 5 ) ##EQU5##
[0114] Table 3 provides the indentation data obtained for the
nanofibers. TABLE-US-00003 TABLE 3 Nanoindentation data of the
nanofibers. PVA.sup.1 PVA + Fe.sub.3O.sub.4.sup.1 PEO.sup.2 PEO +
Fe.sub.3O.sub.4.sup.2 Epoxy.sup.1 Epoxy.sup.2 .DELTA.Z.sub.i (nm)
12.33 .+-. 4.46 14.55 .+-. 2.67 9.36 .+-. 1.41 5.92 .+-. 0.68 22.14
.+-. 1.42 10.92 .+-. 0.35 .DELTA.Z.sub.i (nm) 80.04 .+-. 5.05 51.23
.+-. 4.46 170.32 .+-. 15.65 194.24 .+-. 14.40 67.83 .+-. 1.42 38.94
.+-. 1.87 Modulus 4.8 .+-. 1.73 4.1 .+-. 0.75 0.66 .+-. 0.10 1.04
.+-. 0.12 1.52 .+-. 0.12* 1.52 .+-. 0.12* (GPa) .sup.1Trigger
setpoint of deflection signal of the cantilever is 1.2 V.
.sup.2Trigger setpoint of delfection signal of the cantilever is
0.6 V. *The modulus is determined independently by a triboindenter
using a Berkovich tip.
[0115] .DELTA.Z; represented the total displacement recovered from
P=P.sub.max to P=0. .DELTA.Z.sub.t represented the total
indentation depth, which measures the penetration of the tip into
the sample surface, including both the inelastic and elastic
deformation of the material. The moduli were obtained by comparing
values obtained with reference epoxy sample within each group,
using equation (5). The modulus of the reference epoxy sample was
determined to be 1.52 GPa using a Triboindenter. The indent size,
r, was found to be 40 nm for the PEO and PEO/magnetite nanofibers,
10 nm for PVA and PVA/magnetite nanofibers, 20 nm and 10 nm for
reference epoxy samples under conditions used to test the PVA group
and PEO group, respectively.
[0116] As shown in Table 3, after including magnetite nanoparticles
(8 wt %) within PVA nanofibers, .DELTA.Z.sub.i was statistically
the same, and .DELTA.Z.sub.t decreased. This indicates that the
modulus of the PVA nanofibers was maintained and the inelastic
deformation was decreased due to the reinforcement effect of
magnetite nanoparticles.
[0117] After including (28 wt %) magnetite nanoparticles within the
PEO fiber, .DELTA.Z.sub.i decreased, but .DELTA.Z.sub.t increased
showing that the modulus of PEO nanofibers was increased due to the
reinforcement effect of magnetite nanoparticles. One indication of
the increase in inelastic deformation of the PEO/magnetite
nanofibers was that the short chains of polymeric shell around the
nanoparticles may have been detrimental to the mechanical
properties of the nanofibers, and overwhelmed the effect of
magnetite reinforcements as the concentration of the magnetite
nanoparticles within the fibers increased from 8 wt % to 28 wt
%.
[0118] The superparamagnetic fibers exhibited mechanical properties
comparable to those of the polymer matrix, and were not brittle.
Though the fibers were ceramic, and therefore expected to be very
brittle, and unlikely to deform at all, much less to as large a
strain as produced int the superparamagnetic fibers.
Example 5
Composite Nanofibers Comprising Magnetite Nanoparticles Are
Superparamagnetic
[0119] SQUID tests demonstrated that the magnetite nanoparticles
within the nanofibers were easily magnetized by an external
magnetic field, and that the dipole moments of the nanoparticles
were readily polarized in the direction of the external magnetic
field.
[0120] Since a magnetic dipole experiences a torque force in a
uniform magnetic field and a translational force in a magnetic
field gradient, it was thought that the composite nanofibers
containing magnetite particles in an external magnetic field
gradient, may be deformed by the translational forces experienced
by the embedded nanoparticles (FIG. 10). In response to an electric
field provided by a small laboratory magnet, a strip of
PVA/magnetite nonwoven mat exhibited field-responsive behavior.
[0121] In this example, one end of the nonwoven mat was fastened
onto the surface of a table, while the other end was free to move.
In the absence of the magnetic field, the nonwoven mat lay flat on
the surface of the table (FIG. 10a). When the magnet was placed
above the nonwoven mat, the fabric was deflected by the
translational forces in the direction of increasing magnetic field
as shown in FIG. 10b. As the magnet was brought closer to the
fabric, the greater magnetic field gradients experienced by the
nonwoven mat induced larger translational forces on the magnetite
nanoparticles, causing a greater deflection of the free end of the
mat towards the magnet (FIG. 10c). The PEO/magnetite nonwoven mat
showed similar response behavior to the laboratory magnet as the
PVA/magnetite nonwoven mat. Thus, both superparamagnetic fabrics
produced by the electrospinning techniques exemplified herein,
exhibited field-responsive behavior.
[0122] Superparamagnetic polymer nanofibers ranging in diameter
from 140 to 400 nm obtained via the electrospinning of
polymer-stabilized magnetite nanoparticle suspensions in PEO and
PVA solutions exhibited nanoparticle line up within the fibers in
columns parallel to the fiber axis direction. Both sets of fibers
were superparamagnetic at room temperature, and responded to an
externally-applied magnetic field by deflecting in the direction of
increasing field gradient, with nanoindentation tests -showing
magnetite nanoparticle reinforcement of the mechanical properties
of nanofibers.
Example 6
Composite Nanofibers Comprising Organic-Soluble Polymers
Materials and Methods
Reagents
[0123] Iron(III) acetylacetonate (97%), Benzyl ether (99%), 1-2
hexadecanediol (97%), ethanol, oleic acid (90%) and oleylamine
(70%) were purchased from Sigma Aldrich and used as received.
Synthesis of seeds
[0124] 2 mmol of Iron (III) acetylacetonate, 10 mmol of 1-2
hexadecanediol, 6 mmol of oleic acid and 6 mmol of oleylamine and
20 ml of benzyl ether were mixed in a 3 neck flask and were stirred
continuously under a blanket of nitrogen. The temperature was
ramped up slowly to 200.degree. C. (2.5.degree. C./min) and the
mixture was kept at this temperature for 2 hrs. Finally the mixture
was refluxed at 300.degree. C. for 1 hr. The resulting black
mixture was cooled down to room temperature and ethanol was added
followed by centrifugation at 7000 g to separate out magnetite. The
centrifuged product was re-suspended in hexane and was used for
seed mediated growth.
Synthesis of Bigger Nanoparticles
[0125] 2 mmol of Iron(III) acetylacetonate, 10 mmol of 1-2
hexadecanediol, 2 mmol of oleic acid and 2 mmol of oleylamine, 20
ml of benzyl ether and 80 mg of seeds in 4 ml of hexane were mixed
in a 3 neck flask and stirred continuously under a blanket of
nitrogen. The mixture was kept at 100.degree. C. for 30 mins and at
200.degree. C. for 1 h. Finally the mixture was refluxed at
300.degree. C. for 30 mins. The magnetite was recovered using the
procedure outlined above. The resulting magnetite nanoparticles can
then be used as seeds for subsequent synthesis. In this manner
stable nanoparticles up to 16 nm were synthesized.
Electrospinning
[0126] First, PMMA was directly added into DMF solvent to make 7.5
wt % solution. PMMA was directly added into THF suspension
containing 3.7 wt % of Fe.sub.3O.sub.4. Mixing the two solutions
prepared above at a ratio of 3:1, 1 7.5 wt % PMMA solution
containing 2.78 wt % Fe.sub.3O.sub.4 in a mixture of THF and DMF
(3:1) was prepared for electrospinning. A parallel-plate
electrospinning apparatus was used in this research and has been
described elsewhere by Shin et al [Appl. Phys. Lett. 78, 1149-1151
(2001)] and Fridrikh et al [Phys. Rev. Lett. 90:144502 (2003)].
Measurement and Characterization
[0127] The images of the electrospun fiber were obtained using a
JEOL-6060SEM (JEOL Ltd, Japan), and the fiber diameters were
determined using AnalySIS image processing software (Soft Imaging
System Corp., Lakewood, USA) by measuring 20 randomly selected
fibers for each sample. Transmission Electron Microscopy (TEM) was
performed on the JEOL JEM200 CX TEM microscope (JEOL Ltd, Japan).
The Superconducting Quantum Interference Device (SQUID) test was
conducted using an MPMS XL magnetometer (Quantum Design Inc., San
Diego) for PMMA/magnetite nanofibers.
RESULTS
[0128] In order to determine whether nanoparticles could be
prepared via an organic route, synthesis using organic solvents was
undertaken. The TEM micrographs of magnetite nanoparticles of
different diameters synthesized by the organic route are shown in
FIG. 11. The TEM micrographs confirmed the seed mediated growth. It
was difficult to re-suspend bigger nanoparticles in hexane (>14
nm), possibly due to higher magnetic interaction between the bigger
nanoparticles. Nanoparticles were sonicated to get a suspension in
THF. The sonication tended to disturb the alignment, thereby aiding
in dispersion. The shape of the nanoparticles changed from
spherical to cubical shape with increased particle size, perhaps
attributable to a different amount of surfactants adsorbing on
different faces of the growing crystal.
[0129] Electrospinning of the PMMA/16 nm magnetite nanoparticle
dispersion in THF was not possible as the jet dried too fast at the
capillary tip. The jet became stable, however, following the
addition of a 33% in volume of DMF in THF dispersion, and a uniform
fiber was obtained.
[0130] The preferred processing parameters were 17.5 kV for applied
electrical potential, 0.05 ml/min for flow rate, and 25 cm for the
plate-to-plate distance. The current measured was 72.5 nA and the
diameter of the fiber was around 1.53.+-.0.34 .mu.m. A
representative SEM picture of the electrospun PMMA fiber containing
16 nm magnetite nanoparticles is shown in FIG. 12.
[0131] A SQUID magnetization curve for PMMA fiber containing 37% by
weight of 16 nm magnetite particles is shown in FIG. 13. At room
temperature, a small hysteresis and a small remnant magnetization
at zero field are observed. This is in contrast to the
superparamagnetic behavior observed for the magnetic fibers
containing 7.5 nm magnetite nanoparticles, shown above. These can
be explained by considering the magnetic relaxation of the
nanoparticles. For the particles within the fiber, only the Neel
relaxation mechanism is operative, since the nanoparticles can not
rotate through a Brownian mechanism within the fiber matrix. 16 nm
magnetite nanoparticles have a longer Neel relation time than 7.5
nm magnetite nanoparticles. At room temperature, when the applied
field reached zero, the dipole moments of the 16 nm nanoparticles
were still partially aligned while those of the 7.5 nm
nanoparticles were completely relaxed, and therefore a small
remnant magnetization was observed for fibers containing 16 nm
nanoparticles, but not for containing 7.5 nm nanoparticles.
[0132] In this example, textiles comprised of magnetic fibers
containing 16 nm magnetite nanoparticles have been produced by
electrospinning. The organic synthesis route for nanoparticles
employed here complements the aqueous synthesis route above, and
provides for functionalization of the nanoparticles with an organic
surface coating that is compatible with organic solvents. The
resulting particles can therefore be dispersed in either aqueous or
organic solutions, respectively. The range of polymers that can be
electrospun with magnetic particles to form field-responsive fibers
is thus expanded to include both organic-soluble and water-soluble
polymers.
[0133] The organic route synthesis, in contrast to the aqueous
route presented above also permits a wider range of particle
compositions, includes those with larger intrinsic magnetic
moments. The larger particles produced via organic route result in
a longer Neel relaxation time, as demonstrated by the SQUID test,
where the remnant magnetization at zero field for the 16 nm
particles indicated ferromagnetic behavior, rather than the
superparamagnetic behavior exhibited by the smaller particles
obtained via aqueous synthesis.
[0134] The longer Neel relaxation time provides materials, which
exhibit changes in mechanical properties under a uniform applied
field at conventional rates of deformation. Due to coupling of the
particle magnetic moment with the applied field, deformation of the
magnetic fibers requires additional work, resulting in increased
stiffness and lower strain, compared to the equivalent nonmagnetic
fiber at equal deformation energy.
* * * * *