U.S. patent application number 16/458532 was filed with the patent office on 2020-03-12 for gel-electrospinning process for preparing high-performance polymer nanofibers.
The applicant listed for this patent is Massachusetts Institute of Technology. Invention is credited to Jay Hoon Park, Gregory C. Rutledge.
Application Number | 20200080234 16/458532 |
Document ID | / |
Family ID | 58498845 |
Filed Date | 2020-03-12 |
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United States Patent
Application |
20200080234 |
Kind Code |
A1 |
Rutledge; Gregory C. ; et
al. |
March 12, 2020 |
GEL-ELECTROSPINNING PROCESS FOR PREPARING HIGH-PERFORMANCE POLYMER
NANOFIBERS
Abstract
Disclosed are methods of forming a plurality of fibers, and
nanofibers produced from such a method.
Inventors: |
Rutledge; Gregory C.; (West
Newton, MA) ; Park; Jay Hoon; (Cambridge,
MA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
Massachusetts Institute of Technology |
Cambridge |
MA |
US |
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|
Family ID: |
58498845 |
Appl. No.: |
16/458532 |
Filed: |
July 1, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15290499 |
Oct 11, 2016 |
10344399 |
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16458532 |
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62315289 |
Mar 30, 2016 |
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62239310 |
Oct 9, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D01D 1/09 20130101; D10B
2321/0211 20130101; D01D 5/003 20130101; D01D 5/0038 20130101; D10B
2401/06 20130101; D01F 6/04 20130101 |
International
Class: |
D01D 5/00 20060101
D01D005/00; D01F 6/04 20060101 D01F006/04; D01D 1/09 20060101
D01D001/09 |
Goverment Interests
GOVERNMENT SUPPORT
[0002] This invention was made with Government support under
Contract No. W911NF-13-D-0001 awarded by the Army Research Office.
The Government has certain rights in the invention.
Claims
1-38. (canceled)
39. A nanofiber, wherein the diameter of the nanofiber is about 1
nm to about 1 .mu.m.
40. The nanofiber of claim 39, wherein the yield stress of the
nanofiber is in the range from about 2 GPa to about 100 GPa.
41. The nanofiber of claim 39, wherein the diameter of the
nanofiber is about 10 nm to about 1 .mu.m.
42. The nanofiber of claim 39, wherein the diameter of the
nanofiber is about 100 nm to about 1 .mu.m.
43. The nanofiber of claim 39, wherein the diameter of the
nanofiber is about 10 nm to about 500 nm.
44. The nanofiber of claim 39, wherein the diameter of the
nanofiber is about 100 nm to about 500 nm.
45. The nanofiber of claim 39, wherein the Young's modulus of the
nanofiber is in the range from about 85 GPa to about 1000 GPa.
46. The nanofiber of claim 45, wherein the Young's modulus of the
nanofiber is in the range from about 90 GPa to about 1000 GPa.
47. The nanofiber of claim 45, wherein the Young's modulus of the
nanofiber is in the range from about 95 GPa to about 1000 GPa.
48. The nanofiber of claim 45, wherein the Young's modulus of the
nanofiber is in the range from about to 100 GPa to about 1000
GPa.
49. (canceled)
50. The nanofiber of claim 40, wherein the yield stress of the
nanofiber is in the range from about 3 GPa to about 100 GPa.
51. The nanofiber of claim 40, wherein the yield stress of the
nanofiber is in the range from about 4 GPa to about 100 GPa.
52. The nanofiber of claim 40, wherein the yield stress of the
nanofiber is in the range from about 5 GPa to about 100 GPa.
53. The nanofiber of claim 40, wherein the yield stress of the
nanofiber is in the range from about 6 GPa to about 100 GPa.
54. The nanofiber of claim 40, wherein the yield stress of the
nanofiber is in the range from about 7 GPa to about 100 GPa.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of priority to U.S.
Provisional Patent Application Ser. No. 62/239,310, filed Oct. 9,
2015; and U.S. Provisional Patent Application Ser. No. 62/315,289,
filed Mar. 30, 2016. The contents of each of which are hereby
incorporated by reference in their entirety.
BACKGROUND
[0003] Over the past two decades, electrospinning has attracted
great interest from the academic and industrial scientific
communities due to its capability for continuous fabrication of
ultrafine fibers having diameters from a few nanometers to a few
microns (commonly known as "nanofibers"). Unlike conventional fiber
spinning processes, the fabrication of these sub-micron fibers is
driven by electrical forces rather than mechanical forces, and
often involves in high uniaxial extensional strain rates up to 1000
s.sup.-1. These fibers can be produced from a wide range of organic
and inorganic materials and typically have extremely high specific
surface areas, owing to their nanometer-scale fiber diameters. The
structural and functional versatility of these fibers, in addition
to the economic viability of the process at the laboratory scale,
has allowed their use in a broad range of applications (e.g.,
membranes and filters, battery materials, sensors, biomaterials,
drug delivery). In these applications, the mechanical integrity of
the electrospun material determines whether it will hold up under
end-use conditions that involve stress and strain. Typical Young's
moduli of submicron-diameter electrospun fiber range from about 0.1
GPa to about 7 GPa, which are larger than those of the bulk
material but still less than those of many conventional synthetic
fibers. Moreover, these nanofibers are unable to withstand tearing
or rupture under normal conditions of use (e.g., in apparel).
Indeed, fiber durability has remained one of the biggest
limitations of electrospun fibers for years that has prevented its
use in applications such as chemical and biological protection
membranes, coatings for electromagnetic interference (EMI)
shielding on equipment and personnel, and ultralight-weight
protective gear for soldiers. Use of the ultrafine fibers in high
performance applications, such as transparent composites, soft body
armor, industrial protective clothing or structural cords and
ropes, will benefit from increases in their stiffness, strength,
and/or toughness.
[0004] Thus, there exists a need for nanofibers with improved
mechanical properties, and reliable methods of producing such
nanofibers.
SUMMARY
[0005] In one aspect, disclosed herein is a method of forming a
plurality of fibers, comprising the steps of (i) placing a polymer
solution in a vessel comprising a spinneret; wherein the polymer
solution comprises a polymer and a solvent, the polymer solution
has a gelation temperature and a viscosity, the solvent has a
boiling point, the temperature of the polymer solution in the
vessel is in the range from the boiling point of the solvent to the
gelation temperature, and the viscosity of the polymer solution is
less than about 150 Poise; and (ii) electrostatically drawing the
polymer solution through the spinneret into an electric field,
wherein the temperature of the polymer solution as it is drawn
through the spinneret is in the range from about 15.degree. C.
below the gelation temperature to the gelation temperature, thereby
depositing a plurality of fibers on a collection surface; wherein
the spinneret is separated from the collection surface by a
space.
[0006] In another aspect, the present disclosure relates to
nanofibers made by any of the methods disclosed herein.
BRIEF DESCRIPTION OF THE FIGURES
[0007] FIG. 1 includes two panels (Panels A and B). Panel A shows
an apparatus set-up for gel-electrospinning. T.sub.1=Solution
reservoir temperature, T.sub.2=Extruded jet temperature,
T.sub.3=Space temperature around jet, T.sub.4=collector
temperature. Panel B is a schematic of a molecular organization
within the gel-electrospinning process. As shown in Panel B, the
molecules are dilute and entangled at the extruder exit, but
crystallized and oriented at the collector. At T.sub.2, a
semi-dilute entangled UHMWPE solution is shown. At T.sub.3,
extensional strain of a gel-state UHWPE is shown. At T.sub.4,
highly crystalline submicron PE fibers are shown.
[0008] FIG. 2 includes three panels (Panels A-C). Panels A and B
are plots of oscillatory shear data showing the storage and loss
modulus with respect to temperature at a fixed oscillatory stress
of 0.88 Pa (Panel A) and a fixed strain of 0.05 (Panel B). The
inset plots show the viscosities (open squares) with respect to
temperature. Panel C is a plot showing the mean and standard
deviation of gel-electrospun ultra high molecular weight
polyethylene (UHMWPE) fiber diameters at a various range of
operating temperatures for T.sub.3 from FIG. 1.
[0009] FIG. 3 includes two panels (Panels A and B). Panel A is a
SEM image of a typical gel-electrospun UHMWPE web collected at a
temperature T.sub.3=80.degree. C. The scale bar represents 50
.mu.m. Panel B is a series of TEM images of individual electrospun
UHMWPE nanofibers. The scale bars represent 50 nm, 200 nm, 100 nm,
and 250 nm, respectively starting from the upper left image. Note
that the images presented in FIG. 3, Panel B were collected from
the samples in FIG. 3, Panel A.
[0010] FIG. 4 includes four panels (Panels A-D). Panel A is a plot
showing representative stress-strain curves for UHMWPE fiber
diameters of 0.49 (), 0.73 (.quadrature.), 0.91 (.gradient.), 1.05
(.DELTA.), and 2.31 .mu.m (.largecircle.). Panel B is a plot of
Young's modulus vs fiber diameter. The insert shows the same data
on a log-log scale. The solid line at Young's modulus=0.728 GPa, is
the bulk UHMWPE modulus. The dotted line is an empirically fitted
line (Equation 1). Panel C is a plot of the electrospun fiber
diameter vs the yield stress. The insert shows the same data on a
log-log scale. The solid line at yield stress=0.02 GPa is the bulk
UHMWPE value. Panel D is a plot of WAXD patterns of UHMWPE
nanofiber bundles, with average fiber diameters .about.0.9.+-.0.2
.mu.m.
[0011] FIG. 5 is a plot of Differential Scanning calorimeter (DSC)
data of p-xylene/UHMWPE 1 wt % solution.
[0012] FIG. 6 includes two panels (Panels A and B). Panel A is an
SEM image of an individual gel-electrospun UHMWPE fiber with an
approximate diameter of 350 nm. Panel B is a plot showing the
stress-strain curve of the fiber from Panel A.
[0013] FIG. 7 is a SEM image of a typical gel-electrospun UHMWPE
fiber mat.
[0014] FIG. 8 is a plot of Stacked WAXD traces of the fiber mat
(dashed line, top) and the fiber bundle (solid line, bottom).
[0015] FIG. 9 shows the SAED crystal patterns displayed on the top
row, while the bottom row shows the corresponding individual UHMWPE
fiber. The scale bars represent 2.0 .mu.m, 1.0 .mu.m, and 0.2 .mu.m
from the leftmost column to the rightmost column.
[0016] FIG. 10 is a three-dimensional plot of tensile modulus,
tensile strength, and elongation break for the highest values of an
individual gel-electrospun UHMWPE fiber compared with other
commercial polymer fibers. The shading scheme on the right
corresponds to the z-axis value (elongation at break [%]) of each
data.
[0017] FIG. 11 is a plot of the Differential Scanning calorimeter
(DSC) data of p-xylene/UHMWPE gel-electrospun fiber mat.
DETAILED DESCRIPTION
Overview
[0018] In certain embodiments, the invention relates to a method of
gel-electrospinning. FIG. 1 shows a diagram of an exemplary
gel-electrospinning apparatus. In certain embodiments, the methods
disclosed herein process at the edge of gelation to afford high
elongation and molecular ordering in the electrospun fibers
produced. While not wishing to be bound by theory, this molecular
ordering results in nanofibers with exceptional mechanical
properties.
[0019] To fabricate nanofibers (e.g., UHMWPE nanofibers)
continuously with a high degree of molecular orientation and
crystallinity, in one aspect the method disclosed herein replaced
the hydraulic extrusion process of gel-spinning with the
electrostatically drawn filament-forming process of
electrospinning, and the subsequent mechanical hot drawing stage
with electrostatically driven drawing and whipping processes at
elevated temperature. Unlike conventional electrospinning, which is
often operated at a room temperature, certain embodiments of the
method disclosed herein operate at elevated temperatures chosen to
induce the formation of a gel solution within the filament during
drawing. In certain embodiments, the gel-electrospinning method
disclosed herein operates at a higher extensional strain rate
(.about.1000 s.sup.-1) than that of a conventional gel-spinning
process (.about.1 s.sup.-1). In certain embodiments, the
electrostatically driven hot drawing of a gel polymer solution
occurs predominantly in the whipping region (typically occurs in
T.sub.3 zone of FIG. 1) of an electrospinning process.
[0020] In certain embodiments of the methods disclosed herein,
control over the temperature zones (FIG. 1) and an understanding of
the polymer solution gel rheology are ideal. As disclosed herein,
the range of temperatures for gel-electrospinning may differ from
one temperature zone to another. The four temperature zones, as
labeled in FIG. 1, are: solution reservoir (T.sub.1), the extruded
jet (T.sub.2), the space around the jet (T.sub.3), and the
collector (T.sub.4).
[0021] In certain embodiments, the operable temperature window for
each zone varies based on the gelation temperature (T.sub.gel) of
the solution. T.sub.gel can typically be obtained from rheological
experimental data (see e.g., Example 6 and FIG. 2, Panel A).
[0022] As used herein, the "gelation temperature" is the maximum
temperature at which a polymer solution forms a gel. Above the
gelation temperature, a polymer solution ceases to exist in a gel
state.
[0023] As used herein, a "gel" is a three dimensional cross-linked
network that swells in a solvent to a certain finite extent, but
does not dissolve in even a good solvent.
Exemplary Methods
[0024] In certain embodiments, the invention relates to a method of
forming a plurality of fibers, comprising the steps of:
[0025] placing a polymer solution in a vessel comprising a
spinneret; wherein the polymer solution comprises a polymer and a
solvent, the polymer solution has a gelation temperature and a
viscosity, the solvent has a boiling point, the temperature of the
polymer solution in the vessel is in the range from the boiling
point of the solvent to the gelation temperature, and the viscosity
of the polymer solution is less than about 150 Poise; and
[0026] electrostatically drawing the polymer solution through the
spinneret into an electric field, wherein the temperature of the
polymer solution as it is drawn through the spinneret is in the
range from about 15.degree. C. below the gelation temperature to
the gelation temperature, thereby depositing a plurality of fibers
on a collection surface; wherein the spinneret is separated from
the collection surface by a space.
[0027] In certain embodiments, the viscosity of the polymer
solution in the vessel is less than about 125 Poise or less than
about 100 Poise.
[0028] In certain embodiments, the temperature of the polymer
solution in the vessel is in the range from about 40.degree. C.
above the gelation temperature to the gelation temperature, the
temperature of the polymer solution in the vessel is in the range
from about 35.degree. C. above the gelation temperature to the
gelation temperature, the temperature of the polymer solution in
the vessel is in the range from about 30.degree. C. above the
gelation temperature to the gelation temperature, the temperature
of the polymer solution in the vessel is in the range from about
25.degree. C. above the gelation temperature to the gelation
temperature, the temperature of the polymer solution in the vessel
is in the range from about 20.degree. C. above the gelation
temperature to the gelation temperature, the temperature of the
polymer solution in the vessel is in the range from about
15.degree. C. above the gelation temperature to the gelation
temperature, the temperature of the polymer solution in the vessel
is in the range from about 10.degree. C. above the gelation
temperature to the gelation temperature, from about 5.degree. C.
above the gelation temperature to the gelation temperature, from
about 15.degree. C. above the gelation temperature to about
5.degree. C. above the gelation temperature, from about 15.degree.
C. above the gelation temperature to about 10.degree. C. above the
gelation temperature, or from about 10.degree. C. above the
gelation temperature to about 5.degree. C. above the gelation
temperature.
[0029] In certain embodiments, the temperature of the polymer
solution as it is drawn through the spinneret is in the range from
about 10.degree. C. below the gelation temperature to the gelation
temperature, from about 5.degree. C. below the gelation temperature
to the gelation temperature, from about 15.degree. C. below the
gelation temperature to about 5.degree. C. below the gelation
temperature, from about 15.degree. C. below the gelation
temperature to about 10.degree. C. below the gelation temperature,
or from about 10.degree. C. below the gelation temperature to about
5.degree. C. below the gelation temperature.
[0030] In certain embodiments, the methods disclosed herein further
comprise applying heat to the space between the spinneret and the
collection surface.
[0031] In certain embodiments, the polymer solution is heated in
the vessel.
[0032] In certain embodiments, the polymer solution is heated prior
to being placed in the vessel. In certain embodiments, prior to
being placed in the vessel the polymer solution is heated to a
temperature in the range from its gelation temperature to the
boiling point of the solvent.
[0033] In certain embodiments, the space between the spinneret and
the collection surface is heated to a space temperature in the
range from about 15.degree. C. below the gelation temperature to
the gelation temperature, from about 10.degree. C. below the
gelation temperature to the gelation temperature, from about
5.degree. C. below the gelation temperature to the gelation
temperature, from about 15.degree. C. below the gelation
temperature to about 5.degree. C. below the gelation temperature,
from about 15.degree. C. below the gelation temperature to about
10.degree. C. below the gelation temperature, or from about
10.degree. C. below the gelation temperature to about 5.degree. C.
below the gelation temperature.
[0034] In certain embodiments of the methods disclosed herein, a
positive electrical potential is maintained on the spinneret, and a
negative electrical potential is maintained on the collection
surface.
[0035] In certain embodiments, the polymer solution comprises
ultra-high molecular weight polyethylene (UHMWPE).
[0036] In certain embodiments, the solvent comprises decalin,
o-dichlorobenzene, p-xylene, cyclohexanone, or paraffin oil. In
certain embodiments, the solvent is a mixture of p-xylene and
cyclohexanone. In certain embodiments, the solvent is p-xylene.
[0037] In certain embodiments of the methods disclosed herein, the
collection surface is at a temperature in the range from about
15.degree. C. below the gelation temperature to the gelation
temperature, from about 10.degree. C. below the gelation
temperature to the gelation temperature, from about 5.degree. C.
below the gelation temperature to the gelation temperature, from
about 15.degree. C. below the gelation temperature to about
5.degree. C. below the gelation temperature, from about 15.degree.
C. below the gelation temperature to about 10.degree. C. below the
gelation temperature, or from about 10.degree. C. below the
gelation temperature to about 5.degree. C. below the gelation
temperature.
[0038] In certain embodiments, the invention relates to any one of
the aforementioned methods, wherein the polymer solution further
comprises a salt. In certain embodiments, the salt is tetra-butyl
ammonium bromide (t-BAB) or tetra-butylammonium hydrogen sulfate
(t-BAHS). In certain embodiments, the salt is tetra-butyl ammonium
bromide (t-BAB).
[0039] In certain embodiments, to electrostatically draw the
polymer solution through the spinneret a high voltage is applied to
the polymer solution such that a charged meniscus forms at the
spinneret, which emits a jet when the voltage is above a critical
value. In certain embodiments, the electric voltage is about 1 kV
to about 100 kV.
Exemplary Fibers
[0040] In certain embodiments, the invention relates to a nanofiber
made by any one of the methods disclosed herein.
[0041] In certain embodiments, the diameter of the nanofiber is
about 1 nm to about 1 .mu.m, about 10 nm to about 1 .mu.m, about
100 nm to about 1 .mu.m, about 10 nm to about 500 nm, or about 100
nm to about 500 nm.
[0042] In certain embodiments, the Young's modulus of the fiber is
in the range from about 85 GPa to about 1000 GPa, from about 90 GPa
to about 1000 GPa, from about 95 GPa to about 1000 GPa, or from
about 100 GPa to about 1000 GPa.
[0043] In certain embodiments, the yield stress of the fiber is in
the range from about 2 GPa to about 100 GPa, from about 3 GPa to
about 100 GPa, from about 4 GPa to about 100 GPa, from about 5 GPa
to about 100 GPa, from about 6 GPa to about 100 GPa, or from about
7 GPa to about 100 GPa.
EXEMPLIFICATION
[0044] The invention now being generally described, it will be more
readily understood by reference to the following examples, which
are included merely for purposes of illustration of certain aspects
and embodiments of the invention, and are not intended to limit the
invention.
Example 1--UHMWPE Solution Characterization
[0045] Ultra high molecular weight polyethylene (UHMWPE) with a
molecular weight of 2000 kg mol.sup.-1 was purchased from Ticona.
p-xylene and t-BABs were both purchased from Sigma-Aldrich.
Typically, a solution consisted of 1 wt % UHMWPE with 0.02 t-BABs
dissolved in p-xylene. The solution was mixed at a room temperature
and immediately put on a heated (.about.120.degree. C.) stirrer for
at least 2 hours. The crystallization and melting temperatures of
the polymer in solution were obtained by differential scanning
calorimetry (DSC, TA Instruments). The first cooling cycle began
from 130.degree. C. to 40.degree. C., and the following heating
cycle brought the temperature back up to 130.degree. C. The heating
and cooling rates were fixed at 1.degree. C. min.sup.-1. A
rheometer (AR-2000, TA Instruments) was used to measure the
viscosity of the polymer solution as a function of temperature. To
prevent the loss of the volatile p-xylene solvent during rheometry
at elevated temperature (T>100.degree. C.), a solvent trap
filled with p-xylene was used. A temperature sweep from 120.degree.
C. to 40.degree. C. with a constant shear rate of 1 rad s.sup.-1
was performed. An oscillatory shear with the same temperature range
sweep at a fixed shear rate of 1 rad s.sup.-1 was also performed to
obtain the elastic and storage moduli.
Example 2--UHMWPE Nanofiber Fabrication
[0046] To fabricate high performance nanofibers continuously, the
gel-electrospinning process was divided into four zones. In each
zone, the temperature was chosen judiciously based on knowledge of
the polymer solution gel rheology, and care was taken to control
the temperature within each zone. The four zones are: the solution
reservoir, the extruder exit, the draw zone, which includes both
steady jet and whipping regions, and the collector. FIG. 1, Panel A
shows an apparatus for the gel-electrospinning of UHMWPE. The
temperatures of the zones are labelled T.sub.1 through T.sub.4 in
FIG. 1, Panel A. FIG. 1, Panel B shows a schematic of the molecular
organization within a hypothetical gel-electrospinning process; the
molecules are dilute and entangled at the extruder exit, but
crystallized and oriented at the collector. In the apparatus,
T.sub.1 and T.sub.3 were controlled independently using a ceramic
band heater and a space heater, respectively. T.sub.2 was found to
be equal or slightly below T.sub.1
(T.sub.2-T.sub.1.ltoreq.10.degree. C.). T.sub.3 and T.sub.4 stayed
mostly equal throughout the duration of the experiments, with the
biggest difference observed at any point being
T.sub.3=T.sub.4+5.degree. C.
[0047] To fabricate a UHMWPE Nanofiber, a spinning solution
comprising UHMWPE (1 wt %), p-xylene, and t-BABs (0.2 wt %) was
used. The solution was mixed at room temperature and immediately
put on a heated (.about.120.degree. C.) stirrer for 2 hours. The
solution was then transferred to a pre-heated glass syringe
(Cadence Science, 20 mL). A band heater (Plastic Processing
Equipment) was used to heat the solution-filled syringe. A Macor
ceramic encasing was used as an electrical insulator between the
heater and the needle, while still providing a good thermal
conductivity and ability to withstand a maximum process temperature
of 170.degree. C. A cylindrical ceramic space heater (Omega
Engineering) was used to heat the space around the needle.
[0048] For an optimal electrospinning condition, the temperature of
four process zones (FIG. 1) were set at T.sub.1=T.sub.2=130.degree.
C., while T.sub.3 and T.sub.4 were varied from 20.degree. C. to
130.degree. C. The volumetric flow of the feed solution, controlled
by a syringe pump (Harvard apparatus), was controlled from 0.02
ml/min to 0.2 ml/min. A negative electrical potential (-10 to -15
kV) was used on the collector while a positive potential (+15 to 20
kV) was maintained on the spinneret. The distance from the tip of
the needle to the collector was fixed at 300 mm.
Example 3--Electron Microscopy Characterization
[0049] A JEOL 6010LA scanning electron microscope (SEM) was used to
observe the fiber and mat morphology and to measure the fiber
diameter. Prior to the sample loading, the electrospun fibers were
sputter-coated with Au for 30 seconds. A Tecnai T-12 transmission
electron microscope (TEM) was used to observe the single fiber
structure and diameter. The UHMWPE fibers were placed on a standard
copper grid, and subsequently observed under the TEM.
[0050] FIG. 7 shows a SEM image of a gel-electrospun UHMWPE fiber
mat fabricated over a period of 120 minutes (98 mg total mass).
FIG. 3, Panel A shows a UHMWPE fiber bundle of 8 mg fabricated over
10 minutes with this procedure. FIG. 3, Panel B shows TEM images of
the individual UHMWPE fibers. The mean diameter and distribution of
FIG. 7 were 2.12.+-.0.92 .mu.m, while those of FIG. 3, Panel b were
1.41.+-.0.60 .mu.m. As seen in FIG. 3, Panel B, some of the
individual fibers among the fiber mat are ultra-thin (e.g.,
submicron), ranging from 10's of nm to 200 nm. The smallest fiber
observed here was about 20 nm (e.g., 0.025 .mu.m), which is within
an order of magnitude to a single orthorhombic PE crystal size and
is similar to a core size of polyethylene shish-kebab structures.
Presumably, these particularly thin UHMWPE fibers have undergone
high uniaxial extensional strain rate of .about.1000 s.sup.-1 or
more.
Example 4--Crystal Characterization
[0051] DSC was used to obtain the overall degree of crystallinity.
The following equation was used to calculate the percent
crystallinity, X
X = .DELTA. H m - .DELTA. H c .DELTA. H m .degree. ##EQU00001##
where .DELTA.H.sub.n, was obtained by integrating the melting peak
from the heating cycle, and .DELTA.H.degree..sub.m is the specific
enthalpy of fusion of polyethylene. Since cold crystallization was
not observed, .DELTA.H.sub.c=0. The General Area Detector
Diffraction System (GADDS, Bruker) was used to measure the
wide-angle X-ray diffraction pattern of the fiber bundles. The
degree of crystallinity was obtained by integrating the relative
intensities of the crystalline peaks with amorphous halos.
Example 5--Fiber Mechanical Measurements
[0052] A single-fiber mechanical test was performed using a U9815A
T150 Universal Testing Machine ("Nano-UTM", Agilent Technologies)
which is also known as the Nano-UTM. The tensile test method was
directly adopted from the previous work of Pai et al. on measuring
the single fiber tensile properties of PA(6) T. (See C. L. Pai, M.
C. Boyce, G. C. Rutledge, Polymer 2011, 52, 2295). The force was
measured as a function of the extensional strain for individual
electrospun fibers in uniaxial tension at a strain rate of
10.sup.-3 s.sup.-1. The Young's modulus was determined by linear
regression of the stress-strain curve from the origin to a low
strain of about 0.01. Following Pai et al.'s protocol, the
undeformed section of the fiber was observed under SEM after
sputter-coating to examine its diameter. The diameters of five
different sections were measured to determine the fiber diameter
and its variability within the individual fiber (see FIG. 6). It
should be noted that if the standard deviation of the five
measurements for an individual fiber was greater than 20%, the data
point was discarded.
[0053] FIG. 4, Panel A shows the representative stress-strain
curves for gel-electrospun UHMWPE fibers with diameters of 0.49,
0.73, 0.91, 1.05, and 2.31 .mu.m. As seen here, the linear
regression slope from the origin to a strain of 0.01 mm/mm
increased dramatically for fibers whose diameters were nearly as
small as 1 .mu.m, and was even higher for those whose diameters
were submicron. The Young's moduli are plotted against fiber
diameters in FIG. 4, Panel B which shows a dramatic increase in
Young's modulus as the fiber diameter decreases below one micron.
Many of the sub-micron UHMWPE fibers yielded relatively high
Young's moduli, above 30 GPa, which was expected as the higher
extensional strain obtained by the electrical gel-drawing would
likely induce the smaller fiber diameter. Fibers with d.ltoreq.0.60
.mu.m exhibited moduli above 100 GPa In particular, the Young's
modulus of the 0.35.+-.0.05 .mu.m fiber was 120.+-.24 GPa, which is
the highest reported modulus for a single fiber produced by any
electrostatically-driven jetting process, and is comparable to that
of a commercial high performance Spectra.RTM. (see Table 1). It
should be noted that due to the irregularity of some of the fiber
diameters, the Young's modulus values displayed a relatively
significant margin of error as much as 15%. Since the Young's
modulus is inversely proportional to d.sup.2 a slight variation in
smaller fiber diameters (d<1 .mu.m) significantly affected the
moduli error bar. Despite the slight deviations of the reported
data, the mean Young's modulus of the smaller fibers (d<1 .mu.m)
was 73.+-.4 GPa, which is two orders of magnitude higher than the
bulk modulus of UHMWPE. Up to .about.500.times. improvement of
modulus with the size reduction of fiber from 10.1 .mu.m to 0.35
.mu.m was also observed, which is the largest improvement of
modulus by diameter reduction reported for any
electrostatically-driven jetting process.
[0054] These gel-electrospun fibers also exhibited higher yield
stress as the fiber diameter was decreased, as shown in FIG. 4,
Panel C. The magnitude of yield stress improvement with size
reduction of the largest to the smallest fiber was about
600.times.. The mean yield stress of the smaller fibers (d<1
.mu.m) was 3.5.+-.0.2 GPa, which is two orders of magnitude higher
than the bulk of UHMWPE value and similar to a typical ultimate
tensile strength of a Spectra.RTM. fiber. Since the tensile
strength is generally greater than the yield stress, this implies
that both the fiber strength and modulus of the smallest
gel-electrospun nanofibers are comparable to or higher than those
of a commercial high performance microfiber. In fact, as shown in
Table 1, the ultimate tensile strength of the UHMWPE fibers with
d=0.73 and 0.49 .mu.m were both about 1.5.times. the reported
tensile strength of a Spectra.RTM.. The toughness, on the other
hand, did not show a clear trend of change with respect to
decreasing fiber diameter below 1 .mu.m. Due to its highly
crystalline nature, the elongation at break decreased with
reduction of fiber diameter, or yielded more brittle behavior.
However, the decreased flexibility is still offset by the increased
strength, hence the toughness remained to be approximately 2.0 GPa
in all smaller fibers. These toughness values are three times
greater than the highest toughness reported, and exhibit much
higher strain at break than most other high performance fibers
which does not exceed .about.4%.
TABLE-US-00001 TABLE 1 Mechanical properties for selected
electrospun UHMWPE fibers over a range of diameters, compared with
a typical Spectra .RTM. fiber. Fiber Young's Diameter Modulus
Strength Toughness Strain at (.mu.m) (GPa) (GPa) (GPa) Break 0.49
.+-. 0.05 110 .+-. 16 6.3 .+-. 0.9 2.1 .+-. 0.3 0.36 0.73 .+-. 0.08
72 .+-. 11 5.4 .+-. 0.8 1.7 .+-. 0.3 0.40 0.91 .+-. 0.12 19 .+-. 4
3.5 .+-. 0.7 2.3 .+-. 0.8 0.87 1.05 .+-. 0.03 6.85 .+-. 0.28 1.73
.+-. 0.07 2.33 .+-. 0.09 1.82 2.31 .+-. 0.26 1.68 .+-. 0.27 0.55
.+-. 0.09 0.75 .+-. 0.12 1.85 10.0 133 3.68 -- 0.03 (Spectra
.RTM.)
Example 6--Determination of Temperature Ranges for an Electrical
Gel-Drawing
[0055] To promote gel-drawing in the whipping zone (T.sub.3 of FIG.
1), the polymer solution is in a semi-dilute state, or a gel-state,
in the whipping region. At the same time, the gel viscosity is
around 100 Poise or lower to promote spinnability. The
viscoelasticity of a polymer solution heavily depends on the
solvent, concentration, molecular weight of the solute, and
temperature. From preliminary gel-electrospinning experiments (see
example 8), p-xylene/UHMWPE solution yielded the highest production
rate among the good PE solvents, and relatively monodisperse small
fiber diameter sizes.
[0056] FIG. 2, Panel A, and FIG. 2, Panel B, show the complex
viscoelastic behaviors of 1 wt % p-xylene/UHMWPE solution at a
constant oscillatory stress (0.88 Pa) and a constant strain (5%),
respectively. While cooling down, the differences between storage
(G') and loss moduli (G'') at each temperature were kept fairly
constant until T=84.8.degree. C. (FIG. 2, Panel A) and
T=84.7.degree. C. (FIG. 2, Panel B). At these respective points, a
drastic transition of steepened slopes of storage and loss modulus
was observed, followed by subsequent declines of the slopes at
T=81.7.degree. C. (FIG. 2a) and T=81.4.degree. C. (FIG. 2, Panel
B). Below this temperature, G' was about an order of magnitude
larger than G''. The onset temperatures of the sol-gel transition
observed from the rheological experiments closely matched the onset
transitional temperature of 84.1.degree. C. from the cooling cycle
of p-xylene/UHMWPE solution from DSC (c.f. FIG. 5). Based on these
results, the onset of thermoreversible gel formation, or T.sub.gel,
was determined to be approximately between 84-85.degree. C. The
solution viscosity, .eta., was .eta..ltoreq.100 Pas when
T.gtoreq.80.degree. C. (cf. FIG. 2, Panel A, and FIG. 2, Panel B).
A viscosity of 100 Pas or lower is considered desirable for
continuous fiber spinning.
[0057] Based on these findings, the desired temperature within the
draw zone for gel-electrospinning was determined to be 80.degree.
C..ltoreq.T.ltoreq.85.degree. C. The spinning solution was then
gel-electrospun at various values of T.sub.3 and T.sub.4, while all
of the other parameters were held constant at values unless stated
otherwise. FIG. 2, Panel C shows the mean fiber diameter and its
distribution as a function of T.sub.3. The mean fiber diameter
clearly decreased as T.sub.3 was increased from room temperature to
80.degree. C. This reduction of fiber diameter is due to the
decrease in solution viscosity up to .about.80.degree. C. (c.f.
FIG. 2, Panel A). Above 80.degree. C., relatively similar means and
standard deviations of fiber diameter were observed. Although the
viscosity decreased by an order of magnitude above T=80.degree. C.
(c.f. FIG. 2, Panel A), the solution was no longer in a gel-state,
thus it was difficult to observe any obvious reduction of fiber
diameter due to the viscosity differences between the sol and gel
states. The UHMWPE fibers that were collected at T.sub.3=80.degree.
C. showed the smallest mean fiber diameter and the narrowest fiber
size distribution.
[0058] Thus, for a 1 wt % p-xylene/UHMWPE (MW=2.0.times.10.sup.6
g/mol) solution, suitable processing temperatures of each zones
were found to be T.sub.1, T.sub.2=130.degree. C.,
T.sub.3.about.80.degree. C., and T.sub.4.about.75.degree. C. FIG.
3, Panel A shows typical UHMWPE polymer fibers fabricated from the
UHMWPE/p-xylene (1 wt %) solution, with organic salt (tetra-butyl
ammonium bromide, or t-BABs in short) added (0.2 wt %) to increase
the electrical conductivity of the solution.
[0059] The spinning solution was then gel-electrospun and only
T.sub.3 and T.sub.4 were varied, while all the other parameters
were held constant. Unless stated otherwise, the other processing
parameters were held constant as described in the examples above. A
series of experiments consistently revealed that T.sub.3 and
T.sub.4 stayed mostly equal throughout the duration of the
experiment, with the biggest difference observed at any point being
T.sub.3=T.sub.4+5.degree. C. FIG. 2, Panel B shows the mean
diameter and its distribution as a function of T.sub.3. The
distribution and mean fiber diameter clearly decrease as T.sub.3
was increased from room temperature to 80.degree. C. This was
expected as the solution viscosity decreased when the temperature
was increased up to .about.80.degree. C. (FIG. 2, Panel A). As the
temperature was raised above 80.degree. C., no obvious trend of
fiber diameter nor its distribution was observed. The solution
viscosity stayed relatively constant, on the order of 100 Poise
above 80.degree. C., which resulted in relatively similar fiber
diameters and their distributions. Judging from the suggested
preferred gel-electrospinning window of 79.degree.
C..ltoreq.T.ltoreq.90.degree. C., the UHMWPE fibers that were
collected at T.sub.3=80.degree. C. were gel-electrospun.
Example 7--Empirical Relationship Between Fiber Diameter and
Modulus
[0060] The overall crystallinity of UHMWPE nanofiber mat was around
60%, from analysis of a DSC result. The relatively low degree of
crystallinity was largely a result of the polydispersity in fiber
diameters within a fiber mat, which ranged from submicron (high
crystallinity) to micron (low crystallinity). A wide-angle X-ray
diffraction (WAXD) trace of a fiber bundle of d=0.9.+-.0.2 .mu.m
(FIG. 4, Panel D) yielded 90% crystallinity (orthorhombic PE
crystal). These results provided insights on the trend of
mechanical properties between submicron and micron fibers in Table
1 and FIG. 4, Panels B and c. When d>1 .mu.m, the fiber yielded
low modulus yet a high strain at break, which are typical
mechanical behaviors of a low crystallinity material. When d<1
.mu.m, the fiber behaved like a highly crystalline material,
yielding higher modulus and a relatively lower strain at break.
These results further confirmed that the low degree of
crystallinity observed in a fiber mat was due to the presence of
low crystallinity micron fibers among the highly crystalline
submicron fibers.
[0061] These mechanical enhancements of smaller fibers are the
result of larger growth amplitude of the whipping instability,
which resulted in higher drawing ratio, better molecular
orientation, and thus higher degree of crystallinity. An empirical
correlation between the Young's modulus and the fiber diameter was
derived from FIG. 4, Panel B. The fitted power-law correlation
was
E=14.83(d.sup.-2.22)
which was a good fit for the data, with the R.sup.2=0.96. From this
empirical relationship, it is possible to relate the Hencky strain,
c, with the modulus as well. The Hencky strain is defined as
follows:
= 2 ln ( h 0 h mid ( t ) ) ##EQU00002##
which is an indicator of the extensional strain imposed in the
gel-electrospinning process. h.sub.0 is the initial diameter of the
unstretched fluid filament, assumed to be 100 .mu.m. h.sub.mid(t)
is a time-dependent diameter of the stretched fluid, which was
estimated as the as-spun fiber diameter divided by the square root
of the polymer concentration to approximate the terminal jet
diameter before the solvent evaporation. From these known
parameters, a relationship between the modulus and Hencky strain
was derived,
E=0.0005e.sup.1.11.epsilon.
implying that the modulus increases exponentially as the Hencky
strain increases. This result supports that the higher molecular
orientation was induced as the extensional strain of the gel was
increased with the whipping instability. The high molecular
orientation, which was more pronounced for d<1 .mu.m,
synergistically increased the degree of crystallinity and yielded
an exponential increase of modulus with the reduction of the fiber
diameter.
Example 8--Electrospinning Solution Composition
[0062] Several electrospinning solution compositions were examined
for a solution that yielded a high productivity and small fiber
diameters with a narrow distribution. Table 2 shows the results of
electrospinning solution of 1 wt % UHMWPE in several different
solvents. In each case, 0.2 wt % of tetra-butyl ammonium bromide
(t-BAB) was added to increase the electrical conductivity of the
solution up to .about.0.2 .mu.S/cm; the addition of this salt
facilitated the continuous production of UHMWPE fibers with
acceptable production rate. For these preliminary experiments,
T.sub.1 and T.sub.2 were both set at 130.degree. C., which was
above T.sub.melt and below T.sub.boil of all the solvents used.
T.sub.3 and T.sub.4 were fixed at a room temperature. The
p-xylene/UHMWPE solution yielded the highest production rate among
the good PE solvents tested, and the fiber diameters were
relatively small and monodisperse.
TABLE-US-00002 TABLE 2 Electrospinning assessment of UHMWPE with
different solvents. Productivity Mean Fiber Diameter PE Solvent
(mg/h) (.mu.m) decalin 1.0 6.13 .+-. 2.34 p-xylene 25 2.72 .+-.
1.33 p-xylene:cyclohexanone 5.0 3.26 .+-. 0.74 (1:1 v %)
Example 9--Gel-Electrospun Fibers Crystallinity
[0063] The crystallinity of the gel-electrospun fibers was examined
by DSC, WAXD, and SAED The degree of crystallinity of the UHMWPE
fiber mat was calculated from results of both DSC (see FIG. 11) and
WAXD (FIG. 8), which yielded values of 56% and 58%, respectively.
By contrast, the degree of crystallinity of the fiber bundle having
d=1.41.+-.0.60 .mu.m (FIG. 8) was close to 90%, as determined by
WAXD and confirmed to be the orthorhombic crystal form of PE based
on peak locations. DSC was not used to measure the degree of
crystallinity for the fiber bundle sample due to the small amount
of the sample available.
[0064] FIG. 9 shows representative SAED patterns and the
corresponding TEMs of single UHMWPE fibers with different
diameters. All of the patterns in FIG. 9 are indicative of the
orthorhombic PE crystal, in accord with the WAXD results (FIG. 8).
However, crystal orientation within the fibers became significantly
sharper with decreasing diameter. The thickest fiber, d=1.95 .mu.m,
showed random crystal orientation, as signified by the ring SAED
pattern; other fibers with d>1 .mu.m all displayed such
patterns. The thinner fiber in the second column of FIG. 9 (d=0.42
.mu.m) exhibited an arc-shaped reflection, which corresponded to a
distribution of orientations of the 110 and 200 lattice planes.
Even higher crystal orientation was observed when d=0.11 .mu.m
(third column of FIG. 3d), whose pattern was that typical of a
single crystal.
Example 10--Comparison with Commercial Fibers
[0065] FIG. 10 compares the highest mechanical properties attained
from the methods disclosed herein with those of other commercial
polymer fibers. In general, high performance fibers yielded modulus
well above 50 GPa and tensile strength greater than 2.0 GPa, but
none exhibited elongation at break above 3-4%. By contrast, more
flexible commercial fibers yielded 20-30% strains at break, yet
exhibited modest modulus below 20 GPa and strength below 1.0 GPa.
The gel-electrospun UHMWPE fiber yielded modulus higher than 100
GPa, a common threshold used to identify a high performance fiber,
and remarkably high tensile strength of 6.3 GPa, which even exceeds
that of a high modulus Zyron.RTM. fiber. This tensile strength is
also the highest known among the individual polymer fibers
fabricated by any electrostatically-driven jetting process. Even
with such high strength and modulus, a high strain at break of 36%
was achieved, which is at least a ten-fold increase compared to any
other conventional high performance fiber.
Example 11--Wide-Angle X-Ray Diffraction (WAXD)
[0066] A Bruker D8 with General Area Detector Diffraction System
was used to measure the Wide-Angle X-ray Diffraction (WAXD) trace
of fiber mats and bundles. Two-dimensional X-ray diffraction
patterns were measured, integrated, with a background subtraction
to obtain one-dimensional XRD patterns in
15.0.degree..ltoreq.2.theta..ltoreq.60.0.degree.. The degree of
crystallinity was obtained using X.sub.WAXD=I.sub.xtal
(I.sub.xtal+I.sub.amorph), where I.sub.xtal is the integrated area
of the crystalline peaks and I.sub.amorph is the integrated area of
the amorphous peak. In the case of polyethylene, the crystalline
peaks for the 110 and 200 planes were found at
2.theta.=21.4.degree. and 23.9.degree., respectively. The amorphous
halo was defined as a broad peak in the range
15.0.degree..ltoreq.2.theta..ltoreq.25.0.degree..
INCORPORATION BY REFERENCE
[0067] All of the U.S. patents and U.S. published patent
applications cited herein are hereby incorporated by reference.
EQUIVALENTS
[0068] Those skilled in the art will recognize, or be able to
ascertain using no more than routine experimentation, many
equivalents to the specific embodiments of the invention described
herein. Such equivalents are intended to be encompassed by the
following claims.
* * * * *