U.S. patent application number 15/932506 was filed with the patent office on 2018-09-13 for self-strengthening polymer composites.
The applicant listed for this patent is Iowa State University Research Foundation, Inc.. Invention is credited to Boyce S. Chang, Martin M. Thuo.
Application Number | 20180258235 15/932506 |
Document ID | / |
Family ID | 63446893 |
Filed Date | 2018-09-13 |
United States Patent
Application |
20180258235 |
Kind Code |
A1 |
Thuo; Martin M. ; et
al. |
September 13, 2018 |
SELF-STRENGTHENING POLYMER COMPOSITES
Abstract
A composite material is provided including an elastomeric
polymer matrix and undercooled liquid metallic core-shell particles
disposed in the matrix, wherein the particles each have an outer
shell and a liquid metallic material as a core contained within the
outer shell. The outer shell is frangible such that the liquid
metallic material is released from at least some of the particles
in response to a mechanical load applied to the composite and
solidifies in-situ in the polymer matrix. As a result, the
composite material can be self-strengthening and self-healing and
can be reconfigurable in shape at ambient temperature.
Inventors: |
Thuo; Martin M.; (Ames,
IA) ; Chang; Boyce S.; (Ames, IA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Iowa State University Research Foundation, Inc. |
Ames |
IA |
US |
|
|
Family ID: |
63446893 |
Appl. No.: |
15/932506 |
Filed: |
March 7, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62600979 |
Mar 8, 2017 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C08L 2207/53 20130101;
C08L 83/04 20130101; C08J 3/095 20130101; C08K 3/08 20130101; C08J
2383/04 20130101; B29C 73/22 20130101; C09K 5/06 20130101; C08F
2500/21 20130101; C23C 22/00 20130101; C08J 3/12 20130101; C08L
9/00 20130101; C08J 2300/26 20130101; C08L 83/04 20130101; C09K
5/06 20130101 |
International
Class: |
C08J 3/09 20060101
C08J003/09; C08K 3/08 20060101 C08K003/08 |
Claims
1. A composite material, comprising an elastomeric polymer matrix
and undercooled liquid metallic core-shell particles disposed in
the matrix, said particles each having an outer shell and a liquid
metallic material as a core contained within the outer shell, which
is frangible such that the liquid metallic material is released
from at least some of the particles in response to a mechanical
load applied to the composite and phase-changes in-situ in the
polymer matrix.
2. The composite material of claim 1 wherein the released liquid
metallic material is selected to solidify in-situ in the polymer
matrix.
3. The composite material of claim 1 wherein the elastomeric
polymer comprises a single polymer or a copolymer.
4. The composite material of claim 1 wherein the outer shell
comprises an oxide shell.
5. The composite material of claim 1 wherein the outer shell is
functionalized with an organic moiety.
6. The composite material of claim 1 wherein at least some of the
particles are fused together by inter-particle solidification of
the released liquid metallic material after the mechanical load is
applied.
7. A self-strengthening composite material, comprising an
elastomeric polymer matrix and undercooled liquid metallic
core-shell particles disposed in the matrix, said particles each
having an outer shell and a liquid metallic material as a core
contained within the outer shell, which is frangible such that the
liquid metallic material is released from at least some of the
particles in response to a mechanical load applied to the composite
and solidifies within the polymer matrix to increase the strength
of the composite.
8. The composite material of claim 7 wherein the elastomeric
polymer comprises a single polymer or a copolymer.
9. The composite material of claim 7 wherein the outer shell
comprises an oxide shell.
10. The composite material of claim 7 wherein the outer shell is
functionalized with an organic moiety.
11. A reconfigurable composite material, comprising an elastomeric
polymer matrix and undercooled liquid metallic core-shell particles
disposed in the matrix, said particles each having an outer shell
and a liquid metallic material as a core contained within the outer
shell, which is frangible such that the liquid metallic material is
released from at least some of the particles in response to a
mechanical load applied to the composite to change its shape and
solidifies within the polymer matrix to retain the changed shape of
the composite.
12. The composite material of claim 11 wherein the elastomeric
polymer comprises a single or a copolymer.
13. The composite material of claim 11 wherein the outer shell
comprises an oxide shell.
14. The composite material of claim 11 wherein the outer shell is
functionalized with an organic moiety.
15. A method of making a polymer composite, comprising mixing
undercooled liquid metallic core-shell particles and an elastomeric
polymer and curing the polymer to form the polymer composite.
16. The method of claim 15 wherein the elastomeric polymer
comprises a single polymer or a copolymer.
17. The method of claim 15 wherein the outer shell comprises an
oxide shell.
18. The method of claim 15 wherein the outer shell is
functionalized with an organic moiety.
19. A method of treating a polymer composite having an elastomeric
polymer matrix and undercooled liquid metallic core-shell particles
disposed in the matrix by applying a mechanical load to the
composite in a manner to release liquid metallic material from at
least some of the particles and solidifying the released liquid
metallic material in-situ in the polymer matrix.
20. The method of claim 19 wherein the composite is treated by
applying a tensile mechanical load or a compressive mechanical load
to the composite.
Description
RELATED APPLICATION
[0001] This application claims benefit and priority of provisional
application Ser. No. 62/600,979 filed Mar. 8, 2017, the entire
disclosure and drawings of which are incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The invention relates to a composite material comprising an
elastomeric polymer matrix containing undercooled liquid metallic
core-shell particles that impart self-strengthening, self-healing,
and reconfigurable shaping properties to the composite
material.
BACKGROUND OF THE INVENTION
[0003] Composite materials have drastically evolved since their
first conception, centered on a simple approach of invoking synergy
between different materials. The field has since shifted towards
creating so called responsive composites, which exhibit tunable
properties triggered via an external stimulus. Such materials have
been applied as sensors, actuators and platforms for
multi-functional devices..sup.[1-9] Mechanically reconfigurable
parts are especially desirable in the field of soft robotics,
whereby adaptive components are essential for
locomotion..sup.[10-12] Several forms of activation have been
applied in responsive composites; common examples include
temperature, electromagnetic radiation, electric and magnetic
field..sup.[13-19] Mechanical stimulus, although scarce could
potentially offer important applications such as self-healing,
sound engineering and mechanically adaptive materials. White et al.
demonstrated autonomic healing capabilities in a polymeric
composite by introducing monomer containing microcapsules, which
undergoes polymerization upon contact with the catalyst filled
matrix. Crack formation in the composite presumably breaks the
microcapsules, which allows the monomer to escape into the matrix
and concomitantly polymerizes, thus, preventing further propagation
by filling the crack..sup.[20] Synthesis of such microcapsules,
however, involve time consuming procedures and a variety of
reagents. Furthermore, considering that a catalyst must be
incorporated into the matrix, the type of polymer applicable to
this method might be limited.
[0004] Recently, Tevis et al. developed a simple and low cost
method known as SLICE for producing undercooled liquid metal
core-shell (undercooled LMCS) particles..sup.[21] This method
involves shearing liquid metals as an emulsion to produce
spherical-like particles wherein separation of the particles is
maintained by concomitantly oxidizing the surface of the metal.
inar et al. applied this technique to produce undercooled liquid
metal undercooled particles in a metastable state whereby a
normally solid metal maintains itself as a liquid below its melting
temperature..sup.[22] Interestingly, it was demonstrated that
protective oxide is partly elastic, and further deformation will
eventually lead to solidification of the undercooled metal.
Utilizing this phase transformation phenomena, these investigators
showed that such undercooled liquid metal particles can be used as
heat-free solders, triggered by mechanical deformation.
SUMMARY OF THE INVENTION
[0005] The present invention involves incorporation of metastable,
undercooled liquid metallic core-shell particles into a polymer
matrix to provide a composite material exhibiting strengthening
(stiffening) behavior as a response to mechanical load due to phase
change (e.g. solidification). This behavior can be employed to form
a reconfigurable composite material, which will also exhibit
temperature induced shape memory effect.
[0006] An illustrative embodiment involves a composite material
comprising an elastomeric polymer matrix and undercooled liquid
metallic core-shell particles disposed in the matrix, wherein the
particles each have an outer shell and a liquid metallic material
as a core contained within the outer shell. The outer shell is
frangible (e.g. breakable) such that the liquid metallic material
is released from at least some of the particles in response to a
mechanical load, such as a compressive or tensile stress, applied
to the composite and phase-changes in-situ in the polymer matrix.
As a result, the composite material can be self-strengthening and
self-healing and can be reconfigurable in shape, all at and near
room temperature.
[0007] These and other advantages and objects of the present
invention will become more readily apparent from the following
description taken with the following drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic view showing deformation of a
composite material pursuant to an embodiment of the invention
leading to rupture and solidification of the released undercooled
liquid metallic core of the particles in the polymer matrix.
[0009] FIG. 2a is a schematic of an undercooled liquid metal
core-shell (undercooled LMCS) particle. FIG. 2b shows differential
scanning calorimetry of undercooled FM LMCS particles. FIGS. 2c-2d
show secondary electron images of undercooled Field's metal LMCS
particles and their solidification upon deformation where FM is
Field's metal. FIGS. 2e-2f are backscattered electron images of a
cross-section of a FM LMCS particle-silicone matrix composite.
[0010] FIG. 3a shows tensile testing results at room temperature of
the undercooled FM LMCS particle-silicone matrix composite and the
undercooled EGaIn LMCS particle-silicone matrix composite (both 22%
by volume filler). The dotted line represents the 2.sup.nd loading
on the same sample. FIG. 3b shows dynamic strain with increasing
amplitude (15 volume % FM particles and 22 volume % EGaIn
particles). FIG. 3c shows the change in skewness of the storage
modulus distribution.
[0011] FIG. 4a shows storage modulus versus amplitude of the
undercooled FM LMCS particle-silicone matrix composite of various
compositions (various filler particle volume percents) at room
temperature. FIG. 4b shows total change in stiffness, .DELTA.E' at
each composition. FIG. 4c shows change in skewness of the storage
modulus distribution.
[0012] FIGS. 5a and 5b show twisting deformation mode of the
self-strengthening the undercooled FM LMCS particle-silicone matrix
composite at room temperature. FIGS. 5c and 5d show folding
deformation mode of the self-strengthening undercooled FM LMCS
particle-silicone matrix composite. FIGS. 5e and 5f show an
embossing deformation mode of the self-strengthening undercooled FM
LMCS particle-silicone matrix composite.
[0013] FIGS. 6a-6b show a single lap joint using undercooled FM
LMCS particle-silicone matrix composite samples. FIGS. 6c-6d show
shape memory of the composite sample by thermal actuation.
[0014] FIG. 7a are stress-strain curves for pristine and
pre-compressed .PHI.=50% ST3R composites at low strain (1%) where
.PHI. is the undercooled filler particle volume fraction. FIG. 7b
shows stress-strain curves for pre-compressed .PHI.=50% composites
(note for EGaIn sample .PHI.=30% data is presented since the 50%
samples fractured upon compression). FIG. 7c shows a comparison
between Young's modulus (MPa) of pristine and pre-compressed
.PHI.=50% composites. FIG. 7d shows Young's modulus (0.2% strain)
of composites after compression with increasing .PHI.. FIG. 7e
shows a normalized complex stiffness with increasing cyclic strain
for composites with .PHI.=22%. FIG. 7f is a normalized skewness in
the distribution of complex stiffness. All tests conducted at room
temperature.
[0015] FIG. 8a showsYoung's modulus (MPa) as a function of
increasing pre-compression stress for .PHI.=50% ST3R samples. The
line between data points represents a guide to the eye. FIG. 8b
shows normalized fraction of undercooling for .PHI.=50% ST3R
composites. FIG. 8c shows cooling curve from differential scanning
calorimetry (DSC) of pristine and compressed (33 MPa) ST3R
composites.
[0016] FIGS. 9a and 9b show twist deformation leading to swirled
composite shape.
[0017] FIG. 9c shows recessed wells formed by selective compression
of a composite sample.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0018] The present invention involves a composite material having a
polymer matrix into which metastable, undercooled liquid metallic
core-shell particles are incorporated (dispersed) to impart
self-strengthening (stiffening) and self-healing behavior as a
response to mechanical load due to solidification. The composite
material can be reconfigurable in shape and can exhibit temperature
induced shape memory effect.
[0019] The polymer matrix can be selected from any suitable
elastomeric polymer including, but not limited to, silicone rubber
and its respective analogs, polybutadiene/polyisoprene and its
respective analogs, and other copolymers that exhibit elastomeric
properties. Elastomeric polymers are any polymeric materials that
exhibit large elastic deformation; that is, the ability to be
highly stretched, compressed, deformed etc. (e.g. up to 700% in
some cases) and return to their original shape when stress is
released.
[0020] The metastable, undercooled (supercooled) liquid metallic
core-shell particles can be selected from any suitable metallic
material that can be undercooled by the aforementioned SLICE
process or other process to a metastable state having a liquid
metallic core contained within a protective solid shell near and at
room temperature, which typically can be 20 to 27 degrees C. for
purposes of illustration. Such metallic materials include, but are
not limited to, Field's metal (Bi:In:Sn 32.5:51:16.5 weight %;
melting point-about 62.degree. C.)., EGaIn (Ga:In 75:24.5 weight %;
melting point-15.7.degree. C.), a Bi--Sn alloy (Bi:Sn:58:42 weight
%; melting point about 139.degree. C., Rose's metal (Bi:Pb:Sn
50:25:25 weight %; melting point-about 98.degree. C.), and
others.
[0021] The undercooled LMCS particles preferably are made by the
aforementioned SLICE process, which is an extension of droplet
emulsion technique (DET), although the particles can be made by
other techniques. The SLICE process involves shearing a low T.sub.m
(melting point) molten metal or alloy in the presence of a carrier
fluid together with chemical reaction to produce liquid metallic
core-shell nano-particles or micro-particles. The chemical reaction
typically involves oxidation of the molten metal or alloy droplets
in a manner to form an outer oxide shell in-situ on the liquid
metallic core. Moreover, the outer shell can be functionalized with
an organic moiety, such as an acetate ligand or phosphate.
[0022] The following examples are offered to illustrate, but not
limit, the present invention. Examples:
[0023] Undercooled liquid metallic core-shell (undercooled LMCS)
particles of Field's metal (FM) and eutectic galium-indium (EGaIn)
were synthesized using the SLICE method described by I. D. Tevis,
L. B. Newcomb, M. Thuo, Langmuir 2014, 30, 14308, the disclosure of
which is incorporated herein by reference to this end.
[0024] In particular, the undercooled FM LMCS particles were made
by shearing Field's metal in air in acetic acid-diethylene glycol
solution. In a typical synthesis, 1 gram of Field's metal is placed
into a vial containing 5 ml of 5% v/v acid solution. The vial was
then heated using an oil bath (heating tape can also be used) to
about 160.degree. C. A Teflon head attached to the rotary tool was
used to shear the mixture as described by I. D. Tevis, L. B.
Newcomb, M. Thuo, Langmuir 2014, 30, 14308 incorporated herein
above. The shearing was performed for 10-15 minutes and then
solution was allowed to cool at room temperature. The particles
were then filtered through filter paper and rinsed in ethanol.
[0025] The undercooled EGaIn ULMCS particles were made by shearing
EGaIn in acetic acid-water solution. In a typical synthesis, 3
grams of EGaIn is placed into a laboratory grade blender containing
100 ml of 5% v/v acid solution. The shearing (blending) was
performed for 10-15 minutes. The particles were then filtered
through filter paper and rinsed in ethanol.
[0026] These undercooled LMCS particles were incorporated into the
silicone elastomer matrix (Dow Corning Sylgard.RTM. 184) as filler
particles to form an elastomeric polymer-LMCS particle composite.
The undercooled LMCS particles were incorporated into the silicone
elastomer matrix by drying the particles from ethanol, then
directly introducing them into the pre-cured silicone elastomer.
The mixture is then stirred very gently for several minutes until a
homogeneous blend is formed. Finally, the mixture is outgassed to
removed trapped bubbles and cured at 80.degree. C.
[0027] All mechanical tests were performed via dynamic mechanical
analysis (DMA) (TA Instruments Q800) on thin film samples of the
illustrative composite materials.
[0028] The liquid core nature of the synthesized undercooled LMCS
particles (see FIG. 2a) and their survival upon incorporation into
the elastomeric matrix were evaluated by thermal analysis and
imaging. Differential scanning calorimetry (DSC) (FIG. 2b) of the
undercooled FM LMCS particles clearly depicts the underlooled
nature, considering that the event of freezing is mostly (90%)
centered at 7.degree. C. (T.sub.melt=62.degree. C.). Tiny freezing
peaks observed between 40-60.degree. C. correspond to solidified
particles, in agreement with the melting peak from the heating
curve. FIGS. 2c-2d show the FM ULMCS particles before and after
sweeping with a spatula at room temperature. The particles appears
to fuse with each other, suggesting that they were initially liquid
and upon deformation, flow before undergoing solidification.
[0029] FIGS. 2e-2f illustrate a cross-section of the undercooled FM
LMCS particle-silicone matrix composite. The observation of flow
lines (FIG. 2f) indicates the survival of at least some of the
undercooled LMCS particles during fabrication, although not all of
the incorporated particles may remain undercooled due to their
metastable nature.
[0030] Repeated tensile strain at room temperature were performed
on undrecooled LMCS particle-polymer composites using 1) FM
(Field's metal), which undergoes solidification upon particle
breakage and 2) EGaIn, a liquid metal under standard room
temperature conditions. Despite repeated strain, the pristine
silicon elastomer (Control sample) and EGaIn LMCS particle-silicone
matrix composite showed negligible change in stiffness (see FIG.
3a). The FM LMCS particle-silicone matrix composite on the other
hand experienced an undulation around its initial stress-strain
curve due to Mullin's effect..sup.[23, 24] This effect is described
as a strain softening phenomena observed in elastomeric composites,
often attributed to bond rupture (between filler and matrix),
molecular slip (matrix), and filler rupture among others..sup.[25]
The stiffness of the FM LMCS particle-silicone matrix composite
finally rises above its initial stiffness, suggesting that the
material had undergone strain hardening upon repeated loading.
Hence, dynamic loading testing was performed in order to capture
the change in stiffness of the composite. Dynamic tensile strain
was applied onto the fabricated composite at 1 Hz, with increasing
amplitudes from 500-1600 .mu.m (within the elastic region of the
sample).
[0031] The undercooled FM LMCS particle-silicone matrix composite
showed gradual increments in storage modulus (FIG. 3b), supporting
the observation of a strain hardening material. The rise in modulus
appears to be a result of solidification of the undercooled liquid
metal, induced by deformation of the composite. The storage modulus
of the undercooled EGaIn LMCS particle-silicone matrix composite
displays a minute step increase in modulus also at 700-800 .mu.m,
however, gradually decreases from 500-1600 .mu.m (FIG. 3b). This
observation can be attributed to oxide (shell), which induced a
slight step increase in modulus and after that steady decline due
to the exposed liquid metal core. For the control (pristine)
silicone elastomer, as expected a relatively constant storage
modulus was observed with only partial decline at lower amplitudes
(FIG. 3b). Finally, loading a composite using solidified Field's
metal particles (FM solid line) with equal volume percent showed a
similar behavior with the Control sample, validating that the
change in stiffness in the FM LMCS particle-silicone matrix sample
was induced by the solidication of the undercooled particles.
[0032] The difference in behavior of the composites evaluated could
also be highlighted by tracking the change in skewness with
increasing amplitude (FIG. 3c). The EGaIn LMCS particle-silicone
matrix composite showed a steep rise in skewness at lower
amplitudes and plateaued towards the end, indicating the sample
population shifting towards lower modulus. The Control sample and
FM solid particle sample again showed similar behavior with each
other. Both exhibited gradual increase in skewness (FIG. 3c) and
only differ in terms of magnitude. Convincingly, the FM LMCS
particle-silicone matrix composite showed a decrease in skewness
(FIG. 3c), shifting from positive to negative indicating that the
sample population at low amplitude is biased towards low storage
modulus; however, with increasing amplitude, inverts its
inclination towards high storage modulus. The distribution of
storage modulus as function of amplitude was plotted in the form of
heat maps, which illustrate the change in sample population
suggested by the skewness plots. The standard deviation of FM LMCS
particle-silicone matrix composite appears to increase at higher
amplitude, whereas the EGaIn LMCS particle-silicone matrix
composite decreases. This is likely due to differences in number of
particles that remains undercooled in the FM LMCS particle-silicone
matrix composite. In the case of EGaIn as the particle filler core
material, all particles remain liquid through the mechanical test
and thus, their properties converge.
[0033] The stiffness of undercooled FM LMCS particle-silicone
matrix composite increases at higher particle volume percent (FIG.
4a). At .PHI.=6%, the storage modulus rises moderately at low
amplitudes and depresses steadily later on, similar to the Control
sample and the EGaIn LMCS particle-silicone matrix composite,
indicating that its mechanical properties are matrix dominated. At
the other end of the spectrum, the storage modulus of the FM LMSC
particle-silicone matrix composite sample with .PHI.=22% quickly
rises at the beginning and plateaus until 1200 .mu.m before
continuing to rise. This may be due to the introduction of defects
such as stress concentration areas as more filler were integrated,
which would induce the formation of cracks throughout the
composite, although the inventors do not wish or intend to be bound
by any explanation in this regard. Hence, during the plateau
region, a competition appears to occur between hardening due to
particle solidification and what appears to be `softening`, caused
by fatigue in the material via crack propagation or formation.
Higher filler amounts could lead to greater inhomogeneity of the
composite, creating more of such defects.
[0034] Evaluating the total change in stiffness, .DELTA.E', in the
range of amplitude surveyed (FIG. 4b) supports the observed
stiffness increase. A steady rise in .DELTA.E' was observed up to
composite .PHI.=15%; however, the trend is discontinued at
composite .PHI.=22% and this is followed by a wide spread in the
data. Considering that stress concentration sites generated by the
particles are directly linked to their spatial distribution in the
composite, large variances can develop between samples when higher
filler volume is used. The change in skewness (FIG. 4c) as expected
shifted from positive to negative, once again validating the change
in stiffness of the composite due to phase transformation of the
undercooled FM LMCS particles of the composite.
[0035] Self-strengthening behavior displayed by the undercooled FM
LMCS particle-silicone matrix composites could potentially function
as a reconfigurable composite. FIGS. 5a-5f show the different modes
of deformation that can be applied to the self-strengthening
composite, which results in permanent distortion. FIGS. 5a-5b
illustrate that the shape of the ST3R composite is tunable or
reconfigurable at room temperature, in this case by twisting a
sample.
[0036] Elastomeric composites, which self-strengthen and compile
with an induced load might find emergency field applications that
could benefit from having adaptive materials. For example in FIGS.
5c-5d, simply by folding the the FM LMCS particle-silicone matrix
composite at several points along the long axis, a relatively flat
material now grips onto one's finger. Similarly, FIGS. 5e-5f
demonstrate the collection of water droplets by reshaping an
otherwise ineffective material without the need of any other
stimuli.
[0037] FIGS. 6a-6b demonstrate two separate FM LMCS
particle-silicone mtrix composites connected by a single lap joint,
whereby each side can be deformed individually. Furthermore, the
deformed composite is capable of lifting itself when suspended.
FIG. 6c-6d display shape memory effect of deformed FM LMCS
particle-silicone matrix composite when heated above the melting
temperature of the metal, hence, showing direct evidence that
reconfiguration is supported by solidified metal.
Further Examples
[0038] These examples demonstrate stiffness tunning through
thermodynamics relaxation (abbreviated ST3R hereafter). For
example, a ST3R composite is able to support fifty times its own
weight after compressive shaping in a mold. Such a large change in
stiffness is not observed in the case of the pure polymer matrix or
the pure solid metal, and hence is unique to the ST3R composite.
The mechanically induced phase transformation of metastable
undercooled LMCS particles does not require external heat sources
or electrical stimulation, providing an autonomous approach to
self-stiffening when the material is subjected to mechanical stress
during load-bearing applications.
[0039] The experimental details are set forth as follows: [0040]
Materials: Eutectic indium (51%)--bismuth (32.5%)--tin (16.5%)
(Field's metal) was purchased from Rotometals. Eutectic gallium
(75.5%)--indium (24.5%) (EGaIn) was purchased from Sigma Aldrich.
Slygard 184 polymer was purchased from Ellsworth Adhesives. Ethanol
(>99.2%) was purchased from Decon Laboratories Inc. Glacial
acetic acid (99.7%) was purchased from Fisher Scientific.
Diethylene glycol (99.9%) was purchased from VWR.
[0041] Preparing undercooled liquid metal core-shell (undercooled
LMCS) particles: The SLICE (Shearing Liquids Into Complex
Particles) method was applied to produce all the particles used in
this example. In a typical synthesis, 2.5 grams of Field's metal
was placed into a vial containing 5 ml of solvent (diethylene
glycol containing 5% acetic acid). The vial was then heated using
an oil bath at 145 .degree. C. After 5 minutes, a teflon stirrer
attached to a Dremel tool is placed into the vial, making sure that
contact is made with the molten metal. The solution is stirred with
gradual increase from 0-22,000 rpm. The shearing was maintained at
this speed for 10 minutes, and the vial is subsequently removed
from the oil bath with continuous shearing. Once the vial is
completely out of the oil bath, the speed of the Dremel tool is
gradually reduced back down to 0 rpm. The resulting grey solution
is then filtered and rinsed using ethanol to remove residual
solvent. Based on inspection by SEM, particle size ranged from 1-20
.mu.m.
[0042] ST3R composite preparation: As a general procedure,
undercooled LMCS particles of desired amount were first filtered
and dried on filter paper, and then added into premixed Dow Corning
Sylgard 184 silicone elastomer polymer (10:1 ratio Base: catalyst
ratio) to form the ULMCS particle-elastomer composite (ST3R
composite). In a typical preparation, 8 grams of Field's metal
undercooled particles are added into 1 gram of Sylgard 184 polymer.
The mixture is stirred for at least 10 minutes until a homogeneous
mixture is achieved before outgassing. During this process, it is
expected that some undercooled particles might be triggered and
solidify; however, based on DSC and mechanical data clearly
majoririty are not affected. The same procedure is used for all
other fillers. Curing was performed at 100.degree. C. for
approximately 12 hours in a glass mold (4.0.times.4.0.times.0.1 cm)
sprayed with mold release (Mann.RTM. mold release agent). The
composite was subsequently cut into 4.times.0.5.times.0.1 cm
rectangular strips using a razor. Composites for Dynamic Mechanical
Testing were molded in polystyrene weigh boats; thus, these samples
were cured at 60.degree. C. These samples were cut into
2.0.times.0.5.times.0.1 cm using a razor.
[0043] Differential scanning calorimetry (DSC) was performed using
TA Instruments Q2000 (Heating/Cooling rate=10.degree. C./min). For
example, 3 mg (milligrams) of particles are added into an aluminum
pan, and subsequently placed into the DSC stage, which has a
temperature of 40.degree. C. by default. The temperature is held
constant for 5 minutes to achieve equilibrium before performing
cooling/heating.
[0044] Scanning electron microscopy (SEM) of the undercooled LMCS
particles was obtained using JEOL JSM-6060LV with a secondary
electron detector (ET detector) and accelerating voltage of 20 kV.
Micrographs of the composite were taken using an FEI Inspect F50
with a backscattered electron detector to obtain contrast between
the matrix and fillers. Accelerating voltage was initially set to 8
kV but to further increase Z contrast (to show spinodal
decomposition), it was increased to 30 kV.
[0045] Mechanical testing: Dynamic mechanical analysis (DMA)
measurements in tension mode were performed using TA Instruments
Q800. Dynamic strain was set at 1 Hz and static force at 0.01 N
with increasing amplitude from 500 .mu.m to 1600 .mu.m. Tension
tests were performed on Instron 5944 single column testing system
using pneumatic grips and an extension rate of 1 mm/sec. The
initial gauge length is maintained at 27 mm for both pristine and
pre-compressed samples.
[0046] Sample Pre-compression: Samples were pre-compressed for
testing by placing the composite sample centered between two
acrylic plates (5 cm.times.10 cm) and then compressed between the
acrylic plates using a hydraulic press at the desired compressive
load.
Results:
[0047] To investigate mechanically triggered phase-change driven
stiffness enhancement of the composites, tensile tests were
performed on pristine and pre-compressed ST3R samples. In FIGS. 7a
and 7b, tensile stress-strain curves are shown for pristine (solid
line) and pre-compressed (dashed line) composites at filler volume
fraction, .PHI.=50%, comparing ST3R samples with two similarly
prepared control samples of ambient liquid metal (eutectic gallium
indium, EGaIn) and solid FM particles of comparable dimensions. The
data points of the ST3R samples are denoted ST3 in these
figures.
[0048] It was observed that the pre-compressed ST3R composite shows
a significant increase in initial stiffness (FIG. 7a), while both
EGaIn and solid FM particle-containing samples show an
insignificant change due to compression. The dramatic increase in
stiffness appears to be due to solidification of ULMCS particles;
that is, transformation of the initially liquid particles into a
solid metal, likely with concomitant formation of an interconnected
network due to inter-particle fusion, although the inventors do not
wish or intend to be bound by any explanation in this regard. The
ST3R composite shows a 300% increase in Young's modulus (about 80
MPa), significant transition from a flexible low modulus
as-prepared pristine parent material (FIG. 7c).
[0049] In contrast, the EGaIn particle-containing sample fractured
upon pre-compression while solid FM particle-containing composites
show a statistically insignificant increase in stiffness. To
confirm differences in mechanical properties of the compressed
samples, the materials were subjected to high stresses, wherein
shape change and network formation may differentiate compressed
ST3R composite from solid FM particle samples (FIG. 7b). Besides
the larger (300%) initial stiffness for ST3R, an asymptotic
mechanical response was observed prior to yielding, similar to
plastic deformation in metals (FIG. 7b). The stress-strain curve in
FIG. 7b supports the presence of some interconnected (fused) filler
particles in the matrix, whereby the filler networks break as
strain is increased, leading to the flattening of the stress-strain
curve at higher strains. The filler networks appear to be formed by
inter-particle fusion of the released solidified core material. The
ST3R composite becomes electrically conductive after
compression
[0050] This phenomenon is further investigated by varying the
volume of filler (FIG. 7d). When .PHI.=30%, the precompressed ST3R
and solid FM particle-containing samples gave comparable modulus
(12-15 MPa). At .PHI.=50%, however, a dramatic change in modulus is
observed in the ST3R sample (FIG. 7d). This is consistent with the
formation of filler networks whereby a minimum filler volume is
required.
[0051] Since solidification is a stochastic (thermodynamic phase
transformation) process, the strain-driven changes in the modulus
should lead to significant differences between ST3R and analogous
composites where no phase change occurs. The change in elasticity
should, therefore, manifest as an asymmetry in the distribution of
complex stiffness leading to a unique trend in skewness for the
ST3R composite under dynamic stress compared to static
(nonresponsive) analogous composites (e.g. EGaIn particles, solid
FM particles or glycerol droplets). To confirm this, dynamic
tensile strain were performed on thin rectangular samples
(1.0.times.0.5.times.0.1 cm) at 1 Hz, with increasing amplitudes
from 5-15% strain. The ST3R composite shows an increase in complex
stiffness as strain is increased (FIG. 7e), supporting the
indication of a strain hardening material. The rise in complex
stiffness may be a result of partial solidification of the
undercooled liquid metal, induced by deformation of the composite,
although the inventors do not wish or intend to be bound by this
explanation.
[0052] In contrast, EGaIn filler particles show a small initial
rise in normalized complex stiffness before a decrease as strain is
increased. Additional control experiments of solid FM particles,
glycerol droplets, and PDMS (matrix) further show a decrease in
stiffness as strain is increased, highlighting the unique behavior
of the ST3R (FIG. 7e). A shift in skewness of the distribution of
complex stiffness (FIG. 70 was observed, whereby positive values
represent bias in the mass of the distribution towards lower
stiffness and negative values point towards higher stiffness. It is
therefore evident that only the ST3R composite displayed a shift
towards higher complex stiffness. Although stiffness is enhanced
after mechanical loading, the increase is on the order of 10%.
Analysis of the stressed composite samples (DSC) indicates that
under these experimental conditions, only 14% of undercooled
particles solidified after tensile elongation while total
solidification was obtained with the compressed samples. This low
conversion correlates with modest increase in stiffness, and
highlights that stiffness transformation is limited in tension and
will not occur under small perturbations. Thus, special handling of
the composite was not required under the above-described
experimental conditions.
[0053] To further explore the effect of pre-compression on
metastable particle transformation, a series of experiments on
varying pre-compression stress on .PHI.=50% ST3R samples was
performed. Young's modulus increases with the pre-compressive
stress up to 33 MPa (FIG. 8a). Although 68% strain was produced,
more than half of the deformation springs back, which results in an
approximately 30% permanent deformation. However, further
incremental increase in compression to 70 MPa leads to a decrease
in Young's modulus, likely due to damage to the matrix as suggested
by the transition to higher compressive stiffness due to solid
dominated deformation.
[0054] Complete solidification was observed even under low
compression load irrespective of filler volume fraction (FIG.
8b-8c). This indicates that the continuous rise in Young's modulus
(at low compressions) has a structural component in addition to the
solidification of the metal, which could originate from changes in
volume, shape or texture of the solidified metal and the formation
of an interconnected network. Further evidence of total
solidification is provided from scanning electron microscopy (SEM).
Evaluating differences in particle shape by SEM before and after
compression shows uniform spherical particles in the uncompressed
ST3R sample while oblong/elongated particles are observed in the
compressed ST3R samples. Solid FM composite, in contrast, showed
negligible changes in particle appearance. Solidification of
metallic alloys occurs with concomitant spinodal decomposition and
hence changes in sub-surface composition. Analyzing contrast in the
fillers with SEM using an energy selective backscattered (EsB)
detector shows uniform contrast in the uncompressed ST3R sample.
Upon compression of the ST3R sample, however, spinodal
decomposition is observed on the metallic phase via Z-contrast of
backscattered electrons and differences in elemental composition is
confirmed using energy dispersive xray spectroscopy (EDS).
[0055] As observed in the compressed samples, permanent deformation
due to modulus changes can be achieved by stressing the ST3R
composite material. FIGS. 9a-9b illustrate that the shape of the
ST3R composite is tunable or reconfigurable at room temperature, in
this case by twisting a sample. The strip retains its shape even
after the applied stress is removed (FIG. 9b).
[0056] Furthermore, controlled indentation of a flat ST3R composite
with a blunt object can form micro wells of tunable depth (see
profilometry of an example, insert FIG. 9c). A linear correlation
between applied force and depth of the wells shows that the
compressive deformation is highly tunable. The wells can hold a
liquid, while the undeformed portion of the sample releases the
liquid when tilted. A combination of shaping by twisting/bending
and compression can also lead to fabrication of complex surface
topology as illustrated with a slanting strip that can also hold
water. Since the deformed shape is due to solidification, it can
readily be reversed by an inverse phase transformation (melting).
It has already been demonstrated above that a shaped (curved)
compressed sample of the composite flattens upon heating above the
T.sub.m of the filler material, e.g. see FIGS. 6c-6d. Upon the
first solidification from the metastable released liquid metal core
material to the solid metal core material in the matrix, the ST3R
sample transitions into a metal-elastomer composite whereby its
shape can be reconfigured or changed by melting the metal filler.
Under such circumstances, the elastomer is locked in place when the
released metal core material solidifies. During the melting
process, the ST3R composite can be observed to relax and partially
retain its original shape due to elastic response from the
matrix.
[0057] Practice of the present invention provides a mechanically
triggered stiffness tunable composite material by managing the
interplay between thermodynamic relaxation and response of
metastable liquid metal to mechanical (tensile or compressive)
stress. Stiffness change can be selectively targeted resulting in
the ability to couple material transformation with shape
reconfiguration. Composites with such capabilities could find
unique applications as mechanically adaptive or responsive (smart)
materials where external sources of energy such as heat or
electricity are not available. Significant changes in thermal and
electrical transport properties are expected to simultaneously
occur along with the reported increase in stiffness due to
transformation of the undercooled LCMS filler particles into a
different phase and shape with concomitant fusion to an at least
partially fused network at higher loading. Furthermore, as the
metastable undercooled LMCS particles do not rely on a specific
material chemistry, selfstiffening behavior can be incorporated
into diverse materials and applications ranging from sensors and
functional devices to reconfigurable structures and robotics.
[0058] Although certain illustrative embodiments of the present
invention are described, those skilled in the art will appreciate
that the present invention is not limited to these embodiments and
that changes and modification can be made to the invention without
departing from the spirit and scope of the invention as set forth
in the appended claims.
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