U.S. patent application number 17/460669 was filed with the patent office on 2021-12-16 for programmable nanocomposites.
This patent application is currently assigned to Government of the United States as represented by the Secretary of the Air Force. The applicant listed for this patent is Government of the United States as represented by the Secretary of the Air Force, Government of the United States as represented by the Secretary of the Air Force. Invention is credited to Tyler C. Guin, Timothy J White.
Application Number | 20210388268 17/460669 |
Document ID | / |
Family ID | 1000005811667 |
Filed Date | 2021-12-16 |
United States Patent
Application |
20210388268 |
Kind Code |
A1 |
Guin; Tyler C. ; et
al. |
December 16, 2021 |
PROGRAMMABLE NANOCOMPOSITES
Abstract
A shape-programmable liquid crystal elastomer. The elastomer
comprising cross-linked and polymerized nematic, isotropic monomers
with carbon nanotubes that are organized into a plurality of
voxels. Each voxel has a director orientation such that each voxel
of the plurality has a first state according to the director
orientation and a second state according to cross-linkages of the
polymerized nematic monomers. The elastomer transitions between the
first and second states with exposure to an electric field.
Inventors: |
Guin; Tyler C.; (Aiken,
SC) ; White; Timothy J; (Longmont, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Government of the United States as represented by the Secretary of
the Air Force |
Wright-Patterson AFB |
OH |
US |
|
|
Assignee: |
Government of the United States as
represented by the Secretary of the Air Force
Wright-Patterson AFB
OH
|
Family ID: |
1000005811667 |
Appl. No.: |
17/460669 |
Filed: |
August 30, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16373110 |
Apr 2, 2019 |
11142696 |
|
|
17460669 |
|
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62651667 |
Apr 2, 2018 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C09K 19/3852 20130101;
C09K 19/56 20130101 |
International
Class: |
C09K 19/38 20060101
C09K019/38; C09K 19/56 20060101 C09K019/56 |
Goverment Interests
RIGHTS OF THE GOVERNMENT
[0002] The invention described herein may be manufactured and used
by or for the Government of the United States for all governmental
purposes without the payment of any royalty.
Claims
1. A liquid crystal film comprising: a shape-programmable liquid
crystal elastomer comprising cross-linked and polymerized nematic,
isotropic monomers with nanotubes; and a topographical defect
within the shape-programmable liquid crystal elastomer, the
topographical defect having a first state and a second state,
wherein the topographical defect is configured to assume the first
state with a presence of the electric field and to assume the
second state in an absence of the electric field.
2. The liquid crystal film of claim 1, wherein a nanotube content
ranges from 0.02 wt % to 1.0 wt %.
3. The liquid crystal film of claim 1, wherein the nanotubes are
carbon nanotubes, boron nanotubes, or TMD platelets.
4. The liquid crystal film of claim 1, wherein the nanotubes are
single-wall carbon nanotubes, double-walled carbon nanotubes,
multi-walled carbon nanotubes, or combinations thereof.
5. The liquid crystal film of claim 1, wherein the nanotube content
ranges from 0.02 wt % to 1.0 wt %.
6. The liquid crystal film of claim 1, further comprising: an
auxiliary chemical.
7. The liquid crystal film of claim 6, wherein the auxiliary
chemical is a photoinitator, a chain extender or both.
8. The liquid crystal film of claim 7, wherein the photoinitator is
IRGACURE-651.
9. The liquid crystal film of claim 7, wherein the chain extender
is an ethanedithiol, an aliphatic dithiol, or an amine.
10. The liquid crystal film of claim 1, wherein the nematic
monomers are selected from the group consisting of acrylates,
methacrylates, thiols, vinyl, epoxides, and amines.
11. The liquid crystal film of claim 1, wherein the nematic
monomers include a ratio mixture of a diacrylate and an amine.
12. The liquid crystal film of claim 11, wherein the diacrylate is
1,4-bis-[4-(6-acryloxyloxyhexyloxy)benzoyloxy]-2-methylbenzene and
the amine is n-butyl amine.
13. The liquid crystal film of claim 1, wherein the polymer
includes a mesogen.
14. The liquid crystal film of claim 1, further comprising: a
domain comprising adjacent voxels of the plurality having similar
director orientation.
15. The liquid crystal film of claim 1, wherein each voxel of the
plurality has a length dimension, a width dimension, and a depth
dimension.
16. The liquid crystal film of claim 1, wherein the polymerized,
nematic monomer is polymerized diacrylate functionalized
azobenzene.
Description
[0001] This application is a continuation of U.S. application Ser.
No. 16/373,110 (pending), which was filed on Apr. 2, 2019 and
claimed the benefit of and priority to prior filed Provisional
Application Ser. No. 62/651,667, filed Apr. 2, 2018. The
disclosures of these applications are incorporated herein by
reference, each in its entirety.
FIELD OF THE INVENTION
[0003] The present invention relates generally to liquid crystal
polymer nanocomposites and, more particularly, to programmable
liquid crystal polymer nanocomposites.
BACKGROUND OF THE INVENTION
[0004] One-dimensional, high aspect ratio nanomaterials, such as
carbon nanotubes ("CNTs") possess unique anisotropic electrical,
photonic, and mechanical properties. The addition of these
materials into polymer matrices yields composites that assimilate
the processing and durability of polymers with the distinctive
properties (such as superior electrical or thermal conductivity or
mechanical reinforcement) of the nanoinclusion (that is, the
incorporated CNT). Enforcing alignment upon the nanoinclusion can
further enhance the properties of the nanocomposite by facilitating
cooperative interactions.
[0005] The facile preparation and alignment of nanoinclusions in
polymer nanocomposites is complicated by entropy, which is
minimized in the disordered state. A number of techniques have been
explored to orient one-dimensional nanomaterials. The most common
of these conventional methods has been extrusion, which aligns the
nanomaterial through rheological forces. This approach, along with
other methods that rely on mechanical forces or external fields, is
limited to uniaxial alignment without spatial control of the local
order.
[0006] Liquid crystalline ("LC") materials are inherently
anisotropic and benefit from cooperative interactions when aligned.
LCs are typified by long-range order (orientation), which may be
locally controlled through electric fields, magnetic fields, or
surface alignment. In fact, liquid crystallinity may be retained in
various polymeric forms.
[0007] Arbitrary and complex control of the local orientation of
the LC director within LCEs (LC elastomers) has been demonstrated,
such as in White et al., U.S. Pat. No. 9,902,906, issued Feb. 27,
2018, the disclosure of which is incorporated herein by reference,
in its entirety). For example, localized irradiation of LCE
photoalignment surfaces direct the self-assembly of liquid crystal
monomers to yield a voxelated (3-dimensionally pixelated) LCE upon
polymerization. The localization of the mechanical response of the
materials enables stimuli-responsive transformation from 2-D flat
sheets to 3-D shapes.
[0008] Recently there has been interest in exploring shape
programming in LCEs that mechanically respond to thermally or
photo-induced stimuli. A number of examinations of electrically
induced mechanical responses of polymer nanocomposites have
employed resistive heating. Comparatively few reports detail direct
electromechanical transduction. Most compelling of these results
are electromechanical effects observed in tilted smectic LCEs,
where the mesogen unit of the LCE is free to rotate in an electric
field, resulting in macroscopic shear. However, smectic LCEs,
despite displaying impressive reversible strains (greater than 8%),
are not amenable to command surfaces and, therefore, are not
applicable to topologically complex director orientations.
[0009] Shape programming in LCEs may transform the flat sheets into
pre-determined shapes when exposed to heat, solvent, or light.
These stimuli disrupt the order of the liquid crystalline material
and produce spontaneous and anisotropic dimensional changes that
are defined by the director orientation. Many potential end use
applications of these materials as actuators require response times
not yet achievable with heat or light stimuli. For this and other
reasons, electric fields have long been acknowledged as a desirable
stimulus to trigger shape change or actuation.
[0010] Nematic LCEs have also been mildly sensitized to electrical
control (less than 4.5 kPa blocking force) by incorporating a small
volume fraction of CNTs (that is, far below the percolation
threshold). However, these conventional approaches rely on uniaxial
mechanical stretching to induce alignment of both the LCE and the
nanomaterial. Such uniaxial mechanical stretching prevents utility
as a pixelated LCE. Ideally, an electrically-stimulated
nanocomposite LCE would be conducive to surface alignment via
photopatterning and able to be processed as a one-pot reaction to
facilitate classical LC processing techniques.
[0011] Therefore, there remains a need for electrical control of
topologically imprinted LCEs, which could enable device
implementations primarily associated with swifter response
times.
SUMMARY OF THE INVENTION
[0012] The present invention overcomes the foregoing problems and
other shortcomings, drawbacks, and challenges of topologically
imprinted LCEs that enable electrical control. While the invention
will be described in connection with certain embodiments, it will
be understood that the invention is not limited to these
embodiments. To the contrary, this invention includes all
alternatives, modifications, and equivalents as may be included
within the spirit and scope of the present invention.
[0013] According to one embodiment of the present invention, a
shape-programmable LCE includes cross-linked and polymerized
nematic, isotropic monomers with carbon nanotubes ("CNTs")
organized into a plurality of voxels. Each voxel has a director
orientation such that each voxel of the plurality has a first state
according to the director orientation and a second state according
to cross-linkages of the polymerized nematic monomers. The
elastomer transitions between the first and second states with
exposure to an electric field.
[0014] As to these embodiments, the composition of liquid
crystalline and isotropic monomers, when polymerized, form liquid
crystalline polymer networks and elastomers. The composition is
subject to surface-enforced alignment to program orientation in
discrete volume elements (e.g., voxels). Inclusion of CNTs
sensitizes the polymer nanocomposite to electric fields. The
association of the stimulus response and orientation of the voxel
yields directional strain.
[0015] Other embodiments of the present invention include a liquid
crystal film comprising the shape-programmable liquid crystal
elastomer and a topographical defect. The topographical defect has
first and second states and is configured such that the first state
is assumed with the presence of the electric field and the second
state is assumed with the absence of the electric field.
[0016] Additional objects, advantages, and novel features of the
invention will be set forth in part in the description which
follows, and in part will become apparent to those skilled in the
art upon examination of the following or may be learned by practice
of the invention. The objects and advantages of the invention may
be realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawing(s) will be provided by the Office
upon request and payment of the necessary fee.
[0018] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate embodiments of
the present invention and, together with a general description of
the invention given above, and the detailed description of the
embodiments given below, serve to explain the principles of the
present invention.
[0019] FIG. 1 is a flow chart illustrating a method of preparing
elastomers according to embodiments of the present invention.
[0020] FIG. 2 is perspective view of substrate preparation for use
in the method of FIG. 1.
[0021] FIGS. 3 and 3A are alternative, schematic views of exemplary
systems for conducting a portion of the method of FIG. 1.
[0022] FIG. 4 is a schematic representation of an exemplary
computer suitable for conducting a portion of the method of FIG.
1.
[0023] FIGS. 5 and 6 are perspective and side elevational views (in
cross-section), respectively, of forming and filling a cell
according to the method of FIG. 1.
[0024] FIG. 7 illustrates exemplary monomers suitable for
embodiments of the present invention.
[0025] FIGS. 8A and 8B are polarized optical micrographs of 15
.mu.m thick films prepared in accordance with an embodiment of the
present invention.
[0026] FIGS. 9A-9D are cross-polarized images of 8 .mu.m thick
films of varying SWNT content filling a cell at 100.degree. C.
[0027] FIG. 10 is a photograph of a 12 .mu.m thick film prepared in
accordance with an embodiment of the present invention having the
image of an aircraft embedded therein.
[0028] FIG. 11A is a DSC trace of monomer mixtures, with and
without SWNTs, some monomer mixtures being in accordance with
embodiments of the present invention.
[0029] FIG. 11B is a DSC trace of a neat LCE film and a 0.02%
SWNT-LCE film prepared in accordance with an embodiment of the
present invention.
[0030] FIG. 12A is a Raman spectra of 15 .mu.m thick films, with
and without SWNTs, some films being in accordance with an
embodiment of the present invention.
[0031] FIG. 12B is an enlarged view of the radial breathing modes
of FIG. 12A.
[0032] FIG. 13 graphically illustrates normalized Raman intensity
as a function of polarization angle for identifiable Raman signals
of LCE and SWNT in a 15 .mu.m thick film.
[0033] FIG. 14 graphically illustrates X-ray scattering intensity
at 19.4.degree. 2-theta peak as a function of azimuthal angle.
[0034] FIGS. 15A and 15B are X-ray scattering profiles of 15 .mu.m
thick film, with and without SWNTs, some films being in accordance
with embodiments of the present invention.
[0035] FIG. 16A graphically illustrates average orientation of
SWNTs of a SWNT-LCE film having a +1 defect according to an
embodiment of the present invention.
[0036] FIG. 16B illustrates the film having the +1 defect of FIG.
16A.
[0037] FIGS. 17A and 17B are transmission electron micrographs of a
0.02% SWNT-LCE film prepared in accordance with an embodiment of
the present invention, cut parallel and perpendicular to the
orientation direction, respectively.
[0038] FIGS. 18A and 18B illustrate 15 .mu.m thick films in
accordance to embodiments of the present invention, floating in oil
between ITO plates, and before and after exposure to an electric
field.
[0039] FIG. 19A is a height map difference between a 0.02% SWNT-LCE
film patterned into a +1 topographical defect according to
embodiments of the present invention, before and after exposure to
an electric field.
[0040] FIG. 19B graphically illustrates the height map of FIG.
19A.
[0041] FIG. 20 graphically illustrates the relaxation peak from
dielectric relaxation spectroscopy as a function of inverse
temperature for films with and without SWNTS, some films being in
accordance with embodiments of the present invention.
[0042] FIG. 21A graphically illustrates the shape change of films,
with and without SWNTs, along the director as a function of
temperature, some films being in accordance with embodiments of the
present invention.
[0043] FIG. 21B graphically illustrates birefringence of films,
with and without SWNTS, as a function of temperature, some films
being in accordance with embodiments of the present invention.
[0044] FIGS. 22A and 22B are cross-polarized pictures of films
graphically illustrated in FIG. 21B.
[0045] FIG. 23 graphically illustrates uniaxial tensile testing
curves of 15 .mu.m thick films, with and without SWNTs, with
accompanying Young's modulus, some films being in accordance with
embodiments of the present invention.
[0046] FIG. 24 graphically illustrates the relative length change
along the director of a film according to the present invention at
various temperatures and electric fields.
[0047] FIG. 25 graphically illustrates relative length change along
the director of a film according to the present invention as a
function of time at applied DC fields.
[0048] FIG. 26 graphically illustrates relative length change along
the director of a film according to the present invention as a
function of time in the presence of pulsed electric fields.
[0049] FIG. 27 is a polarized micrograph of a 12 .mu.m 0.02 wt %
SWNT-LCE film having a +1 defect in accordance with embodiments of
the present invention.
[0050] FIGS. 28A-28C are optical images of a film having
topographical feature before heating (FIG. 28A), after heating
(FIG. 28B), and after exposure to an electric filed (FIG. 28C).
[0051] FIG. 29 graphically illustrates loss tangent as a function
of frequency and temperature of films, with and without SWNTs, some
films being in accordance with embodiments of the present
invention.
[0052] FIG. 30 graphically illustrates the stead-state electrical
conductivity as a function of DC voltage for a film in accordance
with an embodiment of the present invention.
[0053] FIG. 31 graphically illustrates real permittivity at 1 Hz as
a function of temperature for films, with and without SWNTs, some
films being in accordance with embodiments of the present
invention.
[0054] It should be understood that the appended drawings are not
necessarily to scale, presenting a somewhat simplified
representation of various features illustrative of the basic
principles of the invention. The specific design features of the
sequence of operations as disclosed herein, including, for example,
specific dimensions, orientations, locations, and shapes of various
illustrated components, will be determined in part by the
particular intended application and use environment. Certain
features of the illustrated embodiments have been enlarged or
distorted relative to others to facilitate visualization and clear
understanding. In particular, thin features may be thickened, for
example, for clarity or illustration.
DETAILED DESCRIPTION OF THE INVENTION
[0055] Referring now to the figures, methods of preparing and using
materials chemistry platforms conducive to the surface-alignment of
liquid crystals are shown and described. The sensitivity of the
materials chemistry to surface-alignment, specifically,
photoalignment surface chemistries, realizes volumetric elements
(also known as "voxels") having discrete directors (or domains of
similar directors) of aligned liquid crystal elastomer ("LCE") or
liquid crystal network ("LCN"). Enabled by the large strain
inherent to LCEs, the sensitivity of the material chemistry to
surface-alignment, and the optical patterning methods, programmable
shape change, and actuation in a monolithic element derived from a
variety of complex director profiles can be achieved.
[0056] As used herein, "LCEs" are defined by the couple of liquid
crystal orientation and viscoelasticity at temperatures below the
glass transition. As exemplified herein, the LCEs are evaluated at
room temperature.
[0057] As used herein, "liquid crystal" or "LC" refers to a state
of matter with local or global orientation of the macromolecular
units.
[0058] As used herein, "mesogen" is a part of a molecule or
compound of a liquid crystal that is responsible for the liquid
crystal properties.
[0059] As used herein, "macromers" are polymerizable molecules
formed from a chain-extension reaction of monomer precursors.
[0060] As used herein, "director" refers to an average molecular
orientation of the mesogens comprising the liquid crystal.
[0061] As used herein, "voxel" refers to a discrete,
three-dimensional area within a liquid crystal elastomer having a
director.
[0062] As used herein, "domain" is a generalized volume of material
with a uniform liquid crystalline orientation.
[0063] As used herein, "acrylates" are salts, esters, and conjugate
bases of acrylic acid and its derivatives.
[0064] As used herein, "methacrylates" are salts, esters, and
conjugate bases of methacrylic acid ("MAA"), CH.sub.3CH.sub.2CCOOH,
and its derivatives.
[0065] As used herein, "thiols" are organosulfur compounds: HSRSH,
wherein R may include alkyl chains, such as ethyl, propyl, or butyl
groups.
[0066] As used herein, "vinyls" are ethenyl functional groups:
--C.sub.2H.sub.3
[0067] As used herein, "epoxides" are cyclic ethers having a
three-atom ring: R.sup.1R.sup.2COCR.sup.3R.sup.4.
[0068] As used herein, "amines" are compounds and functional groups
comprising a basic nitrogen atom, e.g., having a lone pair of
electrons: RNH.sub.2, wherein R may be an alkyl chain, for example,
an n-butyl group.
[0069] As used herein, "diacrylates" are molecules having two
acrylate groups.
[0070] As used herein, "nematic" refers to a liquid crystal phase
in which the mesogens are oriented in parallel, but not in
well-defined planes.
[0071] As used herein, a "smectic" refers to a liquid crystal
having mesogens oriented in parallel and arranged in well-defined
planes.
[0072] As used herein, a "chiral phase" refers to a nematic liquid
crystal possessing a chiral center between well-defined planes.
[0073] As used herein, "defect" refers to a topological pattern of
order within a liquid crystal elastomer. Defects may be
characterized by strength and charge.
[0074] As used herein, "glass transition temperature" or "T.sub.g"
refers the temperature at which glass transition occurs. "Glass
transition," as it is used herein, is a reversible transition of a
material from a "glassy" state to an elastomeric state.
[0075] As used herein, "carbon nanotubes" or "CNT" refers to
tubular-shaped molecular structures comprising carbon rings having
a diameter on the order of nanometers and lengths ranging from the
order of nanometers to millimeters.
[0076] As used herein, "nanotubes" refers to a nanometer-scale
tube-like structure conventionally comprising boron, carbon,
nitrogen, or other elements or mixtures thereof.
[0077] In that regard and with reference now to a method 50
according to an embodiment of the present invention illustrated in
FIGS. 1 and 2, a substrate 52 is prepared (Block 54). Preparation
of the substrate 52 may include various combinations of cleaning,
baking, washing, drying, and so forth, and as would be known by
those of ordinary skill in the art. The substrate 52, itself, may
comprise glass, poly(ethylene terephthalate), or other inert
materials.
[0078] An alignment layer 56 may then be applied to a cleaned
surface 58 of the substrate 52 (Block 60). The alignment layer 56
generally comprising a chromophore that, when illuminated, behaves
as a molecular oscillator until the absorption cross section is
minimized with the final orientation being 90.degree. to the
electric field vector of the incident light. Said another way, the
chromophores of the alignment layer, when exposed to light (such as
light emitted from a laser), having particular polarization,
amplitude, and phase, may so orient themselves with respect to the
surface 58 so as to be orthogonal to the electric field vector of
that light. Suitable alignment layer materials may comprise, for
example, a photochromic dye, an azobenzene polymer, a stilbene
polymer, a linearly polymerizable polymer, or other suitable
photosensitive material know to those of ordinary skill in the art
of liquid crystal alignment. Application of the alignment layer 56
may include dispersion (such as from a pipette or other like
device) or printing, spinning to ensure uniformity, baking to set
the alignment layer 56 and remove residual solvent, and so
forth.
[0079] With the alignment layer 56 applied, the alignment layer 56
may then be optically patterned (continuation of Block 60). An
exemplary system 62 for optically patterning the alignment layer 56
according to one exemplary method of the present invention is shown
in FIG. 3. Generally, the system 62 includes a laser 64 (for
example, a 445 nm laser), a moveable waveplate 66, and a moveable
substrate support 68. The moveable waveplate 66 is operably coupled
to a controller 70, which is configured to move the waveplate 66
with respect to a directionality of the beam 72, which controls a
polarization of the light to be used for patterning the alignment
layer 56 (FIG. 2). Although not specifically shown, movement of the
waveplate 66 may include one or more motors (such as a rotary
motor), which may work in concert with movement of the substrate
support 68 to dynamically control irradiation over an area as small
as 100 .mu.m.sup.2. The system 62 is configured to manipulate the
local surface-alignment of liquid crystalline cells prepared with
the alignment layer 56 (FIG. 2).
[0080] The controller 70 is operably coupled to a computer 74,
which is described in greater detail with respect to FIG. 4, and
which may be considered to represent any type of computer, computer
system, computing system, server, disk array, or programmable
device such as multi-user computers, single-user computers,
handheld devices, networked devices, or embedded devices, etc. The
computer 74 may be implemented with one or more networked computers
76 using one or more networks 78, e.g., in a cluster or other
distributed computing system through a network interface 80
(illustrated as "NETWORK OF"). The computer 74 will be referred to
as "computer" for brevity's sake, although it should be appreciated
that the term "computing system" may also include other suitable
programmable electronic devices consistent with embodiments of the
invention.
[0081] The computer 74 typically includes at least one central
processing unit 82 (illustrated as "CPU") coupled to a memory 84
along with several different types of peripheral devices, e.g., a
mass storage device 86 with one or more databases 88, an
input/output interface 90 (illustrated as "I/O I/F" with associated
display 87 and user input device 89), and the Network I/F 80. The
memory 84 may include dynamic random access memory ("DRAM"), static
random access memory ("SRAM"), non-volatile random access memory
("NVRAM"), persistent memory, flash memory, at least one hard disk
drive, and/or another digital storage medium. The mass storage
device 86 is typically at least one hard disk drive and may be
located externally to the computer 74, such as in a separate
enclosure or in one or more networked computers 76, one or more
networked storage devices (including, for example, a tape or
optical drive), and/or one or more other networked devices 91
(including, for example, a server).
[0082] The CPU 82 may be, in various embodiments, a single-thread,
multi-threaded, multi-core, and/or multi-element processing unit
(not shown) as is well known in the art. In alternative
embodiments, the computer 74 may include a plurality of processing
units that may include single-thread processing units,
multi-threaded processing units, multi-core processing units,
multi-element processing units, and/or combinations thereof as is
well known in the art. Similarly, the memory 84 may include one or
more levels of data, instruction, and/or combination caches, with
caches serving the individual processing unit or multiple
processing units (not shown) as is well known in the art.
[0083] The memory 84 of the computer 74 may include one or more
applications 92 (illustrated as "APP."), or other software program,
which are configured to execute in combination with the Operating
System 94 (illustrated as "OS") and automatically perform tasks
necessary for operating the transducers and/or reconstructing the
images with or without accessing further information or data from
the database(s) 88 of the mass storage device 86.
[0084] Those skilled in the art will recognize that the environment
illustrated in FIG. 4 is not intended to limit the present
invention. Indeed, those skilled in the art will recognize that
other alternative hardware and/or software environments may be used
without departing from the scope of the invention.
[0085] Referring again to FIG. 3, the system 62 may further
comprise a shutter 100, a collimator 102, and a lens 104.
Altogether, the system 62 operates to focus the laser beam 72 onto
each point on the alignment layer 56 (FIG. 2) of the substrate 52
having a desired polarization. Linear polarization angles from
about 0.degree. to about 180.degree. with respect to the beam
propagation direction may be achieved. Exposure dosage is
controlled through the shutter and power of the laser. Dose is
dependent on the alignment layer, for example, for azobenzene dyes
the exposure energy may be 0.1 J/cm.sup.2 and higher.
[0086] As shown, the system 62 may be configured to provide a focal
spot having a maximum dimensions ranging from nanometer scales to
meter scales. More particularly, a maximum dimension of about 100
.mu.m may be easily achieved. As such, a 200.times.200 pixelated
square area (comprising 40,000 pixels, 4 cm.sup.2), each pixel
being 100 .mu.m, and presuming a 10 msec exposure time per pixel,
may take approximately 80 min to pattern.
[0087] Alternatively, and as is shown in FIG. 3A, a system 62' is
shown and is similar to the system 62 of FIG. 3. In the illustrated
system 62', a spatial light modulator 101 replaces the waveplate 66
(FIG. 3). The spatial light modulator 101 imposes a spatial
modulation pattern onto light from the laser 64 by altering at
least one of amplitude, phase, or polarization of the light. The
modulated light may be focused by a first lens 103 to form a
Fourier transform at a plane 105. A second lens 107 focuses the
Fourier transform at the plane 105 to the image to be patterned.
According to an exemplary embodiment, using the spatial light
modulation system 62' enables simultaneous writing of 800.times.600
independent polarizations.
[0088] Using the spatial light modulation system 62' of FIG. 3A, it
is possible to pattern 10.sup.6 pixels, for example, with 15 .mu.m
resolution in about 1 second per square centimeter. As such, the
spatial light modulation system 62' of FIG. 3A, as compared to the
pixel-by-pixel system 62 of FIG. 3, is capable of patterning
substrates at a much higher rate.
[0089] Referring now again to FIG. 1, if necessary or otherwise
desired, alignment of the optically patterned alignment layer 107
(FIG. 2) may optionally be preserved (Block 106). For instance,
polymerizing a thin layer of liquid crystal monomer (generally,
several hundred nanometers thick, such as ranging from 300 nm to
500 nm) atop the patterned alignment layer 107 (FIG. 2) may be
used.
[0090] Turning now to FIGS. 5 and 6, with continued reference to
FIG. 1, a liquid crystal cell 108 may then be constructed (Block
110). In that regard, prepared substrates 52, 52' may be arranged
such that the patterned alignment layers 107, 107' of each are
facing inwardly, separated with spacers 112, and at least partially
secured and/or sealed so as to form a cavity 114 there between.
Additionally, or alternatively, spacers (not shown), such as
micro-sized glass spheres, may be mixed into an adhesive (for
example, an epoxy) such that the layers 107, 107' may be
simultaneously spaced and secured and/or sealed. Size of the
spacers 112 or micro-sized glass spheres may determine the final
thickness of a resultant LC film and may range generally from tens
of microns to hundreds of microns, or more particularly, for
example, spacers 112 having a maximum dimension of 50 .mu.m may be
used.
[0091] The cavity 114 may then be filled with a nematic mixture of
liquid crystal monomers with nanotubes configured to cross-link and
to reversibly shape change according to a complex programming using
surface alignment in accordance with an embodiment of the present
invention (Block 112).
[0092] The nematic mixture generally comprises monomers that are
configured to react via free radian, chain, or step-growth
mechanisms, such as those exemplary chemical compositions shown in
FIG. 7. Further, these may include, for example, acrylates,
methacrylates, thiols, vinyls, epoxides, amines, and others.
Furthermore, monomer solutions that undergo sequential reactions
may be utilized, for example, mixtures of diacrylates, amines,
alkenes, and thiols. According to some embodiments of the present
invention, a thiol-ene-acrylate formulation may be used as such
monomers are amenable to surface alignment and sufficiently
compatible with nanotubes.
[0093] The monomers may be dissolved or suspended in a solvent (for
example, an organic solvent such as acetone). A solution or
suspension of nanotubes may be added to the monomer solution or
suspension. Suitable nanotubes include semi-conductive or
conductive nanotubes comprising carbon (such as single-wall CNTs
("SWNT"), double-walled CNTs, or multi-walled CNTs), boron (boron
nanotubes), or other like structures (transition metal
dichalcogenide ("TMD") platelets). According to some embodiments of
the present invention, SWNT are used. CNTs have a high aspect ratio
(length-to-diameter) and strong .pi.-.pi. interactions, the latter
of which may improve dispersity and interaction with LCE mesogen
units. Exemplary films, without and with SWNT, are illustrated in
FIGS. 8A and 8B, respectively.
[0094] Viscosities of resulting elastomers increases with increased
SWNT concentration (shown in FIGS. 9A-9D). As such, the CNT content
of the solution or suspension may vary, but generally CNT loading
ranges from 0.02 wt % to 0.1 wt % of total material comprising the
nanocomposite. Up to 1.0 wt % may be filled with the aid of
vacuum.
[0095] While not required, a dispersant may be introduced to the
mixture of monomers and CNTs so as to prevent or reduce aggregation
of the CNTs. However, according to some embodiments of the present
invention, the monomers may provide some mild dispersant activity
in this regard. Sonication may also be used to aid in dispersion.
CNT functionalization, including reactions with polymer grafts, may
further improve dispersion.
[0096] The mixture may be heated and, optionally, sonicated or
mixed so as to drive off a portion of the solvents. Additionally or
alternatively, the mixture may be placed in vacuo to drive off
solvents.
[0097] With the solvent removed, auxiliary chemicals may be
introduced. Auxiliary chemical may include those that are
configured to extend polymeric chains, are photoinitator, and so
forth. Exemplary auxiliary chemicals may include, chain extenders,
such as dithiols or amines. Specifically, ethanedithiol, an
aliphatic dithiol, may be used to chain extend di-alkene monomers
via thiol-ene click reactions. Free radical photoinitator may also
be used, such as commercially available IRGACURE-651.
[0098] Optionally, and if CNTs of the mixture are unstable,
aggregate, or both, the mixture may be maintained in an isotropic
state until the mixture is used to fill the cavity 114 (FIG.
6).
[0099] Optionally still, the mixture may be melt-mixed a plurality
of times so as to facilitate the incorporation of photoinitator,
dithiols, or other auxiliary chemicals. Other methods may also be
used; however melt-mixing may be preferred for small batch
preparation.
[0100] Mixing may continue until the mixture appears uniform in
color (oftentimes black) and without clear droplets or white or
gray particulates.
[0101] Resultant SWNT-LCE nanocomposites resulting from embodiments
of the present invention are configured to, and have an ability to
maintain, and image, such as is shown in FIG. 10. Specifically, the
image of FIG. 10 includes a 12 .mu.m thick film of 0.02 wt %
SWNT-LCE.
[0102] The following examples illustrate particular properties and
advantages of some of the embodiments of the present invention.
Furthermore, these are examples of reduction to practice of the
present invention and confirmation that the principles described in
the present invention are therefore valid but should not be
construed as in any way limiting the scope of the invention.
EXAMPLE 1
Material Preparation
[0103] Nanocomposites according to embodiments of the present
invention were prepared by incorporating pristine SWNTs into a
liquid crystal monomer mixture. RM82
(1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene) and
RM2AE (2-methyl-1,4-phenylene bis(4-(3-(allyloxy)propoxy)benzoate)
were purchased from Synthon Chemicals (Wolfen, Germany) and were
used after recrystallization from methanol. Irgacure 651 was
donated by BASF (Ludwigshafen, Germany). Ethanedithiol ("EDT") was
purchased from Sigma-Aldrich (St. Louis, Mo.) and used as received.
Single-wall carbon nanotubes (SWNT'') (length ranging from 100 nm
to 1000 nm) were provided by NanoIntegris (Boisbraind, Quebec) and
contained more than 95% carbon. PAAD-22 was provided by Beam Co.
(Orlando, Fla.) and was diluted to 0.33 wt % in dimethylformamide
before use. Elvamide was provided by DuPont (Wilmington, Del.), and
was dissolved into methanol as a 0.25 wt %/vol % solution.
[0104] RM2AE and RM82 were added at a 2:1 mol ratio to a glass vial
and dissolved in warm acetone. SWNTs were loosely suspended in
acetone, added to the vial, and the mixture was sonicated in a
Branson ultrasonic bath for 1 hr. At all times, the mixture was
shielded from fluorescent lighting. The mesogenic monomers act as a
mild dispersant and prevented the SWNTs from aggregating after
sonication. The mixture was then sonicated in an ultrasonic bath
heated to about 95.degree. C. to remove a majority of the solvent
while keeping the SWNTs dispersed. The mixture was then placed in a
vacuum chamber at 100.degree. C. for 10 min to remove any remaining
solvent. Afterward, EDT (equivalent molar to RM2AE) was added to
the mixture along with 1 wt % IRGACURE 651. The mixture was
melt-mixed a minimum of three times.
[0105] The SWNTs did not stay stably dispersed in the nematic state
and would precipitate over the course of several hours. As such,
the mixture was held in the isotropic state until filling.
[0106] The SWNTs modified a phase behavior of the monomer mixture,
increasing the nematic transition temperature (TNI) from 24.degree.
C. to 37.degree. C. This relatively low transition temperature is
most likely a result from the processing conditions.
[0107] Glass slides were initially cleaned by successive rinses in
acetone and methanol, followed by a 10 min plasma cleaning
treatment (Branson Ultrasonic Cleaner). To apply the photoalignment
layer, PAAD solution was spin-coated onto the glass slides at 4500
RPM, and then baked at 100.degree. C. for 10 min. To apply the
Elvamide buffed alignment layer, Elvamide solution was spin-coated
onto the glass slides at 4500 RPM and allowed to dry in ambient
conditions. The Elvamide alignment layer was then rubbed in one
direction, 30 times, with a felt cloth. Two glass slides were then
glued together using UV-curable epoxy (Epofix 68) and glass spacers
therebetween.
[0108] Incorporation of SWNTs led to an increase in viscosity,
making it difficult to fill liquid crystal cells at loadings higher
than 0.08 wt %. Images of filled cells of various loadings of SWNTs
are shown in FIGS. 9A-9D. These figures are cross-polarized images
of 8 .mu.m thick, homogeneous planar LC cells filled with SWNT-LCEs
with varying concentrations of SWNT (0%, 0.02%, 0.04%, and 0.08%,
respecitvely). The viscosity of the mixture increased with
increased SWNT loading, preventing filling of entire cell at
100.degree. C.
[0109] The mixture was filled into liquid crystal cells via
capillary action at 100.degree. C., cooled to room temperature over
20 min., and then polymerized at room temperature under 365 nm UV
light (about 150 mW/cm.sup.2) for 20 min.
[0110] Alignment of the photopatterning layer was achieved by
either using a vector vortex waveplate (Beam Co.) to produce a +1
defect or by using a spatial light modulator to produce pixelated
patterns of linearly polarized light. In brief, the PAAD-22
photoalignment layer orients orthogonally to the polarization of
the light, which then induces alignment of the liquid crystal
mixture through the cell thickness.
[0111] The film was removed from the cell by soaking in deionized
water for 16 hrs and then by separating the glass slides with a
razor blade.
EXAMPLE 2
Material Characterization
[0112] Phase transitions, birefringence, and film quality were
measured using polarized optical microscopy ("POM") (Nikon) in
transmission mode with temperature controlled by a Mettler Toledo
HS82 heat stage. Birefringence of films was measured at a
wavelength of 600 nm using a Newport 818-UV photodiode and 600 nm
filter attached to the microscope. Shape change of homogenous
planar films, floating on silicone oil and 5 .mu.m glass spacers as
a function of temperature, was also determined using POM.
[0113] Dynamic scanning calorimetry ("DSC") (TA Instrument Q1000)
was performed under nitrogen at temperatures ranging from
-40.degree. C. to 100.degree. C. for monomer mixtures and from
-40.degree. C. to 250.degree. C. for cured films in hermetically
sealed pans. The nematic transition determined from the peak of the
heat flux trace on second cooling, and the glass transition was
determined from the peak of the derivative of the heat flux
trace.
[0114] The LC monomers formed a stable room temperature nematic
mixture having an isotropic transition at relatively low
temperature. DSC traces of the monomer mixtures provided in FIGS.
11A and 11B. Traces in FIG. 11A are for the second heating and
cooling. No crystallization peak was apparent to -40.degree. C.,
and the nematic transition was very broad. Traces in FIG. 11B are
for neat and 0.02% SWNT-LCEs. A clear transition is observed at
4.degree. C., and a pseudo-isotropic transition is observed at
128.degree. C. for the net LCE and 122.degree. C. for the 0.02%
SWNT-LCE.
[0115] The formulations employed here to prepare the nanocomposites
were intended to generate elastomers. FIG. 11B compares DSC traces
of a SWNT-LCE nanocomposite to an LCE prepared with the same
composition. Both materials are elastomeric, with glass transition
temperatures (T.sub.g) at 4.degree. C. The secondary transition
(nematic to isotropic phase transition, TNT) evident in the
SWNT-LCE nanocomposite is suppressed by nearly 6.degree. C. The
suppression of the TNI is evidence for close association and
intercoupling between the SWNT and the LCE host, even at very low
loadings (0.02 wt %). However, despite the similarities in the
baseline thermomechanical properties, the thermally-induced
mechanical deformation apparently in these materials is
distinguished.
[0116] Polarized Raman spectroscopy, shown in FIG. 12A (FIG. 12B
being a close-up of Raman scatter from the radial breathing modes
of the CNT), was performed on 15 .mu.m thick samples with a
Renishaw in Via confocal Raman microscope. Focused light
(100.times. objective, 600 nm spot) from 514.5 nm and 633 nm laser
excitation sources was used to excite the samples at various spots.
The polarization of the incident laser was rotated every 5.degree.,
from -90.degree. to 90.degree. to obtain angle-dependent Raman
scattering from the LCE and the SWNTs. Polarized Raman spectra were
collected at various confocal depths through the thickness of the
films in the case of the twisted nematic films. Order parameter for
Raman spectroscopy was calculated according to conventional methods
at the LCE and G' band peaks as a function of incoming laser
polarization.
[0117] SWNTs exhibit a strong Raman signal, which can enable
accurate determination of orientation at very low loadings through
polarized Raman measurements. The radial breathing modes (RBMs) and
the G and G' bands of the SWNT are apparent in the Raman spectra in
FIG. 12A. Due to overlap in the Raman signals from the G band of
the SWNTs and the LCE composition, the polarization dependence of
the G' band at 2670 cm.sup.-1 was measured and is shown in FIG.
13.
[0118] The signal at 1700 cm.sup.-1 was selected to represent LC
mesogen orientation. The average orientation of the SWNT and the LC
mesogen was determined by rotating the laser polarization and
comparing the relative intensities of the normalized peak areas.
Both the SWNT and the LC mesogens take on the surface alignment of
the cell. The orientation parameter of the LC mesogens was
calculated from these data to be 0.46.+-.0.08, while the SWNT
orientation was found to be slightly higher at 0.51.+-.0.08. The
orientation parameter was in good agreement with the orientation
parameter for the liquid crystalline elastomers measured by
wide-angle X-ray scattering (see FIGS. 14A, 14B, and 15). Notably,
the orientation parameter of the LCE host (without SWNT) was nearly
identical (0.46), indicating that the SWNT are not disrupting the
mesogen alignment.
[0119] Wide angle X-ray scattering was performed using a Rigaku
Ultrax and Cu K.alpha. radiation on a 15 .mu.m thick sample with
uniaxial alignment. Again, order parameter was calculated according
to conventional methods. FIG. 14 illustrates X-ray scattering
intensity at the 19.4.degree. 2-theta peak as a function of
azimuthal angle. FIGS. 15A and 15B are X-ray scattering profiles of
the planar SWNT-LCEW with and without SWNT, respectively. These
profiles are indicative of a well-aligned nematic system.
[0120] To confirm the association of the SWNT and LCE in more
complex topologies, a +1 radial defect was imprinted into an LCE
containing 0.02 wt % SWNT via a photoalignment surfaces. The SWNT
orientation matched the expected director profile (FIGS. 16A and
16B). These results confirm that previously developed methods to
imprint (voxelate) orientation in LCEs can be extended to
arbitrarily align one-dimensional nanomaterials in a monolithic
polymer matrix.
[0121] The 0.02 wt % SWNT-LCE nanocomposite films were fixed in
OsO.sub.4 and embedded into flat molds with Epofix resin so that
the cutting direction would be either parallel or perpendicular to
the LCE/SWNT orientation direction. The blocks were polymerized
overnight at 60.degree. C. and then hand trimmed with razor blades
to form a trapezoid face. The blocks were then ultramicrotomed
using an RMC Ultracut microtome with a 35.degree. Diatome diamond
knife. A 75 nm thick section was collected onto a 400 hex Cu mesh
grid and allowed to dry. Imaging was captured using an FEI CM 200
transmission electron microscope at 200 kV. Digital images were
captured with a CCD camera and a 4Pi system. Exemplary images are
provided in FIGS. 17A and 17B, for parallel and perpendicular
directions, respectively.
[0122] FTIR spectra of monomer polymerization between two 1 cm
thick NaCl slides were collected using a Bruker FTIR in
transmission mode. Upon exposure to UV light, four scans were taken
from 400 cm.sup.-1 to 3200 cm.sup.-1 every 0.5 sec for 30 min.
[0123] Gel fraction was performed by extraction in acetone for 24
hr and drying at 45.degree. C. under vacuum. The gel fraction of
neat LCE and SWNT-LCE sample was found to be 0.78 .+-.0.1.
[0124] Mechanical measurements were performed at 25.degree. C. on
an RSA3 TA Instruments tensile tester in uniaxial strain mode at 2%
strain/min. Samples dimensions were 5 mm.times.2 mm.times.0.015 mm.
All tests were performed a minimum of five times with five
different samples.
[0125] Electromechanical measurements of homogeneous 15 .mu.m films
were imaged using POM while the film floated in silicone oil
between two ITO-coated glass slides spaced 100 .mu.m apart. Strain
was determined by measuring an area of the film as a function of
time using a homemade MATLAB program. The temperature was
controlled using a home-made ITO heat stage and surface
thermocouple. FIGS. 18A and 18B illustrate the floating film before
and after, respectively, exposure to 3.6 VDC/.mu.m electric field.
The film in FIG. 18B bends along the director.
[0126] FIG. 19A is an exemplary heat map of 0.02% SWNT-LCE film,
patterned into a +1 topological defect. The heat map indicates
significant localized shape change throughout the film during
electrical activation. FIG. 19B graphically illustrates a
difference in the film before and after a 1.2 VDC/.mu.m field
applied at 90.degree. C.
[0127] Electromechanical actuation of photopatterned films was
measured using a Keyence optical profilometer 3D scanner. The
12.mu.m thick films, in air, were placed between ITO-coated glass
slides spaced 1 mm apart, and the field applied through the film
thickness. The temperature was controlled using a home-made ITO
heat stage.
[0128] Electrical conductivity of 0.02 wt % SWNT-LCE films was
performed in an ITO-glass liquid crystal cell with homogeneous
Elvamide alignment layers. Voltage was swept from 0 V/.mu.m to 2
V/.mu.m in 0.1 V intervals, and the current measured after a 2 sec
equilibration. The sweep was performed three times, and data
reported is from the second sweep.
[0129] Dielectric impedance and permittivity measurements were
conducted on ITO-glass liquid crystal cells with homogeneous
Elvamide alignment layers using a Novocontrol Alpha Analyzer. The
LC cells were placed inside an oven (Memmert), where temperature
was ramped from 25.degree. C. to 240.degree. C. at a rate of
0.5.degree. C./min. Permittivity was measured at discrete
frequencies, swept over the range 0.5 Hz to 1 MHz at an AC driving
voltage of 1 V; a new scan was initialized every 100 sec.
[0130] FIG. 20 graphically illustrates relaxation peak from
dielectric relaxation spectroscopy as a function of inverse
temperature for samples with and without SWNTs. Fitted curves were
obtained for before and after the para-nematic transition
temperature, and the slope was directly proportional to the dipole
rotational activation energy. E.sub.a=78.1 kJ/mol for films having
no SWNT at low temperature. E.sub.a=141.7 kJ/mol for films having
no SWNT at high temp. E.sub.a=66.8 kJ/mol for films having 0.02%
SWNT at low temperature. E.sub.a=139.5 kJ/mol for films having
0.02% SWNT at high temperature.
[0131] In FIG. 21A, the incorporation of 0.02 wt % SWNT into the
LCE reduces the thermally induced contraction from 140% to 60%. To
clarify whether the difference is attributable to limits in
thermotropic disruption of the order of the materials, the
birefringence of both the neat LCE and SWNT-LCE nanocomposites were
examined over a range of temperatures (FIG. 21B). The temperature
dependence of the neat LCE sample and the SWNT-LCE nanocomposite
are effectively identical with the exception of a small residual
birefringence for the SWNT-LCE sample (FIGS. 22A and 22B).
[0132] Many conventional nanocomposites exhibit enhanced mechanical
properties, such as stiffness, in part derived from anisotropy of
various nanoinclusions. Deformation of the LCE in the planar
orientation parallel to the director orientation exhibited
classical linear stress-strain response of an elastomer. When the
director orientation of the LCE is orthogonal to the stretch
direction, the LCE displayed nonlinear elastic behavior, known as
"soft elasticity." A plateau in the stress-strain curve is
associated with the rotation of the mesogens in the stress field.
Evident in FIG. 23, SWNT-LCEs display a comparatively higher (and
reinforced) modulus parallel to the director compared to an LCE
without SWNT. There was no measurable modulus difference
perpendicular to the director. The modulus increased with the
inclusion of just 0.02 wt % SWNT and was about 24% (Z=5,
P<0.001). However, as evident in the representative
stress-strain curves presented in FIG. 23, the maximum elongation
(strain to failure) of the SWNT-LCE was reduced when the director
is orthogonal to the stretch direction.
[0133] Shape change of the film was monitored between
cross-polarizers. As is evident in a supporting movie available at
https://pubs.acs.org/doi/suppl/10.1021/acsami.7b13814/suppl_file/am07b138-
14_si_002.avi, the disclosure of which is incorporated herein by
reference in its entirety, the film primarily constricted along the
director orientation achieving an 18% reduction in measured length
at 100.degree. C. and 5 V/.mu.m, as shown in FIG. 24. Due to slight
heterogeneity in the dispersion of SWNT across the sample thickness
(TEM micrographs shown in FIGS. 17A and 17B), some bending was also
observed (FIGS. 18A and 18B). The electromechanical responses
reported in FIG. 24 are strongly temperature dependent. The
nanocomposite prepared with this composition is not responsive to
applied DC field at room temperature.
[0134] FIG. 25 examines the electromechanical response of the
SWNT-LCE composite at 100.degree. C. as a function of voltage. The
nanocomposites constrict rapidly, reaching maximum deformation in
less than one second and return to their original state quickly
(less than about 1 sec) upon removing the field. This constriction
and relaxation can be cycled dozens of times (FIG. 26). At
extremely high fields (more than about 4.5 VDC/.mu.m), the films
showed signs of unrecoverable deformation.
[0135] SWNT-LCE nanocomposites, photopatterned such that the
director field is a +1 radial defect (FIGS. 27 and 28A), were then
placed in a 1 mm thick, ITO cell to simulate free-standing
actuation without the need for flexible electrodes or electrical
contacts. Upon heating to 90.degree. C., the films began to form a
very shallow cone (FIG. 28B). At this temperature, a 1.5 V DC/.mu.m
field was applied to electrically induce a rapid shape deformation
(FIG. 28C). The new shape appeared to be a cone-like shape but with
some asymmetry (FIGS. 28A and 28B). When the field was released,
the film quickly returned to the shallow cone evident in FIG. 28B.
As such, a DC electric field was shown to generate a complex and
curved shape. This shape change is smoothly varying across the film
(FIG. 19B). It is not yet clear why the electromechanical
deformation does not form a cone that is apparent in thermal
experiments.
[0136] While not wishing to be bound by theory, it is hypothesized
that the observed electromechanical response may be attributable to
a combination of rotation of the SWNT ascribed to dielectric
mismatch between the SWNT and the LCE and mesogen rotation via
interfacial polarization between the nanoinclusions and the LCE.
The possibility of electro-thermal induced effect may be dismissed
because deformation and recovery of the SWNT-LCE films occurs in
less than 1 sec (resistive heating responses are transient and
comparatively slow). Moreover, resistive heating may be induced by
either AC or DC field; the SWNT-LCE films were responsive only to
low frequency AC fields. Such nature of the electric field
susceptibility of the SWNT-LCE indicates that the interface of the
SWNT and the LCE host may be critical.
[0137] To further explore the electrical properties of these
materials, dielectric relaxation spectroscopy ("DRS") was used to
determine whether LCE dipoles rotated more freely in the electric
field in the presence of SWNT. Neat LCE displayed no dipole
relaxation from 0.5 Hz to 10.sup.6 Hz until heated above
100.degree. C. In contrast, SWNT-LCE films displayed a strong
dipole relaxation in the same frequency range by 65.degree. C.
(FIG. 29). At all measured temperatures, the SWNT-LCE dipole
relaxation occurred at a much higher frequency than in the neat LCE
film. This suggests that the LCE dipoles experience much less
resistance to motion in an electric field and are able to relax
faster. The permittivity of a dielectric material is increased both
from the dipole rotation, which effectively screens the field, and
from additional free surface charge accumulation. From 50.degree.
C. to 150.degree. C., the permittivity of the SWNT-LCE is much
greater than the permittivity of the neat LCE, which indicates a
significant increase in either dipole mobility, charge
accumulation, or both (FIG. 30).
[0138] Dipole rotational activation energy may be determined from
the relaxation peak as a function of temperature (FIG. 20). SWNTs
lowered the dipole rotational activation energy by 15% when the
material was below the TNT.
[0139] To determine whether the SWNTs were reorienting in the
electric field, electrical conductivity through film thickness was
measured as a function of field strength. Evident in FIG. 31, the
SWNT-LCE films displayed highly non-Ohmic conductivity and
increased conductivity with increasing DC electric field. At high
field strength, the SWNT-LCE film became modestly semiconductive
(less than 1.2 .mu.S/m at 2 VDC/.mu.m), which suggests SWNT
orientation changes with increasing field strength. The material
relaxed when the field was removed, becoming insulating again at
low voltages. It is likely that rotation of the SWNTs is not solely
responsible for the electromechanical responses, but it may play a
complementary role that is potentially attributable to interfacial
charge accumulation and which synergistically produce a strong
electrostrictive force.
[0140] According to the various embodiments, liquid crystal
elastomers that retain the order of their liquid crystalline
precursors are described herein. These liquid crystal elastomers
undergo large, reversible deformations in response to electricity.
According to some embodiments, the liquid crystal elastomers may
also under go large, reversible deformations in response to one or
more of heat, light, and solvent. Such liquid crystal elastomers
could enable easier device integration and swifter response
times.
[0141] Preparation of liquid crystal elastomers responsive to
electricity according to various embodiments are described herein.
Some exemplary embodiments are characterized. The liquid crystal
elastomers comprise nanocomposites having CNTs dispersed into the
liquid crystal elastomer matrix such that the CNTs align with an
orientation of the liquid crystalline director. Accordingly, these
nanocomposites are arbitrarily and complexly aligned, with local
control of both the liquid crystalline director as well as the
orientation of the CNTs. Due to the interfacial interactions of the
SWNTs with the polymer host, these nanocomposites exhibit
distinctive electrically-induced shape transformation.
[0142] While the present invention has been illustrated by a
description of one or more embodiments thereof and while these
embodiments have been described in considerable detail, they are
not intended to restrict or in any way limit the scope of the
appended claims to such detail. Additional advantages and
modifications will readily appear to those skilled in the art. The
invention in its broader aspects is therefore not limited to the
specific details, representative apparatus and method, and
illustrative examples shown and described. Accordingly, departures
may be made from such details without departing from the scope of
the general inventive concept.
* * * * *
References