U.S. patent application number 12/761709 was filed with the patent office on 2011-10-20 for assembly for and method of forming localized surface wrinkles.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS, INC.. Invention is credited to Joseph C. Simmer, Ruomiao Wang, Xingcheng Xiao, Tao Xie.
Application Number | 20110253288 12/761709 |
Document ID | / |
Family ID | 44787273 |
Filed Date | 2011-10-20 |
United States Patent
Application |
20110253288 |
Kind Code |
A1 |
Xie; Tao ; et al. |
October 20, 2011 |
ASSEMBLY FOR AND METHOD OF FORMING LOCALIZED SURFACE WRINKLES
Abstract
An assembly including and method of forming arbitrary localized
wrinkles upon a surface utilizing a shape memory polymer substrate
and rigid overlay, wherein the geometrical distribution of the
wrinkles is produced by recovering a lateral strain history within
the substrate and buckling the overlay, and the localized wrinkles
are used to create, among other things, optically three-dimensional
engaging surfaces, structural colors, modified surface texturing,
and haptic alerts.
Inventors: |
Xie; Tao; (Troy, MI)
; Xiao; Xingcheng; (Troy, MI) ; Wang; Ruomiao;
(Lynchburg, VA) ; Simmer; Joseph C.; (Armada,
MI) |
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS,
INC.
Detroit
MI
|
Family ID: |
44787273 |
Appl. No.: |
12/761709 |
Filed: |
April 16, 2010 |
Current U.S.
Class: |
156/64 ; 156/219;
156/459 |
Current CPC
Class: |
Y10T 428/24942 20150115;
B29C 59/02 20130101; B29C 59/18 20130101; B44C 1/005 20130101; Y10T
156/1039 20150115; Y10T 428/24446 20150115 |
Class at
Publication: |
156/64 ; 156/219;
156/459 |
International
Class: |
B29C 59/18 20060101
B29C059/18; B29C 59/00 20060101 B29C059/00; B44C 3/08 20060101
B44C003/08 |
Claims
1. A method of forming localized wrinkles upon a surface, said
method comprising: a. indenting a substrate at least partly formed
of shape memory polymer, so that the substrate defines a first area
experiencing purely compressive strain and a second area adjacent
the first area and experiencing compressive and tensile strains; b.
attaching a relaxed overlay to the substrate, so as to overlay the
first and second areas; and define the surface; c. activating the
polymer, so as to recover the strains; d. causing wrinkles to form
in the overlay, as a result of recovering the tensile strain in the
second area; and e. deactivating the polymer.
2. The method as claimed in claim 1, wherein steps a) and b)
further comprises heating the polymer to a first temperature, and
applying a load to the substrate after the substrate achieves the
first temperature, and cooling the polymer to a second temperature
below the first temperature, while continuing to apply the load, so
as to lock in the strains.
3. The method as claimed in claim 1, wherein step a) further
comprises the steps of applying a press to the substrate, wherein
the press defines an engaging relief.
4. The method as claimed in claim 3, wherein the relief presents
and the first or second area defines, indicia, a logo, an engaging
surface, or a shape.
5. The method as claimed in claim 1, wherein the overlay,
substrate, and strains are cooperatively configured to produce
wrinkles having a wavelength within the visible spectrum.
6. The method as claimed in claim 1, wherein the polymer consists
essentially of an aromatic diepoxide, an aliphatic diepoxide, and
an aliphatic diamine curing agent.
7. The method as claimed in claim 1, wherein the overlay is a
metallic thin film.
8. The method as claimed in claim 7, wherein the overlay is formed
of white gold.
9. The method as claimed in claim 1, wherein step c) further
includes the steps of depositing the overlay.
10. The method as claimed in claim 1, wherein the substrate
presents a first modulus, the overlay presents a second modulus
greater than the first, and the first and second modulus present a
ratio greater than a predetermined threshold ratio.
11. The method as claimed in claim 10, wherein the overlay presents
a critical bucking strain (.epsilon..sub.c), and steps a) and c)
further includes the steps of defining and recovering a tensile
strain greater than the critical bucking strain.
12. The method as claimed in claim 11, wherein the critical bucking
strain is determined in accordance with the formula:
.epsilon..sub.c=[E.sub.s.sup.2/E.sub.f.sup.2].sup.1/3 E.sub.s is
the modulus of the substrate, and E.sub.f is the modulus of the
film.
13. The method as claimed in claim 1, wherein the second area
circumscribes the first area.
14. The method as claimed in claim 1, wherein the overlay presents
a thickness approximately equal to 10 nm.
15. A method of measuring the cracking strain/stress of a
nanoscopically thin overlay, said method comprising the steps of:
a. indenting a substrate at least partly formed of shape memory
polymer presenting a glass transition temperature, so that the
substrate defines a first area experiencing a purely compressive
strain, and a second area adjacent the first area and experiencing
compressive and tensile strains; b. attaching the overlay to the
substrate, so as to cover the first and second areas, and define
the surface; c. activating the polymer, so as to recover the
strains; d. determining the formation of a crack within the
overlay, as a result of recovering the tensile strain in the second
area; and e. observing and measuring the crack relative to the
strains, when the crack is formed.
16. An assembly for forming localized wrinkles within a continuous
surface, said assembly comprising: a substrate at least partially
formed of a shape memory polymer presenting a glass transition
temperature and first elastic modulus when activated, being
plastically indented so as to define a first area experiencing a
purely compressive strain, and a second area adjacent the first
area and experiencing compressive and tensile strains, when the
polymer is deactivated, and operable to recover a the strains when
the polymer is activated; and a relaxed overlay fixedly attached to
and configured to cover the first and second areas, and defining
the surface, a second elastic modulus, and a height, wherein the
second elastic modulus is greater than the first modulus, and the
moduli, height, and strains are cooperatively configured to cause
buckling in the overlay, when the polymer is activated and recovers
the strains.
17. The method as claimed in claim 16, wherein the polymer consists
essentially of an aromatic diepoxide, an aliphatic diepoxide, and
an aliphatic diamine curing agent.
18. The method as claimed in claim 16, wherein the overlay is a
metallic thin film.
19. The method as claimed in claim 18, wherein the overlay is
formed of white gold.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present disclosure generally relates to assemblies for
and methods of producing localized surface wrinkles, and more
particularly, to an assembly for and method of producing localized
surface wrinkles using a shape memory polymer substrate and rigid
overlay.
[0003] 2. Discussion of Prior Art
[0004] Surface wrinkles have been used to effect, modify, or
control various benefits/conditions, including surface adhesion,
texturing, coefficients of friction, structural colors, metrology,
and haptic alerts. Methods of producing surface wrinkles
preexisting in the art include using a stretched substrate overlaid
by a rigid (e.g., metal) film. Wrinkles are instantaneously or
selectively produced in the film, upon the release of energy by the
substrate, if the compressive strain in the film exceeds the
critical bucking strain. As a result, these conventional methods
produce generalized wrinkles that co-extend with the entire surface
defined by the overlay. This method is in fact behind wrinkles
commonly encountered, for example, on human skin and dehydrated
apples. Of particular interest is that the wrinkle geometry is
closely related to the material properties. Precisely controlled
wrinkle structures have found many interesting applications
including nano-metrology, stretchable electronics, biosensors, and
manipulation of material topographic properties.
BRIEF SUMMARY OF THE INVENTION
[0005] The present invention recites a novel assembly for and
method of producing localized wrinkles within a surface, and more
specifically, to an assembly for and method of producing localized
surface wrinkles using a shape memory polymer substrate and rigid
overlay. The present invention is useful for modifying the surface
texture, and/or coefficient of friction of a select portion of a
continuous surface. Where achieving a visible wavelength, the
inventive wrinkling is also useful for producing structural colors
in a predetermined pattern within the continuous surface; and as
such, is further useful to produce a three-dimensional engaging
surface (e.g., indicia, logo, shape, or picture) on a
two-dimensional surface.
[0006] In a first aspect of the invention, a method of forming
localized wrinkles upon a surface is presented. The method includes
indenting a substrate at least partly formed of shape memory
polymer presenting a glass transition temperature, so that the
substrate defines a first area experiencing purely compressive
strain and a second area adjacent the first area and experiencing
both a vertical compressive strain and a lateral tensile strain.
Next, a overlay is attached to the substrate, so as to overlay the
first and second areas. The overlay defines the surface. The
polymer is then activated, so as to recover the strains, and cause
wrinkles to form in the overlay. Finally, the polymer is
deactivated, so as to lock in the wrinkles.
[0007] A second aspect of the invention, includes a method of
determining the cracking strain/stress of a nanoscopicallythin
overlay, which includes observing the wrinkles in the above
process, so as to determine a crack formation occurred for a given
tensile strain.
[0008] Thus, in a third aspect, the invention presents an assembly
for forming localized wrinkles within a continuous surface. The
assembly includes a substrate at least partially formed of a shape
memory polymer presenting a glass transition temperature and first
elastic modulus when activated. The substrate is indented so as to
define a first area experiencing a purely compressive strain, and a
second area adjacent the first area and experiencing both a
vertical compressive strain and a lateral tensile strain, when the
polymer is deactivated. The substrate is operable to recover the
strains when the polymer is activated. The assembly further
includes a relaxed overlay fixedly attached to and configured to
cover the first and second areas. The overlay defines the surface,
a second elastic modulus, and a height, wherein the second elastic
modulus is greater than the first modulus. The moduli, height, and
strains are cooperatively configured to cause buckling in the
overlay, when the polymer is activated and recovers the
strains.
[0009] The disclosure may be understood more readily by reference
to the following detailed description of the various features of
the disclosure and the examples included therein.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0010] A preferred embodiment(s) of the invention is described in
detail below with reference to the attached drawing figures of
exemplary scale, wherein:
[0011] FIGS. 1a-d is a multi-elevation progression showing a method
of forming localized wrinkles upon a surface, wherein a protruding
press is used to indent the substrate, and the wrinkles are further
shown in enlarged caption at FIG. 1d, in accordance with a
preferred embodiment of the invention;
[0012] FIG. 2a-c is a multi-elevation progression showing a method
of forming localized wrinkles upon a surface, wherein a recessed
press is used to form a projection upon the substrate, in
accordance with a preferred embodiment of the invention;
[0013] FIG. 3 is a perspective view of the wrinkles formed in FIGS.
1d, and 3d, in accordance with a preferred embodiment of the
invention; and
[0014] FIG. 4 is a plan view of a surface presenting a plurality of
wrinkles and cracks formed using the inventive method.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The following description of the preferred embodiments is
merely exemplary in nature and is in no way intended to limit the
invention, its application, or uses. As described and illustrated
herein, a novel assembly 10 for and method of forming arbitrary
localized wrinkles (i.e., wrinkle structures) 10a within a surface
12 includes and utilizes, respectively, a locally and plastically
deformed shape memory polymer (SMP) based substrate 14 and a thin,
high modulus overlay 16 (FIGS. 1-4); however, it is certainly
within the ambit of the invention to utilize the benefits of the
assembly 10 with other equivalent selectively activated active
materials exhibiting shape memory effect, and/or in other
applications and configurations discernable by those of ordinary
skill in the art.
[0016] As used herein, the term "shape memory polymer (SMP)" shall
generally refer to a group of polymeric materials that demonstrate
the ability to return to some previously defined shape when
subjected to an appropriate thermal stimulus, as is known in the
art. Shape memory polymers are capable of undergoing phase
transitions in which their shape is altered as a function of
temperature. The previously defined or permanent shape can be set
by curing for thermoset polymers or melting or processing the
polymer at a temperature higher than the highest thermal transition
for thermoplastic polymers. For a thermoplastic shape memory
polymer, a temporary shape can be set by heating the material to a
temperature higher than T.sub.g or T.sub.m of the soft segment, but
lower than the T.sub.g or melting point of the hard segment. For a
thermoset shape memory polymer, a temporary shape can be set by
heating the material to a temperature higher than T.sub.g or
T.sub.m. The temporary shape is set by cooling. The material can be
reverted back to the permanent shape by heating the material above
the shape memory transition temperature.
[0017] The temperature needed for permanent shape recovery can be
set at any temperature between about -63.degree. C. and about
120.degree. C. or above. Engineering the composition and structure
of the polymer itself can allow for the choice of a particular
temperature for a desired application. A preferred temperature for
shape recovery is greater than or equal to about -30.degree. C.,
more preferably greater than or equal to about 0.degree. C., and
most preferably a temperature greater than or equal to about
50.degree. C. Also, a preferred temperature for shape recovery is
less than or equal to about 150.degree. C., and most preferably
less than or equal to about 150.degree. C. and greater than or
equal to about 80.degree. C.
[0018] Suitable shape memory polymers include thermoplastics,
thermosets, interpenetrating networks, semi-interpenetrating
networks, or mixed networks. The polymers can be a single polymer
or a blend of polymers. The polymers can be linear or branched
thermoplastic elastomers with side chains or dendritic structural
elements. Suitable polymer components to form a shape memory
polymer include, but are not limited to, polyolefins, epoxy
polymers, polyphosphazenes, poly(vinyl alcohols), polyamides,
polyester amides, poly(amino acid)s, polyanhydrides,
polycarbonates, polyacrylates, polyalkylenes, polyacrylamides,
polyalkylene glycols, polyalkylene oxides, polyalkylene
terephthalates, polyortho esters, polyvinyl ethers, polyvinyl
esters, polyvinyl halides, polyesters, polylactides,
polyglycolides, polysiloxanes, polyurethanes, polyethers, polyether
amides, polyether esters, and copolymers thereof. Examples of
suitable polyacrylates include poly(methyl methacrylate),
poly(ethyl methacrylate), ply(butyl methacrylate), poly(isobutyl
methacrylate), poly(hexyl methacrylate), poly(isodecyl
methacrylate), poly(lauryl methacrylate), poly(phenyl
methacrylate), poly(methyl acrylate), poly(isopropyl acrylate),
poly(isobutyl acrylate) and poly(octadecyl acrylate). Examples of
other suitable polymers include polystyrene, polypropylene,
polyvinyl phenol, polyvinylpyrrolidone, chlorinated polybutylene,
poly(octadecyl vinyl ether) ethylene vinyl acetate, polyethylene,
poly(ethylene oxide)-poly(ethylene terephthalate),
polyethylene/nylon (graft copolymer), polycaprolactones-polyamide
(block copolymer), poly(caprolactone) dimethacrylate-n-butyl
acrylate, poly(norbornyl-polyhedral oligomeric silsequioxane),
polyvinylchloride, urethane/butadiene copolymers, polyurethane
block copolymers, styrene-butadiene-styrene block copolymers, and
the like.
[0019] In the present invention, pre-patterning on the substrate
overlay 16 is produced to create wrinkle structures 10a (FIGS. 1d,
2c, 3, and 4). The method is based on the principle that the
compressive strain on the surface 12 can be altered by indentation
or pressing. Depending on the shape and dimension of the indenter
or press, the resulting wrinkle structures 10a can be manipulated
in a local and arbitrary fashion.
[0020] More particularly, in an exemplary embodiment, the substrate
14 was at least partially formed of a solid epoxy shape memory
polymer consisting, for example, of an aromatic diepoxide (EPON
826, 3.6 g or 0.01 mol), an aliphatic diepoxide (NGDE, 2.16 g or
0.01 mol), and an aliphatic diamine curing agent (Jeffamine D-230,
2.3 g or 0.01 mol). This mixture was cured at 100.degree. C. for 1
hour and at 130.degree. C. for 1 hour to obtain a shape memory
polymer presenting a glass transition temperature of approximately
40.degree. C., and a permanent default shape. The default shape
preferably defines smooth exterior surfacing (i.e., curved or flat
but having no indentations or protrusions). The substrate 14 may be
rectangular (FIGS. 1-2c), oblong, define a molding, such as an auto
trim, or be of any shape, so long as it is large enough to support
a surface 12 suitable for displaying the intended wrinkle structure
10a. The substrate 14 may include other components, in addition to
a body of SMP, such as an external interface layer (not shown) that
facilitates bonding with the overlay 16, or non-active sectors
where wrinkles are not desired, for example, to better withstand
purely compressive forces.
[0021] To effect the inventive method, a press (e.g., indenter) 18
with a protruded (FIGS. 1a-d) or recessed (FIGS. 3a-d) defining
surface 18a that defines a relief (e.g., three-dimensional logo,
indicia, shape, image, or picture) is pressed either manually or
automatically onto the shape memory polymer substrate 14 until
deformation results (FIG. 1b, 2b). More preferably, the press 18 is
applied after the substrate 14 has been preheated, e.g., at
65.degree. C. for 10 min in the exemplary embodiment, to a
temperature above its glass transition temperature, so as to reduce
the modulus of elasticity of the substrate 14, thereby making it
easier to indent and deform. The relief can be of any arbitrary
shape and dimensions.
[0022] In a first directly indented area 14a, only a compressive
strain in the vertical direction is created (FIGS. 1b, 2b). On the
other hand, no strain is induced in areas sufficiently away from
the indented area 14a. Owing to the material continuity, a second
transition area 14b around the edge of the indented area 14a is
produced. In the transition area 14b, the strain consists of a
compressive component in the vertical direction and a tensile
component in the lateral direction. It is appreciated that the
spatial distribution of the lateral strain in the transition area
14b is highly dependent on the shape of the press 18. As such, the
resulting wrinkle structures 10a can be manipulated by alternating
the shape of the press 18. The substrate 14 is then cooled back to
a temperature below its transition temperature under the load, so
as to lock in the deformation.
[0023] Next, the relatively rigid overlay 16 is securely attached
to the substrate 14, so as to cover the first and second areas
14a,b (FIG. 1c, 2c). In the exemplary embodiment, the deformed
substrate 14 is coated at room temperature with a "white gold" film
(e.g., palladium/gold alloy composition) 16 using a sputtering
system (not shown). Here, the film 16 thickness (e.g.,
approximately 10 nm) is controlled by deposition time and may be
measured directly by a scanning electron microscopic analysis of
the cross-sections.
[0024] After film deposition is complete, and when the formation of
wrinkles are desired, the assembly 10 is heated to activate the
polymer substrate 14 and create wrinkles 10a due to shape recovery
induced local compression (FIGS. 1d, 2c, 3, and 4). As such, it is
certainly appreciated that the overlay (e.g., film) 16 must be
non-reactive at temperatures at least equal to the transition
temperature of the SMP. In the exemplary embodiment, the polymer
may be heated to 90.degree. C. for 10 minutes to ensure complete
activation and shape recovery. In the direct indented area 14a, the
shape recovery simply moves the thin film 16 upwards in the
vertical direction and induces no strain therein. Again, in the
transition area 14a compressive strain is recovered as the thin
film 16 moves upwards. Here, however, tensile strain is also
recovered, which creates lateral compression in the thin film
16.
[0025] If the lateral compression strain exceeds a critical
buckling value defined by the assembly 10, wrinkles 10a will form.
In a preferred embodiment, the critical buckling strain,
.epsilon..sub.c, may be pre-determined according to the following
formula:
.epsilon..sub.c=[9E.sub.s.sup.2/64E.sub.f.sup.2].sup.1/3 (1)
wherein E.sub.s is the modulus of the substrate, and E.sub.f is the
modulus of the film; and accordingly the resultant wrinkle
amplitude, A, may be determined by the following formula:
A=h[(.epsilon./.epsilon..sub.c)-1].sup.1/2 (2)
wherein .epsilon. is the strain currently experienced by, and h is
the thickness of the overlay 16. Thus, it is appreciated that for
rigid substrates, i.e., large E.sub.s critical strain is large,
amplitude is small, and wrinkles are difficult to form. Once the
wrinkles 10a are formed, the substrate 14 is again cooled to a
temperature below the transition temperature of the SMP, so as to
lock in the wrinkles 10a, which makes them more robust than those
conventionally produced by soft substrate assemblies.
[0026] It is appreciated that circularly distributed wrinkles (FIG.
4) are created with a spherical press defining surface 18a,
square-shaped wrinkles would be created by a square shaped press
defining surface (FIGS. 1-2c), and a logo would be created by a
logo-shaped defining surface 18a. By contrast, the wrinkles 10a may
be generated using a Vickers indenter and the first indentation
step may be conducted on a non-preheated shape memory polymer
substrate 14 using a Nano Scratch Tester (CSM Instruments) under a
predetermined load.
[0027] In the exemplary embodiment, the wrinkles 10a were analyzed
using an atomic force microsopy (AFM) due to the microscopic scales
resulting therefrom. AFM characterization of wrinkles was conducted
at room temperature in a contact mode using Dimension 3100
manufactured by Veeco.TM.. The wavelength, a, and amplitude or
height, A, of the wrinkles 10a were obtained by measuring 80-100
individual wrinkles using the section analysis function in the
Nanoscope software (Nanoscope 5.31r1). One sample, presented a
wavelength and amplitude of 800 nm of 80 nm, respectively.
[0028] It is appreciated that the wrinkle wavelength decreases
linearly with strain, whereas wrinkle amplitude is independent of
strain. Increasing the overlay thickness on the other hand,
increases both wrinkle wavelength and amplitude. With respect to
the impact of strain, the classical wrinkle theory based on elastic
energy minimization suggests that wrinkle wavelength should be
strain independent according to the following formula:
.lamda. = 2 .pi. h [ ( 1 - v s 2 ) E f ( 1 - v f 2 ) E s ] 1 / 3 (
3 ) ##EQU00001##
where E, v, h, and .epsilon. represent respectively modulus,
Poisson ratio, film thickness, and compressive strain, and the
subscripts s and f denotes substrate and film (i.e., overlay).
Finally, it is appreciated that the linear dependence between
wavelength and strain under the present invention provides a
benefit in creating localized wrinkles 10a of different wavelength
on the same surface, but deviates from the above relationship, when
finite deformation is considered.
[0029] Where the wavelength falls within the visible spectrum, it
is appreciated that a structural color will result. That is to say
the wrinkles 10a will cause a color to be perceived by altering the
way light travels at different dimensions, as opposed to chemical
colors that rely on the absorption of certain wavelength lights by
pigment molecules. It is appreciated that the colors are highly
angle dependent; that is to say, the viewing angle contributes to
the actual color perceived. It is further appreciated that this
process presents advantages over conventional structural color
forming techniques that require the use of lithographic templates,
which can be relatively expensive and require dedicated equipment
not widely accessible.
[0030] Through the creation of structural colors, arbitrary images
can be captured and displayed using wrinkle based diffraction
colors. For example, where the relief 18a presents a protruded logo
or indicia, the letters can be made to appear to protrude out of
the surface 12 (i.e., three-dimensionally), while in fact the
surface 12 is macroscopically smooth. This illusion results because
the edge of the letters is colored to resemble shading even though
no pigment is introduced in the process. By using a recessed relief
(FIGS. 2a-c), an engaging surface or logo may be produced with the
letters colored, instead of the edges of the letters. With this
change, the transition strain area 14b resides in the letter or
image face.
[0031] As shown in the circularly distributed wrinkles of FIG. 4,
it is appreciated that the wavelength increases with the radial
distance from the center of the indent. In certain instances,
cracks 20 may also be produced in the wrinkle structures 10a (FIG.
4). The existence of cracks corresponds to surpassing a critical
strain above which cracks are formed. Thus, it is also appreciated
that the present invention can be used as a convenient method of
measuring the cracking strain/stress of a nanoscopicallythin film
16, which could otherwise be challenging using conventional
methods.
[0032] This invention has been described with reference to
exemplary embodiments; it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to a
particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
all embodiments falling within the scope of the appended
claims.
* * * * *