U.S. patent application number 13/401606 was filed with the patent office on 2012-08-23 for functionally graded shape memory polymer.
This patent application is currently assigned to SYRACUSE UNIVERSITY. Invention is credited to Andrew M. DiOrio, Xiaofan Luo, Patrick Mather, Pine Yang.
Application Number | 20120213969 13/401606 |
Document ID | / |
Family ID | 46652975 |
Filed Date | 2012-08-23 |
United States Patent
Application |
20120213969 |
Kind Code |
A1 |
Mather; Patrick ; et
al. |
August 23, 2012 |
Functionally Graded Shape Memory Polymer
Abstract
A functionally graded shape memory polymer (SMP) that has a
range of transition temperatures that are spatially distributed in
a gradient fashion within one single article. The SMP is formed by
post-curing a pre-cured glassy SMP in a linear temperature gradient
that imposes different vitrification temperature limits at
different positions along the gradient. Utilizing indentation-based
surface shape memory coupled with optical measurements of
photoelastic response, the capability of this material to respond
over a wide range of thermal triggers is correlated with the graded
glass transition behavior. This new class of SMP offers great
potential for such applications as passive temperature sensing and
precise control of shape evolution during a thermally triggered
shape recovery.
Inventors: |
Mather; Patrick; (Manlius,
NY) ; Yang; Pine; (Syracuse, NY) ; Luo;
Xiaofan; (Cleveland, OH) ; DiOrio; Andrew M.;
(Windham, NH) |
Assignee: |
SYRACUSE UNIVERSITY
Syracuse
NY
|
Family ID: |
46652975 |
Appl. No.: |
13/401606 |
Filed: |
February 21, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61444298 |
Feb 18, 2011 |
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Current U.S.
Class: |
428/156 ;
428/174; 428/217; 522/167; 522/168; 522/174; 522/179; 522/182;
522/187; 522/188; 526/328.5; 526/329.7; 526/344; 526/346; 528/271;
528/308.1; 528/421; 528/44 |
Current CPC
Class: |
C08F 220/14 20130101;
C08L 2205/05 20130101; Y10T 428/24628 20150115; C08L 71/02
20130101; B29C 61/06 20130101; Y10T 428/24479 20150115; C08L 71/02
20130101; C08G 2280/00 20130101; C08L 63/00 20130101; C08L 33/04
20130101; C08F 220/12 20130101; C08L 75/04 20130101; Y10T 428/24983
20150115 |
Class at
Publication: |
428/156 ;
528/271; 526/328.5; 528/421; 528/308.1; 526/346; 528/44; 526/344;
526/329.7; 522/167; 522/182; 522/168; 522/179; 522/188; 522/174;
522/187; 428/217; 428/174 |
International
Class: |
B32B 7/02 20060101
B32B007/02; C08F 220/14 20060101 C08F220/14; C08G 65/08 20060101
C08G065/08; C08G 63/127 20060101 C08G063/127; C08F 12/08 20060101
C08F012/08; C08G 18/06 20060101 C08G018/06; C08F 14/06 20060101
C08F014/06; C08F 220/12 20060101 C08F220/12; C08F 2/48 20060101
C08F002/48; C08J 3/28 20060101 C08J003/28; C08G 63/78 20060101
C08G063/78; C08G 18/08 20060101 C08G018/08; B32B 27/08 20060101
B32B027/08; B32B 15/08 20060101 B32B015/08; B32B 33/00 20060101
B32B033/00; C08G 65/34 20060101 C08G065/34 |
Goverment Interests
STATEMENT OF FEDERALLY SPONSORED RESEARCH
[0002] The U.S. Government may have a paid-up license in this
invention and the right in limited circumstances to require the
patent owner to license others on reasonable terms as provided for
by the terms of Grant No. DMR-0907578 of the National Science
Foundation (NSF) and Grant No. FA9550-09-1-0195 of the Air Force
Office of Scientific Research (AFOSR).
Claims
1. A functionally graded shape memory polymer comprising a shape
memory polymer comprising: a first end; a second end; and a
plurality of shape memory glass transition temperatures (T.sub.g's)
that are spatially located and distributed in a gradient fashion
from said first end to said second end.
2. The functionally graded shape memory polymer of claim 1, wherein
said gradient is linear and one dimensional from T.sub.min at said
first end to T.sub.max at said second end.
3. The functionally graded shape memory polymer of claim 2, wherein
said shape memory polymer is structured to respond to a range of
temperatures T, where T.sub.min<T<T.sub.max, upon application
of substantially spatially uniform external heating yielding a
spatially dependent elastic modulus for a given temperature and a
spatially dependent shape recovery response.
4. The functionally graded shape memory polymer of claim 3, wherein
said response comprises recovery of the shape memory polymer from a
temporary deformed or strained configuration with lower
conformational entropy to a permanent configuration with higher
conformational entropy in a wavelike-fashion upon said spatially
uniform external heating, wherein said recovery begins at said
first end where the transition temperature is lowest and propagates
in the direction of increasing transition temperature towards said
second end.
5. The functionally graded shape memory polymer of claim 3, further
comprising a plurality of portions at least partially separated
from one another and spaced along the transition temperature
direction from said first end to said second end, wherein each of
said plurality of portions comprises a different localized T.sub.g
and is structured to relatively independently recover from a
temporary deformed or strained configuration with lower
conformational entropy to a permanent configuration with higher
conformational entropy upon said spatially uniform external
heating.
6. The functionally graded shape memory polymer of claim 5, wherein
each of said plurality of portions is further structured to recover
from a temporary deformed or strained configuration with lower
conformational entropy to a permanent configuration with higher
conformational entropy when said uniform external heating is
greater than a respective portion's localized T.sub.g.
7. The functionally graded shape memory polymer of claim 1, wherein
said shape memory polymer is a thermoset selected from the group
consisting of P(MMA-co-VP)-PEG semi-IPNs, copolyester,
P(AA-co-MMA)-PEG, corn oil copolymer, PMMA-PBMA copolymers, epoxy,
fish oil copolymers, PET-PEG copolymer, P(MA-co-MMA)-PEG, soybean
oil copolymers with styrene and DVB, styrene copolymer,
thermosetting PU, dehydrochlorinated cross-linked PVC, thermosets
formed by thiol-ene reaction, polyacrylates, and
polymethacrylates.
8. The functionally graded shape memory polymer of claim 3, wherein
said shape memory polymer further comprises a film coating on at
least one surface of said shape memory polymer that is more rigid
than said shape memory polymer.
9. The functionally graded shape memory polymer of claim 8, wherein
said response comprises recovery of the shape memory polymer from a
temporary deformed or strained configuration with lower
conformational entropy towards a permanent configuration with
higher conformational entropy, wherein said surface with said film
coating forms wrinkles upon said spatially uniform external
heating, and wherein said recovery and buckling begins at said
first end where the transition temperature is lowest and propagates
in the direction of increasing transition temperature towards said
second end resulting in a change in color of said surface.
10. The functionally graded shape memory polymer of claim 8,
wherein said film coating comprises a coating selected from the
group consisting of a metallic coating and a polymeric coating.
11. The functionally graded shape memory polymer of claim 10,
wherein said metallic coating comprises gold.
12. The functionally graded shape memory polymer of claim 10,
wherein said polymeric coating comprises a polymer with a modulus
of elasticity at least 10 times greater than that of a rubbery
state of said shape memory polymer.
13. The functionally graded shape memory polymer of claim 12,
wherein said polymeric coating comprises a polymer selected from
the group consisting of polystyrene, polycarbonate, poly(alkyl
methacrylate)s, poly(alkyl acrylate)s, polyimides, and poly(arylene
ether ketone)s.
14. A method of preparing a functionally graded shape memory
polymer, said method comprising the steps of: providing a shape
memory polymer comprising a first end and a second end; applying an
increasing temperature gradient to said shape memory polymer from
said first end to said second end, wherein said application
produces a corresponding increasing gradient in crosslink density
and glass transition temperatures (T.sub.g's) to said shape memory
polymer from said first end to said second end.
15. The method of claim 14, further comprising the step of
photocuring said shape memory polymer by use of a radiation
source.
16. The method of claim 15, wherein said shape memory polymer is a
curable thermoset.
17. The method of claim 16, wherein said curable thermoset is
selected from the group consisting of polyacrylates,
polymethacrylates, thermosets formed by thiol-ene reactions,
polyurethanes, and epoxy resins.
18. A method of preparing a functionally graded shape memory
polymer, said method comprising the steps of: providing a shape
memory polymer comprising a first end and a second end; photocuring
said shape memory polymer by use of a radiation source through a
gradient photomask, wherein said gradient photomask allows an
increasing amount of radiation to reach said shape memory polymer
from said first end to said second end, wherein said photocuring
produces a corresponding increasing gradient in crosslink density
and glass transition temperatures (T.sub.g's) to said shape memory
polymer from said first end to said second end.
19. The method of claim 18, wherein said photocuring is performed
when said shape memory polymer is at a temperature greater than a
maximum glass transition temperature (T.sub.g) allowable by said
shape memory polymer.
20. The method of claim 18, wherein said shape memory polymer is a
radiation-curable thermoset.
21. The method of claim 18, wherein said radiation is ultra-violet
radiation.
22. A method of preparing a functionally graded shape memory
polymer, said method comprising the steps of: providing a shape
memory polymer comprising a first end and a second end; photocuring
said shape memory polymer by use of a radiation source and an
opaque mask placed in between said radiation source and said shape
memory polymer, wherein said opaque mask moves at a predetermined
velocity from said second end to said first end to allow an
increasing amount of radiation to reach said shape memory polymer
from said first end to said second end, wherein said photocuring
produces a corresponding increasing gradient in crosslink density
and glass transition temperatures (T.sub.g's) to said shape memory
polymer from said first end to said second end.
23. The method of claim 22, wherein said photocuring is performed
when said shape memory polymer is at a temperature greater than a
maximum glass transition temperature (T.sub.g) allowable by said
shape memory polymer.
24. The method of claim 22, wherein said shape memory polymer is a
radiation-curable theremoset.
25. The method of claim 22, wherein said radiation is ultra-violet
radiation.
26. A temperature sensor device comprising the functionally graded
shape memory polymer of claim 1, wherein said device is a label
structured to be attached to a surface of an object and is
pre-deformed by stretching, bending, indenting, or embossing.
27. The temperature sensing device of claim 26, wherein said device
is adapted to respond to an environmental temperature, T, in the
range of T.sub.min<T<T.sub.max.
28. The temperature sensing device of claim 27, wherein said device
is capable of generating a spatially dependent recovery when
exposed to the environmental temperature, T, in the range of
T.sub.min<T<T.sub.max.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to U.S. Provisional
Patent Application No. 61/444,298, filed on Feb. 18, 2011, which is
hereby incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to shape memory polymers and,
more specifically, to a shape memory polymer having a range of
transition temperatures that are spatially distributed in a
gradient fashion within one single article.
[0005] 2. Description of the Related Art
[0006] Shape memory polymers (SMPs) are a class of "smart"
materials that can switch between two shapes on command, from a
fixed (temporary) shape to a pre-determined permanent shape upon
the application of an external stimulus such as heat. This shape
memory behavior is generally characterized using programmed, cyclic
thermomechanical tests referred to as the shape memory cycle (SMC).
In a typical SMC, the SMP is first deformed at an elevated
temperature that is higher than its transition temperature,
T.sub.trans (either T.sub.m or T.sub.g). The deformation is elastic
in nature and mainly leads to a reduction in conformational entropy
of the constituent network chains, following the rubber elasticity
theory. Commonly applied deformation modes include tension,
compression, and bending. The deformed SMP is then cooled to a
temperature below its T.sub.trans while maintaining constant the
external strain or stress. During cooling, the material transitions
to a more rigid state (semi-crystalline or glassy), which
kinetically traps or "freezes" the constituent network chains in
this low-entropy state. Macroscopically the material retains, or
"fixes," the temporary strain/shape even when external stress is
released. Shape recovery is finally triggered by heating the
material through T.sub.trans under a stress-free
(unconstrained)--or even loaded (constrained)--condition. By
allowing the network chains (with regained mobility) to relax to
their thermodynamically favored, maximal-entropy state, the
material changes from the temporary to its permanent shape. Two
characteristic ratios, fixing ratio (R.sub.f) and recovery ratio
(R.sub.r), characterize the shape memory performance (shape fixing
and shape recovery) for comparison among different material
systems.
[0007] SMPs have several intrinsic advantages over the
traditionally used shape memory alloys (SMAs) including larger
deformation strains, tunable transition temperatures, low density
and low manufacturing cost. As a result they have attracted a
significant amount of research interest during the past decade.
Novel SMPs have been developed with responsiveness to non-heat
stimuli such as light, electricity, and magnetic field, and with
new recovery behavior including two-way shape memory and
triple-shape memory.
[0008] The stimuli-responsiveness gives SMPs an ability to sense
environmental changes such as an increase of temperature, and
respond in a prescribed manner. However, the application of
conventional SMPs as temperature sensors is still limited, mainly
due to the fact that there is usually only one T.sub.trans
associated with a given material, as determined by its constituent
molecular composition and architecture. In other words,
conventional SMPs only respond to a threshold temperature trigger
and are unable to respond to temperatures over a broad range.
[0009] Following are a number of references that provide background
information to the present invention, each of which is hereby
incorporated by reference: C. Liu, H. Qin and P. T. Mather, J.
Mater. Chem., 2007, 17, 1543-1558; P. T. Mather, X. F. Luo and I.
A. Rousseau, Annu. Rev. Mater. Res., 2009, 39, 445-471; A. Lendlein
and S. Kelch, Angew. Chem. Int. Edit., 2002, 41, 2034-2057; D.
Ratna and J. Karger-Kocsis, J. Mater. Sci., 2008, 43, 254-269; I.
A. Rousseau, Polym. Eng. Sci., 2008, 48, 2075-2089; L. R. G.
Treloar, The Physics of Rubber Elasticity, 3rd Ed., Clarendon
Press, Oxford, 1975; A. Lendlein, H. Y. Jiang, O. Junger and R.
Langer, Nature, 2005, 434, 879-882; Y. J. Liu, H. B. Lv, X. Lan, J.
S. Leng and S. Y. Du, Compos. Sci. Technol., 2009, 69, 2064; X. F.
Luo and P. T. Mather, Soft Matter, 2010, 6, 2146-2149; R. Mohr, K.
Kratz, T. Weigel, M. Lucka-Gabor, M. Moneke and A. Lendlein,
Proceedings of the National Academy of Sciences of the United
States of America, 2006, 103, 3540-3545; H. H. Qin and P. T.
Mather, Macromolecules, 2009, 42, 273-280; T. Chung, A. Rorno-Uribe
and P. T. Mather, Macromolecules, 2008, 41, 184-192; I. Bellin, S.
Kelch, R. Langer and A. Lendlein, Proceedings of the National
Academy of Sciences of the United States of America, 2006, 103,
18043-18047; M. Bell and A. Lendlein, J. Mater. Chem., 2010, 20,
3335-3345; T. Xie, X. C. Xiao and Y. T. Cheng, Macromol. Rapid
Commun., 2009, 30, 1823-1827; T. Pretsch, Smart Mater. Struct.,
2010, 19, 015006; X. F. Luo and P. T. Mather, Adv. Funct. Mater.,
early view online, DOI: 10.1002/adfm.201000052; J. Kunzelman, T.
Chung, P. T. Mather and C. Weder, J. Mater. Chem., 2008, 18,
1082-1086; J. Y. Wong, A. Velasco, P. Rajagopalan and Q. Pham,
Langmuir, 2003, 19, 1908-1913; X. F. Yao, D. L. Liu and H. Y. Yeh,
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291, 1388-1396; B. Y. Wen, G. Wu and J. Yu, Polymer, 2004, 45,
3359-3365; F. M. Gallant, H. A. Brack and A. K. Kola, J. Compos.
Mater., 2004, 38, 1873-1893; K. K. U. Stellbrink, G. Hausser and R.
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Z. Yang and W. King, Polymer, 2007, 48, 3213-3225; F. Yang, E.
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[0010] Description Of the Related Art Section Disclaimer: To the
extent that specific publications are discussed above in this
Description of the Related Art Section, or elsewhere herein, these
discussions should not be taken as an admission that the discussed
publications (for example, technical/scientific publications) are
prior art for patent law purposes. For example, some or all of the
discussed publications may not be sufficiently early in time, may
not reflect subject matter developed early enough in time and/or
may not be sufficiently enabling so as to amount to prior art for
patent law purposes. To the extent that specific publications are
discussed above in this Description of the Related Art Section, or
elsewhere herein, they are all hereby incorporated by reference
into this document in their respective entirety(ies).
BRIEF SUMMARY OF THE INVENTION
[0011] It is therefore a principal object and advantage of the
present invention to provide a shape memory polymer that is able to
respond to temperatures over a broad range.
[0012] In accordance with the foregoing objects and advantages, the
present invention applies the concept of functionally graded
materials (FGMs) to SMPs. Specifically, the present invention
involves SMPs with spatially distributed transition temperatures
and the methods to prepare such materials. The term, FGM, refers to
any synthetic material that has spatially dependent compositions,
microstructures and associated properties. The FGM concept has
engaged a significant amount of research effort since its first
introduction in the 1980s, with applications ranging from aerospace
to tissue engineering. A variety of fabrication and processing
techniques have been developed for polymeric FGMs, including UV
polymerization with patterned photo-filters, photodegradation with
a gradually removed mask, thermal curing in a temperature gradient,
controlled interdiffusion of polymer bilayers, co-extrusion with
specially designed gradient distribution and 2-dimensional mixing
units, and extrusion followed by laminate molding. However none of
these techniques has ever been applied to SMPs and the preparation
of SMPs with functionally graded properties.
[0013] The new shape memory polymers may exist as films, coatings,
or adhesives, and feature a continuous gradient, of shape memory
transition temperatures from one portion of the sample to another.
As a consequence, thermally stimulated recovery of a temporary
configuration to the permanent configuration occurs with spatial
localization. In one embodiment where the special localization of
transition temperature is in the form of a continuous gradient, the
recovery can occur in a wavelike-fashion upon uniform heating with
recovery beginning at the regions where the transition temperature
is lowest and propagating in the direction of increasing transition
temperature. The materials are achieved by one or more of the three
general methods, one of which involves photocuring a glassy shape
memory polymer within a temperature gradient, the researchers
having discovered that the ambient temperature during photocure
determines the final and local glass transition temperature,
T.sub.g. The second method involves photocuring at a temperature
greater than the maximum T.sub.g allowable by the composition and
through a mask with spatial grading of the optical absorbance. In
this manner, the crosslinking (which controls T.sub.g) develops at
a rate that is spatially graded. The third method is similar to the
second: photocuring at a temperature greater than the maximum
T.sub.g allowable by the composition and through a mask that
translates laterally during cure. In this manner, the region of
sample first exposed during mask translation will feature the
longest photocuring time and thus the highest T.sub.g, whereas the
regions further along in the direction of mask translation will
experience less and less exposure time and thus a lower T.sub.g.
This T.sub.g becomes the local transition temperature in the
functionally graded shape memory polymer articles. Envisioned
applications are in simple, electronics-free temperature sensing in
the form of labels or in complex deployment of mechanical
structures wherein wave-like deployment is advantageous or
required.
BRIEF DESCRIPTION OF THE DRAWING
[0014] The present invention will be more fully understood and
appreciated by reading the following Detailed Description in
conjunction with the accompanying drawings, in which:
[0015] FIG. 1 is a schematic and photograph of a temperature
gradient hot-stage according to the present invention;
[0016] FIG. 2 is a schematic and photograph of a micro-indentation
setup;
[0017] FIG. 3 is a graph of the bulk 1WSM cycles (top; the asterisk
indicates experimental onset) and temperature dependent DMA result
(bottom) of cured NOA63 (no post-cure);
[0018] FIG. 4 is a graph of the temperature vs. position plots for
the temperature gradient hot-stage ( ), the glass slide
(.largecircle.) and the T.sub.g's (measured by DSC) on the final
NOA63 film ( ), where the temperature of the "heater" end was set
to be 120.degree. C. (see FIG. 1) while ice-water circulation was
maintained at the "cooler" end;
[0019] FIG. 5 is a graph of the torce vs. depth curves showing the
loading step at 80.degree. C. ( ) and the unloading step at
25.degree. C. (.largecircle.) for sample 9 (see text as well as
FIG. 7);
[0020] FIG. 6 is a series of polarized optical microscope (POM)
images showing the recovery of an indent during heating, where
sample 9 is shown (see text as well as FIG. 5) which has a
DSC-measured T.sub.g of 43.degree. C. and The scale bar represents
200 .mu.m;
[0021] FIG. 7 is a graph of the indent recoveries, shown as the
normalized birefringence intensity (%) vs. temperature (.degree.
C.) plots, for samples 1-10 (see text for details), where the
filled triangles stand for DSC-measured T.sub.g's for all the
samples.
[0022] FIG. 8 is a series of photographs provided a visual
demonstration of the gradient recovery behavior of a functionally
graded NOA63, where the arrow in the first (35.degree. C.) image
indicates the direction of T.sub.g gradient.
[0023] FIG. 9 is a schematic of the dumbbell geometry used for bulk
shape memory characterization, where W: width of narrow section, L:
length of narrow section, G: gage length, WO: width overall, LO:
length overall, D: distance between grips, R: radius of fillet, and
RO: outer radius.
[0024] FIG. 10 is a graph of the temperature-distance plots for
different temperature gradients generated by varying the heater
temperature;
[0025] FIG. 11 is a graph of the indentation force-depth results
for gradient samples 1-10;
[0026] FIG. 12 is a graph of the loading-unloading curves for NOA63
indented at T>T.sub.g;
[0027] FIG. 13 is a schematic and a series of photographs showing
birefringence (photoelasticity) based demonstrations of gradient
recovery.
[0028] FIG. 14 is a schematic representation of a temperature
gradient cure method of preparing functionally graded shape memory
polymer products, according to an embodiment of the present
invention.
[0029] FIG. 15 is a schematic representation of a radiation
gradient cure with a gradient photo-mask method of preparing
functionally graded shape memory polymer products, according to an
embodiment of the present invention.
[0030] FIG. 16 is a schematic representation of a radiation
gradient cure with a moving photo-mask method of preparing
functionally graded shape memory polymer products, according to an
embodiment of the present invention.
[0031] FIG. 17 is a schematic illustration showing the sensing of
environmental temperature using FG-SMP, according to an embodiment
of the present invention.
[0032] FIG. 18 is a schematic representation of a process for
fabricating a wrinkled surface, according to an embodiment of the
present invention.
[0033] FIG. 19 is a graphical illustration of DSC characterization
of functional gradient shape memory polymer, according to an
embodiment of the present invention.
[0034] FIG. 20 are photographs of a functionally graded shape
memory polymer sample heated to 26.degree. C. (left); the sample
recovered at 32.degree. C. (middle), the red arrow points out the
separation line; and the sample recovered at 36.degree. C. (right),
according to an embodiment of the present invention.
[0035] FIG. 21 are AFM height images of different regions on the
sample shown and described with reference to FIG. 20, recovered at
32.degree. C., according to an embodiment of the present
invention.
[0036] FIG. 22a-b are graphical representations of 2D FFT analysis
on 32.degree. C. recovered sample, as shown and described with
respect to FIGS. 20-21, W1, W2, W3 are three distinguished peaks
from the left graph (a).
[0037] FIG. 23 relates to the moving mask method for the
preparation of functionally graded SMPs, and shows is a (A)
schematic illustration of the concept, and (B) graphical
illustration showing T.sub.g vs. position profiles for two graded
NOA63 samples cured at different temperatures, according to an
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0038] Referring now to the drawings, wherein like reference
numerals refer to like parts throughout, SMPs according to the
present invention include a spatially graded glass transition
temperature. SMPs according to the present invention were
technically achieved by post-curing the material (a thiol-ene based
photo-crosslinkable glassy thermoset formulation) in a linear
temperature gradient, allowing vitrification to occur at different
temperatures along the gradient. The resulting material shows a
one-dimensional gradient of glass transition temperatures
(T.sub.g's) from T.sub.min to T.sub.max. Therefore the material can
respond to a range of temperatures, T.sub.min<T<T.sub.max,
yielding a spatially dependent elastic modulus for a given
temperature and a spatially dependent shape recovery response upon
application of spatially uniform external heating. To characterize
the position-dependent shape memory properties, it is apparent that
conventional, bulk characterization methods described above are not
ideal since they are based on macroscopic deformations (tension,
compression, or bending) that do not have required spatial
resolution. Needed are microscopic deformations (with length scales
that are smaller than the characteristic distance for T.sub.g to
change with the gradient, .delta.=.DELTA.T.sub.g/(dT.sub.g/dx),
with .DELTA.T.sub.g being the T.sub.g breadth) that fix and recover
"locally" without interfering with each other. For properties of
the present case, described below, .delta..about.0.5 mm.
Consequently a good candidate for shape memory characterization is
indentation, a method that has been applied to SMP research with
micro- and nano-indentation by several authors. For a large variety
of synthetic chemistries and compositions utilized among these
studies, it was uniformly observed that excellent heat-induced
recovery of vitrified indents occurs for all of the SMPs studied.
Table 1 summarizing these studies is below:
TABLE-US-00001 TABLE 1 Indenter Indentation Reference Materials
Geometry Temperature Observation Gall et al. A commercial, two-part
Vickers indenter Indented at 5 different (1) Complete recovery
epoxy SMP with a T.sub.g = 67.degree. C. (four sided temperatures,
between was achieved (from loss tangent pyramid with a 0.37T.sub.g
and 1.27T.sub.g regardless of peak). The chemical face angle of
(c.a. 25 to 85.degree. C.), then indentation composition is not
136.degree.) cooled to 0.37T.sub.g (25.degree. C.) temperature;
disclosed. for indent "fixing" (2) Recovery temperatures increased
with higher indentation temperatures. Xu et al. Glassy PU (MM5520
Vickers indenter Not mentioned; (1) Complete recovery from
Mitsubishi Heavy (same as above) presumably at ambient was observed
for both Industries) reinforced with temperature neat PU and the
thermally treated nanocomposite; attapulgite clay (2) Nanocomposite
T.sub.g of neat PU = 34.3.degree. C. showed slower T.sub.g of
nanocomposites ~40.degree. C. recovery kinetics. Wornyo et tBA
photo-crosslinked Berkovich Indented at ambient Complete recovery
al. with different amounts of indenter temperature was observed for
all DEGDMA and PEGDMA the samples with (M.sub.w = 550 g/mol)
different crosslink densities; Yang et al. tBA, photo-crosslinked A
custom-made Indented at different tip (1) All indents showed with
DEGDMA cantilever with a temperatures of 150, 192 almost complete
heated tip; tip and 250.degree. C.; two "cold recovery upon
geometry not indents" were also heating; specified introduced by
using a (2) The indents unheated tip formed at room temperature
recovered at lower temperatures. SMP: shape memory polymer; PU:
polyurethane; tBA: tert-butyl acrylate; DEGDMA/PEGDMA:
di-/poly-(ethylene glycol) dimethacrylate; M.sub.w: molecular
weight.
[0039] This led to an expectation that, among glassy SMPs, similar
shape memory fixing and recovery for the indentation geometry
should be possible. The analytical tools that have been used to
monitor and quantify indentation recovery have included either
profilometry or atomic force microscopy (AFM). While these tools
have allowed for high spatial resolution for quantification of
shape recovery, they are relatively slow and do not provide
adequate time resolution of typical recovery events. We
successfully utilized the photoelastic effect, observed using
polarized optical microscopy (POM), to monitor and analyze indent
recovery. Practically speaking, this can serve as a good readout
method for the actual sensing applications.
[0040] Overall, our functionally graded SMP not only meets the
requirements for temperature sensing, but also provides a potential
route for precisely controlling the shape recovery profile; for
example, directional shape recovery from one end to the other.
Following is a description of the experimental protocols used for
preparing and characterizing the functionally graded SMP. Then,
detailed results and analysis showing the spatially graded shape
memory properties are provided. Finally, the gradient shape
recovery behavior of the material according to the present
invention under continuous heating is demonstrated.
[0041] Advantages of the invention are illustrated by the following
Example. However, the particular materials and amounts thereof
recited in these examples, as well as other conditions and details,
are to be interpreted to apply broadly in the art and should not be
construed to unduly restrict or limit the invention in any way.
EXAMPLES
Fabrication of Temperature Gradient Hot Stage
[0042] A custom made temperature gradient hot stage was used to
impart T.sub.g gradient to the SMP system, shown in FIG. 1. The
stage was designed and fabricated following a technical guide
published by NIST. A temperature gradient is produced by heating at
one end (via a heating unit) and maintaining a constant low
temperature (via a cooling unit) at the other end. The heating unit
consists of a cartridge heater (output power=300 W, Mcmaster-Carr)
and a temperature controller (ETR-9090 from OGDEN Manufacturing
Company) with a type T thermocouple. The cooling unit functions by
circulating cold water at a constant flow rate using a submersion
pump (model 1C-MD-1, March MFG., Inc.). Thermal insulation between
the entire apparatus and the laboratory table is provided by two
machined Teflon blocks. Besides varying the temperatures of the
heating and cooling units, the temperature gradient can be further
controlled via adjusting the distance between both units by the use
of two positioning slots. The actual temperature gradient was
verified by measuring the temperatures at different positions using
a thermometer (S1-II from Fluke Corporation).
Preparation of Functionally Graded SMP Samples
[0043] The SMP used is a commercial UV curable glassy thermoset
(Norland Optical Adhesive 63 or NOA63), purchased from Norland
Products, Inc. The liquid formulation is polymerized to a solid
state via thiol-ene step-growth photopolymerization chemistry.
Nevertheless, the exact chemical composition of this commercial
product is unknown. Uncured NOA63 (a clear, viscous liquid) was
first uniformly coated on a glass slide with a controlled thickness
of 0.6 mm using a micrometer-based, doctor-blade film applicator
(Gardco Microm-II from Paul N. Gardner Company, Inc.). Curing was
carried out by exposing NOA63 to 365 nm UV irradiation (Spectroline
SB-100P high intensity UV lamp) at room temperature (r.t.) for 1 h.
This resulted in a NOA63 film with a single, uniform T.sub.g of
c.a. 30.degree. C. (measured by DSC). Although dynamic mechanical
analysis of this polymer has been previously reported to better
understand its use in nano-scale microcontact printing, it has not
been previously reported as an SMP.
[0044] To introduce a T.sub.g gradient, the cured NOA63 film (on a
glass slide) was placed on the temperature gradient plate and
post-cured under the same UV source for an additional time of 1 h.
More details are provided in the next section (Results and
Discussion), as variations thereof caused systematic changes in the
material behavior.
Thermal, Mechanical and Bulk Shape Memory Characterization
[0045] The thermal and mechanical properties of NOA63 were studied
using differential scanning calorimetry (DSC) and dynamic
mechanical analysis (DMA). For the former, a typical sample of 3-5
mg was encapsulated in a Tzero aluminum pan, and examined using a
TA Q200 DSC (TA Instruments, Inc.). The temperature was first
ramped from 40.degree. C. to 80.degree. C., then cooled back to
-60.degree. C., and finally ramped to 80.degree. C. while
collecting the heat flow data. Both heating and cooling rates were
10.degree. C./min. The T.sub.g was determined as the mid-point of
the step transition in heat flow during the 2nd heating. For DMA, a
rectangular film (7.24 mm*3.56 mm*0.19 mm) was loaded under tension
on a TA Q800 dynamic mechanical analyzer (TA Instruments, Inc.). An
oscillatory deformation with an amplitude of 15 .mu.m, a frequency
of 1 Hz, and a "force track" (ratio of static to dynamic force) of
115% was applied while ramping the temperature from -90.degree. C.
to 100.degree. C. at 3.degree. C./min.
[0046] The bulk shape memory of cured NOA63 was characterized using
a well established four-step thermomechanical cycling method,
referred to as the one-way shape memory (1 WSM) cycle. Since this
involved large-strain tensile deformation, a dumbbell geometry
guided by ASTM D638, as seen in FIG. 9, was used successfully
avoiding sample failure at the grips that often plagues thin film
SMP testing. Film thicknesses of 0.16 mm were utilized. Each
sample, loaded under tension, was: (1) stretched to a tensile
strain of 31% by ramping the force to 0.5 N (corresponding to a
stress of 2 MPa) at a constant rate of 0.05 N/min at 70.degree. C.
(T>T.sub.g), (2) cooled back to 20.degree. C. (T<T.sub.g)
followed by an isothermal hold for 10 min, (3) unloaded by ramping
the force to 0.01 N at the same rate of 0.05 N/min to witness
strain fixing and (4) heated to 70.degree. C. to allow strain
recovery under no constraint. The same procedure was then repeated
for two more times to assess the deterioration in shape memory
performance, if any, as a result of thermomechanical cycling. To
further analyze the shape memory results, fixing ratio (Rf) and
recovery ratio (Rr) were calculated according to:
R f ( N ) = u ( N ) r ( N ) .times. 100 % ( 1 ) R r ( N ) = u ( N )
- p ( N ) u ( N ) - p ( N - 1 ) .times. 100 % ( 2 )
##EQU00001##
Here .epsilon.m, .epsilon.u, .epsilon.p and N stand for the strain
before unloading, the strain after unloading, the permanent strain
after heating (strain that is not recovered) and the cycle number,
respectively. For cycle 1 (N=1), .epsilon.p(0) is taken as the
initial strain at the onset of the experiment.
Micro-Indentation of Functionally Graded NOA63
[0047] A post-cured NOA63 was cut evenly into 10 separate pieces
along the length (T.sub.g gradient) direction. Samples were spaced
by 2 mm or a T.sub.g difference of about 0.5.degree. C. Each piece
was then indented on a rheometer (ARG2, TA Instruments) using the
gap-control feature of the instrument and normal force transducer,
along with a custom-assembled indenter setup (FIG. 2). The indenter
tip was made from a Pfanstiehl diamond stylus (352-D7 from KAB
Electro Acoustics) with a well defined conical geometry (Scheme 2;
tip radius .about.25 .mu.m), bonded to the center of a 25 mm
disposable aluminum plate. The sample, placed on the bottom plate,
was indented at 80.degree. C. (T>T.sub.g; temperature controlled
by a thermal chamber, known as the environmental testing chamber or
ETC on the above mentioned rheometer) by bringing the indenter tip
(or top plate) down at a constant speed of 1 .mu.m/s until a
maximum normal force of 0.4 N was reached. Then, the sample was
quickly cooled (10.degree. C./min) to 25.degree. C. (T<T.sub.g)
while holding the normal force constant. The normal force was
finally released by raising the indenter (top plate) away from the
sample at 1 .mu.m/s.
Indent Recovery and Image Analysis Method
[0048] As mentioned above, the strain field induced by indentation
could be visualized semi-quantitatively as birefringence based on
the photoelastic effect. When heated, the birefringence would
disappear in sync with the stress field and (for SMPs with good
recovery) the strain field. At the molecular level this is due to
the oriented polymer chains relaxing back to their
thermodynamically favored random coil confirmations. Experimentally
this was monitored by an Olympus BX51 optical microscope with
crossed polarizer and analyzer, coupled with an Instec HCS402
hot-stage. Digital micrographs (24 bit color) were taken every 30 s
by a QICAM FAST-1394 CCD camera while heating the sample from
25.degree. C. to 65.degree. C. at a linear heating rate of
2.degree. C./min.
[0049] The digital images were then converted to 8 bit grayscale
using Photoshop CS2. The histogram of each image was analyzed to
obtain the average pixel intensity ( ) by dividing the overall
greyscale intensity (integration of the histogram) by the total
number of pixels, using a constant region of interest (ROI) area of
1160 .mu.m*870 .mu.m which covers the entire birefringence zone (as
shown in FIG. 4). The of the last image (the one taken at
65.degree. C.) was used as the background noise (I.sub.B) to
calculate the normalized intensity (I.sub.N):
I N = I _ - I B I o - I B .times. 100 % ( 3 ) ##EQU00002##
where I.sub.o is the of the first (25.degree. C.) image. The
normalized intensity, I.sub.n, was then plotted as a function of
temperature for each sample, quantifying indentation recovery
temperature and breadth with spatial resolution achieved by the
small indenter size.
Demonstration of Gradient Shape Recovery
[0050] To further demonstrate the gradient shape recovery behavior,
a functionally graded NOA63 film was prepared, with dimensions of
7.5 cm (length)*2.3 cm (width)*0.28 mm (thickness). A series of
cuts spaced along the T.sub.g-gradient (length) direction were cut
through the film thickness and along the film width direction using
a razor blade. The cuts started from the edge and ended around the
center of the film width (1.1-1.2 cm long), and were 5 mm apart
from the adjacent ones. The sample was then heated at 80.degree.
C., folded along its "center line" (parallel to the film length),
and cooled to room temperature to fix the deformation, A Pelletier
plate (an accessory of the ARG2 rheometer) was used to uniformly
heat the material and trigger its recovery. For this purpose, the
deformed sample was placed on the Pelletier system, with the
"virgin" (uncut) half-surface actually touching the Pelletier
plate. A glass slide was put on top of the virgin half-surface to
enhance thermal contact. With this configuration, the recovery of
each "finger" (the area between two adjacent cuts) can occur
without much mechanical constraint, or under a relatively
stress-free condition. In other words, the recovery of each finger
is not affected by the recovery of the adjacent fingers, and is
solely determined by its localized T.sub.g (the average T.sub.g of
that finger). The temperature was linearly ramped from 25.degree.
C. to 60.degree. C. at 2.degree. C./min with images taken every
minute (or every 2.degree. C.) using a digital camera,
Results and Discussion of Examples
[0051] Cured NOA63 is a transparent, glassy solid that has
excellent shape memory properties as shown in FIG. 3. In this ease
the material was cured under UV for 1 h at r.t. (the actual
temperature was .about.5.degree. C. higher due to the heating
effect of UV irradiation) without, any further post-cure and shows
a uniform T.sub.g of 29.7.degree. C. (determined from the onset of
E' drop). It is observed from the 1WSM cycles (FIG. 3, top) that, a
large percentage of strain was fixed after unloading at 20.degree.
C., corresponding to an Rf of 98.4% (averaged over three cycles;
the same below for Rr). The fixed strain recovered almost
completely (Rr=99.7%) in a relatively small temperature range
during heating. Furthermore, the shape memory performance showed no
deterioration up to three cycles, in that all the curves follow
almost exactly with each other. This indicates good thermal
stability of cured NOA63.
[0052] The T.sub.g of cured NOA63 was found to increase in response
to post-photocure at higher temperatures. This can be interpreted
based on reaction kinetics. When NOA63 is being photo-cured, the
T.sub.g increases with conversion until it reaches the
environmental temperature, T.sub.e. Vitrification (transition from
rubbery to glassy state) takes place which significantly limits the
reaction rate due to reduced chain mobility/diffusion. When the
environmental temperature is raised to T.sub.e
(T.sub.e>T.sub.g); however, the polymer chains re-enter the
rubbery state and the residual reaction resumes, until the material
T.sub.g reaches T.sub.e or T.sub.u, whichever is lower. Here,
T.sub.u is the ultimate T.sub.g the material can potentially reach
(determined by network chain composition) at 100% conversion.
Therefore the material T.sub.g can be controlled precisely by
controlling T.sub.e, as long as T.sub.e is lower than T.sub.u. In
this sense, the method of the present invention is not expected to
work for semicrystalline networks (Class II SMPs1), which are
thermally or photocured well above the vitrification point. It is
also worth noting that in the specific case of NOA63, we found that
heat and UV irradiation are both required to raise the T.sub.g.
Heat by itself does not change the T.sub.g tangibly. This is
evident from the 1WSM cycles in FIG. 3: if heating were to change
the T.sub.g, the recovery transitions of the second and third
cycles would have shifted to higher temperature, rather than
staying almost constant. This is understandable since NOA63
polymerizes via a free radical mechanism, and UV is the only means
to generate free radicals (by the decomposition of remaining UV
initiators) in the system.
[0053] Based on the above discussions, post-curing NOA63 on a
temperature gradient would therefore introduce a T.sub.g gradient
on the material. For this purpose, a temperature gradient hot-stage
was fabricated. By controlling the heating (via a cartridge heater)
at one end and cooling (via cold water circulation) at the other
end, a series of linear temperature gradients can be easily
produced (FIG. 1). For the post-curing of NOA63, we utilized a
linear temperature gradient from 36 to 65.degree. C. along the
sample length (70 mm; filled circles in FIG. 4). The actual
temperatures at the glass slide surface (temperatures NOA63 was
actually experiencing) were also measured and a large "damping"
effect was observed, which reduced the temperature gradient from
36-65.degree. C. to 33-51.degree. C. (hollow circles in FIG. 4).
The DSC-measured sample T.sub.g's closely matched the glass slide
temperatures and spanned from 30 to 48.degree. C., or a gradient of
2.7.degree. C./cm. This also proved that the reaction was indeed
vitrification-limited.
[0054] To study the functionally graded shape memory properties,
another post-cure was carried out and the resulting film was evenly
cut into 10 samples along the gradient direction, as described
above. The samples are referred to as sample 1 to 10, where the
sample numbers increase with decreasing T.sub.g, as will be shown.
Each sample was indented to a maximum normal force of 0.4 N at
80.degree. C. This resulted in a penetration distance, or an indent
depth of c.a. 120 .mu.m as shown in FIG. 5 (for clarity only the
sample 9 is shown in FIG. 5; but other samples are shown in FIG.
11). Since the material existed in its rubbery state (80.degree.
C.>>T.sub.g), the deformation was primarily elastic. This is
supported by the experimental observation that the loading and
unloading yielded very similar force-depth curves with minimal
hysteresis, as further seen in FIG. 12. Microscopically, this
deformation led to conformational changes (orientation) of the
polymer chain segments. Similar to the fixing of a macroscopic
deformation shown in FIG. 3, the indented sample was cooled to
25.degree. C. while holding the force constant. During cooling, the
polymer went through its T.sub.g and as a result, the
conformational changes of chain segments were "frozen" due to a
significant decrease of mobility and the indent was "fixed". The
latter can be seen from FIG. 5 in that the depth decreased only
slightly from 118 to 110 .mu.m after unloading at 25.degree. C. In
other words 93.2% of the deformation was fixed.
[0055] The indent was then visualized under POM, and a classical
"four-leaf" birefringent pattern could be observed (FIG. 6), which
reflects the strain field surrounding the indent. When heated, both
the intensity and the total area of the pattern decreased gradually
with temperature. The image became eventually dark, indicating the
fact that the strain had fully recovered, and all the chain
segments had relaxed back to their thermodynamically favored random
coil conformations.
[0056] The indent recovery was further studied by image analysis,
in which the normalized intensity of each image was plotted as a
function of temperature for samples 1 to 10 (FIG. 7). The
DSC-measured T.sub.g's for each sample are also indicated on the
graph (black triangles in FIG. 7). For all the samples, a
sigmoidal-like recovery profile similar to the recovery of
macroscopic deformation (FIG. 3) was seen. It is clear that the
indent recovered at higher temperatures with increased T.sub.g's,
and the DSC-measured T.sub.g always corresponded to the temperature
with a normalized intensity of c.a. 60% (or 40% of the intensity
recovery).
[0057] This gradient recovery behavior was further demonstrated in
a macroscopically visible manner. The experimental details were
described above and the result is shown in FIG. 8. The material has
an increasing T.sub.g from left to right, as the arrow in FIG. 8
indicates. For this sample, the T.sub.g varied from ca. 30.degree.
C. on the left-hand side to 50.degree. C. on the right hand side,
while the gradient was "sampled" by slicing along the gradient
direction to give 15 "fingers" along the bottom edge, each marked
on its terminus with a black dot. In this configuration, each
finger featured T.sub.g variation <1.5.degree. C. Uniform
heating was provided by the Pelletier plate on which the sample was
placed. The plate temperature was linearly ramped from 25.degree.
C. to 60.degree. C. at 2.degree. C./min. As anticipated, the
recovery initiated at the left end (where the T.sub.g was lowest)
and propagated to the right with increasing temperature.
[0058] Finally, the potential applications of functionally graded
SMPs are considered for temperature sensing. A material with a
known one-dimensional T.sub.g gradient (such as the graded NOA63
presented in this paper) can be fixed thermomechanically with
localized deformations, such as a series of evenly spaced indents
along the gradient direction. Heating such a specimen to a
temperature T within its T.sub.g range (between T.sub.min and
T.sub.max) would result in the recovery of indents located between
T.sub.min to T but not T to T.sub.max. Therefore examining the
recovery profile by some means would allow the precise
determination of T. Considering d to be the spatial resolution of
indentation recovery detection, the temperature sensing resolution,
.DELTA.T, is then given by either (d.times.dT.sub.a/dx) if
d.gtoreq..delta. (.delta. being the characteristic distance defined
in Introduction), or (.delta..times.dT.sub.a/dx) if d<.delta..
In the former case, the sensing resolution can be enhanced
(lowering .DELTA.T) by reducing the temperature gradient
dT.sub.a/dx. This can, in turn, be controlled by the external
temperature gradient, as shown in FIG. 4. In the latter case, since
.delta.=.DELTA.T.sub.a/(dT.sub.a/dx) (.DELTA.T.sub.g being the
T.sub.g breadth), the above expression becomes at
.DELTA.T=.DELTA.T.sub.a. This indicates that the sensing resolution
is material-limiting when d<.delta.. Therefore the only way to
enhance the resolution would be to reduce .DELTA.T.sub.g.
[0059] Due to the simplicity of the presented material and
fabrication method, the production of low-cost "temperature labels"
are possible that could be utilized to measure temperatures in
areas that are not accessible by conventional methods or not
amenable to continuous monitoring, to indirectly indicate
sterilization completion, or for incorporation into product
packaging (for shipping industry or food storage) to indicate the
maximum temperature of product exposure. For example, temperature
sensing labels wherein packaging for thermally sensitive and
valuable materials (drugs, chemicals, food, etc) may be labeled
with an embossed or otherwise "fixed" functionally graded shape
memory polymer. Visual inspection of the received package label
will indicate the highest temperature that the package experienced
in transit. In addition, the present invention may be used for
complex structure deployment where the prescribed transition
temperature enables activation from one position continuously to
the other for smoothness of operation. Finally, the present
invention may be used for local temperature sensing of surgical
tools during sterilization where, if the sensing label
incorporating the present invention does not indicate a target
sterilization temperature, then a user is alerted that the tools
did not get sterilized.
General Methods for Preparing Functionally Graded Shape Memory
Polymers (FG-SMP)
[0060] In accordance with an embodiment of the present invention,
functionally graded shape memory polymer (FG-SMP) products can be
prepared via one or more of the three general methods depicted in
FIGS. 14-16.
[0061] In the first method shown in FIG. 14, a curable thermoset is
cured on a temperature gradient from T.sub.min to T.sub.max. The
curable thermoset can be any material that polymerizes ("cures")
into a macromolecular network under heat, radiation, curing agents,
or a combination of one or more of them. Examples of curable
thermosets include, but are not limited to,
polyacrylates/polymethacrylates, thermosets formed by thiol-ene
reactions, polyurethanes, epoxy resins, etc.
[0062] The temperature gradient can be applied by various methods,
but is most conveniently achieved by using a temperature-gradient
hot plate such as the one shown in FIG. 1. The temperature gradient
produces a gradient in crosslink density (as the schematic shows),
thus a gradient of glass transition temperatures (T.sub.g's).
Depending on the thermoset chemistry, a radiation source may or may
not be required.
[0063] In the second method shown in FIG. 15, a radiation-curable
thermoset is cured with a gradient photo-mask that attenuates the
radiation differently along one or more directions, resulting in a
gradient in crosslink density and a gradient in T.sub.g. The
radiation source is selected based on the thermoset chemistry. The
most commonly used radiation to induce crosslinking of polymers is
ultra-violet (UV) irradiation, which is defined as any
electromagnetic radiation in the range between 10 nm and 400
nm.
[0064] A third method shown in FIG. 16, is to use an opaque
photo-mask that moves to gradually expose the thermoset during
cure, leading to a gradient in crosslink density and T.sub.g along
the moving direction. The moving velocity is programmed to achieve
the desired gradient profile.
Use and Functionality of Functionally Graded Shape Memory
Polymers
[0065] The use and functionality of FG-SMPs are described herein
below. Utilizing the responsiveness of FG-SMP to a broad range of
temperatures (rather than only one for traditional shape memory
polymers), FG-SMP can be used to produce temperature sensors.
Several possible designs of temperature sensors from FG-SMPs are
presented and discussed as non-limiting examples. In general, a
deformation profile can be applied along the T.sub.g gradient
direction of a FG-SMP. This deformation can be introduced by
indentation, wrinkle formation (see discussion related to the
Functionally Graded Shape Memory Polymer Wrinkle System section,
below) or by macroscopic deformations such as stretching and
bending.
[0066] For example, as shown in FIG. 17, a FG-SMP can be heated to
T>T.sub.max, stretched perpendicular to the gradient direction
and cooled to T<T.sub.min (T.sub.min is higher than room
temperature). The FG-SMP would maintain, or "fix" into this
deformed temporary shape. The material is then sliced along the
gradient direction to give individual "fingers". The purpose of
this is to mechanically isolate the "fingers" so they can recover
relatively independently from each other. When the material is
exposed to environmental temperature T.sub.e (T.sub.e being between
T.sub.min and T.sub.max), recovery will occur to the fingers with
T.sub.g's below T.sub.e but not those with T.sub.g's higher than
T.sub.e. By inspecting the recovery profile of the fingers, the
environmental temperature (or the highest environmental temperature
the material has been exposed to) can be precisely determined. The
design possibility is potentially endless.
Functionally Graded Shape Memory Polymer Wrinkle System
[0067] Wrinkle occurs when a bilayer system consisting of a thick
compliant substrate and a thin rigid film undergoes a compressive
stress, causing the rigid film to buckle atop the compliant
substrate shown in FIG. 18. In a bilayer system, the modulus
mismatch of two layers is necessary for surface buckling to happen.
The rigid skin layer can be introduced onto a prestrained compliant
substrate by deposition, oxygen plasma and etc. Wrinkles will form
upon compressive stress releasing.
[0068] For example, in the FG-SMP wrinkle system in accordance with
an embodiment of the present invention, a sample was 3 cm long with
T.sub.g ranging from 28.degree. C. to 36.degree. C. in and gold
served as a hard layer (see FIG. 19 for DSC characterization of
functional, gradient, shape memory polymer). While gold is a
convenient material to use as a coating for this purpose, any
metallic coating that can be deposited onto the FG-SMP, for example
by thermal evaporation, sputter coating, chemical vapor deposition,
or electroless plating, will function in the desired manner.
Further, polymeric coatings featuring modulus of elasticity at
least 10.times. greater than that of the rubbery state of the
FG-SMP (.about.1 MPa) will function in the desired manner. Such
polymers include polystyrene, polycarbonate, poly(alkyl
methacrylate)s, poly(alkyl acrylate)s, polyimides, and poly(arylene
ether ketone)s. In brief, uniaxial stretching was conducted using
the DMA to fix a strain into the FG-SMP. The sample was first
heated to 80.degree. C. and subsequently loaded until a prescribed
strain was achieved. Upon reaching the prescribed strain the load
was held constant and the sample was cooled to fix the strain into
the substrate. For this experiment, uniaxial strain of 3% was
applied. A gold coating was applied to the substrate via sputtering
under room temperature. A total sputter time of 100 seconds used to
a yield 33 nm thick layer. Gold-coated substrates were placed in an
isothermal oven for 32 and 36.degree. C. respectively for 30
minutes to allow the substrates recover and form wrinkles.
[0069] The result in FIG. 20 shows that at 26.degree. C. the sample
did not show any change macroscopically. When the temperature
increased to 32.degree. C., only part of the sample recovered and
showed visible reflective color (forming wrinkles), and the rest
area remained the same. At 36.degree. C., the whole sample
recovered with flashy color. Close to nano-scale wrinkles were
imaged and analyzed, and the results are shown in FIGS. 4 and 5.
The wavelength decreased along the direction of increasing T.sub.g.
For this temperature sensing device, the surface color change
(wrinkle formation) will move towards high T.sub.g end as
increasing temperature. The temperature range will be adjustable
for this application.
Preparation of Graded SMPs Using a Moving Mask Method
[0070] In accordance with an embodiment of the present invention, a
method for preparing functionally graded SMPs using a moving
photo-mask during UV curing is presented herein below. It is
briefly described below for comparison with the temperature
gradient curing approach.
[0071] The method is shown schematically in FIG. 23A. In brief,
uncured NOA63 was first uniformly coated on a glass slide that is
7.5 cm long. The photo-mask (attached to a custom built motion
system) was set to move under the configuration shown in FIG. 23A
at a constant velocity of 7.5 cm/h, which gradually exposed the
NOA63 to UV light. The curing lasted for a total time of 61 min.
Therefore, the sample had a gradient of exposure times along its
length, with the shortest exposure time (the rightmost position as
shown in the schematic) and longest exposure time (the leftmost
position) being 1 min and 61 min, respectively. A gradient with
increasing T.sub.g's from right to left was anticipated, since more
exposure should lead to more crosslinking reactions thus a higher
T.sub.g.
[0072] FIG. 23B is a graphical illustration showing the T.sub.g vs.
position profiles for two NOA63 samples cured at the room
temperature (with no active heating or cooling) and 55.degree. C.,
respectively. In the former case, the cured sample does not show a
clear T.sub.g gradient. Except the last point (7.25 cm), all other
locations display a very similar T.sub.g at c.a. 30.degree. C. This
reveals that a vitrification limit was imposed by the environmental
temperature. In other words, the temperature during cure was
approximately 30.degree. C. (slightly higher than room temperature
due to the heating effect from UV and also some reaction exotherm);
the reaction quickly proceeded until the sample reached a T.sub.g
that was equal to the environmental temperature (c.a. 30.degree.
C.). The material then vitrified, and the reaction was almost
"terminated" due to limited diffusion. So, no matter how long the
material is exposed to UV, the T.sub.g in this case would remain
approximately at the environmental temperature (.about.30.degree.
C.).
[0073] A second curing with a moving mask was conducted at an
elevated temperature of 55.degree. C. using a hot-stage. In this
case 55.degree. C. is higher than the "ultimate" T.sub.g of NOA63
(the T.sub.g at full conversion; .about.50.degree. C.). This
removes the vitrification limit in the first case discussed above.
As a result, a T.sub.g gradient from 33 to 50.degree. C. could be
generated (FIG. 23B). However, compared to the result from
temperature gradient curing (FIG. 2), the gradient here is not
linear. This is simply-due to the fact that in this system (or in
any other thiol-ene systems) T.sub.g does not increase linearly
with time. One would need to adjust the moving velocity of the
photo-mask (rather than keeping it at a constant velocity) during
the curing process in order to achieve a linear T.sub.g
gradient.
[0074] Comparing these two methods, the temperature gradient curing
is conceptually simpler and practically more feasible. It does not
require much information on the reaction kinetics, and the T.sub.g
gradient is controlled just by the applied temperature gradient.
For the moving mask method, one would need to fully investigate the
reaction kinetics (the relationships among T.sub.g, time and
temperature) to control the final T.sub.g gradient.
[0075] One comment has to be made concerning the amenability of
these two methods to different types of polymerizations. In other
words, can these two methods be applied to any polymerizing system?
For the temperature gradient curing method, it is required that the
polymerization exhibits a vitrification limit. This is usually the
case for glass-forming, step-growth polymerizations, but is not
commonly observed for chain-growth ("free-radical")
polymerizations. This is due to the fundamental difference in
polymerization mechanisms. Step-growth polymerizations proceed via
a step-wise coupling mechanism, i.e., monomers forming dimers,
dimers then forming tetramers, tetramers then forming octamers,
etc. In other words, the molecular weight of the polymerizing
system increases gradually and "uniformly". When vitrification
occurs (T.sub.g=T.sub.e, T.sub.e being the environmental
temperature), the diffusion of the reactive species is
significantly limited due to their high molecular weight, rendering
the reaction almost, stagnant. In the case of chain-growth
polymerizations, the system is composed of a certain number of
growing chains within a vast amount of monomers. The reaction
proceeds by the addition of monomers to the active, growing chains
until they terminate. Under this situation, the reactive species
are the monomers at any time during polymerization. The diffusion
of monomers is quite easy due to their low molecular weight,
regardless of whether the overall system vitrifies
(T.sub.g=T.sub.e) or not. Therefore the effect of vitrification
limit is minimal for chain-growth polymerizations. Considering the
case of NOA63, it polymerizes via UV-initiated thiol-ene
polymerization. It is well known to polymer scientists that
thiol-ene network polymerization proceeds by a step-growth
mechanism enabled by the creating of free radicals thermally or
photochemically. Therefore, the temperature-gradient curing method
worked well. However, it may not be applicable to
chain-growth/free-radical polymerizations based on the mechanistic
analysis above.
[0076] On the contrary, the moving mask method is not based on
vitrification (as far as T>T.sub.u, T.sub.u being the ultimate
T.sub.g of the material) but the change of T.sub.g as a function of
time. Therefore, it should be applicable to both polymerization
types (step-growth and chain-growth), since in both cases the
material T.sub.g would increase with time. However, the precise
control of T.sub.g gradient would require a thorough understanding
of the T.sub.g-time relationship during polymerization (reaction
kinetics). This broader applicability is an advantage of the moving
mask method.
[0077] Finally, the exact condition under which the polymerization
takes place is also important. This is mainly for practical
reasons. Suppose the polymerization is thermally triggered and a
T.sub.g gradient is prepared by the temperature gradient curing
method. Once the graded material is exposed to T>T.sub.g (for
example during shape fixing/recovery), the residue reaction will be
triggered which would further raise the T.sub.g. In other words,
the T.sub.g gradient will change once the material is heated again.
In the current case of NOA63, the polymerization is UV initiated;
heat along cannot trigger the residue reaction (see the discussion
in section 8.4). As a result, the material will maintain its
T.sub.g gradient for repeated use under normal shape memory
(heating/cooling) conditions. Therefore, from a design point of
view the polymerization condition should be different from the
application condition to render the T.sub.g gradient stable over
time.
[0078] Accordingly, a functionally graded SMP encompassing a range
of T.sub.g's distributed in a gradient fashion has been
successfully fabricated by post-curing the material in a linear
temperature gradient. Utilizing indentation-based surface shape
memory, the gradient recovery-properties of the material were
explored and its ability to respond to a broad temperature range
was demonstrated. Further, a macroscopic manifestation of the
functionally graded shape memory phenomenon was demonstrated. Owing
to its simplicity and optical characteristics, this new class of
SMPs offers great potential for material-based temperature sensors
as well as applications where controlled shape evolution during
recovery is desired.
Shape Memory Polymers--Covalently Cross-Linked Glassy Thermoset
Networks as SMPs
[0079] In accordance with an embodiment of the present invention,
the following description of shape memory polymers is contemplated.
See, e.g., C. Liu et al., Review of Progress in Shape Memory
Polymers, J. Mater. Chem., 2007, 17, 1543-1558.
[0080] The simplest type of shape-memory polymer is a cross-linked
glassy polymer featuring a sharp T.sub.g at the temperature of
interest and rubbery elasticity above T.sub.g derived from covalent
cross-links. This class of materials has attractive characteristics
that include excellent degree of shape recovery afforded by rubbery
elasticity due to the nature of permanent (or near permanent)
cross-linking, tunable work capacity during recovery garnered by a
rubbery modulus that can be adjusted through the extent of covalent
cross-linking, and an absence of molecular slippage between chains
due to strong chemical cross-linking. However, since the primary
shape is covalently fixed, once processed (casting or molding)
these materials are difficult to reshape thereafter. An example of
this class is a chemically cross-linked vinylidene random copolymer
consisting of two vinylidene monomers (one being methyl
methacrylate and the other butyl methacrylate) whose homopolymers
show two very different T.sub.g values of 110.degree. C. and
20.degree. C., respectively. The random copolymer itself gives a
single, sharp T.sub.g that is tunable between the two T.sub.g
values of the homopolymers by varying the composition. The work
capacity, dictated by the rubbery modulus, is precisely adjustable
to accommodate each particular application by varying the extent of
cross-linking, in this case achieved by copolymerization with a
tetra-ethylene glycol dimethacrylate. This thermoset shows complete
shape fixing and fast, complete shape recovery in hot water at the
stress-free stage. In addition, this polymer has the advantage of
being castable and optically transparent.
[0081] Copolymerization and chemical cross-linking of renewable
natural oils were worked on, having a high degree of unsaturation,
with styrene and divinylbenzene to obtain random copolymer
networks. These networks show tunable glass transitions and rubbery
properties upon varying the monomer ratio. In this work, broad
glass-transition spans were observed for all of the copolymers and
this, in turn, apparently slowed the shape-recovery speed, though
no shape-recovery speed data was shown. Complete shape fixing and
shape recovery were observed at high temperatures. However, due to
the broad glass-transition span and the coexistence of rigid,
glassy fragments and soft, elastic rubbery segments, incomplete
shape recovery occurs at these transition ranges. While attractive
in their unique composition, an unfavorably broad T.sub.g might
limit the materials as SMPs.
[0082] Besides the chemically cross-linked polymers, polymers with
T.sub.g> room temperature and with ultra-high molecular weight,
>10.sup.6 g mol.sup.-1, may also be included in this category
due to their lack of flow above T.sub.g and good shape fixing by
vitrification. Such polymers feature a significant number of
entanglements per chain (>25) and these entanglements function
as physical cross-links on the time scale of typical deformations
(1 s<t<10 s). Such physical cross-linking forms a three
dimensional network that gives excellent elasticity above the glass
transition, but makes thermal processing difficult; instead
solvent-based processing may be required. These characteristics
make the polymers essentially behave like the thermoset
shape-memory polymers just discussed. An external force applied
above the T.sub.g causes deformation to a secondary shape that can
be fixed when cooled below T.sub.g, which stores the elastic energy
exerted during deformation. The decrease in mobility of PN
molecules at T<T.sub.g maintains the secondary shape. The
recovery of the original shape can be accomplished by reheating
above its T.sub.g, releasing the stored energy. Such polymers show
quite complete shape fixing when vitrified and demonstrate fast and
complete shape recovery due to the sharp glass-transition
temperature and high entanglement density that forms a three
dimensional network, evidenced by a flat rubbery plateau measured
rheologically. However, the disadvantages of such materials are: 1)
the transition temperature cannot, be easily varied; 2) the modulus
plateau, which controls the energy stored when deforming, is low
(.about.1 MPa) and also hard to change; 3) the polymer will creep
under stress at high temperature due to the finite lifetime of the
entanglements; and 4) difficulty of processing because of the high
viscosity associated with high molecular weight polymers. Thus, the
processing of such materials is limited to solvent casting instead
of more desirable thermal processing, such as extrusion, injection
molding, or compression molding.
[0083] In addition to the examples given above, other materials are
reported to be shape-memory materials based on the same mechanism,
such as poly(alky) methacrylate) copolymers, polystyrene
copolymers, filler-modified epoxy networks, chemically cross-linked
amorphous polyurethanes, poly((methyl
methacrylate)-co-(N-vinyl-2-pyrrolidone))-PEG semi-IPNs,
HDI-HPED-TEA network, and biodegradable copolyester-urethane
networks. A list of shape-memory polymers based on glassy
thermosets, along with references, is summarized in Table 2
below.
TABLE-US-00002 TABLE 2 Summary of shape-memory thermosets with the
shape recovery triggered by their glass-transition temperatures
Transition temperature/ Materials .degree. C. Special features
Reference P(MMA-co-VP)-PEG 65 Semi-IPN 66 semi-IPNs Copolyester
48-66 Biodegradable 31 P(AA-co-MMA)-PEG 60 Broad transition 67 Corn
oil copolymer 0-90 Biomaterial 39 PMMA-PBMA copolymers 20-110
Optically 59, 68, 69 transparent Epoxy 50-80 Filled reinforced 40,
41, 65, 70 Fish oil copolymers 30-109 Biodegradable 36 PET-PEG
copolymer Up to 80 -- 71 P(MA-co-MMA)-PEG 50-90 -- 72 Soybean oil
copolymer 30-110 Biomaterial 37, 73 with styrene and DVB Styrene
copolymer -- Optically 74 transparent Thermosetting PU Up to 56
Water swollen 75 Thermosetting PU 0-150 Ester type 76
Dehydrochlorinated 80 -- 77 cross-linked PVC Polynorbornene 40
Sharp T.sub.g 61, 62 High M.sub.w PMMA 105 Deformable 2, 64 below
T.sub.g
[0084] While several embodiments of the invention have been
discussed, it will be appreciated by those skilled in the art that
various modifications and variations of the present invention are
possible. Such modifications do not depart from the spirit and
scope of the present invention.
* * * * *
References