Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Harvey A. Liu et al.
SELF-HEALING OF METAL OXIDE COMPOSITE
MATERIALS
Harvey A. Liu, Bruce E. Gnade, and Kenneth J. Balkus, Jr.
The University of Texas at Dallas, Department of Chemistry and the UTD NanoTech Institute,
Richardson, Texas 75083-0650, USA
972-883-2659
e-mail: balkus@utdallas.edu
A permeation barrier system that could self-heal upon the influx of moisture through defects and subsequently
seal the cracks would find potential applications in multilayer composite materials. Nanoporous polylactic acid
(PLA) spheres and fibers were prepared by electrospraying and electrospinning respectively. The porous spheres
and fibers were exposed to titanium (IV) chloride to introduce the metal oxide precursor into the pores and
subsequently seal by annealing and solvent treatment.
The self-healing ability was demonstrated by the formation of metal oxides during the controlled hydrolysis of
the water-degradable polymer. This method of delivering metal oxides also showed efficacy when encapsulated
in poly(methyl methacrylate) (PMMA). The fibers and capsules synthesized were characterized with SEM,
Raman, UV-Visible spectroscopy, XRD, and EDX.
Keywords: self-healing, polylactic acid, titanium (IV) chloride, electrospinning, electrospray,
porous fibers, OLED, permeation barrier, metal oxide healing
1
Introduction
Permeation barriers are critical components that exclude air and water from flexible organic
electronic devices including organic transistors, conducting polymers, and organic light
emitting diodes (OLED) 1-3. For a standard lifetime of >10 000 hours the generally accepted
water vapor transmission rate (WVTR) is <1x10-6 g/m2 day at 25°C and 40% RH 1, 4, 5.
Permeation barriers accomplish the shielding of these devices through the use of a layered
composite material consisting of metal oxides formed from Al or Si and a low permeability
polymer 2, 3, 6-11.
It was shown that the use of a single layer metal oxide barrier exhibited 2-3 orders of
magnitude improvement over polymer substrates alone, though permeation was still
detectable through defects or pores caused by the deposition of the metal oxide on the rough
substrates 4, 6, 12, 13. Additionally, it was found that since the deposition method replicated the
defects in the underlying layer multiple depositions only showed a slight improvement in
preventing permeation 14-17. This led to the incorporation of a low permeability polymer
smoothing layer, which served to decouple defects as well as introduce a long diffusion path
for existing defects 7, 16-18. While these composite materials show great promise in both
shielding the adverse environment from organic devices as well as providing the flexibility
that make these devices desirable, their use is still limited by the inherent brittleness of the
metal oxide films.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Harvey A. Liu et al.
Unfortunately, excessive flexing of the composite film result in cracks within the oxide layer
allowing the influx of air and moisture and leading to a dramatic reduction in the lifetime of
these devices.
While improvements have been made using multilayered composites, cracking is still a major
source of the reduced lifetime. Therefore, an oxide coating that self heals when a crack or
scratch is initiated would be a major advance and allow the technology of flexible displays to
overcome the obstacle that limit to their lifetimes.
The concept of autonomic healing of structural polymers has previously been demonstrated
using microcapsules containing a healing agent dispersed with a catalyst in a polymer matrix
19-25
. While this study presents a novel method for the autonomic healing of polymer
matrices, the application of this strategy to self-healing metal oxides is not possible. In an
ideal self-healing inorganic system, the structural properties and integrity of the stressed
structure should be restored to its initial state. The use of a polymeric healing agent would
not be able to achieve this feat, while the use of an inorganic oxide would allow the
restoration of the initial structural integrity as well as the initial barrier properties. Though to
date, work showing the encapsulation of a reactive metal oxide precursor into a waterdegradable polymer shell for self-healing has not been explored.
In the current work, we have developed a delivery system for the self-healing of a metal oxide
layer in the form of a free-standing paper that could potentially be integrated in a composite
material with ease. Since a major source of degradation is caused by the influx of water and
oxygen through pinholes and defects, we have employed a water-degradable polymer,
polylactic acid (PLA), as the shell structure to encapsulate the healing agent, titanium (IV)
chloride (TiCl4). The healing agent, titanium (IV) chloride, was chosen because of its rapid
reactivity, volatility, and its ability to propagate repair without the introduction of a catalyst.
Thus, initiation of repair in our delivery system is not only responsive to the physical rupture
of our shell material, it is chemically responsive to the influx of moisture. An additional level
of protection from the influx of water is provided by the absorptive ability of polylactic acid
in the polymer network during hydrolysis as well as the coordination of water in the metal
oxides formed through the hydrolysis of TiCl4.
2
Experimental procedure
2.1
Preparation of hollow polymer shells
Polymer shells of high molecular weight lactic acid polymer were prepared by integrating the
technique of electrospraying with a freeze drying method for generating core-shell structures
reported by Hyuk et al 26. In a typical preparation, high molecular weight polylactic acid was
dissolved in chloroform at room temperature in the range of 1-3% w/w. From the polymer
solution, 2 ml were drawn into a 5 ml plastic syringe, which was controlled by a syringe
pump (7802CO, KD Scientific Inc., USA) feeding at a rate of 0.1 ml/min to a flat tip 20-G
needle. The distance between the needle tip and the grounded collector, an aluminum target,
was adjusted to 20-25 cm. The electrosprayed PLA was collected in a 100 ml Petri dish
containing liquid nitrogen placed directly above the grounded collector plate. Electrospraying
was initiated when 15 kV was applied to the needle from a variable high voltage power
supply (ES50P-5W, Gamma High Voltage Research) nebulizing the polymer solution into a
fine mist.
2
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Harvey A. Liu et al.
The resulting frozen droplets of the polymer solution dispersed in liquid nitrogen were placed
in a vacuum less than 225 torr for at least 5 h. The spheres were allowed to warm up to room
temperature and the chloroform then allowed to evaporate.
2.2
Preparation of porous polymer fibers
Porous PLA polymer fibers were prepared by electrospinning 27-29. High molecular weight
polylactic acid was dispersed in chloroform at room temperature in the range of 7-10% w/w,
producing a more viscous polymer solution than that used in the electrospraying of hollow
polymer spheres. As in the electrospraying process, the electrospinning was initiated by
feeding the polymer solution into the syringe at a rate of 2-3 ml/h with needles ranging from
18-G to 20-G. A voltage of 10-12 kV was applied to the needle using a variable high voltage
power supply to initiate electrospinning. Aligned fibers were obtained by using a grounded
rotating drum positioned a distance of 25 cm from the needle tip, whereas a flat piece of
aluminum foil was used for collecting random fibers.
2.3
Filling and sealing of hollow polymer shells and porous fibers
The filling and sealing of the polymer shell and porous fibers were executed by similar
methods. The polymer shells or porous fibers were transferred into a 40 ml tube equipped
with a vacuum stopcock, then transferred to a vacuum line and pumped for approximately 30
minutes under heating of approximately 40°C. After evacuation, titanium (IV) chloride was
introduced by vacuum transfer to sufficiently immerse the fibers or hollow polymer shells
(~1ml). This process was followed by the vacuum transfer of dry toluene (~2 ml) to seal the
polymer shells and porous fibers. The toluene was allowed to react with the beads and fibers
for approximately 2 hours to allow proper sealing of the polymer holes and pores. The
remaining titanium (IV) chloride-toluene solution was then removed by vacuum transfer and
the beads and fibers stored under a vacuum at room temperature.
3
Testing and results of self-healing system
3.1
Polymer microcapsules
Representative scanning electron microscopy (SEM) micrograph of the hollow polymer shells
are shown in figure 1a-b. This method also allowed the investigation of the size of the
polymer shells as well as the hole on the surface, which ranged from 15 – 40 µm and 5-10 µm
in diameter, respectively. Figure 1b shows two hollow polymer spheres comparable in
dimensions with equal diameters (~35 µm) as well as equal sized holes on their surface (~10
µm). The size of the hollow polymer shells were shown to be dictated by the size of the
droplets which could be controlled by the electrospraying parameters, while the size of the
holes on the polymer shell can be attributed to the solvent used and the conditions at which
the solvent was evaporated 26.
3
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
A
Harvey A. Liu et al.
B
Figure 1: (A) SEM image of hollow polymer shell with controllable hole on surface fabricated via
electrospraying, (B) SEM image of hollow polymer shells comparable in size showing reproducibility of method
Figure 2a-b depicts the result of the toluene solvent treatment on the hollow polymer shells.
Toluene causes the polymer to undergo a swelling process in which the rigid structure and the
integrity of the polymer melts and softens, resulting in the sealing of the hole on the surface of
the polymer shell. This property was utilized in the encapsulation of TiCl4 in the
microcapsules. The size of the sealed microcapsules ranged from ~10-25 µm in diameter,
slightly smaller than the diameter of the hollow polymer shells. The slight decrease in size
was attributed to the softening and subsequent closing of the hollow sphere. Figure 2b shows
a capsule that has been treated toluene to melt and seal the hole on the surface attached to the
remnants of a fractured microcapsule. The fractured capsule shows the smooth interior
morphology of the microcapsule, which suggests a hollow interior.
B
A
Figure 2: (A) SEM image of hollow polymer shells after solvent treatment to seal the hole on the surface (B)
SEM image of hollow polymer shell attached to remnants of a fractured polymer shell
Elemental mapping of the sealed microcapsules in figure 3a-c indicates the presence of a high
concentration of chlorine and titanium in the polymer shell, demonstrating preliminary
evidence for the presence of titanium (IV) chloride. The morphology of the TiCl4 filled
sphere (fig. 4) in contrast to the conventional sphere (fig 2) showed protrusions that can be
seen throughout the surface resulting in an irregular sphere. Theses protrusions are caused by
the hydrolysis of titanium (IV) chloride within the microcapsules due to the influx of
atmospheric moisture through pores and channels, which are formed in the initial stages of the
hydrolytic degradation of the polylactic acid wall. While these initial results show promise in
the encapsulation and potential release of titanium (IV) chloride. The handling and
transferring of the hollow spheres to the vacuum line resulted in aggregation and upon toluene
treatment the core shells either fused together (fig 5a) or were deformed.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Harvey A. Liu et al.
While we continue to work on the isolation and dispersion of the microsphere delivery
system, it was recognized that a fibrous form would be easier to handle. Therefore freestanding papers composed of porous PLA fibers as the carrier of the TiCl4 were fabricated by
electrospinning.
A
B
C
Figure 3: Elemental mapping of hollow polymer shell filled with TiCl4 (A) image, (B) chlorine, (C) titanium
Figure 4: Polymer shell filled with TiCl4 indicating hydrolysis of polymer shell and subsequent hydrolysis of
metal oxide precursor resulting in protrusions
Figure 5: Agglomeration of polymer microspheres after solvent treatment
5
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
3.2
Harvey A. Liu et al.
Free-standing paper fibers
Figure 6a-b shows SEM images of the aligned porous PLA fibers ranging in diameter from 412 µm formed by electrospinning a 10% w/w polymer solution. The pores were oval shaped
with the average pore size on the order of 250 nm in width and 400 nm in length with the long
axis oriented along the fiber axis, as shown in figure 6b. The pore formation on the fibers
resulted from the rapid phase separation between polymer rich and polymer poor regions
during the electrospinning process followed by a rapid solidification of the polymer caused by
the evaporation of the solvent. The elongation of the pores evolved from the whipping action
the fibers experienced between the spinerette and the grounded target in the electrospinning
process.
A
B
Figure 6: (A) SEM micrograph of aligned PLA fibers fabricated through electrospinning, (B) Scanning electron
microscopic images of the fibers showing porosity before encapsulation
The porous PLA fibers were collected on a grounded rotating drum, resulting in a flexible
free-standing paper material consisting of aligned fibers that are easily physically manipulated
without compromising the structural integrity. The fabrication of these fibers as a freestanding, self-supporting paper provided a means of readily transferring them from the
electrospinning apparatus to the vacuum line and then the subsequent transferring from the
vacuum line to characterization, an underlying problem posed by the hollow polymer shells.
The ease of manipulating these fibers as a paper also introduces the potential for the facile
integration into inorganic composite materials.
SEM micrographs of the fibers after filling the pores with TiCl4 and the subsequent treatment
with toluene are shown in figure 7a-b. The polymer fibers ranged in diameter from 5-10 µm.
Figure 7b show the close grouping of theses aligned fibers with no indication of the
problematic agglomeration, in contrast to the fabrication of the TiCl4 filled capsules.
A
B
Figure 7: (A) Scanning electron microscopic images of the fibers after encapsulation showing sealed pores, (B)
SEM images of similar fibers showing no signs of agglomeration
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Harvey A. Liu et al.
Digital images of the aligned fibers as a free-standing paper before and after toluene
treatment, as well as after exposure of the fibers to 80% RH following encapsulation are
shown in figure 8a-c, respectively. Before the encapsulation of TiCl4 within the pores, the
fibers appear as a semitransparent white paper. Upon introduction of toluene into the system,
the titanium (IV) chloride immediately formed a yellow-orange solution, which was
consequently encapsulated within the pores of the fiber, causing the fibers to also take on this
yellow orange color (fig 8a). Upon exposing the titanium (IV) chloride treated fibers to 80%
RH for an extended period of time, the majority of the yellow-orange color dissipated. This
change in color was due to the formation of a charge transfer complex between the aromatic
toluene and the titanium (IV) chloride. UV-visible spectroscopic analysis of the as-prepared
solution of the TiCl4-toluene complex exhibited an absorption maxima (λmax) at
approximately 360 nm, an attribute that was utilized in quantifying the loading of TiCl4 in our
fibers. While a short period of exposure to toluene caused the polymer to swell and
subsequently seal, prolonged exposure altered the fibers into a sticky mass leading to the rapid
release of the titanium (IV) chloride out of the pores after 10 minutes (fig. 9). Through UVvisible spectroscopy it was found that approximately 24.6 mmoles of TiCl4 was encapsulated
per gram of fibrous paper.
A
B
C
Figure 8: (A) Digital image of a free-standing paper composed of the porous PLA before encapsulation of TiCl4,
(B) digital image of the treated fibers showing the encapsulation of yellow-orange TiCl4-toluene complex before
exposure to 80% relative humidity, (C) Digital image of fibers after exposure to 80% relative humidity for 20
days
0,0074
Concentration (mol/L)
0,0072
0,007
0,0068
0,0066
0,0064
0,0062
4
6
8
10
12
14
16
Time (min)
Figure 9: Concentration of TiCl4 release from treated fibers after immersion in anhydrous toluene with respect to
time
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
3.3
Harvey A. Liu et al.
Characterization of exposed fibers
After confirming the encapsulation and subsequent release of titanium (IV) chloride from the
pores of the fibers through UV-vis, Raman spectroscopic analysis was employed to monitor
both the extinction of titanium (IV) chloride as well as the evolution of titanium dioxide
(TiO2), a hydrolytic product of titanium (IV) chloride. The Raman frequencies of the
corresponding compounds are summarized in Table 1. In additional to the vibrational
frequencies of titanium (IV) chloride in the spectral region around 100-500 cm-1, which
correspond with the v2, v4, and v1lines, other frequencies of vibration were detected which
were attributed to the formation of TiO2 30. These frequencies of vibration can be seen as
shoulders appearing at ~160 cm-1 as well as 1410 cm-1. In this case, the PLA paper containing
TiCl4 was exposed to air for approximately 1 hour during acquisition of the spectrum,
therefore some hydrolysis was expected.
UV-visible spectroscopy as well as Raman spectroscopy demonstrated the encapsulation of
titanium (IV) chloride in the fibers and the Raman spectra clearly shows the dissolution of the
PLA fibers and subsequent hydrolysis of TiCl4 in air.
Table 1: Raman Frequencies of TiCl4 and TiO2
a
b
TiCl4 ,a cm-1
TiCl4 in Fibers, cm-1
v2 (E)
121.4-123
121
v4 (T2)
137-142
135
v1 (A1)
388
387
Raman Frequencies of TiCl4 as reported in ref 30
Raman Frequencies of TiO2 as reported in ref 32
Therefore, the TiCl4 treated fibers were exposed to ~80% RH for an extended period of time
to observe the morphological changes as well as their reactivity to water. Time elapsed SEM
micrographs of the fibers show the formation and proliferation of nodules are shown in figure
10a-h. The reactivity of the fibers can be seen from the growth and development of the
nodules on the surface of the fibers, which can be seen as early as 3 days after exposing these
fibers to moisture. These protrusions or nodules have been previously discussed with the
polymer capsules and attributed to the influx of moisture through channels formed by the
hydrolysis of the PLA and the subsequent hydrolysis of titanium (IV) chloride. Table 2
shows the proliferation in the average size of the nodules as a function of time.
Table 2: Growth of Nodules with Respect to Time
Time Exposed (Days)
Avg. Size of Nodules (μm)
0
0.0
3
0.5-2.0
5
2.0-4.0
10
4.0-8.0
15
16.0
20
40.0
8
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Harvey A. Liu et al.
After exposure to ~80 % RH for 15-20 days, theses growths break through the encapsulating
barrier of the fibers and are no longer contained by the polylactic acid (fig 11). Elemental
mapping of theses fibers show the growths are comprised mainly of chlorine (fig 11b) and
traces amounts of titanium (fig 11c), which are ascribed to the formation of the titanium
oxychloride intermediates in addition to titanium dioxide. X-ray diffractive studies on the
exposed fibers confirmed the presence of the titanium oxychloride complex,
[Ti8O12(H2O)24]Cl8•HCl•7H2O, a cubic titanium octamer linked to chloride ions and water
molecules by a complex network of hydrogen bonds, which decomposes upon oxidation or
dissolution in water to form anatase titanium dioxide 31.
The observed Raman vibrational modes are tabulated in table 3, along with the modes of
vibration for anatase TiO2. The three peaks appearing at 411 cm-1, 512 cm-1, and 625 cm1
show a high correlation with that previously reported 32. Additionally, the distinct strong
weak vibrational modes occurring at 156 cm-1 and 203 cm-1 respectively are also in agreement
with that of anatase TiO2 nanocrystals 32. Comparison of the Raman frequencies for the
unexposed TiCl4 treated polymer fibers (Table 1) to the exposed fibers (Table 3)
demonstrated the consumption of TiCl4 and the production of TiO2 giving further evidence of
the reactivity of the encapsulated titanium (IV) chloride filled fibers to the atmospheric
moisture.
A
B
C
Figure 11: Elemental mapping of fibers exposed for 15 days and 20 days, showing the presence of chlorine (B)
and titanium (C) indicating the formation of titanium oxychloride complexes
Table 3: Raman Frequencies of Fibers Exposed to 80% RH
TiO2,a cm-1
Reacted Fibers cm-1
Eg
151 (strong)
156
Eg
196 (weak)
203
B1g
409
411
A1g
515
512
B1g and Eg
633
625
9
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
3.4
Harvey A. Liu et al.
Integration of the reactive fibers in multilayer composite materials
Ideally, the reactive fibers can be embedded in the poly(methyl methacrylate) (PMMA)
planarization layer during its deposition coating. The autonomic healing response of the
fibers will be initiated when the diffusion of oxygen and moisture penetrate through stress
induced cracks of the metal oxide layer. This influx of oxygen and moisture would cause the
hydrolysis of the polylactic acid fibers forming pinholes and channels and exposing the
encapsulated titanium (IV) chloride. Upon exposure of the reactive metal oxide precursor, the
volatile reagent would diffuse into the cracks and hydrolyze, forming solid titanium dioxide,
titanium oxychlorides and its complexes, subsequently filling the cracks. A schematic of our
proposed strategy is illustrated in figure 12. It is also proposed that the response of these
fibers will not only be limited to stress induced cracks, but also to pinhole defects caused by
the deposition technique. A schematic of the integrated aligned fibers in the polymerizing
layer is shown in figure 13a and an SEM micrograph showing a cross-section of the aligned
fibers integrated in a PMMA layer through spin coating is shown in figure 13b. The crosssection shows a good distribution of the fibers throughout the planarization layer through the
facile method of spin-coating, the current method for depositing the polymer planarization
layer. Preliminary results of the fibers reacting within the PMMA layer show the efficacy of
this method of delivery. A freeze-fractured cross-section of the reacted fibers integrated in
the PMMA layer and the corresponding elemental mapping images are shown in figure 14a-b.
The SEM micrograph in figure 14b shows the irregular solid formation of titanium oxides on
the PMMA surface. Further analysis with elemental mapping shows initial evidence of the
reactivity of the encapsulated titanium (IV) chloride to form a solid metal oxide.
Figure 12: Schematic diagram of the proposed healing mechanism. (A) Fibers containing a reactive metal oxide
precursor is embedded in the polymer layer. (B) Pinholes and cracks are introduced in the metal oxide layer; b,
the cracks allow the influx of water into the system; (C), leading to the hydrolysis of the water-degradable
polymer; (D), releasing the metal oxide precursor into the crack; (E), and the subsequent hydrolysis of the metal
oxide precursor forming a solid metal oxide to seal the crack
10
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Harvey A. Liu et al.
A
C
B
Figure 13: (A) Schematic diagram showing the proposed integration of polymer fibers in the planarization layer
of the permeation barrier (B) SEM image of aligned PLA fibers fabricated via electrospinning dispersed within
PMMA by spin coating
A
B
Figure 14: (A) SEM image of PMMA cross-section with aligned PLA fibers dispersed showing reactivity and
release of TiCl4, (B) Elemental mapping of cross section showing the presence of titanium (red) and chloride
(green) in solid formation on cross-section
3.5
Towards nanosized fibers
Currently, our methods have been to characterize fibers in the microscale range to ease the
characterization of the fibers as well as follow their reactivity. Though with the method of
electrospinning smaller fibers can be fabricated and preliminary results show the reactivity of
fibers in the submicron scale are comparable to that of the larger fibers. Figure 15a shows an
SEM micrograph of the submicron sized untreated polylactic acid fibers, which range from
100-300 nm in diameter.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Harvey A. Liu et al.
Figure 15b shows the submicron sized fibers after filling with TiCl4 and the subsequent
prolonged exposure to 80% RH, which have began to react with the moisture in the
environment to form nodules similar to those seen in the larger fibers (fig. 10).
A
A
B
C
C
D
E
E
F
G
G
H
Figure 10: Time elapsed SEM images of PLA fibers exposed to 80% relative humidity showing formation and
growth of nodules. a, b, 3 days; c, d, 5 days; e, f, 10 days; g, 15 days; h, 20 day
4
Conclusion
Better permeation barriers are needed for the advancement of organic electronic devices to
reach the required WVTRs. In this work, the reactivity of these fibers to an accelerated
environment showing the release of the encapsulated titanium (IV) chloride from the pores of
polylactic acid and the subsequent formation of a metal oxide solid has been demonstrated.
The initial work showing the efficacy of theses fibers on the submicron scale has also been
presented. Additionally, the preliminary results of our delivery system within the PMMA
planarization layer show great promise as a first generation self-healing system that may
reduce the permeation of moisture through stress induced cracks and pinhole defects in metal
oxide layers.
ACKNOWLEDGEMENTS
We thank the R.A. Welch Foundation and SPRING for support of this research.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Harvey A. Liu et al.
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