Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Steven D. Mookhoek et al.
THE CREATION OF HIGH ASPECT RATIO
CAPSULES TO IMPROVE THE EFFICIENCY OF
SELF-HEALING POLYMER SYSTEMS
Steven D. Mookhoeka, Patrick J. Colverb, Hartmut R. Fischerc,
Stefan A. F. Bonb and Sybrand van der Zwaaga
a
Delft University of Technology, Faculty of Aerospace Engineering,
Kluyverweg 1, 2629 HS Delft, The Netherlands
e-mail: s.d.mookhoek@tudelft.nl, s.vanderzwaag@tudelft.nl
b
University of Warwick, Department of Chemistry,
Coventry CV4 7AL, United Kingdom
e-mail: p.j.colver@warwick.ac.uk, s.bon@warwick.ac.uk
c
TNO Science & Industry, Department of Materials Performance,
De Rondom 1, 5612 AP Eindhoven, The Netherlands
e-mail: hartmut.fischer@tno.nl
High aspect ratios polymer capsules with a functional monomer as core liquid were synthesised with the aid of
Pickering stabilised droplets in emulsion. Anisometric capsules can be made when stretching particle stabilised
emulsion droplets in the presence of excess latex particles. Due to the strong adhesion of the Pickering
stabilisers, i.e. the latex particles, to the oilwater interface and their successive interfacial jamming, the deformed
droplets will not relax back to spherical shape when they are no longer stressed. The capsules were filled with
dicyclopentadiene monomer, stabilised with polystyrene particles and were reinforced with either a polyurea or
polyamide shell wall that was obtained by interfacial polymerisation. The capsules are currently used in cured
epoxy samples for self-healing experiments that are on-going and will be reported elsewhere [1]
Keywords: self-healing, encapsulation, high aspect ratio capsules, Pickering
emulsion, interfacial polymerization
1
Introduction
Liquid encapsulation was one of the first approaches to generate self-healing in polymer
systems [2]. In this approach spherical capsules, containing reactive resin or monomer, were
embedded as attributes in a matrix material. The ureaformaldehyde system used yields a
capsule with excellent barrier properties, and adequate adhesion to the matrix and a brittle
fracture mode. The spherical shape of the capsules is a direct consequence of their
manufacturing process [3]. When fracture or cracking occurs to the matrix material, the
embedded containers break and liquid will wet the crack-plane where it undergoes a chemical
reaction to crosslink and bridge the two crack surfaces.
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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
Steven D. Mookhoek et al.
This will redistribute the local stresses at the crack-tip and therefore stop the propagation of
damage.
A disadvantage of the spherical capsules relates to the damage handling efficiency of the
system. A random distribution of spherical capsules in a matrix material has a relatively low
sensing capability. The isotropic geometry of the capsules together with their random
distribution results in a low probability of a propagating crack hitting a capsule. Furthermore,
the amount of liquid resin released from the spherical capsule is relatively small. When
changing the geometry of the capsules from spherical to anisotropic structures, e.g. ellipsoidal
or rod-shaped, the fracture of such embedded entities will increase the released volume per
intersection. In contrast to self-healing hollow fibre composites [4, 5], matrices with
anisotropic capsules will retain their isotropic behaviour and the discrete nature of resin
storage in capsules allows better control of the self-healing functionality in case of multiple
cracking.
Various techniques have been described in literature to produce anisometric capsule structures
[6, 7] , but a careful examination of their characteristics indicated that in these procedures the
droplet encapsulation process required either continuously stressing or confining of the
droplets; this to prevent the relaxation of the induced anisotropic shape back to the spherical
shape by surface tension effects. By using so called Pickering [8] stabilised emulsions we
have been able to overcome this shortcoming. [9] This research presents that we are able to
produce anisotropic liquid filled capsules and build shell walls by various methods of
interfacial polymerisation that also give good barrier properties and cause brittle fracture of
the capsule shell. The designed capsules will be used as improvement to the existing liquid
encapsulated self-healing systems.
2
Experimental
2.1
Materials
Laponite clay (RD) is a hectorite synthetic clay and was donated by Rockwell additives ltd.,
UK. Ethylene-maleic anhydride copolymer (100k-500k g/mol), styrene monomer,
divinylbenzene, adipoyl chloride, and toluene diisocyanate, were purchased from SigmaAldrich and were used as received. Azobisisobutyronitril (AIBN), a free-radical initiator was
obtained from Sigma-Aldrich (98%) but was recrystallised from methanol prior to use.
Diethyltriamine (DETA) and dicyclopentadiene (DCPD) (95%) were both purchased from
Acros Organic. DCPD was vacuum distilled before encapsulation.
2.2
Synthesis of laponite clay armoured polystyrene particles
The crosslinked polystyrene particles that were used to form Pickering emulsions were
synthesised via a mini-emulsion polymerisation using either laponite clay or ethylene-maleic
anhydride copolymer (EMA) as dispersing agent [10]. To obtain non-soluble polymer latex
particles, divinylbenzene was copolymerized with styrene by a free radical reaction, initiated
by azobisisobutyronitril (AIBN).
In the experiments 1.00 g of laponite clay or 5.00 ml of 2.5wt% EMA solution in water was
dispersed in 100 ml of demineralised water in a round-bottom flask and was subsequently
sonicated for 1 min whilst stirring applying a Branson digital 450 W sonifier.
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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
Steven D. Mookhoek et al.
After the 1 min sonication sodium chloride was added to obtain a 0.1 M solution, and
sonication was continued for another 3 minutes, with a 30 s interval. Next a mixture of 7.50 g
styrene, 1.50 g of divinylbenzene and 0.22 g of AIBN was added to the water clay suspension.
Another sonication treatment of 5 minutes, in five series of 1 minute using a 30 s interval, was
used to create a stable miniemulsion of the organic phase in the water. Subsequently the flask
containing the mini-emulsion was put into an oil bath under nitrogen flow and heated to 60 ºC
and the monomer mixture was polymerised overnight. Dynamic Light Scattering (DLS)
measurements indicated an average particle size of 340 nm with a polydispersity of 0.15.
In the case of using EMA as dispersing agent the polystyrene latex particles were separated
from the surfactant by centrifuging, decanting and resuspending the latex particles three
times; assuming the surfactant concentration to be negligible.
2.3
Creation of a stable pickering DCPD/water emulsion
A stable microemulsion of DCPD in water was created using (clay armoured) crosslinked
polystyrene latex particles synthesized earlier. For the preparation of the emulsion a glass vial
was filled with 100 g of demineralised water together with 0.30 g of sodium chloride. The vial
was hand-shaken to dissolve the NaCl and subsequently 1.25 g of the polystyrene latex
particle solution (solid content: 3 wt%) was added dropwise to the solution. A sonication
treatment of 3 minutes was applied to redisperse the latex particles in NaCl solution. Parallel
20 g of previously distilled DCPD monomer was mixed with 17 mmol of either adipoyl
chloride or toluene diisocyanate. 20 g of this mixture was added to the latex particle
suspension. Mild stirring (500 rpm) led to the creation of stable DCPD colloidosomes [11] in
water with a droplet size of approximately 100 μm.
In the case of spherical capsules the droplets with the polystyrene particles at the interface
were directly fixated (also referred to as scaffolding [12]) by addition of diethylenetriamine
(DETA) to the emulsion, creating polyamide or polyurea DCPD filled capsules respectively
by interfacial polymerisation.
2.4
Stretching of pickering stabilised emulsion droplets
For the creation of anisotropic capsules we followed the method described by Bon et al. [9]
The non-scaffolded Pickering stabilised DCPD droplets dispersed in water were pushed
(containing an excess of latex particles) through a long and narrow glass capillary (minimum
inner diameter: 50 μm). The droplets reached an anisotropic geometry inside the capillary
during which more latex particles adhere to the droplets newly created interfacial area.
Droplets exiting from the capillary were caught in a stirring solution of water, sodium
chloride and diethyltriamine (DETA), where they were fixated as capsules by the interfacial
polymerisation of trifunctional amine and the DCPD containing adipoyl chloride or
toluenediisocyanate to create a polyamide and polyurea shell wall respectively.
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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
Steven D. Mookhoek et al.
3
Results and discussion
3.1
Capsule anisotropy and capsule properties via pickering systems
The clay armoured and non clay armoured, polystyrene particles could both be used as
Pickering stabilisers for the creation of stable DCPD in water emulsions. Laponite clay was
used to replace a (polymeric) surfactant in the polystyrene latex particle synthesis, this to
warrant complete absence of surfactant which could destabilise the Pickering emulsion
droplets.
Figure 1: Optical microscopy images of DCPD filled polyurea capsules with polystyrene particles in the shell
wall: a) 20x, b) 50x
Pushing the DCPD Pickering stabilised emulsion droplets through a narrow glass capillary
caused the particle stabilised DCPD droplets to deform, adapting to the confinement of the
capillary. Due to the shortage of the stabilising particles at the enlarged interfacial area,
particles in the continuous phase will migrate to the oil/water interface to minimise the
surface energy stabilising the deformed droplet. Finally, the non-spherical shape is retained
since the densely packed saturated interfacial area of the deformed droplet cannot relax back
to a spherical geometry. It is due to the presence of a large energy well at the interface which
prohibits the particles to leave the interface [13]. The adhered particles will achieve a
jammed-state, thereby restricting the relaxation process. The particle jamming compensates
for the interfacial tension and it results in a permanent droplet deformation. This phenomenon
will not be observed when using (polymeric) surfactants, since surfactant molecules can easily
rearrange or desorb to create a minimal droplet interfacial area, resulting in spherical droplets.
When the droplets exited the capillary they maintained anisotropic shape and did not break up
when injected into the reactive DETA/water solution.
In the work of Subramaniam et al. [14, 15] and Bon and co-workers [9] it was shown that
similar Pickering systems, using polystyrene particles, were used to create non-spherical and
even anisotropic bubble and droplet geometries.
Subramaniam et al. mainly focused on the merging of two gas bubbles and they were able to
create anisotropic moieties with aspect ratios higher than 10. Bon et al. however used liquidliquid systems and managed to produce anisotropic liquid droplets from a Pickering stabilised
emulsion in large numbers and in an easy and straightforward manner. The deformed droplets
had aspect ratios near 5.
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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
Steven D. Mookhoek et al.
Interfacial polymerisation through either isocyanate or acid chloride with amine condensation
led to stable capsule walls, encapsulating the spherical and anisotropic particle stabilised
DCPD droplets. The obtained capsules all had a mean wall thickness of around 500 nm,
which is in line with earlier work of Soto-Portas et al. [16], and showed clearly the
embedment of polystyrene nanoparticles in the shell. Embedment of the latex-particles caused
the shell wall to be more rigid than similar capsules that were made with the aid of
surfactants. The presence of particles at the surface of the Pickering capsules generates a
raspberry morphology which might have advancing capsule properties, such as fracture
mechanics and solvent permeability [12].
Fracture mechanical experiments, similar to those in the original work by White et al. [2] and
Brown et al. [17] to test the healing efficiency of epoxy systems, containing spherical and
high aspect ration capsules are on-going and will be reported elsewhere [1] .
ACKNOWLEDGEMENTS
This research was funded by the Dutch Polymer Institute; project #582.
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