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Document ref: 2004-R06
Rev. 1 – January 2005
Client:
ESA-ESTEC
Noordwijk, The Netherlands
Project title: Development
of
Nematic
Elastomer Nanocomposites
Deliverable: Inputs For HTML Page
Prepared by
Silvio Campigli
Eugene Terentjev
Jordi Elvira
Date
3 January 2005
3 January 2005
3 January 2005
Approved by
Filippo Pagliai
Date
4 January 2005
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Document ref: 2004-R06
Rev. 1 – January 2005
DESCRIPTION
PROJECT OBJECTIVES
Inflatable beams supports solar arrays, mirrors, sunshields and solar sails. Low
mass, low storage volume and easy of deployment reduce cost considerably. Three
main rigidization methods exist to extend the lifetime: forming a composite in space;
inflating an
luminium-kapton laminate structure beyond yield stress; using liquid
foam instead of gas. To overcome bottlenecks of inflation and rigidization, a new
concept of deployment is proposed. The approach is based on new two-ways shapememory materials based on nematic elastomers, with a stroke and stored energy far
beyond existing records (strain 400%, recovery stress 1Mpa). However electricallydriven actuation is so far not possible. We propose to develop a composite material
with enhanced dielectric anisotropy by embedding carbon nanotubes with high
mechanical strength (Young modulus ~1Tpa), aligned along the uniaxial director of
the monodomain nematic elastomer network and integrated in textile based
membranes.
The expected results of this study were to demonstrate the feasibility of
electromechanically actuate the membrane. The new solution would enable
predictable path of deployment and ensures that zones of exclusion around the
spacecraft, such as areas populated by solar arrays or instruments, are not violated.
As a fallback option, the same study looked at ordinary PDMS and semi-crystalline
polymer nanocomposites, which may have only one-way actuation stroke but could
still be very useful in deployable structure design.
THE PARTNERSHIP
To address the project objectives three entities with complementary technical
background and market positioning have joined forces, respectively Grado Zero
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Espace (Italy) as developer, the Cavendish Laboratory of the Un. Cambridge (UK) as
inventor and NTE (Spain) as the customer.
MAJOR FINDINGS
Scientists and engineers at Un. Cambridge have reviewed traditional approaches to
dispersing nanofillers (carbon nanotubes and fibers) which subject the aggregates to
high local shear, provided by a high-rpm mixer or ultrasound radiation, and then
prevent re-aggregation by crosslinking the polymer matrix. In many cases a
surfactant was involved to protect the highly polar nanotube surface. One of the tricks
of the trade was to use the high-viscosity suspending liquid, i.e. a polymer solution or
even a melt.
The other aspect, that has emerged during this work, is the choice of filler particles.
There are many very different sources of carbon nanotubes on the market today.
After an extensive searching and testing of purity and reproducibility of samples, the
preferred choice was the nanotubes provided by the Nanostructured & Amorphous
Materials, Inc. (USA). The nanotube-polymer composites were fabricated by first
carefully weighing the desired quantity of nanotubes and polymer. Calculations of
weight percentage took into account the crosslinker, to be later used in the mixture.
The quality of nanotube dispersion was monitored throughout the processing in two
ways. Before crosslinking, aliquot samples of the mix were smeared into very thin
films and examined with, at early stages, the optical microscope and later with a
High-Resolution Scanning Electron Microscope (SEM) as aggregate sizes reduced
well below optical resolution. After crosslinking, the samples were frozen and
microtomed fractured to reveal internal surfaces such as the one shown on the
coverpage.
The second method was to monitor the viscosity of the mix during shearing – as
tubes dispersed from rigid clusters there was a clear evolution of mean viscosity and
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the plateau saturation was taken as the criterion for final state reached. Un.
Cambridge found that a shearing regime lasting 24 hours was suitable in removing
nanotube aggregates due to the inherently high viscosity of the host polymer.
However, nanotubes re-aggregated rapidly, even in a viscous polymer matrix, unless
fast matrix crosslinking was made to prevent their subsequent movement.
Wide angle X-ray diffraction was used as a method to assess the average tube
orientation as a function of increasing applied uniaxial strain. An important
characteristic of nanotube composite (where the tube surface is non-modified and
remains conducting) is the overall conductivity of the material. In the case of nonaligned tubes, this identifies the percolation threshold of long flexible rods. In the
case of increasing tube alignment in the stretched matrix, there is a possibility of
conducting rods in contact (above percolation) to gradually lose this contact, at a
fixed concentration. This is the way to increase the nanotube loading without crossing
the concentration at which the composite becomes an electric conductor and lose its
potential for reversible (equilibrium) electromechanical actuation.
As the concentration is increased the rubbery network becomes stiffer and Young
modulus (the response to static linear extension) generally increases. This is
expected and in line with literature findings [Harris2004]. Another important and
interesting subject of mechanical properties is the dynamical shear response,
characterised by the frequency-dependent spectrum of complex modulus G*=G’+iG’’.
The frequency dependence of the loss factor G'' reveals information about slow
internal relaxation caused by nanotube motion; while the marked difference in
magnitude of the storage modulus G' measured in different orientations of shear with
respect to nanotube alignment axis makes analogies with deformation and soft
elasticity of nematic rubbers [Warner2003].
At very low nanotube loading one might expect that large regions of rubbery network
are still pristine. As a result, the transmission of stress from one side of the sample to
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the other can occur through `channels' with the original PDMS rubber modulus,
making the total response only weakly affected by the sparse inclusions. At higher
loading the concentration dependence becomes non-linear, reflecting the pair
interactions between inclusions, etc. One is tempted to make a connection between
the onset of this non-linear regime and the separately determined electric percolation
threshold, when the whole nanocomposite becomes conducting through nanotube
contacts. By design, our actuation materials stay in the linear loading regime, below
percolation.
The most important focus was the electro-mechanical response, or the actuation
induced by applying voltage to the solvent free composite. The clear and repeatable
response in contracting stress was evident. Indications are that the intensity of effect
is in relation to nanotube alignment (similar findings were observed in response to IR
radiation).
The response was clearly significant, in the range of kPa. The on-cycle shows a fast
rise in exerted stress (the dynamics has been studied separately, demonstrating
possibility of slow/retarded and fast/inertial response in different configurations), then
reaching a plateau level that depends on tube concentration and the field strength.
There is no electromechanical response in elastomers without nanotubes, which
confirms that the observed effect is a new action of nanotube composite.
A relevant aspect to be considered during the development of Carbon Nanotube
membrane actuators is the polymer film interaction with the textile membrane as the
fabric properties has not to limit the deployment performances. Two configurations for
the textile support structure have been investigated. A more traditional knitted
structure and a 3D warp-knitted structure, consisting of three textile layers, which are
simultaneously manufactured and joined together, resulting in a highly functional
textile sandwich composed of a base layer and a cover face with a knitted fabric
comprising vertical and diagonal spacer yarns arranged in-between.
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A pictorial
Document ref: 2004-R06
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representation of a candidate configuration of an uniaxial carbon nanotube
membrane actuator is provided below.
Although this preliminary work was used to validate the overall configuration from a
qualitative point of view, there is evidence that a 3D warp-knitted structure is not
significantly affecting the polymer film mechanical properties.
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