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DESIGN, MODELLING AND
TESTING OF A SYNTHETIC
MUSCLE SYSTEM AS ACTUATION
FOR AN AIRCRAFT CONTROL
SURFACE
BY KUDZAI C.K. MUTASA
STUDENT NUMBER 11001259
MENG (HONS) AERONAUTICAL ENGINEERING
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I) ABSTRACT
An attempt was made to try and improve the aerodynamic characteristics of modern day low speed
aircraft by using shape changing materials to actuate the control surfaces. Theoretically, a morphing
wing design approach was undertaken with major concentration being on the material for actuation.
Shape memory polymers (SMPs) were chosen over shape memory alloys (SMAs) due to their ease of
manufacture, low cost and highly flexible programming.
The fundamental characteristics of SMPs were presented along with their mechanism and
microstructure. These two factors along with preparation are the main influences into the behavior
of the SMP and this was confirmed through experimental work. Thermomechanical and tensile tests
were undertaken to determine glass transition temperature as well as mechanical strength of the
polymer composite above glass transition temperature and the 5% CB sample was selected to be
incorporated into the design.
The main hindrance towards a successful design in the past has been strength of SMPs as well as
means of actuation therefore an electro active polymer of Polyurethane (PU) incorporating Carbon
Black (CB) as the conductive filler was selected in hope that it would not drastically decrease the
mechanical strength of PU.
A mathematical model was also presented beginning from initial assumptions as well as a relation of
the modelling to a typical shape memory process. This enabled determination of constitutive
modelling parameters such as the frozen volume fraction ( ) , shear moduli of the
frozen and active phases and respectively, and volume ratios in the
frozen and active phases ( ) and ( ) respectively.
Calibration of this model was not possible as the material failed before the required strain was
achieved and reasons for this are given. The composition of PU was found to be directly related to
the modelling as well possible improvements for the material were presented. The material was
determined to have contained not enough hard segment content for adequate shape recovery.
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II) DECLARATION
I declare that this dissertation is my own work and effort and that it has not been submitted
anywhere for any award. If other sources of information have been used, they have been
acknowledged.
Signature: …...........................................................................................
Date: …………………………………………………………………………………………………
This dissertation was submitted in partial fulfillment of MEng (Hons) Aeronautical Engineering at the
University Of South Wales, United Kingdom.
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III) ACKNOWLEDGEMENT
It would not have been possible to write this dissertation without the help and support of kind
people around me, all of which are worth mentioning.
Above all I would like to thank my parents for their undying support and financial dedication towards
my education entirely. I wish to thank my brothers and sister for their emotional support
throughout and laughs provided during tough times.
This dissertation would also not have been possible without the help and support of my supervisor,
Dr. Giuliano Claude Premier, whose advice and support have been invaluable on both an academic
and personal level, for which I am extremely grateful.
I would also like to thank the academic and technical staff at the University of South Wales,
Trefforest Campus, as well as all lecturers who have provided me with engineering knowledge to
reach me to such a stage in my studies. The library and computer facilities have also been second to
none which have enabled access to various publications from great minds all over the world.
For any errors and inadequacies that remain in this work, of course, the responsibility is entirely my
own.
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IV) ABBREVIATIONS
, shape fixity rate
, shape recovery rate TPU, thermoplastic
, glass transition temperature
, isotropic temperature
, crystal melting temperature
, transition temperature
AFM, Atomic force microscopy
BD , 1,4-butanediol
CNFs, carbon nanofibers
CNTs, carbon nanotubes
ED, Ethylenediamine
FT-IR, Fourier transform infrared spectroscopy
LC, liquid crystalline
LCE, liquid crystalline elastomer
LiFeP , Lithium Iron Phospate
MDI, 4,4’-diphenylmethane diisocyanate
MWCNTs, multi-walled carbon nanotubes
Ni, Nickel
OM, Optical microscopy
PB, poly(1,4-butadiene)
PCL, polycaprolactone
PCO, polyoctene
PN, Polynorbornene
Polyurethane
POSS, polyhedral oligomeric silsesquioxane
PPO, poly(propylene oxide
PTMO, poly (tetramethyl oxide) glycol
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PVC, poly(vinyl chloride)
SCPs, shape changing polymers
SEM, Scanning electron microscopy
SMAs, shape–memory alloys
SMEs, shape–memory effects
SMF, shape–memory fiber
SMMs, shape–memory materials
SMPs, shape–memory polymers
SMPUs, shape–memory polyurethanes;
SMPUU, shape–memory polyurethane-urea
SPM, Scanning probe microscopy
STBS , styrene-trans-butadiene-styrene
SWCNTs, single-walled carbon nanotubes
TEM, Transmission electron microscopy
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V) LIST OF FIGURES
Figure 1 aircraft control surfaces............................................................................................................1
Figure 2 aircraft wing internal structure courtesy of nomenclaturo.com..............................................2
Figure 3 basic idea behind morphing wings courtesy of Baier and Datashvili (2011) ............................2
Figure 4 pneumatic rubber muscle actuator (Peel, Mejia, Narvaez, Thompson and Lingala (2009)) ....3
Figure 5 various morphing wing mechanisms. Morphing wing skin mechanism(left) and flap activated
by shape memory alloy wire by Kang, Kim, Jeong and Lee (2012) (right) .............................................3
Figure 6 different morphing skin concepts a) sandwich concept with elastomer cover and auxetic
material core (Baier and Datashvili (2011))............................................................................................4
Figure 7 leading and trailing edge mechanisms developed for the f-111 mission adaptive wing
program (Kota, Hetrick, Osborn, Paul, Pendleton, Flick and Tilman (2006))..........................................5
Figure 8 possible solution 1 ....................................................................................................................5
Figure 9 possible morphing wing design.................................................................................................6
Figure 10 the overall architecture of SMPs (Hu and Chen (2010)).........................................................9
Figure 11 strain recovery of a cross-linked, castable shape memory polymer upon rapid exposure to
a water bath at T=80˚C (Liu, Quinn and Mather (2007)) ......................................................................12
Figure 12 schematic depiction of shape fixing and recovery mechanisms of semi-crystalline rubbers.
a) cross linked shape at semi-crystalline stage, b) melted sample of stress free stage (high
temperature), c) deformed shape at melt stage (high temperature) and ; crystal frozen deformed
shape (low temperature) (Lu, Chun, Mather, Zhen, Haley and Coughlin (2002)) ................................14
Figure 13 PU with micro-phase separation structure (Chun, Cho and Chung (2006)) .........................17
Figure 14 four types of shape memory polymers with different shape fixing and shape recovery
mechanisms depicted as a function of their dynamic mechanical behaviour. Tensile storage modulus
versus temperature as measured using a small oscillatory deformation at 1Hz for I) chemically cross
linked glassy thermosets, II) chemically cross linked semi-crystalline rubbers, III) physically cross
linked thermoplastics and IV) physically cross linked block copolymers (Liu, Quinn and Mather
(2007))...................................................................................................................................................18
Figure 15 a) modulus and b) stress at 100% elongation of composites as a function of percentage
MWCNT content (open square : raw, open circle: 90˚C acid treatment, filled circle: 140˚C acid
treatment) (Cho, Kim, Jung and Goo (2005))........................................................................................19
Figure 16 electro-active shape-recovery behaviour of PU-MWCNT composites at 5% content. The
sample undergoes transition from temporary shape (linear left), to permanent (helix, right) within
10s when a voltage of 40V is applied. (Cho, Kim, Jung and Goo (2005))..............................................20
Figure 17 casting mold and machined sample with imbedded electrodes. Glass tape was used at each
end for securing in tensile testing frame. (Rogers and Khan (2012)) ...................................................21
Figure 18 results for carbon black filled polymer (Rogers and Khan (2012)) .......................................22
Figure 19 (Rogers and Khan (2012))......................................................................................................23
Figure 20 DSC results of CB at various compositions (Lan, Leng, Liu and Du (2008))...........................23
Figure 21 sequences of shape recovery of CB 10% by passing as electrical current of 30V (Rogers and
Khan (2012))..........................................................................................................................................23
Figure 22 magnetic field curing (Leng, Huang, Lan, Liu and Du (2008)) ...............................................24
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Figure 23 resistivity vs volume fraction of cb with/without 0.5 vol % ni. red symbol, right after
fabrication; blue symbol, one month later. inset displays how resistance was measured. (Leng,
Huang, Lan, Liu and Du (2008)).............................................................................................................24
Figure 24 evolution of resistivity upon shape memory cycling (Leng, Huang, Lan, Liu and Du (2008))25
Figure 25 storage nodulus versus volume fraction of ni at 0 degrees (Leng, Huang, Lan, Liu, Du, Phee
and Yuan (2008))...................................................................................................................................25
Figure 26 left: values of restistance vs temperature; right: values of restance versus strain for scf-
smp composite (Lu, Yu, Liu and Leng (2010)) .......................................................................................26
Figure 27 morphologies of scf-smp composite specim observed by SEM (2% SCF and 5% CB) a)
morphologies of scf fillers and b) morphologies of cb particles (Lu, Yu, Liu and Leng (2010)) ............26
Figure 28 stress-strain curves of composites filled with various scf contents in tensile mode (Lu, Yu,
Liu and Leng (2010))..............................................................................................................................27
Figure 29 images showing the macroscopic shape memory effect of 5% cb and 2% scf composite .
the permanent shape is a flat strip and the temporary shape a right angle deformation. (Lu, Yu, Liu
and Leng (2010)) ...................................................................................................................................27
Figure 30 morphology characterised with different methods. OM and TEM for PC containing 0.688%
vol CNTs. SEM in charge contrast mode shows the distribution of MWNTs in Polypyrrole matrix and
HAADF-STEM pictures show individual carbon black particles and their clusters in polymer
composites: OM and TEM (Deng, Lin, Ji, Zhang, Yang and Fu (2013))..................................................30
Figure 31 some design strategies for SMPs (Meng and Hu (2009))......................................................32
Figure 32 against CB content (Lan, Leng, Liu and Du (2008)) .........................................................32
Figure 33 schematic diagram of the micromechanics foundation of the 3D shape memory polymer
constitutive model with the existence of two extreme polymer states shown. In this diagram the
polymer is in the glass transition state with a predominant active phase. (Liu, Gall, Dunn, Greenberg
and Diani (2006))...................................................................................................................................34
Figure 34 deformation of an SMP in various states during cooling (Chen and Lagoudas (2008))........39
Figure 35 schematic of SMP thermomechanical cycle showing shape memory effect and constrained
recovery (Atli, Gandhi and Karst (2008)) ..............................................................................................45
Figure 36 schematic representation of results of cyclic thermo-mechanical investigations (Lendlein
and Kelch (2002))..................................................................................................................................49
Figure 37 material properties of PU (ALchemie.ltd).............................................................................51
Figure 38 experimental set up ..............................................................................................................52
Figure 39 extension vs temperature for 10% CB ..................................................................................53
Figure 40 5% CB thermomechanical test..............................................................................................54
Figure 41 Thermomechanical testing veil and 5% CB...........................................................................54
Figure 42 10% CB stress versus strain...................................................................................................55
Figure 43 5% CB stress versus strain.....................................................................................................56
Figure 44 5% CB and veil stress versus strain .......................................................................................56
Figure 45 5% CB thermal strain.............................................................................................................57
Figure 46 5% CB and veil thermal strain...............................................................................................57
Figure 47 thermal strain........................................................................................................................59
Figure 48 active phase stress strain graph............................................................................................60
Figure 49 zero stress cooling curve.......................................................................................................62
Figure 50 frozen volume fraction..........................................................................................................62
Figure 51 fractured test sample displaying crack propagation along regions embedded with wire. ..67
Figure 52 shape memory behaviour study of a) PTMO250 and b) PTMO650 (Lin and Chen (1998)) ..69
Figure 53 shape memory behaviour of soft segment investigation (Lin and Chen (1998)) .................70
Figure 54 chemical structure of pu block copolymer a) bd type and b) ed type..................................71
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Figure 55 mechanical properties of Pu a) maximum stress, b)tensile modulus and c) strain at break
(Chun, Cho and Chung (2006))..............................................................................................................72
Figure 56 shape memory properties vs hard segment content profile of PU chain extended with a)
BD and b) ED after the first test cycle (Chun, Cho and Chung (2006)) .................................................72
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VI) LIST OF TABLES
Table 1 possible benefits and setbacks of design 1................................................................................6
Table 2 (Liu, Quinn and Mather (2007)) .................................................................................................8
Table 3 shape memory thermosets (Liu, Quinn and Mather (2007))...................................................13
Table 4 polymer special features (Liu, Quinn and Mather (2007)).......................................................14
Table 5 summary of physically cross linked copolymer blends (Liu, Quinn and Mather (2007)).........16
Table 6 summary of physically cross-linked semi-crystalline copolymer blends (Liu, Quinn and
Mather (2007))......................................................................................................................................17
Table 7 summary of conductive fillers..................................................................................................33
Table 8...................................................................................................................................................48
Table 9 summary of constitutive parameters.......................................................................................63
Table 10 percentage extension and corresponding values.............................................................66
Table 11 notation and molar compositions of PU when investigation hard segment content. (Lin and
Chen (1998))..........................................................................................................................................68
Table 12 molar compositions of pu when studying soft segment (Lin and Chen (1998)) ....................70
Table 13 composition of PU used (Chun, Cho and Chung (2006))........................................................71
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TABLE OF CONTENTS
I) Abstract........................................................................................................................................... ii
II) Declaration................................................................................................................................. iii
III) Acknowledgement ..................................................................................................................... iv
IV) Abbreviations.............................................................................................................................. v
V) List of Figures ................................................................................................................................ vii
VI) List of Tables ............................................................................................................................... x
VII) List of Equations......................................................................................................................... xi
Table of Contents................................................................................................................................. xiii
Chapter 1.................................................................................................................................................1
1.0 Introduction ................................................................................................................................1
1.1 Morphing airfoil approach......................................................................................................2
1.1.1. Morphing wing skins.......................................................................................................4
1.2.1 Compliant wing System...................................................................................................4
1.2 Possible Wing Designs.............................................................................................................5
1.2.1. Possible Solution 1 ..........................................................................................................5
1.2.2. Possible Solution 2 ..........................................................................................................6
Chapter 2.................................................................................................................................................7
Literature Survey.....................................................................................................................................7
2.1 Fundamentals of Shape memory materials............................................................................7
2.1.1 Shape memory alloys......................................................................................................7
2.1.2 Shape memory polymers................................................................................................8
2.2 General framework of SMPs...................................................................................................9
2.2.1. Thermodynamic behaviour...........................................................................................10
2.2.2. Entropy elasticity...........................................................................................................11
2.3. Structure and Mechanism of SMPs.......................................................................................11
2.3.1. Covalently cross-linked glassy thermoset networks.....................................................11
2.3.2. Covalently cross-linked semi-crystalline networks.......................................................13
2.3.3. Physically cross-linked glassy copolymers ....................................................................15
2.3.4. Physically cross-linked semi-crystalline block copolymers ...........................................16
2.4 Electro-active polymers ........................................................................................................19
2.4.1 SMP filled with carbon nanotubes................................................................................19
2.4.2 SMP filled with Carbon black ........................................................................................20
2.4.3 SMP filled with nickel....................................................................................................24
2.4.4 SMP filled with hybrid fillers.........................................................................................26
2.5. Preparation of Conductive shape memory polymers...........................................................28
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2.5.1. Melt compounding........................................................................................................28
2.5.2. In-Situ polymerisation...................................................................................................29
2.5.3. Solution mixing..............................................................................................................29
2.6 Morphological control of conductive networks in shape memory polymers.......................30
2.6.1 Characterisation of conductive network formation .....................................................30
2.6.2 Morphological control through polymer blends...........................................................31
2.6.3 Influence of filler chemistry on glass transition temperature ......................................32
2.7 Material selection and Manufacturing method....................................................................33
Chapter 3...............................................................................................................................................34
3.1 Preliminary Modelling for Shape Memory Polymer behaviour................................................34
3.1.1 Preliminary assumptions...................................................................................................36
3.1.2 Constitutive Equations......................................................................................................37
3.1.3 Average Scheme................................................................................................................39
3.1.4 The shape memory cycle ..................................................................................................42
3.1.4.1 Constrained recovery....................................................................................................45
3.1.5 Neo-hookean modelling....................................................................................................46
3.1.6 Reduction of constitutive model for Uniaxial tension experiment...................................47
Chapter 4...............................................................................................................................................49
4.1 Experimental Setup...................................................................................................................49
4.1.1 Cyclic characterisation ......................................................................................................49
4.1.2 Sample Fabrication............................................................................................................51
4.1.3 Determination of Glass transition temperature...............................................................52
4.1.4 Extension versus temperature (zero load)........................................................................52
4.1.5 Tensile Testing...................................................................................................................52
Chapter 5...............................................................................................................................................53
5.1 Results.......................................................................................................................................53
5.1.1 Glass transition temperature............................................................................................53
5.1.2 Stress versus Strain ...........................................................................................................55
5.1.3 Thermal Strain measurement ...........................................................................................57
Chapter 6...............................................................................................................................................58
6.1 Model calibration......................................................................................................................58
6.1.1 Determination of Constitutive parameters ......................................................................58
6.2 Model Implementation............................................................................................................64
6.2.1 Stretch controlled process................................................................................................64
Chapter 7...............................................................................................................................................66
7.1 Validation and Discussion .........................................................................................................66
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7.1.1 Influence of modelling on material behaviour .................................................................66
7.1.2 Polyurethane analysis .......................................................................................................68
7.1.2.1 Material relation to modelling......................................................................................68
7.1.2.2 Chemical structure dependance on perfomance .........................................................71
7.1.3 Recommendations............................................................................................................73
Chapter 8...............................................................................................................................................74
8.0 Conclusion.................................................................................................................................74
VIII) References ................................................................................................................................75
IX) Index..........................................................................................................................................79
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CHAPTER 1
1.0 INTRODUCTION
In the modern day of aviation many technological advances have been made in order to increase the
efficiency of aircraft for various flight requirements. One of the factors widely recognized as
influencing the performance of an aircraft are the materials of the mechanisms used for actuation of
aircraft control surfaces. Control surfaces (figure 1) play a vital role as they determine primary and
secondary control of an aircraft and determine most of the aerodynamic characteristics of the
aircraft.
However, hydraulic actuation mechanisms contribute greatly to the weight of an aircraft and this
problem could be improved with the use of a system involving lighter, more responsive materials.
Such materials could lead to a smoother wing and hence more aerodynamically desirable aircraft.
These materials would be of the type which can retain a shape upon temperature modification and
are referred to as shape-memory materials.
FIGURE 1 AIRCRAFT CONTROL SURFACES
The task was then set of designing and testing some sort of synthetic muscle system that may be
applicable to any type of airborne system, beginning with low speed aircraft. Shape memory
material actuation on an airborne system has been attempted before however in this case a
somewhat novel application was proposed. Previous applications have been to actuate the trailing
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edge section of the wing therefore becoming the flap however in this case, a morphing wing design
was attempted.
FIGURE 2 AIRCRAFT WING INTERNAL STRUCTURE COURTESY OF NOMENCLATURO.COM
1.1 MORPHING AIRFOIL APPROACH
FIGURE 3 BASIC IDEA BEHIND MORPHING WINGS COURTESY OF BAIER AND DATASHVILI (2011)
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In order to achieve the morphing airfoil, the shape changing structure would be integrated into the
primary structure of the wing namely ribs and stringers. Figure 2 displays the internal wing structure
for a typical low speed aircraft. Many authors have achieved shape memory actuation of the wing
flap alone. For example, Kang, Kim, Jeong and Lee (2012) achieved a morphing wing mechanism
using an SMA wire actuator and achieved smooth actuation without extension of the wing skin.
Aircraft flap systems consists of discontinuous sections which can possibly cause aerodynamic losses
therefore morphing sections reduce these losses and contribute to aircraft efficiency. It is important
to note that various other setups have been attempted as well.
FIGURE 4 PNEUMATIC RUBBER MUSCLE ACTUATOR (PEEL, MEJIA, NARVAEZ, THOMPSON AND LINGALA
(2009))
Peel, Mejia, Narvaez, Thompson and Lingala (2009) achieved a morphing wing concept by using a
composite skin and pneumatic rubber muscle actuator. James, Menner Bismarck and Iannucci (2009)
proposed a morphing skin as well by using a shape memory polymer as the wing skin. Baier and
Datashvili (2011) provided a cross linking between structures and mechanisms in morphing
aerospace structures in their review paper.
Kang, Kim, Jeong and Lee (2012) are referred to in this last mentioned paper and comment about
how in general, a morphing wing requires a change in length of the wing skin and this requires the
skin to be flexible. At the same time the skin must possess enough stiffness to resist external
aerodynamic pressure. These contradictory characteristics thus prove as a setback in morphing wing
design, which has sparked the need for a novel design. It was also noted that all the previous
research did not include any mechanism of leading edge actuation, this shall also be investigated in
this paper, however as a secondary function.
FIGURE 5 VARIOUS MORPHING WING MECHANISMS. MORPHING WING SKIN MECHANISM(LEFT) AND FLAP
ACTIVATED BY SHAPE MEMORY ALLOY WIRE BY KANG, KIM, JEONG AND LEE (2012) (RIGHT)
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1.1.1. MORPHING WING SKINS
Baier and Datashvili (2011) mention the skins for morphing wings can be a challenging design
element due to the fact that they have to be deformable but at the same time have to take and
transfer high aerodynamic loads. Thill, Etches, Bond, Potter and Weaver (2008) extensively reviewed
concepts of morphing skins such as properly tailored laminates or structural non-isotropy achieved
by corrugation as shown in figure 6. A sandwich morphing skin consists of flexible elastomers as
cover and different types of cores including auxetic materials. This is beneficial in providing relatively
low in-plane stiffness of the skin combined with sufficiently high bending stiffness. It should be
noted that a morphing skin is a mammoth subject on its own and hence it shall be theoretically
assumed that a morphing wing skin is part of the design.
FIGURE 6 DIFFERENT MORPHING SKIN CONCEPTS A) SANDWICH CONCEPT WITH ELASTOMER COVER AND
AUXETIC MATERIAL CORE (BAIER AND DATASHVILI (2011))
1.2.1 COMPLIANT WING SYSTEM
One of the most recent and most successful applications of the morphing wing mechanism has been
achieved by Flexsys.Inc. They have developed the world’s first functional, seamless and hinge-free
wing whose trailing and/or leading edges morph to adapt to different flight conditions. In the
publication by Kota, Hetrick, Osborn, Paul, Pendleton, Flick and Tilman (2006) they refer to the term
a “compliant mechanism’’ which can be defined as a class of mechanism that relies on elastic
deformation of its constituent elements to transmit motion and/or force.
This is a particularly useful mechanism for any morphing wing mechanism as it eliminates the
application of any standard wing internal structure. The primary challenge in a morphing system is to
develop an efficient structure that can distribute local actuation power to the surface of the airfoil to
produce a specified shape change. A compliant mechanism provides a solution to this challenge but
it should be noted that in this case only the leading and trailing edges are able to be modified as
shown by figure 7.
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FIGURE 7 LEADING AND TRAILING EDGE MECHANISMS DEVELOPED FOR THE F-111 MISSION ADAPTIVE WING
PROGRAM (KOTA, HETRICK, OSBORN, PAUL, PENDLETON, FLICK AND TILMAN (2006))
1.2 POSSIBLE WING DESIGNS
It should be noted that the goal was to produce a morphing wing for low altitude, low endurance
and low mach number for application in an unmanned aerial vehicle (UAV).
1.2.1. POSSIBLE SOLUTION 1
FIGURE 8 POSSIBLE SOLUTION 1
A novel design suggested is that shown in the figure below which consists of placing the selected
shape memory material in block fashion around the entirety of the wing rib. The material is thus
lodged between the rib and the wing skin and when the system has been activated, the blocks would
change shape so as to initiate morphing in the material. Another design requirement was to create a
compliant wing to fit within the same space constraints while minimizing the weight and power
requirements.
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Benefits Setbacks
Shape can be changed on any point on the wing
circumferential length
Material needs to absorb flight vibrations
Material blocks can be easily replaced Blocks need to be very stable during morphing
phase
Limitless design capabilities and application Method of fixing blocks to the internal structure
will need to be determined
TABLE 1 POSSIBLE BENEFITS AND SETBACKS OF DESIGN 1
1.2.2. POSSIBLE SOLUTION 2
Another design which incorporates all the requirements is similar to the one has the benefits of a
wing that can be deflected differentially along the span in order vary the deflection and optimize
wing loading. This design has the benefit that the material subparts can be designed to only have
slight differences thus making manufacturing easier. However there is a possibility that the
stabilizing rod could interfere with the mechanical strength of the material.
FIGURE 9 POSSIBLE MORPHING WING DESIGN
Stabilizing rod/spar
SMP
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CHAPTER 2
LITERATURE SURVEY
2.1 FUNDAMENTALS OF SHAPE MEMORY MATERIALS
2.1.1 SHAPE MEMORY ALLOYS
Liu, Quinn and Mather (2007) define a shape memory material as
‘’those materials that have the ability to memorize a macroscopic permanent shape, be manipulated
and fixed to a dormant and temporary shape under specific conditions of temperature or stress, and
then later relax to the original, stress free condition under thermal, electrical or environmental
command.’’
The aforementioned relaxation is associated with an elastic deformation stored within the material
prior to deformation.
The most prominent and widely used shape memory materials are shape memory alloys as Liu,
Quinn and Mather (2007) continue to explain how their shape memory behavior stems from the
existence of two stable crystal structures in the material. SMAs consist of a high temperature
favored austenitic phase and a low temperature favored martensitic phase. Deformations that occur
during the low temperature phase, occurring above a critical stress, are then completely recovered
during the solid-solid transformation to the high temperature austenitic phase.
SMAs come in various combinations but the most common is the Nickel-titanium alloy due to the
combination it possesses of
1. a desirable transition temperature close to body temperature,
2. superelasticity and
3. two way shape memory capability.
Despite these benefits there are also downfalls to SMAs which come in the form of
a) limited recoverable strains of less than 8%,
b) inherently high stiffness,
c) high cost,
d) a comparatively inflexible transition temperature and
e) demanding processing and training conditions.
These limitations encouraged consideration for alternative polymeric shape memory materials. In
general, Liu, Quinn and Mather (2007) state that SMAs achieve pseudo-plastic fixing through
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martensitic de-twinning mechanism, with recovery being triggered by the martenite-austenite phase
transition. This implies that fixing of a temporary shape is accomplished at a single temperature and
recovery occurs upon heating beyond the martensitic transformation temperature.
2.1.2 SHAPE MEMORY POLYMERS
Liu, Quinn and Mather (2007) differentiate these from SMAs in that shape memory polymers
achieve their strain fixing and recovery through a plethora of physical means.
TABLE 2 (LIU, QUINN AND MATHER (2007))
Hu, Zhu, Huang and Lu (2012) refer to how with the rapid development and improvement of SMPs,
the features have become more and more prominent in comparison with SMAs. The advantages of
SMPs are as follows.
1. They can use diverse external stimuli and triggers as compared to SMAs which are only heat
triggered. Diverse stimulation can also result in multi-sensitive materials
2. Highly flexible programming through either single or multi-step processes
3. Broad range of structural designs. Various approaches are possible for designing net points
and switches for the various types of SMPs.
4. They possess tunable properties. SMP properties can be easily and accurately tuned using
composites, blending and synthesis
5. They can be modified to occupy a large space with a small volume in the form of foams. Such
applications have been observed in aerospace configurations and airplane components
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2.2 GENERAL FRAMEWORK OF SMPS
FIGURE 10 THE OVERALL ARCHITECTURE OF SMPS (HU AND CHEN (2010))
Hu and Chen (2010) mention that at the molecular level, shape memory polymers and shape
changing polymers consist of switches and net points as shown by the figure above. Net points
determine the permanent shape of the polymer network and can be of a chemical or physical nature
comprising covalent or non-covalent bonds respectively. The physical cross linking is formed through
the crystals, amorphous hard domains or other forms of entangled chains which will be discussed in
the next section. Switches are the major constituents which are responsible for strain fixation and
partial strain recovery. The switches can either be any of the following
1. the amorphous phase with a low glass transition temperature ( ),
2. semi-crystalline phase with a low melting temperature ( ) or
3. liquid crystalline (LC) phase with a low isotropization temperature ( )
Noted by Hu and Chen (2010), so far the amorphous phase, semi-crystalline phase and
supramolecular entities are used in shape memory polymer (SMP) construction while shape
changing polymers (SCPs) are observed in the liquid crystalline elastomers (LCEs) and cross-linked
polymers with stress-induced crystallization.
25. Design, Modelling and Testing of a synthetic muscle system
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In order for shape memory functionality to be achieved, the polymer network of SMPs must be
temporarily fixed in a deformed state under environmental conditions. Reversible molecular
switches can prevent recoiling of deformed chain segment when the switch is ‘’idle’’ possibly from
re-crystallization of a semi-crystalline soft phase. Under an environmental trigger such as heat or
light, the original shape can be recovered from the deformed shape due to the crystal melting of the
soft phase. However in SCPs the geometry is distinctly mentioned as being governed by the original
three dimensional shape.
For SCPs, the process of deformation and recovery can be repeated several times however shape
geometry change is not possible. This can be explained with the example of LCEs which change their
shape when the temperature is raised above as a result of phase transition from LC phase to
isotropic phase. When the temperature is cooled down the material returns to its original shape by
sampling returning to the LC phase.
In the amorphous state, polymer chains will take up a completely random distribution in the matrix,
with no restriction given by the order of crystallites in semi-crystalline polymers. All possible
conformations of a polymer chain have the same internal energy. Let W represent the probability of
a conformation, which is the state of maximum entropy, represents the most probable state for an
amorphous linear polymer chain according to the Boltzmann equation as follows
EQUATION 1
Where S=entropy, k=Boltzmann constant
2.2.1. THERMODYNAMIC BEHAVIOUR
Lendlein and Kelch (2002) introduce methods for the quantification of shape-memory properties as
well as the corresponding physical quantities based on a description of the macroscopic shape
memory effect. In the glassy state, all movements of the polymer sections are frozen. The transition
from this state to the rubbery elastic state occurs when the thermal activation is increased; meaning
the rotation around the segment bonds becomes increasingly unimpeded. This enables the chains to
take up one of the possible, energetically equivalent conformations without disentangling
significantly.
In the elastic state, a polymer with sufficient molecular weight stretches in the direction of an
applied force and if this tensile stress is applied for a short time interval, entanglements of the
polymer chains with their direct neighbors will prevent a large movement of the chain. If the tensile
stress is applied for an extended period of time, a relaxations process results which is a plastic,
irreversible deformation due to slipping and disentangling of the polymer chains from each other.
26. Design, Modelling and Testing of a synthetic muscle system
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2.2.2. ENTROPY ELASTICITY
The aforementioned slipping or flow of the polymer chains under strains can be stopped almost
completely by cross-linking of the chains as discussed by Lendlein and Kelch (2002). It is described
that the cross-linkage points act as permanent entanglements which prevent the chains from
slipping from each other. The cross links are discussed in more detail in the next section.
Apart from the net points, polymer networks contain amorphous chain segments which are also
flexible components. If the of these segments is below working temperature, the networks will
prove to be elastic, showing entropy elasticity with a loss of entropy. Distance between these
netpoints increases during stretching and they will become oriented thus as soon as the external
force is released, the material returns to its original shape and gains back the previously lost
entropy. Therefore the polymer network maintains mechanical stress in equilibrium.
2.3. STRUCTURE AND MECHANISM OF SMPS
2.3.1. COVALENTLY CROSS-LINKED GLASSY THERMOSET NETWORKS
Liu, Quinn and Mather (2007) refer to this as the simplest type of SMP consisting of a sharp glass
temperature ( ) at the temperature of interest and rubbery elasticity above derived from
covalent crosslinks. Attractive characteristics of this class of materials includes the following
a) Excellent degree of shape recovery due to the rubbery elasticity caused by the occurrence of
permanent cross-linking
b) Tunable work capacity during recovery garnered by a rubbery modulus that can be adjusted
through the extent of covalent cross-linking and
c) An absence of molecular slippage between chains due to the chemical cross linking.
A downside this type of network is that since the primary shape is covalently fixed these materials
are difficult to reshape after casting or molding. An example of this type is chemically cross-linked
vinylidene random copolymer which consists of two vinilydene monomers namely methyl
methacrylate and butyl methacrylate. The homopolymers show two different values of 110˚C and
20˚C which gives the random copolymer a sharp tunable between the two values of the
homopolymers by varying the composition. The work capacity is adjustable by varying the extent of
cross-linking achieved by copolymerization with tetraethylene glycol dimethacrylate. The resultant
performance of the thermoset is complete shape fixing, fast shape recovery at the stress free stage
and is also castable and optically transparent.
27. Design, Modelling and Testing of a synthetic muscle system
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FIGURE 11 STRAIN RECOVERY OF A CROSS-LINKED, CASTABLE SHAPE MEMORY POLYMER UPON RAPID
EXPOSURE TO A WATER BATH AT T=80˚C (LIU, QUINN AND MATHER (2007))
Liu, Quinn and Mather (2007) also include under this category polymers with above room
temperature with ultra-high molecular weight above g/mol due to their lack of flow above
and good shape fixing by vitrification. These polymers are mentioned to possess above 25
entanglements per chain and these entanglements function as physical cross-links on the time scale
of typical deformations which is mentioned to range from 1 to 10 seconds. The physical cross-linking
results in a three dimensional network which gives excellent elasticity above although a downside
is difficult thermal processing which may require solvent-processing. Such characteristics induce
performance results of the just recently discussed polymer type hence their inclusion in this group.
An example is polynorbornene (PN) with and high molecular weight. In this case the
decrease of mobility of PN molecules at temperatures below maintains the secondary shape.
Shape recovery to the original shape is then achieved by heating above the releasing the stored
energy. Performance characteristics were complete shape fixing when vitrified, fast and complete
shape recovery due to the sharp and high entanglement density that forms a three dimensional
network. Disadvantages of such materials were found to be as follows
1. Transition temperature cannot be easily varied
28. Design, Modelling and Testing of a synthetic muscle system
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2. The modulus plateau, responsible for controlling energy stored during deformation, is low
and hard to modify
3. Creep will occur to the polymer under stress at high temperatures due to the finite lifetime
of entanglements
4. Difficulty of processing due to high viscosity associated with high molecular weight polymers
TABLE 3 SHAPE MEMORY THERMOSETS (LIU, QUINN AND MATHER (2007))
2.3.2. COVALENTLY CROSS-LINKED SEMI-CRYSTALLINE NETWORKS
The melting transition of semi-crystalline networks can also be employed to induce a shape recovery
and Liu, Quinn and Mather (2007) mention that it induces a sharper recovery event. In this case the
secondary shape is fixed by crystallization rather than vitrification. The permanent shapes are also in
this case established by chemical cross linking with no reshaping possible after processing.
This class generally proves to be more compliant below the critical temperature and its stiffness is
sensitive to the degree of crystallinity and therefore indirectly to the degree of cross-linking. Shape
recovery speed were also noticed to be faster for the first order transition, usually also with a
sharper transition zone. This class includes the following materials
a) Bulk polymers such as semi-crystalline rubbers and Liquid Crystal Elastomers (LCEs)
b) Hydrogels with phase separated crystalline microdomains
Semi-crystalline rubbers have been considered for shape memory application due to their super
elastic rheological characteristics, fast shape recovery and flexible modulus at the fixed stage. Liu,
Quinn and Mather (2007) successfully developed a chemically cross-linked, semi-crystalline trans-
polyoctenamer (polyoctene, PCO) possessing a trans content of 80%, of -70˚C, of 58˚C and
much better thermal stability for shape memory application. When a strained sample was cooled
below , crystalline domains began to form and ultimately percolated the sample, establishing
strain fixing. When the material was heated above , the crystals melted to an amorphous,
homogenous phase with high mobility, leaving the chemical cross-links to re-establish the primary
shape. PCO has elasticity similar to rubber at temperatures above with easy deformation possible
by an external shape to create a secondary shape. The secondary shape, fixed by crystallization
during the subsequent cooling process, does not change below and as long as the crystals were
not destroyed though was possibly subject to warping.
29. Design, Modelling and Testing of a synthetic muscle system
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TABLE 4 POLYMER SPECIAL FEATURES (LIU, QUINN AND MATHER (2007))
FIGURE 12 SCHEMATIC DEPICTION OF SHAPE FIXING AND RECOVERY MECHANISMS OF SEMI-CRYSTALLINE
RUBBERS. A) CROSS LINKED SHAPE AT SEMI-CRYSTALLINE STAGE, B) MELTED SAMPLE OF STRESS FREE STAGE
(HIGH TEMPERATURE), C) DEFORMED SHAPE AT MELT STAGE (HIGH TEMPERATURE) AND ; CRYSTAL FROZEN
DEFORMED SHAPE (LOW TEMPERATURE) (LU, CHUN, MATHER, ZHEN, HALEY AND COUGHLIN (2002))
Figure 12 displays the discussed shape memory mechanism as it was investigated by Lu, Chun,
Mather, Zhen, Haley and Coughlin (2002). The room temperature stiffness, transition temperatures
and rubbery modulus proved to be able to be tuned independently by blending with rubbery or solid
components which manipulates the tacticity of PCO. Cross linking a semi crystalline material
impedes crystal formation which might cause a lesser degree of crystallinity, broader crystal size
distribution and a lower and broader melting transition temperature span hence slower shape
memory recovery.
30. Design, Modelling and Testing of a synthetic muscle system
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Chernous, Shil’ko and Pleskachevskii (2004) attempted to specifically crosslink the amorphous
fraction but not the crystalline fraction so as to avoid a drop in transition temperature due to cross-
linking. Successful implementation was achieved for a blend composition composed of a semi-
crystalline polymer acting as the reversible phase and a specially functionalized, co-continuous
rubber matrix as the permanent phase. They applied special curing techniques to the rubber matrix
but left the semi-crystalline phase unaffected. Another group applicable are Liquid Crystalline
Elastomers (LCEs) which are discussed later.
2.3.3. PHYSICALLY CROSS-LINKED GLASSY COPOLYMERS
This group is mentioned to solve the issue of ease of processing of shape memory polymers by Liu,
Quinn and Mather (2007) as these polymers display rheological characteristics compliant to
simplistic processing with conventional thermoplastics technology. Here crystalline or rigid
amorphous domains in thermoplastics are able to serve as physical crosslinks affording the super -
elasticity required for shape memory to be developed, which is mainly in the form of phase
separated block copolymers.
It is described that when the temperature exceeds the or of these physical domains,
described as , the material will flow and therefore can be processed and manipulated
physically. Another continuous phase possessing a lower or , which can be represented as
, exists which softens to a rubbery state in the range between the two critical temperatures and
fixes a secondary shape on cooling to a temperature below .
Jeong, Song, Lee and Kim (2001) recognized how for some block copolymers and polyurethanes, the
soft domain displayed a sharp glass-transition which could be tuned for shape memory applications.
Despite this groups’ room temperature stiffness being similar to covalently cross-linked glassy
thermosets, their being only physically cross-linked yields the benefit of being processable above
of the hard domains. An example of this type is one which was investigated by Jeon, Mather
and Haddad (2001) of norbonene copolymerised with a polyhedral oligosilsesquioxane (norbonyl-
POSS) hybrid monomer. This yielded a microphase separated copolymer with fewer repeating units
in the backbone than commercial polynorbonene. The composition improved the thermal
processability and suppressed high temperature yielding of polynorbonene homopolymer, also
enhancing the critical temperature and stored energy during deformation (rubbery modulus). Also
reported results were the broadening of the , which to a certain extent retarded the shape
recovery speed.
Liu, Quinn and Mather (2007) also include in this class some low crystallinity, semi-crystalline
homopolymers, or melt-miscible polymer blends compatible in molten and amorphous states, but
having at least one semi-crystalline component. Liu and Mather (2003) reported that in such a
system, the crystals serve as physical cross-links, or rather hard domains, and the composition
dependent of the amorphous region functions as the transition temperature. For these it was
noted that easy tuning of the glass-transition temperature of the amorphous phase and the work
output during shape recovery was possible by changing the blend composition, akin to the
copolymer thermosets mentioned in the first group discussed.
31. Design, Modelling and Testing of a synthetic muscle system
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Apart from the crystalline and glassy domains, other physical cross-linking techniques include
hydrogen bonding or ionic clusters within the hard domains investigated by Li, Chen, Zhu, Zhang and
Xu (1998) and Kim, Lee, J. S. Lee, Baek, Choi, J. O. Lee and M. Xu (1998) respectively. Existence of
these interactions is said to strengthen the hard domains by decreasing chain slippage during
deformation which therefore increases the extent of shape recovery.
TABLE 5 SUMMARY OF PHYSICALLY CROSS LINKED COPOLYMER BLENDS (LIU, QUINN AND MATHER (2007))
Liu, Quinn and Mather (2007) recognise the diversity in selection of soft domains as examples are
displayed in the table above and conclude that hydrophilic oligomers can be used to create
multiblock copolymers with shape memory properties. These add the benefit of moisture-triggered
shape memory recovery apart from heat stimulation. However slow recovery was reported by
Huang, Yang, An, Li and Chan (2005) due to the relatively slow speed of water diffusion.
2.3.4. PHYSICALLY CROSS-LINKED SEMI-CRYSTALLINE BLOCK COPOLYMERS
It is mentioned by Liu, Quinn and Mather (2007) that for some block copolymers, the soft domain
will crystallize and rather than the , the values will function as shape memory transition
temperatures therefore the secondary shapes are fixed by crystallization of the soft domains. An
example is styrene-trans-butadiene-styrene (STBS) triblock copolymers which feature shape memory
behavior afforded by this mechanism investigated by Ikematsu, Kishimoto and Karaushi (1990).
STBS is referred to as a strongly segregated ABA-type triblock copolymer with a minor component of
polystyrene (PS) segments, ca 10-30 volume percent, serving as A domains at each end of the
macromolecular chains, and a major component of semi-crystalline poly trans-butadiene (TPB)
segments as B-domains in the middle block. As a result of the immiscibility between TPB and PS
blocks, the copolymer phase separates and PS blocks form discontinuous, amorphous micro-
domains having =93˚C. TPB blocks will form a semi-crystalline matrix having a of 68˚C with a
of 90˚C. The rigid PS microdomains are mentioned to remain rigid up to 90˚C which enables them to
serve as physical cross-links whose configuration set the permanent shape when a temperature of
68˚C<T<90˚C was applied, the material became flexible and rubbery due to the melting point of the
TPB crystals but the material will not flow due to the rigid PS microdomains which maintains a
stress-free permanent shape. At his stage the material had a storage modulus similar to rubber
which was dictated by the TPB molecular weight. When cooled below 40˚C, the TPB matrix
32. Design, Modelling and Testing of a synthetic muscle system
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crystallised so that a secondary shape can be fixed by those crystals while energy exerted during
deformation is more or less frozen into the material. Melting of TPB will enable returning to the
original shape via reheating.
Benefits if this polymer are possessing a permanent that can be reprocessed above 100˚C when both
domains flow and is disadvantageous due to the fact that the hard microdomains may creep under
stress when setting the temporary shape near a which limits the extent of recoverable strain.
TABLE 6 SUMMARY OF PHYSICALLY CROSS-LINKED SEMI-CRYSTALLINE COPOLYMER BLENDS (LIU, QUINN AND
MATHER (2007))
Thermoplastic segmented polyurethanes with semi-crystalline flexible segments have also been
investigated as a similar approach. Liu, Quinn and Mather (2007) describe polyurethanes as
conventionally being multiblock copolymers consisting of alternating sequences of hard and soft
segments. Hard segments form the physical cross-links via polar interaction, hydrogen bonding, or
crystallization and these crosslinks are able to resist moderately high temparatures without being
destroyed (≈110˚C). The soft segments capable of crystallization form the thermally reversible phase
with crystallization of the soft segments governing the secondary shape. With regards to the ,
Chun, Cho and Chung (2006) mention that the hard segment generally has much higher than
room temperature while the soft segment has lower than room temperature and endows the
SMP with properties such as high draw ratio, low modulus and high elastic recovery. Polyurethanes
possess the following benefits
1. Easily tunable room temperature stiffness, transition temperature and room temperature
stiffness by manipulating their compositions
2. Biodegradeability for some
3. Can easily be foamed as the foam memory materials CHEM by Sokolowski, Chmielewski,
Hayashi and Yamada (1999)
FIGURE 13 PU WITH MICRO-PHASE SEPARATION STRUCTURE (CHUN, CHO AND CHUNG (2006))
33. Design, Modelling and Testing of a synthetic muscle system
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FIGURE 14 FOUR TYPES OF SHAPE MEMORY POLYMERS WITH DIFFERENT SHAPE FIXING AND SHAPE RECOVERY
MECHANISMS DEPICTED AS A FUNCTION OF THEIR DYNAMIC MECHANICAL BEHAVIOUR. TENSILE STORAGE
MODULUS VERSUS TEMPERATURE AS MEASURED USING A SMALL OSCILLATORY DEFORMATION AT 1HZ FOR I)
CHEMICALLY CROSS LINKED GLASSY THERMOSETS, II) CHEMICALLY CROSS LINKED SEMI-CRYSTALLINE
RUBBERS, III) PHYSICALLY CROSS LINKED THERMOPLASTICS AND IV) PHYSICALLY CROSS LINKED BLOCK
COPOLYMERS (LIU, QUINN AND MATHER (2007))
34. Design, Modelling and Testing of a synthetic muscle system
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2.4 ELECTRO-ACTIVE POLYMERS
For certain applications such as aerospace and automotive, it is not possible to create an external
environment so as to enforce shape memory behavior such as heat, light, pH or water. Liu, Lv, Lan,
Leng and Du (2008) mention how the need to get rid of external heating has led to the application of
electro-conductive fillers in SMPs. This application proves particularly useful with respect to the
desired performance requirements for the morphing wing system
2.4.1 SMP FILLED WITH CARBON NANOTUBES
Cho, Kim, Jung and Goo (2005) investigated shape recovery of Polyurethane (PU) composites by
applying a voltage and not thermal heating. This is a key factor in this study as this would enable the
application of SMPs as smart actuators. In order for electro active shape memory behavior to be
established, multi-walled carbon nanotubes (MWCNTs) were used after being chemically surface
modified in a nitric acid and sulphuric acid mixture. Surface modification was applied in order to
improve the interfacial bonding between polymers and nanotubes as previously investigated.
PU containing 40% hard segments were synthesized by a pre-polymerisation method using
monitored portions of polycaprolactenediol (PCL) as the soft segment and 4-4’-methylene bis
(phenylisocyanate) (MDI) and butane-1,4-diol acting as the hard segments. Composite films were
produced when mixed with the MWCNTs and the electrical conductivity was measured using the
four point probe method, which was in the order of S cm for 5% MWCNT modified content,
which was sufficient to heat the material above 35˚C which is the transition temperature of
polyurethane. In the tensile test, it was noticed that modulus and strength at 100% elongation
increased with increasing surface modified MWCNT content, with elongation at break decreasing as
shown by the figure below.
FIGURE 15 A) MODULUS AND B) STRESS AT 100% ELONGATION OF COMPOSITES AS A FUNCTION OF
PERCENTAGE MWCNT CONTENT (OPEN SQUARE : RAW, OPEN CIRCLE: 90˚C ACID TREATMENT, FILLED CIRCLE:
140˚C ACID TREATMENT) (CHO, KIM, JUNG AND GOO (2005))
35. Design, Modelling and Testing of a synthetic muscle system
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Electrical conductivity was found to increase as the amount of MWCNT content increased, with
surface modification displaying significant results. In the area of surface modification, electrical
conductivity of surface modified MWCNT was lower than that in untreated MWCNT of the same
filler content and Cho, Kim, Jung and Goo (2005) attribute this to the increased defects in the lattice
structure of carbon-carbon bonds formed on the nanotube surface due to the acid treatment. It was
also noticed that severe surface modification lowers mechanical and conductive properties while
modification of nanotubes at optimum conditions could increase the mechanical properties of shape
memory composites. Therefore both mechanical and conducting properties were dependent on the
degree of surface modification of the MWCNTs, with an acid treatment of 90˚C giving desirable
properties for shape memory.
FIGURE 16 ELECTRO-ACTIVE SHAPE-RECOVERY BEHAVIOUR OF PU-MWCNT COMPOSITES AT 5% CONTENT. THE
SAMPLE UNDERGOES TRANSITION FROM TEMPORARY SHAPE (LINEAR LEFT), TO PERMANENT (HELIX, RIGHT)
WITHIN 10S WHEN A VOLTAGE OF 40V IS APPLIED. (CHO, KIM, JUNG AND GOO (2005))
The temperature of the sample was measured using digital multi-meters (M-4660, DM-7241 and
ME-TEX) with a non-contact temperature measuring system. With 60V applied voltage the sample
heated above 35˚C in 8seconds although it was impossible to heat the sample to a temperature
above its transition with less than 40V. Composites with surface modified MWCNTs could show
electro-activated shape memory recovery with an energy efficiency of 10.4% with improved
mechanical properties.
2.4.2 SMP FILLED WITH CARBON BLACK
Rogers and Khan (2012) prepared an electrically conducted SMP through impregnating the resin
conductive carbon black using two dispersion techniques. All filled samples used in this study were
loaded to 10% mass by directly mixing the Carbon Black (CB) into the resin before processing. Higher
mixing percentages were not considered due to increasing difficulty of mixing CB into the resin.
Before curing, copper electrodes were placed in the slurry in order to allow for testing as shown by
the image below. Electrical conductivity tests revealed that high resistivity values of the 2.5% and 5%
systems prevented the attainment of the triggering temperature with the triggering temperature
being easily achieved at 10% content.
36. Design, Modelling and Testing of a synthetic muscle system
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FIGURE 17 CASTING MOLD AND MACHINED SAMPLE WITH IMBEDDED ELECTRODES. GLASS TAPE WAS USED AT
EACH END FOR SECURING IN TENSILE TESTING FRAME. (ROGERS AND KHAN (2012))
The stress-strain temperature curve resulting from a tensile test carried out at room temperature is
shown below. The addition of CB was found to decrease the ultimate tensile strength and
percentage elongation compared to the base resin. Loading curves also indicated little to no changes
in the structure of the polymer when repeated loading was applied. The graphs below also display
the decrease in strength with the temperature just above versus room temperature conditions.
a)
)
b)
)
37. Design, Modelling and Testing of a synthetic muscle system
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FIGURE 18 RESULTS FOR CARBON BLACK FILLED POLYMER (ROGERS AND KHAN (2012))
The use of a filler can adversely affect the properties of the base matrix depending upon factors such
a quality of dispersion, filler-chain interaction and filler surface coatings to mention a few. The figure
above also displays a comparison of the stress-strain behavior of filled and unfilled SMP in which an
increase in flow stress due to the addition of CB is noticeable. Hence a surfactant was also added to
a sample and a comparison of it is displayed below and effect was found to be minimal. There was
little to distinguish the mechanical properties of CB black and surfactant covered CB with higher
electrical resistivity being achieved by the latter.
Deformation based changes in conductivity were linked to reaggregation and/or transformation of
the aggregates. The increase in resistivity of the surfactant covered CB is actually a more
homogenous CB distribution resulting uniform interparticulate and aggregate spacing. The
conductive networks formed in the CB samples provided more efficient pathways for the current. At
small strains, the chain deformation mechanisms such as stretching results in degradation of the
existing pathways thereby increasing resistivity with increasing strain. However, as strain increases,
large segmental motion of the chains results in axial alignment or deformation induced
crystallization. More robust pathways are formed with these effect being more pronounced in
surfactant-covered CB samples where agglomerates are more finer and mobile. It should be noted
however, that in this study, only 30% strain was achievable due to fracture beyond this limit.
a)
b)
c)
)
d)
)
38. Design, Modelling and Testing of a synthetic muscle system
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FIGURE 19 (ROGERS AND KHAN (2012))
Lan, Leng, Liu and Du (2008) investigated a similar blend styrene-based resin and analyzed the
thermomechanical properties using differential scanning calorimetry (DSC). DSC results revealed that
decreases slightly with an increase in CB volume fraction, indicating a slight interaction between
the CB powders and SMP. Electrical resistivity tests revealed similar results to the previous case as
well as temperature vs resistivity results. The percolation threshold was found to be 3% which is
lower than many other polymer-based conductive composites.
FIGURE 20 DSC RESULTS OF CB AT VARIOUS COMPOSITIONS (LAN, LENG, LIU AND DU (2008))
Shape recovery was achieved by applying a voltage of 30V. It took a total of 90 seconds for full shape
memory recovery to take place as shown by the image below.
FIGURE 21 SEQUENCES OF SHAPE RECOVERY OF CB 10% BY PASSING AS ELECTRICAL CURRENT OF 30V
(ROGERS AND KHAN (2012))
39. Design, Modelling and Testing of a synthetic muscle system
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2.4.3 SMP FILLED WITH NICKEL
Leng, Huang, Lan, Liu and Du (2008) achieved significant reduction in the electrical resistivity of PU
filled with randomly distributed CB by adding a small amount of randomly distributed Ni
microparticles (0.5 vol. %). Ni chains, formed in a weak magnetic field before curing, served as
conductive channels to bridge CB aggregations so as to significantly reduce the electrical
conductivity. Other properties were reported to remain relatively the same.
FIGURE 22 MAGNETIC FIELD CURING (LENG, HUANG, LAN, LIU AND DU (2008))
Figure 23 displays the relationship of CB versus electrical resistivity of both SMP/CB/Ni chained and
randomly distributed. The resistivity was also measured one month later and is about the same as
before, which shows that the resistivity of the samples was stable. In order to demonstrate shape
recovery, 30V was applied through Joule heating to the samples, all at 10% CB, and samples were
heated to 80 C. Twenty shape recovery cycles at 20% deformation were also done in order to study
evolution of resistivity and it was discovered that the conductive paths in the Ni chain/CB may be
degraded upon thermomechanical cycling.
FIGURE 23 RESISTIVITY VS VOLUME FRACTION OF CB WITH/WITHOUT 0.5 VOL % NI. RED SYMBOL, RIGHT
AFTER FABRICATION; BLUE SYMBOL, ONE MONTH LATER. INSET DISPLAYS HOW RESISTANCE WAS MEASURED.
(LENG, HUANG, LAN, LIU AND DU (2008))
40. Design, Modelling and Testing of a synthetic muscle system
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FIGURE 24 EVOLUTION OF RESISTIVITY UPON SHAPE MEMORY CYCLING (LENG, HUANG, LAN, LIU AND DU
(2008))
Leng, Huang, Lan, Liu, Du, Phee and Yuan (2008) conducted further experiments on this same setup
using only Ni powder and upon determination of noticed that the it shifted a little bit toward the
low temperature range, which indicated the slight chemical interaction between Ni powders and
SMP. Around 10% volume fraction it was found that the composite was significantly strengthened
and the storage modulus was higher for chained samples.
FIGURE 25 STORAGE NODULUS VERSUS VOLUME FRACTION OF NI AT 0 DEGREES (LENG, HUANG, LAN, LIU,
DU, PHEE AND YUAN (2008))
41. Design, Modelling and Testing of a synthetic muscle system
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2.4.4 SMP FILLED WITH HYBRID FILLERS
Lu, Yu, Liu and Leng (2010) integrated hybrid fillers in the form of a carbon black and short carbon
fiber combination into a styrene-based SMP with sensing actuating capabilities. The results showed a
decrease in resistance with an increase in fiber fraction. Also the fibrous fillers enhanced the
mechanical properties of the SCF-SMP composites more significantly than the particulate fillers
FIGURE 26 LEFT: VALUES OF RESTISTANCE VS TEMPERATURE; RIGHT: VALUES OF RESTANCE VERSUS STRAIN
FOR SCF-SMP COMPOSITE (LU, YU, LIU AND LENG (2010))
The increase in conductivity was attributed to the numerous interconnections between the SCF
fibers and the CB/SMP composite. As figure 27 shows, the particles distributed uniformly into the
SMP matrix, aggregating as clusters instead of absolutely separating from each other. This way, the
CB fibers will act as nodes among the fibers, with local conductive pathways also formed in the
composite. This improves orientation of the short fibers because of the large amount of conductive
channels formed in the composite, which makes resistivity low and stable.
FIGURE 27 MORPHOLOGIES OF SCF-SMP COMPOSITE SPECIM OBSERVED BY SEM (2% SCF AND 5% CB) A)
MORPHOLOGIES OF SCF FILLERS AND B) MORPHOLOGIES OF CB PARTICLES (LU, YU, LIU AND LENG (2010))
Lu, Yu, Sun, Liu and Leng (2010) also investigated the mechanical properties of the same type of SMP
composite but in different compositions and the results are shown in figure 26. The approach
successfully improved the thermomechanical and conductive properties of SMP materials by the
addition of a hybrid filler into the matrix. The maximum fracture strains of the composites were
found to be dependent on the dispersion of the hybrid filler which could cause cracks propagating
42. Design, Modelling and Testing of a synthetic muscle system
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along the boundary of the matrix and filler. SMP recovery behavior was also achieved at 5% CB and
2% SCF as shown by figure 28.
FIGURE 28 STRESS-STRAIN CURVES OF COMPOSITES FILLED WITH VARIOUS SCF CONTENTS IN TENSILE MODE
(LU, YU, LIU AND LENG (2010))
FIGURE 29 IMAGES SHOWING THE MACROSCOPIC SHAPE MEMORY EFFECT OF 5% CB AND 2% SCF COMPOSITE
. THE PERMANENT SHAPE IS A FLAT STRIP AND THE TEMPORARY SHAPE A RIGHT ANGLE DEFORMATION. (LU,
YU, LIU AND LENG (2010))
43. Design, Modelling and Testing of a synthetic muscle system
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2.5. PREPARATION OF CONDUCTIVE SHAPE MEMORY POLYMERS
Deng, Lin, Ji, Zhang, Yang and Fu (2013) elaborate on how the preparation of conductive polymer
composites involves the selection of a suitable mixing method in order to incorporate the filler into
the polymer matrix. Conductive networks must be achieved in order to produce acceptable electrical
conductivity and there are generally three methods for preparation of conductive polymers:
1. Melt compounding
2. In situ polymerisation
3. Melt mixing
2.5.1. MELT COMPOUNDING
Melt compounding is referred to be advantageous by Deng, Lin, Ji, Zhang, Yang and Fu (2013)
because the filler can be directly dispersed into the matrix, no chemical modifications are required
and the fillers are prevented from re-aggregation by the viscous polymer matrix. Apart from this
method fitting seamlessly into industrial practices, a number of studies have displayed successful
application of melt compounding when dispersing conductive fillers into various polymer matrices.
These studies also reveal that processing conditions and filler conditions influence preparation of the
SMPs.
Huang, Ahir and Terentjev (2006) invested the melt compounding of polydimethylsiloxane (PDMS)
with Muilti0walled carbon nanotubes (MWCNTs) with the real part of the composite viscosity being
recorded during the mixture. Viscocity changes were measured as a function of the nanotube-
polymer mixing time and it was observed that every batch with the same concentration tended to
exhibit a similar dispersion level when mixed for a long enough time. Generally the higher the
concentration, the longer the critical time was required to achieve a relatively good dispersion. It
was observed however that in most studies, the same mixing time was used for composites with
different filler contents, which may be too short to achieve good dispersion in some cases.
Villmow, Kretzschmar and Potschke (2010) investigated Carbon nanotube (CNT) and polymer
composites, looking at the effect of different processing parameters on the final properties while
paying particular attention to electrical properties. Results showed that increases in the rotation
speed and the throughput decreased the residence time of the material. It was also observed that
the use of back-conveying elements as well as an extension of the processing lengths produced
opposite results to those just stated. Apart from this, the design of the screw profiles can further
increase filler dispersion.
Another valid factor is the interaction between the filler and the polymer matrix which is crucial
when related to the filler dispersion during melt compounding. Therefore, the chemical polarities of
the polymer matrix and the filler significantly influence the final quality of filler dispersion. An
example is given with the study conducted by Deng, Zhang, Bilotti, Loos and Peijs (2009) which
indicated large aggregates of conductive filler in polypropylene (PP) when melt compounding was
used as the dispersion method. This can be explained by the non-polar nature of the PP polymer
chain. Carbon Nanotubes (CNTs) can be also be easily dispersed in a polyamide 6 (PA6) matrix as a
result of the strong interaction between the PA6 polymer chains and the CNTs as investigated by
Gorrasi, Bredeau, Di Candia, Patimo, De Pasquale and Dubois (2011). These authors also refer to the
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use of a surfactant to improve the interaction between the filler and the matrix, and, thus, the filler
dispersion in a non-polar matrix .
In general, melt compounding is an effective and efficient method to add a conductive filler to a
polymer matrix however particular attention should be paid to the critical mixing time and the shear
stress inside the mixer. High shear stresses are not recommended as they reduce the filler aspect
ratio during processing.
2.5.2. IN-SITU POLYMERISATION
This being another method of conductive filler dispersion in a polymer matrix, Deng, Lin, Ji, Zhang,
Yang and Fu (2013) mention its advantage is that the polymer chain and fillers can be dispersed and
grafted on the molecular scale. Excellent filler dispersion is given by this method and a potentially
good interfacial strength between the filler and the polymer matrix. Successful investigations have
been reported which will be discussed later and a uniform dispersion of the filler was obtained and
improved both electrical and mechanical properties.
Recently, Liu, Chen, Chen, Wu, Zhang, Chen and Fu (2011) used this method to fabricate conductive
polymer composites (CPCs) containing grapheme, using a relatively high temperature during
polymerisation in order to reduce graphene oxide into graphene in the polymer matrix. This enabled
CPCs to be obtained at the end of the process without further processing. However it should be
noted that this method of in-situ polymerisation is difficult to adapt to the preparation of CPCs in
industry.
Deng, Lin, Ji, Zhang, Yang and Fu (2013) state the importance of in-situ polymerisation as an
essential method for the preparation of thermoset and rubber-based polymers. An example is epoxy
which has been investigated as a polymer matrix for a range of conductive polymers. A better
control of the conductive network structure and electrical properties can be achieved depending on
the special preparation method used. Defining a network before polymerisation is achieved through
a variety of methods including using a vacuum bag or fibre lay-up methods.
2.5.3. SOLUTION MIXING
It is difficult to achieve local homogenous dispersion states without breaking down the entangled
fillers using physical techniques such as those previously discussed, hence other methods such as
solution mixing need to be considered. With regards to the organic solvent mixing method, a
homogenous dispersion can be achieved throughout the solvent and therefore the host matrix.
Solution mixing is generally adding the filler directly into the polymer and this has been described in
the form of section 2.4.2-2.4.4.
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2.6 MORPHOLOGICAL CONTROL OF CONDUCTIVE NETWORKS IN SHAPE
MEMORY POLYMERS
2.6.1 CHARACTERISATION OF CONDUCTIVE NETWORK FORMATION
Deng, Lin, Ji, Zhang, Yang and Fu (2013) emphasize the important influence of the morphology of
conductive networks on the electrical properties of shape memory polymers and how it is crucial to
characterize the morphological details of these networks. A range of microscopic methods can be
used for direct observation of conductive networks in nanocomposites as follows
a) Optical microscopy
b) Scanning electron microscopy
c) Transmission electron microscopy
d) Scanning probe microscopy and
e) Atomic force microscopy.
FIGURE 30 MORPHOLOGY CHARACTERISED WITH DIFFERENT METHODS. OM AND TEM FOR PC CONTAINING
0.688% VOL CNTS. SEM IN CHARGE CONTRAST MODE SHOWS THE DISTRIBUTION OF MWNTS IN POLYPYRROLE
MATRIX AND HAADF-STEM PICTURES SHOW INDIVIDUAL CARBON BLACK PARTICLES AND THEIR CLUSTERS IN
POLYMER COMPOSITES: OM AND TEM (DENG, LIN, JI, ZHANG, YANG AND FU (2013))
These methods are mentioned to have been widely used as general microscopic methods to
characterize the morphology of polymer composites from various aspects or from different scales.
Optical microscopy (OM) is often used to study the morphology of a few microns or above and for
46. Design, Modelling and Testing of a synthetic muscle system
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anything below this range, all other methods except for scanning probe microscopy are applicable. It
is known that only information near the surface can be captured using conventional scanning
electron microscopy (SEM) due to secondary electrons having only a relatively shallow escape depth
(5-50mm) due to their rather low energy levels. This was reported by Li, Buschhorn, Schulte and
Bauhofer (2011). Although it was later reported that SEM observation of deeply embedded carbon
nanotubes (CNTs) and overall analysis of the CNT dispersion status were possible using voltage
contrast imaging in CNT/polymer based composites. This contrast mechanism was first reported by
Chung, Reisner and Campbell (1983) and has been used by various research groups since then.
Tkalya, Ghislandi, Alekseev, Koning and Loos (2010) utilised conventional SEM in the charge contrast
imaging mode to investigate the morphology of networks of graphene sheets embedded in
polystyrene matrices. They reported that the charge contrast imaging of conductive networks under
high acceleration voltages could provide three-dimensional information on the structure of the
conductive networks.
SEM despite offering valuable information on the morphologies of nanofillers and their conductive
networks, the actual nanofiller size and detailed information on the conductive network are not very
accurate due to local charging of the polymer matrix around the nanofillers. High-angle annular dark
field scanning transmission electron microscopy (HAADF-STEM) has been investigated successfully as
a tool to obtain reliable quantification of images to enhance the characterization of the conductive
network morphology as investigated by Loos, Sourty, Lu, de With and Bavel (2009). When it comes
to polymer materials, STEM is mentioned to possess several advantages over conventional TEM as
follows.
1. Images are easy to interpret due to a lack of phase contrast
2. Signal intensity is linear with thickness variations
3. A high signal to noise ratio is obtained.
These advantages are more pronounced with use of a high-angle annular dark field (HAADF)
detector capable of single-electron counting. Generally, it is believed that HAADF-STEM can be used
as a powerful tool for obtaining high-resolution images of unstrained polymer systems.
2.6.2 MORPHOLOGICAL CONTROL THROUGH POLYMER BLENDS
By constructing a polymer blend with two or more polymers, the advantages of each polymer can be
integrated and thus, balanced and optimized for the properties in the final material. The phase
morphology of the blends also plays a crucial role in the final properties and hence, polymer blends
of various designs and properties can be fabricated by controlling their morphology.
Meng and Hu (2009) mention how Jeong and Song (2001) developed thermoplastic SMPU blended
with poly(vinyl chloride) (PVC) to vary the switch temperature and improve the mechanical strength
of SMPU. The PVC is also miscible with the soft segment of the SMPU thus the switch temperature of
the blends can be varied smoothly with different component compositions. Zhang, Chen and Zhang
(2009) toughened polylactide using a polyamide elastomer from polyamide-12 and
polytetramethyleneoxide. Both polylactide and the polyamide elastomer are bio-degradable and the
mechanical properties of the polylactide were reportedly improved. Some examples of the
numerous design strategies emplored in SMP design are shown below.
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FIGURE 31 SOME DESIGN STRATEGIES FOR SMPS (MENG AND HU (2009))
2.6.3 INFLUENCE OF FILLER CHEMISTRY ON GLASS TRANSITION TEMPERATURE
Lan, Leng, Liu and Du (2008) during their investigation of the conductivity of SMP filled with CB,
conducted DSC tests on different compositions and it is shown below how the reduces with an
increase in filler content. Despite the fact this was not the primary reason for their study it still gives
adequate insight into the presence of a chemical interaction between the CB and SMP.
FIGURE 32 AGAINST CB CONTENT (LAN, LENG, LIU AND DU (2008))
