review of chemo‐responsive shape change/memory polymers

Review of chemo-responsive shapechange/memory polymers

H.B. Lu

Science and Technology on Advanced Composites in Special Environments Laboratory,Harbin Institute of Technology, Harbin City, China

W.M. HuangSchool of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, and

Y.T. YaoScience and Technology on Advanced Composites in Special Environments Laboratory,

Harbin Institute of Technology, Harbin City, China

AbstractPurpose – The purpose of this paper is to examine the underlying mechanism and physico-chemical requirements of chemo-responsive shape change/memory polymers and to explore the future trend of development and potential applications.Design/methodology/approach – Working mechanism in chemo-responsive shape change/memory polymers is firstly identified. And then thephysico-chemical requirements for the representative polymers are characterized.Findings – The different working mechanisms, fundamentals, physico-chemical requirements and theoretical origins have been discussed. Currentresearch and development on the fabrication strategies of chemo-responsive shape change/memory polymers have been summarised. The future trendand potential applications have been explored and estimated.Research limitations/implications – This review examines physico-chemical requirements and theoretical origins necessary to achieve chemo-responsiveness, and then discusses recent developments and future trends.Practical implications – Shape change/memory polymers can be used in the broad field of bio- and/or medicine.Originality/value – Breakthroughs and rapid development of chemo-responsive shape change/memory polymers will significantly improve theresearch and development of smart materials, structures and systems.

Keywords Chemo-responsive, Shape change polymer, Shape memory polymer, Responsive polymer, Polymers, Chemistry

Paper type General review

Introduction

The rapidly developing field of smart materials has provided a

strong impetus for the development of stimuli responsive

materials that can be designed from a wide range of functional

molecular building blocks (Yerushalmi et al., 2005). Stimuli

responsive materials utilise the functionality of individual

molecular units and the macroscopic properties in many fields,

such as nanotechnology, chemical physics, physical chemistry,

materials science, etc. (Roy et al., 2010). Interest in stimuli-

responsive polymers has persisted over many decades, and a

great deal of work has been dedicated to developing adaptive

structures, self-healing materials, sensing detector, actuator, as

well as liquid crystals that are activated by electric current.

These materials are often designed so that the conformational

changes of the individual subunits are additive, and thus

produce a measurable coherent mechanical response to an

external stimulus. Such stimuli include heat (thermo-

responsive materials), stress/pressure (mechano-responsive

materials), electrical current/voltage (electro-responsive

materials), magnetic field (magneto-responsive materials),

pH-change/solvent/water/moisture (chemo-responsive

materials), light (photo-responsive materials) and ultrasound-

responsive materials (Sun et al., 2012; Li et al., 2012).Stimuli responsive polymers have become a very hot topic in

recent years due to a wide range of potential applications, from

functional nanocomposites to controlled/targeted drug/gene

delivery. Among them, one group of stimuli responsive

polymers is able to change their shape at the presence of the

right stimulus. If the shape change is spontaneous and instant in

the presence of the right stimulus, this is the shape change

material (SCM). Where elastic rubber, electro-active polymer,

piezoelectric material and liquid crystal are four typical

examples of SCM. Liquid crystals exhibit responsivity to

electric fields thatmake them very important in display devices.

Some liquid crystals (nematic liquid crystal) do have the ability

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Pigment & Resin Technology

42/4 (2013) 237–246

q Emerald Group Publishing Limited [ISSN 0369-9420]

[DOI 10.1108/PRT-11-2012-0079]

This work has been financially supported by the “Fundamental ResearchFunds for the Central University (Grant No. HIT. NSRIF. 201157)”, theChina Postdoctoral Science Foundation funded major project (GrantNo. 20110490104), special funding of China Postdoctoral ScienceFoundation (Grant No. 2012T50350) and research fund of NationalKey Laboratory of Science and Technology on Advanced Composites inSpecial Environments, China (Grant No. 01507342).

237

for shape change at the presence of a particular stimulusby means of arranging the long axes of the moleculesin parallel lines. Another type of stimuli-responsive materialshaving the ability of actively changing the shape can bedifferentiated into shape memory materials (SMM)(Behl et al., 2010). Taking shape change polymer (SCP) andshape memory polymer (SMP) for example, in both cases thebasic molecular architecture is a polymer network but themechanisms underlying the active movement are differing.Both polymer concepts are based on functional componentsor stimuli-sensitive domains as switches (Behl and Lendlein,2007; Lu et al., 2011; Kumpfer and Rowan, 2011). It should benoted that the SCP can present a shape memory behaviour andvice versa. That is to say a polymer can both be worked as eithera SCPor a SMP,whilst the variety is determined by the externalenvironment and situation. The movement starts upontriggering those switches by applying a suitable stimulus.SCM and SMM differ in the degree of freedom definingthe geometry of the movement as well as the reversibility ofthe movement. A SCM changes their shape gradually,e.g. shrinkage, bend or stretch, as long as a suitable stimulusis applied. They recover their original shape as the stimulus isterminated. The shape change effect (SCE) can be repeatedseveral times. On the other hand, SMMs can be deformed byapplication of external stress and fixed in a temporary shape.This temporary shape is retained until the shaped body isexposed to an appropriate stimulus, which induces the recoveryof the original shape. Themovement occurring during recoveryis predefined as it reverses the mechanical deformation, whichled to the temporary shape. Figure 1 shows the difference inSCM and SMM from the laws of thermodynamics.Fundamentally from energy point of view, the differencebetween SCM and SMM is due to the storage capabilityof energy. Then change in internal energy of materials can be

used to account for the feature of SMM. In contrary to SMM,there is no change in potential in SCM during deformationprocess. That is to say, the external energy has been transferredinto the change in entropy of SCM.The ability of materials to respond to external chemical

stimuli is of high scientific and technological significance due tothat they can change shape on demand. Chemo-responsivematerials can adapt to surrounding environments, regulatetransport of ions and molecules, change chemical or physicalproperties on external stimuli, or convert chemical signals intooptical, electrical, thermal andmechanical signals (Stuart et al.,2010; Wu et al., 2011). These materials are playing anincreasingly important part in a diverse range of applications,such as drug delivery, diagnostics, tissue engineering,therapeutics and regenerative medicine, as well as biosensors,microelectromechanical systems, coatings and textiles (Yao andTam, 2011; Chen et al., 2010; Nandivada et al., 2010).Although chemo-responsive materials have a number ofadvantages and their potential applications are expected to belarge. Their complex types of mechanisms challenge materialdesign, guided largely by chemo-responsive materials ofactuation. The complex molecular mechanism and designprinciples of chemo-responsive materials, as well as the natureof their responses, is incompletely understood and modelled,and much of the possible responses are not even mapped. Ingeneral, the chemo-responsive SME in a SMP is essentially thesame as that of the thermo-responsive SME. The key issue is tosoften (reducing the transition temperature) or even remove(dissolving) the transition components.Themechanismbehindthis is the decrease in transition temperature and the energystoring capability in the polymer network. A good first steptowards quantitatively understanding the chemo-responsiveSME in SMPs is the prediction of (phase) transitiontemperature lines in the state diagram (Zhang et al., 2000;

Figure 1 Thermodynamic difference between SCM and SMM, and the thermodynamic difference between SMP and SMA, where W denotes externalwork, F is Helmholtz free energy, U is the internal energy, T is the absolute thermodynamic temperature, S is entropy and H is enthalpy

Review of chemo-responsive shape change/memory polymers

H.B. Lu, W.M. Huang and Y.T. Yao


Volume 42 · Number 4 · 2013 · 237–246

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Chen et al., 2005; Lu et al., 2010a, b; Lu andLeng, 2013).With

regarding of the above-mentioned points, this work will

emphasize on both experimental and theoretical approaches

to connect microscopic building-block interactions and desiredchemo-responsive performances. Fundamental mechanisms

andmodelling of SCM and SMMwill be discussed, and variety

of actuation approaches will be mapped.

Chemo-responsive SCP

Fundamentally from energy point of view, the difference

between SCE and shape memory effect (SME) is due to themagnitude of energy barrier between two states. The energy

barrier in SCM is almost zero in the recovery process. If the

shape recovery is under the non-isothermal process, the high

enough energy barrier is originated in the materials. Theenergy barrier that is equal to the change in internal energy

originated from the temperature difference, can be stored by

cooling the materials down to a relative low temperature.Chemo-responsive SCMs change their shape as long as they

are exposed to the right chemical stimulus. SCE in the SCMs

renders their shape recovery can be self-achieved as soon as the

applied stimulus is terminated. The geometry, how such aworkpiece ismoving, is determined by its the original.While the

geometry of the shape change cannot be varied for SCM, the

process of stimulated deformation with subsequent recovery isnormally repeatable. For SCP, they all require an elastic

deformable network, in which the cross-links (or netpoints)

determine the permanent shape. SCP consists of long, coiled-

up polymer chains that are interlinked at netpoints. Between apair of netpoints, each monomer can rotate freely, thus giving

each section of chain leeway to assume a large number of

geometries. This polymer network has to be sufficiently flexibleto enable elastic deformation of the chain segments and store

enough kinetic energy so that each section of chain oscillates

chaotically. Covalent bonds as well as physical cross-links could

be used as netpoints.While covalent cross-linking was obtainedby polymerisation or polyaddition of monomers or oligomers

having twoormore reactive groups, physical cross-linking could

be achieved by physical interactions originating from hydrogen

bonds, crystalline regions, ionic clusters or phase-separatedmicrodomains (Behl et al., 2010).

Chemo-responsive shape change gel

Although all SCPs were capable of changing shape, such asstretch, bend or substantial shrinkage. There are two categories

of shape change gel (SCG) and SCP based on different

mechanisms. SCGs are intelligent gels, which are polymernetworks swollen by a large amount of solvating liquid. Here

stimuli-sensitivity was either used to control cross-link density

or to induce demixing processes. Another approach for shape

change was realised by the transfer of geometric changes fromthe molecular to the macroscopic level. Stimuli sensitivity

towards could be reached if the polymer networks were

modified with stimuli-sensitive switches, which were able to

control the swelling or deswelling of the polymer networkduring actuation. Two mechanisms could be differentiated,

where one mechanism aimed at influencing the number of

additional cross-links as the degree of swelling depends on thecross-link density. For an example, sensitivity towards pH was

realisedwhen ionic groups in themain or side chain, whichwere

counterbalanced with oppositely charged ions, were

incorporated into the polymer network. Here, the aggregation

of the ionic units at the nanometre scale provided the additional

cross-link. The degree of cross-linking could be controlled byadjusting pH value. On the other hand, another mechanism for

obtaining stimuli-sensitivity in SCG is the stimuli induced

change in the miscibility of polymeric segments and solventmolecules. Most SCGs described so far were thermally-

triggered, but changes in volume of hydrogels can also betriggeredby a variation in thepHvalue, the ionic strength, or the

quality of the swelling agent. Furthermore, certain SCG could

be stimulated by the application of biochemical substances,e.g. glucose, glutathione, antigens (Gil and Hudson, 2004;

Lendlein and Kelch, 2002). In hydrogels synthesised fromacrylic acid and acryloyl derivatives of aliphatic v-amino acids

an SCG, which was triggered by certain transition metal ions,

was shown in Figure 2 (Varghese et al., 2001). A cylindrical gelsample could be transformed reversibly into a hollow spherical

or ellipsoidal shape depending on the presence ofmetal ions like

Cu2þ , Pb2þ , Cd2þ , Zn2þ , or Fe3þ . These hydrogels, whenfeaturing a critical balance of hydrophilic and hydrophobic

groups, formed complexes with present metal ions leading to adecreased hydrophilicity and self-organisation of the metal

coordinating sites. The extent of this effect depended on the ion

concentration andpropagated from the surface to the core of thegel resulting in a hollow interior. Since the process is diffusion-

controlled, the time required for the shape change depended on

the initial size of the gel. Themetal ionswerewashed out byHClsolution whereas the initial shape of the gel was restored so that

the concentration of the metal ions acted as stimulus for theSCG.Another alternative strategy, chemo-responsive SCGs are

composed of disulphide cross-links that can be degraded byreduction and restored by subsequent oxidation imparted

shape-memory properties to the gels rendering them responsiveto electron transferring compounds (reducing agents/oxidants).

The bead-shaped microgel particles were cross-linked by

using ethylene glycol dimethacrylate and N,N-bis(acryloyl)cystamine during polymerisation, the latter containing a

disulphide bond. Placing the equilibrium-swollen gels in a

solution of dithiothreitol or tris(2-carboxyethyl)phosphineresulted in significant additional swelling. However,

subsequent reoxidation of the thiol groups by sodiumhypochlorite led to the reestablishment of the disulphide

bridges whereas the gels shrank to their equilibrium-swollen

state. In this way the initial shape was retained.

Chemo-responsive SCP

As one of the most promising SCP, ionic polymer-metal

composites (IPMCs) attract so many attentions from scientific

and technological points of view. Generally, EAPs fall into twomajor categories based on their primary activation mechanisms

into electronic EAPs (driven by electric field andcoulomb forces) and ionic EAPs (driven by the movement of

ions) (Bar-Cohen, 2007; Bar-Cohen and Zhang, 2008). Ionic

EAPs include IPMCs and ionic polymeric gels. Since the ionicpolymer gels have been discussed above, here the ionic EAPs are

chemo-responsive and will be discussed in this section.Electroactive IPMC is made from an ion exchange

polymer, which is chemically surface composited with

conductive medium such as platinum or gold a few micronsdeep within the polymer (Shahinpoor and Kim, 2001, 2002).

Perfluorinated alkenes and styrene-divinylbenzene copolymers




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are the typicallyused ionexchangepolymerswhichcanexchange

ions of an environmental charge with their own ions. Styrene-

divinylbenzene copolymers are highly cross-linked and rigid,

which severely limit their applications as EAPs. Perfluorinated

alkenes have large polymer backbones and side chains. The side

chains are terminated by sulphonate or carboxylate (for cation

exchange) or ammonium cations (for anion exchange) to

interact with a solvent to produce the electro-active effect. As

shown in Figure 3, in the case of cation exchange polymer, the

IPMC bends toward the anode if the ionomeric polymer-metal

strip is subject to DC voltage bias across the strip followed by a

slow relaxation in the opposite direction (Lavu et al., 2005; Lee

et al., 2004). The bending force of IPMCs is from redistribution

of hydrated ions and water.

Chemo-responsive SMM

Work mechanism

SMMs are fascinating materials, which can change their

shape in a predefined manner between/among shapes at the

presence of the right stimulus (Lendlein et al., 2005). Shape

memory is a process that enables the reversible storage and

recovery of mechanical energy through a cyclical change in

shape (Lendlein and Langer, 2002). In a SMP, there are at

least two segments (or domains), one is elastic component,

while the other (transition component) is able to significantly

alter its stiffness, depending on whether the right stimulus is

presented. This dual-component system is essential to enable

the SME in a polymer (Bellin et al., 2006). As a matter of fact

in stimulation method, SMPs have one or more properties

that can be significantly changed in a controllable fashion

upon applying the right external stimuli, such as heat

(thermo-responsive), electricity (electro-activated), light (photo-

responsive), chemical (chemo-responsive), etc. (Leng et al.,2009).This also includes a combination of two ormore responses

at the same time. Furthermore, chemo-responsive capability,

excellent chemical stability, biocompatibility and even

biodegradability (in which the degradation rate may be adjusted

if required) render the SMPs as the right candidate for many bio-

medical applications (Mather et al., 2009).In general, the programming procedure to set up the

temporary shape of a SMP for the chemo-responsive SME is

essentially the same as that for the thermo-responsive SME.

However, instead of heating to above the transition temperature

to activate shape recovery, in a more general sense, the approach

to trigger the chemo-responsive SME is to soften/dissolve the

transition component and thus lower the transition temperature

by means of soften, swelling or dissolving.

Soften induced SME

Polyurethane-based SMPhas been found that the actuation can

be achieved by the water or moisture in the experiment, and

hydrogen bonding was identified as the reason behind this

feature (Huang et al., 2005). In subsequence, the solution

theory of polymer physics had been employed for the effect

of water plasticiser on the transition temperature of SMPs.

And then the Fujita’s diffusion theory was introduced to

qualitatively separate effect of free water and bound water on

theSMP. Itwas summarised that thewater-drivenpolyurethane

SMP was due to the plasticising effect of the solvent

molecule on the polymeric materials through increasing

the flexibility of macromolecule chains, and resulting in

the transition temperature of polymer inductively lowered

Figure 2 (a) Morphological change of the gel in metal salt solutions: (i) Solid cylindrical gel, (ii) Ellipsoidal Cu(II) complexed gel when the L/D ratio ofthe starting gel piece was greater than one, (iii) Spherical Cu(II) complexed gel when the L/D ratio of the starting gel piece was one, (iv) Hollow interiorof the Pb(II) complexed gel; (b) development of the hollow interior of the copper complexed gel over a period of time: (i) 12 hours, (ii) 24 hours,(iii) 72 hours, and (iv) 96 hours

(i) (ii)

1 mm 2 mm

(i) (ii)

2.5 mm 4.5 mm

(iii) (iv)

7 mm4 mm

(iii) (iv)

2 mm6 mm

(a) (b)

Source: Reproduced from Varghese et al. (2001) with permission




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(Leng et al., 2008). Taking for an example, two pieces of

polyurethane SMP wires (indicated as original) are immersed

into 228C water and methanol, respectively, as shown in

Figure 4. The wire (MM3520) immersed in water has a

nominal Tg of 358C, while the other (MM5520) immersed in

methanol has a nominal Tg of 558C, which is 208C higher.

The one immersed into the water (Figure 4(a)), although its

Tg is lower than the other piece, takes about 40 minutes

to straighten; while the other immersed into room

temperature methanol (Figure 4(b)) reacts much faster

(Huang et al., 2010).According to the previous studies (Yang et al., 2005, 2006),

upon immersing into room temperature water, the Tg’s of both

polyurethanes can be gradually lowed due to the plasticising

effect of the absorbed (bound) water on the hydrogen bonding.

Experimental result reveals similar shift in C ¼ O stretching

and N-H stretching in polyurethane (MM5520) after being

immersed in ethanol and methanol. Furthermore, with

immersion time increased in water, Tg reduces further.

Upon gradually heating, the SMP becomes lighter but there is

not much change in Tg at all, until it is heated above a certain

temperature. From then on, with further increase in temperature,

the weight of SMP drops while Tg increases continuously.

Future experimental result reveals that the free water does notaffect Tg, and can be removed upon heating to below 1208C,boundwater has strong influence onTg and can only be removed

uponheating to over 1208C.The latter indicates that after heatingto fully remove the absorbed water, the SMP can return to itsoriginal state/condition.Based on this motivation, some interactive solvents (e.g. N,

N-dimethylformamide (DMF), methanol, etc.) have beenutilised to trigger the chemo-responsive SMP (Lu et al.,2008), in which the shape recovery is driven by the diffusionof the solvent molecules into the polymer network. Theabsorbed solvent molecules work as plasticiser to depress theinteraction force among macromolecules, and then to increase

the flexibility of macromolecule chains, and hence, inductivelydepress the cohesive energy and lower the transitiontemperature of the polymer network (Lu et al., 2009). Aswell known, when a polymer is brought into contact with an

interactive solvent, either chemically plasticising effect orphysical swelling effect will occur between the polymernetwork and the imbibed solvent molecules, originated from

the polymer physics. During this process, a remarkablechange in the transition temperature and/or other propertiesin the SMP is expected. The depression in the transitiontemperature and capability of storing energy results in the

shape recovery.

Swelling induced SCE/SME

Previously, the chemo-responsive SME in polystyrene (PS)

SMP has been achieved by means of immersing into DMFthrough chemical conjunct interaction. When a polymer isbrought into contact with a solvent, the polymer network mayimbibe solvent molecules progressively and swells, resulting in

aggregation of solvent molecules, known as gel. A gel is ableto undergo a large deformation due to long-range migration ofthe solvent molecules, resulting in a significant change in

volume. Simultaneously, a significant change in the transitiontemperature in the polymer is expected. The change in the

Figure 4 Chemo-responsive SME in polyurethane SMP

Note: Top: in 22°C water; bottom: in 22°C methanolSource: Reproduced from Huang et al. (2010) with permission

Figure 3 A Schematic diagram of the typical IPMC and its actuationprinciple

Notes: Typically, the strip of the perfluorinated ionic polymermembrane bends towards the anode under the influence of anelectric potential; this electrophoresis-like internal ion-watermovement is responsible for creating effective strains foractuation




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transition temperature affects the relative motion in thetransition segment, which is the primary mechanism for theSME in SMPs (Lu, 2012a, b).Toluene has been used to trigger chemo-responsive SME in

PS SMP by means of physical swelling effect. As shown inFigure 5(a), a piece of straight SMP strip (original shape) is bentinto a “N”-like shape at 858C (Tg þ 208C), and then cool backto 458C (Tg – 208C). No change in shape recovery is observedafter being kept in the air for more than 6 hours. Afterimmersing into toluene at 458C for 40 minutes, the samplestarts to return to its original shape. After another 90 minutes,the sample becomes straight again. Swelling is revealed inFigure 5(b), which is apparent but not very significant(Lu, 2012a).Dynamic mechanical analysis (DMA) and FTIR tests have

been, respectively, conducted to characterise the thermo-mechanical properties and chemical structures of the SMPafter immersing for a period of time. DMA results reveal alower Tg after immersion. A decrease from 55.778C to35.828C is observed after 60 minutes of immersion.Meanwhile, the modulus gradually depresses. On the otherhand, the result of FTIR spectroscopy indicates no significantchemical interaction between the polymer and solvent, sincethe characteristic peaks of O-H and C ¼ O bonds virtually donot shift. We may conclude that there is not much remarkablechemical and polar interaction between the polymer and

solvent molecules. Note that the solvent molecules contain no

polar group that can interact with the characteristic polar

group in the SMP molecules. It is clear that shape recovery in

this styrene-based SMP upon immersing into the toluene

solvent is the result of physical swelling effect.Apart from experimental verification, the rubber elastic

theory on the Young’s modulus also has been employed to

theoretically investigate the physical swelling effect on the

actuation of SMP. According to the rubber elastic theory,

the force per unit area and the concentration of chain segment in

the polymer network are reduced, when the polymer network

swells in an interactive solvent. These changes result in the

reduction in elastic modulus. On the other hand, the volume of

polymer increases. The elastic modulus of polymer network

decreases gradually with an increase in the expansion

(i.e. stretch) in a bulky piece of SMP. In the case of a cubic

shaped SMP, the effect of stretch on the decrease in the Young’s

modulus is revealed in Figure 6 (Lu et al., 2010a, b).Furthermore, free-energy function is applicable to study

swelling induced deformation and shape recovery in SMPs.

When a piece of polymer (which is a non-ionic solvent-

swollen polymer) is subject to a solvent and then stressed, the

free-energy in the polymer/solvent system (W) is a function of

the stretching free-energy (Ws) and mixing free-energy (Wm),

i.e. (Flory, 1942; Hong et al., 2009):

Figure 5 (a) Shape recovery in a styrene-based SMP (induced by swelling); (b) change in dimensions before and after swollen

(a)

(b)

Source: Reproduced from Lu (2012a) with permission




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242

W ¼ Ws þWm

Ws ¼ 12NkT l21 þ l22 þ l23 2 32 2 log l1l2l3

� �

Wm ¼ 2 kTv

nC log 1þ 1nC

� �þ x

ð1þnCÞ

h i

W ¼ 12NkTðl21 þ l22 þ l23 2 32 2 log l1l2l3Þ

2 kTv

nC log 1þ 1nC

� �þ x

ð1þnCÞ

h i

8>>>>>>>>>>>><>>>>>>>>>>>>:

ð1Þ

where N is the number of chains in the polymer divided by thevolume of the polymer block in the referenced state. k is theBoltzmann’s constant. T is the temperature in the unitof energy. l1, l2 and l3 are the generalised coordinates of acubic block in three directions.n is the volume of solvent. C isthe number of the solvent molecules in the polymericnetwork. x is the Flory-Huggins interaction parameter.Considering a cubic piece of solvent-swollen polymer

subjected to stretching in three directions to the sameamount, i.e. l1 þ l2 þ l3 ¼ l, 1 þ nC may be calculated bysubstituting l into equation (1), namely 1 þ nC ¼ l3. Hence:

W ¼ 3

2NkT ðl2 2 12 2 log lÞ

2kT

vðl3 2 1Þlog l3

l3 2 1

� �þ x

l3

� � ð2Þ

Here, the change in chemical potential of polymer up may beexpressed as:

mp ¼›W

›ð2CÞ ¼ 2NkTv1

l2

1

l3

� �

2 kT logl3 2 1

l3þ 1

l3þ x

l6

� � ð3Þ

Wemay normalise the chemical potential by kT. The value of nfor a representative volume per solventmolecule is 10-28m3. Inthe Flory-Rehner free-energy function, two dimensionlessmaterial parameters, Nn and x are included. Nn ranges from1024 to 1021 (Hong et al., 2008).Here, we assumeNn ¼ 1023.x is a measure of enthalpy change in the mixture, and rangesfrom 0 to 1.2. In an application, which prefers a solvent-swollen

polymerwith a large swelling ratio, amixture with proper values

of Nn and x are used. The chemical potential as a function of

swelling stretch with some typical x values is shown in Figure 7.This simulation result proves that the chemical potential of

the SMP depressed to a lower value in the mixing process.

With an increase in stretch, the potential chemical of SMP

bulk firstly decreases to a minimum value, and then gradually

increases. On the other hand, withx increases from 0 to 1.2,

the chemical potential of SMP decreases to a further lower

value. This result implies that the chemical potential of SMP

will be further decreased when interaction between polymer

and solvent of x becomes larger.

Dissolving induced SME

A polyurethane block copolymer synthesised from polyhedral

oligomeric silsesquioxane molecules and poly(ethylene glycol)

(PEG) is able to achieve shape recovery upon immersing into

water, resulting from dissolution of PEG in water (Xu and

Song, 2010). Such chemo-responsive SME is originated from

Figure 6 (a) Inhomogeneous deformation in two stretches l1 ¼ l2 in a cubic SMP and (b) inhomogeneous deformation in a cubic SMP with stretchesl1 þ l2 þ l3 induced by physical swelling effect in solvent

(a) (b)

Source: Reproduced from Lu et al. (2010a, b) with permission

Figure 7 Chemical potential of a solvent-swollen polymer as a functionof l with some representative x-values

Source: Reproduced from Lu (2012a) with permission




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dissolving the transition component in the SMP, resulting in

the elastic energy stored in elastic component freely released

during programming. It is well known that sodium acetate

trihydrate is highly dissolvable in water and has a melting

temperature at about 608C. As shown in Figure 8, a ring

made of silicone and sodium acetate trihydrate, is

programmed at above 608C into a star shape (Fan et al.,

2011). After being immersed into room temperature water,

the ring-shaped SMP gradually returns its original circular

shape. The actual recovery speed can be controlled by varying

the volume fraction of sodium acetate trihydrate. Different

from that in Figure 8, in Figure 9 the transition component is

cupric sulphate pentahydrate, which in the programming

stage, grows inside a piece of pre-distorted elastic sponge.

Upon immersing into room temperature, cupric sulphate

pentahydrate quickly dissolves and thus, the hole is largely

closed within 6 minutes.Dissolving induced SME may be considered as a kind of

extension of the swelling induced SME, in which the volume

expansion of the transition component is infinite. Although

there are a few research outcomes, this approach is expected

to be very promising for the application of drug delivery.

Conclusions and future perspective

This review examines the different working mechanisms,

fundamentals, physico-chemical requirements and theoretical

origins of typical SCMandSMMwhich can change their shapes

in response to the chemicals. Even though research on chemo-

responsive materials has been rapidly progress and several

fabrication strategies of typical chemo-responsive materials

have been achieved in the past several decades, the research inthis area is far frommaturity. Chemo-responsive SCPand SMP

are starting to have a considerable impact on the design of

materials in many potential applications. Nevertheless, the

current trend is, among others, to search for applicable

approach to enable these chemo-responsive materials in bio-and/or medical fields, and explore their potential in many other

fields. The future trends are summarised as follows:. Researches on complicated movements with controllable

strains, responsive speed, and exact deformation stresswith multiple steps are more preferred.

. Multiple chemo-responsive materials obtained by

combining the different stimulus-active structures into

one.. Theoretical simulations and modelling are needed to provide

power tools and explore complex mechanisms behind

experimental features.. Specific application requirements on themultiple properties

of chemo-responsive SCMandSMMare eager to extend for

better application.

Finally, the successful development will require increased

multidisciplinary partnership among chemists, biochemists,

physicists and engineers.

Figure 8 Shape recovery in a silicone ring filled with 50 vol.% of sodium acetate trihydrate upon immersing into room temperature water

Source: Reproduced from Fan et al. (2011) with permission

Figure 9 Shape recovery (hole closure) of sponge/cupric sulphate pentahydrate hybrid upon immersing into room temperature water

Source: Modified and Reproduced from Fan et al. (2011) with permission




Volume 42 · Number 4 · 2013 · 237–246

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Corresponding author

H.B. Lu can be contacted at: [email protected]




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