revealing triple-shape memory effect by polymer bilayers

Communication

Revealing Triple-Shape Memory Effect byPolymer Bilayersa

Tao Xie,* Xingcheng Xiao, Yang-Tse Cheng

Bilayer polymers that consist of two epoxy dual-shape memory polymers of well-separatedglass transition temperatures have been synthesized. These bilayer epoxy samples exhibit atriple-shape memory effect (TSME) with shape fixities tailorable by changing the ratiobetween the two layers. The triple-shape fix-ities of the bilayer epoxy polymers can beexplained by the balance of stress betweenthe two layers. Based on this work, it is believedthat the following three molecular design cri-terions should be considered in designing tri-ple-shape memory polymers with optimumTSME: 1) well-separated thermal transitions,2) a strong interface, and 3) an appropriatebalance of moduli and relative ratios betweenthe layers (or microphases).

Introduction

Shape memory polymers (SMPs) represent responsive

polymers that can fix deformed temporary shapes and

recover to their permanent (original) shapes upon external

stimuli.[1–7] A conventional SMP can only memorize one

temporary shape in each shape memory cycle, displaying

the so-called dual-shape memory effect (DSME).[1,8] By

comparison, the recently reported polymer triple-shape

T. Xie, X. XiaoMaterials & Processes Laboratory, General Motors Research &Development Center, Warren, MI 48090-9055, USAE-mail: [email protected]. ChengDepartment of Chemical and Materials Engineering, University ofKentucky, Lexington, KY 40506-0046, USA

a : Supporting information for this article is available at the bottomof the article’s abstract page, which can be accessed from thejournal’s homepage at http://www.mrc-journal.de, or from theauthor.

Macromol. Rapid Commun. 2009, 30, 1823–1827

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

memory effect (TSME) refers to the ability of SMPs to

memorize two temporary shapes in a single shapememory

cycle.[8] The discovery of the polymer TSME is significant, as

it provides an additional dimension to the technical

potential of SMPs, which have shown great promise in

many applications such as medical sutures,[9] actua-

tors,[10,11] defect recoverable coatings,[12,13] and smart

adhesives.[14] From a molecular design standpoint, DSME

is realized with the combination of a reversible phase

transition and a freezing mechanism.[1–4] The minimum

requirement to achieve a TSME is to have at least an

additional reversible phase transition.[8] Although SMPs

thatmeet this requirementhavebeen reported,[8,15–18] only

the ones reported by Bellin et al. show a TSME.[8] Thus far, it

has remained unclear what governs the polymer TSME

besides the basic requirement of having two reversible

phase transitions and a freezing mechanism. Here, we

report a method of achieving a TSME using macroscopic

bilayer crosslinked polymer structures with two well-

separated phase transitions. This method differs from the

reported triple-shape memory polymers (TSMPs) based on

DOI: 10.1002/marc.200900409 1823

T. Xie, X. Xiao, Y.-T. Cheng

1824

macroscopically homogeneous systems with two micro-

scopic phases.[8] We note that the microscopically phase-

separated TSMP system relies on polymer grafting chem-

istry to introduce a second phase and tuning the TSME is

thus highly dependent on the availability of appropriate

grafting chemistry.[8] For our macroscopic bilayer TSMP,

however, the two polymer layers involved are decoupled.

The two macroscopic layers can thus be independently

tuned (with very little chemistry constraint) both in terms

of the ratio between themand their corresponding thermal

transition temperatures, which allows the TSME to be

tailored with ease. Using this macroscopic bilayer material

systemas amodel system,we reveal that a strong interface

between the macroscopic layers (or microscopic phases) is

of critical importance for the TSME.

Unlike the TSME, the traditional dual-shape memory

effect (DSME)maybe consideredasan intrinsic property for

polymers.[4] Such an argument is based on the fact that

polymers are intrinsically viscoelastic materials with at

least one thermal reversible phase transition (glass

transition or melting point), although on very rare

occasions the thermal transition temperatures may be

above the thermal decomposition temperatures and are

thus practically non-existent. With proper modifications

such as chemical crosslinking, a freezing mechanism can

always be introduced into polymers to convert them into

dual-shape memory polymers (DSMPs). A seemingly

opposite argument that the DSME is not an intrinsic

propertyofpolymers[1] is presumablybasedon the fact that

many polymers (e.g., non-crosslinked polystyrene), as they

are, do not possess the freezing mechanism required for a

DSME. In truth, both arguments are valid provided that the

underlying bases are taken into account. Alternatively, it

couldbearguedthat theshapememoryeffectofpolymers is

a material capability instead of a property, since it is a

tailored effect that requires the right combination of

intrinsic properties. Nevertheless, many thermoset poly-

mersused inourdaily livesareDSMPs, although their shape

memory properties are not necessarily utilized. Indeed, we

have shown in our earlier work that a common epoxy

polymer system possesses dual-shape memory properties

with a tunable thermal transition temperature that ranges

from 20 to 90 8C.[19] This epoxy DSMP system was used to

fabricate the bilayer TSMP in this work.

Experimental Part

Materials

The diglycidyl ether bisphenol A epoxy monomer (EPON 826) and

the poly(propylene glycol)bis(2-aminopropyl) ether curing agent

(Jeffamine D-230) were obtained from Hexion and Huntsman,

respectively. Neopentyl glycol diglycidyl ether (NGDE) was

purchased from TCI America. All chemicals were used as received.



Synthesis of Epoxy Polymer H and L

EPON 826 was first melted at 75 8C for 15min. It was then mixed

with NGDE and Jeffamine D-230 at a mole ratio of 1.6:0.4:1. The

mixturewaspoured intoanaluminummold, curedat100 8Cfor1 h,andpostcuredat 130 8C for 1h toproduce the epoxypolymerH. The

cured epoxy was demolded and cut into desirable sizes prior to

testing. Epoxy polymer L was produced in the same fashion except

that the mole ratio of EPON 826/NGDE/Jeffamine D-230 was

0.8:1.2:1. In both the compositions of epoxy polymer L and H, the

stoichiometry between active hydrogens and epoxy groups was

kept at 1:1.

Synthesis of Epoxy Bilayer Polymer Samples (BE1–BE4)

The epoxy liquid mixture that corresponded to epoxy polymer H

was cured in analuminummoldat 100 8C for 40min toproduce the

first epoxy layer. The epoxy liquid mixture that corresponded to

epoxy polymer L was poured on top of the cured first epoxy layer,

curedat100 8Cfor40min, andpostcuredat130 8C for1 h. Following

this two step curing process, four bilayer epoxy polymer samples

(BE1–BE4) were produced by varying the ratio between the two

epoxy liquids. Specifically, the thickness ratios between polymer L

andpolymerHinBE1,BE2,BE3,andBE4are2.78,2.61,1.27,and0.44,

respectively.

Thermomechanical Characterization

The dynamic mechanical analysis (DMA) experiments were

performed in a dual cantilever mode using a DMA Q800 (TA

instruments). The testing parameters were: constant frequency¼1Hz, oscillation amplitude¼30mm, and heating rate¼ 1 8C �min�1.

Shape Memory Cycles

All quantitative shape memory cycles were obtained using a DMA

2980 in a three point bending mode. The analysis was conducted

under a static force mode and the heating and cooling rates were

1 8C �min�1. For visual demonstration of the TSME, BE3 with a

rectangular shape (shapeA)washeated inanovenpresetat90 8C for10min. Itwas then deformedmanually after it was taken out of the

oven and immediately immersed into a hot water bath preset at

56.0�0.5 8Cfor1min.ThisyieldedthefirsttemporaryshapeB,which

was photographed outside the bath. Shape B was immersed in the

waterbathforanother1min.Afterwards, itwastakenoutofthebath

again, immediately deformed, and cooled to 22 8C to fix the second

temporaryshapeC.For recovery, shapeCwasputbackintothewater

bath (56.0� 0.5 8C) for 1min,which changed to shape B. Recovery of

shape A was performed by heating shape B to 90 8C for 5min.

Results and Discussion

A rigid aromatic diepoxide (EPON 826), a flexible aliphatic

diepoxide (NGDE), and an aliphatic diamine (Jeffamine

D-230) were used to formulate the epoxy thermoset

DOI: 10.1002/marc.200900409

Revealing Triple-Shape Memory Effect by Polymer Bilayers

Figure 1. DMA curves for the bilayer polymers. a) BE2 and b) BE3.

DSMPs.[19] By varying the ratio between EPON 826 and

NGDE, epoxy DSMPs with transition temperatures any-

where between 20 to 90 8C can be synthesized.[19]

Specifically, the two epoxy SMP polymers (L and H) used

in this study possess glass transition temperatures (Tgs) of

38 and 75 8C, respectively (based on their storagemoduli in

the DMA curves). Here, the sample designations L and H

indicate their low and high Tgs on a relative basis. The

respective average molecular weights between crosslinks

(Mc) for L and H are 389 and 446 g �mol�1, and reflect their

highly crosslinked nature. Both polymer L and H display

DSMPs with shape fixities and recovery of around 100%, as

indicated in their respective dual-shape memory cycles in

Figure S1a and S1b. It should be noted that all the shape

memory cycles reported here were obtained under a

bending deformation mode, the choice of which over a

tension mode was because of its direct relevance to the

visual demonstration of shape memory cycles shown later

in Figure 3. All the bending strains were kept below 10%

because of the constraints of the deformation fixture in the

DMA setup. It should be noted that the strains, although

small for tensile deformation, actually represent quite

noticeable shape changes in a bending deformation, as

observed in the quantitative shape memory cycling

experiments and illustrated later in Figure 3.

The formulations that correspond to polymer L and H

were used to construct bilayer epoxy polymers using a two

step curing process described in the experimental section

and illustrated in Figure S2. A total of four bilayer epoxy

samples (BE1–BE4) with different thickness ratios between

LandH(showninTable1)weresynthesized.TheDMAcurve

for BE2 (Figure 1a) shows two glass transitions that

correspond to epoxy L and H, respectively. These two glass

transitions arewell-separated,which results in a plateau in

storage modulus between 50 and 65 8C, in addition to the

two plateaus below the Tg of epoxy L and above the Tg of

Table 1. Summary of the triple-shape memory properties.

Sample

ID

Ratio

(L/H)

Rf

(A!B)a)Rf

(B!C)a)Rr

(C!B)b)Rr

(B!A)b)

% % % %

BE1 2.78 76.4 96.4 91.5 98.5

BE2 2.61 78.2 93.8 98.3 97.3

BE3 1.27 95.6 83.3 92.8 98.6

BE4 0.44 97.4 71.4 92.5 97.3

H 0 – 100.0 98.6 –

L 1 100.0 – – 100.8

a)The shape fixity (Rf) was calculated based on the definition

Rf(X!Y)¼ (ey� ex)/(eyload� ex)[8]; b)The shape recovery (Rr) was

calculated based on the definition Rr(Y!X)¼ (ey� exrec)/(ey� ex)[8].



epoxy H. The DMA curves for BE3 (Figure 1b), BE1, and BE4

(not shown) display similar features except that their

respective storage moduli that correspond to the mid-

plateau varies depending on the ratios between L and H in

the samples.

All shape memory characterization was conducted in a

three-point bending deformation mode. The triple-shape

memory capability for BE2 is illustrated in Figure 2a. In the

two-step shape fixing process, the permanent shape Awas

first heated to Thigh (90 8C,which is above the Tg of epoxyH)

and deformed. Cooling under stress to Tmid (56 8C, which

falls in themiddleof themid-plateau in theDMAcurve) and

releasing the stress fixed temporary shape B, which

corresponds to eB. In the second fixing step, shape B was

further deformed under a larger stress and cooled to Tlow(20 8C). Releasing the stress after cooling led to temporary

shape C (ec). Here, the glassification of epoxyH at Tmid and L

at Tlow was responsible for fixing shapes B and C,

respectively. For recovery, shape C was heated to Tmid, to

yield the recovered shape B (eBrec). The recovered shape B

remained stable until the temperature was further

increased to Thigh, which leads to the recovered shape A

(eArec). Under the identical thermomechanical conditions,

the triple-shape memory cycle was repeated two more

times using the same sample and no noticeable difference

was observed in the shape memory curves. Qualitatively,

www.mrc-journal.de 1825

T. Xie, X. Xiao, Y.-T. Cheng

Figure 2. Triple-shape memory cycles for the bilayer polymers.a) BE2 and b) BE3.

Figure 3. Illustration of the TSME of BE3: A) original shape, B) firsttemporary shape, and C) second temporary shape.

1826

the bilayer samples BE1, BE3, and BE4 also show triple-

shapememory capability. For comparison, the triple-shape

memory cycle for BE3 is displayed in Figure 2b. A notable

difference between Figure 2a and Figure 2b is that a much

smaller stress was used to deform and fix shape C for BE3,

which was attributed to its lower storage modulus at Tmid

than that of BE2.

The quantitative TSMPs (shape fixity Rf and shape

recovery Rr) for all the bilayer polymers are summarized

in Table 1. The data in this table shows that Rf (A!B)

increases as the ratio between the epoxy L and the epoxy H

decreases (fromBE1toBE4, in thatparticularorder),whileRf(B!C) followsanopposite trend. Such trendscanbe readily

explained by a stress-balancing mechanism. At the first

stage of shape fixing (A!B) at Tmid, the fixing relies on the

freezing of molecular mobility of the H layer while the L

layer tends to retain its original shape and thus disfavors

the shape fixing of the bilayer polymers. The situation



reverses at the second stage of the shape fixing (B!C) that

occurs at Tlow, that is, the fixing relies on the L layer, while

the layer H has a tendency to keep shape B. Overall, the

shapefixities of the bilayer polymers are determined by the

ratio between the two layers and their moduli at the

corresponding shapefixing temperatures. In principle, such

a stress-balancing effect can be quantified by modeling,

which we intend to pursue in the near future. In terms of

shape recovery, all Rc values in Table 1 are higher than 91%,

which indicates that they recover well in all cases.

The TSME is further demonstrated visually in Figure 3.

BE3 in its original rectangular shape A was subsequently

fixed into the shapeof thenumber9 (shapeB) and the shape

of the number 7 (shape C). Reheating recovered shape B and

shape C subsequently. The slight difference in the fixed

shape B and the recovered shape B is visible and consistent

with the non-complete shape recovery (Rc) reported for BE3

inTable1.Weobserved thatall theshapes showninFigure3

were stable at andbelowthe relevant temperatures and the

above demonstration can be repeated multiple times.

We note that the TSME associated with the bilayer

polymers benefited from the strong interface between the

two epoxy layers. During the two-step synthesis of the

bilayer samples, the unreacted epoxy groups or amine

groups on the surface of the first cured epoxy layer

continued to react with the second epoxy liquid poured

onto it, which produces an interface with a strength that

exceeds that of the bulk strength of polymer L (see

supporting information). Without the strong interface,

the bilayer would not have exhibited a TSME, instead,

delamination would have occurred during the shape

memory cycles. In principle, the general approach of

achieving a TSME with a bilayer construction can be

extended to any combination of two DSMPs, provided that

DOI: 10.1002/marc.200900409

Revealing Triple-Shape Memory Effect by Polymer Bilayers

the interface is sufficiently strong. Because of theversatility

of thematerial design, achieving amultiple-shapememory

effect beyond a triple-shape is possible with material

constructions that consist of more than two layers.

Although our TSMPs aremacroscopically heterogeneous,

we believe that the learning is applicable to polymer

systems that are heterogeneous only at microscopic scales.

For instance, itmay explain the ‘‘mystery’’ as towhy not all

polymers that possess two reversible thermal transitions

anda freezingmechanismshowaTSME.[8] In fact,wenotice

that the only macroscopically homogenous TSMPs are the

ones with chemical bonding between the two reversible

microscopic phases, which indicates that the interfaces at

the microscopic scale are strong. In the absence of such

microscopic interfacial chemical bonding, we believe that

the interfacial slippage between the microscopic phases

could at least compromise the TSME. In addition, the stress-

balancing mechanism we proposed is also valid for

macroscopically homogeneous polymers, albeit balancing

of the stress occurs between phases at microscopic scales.

Conclusion

Bilayer polymers that consist of two epoxy DSMPs of well-

separated glass transition temperatures have been synthe-

sized. These bilayer epoxy samples exhibit a TSME, which

canbetailoredeasilybychanging theratiobetweenthetwo

layers. The TSME of the bilayer epoxy polymers can be

explained by a stress-balancing mechanism. Based on this

work, we believe that the following threemolecular design

criterions should be considered to design TSMPs with an

optimum TSME: 1) well-separated thermal transitions, 2) a

strong interface, 3) an appropriate balance of moduli and

relative ratios between the layers (or micro-phases).



Acknowledgements: The authors thank Mr. Bhavesh Shah andDr. Ingrid Rousseau for their assistance in the DMA experiments.

Received: June 10, 2009; Published online: August 4, 2009;DOI: 10.1002/marc.200900409

Keywords: interfaces; shape memory polymers; stimuli-sensitivepolymers; triple-shape memory effect

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revealing triple-shape memory effect by polymer bilayers

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