High Modulus Ratio Shape-Memory Polymers Achieved by Combining

Hydrogen Bonding with Controlled Crosslinking

Yi Pan,1,2 Tuo Liu,1,2 Jing Li,1,2 Zhaohui Zheng,1 Xiaobin Ding,1 Yuxing Peng1

1Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, People’s Republic of China

2Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

Correspondence to: X. Ding (E-mail: xbding@cioc.ac.cn)

Received 30 May 2011; revised 29 June 2011; accepted 29 June 2011; published online in Wiley Online Library

DOI: 10.1002/polb.22317

ABSTRACT: A new type of poly(methyl acrylate)-co-(acrylic acid)

(PMA-AA) networks obtained by combining hydrogen bonding

with controlled crosslinking exhibit full and rapid shape-memory

recovery. The structure, thermal properties, dynamical mechani-

cal properties and shape-memory effects of these networks were

presented. High modulus ratios were achieved for the series of

PMA-AA networks based on intense self-complementary hydro-

gen bonding in poly(acrylic acid) (PAA) segments. This lead to

excellent shape-memory effects with strain-recovery ratio above

99%. Meanwhile, faster recovery speed was achieved by the syn-

ergistic effect of hydrogen bonding and controlled crosslinking

compared to the linear PMA-AA copolymers. VC 2011 Wiley Peri-

odicals, Inc. J Polym Sci Part B: Polym Phys 49: 1241–1245, 2011

KEYWORDS: hydrogen bonding; modulus networks; shape-

memory; thermodynamics

INTRODUCTION Shape-memory polymers (SMPs), which canbe deformed from one shape to another in response to astimulus, represent a promising class of materials and offertremendous potential to the fields of biotechnology, medi-cine, sensors, actuator systems, and textiles.1,2 However, incontrast to shape-memory alloys, some of the major draw-backs of SMPs strongly limit their applications, such as poormechanical properties, inherently low recovery stress, slowrecovery speed, and short cycle life. These limitations haveprovided motivation for the development of new types ofSMPs, for example, hydrogen-bonded shape-memory com-plexes,3–5 with specific structural, composition, and geomet-rical design for specific applications.

According to viscoelastic and thermomechanical models thatcorrelate the viscoelastic/thermal properties to the shape-memory behavior of SMPs, the ratio of glassy modulus torubbery modulus (Eg/Er) plays a key role on shape-memoryperformance.6,7 With a large modulus ratio, deforming at hightemperature is easy, whereas, at low temperature, the resist-ance against deformation is large.8 Up to now, the ways toimprove modulus ratio of SMPs focused on the introductionof crystal components8,9–12 and hydrogen bonding.3–5,13–15

Hydrogen bonds associate at low temperatures, which is ben-eficial to increase the storage modulus and fix the initialshape of the materials, and they dissociate at elevated tem-peratures, which decreases the storage modulus of thematerials.14 However, most of the reports have focused onintroducing hydrogen bonds as molecular switches todevelop reversible shape-memory networks.15 Only a few

cases reported on introducing hydrogen bonding to develophigh modulus ratio SMPs with excellent shape-memoryeffects.3 Meanwhile, it is well known that PAA is a proton-donating polymer which has been extensively studied inhydrogen-bonded complex. And it is also used to prepareshape-memory complexes with another hydrogen bondacceptor.4,16 However, there were no reports about SMPsprepared with PAA containing strong self-complementaryhydrogen bonding.

Meanwhile, the introduction of covalent crosslinking is a gen-eral method to enhance rubber modulus of SMPs, whichwould correspondingly improve the shape-memory recoverystresses.17–21 However, the shape-memory behavior of ther-mal-induced SMPs is generally triggered by soft segments.13

By selecting controlled crosslinking only of hard segments,larger recovery stresses could be obtained and the flexibilityof the soft-segments could be preserved. This would be help-ful to achieve faster shape-memory recovery of the polymernetworks. Herein, we design a series of shape memory poly-(methyl acrylate)-co-(acrylic acid) (PMA-AA) networks basedon hydrogen bonding and controlled crosslinking. 2-Hydrox-yethyl acrylate (HEA) was used as the functional monomerand reacted via esterification with the PAA segments, therebyaddressing the issues discussed above and optimizing theshape-memory properties by the synergistic effect of hydro-gen bonding and controlled crosslinking. To the best of ourknowledge, similar work has not been reported, and thismay broaden the list of SMPs with excellent shape memoryproperties.

Additional Supporting Information may be found in the online version of this article.

VC 2011 Wiley Periodicals, Inc.

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EXPERIMENTAL

MaterialsHEA was purchased from Alfa Aesar and used as received.2,20-Azobis (isobutyronitrile) (AIBN) was of analytical gradeobtained from the Chengdu Kelong Chemical Reagent Factoryand used as a radical initiator. It was recrystallized fromethanol solution. Methyl acrylate (MA) and acrylic acid (AA)were purchased from Chengdu Kelong Chemical Reagent Fac-tory and distilled under reduced pressure before use.

PreparationThe syntheses of these films were accomplished using con-ventional free-radical polymerization. All crosslinked speci-mens were prepared according to the monomer weight ratioMA:AA ¼ 13:7. AIBN was added to 0.5 wt % of the MA andAA monomer total weight, and HEA was added to 1-3 wt %of the MA and AA monomer total weight. Linear copolymerswere synthesized without HEA. All monomers and initiatorswere mixed to form a clear solution and then bubbled withnitrogen for 5 min to remove oxygen. The mixtures werethen injected into glass molds sealed by silicon rubberspacers (1 mm thick). The substrate temperature was con-trolled at 43 �C using an electrically heated drying oven for12 h and then heated to 60 �C for 24 h. Then the specimenswere taken out from the molds, the following esterificationwas carried out by heating the specimens to 140 �C andkeeping them for 4 h in a vacuum oven (Pabs ¼ 5 kPa).

MeasurementsDifferential scanning calorimetry (DSC) measurements wereperformed on a TA instrument Q2000 with LNCS coolingaccessory over the temperature range 0–120 �C at a heatingrate of 10 �C/min. Dynamic mechanical analysis (DMA) intensile loading was carried out to determine the Tg of thenetworks on a TA Q800 DMA. Rectangular samples withdimensions of 1 � 5 � 25 mm3 were cut. The samples werethermally equilibrated at 0 �C for 3 min and then heated to

130 �C at a rate of 3 �C/min, and a frequency of 1 Hz. TheTg was defined at the peak of tan d. Cyclic thermomechanicalexperiments1 were performed on a Tinius Olson H10K-T ten-sile machine at a crosshead speed of 5 mm/min, and a ther-mal chamber with temperature controller and connected toa liquid nitrogen Dewar was used. The sample was stretchedto an extensional strain of e1 ¼ 20%, that is, the ratio ofelongation with respect to the original length of sample, andkept 5 min at 100 �C. While keeping the strain, the samplewas cooled to room temperature (25 �C) and maintained at25 �C for 5 min. After unloading, the length of the samplewas measured, and the fixed strain, e2, was evaluated. Theunloaded sample was heated to 100 �C again, and thedimensional changes were measured to evaluate recoveringstrain, e3. A shape fixity ratio (Rf) and a shape recovery ratio(Rr) were defined by the following equations:

Rf ¼ e2ðNÞe1

� 100%; Rr ¼ e1 � e3ðNÞe1 � e3ðN� 1Þ � 100%

Bending test1 examined the shape-memory effect as follows.A straight strip (60 � 6 � 1 mm3) of the specimen wasfolded at 100 �C and then cooled to room temperature underthe deformation and maintained at 25 �C for 5 min. Afterunloading, the deformed sample was heated again by twomethods. One method is that the deformed sample washeated in a constant heating rate, and another is that thedeformed sample was abruptly heated to the temperature of100 �C. The change of angle (yf), and the recovery time wererecorded. The ratio of the recovery was defined as yf/180.

RESULTS AND DISCUSSION

The syntheses of shape-memory elastic films were accom-plished using conventional free-radical polymerization andcrosslinking by esterification at 140 �C. The chosen mono-mers involve three components: MA, AA, and HEA. PMA was

SCHEME 1 Structural model of the controlled cross-linked PMA-AA networks. Violet: PAA segments; green: PMA segments;

yellow: PHEA segments; blue: hydrogen bonds; black: controlled cross-linking.

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chosen as the soft-segment component because it is a soft,amorphous polymer which exhibits a low glass transition tem-perature (Tg � 6 �C) and has short side groups contributing tothe lower energy loss. PAA was chosen as the hard-segmentcomponent because it exhibits a high glass transition tempera-ture (Tg � 102 �C), which could form strong self-complemen-tary hydrogen bonding22 and offer carboxyl group for the ester-ification. And HEA was chosen as the functional monomerbecause of the ability to accomplish esterification with AA athigh temperature. A structural model for the PMA-AA networkis shown in Scheme 1. PAA acts as the hard segments and formsself-complementary hydrogen bonding. PMA acts as the softsegments and shows good flexibility. Finally, poly(2-hydroxy-ethyl acrylate) (PHEA) segments enable the esterification reac-tion with PAA segments to permit the controlled crosslinking.

The resulting material appears completely homogeneous andtransparent when inspected using optical microscopy. Thethermal properties of PMA-AA networks and linear PMA-AAcopolymers were investigated by DSC (Supporting Informa-tion, Fig. S1). All samples show only a single transition inthe DSC curves indicating that the components are miscibleand amorphous phases have been formed.

The storage modulus and tan d of the crosslinked networksand linear copolymers as a function of temperature areshown in Figure 1. Figure 1(a) shows that PMA-AA linearcopolymers and PMA-AA networks both possess gigapascal

(GPa)-storage moduli at room temperature; however, the lat-ter have a higher glassy modulus and rubbery modulusbelow and above Tg, which can be expected to yield a largerelastic recovery stress. Furthermore, PMA-AA copolymersand the crosslinked networks exhibit similar temperature-de-pendent viscoelastic properties, with the storage modulussharply descending from the GPa-glassy state to a megapas-cal-elastic plateau around their respective glass transition.The modulus ratio (E0

Tg�20�C/E0Tgþ20�C)

3 of the linear PMA-AA is 907 and that of the crosslinked samples H-1, H-2, andH-3 reached 809, 741, and 577, respectively. This may beattributed to the intense hydrogen bonding interaction inPAA segments (Supporting Information, Fig. S2). Therefore,excellent shape-memory properties can be expected.

Interestingly, Figure 1(b) shows narrower glass transition ofthe crosslinked networks compared to the PMA-AA linearcopolymers. This is beneficial to obtain faster shape recovery,because shape memory behavior requires a sharp transitionfrom the glassy state to rubbery state to achieve permanentshape reversion and fast recovery speed.23,24 Given that linearPMA-AA copolymers and the crosslinked networks had similarpolymer chain compositions, the observed difference in ther-momechanical properties was attributed to the controlledcrosslinking in PAA hard segments of PMA-AA networks.

Cyclic thermomechanical experiment and bending test werethen utilized to study the shape memory behavior of these

TABLE 1 Summary of the Thermal–Mechanical Properties and Shape-Memory Properties of H-0, H-1, H-2, and H-3

Samples

HEA

(wt %)

TDMAg

(�C)aE0

high

(MPa)b me (mol/L)cE0

Tg�20�C/

E0Tgþ20�C

Rf (%)d Rr (%)d

First Second Third First Second Third

H-0 0 70 1.468 0.16 907 91 94 95 97 98 98

H-1 1 78 3.073 0.32 809 87 91 92 99 99 100

H-2 2 80 3.142 0.33 741 86 89 91 99 100 100

H-3 3 82 3.852 0.40 577 83 85 86 99 100 100

a Obtained from DMA results.b At the temperature of TDMA

g þ 30 �C.c Calculated from me ¼ E0

high/3RThigh (Thigh ¼ TDMAg þ 30 þ 273 K).

d Measured on cyclic thermomechanical experiment of the first three

cycles.

FIGURE 1 (a) Storage modulus-temperature curves of H-0, H-1, H-2, and H-3; (b) loss angle (tan d)-temperature curves of H-0, H-1,

H-2, and H-3.

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linear copolymers and PMA-AA networks, respectively. Thetemperature of deformation was selected to be 100 �C, andthe deformation strain was selected to be 20% for all samplesowing to the low elongation at break of H-0, which was main-tained by cooling the samples to the room temperature. Theresults as shown in Table 1, both crosslinked specimens andlinear copolymers show excellent shape-memory recoverywith Rr of above 97%, which is in accordance to the highmodulus ratio. Shape-memory recovery ratio of H-0 is lowerthan that of the crosslinked samples owing to greater creep.However, in comparison with the linear copolymers, the cross-linked specimens show lower shape fixity. As mentionedabove, crosslinked PMA-AA networks exhibited a higher glassymodulus and rubbery modulus below and above Tg, respec-tively, which would be under relatively larger stress duringthe loading and subsequent cooling process to cause a largeshrinkage on removing the grip from the samples.

Furthermore, as shown in Figure 2, the crosslinked speci-mens show a much faster recovery speed than that of thelinear PMA-AA copolymers, even though the latter have alower Tg. The shape recovery speed of the crosslinked sam-ples was accelerated when the crosslinking density (me)

25

was increased as shown in Table 1, which may be due to thelarger residual stress owing to the higher glass storagemodulus, adopted by this sample during the programming

process. And another possibility may be that the controlledcrosslinking of the PAA hard segments results in the preser-vation of flexibility in soft segments, hence a steep and nar-rower thermomechanical transition. Especially, the sampleH-3 with the largest crosslink density shows surprising fastrecovery speed in water, where 2.32 s was necessary toachieved the permanent shape at 90 �C (Supporting Infor-mation, Fig. S3).

CONCLUSIONS

In summary, we present a method to prepare SMP networksby combining hydrogen bonding with controlled crosslinking.This approach provides an opportunity to fabricate highmodulus ratio shape-memory materials with excellent shape-memory effects and rapid shape-memory recovery by thesynergistic effect of hydrogen bonding and selective cross-linking. It is expected that this demonstration will stimulatefurther work on the development of application-specific,high-performing SMPs.

ACKNOWLEDGMENTS

This workwas supported by the National Natural Science Foun-dation of China (Grant No. 20874103, 50973111).

FIGURE 2 (a) Shape memory effects of H-0, H-1, H-2, and H-3 when the samples were heated from 50 to 100 �C with a heating rate

of 3 �C/min measured by bending test; (b) recovery time of H-0, H-1, H-2, and H-3 at 100 �C measured by bending test; (c) shape

recovery rates of H-0 versus H-3 from an identical zigzag-shaped temporary shape to fully extended strip at 100 �C.

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