One-Way Multishape-Memory Effect and Tunable Two-Way Shape ...

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One-Way Multishape-Memory Effect and Tunable Two-Way Shape Memory Effect of Ionomer Poly(ethylene-co-methacrylic acid) Lu Lu and Guoqiang Li* Department of Mechanical and Industrial Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States ABSTRACT: Reversible elongation by cooling and contraction by heating, without the need for repeated programming, is well-known as the two-way shape-memory effect (2W-SME). This behavior is contrary to the common physicscontraction when cooling and expansion when heating. Materials with such behavior may find many applications in real life, such as selfsufficient grippers, fastening devices, optical gratings, soft actuators, and sealant. Here, it is shown that ionomer Surlyn 8940, a 50-year old polymer, exhibits both one-way multishapememory effects and tunable two-way reversible actuation. The required external tensile stress to trigger the tunable 2W-SME is very low when randomly jumping the temperatures within the melting transition window. With a proper one-time programming, “true” 2W-SME (i.e., 2W-SME without the need for an external tensile load) is also achieved. A long training process is not needed to trigger the tunable 2W-SME. Instead, a proper one-time tensile programming is sufficient to trigger repeated and tunable 2W-SME. Because the 2W-SME of the ionomer Surlyn is driven by the thermally reversible network, here crystallization and melting transitions of the semicrystalline poly(ethylene-comethacrylic acid), it is believed that a class of thermally reversible polymers should also exhibit tunable 2W-SMEs. KEYWORDS: actuators, polymeric materials, shape-memory materials, stimuli-responsive materials, ionomer Surlyn

1. INTRODUCTION

Using amorphous one-way shape-memory polymer as an example, in a typical shape-memory cycle, the shape of the material is programmed by deforming at a temperature above the glass transition temperature (Tg), followed by cooling below Tg and removing the load to fix the temporary shape. The shape can be recovered by increasing the temperature of the material above Tg due to entropy recovery. Of course, programming can also be conducted in the glassy state, locking reversible plastic deformation, and when heated above Tg, the shape-memory polymer recovers to its original shape driven by the stored energetic force.20,21 This programming and recovery can be repeated for many cycles; however, going from the original shape to the temporary shape always requires additional programming step. Such shape-memory behavior is referred as one-way shapememory effect. Demands have arisen for reversible actuation in SMPs without the need of repeated external mechanical manipulation. Twoway shape-memory polymers (2W-SMPs) were found to be able to achieve the reversible actuation with only one-time programming or even without a particular programming but with a constant tensile load. Polymers that can form crystalline domains are a major class of materials that display two-way shape-memory effect (2W-SME). To trigger the 2W-SME, the SMP is stretched under a constant applied load at a temperature above the phase transition temperature. The polymer chains then align in the direction of the

Shape-memory polymers (SMPs) are smart materials, which can memorize temporary shapes and restore their permanent shapes by external stimuli on demand.1−3 This unique programmable shape-memory behavior together with other merits such as excellent structural versatility, low manufacturing cost, easy processing, large recoverable deformation, tailorable recovery temperature, and possible biocompatibility and biodegradability, make SMPs ideal for a broad range of potential applications such as civil engineering materials,1 deployable aerospace structures,4 biomedical devices,5 smart textiles,6 self-healing,7 and so on. Conventional SMPs can only remember one temporary shape in each shape-memory cycle, which is named as dual-shape-memory effect or one-way shape-memory effect. The triple-shape-memory effect, which suggested that SMPs can memorize two temporary shapes in a shape-memory cycle, was introduced by Lendlein et al. in 2006.8 It offers an additional dimension to SMPs, which opens up many applications such as medical sutures,9 actuators,10 and smart adhesives.11 Subsequently, epoxy bilayer,12 acrylate copolymer containing supramolecular hydrogen bonding,13 Nafion,14 and so on were investigated for their triple-shape-memory effect. Some systems even displayed more than two temporary shapes such as quintuple-shape-memory polymers.15−18 Yu and Qi et al. discussed the mechanisms of multishape-memory effects and associated energy release in shape-memory polymers.19 However, regardless of dual-shape-memory effect or multishape-memory effect, a new round of programming is needed for each new thermomechanical cycle because these SMPs are one-way SMPs. © 2016 American Chemical Society

Received: April 7, 2016 Accepted: May 18, 2016 Published: May 18, 2016 14812

DOI: 10.1021/acsami.6b04105 ACS Appl. Mater. Interfaces 2016, 8, 14812−14823

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curious whether one single ionomer (such as Surlyn 8940) possesses both one-way multishape-memory effect and 2W-SME. If it has both properties, it would greatly enhance the scope of applications of this type of smart polymers. Our basis is that Surlyn 9520 has demonstrated one-way multishape-memory effect and another commercially available ionomer perfluorosulfonic acid (i.e., Nafion) was found to display thermoreversible actuation.37 In addition to exploring the reversible actuation mechanism of Surlyn, it is desired to know whether the 2W-SME of Surlyn is tunable within its broad melting transition because the reported 2W-SMEs for Nafion have specific working temperature ranges. The objective of this study is thus to experimentally investigate the one-way multishape-memory effect and tunable 2W-SME of ionomer Surlyn 8940.

applied load. Afterward, the SMPs are cooled below the phase transition temperature while maintaining the external load. Crystalline domains will form along the loading direction, leading to an increase in materials stretch during cooling. Heating the material above the phase transition temperature causes the recovery to its preload shape due to the melting of crystalline domains, leading to heating-induced contraction. Under the constant applied load, the shape changes can be repeated without additional programming. Here, we would like to clarify programming and the constant external load during two-way shape-memory cycle. When one conducts programming, the programming load needs to be removed at the end of the programming process; on the contrary, the constant external load is maintained in each thermomechanical cycle for 2W-SMPs. The two-way shape-memory effect was first reported for liquid crystalline elastomers.22 Many other polymer systems also displayed 2W-SMEs. For example, poly(ε-caprolactone),23−32 oligo(pentadecalactone),33,34 polyethylene,35,36 perfluorosulfonic acid ionomer (Nafion),37 poly(octylene adipate),38−40 polycyclooctene,41−43 epoxy,44,45 poly(ethylene-co-vinyl acetate),46,47 polyurethane,48−53 polyester,54 liquid crystalline elastomer nanocomposite,55 among others, have been demonstrated to display 2W-SMEs with varying actuation abilities and working temperature range. Crystalline poly(ε-caprolactone) and elastomeric poly(tetramethylene ether) glycol (PTMEG) formed an interpenetrated network for 2W-SME.56 In this network, poly(ε-caprolactone) is responsible for the reversible shape shrinkage while PTMEG provides the driving force for shape expansion. A polymer nanocomposite prepared with crosslinked poly(ε-caprolactone) with allyl alcohol as matrix and Fe3O4 nanoparticles decorated carbon nanotubes as responsive source can also display multistimuli and two-way shape-memory effect.57 Several works on 2W-SME without external tensile load, i.e., “true 2W-SME”, by introducing internal tensile stress to the stable network through tensile programming were reported recently.26,47,53,58 The reversible bidirectional shape-memory effect enabled applications such as self-sufficient grippers, fixator, fastening devices, cell encapsulation, swimmers, optical gratings, soft actuators, morphing structures, self-healing materials, sealant, and so on.7,29,38,53 Ionomers, which are polymers with bonded ionic species, possess strong physically cross-linked network due to nanophase separated ion-rich domains. Surlyn is a commercial thermoplastic ionomer resin synthesized by DuPont in the early 1960s. It is a semicrystalline random copolymer consisting of poly(ethyleneco-methacrylic acid), partially neutralized with metal ion. The permanent cross-linked network is provided by ionic clusters in the ionomer, whereas the temporary network is offered by polyethylene crystalline domains. The use of ionic cross-links retained the melting processability of ionomers. Known for its outstanding clarity, toughness, chemical resistance, and lightweight, ionomer Surlyn is reliable and cost-effective for use in food, cosmetic, medical device, and stretch packaging. Usually the multishape-memory polymers containing N temporary shapes possess N-1 well separated phase transitions. Tuning the shape-memory effect of such systems cannot be achieved without modifying the composition of the materials. With respect to Surlyn, owning to its broad melting transition, the N shapes of multishape-memory effect can be simply achieved by selecting N-1 transition temperatures within the transition window. It was also discovered that partially zinc-neutralized ionomer poly(ethylene-co-methacrylic acid) (PEMA) (also known as Surlyn 9520), possesses multishape-memory effect.59 We are

2. MATERIALS AND METHODS Surlyn 8940, which is an ionomer manufactured by DuPont, was used throughout the study. We are interested in this ionomer due to its outstanding mechanical properties and wide scope of applications. It is a random copolymer poly(ethylene-co-methacrylic acid), with 5.4 mol % methacrylic acid (30% of the 5.4 mol % methacrylic acid groups is neutralized with sodium). The Surlyn pellets were melted at 130 °C and pressed into films with thickness around 0.5 mm. These clear Surlyn films were used throughout the investigation. The dynamic mechanical analysis (DMA) studies were conducted in a tensile mode using model Q800 DMA (TA Instruments). The DMA curves was obtained in a controlled force mode. The preload is 0.001 N. The frequency is 1 Hz. The heating and cooling rate is 10 °C/min. The differential scanning calorimeter (DSC) studies were conducted using model DSC 4000 by PerkinElmer. A piece of Surlyn film of about 6 mg was placed in an aluminum pan and scanned between −50 and 180 °C with heating and cooling rates of 10 °C/min. The purging rate of nitrogen is 30 mL/min. Two cooling and heating cycles were conducted. The X-ray diffraction (XRD) analysis was performed on a Panalytical Empyrean diffractometer by using Cu as the anode material. It scanned from 4° to 90° in a step size of 0.026° at generator voltage of 40 kV and current of 40 mA at room temperature. 3. RESULTS AND DISCUSSION 3.1. Characterization of Ionomer. The Surlyn film was scanned by DMA for its thermomechanical property in the temperature range of −50 to 100 °C (Figure 1a). The glass transition temperatures of Surlyn are labeled on the basis of the onset of storage modulus, peak of loss modulus, and peak of tan delta. There is a slight difference between the three Tg values, which is a common phenomenon. The storage modulus of Surlyn 8940 is 54 MPa at 25 °C and it quickly drops to 33 MPa at 75 °C; this reduction in modulus suggests its good shapememory capability. DSC studies of Surlyn 8940 with two heating and cooling cycles were conducted between −50 to 180 °C (Figure 1b). Two endotherms appear in the first heating cycle. The first one at 47.9 °C is due to the order to disorder transition where the ionic clusters disorder upon heating.60 The recovery for the ionic clusters to the ordered state is a slow process which leads to no peak around 47.9 °C in the second cycle.60 The second endotherm is due to the melting of crystallites in the temperature range of 56−100 °C, with a maximum melting rate at 89.2 °C, which is the melting point of Surlyn 8940. The broad 14813

DOI: 10.1021/acsami.6b04105 ACS Appl. Mater. Interfaces 2016, 8, 14812−14823

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Figure 1. Characterization of as-prepared Surlyn 8940 films using DMA (a) and DSC (b). (a) Tg measurements using storage modulus, loss modulus, and tan delta curves. Tan delta is the ratio between loss modulus and storage modulus. (b) DSC cooling and heating thermograms of Surlyn 8940 film at temperature range of −50 to 180 °C.

Figure 2. Multishape-memory effect of Surlyn 8940. FSi→Sj: shape fixity ratio from shape i to shape j; RSj→Si: shape recovery ratio from shape j to shape i; Td: programming (deformation) temperature; Tr: recovery temperature. (a) Dual-shape-memory cycle at Td = Tr = 65 °C. F: 89.3%, R: 59.9%. (b) Triple-shape-memory cycle at Td1 = Tr2 = 71 °C, Td2 = Tr1 = 55 °C. FS0→S1: 66.8%, FS1→S2: 72.8%, RS2→S1: 67.1%, RS1→S0: 24.5%. (c) Quadrupleshape-memory cycle at Td1 = Tr3 = 85 °C, Td2 = Tr2 = 75 °C, Td3 = Tr1 = 65 °C. FS0→S1: 77.8%, FS1→S2: 46.9%, FS2→S3: 70.5%, RS3→S2: 87.9%, RS2→S1: 36.9%. RS1→S0: 15.0%. (d) Quintuple-shape-memory cycle at Td1 = Tr4 = 92 °C, Td2 = Tr3 = 85 °C, Td3 = Tr2 = 75 °C, Td4 = Tr1 = 65 °C. FS0→S1: 61.1%, FS1→S2: 59.5%, FS2→S3: 40.5%, FS3→S4: 78.2%, RS4→S3: 89.1%, RS3→S2: 30.4%. RS2→S1: 17.9%, RS1→S0: 24.5%.

the polyethylene domains within the ionomer (crystallization temperature Tc = 55.3 °C). 3.2. One-Way Multishape-Memory Effect of Ionomer 8940. One-way multishape-memory effects of the Surlyn films with increasing number of temporary shapes are shown in Figure 2.

melting window indicates a distribution of crystallite sizes. In the second heating cycle, the glass transition temperature (Tg) of 53.1 °C (comparable with values obtained from Figure 1a) and a broad melting transition were detected. The only exotherm appeared in both cycles represents the crystallization process of 14814

DOI: 10.1021/acsami.6b04105 ACS Appl. Mater. Interfaces 2016, 8, 14812−14823

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ACS Applied Materials & Interfaces The changes of temperature and stress with time are preset in the software. The strain changes are sensitively monitored by the instrument and plotted in the diagram as shown in Figure 2. The shape-memory effect can be quantified by the percentage of shape fixation (F) and shape recovery (R) using eqs 1 to 4. All of the F and R values are calculated on the basis of the first heating and cooling cycle unless indicated. Both F and R ratios for the dualshape-memory effect are calculated using eqs 1 and 2, respectively, and F and R ratios for the triple-, quadruple-, and quintuple-shapememory effects are calculated using eqs 3 and 4, respectively. F = 100% × εf /εs

(1)

R = 100% × (εf − εr)/εf

(2)

FSi → Sj = 100% × (εfj − εfi)/(εsj − εfi)

(3)

R Sj → Si = 100% × (εfj − εri)/(εfj − εfi)

(4)

where εs is the maximum strain under load, εf is the fixed strain after cooling and load removal, εr is the recovered strain, and i and j represent the different temporary shapes. One-way dual-shape-memory effect of Surlyn 8940 (Figure 2a) was conducted at Td = Tr = 65 °C (Td, deformation temperature; Tr, recovery temperature), with F = 89.3% and R = 59.6%. The programming stress was 1.25 MPa. The one-way triple-shapememory effect of Surlyn is demonstrated in Figure 2b, by deforming and fixing the film into two temporary shapes, 1 and 2, at 75 and 65 °C programmed by external loads of 0.59 and 1.55 MPa, respectively. Upon heating to 65 °C, the shape recovered to temporary shape 1. Upon heating to 75 °C, the shape returned to the permanent shape. The shape fixity and recovery ratios are close to and comparable with reported values.59 The one-way quadruple- and quintuple-shape-memory effect of Surlyn were demonstrated in Figure 2c,d with memorizing three and four temporary shapes, respectively. The programming stress for each temporary shape is also shown in Figure 2. The shape fixity and recovery ratios decrease when more temporary shapes are involved because the shape fixation temperatures will be close to the corresponding deformation temperature, and thus, not much strain can be fixed. Xie indicated that for commercially available perfluorosulfonic acid ionomer (such as Nafion), which has a broad reversible phase transition, the multishape effect of Nafion is achievable, as long as selecting arbitrary temperatures above the onset of the glass transition and the temperatures are not too close.61 Xie selected transition temperatures that far apart (temperature intervals >30 °C) and conducted the dual-, triple-, and quadruple-shape-memory effect tests. However, we noticed that for ionomer Surlyn, the quintuple shape-memory effect can be realized by a narrower temperature intervals of 6−10 °C at 65, 75, 85, 91 °C (Figure 2d). 3.3. Effect of Cyclic Thermomechanical Cycles on Shape Fixity and Shape Recovery Ratios. The shape recovery of the ionomer Surlyn is fair during the first cycle. However, improvement after the initial shape-memory cycle has been observed by running one-way dual-shape-memory tests with three cycles (Figure 3). From the shape fixity and recovery ratios listed beside Figure 3, we can see a dramatic increase in the recovery ratio in the second cycle, regardless of the deformation and recovery temperatures. This phenomenon has also been noticed by other SMP systems.62 Two programming temperatures and three cycles are selected. It is obvious that the shape fixity ratio decreases and shape recovery ratio increases as the thermomechanical cycles increases, and the ratios seem to stabilize

Figure 3. Three cycles of dual-shape-memory studies at (a) Td = Tr = 65 °C and (b) Td = Tr = 80 °C. The shape fixity and recovery ratios are shown beside the figure.

at the third cycle. The fact that the shape recovery ratio is 100% in the third cycle, for both programing temperatures, suggests that the ionomer is an excellent one-way shape-memory polymer. 3.4. Two-Way Shape-Memory Effect (2W-SME) of Ionomer Surlyn 8940. Reversible bidirectional SME (2W-SME) was quantified by cyclic, thermomechanical tensile tests. These tests consisted of an initial programming and several reversible cycles. Programming is a process for obtaining the chain segment orientation and the macroscopic shape shifting geometry.6 The 2W-SME of the Surlyn film was studied by imposing selected tensile stresses and cyclic switching temperatures between a high temperature and a low temperature, at intervals of 25−71 °C (Figure 4). After a hot programming at 72 °C with 0.86 MPa load, the Surlyn film was stretched to 79% strain (Figure 4a). When testing its 2W-SME between 25 and 71 °C, external loads were gradually increased from 0.48 to 1.13 MPa, and the reversible actuation of strain increases with ramping up the loads. With 1.13 MPa external load, the contraction of the Surlyn film upon heating increased to 3.2%, whereas the elongation upon cooling is 9.0% (average values of five cycles). The creep effect was noticed in the plot (strain range shifted to higher values as the cycle number increased), and it is a common behavior for polymers. Clearly, the obvious 2W-SME of Surlyn did not appear until the external stress was 1.13 MPa in Figure 4a. The reason may be because the previous loads were too small to trigger the 2W-SME. The training history, which is presumably needed for displaying the reversible actuation, actually plays an insignificant role. To verify this, an experiment, which directly applies 1.13 MPa external load to the Surlyn film after the one-time programming by the 0.86 MPa tensile load, but without the gradual training process in 14815

DOI: 10.1021/acsami.6b04105 ACS Appl. Mater. Interfaces 2016, 8, 14812−14823

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Figure 4. Strain-response in thermomechanical cycles within 25−71 °C indicating the two-way shape-memory effect of the Surlyn films. (a) Training history with gradually ramping up the external stresses. (b) Direct loading with optimized stress based on (a). The selected stresses applied to the Surlyn films which are discussed in the text, are labeled in the figure for clarity.

Figure 5. Strain-response in thermomechanical cycles within (a) 25−74 °C and (b) 25−80 °C at selected loading stresses. The selected stresses applied to the Surlyn films, which are discussed in the text are labeled in the figure for clarity.

Figure 4a, has been conducted (Figure 4b). The 2W-SME appears immediately with contraction of 3.3% upon heating and elongation of 13.2% upon cooling (average values of three cycles). This indicates that the training history in Figure 4a does not play a significant role in inducing the 2W-SME. As long as a “correct or optimal” external load is applied, here 1.13 MPa, the 2W-SME can occur, regardless of the training history. The only role that the long training history may play is probably in determining the “right” external load. It helps find the “right” external load by gradually increasing the load, avoiding a possible overload which may damage the specimen with one-time loading. From this point of view, we believe that a training history may be helpful when determining the “right” external load to trigger the 2W-SME, particularly for new polymers that lack information on their thermomechanical properties. It is interesting to know that once the maximum external load has been applied to the specimen, here 1.13 MPa, the 2W-SME can still be seen even when ramping down the external load gradually. For example, at 0.44 MPa external load, the specimen shows contraction of 11.2% upon heating and elongation of 4.6% upon cooling. This suggests that as long as the specimen has experienced a big stress history, here 1.13 MPa, 2W-SME is ensured even though a much smaller external stress (0.44 MPa) is used. The 2W-SME of the Surlyn films was also studied at other temperature intervals of 25−74 °C (Figure 5a) and 25−80 °C (Figure 5b). By tuning the external loads, the 2W-SME can be

amplified, which are displayed in the purple boxes in Figure 5a and 5b. At temperature range of 25−74 °C (Figure 5a), the reversible strain actuation is increased with the increase in the external load. With 0.83 MPa load, the contraction upon heating is 3.2%, and the elongation upon cooling is 12.7%. At a temperature range of 25−80 °C (Figure 5b), the contraction upon heating is 2.9%, and the elongation upon cooling is 14.2% (average 0.64 MPa load). Then the load is gradually decreased to continuously monitor the 2W-SME. With 0.51 MPa load, the contraction upon heating is 10.2%, and the elongation upon cooling is 22.7%. This strong actuation was not achieved when gradually increasing the load to tune the 2W-SME. It indicates that the 2W-SME needs to be triggered first with a relatively large load, and then the force can be decreased to maintain the reversible actuation. When the external load is decreased to 0.42 MPa, the contraction is 9.6% upon heating, and the elongation is 7.0% upon cooling, where the effect is not very obvious when the external load is 0.42 MPa at the beginning of the 2W-SME testing. The strain changes of the bidirectional actuation from the 2W-SME are comparable with reported values for other systems.28,37 The coefficient of thermal expansion (CTE) study of the Surlyn film shown in Figure 6 exhibits that the material expands upon heating and contracts upon cooling with 1.0% strain change within the temperature range. This indicates that the 2W-SME of Surlyn should be 1.0% more than the values obtained from the 14816

DOI: 10.1021/acsami.6b04105 ACS Appl. Mater. Interfaces 2016, 8, 14812−14823

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strain level, the more efficient the 2W-SME (larger reversible actuation strain), when almost zero external stress was applied during the test. When Surlyn film was programmed with an even larger prestrain, 250% in Figure 8, true 2W-SMEs (i.e., 2W-SME with zero external load) were displayed. It is also found that bigger reversible actuation were obtained with larger temperature intervals. Similarly, under different programming prestrains, the force needed for inducing the 2W-SME and the strain changes of the reversible actuation are compared side-by-side; see Figure 9 for temperature range 25−79 °C and Figure 10 for temperature range of 25−76 °C. It is seen that if a large programming prestrain is used during the tension programming, the external tensile stress needed to induce the 2W-SME is reduced and the reversible actuation is increased. In other words, “true 2W-SME” can be achieved through a one-time large-tension programming, as shown in Figure 8. 3.6. Tunable 2W-SME of Ionomer Surlyn. After the systematic studies of one-way multishape-memory effect and two-way shape-memory effect of the Surlyn film, we were curious about the possibility to combine the two properties together (i.e., multishape 2W-SME or tunable 2W-SME). Is the 2W-SME of ionomer Surlyn tunable? That is, does the 2W-SME always exist when jumping temperatures randomly within the melting transition window? This experiment was conducted, and the results are shown in Figure 11. The Surlyn film first experienced a hot tensile programming with a prestrain of 207%. The 2W-SMEs, in other words, contraction upon heating (CUH) and elongation upon cooling

Figure 6. Coefficient of thermal expansion study of the Surlyn film. External load is 3.5 × 10−4 MPa (small enough and readable for the instrument). The Surlyn film elongates upon heating and contracts upon cooling with 1.0% of the strain change. The strain changes of the first two cycles are not included in the calculations to eliminate the influence of the materials’ training history.

thermomechanical cycles in order to compensate for the positive CTE effect. 3.5. Effect of Programming on 2W-SME. The programming step has a strong influence on the 2W-SME of the Surlyn films. As shown in Figure 7, the larger the initial programming

Figure 7. Two-way shape-memory effect with almost zero external stress (0.02 MPa). The specimens were one-time programmed by different prestretching strains. (a) 204%. (b) 150%. (c) 106%. (d) 42%. The strain reversible actuation under these four conditions are (a) 6.7%. (b) 3.9%. (c) 3.4%. (d) 0% (no two-way effect). The working temperature range is 25−77 °C. 14817

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Figure 8. True two-way shape-memory effect under different temperature intervals with zero external stress after training by 250% pretension. The strain reversible actuation under these four temperature intervals are listed in the inset table.

Figure 9. Two-way shape-memory effect at 25−79 °C with different programming strains. (a) 201.8%. (b) 80.2%. (c) 0% (without hot programming). (d) Side-by-side comparison of panels a, b, and c.

(EUC) were consistently observed in the cyclic thermomechanical tests when jumping randomly within different temperature

intervals, which suggests different actuation levels (or shapes) (Figure 11). When returning back to the temperature intervals to 14818

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The Surlyn film was deformed and fixed into four temporary shapes, at 92, 85, 75, and 65 °C with external loads of 0.02, 0.08, 0.46, and 1.14 MPa, respectively. Upon heating to 65, 75, 85, and 92 °C, the shape of the Surlyn film gradually recovered to each temporary shape and finally the permanent shape. The shape fixity and recovery ratios are shown in the caption of Figure 12, which were calculated on the basis of eqs 3 and 4. After the quintuple-shape-memory testing, the Surlyn film experienced a hot programming and was stretched to a prestrain of 152% in Figure 12a. The tunable 2W-SME was then examined by jumping randomly at different temperature intervals. Elongation upon cooling and contraction upon heating can be repeatedly visualized at randomly selected temperature intervals. The external loads and the strain fluctuations of the reversible actuation are listed in Table 1. For the very last two cycles, the Surlyn film was cooled to −21 °C and heated to 74 °C to test the 2W-SME. The elongation upon cooling is 5.7% and contraction upon heating is 3.4%. This indicates that the 2W-SME can be enabled under a wide working temperature range, even well below the freezing point of water. For comparison, in Figure 12b, no programming was conducted and the Surlyn film was subjected to 2W-SME testing immediately after quintuple shape-memory cycle (the training zone). On the basis of the values given in Table 1, we can conclude that the reversible actuation in Figure 12a is larger than that in Figure 12b, even with smaller external tensile stresses in Figure 12a. This suggests that a hot tensile programming with a large prestrain of over 80% can store a sufficient amount of internal tensile stress, so that the 2W-SME can have a relatively large reversible actuation with a relatively small external tensile stress. In other words, in order to trigger tunable 2W-SME, a long time period of cyclic training using small external stress is not needed. As long as a one-time tensile programming has a sufficiently large pretension, tunable 2W-SME can be endowed to the Surlyn film. It is seen that the tunable bidirectional reversible actuations always exist, regardless of the training history. However, with a programming process, less external tensile stress is required to trigger the 2W-SME with higher reversible actuation (Table 1). We believe that this is attributed to a higher degree of crystallinity of the Surlyn film with a larger programming strain, which stored a sufficient amount of internal energy through stress induced crystallization. Therefore, the tunable 2W-SME of the ionomer Surlyn exists, regardless of the training process. Only a one-time tensile programming with a sufficiently large prestrain is needed. 3.7. XRD Study on Surlyn Films. One as-prepared and two stretched Surlyn films with programming strains of 31% and 69% were characterized with XRD. The samples were scanned from 4° to 90° angles, and only 4° to 50° angles were plotted because no prominent peaks were observed in the higher scattering angle area. The intensity versus scattering angle for all the three samples are plotted in Figure 13. We can see that with the increase in the programming strain of the Surlyn films, the intensities of the peaks around 20° are increased, and the peaks are becoming sharper. The crystallinities of the Surlyn films are calculated on the basis of eq 5.

Figure 10. Side-by-side comparison of the 2W-SME of the Surlyn film with (a) and without (b) hot programming. The working temperature interval is 25−76 °C. The tensile stress to induce 2W-SME, the elongation upon cooling, and contraction upon heating for both cases are listed in the inset table for comparison. The elongation upon cooling and contraction upon heating are average values of five cycles and the last four cycles for a and b, respectively.

which it was subjected previously, the Surlyn film displayed the 2W-SME at a similar strain actuation level. A slight increase in the strain actuation was noticed, which is due to the creep effect of the material. Therefore, the fact that, with the same temperature interval and under the same external load, the Surlyn ionomer shows similar reversible strain actuations, regardless of its thermomechanical history, proves its tunability. In Figure 11, the tunable 2W-SME of the Surlyn film appears immediately after a hot programming process without the long training process. In Figure 4a, we have shown that the training process is not needed to trigger the 2W-SME, as along as the specimen experienced a one-time programming with sufficient prestrain. Here, we show that the role played by the training process on the tunable 2W-SME is also insignificant. To prove this statement, in Figure 12, a training process− quintuple shape-memory cycle has been employed prior to the tunable 2W-SME test (the training zones in Figure 12a,b).

%crystallinity = (100% × area of crystalline peaks) /(areas of crystalline peaks + amorphous peaks) (5)

The values are given in Table 2, and the trend follows our prediction; that is, a tension programming leads to an increase in crystallinity (stress induced crystallization) and internal energy 14819

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Figure 11. Tunable two-way shape-memory effect of the ionomer Surlyn. The working temperature interval (ΔT), tensile stress to trigger the 2W-SME (F), the contraction upon heating (CUH), and elongation upon cooling (EUC) are displayed in each interval of the figure.

result, the melting and crystallization of these crystallites are more repeatable and tunable. Furthermore, the higher crystallinity suggests that more polymer chains are involved in the melting and crystallization process, leading to larger 2W-SME for tensile programmed samples. The proposed mechanism of the tunable 2W-SME is also schematically shown in Figure 14. A hot programming process leads to an increase in the crystallinity of the Surlyn film and the formation of crystals with varying sizes. During the tunable 2W-SME testing, smaller crystals melt and bigger crystals decrease in size upon heating to their corresponding melting temperature, for example, 65 °C. The molten crystals recrystallize, and smaller crystals increase in size upon cooling to a temperature below Tg, such as 25 °C. Upon heating to an even higher temperature, for example, 85 °C, the crystallites of various sizes melt or decrease in size first with melting temperatures corresponding to 25−65 °C, followed by those corresponding to melting temperatures of 65−85 °C. When cooling to 25 °C, all the crystallites melted in 25−85 °C recrystallize (i.e., returning to their initial status). Therefore, when heated up again from 25 to 65 °C, the actuation behavior seen in the first cycle reappear, leading to tunability. Melting of crystallites leads to contraction and recrystallization leads to elongation (i.e., 2W-SME).

storage. Domain sizes of the crystallites are calculated on the basis of eq 6 τ=

Kλ β cos θ

(6)

where K is a dimensionless shape factor, which is assumed to be 1 since the Surlyn film is rectangular in shape. λ is the X-ray wavelength. β is the line broadening at half of the maximum intensity. θ is the Bragg angle. Each area covered by every broad peak around the 20° angle is fitted by three Gaussian functions. The lattice parameters for all the three samples are comparable with each other. However, the domain sizes of the crystallites show a clear increase with programming strain level. 3.8. Mechanism for Tunable 2W-SME. The 2W-SME of Nafion is based on the order−disorder transitions of the ionic clusters.12 We believe that for Surlyn, it is a crystallizationmelting driven process. As shown in Figure 1b, the peak at 47.9 °C is due to the order to disorder transition where the ionic clusters disorder upon heating.60 A slow relaxation is needed for the ordering of ionic clusters from the disordered state,60 leading to the disappearance of the endotherm peak in the second heating cycle. This suggests that the order−disorder transitions cannot be the mechanism for the 2W-SME of ionomer Surlyn because we observed the 2W-SME repeatedly with cyclic heating and cooling. The broad melting peak (56−100 °C) and the crystallization peak (55.3 °C) repeatedly appear in these two cycles, which indicate that the 2W-SME of this Surlyn ionomer is driven by the melting and crystallization mechanism. The existence of the tunability of the 2W-SME of the ionomer Surlyn within the broad melting transition window is due to the existence of the melting and crystallization transition, regardless of the width of the temperature interval and the amount of the crystallites involved. However, larger reversible actuation can be achieved with wider temperature interval. This mechanism is also supported by the XRD results. From Table 2, programming clearly increases the crystallinity and crystal domain size, suggesting larger and more perfect crystallites. As a

4. CONCLUSIONS Ionomer poly(ethylene-co-methacrylic acid) (commercial product Surlyn 8940) has been established to have both oneway multishape-memory effect and tunable two-way shapememory effect. Tensile programming is noticed to significantly reduce the external tensile stress needed to trigger the two-way shape-memory effect because crystallinity of the material can be increased by hot programming. With the largest programming strain of 250%, “true 2W-SME”, that is, 2W-SME without external load, is achieved. For both 2W-SME and tunable 2W-SME, a long time period of training process, for example, gradually ramping up the external load, is not needed to trigger the 2W-SME. As long as a one-time tensile 14820

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Figure 13. XRD intensity profiles of the three samples with different programming strains. Sample 1 is as-prepared; sample 2 and 3 are prestretched to 31% and 69% strain, respectively.

Table 2. Crystallinity, Domain Sizes, and Lattice Parameters Acquired with XRD

programming strain crystallinity domain size (three directions) lattice parameter (three directions)

Figure 12. Multishape and tunable two-way shape-memory effect with (a) or without (b) programming before jumping temperature randomly at different intervals. (a) Quintuple-shape-memory cycle at Td1 = Tr4 = 92 °C, Td2 = Tr3 = 85 °C, Td3 = Tr2 = 75 °C, Td4 = Tr1 = 65 °C. FS0→S1: 76.5%, FS1→S2: 54%, FS2→S3: 40.2%, FS3→S4: 76.4%, RS4→S3: 85.9%, RS3→S2: 38.4%. RS2→S1: 24.4%, RS1→S0: 4.2%. (b) Quintuple-shapememory cycle at Td1 = Tr4 = 92 °C, Td2 = Tr3 = 85 °C, Td3 = Tr2 = 75 °C, Td4 = Tr1 = 65 °C. FS0→S1: 60.0%, FS1→S2: 55.6%, FS2→S3: 42.3%, FS3→S4: 85.7%, RS4→S3: 87.3%, RS3→S2: 51.5%. RS2→S1: 43.6%, RS1→S0: 68.8%.

sample 1 (as-prepared film)

sample 2 (31% strain)

sample 3 (69% strain)

0 42.4% 5.3 nm 5.0 nm 10.8 nm 0.49 nm 0.42 nm 0.40 nm

31% 44.7% 3.7 nm 18.5 nm 19.5 nm 0.44 nm 0.42 nm 0.38 nm

69% 50.1% 4.0 nm 20.8 nm 19.8 nm 0.43 nm 0.42 nm 0.38 nm

Table 1. Tunable Two-Way Shape-Memory Effect at Different Temperature Intervals with (a) or without (b) a Large Hot Programming with programming tensile stress to trigger 2W-SME (MPa) elongation upon cooling (%) contraction upon heating (%) without programming tensile stress to trigger 2W-SME (MPa) elongation upon cooling (%) contraction upon heating (%)

25−79 °C 25−71 °C

25−76 °C 25−69 °C

0.46

0.80

0.62

0.91

7.6

5.8

8.4

6.0

5.9

4.9

6.6

4.9

25−76 °C 25−70 °C

25−79 °C 25−73 °C

0.83

1.13

0.64

0.83

8.3

3.9

5.0

3.6

1.8

2.4

3.5

3.0

Figure 14. Proposed mechanism for the tunable 2W-SME of the Surlyn film.

is a crystallization-melting-driven process. On the basis of the experimental observation and our proposed mechanism, we believe that a class of thermally reversible polymers should also exhibit tunable 2W-SMEs and may find applications in real life such as actuators, fixator, sealant, artificial muscles, and so on.

programming with sufficiently large prestrain is conducted, the Surlyn film exhibits 2W-SEM and tunable 2W-SME. The XRD results show that the crystallinity and crystal domain size are increased by tension programming. The mechanism of the tunable two-way shape-memory effect is proposed, which 14821

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 001-225-578-5302. Fax: 001-225-578-5924. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors gratefully acknowledge the financially support by National Science Foundation under grant number CMMI 1333997, and Army Research Office under grant number W911NF-13-1-0145. The authors thanks Dr. Xin Li for helpful discussion on XRD characterization for Surlyn film.

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