Macromolecules 2011, 44, 175–180
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DOI: 10.1021/ma102279y
Significant Impact of Thermo-Mechanical Conditions on Polymer Triple-Shape Memory Effect Junjun Li and Tao Xie* Chemical Sciences & Materials Systems Laboratory General Motors Research & Development Center, Mail Code: 480-106-710, 30500 Mound Road, Warren, Michigan 48090-9055, United States Received October 4, 2010
ABSTRACT: A polymer triple-shape memory effect represents one of the most recent discoveries in the rapidly expanding field of shape memory polymers. It refers to the capability of a polymer to memorize two temporary shapes and subsequently recover them, all in one shape memory cycle. Although several examples of triple-shape polymers had been reported in the literature, they were notably evaluated under very different thermo-mechanical conditions. In this study, the effect of various thermo-mechanical conditions on the polymer triple-shape properties was investigated using Nafion as a model material. The choice of the programming and recovery heating methods in constructing triple-shape cycles was found to have a profound impact on the triple-shape properties. As such, the results of this study provided useful reference for future development of triple-shape polymers.
Introduction A conventional shape memory polymer can fix one temporary shape and recover to its permanent shape when exposed to an appropriate external stimulus.1-10 With a total of two shapes involved in each shape memory cycle, this effect can be called the dual-shape memory effect. Such a concept of polymer dual-shape memory effect has been known for at least half a century and is the basis for most SMP applications known today.11-20 Within the past decade or so, the field of shape memory polymers has grown in an accelerated pace. Among the most recent and exciting discoveries are the multishape memory effects, which refer to the capability of a polymer to memorize multiple temporary shapes and subsequently recover them, all in one shape memory cycle.21-29 At the molecular scale, the dual-shape memory effect requires the combination of a reversible thermal transition and a mechanism for setting the permanent shape. The former is referred to as a shape memory transition and is commonly a melting or glass transition, while the latter can be physical or chemical crosslinking.1-10 As such, the expansion from dual- to triple-shape memory effect follows the logic that an additional distinctive shape memory transition allows a second temporary shape to be fixed in the shape memory cycle.22-29 For this type of triple-shape polymers, the two temporary shapes are introduced and later recovered above and in between the two shape memory transition temperatures (Ttrans’s), respectively.22-29 Accordingly, tuning the triple-shape memory effect can be accomplished by changing the Ttrans’s and/or the ratio between the two shape memory transition phases. Representing significant deviation from the above strategy is our recent report of the tunable multishape memory effect.21 It refers to the phenomenon that a polymer with a single yet sufficiently broad thermo-mechanical transition can exhibit multishape memory effects at arbitrarily chosen deformation and recovery temperatures (within the broad transition).21 In other words, the multishape memory effect can be tuned without changing the material composition, which is in sharp contrast to the triple-shape polymers with two distinct thermal transitions. *Corresponding author. E-mail:
[email protected]. r 2010 American Chemical Society
Putting aside the tunability and different strategies to achieve multishape memory effects, only triple- and quadruple-memory effects have been experimentally demonstrated thus far, although the potential beyond quadruple- is feasible.21 For simplification, however, we hereby focus on the triple-shape memory effect, noting in particular that the issues discussed hereafter are also applicable to multishape memory effects beyond triple-. In general, all shape memory effects can be quantitatively evaluated in appropriate thermo-mechanical shape memory cycles.1-10,21-26 For the dual-shape memory effect, this task is relatively straightforward. It is usually sufficient to determine the extent of shape fixing (shape fixity, Rf) and the degree of recovery (shape recovery, Rr), both of which can be extracted conveniently from a dual-shape memory cycle.1-10 In such a cycle, deformation (or programming) can be conducted by applying a constant force (i.e., a stress controlled mode) or a target strain (i.e., a strain controlled mode). The stress free shape recovery, on the other hand, can be carried out by either continuously heating or holding the polymer at a constant temperature above its shape memory transition (i.e., isothermal) until the strain reaches an equilibrium value. Although the impact of various thermo-mechanical conditions on dual-shape memory properties have been investigated,30-34 the specific distinction between different deformation and recovery modes is typically overlooked. Presumably, this is due to the fact that these two factors are not expected to noticeably impact the evaluation of Rf and Rr. For triple-shape polymers, a careful examination of the literature reveals significant differences in thermo-mechanical conditions under which triple-shape memory cycles were constructed.21-29 The most notable ones lie in the deformation modes (stress versus strain controlled) and shape recovery conditions (continuous versus staged heating). Since triple-shape memory cycles are intrinsically more complicated than dual-shape memory cycles, it is not clear how much these differences may impact the evaluation of the polymer triple-shape memory effect. In this work, the tripleshape memory effect of Nafion is investigated under carefully designed experimental conditions. We show that, unlike the dualshape memory effect, the thermo-mechanical conditions greatly impact the triple-shape effect. In the context of the broader Published on Web 12/13/2010
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literature on triple-shape polymers, we demonstrate that the protocols employed to construct triple-shape cycles could even challenge the very definition of triple-shape polymers. Although only a few triple-shape memory polymer systems have been reported thus far, we fully anticipate that many more will be developed in the near future. This study thus provides valuable guidelines which will benefit this emerging area of triple-shape polymers. Experimental Section Materials. Nafion (acid form, equivalent weight of 1000, and thickness of 0.08 mm) was obtained from DuPont. It was annealed at 140 °C for 25 min prior to investigation. Shape Memory Characterization. All the thermo-mechanical analysis (TMA) experiments were conducted in a tensile mode using a DMA Q800 (TA Instruments). Stress controlled deformation was performed by applying a deformation force at a target deformation temperature (Td) and the force was maintained constant during the subsequent cooling stage. By contrast, strain controlled deformation was conducted by ramping the strain at Td at 10%/min to a target strain, which was maintained constant during subsequent cooling. All strain recovery experiments were carried out under stress free condition (note: a minimum static force of 0.001 N was used). The strain recovery upon staged heating was conducted by holding the polymer at the target temperatures (recovery temperatures, Tr’s) equal to the corresponding deformation temperatures (Td’s) for 25 min. For strain recovery under continuous heating conditions, the experiments were performed by linearly ramping the temperature to 140 °C. Unless otherwise noted, a temperature ramping rate of 5 °C/min was used for both heating and cooling in all experiments.
Results and Discussion In our previous work, we demonstrated in a broad sense that Nafion possesses tunable multishape memory and temperature memory effects.21 Of relevance to the current study is that Nafion shows the triple-shape memory effect at any two sufficiently separated deformation temperatures (Td1 and Td2) across its broad thermo-mechanical transition (from 55 to 130 °C).21 This allows its triple-shape memory effect to be tuned based on the selection of Td’s, instead of changing the material composition. Since the focus of the current investigation is not on tunability of the shape memory effect, we concentrate primarily on two deformation temperatures (Td1 =100 °C and Td2 = 60 °C) in this investigation. A triple-shape memory cycle constructed under strain controlled programming and staged heating recovery conditions is shown in Figure 1. In the first deformation (or programming) step, the polymer is deformed at Td1 to a target strain (ε1,load) by linear strain ramping. The strain is maintained constant during subsequent cooling (i.e., a strain controlled mode). At this step, stress relaxation is observed. Once the temperature reaches Td2 and is equilibrated, the stress is removed. The stress removal leads to an instantaneous strain recovery (i.e., spring back). Isothermal and stress free holding at Td2 leads to an equilibrium strain (ε1). This completes the first programming step and ε1 is regarded as the first fixed temporary shape. The second programming step proceeds in a similar fashion except that the deformation temperature is Td2 and the isothermal and stress free holding temperature is 25 °C. ε2,load and ε2 are similarly obtained. The first and second shape fixities (Rf1 and Rf2) are 63.8%, and 98.1%, as calculated using ð1Þ Rf 1 ¼ 100%ðε1 =ε1, load Þ and Rf 2 ¼ 100%ðε2 - ε1 Þ=ðε2, load - ε1 Þ
ð2Þ
Figure 1. Triple-shape memory cycle obtained under strain controlled programming and staged heating recovery conditions. Td1 = Tr2 = 100 °C, Td2 = Tr1 = 60 °C. Key: solid line, strain; dotted line, stress; dashed line, temperature.
After programming, the polymer is heated under a stress free condition to induce the strain recovery. In the first recovery step, the temperature is ramped to Td2 and held constant until the strain reaches an equilibrium value, which is taken as ε1,rec. Further heating to Td1 followed by isothermal holding leads to a second equilibrium strain of ε0,rec. The two shape recoveries (Rr2 and Rr1) are 95.1%, and 106.0%, based on Rr2 ¼ 100%ðε2 - ε1, rec Þ=ðε2 - ε1 Þ
ð3Þ
Rr1 ¼ 100%ðε1, rec - ε0, rec Þ=ðε1 - ε0 Þ
ð4Þ
and
Here, Rr1 above 100% is due to the incomplete recovery of the previous recovery event (Rr2 = 95.1%). The portion that was not recovered in the first recovery event was recovered at the second recovery event, thus leading to a Rr1 value above 100%. Close examination of the strain evolution curve in Figure 1 revealed that, in the second strain ramping step from ε1 to ε2,load, the slop of the strain curve undergoes a sudden change. This is rather surprising as a single strain ramping rate was designated in the corresponding TMA method. In addition, this change appears to occur when the strain reaches a value close to the previous ε1,load. To investigate whether such a correlation is a coincident or not, two experiments were conducted in which only the target value of ε1,load was varied from 60% to 40% and 80%, respectively. The results (shown in Figure 2a and 2b) confirm that, regardless of the ε1,load, this abrupt slope change in the strain curve always occurs at a strain in the vicinity of the corresponding ε1,load. An experiment was further carried out under conditions identical to Figure 1 except that Td1 was changed to 120 °C. The strain evolution curve (Figure 2c) shows again the same slope change at the ε1,load. The above results imply that the instantaneously recovered strain (or nonfixed strain, ε1,load - ε1), while erased, shows a quantitative impact at a later stage. We refer to it a strain history memory effect. Such an effect is in sharp contrast to the traditional understanding of shape memory polymers, for which the recovered strain cannot be traced. Here, the entropy corresponding to the strain (ε1,load - ε1) was lost instantaneously upon load removal, but the contributing molecular segmental movement remained at an “activated” state mechanically. Surprisingly, this “activated” state appears to be thermally stable as the observed phenomenon did not diminish even when the isothermal holding time between ε1 to ε2,load was increased from 5 min (Figure 2a) to 1 h (Figure 2d). Together with the tunable multishape memory and temperature memory effects recently reported,21,35 this strain history memory effect suggests that that a polymer memory effect can be more broadly understood as a
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Figure 3. Effect of ΔTd on Rfs. In all cases, Td2 = 60 °C and ε1,
load=60%.
Figure 2. Thermo-mechanical curves for strain controlled programming (Td2 = 60 °C in all cases): (a) Td1 = 100 °C, ε1,load=40%, ε2,load = 85%; (b) Td1 = 100 °C, ε1,load = 80%, ε2,load = 100%; (c) Td1 = 120 °C, ε1,load = 60%, ε2,load = 85%; (d) condition same as part a except the isothermal holding time (1 h).
thermo-mechanical memory effect rather than just strain (or shape) memory. On a broader basis, the strain history effect uncovered above implies that the thermo-mechanical history of a polymer could have a hidden effect on its thermo-mechanical properties. Under various experimental conditions studied, Rr1 and Rr2 for Nafion were generally close to 100%, but the difference between the two Td’s (ΔTd) had a strong impact on Rf1.21 Under the stress controlled programming conditions previously used in our previous work,21 it was difficult to reach the same ε1,load for different experiments, we thus did not attempt to quantify the impact of ΔTd on Rf1 due to this uncertainty. The strain controlled programming method currently employed allows quantification of such an impact at the same ε1,load value. The results (Figure 3) show that, whereas Rf2 is independent of ΔTd and generally above 95%, Rf1 has a strong dependence on ΔTd. Specifically, Rf1 is around 50% for ΔTd of 30 °C and increased gradually to 70% for ΔTd of 60 °C (Figure 3). This trend can be explained by an analogy we used to explain the tunable multishape memory effect. On the basis of such an analogy, Nafion’s broad thermal transition can be viewed as the collective contribution of an infinite number of transitions, which can be further considered as individual memory elements corresponding to infinitely sharp transition temperatures continuously distributed across the broad transition. Using this analogy, if the ΔTd is higher, a higher population of memory elements is “frozen” upon cooling, yet the number of “non-frozen” memory elements remains the same since Td2 is identical. The frozen memory elements act against the nonfrozen memory elements to fix the strain. Consequently, a higher ΔTd leads to a higher Rf1 value. Here, we note that the dependence of Rf1 on ΔTd may also be attributed to Td1, the change of which led to different ΔTd values in the series of the experiments summarized in Figure 3. For comparison purposes, two triple-shape memory cycles were obtained under a stress controlled condition (Figure 4). Here, Td1’s for Figures 4a and 4b are 100 and 140 °C, respectively, whereas Td2’s are kept identical at 60 °C. In these stress controlled experiments, the deformation stress was applied at the target deformation temperature (Td1). While the stress was held constant during cooling, strain continued to increase during cooling (i.e., creep). In other words, the strain was actually introduced across a temperature range instead of at a single temperature in a strain controlled programming. The creep upon cooling led to strain development at temperatures lower than the designated Td, which would be recovered at lower temperatures during the corresponding recovery step(s). Here, noticeable levels of creep are observed in the first deformation steps in both Figures 4a and 4b, but not in their second deformation steps. Relatively speaking, the creep in Figure 4b is much more pronounced than Figure 4a.
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Figure 4. Triple-shape memory cycle obtained under stress controlled programming and staged heating recovery conditions. (a) Td1 = Tr2 = 100 °C, Td2 = Tr1 = 60 °C; (b) Td1 = Tr2 = 140 °C, Td2 = Tr1 = 60 °C. (c) Conditions similar to part b except that a faster cooling rate of 10 °C/min was used in the first fixing step. Key: solid lines, strain; dotted lines, stress; dashed lines, temperature.
The creep upon cooling leads to the much broader second shape recovery event in Figure 4b, when compared to the first recovery in the same figure or the second shape recovery in Figure 4a. The extent of creep upon cooling was reduced to 34% of the total ε1,load when a faster cooling rate of 10 °C/min was employed (Figure 4c), compared to 40% for a cooling rate of 5 °C/min (Figure 4b). The above stress controlled programming was widely utilized by different groups.21-25,29 For our own amorphous epoxy based triple-shape memory systems reported earlier, however, the creep upon cooling under the constant load is rather minimal, if any.23 This is due to the very tightly cross-linked nature of the epoxy network that resists the creep. Consequently, the selection of the programming method is unimportant for such a system. The triple-shape systems by Lendlein et al are lightly cross-linked networks comprising of at least one crystalline phase.22,24,25 Under a constant stress, a crystalline phase is known to undergo rearrangement upon temperature changes, leading to significant strain
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change. In fact, such is the basis for the two way shape memory effect for semicrystalline polymers.26,36 As for the triple-shape memory effect discussed here, significant strain changes upon cooling were indeed observed in the semicrystalline based tripleshape systems.22,25 Consequently, we anticipate that the choice of the programming method would affect the recovery behavior for such systems, although the exact impact has yet to be reported. Nevertheless, the comparison between Nafion and our previously reported epoxy system is sufficient to conclude that the selection of the programming method may significantly affect the multirecovery behavior, depending on the specific molecular structures. Here, we should note that the isothermal and stress free holding between the two programming steps (Figures 1 and 4) is sometimes omitted in the literature. Regardless of the programming methods, this step is necessary to determine ε1, which is a critical parameter in the complete quantification of the triple-shape performance (eqs 1-4). In the context of different programming methods for the tripleshape memory effect, the so-called one-step programming process should also be mentioned. It refers specifically to the phenomenon that only one programming step is needed for a multishape capable polymer to exhibit multiple shape recovery events upon heating.21,24 Fundamentally, the multiple recovery events arise from the fact that a single strain introduced at a high temperature in a one-step programming process is in fact differentiated as multiple strains occurring to different memory elements in Nafion21 or two distinct phases in other polymers.24,25 Notably, the examples of one-step programming multishape effect were all realized through stress control.21,24,25 Under such a programming condition, if creep occurs, the memory elements corresponding to transition temperatures lower than the designated Td (or the lower temperature phase) could sustain a larger share of the total strain. This could favor more strain recovery at lower temperatures, relative to a situation in which a strain controlled one-step programming is used. As for the shape recovery, Figures 1 and 4 were both obtained by staged heating, which is a condition used for our triple-shape epoxy polymers23 and the triple-shape poly(ester urethane) by Pretsch.27 This staged heating method is analogous to the isothermal recovery condition for dual-shape memory polymers except that the two isothermal recovery events are needed for triple-shape memory polymers. Other triple-shape systems reported in the literature adopted a continuous heating method to evaluate triple-shape properties.21,25,28,29 Whereas the distinction between the two heating methods for dual-shape memory polymers is typically overlooked, the difference in the recovery heating method for triple-shape polymers should be carefully considered. Here, two critical questions arise: is it possible that a polymer displays two separate recovery events under a staged heating condition, but not under a continuous heating condition? If the answer is yes, then the next question would be: could both heating methods be accepted in qualifying a triple-shape polymer? The answers to these questions may impact the very definition of a triple-shape polymer. We believe that both the staged heating and continuous heating methods are valid methods for triple-shape evaluation. In fact, even for triple-shape polymer systems quantitatively evaluated under continuous heating recovery conditions, it appears that the actual visual demonstration of the triple-shape memory effect was carried out by “subsequent heating”.22,25 Presumably, this is due to the fact that, in a practical setting, it is easier to achieve staged heating than finely controlled continuous heating. Fundamentally, however, the question on whether or not Nafion can display multiple recovery events by continuous heating remains interesting, given the fact that its triple-shape behavior rely on one broad thermal transition rather than two
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Figure 5. Strain recovery behavior under continuous heating conditions: (a) strain evolution; (b) strain derivative curve.
distinctive ones. A series of experiments were thus conducted in which the two programming steps were identical to Figure 1, but the subsequent recovery was performed by continuous heating at different heating rates. The corresponding recovery curves are shown in Figure 5a. At a heating rate of 5 °C/min, only vaguely separated two recovery events can be identified. Such a separation becomes much more evident when the heating rate is reduced to 1 °C/min. Further reduction of the heating rate to 0.5 °C/min yields only slight difference. Unlike Figures 1 and 4, no strain plateau is observed between the two recovery events in Figure 5a. We note that this is an issue that is applicable to all other known triple-shape memory polymer systems under continuous heating conditions. The existence of two recovery events is more clearly illustrated by plotting the strain derivatives (i.e., instantaneous recovery rate) against the temperature (Figure 5b). Two maxima are clearly visible regardless of the heating rate, although the relative peak height and positions do vary. We believe that this dependence between the recovery behavior and the heating rate is linked to the recovery kinetics. At a high heating rate, the first recovery event does not have sufficient time to complete before the temperature raises high enough to trigger the next recovery event (i.e., recovery overlap). The relative separation between the recovery events is dependent on the overlap, which is reduced at a lower heating rate. Here, it is noteworthy that a heating rate of 1 °C/min was used for the shape recovery in the literature despite the fact that a faster temperature ramping rate of 5 °C/min was employed in all other stages in the same triple-shape cycles.22,24,25 This seems to indicate a similar need for a low heating rate for those material systems. Comparing the recovery curve in Figure 1 to those in Figure 5a, it is also evident that it is easier to obtain the plateau strain values using staged heating. This method is thus suitable for evaluating
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Figure 6. Comparison of shape recovery curves under continuous heating conditions for two deformation temperatures Td1’s (all the other themomechanical conditions are identical to Figure 1). Key: (a) strain evolution; (b) strain derivative curve.
the maximum recovery capability. Recovery by continuous heating, on the other hand, is affected by both the maximum recovery capability and the recovery kinetics, the latter of which suggests heating rate dependence. In principle, the recovery overlap under a continuous heating condition may be reduced if the recovery events can be further distinguished. For a triple-shape polymer enabled by two distinct thermal transitions, one approach is to widen the difference between the two transition temperatures through material compositional tuning. For Nafion, owing to its tunable nature, this can be conveniently achieved by selecting two T d’s that are further apart (i.e., a larger ΔTd). To verify this hypothesis, two triple-shape cycles were constructed under identical thermo-mechanical conditions except the Td1. The corresponding recovery behaviors (Figures 6a and 6b) clearly show that the two recovery events obtained for the higher Td1 (thus greater ΔTd) are indeed better separated. As a separate but important note, we should point out that, despite its versatile shape memory properties, a significant limitation for Nafion lies in its poor processability. As a result, Nafion typically exists in thin film form, which prohibits its use as a bulk shape memory material. Nevertheless, the fact that Nafion’s multishape memory properties arise from a single thermal transition suggests an alternative platform on which future more processable multishape memory polymers can be based. Conclusion We show in this study that the impact of various thermomechanical conditions on the polymer triple-shape memory effect is much more complex than for the more conventional dual-shape
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memory effect. While material chemistry (two distinct thermal transitions versus a single broad transition) is critical, the thermomechanical treatment is just as important in determining polymer triple-shape functions. Here, the material chemistry provides the basis, whereas significant variations in memory functions can be expected by selecting the appropriate thermo-mechanical treatment. In particular, the choices of the programming method (stress versus strain controlled) and recovery heating method (staged versus continuous heating) were found to greatly affect the evaluation of triple-shape polymers. The results further support that staged and continuous heating are both valid methods for triple-shape evaluation. The staged heating method allows better evaluation of the equilibrium recovery behavior. Recovery behavior under a continuous heating condition may strongly depend on the recovery kinetics and a low heating rate is thus preferred to realize the multiple recovery events. Overall, this study suggests that, relative to the conventional dual-shape memory properties, greater care should be taken to achieve and/or evaluate triple-shape performance. Acknowledgment. The authors thank Prof. Patrick Mather at Syracuse University for his comments on the relevance of the different recovery heating methods for triple-shape polymers. References and Notes (1) Mather, P. T.; Luo, X.; Rousseau, I. A. Annu. Rev. Mater. Res. 2009, 39, 445. (2) Lendlein, A.; Kelch, S. Angew. Chem., Int. Ed. 2002, 41, 2034. (3) Liu, C.; Qin, H.; Mather, P. T. J. Mater. Chem. 2007, 17, 1543. (4) Xie, T.; Rousseau, I. A. Polymer 2009, 50, 1852. (5) Ratna, D.; Karger-Kocsis, J. J. Mater. Sci. 2008, 43, 254. (6) Koerner, H.; Price, G.; Pearce, N.; Alexander, M.; Vaia, R. Nat. Mater. 2004, 3, 115. (7) Lendlein, A.; Jiang, H.; Junger, O.; Langer, R. Nature 2005, 434, 879. (8) Rousseau, I. A. Polym. Eng. Sci. 2008, 48, 2075.
Li and Xie (9) Weiss, R.; Izzo, E.; Mandelbaum, S. Macromolecules 2008, 41, 2978. (10) Li, J.; Viveros., J. A.; Wrue, M. H.; Anthamatten, M. Adv. Mater. 2007, 19, 2851. (11) Lendlein, A.; Langer, R. Science 2002, 296, 1673. (12) Xie, T.; Xiao, X. Chem. Mater. 2008, 20, 2866. (13) Xiao, X.; Xie, T.; Cheng, Y. T. J. Mater. Chem. 2010, 20, 3508. (14) Gall, K.; Kreiner, P.; Turner, D.; Hulse, M. J. MEMS. 2004, 13, 472. (15) Wang, R.; Xie, T. Langmuir 2010, 26, 2999. (16) Wang, R.; Xie, T. Chem. Commun. 2010, 46, 1341. (17) Wang, R.; Xiao, X.; Xie, T. Macromol. Rapid Commun. 2010, 31, 295. (18) Kunzelman, J.; Chung, T.; Mather, P. T.; Weder, C. J. Mater. Chem. 2008, 18, 1082. (19) Kim, S.; Sitti, M.; Xie, T.; Xiao, X. Soft Matter 2009, 5, 3689. (20) Hu, J. Shape memory polymers and textiles; CRC Press, Woodhead Publishing Limited: Boca Raton FL, Boston, MA, New York, and Washington, DC, 2007. (21) Xie, T. Nature 2010, 464, 267. (22) Bellin, I.; Kelch, S.; Langer, R.; Lendlein, A. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 18043. (23) Xie, T.; Xiao, X.; Cheng, Y. T. Macromol. Rapid Commun. 2009, 30, 1823. (24) Behl, M.; Bellin, I.; Kelch, S.; Wagermaier, W.; Lendlein, A. Adv. Func. Mater. 2009, 19, 102. (25) Behl, M.; Lendlein, A. J. Mater. Chem. 2010, 20, 3335. (26) Zotzmann, J.; Behl, M.; Hofmann, D.; Lendlein, A. Adv. Mater. 2010, 22, 3424. (27) Pretsch, T. Smart Mater. Struct. 2010, 19, 015006. (28) Kolesov, I. S.; Radusch, H. J. eXPRESS Polymer Lett. 2008, 2, 461. (29) Luo, X.; Mather, P. T. Adv. Funct. Mater. 2010, 20, 2649. (30) Rousseau, I. A.; Xie, T. J. Mater. Chem. 2010, 20, 3431. (31) Yakacki, C. M.; Willis, S.; Luders, C.; Gall, K. Adv. Eng. Mater. 2008, 10, 112. (32) Gall, K.; et al. J. Biomed. Mater. Res. A 2005, 73A, 339. (33) Yakacki, C. M.; et al. Biomaterials 2007, 28, 2255. (34) Wong, Y. S.; Venkatraman, S. S. Acta Mater. 2010, 58, 49. (35) Miaudet, P.; et al. Science 2007, 318, 1294. (36) Chung, T.; Rorno-Uribe, A.; Mather, P. T. Macromolecules 2008, 41, 184.
