Hidden Thermoreversible Actuation Behavior of Nafion and Its

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Hidden Thermoreversible Actuation Behavior of Nafion and Its Morphological Origin Tao Xie,*,† Junjun Li,‡ and Qian Zhao† †

State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, P. R. China ‡ Eaton Corporation Global Research & Technology, 26201 Northwestern Highway, Southfield, Michigan 48076, United States ABSTRACT: Recent morphological characterization of Nafion as correlated to its proton transport properties has led to the conclusion that the structural ordering in the ionic phase of Nafion may be quite similar to liquid crystalline polymers. Liquid crystalline polymers, on the other hand, are well-known for their thermoreversible actuation (or two-way shape memory) behavior. This has motivated us to design and conduct thermomechanical experiments to probe Nafion’s potential liquid crystalline characteristics from the standpoint of thermoreversible actuation. Our experimental results surprisingly revealed the thermoreversible actuation behavior of Nafion, buried alongside the irreversible creep behavior commonly observed for some polymers. As such, the current study provides evidence supporting Nafion’s structural similarity to liquid crystalline polymers.

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INTRODUCTION Nafion, owing to its exceptional combination of chemical durability and proton conductivity, has been the state of the art material of choice for use as fuel cell proton exchange membranes.1 In particular, the desire for ever better proton conductivity has driven intense interest in understanding its morphology as related to its proton transport properties.2−4 An interconnected spherical reverse micelle structure was proposed in the early days.1 Increasing evidence from recent literature, however, has pointed to a microfibrillar structure.2 A study by Shmidt-Rohr et al. in 2008 further suggested that the ionic phase is composed of locally parallel cylindrical hydrophilic channels that are, on a more global scale, randomly distributed in its fluorinated matrix.3 In 2011, Li et al. reported a linear correlation between the proton transport anisotropy of Nafion and its degree of macroscopic alignment, leading to an intriguing conclusion that its hydrophilic ionic domains have an inherent nematic liquid crystalline-like character.4 On a seemingly unrelated subject, we have recently discovered that Nafion, by virtue of its broad thermal transition between ca. 55 and 135 °C, possesses surprisingly versatile multishape memory effect and temperature memory effect.5−7 This discovery was rather unexpected as Nafion, while not originally intended for this purpose, has been studied intensively for about half a century. It is worth noting that the type of shape memory behaviors reported so far for Nafion belong to the so-called one-way shape memory effect.8−14 The word “one way” emphasizes the irreversible nature of the shape memory effect; that is, without a reprogramming step, the recovered permanent shape cannot go back to the previously programmed temporary shape.8 By contrast, the two-way (or reversible) shape memory effect refers to the behavior that a © 2014 American Chemical Society

polymer can switch back and forward between two shapes without any change to the imposed external mechanical conditions such as reprogramming.8 This two-way shape memory behavior was at first reported for liquid crystalline elastomers (LCE),15−17 although recent developments have shown that cross-linked semicrystalline polymer networks are also capable of that.18−21 Specifically, such a thermoreversible actuation behavior is reflected as cooling-induced-elongation (CIE) and heating-induced-contraction (HIC), while the externally imposed stress (typically nonzero) remains unchanged.15−21 Overall, change in temperature alone leads to a reversible shape change event. Mechanistically, CIE originates from the cooling-induced formation of liquid crystalline (or crystalline) domains preferentially aligned in the stress direction, while HIC is due to the erasing (or melting) of the liquid crystalline (or crystalline) anisotropy.15−20 It is important to note that the typical nonzero external stress required for the two-way shape memory effect can be replaced by an elegantly introduced internal stress, thus achieving two-way shape memory behavior free of any external stress.22 In our early study of Nafion’s one-way shape memory behaviors, we observed that, while the stress remained constant in the programming step, the strain continued to increase during cooling (see Figure 1c of ref 5). Such a behavior is similar to the CIE essential for the two-way shape memory effect described above, although the structural origin of the CIE for Nafion was unknown (in fact, it may just be the creep sometimes observed for other shape memory polymers23). Received: October 26, 2013 Revised: January 12, 2014 Published: January 29, 2014 1085

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We suspected that the irreversible strain shifting arises from the creep. To verify that, two experiments were conducted under conditions (e.g., stress) identical to those for Figure 1 except that the temperature within each experiment was kept constant at 80 and 120 °C, respectively. The strain evolutions under such conditions represent creep and are recorded as the bottom short dashed curve and upper long dashed curve in Figure 2, respectively. By averaging these two curves, the

Nevertheless, we were quite intrigued by this observation. The morphological analogy between Nafion and liquid crystalline network as proposed in the fuel cell community4 further stimulated us to probe Nafion’s CIE in search for clues for its potential liquid crystalline like two-way shape memory behavior. Such an effort is described in this paper.

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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 30 min prior to investigation. Shape Memory Characterization. All the thermomechanical analysis (TMA) experiments were conducted in a tensile mode using a DMA Q800 (TA Instruments).

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RESULTS AND DISCUSSION A protocol similar to the two-way shape memory cycle15−21 was utilized to probe the thermoreversible actuation response of Nafion. In this protocol, the sample was first heated to a high temperature (TH) and a stress was subsequently imposed. This stress was maintained constant throughout the rest of the experiment while the strain response was recorded as the temperature was repeatedly switched between TH and a lower temperature (TL) at a constant temperature ramping rate of 10 °C/min. In particular, TL was chosen above the onset of the socalled α transition (between 55 and 135 °C). Somewhat coincidental yet nontrial is that the projected operating temperatures of fuel cells also falls within this α transition. Figure 1 shows the strain response upon consecutive temperature cycling between 120 and 80 °C under a constant

Figure 2. Comparison of strain responses in various thermomechanical cycles (stress is constant at 0.47 MPa for all curves).

middle solid smooth curve was obtained, which represents approximately the creep in the temperature cycling experiment. The saw-shaped strain curve Figure 1 was replotted in Figure 2 for comparison. As such, Figure 2 reveals a good match between the middle curve and the irreversible strain upshifting in the saw-shaped curve. This confirms that indeed the irreversible strain shifting does originate from creep, which is a common behavior for polymers. Here, we proceeded further to investigate the impact of experimental parameters on the extent of reversibility. To quantify the extent of reversibility, it is necessary to first define the cycles. Here, we consider that the nth cycle starts at TH, corresponding to a low strain of εHT(n). Cooling leads to elongation and the highest strain of εLT(n) is reached at TL. Further heating results in contraction, reaching the next lowest strain of εHT(n + 1) at TH, marking the completion of the nth cycle, which is also the beginning of the (n + 1)th cycle. Based on these definitions, within the nth cycle, the reversible strain and the overall strain (irreversible plus reversible) are εLT(n) − εHT(n + 1) and εLT(n) − εHT(n), respectively. Thus, the extent of strain reversibility (or reversibility ratio R) for the nth cycle can be calculated as

Figure 1. Strain response in thermomechanical cycles within 80−120 °C under constant stress 0.47 MPa.

stress of 0.47 MPa. In the first cycle, the application of the stress at 120 °C led to an instantaneous strain of 25%. The strain continued to rise upon cooling (CIE), reaching 35% at 80 °C. When cooling was switched to heating, the sample started to contract (strain decrease) instantaneously. This heating-induced-contraction (HIC) continued throughout the heating to 120 °C, completing the first cycle. For subsequent temperature cycles, similar CIE and HIC were also observed, resulting in a saw-shape strain response curve. Comparison between different cycles revealed that the strain range shifted to higher values as the cycle number increased. The overall strain response appeared to suggest that two concurrent molecular processes may be responsible for the reversible strain change (CIE and HIC) and the irreversible strain change (the upshift of the strain range between cycles).

R(n) =

εLT(n) − εHT(n + 1) × 100% εLT(n) − εHT(n)

(1)

Accordingly, data in Figure 1 were utilized to calculate R(n) values. Figure 3 summarizes that the evolution of reversibility ratio upon cycling, showing that the reversibility ratio improves with cycling, eventually reaching a plateau value. This is largely due to the fact that the irreversible strain (creep) slows down upon consecutive cycling (see the middle smooth curve in Figure 2), whereas the reversible strain remains relatively unchanged upon cycling. Due to the relatively independent nature of the reversible and irreversible strain components, we anticipate that various experimental parameters may affect the two strain components 1086

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Figure 3. Evolution of reversibility ratio upon consecutive cycling. Note: due to the initial application of stress in the first cycle, εHT(1) cannot be reliably obtained. R(1) was thus not calculated.

Figure 5. Impact of TH on the reversibility ratio. In all the experiments, TL and stress were identical at 60 °C and 0.36 MPa, respectively.

differently. Given the fact that the phenomenon is quantitatively dependent on cycling, we hereafter focus on data obtained for the third cycle to evaluate the impact of stress and TH. Here, the choice of the third cycle is to ensure comparable data without running overly long experiments. Accordingly, three experiments were run at identical TH (110 °C) and TL (60 °C) under different stresses. Here, the stress was maintained constant within each experiment but varied from one experiment to the other. Under these conditions, both the reversible and irreversible strains increase with the stress, leading to upper shifting of the entire strain curve. However, the extent of increase for the irreversible strain is more pronounced than the reversible strain. Thus, a lower stress is favored for a higher reversibility ratio (Figure 4),

Figure 6. Strain development in thermomechanical cycles within 60− 90 °C under constant stress 0.47 MPa.

the thermoreversible strain component (i.e., CIE and HIC) is rather unexpected for Nafion. We emphasize that the thermoreversible strain switching should be distinguished from the well-known reversible bending (i.e., strain change) of ionic polymer metal composites (IPMCs). IPMCs can only operate in aqueous solutions via the reversible redistribution of mobile counterions and water in response to electrical stimulation.25 In contrast, the reversible strain switching here is driven solely by temperature change. It is also important to point out that all the experiments reported here were conducted at ambient humidity (relative humidity around 30%). Although the lowest temperature used in this study is 60 °C, majority of the reversible strain switching occurred at temperature above 70 °C. Under ambient humidity, the free water activity is believed to be negligible at temperatures above 70 °C.26,27 Thus, the thermoreversible strain switching does not originate from the difference in free water activity induced by temperature change. On the other hand, dry Nafion always contains strongly bonded water due to the extremely hydrophilic nature of the SO3H groups. However, the bonded water cannot be removed under realistic experimental conditions. For instance, Nafion would thermally decompose before the bonded water is completely removed.26,27 Thus, it is practically impossible to prove/disprove the contribution of bonded water, as is the case for numerous studies conducted on Nafion. Although the thermoreversible strain switching has not been previously reported for Nafion, such a behavior is typical for liquid crystalline elastomers (LCEs). Under a constant stress, LCE exhibits the cooling-induced-elongation (CIE) and melting-induced-contraction (MIC) when the temperature is

Figure 4. Reversibility change with respect to stress, showing that lower stress favors better reversibility.

although it does adversely impact the amplitude of the reversible strain. Another set of experiments were run to understand the impact of TH. In these experiments, the stress and TL were kept identical at 0.36 MPa and 60 °C, respectively. The results, as summarized in Figure 5, show that a higher TH leads to (1) upper shift of the entire strain curve, (2) higher reversible and irreversible strain, and (3) a higher reversibility ratio. We note that the reversible strain switching with temperature was observed with TH as low as 100 °C, but we failed to observe this phenomenon with a TH of 90 °C. Temperature cycling between 90 and 60 °C yields only the irreversible creep (Figure 6). Thus, the onset temperature (TH) for the reversible strain switching is around 100 °C. In the thermomechanical behavior described above, the irreversible strain due to creep is common for polymers,23 but 1087

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reversibly switched between TH and TL (TH > Tcl > TL and Tcl being the liquid crystalline clearing temperature). At the microscopic level, the CIE arises from the stress-induced anisotropic alignment of its liquid crystalline phase, the melting of which is responsible for MIC. We should note that CIE and MIC have also been demonstrated for semicrystalline polymer networks. For ease of discussion only, however, we hereafter refer exclusively to LCE. The choice of this language is because Nafion’s structural similarity with LCE as suggested by others3,4 was the original motivation for this study and that LCE was the best known material showing the thermoreversible actuation. This context is important since other non-LCE materials such as cross-linked semicrystalline polymers also exhibit the thermoreversible actuation behavior. Thus, in the absence of other supporting evidence, a tighter connection between LCE and Nafion beyond the above context is not recommended. The morphological characteristics of Nafion are much more complicated than typical LCE. Understanding the molecular origin of its thermoreversible strain switching is only possible in the larger context of its complex morphology and molecular dynamics. Nafion has a complex multiphase morphology: (1) a hydrophobic continuous polytetrafluoroethylene (PTFE) phase with crystalline domains; (2) a hydrophilic dispersed phase consisting of the sulfonic acid groups and water (i.e., ionic phase). The melting point of the crystalline domains in Nafion is around 240 °C,24,29 well above the temperature range of the current study. Thus, the crystalline domains do not contribute to the phenomenon described above. The nature of the ionic phase has been hotly debated. Although the ionic phase was believed to exist in the form of isotropic spherical micelles, recent evidence points to anisotropic elongated structures (microcylinders, microfibrils, or microchannels).2,3 More details on the elongated structures have emerged recently in the literature. On the basis of theoretical simulation in comparison with experimental smallangle X-ray scattering (SAXS) data, Schmidt-Rohr et al. suggested that the ionic phase can be further described as the parallel cylindrical water nanochannels.3 In particular, the word “parallel” emphasizes the local ordering of the ionic phase. Another recent study by Li et al. showed that the transport properties of Nafion (ion conduction) are linearly coupled to the membrane stretching.4 Accordingly, they concluded that stretching led to orientation of the anisotropic ionic domains without affecting their individual dimensions. This is particularly noteworthy as it suggests the local ordering of the ionic phase proposed Schmidt-Rohr is in fact similar to the liquid crystalline ordering in nematic elastomers. The studies by Schmidt-Rohr et al. and Li et al. utilized significantly wet Nafion (e.g., 20 vol % of free water), which is different from the dry Nafion used in the current study. On the other hand, they appeared to imply that water swelling alters the dimension of ionic channels, but not the overall local ordering or liquid-crystalline like structure. With this in mind, the thermomechanical strain switching for dry Nafion can be potentially explained by its liquid crystalline like ionic phase. The fact that the phenomenon is only observed when TH is above 100 °C is also intriguing, given that the α transition occurs in a much broader temperature range from 55 to 135 °C. A small-angle neutron scattering (SANS) study by Page et al. suggested that the molecular dynamics of Nafion is drastically different at temperatures below and above its Tα (ca. 100 °C).24,28,29 The strong binding between the ionic groups in Nafion establishes an electrostatic network that

functions as physical cross-links. At temperatures between 55 and 100 °C, the molecular motions are facilitated through short-range segmental dynamics confined by the static electrostatic network.24,28,29 At above 100 °C, an ion-hopping process is activated, leading to the significant destabilization of the electrostatic network.24,28,29 As such, longer range molecular motions occur in the dynamic electrostatic network above the Tα. The molecular dynamics proposed by Page et al. was supported by our own recent study dedicated to the temperature memory effect of Nafion.6 Here, the temperature memory effect refers to the ability of a polymer to quantitatively memorize deformation temperatures. For Nafion, whereas the temperature memory effect was observed at temperatures below 100 °C, it became invalid above 100 °C due to the much faster molecular relaxation facilitated by the dynamic electrostatic network. In analogous to the melting of the liquid crystalline phase for LCE, we suggest that the destabilization of the electrostatic network at above the Tα of Nafion can be viewed as the “melting” of the locally ordered ionic phase. This view is consistent with the notion that Nafion undergoes an order−disorder transition at above 100 °C.24 In the context of the localized structural ordering and the dynamic ionic network structure reported in the literature and summarized above, we now proceed to explain the reversible strain switching for Nafion in more detail. Here, refs 3 and 4 both show strong evidence that the hydrophilic domains of Nafion consist of cylindrical hydrophilic channels that are locally parallel but macroscopically randomly distributed. At the initial state when Nafion is heated above the α transition of 100 °C, the ionic domains are in a dynamically disordered state due to the ion-hopping process.24,28,29 Applying a stress at this point leads to some (albeit limited) orientation in the polymer. Upon cooling, ion-hopping decreases and local structural ordering mentioned above (i.e., locally parallel hydrophilic channels) is formed. Let us consider each bundle of the locally parallel hydrophilic channels as a single domain. Due to the presence of the stress during cooling, these domains are aligned with respect to each other in the stress direction. It is this stress-induced domain to domain alignment that leads to cooling induced elongation. During heating under the same stress, the domain to domain alignment is erased due to the loss of structural ordering within each domain, leading to heating induced contraction. The above process bears some similarity to what is responsible for the reversible actuation behavior of LCE if each bundle of the parallel hydrophilic channels can be viewed as a liquid crystalline mesogen.4 The above discussion centers along mechanistic interpretation of the reversible strain switching of Nafion. For the irreversible creep, the work by Benziger’s group suggests that the creep following the initial fast strain development (i.e., delayed elastic strain; see the curves in Figure 2) arises from the uncoiling and rearrangement of the ionic cross-links.26,27 Presumably, this type of molecular relaxation occurs at such a localized length scale that it does not affect the overall elongated structure. We also emphasize that the creep reported in this work is irreversible only on a relative basis. That is, it is irreversible only when the constant stress is maintained. Under a stress-free condition, we found that all the strain components reported here can be recovered by annealing the sample at 140 °C. The fully recoverable nature of the strains is notably different from the results obtained by Benziger et al., which showed that, even under a zero stress condition, a portion of creep is irrecoverable (or permanent) due to viscous flow.26,27 1088

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(8) Xie, T. Polymer 2011, 52, 4895. (9) Behl, M.; Lendlein, A. Mater. Today 2007, 10, 20. (10) Mather, P.; Luo, X.; Rousseau, I. Annu. Rev. Mater. Res. 2009, 39, 445. (11) Gunes, I.; Jana, S. J. Nanosci. Nanotechnol. 2008, 8, 1616. (12) Behl, M.; Razzaq, M.; Lendlein, A. Adv. Mater. 2010, 22, 3388. (13) Hu, J.; Zhu, Y.; Huang, H.; Lu, J. Prog. Polym. Sci. 2012, 37, 1720. (14) Liu, Y.; Lv, H.; Lan, X.; Leng, J.; Du, S. Compos. Sci. Technol. 2009, 69, 2064. (15) Thomsen, D.; Keller, P.; Naciri, J.; Pink, R.; Jeon, H.; Shenoy, D.; et al. Macromolecules 2001, 34, 5868. (16) Tajbakhsh, A. R.; Terentjev, E. M. Eur. Phys. J. E 2001, 6, 181. (17) Qin, H.; Mather, P. Macromolecules 2009, 42, 273. (18) Li, J.; Rodgers, W.; Xie, T. Polymer 2011, 52, 5320. (19) Chung, T.; Rorno-Uribe, A.; Mather, P. Macromolecules 2008, 41, 184. (20) Zotzmann, J.; Behl, M.; Hofmann, D.; Lendlein, A. Adv. Mater. 2010, 22, 3424. (21) Pandinia, S.; Passeraa, S.; Messorib, M.; Padernib, K.; Tosellic, M.; Gianoncellid, A.; Bontempid, E.; Riccòa, T. Polymer 2012, 53, 1915. (22) Behl, M.; Kratz, K.; Zotzmann, J.; Nöchel, U.; Lendlein, A. Adv. Mater. 2013, 25, 4466. (23) Weiss, R.; Izzo, E.; Mandelbaum, S. Macromolecules 2008, 41, 2978. (24) Page, K. A.; Cable, K. M.; Moore, R. B. Macromolecules 2005, 38, 6472. (25) Park, J. K.; Jones, P. J.; Sahagun, C.; Page, K. A.; Hussey, D. S.; Jacobson, D. L.; Morgan, S. E.; Moore, R. B. Soft Matter 2010, 6, 1444. (26) Majsztrik, P. W.; Bocarsly, A. B.; Benziger, J. B. Macromolecules 2008, 41, 9849. (27) Satterfield, M. B.; Benziger, J. B. J. Polym. Sci., Polym. Phys. 2009, 47, 11. (28) Page, K. A.; Landis, F. A.; Philips, A. K.; Moore, R. B. Macromolecules 2006, 39, 3939. (29) Page, K. A.; Park, J. K.; Moore, R. B.; Sakai, V. G. Macromolecules 2009, 42, 2729.

Most likely, this discrepancy is due to the difference in the crystalline domains between our samples and their samples. In fact, the SANS analysis of our samples reported earlier does show a pronounced crystalline peak, indicating its relatively high crystallinity.6 The higher crystallinity establishes more effective physical cross-linking domains that prevent viscous flow. This in turn emphasizes the importance of the crystalline phase on the thermomechanical stability of Nafion. Last, but not least, although the experiments reported here were conducted under conditions for which the free water activity can be neglected, we do anticipate that purposely introduced water will have a large impact on the phenomenon. This remains an interesting subject to explore in the future.

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CONCLUSION In this work, the thermomechanical response of Nafion was investigated under cyclic heating and cooling conditions while the material was placed under a constant external stress. The results revealed that the overall strain response was the superposition of an irreversible component and a reversible component. The former is a classical polymer creep behavior and deemed to be detrimental to long-term durability of the material. In contract, the latter (i.e., elongation upon cooling and contraction upon heating) was quite intriguing as such a behavior is best known for liquid crystalline elastomers (LCE), but not for ionomers. The results support the claim in the literature that there is indeed some structural similarity between Nafion and LCE. However, as a reversible actuator, Nafion is not as competitive as typical LCE due to the small reversibility ratio as well as the irreversible creep. Thus, the reported behavior is really more an interesting manifestation of its morphology. On the other hand, this study does point to a potential new way of making ionomeric reversible actuators if the small reversibility ratio and the irreversible creep can be resolved with new ionomer design. Practically for Nafion, its thermomechanical strain response reported in this work may have implications to its temperature and mechanical cycling stability for fuel cell applications. As such, the thermomechanical protocol employed in this study represents a potentially useful tool for future evaluation of alternative ionomeric materials for those applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.X. thanks the Chinese central government’s Recruitment Program of Global Experts (also known as thousand talent program) and 985 program for the startup funding.

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REFERENCES

(1) Mauritz, K. A.; Moore, R. B. Chem. Rev. 2004, 104, 4535. (2) Rubatat, L.; Rollet, A. L.; Gebel, G.; Diat, O. Macromolecules 2002, 35, 4050. (3) Schmidt-Rohr, K.; Chen, Q. Nat. Mater. 2008, 7, 75. (4) Li, J.; Park, J. K.; Moore, R. B.; Madsen, L. A. Nat. Mater. 2011, 10, 507. (5) Xie, T. Nature 2010, 464, 267. (6) Xie, T.; Page, K.; Eastman, S. Adv. Funct. Mater. 2011, 21, 2057. (7) Li, J.; Xie, T. Macromolecules 2011, 44, 175. 1089

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