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Dominik Quitmann, Nikola Gushterov, Gabriele Sadowski, Frank Katzenberg, and Joerg C. Tiller* Responsive materials will play a major role in the future, since they are able to recognize environmental signals and react to them by self-healing, changing their physical properties, or adapting their shape.[1–3] Generally, the responsiveness of such materials is implemented or programmed prior to their application.[4–6] In the case of reversible actuation this responsiveness is not altered during the recognition step or the material response, with the exception of pH-sensitive hydrogels.[7–9] In this study we describe an example of lightly crosslinked natural rubber where stretched and constrained semi-crystalline polymer networks are capable of responding to different solvent gas concentrations with stress and simultaneously memorize the concentration and the chemical nature of the solvent itself in their microstructure (concept illustrated in Scheme 1). This written solvent signature can even be deleted by temperature. In principle nearly all lightly crosslinked polymer networks are responsive materials showing at least a shape memory effect and responding to suitable solvents by swelling.[10] The set of properties can be greatly enhanced by blending,[11–15] functional groups,[16–18] and additives.[19] Particularly interesting are polymer networks that not only respond to but also memorize an environmental signal. One way to achieve this is to design a material that allows re-arrangement of its microstructure by such an external stimulus, which then stays intact after removing the stimulus. We hypothesize that this behavior might be found for those semi-crystalline networks where the change of the amorphous phase has a significant impact on the stability of the crystals. We have recently introduced lightly crosslinked natural rubber as a shape-memory natural rubber (SMNR), a material that might possess the prerequisites for such behavior.[20,21] It was found that whenever NR is vulcanized below a critical degree of crosslinking, it generates crystals upon strain that are stable at room temperature and can thus stabilize a programmed shape. These crystals can be molten or further stabilized by applying an external mechanical stress.[22,23] Further, programmed SMNR strongly responds to liquids and gases by resuming its original shape. All those responses to mechanical and chemical stimuli are examples of the influence of the amorphous phase on the stability of the crystals. D. Quitmann, Dr. F. Katzenberg, Prof. Dr. J. C. Tiller Chair of Biomaterials & Polymer Science Department of Bio & Chemical Engineering TU Dortmund, D-44221 Dortmund, Germany E-mail: [email protected] N. Gushterov, Prof. Dr. G. Sadowski Laboratory of Thermodynamic Department of Bio & Chemical Engineering TU Dortmund, D-44221 Dortmund, Germany
DOI: 10.1002/adma.201305698
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Investigation of the solvent sensitivity of programmed SMNR under constrained conditions, i.e., the sample is fixed between two clamps and cannot recover its original shape, revealed that the material builds up tensile-stress upon solvent gas triggering.[24] This is most uncommon because a conventional polymer network expands under these conditions, pressing the clamps apart. We found that after solvent removal the stressanswer of the constrained specimen does not return to its initial value. The tensile-stress-answer is composed of a reversible part Δσrev that directly follows the gas pressure pi, and an irreversible part Δσirr that remains after solvent removal (Figure 1). This is in contrast to known solvent-sensitive polymer systems which respond to solvent exposure by swelling, mass increase,[25,26] or, rarely, by building up stress analogously to thermo-elastic inversion found for highly stretched rubbers.[27] We supposed that Δσirr might contain stored information on the concentration of solvent exposure. In order to explore this, the dependence of the irreversible stress Δσirr of constrained SMNR on the solvent gas pressure pi was determined. We chose an SMNR with a degree of crosslinking (percent of cross-linked monomeric units) of 0.2% for these experiments. All samples were identically programmed (fixed strain of 900%, trigger temperature of 33 ± 0.5 °C) and stress-free fixed between two clamps in a custom-made tensile creep apparatus that allowed measurement and adjustment of the solvent gas pressure, temperature, and stress-answer without dimension change along the axis of elongation.[28] The stress-response of constrained SMNR was measured depending on cyclically applied and evacuated solvent gas while temperature was kept constant at 20 °C. Initial experiments were performed with toluene. As seen in Figure 1a, when applying a toluene gas pressure of p1,max = 12.8 × 102 Pa in the first cycle the sample stress increases to 1.48 MPa and does not change as long the toluene gas pressure is kept constant. After evacuating, the sample stress decreases to a Δσ1,irr of 0.94 MPa (indicated by the lowest red dashed line). This value represents the irreversible part of the stress-answer of the SMNR to toluene gas pressure. In the next cycle, the increase to a higher toluene gas pressure p2,max = 18.7 × 102 Pa results in an irreversible stress-answer Δσ2,irr of 1.62 MPa after evacuation. The third pressure cycle with the lower toluene gas pressure p1,max results not in the expected Δσ1,irr but in Δσ2,irr. This indicates that the memorized stress cannot be altered by a toluene gas with a lower pressure than a previously applied one. In the fourth cycle where p2,max was applied, Δσ2,irr was found again. Further increasing the toluene gas pressure to 25.3 × 102 Pa in the fifth cycle resulted in a higher Δσ3,irr of 2.09 MPa. It is obvious from this experiment that Δσirr is indeed depending on solvent pressure. Furthermore, the irreversible stress-answer Δσirr is not influenced by pressure cycles that apply a solvent pressure lower than the one that induced this irreversible
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indicates that no triggering and thus no solvent-signature writing of the programmed, constrained SMNR sample takes place below a minimal solvent gas pressure. Since it is generally accepted that swellable polymers show a solvent–temperature superposition,[25,26] i.e., the reaction on solvents and on temperature is comparable, we were wondering if this holds also true for Δσirr. To this end an experiment similar to that above was performed with temperature cycles instead of solvent gas pressure cycles (curves are given in the Supporting Information in Figure S3); we found that the stressScheme 1. Schematic illustration of the solvent memory effect of a semi-crystalline, solvent- answer of constrained SMNR is indeed responsive polymer network. dependent on temperature similar to solvent gas. This is consistent with previous findings of melting temperatures in a broad range from 50 to 90 °C stress, i.e. Δσirr is an indicator for the maximal solvent gas presunder constrained conditions.[22] Furthermore, an irreversible sure applied to the programmed and constrained SMNR in its lifetime. As seen in Figure 1b, Δσirr depends linearly on the stress-answer Δσirr remains after cooling to room temperature. maximal toluene gas pressure ptoluene,max. Accordingly, the conHowever, Δσirr is not dependent on temperature level but is strained SMNR represents an analog non-volatile information constant in the whole range of investigated temperatures. This storage device for applied solvent gas pressures in the sense of a intriguing behavior might offer the opportunity to delete the “programmable read-only memory” (PROM). The stored solvent irreversible stress-answer Δσirr, written in by the solvent gas. gas pressure correlates to the highest applied value. This was tested by repeated alternating solvent gas pressure In order to broaden the concept, gases of cyclohexane and and temperature cycles for cyclohexane (Figure 4). tetrahydrofuran (THF), respectively, were brought in contact As seen in Figure 3, the first section shows the typical solwith constrained SMNR using the above-described procevent response to cyclohexane gas pressure increase. Then the dure. The respective pressure–stress curves are given in the temperature was raised to 60 °C, which induced a large tensile Supporting Information (see Figure S1 and S2, Supporting stress-answer. When lowering the temperature to 20 °C, a new Information). As seen in Figure 2, which depicts the irreversΔσirr lower than that caused by cyclohexane was found. The ible stress answer Δσirr as function of the respective maximal next cyclohexane gas cycles induced a lower stress-answer than that found for the originally programmed constrained SMNR applied solvent vapor pressures, all three solvents show a linear sample. In the following section the temperature cycle and the dependency between Δσirr and pi,max. In all cases Δσirr represolvent gas pressure cycles were repeated, this time resulting in sents the highest applied solvent gas pressure. fully reproducible stress-answer and Δσirr values. This experiIt is worth noting that the regression lines do not intersect the origin of the diagram but have different x-intercepts. This ment clearly proves that the selective solvent gas pressure
Figure 1. a) Temporal progress of the stress-answer σ of programmed constrained SMNR with changing solvent gas pressure pi. b) Schematic illustration of the irreversible stress-answer Δσirr caused by solvent gas pressure pi.
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Figure 2. Plot of the irreversible stress-answer Δσirr of constrained SMNR against maximal solvent gas pressure pi of toluene (䊏), cyclohexane (䊉), and THF (䉱).
memory represented by Δσirr can be deleted by temperature treatment and that it is fully reproducible and repeatable after an initial conditioning (first temperature cycle). Thus constrained SMNR behaves in the sense of an erasable programmable read only memory (EPROM) because the information of the solvent gas pressure is not only programmed into the material but can also be erased or deleted. The experiment described in Figure 3 was repeated with the solvent gases toluene, cyclohexane, and THF with temperature cycles at 60 and 80 °C, respectively. The stress-answer curves of these measurements are given in the Supporting Information (Figure S4, S5, and S6). In all cases the temperature treatment resulted in deletion of the solvent gas memory. All experiments were fully repeatable. The solvent gas-induced Δσirr (pi) values after the first temperature cycle are depicted in Figure 4. All three chemical compounds show a linear dependency between
Figure 4. Plot of irreversible stress-answer Δσirr of programmed constrained SMNR versus applied maximal solvent gas pressure for toluene (䊏,䊐), cyclohexane (䊉,䊊), and THF (䉱,䉭) after temperature-induced deletion at 60 °C (full symbols) and 80 °C (hollow symbols). Stars (夹) mark solvent gas pressures of toluene, cyclohexane, and THF that result in identical degrees of swelling.
Δσirr and gas pressure pi. Clearly the sensitivity of the material after temperature treatment is solvent specific, being the highest for toluene and the lowest for THF. As seen in Figure 4 temperature treatment at 80 °C of all solvent-treated samples lowers Δσirr to the same minimal value Δσirr,min. This value is identical to the Δσirr measured after thermal treatment of a programmed constrained SMNR prior to solvent gas treatment. It indicates that the solvent signature is fully deleted when reaching Δσirr,min after heating. While the solvent signatures of cyclohexane and THF, respectively, are fully deleted at 60 °C, the signature of toluene can only be deleted at 80 °C (Figure S6 and S7 in the Supporting Information). Since the only thermally addressable structural elements of NR are the crystals, we presume that the solvent gas somehow alters the morphology of programmed constrained SMNR comprising crystal size, distribution, amount, and orientation. In other words, the solvent gas seems to reorganize the microstructure of the material in a manner different from the effect of temperature. This could be due to the fact that the essential controlling mechanisms of crystal formation, being nucleation- and growth-rate, are differently influenced by solvent gas and temperature. According to Figure 4 the different slopes Δσirr/Δpi are solvent-specific, indicating that not only the solvent gas pressure but also the kind of solvent might be remembered by the material. It is also possible that the degree of swelling is the major factor for controlling Δσirr. In order to explore this, the degree of swelling QV,15 (achieved Figure 3. Temporal progress of the stress-answer σ of programmed constrained SMNR with during solvent exposure for 15 min) at difchanging cyclohexane gas pressure pi and temperature T. The arrows indicate the temperature- ferent gas pressures was determined by sorption measurements (data for toluene are given induced solvent memory deletion of SMNR, represented by the reset of Δσirr to Δσirr,min.
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as an example in the Supporting Information, Figure S8) and related to the respective Δσirr. Data points for similar degrees of swelling of QV,15 = 1.21 are depicted as stars in Figure 4. It is clearly seen that the degree of swelling is not the only factor controlling Δσirr. Obviously the molecular structure of the solvent plays a great part in reorganizing the microstructure; thus, not only the quantity but also the kind of solvent is memorized. In conclusion we have shown an example in which responsive polymer networks under certain conditions are capable of not only reacting to an environmental signal but are also of memorizing it. Given the right conditions other chemically different, semicrystalline responsive networks might be able to react to and also memorize other environmental signals, such as different chemicals (artificial olfaction),[29] light, and magnetism.
Experimental Section Synthesis: Natural Rubber (Standard Malaysian Rubber, SMR10) with an initial molecular weight of about 3 000 000 g mol−1 was masticated for 10 min using a heatable double-roller operated at 80 °C, subsequently mixed with dicumylperoxide (DCP) for 5 min, and cross-linked in a heating press at 160 °C for 35 min. Using 0.2 parts per hundred rubber DCP resulted in an Mc of 34 000 g mol−1 (0.2% degree of cross-linking), measured according to the Mooney-Rivlin theory.[30] The stretching of the specimen was performed at 60 °C to a strain of 950% with a fixed strain of 900%. Analysis: The measurements of solvent gas-induced stress were carried out in an air thermostatted chamber with an operating range from 0 to 100 °C. The programmed SMNR was fixed between two film-tension clamps. One clamp was connected to a force transducer with a measuring range from 0.003 to 20 N and a reproducibility of ±0.003 N.[28] After reaching the respective vapor pressure, it was kept constant for 15 min. In each case more than 90% of the solvent mass was evacuated according to respective sorption experiments. Solvent sorption measurements under constrained conditions were carried out in an air thermostatted chamber with an operating range from 20 to 150 °C with a magnetic suspension balance operating in the range from 0.01 to 30 g with a reproducibility of ± 0.03 mg.[31] The degree of swelling was determined after 15 min solvent exposure and is not the equilibrium swelling. All used solvents were of analytical grade or purer and were obtained from Sigma Aldrich.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors thank Continental Reifen Deutschland GmbH, in particular Dr. Fred Waldner, for providing non-vulcanized natural rubber. Received: November 18, 2013 Revised: January 24, 2014 Published online: March 14, 2014
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[1] P. Cordier, F. Tournilhac, C. Soulié-Ziakovic, L. Leibler, Nature 2008, 451, 977–980. [2] T. Xie, Nature 2010, 464, 267–270. [3] C. C. Wang, Y. Zhao, H. Purnawali, W. M. Huang, L. Sun, React. Funct. Polym. 2012, 72, 757–764. [4] L. Sun, W. M. Huang, Soft Matter 2010, 6, 4403–4406. [5] C. Liu, H. Qin, P. T. Mather, J. Mater. Chem. 2007, 17, 1543– 1558. [6] R. Hoeher, T. Raidt, M. Rose, F. Katzenberg, J. C. Tiller, J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1033–1040. [7] A. Lendlein, S. Kelch, Angew. Chem. Int. Ed. 2002, 41, 2034– 2057. [8] M. A. C. Stuart, W. T. S. Huck, J. Genzer, M. Müller, C. Ober, M. Stamm, G. B. Sukhorukov, I. Szleifer, V. V. Tsukruk, M. Urban, F. Winnik, S. Zauscher, I. Luzinov, S. Minko, Nat. Mater. 2010, 9, 101–113. [9] A. Lendlein, V. P. Shastri, Adv. Mater. 2010, 22, 3344–3347. [10] P. J. Flory, J. Chem. Phys. 1942, 10, 51–61. [11] Y. Zhu, J. Hu, H. Luo, R. J. Young, L. Deng, S. Zhang, Y. Fan, G. Ye, Soft Matter 2012, 8, 2509–2517. [12] I. S. Kolesov, K. Kratz, A. Lendlein, H.-J. Radusch, Polymer 2009, 50, 5490–5498. [13] R. Hoeher, T. Raidt, C. Krumm, M. Meuris, F. Katzenberg, J. C. Tiller, Macromol. Chem. Phys. 2013, 214, 2725–2732. [14] M. Behl, A. Lendlein, J. Mater. Chem. 2010, 20, 3335–3345. [15] W. M. Huang, Y. Zhao, C. C. Wang, Z. Ding, H. Purnawali, C. Tang, J. L. Zhang, J. Polym. Res. 2012, 19, 1–34. [16] J. Li, J. A. Viveros, M. H. Wrue, M. Anthamatten, Adv. Mater. 2007, 19, 2851–2855. [17] A. Lendlein, H. Jiang, O. Jünger, R. Langer, Nature 2005, 434, 879–882. [18] M. S. Baker, V. Yadav, A. Sen, S. T. Phillips, Angew. Chem. Int. Ed. 2013, 52, 10295–10299. [19] H. Koerner, G. Price, N. A. Pearce, M. Alexander, R. A. Vaia, Nat. Mater. 2004, 3, 115–120. [20] F. Katzenberg, B. Heuwers, J. C. Tiller, Adv. Mater. 2011, 23, 1909–1911. [21] B. Heuwers, A. Beckel, A. Krieger, F. Katzenberg, J. C. Tiller, Macromol. Chem. Phys. 2013, 214, 912–923. [22] B. Heuwers, D. Quitmann, R. Hoeher, F. M. Reinders, S. Tiemeyer, C. Sternemann, M. Tolan, F. Katzenberg, J. C. Tiller, Macromol. Rapid Commun. 2013, 34, 180–184. [23] B. Heuwers, D. Quitmann, F. Katzenberg, J. C. Tiller, Macromol. Rapid Commun. 2012, 33, 1517–1522. [24] D. Quitmann, N. Gushterov, G. Sadowski, F. Katzenberg, J. C. Tiller, ACS Appl. Mater. Interfaces 2013, 5, 3504–3507. [25] R. M. Barrer, G. Skirrow, J. Polym. Sci. 1948, 3, 564–575. [26] J. A. Barrie, B. Platt, J. Polym. Sci. 1961, 49, 479–493. [27] U. W. Gedde, Polymer Physics, Kluwer Academic Publishers, Stockholm 2001. [28] F. Mueller, B. Heuwers, F. Katzenberg, J. C. Tiller, G. Sadowski, Macromolecules 2010, 43, 8997–9003. [29] K. J. Albert, N. S. Lewis, C. L. Schauer, G. A. Sotzing, S. E. Stitzel, T. P. Vaid, D. R. Walt, Chem. Rev. 2000, 100, 2595–2626. [30] M. Mooney, J. Appl. Phys. 1940, 11, 582–592. [31] K.-M. Krueger, G. Sadowski, Macromolecules 2005, 38, 8408– 8417.
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Dominik Quitmann, Nikola Gushterov, Gabriele Sadowski, Frank Katzenberg, and Joerg C. Tiller* Responsive materials will play a major role in the future, since they are able to recognize environmental signals and react to them by self-healing, changing their physical properties, or adapting their shape.[1–3] Generally, the responsiveness of such materials is implemented or programmed prior to their application.[4–6] In the case of reversible actuation this responsiveness is not altered during the recognition step or the material response, with the exception of pH-sensitive hydrogels.[7–9] In this study we describe an example of lightly crosslinked natural rubber where stretched and constrained semi-crystalline polymer networks are capable of responding to different solvent gas concentrations with stress and simultaneously memorize the concentration and the chemical nature of the solvent itself in their microstructure (concept illustrated in Scheme 1). This written solvent signature can even be deleted by temperature. In principle nearly all lightly crosslinked polymer networks are responsive materials showing at least a shape memory effect and responding to suitable solvents by swelling.[10] The set of properties can be greatly enhanced by blending,[11–15] functional groups,[16–18] and additives.[19] Particularly interesting are polymer networks that not only respond to but also memorize an environmental signal. One way to achieve this is to design a material that allows re-arrangement of its microstructure by such an external stimulus, which then stays intact after removing the stimulus. We hypothesize that this behavior might be found for those semi-crystalline networks where the change of the amorphous phase has a significant impact on the stability of the crystals. We have recently introduced lightly crosslinked natural rubber as a shape-memory natural rubber (SMNR), a material that might possess the prerequisites for such behavior.[20,21] It was found that whenever NR is vulcanized below a critical degree of crosslinking, it generates crystals upon strain that are stable at room temperature and can thus stabilize a programmed shape. These crystals can be molten or further stabilized by applying an external mechanical stress.[22,23] Further, programmed SMNR strongly responds to liquids and gases by resuming its original shape. All those responses to mechanical and chemical stimuli are examples of the influence of the amorphous phase on the stability of the crystals. D. Quitmann, Dr. F. Katzenberg, Prof. Dr. J. C. Tiller Chair of Biomaterials & Polymer Science Department of Bio & Chemical Engineering TU Dortmund, D-44221 Dortmund, Germany E-mail: [email protected] N. Gushterov, Prof. Dr. G. Sadowski Laboratory of Thermodynamic Department of Bio & Chemical Engineering TU Dortmund, D-44221 Dortmund, Germany
DOI: 10.1002/adma.201305698
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Investigation of the solvent sensitivity of programmed SMNR under constrained conditions, i.e., the sample is fixed between two clamps and cannot recover its original shape, revealed that the material builds up tensile-stress upon solvent gas triggering.[24] This is most uncommon because a conventional polymer network expands under these conditions, pressing the clamps apart. We found that after solvent removal the stressanswer of the constrained specimen does not return to its initial value. The tensile-stress-answer is composed of a reversible part Δσrev that directly follows the gas pressure pi, and an irreversible part Δσirr that remains after solvent removal (Figure 1). This is in contrast to known solvent-sensitive polymer systems which respond to solvent exposure by swelling, mass increase,[25,26] or, rarely, by building up stress analogously to thermo-elastic inversion found for highly stretched rubbers.[27] We supposed that Δσirr might contain stored information on the concentration of solvent exposure. In order to explore this, the dependence of the irreversible stress Δσirr of constrained SMNR on the solvent gas pressure pi was determined. We chose an SMNR with a degree of crosslinking (percent of cross-linked monomeric units) of 0.2% for these experiments. All samples were identically programmed (fixed strain of 900%, trigger temperature of 33 ± 0.5 °C) and stress-free fixed between two clamps in a custom-made tensile creep apparatus that allowed measurement and adjustment of the solvent gas pressure, temperature, and stress-answer without dimension change along the axis of elongation.[28] The stress-response of constrained SMNR was measured depending on cyclically applied and evacuated solvent gas while temperature was kept constant at 20 °C. Initial experiments were performed with toluene. As seen in Figure 1a, when applying a toluene gas pressure of p1,max = 12.8 × 102 Pa in the first cycle the sample stress increases to 1.48 MPa and does not change as long the toluene gas pressure is kept constant. After evacuating, the sample stress decreases to a Δσ1,irr of 0.94 MPa (indicated by the lowest red dashed line). This value represents the irreversible part of the stress-answer of the SMNR to toluene gas pressure. In the next cycle, the increase to a higher toluene gas pressure p2,max = 18.7 × 102 Pa results in an irreversible stress-answer Δσ2,irr of 1.62 MPa after evacuation. The third pressure cycle with the lower toluene gas pressure p1,max results not in the expected Δσ1,irr but in Δσ2,irr. This indicates that the memorized stress cannot be altered by a toluene gas with a lower pressure than a previously applied one. In the fourth cycle where p2,max was applied, Δσ2,irr was found again. Further increasing the toluene gas pressure to 25.3 × 102 Pa in the fifth cycle resulted in a higher Δσ3,irr of 2.09 MPa. It is obvious from this experiment that Δσirr is indeed depending on solvent pressure. Furthermore, the irreversible stress-answer Δσirr is not influenced by pressure cycles that apply a solvent pressure lower than the one that induced this irreversible
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indicates that no triggering and thus no solvent-signature writing of the programmed, constrained SMNR sample takes place below a minimal solvent gas pressure. Since it is generally accepted that swellable polymers show a solvent–temperature superposition,[25,26] i.e., the reaction on solvents and on temperature is comparable, we were wondering if this holds also true for Δσirr. To this end an experiment similar to that above was performed with temperature cycles instead of solvent gas pressure cycles (curves are given in the Supporting Information in Figure S3); we found that the stressScheme 1. Schematic illustration of the solvent memory effect of a semi-crystalline, solvent- answer of constrained SMNR is indeed responsive polymer network. dependent on temperature similar to solvent gas. This is consistent with previous findings of melting temperatures in a broad range from 50 to 90 °C stress, i.e. Δσirr is an indicator for the maximal solvent gas presunder constrained conditions.[22] Furthermore, an irreversible sure applied to the programmed and constrained SMNR in its lifetime. As seen in Figure 1b, Δσirr depends linearly on the stress-answer Δσirr remains after cooling to room temperature. maximal toluene gas pressure ptoluene,max. Accordingly, the conHowever, Δσirr is not dependent on temperature level but is strained SMNR represents an analog non-volatile information constant in the whole range of investigated temperatures. This storage device for applied solvent gas pressures in the sense of a intriguing behavior might offer the opportunity to delete the “programmable read-only memory” (PROM). The stored solvent irreversible stress-answer Δσirr, written in by the solvent gas. gas pressure correlates to the highest applied value. This was tested by repeated alternating solvent gas pressure In order to broaden the concept, gases of cyclohexane and and temperature cycles for cyclohexane (Figure 4). tetrahydrofuran (THF), respectively, were brought in contact As seen in Figure 3, the first section shows the typical solwith constrained SMNR using the above-described procevent response to cyclohexane gas pressure increase. Then the dure. The respective pressure–stress curves are given in the temperature was raised to 60 °C, which induced a large tensile Supporting Information (see Figure S1 and S2, Supporting stress-answer. When lowering the temperature to 20 °C, a new Information). As seen in Figure 2, which depicts the irreversΔσirr lower than that caused by cyclohexane was found. The ible stress answer Δσirr as function of the respective maximal next cyclohexane gas cycles induced a lower stress-answer than that found for the originally programmed constrained SMNR applied solvent vapor pressures, all three solvents show a linear sample. In the following section the temperature cycle and the dependency between Δσirr and pi,max. In all cases Δσirr represolvent gas pressure cycles were repeated, this time resulting in sents the highest applied solvent gas pressure. fully reproducible stress-answer and Δσirr values. This experiIt is worth noting that the regression lines do not intersect the origin of the diagram but have different x-intercepts. This ment clearly proves that the selective solvent gas pressure
Figure 1. a) Temporal progress of the stress-answer σ of programmed constrained SMNR with changing solvent gas pressure pi. b) Schematic illustration of the irreversible stress-answer Δσirr caused by solvent gas pressure pi.
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Figure 2. Plot of the irreversible stress-answer Δσirr of constrained SMNR against maximal solvent gas pressure pi of toluene (䊏), cyclohexane (䊉), and THF (䉱).
memory represented by Δσirr can be deleted by temperature treatment and that it is fully reproducible and repeatable after an initial conditioning (first temperature cycle). Thus constrained SMNR behaves in the sense of an erasable programmable read only memory (EPROM) because the information of the solvent gas pressure is not only programmed into the material but can also be erased or deleted. The experiment described in Figure 3 was repeated with the solvent gases toluene, cyclohexane, and THF with temperature cycles at 60 and 80 °C, respectively. The stress-answer curves of these measurements are given in the Supporting Information (Figure S4, S5, and S6). In all cases the temperature treatment resulted in deletion of the solvent gas memory. All experiments were fully repeatable. The solvent gas-induced Δσirr (pi) values after the first temperature cycle are depicted in Figure 4. All three chemical compounds show a linear dependency between
Figure 4. Plot of irreversible stress-answer Δσirr of programmed constrained SMNR versus applied maximal solvent gas pressure for toluene (䊏,䊐), cyclohexane (䊉,䊊), and THF (䉱,䉭) after temperature-induced deletion at 60 °C (full symbols) and 80 °C (hollow symbols). Stars (夹) mark solvent gas pressures of toluene, cyclohexane, and THF that result in identical degrees of swelling.
Δσirr and gas pressure pi. Clearly the sensitivity of the material after temperature treatment is solvent specific, being the highest for toluene and the lowest for THF. As seen in Figure 4 temperature treatment at 80 °C of all solvent-treated samples lowers Δσirr to the same minimal value Δσirr,min. This value is identical to the Δσirr measured after thermal treatment of a programmed constrained SMNR prior to solvent gas treatment. It indicates that the solvent signature is fully deleted when reaching Δσirr,min after heating. While the solvent signatures of cyclohexane and THF, respectively, are fully deleted at 60 °C, the signature of toluene can only be deleted at 80 °C (Figure S6 and S7 in the Supporting Information). Since the only thermally addressable structural elements of NR are the crystals, we presume that the solvent gas somehow alters the morphology of programmed constrained SMNR comprising crystal size, distribution, amount, and orientation. In other words, the solvent gas seems to reorganize the microstructure of the material in a manner different from the effect of temperature. This could be due to the fact that the essential controlling mechanisms of crystal formation, being nucleation- and growth-rate, are differently influenced by solvent gas and temperature. According to Figure 4 the different slopes Δσirr/Δpi are solvent-specific, indicating that not only the solvent gas pressure but also the kind of solvent might be remembered by the material. It is also possible that the degree of swelling is the major factor for controlling Δσirr. In order to explore this, the degree of swelling QV,15 (achieved Figure 3. Temporal progress of the stress-answer σ of programmed constrained SMNR with during solvent exposure for 15 min) at difchanging cyclohexane gas pressure pi and temperature T. The arrows indicate the temperature- ferent gas pressures was determined by sorption measurements (data for toluene are given induced solvent memory deletion of SMNR, represented by the reset of Δσirr to Δσirr,min.
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as an example in the Supporting Information, Figure S8) and related to the respective Δσirr. Data points for similar degrees of swelling of QV,15 = 1.21 are depicted as stars in Figure 4. It is clearly seen that the degree of swelling is not the only factor controlling Δσirr. Obviously the molecular structure of the solvent plays a great part in reorganizing the microstructure; thus, not only the quantity but also the kind of solvent is memorized. In conclusion we have shown an example in which responsive polymer networks under certain conditions are capable of not only reacting to an environmental signal but are also of memorizing it. Given the right conditions other chemically different, semicrystalline responsive networks might be able to react to and also memorize other environmental signals, such as different chemicals (artificial olfaction),[29] light, and magnetism.
Experimental Section Synthesis: Natural Rubber (Standard Malaysian Rubber, SMR10) with an initial molecular weight of about 3 000 000 g mol−1 was masticated for 10 min using a heatable double-roller operated at 80 °C, subsequently mixed with dicumylperoxide (DCP) for 5 min, and cross-linked in a heating press at 160 °C for 35 min. Using 0.2 parts per hundred rubber DCP resulted in an Mc of 34 000 g mol−1 (0.2% degree of cross-linking), measured according to the Mooney-Rivlin theory.[30] The stretching of the specimen was performed at 60 °C to a strain of 950% with a fixed strain of 900%. Analysis: The measurements of solvent gas-induced stress were carried out in an air thermostatted chamber with an operating range from 0 to 100 °C. The programmed SMNR was fixed between two film-tension clamps. One clamp was connected to a force transducer with a measuring range from 0.003 to 20 N and a reproducibility of ±0.003 N.[28] After reaching the respective vapor pressure, it was kept constant for 15 min. In each case more than 90% of the solvent mass was evacuated according to respective sorption experiments. Solvent sorption measurements under constrained conditions were carried out in an air thermostatted chamber with an operating range from 20 to 150 °C with a magnetic suspension balance operating in the range from 0.01 to 30 g with a reproducibility of ± 0.03 mg.[31] The degree of swelling was determined after 15 min solvent exposure and is not the equilibrium swelling. All used solvents were of analytical grade or purer and were obtained from Sigma Aldrich.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors thank Continental Reifen Deutschland GmbH, in particular Dr. Fred Waldner, for providing non-vulcanized natural rubber. Received: November 18, 2013 Revised: January 24, 2014 Published online: March 14, 2014
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