Rheological Study on Rapid Recovery of Hydrogel Based on

J. Phys. Chem. B 2008, 112, 11537–11541

11537

Rheological Study on Rapid Recovery of Hydrogel Based on Oligomeric Electrolyte Shyamal Kumar Kundu,† Takuro Matsunaga,† Masaru Yoshida,‡ and Mitsuhiro Shibayama*,† Institute for Solid State Physics, The UniVersity of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan, Nanotechnology Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed: April 30, 2008; ReVised Manuscript ReceiVed: July 9, 2008

The hydrogel consisting of an oligomeric electrolyte, poly[pyridinium-1,4-diyl-iminocarbonyl-1,4-phenylenemethylene chloride] (1-Cl) underwent self-healing at temperatures lower than its gelation temperature after destruction of the gel network in a shear flow. The self-healing mechanism was investigated by rheological measurements on three different kinds of gels including a low-molecular weight organogelator and a polymeric hydrogelator. Although all of the three gels exhibited thermo-reversible hysteresis loops in the shear moduli, only 1-Cl hydrogel recovered its mechanical properties after vigorous agitation. It is conjectured that the self-healing is due to formation of network structure via a chlorine ion mediated hydrogen bond for which the activation energy is on the order of 10 kJ/mol. Introduction Hydrogels are cross-linked hydrophilic polymers, which can hold substantial amounts of water.1 Investigations of the physical properties of hydrogels with varying water contents have been acquired special significance, both from fundamental and application points of view. The broad utility of gels has made the search for new gelators, a very active area of science. A variety of organo- and hydro-gelators capable of immobilizing organic fluids and/or water have been proposed, in which various types of intermolecular interactions, such as hydrogen (H)-bond, π-π, cation-π, and electrostatic interactions, play a significant role. These interactions bestow unique properties to polymer gels, which are readily applied to biological, pharmaceutical products and medical applications. Sol-gel transition is related to connectivity correlation of which characteristic length diverges at the gel point. The behavior of low-molecular-weight gelators is a subject of special interest2-10 because of its wide application including oil-recovery. Nowak et al.11 designed and synthesized diblock copolypeptides with poly-(L-lysine) or poly(L-glutamic acid) as a hydrophilic block and poly(L-leucine), poly(L-valine) or poly(D/Lleucine) as a hydrophobic block. They showed that these low-molecular weight diblock copolymers self-assembled to hydrogels even at a low polymer concentration (≈ 0.25 wt %). The gels could maintain their mechanical strength up to temperatures of about 90 °C and recovered rapidly after stress releasing. The transient properties of the copolypeptide hydrogel were studied by subjecting the samples to oscillatory strains with a sufficiently large amplitude to break down the gel network, followed by measuring the storage (G′) and loss (G′′) shear moduli during the recovery process with a small strain amplitudes.11,12 However, it is naively expected that gelrecovering process may not be properly detected by such a mechanical-oscillatory measurement. This is because such a measurement may lead to a misunderstanding of gel-recovery * To whom correspondence should be addressed. E-mail: sibayama@ issp.u-tokyo.ac.jp. † Institute for Solid State Physics, The University of Tokyo. ‡ Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST).

capability of the gel. That is, oscillatory strains with large amplitude may simply break a piece of gel to small fragmented pieces, but thus-formed tightly assembled fragmented gel pieces could respond elastically against external mechanical stimuli. As a matter of fact, Deming13 reported that local gel networks were not disrupted under a large strain field, but only interconnections between gel domains were broken. Hence, a more elaborate method capable of detection of gel-healing is desired, which will put in favorable industrial applications. In this paper, we will discuss the sol-gel transitions and hysteresis behaviors of an ionic gelator, a low-molecular weight organogelator, and a polymeric hydrogelator by rheological methods. We will also discuss the gel recovery process after mechanical agitation with a high shear rate. We will compare gel-recovery processes of a novel ionic gelator, poly[pyridinium1,4-diyl-iminocarbonyl-1,4-phenylene-methylene chloride] (1Cl)14 with those of other gelators, i.e., a low-molecular weight organogelator and a high-molecular weight hydrogelator. We will demonstrate that 1-Cl exclusively undergoes self-recovering. An oligomeric electrolyte, 1-Cl, was synthesized as an ionic gelator by simple mixing of aminopyridine and chloromethyl benzoyl chloride in dichloromethane with slight excess of triethylamine.14 The ampholytic amide intermediate having both the nucleophilic and the electrophilic parts at the termini was not isolated because the polymerization is very fast and occurs immediately after the condensation reaction (Scheme 1a). The weight average molecular weight (Mw) and the degree of polymerization were 3 × 103 Da and 13, respectively. 1-Cl is one of the rare examples as a main-chain polyelectrolyte synthesized by “self-condensation” and it has some notable characteristics, for example, (i) very simple one-pot preparation including condensation and intermolecular quaternization reaction, (ii) relatively high ionic conductivity of the ionic liquid gel, (iii) a fast recovery of its rheological properties, (iv) resistance to acid in contrast to the some natural gelator (e.g., agarose and gelatin), etc. 1-Cl carries a positive charge on the main chain15,16 and its structure was determined by NMR spectroscopy in D2O. 1-Cl is soluble only in water and it

10.1021/jp803774r CCC: $40.75  2008 American Chemical Society Published on Web 08/22/2008

11538 J. Phys. Chem. B, Vol. 112, No. 37, 2008

Kundu et al.

SCHEME 1: Chemical Structures of (a) Poly[pyridinium-1,4-diyliminocarbonyl-1,4-phenylenemethylene chloride] (1-Cl), and (b) Nε-lauroyl-Nr-stearylaminocarbonyl-L-lysine ethyl ester (LEE)

produces a translucent gel at concentrations above 7.5 g/L determined by visual evaluation. For comparison, we used two types of gelators. One was a low-molecular weight organogelator, Nε-lauroyl-NR-stearylaminocarbonyl-L-lysine ethyl ester (LEE),17 and the other was alkalitreated gelatin (hydrogel). LEE was prepared by esterification of commercially available Nε-lauroyl-L-lysine, followed by addition of n-octadecyl isocyanate (Scheme 1b). It is a low molecular weight gelator (Mw ) 652 Da) and it forms organogels in a wide variety of organic solvents, e.g., alcohols, ethers, toluene, etc.17 Since it carries amide and ureylene groups, it forms a transparent and thermoreversible physical gel of toluene via H-bonding between the functional groups. As a highmolecular-weight gelator, alkali-treated gelatin (Mw ) 1.45 × 105 Da), extracted from calf bone type B, Nitta Gelatin Co., Japan, was used. Although the chemical structure of gelatin depends upon its origin, it always contains large amounts of proline (Pro), hydroxyproline (Hyp), and glycine (Gly).18 Sequences of (Gly-X-Pro) and (Gly-X-Hyp), in which X is another amino acid, are the commonest. The iso-electric point is pH 4.97. The gelatin formed a transparent and elastic physical gel.18-25 The sol-gel transition is known to occur mainly via random coil-helix transition,18 of which details are not the scope of this work. Experimental Section An oligomeric electrolyte, 1-Cl and 1-TFSI [TFSI: Bis(trifluoromethanesulfonyl)imide (N(SO2CF3)2)] were prepared in a manner similar to the previous report.14 A powder of 1-Cl (10 g/L) was immersed in distilled water and the mixture was sonicated for 30 min. Then the dispersion was heated at around 97 °C by a heat gun to form a clear isotropic sol phase (1-Cl/ water). LEE compound (10 g/L) was immersed in toluene and then the solution was kept in a hot water bath at 80 °C for an hour to form a clear isotropic sol phase (LEE/toluene). Gelatin (10 g/L) was immersed in distilled water and then the solution was kept in hot water bath at 50 °C for an hour to form a clear isotropic sol phase (gelatin/water). Rheological measurements were carried out by MCR501, Anton Paar, Austria. We employed a double cylinder geometry with 26.7- and 28.9- mm diameter cylinders for the observation of both the sol-gel transition and recovery process. For observation of the hysteresis behavior, the experiments were performed by heating and cooling processes with a rate of 0.12 °C/min. The gel-recovery process was observed at 25 °C. 1H NMR spectrum was recorded on a Varian VXR-300 (300 MHz). Chemical shifts are denoted in δ-unit (ppm) relative to DMSO-d6.

Figure 1. Hysteresis of the storage (G′) and loss (G′′) moduli for (a) 10 g/L 1-Cl/water, (b) 10 g/L LEE/toluene, and (c) 10 g/L gelatin/ water on heating and cooling processes.

Result and Discussion Sol-gel transition is defined as a point where connectivity correlation diverges. Rheologically, it corresponds to divergence of viscosity, appearance of steady state modulus, and a crossover of the storage and loss moduli with a power-law behavior, i.e., G′(ω) ) G′′(ω) ∼ ωn, where ωn and n are, respectively, the angular frequency and the viscoelasticity exponent.19,26 Other characteristic features of gels include nonergodicity, frozen inhomogeneities and divergence of connectivity correlation.19,27,28 Figure 1 shows that the variations of G′ and G′′ with temperature for (a) 10 g/L 1-Cl/water, (b) 10 g/L LEE/toluene, and (c) 10 g/L gelatin/water on heating and cooling processes as indicated with arrows. The strain, γ, and the angular frequency, ω, were chosen to be γ ) 0.5%, ω ) 2 rad/s in the transition region and the gel state. On the other hand, γ and ω were increased to be 100% and 2 rad/s in the sol state, respectively, so as to ensure the linear response regime of G′ and G′′. As shown in the figure, each system exhibited a large hysteresis loop during a temperature cycle. Hence, all the systems are thermally reversible. The gelation temperature, Tgel, during cooling process and the gel melting temperature, Tmelt, during heating process for 1-Cl/water, LEE/toluene, and the gelatin/water were determined as the point where G′ was equal to G′′ and are listed in Table 1. 1-Cl/water has the highest Tgel and Tmelt among the three systems and the largest difference between Tgel and Tmelt, i.e., ∆T ) Tmelt - Tgel. It should be mentioned here that the observed Tgel for gelatin is

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Figure 3. (a) Variation of G′ with time of 1-Cl hydrogel observed at different temperatures after cessation of the large strain (t ) 50 min). The solid and dashed curves are, respectively, the experimental data and fitted curves with eq 1. (b) Arrhenius plot of the relaxation time, τ, for 10 g/L 1-Cl/water.

Figure 2. Recovery processes of the gel strength as a function of time for (a) 10 g/L 1-Cl/water, (b) 10 g/L LEE/toluene, and (c) 10 g/L gelatin/water at 25 °C.

TABLE 1: Observation of Gelation and Gel Melting Temperatures for 10g/L 1-Cl/Water, 10g/L LEE/Toluene, and 10g/L Gelatin/Water Mixtures by Rheology materials

Tgel (°C)

Tmelt (°C)

∆T (°C)

1-Cl/water LEE/toluene gelatin/water

54.7 45.7 11.5

77.3 58.4 25.8

22.6 12.7 14.3

3.5 °C lower than the reported21 value. This is due to isotope effect. The hydrogen bonding is weaker in H2O than in D2O, so that the Tgel of gelatin in H2O was observed to be lower than that in D2O. Figure 2 shows the variations of G′ and G′′ as a function of time for (a) 10 g/L 1-Cl/water, (b) 10 g/L LEE/toluene, and (c) 10 g/L gelatin/water at 25 °C. At first, we applied a lowamplitude oscillation (γ ) 0.5%, ω ) 2 rad/s) for 20 min for the observation of the equilibrium gel strength, and then deformed it by a high shear rate (γ˙ ) 1000 s-1) for 30 min (see the blank portion indicated by the both-headed arrow). Then, we applied a low-amplitude oscillation (the same as the initial values) so as to measure the mechanical response. The gel recovery process is well demonstrated by this method, i.e., a rheological measurement with a series of strain with (i) an oscillatory-strain with small strain, (ii) a continuous flow with a high strain rate, and (iii) the same oscillatory strain as part i. Now, let us discuss the results: In the case of 1-Cl (Figure 2a), G′ became smaller than G′′ (i.e., G′ < G′′) when the high strain was switched off. Within

0.6 min, a crossover between G′ and G′′ appeared, indicating that a sol-gel transition took place, followed by gel-strength recovering. The original mechanical properties were recovered approximately in 220 min after cessation of the large strain (t ≈ 270 min). It should be noted that the observation temperature was 25 °C, which was much lower than the gelation (Tgel ) 54.7 °C) and gel melting temperatures (Tmelt ) 77.3 °C) of 1-Cl, i.e., Tobs , Tgel, Tmelt. This means that the network structure of 1-Cl hydrogel was self-healed at a temperature much below its gelation temperature. Such a recovery of gel structure observed in 1-Cl is quite surprising. Note that a similar self-healing behavior was recently found by Leibler and co-workers in thermoreversible rubber made with supramolecular assembly.29 Because the rubber network is built with H-bonding, the rubber is self-healed even at room temperature by tuning the glass transition temperature of the rubber. In the case of LEE system (Figure 2b), on the other hand, both G′ and G′′ dropped down by more than two magnitude as a result of fragmentation of the gel. Similarly to 1-Cl, a crossover of G′ and G′′ occurred at t ≈ 56 min after cessation of the large strain, and then both G′ and G′′ increased gradually with time. However, this crossover was found to be due to precipitation of small LEE crystals. Hence, it can be concluded that no gel-recovery took place in LEE/toluene. Furthermore, no recovery in G′ to a value comparable to that in the gel state was observed in LEE/toluene. Such a behavior seen in LEE/ toluene is typical for organogelators. It is known that gelation of LEE in an organic solvent undergoes via hydrogen bonding (H-bonding) between the amide and ureylene groups of LEE.17,30 However, in order to form H-bonding, LEE compound has to be dissolved in a solvent at a temperature much higher than Tmelt. As a matter of fact, Tobs was much lower than Tgel and

11540 J. Phys. Chem. B, Vol. 112, No. 37, 2008

Figure 4.

1

Kundu et al.

H NMR spectrum of 1-TFSI (300 MHz, DMSO-d6).

cessation of the large strain (t ) 50 min). G′ seems to be an increasing function of t. The dashed curves in Figure 3a are the fits with the creep-type function given by

G′ ) G∞{1 - exp[-(t - 50)/τ]}

Figure 5. One of the possible forms for the chlorine-ion mediated H-bonding structure of 1-Cl in water.

Tmelt; i.e., Tobs , Tgel, Tmelt. Therefore, it is quite reasonable why gel-recovery was not observed at 25 °C. For gelatin (Figure 2c), the plots showed no crossover between G′ and G′′ and showed liquid like behavior, i.e., G′′ . G′, independent of aging time (t > 50 min). This means that the gelatin was in liquid state although Tobs was slightly lower than Tmelt. It should be mentioned here that the gelatin did not show any gel-recovery behavior even a temperature below Tgel. As a matter of fact, the gelatin gel was broken to small fragmented pieces by oscillatory strains with large amplitude. Then, the fragmented pieces of gelatin gel came out from the cylindrical cell. It is well-known that the mechanism of gelation for gelatin has been extensively investigated,18-25 and helix formation via H-bonding is responsible for gelation. As a matter of fact, a broken gelatin gel has to be heated to a temperature above Tgel in order to recover the gel strength. Another interesting difference can be found in the gel strength. In Figure 2, G′ for 1-Cl (G′ ≈ 90 Pa) is higher than that of gelatin gel (G′ ≈ 2 Pa), but significantly lower than that of LEE (G′ ≈ 1 kPa). Note that the concentrations of the gelators were the same. In the case of physical gels, the difference of G′ corresponds to the gel network and the interaction between the molecules. LEE system forms a brittle gel consisting of a fibrillike network with direct H-bonding between amide and ureylene groups of LEE. The lowest value of G′ in gelatin may be ascribed to the polymeric nature of gelatin. Due to rubberlike elasticity, the gelatin gel behaves as an elastic gel with a lower G′ than the other gels. The flexibility of polymeric gelatin chains leads to a relatively low value of G′. On the other hand, the value of G′ for 1-Cl is in between the two. As was shown in Figure 2a, self-healing of 1-Cl hydrogel after cessation of the large strain is also dependent on temperature. Figure 3 shows the variation of G′ for 1-Cl hydrogel healed at different temperatures. The solid lines show the variation of G′ with time for four different temperatures after

(1)

where G∞ is the value of G′ at t f ∞ and τ is the characteristic time for gel-recovering. Figure 3b shows that the variation τ with inverse of absolute temperature, T, for 10 g/L 1-Cl/water. The temperature dependence of the relaxation time could be described by an Arrhenius-type function:

(

τ ) τ0 exp -

∆EH RT

)

(2)

where τ0 is a constant, R is the gas constant, and ∆EH is the activation energy for gel-recovery. The value of ∆EH was evaluated to be ∆EH ) -10.4 ( 0.6 kJ/mol. The negative sign indicates that the gel-recovery is an exothermic process. On the other hand, the absolute value of ∆EH is about half of the bonding energy for H-bonding (≈20 kJ/mol), and is somewhat larger than the H-bonding in liquid hydrogen fluoride (≈7.9 kJ/mol).31 This value is also smaller than H-bonding in H2O (≈16 kJ/mol).31 It is also interesting to compare the value of ∆EH with the enthalpy of cross-link formation in “slime” gels, i.e., poly(vinyl alcohol)-borate gels (PVA-borate gel). In the case of PVA-borate gels, gelation takes place via borate-ion mediated H-bonding, and the enthalpy necessary for dissociation is estimated to be ca. -30 kJ/mol.32 This comparison suggests that ion-mediated hydrogen bonding can increase or decrease the bonding strength. For better understanding the mechanism of gelation and selfhealing process of 1-Cl, we propose a possible assembly of the supra-molecular structure of 1-Cl in water. Since 1-Cl molecule carries amide group, it is naively anticipated that intermolecular H-bonding may be formed between neighboring amide groups. Furthermore, the phenyl group may participate in intermolecular stacking to form a network structure. However, due to strong repulsive electrostatic interaction induced by the positive charges on the main chain, neither π-π staking nor direct H-bonding between the amide groups of 1-Cl molecules are expected to form. Instead, gel formation may take place via chloride ions and with water molecules. Although, it is very difficult to directly detect the interaction between the amide group and the chlorine ion by NMR spectroscopy33 due to a fast H/D exchange of amide proton in D2O, it has been found that the anion exchanged derivative, i.e., 1-TFSI shows an amide proton signal at very low magnetic field position (δ ) 11.7 ppm) in DMSOd6 as shown in Figure 4. The observed peak position is more

Rapid Recovery of Hydrogel than 1 ppm lower than that of the neutral N-(pyrid-4-yl)benzamide (δ ) 10.6 ppm)34 in the same solvent and the significant low field shift is consistent with the nature of anion-binding amide group.35 In addition, the X-ray crystallography of the model compound of 1-Cl recrystallized from water revealed that chloride ion is bound to the hydrogen of the amide group. There was no direct hydrogen bond interaction between the amide groups inconsistent with the typical amide compounds e.g., LEE, probably due to strong repulsive electrostatic interaction between cationic gelator molecules. Therefore, a possible interaction scheme between 1-Cl molecules, on the basis of X-ray crystallography, is depicted in Figure 5. Here, the amide proton is bonded to a water proton via a chlorine ion. This “chlorine-ion mediated H-bonding” can lead to formation of a diamond-like cross-linking structure. In the real gel system based on 1-Cl, the gel network should have more dynamic nature compared to a static structure determined by the X-ray crystallography. Thus not only the diamond structure but also many other cross-linking manners via “chlorine-ion mediated H-bonding” can be apparently considered. On the other hand, when the external perturbation was applied, the chlorine-ion mediated H-bonding is broken, resulting in fragmentation of 1-Cl gel to small pieces. After removal of external perturbation, the chlorine-ion mediated H-bonding recovers to form an infinite network. This interaction is so fast that a gel is formed within a few minutes and then the gel structure is strengthened by aging. Note that LEE did not show such a rapid recovery behavior probably due to the poor solubility of LEE itself. The chlorine ions and water molecules in 1-Cl can freely move within the medium and likely to find out the appropriate positions to regenerate the interconnections between the transiently broken gel domains. Similar nature of such an ion-clustered structure through the amide groups has been examined by X-ray analysis or theoretical calculations in different systems.33,35-38 Conclusion The gel-healing process of hydrogels consisting of an oligomeric electrolyte gelator, poly[pyridinium-1-4-diyliminocarbonyl-1,4-phenylene-methylene chloride] (1-Cl) was investigated by rheological measurements in comparison with a low-molecular weight organic gelator (LEE) and a polymeric hydrogelator (gelatin). Each system exhibited a large hysteresis loop during a temperature cycle. By employing a set of strains, i.e., (i) a small oscillatory strain, (ii) a large shear flow, and (iii) the same small oscillatory strain, the recovery process was exclusively observed for 1-Cl. Self-healing of 1-Cl hydrogel at 25 °C, a temperature much lower than its gelation temperature, was attained in τ ≈ 95 min after a large shear flow. The activation energy for gel-recovery was estimated to be ca. -10 kJ/mol. Here, minus sign means the gelation is an exothermic process. Such a healing behavior was not observed either in LEE or gelatin. A possible gelation mechanism via chlorine ion mediated hydrogen bonding is proposed. Acknowledgment. This work was partly supported by the Ministry of Education, Science, Sports, and Culture, Japan for Scientific Research on Priority Areas, 2006-2010, No. 18068004,

J. Phys. Chem. B, Vol. 112, No. 37, 2008 11541 and the Industrial Technology Research Grant Program (05A25710a) from the New Energy and Industrial Technology Development Organization. The authors are indebted to Prof. K. Hanabusa, Shinshu University, for his kind supplement of LEE. The authors are grateful for fruitful discussions with T. Koga, Kyoto University. References and Notes (1) Moynihan, H. Hydrogels in Medicine and Pharmacy; Wiley: New York, 1987. (2) Estroff, L. A.; Hamilton, A. D. Chem. ReV. 2004, 104, 1201. (3) Shirakawa, M.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 4164. (4) Mohmeyer, N.; Schmidt, H. W. Chem. Eur. J. 2005, 11, 863. (5) Abdallah, D. J.; Weiss, R. G. AdV. Mater. 2000, 12, 1237. (6) Moniruzzaman, M.; Sundararajan, P. R. Langmuir 2005, 21, 3802. (7) George, M.; Snyder, S. L.; Terech, P.; Glinka, C. J.; Weiss, R. G. J. Am. Chem. Soc. 2003, 125, 10275. (8) Bhuniya, S.; Park, S. M.; Kim, B. H. Org. Lett. 2005, 7, 1741. (9) Trivedi, D. R.; Ballabh, A.; Dastidar, P. J. Mater. Chem. 2005, 15, 2606. (10) Schmidt, R.; Adam, F. B.; Michel, M.; Schmutz, M.; Decher, G.; Mesini, P. J. Tetrahedron Lett. 2003, 44, 3171. (11) Nowak, A. P.; Breedveld, V.; Pakstis, L.; Ozbas, B.; Pine, D. J.; Pochan, D.; Deming, T. J. Nature 2002, 417, 424. (12) Breedveld, V.; Nowak, A. P.; Sato, J.; Deming, T. J.; Pine, D. J. Macromolecules 2004, 37, 3943. (13) Deming, T. J. Soft Matter 2005, 1, 28. (14) Yoshida, M.; Koumura, N.; Misawa, Y.; Tamaoki, N.; Matsumoto, H.; Kawanami, H.; Kazaoui, S.; Minami, N. J. Am. Chem. Soc. 2007, 129, 11039. (15) Laurent, D.; Schlenoff, J. B. Langmuir 1997, 13, 1552. (16) Ichinose, I.; Kunitake, T. AdV. Mater. 2002, 14, 344. (17) Hanabusa, K.; Nakayama, H.; Kimura, M.; Shirai, H. Chem. Lett. 2000, 9, 1070. (18) Guenet, J. M. ThermoreVersible Gelation of Polymers and Biopolymers; Academic Press: New York, 1992. (19) Matsunaga, T.; Shibayama, M. Phys. ReV. E, Rapid Commun. 2007, 76, 030401. (20) Shibayama, M.; Okamoto, M. J. Chem. Phys. 2001, 115, 4285. (21) Okamoto, M.; Norisuye, T.; Shibayama, M. Macromolecules 2001, 34, 8496. (22) te Nijenhuis, K. AdV. Polym. Sci. 1997, 130, 1. (23) te Nijenhuis, K. Colloid Polym. Sci. 1981, 259, 522. (24) Djabourov, M.; Leblond, J.; Papon, P. J. Phys. (Paris) 1988, 49, 319. (25) Djabourov, M.; Leblond, J.; Papon, P. J. Phys. (Paris) 1988, 49, 333. (26) Winter, H. H.; Chambon, F. J. Rheol. 1986, 30, 367. (27) Shibayama, M.; Norisuye, T. Bull. Chem. Soc. Jpn. 2002, 75, 641. (28) Winter, H. H.; Mours, M. AdV. Polym. Sci. 1997, 134, 167. (29) Cordier, P.; Tournilhac, F.; Soulie´-Ziakovic, C.; Leibler, L. Nature 2008, 451, 977. (30) Okabe, S.; Hanabusa, K.; Shibayama, M. J. Polym. Sci., Part B: Polym. Phys. Ed. 2005, 43, 3567. (31) Angelini, R.; Giura, P.; Fioretto, D.; Monaco, G.; Ruocco, G.; Sette, F. Phys. ReV. B 2004, 70, 224302. (32) Shibayama, M.; Yoshizawa, H.; Kurokawa, H.; Fujiwara, H.; Nomura, S. Polymer 1988, 29, 2066. (33) Chmielewski, M. J.; Jurczak, J. Chem. Eur. J. 2005, 11, 6080. (34) Joˇz¨wiak, A.; Brzezin´ski, J. Z.; Płotka, M. W.; Szczeœˇniak, A. K.; Malinowski, Z.; Epsztajn, J. Eur. J. Org. Chem. 2004, 15, 3254. (35) Kavallieratos, K.; Bertao, C. M.; Crabtree, R. H. J. Org. Chem. 1999, 64, 1675. (36) Sessler, J. L.; An, D. Q.; Cho, W. S.; Lynch, V.; Yoon, D. W.; Hong, S. J.; Lee, C. H. J. Org. Chem. 2005, 70, 1511. (37) Ca`jan, M.; Stibor, I.; Kocˇa, J. J. Phys. Chem. A 1999, 103, 3778. (38) Chmielewski, M. J.; Jurczak, J. Chem. Eur. J. 2006, 12, 7652.

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