Article pubs.acs.org/Macromolecules
Supramolecular Organogel Based on Crown Ether and Secondary Ammoniumion Functionalized Glycidyl Triazole Polymers Dian Liu,† Dapeng Wang,† Miao Wang,† Yijun Zheng,† Kaloian Koynov,† Günter K. Auernhammer,† Hans-Jürgen Butt,† and Taichi Ikeda†,‡,* †
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany Polymer Materials Unit, National Institute for Materials Science, Namiki 1-1, 305-0044 Tsukuba, Japan
‡
S Supporting Information *
ABSTRACT: A supramolecular organogel was prepared by mixing the glycidyl triazole polymers (GTP) functionalized with crown ether and secondary ammoniumion at the side groups. The polymers form an organogel above a concentration of 3 wt % via physical cross-links of the inclusion complex. The organogel responds to multiple stimuli, e.g., temperature, acid/base, and chemical species. The number of the effective cross-links estimated from the storage modulus and the affine network model suggests that some part of the binding sites could not work as the physical cross-links. Rheological measurement under large deformation showed that the storage modulus was constant up to 250% strain and larger than the loss modulus up to 600% strain. The high elasticity of the gel is attributable to the material design based on the high-molecular-weight flexible glycidyl polymers with many binding sites in the single polymer chain. The organogel also showed nice self-healing behavior. The molecular diffusion in the gel network was characterized by fluorescence correlation spectroscopy. Although the cross-link of the organogel has dynamic nature due to inclusion complexation, the diffusion behavior of the low-molecular-weight fluorescence tracer was similar to that observed in chemically cross-linked gels.
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and guest units.9,12,14,34−39 The other way is based on the hostand guest-functionalized polymers, in which the host and guest units are introduced as the polymer side groups.3,17,18,23 In the case of the DB24C8 and DBAS host−guest pairs, the former approach has been more popular.12,14,34−39 In the cases of other host−guest pairs, however, many research groups have applied the latter approach.3,17,18,23 There are some advantages. (1) Higher molecular weight polymer leads to better mechanical properties of the gel, which allows to create tough and elastic materials. (2) Cross-link density in the gel can be easily modulated by the degree of functionalization, which gives a handle to control the mechanical properties of the gel. In the case of the supramolecular polymer, the number of the binding sites in the single molecule is limited. (3) One can reduce the amount of host and guest units in the gel, which reduces the time and cost of the synthesis. A. Harada et al. have developed various kinds of HG supramolecular hydrogels using cyclodextrin-functionalized polymers.3,17,18 The binding constant of the cyclodextrins is the order of 102−105 M−1.40 In order to improve the mechanical properties of the gels, some groups used the host−guest pair of cucurbit[8]uril and viologen + naphthalene or diaminohexane derivatives,8,13,23 because they can form 1: 2 inclusion complexes with larger binding constants
INTRODUCTION Supramolecular gels based on host−guest complexation (HG supramolecular gel) are emerging smart materials.1−10 In HG supramolecular gels, the polymers are cross-linked by the inclusion complex between the macrocyclic host and guest units. Since inclusion complexation is controllable by external stimuli, one can easily create smart materials11 exhibiting sol− gel transitions in response to the changes of temperature,4,6,9,12−15 acid/base,6,12,14−16 chemical species,13−15 light,17 redox reaction,13,18 etc. In addition, the HG supramolecular gel is a promising self-healing material19−21 on account of the reversible nature of inclusion complexation.13,16,18,22 These intriguing properties lead to many potential applications such as sensors, controlled release systems, tissue engineering, etc.10,14,15,23,24 For practical application, improving the mechanical properties of the HG supramolecular gel is an important issue. In this study, we use the host−guest pair of a crown ether, dibenzo[24]crown-8 (DB24C8), and a secondary ammonium ion, dibenzylammonium salt (DBAS). Since the first synthesis by C. J. Pedersen,25 this host−guest pair has been widely used to construct supramolecular assemblies26−29 and interlocked compounds,30−33 because of the easy and well-established synthetic protocols and the expandability of the molecular design. Two approaches have been proposed for designing HG supramolecular gels. One is based on supramolecular polymers using low-molecular-weight compounds consisting of the host © XXXX American Chemical Society
Received: February 25, 2013 Revised: May 21, 2013
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Scheme 1. Synthesis of Crown Ether and Secondary Ammoniumion Functionalized GTPs and Preparation of HG Supramolecular Gel
of 1011−1012 M−2.13,23,41 Since the binding constant of DB24C8 and DBAS host−guest pair is not so high (102−104 M−1)26 as that of the cucurbit[8]uril, the effective approaches for improving the mechanical properties are increasing the cross-link density and molecular weight of the host- and guestfunctionalized polymers. However, there are only few reports on HG supramolecular gels consisting of crown ether and secondary ammoniumion functionalized polymers. Some groups have reported polymer gels using bifunctional crosslinkers, bis-ammonium derivatives4,6,16,24 or bis-crown ether derivatives.15 The strategy using bifunctional cross-linkers is, however, not desirable for creating mechanically tough gels. Since the bifunctional cross-linker can cross-link the polymers only when each binding site catches different polymers,4 a large amount of bifunctional cross-linker is required to obtain a mechanically tough gel.15 If the gel were immersed in the solution, a bifunctional cross-linker would be released from the gel and the network structure of the polymers would be collapsed. Therefore, it is highly attractive to develop HG supramolecular gels based on crown ether and secondary ammoniumion functionalized polymers. In such a gel, the polymers can hold each other tightly with many cross-links. Recently, we have reported the synthesis of “glycidyl 4functionalized-1,2,3-triazole polymer” (functionalized GTP),42 which is synthesized from a glycidyl azide polymer (GAP) by azide−alkyne Huisgen cycloaddition. Although click-functionalization of low-molecular-weight GAP (Number-averaged molecular weight, Mn = 1.2 kDa) has been reported by Y.-G. Lim et al.,43 we have proved that various kinds of highmolecular-weight functionalized GTPs (Mn > 100 kDa) could b e p r e p a re d fr o m c o m m e r c i a l l y a v a i l ab l e p o l y -
(epichlorohydrin). Long and flexible polyether main chain of our functionalized GTP is expected to be suitable for creating tough and elastic gel materials. Therefore, functionalized GTP is a promising scaffold to synthesize DB24C8- and DBASfunctionalized polymers. Here, we report a HG supramolecular organogel based on the DB24C8- and DBAS-functionalized GTPs. These polymers are abbreviated as GTP-DB24C8 and GTP-DBAS, respectively (Scheme 1). We succeeded to prepare an organogel with higher elasticity than conventional HG supramolecular gels. The organogel showed multiple stimuli responsiveness (temperature, acid/base, and chemical species) and self-healing ability. The mechanical properties and self-healing ability of the gel were characterized by the rheometers. The molecular diffusion inside the gel network was characterized by fluorescence correlation spectroscopy (FCS).
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EXPERIMENTAL SECTION
Reagents. All solvents, poly(epichlorohydrin) (Mw = 700 kDa), 3,3-dimethyl-1-butyne, tetrakisacetonitrile copper(I) hexafluorophosphate (Cu(MeCN)4·PF6), cation exchange resin Dowex Marathon MSC hydrogen form, ethylenediaminetetraacetic acid two sodium salt (EDTA·2Na), and hexafluorophosphoric acid (HPF6) were purchased from Sigma-Aldrich Co. Glycidyl azide polymer (GAP), acetyleneterminated DB24C8 and DBA were synthesized according to the literature.6,34,44−48 Instruments. NMR spectra were recorded at 293 K on a Bruker spectrospin 250 (250 and 62.5 MHz for 1H and 13C nuclei, respectively). Gel permeation chromatography (GPC) was carried out at 60 °C on a SECcurityGPC system (Polymer Standards Service) equipped with column GRAM 1000, GRAM 100, and Shodex RI101 as the detector. Thermogravimetric analysis (TGA) was performed on B
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a TGA 851 (Mettler Toledo) from room temperature to 800 °C at the heating rate of 10 °C min−1 under N2. Differential scanning calorimetry (DSC) was performed on a DSC 822 (Mettler Toledo) from −140 to +220 °C at the aheating rate of 10 °C min−1 under a N2 flow. Samples were first heated from −140 to +220 °C then cooled to −140 °C, and the onset temperature was taken as the glass transition temperature (Tg). GTP-DB24C8. GAP (0.3 g, 3.0 mmol azido group) was dissolved completely in dry N,N-dimethylformamide (DMF) (30 mL) with stirring at 40 °C. Caution! Never heat up too much and never agitate by sonication. The acetylene-terminated DB24C8 (84 mg, 0.15 mmol) and the copper catalyst (Cu(MeCN)4·PF6, 56 mg, 0.15 mmol) were added to the solution. The mixture was stirred at 35 °C under Ar for 2 h. Then 3,3-dimethyl-1-butyne (0.74 mL, 6.0 mmol) was injected into the mixture. The mixture was stirred for another 24 h. After the treatment with cation exchange resin (15 g), the solution was filtered and concentrated. The polymer was precipitated by adding the DMF solution dropwise to diethyl ether. After removing the solvent by decantation, the polymer was dissolved in a small amount of dichloromethane (DCM). The polymer was precipitated again by adding the DCM solution dropwise to diethyl ether. The precipitated polymer was dissolved in DCM (200 mL) and washed with 2N EDTA·2Na aqueous solution (200 mL). The organic phase was dried with MgSO4, filtrated, and concentrated. The polymer was recovered by precipitation in diethyl ether and dried under vacuum. Yield: 0.53 g (96%). 1H NMR (250 MHz, DMSO-d6): δ = 1.10, 3.41, 3.55, 3.64, 3.94, 4.17, 6.77, 7.60−7.63. GTP-DBA. GTP-DBA was synthesized with the same procedure as GTP-DB24C8. Yield: 0.73 g (89%). 1H NMR (250 MHz, DMSO-d6): δ = 1.08, 3.39, 3.64, 4.15, 6.73, 7.20, 7.57−7.62. GTP-DBAS. GTP-DBA (0.6 g) was dissolved in DMSO (20 mL), and HPF6 (65 wt % aqueous, 60 μL) was added dropwise. The solution was stirred for 30 min. The polymer was purified by precipitation twice in water followed by precipitation in methanol and diethyl ether, then dried under vacuum. Yield: 0.58 g (94%). 1H NMR (250 MHz, DMSO-d6): δ = 1.18, 3.49, 3.74, 4.25, 6.94, 7.44, 7.67− 7.72. Gel Preparation. GTP-DB24C8 and GTP-DBAS were added to the solvent (CHCl3 or C2H2Cl4). These polymers were dissolved completely by heating. A transparent gel was obtained after cooling to room temperature. Rheology Measurement. The mechanical properties of the gel were investigated with a homemade piezorheometer.49 A small amount (typically 15−25 μg) of the gel sample was placed between two glass slides (area 2 cm2) with a gap of 100 μm between them. Using the density of the solvent C2H2Cl4 (ρ = 1.59 g cm−3), the volume of the gel was calculated from the weight. Since the evaporation of C2H2Cl4 from the solution is slow at room temperature (boiling point, 147 °C; vapor pressure, 8 mmHg at 20 °C), the concentration change is negligible in the time scale of the experiment, typically 10 min. In the measurement, a defined shear deformation was applied with a piezoactuator and the stress transmitted through the sample was measured with a second piezoactuator. Rheological properties were obtained from the applied strain and measured resulting stress. Frequency sweeps were performed at 23 °C and 0.32% strain from 1.58 to 2000 rad s−1. The small amplitudes guaranteed that the sample was measured in the linear response regime. The strain-amplitudesweep and step-strain experiments were carried out on a rotational ARES rheometer (Rheometric Scientific) equipped with parallel plates (diameter: 13 mm). Fluorescence Correlation Spectroscopy (FCS). The measurements were carried out on a commercial setup (Carl Zeiss, Jena, Germany) comprising the module ConfoCor2, an inverted microscope model Axiovert 200 and a 40 × /NA0.9 Plan Neofluar multiimmersion objective with oil as an immersion liquid. The fluorescent molecules were excited by a HeNe laser (633 nm), and the emission was collected after filtering with LP650 long pass filter. An Attofluor cell chamber (Invitrogen, Leiden, Netherlands) with a microscope glass slide, with diameter of 25 mm and thickness of approximately 0.15 mm, was employed as a sample cell. The temporal fluctuations of
the detected fluorescence intensity, δF(t′), caused by fluorescent species diffusing through the confocal observation volume (laser focus) were evaluated in terms of an autocorrelation function, G(t) = ⟨δF(t′)δF(t′ + t)⟩/⟨F(t′)⟩2. For each sample, 3 or 4 independent autocorrelation functions were measured (3−5 min each). Since the FCS experiments are very sensitive to the presence of large aggregates, the data sets influenced by large aggregates were excluded. The calibration of the observation volume was done using a tracer with known diffusion coefficient, i.e., TDI in toluene (D = 4.6 × 10−10 m2 s−1).50 Scanning Electron Microscopy (SEM). The measurement was carried out on a LEO 1530 Gemini (Zeiss) microscopy under 3 kV. The gel sample was prepared using chloroform, then it was cryo-dried and broken in liquid nitrogen and lyophilized overnight. The sample was coated by gold sputtering (10 nm) before the measurement.
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RESULTS AND DISCUSSION Synthesis of Functionalized GTPs. GTP-DB24C8 and dibenzylamine-functionalized GTPs (GTP-DBA) were synthesized from GAP by simultaneous click reactions of 3,3dimethyl-1-butyne and acetylene-terminated DB24C8 or DBA derivatives (Scheme 1). In our previous study,42 it was confirmed that the functionalized GTP easily aggregates each other, and some functionalized GTPs form a gel. Therefore, the design of the side group was very important. In fact, we faced the solubility issue of GTP-DBA. Some possible molecular interactions between GTP-DBAs can be pointed out, e.g., hydrogen bonds between the DBA units, DBA and the triazole ring, or DBA and the ether oxygen of the glycidyl unit. We selected tert-butyl (t-Bu) group as a side group. Since t-Bu group is short and bulky, it can suppress the molecular interactions between the DBA and glycidyl triazole units, but does not interfere the host−guest interaction between DB24C8 and DBAS units. GTP-DB24C8 and GTP-DBA were characterized by 1H NMR (Figure 1). All peaks were broad due to high molecular
Figure 1. 1H NMR spectra of (a) GTP-DB24C8 and (b) GTP-DBA. 250 MHz, DMSO-d6.
weight of the copolymer. The peak of the triazole group was confirmed at 7.7 ppm.42 In the case of GTP-DB24C8, the aromatic protons of the crown ether were observed as a broad peak at 6.8 ppm. The peaks of glycidyl unit and methylene protons of the crown ether were overlapped at 3.2−4.4 ppm. The t-Bu group gave an intense peak at 1.2 ppm. As for GTPDBA, the signal of the DBA aromatic protons appeared at 6.8− 7.5 ppm. From the integral ratio of the proton signals for the aromatic and triazole rings, the contents of the DB24C8- and C
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units is a key to form the gel. The critical gelation concentration of the polymer was ca. 3 wt %. As reported by some groups,4,6,12,14,15 our gel also showed multiple stimuli responsiveness (Figure 2). The gel is thermo-
DBA-functionalized glycidyl monomer units in the polymers were calculated to be ca. 5% and 4%, respectively. Since the contents of the host and guest units in the polymer are low, the physical properties of GTP-DB24C8 and GTPDBA are considered to be comparable to that of the model polymer, t-Bu-functionalized GTP (GTP-t-Bu). Molecular weights of the polymers were characterized by GPC using DMF as an eluent with polystyrene as a standard. In the both cases of GTP-DB24C8 and GTP-DBA, GPC chart gave a broad peak with a shoulder at higher molecular weight region (Figure S11, Supporting Information). The shoulder of the peak is attributable to the self-aggregation of the polymer. Although the self-aggregation makes it difficult to obtain reliable molecular weights of GTP-DB24C8 and GTP-DBA (Table 1), the result of our previous study indicated that the Table 1. Characterization of Functionalized GTPs polymer
Mw (kDa)a
Mw (kDa)b
PDIc
Tg (°C)d
Td (°C)e
GTP-DB24C8 GTP-DBA GTP-t-Bu
257 260 164
128 123 120
3.10 2.76 1.89
45.1 57.5 54.6
353 342 366
Figure 2. Multiple stimuli-responsiveness of HG supramolecular organogel. Sol−gel transition can be induced by temperature change, addition of acid/base, and low-molecular-weight competitive binder. Polymer concentration: 6 wt %. Solvent: C2H2Cl4. TEA: triethylamine. TFA: trifluoroacetic acid. DB24C8: acetylene-terminated dibenzo[24]crown-8. DBAS: acetylene-terminated dibenzylammonium salt.
a Average molecular weight determined by GPC. Solvent: DMF. bPeak top value of the GPC curve. cPolydispersity index. dGlass transition temperature (onset value). eThermal decomposition temperature (onset value).
responsive. By heating, the gel turned to the solution, then it came back to the gel after cooling. This process was repeatable. The gel is also acid/base-responsive. When GTP-DBAS was deprotonated by adding triethylamine (TEA), the gel turned to a solution. The gel could be recovered by adding excess trifluoroacetic acid (TFA). Though this process was also repeatable, the gel became translucent after some sol−gel transitions because of the triethylamine salt precipitation in the gel. The gel is also chemical species-responsive. When a lowmolecular-weight acetylene-terminated DB24C8 (or DBAS derivatives) was added as a competitive binder, the gel turned to a solution. This process was not reversible. Mechanical Properties. Rheological measurements were conducted using a homemade piezorheometer which can perform the shearing under a low strains. Figure 3 shows the frequency-dependent storage modulus of the gel with different polymer concentrations (4, 6, 11 wt %). All curves demonstrated the gel behavior, i.e., a low frequency depend-
functionalized GTP has ca. 700 repeating units.42 It should be noted that we used the same GAP sample as our previous study for the synthesis of GTP-DB24C8 and GTP-DBA. Therefore, the molecular weights of GTP-DB24C8 and GTPDBA are estimated to be ca. 140 kDa and 130 kDa, respectively, which are comparable to the peak-top molecular weights of the GPC charts. The numbers of the DB24C8 and DBA units in the single polymer chain are estimated to be ca. 35 and 28, respectively. These properties, high-molecular-weight and many binding sites in the single polymer chain, are advantage for creating a mechanically tough gel. Thermal properties of the polymers (glass transition temperature: Tg, and decomposition temperature: Td) were characterized by DSC and TGA (Table 1 and Figure S12, Supporting Information). The results are comparable to those of GTP-t-Bu (Table 1).42 The relatively low Tg is partly due to the flexible main chain of the glycidyl polymer. The high mobility of the polymer segments facilitates the appended host and guest units to find each other and form an inclusion complex. This is an advantage as compared to conventional HG supramolecular gels consisting of functionalized polyolefins.3,8,13,17,18 Gel Preparation and Stimuli-Responsive Behavior. Before gel preparation, GTP-DBA was protonated to GTPDBAS by HPF6 (Scheme 1). A transparent gel was obtained by mixing the solid samples of GTP-DB24C8 and GTP-DBAS in chloroform (CHCl3) or 1,1,2,2-tetrachloroethane (C2H2Cl4). Mixing the solutions of GTP-DB24C8 and GTP-DBAS also led to gel formation. Because of low contents of the host and guest units in the solution and high viscosity of the mixture solution, it was impossible to characterize inclusion complexation by NMR.12,34,35,37−39 We confirmed that the combinations of GTP-DB24C8/GTP-t-Bu or GTP-DBAS/GTP-t-Bu did not form gels under the same condition. This result supports that the inclusion complexation between the DB24C8 and DBAS
Figure 3. Frequency-dependent storage modulus G′ of the gel with different polymer concentrations performed at 0.32% strain. Solvent: C2H2Cl4. Temperature: 25 °C. D
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ence of the storage modulus G′. Since the cross-link density increases with the polymer concentration, the gel with higher polymer concentration afforded higher storage modulus. The frequency range of the storage modulus plateau also relates to the cross-link density. In the case of a 4 wt % gel, it can be seen that its storage modulus stays as a constant up to 100 rad s−1, but increases with the frequency above 100 rad s−1, which indicates the internal degrees of freedom of the polymer chains.51 Because of the relatively low cross-link density, this mode appeared at higher frequency region. Once the polymer concentration was increased to 6 wt %, a stable and wide plateau was observed even at very high frequency, which indicates the formation of stable three-dimensional network in the gel. The G′ value data allowed us to estimate the number of the effective cross-links per unit volume (cross-link density) with the aid of simple affine network model (G′ = maffinekBT; maffine, cross-link density; kB, Boltzmann constant; T, absolute temperature).52 The estimated cross-link density from the G′ value (maffine) was compared to the expected cross-link density based on the chemical composition (mchem), assuming 100% DBAS units could form inclusion complexes with DB24C8 units (Figure 4). The maffine value is significantly smaller than
Figure 5. Plateau value of storage modulus G′ with different mixing ratio of GTP-DB24C8 and GTP-DBAS. The plateau value was obtained as an average value of the storage modulus G′ in the frequency range from 1.58 to 100 rad s−1. Total polymer concentration: 6 wt %. Solvent: C2H2Cl4.
obtained at a mixing ratio of 1: 1. Ideally, the G′ plateau value should decrease symmetrically with enriching one component against another (GTP-DB24C8 or GTP-DBAS). However, the plot was asymmetric. The GTP-DBAS-rich gel gave a higher G′ plateau value than the GTP-DB24C8-rich gel. This might be due to molecular interactions between GTP-DBASs, which are considered to be larger than those between GTP-DB24C8s. This is supported by the fact that the GTP-DBAS solution had higher viscosity than GTP-DB24C8 solution (Figure S13, Supporting Information). Figure 6a shows strain-amplitude dependency of G′ and G″, which was done on a rotational parallel-plate rheometer. The G′ and G″ data demonstrate a remarkably wide linear response zone up to 150% strain amplitude and moderate hardening (increase of G′) on a further increase of strain amplitude to 300%. As discussed above, some part of the binding sites are considered not to work as the physical cross-links, which result in the long distance between the cross-links. This is probably responsible for the wide linear response zone and the successive moderate strain hardening. The decrease in G′ at very large strain is due to the balance between the rate of bond breaking by the external force and the bond reformation due to newly formed inclusion complexes. The G′ value was larger than G″ up to 600% strain. This is a remarkable toughness compared with the HG supramolecular gels reported previously.13,16,18 These results prove the advantage of our material design using the functionalized GTP. Presumably, flexibility of the glycidyl polymer chain, high-molecular-weight polymeric structure, many binding sites in the single polymer chain, and dynamic nature of the physical cross-links might contribute to keep the cross-linked network structure under large deformation. Self-Healing Behavior. Self-healing behavior was characterized by step-strain experiment of the storage modulus (Figure 6b). After applying a high-magnitude strain (750%), the G′ value decreased significantly because the network structure was broken. When reducing the strain to 1%, the G′ value recovered in less than 10 s, indicating the reconstruction of the network structure. Fast recovery of the mechanical property originates from fast inclusion complexation between DB24C8 and DBAS units53 in the gel. After the first step-strain, G′ did not completely recover to the original value, due to the loss of the gel sample during large rotational strain. We confirmed that a part of the gel sample came out from the gap between two parallel plates of the rheometer.
Figure 4. Estimated cross-link density by affine network model (maffine) from G′ data (solid square) and calculated cross-link density from chemical composition (mchem) assuming that all DBAS units form the inclusion complex with DB24C8 units (solid triangle).
the mchem value, suggesting that some part of the DBAS units could not form inclusion complexes. Since the diffusion of the DBAS and DB24C8 units is strongly restricted by the polymer main chain, smaller effective cross-link density than the mchem value should be reasonable. This result implies that the HG supramolecular gel would require many binding sites in the single polymer chain to form stable three-dimensional network. It should be noted that the affine network model would be too simple to describe the system of HG supramolecular gel. Furthermore, loop formation and the dynamic nature of the inclusion complexes might also contribute to the small maffine values. Considering that GTP-DB24C8 and GTP-DBAS have almost the same binding units for inclusion complexation, one can predict that the best mixing ratio for obtaining a tough gel would be 1: 1, because this ratio should give highest crosslink density in the gel. Figure 5 shows the relationship between the plateau value of G′ and the mixing ratio of GTP-DB24C8 and GTP-DBAS. Total concentration of the polymers was fixed to be 6 wt %. As expected, the highest G′ plateau value was E
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Figure 7. Macroscopic self-healing behavior of HG supramolecular organogel. (a) Gel was cut into two pieces, attached together, then picked up with tweezers. (b) Four pieces of gels were connected linearly. Methylene blue was used for visibility.54 Polymer concentration: 11 wt %. Solvent: C2H2Cl4.
Figure 6. (a) Storage (G′) and loss moduli (G″) of the gel in strain sweep experiment. Total polymer concentration: 6 wt %. Solvent: C2H2Cl4. (b) Time-course-change of storage modulus (G′) in stepstrain experiment. 750% strain (duration: 30 s) and 1% strain (duration: 120 s) were applied on the gel alternatively. Polymer concentration: 6 wt %. Solvent: C2H2Cl4. Scanning frequency: 10 rad s−1.
We also demonstrated the macroscopic self-healing behavior by cutting the gel into two pieces followed by connecting them back to the original shape (Figure 7a). Self-healing started immediately after connecting the cut faces. We could connect four pieces of the gel in a raw (Figure 7b).54 After 5 min, the combined gel was strong enough to be held vertically or suspended horizontally. Molecular Diffusion Inside the Gel. Understanding the diffusion of low-molecular-weight compounds in the polymer network of the HG supramolecular gel is important, e.g., for applications as drug release systems. Furthermore, the diffusion data may provide information for the homogeneity of the swollen gel structure on different length scales. Such information is complementary to the results obtained by SEM image of the cryo-dried gels (Figure 8). Dried organogels showed a network-like structure with typical pore diameters of 100−300 nm and fiber diameters of 10−30 nm. The fibers might consist of many polymers which had collapsed during the drying process. The diffusion of a small fluorescent tracer, terylenediimide derivative55,56 (TDI, inset of Figure 9a), in the gel was characterized by FCS. FCS allows the determination of the tracer diffusion with high sensitivity down to the single molecule level.57 The method is based on monitoring and recording the fluctuations of the fluorescent intensity signal caused by the diffusion of the fluorescent tracer through a very small observation volume, typically the focus of a confocal
Figure 8. SEM images of cryo-dried and lyophilized sample of gel. The sample surface was coated by the gold (10 nm thickness). Polymer concentration: 6 wt %.
microscope. These fluctuations are evaluated in terms of an experimental autocorrelation curve. In the case of Fickian diffusion of the tracers, the FCS autocorrelation curve can be represented by the following equation:57 −1/2 −1 1 ⎛ τ⎞ ⎛ τ ⎞ G (τ ) = 1 + ⎜1 + ⎟ ⎜1 + 2 ⎟ N* ⎝ τD ⎠ ⎝ S τD ⎠
(1)
Here N* is the average number of fluorescent tracers in the observation volume, τD is their diffusion time. The so-called structure parameter is defined as S = z0/x0, where z0 and x0 are the axial and radial dimensions of the observation volume, respectively. The diffusion coefficient of the tracers is related to their diffusion time through D = x0/4τD and can be obtained by fitting the experimental autocorrelation curve with eq 1. F
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from similar studies in chemically cross-linked PNIPAM hydrogels.58 These master curves can be represented by a stretched exponential function of eq 2. D/D0 = exp( −αφ β )
(2)
In the case of polymer solution, α and β values are 9.9 and 1.35, respectively (solid curve in Figure 8b).59 For chemically cross-linked PNIPAM hydrogel, α and β values are 20.5 and 1.35, respectively (dashed curve in Figure 9b).58 Two master curves in Figure 9b indicate much slower tracers’ diffusion in chemically cross-linked PNIPAM hydrogels than in polymer solution. In the case of polymer solutions, only dynamic entanglement of the polymers exists and the tracer diffusion is not affected by the presence of permanent cross-links. Slower diffusion in chemically cross-linked gels should be related to the existence of permanent cross-links. The results obtained from HG supramolecular gel are on the master curve of the chemically cross-linked PNIPAM hydrogels (open circles). Although the cross-link of the HG supramolecular gel has dynamic nature and it is not a permanent one, the diffusion behavior inside HG supramolecular gel is close to that for chemically cross-linked PNIPAM gel. This indicates that the exchange rate of the cross-links in the HG supramolecular gel is significantly slower than the time scale of the low-molecular weight tracer diffusion through the FCS observation volume, which is in the order of 500 μs.
Figure 9. (a) Normalized experimental autocorrelation curves (symbols) measured with TDI diffusing in HG supramolecular organogel with different polymer concentrations. The solid curves represent the corresponding fits with eq 1. The inset shows the chemical structure of TDI. (b) Relationship between normalized diffusion coefficient of TDI in HG supramolecular organogel and polymer volume fraction (open circles). For comparison, universal master curves obtained from the diffusion data in polymer solutions (solid curve) and in PNIPAM hydrogels (dashed curve) are also shown.
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CONCLUSION
We successfully prepared a HG supramolecular organogel based on crown ether and secondary ammoniumion functionalized GTPs. Since the functionalized GTPs can be easily synthesized from commercially available poly(epichlorohydrin), our approach opens a way for facile preparation of HG supramolecular gels. Since the number of the effective crosslinks estimated from the G′ value and the affine network model suggests that some part of the binding sites could not work as the physical cross-links, many binding sites in the single polymer chain should be important to form stable threedimensional network. Rheological measurement showed that the storage modulus was constant up to 250% strain and larger than the loss modulus up to 600% strain. The high elasticity under large deformation is attributable to high-molecularweight of the polymer (over 100 kDa), flexibility of the glycidyl polymer main chain, many binding sites in the single polymer chain, and dynamic nature of the physical cross-links. The mechanical properties of the gel were controllable by changing the polymer concentration or mixing ratio of the host and guest-functionalized GTPs. The organogel showed multiple stimuli responsiveness (temperature, acid/base and chemical species), and nice self-healing ability. Fluorescence correlation spectroscopy revealed that the diffusion coefficient of a lowmolecular-weight fluorescence tracer in the HG supramolecular gel is close to that observed in a chemically cross-linked gel. Some questions remain on the rheological property of the HG supramolecular gel, e.g., the exchange rate of the cross-link, the relationship between the effective cross-link density and the degree of the inclusion complex formation, time evolution of mechanical toughness in the self-healing process, etc. These will be attractive theme for future study.
Experimental FCS autocorrelation curves for TDI tracers diffusing in the gels could be adequately fitted with eq 1 (Figure 9a), indicating single component Fickian diffusion of TDI in the HG supramolecular gels. This means that the gel’s structure was homogeneous at the length scale of the observation volume (
