Calix[4]arene-based low molecular mass gelators to form gels in organoalkoxysilanes

Abstract

Three calix[4]arene derivatives (CMA, CDA and CTA) appended with one, two or four carboxyl acid structures were prepared. Gelation behaviours of the compounds and calix[4]arene itself in ten common organoalkoxysilanes were studied. It was found that at a concentration of 2% (w/v), only the compound with the most carboxyl groups (CTA) functions as a gelator, and only this compound gels with three of the liquids tested; however, the as created gels possess smart thixotropic and thermo-reversible phase transition properties. In particular, the CTA/trimethoxyphenylsilane (PTMS) gel exhibits superior mechanical strength with a storage modulus (G′) greater than 1.9 × 10⁶ Pa and a yield stress exceeding 3600 Pa at a concentration of 6.0% (w/v). Further testing demonstrated that the gel could be used as a substrate for sensing film fabrication, injection molding and melting-free deposition molding. Moreover, the objects from the molding and fabrication could be turned into permanent structures through further hydrolysis and condensation reactions. It is believed that the LMMGs based organoalkoxysilane gels have the potential to be used as smart materials for 3D printing and pre-cursors to a functionality-oriented solid matrix.

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Introduction

As a burgeoning field of soft matter science, molecular/supramolecular gels based on low molecular mass gelators (LMMGs) have attracted great attention during the last few decades for their great potential in numerous applications.^1–9 It is well known that within molecular gels, the molecules of LMMGs interact with each other via hydrogen bonding, π–π stacking, van der Waals forces, hydrophobic interaction, coordination interaction, dipole–dipole interaction, electrostatic interaction, and host–guest interaction, etc., in the solvents to form self-assembled 3D networks that immobilize solvents and result in gelation.^10,11 The stability and rheological properties of a gel as formed are dependent on the supramolecular interaction nature of the gel networks and their interaction with the solvent. It is the non-covalent nature of the interaction that makes the gels sensitive to external stimuli, such as temperature, shear-force, sonication, illumination and chemicals, etc.^12–14 Thus, molecular gels are highly desirable for applications in drug delivery,¹⁵ sensing,^16–18 catalysis,¹⁹ crystallization,²⁰ 3D printing,²¹ and pollutant removal,^22–24 etc.

Calixarenes are recognized as a new generation of host molecules, and are frequently used as building blocks in supramolecular assemblies.^25–55 As for the creation of molecular gels, Shinkai and co-workers were the first to use calix[n]arene (n = 4, 6, 8) derivatives as LMMGs to gel apolar solvents.^25,26 Subsequently, more interesting calix[n]arene based molecular gels were reported.^27–55 For example, Xu and co-workers used 3-pyridine-azo-calix[4]arenes as chelates and built highly stable metallogels, which could uptake non-ionic organic molecules from aqueous or gas phases, and also showed catalytic activities.^27–29 Park and co-workers found that with the assistance of K⁺ and Rb⁺, 1,3-alternate calix[4]arene tetra-acetate could gel a 1 : 1 MeOH/water mixture, and revealed the amazing structures of the gel networks at the molecular level.³⁰ Jung et al. prepared some molecular gels via combination of 1,3-alternate tetracarboxylic acid-appended thiacalix[4]arene with Co²⁺, and it was reported that one of the gels could be used for the sensing of volatile gases containing chlorine atoms (VGCl), in a visualized way.³¹ The same group also synthesized another 1,3-alternate calix[4]arene derivative bearing four terpyridine at its lower rim, and discovered that in the presence of Pt²⁺, the compound gels the mixtures of DMSO and H₂O with different compositions.³² The as produced metallogels show strong luminescence, and the brightness of the emission is solvent composition dependent. Recently, the authors found that the gelation behaviour of the 1,3-alternate tetracarboxylic acid-appended calix[4]arene derivative is process-dependent.³³ For instance, the addition of amines and acid in different sequences could result in completely different results. Moreover, the cationic derivative of calix[4]arene bearing four guanidinium units could effectively bind the anionic type clay nanosheet (CNS) surrounded by sodium polyacrylate via electrostatic interaction in water to afford supramolecular hydrogels.³⁴ Recently, Jung and co-workers prepared a super strong gel (with a tensile strength greater than 40 MPa) via the introduction of dynamic covalent bonds, of which a 1,3-alternate calix[4]arene derivative bearing four hydrazide moieties and diphenyl terephthalate-derivative with two aldehyde groups were employed as the reactants.³⁵ Our group prepared three cholesteryl derivatives of calix[4]arene and tested their gelation behaviour in common organic solvents, and it was found that some of the gels as produced exhibited fast and fully reversible thixotropic properties, strong mechanical strength and exceptional thermo-stability, etc.^36–38 However, unlike studies on common organic solvents and water, studies on the gelation behaviour of calixarene derivatives in reactive liquids are extremely rare.¹² Very recently, we found that p-tert-butyl-calix[4]arene tetracarboxylic acid (CTA) could gel some organic monomers, and the gels as formed could be emulsified with water, resulting in reactive gel-emulsions.³⁹ Their polymerization resulted in porous monoliths with extremely low densities. The as produced monoliths could be used to absorb organic liquids and some heavy metal ions, such as Cr(III), from the aqueous phase.

Organoalkoxysilanes are silicon-containing organic liquids. Unlike others, these liquids are reactive and possess high boiling points, which make them usable or even unique in the preparation of a variety of inorganic and inorganic–organic composite materials via the well-known sol–gel process.^56,57 However, as liquids, they are rarely used in molecular gel studies.¹² As for calixarenes and their derivatives, they have never been reported for the gelation of liquids until the writing of this paper. The only recent publication relevant to the liquids and the compounds is related to the development of a new esterification catalyst,⁵⁸ but molecular gels are not involved. Therefore, the gelation behaviour of the known carboxyl acid-appended calix[4]arene derivatives (CMA, CDA and CTA) in common organoalkoxysilanes were explored, and the rheological properties and possible applications of the as obtained gels, were systematically studied. It was found that the CTA/trimethoxyphenylsilane (PTMS) gel exhibits superior mechanical strength and fast and fully reversible thixotropic properties, which allow the gel to be applicable in injection molding and 3D printing. This paper reports the details.

Experimental

Reagents and materials

All organoalkoxysilanes (>99.0%) and 4-tert-butylphenol (>98.0%) were purchased from TCI Tokyo Chemical Industry Co. Ltd., but ethyl chloroacetate (99%) and diphenyl ether (99%) were obtained from J&K Technology Co. Ltd. These reagents were used without further purification. Other reagents were of analytical grade and used as received, except otherwise mentioned. The organoalkoxysilanes tested in the present study are methyltrimethoxysilane (MTMS), methyltriethoxysilane (MTES), trimethoxy(phenyl)silane (PTMS), triethoxy(phenyl)silane (PTES), trimethoxy(vinyl)silane (VTMS), triethoxy(vinyl)silane (VTES), ethyltrimethoxysilane (ETMS), trimethoxy(n-octyl)-silane (n-OTMS), and triethoxy(n-octyl)silane (n-OTES), and trimethoxy(n-propyl)silane (n-PTMS), respectively.

The three derivatives of calix[4]arene (CMA, CDA and CTA), and the control of p-tert-butyl-calix[4]arene (C4) were synthesized with a slightly improved literature method.The synthesizing processes could be described by Scheme 1 and the details of the syntheses as well as the corresponding characterization data were provided in ESI.† The fluorescent compounds used in the present study are products of our laboratory; their structures are depicted in Fig. S1.†

Scheme 1 The synthetic routes for compounds CMA, CDA and CTA.

Gelation tests

Typically, a weighed amount (0.01 g) of the tested compound and a measured volume (0.5 mL) of selected pure liquid were placed into a sealed test tube (diameter: 1.0 cm; height: 2.5 cm) and the system was shaken for a while, then the test tube was heated until the solid was completely dissolved, and then the system was cooled to room temperature. Finally, the system was checked again to see if it was gelled or not. The systems with gel formation were denoted as “G” (gel), but those with only solution remaining were denoted as “S” (solution). The systems in which the potential gelators were substantially in-dissolvable, even at the boiling points of the liquids, were denoted as “I”. In some cases, clear solutions were obtained during heating, but precipitation or floating occurred during cooling, and this kind of system was labelled “P”. Critical gelation concentrations (CGCs) of the relevant gel systems were measured by considering them as the minimum concentrations of the gelators needed for gelation of the relevant liquids.

Gel–sol phase transition temperatures (T_gel)

The falling ball method was employed to measure T_gel. Specifically, a small glass ball with a smooth and regular shape (diameter ∼ 3 mm, weighing ∼ 0.1 g) was gently put onto the top of a gel (∼0.5 mL) to be studied, which was pre-prepared in a sealed test tube with a diameter of 1.0 cm and a height of 2.5 cm. The tube was then slowly heated at ca. 0.4 °C min⁻¹ in a water bath. The temperatures at which the glass ball just began to fall and just sank to the bottom of the tube were recorded, respectively. The average value of the two temperatures was taken as the T_gel of the gel under test. This experiment was repeated three times, and the data reported are the averages.

Morphological studies

Scanning electron microscopy (SEM) observation. SEM images of the xerogel were taken on a TM 3000 Tabletop microscope (Hitachi Limited). The accelerating voltage was 15 kV and the emission current was 10.0 mA. The xerogel for the measurement was prepared by freezing the corresponding gel in liquid nitrogen, and freeze-drying for 12–24 h. Prior to examination, the xerogel was attached to a copper holder by using conductive adhesive tape, followed by a thin coating of gold.

Fluorescence microscopy observation. Fluorescence images of the gels were taken on an optical microscope (Axio Observer-A1). A perylene bisimide derivative 1 (cf. Fig. S1a†) was adopted as a fluorescent probe since it possesses high fluorescence quantum yield and dissolves well in both PTMS and PTES. The excitation and emission wavelengths adopted in the measurements were 546 and 575–640 nm, respectively. The samples used for the observation were prepared by dropping a small volume of the hot solution of the gel to be tested onto a glass slide surface at room temperature, then the slide with the sample was put into a desiccator to allow the solution to gel. Finally, the thin layer of the as obtained gel was used for examination.

Rheological measurements

Rheological measurements were carried out by using a stress-controlled rheometer (TA instrument, AR-G2) equipped with a 20 mm diameter steel-coated parallel-plate geometry. The gap distance was fixed at 500 μm. The following procedure was used to load the fresh gel sample: 2.0 mL of solution containing solvent and gelator was heated until the gelator dissolved, then closed in a tight cone, and then cooled down to 20 °C. The measurements were started two hours after the gel formation. A solvent-trapping device was placed above the plate to avoid evaporation. All measurements were conducted at 20 °C.

A stress sweep at a constant angle frequency (6.28 rad s⁻¹) was performed, which provides information about the mechanical strength of the gel sample. The frequency sweep was obtained in an angle frequency range of 0.0628–560.0 rad s⁻¹ at a shear stress of 5 Pa, which is well within the linear viscoelastic region of the gel samples, causing small strains in the tested materials.

To further examine the thixotropic properties, the following tests were conducted. First, deformation, where the gel under examination was sheared at a constant oscillatory shear stress, which was well beyond its yield stress, for two minutes to make sure that the gel was destroyed. Second, the modulus recovery in a time sweep, where the storage modulus G′ and the loss modulus G′′ of the system were monitored as functions of time by applying a low shear stress of 5 Pa, which is much lower than the yield stress of the gel sample, onto the above destroyed gel.

FTIR measurements

All FTIR measurements were performed on a Bruker VERTEX70 V infrared spectrometer. The testing scale was from 400 to 4000 cm⁻¹ with 128 scans for each sample. The KBr pellet was obtained by mixing a small amount of the sample and anhydrous KBr powder. The FTIR measurements were carried out at room temperature.

Results and discussion

Gelation behaviours

The gelation behaviours of the three calix[4]arene derivatives and the control, C4, in the ten aforementioned organoalkoxysilanes were tested at a standard concentration of 2.0% (w/v), and the results are shown in Table 1. It can be seen that for the liquids and at the concentrations tested, (1) only CTA, which is the one possessing the most carboxyl structures, functions as a gelator, and it only gels PTMS, PTES and VTES; (2) CMA, CDA and CTA dissolve in MTMS, MTES, ETMS, n-OTMS, n-OTES and n-PTMS, respectively, provided the mixtures were heated, whereas the control, C4, does not dissolve in all the liquids tested. Similarly, CMA is also in-dissolvable in VTMS. These findings demonstrate the importance of the substitute, carboxyl structure, and its number in the low rim of the primary calix[4]arene for their gelation of the liquids and for their dissolution in the liquids. To our surprise, it seems that the introduction of the substitute enhances the dissolution of the calix[4]arene derivatives in the liquids, which should promote gelation of the liquids. This is because molecular gel formation is a result of the balance of competition between aggregation and disaggregation of a gelator in a liquid. It is seen that unlike C4, CMA and CDA dissolve in most of the liquids tested, which suggests that CTA should be more soluble in the liquids tested. However, only CTA shows gelation ability, definitely a rather surprising and un-expected result, which needs to be further investigated in the future.

Table 1 Gelation properties, T_gel (°C) at a concentration of 2.0% (w/v), and CGCs (%; in parentheses) of the calix[4]arene derivatives in different organoalkoxysilanes and CCl₄^a

Liquids	C4	CMA	CDA	CTA
a Note: (1) I: in-dissolvable, P: precipitation or floating, S: solution, G: gelation via heating–cooling cycle, C4: p-tert-butyl-calix[4]arene. (2) Only the four liquids listed in the table can be gelled by one of the calix[4]arene derivatives as synthesized, and the compounds either dissolve or are indissolvable in the other seven organoalkoxysilanes under test. (3) The information for the systems with CCl4 as the solvent was adopted from an earlier publication (Chin. J. Appl. Chem. 2016, 33, 633–640).
Trimethoxyphenylsilane (PTMS)	I	S	S
Triethoxyphenylsilane (PTES)	I	S	S
Triethoxyvinylsilane (VTES)	I	S	S
Carbon tetrachloride (CCl4)	I	I	P

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The T_gel and CGCs, which are the fundamental properties of the gels, were measured, and the results are also listed in the Table 1. With reference to the results shown in the table and the observations acquired during the study, it is known that for each of the three gels (CTA/PTMS, CTA/PTES and CTA/VTES), the corresponding T_gel and CGC are 56 °C, 1.0% (w/v); 74 °C, 0.5% (w/v); and 69 °C, 1.8% (w/v), respectively. Moreover, the gels of CTA/PTMS and CTA/PTES (2.0%, w/v) are stable at room temperature for more than one month, provided they are kept in sealed tubes (cf. Fig. S2†). Concentration effect studies revealed that the T_gel values of the two gels increase linearly along with increasing the gelator concentration (cf. Fig. 1), which means that the molecules of the gelator participate in the gelation process in the same way, even though they may enter the gel networks at different times; in other words, the gel networks must possess uniform structures.^59,60

Fig. 1 Plots of the T_gel values of CTA/PTMS and CTA/PTES gels as functions of CTA concentration.

Morphological studies

Scanning electron microscopy (SEM) and optical microscopy were employed to investigate the microstructures of the gels as obtained. The results for the gel systems of CTA/PTMS and CTA/PTES are shown in Fig. S3a and b,† respectively. Clearly far from what was expected, the xerogels from both gels under study lack network structures. The reasons behind this might be the high boiling points of the organoalkoxysilanes, which must increase the difficulty of their evaporation from the wet gels, resulting in collapsed gel networks, mainly due to capillary effects.¹² To further rationalize this, the network structures of the CAT/CCl₄ gel were examined as a control system, and the result is shown in Fig. S3c.† It is seen that CAT aggregated into rod like structures within the gels, suggesting that the morphological structure of a xerogel is determined not only by the structure of the gelator itself, but also by the interactions between the gelator and the liquids.³⁸

As the information acquired from SEM studies is limited, fluorescence microscopy was employed to interrogate the gel network structures of the CTA/organoalkoxysilane systems, because unlike SEM, fluorescence microscopy could be used to examine the micro-structures of the gels in the wet state. Accordingly, a soluble derivative of perylene bisimide 1 was adopted as a fluorescent probe (cf. Fig. S1a†), of which the concentration used was ∼5.9 × 10⁻⁵ mol L⁻¹. The probe was doped in the wet gels of CTA/PTMS and CTA/PTES with different CTA contents, and then used for fluorescence microscopy studies. Fig. 2 depicts some typical results from the measurements, revealing that (1) for the systems of CAT/PTMS (a, b and c), with increasing CTA concentration, the structures of the gel networks changed from loosely packed clusters consisting of needle-like aggregates, to densely packed and entangled clusters with similar internal structures; (2) for the system of CAT/PTES (d, e and f), however, the aggregate structure of CAT is very different from that observed in PTMS, and is characterized by short fibers or rods packed in cluster form. With increasing the gelator concentration, the cluster density increases. It is the difference in the aggregate structures of CAT in the two gels that may explain why the CAT/PTMS gel is more transparent than that of the CAT/PTES gel (cf. Table 1).

Fig. 2 Fluorescence micrographs of the PTMS gel of CTA at different concentrations ((a) 2.0%; (b) 4.0%; (c) 6.0%; w/v) and the gel of CTA in PTES at different concentrations ((d) 0.5%; (e) 1.0%; (f) 2.0%) with a perylene bisimide derivative 1 as a fluorescent doper (5.9 × 10⁻⁵ mol L⁻¹).

To make sure that the aggregate observed does come from CAT, rather than from the fluorescent probe itself, a control experiment was conducted. In the test, a drop of the PTMS solution of the probe (∼5.9 × 10⁻⁵ mol L⁻¹) was casted on a glass plate, and the morphology of the liquid film was examined in the same way. It was shown that there was no observable aggregate in the control, indicating that the aggregates as described do originate from CAT. Moreover, the concentration dependent morphology study also demonstrates that above the CGCs of the gels, an increase in gelator concentration shows little effect upon the aggregate structures, suggesting that the main factors affecting the aggregation are the gelator–gelator and gelator–liquid interactions, and the aggregate–aggregate interaction has little effect upon the morphologies of the gel networks.

Rheological studies

It is well known that the rheological properties of a gel system are determined by its network structure, and therefore, it should be reasonable to anticipate that the gels under study might exhibit very different rheological properties, since they possess very different gel network structures.⁶¹ This study is important because it also yields useful information about the structures of the gel networks (sizes and cross-linking densities), their formation and dis-formation dynamics and their self-assembly mechanisms.⁶² Moreover, the data as obtained from the studies also provide useful information for them to find real-life applications.³⁸

To examine the influence of solvent nature on the viscoelasticity and flow behaviour of the gels, the storage modulus (G′) and loss modulus (G′′) of the gel systems (2.0%, w/v) in PTMS, PTES, VTES and CCl₄, were measured as functions of shear stress at an angular frequency of 6.28 rad s⁻¹. The results are shown in Fig. 3a. It can be seen that the gel of CTA/PTMS possesses high storage modulus (G′, 1.2 × 10⁵ Pa) and yield stress (σ_y, 860 Pa), but the values for the gel of CTA/PTES are only 1.6 × 10⁴ Pa and 92 Pa, respectively, nearly one order lower than the corresponding PTMS gel, indicating that the gel networks of CTA/PTMS are much stronger than the corresponding CTA/PTES gels. However, with reference to the data shown in Table 1, it is seen that the gel of CTA/PTMS looks much more transparent and shows significantly lower T_gel, suggesting that the thermo-stability of CTA/PTES is superior to that of CTA/PTMS, inconsistent with that observed in the rheological studies as described earlier. The as observed inconsistence might be a result of differences in the homogeneity of the gels under study. As depicted in the table, the gel of CTA/PTMS looks much more homogeneous than that of CTA/PTES, as the former is more transparent. Clearly, for the PTES gel, microscopically, the strengths of the gel networks are not uniform, and there must be some weak points or parts that are sensitive to shear stress, resulting in a collapse of the gel networks. To further explore the influence of CTA concentration on the mechanical properties of the PTMS gels, G′ and G′′ of the gels were measured at different CTA concentrations. The results are collectively shown in Fig. 3b. It is seen that with increasing CTA concentration from 2.0% to 4.0%, and then to 6.0% (w/v), the value of G′ increased from 1.2 × 10⁵ Pa to 5.8 × 10⁵ Pa, and then to 1.9 × 10⁶ Pa, and the yield stress increased from 860 Pa to 1660, and then to 3630 Pa. A similar change was also observed in the gel system of CTA/PTES (Fig. S4†). These results suggest that the gels under study should possess good free standing and load bearing properties.⁵⁶

Fig. 3 (a) Evolution of G′ and G′′ as functions of the applied shear stress in different solvents (2.0%, w/v). (b) Evolution of G′ as a function of the applied shear stress at different concentrations of CTA in PTMS.

To look at the tolerance of the CTA/PTMS gel (2.0%, w/v) to external forces, an angle frequency sweep was also conducted at a shear stress of 5 Pa, which is well within the linear region of the gel sample, and the result is shown in Fig. 4a. As can be seen from the figure, with the angle frequency increased from 0.0628 rad s⁻¹ to 560.0 rad s⁻¹, the values of G′ are always higher than those of G′′, suggesting no phase transition during the sweep process. Furthermore, both G′ and G′′ were kept relatively stable within the whole frequency region swept, indicating that the gel possesses excellent tolerance toward external forces.

Fig. 4 (a) Evolution of G′ and G′′ as functions of the angle frequency sweep at a 2.0% (w/v) of CTA in PTMS. (b) Reversibility of thixotropic properties of the CTA gel in PTMS (4.0%).

To examine the thixotropic properties of the CTA/PTMS gel and its reversibility, G′ and G′′ of the gel were measured as functions of time (time sweep). The CTA concentration of the sample used for the test was 4.0% (w/v), and the measurement was conducted at a constant frequency of 10 rad s⁻¹ at 20 °C. The results as obtained are shown in Fig. 4b. To conduct the test, a high, constant oscillatory shear stress of 1700 Pa, which is well beyond the yield stress of the gel sample and is enough to destroy it, was applied to the gel for 2 min, and then the destroyed gel was switched to a low shear stress of 5 Pa for another 2 min to monitor the recovery of the gel. The destruction and recovery cycle was repeated another five times. As expected, the gel recovered immediately after removing the high shear stress (cf. Fig. 4b), indicating that the gel under study possesses smart, reversible thixotropic properties.

Considering the good mechanical and rheological properties of the CTA/PTMS gel, it is anticipated that the gel may find applications in injection molding and 3D printing, which will be presented in the last part of this paper.

FTIR studies

Hydrogen bonding may function as an important driving force in the formation of the CTA gels. Accordingly, FTIR measurements of the relevant samples were conducted since this technique could reveal the presence of H bonding.⁶³ Fig. 5 shows the FTIR spectra of the xerogels of CTA/PTMS and CTA/PTES, as well as a pure CTA sample from its THF solution, and it can be seen that the broad band assignable to the absorption of the O–H stretching vibration of CTA appeared in the region around 3000 cm⁻¹. At the same time, some new signals appeared in 2842 cm⁻¹. Moreover, the appearance of a series of peaks between 2700 and 2500 cm⁻¹ might be an indication of the formation of the associates of the carboxylic acid structures of the gelator during the gelation process.⁶⁴ The FTIR signal changes between free CTA and networked CTA reveal that the hydroxyl groups of the gelator were involved in some interactions due to gel network formation.⁶⁵ With further interrogation of the FTIR spectra shown in the figure, it can be also seen that the strong absorption belonging to the C=O stretching vibrations of free CTA appeared at 1755 cm⁻¹, and the absorption shifted upon gelation to 1731 cm⁻¹ for CTA/PTMS, and 1733 cm⁻¹ for CTA/PTES, which is strong evidence supporting its involvement in the intermolecular interactions relevant to the gelation of the systems.^66,67 Considering the changes in the FTIR signals of the gelator before and after gelation, it is reasonable to conclude that intermolecular hydrogen bonds form during the gelation of the CTA/PTMS and CTA/PTES systems, which could be the main driving force to promote the gelation.

Fig. 5 FTIR spectra of CTA in THF solution (a), and in PTMS gel (b) and PTES gel (c) (2.0%, w/v).

Example applications of the CTA gels

Molecular-gels are known to be used as substrate films for sensor fabrication, thanks to their unique porous network structures.⁶⁸ To demonstrate the applicability of the CTA gels as created, a NBD derivative 2 (cf. Fig. S1b†), a fluorescent compound recently synthesized and characterized by us,⁶⁹ was doped in a sample CTA/PTMS gel (6.0%, w/v) with a concentration of 3.0 × 10⁻⁴ mol L⁻¹. The sensing performance of the film to Ag⁺ was examined, and the result is shown in Fig. 6. It is clearly seen that a colour change (under illumination of UV light, 365 nm) was induced with the addition of a drop of an aqueous solution of AgNO₃ (5.0 × 10⁻⁴ mol L⁻¹), but no change was observed in the control (blank, water), indicating the applicability of the gel networks as substrates for sensing film fabrication.

Fig. 6 Sensing of Ag⁺ with a NBD derivative 2 (3.0 × 10⁻⁴ mol L⁻¹) doped CTA/PTMS gel (6.0%, w/v) on filter paper.

Since the formation of the gels is thermo-reversible and they are mechanically strong (free standing) and possess fast and fully reversible thixotropic properties (cf. Fig. 7a and b), it is anticipated that they may be used as smart materials for injection molding and 3D printing. To verify this, the CTA/PTMS gel was pre-doped with a small amount of a fluorophore, and then used for injection molding (cf. Fig. 7c and Movie S1,† ESI) and drawn (cf. Fig. 7d) through a syringe. Moreover, the gel was also allowed to be made into a “duck” and a “leaf” through melting-free deposition molding, of which the gels were pre-doped with different fluorophores (Fig. 8). Interestingly, the “duck” and the “leaf” could be further transformed into permanent structures through hydrolysis and condensation reactions. As depicted in Fig. 8, the NBD-based probe dispersed in the solid matrix was still fluorescence active, indicating that the dissolution of organic fluorophores within the solid matrix could be realized via the molding, hydrolysis and condensation process. This is because the compound is actually fluorescence inactive when existing in the solid state. Therefore, the gels as created could be used as materials for melting-free deposition modeling, 3D printing, and pre-cursors to a functionality-oriented solid matrix.

Fig. 7 (a) Images of the reversible gel–sol phase transition of CTA in PTMS (6.0%, w/v), (b) free-standing CTA/PTMS gel (6.0%, w/v), (c) photographs of injection molding, and (d) writing on paper using a syringe. The green colour is the fluorescence upon illumination with UV-light, and gel samples used were doped with a NBD derivative 2 (5.0 × 10⁻⁵ mol L⁻¹).

Fig. 8 Images of the model duck (a and c) and leaf (b) made from the PTMS gel of CTA (6.0%, w/v) before and after one month storage. The gels (a) and (c) were doped with a perylene bisimide derivative 1, but (b) was doped with a NBD derivative 2 (5.0 × 10⁻⁵ mol L⁻¹).

Conclusions

Three carboxyl acid-appended calix[4]arene derivatives (CMA, CDA and CTA) and calix[4]arene itself (C4) were employed for the gelation of organoalkoxysilanes. It was found that for the liquids and the concentrations tested, only CTA functions as a gelator. Even at a concentration of 2% (w/v), CTA gels with three of the ten organoalkoxysilanes. However, the as created gels, in particular the CTA/PTMS gel, possess unusual rheological properties, showing not only thermo-reversible phase transition properties, but also fast and fully reversible thixotropic properties at room temperature. Moreover, the CTA/PTMS gel containing 6% of the gelator (w/v) is stable for more than four months at room temperature, provided it is kept in a dry environment. Based upon the findings, the gel was successfully used as a substrate for film sensor fabrication, for injection molding and for melt-free deposition molding. Considering the reactivity of organo-alkoxysilanes, the LMMGs based molecular gels may represent a new class of smart materials for molding and printing.

Acknowledgements

This work was supported by the NSFC (21273141, 21527802, 21673133), the 111 project (B14041), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT-14R33).

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Footnote

† Electronic supplementary information (ESI) available: The preparation of compounds CMA, CDA and CTA, supplementary figures, preparation of test paper for Ag⁺ detection, preparation of gel model, ¹H spectra for compounds CMA, CDA and CTA, MS spectra for compounds CMA, CDA and CTA. See DOI: 10.1039/c6ra22731g

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