Supramolecular Liquid-Crystalline Polymer Organogel: Fabrication

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A Supramolecular Liquid-Crystalline Polymer Organogel: Fabrication, Multiresponsive and Holographic Switching Properties Yue Ni, Xiao Li, Jing Hu, Shuai Huang, and Haifeng Yu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00551 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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Chemistry of Materials

A Supramolecular Liquid-Crystalline Polymer Organogel: Fabrication, Multiresponsive and Holographic Switching Properties Yue Ni, Xiao Li, Jing Hu, Shuai Huang, and Haifeng Yu* Department of Material Science and Engineering, College of Engineering and Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing, 100871, China. ABSTRACT: Stimuli-responsive supramolecular organogels are fascinating for their dynamically controllable features, but it is difficult for most versatile linear vinyl polymers to construct suitable cross-linking points in oils or organic solvents. Here, a mesomorphic organogelator was fabricated by self-assembly of one azopyridine-containing polymer and oleic acid for the first time. Particularly, oleic acid acts as not only one important part to construct mesogenic gelator but also the solvent entrapped in the interstices of the physically crosslinked three-dimensional (3D) network. The resulting organogel shows a macroscopic gel–sol transition upon external triggers of temperature, light and organic metal ion. Accordingly, holographic gratings were successfully inscribed in the organogel, whose switching behaviors were obtained by manipulation of the three external stimuli. This provides a simple and elegant strategy to construct multiresponsive supramolecular liquid-crystalline polymer organogels with promising applications in optics, data storage and sensors.

INTRODUCTION

using block copolymers as gelators to construct 3D networks or crosslinking several different stimuliresponsive polymers, which are time-consuming, laborious and costly.19 Therefore, multiresponsive physical organogels with linear polymers as gelators are still limited in scope and it is urgent to develop a more simple and convenient fabrication strategy.8,11,16-18,20

Recently, stimuli-responsive materials have aroused great interests for their dynamically controllable features, leading to a wide range of applications in sensors, actuators, biomedical fields, etc.1-7 Among them, multiresponsive physical organogels have been intensively explored as Azopyridine derivatives are among the most promising novel smart soft materials because of their dramatic phase candidates to prepare stimuli-responsive organogels for 8-14 transformation when exposure to external stimuli. Their their elegant combination of photoresponsive features gelation driving forces generally benefit from dynamic related to azobenzenes and supramolecular self-assembly non-covalent interactions so that they are endowed with brought by pyridyl groups.21-26 Meanwhile, tightly-packed tunable features which are usually lacking in the aggregates of chromophores induced by aromatic π-π chemically-crosslinked polymeric gels.15 However, so far stacking are helpful to form supramolecular gels.15 most of these physical organogels have been obtained with Organogels have thrived not only as academic interests but low molecular-weight organogelators (LMWOGs). On one also in industrial fields, such as textile, cosmetics, foods, hand, a number of low molecular-weight units are linked health care and oil technology.17 Accordingly, oleic acid to each other by non-covalent bonds to show polymeric (OA) was chosen to fabricate supramolecular organogels properties in solution or in bulk.16,17 On the other hand, for its suitable physical and chemical properties, which has they can be reversibly converted from supramolecular been widely used as an industrial solvent and surfactant.27 polymers to low molecular-weight units easily by external Firstly, it is widely existed in living bodies of plants and stimuli. And yet, many low molecular-weight molecules animals, assuring the organogels to be more biocompatible are not environmentally friendly or biocompatible. Some and environmentally friendly. Secondly, OA is a long-chain versatile polymers combine the tailorable mechanical fatty acid, remaining liquid at room temperature. 14 property with acceptable biocompatibility. Nevertheless, Supramolecular liquid-crystalline polymers (SLCPs) have it is challenging for most linear vinyl polymers to generate been fabricated by using azopyridine-containing polymers suitable crosslinking points or three-dimensional (3D) with appropriate carboxylic acids through self11,16-18 networks in oils or organic solvents. In very rare cases, assembly,28,29 thus the introduction of OA is expected to vinyl polymers such as polystyrene and poly(methyl endow the organogel system with liquid crystalline (LC) methacrylate) with a high stereoregularity are able to form properties. supramolecular crosslinking points under critical Herein, we report fabrication of multiresponsive SLCP conditions, but it is difficult for them to acquire multiple organogel with a long side-chain azopyridine-containing responsiveness. Other methods have been proposed to polymer (PM 11AzPy) through self-assembly with OA as develop multiresponsive polymer organogels including ACS Paragon Plus Environment

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organogelator (Figure 1a). It is noteworthy that OA here is more than the hydrogen-bonding donor in supramolecular self-assembly which plays an important role in developing 3D network, and it also serves as the liquid (major component) entrapped in the interstices of the 3D matrix (minor component). The obtained organogel shows obvious sol-gel transformation upon external triggers including temperature, light and organic metal ion. To our best knowledge, there are no reports about such multiresponsive physical organogel where its organogelator is SLCP obtained by self-assembly of one linear polymer and the solvent of this gel system. Furthermore, multiresponsive holographic gratings are recorded in the resulting SLCP organogel for the first time.

Figure 1. Materials and characterizations used in this work. (a) Chemical structures of the azopyridine-containing polymer (PM11AzPy) and oleic acid (OA); (b) FTIR spectra of OA, PM11AzPy and PM11AzPy-OA (1:1); (c) DSC curves (the second cooling scan) of OA, PM11AzPy and PM11AzPy-OA (1:1); (d) One POM image of the compound PM11AzPy-OA (1:1) annealed at 46 °C. Scale bar: 10 μm; (e) Possible LC structure of the compound PM11AzPy-OA (1:1).

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was chosen to form supramolecular hydrogen bonding with PM11AzPy, as confirmed by Fourier transform infrared spectroscopy (FTIR, Figure 1b).30 Two additional peaks at 1927 cm-1 and 2521 cm-1 appeared in FTIR of PM11AzPy-OA (1:1) compared with PM11AzPy, which can be assigned to the fermi resonance bands and the vibration of hydroxyl groups, respectively.31 The peak at 930 cm-1 in FTIR of OA, corresponding to the out-of-plane O–H wagging vibration of the carboxylic acid dimers, disappeared completely in that of PM11AzPy-OA (1:1), strongly indicating the formation of hydrogen bonding between the carboxylic acid and the pyridyl group.32 The complex PM11AzPy-OA exhibited different thermal properties from pure PM11AzPy, and their differential scanning calorimetry (DSC) results are given in Figure 1c and Figure S6. On cooling, PM11AzPy exhibited a glass transition (Tg) at about 43 °C and a melting point at 52 °C. By contrast, two peaks at 38 °C and 58 °C appeared in DSC of PM11AzPy-OA (1:1), which can be assigned as the melting point and the LC to isotropic phase transition (or the clearing point). In addition, its Tg shifted to 47 °C. All the PM11AzPy-OA complexes with different molar ratio of azopyridyl to OA showed LC phases from 38 °C to 58 °C (Figure S6). Observed with polarizing optical microscopy (POM), PM11AzPy-OA (1:1) demonstrated a typical Schlieren texture (Figure 1d). It has been reported that polymers with azopyridine-containing pendant groups connected by a long spacer were able to self-assemble with aliphatic carboxylic acid, and most of them may tend to develop smectic LCs.28 In addition, there is a general rule that Schlieren texture is frequently found in smectic C phase rather than smectic A phase.33,34 Small-angle X-ray scattering (SAXS) can offer further information about the molecular arrangement and packing mode of the complex (Figure S4),35,36 indicating the smectic C phase with a possible mesophase structure shown in Figure 1e. We also investigated the difference of the orientation behaviors between the SLCP and PM11AzPy though a rubbed method (Figure S7). The SLCP showed good orientation, whereas pure PM11AzPy was hardly aligned.

RESULTS AND DISCUSSION Fabrication of SLCP via self-assembly. The polymer PM11AzPy (its synthetic details are given in Supporting Information) is not mesogenic due to the absent end substituent in the azopyridyl group.28,29 However, the existence of a lone pair electrons on the nitrogen atom enables formation of various noncovalent bonding interactions, such as hydrogen, coordination, halogen and ionic bonds.22 Especially, self-assembly via hydrogen bonding is a simple but widely adopted way to fabricate SLCPs.28 Here, a long-chain aliphatic carboxylic acid (OA)

Figure 2. SLCP gelation behaviors. (a) Photos of different molar ratio of PM11AzPy and OA compounds; (b) Plots of Tgel against the concentration of PM11AzPy in OA with the heating rate 0.1 °C/min at heating stage. (AzPy: OA= 1:10, 1:20, 1:30, 1:40, 1:50).

Gelation behaviors. Generally, equimolar pyridyl and carboxylic groups are often utilized to fabricate hydrogen-

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Chemistry of Materials bonded SLCPs.37,38 In this work, we just multiplied the amount of OA, and a series of SLCP organogels were easily obtained. Even the molar equivalent of OA was up to 40 times more than the pyridyl groups, the organogel didn’t collapse at room temperature, as shown in Figure 2a. The gel was found to be stable for months at room light. While in the obtained SLCP organogel, OA acted as both one important part to construct mesogenic gelator and the solvent entrapped in the interstices of the 3D networks. The organogel with different amounts of PM11AzPy had distinct LC characteristics (Figure S6, Figure 3a). We also measured the gel-sol phase transition temperatures (Tgel) of the obtained organogels. All the Tgel were at around 50 °C and rose slightly with the increasing concentration of PM11AzPy (Figure 2b), which is coincident with the results that the network of the organogel became thicker as the PM11AzPy ratio increased (Figure 3b).

Figure 3. Characterizations of the fabricated SLCP organogel. (a) POM images of the PM11AzPy-OA gel with different molar ratio annealed at 46°C. Scale bar: 10 μm; (b) FESEM images of the PM11AzPy-OA xerogel with different molar ratio. Scale bar: 500 nm. (AzPy: OA= 1:1.2, 1:10, 1:20, 1:30, 1:50).

Driving forces of gelation. It has been reported that polymers are normally used as hydrogelators instead of organogels because it is difficult for most common vinyl polymers to construct suitable cross-linking points or a 3D network in oils or organic solvents,11,16-18 which promotes us to explore the driving forces in the present SLCP gelation system. Obviously, the formation of hydrogen bond between PM11AzPy and OA helped to increase the solubility of the polymer in OA by capping azopyridine group with the fatty acid, which made it possible for azopyridines to move and assemble. Besides, the π-π staking interaction was supposed to play a critical role in the gelation process, which was further proved by UV-vis absorption spectroscopy. Usually, azobenzene moieties tend to aggregate because their transition dipole moments prefer to parallel with their molecular long axis,8,20,39,40 which is often recognizably indicated by the shift in UV-vis absorption spectra.15,41,42 Figure 4a shows UV-vis absorption spectra of PM11AzPy-OA (1:20) organogel in different states. A maximum absorbance peak at 358 nm was observed in its tetrahydrofuran (THF) solution,

corresponding to the π-π* transition of “isolated” chromophores.41 Whereas a hypsochromic shift of 17 nm occurred in its gel state, demonstrating the formation of Haggregation. Furthermore, the temperature-dependent UV-vis absorption spectra also indicated that Haggregation was closely related to the gelation process. As shown in Figure 4b, the maximum absorption band shifted from 338 nm to 358 nm at elevated temperatures, exhibiting the occurrence of transition of azopyridyl chromophores from aggregates to free states. This change was observed between 50 °C and 60 °C in accordance with the Tgel of the organogel PM11AzPy-OA (1:20) (50.7 °C). All these results suggest that the aggregation of azopyridines should be the key driving force for the SLCP gelation.

Figure 4. UV-Vis spectra of the organogel PM11AzPy-OA (1:20). (a) in different states, black: solution state; red: gel state; (b) at elevated temperatures, from 20 °C to 60 °C; (c) upon irradiation of UV light and (d) relaxed in dark after UV irradiation.

Multiresponsive behaviors of the organogel. Like most supramolecular gels, the resulting PM11AzPy-OA organogel was thermoresponsive owing to the inherent properties of noncovalent bonds (Figure 6b). When being heated up to 50 °C, the organogel started to collapse. In general, both hydrogen bonding and π-π staking interactions are sensitive to temperature. Figure 4b shows the change in UV-vis absorption spectra of the organogel at increasing temperatures. The bathochromic shift of 20 nm occurred at 50 °C, demonstrating that the break of Haggregation almost happened simultaneously with the gelsol transition. The thermal stability of hydrogen bonding was further characterized by in-situ measurement of temperature-dependent FTIR. As shown in Figure 5, the characteristic peaks of hydrogen bonding (1927 cm-1 and 2521 cm-1) didn’t change significantly at 65 °C in FTIR spectra of the organogel even the gel-sol transition occurred, which still maintained stable up to 85 °C on heating. Thus, it can be concluded that the break of H-



aggregation along with enhanced molecular motion should be the dominant factor of thermally-induced gel-sol phase transition (Figure 6a). Obviously, this gel-sol transition was completely reversible upon cooling.

Figure 5. In situ temperature-dependent FTIR spectra of the organogel PM11AzPy-OA (1:20).

In addition to the thermoresponsive property, the azopyridine moieties brought about photoresponsiveness to the obtained organogel. As a result, gel-sol transition was successfully manipulated by light, as shown in Figure 6. Upon photoirradiation of UV light, the organogel gradually collapsed into a transparent solution. As shown in Figure 4c, the maximum absorption peak shifted from 340 nm to a longer wavelength of about 359 nm with increasing the photoirradiation time, indicating the break of H-aggregation. This could be attributed to the trans-cis photoisomerization of azopyridines, which increased the free volume of azopyridines in bent cis-isomers and caused the dissociation of aggregates (Figure 6a).15 It was found the complete recovery of the organogel was a slow process although the gel-sol transition could be triggered by UV light quickly. Generally, photoisomerization process from cis to trans is fast. But the time of reconstructing organogel mismatched the fast cis-trans back isomerization process. This difference could be explained based on the fact that the recovery of the organogel consists of two steps.15 Firstly, the cis-azo isomers turn into trans-azo isomers by back isomerization. Then the resulting trans-azo isomers in free states stack or aggregate through π-π staking interactions. But the cis-azo isomers can’t change into trans-azo isomers all at once and the photoisomerization is a process of dynamic equilibrium. The existence of cis-azo isomers might disturb the formation of H-aggregation. At the same time, the formation of H-aggregation in the gelation process also increases the viscosity of the system greatly, which influences the back cis-trans isomerization and restricts the trans-azo isomers to move and aggregate. As a result, the restoration of the organogel depended on not only cis-trans back isomerization process but also the formation of H-aggregation. It took several days to be kept in dark for the complete recovery of the organogel and restoration of H-aggregation. (Figure 4d)

Due to the existence of pyridine groups, the organogel was also sensitive to organic silver ion (Ag+). After adding a few drops of ethanol solution of Ag+ (silver trifluoromethanesulfonate) and shaking slightly, the organogel was destroyed, producing grainy yellow precipitates (Figure 6b). To further confirm the Ag+induced gel-sol transition, field emission scanning electron microscopy (FESEM) was used to investigate the morphological changes. As shown in Figure 3b and Figure 6c, fibrous interconnected 3D networks were clearly observed, which should contribute mainly to the gelation. However, the networks were broken and spherical microparticles with wrinkled surface (precipitates) were obtained after addition of Ag+ (Figure 6d). Obviously, the coordination between the pyridine groups and Ag+ should be responsible for these morphological changes, since the coordination was much stronger than the hydrogenbonding interaction between PM11AzPy and OA. Here, Ag+ acted as one crosslinker between two azopyridyl groups,43,44 leading to the deposition of polymers. The organogel was unable to recover even whatever treated by heating or adding solution. The resultant microparticles were hardly dissolved in THF which is a good solvent for PM11AzPy. In comparison with PM11AzPy, the resultant precipitates exhibited a significant change in the thermal property (Figure S8) because of the strong coordination interaction existing between Ag+ and PM11AzPy.

Figure 6. Multiresponsive behaviors of the organogel. (a) A schematic illustration of thermal and photo response of the organogel; (b) photos of the organogel in response to heating, UV light and adding Ag+; (c) FESEM image of PM11AzPy-OA (1:20) xerogel. Scale bar: 500 nm; and (d) FESEM image of the precipitates from the organogel after adding Ag+. Scale bar: 2 μm.


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Chemistry of Materials

+ Figure 7. POM images of the gratings in response to (a) heating; (b) UV light and (c) Ag ion. Scale bar: 10 μm.

Recording and switching of holographic gratings. Since azo materials have been intensively studied for holographic gratings,45,46 it is expected for the multiresponsive azopyridine-containing SLCP organogel to be applied in recording holographic gratings with switching performance. As shown in Figure S9, phase-type gratings with a periodicity of 3 μm were successfully inscribed in an LC cell filled with the organogel upon irradiation of one interference pattern (the details are given in Supporting Information). When one laser beam (633 nm) was incident vertically to the grating, the first order diffraction beams were clearly observed. To the best of our knowledge, this is the first example to fabricate diffraction gratings employing multistimuli-responsive physical organogel.39,40,45 Just as expected, the switching of gratings can be manipulated by the external stimuli due to the inherent nature of the organogel, as indicated by the POM observations. The recorded grating pattern remained stable at room light, which were significantly influenced by temperature, as observed under POM (Figure 7a). When heated up to 40 °C from room temperature, the grating pattern became more prominent because of the enhanced birefringence at the LC phase (the mesomorphic range is from 38°C to 58 °C). As the temperature rose above Tgel, the grating structures deformed but remained bright field of view (55 °C), and then finally turned into dark at the isotropic temperature (60 °C). The disappeared grating pattern can be re-inscribed again. As the heating process was able to introduce a striking modification of brightness and patterned structures to the holographic gratings, such

unique features can be designed as sensors to monitor thermal history subjected by materials. As shown in Figure 7b, the switching of the gratings was also remotely triggered by photoirradiation of actinic UV light. As mentioned above, the photoisomerization of azopyridines broke their H-aggregation in the organogel state since the bent cis-isomer increased the distance between chromophores and destabilized the supramolecular mesogenic phase. Thus, the organogel collapsed and the birefringence weakened consequentially, making the grating pattern gradually dim and then completely disappeared. Noteworthily, the grating pattern can be re-recorded in the same sample in its organogel state after thermal annealing treatment. Interestingly, the obtained gratings were greatly influenced as well after a drib of ethanol solution of Ag+ was introduced (Figure 7c). The bright pattern of the gratings faded away over time and disappeared completely in the end, so did the first diffraction beams. Just as the Ag+-responsive organogels, strong coordination interaction formed between the azopyridines and Ag+, thus making the grating structures broken and the birefringence lost. The disappeared gratings were not able to recover even after thermal annealing. As comparison, the gratings remained clear after adding a little drop of pure ethanol (Figure S10). This intriguing finding possibly paves a new way of chemical sensing for Ag+.10,47,48



CONCLUSION In summary, a series of SLCP organogel was successfully fabricated. The gelator was obtained by supramolecular self-assembly of one azopyridine-containing polymer (PM11AzPy) and a judiciously chosen long-chain fatty acid (OA). The aggregation of chromophores mainly contributed to the gelation behaviors. Remarkably, OA acted as not only the liquid entrapped in the interstices of the 3D matrix but also the donor of the hydrogen bond to construct the 3D SLCP networks. Significant phase transition of the organogel in response to external triggers including temperature, light and Ag+ ion was systematically investigated. Holographic gratings were recorded in the fabricated SLCP organogel, presenting multiple switching behaviors to these stimuli due to the inherent nature of the multiresponsive organogel. This provides a simple, elegant and ingenious strategy to design multiresponsive physical polymer organogels with promising applications in optics, data storage and sensors.

ASSOCIATED CONTENT Supporting Information. Experimental details and additional data including SAXS patterns, UV-vis absorption spectra, POM images and DSC curves (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (Grant No.s 51573005, 51773002) and the National Key R&D Program of China (2018YFB0703702).

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Supramolecular Liquid-Crystalline Polymer Organogel: Fabrication

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