Observation of Morphology and Structure Evolution during Gelation of

Aug 28, 2017 - †Key Laboratory for Automobile Materials (JLU), Ministry of Education, College of Materials of Science and Engineering, and ‡Key La...
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Observation of Morphology and Structure Evolution during Gelation of a Bis(Anhydrazide) Derivative Wei Li,† Xiangyang Che,† Fangyi Chen,† Chunxue Zhang,† Tianren Zhang,† Haitao Wang,† Binglian Bai,‡ and Min Li*,† †

Key Laboratory for Automobile Materials (JLU), Ministry of Education, College of Materials of Science and Engineering, and ‡Key Laboratory of Coherent Light and Atomic and Molecular Spectroscopy of Ministry Education, College of Physics, Jilin University, Changchun 130012, P. R. China S Supporting Information *

ABSTRACT: A new bis(anhydrazide) derivative containing cyclohexyl terminal groups (compound 1) was synthesized, and its gelation process was investigated. Compound 1 showed both thermal-induced gelation (T-gel) and sonication-induced gelation (S-gel) in alcohols. We investigated the gelation process of compound 1 in ethanol by different techniques. It was demonstrated that gelator 1 in ethanol underwent a transition from a clear solution through a turbid suspension to an opaque gel. Scanning electron microscopy (SEM) observations indicated that the turbid suspension consisted of separated clew-like spheres, connected spheres, and short nanorods, whereas the opaque gel consisted of fibers or bundles of fiber networks. Molecules packed loosely into an unknown phase in the spheres, whereas they packed tightly into a hexagonal columnar phase with a = 1.62 nm in the fibers. Intermolecular H-bonding between CO and NH was revealed to be the driving force for gelation, and the strength of the H-bonding became stronger in the fibers than in the spheres. We propose that the gel of compound 1 in ethanol consisting of fibers is a stable phase compared to the turbid suspension consisting of spheres or short nanorods, which is considered to be metastable. The kinetics of gelation of gelator 1 in ethanol under sonication suggest that the gelation process is a two-stage kinetic pathway with fractal values of 1.27 and 0.84. Our study hence provides new insights into the formation of fibers and the structural evolution of the gelation process and can be exploited to achieve a detailed understanding of gels.

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gelation process depend on this vital parameter.11 In addition, the structural evolution and kinetics of gelation might play key roles in determining the final properties of the gels during gelation. Thordarson and co-workers investigated the structural evolution of pyromellitamide self-assembled gels by small-angle neutron scattering (SANS) and confirmed that the structural changes occurred over a long period of time during gelation and that the molecular structure of the gelator could influence the molecular packing on the single- to few-fiber bundle stage.8 Their results shed light on the process of gelation and the origin of the macroscopic properties of gels such as mechanical and rheological properties. Liu and co-workers published a series of works on the kinetics of gelation in an effort to understand the gelation mechanism.12−18 They proposed that gelation is a supersaturation-driven crystallographic mismatch branching (CMB) process, in which different factors such as additives, temperature, and gelator saturation could affect the fiber dimensions and, thus, the gel network. Huang et al. investigated the isothermal kinetic pathways of gelation by

INTRODUCTION Low-molecular-weight organogels (LMWGs) are a type of soft material with nanoscaled three-dimensional networks formed through weak noncovalent interactions, such as hydrogen bonding, π−π stacking, van der Waals forces, and hydrophobic interactions.1,2 They have attracted increasing interest because of their potential applications in template materials,3 sensors,4 and gene delivery devices.5 The design and synthesis of molecules showing organogelation has advanced rapidly over the past two decades. It is not difficult for researchers to design a gelator that gels certain solvents based on previous experiments. However, understanding the process of gelation and determining experimental evidence for a number of speculations has remained a challenge, and up to now, there is still no model to describe organogels.6−9 Many extensive and dedicated works have been published concerning the mechanism of gelation. Meijer and co-workers investigated the self-assembly stages of oligo(p-phenylenevinylene) derivatives.10 They confirmed that the self-assembly was a highly cooperative process in which solvent molecules played an important role in controlling the self-assembly process. Hirst and Smith demonstrated that the solubility of the gelator plays a key role in the gelation process and that many facets of the © XXXX American Chemical Society

Received: July 15, 2017 Revised: August 24, 2017 Published: August 28, 2017 A

DOI: 10.1021/acs.jpcb.7b06965 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B different techniques and revealed the extreme sensitivity of the self-assembled fibrillar networks in the gels and viscoelastic properties of such organogels to slight modifications in gelator structure and sample history. 19 They revealed gelation processes at different length scales and steps in the aggregation/self-assembly process. These excellent studies have shed light on the process of gelation and the origins of macroscopic properties such as mechanical and rheological properties. The gelation process is actually a phase-separation process and can be accompanied by phase transitions. Several interesting phase-transition phenomenon during gelation of different types of gelators have been reported.20−25 Zhang et al. reported morphological exchange between a three-dimensional gel network and spherical vesicles under sonication or a heating−cooling process and revealed that gelator molecules are arranged as lamellar structures either in networks or in vesicles.20 Ultrasound was considered to disturb the long arrangement of the lamella by inserting ethanol molecules into the self-assembly of the gelators. Datta and Bhattacharya reported a Ag+-induced reverse vesicle-to-helical fiber transformation in the self-assembly of a pyridine-appended chiral salicylideneaniline. They demonstrated that a keto−enol tautomerism induced the gel-to-sol transition.21 Higashiguchi et al. investigated an amphiphilic diarylethene gelator that showed a photoinduced reversible morphological change between colorless microspheres and colored fibers in water.22 They revealed that the reversible photoisomerization of the core diarylethene gave rise to the reversible morphological transformation. In this article, we report the gelation behavior of a bis(anhydrazide) derivative containing cyclohexyl terminal groups (gelator 1). Although gelator 1 is not an efficient gelator for organic solvents, it can gel only alcohols, and it takes a long time for gelation. Fortunately, we observed the morphological and structural evolution during the gelation process of 1 in ethanol. Our results demonstrated that the gelation of gelator 1 in ethanol proceeds from a clear solution through a turbid suspension and then finally to a gel, during which at least three stages of structural evolution were confirmed, namely, separated clew-like spheres, connected spheres, and short nanorods and fibers or bundles of fiber networks. We propose that the gelation of gelator 1 develops through a two-stage pathway that is obviously different from the widely accepted points that fibers develop and grow through a one-stage pathway. The kinetics of gelation of gelator 1 in ethanol under sonication suggest that the gelation process is a two-stage kinetic pathway with fractal values of 1.27 and 0.84. We demonstrate that the packing modes of the molecules are different, namely, that the molecules in the spheres are arranged in an unknown phase, whereas they are packed into a hexagonal columnar unit with a = 1.62 nm in the fibers. Intermolecular H-bonding between CO and NH is the driving force for gelation, and its strength is higher in the fibers than in the spheres. We propose that the gel consisting of fibers is a stable phase compared to the turbid suspension consisting of spheres or short nanorods, which is metastable. Our study hence provides new insights into the gelation process and might deepen the understanding of arrested gels.

Figure 1. Molecular structure of compound 1.

5.15 mmol) and isocyanatocyclohexane (1.3 mL, 10.30 mmol) were stirred in dry N,N-dimethylformamide (DMF) for 3 h at room temperature. Then, the mixture was poured into ice water, and the crude product was purified by recrystallization from methanol, yield = 35.41%. mp: 216−218 °C. 1H NMR [300 MHz, deuterated dimethyl sulfoxide (DMSO-d6)]: δ 10.19 (d, J = 2.0 Hz, 2H), 8.38 (s, 1H), 8.03 (dd, J = 7.8, 1.5 Hz, 2H), 7.80 (d, J = 2.2 Hz, 2H), 7.59 (t, J = 7.8 Hz, 1H), 6.30 (d, J = 8.1 Hz, 2H), 3.40 (s, 2H), 1.77−1.52 (10H), 1.229− 1.10 (10H). 13C NMR (75 MHz, DMSO-d6): δ 165.65 (s), 157.33 (s), 132.98 (s), 130.29 (s), 128.39 (s), 126.96 (s), 48.13 (s), 32.98 (s), 25.24 (s), 24.56 (s). Anal. Calcd for C22H32N6O4 (%): C, 59.44; H, 7.26; N, 18.91. Found (%): C, 59.83; H, 7.20; N, 18.58. Characterization. 1H NMR and 13C NMR spectra were recorded with a Varian Unity 300 spectrometer at 300 and 75 MHz, respectively, using deuterated dimethyl sulfoxide as the solvent and tetramethylsilane (TMS) as an internal standard (δ = 0.00). Field-emission scanning electron microscopy (FESEM) images were taken with a JSM-6700F scanning electron microscope. Transmission electron microscopy (TEM) images were taken with a JEM-2100F microscope, operating at an acceleration voltage of 200 kV. X-ray diffraction was carried out with a Bruker Avance D8 X-ray diffractometer. FTIR spectra were recorded on a Perkin-Elmer spectrometer (Spectrum One B). Sonication was performed on a KQ-50DA apparatus. Rheological properties were studied on a TA Instruments AR2000 rheometer equipped with a 40-mm-diameter stainless steel plate. The samples were sandwiched between the two plates with a gap of 0.3 mm throughout the experiments. Optical microscopy photographs were obtained under a Leica DMLP microscope. Preparation of Organogels. The gelation behavior of gelator 1 in organic solvents was examined in a simple and reproducible way. The gelator and solvent were put into a septum-capped test tube and heated until the solid dissolved. Then, the solution was cooled to room temperature (this is called the prepared sample). If the prepared sample could form a gel after being left for a certain period of time in the given solvent at room temperature, it was denoted as a T-gel. In contrast, it was called an S-gel-n (where n indicates the sonication time in minutes) if the prepared sample was subjected to ultrasound (50 W, 80% power output, 40 kHz) for several minutes at room temperature, and was then left at that temperature and gelation finally occurred. The gel was considered successfully formed by the “inverse flow” method.9 The kinetics of gelation of the S-gels was measured by rheometry. After being sonicated for a certain time at room temperature, the solution of compound 1 was transferred to rheometer plates. The samples were sandwiched between two plates with a gap of 0.3 mm throughout the experiments, and the temperature of the two plates was set at 5 °C to minimize evaporation of the solvent. Measurements were conducted at a low strain (0.1%) well within the linear viscoelastic (LVE) regime.

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EXPERIMENTAL METHODS Synthesis of Compound 1. Compound 1 (Figure 1) was prepared by one-step reaction. Isophthalic dihydrazide (1 g, B

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The Journal of Physical Chemistry B Table 1. Gel Behavior of 1 and Its Critical Gelation Concentrations (CGC, mg/mL) in Different Solvents state(s)a

solvent ethanol methanol ethylene glycol 1-butyl alcohol 1-pentanol a

T-gel (10) P PG P P

S-gel (3.3) S-gel (15) S-gel (13)

solvent

statea

dichloromethane chloroform acetone tetrahydrofuran N,N-dimethylformamide

I I I P P

G, gel (values denote critical gelation concentration necessary for organogelation); S, solution; P, precipitate; PG, partial gel; I, insoluble.

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RESULTS AND DISCUSSION Gelation Behavior and Gel Properties of Compound 1. The gelation properties of 1 were examined in 10 different organic solvents, and the results and the corresponding critical gel concentrations are summarized in Table 1. As shown in Table 1, compound 1 was able to dissolve in alcohols upon heating and precipitated upon being cooled to room temperature, except for ethanol and ethylene glycol, in which 1 formed a gel and a partial gel, respectively. Its CGC in ethanol was 10 mg/mL, and it took about 60 h for compound 1 to gel ethanol at room temperature. Thus, compound 1 is not an efficient gelator for the gelation of organic solvents; however, its slow gelation feature allowed us to follow its gelation process. It is widely accepted that sonication can influence the gelation ability and gelation behavior of a given gelator. Sonication-induced gels show shorter gelation times and lower critical gelation concentrations (CGCs) than those without sonication.26−28 In our case, the CGC for T-gel in ethanol decreased from 10 to 3.3 mg/mL for S-gel-20 in ethanol whose solution was subjected to sonication for 20 min at 25 °C. 1 could gel ethylene glycol after sonication for 10 min at a concentration of 15 mg/mL. Moreover, gelation was observed for compound 1 in methanol at 15 mg/mL after the solution of 1 in methanol had been subjected to sonication for 30 min; otherwise, 1 precipitated at room temperature without sonication (Figure S1). Thus, compound 1 showed sonication-induced gelation in methanol and ethylene glycol. The gel−sol transition temperature (Tgel) of both T-gel and S-gel in ethanol were measured by a simple method called the “inverted test tube” method. Figure 2 shows the Tgel values of both T-gel and S-gel as functions of concentration. It can be seen that Tgel increased nonlinearly with increasing concentration of gelator 1, until a relatively steady value was reached. In addition, S-gel showed higher Tgel values than T-gel at the

same concentration, suggesting that the S-gel was more stable than the T-gel at the same concentration. The mechanical properties of both T-gel and S-gel were determined by dynamic oscillatory measurements (Figure 3).

Figure 3. Amplitude dependencies of the storage modulus (G′) and the loss modulus (G″) of T-gel and S-gel of 1 in ethanol (10 mg/mL). The frequency was 1 Hz.

The linear viscoelastic region (LVR) of the gel as a function of increasing amplitude of deformation on shear was determined with strain amplitudes ranging from 0.01% to 100% at 6.28 rad s−1. The storage modulus (G′) and the loss modulus (G″) remained nearly constant up to approximately 0.1% strain (G′ > G″), an outcome that defines the uppermost boundary of the LVR, indicating its true gel feature when a small strain was imposed (linear region). Furthermore, the S-gels showed larger G′ and G″ values than the T-gels, and the longer the applied sonication time, the larger the values of G′ and G″. The different mechanical strengths and thermal stabilities of the Sgel and T-gel were attributed to their different morphologies, as confirmed below. The xerogels of the T-gels showed the coexistence of spheres and fibers, whereas mostly fibers and no spheres were observed in those of the S-gels. Morphology and Structure Evolution of Compound 1 during Gelation. As shown in Figure 4, the xerogel of the Tgel in ethanol consisted of two kinds of aggregates, namely, well-defined fibers with widths of 0.2−0.7 μm and solid spheres with diameters of 0.2−1.5 μm that were not hollow, as evidenced by TEM (Figure S2). Magnified images of the spheres showed clew-like features that were not smooth (Figure S3). The xerogels of S-gel-2 and S-gel-5 of 1 showed the coexistence of spheres and fibers, whose amounts were dependent on the ultrasound irradiation time. Longer-time ultrasound irradiation of the solution caused the numbers of solid spheres to decrease and the numbers of fibers to increase. It is obvious that a well-defined fiber developed from the clewlike spheres (Figure 4c). Almost no clew-like aggregates were observed for S-gel-10 (Figure 4d). Ultrasound is known to

Figure 2. Gel−sol transition temperature (Tgel) of T-gel and S-gel of gelator 1 in ethanol. C

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Figure 4. SEM images of xerogels of (a) T-gel and (b−d) S-gels of 1 from ethanol (10 mg/mL) under ultrasound irradiation for different times: (b) 2, (c) 5, and (d) 10 min.

Figure 5. Photographs of compound 1 in ethanol (10 mg/mL) after sonication for 10 min then aging for different times at room temperature.

Figure 6. SEM images of films casting from solution of 1 in ethanol (10 mg/mL) under sonication for 10 min and then being aged at room temperature for different time: (a) 0 min; (b)10 min; (c) 15 min; (d) 30 min; (e) 50 min; (f-g) 60 min; (h) 180 min.

(10 mg/mL) under sonication for 10 min and then aged at room temperature for different time period were performed. Figure 6 shows SEM images of S-gel-10 during the sol−gel transition. Obviously, different morphologies during gelation were observed for gelator 1 in ethanol. At the early stage, the casting films (Figure 6a) showed only isolated clew-like architectures with diameters of 0.2−1.5 μm, corresponding to the relatively transparent solution states. Later on, the clew-like aggregates were connected by short fibers along different directions, and some typical “8”-shaped aggregates were observed (Figure 6b,c). Further, the “8”-shaped aggregates grew along the fusion direction to form short rod-like aggregates with lengths of several micrometers (Figure 6d). The short rods showed extension growth, and the width of the nanorods decreased; meanwhile, accompanied by some spheres attached to the nanorods through branching, corresponding to the turbid suspension (Figures 6e and S3f). Subsequently, the shorter rods evolved into longer ones, and meanwhile, the number of spheres decreased, and the number of fibers

stimulate the primary nucleation process and help to break the seeds and disperse them uniformly in solution. In addition, the cavitation and streaming effects of the ultrasound waves ensure the bulk-phase mass transfer of solute to the surface of growing crystals, thereby promoting the secondary nucleation process.29−33 Based on this knowledge and the morphologies shown in Figure 4, we speculated that the fibers developed from the clew-like aggregates and ultrasound irradiation of the solution of 1 in ethanol could accelerate the growth from clew-like spheres to fibers. Figure 5 shows the development of gelation of compound 1 in ethanol. It can be seen that it took 180 min for the sols to develop from a transparent solution to a turbid suspension and then finally to a gel. Thus, the turbid suspension could be considered as a metastable phase, whereas the gel can be considered as a stable phase. To understand the morphological evolution of gelator 1 in ethanol and to investigate the mechanism of the sphere-to-fiber transformation, morphological observations of films cast from a solution of 1 in ethanol D

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Figure 7. (a) XRD patterns of xerogels of 1: (a) T-gel (10 mg/mL), (b) S-gel-10 (10 mg/mL). (b) FT-IR spectra of clew-like spheres and fibers developed from ethanol.

Figure 8. (a) Dynamic rheological data for a 10 mg/mL S-gel-8 undergoing gelation at 5 °C. The main figure shows the evolutions of the elastic modulus G′ and the viscous modulus G″ recorded at a frequency of 1 Hz and a strain of 0.1%. The time point 0 s is defined as the first measurement point. Inset: tan δ versus time during the gelation process. (b) Fractal analysis of the kinetic data acquired by dynamic rheological measurements and in terms of the Dickinson model (eq 1) for S-gel-8 (5 °C, 10 mg/mL; first stage, Df = 1.27; second stage, Df = 0.84).

the one set of the diffractions of the T-gel. Considering the fact that the T-gel showed the coexistence of fibers and clew-like spheres whereas the S-gels consisted of almost all fibers and very few spheres, depending on the sonication time, we propose that fibers in either the S-gels or T-gels exhibit hexagonal columnar structures with a unit cell parameter of a = 1.62 nm, whereas the clew-like spheres exhibit an unknown aggregated structure for which we cannot give the exact unit cell parameters because of the limited diffractions. Diffractions at 2θ = 6.32°,11.8°, 12.8° and 15.6° are sharper and stronger, suggesting that molecular packing in the fibers is well arranged compared to that in the spheres. In addition, we could reasonably deduce that, during the process of gelation, molecules of 1 pack loosely in the clew-like spheres, whereas they arrange slightly closer in the fibers, resulting in transformation from an unknown phase to a hexagonal columnar phase. It is known that free NH stretching vibrations (νN−H) and free CO vibrations (νCO) of amide derivatives are usually observed at wavenumbers larger than 3400 cm−1 and at 1680 cm−1, respectively. To address the intermolecular interactions during gelation, films consisting of almost all fibers and those of clew-like spheres were prepared, and their FTIR spectra are shown in Figure 7b. It can be seen that the clew-like spheres showed νN−H at 3322 cm−1 (wide), νCO at 1653 cm−1, and δN−H at 1545 cm−1 that overlapped with νC−H of the benzene ring. In contrast, νN−H, νCO, and δN−H of the fibers were located at 3310 cm−1 (sharp), 1650 cm−1, and 1579 cm−1, respectively. These results suggest intermolecular H-bonding between NH and CO in the spheres and in the

increased with time, resulting in the opaque gel state (Figure 6f,g). Further storage of the gel caused the extended nanorods to develop into bundles of fibers with widths of 100−700 nm, leading to a more opaque gel. Based on the above observations, we propose that the gelation of compound 1 in ethanol could be described as a progressive process in which a macroscopically homogeneous solution was transformed into a more stable gel phase through a turbid metastable suspension state.34 During the sol−gel transformation, at least three different microscopic stages can be envisioned, namely, isolated clew-like spheres, connection or extension of clew-like spheres, and fibers and/or networks of fibers. Ultrasound irradiation of the solution of 1 could accelerate the growth from clew-like spheres to fibers. The morphological evolution of gelator 1 in ethanol during gelation was also observed by optical microscopy (Figure S4), which was consistent with the above observations. To explore the aggregation structures of the xerogels of both T-gel and S-gel and the structures of the fibers and the spheres, powder X-ray diffraction (XRD) of the xerogels was performed (Figures 7a and S5). The xerogels of the T-gels exhibited two sets of diffraction peaks at low angles of 2θ = 5.08°, 7.78°, 10.3°, and 14.13° corresponding to d = 1.75, 1.13, 0.86, and 0.63 nm, respectively, and of 2θ = 6.32°, 11.8°, 12.8°, and 15.6°, corresponding to d = 1.40, 0.77, 0.70, and 0.57 nm in a ratio of 1:1/√3:1/2:1/√7, assigned to the hexagonal columnar structure with the unit cell parameter a = 1.62 nm, indicating there are two different packing modes in the Tgels.35,36 The xerogels of the S-gels showed diffraction peak at 2θ = 6.32°, 11.8°, 12.8°, and 15.6°, which matched well with E

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The Journal of Physical Chemistry B fibers.37 The shifts of νN−H and νCO to lower wavenumbers and the shift of δN−H to higher wavenumbers in the fibers compared to the spheres indicates that H-bonding in the fibers was stronger than that in the spheres.29 Thus, intermolecular H-bonding between CO and NH is the driving force for the sol−gel transition and that its strength increases during sphere−fiber evolution. Kinetics of Gelation Based on Dickinson Model. During the gelation process, the time evolution of the rheological properties, namely, the storage modulus G′, provides important kinetic information for the development of a fiber network.38 Figure 8a shows a typical kinetic process for S-gel-8 as observed by rheological measurements. The rheological results of S-gel-10 are provided in the Supporting Information (Figure S6). The nucleation and growth of crystalline fibers occurs at the induction stage. In this study, we used the sonication method to accelerate the induction stage and to minimize the effects of the low gelation rate and the evaporation of ethanol on the measurements. Measurements started at the appearance of a turbid suspension similar to that shown in Figure 5 at 15 min. Therefore, no induction stage can be observed in Figure 8a. With the increase of the fiber content (i.e., the volume fraction of the fibers) to a critical value, the onset of the storage modulus G′ appears. We refer to this point as the starting point of gelation tg (tg = 300 s in Figure 8a). After that, G′ increases quickly by a few orders of magnitude, corresponding to the rapid growth of the volume fraction of fractal fibers until the space is filled with fiber networks. Then, G′ increases slowly to reach a quasiequilibrium state, indicating gelation. During gelation, the change in tan δ (where δ = G″/G′) with time can be divided into two stages: In the first stage, tan δ decreases rapidly with time, whereas in the second stage, tan δ decreases slowly with time, until it remains almost unchanged (tan δ = 0.34), finally indicating the formation of a cross-linked fiber network. Analysis of the fractal nature of our molecular organogels is based on a kinetic model that was first proposed for dilute systems by Dickinson39 and was initially used to describe the crystallization of polymer melts;36,40,41 in this work, this model was used to measure the fractal structure of the nanostructure networks during the gelation process. According to the Dickinson model, the time-dependent increase in the volume fraction of a gel network in the gelation stage can be expressed as ln

3 − Df G′(t ) − G′(0) ln(t − tg) =C+ G′(∞) − G′(0) Df

relatively smaller Df value in the second stage corresponds to elongation of the fused balls along a given direction; these results are consistent with those derived from the SEM observations (Figures 6 and S3).

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CONCLUSIONS In summary, we have designed and synthesized a novel kind of anhydrazide derivative showing organogelation in alcohols. Investigation of the gelation process for gelator 1 in ethanol suggested that its sol−gel transition takes place through a metastable suspension state and includes a concomitant morphological evolution from clew-like spheres to fibers. Correspondingly, the molecule packing modes change from an unknown phase in the spheres to a hexagonal columnar phase with a = 1.62 nm in the fibers. Intermolecular hydrogen bonding between CO and NH was found to be the driving force for the formation of fibers and spheres, and its strength in the fibers was was found to be greater than that in the clew-like spheres. The kinetics of the gelation of gelator 1 in ethanol under sonication showed a two-stage kinetic pathway with different fractal values of 1.27 and 0.84. Our study hence provides new insights into the formation of fibers and the evolution of the structure during gelation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b06965. Reversible transformations among different states of gelator 1, additional gelation process of gelator 1 in ethanol, XRD patterns of T-gel and S-gel, kinetics analysis of gelation, and characterization of compound 1 to substantiate the content (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiangyang Che: 0000-0002-5238-470X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Science Foundation Committee of China (Projects 51073071, 21072076, and 51103057) and Project 985-Automotive Engineering of Jilin University for financial support of this work.

(1)

where G′(0), G′(t), and G′(∞) are the storage moduli at times 0, t, and infinity, respectively. tg is defined as the time at which G′ starts to appear. In the following calculations, the time zero point is defined as the point where t − tg = 0, G′(0) is the storage modulus at tg, and G′(∞) is the average of the points of the quasi-equilibrium state that show negligible changes in G′ as time increases. The fractal dimension Df is a measure of the extent of the compactness of a fiber network microstructure. As shown in Figure 8b, the Df values of S-gel-8 showed two different growth modes with values of 1.27 and 0.84, respectively, suggesting that both are one-dimensional growth mechanisms. The Df value of the first stage was larger than that of the second stage, indicating more of a branching growth mode in the first stage (e.g., separated balls fuse and connect with surrounding balls along different directions), whereas the

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REFERENCES

(1) Terech, P.; Weiss, R. G. Low Molecular Mass Gelators of Organic Liquids and the Properties of Their Gels. Chem. Rev. 1997, 97, 3133− 3160. (2) Sangeetha, N. M.; Maitra, U. Supramolecular Gels: Functions and Uses. Chem. Soc. Rev. 2005, 34, 821−836. (3) Ajayaghosh, A.; George, S. J. First Phenylenevinylene Based Organogels: Self-Assembled Nanostructures via Cooperative Hydrogen Bonding and π-Stacking. J. Am. Chem. Soc. 2001, 123, 5148−5149. (4) Matsumoto, A.; Yoshida, R.; Kataoka, K. Glucose-Responsive Polymer Gel Bearing Phenylborate Derivative as a Glucose-Sensing Moiety Operating at the Physiological pH. Biomacromolecules 2004, 5, 1038−1045.

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DOI: 10.1021/acs.jpcb.7b06965 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B (5) Chatterjee, J.; Haik, Y.; Chen, C. J. Biodegradable Magnetic Gel: Synthesis and Characterization. Colloid Polym. Sci. 2003, 281, 892− 896. (6) van Esch, J. H. We Can Design Molecular Gelators, But Do We Understand Them? Langmuir 2009, 25, 8392−8394. (7) Weiss, R. G. The Past, Present, and Future of Molecular Gels. What Is the Status of the Field, and Where Is It Going? J. Am. Chem. Soc. 2014, 136, 7519−7530. (8) Jamieson, S. A.; Tong, K. W. K.; Hamilton, W. A.; He, L. L.; James, M.; Thordarson, P. Small Angle Neutron Scattering (SANS) Studies on the Structural Evolution of Pyromellitamide Self-Assembled Gels. Langmuir 2014, 30, 13987−13993. (9) Weiss, R. G.; Terech, P. Molecular Gels: Materials with SelfAssembled Fibrillar Networks; Springer: Dordrecht, The Netherlands, 2006. (10) Jonkheijm, P.; Schoot, P.; Schenning, A.; Meijer, E. W. Probing the Solvent-Assisted Nucleation Pathway in Chemical Self-Assembly. Science 2006, 313, 80−83. (11) Hirst, A. R.; Smith, D. K. Solvent Effects on Supramolecular Gel-Phase Materials: Two-Component Dendritic Gel. Langmuir 2004, 20, 10851−10857. (12) Liu, X. Y.; Sawant, P. D. Formation Kinetics of Fractal Nanofiber Networks in Organogels. Appl. Phys. Lett. 2001, 79, 3518− 3520. (13) Liu, X. Y.; Sawant, P. D. Mechanism of the Formation of SelfOrganized Microstructures in Soft Functional Material. Adv. Mater. 2002, 14, 421−426. (14) Liu, X. Y.; Sawant, P. D.; Tan, W. B.; Noor, I. B. M.; Pramesti, C.; Chen, B. H. Creating New Supramolecular Materials by Architecture of Three-Dimensional Nanocrystal Fiber Networks. J. Am. Chem. Soc. 2002, 124, 15055−15063. (15) Li, J. L.; Liu, X. Y.; Strom, C. S.; Xiong, J. Y. Engineering of Small Molecule Organogels by Design of the Nanometer Structure of Fiber Networks. Adv. Mater. 2006, 18, 2574−2578. (16) Wang, R. Y.; Liu, X. Y.; Narayanan, J.; Xiong, J. Y.; Li, J. L. Architecture of Fiber Network: From Understanding to Engineering of Molecular Gels. J. Phys. Chem. B 2006, 110, 25797−25802. (17) Wang, R. Y.; Liu, X. Y.; Xiong, J. Y.; Li, J. L. Real-Time Observation of Fiber Network Formation in Molecular Organogel: Supersaturation-Dependent Microstructure and Its Related Rheological Property. J. Phys. Chem. B 2006, 110, 7275−7280. (18) Xiong, J. Y.; Liu, X. Y.; Li, J. L.; Vallon, M. W. Architecture of Macromolecular Network of Soft Functional Materials: from Structure to Function. J. Phys. Chem. B 2007, 111, 5558−5563. (19) Huang, X.; Terech, P.; Raghavan, S. R.; Weiss, R. G. Kinetics of 5α-Cholestan-3β-yl N-(2-Naphthyl) carbamate/n-Alkane Organogel Formation and Its Influence on the Fibrillar Networks. J. Am. Chem. Soc. 2005, 127, 4336−4344. (20) Zhang, M. M.; Meng, L. Y.; Cao, X. H.; Jiang, M. J.; Yi, T. Morphological Transformation between Three-Dimensional Gel Network and Spherical Vesicles via Sonication. Soft Matter 2012, 8, 4494−4498. (21) Datta, S.; Bhattacharya, S. Ag+-Induced Reverse Vesicle to Helical Fiber Transformation in a Self-Assembly by Adjusting the Keto−Enol Equilibrium of a Chiral Salicylideneaniline. Chem. Commun. 2015, 51, 13929−13932. (22) Higashiguchi, K. J.; Taira, G.; Kitai, J. I.; Hirose, T.; Matsuda, K. J. Photoinduced Macroscopic Morphological Transformation of an Amphiphilic Diarylethene Assembly: Reversible Dynamic Motion. J. Am. Chem. Soc. 2015, 137, 2722−2729. (23) Ke, D. M.; Zhan, C. L.; Li, A. D. Q.; Yao, J. N. Morphological Transformation between Nanofibers and Vesicles in a Controllable Bipyridine-Tripeptide Self-Assembly. Angew. Chem., Int. Ed. 2011, 50, 3715−3719. (24) Xing, P. Y.; Chen, H. Z.; Bai, L. Y.; Zhao, Y. L. Photo-Triggered Transformation from Vesicles to Branched Nanotubes Fabricated by a Cholesterol-Appended Cyanostilbene. Chem. Commun. 2015, 51, 9309−9312.

(25) Cai, X. Q.; Wu, Y.; Wang, L. Y.; Yan, N.; Liu, J.; Fang, X. H.; Fang, Y. Mechano-Responsive Calix[4]Arene-Based Molecular Gels: Agitation Induced Gelation and Hardening. Soft Matter 2013, 9, 5807−5814. (26) Zhang, Y.; Ding, H.; Wu, Y.; Zhang, C. X.; Bai, B. L.; Wang, H. T.; Li, M. Ultrasound-Induced Controllable Morphology and Growth Dimension in a Dihydrazide-Based Self-Assembly System. Soft Matter 2014, 10, 8838−8845. (27) Wu, J. C.; Yi, T.; Shu, T. M.; Yu, M. X.; Zhou, Z. G.; Xu, M.; Zhou, Y. F.; Zhang, H. J.; Han, J. T.; Li, F. Y.; et al. Ultrasound Switch and Thermal Self-Repair of Morphology and Surface Wettability in a Cholesterol-Based Self-Assembly System. Angew. Chem., Int. Ed. 2008, 47, 1063−1067. (28) Das Mahapatra, R.; Dey, J. Ultrasound-Induced Gelation of Organic Liquids by L-Cysteine-Derived Amphiphile Containing Poly (ethylene glycol) Tail. Langmuir 2015, 31, 8703−8709. (29) Malicka, J. M.; Sandeep, A.; Monti, F.; Bandini, E.; Gazzano, M.; Ranjith, C.; Praveen, V. K.; Ajayaghosh, A.; Armaroli, N. Ultrasound Stimulated Nucleation and Growth of a Dye Assembly into Extended Gel Nanostructures. Chem. - Eur. J. 2013, 19, 12991−13001. (30) Thompson, L. H.; Doraiswamy, L. K. The Rate Enhancing Effect of Ultrasound by Inducing Supersaturation in a Solid−Liquid System. Chem. Eng. Sci. 2000, 55, 3085−3090. (31) Ruecroft, G.; Hipkiss, D.; Ly, T.; Maxted, N.; Cains, P. W. Sonocrystallization: The Use of Ultrasound for Improved Industrial Crystallization. Org. Process Res. Dev. 2005, 9, 923−932. (32) Luque de Castro, M. D.; Priego-Capote, F. Ultrasound-Assisted Crystallization (Sonocrystallization). Ultrason. Sonochem. 2007, 14, 717−724. (33) Wang, R. Y.; Liu, X. Y.; Li, J. L. Engineering Molecular SelfAssembled Fibrillar Networks by Ultrasound. Cryst. Growth Des. 2009, 9, 3286−3291. (34) Furman, I.; Weiss, R. G. Factors Influencing the Formation of Thermally Reversible Gels Comprised of Cholesteryl 4-(2-anthryloxy) Butanoate in Hexadecane, 1-Octanol, or Their Mixtures. Langmuir 1993, 9, 2084−2088. (35) Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; Hagele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.; Tosoni, M. Discotic Liquid Crystals: From Tailor-Made Synthesis to Plastic Electronics. Angew. Chem., Int. Ed. 2007, 46, 4832−4887. (36) Zhang, C. X.; Zhang, T. R.; Ji, N.; Zhang, Y.; Bai, B. L.; Wang, H. T.; Li, M. Gelation Behaviour of a Bent-Core Dihydrazide Derivative: Effect of Incubation Temperature in Chloroform and Toluene. Soft Matter 2016, 12, 1525−1533. (37) Pang, D. M.; Wang, H. T.; Li, M. Smectic A Liquid Crystals from Dihydrazide Derivatives with Lateral Intermolecular Hydrogen Bonding. Tetrahedron 2005, 61, 6108−6114. (38) Huang, X.; Raghavan, S. R.; Terech, P.; Weiss, R. G. Distinct Kinetic Pathways Generate Organogel Networks with Contrasting Fractality and Thixotropic Properties. J. Am. Chem. Soc. 2006, 128, 15341−15352. (39) Dickinson, E. On Gelation Kinetics in a System of Particles with Both Weak and Strong Interactions. J. Chem. Soc., Faraday Trans. 1997, 93, 111−114. (40) Avrami, M. Kinetics of Phase Change. I General Theory. J. Chem. Phys. 1939, 7, 1103−1112. (41) Avrami, M. Kinetics of Phase Change. II Transformation-Time Relations for Random Distribution of Nuclei. J. Chem. Phys. 1940, 8, 212−224.

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DOI: 10.1021/acs.jpcb.7b06965 J. Phys. Chem. B XXXX, XXX, XXX−XXX