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Gelation behaviours and gel properties of a dihydrazide derivative
Accepted Manuscript Gelation behaviours and gel properties of a dihydrazide derivative
Xiangyang Che, Min Li, Chunling Zhang, Chunxue Zhang, Haitao Wang, Binglian Bai PII: DOI: Reference:
S0167-7322(17)30162-9 doi: 10.1016/j.molliq.2017.06.036 MOLLIQ 7482
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
13 January 2017 28 April 2017 7 June 2017
Please cite this article as: Xiangyang Che, Min Li, Chunling Zhang, Chunxue Zhang, Haitao Wang, Binglian Bai , Gelation behaviours and gel properties of a dihydrazide derivative, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.06.036
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ACCEPTED MANUSCRIPT Gelation behaviours and gel properties of a Dihydrazide derivative Xiangyang Che a, Min Li *a, Chunling Zhang a, Chunxue Zhang a, Haitao Wang a and
a
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Binglian Bai b Key Laboratory of Automobile Materials, Ministry of Education, Institute of
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Materials Science and Engineering, Jilin University, Changchun 130012, People’s Republic of China.
College of Physics, Jilin University, Changchun 130012, PR China.
*
Corresponding author
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b
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E-mail: [email protected]
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Key words: dynamics of gelation, dihydrazide, properties.
ABSTRACT
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The gelation behaviour and gel properties of the organogels based on a twin-tapered
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dihydrazide derivative gelator, oxalyl acid N’,N’-di (3,4,5-tripentyloxybenzoyl) hydrazide (FH-T5) in mixtures of ethanol and ethylene glycol were investigated. It
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was demonstrated that the xerogels in the mixtures of ethanol and ethylene glycol showed the co-existence of sphere-like particles or short rod-like aggregates and
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fibers, which showed Colh phase. The more ethylene glycol in the mixtures, the shorter the gelation time of FH-T5 when the volume contents of ethylene glycol increased from 0% to 30%, and the kinetics of gelation of FH-T5 showed that Df value of the fibers increased from 1.302 in ethanol to 1.426 in the mixture with 30% ethylene glycol. Gels from the mixture of ethanol and ethylene glycol showed reduced storage modulus, higher Tg and the xerogels obtained from the mixtures exhibited higher wetting angle compared to that from ethanol.
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ACCEPTED MANUSCRIPT 1. Introduction Organogel have received much attention due to their potential applications such as template materials [1-3], sensors [4,5], and light harvesting [6], etc. As has been proved that gelation behaviour and gel properties were affected by many factors such as solvent [7,8], temperature [9,10], molecular structure of gelators [11]. Among the
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factors, solvent played an important role in aggregation of a gelator [12,13], kinetics process of gelation [14] and final properties [15] of the gels. Introduction of a second
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solvent to a solvent could be a useful method to tune the polarity of the solvent and thus modulated the gelation behaviour and gel properties. Li and co-workers reported
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gelators (diphenylalanine derivatives) aggregated into gels in toluene and assembled into microcrystal further by introducing ethanol to toluene [16]. Fan and co-workers
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investigated the changes of the gelation behaviour, morphology structure and gel-to-sol transition temperature based on two-component gel system (melamine (M)
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and 2-ethylhexylphosphoric acid mono-2-ethylhexyl ester) in the mixtures of methanol and water [17]. Zhang and co-workers investigated the photoresponse and
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speed of gelation of a dicholesterol-linked azobenzene organogelator in the mixtures
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of methanol and cyclopentanone and results indicated that they were influenced by the polarity of the solvents [18]. Except for exploring the influence of solvent polarity, there were relevant reports focusing on the interaction between gelators and solvent.
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As Dordick and coworkers reported, good gelation could be achieved by using a solvent that has limited interaction with the gelators. When the gelators (trehalose
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6,6’-didecanoate and 6,6’-dimyristate) and the solvent were unable to interact via hydrogen bond and then thin, entangled fibers formed. On the other hand, if the solvent interacted strongly with the gelators, then thick and highly clustered fibers arised [19]. In addition, the solvent parameters such as Hansen parameters [20,21] and kamlet-Taft parameters [22] were also testified to affect the gelation. Smith and co-workers investigated the relationship between gelation ability of gelator and Kamlet-Taft parameters. The result shown that gelation behavior was close to the hydrogen bond donor ability (α) of solvent. If α=0, the system could gelation. While if α >0.19, the system would keep solution state [23]. Furthermore, solvent structure and 2
ACCEPTED MANUSCRIPT solubility of gelators in solvent might cause inversion of chirality, investigated by Jamart-Gregoire’s and Xue’s teams, respectively [24,25]. Moreover, it was very important for nucleation behavior because of its strong effects on structural features including fiber size and morphology and the spatial distribution [26]. Rogers and coworkers reported both the nucleation and
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crystallization of 12-hydroxystearic acid based gelator such as induction time, fiber length and other structural parameters of the gels were dependent on the solvent [14].
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Liu and co-workers revealed that adding ethylene/vinyl acetate copolymer (EVACP) as surfactant could prolong the nucleation time and tuned the arrangement of gelators
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and gel properties in a two-component system (lanosta-8,24-dien-3β-ol : 24,25-dihydrolanosterol=56 : 44) [27]. In his another paper, Liu and co-workers
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enabled to tune the network from the spherulitic domains pattern to the extensively interconnected fibrillar network in N-lauroyl-L-glutamic acid di-n-butylamide
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organogel system by changing the incubation temperature of nucleation period. As he reported, the early stage aggregates in solution could affect the growth of the fiber in
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fiber were rarely reported.
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the process of gelation [10]. However, co-solvent induced nucleation and growth of
Our present paper report the gelation behaviour and gel properties based on oxalyl acid N’,N’-di (3,4,5-tripentyloxybenzoyl) hydrazide (FH-T5) in the mixture of ethanol
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and ethylene glycol. It was demonstrated that the xerogels showed coexistence of sphere-like particles or rod-like aggregates and fibers in the mixtures of ethanol and
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ethylene glycol. The gelators self-assembled through hydrogen bond between N-H and C=O to result in Colh phase. Both the gelation time and dimension of the fibers were dependent on the volume ratios of ethanol and ethylene glycol. Gels from the mixture of ethanol and ethylene glycol showed reduced storage modulus, higher Tg and the xerogels obtained from the mixtures exhibited higher wetting angle compared to that from ethanol.
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Figure 1 Molecular structure of FH-T5 2. Results and discussion
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The synthesis of FH-T5 was reported in our previous work and its molecular structure was shown in Figure 1 [28]. It was demonstrated that FH-T5 could gel
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ethanol with critical gelation concentration (CGC) of 4.5 mg/mL. We observed a change from gelation to turbid solution of FH-T5 at the volume fraction of ethylene
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glycol larger than 50% in the mixtures of ethanol and ethylene glycol (Figure 2).
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Figure 2 Photos of FH-T5 (10 mg/mL, 20℃) in (a) ethanol, (b-e) mixtures of ethanol and ethylene glycol with different ethylene glycol volume contents: (b) 15%, (c) 25%, (d) 30%, (e) 50% and (f) ethylene glycol
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2.1 Characterization of the gels
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ACCEPTED MANUSCRIPT Figure 3 SEM images of xerogels of FH-T5 (10 mg/mL, 20℃) prepared in (a) ethanol, (b-d) mixtures of ethanol and ethylene glycol with different ethylene glycol volume contents: (b) 15%, (c) 25%, (d) 30% and film casted from FH-T5 (10 mg/mL, 20℃) in the mixtures with different ethylene glycol volume contents: (e) 50%, (f) 100%.
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In order to obtain a visual insight into structures of the resultant gels, xerogels of FH-T5 obtained from ethanol, ethylene glycol and their mixtures were investigated by
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SEM. The xerogels obtained from ethanol exhibited well-defined 3D networks consisting of fibers entangled which were about 0.1-0.3 μm in width and more than
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100 μm in length (Figure 3a). In contrast, xerogels obtained from the mixture of ethanol and ethylene glycol (volume content: 15%) consisted of fibers with the width
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of 0.4-0.6 μm and sphere-like aggregates with the diameter of 0.5-1 μm (Figure 3b). Both sphere-like particles and fibers with the width of 0.5-0.7 μm and ca.50 μm in
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length were observed for xerogels obtained from the mixture containing 25% ethylene glycol (Figure 3c). While rod-like aggregates with the width 0.4-0.6 μm and length
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1.5-2.5 μm and fibers with 0.6-1 μm in width were observed for the xerogels from the
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mixture of 30% ethylene glycol and 70% ethanol (Figure.3d). Sphere-like particles and thick rod-like aggregates were observed for the suspension from the mixture containing 50% ethylene glycol (Figure 3e). FH-T5 suspension in ethylene glycol
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exhibited coexistence of sphere-like particles and short rod-like aggregates morphology (Figure 3f). In addition, the xerogels of the gels that were obtained from
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lower temperature (5℃ and -20℃) exhibited slightly different morphology, i.e. more straight in length and coarse in width compared that at 20℃ (Figure S1-2). On the other hand, with the decrease of temperature, amounts of spherical or short rod-like aggregates increased. The above discussion demonstrated that the morphology of xerogels dependent not only on the volume contents of ethylene glycol in the mixture, but also on the difference in degree of supercooling.
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Figure 4 XRD patterns of xerogels of FH-T5 (10 mg/mL, 20℃) prepared in ethanol
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(0%) and in mixtures of ethanol and ethylene glycol with different ethylene glycol volume contents: 15%, 25%, 30%.
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XRD patterns of the xerogels of FH-T5 were shown in Figure 4. It could be seen that the xerogels prepared in different mixtures showed similar diffractions suggesting
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molecular arrangement in sphere-like particles, rod-like aggregates and fibers were
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almost the same (Figure S3). As reported previously, diffractions of the xerogels of FH-T5 could be assigned to (100), (110) and (200) and thus the xerogels showed Colh phase [28]. The slight gradual increase of d-spacings of the xerogels with the increase
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of content of ethylene glycol could be explained as the solvents penetration inside the
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structure unit [34].
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Figure 5 FT-IR spectra of xerogels of FH-T5 (10 mg/mL, 20℃) prepared in ethanol
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and in mixtures of ethanol and ethylene glycol with different ethylene glycol volume contents: 15%, 25%, 30% and precipitates from the mixtures containing ethylene
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glycol volume contents: 50%, 100%.
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Table 1 Assignments of infrared frequencies for FH-T5 xerogels or precipitates in
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ethanol, in ethylene glycol and in the mixtures of ethanol and ethylene glycol (EG)
EG
0%
15%
25%
30%
50%
100%
ν(N-H)
3209.75
3200.61
3197.61
3204.80
3220.27
3247.36
ν(C=O)
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with different volume contents.
1623.90
1623.34
1624.29
1625.78
1639.50
1624.34
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To obtain the insight into the information about interaction between gelators, FT-IR spectra of FH-T5 xerogels or precipitates extracted from ethanol, ethylene glycol and their mixtures were measured and shown in Figure 5 and Table 1. FH-T5 gel in ethanol showed N–H stretching bands at 3209.75 cm-1 and C=O stretching bands at 1624.34 cm-1 which were lower than those in their free state indicating that the N–H groups were associated with C=O groups via strong hydrogen bond interactions, and thus the hydrogen bond was considered to be the driving force for self-assembly and gelation [29]. The stretching bands of N–H and C=O of the 7
ACCEPTED MANUSCRIPT xerogels shifted to lower wavenumbers, indicating stronger hydrogen bond of FH-T5 in the mixtures（with ethylene glycol lower than 30% ) compared to that in ethanol or in ethylene glycol. In contrast, FH-T5 could not gel the mixtures with ethylene glycol higher than 50%, in which hydrogen bond were relatively weak due to the increase of polarity of the mixture [15] and thus hindering the directional of hydrogen bond
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among FH-T5 molecules and the formation of fibers which were essential for
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gelation.
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2.2 Kinetics of gelation
Figure 6 Optical micrographs of the dynamics process of FH-T5 in mixtures of ethanol and ethylene glycol with the content of ethylene glycol was 15% at different
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temperatures (a) 40℃, (b) 25℃. The scale bar is 20 μm.
Figure 7 (a) Time-dependent fluorescence spectra of FH-T5 (10 mg/mL) in ethanol and in mixtures of ethanol and ethylene glycol from 40℃ to 20℃ (λ ex=270 nm, λ em=370
nm). (b) Kinetics plots for gelation of sols of FH-T5 (10 mg/mL) in ethanol
and in mixtures of ethanol and ethylene glycol based on Dickinson mode. (Volume contents of ethylene glycol in the mixtures were shown inset). The fluorescent microscopic images of FH-T5 in mixtures of ethanol and 15% (volume %) ethylene glycol at different temperatures shown in Figure 6, suggested that the particles developed first upon cooling from hot solution (Figure 6a) and then 8
ACCEPTED MANUSCRIPT fibers at lower temperatures (Figure 6b). To detect the mechanism of fiber growth in the mixtures with different volume fraction of ethylene glycol, the kinetics of gel formation of FH-T5 in mixtures of ethanol and ethylene glycol were investigated by time-resolved fluorescence spectroscopy and the results were shown in Figure 7a. Considering that FH-T5 showed aggregation induced enhanced emission (AIEE)
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[30], the kinetics of gelation process of FH-T5 was monitored by measuring its fluorescence intensity changes with time during gelation. It could be seen that FH-T5
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in ethanol showed a very weak fluorescence intensity at the early stage of gelation, corresponding to a process of nucleation (induction time), and then, fluorescence
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emission intensity increased drastically until a maximum value was reached. The time at which the intensity began to increase rapidly was defined as the gelation, and the
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time at which the maximum intensity reached corresponded to the accomplishment of gelation. So we could easily get the conclusion that the speed of gelation of FH-T5
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was relatively slow in ethanol compared to those in mixtures of ethanol and ethylene glycol. The fluorescence emission intensities of FH-T5 in the mixtures were higher
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than that in ethanol at early stage, which was attributed to the existence of sphere-like
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particles due to introduction of contain amount of ethylene glycol (Figure S4). Moreover, with the increase of ethylene glycol content, fluorescence emission intensity at onset points augmented which were owing to the increase of sphere-like
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particles or short rod-like aggregates in the mixtures (Table 2). So the sphere-like particles or short rod-like aggregates could serve as a heterogeneous nucleation points
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to accelerate the gelation of FH-T5 and the more sphere-like particles or short rod-like aggregates in the mixtures, the shorter gelation time when the contents of ethylene glycol increased from 0% to 30%.
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ACCEPTED MANUSCRIPT Table 2 t0 for onset points, intensity for induction time and Df data in ethanol (0%) and in the mixtures of ethanol and ethylene glycol with the volume contents of ethylene glycol: 15%, 25%, 30%. EG
0%
15%
25%
30%
2155
955
204
97
2
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32
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Df
1.302
1.385
1.403
1.426
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t0 Intensity
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In order to analysis of the fractal nature of the series of organogel system, Dickinson (eq.1) model which was initially used to describe the crystallization of
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polymer melts to measure the fractal structure of nanostructure networks during gelation formation process was used to explore the information about fiber growth
X (t ) X (0) C (3- D f ) / D f ln t X () X (0)
(1)
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ln
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[31,32].
The fractal dimension parameter Df indicates the growth mode of the fiber network. In
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our fluorescence method, the excluded volume of the aggregate (X(t)) was related to
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the fluorescence intensity. While t was time, Df was the fractal dimension, X(t) was the volume fraction of the gel phase at time t, expressed fluorescence intensity at time = 0, t, and ∞ (X(0), X(t), and X(∞)) in the present study, zero-time was defined as
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fluorescence intensity starts to increase rapidly. So the Df value of fiber obtained in ethanol and in mixtures could be easily achieved and shown in Figure 7b. The Df
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value was 1.302 for the FH-T5 gel in ethanol, indicating the two-dimensional growth pattern of the fiber. Df values and the t0 (onset points) for gels in different solvents were given in Table 2 Obviously, the Df values increased while t0 decreased with the increase of ethylene glycol content in the mixtures. Thus introduction of ethylene glycol in ethanol resulted in a more branching or 3D networks of fibers in FH-T5 gel and meanwhile accelerated the gelation [32,33]. In addition, time-dependent fluorescence spectra and kinetics plots for aggregation of sols of FH-T5 in ethylene glycol based on Dickinson mode were detected and shown in Figure S5. Obviously, Dickinson based kinetics analyses of dynamic fluorescence data revealed a 10
ACCEPTED MANUSCRIPT two-phrase kinetic process [34], featured by Df = 2.727 and followed by Df = 1.851, Two types of kinetics growth mode could be understood for FH-T5 aggregation process in ethylene glycol: one was three-dimensional growth (Df = 2.727) for spherical particles and another was two-dimensional growth (Df = 1.851) for rod-like aggregates.
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2.3 Thermostability of the gels
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Figure 8 Plots of gel-sol phase transition temperature (Tgel) of FH-T5 (10 mg/mL) gels obtained at different temperatures (20℃, 5℃, -20℃) versus the volume fraction of ethylene glycol: 0%, 15%, 25%, 30%.
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To assess the stability of the gels formed in different solvent and at different temperature, the gel-sol transition temperature (Tgel) was measured by the so-called
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“inverted test tube” method [35]. Figure 8 showed the Tgel of the gels as a function of the content of the ethylene glycol. For the gel from solvents with the same ethylene glycol content, the higher the temperature at which the gels formed, the higher the Tgel. In contrast, Tgel increased with the increase of ethylene glycol content in the gels which was attributed to the stronger H-bond as the increase of ethylene glycol [15].
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Figure 9 (a) Amplitude dependencies of the storage modulus (G’) and loss modulus (G”) of the FH-T5 gel samples (10 mg/mL, 20℃) in ethanol and in mixtures with the
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contents of ethylene glycol: 15%, 20% and 30%. The frequency is 1 Hz. (b)
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Frequency dependency of the storage modulus (G’) and loss modulus (G”) of the FH-T5 gels (10 mg/mL, 20℃) in ethanol and in mixtures of ethanol and ethylene
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glycol with the content of ethylene glycol: 15%, 20% and 30%. The strain amplitude was kept constant at 0.1%.
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The mechanical property of the gels was determined by rheology either testing the strain amplitude from 0.1% to 10% (Figure 9a) or sweeping the frequency under a constant strain of 0.1% (Figure 9b). Obviously, the gels showed solid behavior, i.e. the
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storage modulus was larger than the loss modulus over the entire frequency range. In
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addition, both G’ and G” exhibited frequency-independent within the experimental frequency range, which was considered to be their viscoelastic behavior. The storage
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modulus of the gels decreased from 20 kPa in ethanol to 2600 Pa in the mixture of 30% volume content of ethylene glycol, suggesting that ethylene glycol in ethanol
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resulted in the decrease of the mechanical strength of the gels.
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2.5 The surface wettability of the gels
Figure 10 Photographs of water droplets on silicon slides coated with FH-T5 xerogels
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(10 mg/mL, 20℃) obtained from (a) ethanol and from mixtures of ethanol and ethylene glycol with different ethylene glycol volume contents: (b) 15%, (c) 25%, (d)
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30%. All contact angles were analysed by Laplace–Young fitting.
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Taking these different micro-structures into consideration, the wetting properties of films of xerogels were investigated. As shown in Figure 10, the static contact angle (CA) of water on the films of xerogels was 132.8º in ethanol (Figure 10a), while the
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CA values were 155.2º, 164.2º and 144.1º for the films prepared from the mixtures with the ethylene glycol contents of 15%, 25% and 30% (Figure 10b-d). The surface
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wettability results suggested that the hydrophobic surfaces could be obtained and the CA values could be simply manipulated by changing volume content of ethylene glycol.
3. Conclusion In summary, we investigated the gelation behavior and gel properties of a twin-tapered dihydrazide derivative (FH-T5) in the mixtures of ethanol and ethylene glycol. The results showed that the xerogels extracted from mixtures showed the coexistence of the sphere-like particles or rod-like aggregates and fibers, which 13
ACCEPTED MANUSCRIPT exhibited similar molecular packing (Colh). In the process of cooling (from 130℃ to 20℃) for mixtures, the sphere-like particles or rod-like aggregates which aggregated first could serve as a heterogeneous nucleation point to speed up the growth of fiber and accelerated the gelation of FH-T5 when the contents of ethylene glycol increased from 0% to 30%. The more sphere-like particles or short rod-like aggregates in the
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mixtures, the shorter gelation time. Meanwhile a slight increase of the growth dimension (Df value) for the fibers from 1.302 in ethanol to 1.426 in the mixture with
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30% ethylene glycol was confirmed by Dickinson mode. FH-T5 could not gel the mixture with more than 50% ethylene glycol. In addition, Gels from the mixture of
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ethanol and ethylene glycol showed reduced storage modulus, higher Tgel compared to that from ethanol. Moreover, higher hydrophobic properties of xerogels were obtained
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mixtures of ethanol and ethylene glycol.
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and could be regulated through changing the volume fraction of ethylene glycol in the
4. Materials and methods
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The sample FH-T5 was synthesised by our team before and the chemical structure
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and the content of elements met the international standard. We have measured FH-T5 behaviour in the mixture of ethanol and ethylene glycol with the volume content of ethylene glycol: 5% and 10%. It showed almost the same
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behaviour as that in ethanol and were not given.
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An organogel was prepared by mixing a predetermined amount of FH-T5 with solvent in a sample vial with a crew cap and the whole system was heated until the solid were completely dissolved (nearly 130℃). The hot solution was quenched to 20℃, 5℃ or -20℃ for gelation. Gelation was considered to occur when the test tube was turned upside down with no fluid running down the wall The xerogels used for texts of FT-IR spectra, X-Ray diffraction and SEM were obtained by using the method of freeze-dried in freeze drying machine and we used TGA to confirm that where was no solvent left in the xerogels (Fig.S6). 14
ACCEPTED MANUSCRIPT The Fluorescence measurements were performed using a Perkin Elmer LS 55 fluorescence spectrophotometer with a 1.0cm length quartz cell. The fluorescence measurements were done at 20℃. FT-IR spectra were recorded with a Perkin-Elmer spectrometer (Spectrum One B). The samples were pressed tablets with KBr.
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X-Ray diffraction was carried out with a Bruker Avance D8 X-ray diffractometer. Data reduction were done by using MDI JADE 5.0 with PDF 2004 database.
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Scanning electron microscopy (SEM) observations were recorded with a SSX-550 apparatus. Samples for this measurement were prepared by casting the organogel on a
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silica substrate and all the samples were dried at room temperature, then coated by gold before observation.
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The rheological properties were studied on a TA instrument (AR2000 Rheometer) equipped with a stainless steel plate of 40mm diameter. The samples were sandwiched
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between the two plates with a gap of 0.5 mm throughout the experiments. The films of xerogels used for the text of static contact angle (CA) of water were
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obtained by dosing the hot solution to the silicon wafer. After several minutes, the gel
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formed. Afterwards, continuing to dose the hot solution to the surface and repeat this process several times until the silicon wafer surface was covered by the gel
the samples.
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completely. Then, we took the silicon wafer to freeze drying machine and freeze-dried
Surface wettability was characterized using a commercial video-based,
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software-controlled contact angle analyzer (Kruss DSA 30) on the dry sample film at room temperature. The water droplet size used for measurements was 20 μL. At least five different positions were measured and averaged to get a reliable value for the same sample.
Acknowledgements The authors are grateful to the National Science Foundation Committee of China (Project no. 51073071, 21072076 and 51103057) and Project 985-Automotive Engineering of Jilin University for financial support of this work. 15
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(31) E. Dickinson, J. Chem. Soc., Faraday Trans. 93 (1997) 111-114. (32) R. Wang, P. Wang, J. Li, B. Yuan, Y. Liu, L. Li, X. Liu, Phys. Chem. Chem. Phys.
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15 (2013) 3313-3319. (33) D.J. Cornwell, B.O. Okesola, D.K. Smith, Angew. Chem. Int. Ed. 53 (2014) 12461-12465. (34) C. Zhang, T. Zhang, N. Ji, Y. Zhang, B. Bai, H. Wang, M. Li, Soft Matter 12 (2016) 1525-1533. (35) M. George, G.P. Funkhouser, P. Terech, R.G. Weiss, Langmuir 22 (2006) 7885-7893.
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ACCEPTED MANUSCRIPT Graphical abstract The FH-T5 xerogels from the mixtures of ethanol and ethylene glycol showed the co-existence of the sphere-like particles or rod-like aggregates and fibers, which exhibited similar molecular packing (Colh). The FH-T5 gels exhibited shorter gelation
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time and larger growth dimension of fibers (Df value) compared to that in ethanol.
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ACCEPTED MANUSCRIPT Highlights 1 Co-solvent (ethanol and EG) induced nucleation and growth of fibers are reported. 2 Exploring the different growth dynamics of fibers in mixtures and in single system. 3 Different morphologies of xerogels are found with the change of EG content.
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4 The gels in the mixtures exhibit different properties compared to that in ethanol.
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Xiangyang Che, Min Li, Chunling Zhang, Chunxue Zhang, Haitao Wang, Binglian Bai PII: DOI: Reference:
S0167-7322(17)30162-9 doi: 10.1016/j.molliq.2017.06.036 MOLLIQ 7482
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
13 January 2017 28 April 2017 7 June 2017
Please cite this article as: Xiangyang Che, Min Li, Chunling Zhang, Chunxue Zhang, Haitao Wang, Binglian Bai , Gelation behaviours and gel properties of a dihydrazide derivative, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.06.036
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Gelation behaviours and gel properties of a Dihydrazide derivative Xiangyang Che a, Min Li *a, Chunling Zhang a, Chunxue Zhang a, Haitao Wang a and
a
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Binglian Bai b Key Laboratory of Automobile Materials, Ministry of Education, Institute of
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Materials Science and Engineering, Jilin University, Changchun 130012, People’s Republic of China.
College of Physics, Jilin University, Changchun 130012, PR China.
*
Corresponding author
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b
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E-mail: [email protected]
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Key words: dynamics of gelation, dihydrazide, properties.
ABSTRACT
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The gelation behaviour and gel properties of the organogels based on a twin-tapered
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dihydrazide derivative gelator, oxalyl acid N’,N’-di (3,4,5-tripentyloxybenzoyl) hydrazide (FH-T5) in mixtures of ethanol and ethylene glycol were investigated. It
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was demonstrated that the xerogels in the mixtures of ethanol and ethylene glycol showed the co-existence of sphere-like particles or short rod-like aggregates and
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fibers, which showed Colh phase. The more ethylene glycol in the mixtures, the shorter the gelation time of FH-T5 when the volume contents of ethylene glycol increased from 0% to 30%, and the kinetics of gelation of FH-T5 showed that Df value of the fibers increased from 1.302 in ethanol to 1.426 in the mixture with 30% ethylene glycol. Gels from the mixture of ethanol and ethylene glycol showed reduced storage modulus, higher Tg and the xerogels obtained from the mixtures exhibited higher wetting angle compared to that from ethanol.
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ACCEPTED MANUSCRIPT 1. Introduction Organogel have received much attention due to their potential applications such as template materials [1-3], sensors [4,5], and light harvesting [6], etc. As has been proved that gelation behaviour and gel properties were affected by many factors such as solvent [7,8], temperature [9,10], molecular structure of gelators [11]. Among the
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factors, solvent played an important role in aggregation of a gelator [12,13], kinetics process of gelation [14] and final properties [15] of the gels. Introduction of a second
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solvent to a solvent could be a useful method to tune the polarity of the solvent and thus modulated the gelation behaviour and gel properties. Li and co-workers reported
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gelators (diphenylalanine derivatives) aggregated into gels in toluene and assembled into microcrystal further by introducing ethanol to toluene [16]. Fan and co-workers
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investigated the changes of the gelation behaviour, morphology structure and gel-to-sol transition temperature based on two-component gel system (melamine (M)
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and 2-ethylhexylphosphoric acid mono-2-ethylhexyl ester) in the mixtures of methanol and water [17]. Zhang and co-workers investigated the photoresponse and
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speed of gelation of a dicholesterol-linked azobenzene organogelator in the mixtures
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of methanol and cyclopentanone and results indicated that they were influenced by the polarity of the solvents [18]. Except for exploring the influence of solvent polarity, there were relevant reports focusing on the interaction between gelators and solvent.
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As Dordick and coworkers reported, good gelation could be achieved by using a solvent that has limited interaction with the gelators. When the gelators (trehalose
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6,6’-didecanoate and 6,6’-dimyristate) and the solvent were unable to interact via hydrogen bond and then thin, entangled fibers formed. On the other hand, if the solvent interacted strongly with the gelators, then thick and highly clustered fibers arised [19]. In addition, the solvent parameters such as Hansen parameters [20,21] and kamlet-Taft parameters [22] were also testified to affect the gelation. Smith and co-workers investigated the relationship between gelation ability of gelator and Kamlet-Taft parameters. The result shown that gelation behavior was close to the hydrogen bond donor ability (α) of solvent. If α=0, the system could gelation. While if α >0.19, the system would keep solution state [23]. Furthermore, solvent structure and 2
ACCEPTED MANUSCRIPT solubility of gelators in solvent might cause inversion of chirality, investigated by Jamart-Gregoire’s and Xue’s teams, respectively [24,25]. Moreover, it was very important for nucleation behavior because of its strong effects on structural features including fiber size and morphology and the spatial distribution [26]. Rogers and coworkers reported both the nucleation and
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crystallization of 12-hydroxystearic acid based gelator such as induction time, fiber length and other structural parameters of the gels were dependent on the solvent [14].
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Liu and co-workers revealed that adding ethylene/vinyl acetate copolymer (EVACP) as surfactant could prolong the nucleation time and tuned the arrangement of gelators
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and gel properties in a two-component system (lanosta-8,24-dien-3β-ol : 24,25-dihydrolanosterol=56 : 44) [27]. In his another paper, Liu and co-workers
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enabled to tune the network from the spherulitic domains pattern to the extensively interconnected fibrillar network in N-lauroyl-L-glutamic acid di-n-butylamide
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organogel system by changing the incubation temperature of nucleation period. As he reported, the early stage aggregates in solution could affect the growth of the fiber in
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fiber were rarely reported.
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the process of gelation [10]. However, co-solvent induced nucleation and growth of
Our present paper report the gelation behaviour and gel properties based on oxalyl acid N’,N’-di (3,4,5-tripentyloxybenzoyl) hydrazide (FH-T5) in the mixture of ethanol
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and ethylene glycol. It was demonstrated that the xerogels showed coexistence of sphere-like particles or rod-like aggregates and fibers in the mixtures of ethanol and
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ethylene glycol. The gelators self-assembled through hydrogen bond between N-H and C=O to result in Colh phase. Both the gelation time and dimension of the fibers were dependent on the volume ratios of ethanol and ethylene glycol. Gels from the mixture of ethanol and ethylene glycol showed reduced storage modulus, higher Tg and the xerogels obtained from the mixtures exhibited higher wetting angle compared to that from ethanol.
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Figure 1 Molecular structure of FH-T5 2. Results and discussion
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The synthesis of FH-T5 was reported in our previous work and its molecular structure was shown in Figure 1 [28]. It was demonstrated that FH-T5 could gel
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ethanol with critical gelation concentration (CGC) of 4.5 mg/mL. We observed a change from gelation to turbid solution of FH-T5 at the volume fraction of ethylene
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glycol larger than 50% in the mixtures of ethanol and ethylene glycol (Figure 2).
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Figure 2 Photos of FH-T5 (10 mg/mL, 20℃) in (a) ethanol, (b-e) mixtures of ethanol and ethylene glycol with different ethylene glycol volume contents: (b) 15%, (c) 25%, (d) 30%, (e) 50% and (f) ethylene glycol
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2.1 Characterization of the gels
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ACCEPTED MANUSCRIPT Figure 3 SEM images of xerogels of FH-T5 (10 mg/mL, 20℃) prepared in (a) ethanol, (b-d) mixtures of ethanol and ethylene glycol with different ethylene glycol volume contents: (b) 15%, (c) 25%, (d) 30% and film casted from FH-T5 (10 mg/mL, 20℃) in the mixtures with different ethylene glycol volume contents: (e) 50%, (f) 100%.
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In order to obtain a visual insight into structures of the resultant gels, xerogels of FH-T5 obtained from ethanol, ethylene glycol and their mixtures were investigated by
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SEM. The xerogels obtained from ethanol exhibited well-defined 3D networks consisting of fibers entangled which were about 0.1-0.3 μm in width and more than
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100 μm in length (Figure 3a). In contrast, xerogels obtained from the mixture of ethanol and ethylene glycol (volume content: 15%) consisted of fibers with the width
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of 0.4-0.6 μm and sphere-like aggregates with the diameter of 0.5-1 μm (Figure 3b). Both sphere-like particles and fibers with the width of 0.5-0.7 μm and ca.50 μm in
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length were observed for xerogels obtained from the mixture containing 25% ethylene glycol (Figure 3c). While rod-like aggregates with the width 0.4-0.6 μm and length
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1.5-2.5 μm and fibers with 0.6-1 μm in width were observed for the xerogels from the
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mixture of 30% ethylene glycol and 70% ethanol (Figure.3d). Sphere-like particles and thick rod-like aggregates were observed for the suspension from the mixture containing 50% ethylene glycol (Figure 3e). FH-T5 suspension in ethylene glycol
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exhibited coexistence of sphere-like particles and short rod-like aggregates morphology (Figure 3f). In addition, the xerogels of the gels that were obtained from
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lower temperature (5℃ and -20℃) exhibited slightly different morphology, i.e. more straight in length and coarse in width compared that at 20℃ (Figure S1-2). On the other hand, with the decrease of temperature, amounts of spherical or short rod-like aggregates increased. The above discussion demonstrated that the morphology of xerogels dependent not only on the volume contents of ethylene glycol in the mixture, but also on the difference in degree of supercooling.
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Figure 4 XRD patterns of xerogels of FH-T5 (10 mg/mL, 20℃) prepared in ethanol
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(0%) and in mixtures of ethanol and ethylene glycol with different ethylene glycol volume contents: 15%, 25%, 30%.
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XRD patterns of the xerogels of FH-T5 were shown in Figure 4. It could be seen that the xerogels prepared in different mixtures showed similar diffractions suggesting
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molecular arrangement in sphere-like particles, rod-like aggregates and fibers were
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almost the same (Figure S3). As reported previously, diffractions of the xerogels of FH-T5 could be assigned to (100), (110) and (200) and thus the xerogels showed Colh phase [28]. The slight gradual increase of d-spacings of the xerogels with the increase
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of content of ethylene glycol could be explained as the solvents penetration inside the
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structure unit [34].
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Figure 5 FT-IR spectra of xerogels of FH-T5 (10 mg/mL, 20℃) prepared in ethanol
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and in mixtures of ethanol and ethylene glycol with different ethylene glycol volume contents: 15%, 25%, 30% and precipitates from the mixtures containing ethylene
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glycol volume contents: 50%, 100%.
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Table 1 Assignments of infrared frequencies for FH-T5 xerogels or precipitates in
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ethanol, in ethylene glycol and in the mixtures of ethanol and ethylene glycol (EG)
EG
0%
15%
25%
30%
50%
100%
ν(N-H)
3209.75
3200.61
3197.61
3204.80
3220.27
3247.36
ν(C=O)
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with different volume contents.
1623.90
1623.34
1624.29
1625.78
1639.50
1624.34
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To obtain the insight into the information about interaction between gelators, FT-IR spectra of FH-T5 xerogels or precipitates extracted from ethanol, ethylene glycol and their mixtures were measured and shown in Figure 5 and Table 1. FH-T5 gel in ethanol showed N–H stretching bands at 3209.75 cm-1 and C=O stretching bands at 1624.34 cm-1 which were lower than those in their free state indicating that the N–H groups were associated with C=O groups via strong hydrogen bond interactions, and thus the hydrogen bond was considered to be the driving force for self-assembly and gelation [29]. The stretching bands of N–H and C=O of the 7
ACCEPTED MANUSCRIPT xerogels shifted to lower wavenumbers, indicating stronger hydrogen bond of FH-T5 in the mixtures（with ethylene glycol lower than 30% ) compared to that in ethanol or in ethylene glycol. In contrast, FH-T5 could not gel the mixtures with ethylene glycol higher than 50%, in which hydrogen bond were relatively weak due to the increase of polarity of the mixture [15] and thus hindering the directional of hydrogen bond
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among FH-T5 molecules and the formation of fibers which were essential for
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gelation.
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2.2 Kinetics of gelation
Figure 6 Optical micrographs of the dynamics process of FH-T5 in mixtures of ethanol and ethylene glycol with the content of ethylene glycol was 15% at different
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temperatures (a) 40℃, (b) 25℃. The scale bar is 20 μm.
Figure 7 (a) Time-dependent fluorescence spectra of FH-T5 (10 mg/mL) in ethanol and in mixtures of ethanol and ethylene glycol from 40℃ to 20℃ (λ ex=270 nm, λ em=370
nm). (b) Kinetics plots for gelation of sols of FH-T5 (10 mg/mL) in ethanol
and in mixtures of ethanol and ethylene glycol based on Dickinson mode. (Volume contents of ethylene glycol in the mixtures were shown inset). The fluorescent microscopic images of FH-T5 in mixtures of ethanol and 15% (volume %) ethylene glycol at different temperatures shown in Figure 6, suggested that the particles developed first upon cooling from hot solution (Figure 6a) and then 8
ACCEPTED MANUSCRIPT fibers at lower temperatures (Figure 6b). To detect the mechanism of fiber growth in the mixtures with different volume fraction of ethylene glycol, the kinetics of gel formation of FH-T5 in mixtures of ethanol and ethylene glycol were investigated by time-resolved fluorescence spectroscopy and the results were shown in Figure 7a. Considering that FH-T5 showed aggregation induced enhanced emission (AIEE)
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[30], the kinetics of gelation process of FH-T5 was monitored by measuring its fluorescence intensity changes with time during gelation. It could be seen that FH-T5
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in ethanol showed a very weak fluorescence intensity at the early stage of gelation, corresponding to a process of nucleation (induction time), and then, fluorescence
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emission intensity increased drastically until a maximum value was reached. The time at which the intensity began to increase rapidly was defined as the gelation, and the
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time at which the maximum intensity reached corresponded to the accomplishment of gelation. So we could easily get the conclusion that the speed of gelation of FH-T5
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was relatively slow in ethanol compared to those in mixtures of ethanol and ethylene glycol. The fluorescence emission intensities of FH-T5 in the mixtures were higher
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than that in ethanol at early stage, which was attributed to the existence of sphere-like
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particles due to introduction of contain amount of ethylene glycol (Figure S4). Moreover, with the increase of ethylene glycol content, fluorescence emission intensity at onset points augmented which were owing to the increase of sphere-like
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particles or short rod-like aggregates in the mixtures (Table 2). So the sphere-like particles or short rod-like aggregates could serve as a heterogeneous nucleation points
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to accelerate the gelation of FH-T5 and the more sphere-like particles or short rod-like aggregates in the mixtures, the shorter gelation time when the contents of ethylene glycol increased from 0% to 30%.
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ACCEPTED MANUSCRIPT Table 2 t0 for onset points, intensity for induction time and Df data in ethanol (0%) and in the mixtures of ethanol and ethylene glycol with the volume contents of ethylene glycol: 15%, 25%, 30%. EG
0%
15%
25%
30%
2155
955
204
97
2
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32
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Df
1.302
1.385
1.403
1.426
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t0 Intensity
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In order to analysis of the fractal nature of the series of organogel system, Dickinson (eq.1) model which was initially used to describe the crystallization of
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polymer melts to measure the fractal structure of nanostructure networks during gelation formation process was used to explore the information about fiber growth
X (t ) X (0) C (3- D f ) / D f ln t X () X (0)
(1)
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ln
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[31,32].
The fractal dimension parameter Df indicates the growth mode of the fiber network. In
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our fluorescence method, the excluded volume of the aggregate (X(t)) was related to
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the fluorescence intensity. While t was time, Df was the fractal dimension, X(t) was the volume fraction of the gel phase at time t, expressed fluorescence intensity at time = 0, t, and ∞ (X(0), X(t), and X(∞)) in the present study, zero-time was defined as
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fluorescence intensity starts to increase rapidly. So the Df value of fiber obtained in ethanol and in mixtures could be easily achieved and shown in Figure 7b. The Df
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value was 1.302 for the FH-T5 gel in ethanol, indicating the two-dimensional growth pattern of the fiber. Df values and the t0 (onset points) for gels in different solvents were given in Table 2 Obviously, the Df values increased while t0 decreased with the increase of ethylene glycol content in the mixtures. Thus introduction of ethylene glycol in ethanol resulted in a more branching or 3D networks of fibers in FH-T5 gel and meanwhile accelerated the gelation [32,33]. In addition, time-dependent fluorescence spectra and kinetics plots for aggregation of sols of FH-T5 in ethylene glycol based on Dickinson mode were detected and shown in Figure S5. Obviously, Dickinson based kinetics analyses of dynamic fluorescence data revealed a 10
ACCEPTED MANUSCRIPT two-phrase kinetic process [34], featured by Df = 2.727 and followed by Df = 1.851, Two types of kinetics growth mode could be understood for FH-T5 aggregation process in ethylene glycol: one was three-dimensional growth (Df = 2.727) for spherical particles and another was two-dimensional growth (Df = 1.851) for rod-like aggregates.
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2.3 Thermostability of the gels
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Figure 8 Plots of gel-sol phase transition temperature (Tgel) of FH-T5 (10 mg/mL) gels obtained at different temperatures (20℃, 5℃, -20℃) versus the volume fraction of ethylene glycol: 0%, 15%, 25%, 30%.
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To assess the stability of the gels formed in different solvent and at different temperature, the gel-sol transition temperature (Tgel) was measured by the so-called
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“inverted test tube” method [35]. Figure 8 showed the Tgel of the gels as a function of the content of the ethylene glycol. For the gel from solvents with the same ethylene glycol content, the higher the temperature at which the gels formed, the higher the Tgel. In contrast, Tgel increased with the increase of ethylene glycol content in the gels which was attributed to the stronger H-bond as the increase of ethylene glycol [15].
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Figure 9 (a) Amplitude dependencies of the storage modulus (G’) and loss modulus (G”) of the FH-T5 gel samples (10 mg/mL, 20℃) in ethanol and in mixtures with the
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contents of ethylene glycol: 15%, 20% and 30%. The frequency is 1 Hz. (b)
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Frequency dependency of the storage modulus (G’) and loss modulus (G”) of the FH-T5 gels (10 mg/mL, 20℃) in ethanol and in mixtures of ethanol and ethylene
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glycol with the content of ethylene glycol: 15%, 20% and 30%. The strain amplitude was kept constant at 0.1%.
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The mechanical property of the gels was determined by rheology either testing the strain amplitude from 0.1% to 10% (Figure 9a) or sweeping the frequency under a constant strain of 0.1% (Figure 9b). Obviously, the gels showed solid behavior, i.e. the
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storage modulus was larger than the loss modulus over the entire frequency range. In
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addition, both G’ and G” exhibited frequency-independent within the experimental frequency range, which was considered to be their viscoelastic behavior. The storage
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modulus of the gels decreased from 20 kPa in ethanol to 2600 Pa in the mixture of 30% volume content of ethylene glycol, suggesting that ethylene glycol in ethanol
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resulted in the decrease of the mechanical strength of the gels.
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2.5 The surface wettability of the gels
Figure 10 Photographs of water droplets on silicon slides coated with FH-T5 xerogels
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(10 mg/mL, 20℃) obtained from (a) ethanol and from mixtures of ethanol and ethylene glycol with different ethylene glycol volume contents: (b) 15%, (c) 25%, (d)
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30%. All contact angles were analysed by Laplace–Young fitting.
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Taking these different micro-structures into consideration, the wetting properties of films of xerogels were investigated. As shown in Figure 10, the static contact angle (CA) of water on the films of xerogels was 132.8º in ethanol (Figure 10a), while the
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CA values were 155.2º, 164.2º and 144.1º for the films prepared from the mixtures with the ethylene glycol contents of 15%, 25% and 30% (Figure 10b-d). The surface
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wettability results suggested that the hydrophobic surfaces could be obtained and the CA values could be simply manipulated by changing volume content of ethylene glycol.
3. Conclusion In summary, we investigated the gelation behavior and gel properties of a twin-tapered dihydrazide derivative (FH-T5) in the mixtures of ethanol and ethylene glycol. The results showed that the xerogels extracted from mixtures showed the coexistence of the sphere-like particles or rod-like aggregates and fibers, which 13
ACCEPTED MANUSCRIPT exhibited similar molecular packing (Colh). In the process of cooling (from 130℃ to 20℃) for mixtures, the sphere-like particles or rod-like aggregates which aggregated first could serve as a heterogeneous nucleation point to speed up the growth of fiber and accelerated the gelation of FH-T5 when the contents of ethylene glycol increased from 0% to 30%. The more sphere-like particles or short rod-like aggregates in the
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mixtures, the shorter gelation time. Meanwhile a slight increase of the growth dimension (Df value) for the fibers from 1.302 in ethanol to 1.426 in the mixture with
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30% ethylene glycol was confirmed by Dickinson mode. FH-T5 could not gel the mixture with more than 50% ethylene glycol. In addition, Gels from the mixture of
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ethanol and ethylene glycol showed reduced storage modulus, higher Tgel compared to that from ethanol. Moreover, higher hydrophobic properties of xerogels were obtained
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mixtures of ethanol and ethylene glycol.
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and could be regulated through changing the volume fraction of ethylene glycol in the
4. Materials and methods
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The sample FH-T5 was synthesised by our team before and the chemical structure
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and the content of elements met the international standard. We have measured FH-T5 behaviour in the mixture of ethanol and ethylene glycol with the volume content of ethylene glycol: 5% and 10%. It showed almost the same
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behaviour as that in ethanol and were not given.
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An organogel was prepared by mixing a predetermined amount of FH-T5 with solvent in a sample vial with a crew cap and the whole system was heated until the solid were completely dissolved (nearly 130℃). The hot solution was quenched to 20℃, 5℃ or -20℃ for gelation. Gelation was considered to occur when the test tube was turned upside down with no fluid running down the wall The xerogels used for texts of FT-IR spectra, X-Ray diffraction and SEM were obtained by using the method of freeze-dried in freeze drying machine and we used TGA to confirm that where was no solvent left in the xerogels (Fig.S6). 14
ACCEPTED MANUSCRIPT The Fluorescence measurements were performed using a Perkin Elmer LS 55 fluorescence spectrophotometer with a 1.0cm length quartz cell. The fluorescence measurements were done at 20℃. FT-IR spectra were recorded with a Perkin-Elmer spectrometer (Spectrum One B). The samples were pressed tablets with KBr.
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X-Ray diffraction was carried out with a Bruker Avance D8 X-ray diffractometer. Data reduction were done by using MDI JADE 5.0 with PDF 2004 database.
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Scanning electron microscopy (SEM) observations were recorded with a SSX-550 apparatus. Samples for this measurement were prepared by casting the organogel on a
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silica substrate and all the samples were dried at room temperature, then coated by gold before observation.
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The rheological properties were studied on a TA instrument (AR2000 Rheometer) equipped with a stainless steel plate of 40mm diameter. The samples were sandwiched
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between the two plates with a gap of 0.5 mm throughout the experiments. The films of xerogels used for the text of static contact angle (CA) of water were
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obtained by dosing the hot solution to the silicon wafer. After several minutes, the gel
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formed. Afterwards, continuing to dose the hot solution to the surface and repeat this process several times until the silicon wafer surface was covered by the gel
the samples.
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completely. Then, we took the silicon wafer to freeze drying machine and freeze-dried
Surface wettability was characterized using a commercial video-based,
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software-controlled contact angle analyzer (Kruss DSA 30) on the dry sample film at room temperature. The water droplet size used for measurements was 20 μL. At least five different positions were measured and averaged to get a reliable value for the same sample.
Acknowledgements The authors are grateful to the National Science Foundation Committee of China (Project no. 51073071, 21072076 and 51103057) and Project 985-Automotive Engineering of Jilin University for financial support of this work. 15
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ACCEPTED MANUSCRIPT Graphical abstract The FH-T5 xerogels from the mixtures of ethanol and ethylene glycol showed the co-existence of the sphere-like particles or rod-like aggregates and fibers, which exhibited similar molecular packing (Colh). The FH-T5 gels exhibited shorter gelation
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time and larger growth dimension of fibers (Df value) compared to that in ethanol.
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ACCEPTED MANUSCRIPT Highlights 1 Co-solvent (ethanol and EG) induced nucleation and growth of fibers are reported. 2 Exploring the different growth dynamics of fibers in mixtures and in single system. 3 Different morphologies of xerogels are found with the change of EG content.
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4 The gels in the mixtures exhibit different properties compared to that in ethanol.
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