Self-Assembly of a Strong Polyhedral Oligomeric Silsesquioxane Core

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Self-assembly of a strong POSS core based aspartate derivative dendrimer supramolecular gelator in different polarity solvents Huiwen He, Si Chen, Xiaoqian Tong, Zhihang An, Meng Ma, Xiaosong Wang, and Xu Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02893 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 29, 2017

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Self-assembly of a strong POSS core based aspartate derivative dendrimer supramolecular gelator in different polarity solvents Huiwen He,a‡ Si Chen a‡*, Xiaoqian Tong a, Zhihang An a, Meng Ma a, Xiaosong Wang b and Xu Wang a* a

College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou

310014, China. b

University of Waterloo, Department of Chemistry, Waterloo, Ontario, Canada.

ABSTRACT

Aromatic groups are introduced into the end peripherals of polyhedral oligomeric silsesquioxane (POSS) core based organic/inorganic hybrid supramolecules to get novel dendrimer gelator POSS-Z-Asp(OBzl) (POSS-ASP), which have eight aspartate derivative arms to make full use of mighty π-π stacking forces in order to get strong supramoolecular gels addition to multiple hydrogen bindings and van der Waals interactions. POSS-ASP can self-assembly into threedimensional nanoscale gel networks to provide the hybrid physical gels especially strong mechanical properties and fast-recovery behaviors. Two totally different morphology of connected spherical particles structures and banded ultra-long fibres are observed according to

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the polarity of solvents confirmed by SEM, POM and TEM, expecting the existing various selfassemble model and illustrating the peripherals of dendrimer and polarity of solvents having huge influences in supramolecular self-assemble mechanism. What's more, the thermal stability, rheological properties and network architecture information also be investigated via tubeinversion, rotational Rheometer, and power X-ray diffraction, the results of which confirming the two different gel formation mechanism that makes POSS-ASP presents two totally different thermal and mechanical property. Such a study reports a new gelation system in organic or organic/aqueous mixed solvents, which can be helpful for investigating the relationship of dendritic supramolecular gelation and different polarity solvents during the supramolecular selfassembly process of gelators.

INTRODUCTION Supramolecular gels are colloidal soft materials consisting of self-assembled fiber networks that have attracted significant attention in recent years due to their all kinds of potential applications in various fields1-6, including pollutant removal, drug delivery, biosensors, catalysis, optoelectronics, enzyme immobilization, solar cells and so on. The well-defined supramolecules always achieves a reversible sol-gel phase transition by means of the non-covalent nature of the interactions including hydrogen bonding, π-π stacking, ion-ion, dipole-dipole, vander Waals force, host-guest and ion coordination in the supramolecular network to form supramolecular gels.7-11


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As a kind of soft material, the poor heat resistance, mechanical properties and self-healing performance of gels always act as the primary obstacle to its applications in daily life.12-15 Polymer gels16-18 always have supramolecular network structure formed by the long molecular chains and tend to have good mechanical properties and heat resistance properties, while the needs of high concentration of gelator (>10%) often limits its application. Low molecular weight gels19-21 have simple chemical structures, low gel concentration, excellent self-healing properties that make sure it can be used in some biosensors, optoelectronics et al. while the poor hardness and toughness limit its applications in fields of support materials. Dendrimers22-25 filling the gap between polymers and small molecules are nanoscopic hyperbranched macromolecules with well-predictable three-dimensional shapes and potential building blocks for the construction of organized functional materials which have attracted great attention by many researchers for its supramolecular gels applications. In recent years, some dendrimers and dendrons are found to be good candidates for constructing gel-phase materials in organic or aqueous media due to the advantages of significant steric impact and the capability to form multiple noncovalent interactions.26-28 The first examples of a dendritic physical gel was designed and synthesised by Aida and co-workers, which formed by self-assembly of dipeptide core poly(benzyl ether) dendrimers in organic solvents and hierarchically self-organization to form a micrometerscale fibrous assembly of nanoscale dendritic fibrils.29 Later, a series of functional dendritic supramolecular gels consisting of dendrons or dendrimers with different chemical structures sprang up quest for new materials and new applications30-32. However, advances in this area often rely on serendipity or discovery-based screening strategies, and the rational design of new types of dendritic functional organogelators with excellent mechanical properties has been a great challenge. Recently, we have reported a series of 3D radial symmetric


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organic/inorganic hybrid dendrimer supramolecular gelator POSS-Lys33, POSS-Cys and POSSHpy34 which could form a unique loofah-like network structure and macroscopically manifest itself with a high efficiency in gelation, with a strong ability to resistant the external stress and exhibiting a distinct viscous flow state. Such novel POSS core based dendritic supramolecular gelators can be connected with a lot of functional groups and have great potential in new soft materials. During our researches, although the low storage modulus (G') of POSS-Cys gels can be overcome and improved up to 100 times via introduction of AuNPs, the applications of original POSS-Lys and POSS-Cys gels been restricted by low G'. Expose the mysterious secrets about self-assemble of POSS based dendritic supramolecules, and design new molecules with excellent mechanical properties of its original gels is still a critical issue.

Scheme 1. Chemical structure of POSS core based dendrimer POSS-ASP. In this paper, POSS-ASP as scheme 1 shows, a novel 3D radial symmetric organic/inorganic hybrid aspartate derivative dendrimer, who have eight arms with aromatic groups as the end peripherals was designed and synthesized. POSS-ASP present a strong gel


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ability with a high G' (>100 kPa) and can form quite different network structure with macroscopically manifest itself with a high efficiency gelation in both nonpolar and polarity organic solvents. As a new POSS based dendrimer gelator, no loofah-like network structure with no-physical cross-linked, continuous and integral network can be obtained as a result of the replacement of the arm structure. What’s more, the gelation ability, thermal stability, rheological properties, microcosmic morphology, network architecture information and driving force for gelation of POSS-ASP are investigated systematically. The results turn out that POSS-ASP can self-assembly into three-dimensional nanoscale gel network through strong π-π stacking, multiple hydrogen bindings and van der Waals interactions and so that the hybrid physical gels have fast-recovery behaviors. Moreover, the morphology of POSS-ASP self-assemble in a transparent methyl methacrylate (MMA) observed a number of spherical particles with a diameter between 40-60 µm connected to each other are formed via many hexagonal columnar structure nanofibers aggregated together to form thick and spherical fiber bundles, while in Tetrahydrofuran (THF) and THF/water a sparse and banded ultra-long fibers with a wide of 50200 nm formed via plenty of square columnar structure nanofibers aggregated together were observed, demonstrates solvent polarity could dramatically influence the different self-assemble mechanism of such an organic/inorganic hybrid gelator, with totally different self-assemble model. The results reported here are helpful for developing a quite strong organogel by introduction of π-π stacking and investigating the relationship of supramolecular gelation between different polarity solvents during the supramolecular self-assembly process of gelators.

EXPERIMENTAL AND METHODS


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Design and synthesis of POSS-ASP. In order to form a contrast with POSS-Lys33 who used eight Boc- protected L-Lys as the arm structure, we carefully selected the similar topological structure Z- protected aspartate derivative Z-Asp(OBzl)-OH as the arm structure. The dendrimer POSS-ASP was synthesised by converged method as we reported and the details were presented in section 1 in support information. Gelation tests and gel-sol transition temperature (Tgel). A known weight of a tested compound and a measured volume (1 mL) of a chosen liquid were placed into a 3 mL glass vial as a test tube and the system was ultrasonic treated for 10 min at room temperature, then the test tube was inversed to observe if a gel had been formed. If the result was negative, the test tube was heated until the solid was dissolved completely, and then the system was cooled to room temperature. Finally, the test tube was inversed to observe the state of the materials inside. The gel-sol transition temperature (Tgel) was measured by using a simple tube-inversion method35. We defined the temperature at which the gels began to flow the gel-sol transition temperature (Tgel) as the gel point. Considering such a simple tube-inversion method was not a rigorous test to examine the accurate gel point as Winter et al.36 and Negi et al.37 described, it was a working definition rather than the stricter definition for utilization. Rheology. Rheological characterizations of gels were performed on an Anton Paar MCR302 Rheometer with plate geometry (PP 25). The gap distance between plate and plate was fixed at 0.5 mm. Rheological measurements were carried out on freshly prepared gels and were scooped onto the plate of the Rheometer. The following tests were performed: the samples were submitted to this parallel-plate very quickly to minimize solvent evaporation. Dynamic strain sweep tests were increased amplitude of oscillation from 0.1% up to 100% apparent strain shear (kept a frequency ω=10 rad·s-1) at 25 oC. Dynamic frequency sweep tests were between 0.1 and


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100 rad·s-1 (kept a strain γ=0.1%) at 25 oC, Oscillatory frequency sweep experiments were performed in the linear viscoelastic region to ensure that calculated parameters correspond to an intact network structure. Dynamic time sweep tests were first given a small strain γ= 0.1% for 100 s and then subjected to a strong strain γ= 200% for 200 s, following a small strain γ= 0.1% for its recovery. Then repeated twice to detect its reversibility, and frequency was kept in ω= 10 rad·s-1. Dynamic temperature sweep test was start at 25 oC in continuous step strain and frequencies (kept strain γ=0.1%, frequency ω=10 rad s-1) with a heating rate of 5 oC·min-1. Field emission scanning electron microscopy (FESEM). The gels were carefully scooped onto the conducting resin on the platinum stubs and were allowed to dry overnight in the air. The samples were further dried in vacuum for 8 h. Then 10 nm-thick platinum films were deposited on the gels. Finally, the morphology of the gels was investigated by using a Hitachi S-4700 FESEM operated at 15 kV. Transmission electron microscopy (TEM). For the gel, the material as a MMA sol was drop coated on amorphous carbon-coated Cu grids. In both cases, the concentration of the gelators were below the minimum gelator concentration, due to the resultant aggregates were too thick to get good-quality micrograph at higher concentrations. TEM images were recorded by using a JEM-1230 microscope at an accelerating voltage of 80 kV. FTIR analysis. FTIR spectra of the xerogel in MMA were collected on a Nicole 6700 spectrophotometer by loading the samples into a KBr cuvette. The sample was prepared by using MMA as a solvent. Wide angle X-ray diffraction. The X-ray diffraction patterns were obtained using a Shimadzu XRD-6000 diffractometer with a Ni filter and CuKα (l = 1.54056 Å, voltage = 40 kV, and


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current = 40 mA). Each sample was scanned over the range from 3o to 40o with a sweep speed 10o· min-1. Fluorescence Spectroscopy. Fluorescence spectra of peptide gel samples were recorded on a Hitachi Spectrofluorimeter (model 650-40). The fluorescence spectrum of peptide gel samples was analysed with pyrene as fluorescent probe. The fluorescence emission spectrum of pyrene, upon excitation at 335 nm, shows five vibrancy peaks. The polarity of the interior of peptide gels was studied by comparing the relative intensity of peak I to peak III of pyrene monomer fluorescence. RESULTS AND DISCUSSION Gelation ability and thermal stability The gelation ability of the hybrid supermolecule POSS-ASP was examined in various organic solvents (Table 1). Kamlet-Taft model was used in order to better understand the polarity effect of solvents on gelation38. It evidenced that POSS-ASP can form gels best at not only nonpolar or weak polar solvents where α parameter is zero, β parameter is low meanwhile π* parameter is high — i.e., toluene, benzene, chlorobenzene, styrene, but also in some strong polar solvents with α or β parameter is high, — i.e., THF (0.00, 0.55, 0.58, α, β, π*), dichloromethane (DCM) (0.44, 0.00, 0.73) et al. What’s more, most of these gels are highly transparent and thermally stable for a few months at room temperature (Figure S3). What a quite unnormal phenomenon is that the minimal gel concentration (MGC) of these gels are all below 20 mg/mL (2% w/v), especially in chlorobenzene, benzene and ethyl acetate are below 7.0 mg/mL (0.7% w/v), indicating high efficiency of gelating solvents for such POSS based dendrimers, further


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demonstrating the polarity of solvents would not affect the formation of hydrogen bonds or π-π stackings in POSS-ASP dendrimer self-assembly process. Table 1. Gelation behaviour of POSS-ASP in various solventsa Solvents n-hexane cyclohexane toluene benzene chlorobenzene xylene styrene methyl methacrylate ethyl acetate butyl acetate acetone THF chloroform dichloromethane methanol

α 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.08 0.00 0.44 0.30 0.93

β 0.00 0.00 0.11 0.10 0.07 0.03 N/A N/A 0.45 N/A 0.48 0.55 0.00 0.00 0.62

π* -0.08 0.00 0.54 0.59 0.71 0.80 N/A N/A 0.55 0.46 0.71 0.58 0.69 0.73 0.60

POSSASP P P TG(13.4) TG(6.7) TG(4.5) TG(10.5) TG(10.0) OG(10.0) OG(6.5) OG(14.0) S OG(20.0) S OG(20.0) S

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P = precipitate, TG = transparent gel, OG = Opaque gel, S = solution. α, β, π* are the Kamlet-Taft parameters relating to the ability of the solvent to donate and accept hydrogen bonds and a generalized polarity parameter, respectively, N/A = not available. The data in brackets represents the minimal gel concentration (MGC), mg/mL.

POSS-ASP, who has a similar topological molecule structure with POSS-Lys33 while shown a better solvent adaptability gelation ability in DCM and THF, which illustrates the huge influence on supramolecular self-assembly by Z- peripheral groups. Considering the good solubility between THF and water, the gelation ability has also been tested in THF/water hybrid solvents with keeping POSS-ASP in 4% w/v. As a poor solvent for POSS-ASP, the unabridged gel can be formed with volume of water below 40% v/v, however, only partial gel can be obtained after the volume fraction of water was more than 40% (Figure S4), which proving the balance between dissolution, precipitation and gelation of POSS-ASP would not be broken with


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a low water content. Such an aqueous organogel have huge potential applications in drug delivery and catalyst support.

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Figure 1. Phase diagrams of POSS-ASP gels in different solvents. Like the most traditional low molecular weight gels, as the concentration increased, the Tgel values of POSS-ASP increase until a plateau region is reached as Figure 1 shows, confirming the Tgel of POSS-ASP gel in 1,2-Dichlorobenzene (DCB) (108 oC) is the higher than in MMA (78 o

C) and THF (73 oC), demonstrating the ultra-strong thermal stability of POSS-ASP gels and the

boiling point of solvent or the self-assemble ability play a decisive role in thermal stability of gels. In addition, no viscous flow state can be observed when the temperature increased to the Tgel point, as in many low molecular weight gel systems, the sample-spanning gel networks are broken down to amounts of small aggregates which are dissolved in liquid phase at the Tgel point.39 Such a different phenomenon from the similar structure reported before33,34, seems like that the 3D network POSS-Asp formed cannot hold itself in a solid-like state at the Tgel points for having no cross-linked nanofibres.


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According to the gelation ability and thermal stability study, we can confirm that POSSASP is an efficient dendrimer gelator in polar, no-polar and aqueous solvent which has never seen in POSS based gelators, in addition, the impressive performances in thermal stability make POSS-ASP gels have more selections in potential applications. Mechanical property of supramolecular gels The mechanical properties of the gels are particularly important parameters for the actual applications, which are strongly depended by their supramolecular aggregations of gelators40. Rheological measurements are carried out to investigate the mechanical properties, self-recovery properties and thermal properties of these gels. The viscoelastic behaviour of a gel is determined by two key parameters, storage modulus (G') and loss modulus (G''), which represented the ability of a deformed gel to return to its original geometry and the tendency of a material to flow under exerted stress. The G' (elasticity) and G'' (viscosity) as a function of ω for gels within linear deformation range are illustrated. From Figure 2a, 2e it can be clearly observed that POSS-ASP gels have ultra-high G' and G'', and essentially followed a similar trend and good linearity at test frequency with the elastic character act as the dominant factor (G' > G''), the gels exhibited a typical solidlike character. In MMA, G', G'' value reach to 20kPa and 3kPa which is 10 times high than POSS-Lys gel, while in THF mixed solvent, the G', G'' value reach to 200kPa and 30kPa which is 100 times high than POSS-Lys gel with gelator in a concentration (4%) lower than most dendritic organogelators41,42, indicated a supra-strong gel material formed by POSS-ASP. The results show that the moduli curves of these of gels are scarcely independent of ω throughout the whole ω range evidenced the formation of supra-strong and stable gel network structure.


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Dynamic strain sweep data shows that the uppermost boundary of the linear viscoelastic region (LVR) of POSS-ASP MMA gel is about 1% strain, and could keep its elasticity (G' > G'') to an extremely large strain of 9 %, a value of which is a substantially declined than the loofahlike network structure as reported before33,34, and is the same as most of the traditional lowmolecular weight gels (Figure 2a, 2f). The γc of POSS-ASP MMA gels are sensitive to the concentration of gelators. The increasing of gelator’s concentration have obvious influence on G', G'' and γc, the G', G'' of MMA gels are increased from 1 kPa and 100 Pa (2%) to 20 kPa and 3 kPa (4%). While γc is greatly decreased from 9.0 % to 0.6 %, which means that the POSS-ASP self-assembled into more dense and stable nanofiber networks in solvent by increasing the concentration of gelators to enhance the mechanical strength, while weaken the ability to resistant the external stress25. The results show that although POSS-ASP MMA gels have a high strength and high rigidity while there have a certain degree of sacrifice in their toughness. As a complement, the dynamic strain sweep test also been carried out on POSS-ASP THF/water hybrid gels with 4%wt of gelator and a changed water content as Figure 2f shows. The POSSASP THF/water hybrid gels can keep its elasticity to an extremely large strain of 15.8 % and a rather high G' and G'' (300 KPa and 20 KPa), values of which are higher than DCB and MMA gels. What’s more the G' and G'' is increased from 20 KPa and 3 KPa to 300 KPa and 20 KPa with the water content increased from 0% to 30%, which due to the possible existence of different self-assembly ability in different polarity solvents. The self-recovery is known as one of the most amazing properties in nature; an ability of biological or artificial systems spontaneously repair their damage and restore their original state43,45. POSS-ASP gels exhibit very rapid recovery of its mechanical properties after a largeamplitude oscillatory break-down, known as thixotropic nature (Figure 2c). Under the applicati-


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Figure 2. Charts of rheological measurements of POSS-ASP gels. (a), (e) Frequency dependence of G' and G'' for MMA gels and THF/water gels respectively; (b), (f) Strain dependence of G' and G'' for MMA gels and THF/water gels respectively; (c) The G' and G'' dependence of time in continuous step strain measurements for MMA gels; (d) The G' and G'' dependence of temperature in continuous step strain and frequency measurements for DCB gels, gelators in 40 mg/mL. MMA gels have concentrations of gelators in 20 mg/mL and 40 mg/mL in (a) and (b) while 40 mg/mL in (c); THF/water gels keep a concentration of gelators in 40 mg/mL and have various water contents from 0% to 30%.


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on of a large-amplitude oscillatory force (strain, γ = 200%; frequency, ω = 6.28 rad/s) the G' value decreased from 20 KPa to 5 Pa (POSS-ASP = 4 %w/v), resulting in a quasi-liquid state (tan δ = 0.5; G''/G'< 3.0–4.0). However, when the amplitude decrease (γ = 0.1%) at the same frequency, G' recover to 14 KPa a value 70% of its initial value immediately at first circulation and the system returned to a quasi-solid state (tan δ = 0.2 < 0.4–0.5). The reduced of G' shows a certain degree of destroy of network structure under large-amplitude oscillatory force while retain the majority of the network structure to maintain the self-recovery of gel. The temperature sensitive property of POSS-ASP is also expressed on the rheological properties. The relationship between G', G'' and temperature of POSS-ASP DCB gel (POSS-ASP = 4 %w/v, consider the high boiling point and less volatile of DCB) is shown in Figure 2d. We can see when the temperature is below 78 oC, the G' and G'' of POSS-ASP gel is as high as 2 kPa and 1 kPa. But when the temperature is higher than 78 oC, the G' drop rapidly to 1 Pa and with a largest decline rate at 82 oC which a temperature same as Tgel measured by tube-inversion method, indicating the elasticity of the gel is totally lost. The whole rheological curve turns out to be a special L shape and shows non-gradually drops corresponding to POSS-Lys/DCB gel32, which forebode a quite different self-assembly structure in POSS-Asp gels. Unlike the loofah-like structure self-assembled by POSS-Lys who has a stronger ability to resist external stress and a more complex hierarchical self-assemble structure, the gel formed by POSS-ASP have high strength and good self-recovery while a poor ability to resist external stress and a simplex hierarchical self-assemble structure. The great differences of mechanical properties and thermal properties between POSS-ASP and POSS-Lys gels, demonstrate the huge influence of peripheral groups of dendrimers and the great difference in self-assembly ability.


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What’s more, the G' in THF system are higher than in MMA system which indicate a denser network formed in THF. Morphology of supramolecular gels The morphology of POSS-ASP xerogels obtained from different solvents were investigated by SEM, as shown in Figure 3, quite different self-assemble morphologies can be seen from different polarity solvents. What’s more, there has no loofah-like network structure with nophysical cross-linked, continuous and integral network can be seen as reported before33,34, this demonstrated that as a result of the transformation of organic ‘‘arms’’ of the POSS dendrimer, great changes have taken place in the gel networks architecture. The Figure 3A bring out a number of spherical particles with a diameter between 40-60 µm connected to each other as the morphology of POSS-ASP self-assemble in MMA. Interestingly, these spherical particles are composed of many folds (Figure 3B, 3C) which formed from many intertwined fibers with a diameter between 50-100 nm (Figure 3D). Particularly, from the POM images as shown in Figure 4, many obvious right-angled intersection extinction phenomena as spherocrystal with a 40-60 µm diameter can be seen. Such an amazing consistent result demonstrating these spherical particles are spherocrystals, further illustrating the gel formation is described as being a balance between amorphous aggregation and well-ordered crystallization.46,47 The morphology of the xerogel obtained from MMA has also been investigated by TEM as Figure 5 shows. The TEM images as shown in Figure 5A present a dynamic process of assembly and revealed that the organogels in MMA consisted of individual nanofibers which further aggregated together to form thick and spherical fiber bundles with diameters of 5 µm. These bundles further develop into 40-60 µm spherocrystal as the spherical nuclear, eventually, a stable nanostructure is formed. What more amazing is that the morphology obtained from THF (Figure 3E, 3F) have a


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quite different performance, POSS-ASP self-assembled into sparse and band-like ultra-long fibers with a wide of 50-200 nm. The morphology from THF/water mixed solvents with different water content have also been observed (Figure S5), band-like ultra-long fibers also can be found as the water added. The fibers obtained from THF/water mixed solvents still keep it’s wide in 50-200 nm, and the self-assembled morphology seems no obvious changes by increasing the water content which indicated that the water a poor solvent would not disturbed the accurate selfassembly of POSS-ASP in THF. From the Figure 3G, 3H, the morphology obtained from DCM have a similar performance like from MMA while there has no obvious phenomenon of crystallization. The results show two quite different self-assemble model of POSS-ASP in different solvents due to the polarity of solvents, and the existent of water can’t affect the selfassembly expression of POSS-ASP.

Figure 3. SEM images of xerogels obtained from different solvents. (A), (B), (C) (D) are obtained from MMA with different magnifications, the concentrations of the gelator is 10 mg/mL; (E), (F) are obtained from THF with different magnifications, the concentrations of the gelator is 30 mg/mL; (G), (H) are obtained from DCM with different magnifications, the concentrations of the gelator is 30 mg/mL.


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Figure 4. POM images of POSS-ASP gel obtained from MMA with different magnifications, the concentrations of the gelator is 30 mg/mL.

Figure 5. TEM images of POSS-ASP xerogel obtained from MMA with different magnifications, the concentrations of the gelator is 1 mg/mL. The possible mechanism of gel formation Hydrogen bonding is considered as one of the major driving forces for the supramolecular self-aggregation of gelators40, which always can be proved by Fourier transform infrared spectroscopy48 and temperature dependent 1H NMR49. Figure 6a shows the FTIR spectra of


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POSS-ASP powder and its MMA xerogel, illustrating the essential characteristic IR peaks for specific hydrogen bonds and clear changes in hydrogen bonds during the gelation. As shown in Figure 6a, the three main bands at 3310, 1670, and 1540 cm-1 in POSS-ASP corresponded to the non-hydrogen-bonded NH band and the amide I and II bands frequencies50, which shifted slightly to 3300, 1660, and 1530 cm-1 for the xerogel. The slightly shift to lower wave number in xerogel suggesting the presence of hydrogen bonding interactions between the carbonyl group and the amide NH group, further indicated that the hydrogen bonds play a key role in the formation of gels. Additionally, to get a quantitative result about the assembly of gelator molecules via hydrogen bonding interactions, a systematic temperature-dependent

1

H NMR study was

performed by using the solution of POSS-ASP in DMSO-d6. 1H NMR measurements of these compounds indicate that the amide protons are sensitive to changes in temperature. At 25 oC two amide protons of POSS-ASP appeared at δ = 7.93 and δ = 7.53 ppm, respectively. With the gradual increase in temperature, these two protons experienced a notably upfield shifts and at 80oC the two amide protons appeared at δ = 7.61 and δ = 7.10 ppm, respectively (Figure 6b). These results indicated that in the gel state at room temperature intermolecular N–H…O hydrogen bonding was present between the neighbouring amide groups. The observed upfield shift of amide protons with an increase in temperature is due to the disruption of intermolecular hydrogen bonds between the amide functionalities leading to the phase transition from gel to sol.49


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(a) Transmittance (%)

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4000

POSS-ASP xerogel 3300

1530 1660 POSS-ASP

3310

3500

3000

1670 1540

2500

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Wave (nm)

Figure 6. (a) FT-IR spectra of the xerogel obtained from MMA and pure POSS-ASP solid, respectively. (b) 1H NMR spectra of POSS-ASP in DMSO-d6 with varying temperature, the concentration of hybrid gelator was 0.5 % w/v (2.5 mg, 0.5 mL). From the above results from FT-IR and 1H NMR, it is an overwhelming evidence that both the amide protons from each arm of the POSS-ASP molecules were involved in the formation of N–H…O hydrogen bond which is considered as one of the most significant driving forces for the


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supramolecular aggregation of POSS-ASP molecule to immobilize the large volume of solvents to form a gel. Aromatic groups of gelator molecules51 always provide a π-π staking interactions, which play a key role in the formation of gels. In order to confirm π-π staking interactions between the molecules, fluorescence emission spectra of POSS-ASP in the solution phase was obtained (Figure S6). The observed changes in the I1/I3 value demonstrated the formation of π-π staking34. Apart from the hydrogen bonding among the carboxylic acid/amide units, other driving forces for the formation of fibers can be π-π stacking interactions. The composite driving force of POSS-ASP provides the linkages required for forming the extended network structures during gel formation. Other possible intermolecular interactions between POSS-ASP molecules was investigated by 2D nuclear overhauser effect spectroscopy (NOESY)52,53 with POSS-ASP (1.5%, w/v) in DMSO-d6 and also in the presence of 50% CDCl3 as shown in Figure S7. As expected, no offdiagonal cross-peak was recorded for the non-gelated state of POSS-ASP in DMSO. However, in 50% CDCl3, several off-diagonal cross-peaks can be observed between protons of the amino and methylene protons close to the POSS core as well as the methine proton between the chiral protons of the amino. In addition, other off-diagonal cross-peaks described the obvious interaction between methylene protons close to the POSS core. This suggests that a plane-toplane stacking interaction between the protons of two adjacent molecules also plays a crucial role in self-assembled gelation.


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Figure 7. XRD pattern of the POSS-ASP MMA xerogel and THF xreogel. Concentration of MMA gel is 30 mg/mL and THF gel is 40 mg/mL. In order to understand the molecular arrangement of the gels from different solvents, wide angle X-ray diffraction (XRD) measurements were performed. As shown in Figure 7, the XRD patterns of the xerogels prepared from MMA and THF show very different series of reflections, which shows a different molecular arrangement in MMA and THF. Based on Brag’s equation, the obtained d-spacing of 4.9 Å at 2θ = 18.0 o from MMA and 4.3 Å at 2θ = 20.6o from THF, which suggested the existence of π-π stacking between aromatic group54,55. In MMA series, the three major diffraction peaks in the low angle region with the d-spacings of 25.2, 15.8, 9.1 Å corresponding to 2θ = 3.5o, 5.6o, 9.7o, respectively, form a quit close to the ratio of 1:1/√3:1/√7 reasonably indexed as (100), (110), (210) diffractions, a feature that strongly suggests a hexagonal columnar structure56. In the wide-angle region, the broad peak with d-spacings at 4.9 Å (2θ = 18.0o) and 4.6 Å (2θ = 19.5o) are presumably attributed to crystal packing of spherical segments within aromatic groups57. Therefore, the obtained d-spacing at 25.2 Å (2θ = 3.5o) is supposed to form hexagonal columnar structure with column diameter of 29.1 Å (Figure S8). Considering the molecular dimension of POSS-ASP (30.5 Å in fully extended length), it is most


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likely that the single column is formed by stacking POSS-ASP with a plane-to-plane model with cubic POSS cages fixed in the middle of column with arms stretch outward which the POSS core distance in 0.46 nm corresponding to the peak at 19.5o (Figure S8, Scheme 2). While in THF series, the major spike diffraction peaks with 2θ values of 3.9, 5.6, 8.0, 8.9, 12.2, 16.9, 20.6 and 23° respectively, and these values are corresponding to d-spacing (as d1-d8) of 22.6, 15.8, 11.0, 9.9, 7.3, 5.2, 4.3 and 3.9 Å. The d-spacing of d1 to d6 has a certain ratio of d1:d2:d3:d4:d5:d6=1:1/√2:1/2:1/√5:1/√10:1/4:1/√20, a feature that strongly suggests a columnar square structure58 with column diameter of 31.9 Å. Considering the molecular dimension of POSS-ASP, it is also likely that the single column is formed by stacking POSS-ASP with a plane-to-plane model with cubic POSS cages fixed in the middle of column with arms stretch outward which the POSS core distance in 0.39 nm corresponding to the peak at 23o (Figure S8, Scheme 2). From the above, these results of XRD we can confirm that POSS-ASP form a hexagonal columnar structure in MMA, while in form a columnar square structure THF. The molecular stacking model in THF/Water mixed solvents were also been carried out by XRD and the results showed no obvious difference with THF gel (Figure S9). Consequently, the obtained XRD results support the morphology self-assembled from THF/water mixed solvents seems no obvious changes by increases with the water content. As previously reported, the aggregation of gelator molecules builds the primary structure, further assembly to form fibrous objects such as rods, tubes, or sheets as the secondary structure.59 On the basis of the above findings and discussions, we wish to propose a model for the self-organization of POSS-ASP in MMA and THF. The formation process of gels simulated as Scheme 2 shows, colour pink representing the cubic POSS core and the colour blue representing the aspartate derivative arms as shown in Figure S10. Presumably, in the initial


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time, POSS-ASP promotes a unidirectional stacking of the building blocks because this can lead to a more favourable alignment of hydrogen bonds.16 Moreover, the specific cubic core, terminal π-π staking interactions and multiple hydrogen bonds of POSS-ASP would endow the gelator a unique and strong plane-to-plane stacking to from single columns as the primary structure. In MMA, single columns aggregate to form a hexagonal columnar structure so as to achieve the thermodynamic stable state to form single nanofibers with a diameter of 20 nm. At the same time, the individual nanofibers aggregate together to form thick and spherical fiber bundles which considered as the crystal nucleus of the gel. After the crystallization process, a number of spherical particles with a diameter between 40-60 µm connected to each other as the secondary structure to entrapment of the flowing solvents and form a MMA gel in the end. In THF, things become different, single columns aggregate to form a columnar square structure so as to from the band-like and ultra-long fibers. At last the network structures knitted by band-like fibers entrap the flowing solvents to form a THF gel. As a newly-developed POSS core based organic/inorganic hybrid gelator who has a same topological structure and quite different end peripherals as we report before33,34, have the quite different gel performance in rheological properties, self-assembly network architecture and selfassembly mechanism. As expected, the introduction of the peripheral aromatic groups units can bring π-π stacking to promote the supramolecular self-assembly in polar and nonpolar organic which caused the two quite different self-assembly mechanism in MMA and THF. Though the stacking model of POSS-ASP is same as POSS-Lys in MMA, the self-assembly morphology is quite different with no loofah-like network structure with no-physical cross-linked, continuous and integral network can be obtained as a result of the replacement of the arm structure which bring complicated multiple hydrogen bonding and π-π stacking directly. These results indicate


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that the functionalization of POSS based dendrimer gelator by peripheral aromatic groups promote the diversification of supramolecular self-assembly in different solvents.

Scheme 2. Proposed mechanism of POSS-ASP self-assembled in MMA and THF.

CONCLUSIONS A novel kind of POSS based dendritic supramolecular gelator with Z- protected peripheral groups POSS-ASP have been successfully synthesized and the gelation ability, morphology, rheological property, and the possible mechanism of gel formation have been investigated and presented. The peripheral aromatic groups units can promote the supramolecular self-assembly of this gelator in polar and nonpolar organic solvents via π-π stacking, multiple hydrogen bonding and van der Waals interactions and present a strong enough mechanical performance (G'=200kPa). What's more, for the system in MMA, the formation of organogel undergoes a


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process of supramolecules to form hexagonal columnar structure with column diameter of 30.0 Å. While in THF systems, the formation of organogel undergoes a process of supramolecules to form square columnar structure, this is why two quite different morphologies can be observed in different polarity solvents. We hope that such a study reports a new gelation system in organic, which could be helpful for investigating the relationship of dendrimer supramolecular gelation and different polarity solvents during the supramolecular self-assembly process of gelators. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxx. The synthesis scheme for POSS-ASP (Scheme S1), 1H NMR spectra of POSS-ASP in DMSO-d6 (Figure S1), Matrix-Assisted Laser Desorption/ Ionization Time of Flight Mass Spectrometry (MALDI-TOF MS) and Time of Flight Mass Spectrometry (ESI-TOF MS) of POSS-ASP in methanol (Figure S2), Photograph of POSS-ASP gels in different solvents (Figure S3), Digital photos of POSS-ASP gels from THF/Water mixed solvent with 0-40% water contained (Figure S4), FE-SEM images of POSS-ASP xerogels from THF/Water mixed solvent (Figure S5), Fluorescence emission spectra of POSS-ASP (Figure S6), 2D NOESY spectra of POSS-ASP (1.5% w/v) in DMSO-d6 and with 50% CDCl3 (Figure S7), Value statistics of the XRD and packing models of POSS-ASP MMA xerogel and THF xreogel (Figure S8), XRD pattern of the POSS-ASP xerogel from THF/Water mixed solvent (Figure S9), Molecular model of POSS-ASP (Figure S10).


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AUTHOR INFORMATION Corresponding Author *Email:[email protected], [email protected]. Phone: 86-0571-88320855, Fax: 86-0571-88320855. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no compering financial interest. ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (Grant No: 51773180), Zhejiang Provincial Natural Science Foundation of China (Grant No.LY14E030003, LY14E030004) and China Scholarship Council (CSC) (No. 201708330289) are gratefully acknowledged. REFERENCES (1) Okesola, B. O.; Smith, D. K.; Applying low-molecular weight supramolecular gelators in an environmental setting–self-assembled gels as smart materials for pollutant removal. Chem. Soc. Rev., 2016, 45, 4226-4251. (2) Piepenbrock, M. O. M.; Lloyd, G. O.; Clarke, N.; Steed, J. W. Metal-and anion-binding supramolecular gels. Chem. Rev., 2009, 110, 1960-2004. (3) Amabilino, D. B.; Smith, D. K.; Steed, J. W. Supramolecular materials. Chem. Soc. Rev., 2017, 46, 2404-2420. (4) Chen, S.; Luo, X.; He, H.; Tong, X.; Wu, B.; Ma, M.; Wang, X. Stretchable light scattering display based on super strong liquid crystalline physical gels with special loofah-like 3D gel networks. J. Mater. Chem. C., 2015, 3, 12026-12031.


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(59) 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, 2579725802.


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Table of Contents Graphic (TOC)

In this paper, aromatic groups are introduced into the end peripherals of polyhedral oligomeric silsesquioxane (POSS) core based organic/inorganic hybrid supramolecules to get novel dendrimer gelator (POSS-ASP), which have eight aspartate derivative arms to make full use of mighty π-π stacking forces in order to get strong supramolecular gels addition to multiple hydrogen bindings and van der Waals interactions. While in the gel system, two totally different morphology of connected spherical particles structures and banded ultra-long fibres with different thermal stability, rheological properties expecting the existing various self-assemble model and illustrating the peripherals of dendrimer and polarity of solvents having huge influences in supramolecular self-assembly mechanism. Such a study reports a new POSS based dendrimer gelation system in organic or organic/aqueous mixed solvents, which can be helpful


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for investigating the relationship of dendritic supramolecular gelation and different polarity solvents during the supramolecular self-assembly process of gelators.


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Self-Assembly of a Strong Polyhedral Oligomeric Silsesquioxane Core

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