Article pubs.acs.org/Langmuir

Small Angle Neutron Scattering (SANS) Studies on the Structural Evolution of Pyromellitamide Self-Assembled Gels Scott A. Jamieson,† Katie W. K. Tong,† William A. Hamilton,‡,§ Lilin He,∥ Michael James,†,‡,⊥ and Pall Thordarson*,† †

School of Chemistry and the Australian Centre for Nanomedicine, The University of New South Wales, Sydney, NSW 2052 Australia ‡ Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia § Instrument and Source Design Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Biology and Soft Matter Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ⊥ Australian Synchrotron, 800 Blackburn Road, Clayton 3168, Australia ABSTRACT: The kinetics of aggregation of two pyromellitamide gelators, tetrabutyl- (C4) and tetrahexyl-pyromellitamide (C6), in deuterated cyclohexane has been investigated by small angle neutron scattering (SANS) for up to 6 days. The purpose of this study was to improve our understanding of how self-assembled gels are formed. Short-term (< 3 h) time scales revealed multiple phases with the data for the tetrabutylpyromellitamide C4, indicating one-dimensional stacking and aggregation corresponding to a multifiber braided cluster arrangement that is about 35 Å in diameter. The corresponding tetrahexylpyromellitamide C6 data suggest that the C6 also forms one-dimensional stacks but that these aggregate to a thicker multifiber braided cluster that has a diameter of about 62 Å. Over a longer period of time, the radius, persistence length, and contour length all continue to increase in 6 days after cooling. These data suggest that structural changes in self-assembled gels occur over a period exceeding several days and that fairly subtle changes in the structure (e.g., tail-length) can influence the packing of molecules in self-assembled gels on the singleto-few fiber bundle stage.

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INTRODUCTION The development of self-assembled gels is a burgeoning field due to their unique properties and similarities to materials found in cells and the human body.1,2 Their applications in medicine and biomaterials are extensive, from targeted drug delivery3 to artificial muscle fibers4 and even as a scaffold for the regeneration of new spinal cords.5 The design and synthesis of organic-molecule-based self-assembling gels has advanced rapidly over the last two decades, and we are beginning to see the incorporation of more than simply structural motifs in the gel molecule architecture.6 This increased development has been facilitated by improved knowledge of the types of molecules and systems that are able to form gels; however, as van Esch pointed out, “we can design molecular gelators, but do we understand them?”7 As it stands, there is no complete model for the physical process by which low-molecular weight organic gelator (LMOG) molecules self-assemble into a fully-formed noncovalent gel fiber matrix. This is not limited to our own gel system, but to the majority of self-assembled gels, where previous theories for their formation were based on polymer models for hierarchical8,9 or co-operative assembly.10 However, organic gelators are distinctly different to polymer gels in their © XXXX American Chemical Society

physical and thermodynamic properties, and new methods may be required to study and or characterize these systems.11 We recently reported a new family of molecules, the pyromellitamides that form organogels in nonpolar solvents such as cyclohexane at around 1% w/w.12−14 Previously, we demonstrated that intermolecular hydrogen bonds and π−π interactions cause these molecules to aggregate to form onedimensional fibers. In the case of tetrahexylpyromellitamide C6, these fibers then form higher-order hierarchical assemblies including twisted helical fibers and eventually a threedimensional network resulting in gel formation.13 Interestingly, viscosity studies showed that when dilute samples (0.1%) of C6 were dissolved and then allowed to cool to ambient temperature, the viscosity of the resulting solution continued to increase exponentially over a 2 day period after the viscous solution of C6 was formed. These results suggest that the kinetics of gelation may be quite complex and play a crucial role in determining the final properties of the self-assembled gels formed. Escuder and coReceived: June 28, 2014 Revised: October 24, 2014

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chain. Previous NMR studies have suggested that the side chains remain mobile at lowered and elevated temperatures.27

workers have also shown that when forming self-assembled organogels, the cooling rate can drastically influence the initial structures formed,15 where gels formed by rapid cooling (kinetically trapped) revert slowly to structures that are identical to gels formed by slow controlled cooling (thermostable formation). Neutron scattering spectra of gel samples are able to provide information on the behavior in terms of kinetics of the structures comprising a gel network and can be used to resolve the individual fibers within a system.6,16−18 Comparison of SANS spectra at multiple time points can also allow for identification of structural changes which may be the result of phase transitions or long-term formation of hierarchical structures.19−21 In other areas of gel research, neutron scattering has provided structural information on the packing arrangement in the fiber network. SANS profiles of a fluorenylmethyl chloroformate (Fmoc) peptide gel from Chen and co-workers indicated that the structure of fibers in the gel−solvent matrix was tunable depending on the concentration of dimethyl sulfoxide present in the system, and this related back to complementary rheology measurements.22 Horkay and coworkers found that multiple models could be fitted to the same spectra to elucidate details of the clustering of the individual fibers together in solution.23 From complex modeling, Hule and coworkers revealed the orientation and compactness of hairpinshape fibers in a peptide-based hydrogel system.24 The stacking arrangement of molecules within the individual fibers and how these fibers then pack are both important for understanding self-assembled gels. Scattering methods such as small-angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) can be used to define the packing arrangements in one-dimensional gel fibers. Since the pioneering work of Terech and coworkers with cholesteryl gels in organic solvents,17,18 there have been a number of SANS studies on self-assembled gels;17,21,25,26 however, SANS has until now not been used in long-term kinetic studies on the kinetic evolution of self-assembled gels. Here we report our small-angle neutron scattering studies on the time-dependent structural evolution of gels formed by tetrahexyl- (C6) and tetrabutyl-pyromellitamide (C4) shown in Chart 1. These gels have been previously identified as a model

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EXPERIMENTAL SECTION

Reagents and Materials. Tetrahexylpyromellitamide C613 and pyromellitamide chloride28 were synthesized according to literature reports. n-Butylamine (Sigma-Aldrich) was used as received without further purification. Pyridine (Ajax Finechem) was purified by distillation and dried over potassium hydroxide. Anhydrous, dichloromethane, and tetrahydrofuran were dried and deoxygenated using a Pure Solv PS-MD-7 (Innovative Technology Inc.) solvent purification system. Acetone (Ajax Finechem) and methanol (Univar) were used as received without further purification. Deuterated solvents for NMR and SANS were purchased from Sigma-Aldrich and Cambridge Isotope Laboratories and were used as received. Tetrabutylpyromellitamide C4. A solution of n-butylamine (1.39 g, 18.9 mmol) in dry dichloromethane (150 mL) and dry pyridine (3.0 mL) was stirred at room temperature for 20 min under nitrogen. To this mixture was added a solution of pyromellitoyl chloride28 (1.24 g, 3.78 mmol) in dry tetrahydrofuran (20 mL) dropwise at 0 °C for 1 h. The reaction mixture was stirred for 24 h at room temperature under nitrogen, and the solvent was then evaporated in vacuo to give the crude product as a yellow gel-like material. This crude product was washed with acetone and methanol, filtered, and dried in vacuo to give tetrabutylpyromellitamide C4 (1.08 g, 60%) as a white gel-like paste; mp >250 °C (decomposes); 1H NMR (300 MHz, (CD3)2SO): δ 0.90 (t,12H, J = 7.1 Hz), 1.28−1.40 (m, 8H), 1.43−1.52 (m, 8H), 3.15−3.21 (m, 8H), 7.48 (s, 2H), 8.28 (t, 4H, J = 5.7); 13C NMR data could not be obtained due to poor solubility of C4; FT-IR (ATR: ν = 3230, 3068, 2956, 2929, 2871, 2862, 1629, 1603, 1562, 1463 cm−1; MS (ESI): m/z 473.35 ([M − H]− requires 473.31). HR-ESMS (m/z): m/z 509.2826 ([M + Cl]−, C26H42N4O4Cl requires 509.2895). Anal. Calcd for C26H42N4O4Cl: C, 65.79; H, 8.92; N, 11.80. Found: C, 65.72; H, 8.52; N, 11.52. Small Angle Neutron Scattering (SANS) Measurements and Sample Preparation. Two sets of SANS experiments were completed as part of this study. One was completed at the Bragg Institute (ANSTO, Sydney, Australia) using the QUOKKA beamline for the long-term gel evolution, and the other was completed at the High Flux Isotope Reactor (ORNL, Oak Ridge National Laboratories, TN, U.S.A.) using the GP-SANS beamline for the short term kinetics. Samples consisted of 1.0% w/w of gelator in d12-cyclohexane, heated to 65 °C for 1 h in sealed glass vials, before pouring a heated, fully-solvated gel solution into 1 mm path-length quartz cells. For the measurements on QUOKKA, the neutron wavelength (λ) was 4.94 Å with a wavelength resolution (Δλ/λ) of 6% fwhm (the full width at half-maximum of the wavelength distribution; this distribution contributes to the Q resolution in 1D data). Two-dimensional SANS spectra were collected at distances of 2, 8, and 20 m providing an overall Q-range of 0.04 to 0.4 Å−1, where the Q is the scattering variable defined by the neutron wavelength (λ) and the scattering angle (θ) in the equation Q = 4πsin(θ/2)/ λ, or approximately 2πθ/λ at the small angles in our measurements. Data reduction was performed using the NIST reduction macros including QUOKKA specific extensions.29 The empty cell background was subtracted from the spectra after the data reduction process. All experiments were conducted at 22 °C. For the measurements on GP-SANS, the neutron wavelength (λ) was 6.0 Å with a wavelength resolution (Δλ/λ) of 13%. Twodimensional SANS spectra were collected at distances of 1 m (beam trap 101 mm) and 18.5 m (beam trap 76 mm) providing an overall Qrange of 0.003 to 0.54 Å−1. Data reductions were performed using the NIST reduction macros including HFIR specific extensions.30 The empty cell background was subtracted from the spectra after the data reduction process. The instrument was fitted with a heating stage allowing for a sample temperature between 25−65 °C. The time-resolution for measurements taken for short-term kinetics measurements in the high-Q region (short detector lengths) for was approximately 10 min and for the additional range in the long term

Chart 1. Pyromellitamides C4 (MW = 474.65 g/mol) and C6 (MW = 586.86 g/mol)

gel system, where the pyromellitic core of the gelator exhibits hydrogen and π−π bonding between molecules in a proposed one-dimensional stacking motif.13 The short alkyl carboxamide side chains extend around the core and allow for gelation in nonpolar solvents such as cyclohexane. Partly due to solubility issues, the focus of this work was on the C6 gelator, but when possible, the properties of C4 were also investigated in comparison to the C6 to gain insight into the role of the side B

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regression in Igor. For the data sets that we fitted also to eq 2, a simple linear regression method in the Matlab program was used to the fit the data to the usual Guinier linearized version of eq 2.

measurements in the low-Q region (long detector lengths) the resolution was approximately 30 min. Samples were loaded into the detector between measurements to ensure they were all measured in close time approximation as possible. Data Fitting. Fitting for all SANS spectra was in most cases completed using a model developed by Pedersen and Schurtenberger specifically for self-assembling worm-like fibers built up of segments and existing in a network29 They employed discrete Monte Carlo simulations to empirically parametrize the classic worm-like chain model of Kratky and Porod.31 As later modified by Chen,32 this model is implemented as “Monodisperse Flexible Cylinder” in the Igor software program as part of the NIST NCNR SANS Analysis package33 in a version which allows for excluded volume effects (corresponding to Pedersen and Schurtenberger’s “Method 3”). The structural scattering function generated for this model is

S(Q , L , Lp) =

a1(Q , L , Lp) 4.95

(0.5QLp)

+

a 2(Q , L , Lp) (0.5QLp)5.29

+

π QL

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RESULTS AND DISCUSSION Synthesis and Gel Formation. The synthesis of tetrahexyl pyromellitamide C6 has been reported previously.13 The tetrabutyl pyromellitamide C4 was synthesized using a similar approach to that used for C6 by reacting n-butylamine with pyromellitoyl chloride28 in a mixture of dichloromethane and pyridine to yield C4 in 60% yield. Although C6 is soluble in a range of organic solvents, C4 is only soluble in cyclohexane, toluene, chloroform, and dimethyl sulfoxide and unlike C6, C4 does not dissolve or form gels in hexane and diethyl ether. Both C6 and C4 readily form gels in cyclohexane and toluene. The minimum concentration required for both compounds is around 11−12 mM (≈ 0.5−0.7% w/v) in cyclohexane, whereas in toluene, the minimum concentration required for C4 is 22 mM (≈ 1% w/v) vs 124 mM (≈ 7% w/v) for C6. The gels are transparent, but in more polar solvents, such as propanol and butanol, partially opaque gels are formed. Short-Term Kinetics in High-Q Region. To probe the structural changes in the gel/fiber network during the first hour after gelation, SANS spectra of C6 were recorded over the first few hours transitioning from a hot solution above TG (330 K) to room temperature (298 K). Although it was initially assumed that the sample had reached TG, the spectra (Figure 1)

(1)

representing the scattering function of randomly oriented semiflexible chains within a network. Given that the fiber network is composed of stacked molecules, the model determines the statistical scattering segment of the fiber as Lp, its persistence length, the length along the flexible cylinder over which it can be approximated as a rigid rod (The Kuhn length for this model would be twice this persistence length). The total contour length of the stacked molecules within a strand is represented by the parameter L. Although it must be noted that this is a minimum value as junction zones can interfere with the determination of this parameter. For both a1 and a2 (rather complex functions of Q, L, Lp), exact mathematical expressions may be found in Chen’s Supporting Information.32 This approach incorporates scattering from segments of the network (within a1 and a2) using the standard modified Guinier formula for extended rod-like objects:34,35

QI(Q ) = K exp(− Q 2R G , CS 2/2)

(2)

where I is the coherent scattering intensity and RG,CS the objects’ crosssectional radius of gyration, the weighted mean of the scattering length density contrast. The radius of a cylindrical segment of constant scattering length density will be related to the radius of gyration by

R CS = √2R G , CS

(3)

while K (scattering power per unit length of the segment) is given by K = cπ

N L

(bm − Vmρs )2 W

Figure 1. SANS profiles (I(Q) vs Q on a log−log scale) for C6 gel (1% w/w in d12-cyclohexane), monitored over the first 3 h of gelation. The arrows indicate changes in scattering intensity from t = 0 to t = 3 h.

(4)

where c is the gelator concentration, ⟨N/L⟩w is the weight-average aggregation number per unit segment length, and the scattering contrast of the monomer is given by bm, the sum of the neutron scattering length in each monomer, subtracting Vmρs, the product of the volume of the monomer, and ρs, the scattering length density of the (displaced) solvent. The fit according to the full model also allows for a constant incoherent scattering background, but it is important to note that it does not incorporate any peaks in the high-Q regions of our data as structures at these shorter length scales are beyond the scope of this large scale structure model.32 We note here that the Chen32 version of eq 2 omits the division by 2 inside the exponential. As a consistency check and to verify that this error was not reflected in the full model fitting code we also fitted some of the data sets to the modified Guinier formula, over an appropriate intermediate Q-range, after subtracting a flat incoherent scattering background determined at high Q. We used the full model to obtain fits to the long-term scattering profiles, at time points of 4 h, 1 day, 3 days, and 6 days after gelation. As described, the model returns three structural parameters: the radius of the one-dimensional fibers (assuming a cylindrical cross-section), as well as the persistence length and contour length (defined as the maximum end-to-end distance of the individual fibers). In each scattering profile, the best fit was reached by nonlinear least-squares

suggested that cooling had already taken place before the first measurement. Further investigation of the experimental setup revealed that the temperature of the sample window was not exceeding TG; however, the sample had been transferred from an oven directly to the stage, so only the first few minutes would have been lost. Given the start-up and acquisition time for these types of measurements, when the sample was loaded into the instrument or delays in moving from detector length to another, it was difficult to examine in detail the first few minutes during which the molecular self-assembly process (one-dimensional stacking) occurs. During the first hour, the spectra for the C6 gel (Figure. 1) showed increasing intensity of a correlation peak at 13.13 Å (calculated from Q value at peak maximum). After the first hour, the peak remained at the same intensity for the next few hours. A decrease in the solution intensity (main slope) over time indicates that separate phases that are present in the hot solution, but after 2 h, only one phase remains. Between 2 and C

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3 h, the spectra do not show significant change, confirming that multiple phases are only present in the first 2 h.36 Although the C6 showed a distinct correlation peak in the high-q region, the C4 gel did not exhibit the same feature within the limits of our measurement (Figure 2), as the correlation peak would be expected to appear at a larger value of Q corresponding to a smaller d-spacing (as C4 dimensions are smaller than C6).

Figure 2. SANS profiles (I(Q) vs Q on a log−log scale) at 3 h of aging time (1% w/w in cyclohexane-d12) for C4 gel and C6 gel. The curves have been offset by a factor of 10−n, where n = 0 and 1 for the C4 gel C6, respectively. Fits to the full flexible cylinder model are also shown.

Figure 3. SANS profiles of C6 gel over 6 days of aging (1% w/w in d12-cyclohexane). (a) Showing [I(Q)] vs Q (on log scale) together with the model fits according to the full model and the resulting calculated fiber measurements and quality of fits are shown in Table 1. The curves have been offset by a factor of 10−n, where n = 0, 1, 2, and 3 for the 6 day, 3 day, 1 day, and 4 h data, respectively. (b) On a fiberlike modified Guinier plot showing ln[Q*(I(Q) − Ibckgr)] (with Ibckgr = incoherent background values shown with the marker labels) vs Q2 together with the fitted curves according to eq 2 in the intermediate to high Q region (Q2 between 0.005 and 0.03 Å2). The curves have been offset by a factor of A = 0, −0.5, −1 and −1.5, for the 6 day, 3 day, 1 day and 4 h data, respectively.

Long Term Structural Evolution in the Low-Q Region. To examine long-term growth and changes in the structure, the protonated C6 gel was monitored over the course of a week (Figure 3). Over 6 days, the gel fibers undergo significant structural changes, observable in three properties returned by the full model (Table 1). The initial cross-sectional radius RCS is 9.56 Å, which extends to 9.97 Å after 3 days before dipping slightly at 6 days to 9.87 Å. These values are in good agreement with molecular modeling for the C6 molecule, which gave dimensions for the extended molecule at 20.4 Å in length and 11.9 Å in width. The increase in radius over time may be due to the side-chains in the molecule effectively stretching out to reach a more energetically favored position and may affect other parts of the fiber structure.37 It is noteworthy that the data in Table 1 shows an excellent agreement between the comprehensive nonlinear full model and the modified Guinier plot (eq 2) in terms of the radius of the fiber. While there is a significant calculated error range for the contour length based on eq 1, the data still suggest a significantly large size of each one-dimensional fiber. For the measurements at 4 h and 1 day after gelation, the contour lengths are 125 and 59 nm, respectively. The variation in contour length (L) at subsequent time points may simply be due to the fibers becoming so long (several, even tens of μm) and polydisperse that the resulting rod-like Guinier plot would stretch to very low Q our data. The robustness of the nonlinear full model fitting based on eq 1 (Figure 3a), as further verified by its agreement with the modified Guinier plot (Figure 3b) in terms of the fiber radius suggests that although L gives unrealistic values for the longer time scales, the calculated persistence length (LP) may still be reliable, given also the fairly modest uncertainties on LP (< ± 17%). The calculation of L in eq 1 depends on the size of the other variables in the equation relating to the physical properties of the fibers, including persistence length (LP). For

small values of LP, the model is dependent on the L variable, but at larger sizes, the model is not reliant on L, allowing for a large range of possible L values. For the persistence length at each time point, eq 1 holds that the values of LP are for individual fibers as opposed to the bundles. Persistence Length. The persistence length (Lp) of polymer and self-assembling gel chains describes the flexibility or curvature of the fibers, below which the fiber is considered rigid.36 In this system, the persistence length is increasing at each time point over 6 days (Figure 3), going from 83 nm up to 411 nm; effectively the fibers undergo a 4-fold increase in fiber segment size (decrease in overall curvature). The decreasing flexibility or curvature of the one-dimensional fibers in the C6 system, as described by the changes in persistence length of the fibers over time (Figure 4), leads to different confirmations as the gel ages, which may be affecting the larger system, as the larger fiber network can also respond to these changes. This leads to morphological changes in the three-dimensional fiber network, where the larger fiber bundles separate into smaller, worm-like fibers, as the growth in persistence length leads to less flexibility or curvature and thus the individual fibers may not allow for that shape of fiber bundle.24 It is also worth noting the gradual but significant increase in the incoherent background scattering (Ibckgr) over time (Figure 3b) from Ibckgr = 0.043 cm−1 and 0.076 cm−1 at 4 h and 1 day, D

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Table 1. Calculated Fiber Measurements from Pedersen and Schurtenberger Model Fitting (eq 1) and the Modified Guinier Formula eq 2 for Long-Term Measurements on C6a aging time 4 1 3 6

h day days days

radius (Å) from fitting to: full model

b

9.56±0.08 9.62±0.05 9.97±0.06 9.87±0.06

persistence c

eq 2

length (Å)

9.84±0.22 9.30±0.40 10.23±0.25 9.84±0.49

83±5 157±30 327±36 411±70

d

reduced √χ2

contour length (Å)

(for fit)f

e

1.25±0.08 × 10 5.9±1.2 × 102 (4±9 × 103)g (6+21 × 106)g

3

1.94 1.31 0.99 1.19

The uncertainties shown are asymptotic errors38 from the SANS data fitting process. bRadius calculated as RG,CS according to eq 3 from full model fitting. cRadius calculated as RG,CS according to eq 3 from the fit to the modified Guinier eq 2. dPersistence length (LP) is half of the Kuhn length (b) from eq 1. eCalculated as L in eq 1. fQuality of fit for the full model fit. A reduced chi-squared (√χ2) around 1 signifies that the data fits within one standard deviation of each data point. The linear fits to the modified Guinier eq 2 (Figure 3) all had a coefficient of determination (R2) between 0.9 (6 day data) and 0.98 (4 h data). gDecreased dependence of L in eq 1 leads to a range of possible values for L to exceed 100% estimated uncertainty for the entries shown in brackets. a

Figure 4. Growth of persistence length (LP) of C6 fibers over 6 days (1% w/w in d12-cyclohexane) from full model fitting.

Figure 5. SANS profile (I(Q) vs Q on a log-log scale) of the scattering regions in C6 at 3 h aging time (1% w/w in d12-cyclohexane) with full model fits applied over limited low Q region (reduced √χ2 for fit =0.49) and higher Q-range (reduced √χ2 for fit = 1.46). See text for details.

respectively, to Ibckgr = 0.103 cm−1 and 0.101 cm−1 after 3 and 6 days, respectively. This increase in Ibckgr would usually indicate that the density of disordered hydrogenated material in the SANS beam is increasing as the gel ages. The stiffening of the gel fibers, as suggested by the increase in persistence length with time, could expel some of the fiber-bound deuterated solvent from the neutron beam, which would then explain the increase in the incoherent background scattering (Ibckgr). Fiber Arrangement. The clustering of fibers leading to bundles in the gel can be identified in the low-Q-region of the spectra, with the Pedersen and Schurtenberger model fitting selectively applied. For the C6 gel at 3 h after gelation, the lowQ range modeling (Figure 5) showed a clustering feature with a radius of 30.9 Å (diameter of ≈62 Å) and contour length of 228 nm. This cluster size can be compared against the full-spectra model fit, which showed one-dimensional fibers formed by stacking of the molecules and interaction with solvent, with a radius of 9.8 Å (diameter 18.6 Å) and correlation length ξ of 13.1 Å (measured directly from correlation peak in spectra).39 The possible arrangement of fibers within the bundle can be assumed to be a one-dimensional repeating molecular stack with lamellar spacing of 13.1 Å,40,41 which is reasonable agreement with the 11.9 Å in width of C6 based on simple molecular modeling as mentioned above. The cross-sectional diameter of the clustered C6 fibers at 3 h (62 Å) from the lowQ application of the full model suggests a nonsymmetrical multifiber braided structure, for which several arrangements are possible (Figure 6). These results show another level of hierarchy in the selfassembly of C6 that was not evident in our earlier microscopy based studies on C6.13 In our previous work, gelation appeared

Figure 6. Diagram for transition of the C6 fibers cluster (left) and one possible scenario on how the fibers may pack together in a multifiber clusters based on the correlative peak at Q = 0.47 Å−1 (d = 13 Å) and the clustering feature obtained from the low-Q application of the full model around d = 62 Å. Note that the cluster at 62 Å does not have any particular symmetry based on the lack of additional correlative peaks in the SANS spectra at a lower Q than Q = 0.47 Å−1.

to proceed from single-molecular fibers to thick combined fibers, and the results from this work and our previous studies suggest that gel evolves from one-dimensional supramolecular noncovalent polymers with diameter around 20 Å to hollow tubes ca. 200−500 Å in diameter and then to larger 3D gel networks. The SANS data in this work suggests that the onedimensional stacks first form into a multifiber braided cluster with diameter of 62 Å before going on to form the larger 200− E

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Australian Nuclear Science and Technology Organisation (ANSTO) for access to the Quakka SANS beamline (Proposal 1224) and the Neutron Science Directorate at Oak Ridge National Laboratory (ORNL) for access to the GP-SANS beamline (proposal IPTS-7319). Part of the SANS data collection conducted at ORNL’s High Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. We would also like to thank Jaimie Werner at ORNL for assistance.

500 Å tubes observed by transmission electron microscopy (TEM).13 For the C4 gel fibers (at 3 h), the model gave a clustered fiber structure with a radius of 17.5 Å and contour length of 330 Å. Compared to the full-spectra model fit, which gave a radius of 8.6 Å, this also suggest a multifiber braided fiber arrangement for C4 but probably containing fewer individual fibers than the corresponding braided clusters for C6. The size of the C4 clusters is much smaller than the C6, so the clusters are more flexible or have greater curvature and closer in form to the one-dimensional fibers. The diameter and entanglement motif is different based on the length of the alkyl tails, and this behavior may be affecting the nanoscale morphology and ultimately the larger macroscale gel properties and behavior.

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CONCLUSIONS A minor difference in alkyl tail chain length of two pyromellitamide-based self-assembling gelator molecules affects the properties of the individual fibers and of the aggregate structures (fiber bundles). Modeling of SANS spectra at 3 h shows that the smaller gelator molecule C4 exhibits onedimensional stacking and aggregation corresponding to a 35 Å width fiber cluster. Additional investigation of the larger gelator molecule C6 revealed that multiple phases are present during the first few hours after cooling. The C6 gelator molecules also showed onedimensional stacking but a larger aggregation equating to a 62 Å fiber cluster that had not been observed in earlier microscopy-based studies.13 Over a longer period of time, the radius, persistence length, and contour length all continue to increase in 6 days after cooling, indicating aging and morphological changes to the gel matrix. The results here suggest that self-assembled gels can undergo considerable structural changes over periods spanning several days. It is also noteworthy that the fairly subtle change in tail length from the shorter C4 to the longer C6 gelator molecules appears to change the fiber arrangement from forming multifiber braided bundles. Combined, these observations suggests that the characteristics of self-assembled gels on the nanoscale are not easily predictable and that their structure is exist in a dynamic state over a very broad range of length-andtime scales. Given the strong relationship between gel structure and gel properties such as stiffness and stability, understanding these kinetic processes better is clearly important for advancing the applications of self-assembled gels.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected] (P.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the facilities of the Mark Wainwright Analytical Centre of the University of New South Wales. We acknowledge the Australian Research Council (ARC) for Discovery Project Grants (DP09855059 and DP 13 01 01 51 2) and an ARC F utu re F ell owshi p (FT120100101) to P.T and a Scholarship to S.A.J. and The Universities of Sydney and New South Wales for a UPA Scholarship to K.W.K.T. We would also like to thank the F

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