Strong Physical Hydrogels from Fibrillar Supramolecular Assemblies

Article pubs.acs.org/Macromolecules

Strong Physical Hydrogels from Fibrillar Supramolecular Assemblies of Poly(ethylene glycol) Functionalized Hexaphenylbenzenes J. Marakis,†,‡ K. Wunderlich,§ M. Klapper,§ D. Vlassopoulos,*,†,‡ G. Fytas,*,†,‡,§ and K. Müllen§ †

FORTH, Institute of Electronic Structure & Laser, N. Plastira 100, 70013, Heraklion, Greece Department of Materials Science & Technology, University of Crete, P.O. Box 2208, 71003 Heraklion, Greece § Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany ‡

S Supporting Information *

ABSTRACT: Gel formation without chemical cross-links requires strong physical bonds, as observed in diverse molecular and macromolecular systems or supramolecular assemblies above a critical concentration. Here, we present a new molecular amphiphile of propeller-like hexaphenylbenzene derivative bearing two short poly(ethylene glycol) (PEG) chains which forms reversible physical gels comprising long (∼2.2 μm) bundles of hydrogel fibrils in coexistence with spherical micelles. Combination of topological (entanglement-like) interactions between the fibers and specific hydrophobic interactions due to the helicity of the PEG chains is proposed to account for the hydrogelation in the semidilute regime. The present supramolecular hydrogels, which are based on small molecules, exhibit extraordinary properties with an exceptionally strong dependence of the shear modulus on concentration, akin to the behavior of ultrahigh molecular weight cellulosebased fibers. characterized by the bending modulus κ or the persistence length lp (= κ/kBT, with kB being Boltzmann’s constant and T the absolute temperature) that typically varies between 10 and 30 nm).38 Because of their high length, gelation occurs at low values of the micellar volume fraction (ϕ), depending on the relation of the reptation time of an hypothetically unbreakable chain and the scission time, i.e., the average time required for a WLM to break into two pieces.38,39 WLM networks exhibit gellike rheological response with the high frequency storage modulus scaling as G′ ∼ ϕα where the value of the exponent (α) depends on the interactions.40,41 The high molecular weight analogues of molecular surfactants, amphiphilic block copolymers, also form micelles in selective solvents and often result in physical gels.42 Tuning the copolymer structure parameters (composition or block length) and interactions (amphiphilicity, crystallinity) has enabled systematic exploration of self-assembly (spherical, worm-like, and lamellar structures).43−46 From the few reported amphiphilic block copolymers, diblock44 and triblock copolymers45 self-assemble to WLM and form hydrogels. In spite of qualitative similarities between the classical polymer solutions (scaling exponent α = 9/4) and the different supramolecular structures (α ≥ 9/4), the inherent local microstructure and the strength of nonconvalent bonding

I. INTRODUCTION Supramolecular polymer networks consisting of either small molecules (e.g., surfactants) or macromolecules represent an emerging class of responsive soft materials with significant technological implications.1−9 They are akin to physical (transient) gels and usually responsive to external stimuli such as temperature, pH, solvent quality, mechanical, or electromagnetic fields.2,3,10−16 The fraction and strength of noncovalent interactions (such as metal−ligand, hydrogen bonds, electrostatic, enthalpic, π−π stacking, physical adsorption in nanohybrids) control the static and dynamic properties of the networks2,14−30 offering ample opportunities for designing tunable networks which can adapt to different environments. In particular, the ability to selectively tailor their rheology from liquid-like to solid-like has recently received a great deal of attention.5,15,31−36 Moreover, these networks exhibit self-healing properties and unique rheological behavior such as shear thinning and/or shear thickening, depending on the bonding interactions, which can facilitate their processing and handling.1,2,31−35 The key for understanding the relevant structure−dynamics interplay is the synergy of chemistry, metrology and modeling. Below, we briefly outline the key features of the most popular self-assembled structures. An archetype of a noncovalent supramolecular structure, the wormlike micelles (WLM) resulting from the self-assembly of surfactant molecules have been known for more than 60 years,37 and their structure and rheology have been extensively studied. Since end-caps formation costs (scission) energy, the growth of long chains is favored. The worm-like structure is © XXXX American Chemical Society

Received: March 15, 2016 Revised: April 12, 2016

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DOI: 10.1021/acs.macromol.6b00528 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Schematic representation of the supramolecular fibers of 1 in aqueous solutions17 along with the molecular structure of 1 bearing two PEG (PEG-750) chains (a) and simplified model of 1 used to estimate the packing parameter of 1 (b) (see text).

hydrogel fibers of molecule 1 due to strong interactions transmitted through the hydrophobicity of the helical PEG chains57 and assisted by the HPB packing. Topologically entangled fiber solutions are needed for the gel to form, and the hydrogels show an exceptionally strong dependence of the shear modulus on concentration. Notably, this dependence compares to that of high molecular weight cellulose-based fibers.58 The mechanics of hydrogels can be tuned by tailoring the molecular structure. We present the self-assembled systems and the techniques used in section II, whereas section III includes the experimental results and a detailed discussion. The key conclusions are summarized in section IV.

interactions are expected to quantitatively modify the gel behavior.44,46 Supramolecular fibers can be produced naturally (e.g., from spider silk proteins) or synthetically and are particularly important in the formation of a wide range of hydrogels. The strength of these hydrogels depends on the interactions and molecular characteristics (e.g., microcystals inside cellulose fibrils), and the scaling exponent α exceeds 4.47 We recently demonstrated such a possibility using amphiphiles of hexaphenylbenzene (HPB) hydrophobic “cores” and poly(ethylene glycol) PEG hydrophilic grafts. Fiber formation of these HPB−PEG derivatives (Figure 1) is a consequence of the tendency of the propeller-like HPB to associate in water via π−π interactions which occur at very low concentrations.17,48,49 Amphiphiles consisting of aromatics as hydrophobic part and poly(ethylene glycol) (PEG) as hydrophilic part have been rarely studied;50−53 there are a few examples consisting of PEG and aromatics that form gels.54,55 However, the impact on the rheology is unknown, and the question whether the topology of the entangled semidilute solutions is a prerequisite for gel formation remains unanswered. Though the above developments demonstrate the ability to form superstructures from diverse amphiphiles, the incomplete understanding of the system-dependent gelation prohibits a reliable prediction of either the hydrogel formation concentration or their mechanical strength and its concentration dependence. We address this delicate point in the present contribution. In particular, we extend our earlier work17 and investigate the HPB−PEG derivative at high concentrations anticipating hydrogelation of the long fibers of molecule 1 (Figure 1) due to the synergetic effects of the small macromolecule and the propensity for self-assembly of the aromatic part. The bulky propeller-like aromatic HPB may increase the scission time of the individual fibers and lead to rigid contacts between entangled fibers. The predetermined structure of the two PEG chains (angle of 60°) in molecule 1 impacts the curvature in the assemblies resembling gemini amphiphiles56 that exhibit assembly characteristics (e.g., lower critical micelle concentration (cmc) and longer micellar lifetimes) superior to single chain surfactants. We investigate molecule 1 in aqueous solutions, at concentrations well above cmc (∼2 × 10−3 g/L) in the semidilute and entangled regime, with the aim of exploring the network formation and understanding its origin and strength. To this end, we use a powerful combination of experimental probes, i.e., dynamic light scattering, cryo-TEM, and shear rheometry. We report on the reversible physical gelation of

II. EXPERIMENTAL SECTION II.A. Materials. The molecular and supramolecular structures along with the packing arrangement of the amphiphilic hexaphenylbenzene derivative (1) are illustrated schematically in Figure 1.17 The chemical structure of molecule 1 was proven by 1H NMR spectroscopy, 13C NMR spectroscopy, and MALDI-TOF mass spectrometry. Full NMR spectra and MALDI-TOF spectra are provided in the Supporting Information (Figures S1−S3). The molecular weight is Mn = 2300 g/ mol with a polydispersity index Mw/Mn = 1.02, while the PEG precursor has Mn = 750 g/mol and Mw/Mn = 1.02. Self-assembly of 1 in water leads to worm-like fibers consisting of six, on the average, bundles of single fibrils of 1 (Figure 1a). II.B. Techniques. Cryogenic Transmission Electron Microscopy (Cryo-TEM). For cryofixation, the aqueous solution of sample 1 was dropped onto a TEM grid, and the excess of liquid was blotted off with filter paper. Subsequently, the sample was frozen in liquid ethane (−89 °C) and transferred to the electron microscope. A Tecnai F20 TEM of FEI Co. (USA) was used. Photon Correlation Spectroscopy (PCS). This time-resolved light scattering technique records the intensity autocorrelation function G(q,t) ≡ ⟨I(q,t)I(q)⟩/|I(q)|2 over broad time range (10−7−103 s) with ALV-5000 goniometer/correlator setup (ALV, Germany) using an Nd:YAG laser (Coherent, Germany) at λ = 532 nm. The scattering wave vector has a magnitude of q = (4πn/λ) sin(θ/2) with n and θ on the solution’s refractive index and the scattering angle, respectively. We have performed both polarized (VV) and depolarized (VH) PCS experiments using polarized vertically (V) incident laser beam and selected the scattered light polarized either vertically (VV-configuration) or horizontally (VH-configuration) with respect to the (horizontal) scattering. Under homodyne beating conditions, the desired concentration relaxation function C(q,t) is computed from the experimental G(q,t): C(q,t) = [G(q,t) − 1]/f *]1/2, where f * ≤ 1 is an instrumental coherence factor. In this work, the analysis of C(q,t) proceeded by means of its inverse Laplace transformation (ILT): C(q , t ) =

∫ L(ln τ) exp(−t /τ) d ln τ

(1)

with L(ln τ) being the distribution of relaxation times. B


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Figure 2. (a) Relaxation function C(q,t) describing the two-step diffusion process in a 1.66 wt % (∼17 g/L) aqueous solution of derivative 1 (Figure 1) at two values of the scattering wave vector (q = 0.0304 m−1, black, and q = 0.00815 nm−1, red symbols) and 20 °C, as revealed by the inverse Laplace tranformation of C(q,t) (black and red curves, respectively). (b) The absolute Rayleigh scattering intensities associated with the large (slow) component (blue inverse triangles) and small (fast) component (green triangles) obtained from the average scattering intensity and the amplitudes of the two processes in C(q,t). The solid line represents the Debye−Bueche q-dependence (see text). (c) The respective fast (upper) and slow (lower panel) diffusion coefficients. The lines are drawn to guide the eye. In dilute solution, 1 was found17 to self-assemble in long wormlike structures with average length L = 2.3 μm, persistence length lp ∼ 30 nm, and diameter d ∼ 30 nm. As the latter exceeds the size (8.9 nm) of the individual molecules 1, the supramolecular assembly consists of bundles of micelles per cross section, as schematically represented in Figure 1. The association of 1 to wormlike fibers can be rationalized through the estimation of the packing parameter, p = v/(la) where v, l, and a are the volume occupied by hydrophobic chain, the hydrophobic chain contour length, and the surface area of hydrophilic block, respectively;59−61 wormlike structures are expected for 1/3 < p < 1/2. Representing the hydrophobic part with a rectangular prism, v = (1.1 nm × 1.3 nm × 0.2 nm) = 0.29 nm3 and assuming for the length of the hydrophobic part, l = 1.3 nm, the packing parameter falls in the range of 1/3 < p < 1/2 for an effective surface area of the hydrophilic PEG between 0.44 nm2 and 0.66 nm2, i.e., d = 0.20−044 nm and e = 1.50 nm in Figure 1b. Shear Rheometry. This allows probing the linear and nonlinear viscoelastic response of a test sample in rheometric simple shear flows. Rheological measurements were performed on different aqueous solutions of the hydrogel fibers with a strain-controlled ARES 100 FRTN1 rheometer (TA, USA) and a stress-controlled Physica MCR501 rheometer operating in strain-controlled mode (Anton Paar, Austria). With both rheometers, homemade cone−plate stainless steel geometries were utilized (4 mm diameter, 0.04 rad cone angle, in order to accommodate the minimal amounts of sample available, of the order of 50 mg) along with Peltier temperature control (at 20 ± 0.2 °C) and homemade solvent traps that saturate the atmosphere with solvent vapor and thus minimize the risk of evaporation. They were shown to be particularly efficient for these aqueous solutions for at least 2 h. The experiments included the following dynamic oscillatory measurements: (a) Frequency sweep following an oscillatory strain γ = γ0 sin ωt in order to probe the viscoelastic relaxation spectrum, i.e., the frequency-dependent storage (G′) and loss (G″) moduli in the range 0.1−100 rad/s. The stress response is σ = σ0 sin(ωt + δ) = γ0(G′ sin ωt + cos ωt) with σ0 being the stress amplitude, ω the frequency, and δ the phase angle (δ = G″/G′). (b) Strain sweep with a duration of about 8 min to determine the linear viscoelastic regime. It involves oscillations at constant frequency ω = 1 rad/s and continuously increasing strain amplitude γ0 from 10−3 to 3 strain units. (c) Time sweep at 1 rad/s and a linear strain amplitude, with typical duration of

37 min to ensure that steady state is reached. For solutions of 1 at high concentrations (>10 g/L in the solid-like regime), these tests were proceeded by a time sweep at large strain amplitude in the nonlinear regime in order to shear-melt the solid (rejuvenation process) and a subsequent linear time sweep in order to reach the steady state (aging). This protocol allowed erasing the sample’s history (possible residual stresses during sample preparation and loading) and ensuring reproducible initial conditions for the measurements. For the viscous samples at concentrations lower than 10 g/L, the dynamic signal was too weak and only the zero-shear viscosity was determined from steady shear measurements (rate sweep).

III. RESULTS III.A. Structure and Dynamics in the Dilute and Semidilute Regime. In the dilute regime at concentrations c < 0.1 g/L, C(q,t) are single decay functions representing the translational diffusion of the supramolecular structures (long fibers in Figure 1a) probed over distances of the order of 2π/q (in the range 200−1500 nm). In this concentration regime, the virtually constant Rayleigh ratio RVV(q→0)/c suggests the presence of stable fibers with constant size, in contrast to the respective c1/2 dependence reported for nonionic WLMs.39 However, this changes with increasing concentration due to spatial overlapping of these long fibers. As shown in Figure 2a, the relaxation function C(q,t) at c = 16.6 g/L and 20 °C deviates from the single decay of lower concentrations. In fact, two processes become discernible and clearly revealed by the ILT at high q’s (solid lines). A two-step decay of C(q,t) constitutes a departure from the signature of the dynamics in semidilute solutions of conventional monodisperse homopolymers, where the chain translational diffusion, D, crosses over to cooperative diffusion Dc of the transient network above the overlapping concentration c* (∼0.5 g/L) with the ratio of the diffusion coefficients following Dc /D ∼ (c /c*)ν /(3ν− 1) C

(2) DOI: 10.1021/acs.macromol.6b00528 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules where the scaling exponent ν = 0.5 for Gaussian and about 0.6 for swollen chains.62 From the two diffusion coefficients in Figure 2c, it is the slow Ds(q=0) that is comparable to D (∼1.5 × 10−8 cm2/s) of the fibers in the dilute regime.17 Hence, it is assigned to their cooperative diffusion coefficient at higher concentration; the observed q-dependence of Ds in Figure 2c (unlike the situation for Dc in flexible polymers) is attributed to the contribution of internal dynamics of the fibers at high q’s.63 The new, fast diffusion coefficient Df is faster than D by one order of magnitude and reflects the presence of much smaller species, e.g., small micelles. The proposed assignment of the two diffusive processes above is further supported by their contribution to the total light scattering intensity of the solution. The absolute intensities RVV(q)/c associated with the two processes are obtained from their amplitude in C(q,t) and the total light scattering intensity of the solution. The q-independent contribution of the fast process (solid triangles in Figure 2b) conforms to the small micellar size, Rm = kBT/(6πη0Df) = 12 nm, relative to the wavelength of the probe laser light (η0 denotes the solvent viscosity). On the other hand, the slow process intensity, RVV(q), exhibits a q-dependence (Figure 2b, inverse triangles), indicating a much larger dynamic size, ξ = kBT/(6πη0Dc) ≈ 200 nm. Based on the theory of semidilute solutions of flexible homopolymers which form transient physical networks,62 the static mesh size ξs is proportional to the dynamic size, depends on the concentration and is expressed in analogy to eq 2 as ξs(c) ∼ R g(c /c*)−ν /(3ν − 1)

Figure 3. (a) Rayleigh ratio, i.e., reduced absolute polarized scattering intensity RVV(q→0)/c, and depolarized intensity IVH/c (inset) as functions of concentration. (b) Fast and slow diffusion coefficients Df (red) and Ds(q→0) (black symbols), respectively, associated with the two species in the aqueous solution of derivative 1, as a function of concentration at 20 °C The shaded areas indicate the crossover regions to semidilute and strong interacting regimes.

(3)

where Rg represents the chain’s radius of gyration. For large values of Rg, the static mesh size of the network near and above c* can still be large,64 hence justifying the q-dependent RVV(q) of this process. However, the Ornstein−Zernike q-dependence65 RVV(q)/c ∼ [1 + q2ξs2]−1, which is commonly used, fails in the present case. Instead, the Debye−Bueche q dependence,66 RVV(q)/c ∼ [1 + q2ξs2]−2, represents (solid line in Figure 2b) the experimental intensity pattern of the 17 g/L (c/ c* ∼ 35) solution with an extracted fit parameter ξs = 90 nm. The shape of RVV(q) is further justified by the thickness of fiber (with diameter d ≈ 30 nm) of the physical network (Figure 1) compared to homopolymer semidilute solutions conforming to Ornstein−Zernike RVV(q). An additional distinct feature of the present network is the disparity between the values of the static and dynamic mesh sizes, ξs and ξ, respectively (for 17 g/L, ξ = 200 nm > ξs = 90 nm), which might also relate to the Debye− Bueche q-dependence of RVV(q). Structural changes upon increase of the fiber concentration are evident in the variation of RVV(q→0)/c with c and the accompanied D(c) plot in the thermodynamic limit (q → 0), both shown in Figure 3. Based on the data of Figure 3a, the crossover from dilute to semidilute regime occurs in the vicinity of 0.1 g/L (i.e., close to the static c* value of 0.5 g/L) which is marked by the drop of the scattering intensity, due to negative interference (of the scattered light) between overlapping fibers. This crossover is hardly discernible in the dynamic plot (Figure 3b), probably because of size polydispersity and the associated subtle transition to cooperative diffusion at c ≈ c*. Instead, it is the presence of the fast process which is now experimentally resolved due to the drop of the intensity of the main slow process above c* that provides unambiguous evidence of the transition to semidilute regime. For c ≈ 2 g/L and beyond, i.e.,

well above c*, the slow diffusion Ds exhibits a clear increase due to the decreased mesh size; at the highest concentration measured, c = 47 g/L, the physical network is characterized by a dynamic mesh size ξ ≈ 50 nm. The decrease of ξ (from 200 to 50 nm) in the concentration range 2−47 g/L (Figure 3) is clearly weaker than expected from eq 3, for either swollen (ξ ∼ c−3/4) or ideal Gaussian (ξ ∼ c−1) chains. The concentration range (2−47 g/L) explored in the PCS experiment falls into the low range of the semidilute regime (onset and beyond) with a rheological behavior akin to a weak viscoelastic liquid (section III.B). Summarizing the PCS investigations, the emerging structure at c > c* is schematically shown in Figure 4a. The long fibers appear to form a physical network already at low concentrations (∼0.1 g/L). The network is isotropic as indicated by the weak depolarized light intensity IVH/c shown in the lower inset of Figure 3a. At low concentrations c < c*, IVH/c is high due to the self-assembly of the constituent molecule 1 in the individual fibers. However, it drops near c* due to the randomization of the orientation; formation of liquid-like and/or nematic order67 would have enhanced IVH/c. Confirmation of the presence of fibers by cryo-TEM became feasible at high c = 31 g/L, well above c* as shown in Figure 4b. This justifies the proposed scenario and moreover reveals the presence of the small micelles as well. III.B. Rheology of the Transient Supramolecular Network. For concentrations c > 100 g/L (i.e., c > 200c* based on the above discussion), the solutions are well into the D


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exponent of about 2.5, signaling a behavior akin to semidilute solutions of unentangled (Rouse) flexible linear polymers.62 In fact, the entangled semidilute solution regime is characterized by a much stronger concentration dependence of the solution viscosity (η ∼ c3/(3ν−1) ∼ c3.9 for good solvent conditions). This already suggests that the present supramolecular fiber solution is distinct from that of flexible polymers. Based on the latter and the above discussion, the entanglement regime starts at 5 g/L but behaves like semidilute Rouse regime. Hence, the experimental signatures of c* (from PCS and dynamic response) (Figure 5) do not match the behavior of simple flexible polymers. Rheologically, there is one notable transition, from solution to gel, at about 100−150 g/L, and is marked by the shaded area in Figure 5b. This change of behavior is rather abrupt. In the gel regime, the plateau modulus G′p (Figure 5b) follows a power-law exponent of about 6 with hydrogel concentration, which is much larger than the value of 2.25 of linear flexible polymers and semidilute and entangled regimes62 or that for WLM systems discussed in section I. On the other hand, this exponent is similar to that reported for cellulosic fibril gels,47,58 which was explained on the basis of bending being at the origin of the elastic energy of these fibrils.47 The formed strong hydrogels were further tested for reversibility and mechanical stability. The term “strong hydrogels” here is justified in view of the steep dependence of the shear modulus on concentration and in accordance with earlier reports, in which reversible gels with moduli of 104 Pa are also called strong gels.68 We note that a measure of the gels strength is their resistance to an external force, and the point of mechanical breaking is known as yielding. We used an established yielding protocol, consisting of oscillatory strainamplitude sweeps at a given frequency.69 A typical example is depicted in Figure 6a, where the recorded moduli G′ and G″ are plotted against the imposed strain amplitude γ0 at an oscillatory frequency of 1 rad/s for a 311 mg/mL aqueous HPB−PEG derivative 1 solution. The crossover of G′ and G″ signifies the yield strain γy and yield stress σy = (G′2 + G″2)1/2γy for the given frequency and concentration. At strain amplitudes γ0 < γy , the material exhibits solid-like response (G′ > G″), whereas for γ0 > γy it exhibits liquid-like response (G″ > G′). The yield stress (σy) and yield strain (γy) values are plotted against concentration in the inset of Figure 6a. In the range of concentrations investigated, γy is virtually independent of concentration having a value of about 20%, which is typical of gels with ductile response, for example, in depletion gels from soft colloid−polymer mixtures.70 Concomitantly, the strong dependence of σy on concentration (σy ∼ c8.3) is reminiscent of flocculated colloidal dispersions.68 These results

Figure 4. (a) Schematic illustration of the network-like structure of long hydrogel fibers marked in blue, coexisting with small micelles of derivative 1 (small circles with red center and green periphery). The hydrophilic part of 1 is marked in green, and the hydrophobic part is marked in red. (b) Cryo-TEM micrograph of fibers of 1 in aqueous solution at 30.7 g/L.

entanglement regime (its onset being at about 10c*)60 exhibit solid-like behavior as shown by the viscoelastic spectra in Figure 5a. It is interesting that for all concentrations, G′ ≫ G″ and G′ ∼ ω0 throughout the whole frequency range (encompassing 2 decades), whereas G″ increases weakly as the frequency is reduced. Such a behavior reflects the response of an entanglement-like network of fibers, which is expected for micrometer long fibers at these relatively low concentrations. We assign this network to a physical gel with G′ ≈ kBT/ξrheo3 displaying strong concentration dependence. Doubling of concentration from 150 to 311 g/L increases G′ which results in a tenfold decrease of the average viscoelastic mesh size ξrheo from 220 to 23 nm. Notably, this concentration effect is much stronger compared to that expected from the scaling of the equilibrium semidilute solutions (eq 3). However, the semidilute solutions are viscoelastic liquids only for concentrations up to about 100 g/L, for which the PCS study was performed (section III.A). In the concentration range 110−140 g/L, the viscosity essentially diverges (Figure 5b) and the solution of HPB-PEG derivative 1 behaves as a viscoelastic solid and develops a plateau modulus (Figure 5a). Thus, the aforementioned strong decrease of ξrheo occurs in the gel regime, suggesting that other interactions in addition to the topological entanglement constraints contribute to the gel formation well above c*. The discussion of experimental results in section III.C provides support to the assignment of this viscoelastic solid to gel. The increase of the solution viscosity (η) with concentration in the viscoelastic solutions (Figure 5b) conforms to a scaling

Figure 5. (a) Dynamic storage (G′, solid symbols) and loss (G″, open symbols) moduli of three solutions of the HPB−PEG derivative 1 in the gel regime at 20 °C. The blue, black, purple, and red points represent the data extracted from 311, 255, 200, and 150 g/L concentrations, respectively. (b) Respective concentration dependence of the zero-shear viscosity and effective plateau modulus. Lines are drawn to guide the eye (see text). E


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Figure 6. (a) Dynamic strain amplitude sweep with a 311 g/L aqueous HPB−PEG derivative 1 solution at 1 rad/s. Symbols are the same as in Figure 5. The vertical arrow indicates the yield strain γy. Inset: concentration dependence of the yield stress and strain (left axis) and the plateau modulus (right axis). (b) Respective typical Lissajous−Bowditch representation of stress vs strain for different strain amplitudes (100%, 30%, 15%, and 3% in the direction inward) obtained from oscillatory measurements at 1 rad/s. Numbers in circles indicate different processes in sequence during nonlinear oscillatory cycle (see text).

suggest, again, the presence of strong interactions which are responsible for variation in the distribution of contacts with fiber concentration. Qualitatively similar yielding response, i.e., nearly c-independent γy and strongly c-dependent σy, was reported for cellulose nanofibril gels.47 The mechanical stability of the hydrogels can be further assessed by continuous oscillatory cycles under large (nonlinear) strain deformation. The results of such a test are typically discussed in terms of the Lissajous−Bowditch (LB) representation of stress vs strain during oscillatory cycles at constant frequency and different values of strain amplitude.71 This is shown in Figure 6b for the hydrogel of HPB−PEG derivative 1 with concentration 311 g/L, at a frequency of 1 rad/s. We observe that for small strain amplitudes the material exhibits a viscoelastic solid response, whereas with increasing strain amplitude viscous dissipation becomes more significant. For γ0 above about 30%, the different cycles used for obtaining the LB plots do not superimpose, and the nonlinearity is much more pronounced at the highest γ0 = 100%. These plots reflect a sequence of processes starting from elastic deformation at stresses σ > 0 (circle 1 in the figure), with the effective modulus Geff = dσ/dγ (at σ → 0) being virtually identical for all strain amplitudes, continuing with weakening of the gel and yielding (circle 2) and enhanced viscous dissipation (circle 3), and closing the cycle with sudden decrease of the instantaneous rate, going back to solid-like response (circle 4) and so on.70 A further, necessary test for assessing the shear-induced structure breakup and re-formation of this gel is the thixotropic response.72 This can be better appreciated in Figure 7, where the modulus Gp′ of the 311 g/L hydrogel of HPB−PEG derivative 1 is depicted as a function of time during a dynamic strain sweep, starting at γ0 = 0.1% and reaching a maximum strain amplitude of γ0 = 300% at a frequency of 1 rad/s; this is the same test shown in Figure 6a, but here it is plotted against time (with respective strain amplitude values shown on top of the plot for clarity). The sharp decrease of Gp′ by more than 2 orders of magnitude signifies the breakup of the gel, i.e., yielding (see also Figure 6a). Figure 7 suggests that upon oscillatory shearing for about 500 s the material remains in the linear viscoelastic regime; then for another 1720 s it undergoes nonlinear deformation with yielding and modulus decrease. Then, upon flow cessation its structure builds up very fast as evidenced by the increase of the storage modulus to a steady state. This increase reflects structural evolution of the broken hydrogel and is materialized with a dynamic time sweep which is performed at small strain amplitude of 1% and frequency of 1

Figure 7. Apparent plateau modulus Gp′ as a function of time during a dynamic strain sweep of a 311 g/L HPB−PEG derivative 1 solution in water, starting at γ0 = 0.1% and reaching a maximum strain amplitude of γ0 = 300% in 2200 s (black squares). Subsequently, upon completion of the nonlinear strain sweep, a linear dynamic time sweep is performed at 1% strain and 1 rad/s, reaching a steady state value (blue triangles). The horizontal arrow indicates the time regime where structural evolution is probed. Inset: magnification of the structural re-formation G(t) at early times and fit (red line) in order to obtain the characteristic gel re-formation time τgel after shear-induced breakup (see text).

rad/s. The gel re-formation time can be estimated by representing this time evolution of the modulus as G′ = Gp′[1 − exp(−t/τgel)], which is shown by the solid line in the inset plot of Figure 7; the steady state G′p is the original value of the elastic modulus (Figure 5a). At the same time, the extracted characteristic gelation time τgel= 50 s is found to be rather short and more remarkably, virtually independent of concentration in the range 120−311 g/L (data not shown). This suggests that the elasticity of the fibrils and the related reengagement of PEG segments at junction points may be at the origin of this fast recovery time (see also discussion in section III.C). This time is similar to the respective times reported for ionic multiresponsive hydrogels, due to charge interactions73 or amyloid hydrogels, due to fast bonding of hydrophobic groups in water environment,74 or nanohybrid hydrogels consisting of silica particles immersed in a poly(N,N-dimethylacrylamide) matrix.30 At the same time, it is much faster (by more than a decade) than recovery times reported for other systems, e.g., soft colloidal gels,64 due to slow thermal rearrangements during aging or clay-based hydrogel nanocomposites75 due to polymer−clay specific interactions. Hence, the specificities of the systems (molecular interactions) control the recovery time. F


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Macromolecules To complete the discussion of structure formation and related associations, we note that enthalpic interactions may be possible at higher temperatures. In particular, this is known from literature that PEG has a lower critical solution temperature (LCST).76 However, this was not studied here. III.C. Discussion and Tentative Interpretation. The HPB−PEG derivative (1) forms micrometer long (∼2.2 μm) fibers (Figure 1) at concentrations as low as 10−4 g/L at ambient conditions.17 Their crowding (steric constraints) and interactions have strong impact on the solution rheology. The main experimental observations can be summarized as follows: (i) With increasing concentration above c* (= 0.5 g/L), dynamic light scattering in the range 2−47 g/L (Figure 3) reveals a stable physical network with relatively large mesh size (Figure 4). Up to about 100 g/L, the system conforms to an untangled semidilute viscoelastic liquid with shear viscosity η ∼ c2.5 (Figure 5b). (ii) With further increase of the concentration, the fiber physical network undergoes a transition to a hydrogel within a narrow concentration range around 110 g/L. In this state, the increase of the elastic plateau modulus, Gp′ ∼ c6 (Figure 5b), is beyond that expected from viscoelastic entangled polymers,62 but rather conforms to scaling reported for microfibrillar cellulose gels.58 (iii) The formed hydrogels exhibit yielding response analogous to that of cellulosic fibrils.47 (iv) After yielding and shear flow cessation, the broken hydrogels re-form (Figure 7) with a very fast structural reformation time. The present study unequivocally shows that the hydrogel formation occurs well above c* where entanglement topology dominates the dynamics. Yet, this is not a sufficient condition for hydrogels to form based on the observation (ii) above. Additional interactions should be at work, as suggested by the very strong concentration dependence of Gp′. Actually, concerning this G′(c) dependence, high values of power-law exponents have been reported in the literature for diverse systems ranging from spherical micelles of oxyethylene/ oxybutylene block copolymers (G′ ∼ c5.3 for E92B18)77 and aqueous suspension of thermosensitive core/shell particles (G′ ∼ c4)78 and from low molecular weight PEG solutions or silicate-cross-linked PEG nanocomposites (G′ ∼ c7)57,79 to microfibrillar cellulose fibers (G′ ∼ c4)58,80 and agarose hydrogels.81 The minimum concentration needed to form gels is different and related to the strength and nature of the interactions. The latter vary from liquid-crystalline packing and inter- and intrachain association with different origins of attraction, e.g., hydrogen-bonding,82 hydrophobic,57 and depletion forces;77 this explains why the power-law exponent for these systems ranges from 4 to 7. In the present case, not only spherical particles (Figures 3 and 4) but also bundles of fibrils, both bearing short PEG chains with helical conformation, can contribute to the possible interactions leading to hydrogel formation at a critical gel transition concentration. Triggered by interaction mechanisms reported in the literature, a proposed plausible scenario for the observed gelation (without chemical cross-links) in the present system is illustrated in Figure 8. Attractive interactions due to the hydrophobicity become important through the interpenetration of PEG chains on the fibers, facilitated by the HPB packing (steric constraints and/or π−π stacking) that presumambly leads to rigid fibers and contacts (see also Figure 1b). This implies that the elastic energy of the hydrogel originates from the bending energy between rigid contact (entanglement) points of interacting fibers, implying enthalpic

Figure 8. Schematic illustration of supramolecular fibers formed from the self-assembly of the HPB-PEG derivative 1 (Figure 1); here, the small spherical micelles of Figure 4a are not shown for clarity. The entangled fiber solution (middle) interacts through hydrophobic interactions of the helical PEG chains (green) and interlocking of the rigid HPB (right).

elasticity.83,84 Following a recent theoretical analysis,79 Gp′ depends on the bending energy density, which in the presence of strong interactions is a nonlinear function of concentration. Hence, the relationship Gp′ ∼ c11/3(1 + αc10/3), with α a constant and the second term accounting for the effect of the two-body interactions, predicts power-law exponents spanning a wide range from 3.7 to 7. Hence, a high concentration of fibers is needed to form a topological network, which will then become stronger with the extra bonding brought about by molecular interactions and here in particular PEG-specific interlocking at the fiber contact points. Further, shearing will disengage the interlocked segments and the gel will break, while upon flow cessation the network will re-form fast due to the elasticity of the fibers. This overall behavior reflects the strong interplay of shear and structural time, or else thixotropy in the system, which can be understood in terms of bonding and debonding (mediated by the molecular interlocking of the PEG segments). It indicates full reversibility of the process in this fibril network gel. The above information from linear response, large amplitude oscillatory shear (LAOS), and thixotropy also suggests that the present network is responsive and functional, with tailored physical, chemical, and biological properties, and is therefore of potential interest in applications. Specific examples include superabsorbants or agricultural films for holding moisture, tissue engineering, controlled drug delivery, and regenerative medicine.

IV. CONCLUDING REMARKS In this study, we have examined aqueous solutions of small pphenylene-based amphiphilic molecules (1) at high concentrations by dynamic light scattering, cryo-TEM, and shear rheometry. We have identified three different regimes: very dilute, semidilute, and concentrated. In dilute solutions, the molecules self-assemble into long worm-like structures (fibers) consisting of bundles of micelles (cross section) as schematically represented in Figure 1.17 In the semidilute regime, the overlapping fibers coexist with spherical micelles as verified by dynamic light scattering and cryo-TEM. In the concentrated regime reversible physical gelation of hydrogel fibers of molecule 1 was detected, due to strong interactions transmitted through the hydrophobicity of the helical PEG chains. Topological (entanglements) interactions between the fibers and hydrophobic interactions at the point of overlap are G


Article

Macromolecules

(9) Dutertre, F.; Gaillard, C.; Chassenieux, C.; Nicolai, T. Branched Wormlike Micelles Formed by Self-Assembled Comblike Amphiphilic Copolyelectrolytes. Macromolecules 2015, 48, 7604−7612. (10) Dong, S.; Luo, Y.; Yan, X.; Zheng, B.; Ding, X.; Yu, Y.; Ma, Z.; Zhao, Q.; Huang, F. A Dual-Responsive Supramolecular Polymer Gel Formed by Crown Ether Based Molecular Recognition. Angew. Chem., Int. Ed. 2011, 50, 1905−1909. (11) Jan, X.; Wang, F.; Zheng, B.; Huang, F. Stimuli-responsive supramolecular polymeric materials. Chem. Soc. Rev. 2012, 41, 6042− 6065. (12) Angelos, S.; Yang, Y.-W.; Patel, K.; Stoddart, J. F.; Zink, J. I. pHresponsive supramolecular nanovalves based on cucurbit[6]uril pseudorotaxanes. Angew. Chem., Int. Ed. 2008, 47, 2222−2226. (13) Zhao, Y.; Beck, J. B.; Rowan, S. J.; Jamieson, A. M. Rheological behavior of shear-responsive metallo-supramolecular gels. Macromolecules 2004, 37, 3529−3531. (14) Guillet, P.; Mugemana, C.; Stadler, F. J.; Schubert, U. S.; Fustin, C.-A.; Bailly, C.; Gohy, J.-F. Connecting micelles by metallosupramolecular interactions: towards stimuli responsive hierarchical materials. Soft Matter 2009, 5, 3409−3411. (15) Goldansaz, H.; Voleppe, Q.; Piogé, S.; Fustin, C.-A.; Gohy, J. F.; Brassinne, J.; Auhl, D.; van Ruymbeke, E. Controlling the melt rheology of linear entangled metallo-supramolecular polymers. Soft Matter 2015, 11, 762−774. (16) Guo, M.; Pitet, L. M.; Wyss, H. M.; Vos, M.; Dankers, P. Y.; Meijer, E. W. Tough Stimuli-Responsive Supramolecular Hydrogels with Hydrogen-Bonding Network Junctions. J. Am. Chem. Soc. 2014, 136, 6969−6977. (17) Wunderlich, K.; Larsen, A.; Marakis, J.; Fytas, G.; Klapper, M.; Müllen, K. Controlled Hydrogel Fiber Formation: The Unique Case of Hexaphenylbenzene-Poly(ethylene glycol) Amphiphiles. Small 2014, 10, 1914−1919. (18) Joung, Y.-K.; Ooya, T.; Yamaguchi, N.; Yui, N. Modulating rheological properties of supramolecular networks by pH-responsive double-axle intrusion into γ-cyclodextrin. Adv. Mater. 2007, 19, 396− 400. (19) Pensec, S.; Nouvel, N.; Guilleman, A.; Creton, C.; Boué, F.; Bouteiller, L. Self-Assembly in Solution of a Reversible Comb-Shaped Supramolecular Polymer. Macromolecules 2010, 43, 2529−2534. (20) Catrouillet, S.; Fonteneau, C.; Bouteiller, L.; Delorme, N.; Nicol, E.; Nicolai, T.; Pensec, S.; Colombani, O. Competition Between Steric Hindrance and Hydrogen Bonding in the Formation of Supramolecular Bottle Brush Polymers. Macromolecules 2013, 46, 7911− 7919. (21) Erk, K. A.; Martin, J. D.; Hu, Y. T.; Shull, K. R. Extreme Strain Localization and Sliding Friction in Physically Associating Polymer Gels. Langmuir 2012, 28, 4472−4478. (22) ten Brinke, G.; Ruokolainen, J.; Ikkala, O. Supramolecular materials based on hydrogen-bonded polymers. Adv. Polym. Sci. 2007, 207, 113−177. (23) Nair, K. P.; Breedveld, V.; Weck, M. Complementary HydrogenBonded Thermoreversible Polymer Networks with Tunable Properties. Macromolecules 2008, 41, 3429−3438. (24) Sprakel, J.; van der Goucht, J.; Cohen Stuart, M. A.; Besseling, N. A. M. Brownian particles in transient polymer networks. Phys. Rev. E 2008, 77, 061502/1−061502/10. (25) Yan, T.; Schröter, K.; Herbst, F.; Binder, W. H.; ThurnAlbrecht, T. Nanostructure and Rheology of Hydrogen-Bonding Telechelic Polymers in the Melt: From Micellar Liquids and Solids to Supramolecular Gels. Macromolecules 2014, 47, 2122−2130. (26) Feldmann, K. F.; Kade, M. J.; Meijer, E. W.; Hawker, C. J.; Kramer, E. J. Model transient Networks from Strongly HydrogenBonded Polymers. Macromolecules 2009, 42, 9072−9081. (27) Vermonden, T.; von Steenergen, M. J.; Besseling, N. A. M.; Marcelis, A. T. M.; Hennink, W. E.; Sudhoelter, E. J. R.; Cohen Stuart, M. A. Linear Rheology of Water-Soluble Reversible Neodymium(III) Coordination Polymers. J. Am. Chem. Soc. 2004, 126, 15802−15808. (28) Shedge, A.; Colombani, O.; Nicolai, T.; Chassenieux, C. Charge Dependent Dynamics of Transient Networks and Hydrogels Formed

proposed to account due to the helicity of the PEG chains. These hydrogels show an exceptionally strong dependence of the shear modulus on concentration, comparable to that of very high molecular weight cellulose-based fibers. However, in contrast to the latter, we have prepared a material with similar mechanical properties but based on small molecules, due to very strong noncovalent interactions (hydrophobic interactions, hydrogen bonding). Hence, there are several possibilities for designing hydrogels with tunable mechanical properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00528. Figure S1: 1H NMR spectrum (300 MHz, CD2Cl2) of molecule 1; Figure S2: 13C NMR spectrum (75 MHz, CD2Cl2) of molecule 1; Figure S3: MALDI-TOF spectrum of molecule 1 (dithranol in dichloromethane solution) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (G.F.). *E-mail [email protected] (D.V.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mrs Antje Larsen for technical support. We acknowledge financial support from the EU under the European Soft Matter Infrastructure (ESMI, FP7-GA-262348) and Greek General Secretariat for Research and Technology in the framework of the program Thalis (projects METAASSEMBLY and COVISCO) ) and the European Social Fund and National-Greece, through the KRIPIS-PROENYL (MIS448305) project.

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REFERENCES

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Strong Physical Hydrogels from Fibrillar Supramolecular Assemblies

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