Microengineering of Supramolecular Soft Materials by Design of the

May 12, 2010 - Jing-Liang Li‡§, Bing Yuan‡, Xiang-Yang Liu*†‡ and Hong-Yao Xu†. † College of Material Science and Engineering & State Key...
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DOI: 10.1021/cg100188w

Microengineering of Supramolecular Soft Materials by Design of the Crystalline Fiber Networks

2010, Vol. 10 2699–2706

Jing-Liang Li,‡,§ Bing Yuan,‡ Xiang-Yang Liu,*,†,‡ and Hong-Yao Xu† †

College of Material Science and Engineering & State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, China, ‡Department of Physics and Department of Chemistry, National University of Singapore, 2 Science Drive 3, Singapore, 117542, and §Centre for Micro-Photonics, Faculty of Engineering and Industrial Science, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia Received February 6, 2010; Revised Manuscript Received March 28, 2010

ABSTRACT: Crystalline spherulitic fiber networks are commonly observed in polymeric and supramolecular functional materials. The elasticity of materials with this type of network is low if interactions between the individual spherulites are weak (mutually exclusive). Improving the elasticity of these materials is necessary because of their important applications in many fields. In this work, the engineering of the microstructures and rheological properties of this type of material is carried out. A small molecule organogel formed by the gelation of N-lauroyl-L-glutamic acid di-n-butylamide (GP-1) in propylene glycol (PG) is used as an example. The elasticity of this material is improved by controlling the thermodynamic driving force, the supersaturation of the gelator, and by using a selected copolymer additive to manipulate the primary nucleation of GP-1. Because of the weak interactions between the GP-1 spherulites, with the same fiber mass, the elasticity of GP-1/PG gel is less than half of those of the other two gels formed by GP-1 and 2-hydroxystearlic acid in solvent benzyl benzoate (BB), which are supported by interconnecting spherulitic fiber networks. This work develops a robust approach to the engineering of supramolecular functional materials especially those with mutually exclusive spherulite fiber networks.

1. Introduction Supramolecular functional materials consisting of selfassembled fiber networks are a class of soft functional materials that have attracted significant attention in recent years.1,2 A physical gel, as one of the examples, is formed when a hot solution of a small molecular mass organic compound is cooled to a certain temperature, namely, Tg. The solute molecules, at a concentration in excess of the equilibrium concentration at Tg, will self-organize into a three-dimensional (3D) fibrous network, which can effectively immobilize and entrap the liquid through capillary force.2,3 In recent years, there has been a rapidly growing interest in such materials, motivated by the many potential applications in photographic, cosmetics,4,5 food,6-8 petroleum industries, drug delivery,9-11 lithography, catalyst supporters, fabrication of nanostructures,12-14 etc. For example, a fiber network with uniform and nano-sized pore can act as a template to fabricate nanostructures and control nanoparticle size and shape; a drug delivery process needs an effective holder to retain and release drugs in a controllable manner by means of the desired network structure. In order to obtain such materials with desired macroscopic properties, much attention has focused on the identification of new compounds that are capable of forming a 3D interconnected fiber network structure in organic and/or aqueous solutions. This is a tedious job as it includes the screening of a large number of compounds, as well as suitable solvents capable of forming these structures. Actually, this is a very timeconsuming and an inefficient approach. Furthermore, after the continuous attempts for such a long period, it becomes increasingly difficult to identify new agents. In particular, in some cases, because of the limitation in the choice of materials, such *Corresponding author. E-mail: [email protected]. r 2010 American Chemical Society

screening becomes impossible. On the other hand, if interconnecting 3D micro- or nanofiber networks with the required organization can be constructed, “new” functional materials with the desired functionalities can be produced. This approach based on the reconstruction of the micro/nano fibril structure of gels will become a new alternative route in producing new functional materials. We have devoted much effort to the engineering of supramolecular soft materials by adopting various stimuli such as chemical additive,1,15-17 thermal18-22 and ultrasonic treatment,23 etc. The focus was on the fabrication and engineering of 3D “single” nanofiber networks. In such a network, the nanofibers interpenetrate into each other and the individual sets of fiber that make the whole network can not be identified. Thus, such a network behaves macroscopically like a single fiber network due to the good interconnectivity between fibers. In practice, we also encounter multidomain network systems frequently. In such a system, the entire network consists of a number of identifiable individual fiber networks. A typical example is the one consisting of several individual spherulitic fiber networks (cf. the micrographs of the three materials given in Figure 2b-d). For a material constructed by multidomain networks, not only the structure of the individual fiber networks, but also the interaction between the adjacent fiber networks is important in determining the macroscopic properties of the material. In this paper, we report a robust approach to reconstructing supramolecular functional materials with multidomain fiber networks. This is achieved by manipulating the supersaturation of the system or with the aid of a so-called topological modifier. In this connection, a supramolecular functional material formed by N-lauroyl-L-glutamic acid di-n-butylamide (GP-1) in propylene glycol (PG) will be examined. A copolymer, poly(methyl methacrylate comethacrylic acid) (PMMMA), is Published on Web 05/12/2010

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Li et al. Scheme 1. Illustration of Fiber Network Formation (a) and Engineering of the Fiber Network of Supramolecular Materials by Controlling the Primary Nucleation (b)

Figure 1. Illustration to show fiber networks of soft materials. (A) Spherulite (right), a special class of the Cayley tree-like network (left), and networks with mutually exclusive spherulites (B) and interpenetrating spherulites (C). Compared with a material with an interpenetrating spherulitic fiber network, a material with mutually exclusive spherulites is weak due to the presence of a mechanically weak boundary area between neighboring spherulites.

utilized to modify the topological structure of the overall fiber networks, which gives rise to a significant modification of the rheological properties of the material. 2. Strategy of Overall Networks Reconstruction Spherulites, as illustrated by Figure 1A, can be classified as a special type of fiber networks (Cayley tree-like networks) and commonly identified in soft materials. Depending on the compactness of the spherulites, the adjacent spherulites in a system can be mutually exclusive (not able to penetrate into each other) (Figure 1B) or interpenetrating (Figure 1C). Macroscopic properties, in particular, the mesh size and the corresponding rheological properties of these materials, are determined by the micro/nanostructure of fiber networks. If the fiber networks are mutually interpenetrated and interlocked with other, they will behave not much differently from a single fiber network. Correspondingly, the elasticity of the materials will be strong. On the other hand, if the fibers from one fiber network cannot penetrate into the adjacent fiber networks, the elasticity of the materials is weak due to the presence of the boundary areas. Considering that the mutually exclusive spherulites are commonly observed in soft functional materials, it is important to develop methods to improve the elasticity of these materials to make them suitable to applications in various conditions. So far, many works have been reported on the formation mechanism of low molecular organogels and on improving the properties and structures of these materials. In this context, the self-assembly models can explain how the fiber network is architected by the self-assembly of low mass molecules through noncovalent interactions such as hydrogen bonding, hydrophobic interactions, etc.24-26 Despite some success in interpreting the experimental observation, these models are incapable of providing a global view on fiber network formation and engineering. Alternatively, the models

based on the nucleation-mediated branching and growth mechanism have successfully predicted the fibril network formation in a quantitative manner.16 It follows that the supersaturation of a solute in the solution, impurities, and cooling rate are important parameters that control the fiber growth rate, topology, and microstructure of the fibril networks.16,18,19,27 A comprehensive understanding of the correlation between the rheological properties and the topology and fibril structures of soft functional materials remains to be achieved. According to the optical microscopic in situ observation, the formation of crystalline nanofiber networks initiates from primary nucleation and followed by the growth and branching of fibrous arms (Scheme 1a).22,28 On the basis of the nucleation-growth mechanism, the number of active nucleating centers will determine the number of spherulites occurring in the system. In other words, controlling the number of spherulitic fiber networks can be achieved by removing or inactivating the nucleating centers (Scheme 1b). According to the current nucleation theories,29 the nucleation rate of the spherulites from nucleating centers can be given by   ΔG ð1Þ J∼Nf } ð f Þ1=2 B exp kT where ΔG ¼

16πγcf 3 Ω2 3ðkTÞ2 ½Δμ=kT2

Δμ=kT ¼ lnð1 þ σÞ =

f

ΔHdiss e ðT - TÞ kT e

ð2Þ

ð3Þ

with σ ¼ ðX - X e Þ=X e

ð4Þ

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where J denotes the number of spherulitic nuclei occurring in unit time-volume, N denotes the number of active nucleating centers, ΔG* is the nucleation energy barrier, B is the kink kinetics coefficient, f 00 and f ( f 00 e 1, f > 0) are factors describing the correlation between the substrates and the nucleation phase, k is the Boltzmann constant, Ω is the volume of the growth units, γcf denotes the interfacial free energy between the fibers and the fluid phase, ΔHdiss denotes the molar dissolution enthalpy of the nucleating phase, T is temperature, Δμ denotes the chemical potential difference between solute molecules in the fiber state and in the liquid, and Δμ/kT denotes the thermodynamic driving force, which can be correlated to supercooling ΔT (ΔT = T e - T, T e: equilibrium temperature of solution) as defined by eq 3, respectively. Obviously, the lower the nucleation rate J, the fewer the spherulites (or fiber networks) will be acquired. At a given concentration, this implies that the acquired spherulites (or fiber networks) will be bigger. The eq 1 indicates that for a fixed mass of solute, the nucleation rate J of spherulites can be controlled by producing the material at different temperatures. For example, at an elevated temperature (a lower supersaturation Δμ/kT), a lower nucleation rate corresponding to a lower number density of spherulites will be produced, which will eventually lead to the formation of larger spherulites (Scheme 1b). Consequently, the interconnectivity and hence the elasticity of the entire fiber network will be improved. Equation 1 also indicates that the nucleation kinetics can be changed by manipulating the kink kinetics coefficient and the correlation between the substrates and the nucleation phase. It follows that suitable additives can shield the active nucleation centers (i.e., dust particles, bubbles, metaphases, etc.) and reduce the number of active nucleating centers.29 As illustrated by Scheme 1b, the number of spherulites will also be reduced. Therefore, suitable additives can also be used to control the nucleation kinetics and thus to modify the topological structure of the fiber networks. The selection of additives should follow the criteria: the additives should effectively adsorb onto the nucleating centers (i.e., dust particles) and be capable of disrupting the interaction between the nucleating phase and the nucleating centers. Then, one of the key questions to be addressed is how to design or select the additives. On the basis of the results obtained from theoretical calculations,30-35 we will provide some guidelines as follows: (1) Large molecule with a relatively rigid basic structure. The rigidity of additive molecules can result from a variety of molecular features, such as the intramolecular bonding (i.e., hydrogen bonds, double or triple covalent bonds) and the presence of bulky functional groups in the backbone of the molecules. On the basis of both energetic and entropic considerations, for different molecules of similar types, larger molecules with somewhat rigid structures are easily adsorbed at interfaces.31,32 (2) Stronger interaction between additives and the substrate will lead to a stronger adsorption at the surface.30-35 Since the surface of crystals is highly ordered and stiff, to obtain the maximal interactions by matching the structure of the substrate, it is desirable to have short and relatively flexible functional groups attached to the backbone of additives molecules so that they can adjust their positions to obtain optimal interactions with the solid molecules at the surface of crystals.

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(3) The adsorbed additives should interrupt the growth of crystal layers along the substrate.30-35 The repulsions can originate from steric, electrostatic, polar/nonpolar, or hydrophilic/hydrophobic forces, and can be achieved by attaching some functional groups to the backbone of the molecules of additives. (4) The concentration of additives also plays a certain role.30-35 For a given system, the excess in the concentration may not enhance the adsorption; it sometimes can even cause a decrease in the adsorption. This is attributed to the adsorption synergism by mixing additives with other structural units. It is noteworthy that all the above-mentioned factors are correlated to each other. Therefore, considerations should be given from different perspectives. Detailed guidelines for the selection of additives can be found in a previous publication.16 On the basis of the above criteria, a copolymer, PMMMA was adopted to control the nucleation of GP-1 molecules. The molecule of a polymer such as PMMMA with a rigid backbone has been proven effective to adsorb on the surface of fatty or hydrophobic dust particles.15,16 PMMMA has the following molecular structure:

Obviously, the structure of this copolymer satisfies all the structural requirements: (-COOCH3) and COOH in the molecules fulfills the requirements of (1) and (2) and (C2H4)y satisfies the requirement of (3). In addition to the number of spherulites, the branching density or the compactness of the networks is very important, as it will determine whether the growing fibers can interpenetrate into the adjacent fiber networks. In this regard, the principles of controlling the branching density can be referred to from our previous works.16,19,27 As a comparison, the microstructure dependent elasticity of two other materials formed by the gelation of GP-1 in benzyl benzoate (GP-1/BB) and 12-hydroxystearic acid in benzyl benzoate (HSA/BB) consisting of interpenetrating spherulitic fiber networks will also be presented. 3. Materials and Methods 3.1. Chemicals. PMMMA copolymer with a methyl methacrylate (MM) to methacrylic acid (MA) molar ratio of 1:0.016, 1,2-propanediol/propylene glycol (PG), 12-hydroxystearic acid (HSA), and benzyl benzoate were obtained from Sigma. N-Lauroyl-L-glutamic acid di-n-butylamide (GP-1) was obtained from Kishimoto Sangyo Asia. The molecular structures of HSA and GP-1 are shown below.

3.2. Microscopic Observation of Gel Microstructure. For the optical observations, thin sample films (0.1 mm) were prepared by

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sealing the hot solute solution in a self-made glass cell. A microscope (Olympus BX50) with a heating/cooling temperature controller (Linkam Scientific Instrument, THMS600) at the sample stage was used. The temperature ramp rate was set at 30 °C/min with an accuracy of (0.1 °C. The sol-to-gel transition was monitored by a video system. The images from the microscope were converted to digital images through a JVC KY-F55B 3-CCD color video camera. A series of images were obtained during the gelation process and analyzed by image processing software (analySIS version 3.2). The number density of spherulite was averaged from five micrographs taken at different places of a sample. For the sake of simplicity, the concentrations of GP-1, HSA, and PMMMA (weight percentage, wt%) all through this work are given as %. 3.3. Rheological Study. The rheological properties of the organogels were measured by an advanced rheological expansion system (ARES-LS, Rheometric Scientific). Dynamic temperature ramp tests were carried out to obtain the storage modulus G0 (a measure of elasticity), loss modulus G00 (a measure of viscosity), and complex modulus G* (viscoelasticity, G*= [(G0 )2 þ (G00 )2]1/2) as a function of time. The sol-gel process was performed in situ between two plates with a gap of 0.85 mm. The samples were subjected to sinusoidal oscillation by moving both the upper (with a diameter 25 mm) and the lower circular plates. The amplitude of the oscillation was controlled to obtain a strain of 0.05% in the sample. The oscillation frequency was set at 0.1 Hz and the temperature ramp rate was 30 °C/min. Table 1. Summary of Thermotropic Properties of the Three Supramolecular Functional Materials gels

ΔHdiss (kJ mol-1)

ΔSdiss (kJ mol-1 K-1)

HSA/BB GP-1/BB GP-1/PGa

59.0 60.9 59.3

0.21 0.13 0.13

a

From ref 22.

Li et al.

4. Results and Discussion 4.1. Thermotropic Properties of Solutes in Solvents. Supersaturation is the thermodynamic driving force for the formation of soft materials. To determine this driving force, the solubility data are essential. According to van’t Hoff’s equation, the solubility of solute in a given solvent can in principle be described by ΔHdiss ΔSdiss ð5Þ þ ln X e ¼ RT e R where X e is the equilibrium concentration at a certain temperature; ΔSdiss denotes the molar dissolution entropy. According to eq 5, within a certain concentration range, ln X e can be expressed as a linear function of 1/T e. On the other hand, the equilibrium concentration X e as a function of temperature can be obtained if ΔHdiss and ΔSdiss are known. Thus, the supersaturation can be calculated from eq 4. The method used to determine X e and T e has been reported.18 Table 1 summarizes the dissolution enthalpy ΔHdiss and the entropy ΔSdiss of the three systems. 4.2. Microstructure-Dependent Elasticity of the Three Supramolecular Functional Materials. Figure 2A gives the evolution of the elasticity G0 of the three materials. To eliminate the effects of fiber mass on the value of G0 , the final fiber mass of the soft materials is tuned to the same by manipulating the solute concentrations and the temperature at which the gels form. The result indicates that elasticity of the GP-1/BB and HSA/BB gels are more than twice that of the GP-1/PG gel. The corresponding micrographs of the three materials are given in Figure 2B-D. It follows that GP-1 forms more compact spherulites in PG. The spherulites are mutually exclusive as evidenced from the clear boundary between

Figure 2. G0 of the three soft materials with the same fiber mass. 5% GP-1/BB, 70 °C; 6% HSA/BB, 50 °C; and 5% GP-1/PG, 50 °C, (A) and the corresponding microstructures of the three soft materials (B-D). Scale bars: 100 μm.

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Figure 3. Micrographs of soft materials formed by 5 wt % GP-1 in PG (A) and EG (B) at 20 °C. The scale bars are 50 μm.

adjacent spherulites. In BB, it forms less compact and interpenetrating spherulites. HSA also forms the similar fiber network in BB. The differences in the microstructures of the materials explain the differences observed in their elasticity. For the GP-1/BB and HSA/BB gels, the interpenetration (and perhaps interlocked) of the fibers in one spherulite into the neighboring spherulites makes them behave like single fiber networks with higher elasticity. In contrast, the incidence of boundary between the spheruites of GP-1 formed in PG weakens the material. Therefore, to make a stronger material, the boundary has to be reduced. It is interesting to observe that the same gelator (GP-1) can form spherulites with different compactness when the solvent is varied (PG and BB). The compactness of a spherulite depends on the branching density of the fibers, which for a certain system is decided by the supersaturation of the solute.18,19 A higher supersaturation is conducive to the mismatched nucleation-mediated fiber branching.18,19 For soft materials formed by a certain gelator in different solvents, the type of solvent will affect not only the supersaturation of the gelator but also the arrangement of the gelator molecules during the crystallization process. For solvents of similar molecular structures, their effects on the molecular arrangement of the gelator are similar and the supersaturation effects are dominant in deciding the structure of the fiber networks. To demonstrate this, the microstructure of the GP-1 fiber network formed in a more polar solvent, ethylene glycol (EG) was examined. As shown in Figure 3, more compact spherulites of GP-1 formed in EG. GP-1 is less soluble in EG than in PG. Therefore, at a given concentration of GP-1 and at the same temperature, the supersaturation of GP-1 is higher in EG. Although the dissolution enthalpy of GP-1 in BB is higher than that in PG, the spherulite formed in BB is less compact. As shown by Figure 2B, the centers of spherulite of GP-1 formed in BB are elongated, which is in contrast to the spherical centers of GP-1 spherulite formed in PG. The elongation of the center means that the chemical potential and hence the growth of GP-1 fiber are not homogeneous on all directions. Because of the high hydrophobicity of BB, its molecules have a higher affinity to the hydrophobic chain of the GP-1 molecules. The interaction of BB molecules may have a strong effect on the packing of the GP-1 molecules, influencing the fiber branching and compactness of the final fiber network. The effects of solvent on the microstructure of fiber network have been investigated by researchers.36,37 4.3. Reducing the Boundary Effects by Controlling the Supersaturation of the Gelator. The effects of supersaturation on the microstructures of the three materials were investigated. Figure 4 shows the micrographs of the GP-1/PG

Figure 4. Micrographs of GP-1 fiber networks formed in PG at different temperatures (A-C) and HSA fiber networks formed in BB (D-E). Temperature: (A) 20 °C, (B) 40 °C, (C) 50 °C, (D) 20 °C, (E) 50 °C, (F) 55 °C. The concentrations of GP-1 and HSA are 3% and 6%, respectively. The scale bars are 100 μm.

(Figure 4A-C) and HSA/BB gels (Figure 4D-F). The micrographs demonstrate that the spherulites in both of the two materials become larger when the temperature for gelation was elevated (or the supersaturation is lowered, i.e., eq 3). At a higher temperature, the supersaturation of the gelator in the solvent is lower since more gelator molecules are dissolved in the solvent. A lower supersaturation reduces the thermodynamic driving force (i.e., eq 3) and the nucleation rate, leading to the formation of fewer and larger spherulites. In the case of the GP-1/PG gel, the boundary between the spherulites is reduced. The effect of supersaturation on the fiber network formation and the fibrous network structures of GP-1 in BB (not shown) follows the same trend as the HSA/BB gel. The supersaturation dependence of G0 is plotted in Figure 5. It shows that for GP-1/BB and HSA/BB gels, the elasticity of the materials (G0 ) increases with the increase in supersaturation (decrease in temperature) and can be correlated linearly with ln(σ). However, the G0 of GP-1/PG gel shows a different trend as the superaturation changes. The G0 of this material increases with supersaturation initially and then drops almost linearly with the further increase in supersaturation. The initial increase should be attributable to the increase in fiber mass. For a gel system, when the supersaturation of the solute is below a critical value, a self-supporting material

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Figure 5. Supersaturation dependence of the elastic modulus of soft materials. (A) GP-1/BB, (B) HSA/BB, and (C) GP-1/PG.

cannot be formed due to the lack of a fiber network that is dense enough to trap the solvent. The linear drop above a certain supersaturation is due to the increase in the incidence of boundary between the spherulites. The results indicate that for a pair of solute/solvent, the rhelogical properties of the material can be simply tuned by adjusting the supersaturation of the solute, which offers a convenient approach toward the engineering of the material. For a material with mutually exclusive spherulites, the elasticity can be improved by reducing the supersaturation of solute. In contrast, for a soft material with interpenetrating spherulites, increasing supersaturation increases its elasticity. 4.4. Reducing the Boundary Effects by Additive-Modified Nucleation. Although reducing the supersaturation by increasing the temperature is a convenient way to engineer a soft material with mutually exclusive spherulitic fibers, a higher processing temperature or a low supersaturation will on the other hand correspond to a low fiber mass. More importantly, it is infeasible to process a gel product if temperature sensitive components (such as biomolecules) are to be incorporated into the fiber matrix when soft material forms. Therefore, it is important if the elasticity of such a soft material can be improved at a higher supersaturation (lower temperature). In this regard, the additivemodified nucleation is superior. The micrographs of the fiber GP-1 networks obtained at different PMMMA concentrations are given in Figure 6A-D. The presence of PMMMA effectively reduced the number of spherulites (nucleation rate), which resulted in the formation of larger sphrulites.

Figure 6. Micrographs of GP-1 fiber network (A-D) and effects of PMMMA on the elasticity of the GP-1/PG gel formed at 20 °C (E). PMMMA concentrations: A: 0, B: 0.02%, C: 0.04%, D: 0.06%. The scale bars are 50 μm.

When the polymer concentration is above 0.04%, a noticeable change in size of the spherulites was not observed. The effect of PMMMA on the storage modulus G0 was characterized. The values of G0 of the GP-1/PG gel at 20 °C are shown in Figure 6E. The G0 displays significant increases when the PMMMA concentration is below 0.02%, which gradually levels off with a further increase in the polymer concentration. When the PMMMA concentration is 0.06%, the G0 of the material is about 75 000 N/m2, which is 5 times that obtained in the absence of the polymer. This value is close to the maximal G0 obtained by tuning the supersaturation of the solute. The size of the spherulites formed in the presence of 0.06% PMMMA is similar to that of those formed at 40 °C in the absence of the additive. However, the G0 of the material is only about 50 000 N/m2 in the later case, which is due to the lower fiber mass. The polymer molecules may also strengthen the interactions between neighboring spherulites, reducing the boundary effect and improving the elasticity of the material. This will be discussed later. To obtain a more in-depth understanding on the effects of supersaturation and polymer on the rheological properties of the GP-1/PG gel, dynamic strain analysis was carried out. Apart from G0 , one important parameter to characterize the rheological properties of a soft material is the critical strain

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Figure 8. Schematic description of the interaction of PMMMA molecules with neighboring fibers of a single spherulite and neighboring fibers on two adjacent spherulites. The bridging effects of PMMMA can make the fiber network more resistant to an external strain.

Figure 7. Critical strain analysis of GP-1/PG soft material formed at different temperatures and the influence of additive. (A-C) without additive; (D) with 0.06% PMMMA.

γc (γc is defined as the minimal strain required to break down partially the network structure of the soft material). The determination of γc is shown in Figure 7A. The γc of the material formed at 20, 40, and 50 °C in the absence of the additive was found to be 0.7, 0.4, and 0.3%, respectively. This observation indicates that although the elasticity (G0 ) of the material can be improved, the network becomes more brittle especially when the fiber network is formed at a high temperature. Although the presence of additive also enlarged the size of spherulites, the γc of the material was not compromised. The γc of the material obtained at 20 °C in the presence of 0.04% and 0.06% (Figure 7D) are 1.5 and 2.0%, respectively. The results show that the presence of the polymer significantly reduced the brittleness of the material. As shown in Figure 7, the higher brittleness of the fiber network formed at higher temperatures was also evidenced by the decrease in the γ0. γ0 is defined as the strain at which G0 equals to G00 . When the strain is below this value, the material is solid-like (G0 > G00 ), while it becomes fluidic (G0 < G00 ) when the strain is above this critical value. At 20 °C and in the absence of additive, this point was not observed when the strain was as high as 20% (i.e., γ0 > 20%) (Figure 7A). It was reduced to 8.0% (Figure 7B) and 1.5% (Figure 7C) for the fiber networks formed at 40 and 50 °C, respectively. In the presence of the polymer additive, the γ0 was not compromised even at the strain of 20% (Figure 7D). This implies that although the increase in the size of spherulites makes the fiber network more brittle, the presence of the additive compromises this negative effect. It was demonstrated in our recent work that PMMMA molecules can reduce the nucleation rate and retard the fiber growth of GP-1 by adsorbing on the surface of the nucleating centers and growing fiber tips.38 The strong interaction between PMMMA and GP-1 molecules can be understood on the basis of the energetic and entropic considerations. The rigid PMMMA molecule with functional groups on the repeating unit makes it possible to adsorb on the neighboring branches of GP-1 fiber and interact with fibers on neighboring spherulites in close vicinity. This “bridging” effect will

make the fibers more resistant to the external forces and improve the critical strain to break the fiber structure (Figure 8). This can also be interpreted on the molecular scale. The presence of the carboxylic hydrogen on the PMMMA molecule makes it possible to form hydrogen bonds with the carbonyl oxygen on the GP-1 molecule. Hydrogen bond can also form between the carbonyl oxygen on the PMMMA molecule and the amide hydrogen on the GP-1 molecule. These interactions together with the long chain of the polymer molecule make the bridging between the neighboring fibers of a same spherulites or adjacent spherulite possible. Nevertheless, more detailed work needs to be carried out to prove it. 5. Conclusions In summary, the supersaturation dependence of the elasticity of three supramolecular functional materials formed by GP-1 in PG, GP-1 in BB, and HSA in BB was investigated. All of the three soft materials consist of spherulitic fiber networks. GP-1/PG soft material behaves differently from the other two soft materials. Because of the mutually exclusiveness of the spherulites, the elasticity of this material drops linearly with an increase in supersaturation in contrast to the linear increase observed for the other two materials. The high incidence of boundary which is the mechanically weak fraction of the GP-1/PG material contributes to its low elasticity and the drop in elasticity with an increase in supersaturation. The boundary effect was reduced by lowering the supersaturation and by introducing a suitable copolymer additive. The introduction of additive is a better approach in that the gelation can be processed at a low temperature which is desired for many temperature-sensitive processes. In addition, the presence of the additive molecules also reduced the brittleness of the fiber network of this material. This work provides a robust approach to the engineering of a soft functional with spherulitic fiber network. Acknowledgment. X.Y.L. gratefully acknowledges the financial support from Singapore ARC MOE funding (Project No. T13-0602-P10), the China NRF Grant (Project No. 50928301) and the China Chang Jiang Chair Professorship Program (Dong Hua University).

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