ARTICLE pubs.acs.org/crystal
Kinetically Controlled Homogenization and Transformation of Crystalline Fiber Networks in Supramolecular Materials Jing-Liang Li,†,‡ Bing Yuan,†,^ Xiang-Yang Liu,*,†,§ Xun-Gai Wang,‡ and Rong-Yao Wang†,|| †
Department of Physics and Department of Chemistry, National University of Singapore, 2 Science Drive 3, Singapore 117542 Centre for Material and Fiber Innovation, Institute for Technology Research and Innovation, Deakin University, Waurn Ponds, Victoria 3216, Australia § College of Material Science and Engineering & State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, China ^ National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China Key Laboratory of Cluster Science of Ministry of Education, Beijing Institute of Technology, Beijing 100081, P. R. China
)
‡
ABSTRACT: Supramolecular materials with three-dimensional fiber networks have applications in many fields. For these applications, a homogeneous fiber network is essential in order to get the desired performance of a material. However, such a fiber network is hard to obtain, particularly when the crystallization of fiber takes place nonisothermally. In this work, a copolymer is used to kinetically control the nucleation and fiber network formation of a small molecular gelling agent, N-lauroyl-L-glutamic acid di-nbutylamide (GP-1) in benzyl benzoate. The retarded nucleation and enhanced mismatch nucleation of the gelator by the additive leads to the conversion of a mixed fiber network into a homogeneous network consisting of spherulites only. The enhanced structural mismatch of the GP-1 during crystallization is quantitatively characterized using the rheological data. This effect also leads to the transformation of an interconnecting (single) fiber network of GP-1 into a multidomain fiber network in another solvent, isostearyl alcohol. The approach developed is significant to the production of supramolecular materials with homogeneous fiber networks and is convenient to switch a single fiber network to a multidomain network without adjusting the thermodynamic driving force.
’ INTRODUCTION Supramolecular soft functional materials such as small-molecule gels (SMG) supported by three-dimensional fiber networks have important applications in many fields, such as cosmetics, pharmaceutics, food engineering and bioseparation, preservation of artworks, and preparation of nanomaterials.16 A SMG forms when a hot solution of a gelator is cooled to a temperature below the critical gelation temperature tg.7 The micro- or nanometer structure of the fiber network of a supramolecular material determines its macroscopic properties and performance.8 The formation of supramolecular architectures in SMGs has been generally considered to be a self-assembly process of gelator molecules through noncovalent forces.9,10 Due to the strong solvent dependence of the gelling capacity and fiber structure,11,12 architecture design through the development of novel gelators is a labrious approach. Interestingly, it has been verified recently that the formation of the fiber network in SMGs is thermodynamically controlled. A fiber network in such a material is initiated by the primary nucleation of the gelator molecules, followed by the growth and branching of fibers.8,13,14 As a result, an entire fiber network of a gel is composed of a collection of individual fiber networks. The crystalline nature of fibers including those formed by the gelator in this work has been demonstrated.15,16 The nucleationgrowth mechanism makes it possible to engineer r 2011 American Chemical Society
the fiber network structures of the existing systems by kinetically controlling the nucleation and growth of fibers. In most of the gelling systems, the fiber formation and gelation is a nonisothermal process due to the insufficient cooling rate or speed. That is, the temperature changes as the fibers crystallize. In other words, for such a system, the nucleation and growth of fibers take place (at T2) before a hot solution (at T1) is cooled to a settling temperature T3 (for example, environmental temperature as in most industrial manufacturing processes) (T3 < T2) (Scheme 1a). Due to the difference in supersaturation at T2 and T3, fiber networks of different morphologies normally form at these temperatures.17 At the higher temperature T2 or a lower supercooling, (supercooling ΔT = Teq T, Teq = equilibrium temperature of the gelator solution, T = actual temperature), less branched fibers normally form, while at a lower temperature, the corresponding higher supercooling can lead to the formation of highly branched fibers (i.e., spherulites) due to enhanced mismatch nucleation.17 Consequently, the entire fiber network is often a heterogeneous (mixed) fiber networks of different types Received: April 20, 2011 Revised: May 22, 2011 Published: June 02, 2011 3227
dx.doi.org/10.1021/cg200501h | Cryst. Growth Des. 2011, 11, 3227–3234
Crystal Growth & Design Scheme 1. A Schemetic Description of the Homogenization of Fiber Network Formed during a Non-Isothermal Crystallization Process by Kinetically Controlling the Nucleation of Fibersa
ARTICLE
formed by a low molecular weight gelator, N-lauroyl-L-glutamic acid di-n-butylamide (GP-1) in benzyl benzoate is used as a model material. GP-1 is an amino acid derivative, which consists of a hydrophobic part and a hydrophilic part (see molecular structure of GP-1 in Materials and Methods). An ethylene/vinyl acetate copolymer (EVACP) additive, which has been shown to adsorb strongly on the surface of fibers,13 is employed to kinetically control the nucleation and growth of GP-1 fibers. It is interesting to observe that the inhibition of GP-1 nucleation by this polymers prevents the formation of the intermediate fiber network at the early cooling stage. This effect contributes to the formation of fiber networks of spherulites only in contrast to the mixed fibrillar and spherulitic fiber networks formed in the absence of this additive. To demonstrate quantitatively the capacity of this polymer to retard the nucleation of GP-1, its effects on the fiber network and gel formation of GP-1 in another solvent, namely, isostearyl alcohol, is investigated instead. In this solvent, GP-1 nucleates and crystallizes isothermally, making it convenient to follow the induction time by using rheological data. Due to the limited solubility of EVACP in these two solvents, we did not examine the effects of this additive at higher concentrations.
’ MATERIALS AND METHODS
(a) A heterogeneous fiber network formed due to the nucleation and growth of fibers at different temperatures (T2 and T3) during the cooling period; (b) by kinetic inhibition of the nucleation of fibers to suppress the fiber formation during the early stage of a cooling process (T2), a homogeneous fiber network can be formed (at T3). a
(Scheme 1a). This is not desired in many important applications such as separation, when a homogeneous (pure) fiber network with uniform pore sizes is required. Therefore, it is important that the formation of fiber networks in such a material can be controlled to prevent the formation of a heterogeneous fiber network. In addition, as the nonisothermal process affects significantly the morphology, size, and spatial distribution of fibers, as well as the mechanical and solvent binding capacity of the gels,18,19 it is practically significant to control the nonisothermal effects. On the basis of the nucleation growth mechanism, it is feasible that by kinetically inhibiting the primary nucleation of the gelator molecules to prevent the occurrence of fibers formed at the early stage of cooling, a system consisting of a uniform fiber network can be created (Scheme 1b). It is known that primary nucleation is activated by nucleation centers, that is, dust particles, bubbles, etc., under normal circumstances.20,21 It has been reported that the presence of certain additives particularly large polymer molecules with a rigid molecular structure can inactivate the nucleation centers by enhancing the structural mismatch between the nucleating phase and the substrate.13,16,22 This gives rise to retarded nucleation in the systems. Although growing interest is shown in using small molecular additives such as inorgnic salts to alter the gelling capacity of some gelators,2326 the feasibility to finely tune the nucleation and microor nanometer structure of fiber networks by these additives has not been demonstrated. The aim of this work is to develop a simple approach to homogenizing the fiber networks formed nonisothermally using a suitable polymer additive to retard the nucleation of a gelator. A gel
Chemicals. Benzyl benzoate was obtained from Sigma. N-Lauroyl-L-glutamic acid di-n-butylamide (GP-1) and isostearyl alcohol (ISA) were obtained from Kishimoto Sangyo Asia. The ethylene/vinyl acetate copolymer (EVACP) (MW 100 000, 40% vinyl acetate) was obtained from Scientific Polymer Products, Inc. All the chemicals were used as received. The molecular structures of GP-1 and EVACP are given below.
Microscopic Observation of Gel Microstructure. Thin sample films (0.1 mm) were prepared by sealing the hot solutions of GP-1 in the solvents in self-made glass cells. A microscope (Olympus BX50) with a heating/cooling temperature controller (Linkam Scientific Instrument, THMS600) was used. The temperature ramp rate was set at 50 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 3228
dx.doi.org/10.1021/cg200501h |Cryst. Growth Des. 2011, 11, 3227–3234
Crystal Growth & Design
ARTICLE
through a JVC KY-F55B 3-CCD color video camera. A series of images were obtained during the gelation process. For the sake of simplicity, the concentrations of GP-1 and EVACP (weight percentage, wt %) all through this work are given as %. 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 + (G00 )2]1/2) as a function of time. The solgel process was performed in situ between two circular 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 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 50 C/min.
’ RESULTS AND DISCUSSION It was observed that without any additive, GP-1 forms a heterogeneous fiber network consisting of less branched (elongated) fibers (formed at a higher temperature) and spherulitic fiber networks (at a lower temperature) in benzyl benzoate (Figure 1a). The addition of a tiny amount of EVACP, on the other hand, suppresses the formation of the elongated fibers at the early cooling stage and leads to formation of a uniform network consisting of GP-1 spherulites only (Figure 1b and 1c), which is evidenced by the clear boundary between the neighboring spherulites. In comparison, such a clear bounday was not found in the systems without EVACP (Figure 1a). In addition, the spherulites formed in the presence of EVACP are much bigger than those formed in the absence of this additive. The size enlargement can be attributed to the reduced nucleation rate of primary nucleation of GP-1 in the presence of this polymer, which is to be discussed in more detail. In a fixed volume, the reduced primary nucleation rate implies the formation of a smaller number of individual fiber networks (spherulites in this case). The size of the spherulites are hence larger.22 The in situ obsevation of the early stage of GP-1 fiber network formation in benzyl benzoate was carried out in order to understand the effects of EVACP on the GP-1 fiber network formation in this solvent. Figure 2 demonstrates a few micrographs taken during the cooling of a hot solution of GP-1. When a hot solution (120 C) of 2% GP-1/BB is cooled to 77.6 C, small fibrils appear (Figure 2a). With further reduction in temperature, fiber growth and branching occur. When the temperature is reduced to 75.2 C, (Figure 2b), the whole volume is occupied by more branched fibrils. Due to the low supersaturation, fiber branching takes place primarily on the side surface of fibers (side branching). Further cooling leads to the creation of spherulitic fiber networks, which finally evolve into a network like the one shown in Figure 1a. The formation of a heterogeneous fiber network is demonstrated more clearly when the concentration of GP-1 is increased to 5% (Figure 3). At this high concentration, when a hot solution of GP-1 dissolved in BB is cooled to about 87 C, a network of GP-1 fibrils forms first (Figure 3a). With the further decrease in temperature, spherulitic fiber networks form subsequently (Figure 3b). The thin fibrils are not discernible in the final micrograph. The formation of a heterogeneous fiber network of GP-1 in BB is due to the nonisothermal nucleation and crystallization GP-1 in this solvent. In an isothermal process, the nucleation, crystallization, and fiber network formation occurs at a constant tempera-
Figure 1. Fiber networks of GP-1 (2%) formed in the absence (a) and presence of 0.005% (b) and 0.01% EVACP (c). Clear boundaries between individual spheruites are discernible when the polymer is present. With increase in polymer concentration, the spherulites becomes larger, indicating inhibited nucleation of GP-1 at a higher polymer concentrations. The scale bar is 100 μm. The images are on the same scale.
ture (Figure 4a) during the whole process. In such a process, the supersaturation of the system gradually decreases due to the reduction of gelator concentration as crystallization goes on. For a nonisothermal process, nucleation and crystallization start before the solution is cooled to the final temperature (T3 in Figure 4b), due to insufficient cooling speed. This brings two opposite effects on the supersaturation of the system. On one hand, the decrease in temperature increases the supersaturation, and on the other hand, the nucleation and crystallization consume the gelator molecules, which decreases supersaturation. That is, the overall supersaturation of the system is the net outcome of these two processes. In a wellmixed system where mass and heat transfer are not limited, supersaturation can remain constant. However, in a viscous gel system, the transfer can be a limiting factor, particularly for a nonisothermal 3229
dx.doi.org/10.1021/cg200501h |Cryst. Growth Des. 2011, 11, 3227–3234
Crystal Growth & Design
Figure 2. Real-time observation of the early stage GP-1 fiber formation in BB in the absence of an additive. Micrographs were recorded when the sample was cooled from 120 C at a cooling rate of 50 C/min. The corresponding temperatures for panels a and b are 77.6 and 75.2 C, respectively. Enhanced side branching of fibers at a lower temperature is illustrated by the circled. The scale bar is 100 μm. The two images are on the same scale.
crystallization process.27 This leads to the quick buildup of local supersaturation. Therefore, it is reasonable to assume that a maximal supersaturation may exist for such a system. Due to the enhanced mismatch nucleation with an increase in supersaturation, more branched fibers and even fiber spherulites can form.17 Interestingly, the enhanced structural mismatch has been monitored by synchrotron Fourier transform infrared spectroscopy on a molecular level in some nonisothermal fiber crystallization processes.28 It is interesting to observe that in the presence of 0.01% EVACP, the primary nucleation of GP-1 is retarded. The primary nucleation in this case takes place when the hot solution of GP-1 is cooled to about 59 C (Figure 5a), which is 18.6 C lower than that (77.6 C) when this polymer is absent. This means that EVACP inhibits the nucleation of GP-1 fibers. Due to the lower nucleation temperature (or higher supercooling), more branched fiber networks (spherulites) grow from the nucleation centers (Figure 5a,b). It has been demonstrated in our previous works that a higher driving force (supersaturation) promotes fiber branching.29 When the driving force is low, fibers tend to grow with a lower degree of branching, which leads to the formation of elongated fibers. In such a case, nucleation is kinetically favored at some kink sites existing on the surface of
ARTICLE
the substrate (Scheme 2a). Apart from the thermodynamic effects, during the primary nucleating stage, the adsorption of EVACP molecules can homogenize the surface of the substrate and physically hinder the integration of the nucleating molecules, which can also enhance the structural mismatch nucleation, leading to highly branched fibers (spherulites in this instance) (Scheme 2b). This promotes the formation of a multidomain fiber network if the individual spherulites are compact enough (Scheme 2c). Because the microstructure of the fiber network affects the rheological properties, particularly the elasticity of a gel, the evolution of elastic modulus of the GP-1/BB gel in the absence and presence of EVACP is monitored. The nonisothermal process is also demonstrated by the rheological data. As shown in Figure 6, when the temperature of a hot solution at 120 C is cooled to 20 C, the storage modulus G0 , which characterizes the elasticity of the gel, almost reaches its maximum. The presence of 0.005% EVACP does not have a significant effect on G0 and the addition of 0.01% EVACP doubled the G0 (from ca. 17 000 to 30 000 N/m2). As discussed, the fiber network of a gel with a fixed volume consists of a number of individual fiber networks, with each originating from a primary nucleation center. The macroscopic properties, particularly the elasticity of a gel, are determined by the strength of the individual fiber networks and the interactions between them. If the individual fiber network is not dense enough and the fibers from each fiber network can penetrate into neighboring networks, the entire fiber network behaves like a single fiber network.8,16 In contrast, if the individual fiber networks are compact and fibers from each network cannot enter into the neighboring networks, clear boundaries exist between individual fiber networks. The entire fiber network can thus be considered as a multidomain network.8,16 The typical example is a network comprised of compact spherulites.22 Due to the weak interaction between the individual fiber networks, the elasticity of a multidomain network is normally inferior to that of an interconnecting fiber network with a same fiber mass.16 For a gel with a multidomain network, its elasticity has been demonstrated to be affected by the size of the spherulites.22 In a fixed volume, an increase in the size of spherulites (corresponding to a reduction in primary nucleation rate) means the boundary area between the spherulites, which is mechanically weak compared with the parts occupied by the fiber networks, is reduced. This will lead to improvement in elasticity of the gel. As has been shown in Figure 1, the size of the spherulites is enlarged when the concentration of EVACP is increased from 0.005% to 0.01%, enhancing the G0 . Although at the EVACP concentration of 0.005%, the sizes of the spherulites (Figure 1b) are also larger than those in the absence of this additive (Figure 1a), the fibrils coexisting with the spherulites (mixed fiber network) in the latter case reduce the boundary effects, which offset negative effects from smaller spherulites. As a result, no obvious difference in G0 is observed. In other words, for a gel with a fiber network consisting both fibrillar network and spherulitic fiber networks, its elasticity is contributed by both types of networks. However, due to the low fiber mass produced at the high temperature (low supersaturations), the fibrils do not have a significant effect on the overall elasticity of the gel. The elasticity of the gel with a mixed fiber network is still lower than that of the gel formed in the presence of 0.01% EVACP. A structural transition from a less branched interconnecting fibrillar network of GP-1 into a network consisting of spherutes (multidomain) was observed when benzyl benzoate was replaced 3230
dx.doi.org/10.1021/cg200501h |Cryst. Growth Des. 2011, 11, 3227–3234
Crystal Growth & Design
ARTICLE
Figure 3. Real-time observation of GP-1 fiber network formation in the absence of an additive at a GP-1 concentration of 5 wt %. Cooling rate is 50 C/min. The scale bar in panel a is 100 μm. All the images are on the same scale.
with another solvent, isostearyl alcohol (ISA) (Figure 7). This transformation is also due to the strong adsorption of GP-1 molecules on the fibers in this solvent, which homogenizes the surface of the embryo during primary nucleation and promotes mismatch nucleation during fiber growth. GP-1 has a higher solubility in ISA, with a minimal (critical) gelation concentration of ca. 3.5% at room temperature (ca. 20 C), which is lower than that (ca. 0.5%) in benzyl benzoate. The gel forms slowly and takes a few hours to complete. Therefore, a higher concentration of GP-1 in ISA, which is 6%, was used. Despite the difference in the concentration of GP-1 in these two solvents, the supersaturation of GP-1 in them is similar. The conversion of an interconnecting fibrillar network (or a single fiber network as boundaries between individual networks are not discernible) to a multidomain network is an interesting observation. For a given system, this has been achieved by adjusting the thermodynamic driving force of the system.17,30 However, this takes place generally at extremely low supersaturations (high temperatures for a fixed mass of gelator). In contrast, in this work, the introduction of the polymer makes the conversion achievable at normal temperatures. That is, it also provides a convenient approach for the microstructure transformation from a single fiber network to a multidomain fiber network. The fiber network formation of GP-1 occurs isothermally in ISA due to the slow gelation (fiber formation), which makes it possible to measure the induction time, tg, of GP-1. It was observed that the presence of EVACP can dramatically retard the formation of GP-1 fiber network in ISA. The enhancement of structural mismatch by the EVACP molecules is also quantified. Figure 8a shows the
evolution of G0 of GP-1/ISA gel as a function of time. It demonstrates that the presence of EVACP retards formation of the gel, as indicated by the longer induction time, tg, in the presence of this additive. In addition, the presence of EVACP reduced the elasticity of the gel. The values of G0 are 1.3 106, 5.9 105, and 3.2 105 N/m2 in the absence and in the presence of 0.005% and 0.01% EVACP, respectively. The decrease in G0 is due to the transformation of an interconnecting (single) fiber network into a multidomain network as shown in Figure 7. Although EVACP also promotes branching of fibers, as evidenced by the denser fiber networks in the individual spherulites (Figure 7c), the boundary effect dominates the elasticity, leading to an overall decrease in G0 . Despite the adverse effects, a strong self-supporting material, which is characterized by the large G0 even in the presence of 0.01% EVACP, still forms with the addition of EVACP. In general, the primary nucleation in a solution takes place as a heterogeneous process, in which foreign bodies such as dust particles or air bubbles act as substrates. Homogeneous nucleation where no substrate is involved only occurs when the supersaturation of the solute is extremely low. The presence of a substrate can generally lower the nucleation barrier.31,32 The nucleation barrier, ΔG, for homogeneous and heterogeneous nucleation can be written as
ΔGhetero ¼ f ΔGhomo
ΔGhomo ¼ 3231
16πγcf3 Ω2 3ðkTÞ3 ðΔμ=ðkTÞÞ2
ð1Þ ð2Þ
dx.doi.org/10.1021/cg200501h |Cryst. Growth Des. 2011, 11, 3227–3234
Crystal Growth & Design
Figure 4. Illustration of the changes in temperature, supersaturation, and elastic modulus with time in an isothermal nucleation and crystallization process (a) and a nonisothermal process (b) for fiber network formation in a supramolecular soft material. In an isothermal process, the supersaturation decreases monotonically during the fiber formation. In a nonisothermal process, during the period of t1 and t2, the supersaturation either remains constant or fluctuates. If mass transfer is limited (kinetically controlled), a maximal supersaturation may exist during this period (dotted lines). In panel b, t1 corresponds to the time when the hot solution (at temperature T2) is cooled to the temperature (Tc) at which the primary nucleation occurs and t2 corresponds to the time when the solution is cooled to the final temperature T3. (gelation temperature). The critical temperature, Tc, in an isothermal process (panel a) does not exist. T1 is the temperature of the gelatorsolvent blend before heating (to dissolve the gelator) starts, which is normally the ambient temperature.
with Δμ/(kT) = ln(X/Xe) = ln(1 + σ) where f is a factor describing the lowering of the nucleation barrier due to the presence of a substrate, Ω is the volume of the growth units, k is Boltzmann’s constant, T is temperature, and Δμ denotes the chemical potential difference between nucleating molecules in the crystal and in the liquid phase; X and Xe are molar fractions of the nucleating molecules in the solution and at equilibrium conditions, respectively; σ is supersaturation of the system (σ = (X Xe)/Xe). In general, for a certain system, f is between 0 and 1, which means the primary nucleation is normally governed by heterogeneous nucleation. In the case of a perfect match, f f 0. This implies that the heterogeneous nucleation barrier vanishes almost completely. This occurs when the new layer of crystal is well ordered and oriented with respect to the structure of the substrate. When there is no correlation between the substrate and the nucleating phase, f f 1. In this case, the substrate almost exerts no influence on nucleation, which is equivalent to homogeneous nucleation. The emerging nuclei in this case are completely disordered, bearing no correlation to substrate. The structural match between the nucleating phase and a substrate can be modified by the presence of adsorptive molecules, particularly large molecules such as polymers and
ARTICLE
Figure 5. Real-time observation of early stage GP-1 fiber formation in BB in the presence of 0.01% EVACP. Micrographs were recorded when the sample was cooled from 120 C at a cooling rate of 50 C/min. The corresponding temperatures for panels a and b are 59.1 and 55.4 C, respectively. The scale bar is 100 μm. The images are on the same scale.
macromolecules, based on energy and entropy consideration.8,13 Adsorptive molecules with strong interactions with the nucleating agent can potentially increase the structural correlation, promoting primary nucleation. When the interaction is weak, the adsorption of large molecules on a substrate will most likely hinder the integration of nucleating molecules onto the surface of the substrate, which retards the primary nucleation process. The strength of adsorption depends on many factors. Generally, molecules with rigid structures and multiple interacting points with a substrate can adsorb strongly on the substrate.13 The molecule of EVACP satisfies all the structural requirements for strong interfacial adsorption.13 Its presence increases the structural mismatch during GP-1 fiber formation, which retards the nucleation of GP-1. The enhanced structural mismatch can also lead to the formation of more branched fibers. These effects are demonstrated more clearly when the solvent benzyl benzoate is replaced with isostearyl alcohol. The effects of this polymer on the correlation between the substrate and GP-1 molecules can be characterized from dynamic rheological measurement. Because the fiber network formation is initiated by the nucleation of the gelator, according to 3D nucleation models, the nucleation rate J, the number of critical nuclei generated per unit time-volume at the substrate can be expressed as31,33 " # 16πγcf3 Ω2 1=2 J ¼ f 0f B exp f ð3Þ 3ðkTÞ3 ðΔμ=ðkTÞÞ2 3232
dx.doi.org/10.1021/cg200501h |Cryst. Growth Des. 2011, 11, 3227–3234
Crystal Growth & Design
ARTICLE
Scheme 2. A Schematic Illustration of the Surface Modification of a Substrate (Nucleation Center) with EVACP To Control the Primary Nucleation of the Gelator and the Transformation from a Single Fiber Network in to a Multidomain Fiber Networka
When the supersaturation is low, fewer branched fibers form due to the lower structural mismatch (a); molecules of EVACP adsorbed on the substrate can homogenize the surface of the substrate and enhance the structural mismatch between the substrate and the nucleating phase, which inhibits nucleation and leads to the formation of more branched fiber networks (spherulites) (b); transformation from an interconnecting (single) fiber network into a multidomain fiber network due to enhanced mismatch branching in the presence of the polymer (c). a
Figure 7. Effects of EVACP on the fiber network of GP-1 formed in isostearyl alcohol in the absence (a) and presence (b) of 0.01% EVACP. Panel c is a magnified image of panel b. The presence of EVACP promotes the formation of spherulitic fiber networks. The concentration of GP-1 is 6%. The scale bars are 200 μm. Figure 6. Evolution of elastic modulus of GP-1/BB gel at 15 C as a function of time.
where J is the nucleation rate defined as the number of critical nuclei generated per unit time-volume and B is kink kinetic coefficient and is constant for a given system. If the formation of the fiber network is initiated by nucleation, one should have tg ≈ 1/J. Hence, according to eq 3, for such a
system, a linear relationship between ln(tg) and 1/[(Δμ/(kT))2T3] can be obtained. The slope of the line corresponds to f. A lower f means that a better structural match exists between the substrate and the fluid phase. The plots of ln(tg) ≈ 1/[(Δμ/(kT))2T3] in the absence and presence of the additives for GP-1/ISA gel are given in Figure 8b. Linear correlations are obtained. The slopes of the plots in the absence and presence of 0.005% and 0.01% EVACP are 0.61C, 0.65C, and 0.76C (C is a constant), respectively. The increase in the slope in the presence of EVACP indicates that the adsorption of this 3233
dx.doi.org/10.1021/cg200501h |Cryst. Growth Des. 2011, 11, 3227–3234
Crystal Growth & Design
ARTICLE
’ ACKNOWLEDGMENT This work is supported by Singapore AcRF funding (Grant R-144-000-264-112) and Chinese MOE Chang Jiang Scholarship Chair Professorship (Dong Hua University). ’ REFERENCES
Figure 8. (a) Evolution of elastic modulus of GP-1/ISA gel at 15 C as a function of time and (b) the ln(tg) as a function of thermodynamic driving force. The values of tg are obtained from G* ≈ t at different temperatures. C is a constant. Cooling rate is 50 C/min.
molecule enhances the mismatch between the substrate and the nucleating phase, retarding the primary nucleation of GP-1.
’ CONCULSIONS In summary, by kinetic retardation of the nucleation of gelator N-lauroyl-L-glutamic acid di-n-butylamide with a suitable polymer additive (ethylene/vinyl acetate copolymer), the homogenization of mixed fiber networks formed by this gelator in benzyl benzoate due to the nonisothermal crystallization was achieved. The kinetically retarded nucleation is quantitatively characterized when benzyl benzoate is replaced with isostearyl alcohol. The introduction of this polymer also makes it feasible to convert an interconnecting (single) fiber network into a multidomain network. This provides a robust approach to the creation and engineering of a fiber network with a desired micro- or nanometer structure, which is important to the production of materials with desired performance in advanced applications such as bioseparation and controlled drug release. Compared with the design and synthesis of new gelators, this approach is simpler because it is based on the current commerically available ones with well-known gelling capacity in different solvents. With such a gelator, suitable additives can be selected on the basis of energy and entropy considerations or with the aid of molecular dynamics study. The general criteria for a good additive is strong interfacial adsorption capacity. Molecules of surfactants and polymers with rigid molecular structures should be the first priorities.
(1) Krotscheck, U.; Boothe, D. M.; Boothe, H. W. Vet. Ther. 2004, 5, 238–238. (2) Vintiloiu, A.; Leroux, J. C. J. Controlled Release 2008, 125, 179–192. (3) Hughes, N. E.; Marangoni, A. G.; Wright, A. J.; Rogers, M. A.; Rush, J. W. E. Trends Food Sci. Technol. 2009, 20, 470–480. (4) Terech, P.; Sangeetha, N. M.; Maitra, U. J. Phys. Chem. B 2006, 110, 15224–15233. (5) Carretti, E.; Bonini, M.; Dei, L.; Berrie, B. H.; Angelova, L. V.; Baglioni, P.; Weiss, R. G. Acc. Chem. Res. 2010, 43, 751–760. (6) Fu, X. J.; Yang, Y.; Wang, N. X.; Wang, H.; Yang, Y. J. J. Mol. Recognit. 2007, 20, 238–244. (7) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3159. (8) Li, J. L.; Liu, X. Y. Adv. Funct. Mater. 2010, 20, 3196–3216. (9) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352–355. (10) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Science 2001, 294, 1684–1688. (11) Zhu, G. Y.; Dordick, J. S. Chem. Mater. 2006, 18, 5988–5995. (12) Bielejewski, M.; Lapinski, A.; Luboradzki, R.; Tritt-Goc, J. Langmuir 2009, 25, 8274–8279. (13) Liu, X. Y.; Sawant, P. D.; Tan, W. B.; Noor, I. B. M.; Pramesti, C.; Chen, B. H. J. Am. Chem. Soc. 2002, 124, 15055–15063. (14) Liu, X. Y.; Sawant, P. D. Adv. Mater. 2002, 14, 421–426. (15) Li, J. L.; Liu, X. Y.; Strom, C. S.; Xiong, J. Y. Adv. Mater. 2006, 18, 2574–2578. (16) Li, J. L.; Yuan, B.; Liu, X. Y.; Xu, H. Y. Cryst. Growth Des. 2010, 10, 2699–2706. (17) Wang, R. Y.; Liu, X. Y.; Narayanan, J.; Xiong, J. Y.; Li, J. L. J. Phys. Chem. B 2006, 110, 25797–25802. (18) Rogers, M. A.; Marangoni, A. G. Cryst. Growth Des. 2008, 8, 4596–4601. (19) Rogers, M. A.; Wright, A. J.; Marangoni, A. G. Soft Matter 2008, 4, 1483–1490. (20) Liu, X. Y. J. Chem. Phys. 2000, 112, 9949–9955. (21) Salam, A.; Lohmann, U.; Crenna, B.; Lesins, G.; Klages, P.; Rogers, D.; Irani, R.; MacGillivray, A.; Coffin, M. Aerosol Sci. Technol. 2006, 40, 134–143. (22) Li, J. L.; Liu, X. Y. J. Phys. Chem. B 2009, 113, 15467–15472. (23) Lloyd, G. O.; Steed, J. W. Nat. Chem. 2009, 1, 437–442. (24) Fages, F. Angew. Chem., Int. Ed. 2006, 45, 1680–1682. (25) Piepenbrock, M. O. M.; Lloyd, G. O.; Clarke, N.; Steed, J. W. Chem. Rev. 2010, 110, 1960–2004. (26) Maeda, H. Chem.—Eur. J. 2008, 14, 11274–11282. (27) Lam, R. S. H.; Rogers, M. A. CrystEngComm 2011, 13, 866–875. (28) Lam, R.; Quaroni, L.; Pederson, T.; Rogers, M. A. Soft Matter 2010, 6, 404–408. (29) Li, J. L.; Wang, R. Y.; Liu, X. Y.; Pan, H. H. J. Phys. Chem. B 2009, 113, 5011–5015. (30) Huang, X.; Terech, P.; Raghavan, S. R.; Weiss, R. G. J. Am. Chem. Soc. 2005, 127, 4336–4344. (31) Liu, X. Y. J. Phys. Chem. B 2001, 105, 11550–11558. (32) Brown, G.; Chakrabarti, A. J. Chem. Phys. 1994, 101, 3310–3317. (33) Liu, X. Y. In Advances in Crystal Growth Research; Sato, K., Nakajima, K., Furukawa, Y., Eds.; Elsevier: Amsterdam, 2001; p 42.
’ AUTHOR INFORMATION Corresponding Author
*E-mail address:
[email protected]. 3234
dx.doi.org/10.1021/cg200501h |Cryst. Growth Des. 2011, 11, 3227–3234
