Microengineering of Soft Functional Materials by Controlling the Fiber

Nov 2, 2009 - Department of Physics, National University of Singapore, 2 Science Drive 3, .... was averaged from five micrographs taken at different p...
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J. Phys. Chem. B 2009, 113, 15467–15472

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Microengineering of Soft Functional Materials by Controlling the Fiber Network Formation Jing-Liang Li and Xiang-Yang Liu* Department of Physics, National UniVersity of Singapore, 2 Science DriVe 3, Singapore 117542 ReceiVed: August 17, 2009; ReVised Manuscript ReceiVed: October 4, 2009

The engineering of soft functional materials based on the construction of three-dimensional interconnecting self-organized nanofiber networks is reported. The system under investigation is an organogel formed by N-lauroyl-L-glutamic acid di-n-butylamide (GP-1) in propylene glycol. The engineering of soft functional materials is implemented by controlling primary nucleation kinetics of GP-1, which can be achieved by both reducing thermodynamic driving force and/or introducing a tiny amount of specific copolymers (i.e., poly(methyl methacrylate comethacrylic acid)). The primary nucleation rate of GP-1 is correlated to the number density of GP-1 spherulites, which determines the overall rheological properties of soft functional materials. The results show that the presence of a tiny amount of the polymer (0.01-0.06%) can effectively inhibit the nucleation of GP-1 spherulites, which leads to the formation of integrated fiber networks. It follows that with the additive approach, the viscoelasticity of the soft functional material is significantly enhanced (i.e., more than 1.5 times at 40 °C). A combination of the thermal and additive approach led to an improvement of 3.5 times in the viscosity of the gel. 1. Introduction Small molecule organogels (SMOGs) form a class of soft functional materials that possess three-dimensional (3D) fiber network structures, entrapping liquids in the micro-/nanopores by capillary force. Its application can be found in a great variety of fields including drug delivery,1-4 templated fabrication of nanostructures,5-8 food industry,9-11 and cosmetics,12,13 etc. In general, the fiber network of this type of material determines the macroscopic properties, especially the viscoelastic properties of the materials. Both the micro-/nanostructure and the viscoelastic properties of organogels determine their in-use properties.14 Recent results show that the network formation in many organogels is controlled by a nucleation-mediated branching process, which consists of the primary nucleation of the gelators and the subsequent growth and branching of fibers.14,15 On the basis of this mechanism, the entire gel network is composed of a number of smaller subunits, with each subunit originating from a nucleation center, as has been demonstrated in many organogel systems.15,16 The crystalline nature of the fibers of SMOGs have also been proven.17-21 The spherulitic fiber structure, which can be considered as a type of network subunit, is commonly observed in SMOGs.16,22-24 A gel supported by fibers with such a structure is always weak in terms of viscoelasticity due to the high incidence of boundaries between the subunits (or spherulites). Therefore, to create a stronger gel, it is important to reduce the boundary area and improve the integration of the networks. The integration of the network is defined as the total gel volume divided by the total number of spherulites, which equals the reverse of the number density of spherulites (or nucleation rate). The higher the integration (or the lower the nucleation rate), the lower the incidence of boundaries. The nucleation-branching mechanism indicates that the number of spherulites in a fixed volume of a gel can be manipulated by controlling the primary nucleation rate. For a fixed mass of gelator, a higher nucleation rate leads to the formation of a larger number but smaller spherulites. In * Corresponding author. E-mail: [email protected].

contrast, a lower nucleation rate will lead to the formation of a smaller number but larger spherulites.15 In this way, the entire network and the macroscopic properties of the materials can be tailored. Therefore, an effective way to improve the integration of this type of fiber network is to inhibit the nucleation rate of the gelators, which leads to the formation of an entire network with improved integration. According to the current nucleation theories,14,25 the nucleation rate of the spherultes

[

J ) f''[f]1/2B exp -

∆G* kT

]

(1)

where

∆G* )

16πγ3cfΩ2 3(kT)2[∆µ/kT]2

∆µ/kT ) ln(1 + σ) =

f

∆Hdiss (T - T) kTeq eq

where ∆G* is the nucleation energy barrier; B is the kink kinetics coefficient; f′′ and f (f′′ 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; ∆Hdiss denotes the molar dissolution enthalpy of the nucleating phase; Teq is the equilibrium dissolution temperature; ∆µ denotes the chemical potential difference between gelator molecules in the fiber state and in the liquid; and σ is supersaturation, defined as σ(T) ) (C - Ceq(T))/Ceq(T), where C and Ceq(T) are the actual molar fraction and the equilibrium molar fraction of solute in solution, respectively.

10.1021/jp907963t CCC: $40.75  2009 American Chemical Society Published on Web 11/02/2009

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SCHEME 1: Schematic Illustration of the Strategy to Produce a Strong Gel with Spherulitic Fiber Network Structure by Controlling the Primary Nucleation Rate of the Gelatora

a The dashed curves dedicate the boundary between individual spherulites, which is the mechanically weak area of a gel network. The additive-mediated approach is superior to the supercooling controlled approach in terms of fiber mass conservation and hence improvement in the macroscopic properties of the materials. ∆T is the degree of supercooling.

Equation 1 indicates that the nucleation rate J of spherulites can be reduced by producing the gel at a higher temperature, which reduces the thermodynamic driving force or the degree of supercooling ∆T (∆T ) Teq - T; Teq, equilibrium temperature of solution; T, experimental temperature). A lower nucleation rate corresponds to a lower number density of spherulites, leading to the formation of larger spherulites. However, a lower degree of supercooling means that a larger fraction of the gelator is dissolved in the solvent, reducing the thermodynamic driving force and thus the final fiber mass in the gel. This will also compromise the viscoelasticity of the gel. Hence, it is important to reduce the nucleation rate and improve the integration of the fiber network without sacrificing the fiber mass. According to eq 1, the nucleation kinetics can also be changed by manipulating the kink kinetics coefficient and the correlation between the substrates and the nucleation phase. Suitable additives have been observed to be effective in manipulating the nucleation kinetics of molecules.25 Therefore, the adoption of suitable additives can qualify as a superior approach to control the nucleation rate and the fiber network formation in an organogel, without changing the hydrodynamic driving force of the system. This will hopefully lead to the formation of gels with further improved macroscopic properties (Scheme 1). The selection of additives should be based on the following considerations. The selection criteria of additive has been discussed in detail in a previous work.14 (a) The additives should have strong adsorption on the solid fiber surface. To satisfy this criterion, polymers with rigid structures are better than small molecules. Polymers with repeating structural units as a whole have more interacting points with the crystal surface than single monomers. Therefore, the desorption of polymers is more difficult. (b) The additives should only have strong physical interactions with the gelator molecules on the crystal surface. Since the

Li and Liu surface of the crystal fibers is highly ordered and stiff, it is desirable to have short and relatively flexible functional groups attached to the backbone of the additive molecules to obtain the optimal interactions with the gelator molecules on the surface of the crystals. (c) Solubility of the additives in the solvent of a gel system. For additives with similar structures, one with a limited solubility in the solvent should adsorb on the crystal surface more easily. An additive with a high solubility will have a stronger interaction with the solvent and hence can be more easily desorbed. On the basis of the above considerations, the effect of a selected copolymer poly(methyl methacrylate comethacrylic acid) (PMMMA, molar ratio of methyl methacrylate (MM) to methacrylic acid (MA) is 1:0.016) on the primary nucleation of N-lauroyl-L-glutamic acid di-n-butylamide (GP-1) in 1,2propanediol/propylene glycol (PG) and on the formation of the fiber networks is examined in this work. 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. A hydrogen bond can also form between the carbonyl oxygen on the PMMMA molecule and the amide hydrogen on the GP-1 molecule. The rigid structure of PMMMA will also contribute to its strong physical adsorption on the surface of solid GP-1. The low fraction of methacrylic acid unit contributes to the low solubility of the polymer in the solvent PG. It was observed that the polymer at a concentration as high as 0.06% could be dissolved in PG at a high temperature (100 °C). However, the solutions with a polymer concentration above 0.02% became turbid when the solution was cooled to room temperature (20 °C). This indicates the limited solubility of this polymer in PG. In addition, selection of this polymer is also based on its wide commercial availability. The copolymer can also be synthesized with defined molar ratio of the two monomers. GP-1 has been found to form a typical spherulitic fiber structure in PG.15 The influence of this polymer on the rheological properties of the organogel was also investigated. To examine the effects of MM:MA molar ratio on the microstructure of GP-1 fiber network, another PMMMA copolymer with a higher MM:MA molar ratio of 1:0.16 was also used. 2. Experimental Section Chemicals. Two poly(methyl methacrylate comethacrylic acid) (PMMMA) copolymers, with a methyl methacrylate (MM) to methacrylic acid (MA) molar ratio of 1:0.016 and 1:0.16, and 1,2-propanediol/propylene glycol (PG) were obtained from Sigma. N-Lauroyl-L-glutamic acid di-n-butylamide (GP-1) was obtained from Kishimoto Sangyo Asia. The molecular structures of GP-1 and PMMMA are shown below.

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

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by sealing the hot GP-1/PG 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 and PMMMA (weight percentage, wt %) throughout this work are given as %. Supercritical CO2 Extraction. The xerogels were obtained by extracting the solvent, captured in the network with CO2 supercritical fluid extraction equipment (Thar Design). The flow rate of CO2 was 20 g/min, and the extraction time was 1.5 h. The xerogel powder was used for XRD analysis. Rheological Study. The rheological properties of the organogel were measured by an advanced rheological expansion system (ARES-LS, Rheometric Scientific). Dynamic temperature ramp tests were carried out to obtain the storage modulus G′ (a measure of elasticity), loss modulus G′′ (a measure of viscosity), and complex modulus G* (viscoelasticity, G* ) [(G′)2 + (G′′)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. 3. Results and Discussion 3.1. Real Time Observation of GP-1 Fiber Network Formation. A few micrographs taken during the GP-1 fiber network formation are given in Figure 1A-1D, which demonstrates that the formation of the fiber network is initiated with the nucleation of GP-1, followed by the growth and branching of the fibers. The XRD patterns of the gel fiber and the gelator powder were measured and given in Figure 1E. It shows that the fiber and pure gelator powder have the same crystalline structure. This also suggests that the fiber network formation is a process of GP-1 crystallization in the solvent. 3.2. Effects of Temperature and PMMMA (0.01%, MM: MA ) 1:0.016) on the Microstructure of the GP-1 Fiber Network. Figures 2A and 2B show the optical micrographs of the GP-1 fiber networks formed at 25 and 40 °C, respectively. At a higher temperature, fewer but larger spherulites occurred. This is due to the decrease of the thermodynamic driving force. The corresponding images of the gel formed with the introduction of a tiny amount of PMMMA (0.01%) are shown in Figure 2C and 2D. The presence of PMMMA led to the formation of fewer but larger spherulites in a fixed volume at the same temperatures, indicating the inhibitory effect of PMMMA on the primary nucleation of GP-1 in PG. 3.3. Effects of PMMMA (MM:MA ) 0.016) on the Nucleation Rate and Fiber Growth Rate. The number densities of spherulite in the absence and presence of the additive were calculated using the micrographs obtained. Figure 2E shows that the presence of the polymer reduced the number density of the spherulites (or the nucleation rate) to value from less than half (25 °C) to one tenth (40 °C) of that when it was not introduced. Consequently, the presence of PMMMA gives rise to the formation of more structurally integrated fiber

Figure 1. Real time observation of the fiber network formation process of GP-1 in PG (A-D) and XRD pattern of the fiber and gelator powder (E). (A)-(D) were a series of micrographs taken after a hot GP-1/PG solution (100 °C) was cooled to 40 °C at a cooling rate of 30 °C/min. (A) 10 s, (B) 19 s, (C) 48 s, and (D) 84 s. That circled in (A) demonstrates a GP-1 nucleus. The scale bar is 50 µm. All the images of (A)-(D) are on the same scale.

networks. It is expected that the viscoelasticity of the gel can be improved. The inhibition of PMMMA on the nucleation of GP-1 can be attributed to the adsorption of the polymer molecules on the embryo surface of GP-1 (Figure 3A). The adsorption of polymer molecules interrupts the integration of GP-1 molecules on the surface of the embryo, suppressing the nucleation by increasing the kink kinetics coefficient B or by reducing the correlation (f, f′′) between the nucleating phase and embryo surface (eq 1).25 As a consequence, the nucleation rate or the number of spherulites in the system can be reduced. The strong adsorption of the polymer molecules on the embryo surface could be due to the formation of hydrogen bonds between the molecules of the two compounds. Apart from this, based on entropy and energy considerations, the high rigidity of the long polymer chain also contributes to its strong adsorption on the surface.14 If the polymer molecules adsorb on the embryo surface, they will also adsorb on the tip surfaces of the growing fibers. If this happens, it is expected that the fiber growth can be retarded. The spherulitic structure in this gel makes it possible to measure the fiber growth rate. Figure 3B gives the fiber length R as a function of time at 35 °C. The fiber growth rate RGR, the slope of the curve, was reduced from 2.06 to 1.48 µm/s by the addition of the polymer. It can be concluded that the presence of this polymer greatly reduced the fiber growth rate, which

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Figure 2. Micrographs of 3% GP-1/PG gel (A-D) and the number density of spherulite (D). (A) 25 °C, no additive; (B) 40 °C, no additive; (C) 25 °C, with 0.01% PMMMA; and (D) 40 °C, 0.01% PMMMA. Scale bars: (A) and (C) 20 µm; (B) and (D) 100 µm.

suggests that growth of fiber is interrupted by the polymer molecules. Figure 3C shows fiber growth rates at different temperatures. With increasing temperature, the growth rate increases in the absence of the polymer due to the reduced viscosity of the liquid phase. This effect, together with the decreased nucleation rate at a higher temperature, gives rise to a lower concentration depletion of the solute at the growing front of the fibers. This in turn contributes to a faster mass transfer across the solid-liquid interfacial layer and gives rise to a higher growth rate. On the other hand, an increase in temperature lowers the overall thermodynamic driving force of the system, which may give rise to a reduction of the growth rate. The observed fiber growth rate is a combination of these two contrasting effects. Nevertheless, with a further increase in the temperature approaching the critical gelling temperature (ca. 65 °C, the highest temperature at which the gel can form), a drop in the fiber growth is expected due to the depletion of the gelator molecules available for gelation. In addition, the overall fiber growth is also affected by the branching density of the

Li and Liu

Figure 3. Schematic illustration of adsorption of PMMMA molecules on the surface of a GP-1 embryo (A), fiber length as a function of time at 35 °C (B), and the influence of 0.01% PMMMA on the growth rate of GP-1/PG fibers (C).

fibers, which is controlled by the structure mismatch between the nucleating phase and the fiber tip surface. A change in the thermodynamic driving force and the presence of additives can affect the mismatch and thus the fiber branching. Enhanced fiber branching at a higher thermodynamic driving force or in the presence of additives has been observed in our previous research.20,26-28 An accelerated fiber growth with increasing temperature was also observed in the presence of the polymer when the temperature was below 35 °C. It is interesting to observe that at 40 °C a sharp drop in the growth rate was observed. Since this was not observed in the control (absence of the polymer) at the same temperature, the decrease in the thermodynamic driving force is not enough to explain this phenomenon. Naturally, a more in-depth investigation into the influence of the polymer molecules on the nucleation kinetics and fiber branching is needed to obtain a better understanding. 3.4. Effects of Temperature and PMMMA (0.01%, MM: MA ) 0.016) on the Viscoelasticity of GP-1/PG Gel. Figure 4 shows the G* of the gel formed at different temperatures.

Microengineering of Soft Functional Materials

Figure 4. Effects of PMMMA (0.01%) on the viscoelasticity of 3% GP-1 PG gel at different temperatures (A) and fraction of fiber mass as a function of temperature (B).

Increasing the temperature from 25 to 30 °C, G* is improved significantly. G* does not show significant improvement with a further increase in temperature. Although increasing the temperature improves the integration of the fiber network by forming fewer and larger spherulites, the overall viscoelasticity is also compromised by the lower fiber mass due to the increase in GP-1 solubility. This explains the level off of the G* curve at high temperatures. The fraction of fiber mass (φ) as a function of temperature is shown in Figure 4B. φ is defined as the ratio of the mass of gelator molecule in fibers to the total mass of gelator molecules present in the gel system (the sum of the dissolved and crystallized gelator molecules). With the temperature approaching the critical temperature, φ drops quickly and approaches zero. Therefore, improving the gel elasticity by increasing temperature is limited particularly when the temperature approaches the critical point. Interestingly, the presence of PMMMA can further improve the viscoelasticity of the gel. At a same temperature (same fiber mass), GP-1 forms a more integrated fiber network in the presence of PMMMA, leading to an increase of more than 20% in G*. The advantages of using an additive is that its presence does not change the thermodynamic driving force of the gel system and hence it helps to conserve the fiber mass. 3.5. Effects of PMMMA (MM:MA ) 0.016) Concentration on the Microstructure and Viscoelasticity of GP-1/PG Gel Formed at 40 °C. The microstructures of GP-1 fiber network formed at 40 °C in the presence of PMMMA at different concentrations are shown in Figure 5A-5D. With an increase in the polymer concentration, less and larger spherulites are formed, indicating the enhanced nucleation inhibition at a higher polymer concentration. The G* of the gel at different PMMMA concentrations is shown in Figure 5E. The G* shows

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Figure 5. Effects of PMMMA concentration on the microstructure (A-D) and viscoelasticity (E) of 3% GP-1 PG gel formed at 40 °C. The scale bars are 100 µm.

significant increase when the PMMMA concentration is below 0.04%, which almost levels off with further increase in the polymer concentration. The G* in the presence of 0.06% PMMMA (72000 N/m2) is about 1.5 times that of the gel formed in the absence of the additive (45600 N/m2). A comparison of Figure 5E and 4A indicates that a combination of the thermal and additive approach can be adopted as a more effective way to improve the viscoelasticity of the material. By elevating the temperature for gel formation to 40 °C with the presence of the additive at a concentration above 0.04%, the G* can be improved three and half times as compared with the G* obtained at 25 °C in the absence of the additive (20500 N/m2). 3.6. Effects of the MM to MA Molar Ratio on the Microstructure of the GP-1 Fiber Network. To examine the role of methyl methacrylate to methacrylic acid molar ratio of the PMMMA copolymer on the formation of the GP-1 fiber network, another PMMMA copolymer with a methyl methacrylate to methacrylic acid molar ratio of 0.16 was used. It was observed that this polymer was less efficient in inhibiting the nucleation of GP-1. Figure 6 shows the microstructure of the GP-1 fiber network formed at 40 °C with the presence of this copolymer at a concentration of 0.05%. A comparison between Figures 5 and 6 shows that the addition of this copolymer at this concentration is less effective than the one with the lower molar ratio at the concentration of 0.01%. The reduced effect is attributable to the high solubility of this copolymer in PG. The high solubility is due to the presence of a larger fraction of methacrylic acid on the polymer molecule. Before a polymer molecule adsorbs onto the crystal surface, the solvent layer around the molecule and on the crystal surface has to be liberated, resulting in the increase in entropy and strong

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Li and Liu the efficiency of the additive in tuning the nucleation of the gelator molecules since it determines the relative strength of two pairs of interactions: one is the interaction between gelator and additive molecules, and the other is between the solvent and additive molecules. A high solubility of the additive in the solvent means that it has a strong interaction with the solvent, which weakens the adsorption of the additive on the surface of the gelator crystal. In addition, for a gel system, a suitable additive is not limited to one polymer or a class of structurally similar polymers. Any polymer or macromolecule that satisfied the criteria can be considered. Certainly, to select one with the optimal performance, a screening of the suitable additives is needed. Acknowledgment. This work is supported by ARF funding T13-0602-P10.

Figure 6. Micrograph of GP-1/PG gel formed at 40 °C in the presence of 0.05% PMMMA with a MM to MA molar ratio of 1:0.16. The scale bar is 100 µm.

adsorption of the polymer molecule. The strong interaction between the solvent molecules and the polymer molecules slows down the desolvation of the polymer molecules and hence retards the integration of the polymer molecules onto the crystal surface. It is also not beneficial to entropy gaining.14 Therefore, we can conclude that while a certain degree of solubility of the polymer in the solvent of a gel system should be satisfied a high solubility will render the polymer less effective. A high solubility means that the polymer molecules adsorbed on the surface of gelator embryos or fiber tips can be more easily desorbed. On the basis of this preliminary observation, detailed work on the effects of PMMMA with this MM to MA molar ratio on the microstructure and rheological properties of GP1/PG gel was not performed. 4. Conclusions In summary, this work reported a robust approach to the microengineering of small molecule organogels by introducing a suitable copolymer additive PMMMA (MM:MA ) 1:0.016) or/and by reducing the thermodynamic driving force to tailor the nucleation of the gelator molecules. Both a reduction in the thermodynamic driving force and the presence of the copolymer molecules inhibited the nucleation of the gelator GP-1 molecules in the solvent PG. The effects of polymer could be due to the adsorption of the polymer molecules on the surface of the GP-1 embryos. The adsorption of polymer molecules was evidenced by the reduced fiber growth. As a result, the presence of the copolymer PMMMA induced the formation of more integrated fiber networks of GP-1 in PG, leading to the creation of a gel with improved viscoelasticity. A combination of the thermal and additive approach was observed to be more effective in improving the viscoelasticity of the gel. With the addition of the polymer at a concentration of above 0.04% and a processing temperature of 40 °C, the G* of the gel was improved three and half times compared with that of the gel formed at 25 °C in the absence of the additive. A PMMMA copolymer with a higher methyl methacrylate to methacrylic acid molar ratio (1: 0.16) was found to be less effective than the one with the lower ratio of 1:0.016. For a certain gel system, the solubility of the additive in the solvent should be a critical parameter that decides

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