CRYSTAL GROWTH & DESIGN
Spherulite Formation of Lipophilic Surfactant Induced by Noncrystalline Amphiphilic Diblock Copolymer
2008 VOL. 8, NO. 12 4589–4595
Shuizhu Wu,* Boling Ma, Fang Zeng,* Jian Chen, Jianqing Zhao, Zhen Tong, and Yulan Luo College of Materials Science and Engineering, South China UniVersity of Technology, Guangzhou 510640, China ReceiVed June 25, 2008; ReVised Manuscript ReceiVed August 13, 2008
ABSTRACT: As the lipophilic surfactant tetraoctylammonium bromide (TOAB) was cast from its organic solutions, needle-shape crystals formed. However, with the addition of an amphiphilic noncrystalline diblock copolymer [(poly(acrylic acid-b-styrene)] into the TOAB solutions, spherulites formed in the solid films cast from the solutions. Under preferable conditions, millimeter-sized spherulites could be obtained. It has been found that some factors such as the type of solvent, the film-forming temperature, and the ratio of polymer to surfactant can affect the spherulite formation. Small angle X-ray scattering and wide-angle X-ray diffraction investigation suggests that the formation of the spherulites and the pure TOAB crystals are organized by closely packed lamellar structure, while the addition of diblock copolymer decreased the degree of order of TOAB crystals. Dynamic light scattering study reveals that, in organic solvents such as tetrahydrofuran, TOAB formed molecular-disperse solution, while the amphiphilic copolymer chains formed micelles with or without the presence of TOAB molecules in the solution. We suppose that the morphology change of TOAB crystal is induced by the diblock polymer chains: During the solvent-evaporating film formation, the hydrophobic PS blocks of the amphiphilic copolymer resided between some of the TOAB lamellar crystallites, which might cause the splaying and branching of the surfactant crystallites during the crystal growth and eventually lead to spherulite formation. This result could provide a useful way for spherulite formation, and open interesting opportunities for adjusting the crystal morphology and/or properties of lipophilic surfactant.
1. Introduction Understanding the morphology of crystals is, apart from a scientific challenge, important in practical applications.1-3 Crystal morphology can influence the properties of the materials; for example, for pharmaceutical materials, many downstream pharmaceutically relevant properties, such as filterability, flowability, syringeability, compactability, and dissolution profile may be affected by crystal morphology.4,5 Therefore, proper crystal morphology is essential for various technological applications. Many routes have been reported to control the crystal growth and eventually modify the morphology of the crystals.6-10 For crystal-morphology modification, crystals are grown in the presence of various additives.11-15 The crystal-morphology modifiers may be of a very diverse nature, such as multivalent cations, complexes, surface active agents, soluble polymers, biologically active macromolecules, fine particles of sparingly soluble salts, and so on.2,16-20 These crystal modifiers often adsorb selectively onto different crystal faces and retard their growth rates, thereby influencing the final morphology of the crystals. Amphiphilic block copolymers have attracted major scientific interest in recent years. The continued interest in amphiphilic block copolymers arises due to their unique solution properties, including their ability to self-associate in selective solvents to form micelles.21,22 The presence of two dissimilar functionalities within the same molecule while being separated from one another allows for greater flexibility in modulating the properties of amphiphilic block copolymers. Amphiphilic triblock copolymers have been shown to reduce the particle size and change the crystal habit of ethyl p-hydroxybenzoate.23,24 More recently, double hydrophilic block copolymers have been shown to be effective in engineering the crystal habit of some * To whom correspondence should be addressed. Phone: +86-20-22236262. Fax: +86-20-22236363. E-mail:
[email protected] (S.W.); mcfzeng@ scut.edu.cn.
inorganic materials.25,26 Because of the surface activity of amphiphilic diblock copolymers, amphiphilic diblock copolymers could preferentially adsorb onto the different growing crystal faces and thus be effective in the crystal morphology modification for lipophilic surfactant. In this paper, we investigated the significant changes of crystal formation in the lipophilic surfactant tetraoctylammonium bromide (TOAB) induced by the amphiphilic noncrystalline diblock copolymer [(poly(acrylic acid-b-styrene) or partly hydrolyzed poly(n-butyl acrylate-b-styrene)]. TOAB, a four-tail surfactant, is widely used as stabilizer and phase transfer agent for the preparation of metal nanoparticles in organic solvents. In the presence of a certain amount of the amphiphilic diblock copolymer in the TOAB tetrahydrofuran solution, significant changes in crystal morphology occurred in the solid films cast from the solution. In the absence of the block copolymer, needle-shaped crystals formed in the solid film, which consist of closely packed multiple-layer TOAB crystallites (crystalline lamellar structure), while with the addition of an appropriate amount of the block copolymer, spherulites formed in the solid film. Needle-shaped crystals are usually not desired, since the needle morphology can give rise to problems in handling and product quality issues such as blinding filters.27 On the other hand, spherulite materials have found many applications such as structural materials such as plastics, encapsulating systems, and chemical microreactors, and many potential applications of spherulites are currently well sought-after.28,29 This study could offer a new and useful approach for spherulite formation, as well as open up interesting opportunities for adjusting the crystal morphology and/or properties of lipophilic surfactant.
2. Experimental Section 2.1. Materials. tert-Butyl acrylate (tBA) and n-butyl acrylate (nBA) (Aldrich) were extracted three times with 5% aqueous NaOH and then
10.1021/cg800674d CCC: $40.75 2008 American Chemical Society Published on Web 10/22/2008
4590 Crystal Growth & Design, Vol. 8, No. 12, 2008 washed with distilled water. After drying over CaCl2 and filtering off the drying agent, the monomer was distilled under a vacuum (60 °C/ 60 mmHg). Styrene (Acros) was dried over CaH2 and then distilled under a vacuum (65 °C/35 mmHg). Methyl 2-bromopropionate (MBP) (Acros) was distilled before use. CuBr (Acros) was purified by washing with glacial acetic acid, followed by absolute ethanol and ethyl ether, and then dried under a vacuum. N,N,N′,N′′,N′′-Pentamethyldiethylenetriamine (PMDETA) (Acros, 99+%) was used as received. Xylene was dried over CaH2 and then distilled under reduced pressure before use. Tetraoctylammonium bromide (TOAB, purity g99.0%) was purchased from Fluka and used as received. 2.2. Synthesis. The amphiphilic copolymer (poly(acrylic acid-bstyrene) (PAA-b-PS) or partly hydrolyzed poly(n-butyl acrylate-bstyrene) (PnBA-b-PS) was synthesized according to the literature,30 and the synthesis process was briefly described as follows. Xylene (5 mL), CuBr (2.7 × 10-4 mol), and MBP (5.3 × 10-4 mol) were added to a round-bottom flask. The flask was degassed and back-filled with nitrogen three times through freeze-thaw cycles before introducing deoxygenated tBA (7 × 10-2 mol) (or nBA). Then PMDETA (2.7 × 10-4 mol) was added. The flask was kept at 60 °C for 7 h. The P(tBA) was purified by dissolving the reaction mixture in approximately 300 mL of additional acetone and filtering the solution through a column of alumina. The acetone was removed by evaporation, and the polymer was then dissolved in diethyl ether, and then it was precipitated into a 10-fold excess of a 50:50 v:v water/MeOH solution. After the solvent was decanted off, the polymer was redissolved in diethyl ether, and the precipitation procedure was repeated twice. The final polymer was dried under a vacuum. The bromo-terminated P(tBA) had a Mn ) 1.55 × 104 and a Mw/Mn ) 1.09. The above P(tBA) (3.5 × 10-4 mol), xylene (5 mL), and CuBr (3.5 × 10-4 mol) were added to a round-bottom flask, which was degassed and back-filled with nitrogen three times through freeze-thaw cycles. Deoxygenated styrene was added (4.0 × 10-2 mol). Then PMDETA was introduced (3.5 × 10-4 mol). Then the flask was kept at 100 °C for 6 h. The polymer was dissolved in about 50 mL of THF, and then the solution was filtered through alumina. The polymer was precipitated into a 10-fold excess of MeOH and then isolated by vacuum filtration. Two additional precipitation cycles were performed. Finally, the polymer was dried under a vacuum. The copolymer P(tBA-b-St) had a Mn ) 3.04 × 104 and a Mw/Mn ) 1.12. The above P(tBA-b-St) was dissolved in dioxane, trifluoroacetic acid was added, and the solution was refluxed for 6 h. Afterward, the excess reagents were removed by evaporation under a vacuum. The solid was rinsed well with hexane and dried under a vacuum. Finally, the amphiphilic copolymer PAA-b-PS was obtained. 2.3. Film Preparation. The film was prepared using the procedure as follows: the surfactant TOAB or the surfactant and the block polymer PAA-b-PS at a certain ratio were dissolved in an organic solvent (e.g., tetrahydrofuran) to form a solution with a concentration of about 12% by weight; a clear solution resulted, and then the solution was filtered through a 0.50 µm Teflon filter. This solution was cast on a glass substrate, and then a culture dish was covered over the glass substrate; afterward, the films were allowed to dry in a thermostat at a constant temperature (e.g., 15 °C). 2.4. Measurements. 1H NMR spectra were recorded on a Bruker Avance 400 MHz NMR spectrometer. Molecular weight and molecular weight distribution was determined by Waters Gel Permeation Chromatography with the 2410 RI detector, and polystyrenes were used as the standard. Polarizing micrographs were taken on a Zeiss Axiolab Polarizing microscope. Small-angle X-ray scattering (SAXS) and wideangle X-ray diffraction (WAXD) measurements were performed on Philips X’Pert PRO X-ray diffractometer (radiation: Cu KR, wavelength: 0.1542 nm) (scattering vector q ) (4π/λ) × sin θ, 2θ is the angle between the incident light and the scattered light). The measurement of hydrodynamic diameters for neat diblock polymer, neat TOAB and TOAB/PAA-b-PS solutions were determined on a MALVERN Nano-ZS90 particle size analyzer.
3. Results and Discussion 3.1. Polarizing Optical Micrographs. The block copolymer PAA-b-PS was synthesized and purified according to the literature.30 First, poly(tert-butyl acrylate-b-styrene) was prepared through atom transfer radical polymerization (ATRP), and
Wu et al.
then the hydrolysis of the ester functionality created the amphiphilic copolymer PAA-b-PS. The diblock copolymer contains 143 styrene repeat units and 121 acrylic acid repeat units with the polydispersity of 1.12 (the characterizations for PAA121-b-PS143 are shown in Figures S1-S4, Supporting Information). The amphiphilic block copolymer PAA121-b-PS143 is noncrystalline (Figures S5 andS6, Supporting Information), and we could not observe spherulitic texture in its organic solutions of varied concentrations and in its solid films. When the surfactant TOAB was dissolved in organic solvent (e.g., toluene, chloroform, THF, ethyl methyl ketone, acetone and ethanol), and then cast from the organic solution, the surfactant TOAB itself formed needle-shaped crystals as shown in Figure 1a, and we found in the above-mentioned low-polar solvents that the neat TOAB could not form spherulite at the concentration from 1.0 g/L to the saturated concentrations and under the temperature from 5 °C to higher temperature (just below boiling temperature of the solvents). However, when the amphiphilic diblock copolymer PAA-b-PS was added into the TOAB - THF solution (polymer to TOAB at a molar ratio of 0.024:1), and then the solution (with the concentration of ca. 12 wt%) was cast onto a glass substrate at 15 °C, the spherulites formed in the final solid film upon controlled evaporation of THF; the polarizing optical micrographs of the spherulites formed in the films are shown in Figure 1b,c. One can find that the spherulites, with the fibrillar texture and the clear Maltese crosses with a λ-plate (Figure 1d), it can be found the spherulite texture is optically negative; that is, the refractive index parallel to the radial direction is smaller than that perpendicular to it. To obtain well-developed spherulites, control over the evaporation speed of the solvent is needed; during the film formation we used a culture dish to cover the samples to control the evaporation speed. We found that the solutions cast at lower temperature (∼15 °C) can produce fine and larger spherulites; at higher temperatures, the solvent evaporates more quickly, the mobility of polymer chains were frozen due to the evaporation of solvent, and the fibrils could not fully develop into wellformed spherulites. We also found that the molar ratio of polymer to TOAB affected the spherulite formation, and large and fine spherulites could be obtained at a molar ratio of around 0.024:1. When the ratio is too high (>0.2:1) or too low (
