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Thermoresponsive Colloidal Crystals Built from Core-Shell Poly(styrene/r-tert-butoxy-ω-vinylbenzylpolyglycidol) Microspheres Nebewia Griffete,† Monika Dybkowska,‡ Bartosz Glebocki,‡ Teresa Basinska,‡ Carole Connan,† Agnes Maıˆ tre,§ Mohamed M. Chehimi,† Stanislaw Slomkowski,‡ and Claire Mangeney*,† ‡
† ITODYS, Universit e Paris Diderot and CNRS (UMR 7086), 15 rue Jean de Baı¨f, 75013 Paris, France, Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland, and §INSP, Universit e Pierre et Marie Curie and CNRS (UMR 7588), 140 rue de Lourmel, 75015 Paris, France
Received February 5, 2010. Revised Manuscript Received May 5, 2010 Core-shell particles of poly(styrene/R-tert-butoxy-ω-vinylbenzylpolyglycidol) P(S/PGL) were used as new building blocks for the assembly of a colloidal crystal. The added-value properties of these particles for photonic crystal architectures are their high hydrophilicity together with their thermoresponsivity. Indeed, the poglycidol-rich shell undergoes a phase transition above 45 C, which leads to its collapse at the particle surface accompanied by a decrease in the particle diameter. The threedimensional crystalline arrays display Bragg diffraction properties, as judged by angle-resolved reflectance spectroscopy. The thermoresponsivity of the colloidal assemblies was observed through modifications of their optical properties with respect to the temperature used during the assembly process. The wetting properties of the crystalline material were also shown to reversibly switch from hydrophilic to hydrophobic as a function of the assembly temperature, thus evidencing the reorganization of the surface polyglycidol chains during the polymer phase transition. This work shows conclusively that P(S/PGL) particles are promising alternatives to poly(N-isopropylacrylamide) and poly(ethylene glycol) particles for the elaboration of thermoresponsive colloidal crystals, with a phase transition situated in between those of these two polymers.
1. Introduction Planar colloidal crystals (CCs) obtained by spontaneous selfassembly of nanoscale beads have become an important and popular area of research with a broad spectrum of applications.1 These applications range from colloidal lithography for nanopatterns2 to superhydrophobic surfaces with self-cleaning properties3,4 to hierarchically structured porous materials with high surface-to-volume ratios for catalyst supports5 or photonic crystals showing a stop-band or a pseudo-gap.6 In each of these fields, it is still challenging to design new microsphere building units able to impart improved macroscopic properties to the final colloidal crystal, such as better mechanical stability, higher long-range ordering, self-healing properties, or stimuliresponsive behavior. To respond to this demand, smart hydrogel microspheres of poly(N-isopropylacrylamide)7-9 or poly(ethylene glycol)10 were self-assembled, affording photonic crystals with stimuli-responsive properties. Furthermore, it was shown very recently that such assemblies are intrinsically defect-tolerant because of their ability to dissipate defect energies over long distances through the lattice.11 *Corresponding author: Ph þ33 157 27 68 78; e-mail mangeney@ univ-paris-diderot.fr. (1) Seung-Man, Y.; Shin-Hyun, K.; Jong-Min, L.; Gi-Ra, Y. J. Mater. Chem. 2008, 18, 2177. (2) Yang, S. M.; Jang, S. G.; Choi, D. G.; Kim, S.; Yu, H. K. Small 2006, 2, 458. (3) Synytska, A.; Ionov, L.; Dutschk, V.; Stamm, M.; Grundke, K. Langmuir 2008, 24, 11895. (4) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y.; Wang, Z. J. Mater. Chem. 2008, 18, 621. (5) Liew Kai Hoa, M.; Lu, M.; Zhang, Y. Adv. Colloid Interface Sci. 2006, 121, 9–23. (6) Zhang, J.; Sun, Z.; Yang, B. Curr. Opin. Colloid Interface Sci. 2009, 14, 103– 114. (7) McGrath, J. G.; Bock, R. D.; Cathcart, J. M.; Lyon, L. A. Chem. Mater. 2007, 19, 1584–1591. (8) Kumoda, M.; Watanabe, M.; Takeoka, Y. Langmuir 2006, 22, 4403–4407. (9) Huang, G.; Hu, Z. Macromolecules 2007, 40, 3749–3756. (10) Cai, T.; Wang., G.; Thompson, S.; Marquez, M.; Hu, Z. Macromolecules 2008, 41, 9508–9512.
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However, since they are composed of hydrogels, the crystals cannot be produced as dried, freestanding materials such as those obtainable from hard spheres. An attractive scheme for colloidal crystal formation can be achieved by taking advantage of particles that possess two beneficial properties: soft interactions for particle assembly and hard-sphere properties that enhance crystalline stability. Core-shell microsphere building units can meet these requirements12 while proposing a large choice regarding the chemical nature of the core and the shell. For example, the assembly of core-shell particles of poly(styrene-methyl methacrylate-acrylic acid) provided colloidal crystal films with bright and monochromatic colors while the strong physical connection and hydrogen bond linkages among the particles13 afforded a material with tough mechanical strength. Other used hydrophobic polystyrene cores coated with hydrophilic shells made of polyacrylamide or poly(acrylic acid) (PAA) in order to form three-dimensional CCs in water. However, the formation of CCs by the P(S-AA) spheres can be disrupted by lowering the pH below the pKa value of the COOH groups, when the shell becomes not elastic enough to stabilize the ordered state of the spheres.14 A recent self-assembly approach addressed mechanical toughness by core-shell polymer dispersions15 with a film-forming shell. In that approach, an elastomeric shell enhanced adhesion between neighboring polymer spheres, such that the entire structure withstands nanometer to micrometer deformations due to mechanical impact. The filled-pore colloidal crystals can withstand elongation by 100%, but the optical brilliance of the (11) Iyer, A. St. J.; Lyon, L. A. Angew. Chem., Int. Ed. 2009, 48, 4562–4566. (12) Kalinina, O.; Kumacheva, E. Macromolecules 1999, 32, 4122–4129. (13) Wang, J.; Wen, Y.; Ge, H.; Sun, Z.; Zheng, Y.; Song, Y.; Jiang, L. Macromol. Chem. Phys. 2006, 207, 596–604. (14) Meng, Q.; Li, Z.; Li, G.; Zhu, X. X. Macromol. Rapid Commun. 2007, 28, 1613–1618. (15) Wohlleben, W.; Bartels, F. W.; Altmann, S.; Leyrer, R. J. Langmuir 2007, 2961–2969.
Published on Web 05/18/2010
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Figure 1. Scheme of the core/shell P(S/PGL) microsphere morphology.
material is compromised by the low refractive index contrast between the core and the shell. More recently, hybrid core-shell particles were used as elementary building blocks for 2D colloidal crystals. It consisted of poly(butyl acrylate) brush-coated silica particles displaying homogeneous smoothness particle surface associated with a strong adhesive property. However, the optical properties of the resulting ordered material were not published.16 As far as smart materials are required, amphiphilic particles from poly(styrene-n-butyl acrylate-acrylic acid) were self-assembled into colloidal-crystal films affording a wettability transition temperature from superhydrophilic to superhydrophobic.17 Stimuli-responsive copolymer particles of polystyrene-co-poly-Nisopropylacrylamide7 were also used to construct dried colloidal crystals that exhibit Bragg diffraction and crystalline stability. However, the responsive properties of this material were not reported. A few years ago, Basinska et al. developed the synthesis of coreshell poly(styrene/R-tert-butoxy-ω-vinylbenzylpolyglycidol) P(S/ PGL) microspheres with interfacial layer rich in R-tert-butoxy-ωvinylbenzylpolyglycidol (PGL).18 The main chains of PGL and poly(ethylene oxide) are identical, but PGL contains additionally hydroxymethyl groups in each repeating unit (see Figure 1), which yields highly hydrophilic surfaces. Furthermore, these particles suspended in aqueous media exposed to heating were demonstrated to display thermoresponsive properties with a chainglobule transition resulting in the swelling of interfacial layer below 45 C and its collapse above this temperature.19 These properties together with a low dispersity index and softness of the shell polymer chains are promising assets for providing long-range ordered colloidal crystals with enhanced interparticle adhesion and tunable Bragg diffraction. In a recent short paper, we showed that these P(S/PGL) particles readily self-assemble into a 3D network with Bragg diffraction properties.20 In the present paper, we explore for the first time the thermoresponsive properties of these colloidal crystal built from core-shell poly(styrene/R-tert-butoxy-ω-vinylbenzylpolyglycidol) P(S/PGL) microspheres. We first evidenced the core-shell morphology of these particles by TEM and then characterized the 3D periodic arrays by SEM and AFM. In a second step, we studied the optical and wetting properties of the colloidal crystals using angleresolved reflectance spectroscopy and contact angle measurements, (16) Deleuze, C.; Delville, M. H.; Pellerin, V.; Derail, C.; Billon, L. Macromolecules 2009, 42, 5303–5309. (17) Wang, J.; Wen, Y.; Hu, J.; Song, Y.; Jiang, L. Adv. Funct. Mater. 2007, 17, 219–225. (18) Basinska, T.; Slomkowski, S.; Dworak, A.; Panchev, I.; Chehimi, M. M. Colloid Polym. Sci. 2001, 279, 916–924. (19) Basinska, T.; Slomkowski, S.; Kazmierski, S.; Chehimi, M. M. Langmuir 2008, 24, 8465–8472. (20) Basinska, T.; Kergoat, L.; Mangeney, C.; Chehimi, M. M.; Slomkowski, S. e-Polym. 2007, 87.
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respectively. The response of these materials to variations of temperature allowed us to evidence their thermoresponsive behavior and therefore to underline the propensity of polyglycidol-based particles to be a valuable alternative to the more commonly used thermoresponsive poly(N-isopropylacrylamide) and poly(ethylene glycol) particles. Furthermore, thanks to the remarkable properties of polyglycidol (hydrophilicity, chain mobility, thermoresponsivity), we provide in this paper the first example of thermoresponsive colloidal crystal elaborated from core-shell particles and not from microgels.
2. Experimental Part 2.1. Materials. Styrene (Aldrich) was purified from the stabilizer (4-tert-butylcatechol) by distillation at 30 C under reduced pressure. R-tert-Butoxy-ω-vinylbenzylpolyglycidol macromonomer (shown in Figure 1) was obtained by anionic polymerization of 1,1-ethylethoxyglycidyl ether and terminated by p-chloromethylstyrene with potassium tert-butoxide and subsequent hydrolysis of ethyl ethoxy groups by oxalic acid. Potassium persulfate (K2S2O8, from Fluka) was used without further purification. Water for synthesis of microspheres was distilled three times and pH was adjusted to 6.8 with KHCO3. Glass plates were treated by 2-propanol:KOH solution (5:1). 2.2. Preparation of PS and P(S/PGL) Microspheres. Poly(styrene/R-tert-butoxy-ω-vinylbenzylpolyglycidol) microspheres were prepared by a free radical emulsion copolymerization of styrene and R-tert-butoxy-ω-vinylbenzylpolyglycidol (PGL) macromonomer in water with potassium persulfate used for initiation, according to a procedure described previously by Basinska et al.18 The synthesis of PGL macromonomer was developed by Dworak et al.21 The molecular weight of PGL prepared according to this recipe and used for preparation of microspheres was 2700 and dispersity Mw/Mn = 1.05. Shortly, the polymerization mixture consisted of styrene (10 g, free from stabilizer 4-tert-butylcatechol, removed by distillation under reduced pressure), PGL macromonomer (1.0 g), and water three times distilled (125 g). The mixture was loaded to glass reactor equipped in cooler, nitrogen inlet and immersed in thermostatic bath operated at 65 C. The mixture was purged with nitrogen for 45 min at room temperature, and then the temperature was raised to 65 C, followed by addition of potassium persulfate (0.2 g). Polymerization was continued for 28 h at 65 C with continuous stirring at 600 rpm. Thereafter, purification of particles was performed by removing of styrene residues by steam stripping and repeated washing of particles with fresh portions of distilled water by centrifugation. The composition and procedure for preparation of polystyrene microspheres were identical with the description presented above, except addition of macromonomer. The diameters of polystyrene (PS) and P(S/PGL) microspheres deposited on glass plates were determined by scanning electron microscopy (JEOL 5500LV apparatus) from registered microphotographs. The average diameter and diameter dispersity was (21) Dworak, A.; Panchev, I.; Trzebicka, B.; Walach, W. Polym. Bull. 1998, 40, 461.
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calculated from ca. 700 particles taken for measurements in various pictures. The concentration of acidic groups, introduced during initiation with K2S2O8 on the surface of P(S/PGL) particles, was determined by conductometric titration with KOH. Before titration, the known amount of microspheres was prefiltered through a Dowex 50WX4 ion-exchange resin. Polystyrene particles with number-average diameter Dn = 220 nm and dispersity Dw/Dn = 1.006 were obtained.
2.3. Determination of the Hydrodynamic Diameters of P(S/PGL) Microspheres (Dh). Hydrodynamic diameters of P(S/PGL) microspheres suspended in three times distilled and filtered through 0.2 μm filter water were determined from measurements by photon correlation spectroscopy (PCS) using Zetasizer Nano-ZS (Malvern Instruments). The apparatus was equipped with a HeNe gas laser emitting light at 633 nm and avalanche photodiode detector recording intensity of scattered light at 173. The hydrodynamic diameters of P(S/PGL) microspheres were measured at various temperatures in the range 20-70 C. In each temperature a sample of P(S/PGL) microspheres in water was equilibrated within 10 min prior to measurement. Determination of diffusion coefficient of spherical particles measured by PCS allows calculation of hydrodynamic diameter (Dh) according the Einstein-Stokes equation D ¼ kT=3πηDh
ð1Þ
where D denotes diffusion coefficient, Dh the hydrodynamic diameter of microspheres, k the Boltzmann constant, T the temperature, and η the viscosity of medium. Data were analyzed using a cumulants method on the basis of 15 measurements for each sample at given temperature. According to ISO13321, the cumulants method is recommended for determination of hydrodynamic diameters of spherical particles with very narrow size distribution.
2.4. Determination of Electrophoretic Mobilities (μ) of P(S/PGL) Microspheres. Electrophoretic mobilities (μ) of P(S/ PGL) microspheres were measured in cells equipped with electrodes to which a controlled potential was applied (Zetasizer NanoZS, Malvern). The electrophoretic mobility measurements were performed for 1.0 mL samples of particles suspended in 10-3 M NaCl water solution in the temperature range 10-65 C. Viscosity and temperature dependence of dielectric constant of water was taken into account during data treatment. In a typical measurement, the mean of 10 measurements was registered.
2.5. Preparation of Photonic Crystals from P(S/PGL) Microspheres. First, the glass plates cut into pieces of 2 1 cm were immersed in 2-propanol:KOH solution (5:1 by weight) for 24 h at room temperature. Then, the plates were washed repeatedly with distilled water and dried in the oven at 120 C for 2 h. The assemblies of microspheres were prepared separately at two temperatures: 20 and 70 C.
2.6. Preparation of P(S/PGL) Microspheres Assemblies at 20 C. The microspheres suspended in water (6% aqueous suspension) at room temperature were deposited on glass plates freshly prepared by placing a drop (usually 40 μL) of the suspension on the plate and spreading it to fully cover the substrate surface. Then, the plates were stored at room temperature until the microspheres assemblies become dried.
2.7. Preparation of P(S/PGL) Microspheres Assemblies at 20, 30, 60, 70, 80, and 100 C. 100 μL of aqueous suspension of microspheres (6 wt %) and glass plates were incubated in a dryer chamber for 45 min at a fixed temperature (in the range 30-100 C). Then, a drop of suspension (usually 40 μL) was placed on the heated glass plate (to the same temperature as particle were) and spread on it without cooling the particles. The plate covered with the particles was incubated at given temperature until become dried and thereafter cooled to room temperature. 11552 DOI: 10.1021/la100537v
2.8. Analytical Techniques. Morphology of the microspheres and distances between the particles in particles assemblies were investigated by means of atomic force microscopy (AFM) using a Nanoscope IIIa Multimode scanning probe microscope operated at tapping mode. Transmission electron microscopy (TEM) micrographs were obtained using a JEOL JEM 100CXII UHR operating at 100 kV. Solutions containing the particles were cast onto Formvar-coated copper grids, and the solvent was allowed to evaporate. Chemical composition of interfacial layer of the P(S/PGL) microspheres was determined by X-ray photoelectron spectroscopy registered for dry particles. Spectra were registered using a Thermo VG Scientific ESCALAB 250 system equipped with a monochromatic Al KR X-ray source (1486.6 eV) and magnetic lens. The atomic ratio of carbon and oxygen atoms was calculated from the ratio of the intensity of corresponding XPS signals (for carbon atoms in a range from 285 to 291.6 eV, including a shakeup signal due to the polystyrene aromatic ring, for oxygen atoms at 532 eV) corrected for the relevant sensitivity factors. The details of calculations of fractions of hydrophilic components were described elsewhere.18 Water drop static contact angles were measured by Rame-Hart NRL goniometer (10000-230 model) equipped with video camera system and light system for visualization of the drop deposition on the analyzed surfaces plates. The glass plates were immobilized on the stable horizontal support in a chamber thermostated at 20 C. Contact angles were registered at both sides (left and right) and considered to be equal within the standard error. Drops of 5 μL of distilled water were applied on the top of the microspheres assemblies deposited on glass plates and pictures were registered. Then, an additional 5 μL of water was dropped at the top of existing drop, and the picture was stored again (for advancing contact angle). The receding contact angle was obtained after removing 5 μL of water from the existing drop. Static contact angles were taken as the average values from at least five measurements registered at 20 C. The angle-resolved UV-vis spectra were registered by using SPECORD S600 Analytik Jena spectrophotometer equipped with a variable angle reflectance attachment operated in the range 11-60.
3. Results and Discussion 3.1. Morphology and Properties of Monodispersed Core/ Shell P(S/PGL) Particles. The monodisperse P(S/PGL) microspheres were prepared in one step by surfactant-free emulsion copolymerization of styrene and R-tert-butoxy-ω-vinylbenzylpolyglycidol macromonomer. The hydrophilicity of the comonomer is the main polymerization parameter that influences the particle size and morphology. High concentrations of macromonomer in the polymerization mixture afford particles with low diameters and vice versa. Furthermore, the outermost layer of the sphere is different from its interior, being enriched with polyglycidol in the range 0-42 mol % (depending on the fraction of macromonomer in the polymerization mixture) and therefore in chains having polar groups.18 For the particles studied in this paper, the molar fraction of polyglycidol monomeric units in the shells (determined by XPS) was equal 0.423, i.e., was much higher than the average molar fraction of polyglycidol monomeric units in whole particles equal 0.048 (see details in refs 18 and 19). It is worth noting that P(S/PGL) microspheres synthesized using K2S2O8 initiator are negatively charged due to the presence of sulfate end groups in polymer chains formed during initiation. The concentration of acidic surface anions was equal 4.85 10-7 mol/m2, which indicates that the particles are negatively charged. Results of sulfate groups determination by conductometric titration shown that the anionic groups in the surface layer of P(S/ PGL) particles can be partially screened by hydrophilic polyglycidol chains or can be exposed to the medium, depending on the molecular mass of macromonomer and concentration of Langmuir 2010, 26(13), 11550–11557
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Figure 2. Typical TEM images of core-shell P(S/PGL) particles.
Table 1. Characteristics of P(S/PGL) Microspheres techniques
diameter at 20 C (nm)
diameter at 70 C (nm)
Figure 3. Dependence of hydrodynamic diameters of P(S/PGL) microspheres suspended in water as a function of temperature.
PCS 269 ( 2 254 ( 2 252 (22)a ( 2 TEM 265 (38)a ( 2 AFM 275 ( 1 263 ( 1 232 (1.66)b ( 1 UV-vis 262 (1.56)b ( 1 a Shell thicknesses estimated from TEM images. b Refractive indexes determined from curve fitting with Bragg’s law.
macromonomer in the polymerization mixture.18 The final morphology of the composite microsphere was expected to be a core-shell, as shown in Figure 1. The core-shell architecture of the particles was confirmed by TEM. Indeed, Figure 2 shows images with dark cores representing the PS-rich regions, surrounded by lighter shells of polyglycidol-rich domains. The particles spontaneously self-assemble into a well-organized hexagonal 2D network. One observes interparticle adhesion occurring among conjoint parts due to the hydrophilic polyglycidol peripheries. These images suggest that the particle shells are somewhat soft and amenable to stretching/ adhesion or compression. This adhesive property may be an effect of interpenetration of the swollen polymer networks during the drying process. Average particle diameter, assigned by TEM, was Dn = 265 nm at room temperature, with a low diameter dispersity index Dw/Dn = 1.01. It fell to 252 nm when the particles were dried at 70 C (see Table 1). TEM photographs indicate that the diameter decreases mostly due to a thinning of the shell, from 38 to 22 nm. In contrast, the PS core remains virtually the same. Such behavior highlights the thermoresponsive properties of the P(S/PGL) particles, the shell of which undergoes a phase transition above 45 C. The temperature-dependent average hydrodynamic diameters (Dh) of these core-shell particles in water were characterized with photon correlation spectroscopy measurements, and the results are presented in Figure 3. At room temperature the interfacial layer of P(S/PGL) microspheres is swollen with water, and the hydrodynamic diameters of P(S/PGL) microspheres essentially do not change when temperature is raised from 15 to about 40 C. With increased temperature, exceeding critical temperature (in the range from 45 to 50 C), diameters of P(S/PGL) particles decrease (from 269 down to 254 nm) due to deswelling of polyglycidol-rich interfacial layer. Further increase of temperature (in the range from 47 to 65 C) did not change diameters of microspheres. Swelling and deswelling processes are fully reversible, indicating efficient swelling at low temperatures and deswelling at high temperatures; it is the behavior typical for polymers with LCST transition. It is to note that the thermotransition is relatively large as compared to more classical thermoresponsive polymers such as Langmuir 2010, 26(13), 11550–11557
Figure 4. Temperature dependence of electrophoretic mobilities (μ) of P(S/PGL) particles suspended in water containing 10-3 M NaCl.
poly(N-isopropylacrylamide) (PNIPAM). Actually, poly(N-isopropylacrylamide) is uniform with respect to its chemical composition while shells of P(S/PGL) microspheres are enriched in polyglycidol but contain also polystyrene segments (fraction of polyglycidol equals 34 mol %). Polystyrene and polyglycidol are not miscible, and only one of them is water-soluble (polyglycidol); therefore, the phase separation results in a porous structure with polystyrene walls and water penetrable soft polyglycidol filling the pores. Such a diblock structure is probably responsible for the broaden transition observed in P(S/PGL) compared to classical thermoresponsive homopolymers. Increase of temperature affects not only the diameter of microspheres but also electrophoretic mobility. Figure 4 shows the dependence with temperature of electrophoretic mobility (μ) of particles suspended in water containing 1 10-3 M NaCl. It is worth noting that the absolute value of electrophoretic mobility μ is negative and decreases with temperature increase. This behavior could be attributed to changes of viscosity. Nevertheless, it is well-known from the literature that electrophoretic mobility of colloidal particles depends not only on such parameters as particle diameter and charge (including charge of ions in the double layer)22a but also on the extent of solvent penetration into the “spongy” interfacial layer,22b reflecting deviation of many types of real systems from the simple hard-sphere model. For instance, it is known that ionic strength and temperature affect thickness of double layer and therefore also hydrodynamic radius of particles. Analysis becomes even more complex in the case of particles that consist of block copolymers containing DOI: 10.1021/la100537v
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Figure 5. Typical SEM images of as-prepared colloidal-crystal films made from P(S/PGL) particles assembled at 70 C.
Figure 6. AFM image of the colloidal crystals composed of P(S/PGL) microspheres, assembled at (a) 20 and (b) 70 C.
blocks with different hydrophilicity (e.g., hydrophobic polystyrene and hydrophilic polyglycidol) and electric charge. Tentatively, we can assume that the temperature increase results in the rearrangement of the polymer chains at the surface layer, which become more “spongy like” and permeable for diffusion of counterions from solution. Fully dense FCC colloidal crystals were prepared by placing a drop of P(S/PGL) particle suspension on a glass coverslip and allowing drying completely at temperatures of 20 and 70 C. Typical scanning electron microscopy (SEM) images of prepared colloidal-crystal films are presented in Figure 5. The structure has cubic close-packed [111] planes oriented parallel to the substrate. The close-packed arrangement could extend over a large area (hundreds of μm2). The formation of a fully dense structure can be attributed to both the viscoelastic deformation of the shell and hydrogen bond linkages among the spheres. Indeed, the presence of a hydrophilic PGL-rich shell with abundant OH groups promotes the formation of hydrogen-bonding interaction, which enhances the linkage among particles and accelerates the self-assembly procedure. Nevertheless, the absence 11554 DOI: 10.1021/la100537v
of covalent linkage between particles prevents any free-standing film formation. The monodispersity of the microspheres is also a crucial parameter as it keeps the well-ordered arrangement of the particles during the drying step while the rigid and hydrophobic cores maintain the crystalline ordering of the resultant film. These factors accelerate the well-ordered arrangement and contribute to homogeneous photonic crystals with large scale. The influence of temperature on the structural properties of P(S/PGL) colloidal crystals was investigated using AFM. Figure 6 displays the surface topology of the colloidal crystal dried at 20 and 70 C. It shows a regular arrangement of particles with hexagonal-like packing. The colloidal crystal obtained at 20 C exhibits a particle diameter of 275 nm measured at bottom-tobottom distance. It is worth to stress that the particles are in close contact with each other. The surface polyglycidol chains are probably fully deployed and partly tangled. In contrast, when the colloidal crystal is prepared at 70 C, this value falls to 263 nm for the particle diameter measured at the same horizontal level. This evidence a 12 nm diameter decrease, accompanied by a slight Langmuir 2010, 26(13), 11550–11557
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modification of the lattice parameters, in agreement with the P(S/PGL) chains collapse above their phase transition. Angle-resolved reflection spectroscopy was employed to determine the wavelength of Bragg diffraction for these crystalline materials. Figure 7 shows the temperature dependence of the reflection spectra recorded at an angle of incidence of 20 from the normal to the (111) planes: one observes a 25 nm shift by varying the assembly temperature from 20 to 70 C as well as a strong modification of the Bragg peak intensity. The progressive decrease in λmax as the temperature increases is in line with a decrease in particle diameter above the P(S/PGL) transition temperature while the modification of the peak intensity reveals variations of the index contrasts in the colloidal crystal. Figure 8a shows the angle dependence of the reflection spectra of the colloidal crystal assembled at 70 C. Increasing the angle of incidence θ relative to the surface normal shifts the position of the reflectance peak to shorter wavelengths. If one assumes that the sample surface normal is aligned along a Æ111æ direction and that reflections of incident light occur from the (111) planes (the most densely packed), constructive interferences and maximum reflecting intensity can be obtained for
Figure 7. Angle-resolved reflection spectra (recorded at an angle of incidence of 20 from the normal to the (111) planes) of closepacked colloidal crystalline arrays made of P(S/PGL) microspheres dried at various temperatures: 20 C (dotted line), 50 C (dashed line), and 70 C (full line).
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wavelength (in free space) λmax obeying the equation λmax ¼ 2d111 ðneff 2 - sin2 θÞ1=2
ð2Þ
where θ is the external angle measured from the normal to the (111) planes and d111 is the interplanar spacing between (111) planes. It can be expressed in terms of the unit cell parameter a and the Miller indices by d111 = a/(h2 þ k2 þ l2)1/2 and is equal to (2/3)1/2D, where D is the sphere diameter. neff is the effective refractive index inside the opal (considered as an homogeneous model) and can be expressed as neff ¼ ðεparticle Φþεair ð1 - ΦÞÞ1=2
ð3Þ
where Φ is the filling fraction of the volume occupied by the particles (Φ = 0.74 for the close-packed fcc structure) and εparticle and εair represent the dielectric constants equal to the square root of the refractive indices of particle and air, respectively. Therefore, a plot of λ2 vs sin2 θ (from eq 2) should produce a straight line yielding neff and d111. The very close fit between experimental and theoretical results (see Figure 8b) confirms that these structures exhibit Bragg diffraction. From the fitting of eq 2, the refractive index neff of the colloidal crystalline arrays of P(S/PGL) particles was found to
Figure 9. Static water CAs on P(S/PGL) (b) and PS (2) films as a function of the deposition temperature. Inset shows the variation of CAs following heating (80 C) and cooling cycles (at 20 C).
Figure 8. (a) Angle-resolved reflection spectra of close-packed colloidal crystalline arrays made of P(S/PGL) microspheres. (b) Plot of square of free-space wavelength of light (reflection maxima) vs sin2 θ of angle of incidence to the normal for P(S/PGL) colloidal crystals at 20 C (2) and 70 C (b) and for PS colloidal crystals at 20 C (1) and 70 C (9). The dotted lines are the fits obtained using eq 2. Langmuir 2010, 26(13), 11550–11557
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Figure 10. Schematic illustration of the phase inversion on the surface of particle during the temperature increase.
increase with temperature (from 1.44 at 20 C to 1.52 at 70 C) while the sphere diameter D decreases (from 262 to 232 nm). If Φ = 0.74 for both samples, the refractive indices of P(S/PGL) particles increase from 1.56 at 20 C to 1.66 at 70 C. In order to emphasize the role of the P(S/PGL) copolymer chains in this thermoresponsive behavior, the same experiment was done on colloidal crystals of pure PS particles (see Figure 8b). Fitting to eq 2 reveals a slight decrease of the particle diameter from 171 to 166 nm, significantly smaller than that obtained for P(S/PGL) particles, while the effective refractive index remains roughly the same: 1.55 at 20 C and 1.56 at 70 C and corresponds to refractive index of the particles of 1.70 and 1.71, respectively. From these results, it follows that the optical properties of P(S/ PGL) colloidal crystals can be tuned with temperature due to an opposite evolution of the two parameters that are refractive index and particle diameter. Increasing the temperature leads to a decrease in particle diameter, in agreement with the phase transition observed by photon correlation spectroscopy and to an increase in refractive index, consistent with the collapse of P(S/PGL) chains at the surface of the particles and expulsion of water from the particle. The wettability of the colloidal crystal film was then studied by contact angle (CAs) measurements. For this experiment, colloidal crystal films were dried at various temperatures (from 20 to 100 C) and contact angles were recorded ex situ in a chamber maintained at 20 C. The results showed a dramatic change as a function of the temperature used during the formation of assemblies. As shown in Figure 9, the water CAs of the films increase with the rise of the assembly temperature, from ca. 50 at 20 C to ca. 110 at 70 C. It has been suggested that the water contact angle hysteresis plays an important role for the sliding behavior of water droplets.22 To measure the dynamic contact angle, advancing and receding angles were recorded by increasing or decreasing the drop volume until the three-phase boundary moved over the surfaces. Both advancing and receding water contact angles are listed in Table 2 as a function of the assembly temperature. The measured water contact angle hysteresis is relatively large. A similar hysteresis was observed by another group using micrometer-size square postarray.22c The reason for such large hysteresis can be attributed to the well-ordered nature of our system. Indeed, it was observed that the three-phase contact line for such a well-ordered system is more stable than the randomly rough surfaces, which are tortuous in three dimensions. For sake of comparison, the wetting properties of colloidal crystals of pure PS particles were also studied as a function of (22) (a) Yoshimitsu, Z.; Nanajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818. (b) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395. (c) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777.
11556 DOI: 10.1021/la100537v
Table 2. Dynamic Water Contact Angle Data for P(S/PGL) Colloidal Crystal Films Prepared at Various Assembly Temperatures assembly temperature (C) 20 30 60 80 100 a Standard errors in parentheses.
θA (deg)a
θR (deg)a
62.84 (4.28) 59.07 (2.65) 67.96 (2.07) 87.48 (1.96) 91.53 (1.18)
36.31 (2.95) 35.59 (2.42) 41.52 (2.33) 75.75 (1.42) 77.43 (1.79)
temperature. It is observed that, contrarily to colloidal crystals built from P(S/PGL) microspheres, the contact angles remain almost constant over the whole temperature range (see Figure 9). The particular wettability behavior of P(S/PGL) colloidal crystals probably arises from the phase separation of polymer segments driven toward minimum interfacial energy. Indeed, the particle has a hydrophobic PS core and a hydrophilic P(S/PGL) shell with hydroxyl groups anchored on the surface. Thus, the resulting colloidal crystal film is hydrophilic when fabricated at a lower temperature. With an increase in assembly temperature, thermodynamics drive the polymer chains toward a common minimum energy: the hydrophilic glycidol groups tend to shield from the apolar air and shrink toward the interior of the sphere, while hydrophobic styrene segments groups prefer to extend toward the apolar air as the water evaporates during the assembly procedure, so the resulting film is water-repellent. A possible illustration of the chemical component change of the particle surface during the assembly procedure is shown in Figure 10. Furthermore, the cooperation of the hydrophobic material and the surface roughness that stems from the periodic structure enhances the hydrophobicity of the surface. The multilayers of particles are therefore predominantly hydrophilic at lower temperatures and more hydrophobic at higher temperatures. According to the Wenzel’s formulation, the roughness factor (rs) for close-packed sphere geometry equals 1.9.23 As this factor should be constant whatever the assembly temperature, surface chemical composition and surface energy of the assembled latex spheres are more probably responsible for the changes of the final surface wettability of the resulting films. The reversibility of the system was investigated by an ex-situ multistep procedure which consists of (i) drying a colloidal crystal film at 80 C and recording CAs ex situ in the chamber maintained at 20 C; (ii) incubating the same colloidal crystal in water solution at 20 C and drying it at 20 C before recording the new CAs; (iii) repeating the procedure for five heating/cooling (23) (a) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (b) Nakae, H.; Inui, R.; Hirata, Y.; Saito, H. Acta Mater. 1998, 46, 2313.
Langmuir 2010, 26(13), 11550–11557
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Article
cycles. Results shown in the inset of Figure 9 demonstrate that the thermoresponsive behavior is fully reversible, with the possibility to switch the colloidal crystal surface from hydrophilic to hydrophobic and vice versa, by heating or cooling the sample on both sides of the transition temperature.
4. Conclusions In this work, we have studied a new type of colloidal crystal built from thermoresponsive core-shell particles of poly(styrene/ R-tert-butoxy-ω-vinylbenzylpolyglycidol). Because of the polymerization conditions, the outer layer of these particles differed significantly from the inner part of particles. Namely, the surface layer was enriched with polyglicidol, while the particles core was composed mainly from polystyrene. The core-shell structure of the individual particles was evidenced by TEM as well as the thermoresponsive behavior of the PGL-rich shell. A close packing of the particles was observed in the three-dimensional fcc colloidal array, with interparticle adhesion occurring among conjoint parts due to the hydrophilic polyglycidol peripheries. Bragg diffraction was studied using angle-resolved reflectance spectroscopy.
Langmuir 2010, 26(13), 11550–11557
The temperature-stimulated optical response of these photonic crystals can be explained on the basis of a decrease in particle diameter as the temperature rises above the P(S/PGL) phase transition accompanied by an increase in their refractive index due to the collapse of the PGL-rich shell and expulsion of water from the microsphere. Wetting properties of the colloidal crystals can also be switched by changing the assembly temperature, from hydrophilic (contact angle of ca. 50) under the particle transition temperature to hydrophobic (contact angle of ca. 110) above this temperature. The current work on these optical and chemical-switching materials opens doors to the use of a new type of thermoresponsive polymer derived from polyglycidol and suggests exciting possibilities for the creation of advanced thermoresponsive colloidal crystalline arrays from smart colloidal spheres. Acknowledgment. The presented work was supported by the Polish and French Ministry of Science and Higher Education Research Project POLONIUM for 2008-2009 and Grant for 2006-2009.
DOI: 10.1021/la100537v
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