Surface modified latex particles: Synthesis and self ... - CiteSeerX

Colloids and Surfaces A: Physicochem. Eng. Aspects 298 (2007) 27–33

Surface modified latex particles: Synthesis and self-assembling into photonic crystals A.Yu. Menshikova a,∗ , B.M. Shabsels a , N.N. Shevchenko a,c , A.G. Bazhenova b , A.B. Pevtsov b , A.V. Sel’kin b , A.Yu Bilibin c a b

Institute of Macromolecular Compounds, Russian Academy of Sciences, Bolshoy pr. 31, St. Petersburg 199004, Russia Ioffe Physico-Technical Institute, Russian Academy of Sciences, Politekhnicheskaya 26, St. Petersburg 194021, Russia c Saint Petersburg State University, Institute of Chemistry, Universitetskiy Pr. 26, St. Petersburg 198504, Russia Available online 8 December 2006

Abstract The capability of monodisperse carboxylated particles of poly(methyl methacrylate) and of styrene copolymers with glycidyl methacrylate or with methacrylic acid for self-assembling into thin films exhibiting properties of photonic crystals was studied in relation to their polymer nature, carboxyl group distribution, and to the compositions of the dispersion medium. These particles were capable of self-assembling from their dispersions and forming close-packed three-dimensional ordered arrays on glass slides. The poly(styrene-co-methacrylic acid) particles revealed to be the best choice as the building blocks for photonic crystals formation. To ensure the appropriate surface structure of the particles and narrow particle size distribution in the submicron range the effects of reaction mixture pH, chain-transfer agents, and initiator concentration were investigated subsequently. The Bragg reflection spectra from the (1 1 1) planes of their ordered structures showed a predominant high-reflectance peak corresponding to photonic band gap caused by 3D periodicity of refractive index. The experimental spectra obtained for polarized light at different angles of incidence were well simulated in the theoretical calculations. © 2006 Elsevier B.V. All rights reserved. Keywords: Self-assembling; Monodisperse (co)polymer particles; Carboxylic groups; Photonic crystals

1. Introduction The current trend of miniaturization of existing devices such as chemical sensors, optical limiters or switches, and data storage devices dictates the ever-increasing need for improved performance of nanostructured materials with advanced properties. Synthesis and fabrication of nanocomposites with periodically modulated structure is a new promising step toward development of such materials [1–13]. Composites based on synthetic opals with a face-centered cubic (fcc) lattice formed by close-packed monodisperse submicron-sized spheres of amorphous silica are considered as promising materials for obtaining the required photonic crystallic properties [4–7]. However, in this case, obtaining a perfect three-dimensional (3D) lattice ensuring a high density of structural components involves serious technological problems. The high density of SiO2 results in particle sedimentation, giving rise to stresses and defects

∗

Corresponding author. Tel.: +7 812 3235025; fax: +7 812 3286869. E-mail address: [email protected] (A.Yu. Menshikova).

0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.12.023

in the ordered arrays. Furthermore, the surface non-uniformity of amorphous SiO2 microspheres formed by aggregation of nanometer-sized particles results in considerably increased diffuse scattering of electromagnetic waves at the phase boundary and in suppressed photonic crystallic effects [7]. Methods of polymer chemistry allow preparation of monodisperse surfaceuniform particles of a lower density compared with inorganic materials. Therefore one of new strategies in this field is to apply such polymeric particles as functional building blocks to produce optically responsive materials [8–13]. These particles are capable of self-assembling from their dispersions (latexes) and forming close-packed arrays on solid supports. These ordered structures can demonstrate the properties of photonic crystals (PhC) if particle diameters are comparable with a wavelength in a visual range. Polymeric PhC obtained by this technique can be used as models for studding light interaction with crystal lattices [11] or as templates for fabricating inverse PhC based on semiconductor materials [12,13]. To form perfect PhC of large extension with a few defects, it is necessary to find out the requirements for the particles (their size, polymer nature and surface characteristics) in regard to the

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A.Yu. Menshikova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 298 (2007) 27–33

Table 1 Characteristics of latex samples and thin-film formation conditions Latex

1 2 3

Polymer nature

PMMA P(St-co-GMA) P(St-co-MAA) a b

D (nm)

330 300 250

σ (%)

K

1.00060 1.00015 1.00001

2.45 1.24 0.69

[COOH] (␮mol/m2 )

Solvent

0.76 0.73 3.01

Water Ethanol Ethanol

Characteristics of thin films Number of particle layersa

PBGb , λmax (nm)

63/43 39/44 26/26

637 712 580

Numerator, calculation from the perfect fcc lattice structure and film thickness; denominator, examination of film chips by SEM. Angle of light incidence was 10◦ .

formation of primary polymeric matrix and also to its structure and optical properties. For this goal we studied monodisperse particles with carboxylic groups on the surface because of their ability to vary surface charge density with pH and ionic strength of dispersion medium. These factors make it possible to influence the particle interaction during lattice formation, which may lead to improved ordered structures. 2. Material and methods 2.1. Materials Styrene (St), methyl methacrylate (MMA), methacrylic acid (MAA), tioglycolic acid (TGA), mercaptoethanol (ME), dimethylformamide (DMF), toluene, ethanol, and dioxane were purified by distillation using standard techniques. Water was distilled twice. Potassium persulfate K2 S2 O8 and 4,4 azobis(4-cyanoisovaleric acid) (ACVA) were purified by recrystallization from distilled ethanol. Glycidyl methacrylate (GMA), methacrylic acid (MAA), NaOH, HCl, and Na2 HPO4 , all of commercial grade, were used without additional purification. A chain transfer agent o-diaminodiphenyldisulfide (o-DA) was prepared and purified according to Ref. [14].

maintained during the process. At first, carboxylated latex particles were synthesized by polymerization of methyl methacrylate at 75 ◦ C according to Ref. [15] and copolymerization of styrene and GMA (7.5 mol%) at 80 ◦ C according to Ref. [16] using ACVA as carboxyl-containing initiator. Synthesis conditions and characteristics of the particles, as well as properties of the thin films fabricated from them are presented in Table 1. These particles had rather smooth surface and differed in their hydrophobicity. Monodisperse particles based on styrene and methacrylic acid (MAA) copolymer P(St-co-MAA) with hairy surface carboxylated layer were also prepared using K2 S2 O8 at 80 ◦ C. In order to vary the diameter of P(St-co-MAA) particles in a submicron range and achieve a narrow particle size distribution (PSD) in each case, the effects of the initial pH of reaction mixtures (pH0 ) varied in the range of 9.74–11.50, K2 S2 O8 concentration in the range of 0.092–0.167 wt.% with respect to water, the nature of chain transfer agents (o-DA, TGA, ME) at their concentration of 0.115 mol% with respect to monomer mixture, and o-DA concentration in the range of 0.14–1.34 mol% were investigated subsequently. The synthesis conditions and characteristics of the P(St-co-MAA) particles are given in Table 2. 2.3. Latex particle characterization

2.2. Latex preparation Latexes were prepared by emulsifier-free emulsion polymerization carried out in a four-necked glass reactor with a glass paddle-type stirrer, a condenser, a nitrogen inlet, and a temperature controller. Continuous stirring at a rate of 300 rpm was

Latex particle size was measured by transmission electron microscopy (JEOL JEM 100 S microscope, Japan). For a set of more than 600 particles, the root-mean-square deviation (σ) and polydispersity coefficient K were determined (K = Dm /Dn , where Dm and Dn are the weight-average and number-average

Table 2 Composition of reaction mixtures and characteristics of P(St-co-MAA) particles Latex

4 5 6 7 8 9 10 11 12 a

Reaction mixturea

[COOH] (␮mol/m2 )

(St + MAA)/H2 O (wt.%)

MAA/St (wt.%)

K2 S2 O8 /H2 O (wt.%)

pH0

5.5 5.5 5.5 10 10 5.5 5.5 5.5 10

6.6 6.6 6.6 3.6 3.6 6.6 6.6 6.6 3.6

0.092 0.092 0.092 0.167 0.150 0.092 0.092 0.092 0.130

11.5 11.5 10.7 11.5 11.3 11.5 11.6 9.6 11.4

Chain-transfer agents: o-DA (5–8, 11, 12), TGA (9), ME (10) 0.115 mol% with respect to St.

2.78 1.49 2.87 2.15 3.56 1.74 2.90 1.42 1.78

Dispersity Dn (nm)

K

σ (%)

323 250 251 347 414 331 310 230 260

1.01080 1.00001 1.00036 1.00033 1.00100 1.00040 1.00640 1.00010 1.00020

10.4 0.84 1.76 1.68 3.23 2.05 8.01 1.14 1.58


diameters, respectively). Changing in the particle diameters in aqueous dispersions of different pH were determined by light scattering according to Ref. [17]. The surface concentration of carboxylic groups was determined by conductometric titration according to Ref. [17] after threefold washing of the latexes to remove water-soluble impurities by successive centrifugation followed by redispersion in double-distilled water. For analyzing of terminal aminophenylsulfide groups originated from o-DA as well as initial o-DA on the particle surface, they were coupled with diazo cation obtained from p-nitroaniline by standard technique described elsewhere [18]. After drying the latex particles, hydrophilic and hydrophobic fractions of P(St-co-MAA) copolymer obtained were divided by ethanol addition to copolymer solution in dioxane. Hydrophilic chains were counted as the ones that remained in solution. The whole content of carboxyl groups in the particles, and also their distribution between hydrophilic and hydrophobic chains were determined after dissolution of polymeric fractions in a mix toluene–ethanol (10:1, v/v) by potentiometric titration with KOH solution in isopropanol. Molecular weight distribution (MWD) of the copolymers obtained was studied in toluene solutions by gel permeation chromatography using polystyrene standard samples (Waters, USA and Polymer Laboratories Ltd., USA). The electrophoretic mobility of the particles was studied using standard microelectrophoresis in 10−4 mol/l NaCl solu-

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tions over a wide pH range of 5.0–10.5. Before measurements, the equilibrium between particle surface and NaCl solutions of different pH was achieved for 24 h. 2.4. Thin films fabrication Thin films of the particles were fabricated on clean glass slides from water or ethanol dispersions of the polymer particles. The detailed techniques for film preparation are given elsewhere [10]. 2.5. Characterization of fabricated thin films The structure of the thin films obtained was examined by scanning electron microscopy (JEM 100 C JEOL microscope, Japan) and atomic force microscopy (SPM system Solver P47H-PRO, Russia) using semicontact AFM mode. For nondestructive monitoring of the ordered structures and estimation of photonic crystallic properties of the films obtained, the Bragg light reflection spectra from (1 1 1) growth surfaces of the thin films in the visible and IR ranges were studied from surface spots of a size corresponding to a characteristic monodomain, 10–50 ␮m (after the technique suggested in Ref. [19]). The experimental spectra were obtained for polarized light at different angles of incidence θ in the range of 10–36◦ . Numerical calculation of the spectra were performed using planar approximation and Bloch mode formalism according to a new approach

Fig. 1. SEM micrographs of thin films formed from latex particles listed in Table 1: (a) 1, (b) 2 and (c and d) 3 (side view).

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Table 3 Carboxylic group distribution in P(St-co-MAA) particles Latex

5 6 7 8 9

MAA loading (mol%)

8.0 8.0 4.2 4.2 8.0 a

[MAA]p a (%)

17.0 15.0 43.0 20.3 22.0

[MAA]p (mol%)

1.36 1.20 1.82 0.85 1.76

[COOH] distribution in the particles (%) Hydrophilic chains

Hydrophobic chains

31.5 19.0 70.0 17.8 54.0

68.5 81.0 30.0 82.2 46.0

[COOH] on the surface (%)

28.2 63.0 20.8 57.4 20.1

MAA units in the particles with respect to MAA loading in the reaction mixture.

developed for analyzing Bragg reflection spectra formation in the case of 3D PhC and described elsewhere [11,20]. Their results were used as the new effective method of structural characterization of opal-like PhC systems based on polymeric microspheres. 3. Results and discussion Based on the previously revealed relationships between the polymerization parameters and characteristics of polymer particles (diameter, PSD, surface properties) [15,16], we prepared monodisperse latexes of poly(methyl methacrylate) (PMMA) and of P(St-co-GMA) and P(St-co-MAA) copolymers, which could be applied as building blocks of 3D ordered lattices exhibiting photonic crystallic properties. For preliminary examination of the influence exerted by the polymer nature of monodisperse polymeric particles on their capability for self-

Fig. 2. Reflection spectra of thin films fabricated (a) from the particles listed in Table 1 and (b) from the P(St-co-MAA) particles listed in Table 2 (samples 5 and 8, 9, 11).

assembling, three latexes of the narrowest PSD having the particle diameters Dn in a narrow range of 250–330 nm were chosen and the optimal self-assembling conditions were found for each latex (Table 1). In the case of styrene copolymers with GMA or MAA, the alcoholic dispersions, whose drops spilt over the whole glass surface, formed more uniform thin films with a few defects in comparison with water-based latexes. Thus, passing from aqueous to ethanol media decreases the surface tension forces in the drops and favors self-assembling polymer particles to a more ordered structure under the conditions closed to equilibrium. On the contrary, in the case of PMMA particles, which are less hydrophobic than those of styrene copolymers, the best thin films were obtained by applying aqueous dispersion on glass slides hydrophilized by alkali treatment. SEM micrographs showed that all particles studied under the empirically chosen conditions formed 3D ordered structures which thickness runs up to 50–60 layers (Fig. 1). The Bragg reflection spectra from (1 1 1) planes of the structures displayed a predominant high-reflectance peak corresponding to photonic band gap (PBG) accompanied by Fabry-Perot oscillations due to multiple light reflection from the front and back sides of the films (Fig. 2a). PBG originating from the Bragg diffraction of electromagnetic waves on 3D ordered structures are evidence of the self-assembling monodisperse latex particles into PhC lattice of a large-scale period. The polymer nature characterized by determined refractive index value, along with the diameter of the monodisperse particles, significantly affects the PBG position.

Fig. 3. MWD of P(St-co-MAA) copolymers in the latex particles prepared without chain transfer agents (1) or in the presence of 0.058 mol% o-DA (2), 0.115 mol% o-DA (3), TGA (4) and ME (5).


However, in the case of the films prepared from PMMA and P(Stco-GMA) particles, direct estimation of the number of layers in the film from SEM micrographs and calculation of these values from the film thickness (measured by optical microscopy) and the particle diameter taking into account fcc lattice structure gave different results. The values obtained were close or coinciding only for the film formed from P(St-co-MAA) particles. This fact, along with the SEM micrograph (Fig. 1c), proved that P(St-coMAA)-based film had more perfect structure. In order to vary optical properties of PhC films based on P(Stco-MAA) particles and particularly PBG position, latex series of narrow PSD and various particle diameters in the submicron range was obtained and effects of reaction mixture composition were investigated subsequently to supervise the latex dispersity (Table 2). In particular, the narrowest PSD was achieved in the presence of o-DA or TGA in the reaction mixture. These chain transfer agents having amino or carboxylic groups promoted the formation of surface active copolymer chains ensuring smaller particle size, latex stability and σ about 1–2%. The appearance of such copolymer chains of MW lower than 104 was observed in Fig. 3, curves 2–4. Moreover, estimation from absorption spectra of azo dyes synthesized on the particle surface (samples 5–9) given in Ref. [21] showed that there were 10–40% of the terminal aminophenylsulfide groups originated from o-DA on the surface with respect to the initial o-DA content in the reaction mixtures. The narrowest samples of the particles were used for film fabrication. Bragg reflection spectra from the (1 1 1) planes of the films obtained actually showed shift of PBG positions toward the red light range due to increase in particle diameters (Fig. 1b). Along with this finding the effect of MAA units distribution inside the particles (Table 3) on the PhC features of the thin films was observed. In particular, the best PhC characteristics were demonstrated by the films fabricated from the particles of latex 5. These particles had the lowest surface concentration of carboxylic groups (Table 2) and MAA units were mostly contained in the hydrophobic chains (Table 3). It is well known that emulsion copolymerization of hydrophobic monomers with water-soluble carboxyl-containing monomers yields polymer particles whose surface layer is

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Fig. 4. pH effects on (a) the electrophoretic mobilities U and (b) diameters of the particles listed in Tables 2 and 3.

enriched in copolymer chains with increased content of carboxyl-containing hydrophilic units [22]. Such layers can change their volume depending on the salt composition and pH of the dispersion medium. For example, changing of diameters of the latex particles listed in Table 3 depending on pH of the dispersion medium was noticeable at an electrolyte concentration of 10−3 M (Fig. 4b). The particle diameter appreciably increased in an alkaline medium, in parallel with an increase in the electrophoretic mobility and, hence the negative charge of

Fig. 5. AFM images of thin films formed from aqueous dispersions of P(St-co-MAA) particles samples: (a) 5 and (b) 9 listed in Table 2.

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the surface (Fig. 4a). In the case of particle samples 6–9, swelling of the hydrophilic surface layer due to mutual repulsion of ionized carboxylic groups results in non-uniform contraction of the particles during self-assembling process, which led to partial disordering of PhC structure. Only in the case of latex sample 5, the restricted surface swelling due to distribution of MAA units in hydrophobic chains gave the opportunity for uniform shrinkage occurring during particle packing upon evaporation of the dispersion medium. As a result, the structure of the multilayer fcc lattice formed from such particles became more perfect. AFM image of the film based on sample 5 particles displayed more compact and dense surface layer in comparison with the other film based on the particles having more hydrophilic surface (Fig. 5). To find out effects of carboxylic groups on a rather smooth surface of the latex particles (sample 5) on their capability of selfassembling from ethanol dispersions to form 3D PhC arrays, their ionisation degree was varied by HCl or NaOH addition. HCl suppressed ionisation and arrays seemed to be polydomain and rather disordered (Fig. 6a). Increase in NaOH concentration promoted formation of perfectly ordered structures (Fig. 6b–e). This finding shows the importance of the electrostatic repulsion between particles, which led them during self-assembling process to the points of lattice where their energy achieved the minimum. This repulsion is ensured by overlapping of electric double layers of the particles in their close-packed 3D structures. Polymeric PhC having a high degree of ordering (Fig. 6d and e) was considered as a model object for studying the interaction of light with a 3D fcc structure. Experimental data obtained by scanning spectroscopy were used as a basis for development of a theoretical model describing the specific features of photon propagation in a 3D large-scale periodic lattice [20]. In this model a 3D fcc lattice is approximated by a periodic layered planar medium with the dielectric permittivity modulated along the [1 1 1] direction. The specific profile of the dielectric function was calculated with its averaging within the unit cell in the plane perpendicular to the [1 1 1] direction. The results of numerical calculations by the method of transfer matrices gave good demonstration of principle features observed in the experimental reflection spectra measured at different incidence angles in the range of 15–36◦ (Fig. 7). The theoretical calculations also allowed independent determination of optical constants of the structures synthesized and of geometric parameters of polymer particles in PhC film. The refractive index of the particles is similar to that of bulk polystyrene (1.59 at 20 ◦ C) in accordance with low MAA content in the P(St-co-MAA) particles (Table 3). The calculated particle diameter, 252.4 nm, is very close to the value determined by TEM in the case of sample 5 (250 nm). The model developed takes also into account the effects of particle shrinkage, arising in the course of formation of the bulk material. The calculated shrinkage coefficient in the case of sample 5 was only 3%. Hence, particles shrinkage in the PhC films can be about 7.5 nm. This value seems to be quite reasonable and comparable with the weak changing of the particle diameter due to surface layer swelling (Fig. 4). Thus, the theoretical model is in good agreement with the experiment, which indicates that, on the one

Fig. 6. SEM micrographs of thin films formed from ethanol dispersions of P(St-co-MAA) particles (sample 5) at (a) 8.3 × 10−5 mol/l HCl or NaOH concentrations, (b) 1.6 × 10−4 , (c) 4.0 × 10−4 , and (d and e) 5.5 × 10−4 mol/l. (a, b, and e) Top view and (c and d) side view.


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Program “Nanostructures in Polymer Systems, Promising for Optoelectronics” of the Presidium of the Russian Academy of Sciences, to the Scientific Program “Development and Study of Macromolecules and Macromolecular Structures of New Generations” of the Division of Chemistry and Materials Science of RAS, and to the Program of the St. Petersburg Scientific Center of RAS for the year 2005. References

Fig. 7. Effect of incidence angle on reflection spectrum of the thin film fabricated from ethanol dispersion of P(St-co-MAA) particles (sample 5) at 5.5 × 10−4 mol/l NaOH. Points, experimental data; solid line, theoretical calculation.

hand, the thin film polymer periodic structures obtained are perfect and, on the other hand, these structures can be used as model objects in theoretical studies of interaction of light with the 3D lattice of photonic crystals. 4. Conclusions The results of this research prove that monodisperse (co)polymer particles of submicron size can serve as excellent building blocks for formation of photonic crystals promising for both experimental and theoretical studies of non-linear optical effects. The surface structure and surface charge of monodisperse polymer particles strongly influences photonic crystallic features. In particular, photonic crystals based on P(St-co-MAA) particles had the best quality. Their capability of self-assembling into PhC films of perfect 3D structure increases if the surface layer is less hydrophilic but rather smooth and stable. Besides, defects quantity depends on composition of dispersion media during self-assembling process, as surface tension and electrostatic forces act together and have effect on the particles position and shrinkage in the PhC arrays. These factors are of great importance since the shrinkage uniformity could promote formation of the perfectly ordered structures. Acknowledgments The authors are grateful to Russian Foundation for Basic Research (projects 04-03-33080, 05-02-17776), to the Scientific

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