Citation

Wei Deng, Hua-Chao Guo, Guo-An Li, Cheng-You Kan. Fabrication of polymeric-Laponite composite hollow microspheres via LBL assembly[J]. Chin. Chem. Lett., 2017,28(2): 367-371  

Wei Deng^a, Hua-Chao Guo^a, Guo-An Li^a, Cheng-You Kan^b     

Microspheres with particular structure and function, especially hollow microspheres have stimulated great attention since the concept of particle design was proposed by Okubo in the 1980s. Due to their unique morphology and fascinating physical properties including large specific surface area, low density, excellent light scattering and accommodation for object, hollow microspheres displays broad application prospects in coating , electrode [2], catalysis [3], biomedical [4], and so on [5]. The combination of hollow structure with different materials such as polymeric, inorganic, metallic and biological materials has been intensively investigated [6-10]. Methods of preparing polymeric hollow microspheres reported over the past few decades ranged from osmotic swelling [11-13], assembly method [14, 15], to SPG emulsification technique [16], as well as other polymerizationbased approaches [17-20]. Among them, osmotic swelling technique based on emulsion polymerization is the earliest and predominant method, and its typical processes involves the synthesis of core/shell latex particles and the expansion of these particles through creating ions in the core under the appropriate conditions. However, the most literature of osmotic swelling technique is limited in patents due to the strong commercial interest of polymeric hollow microspheres, and it still needs improvement and development with the aim that polymer microspheres can be designed with unique hollow structure and controllable size. Sacrificial template is usually utilized to build inorganic hollow microspheres, and a final etching or calcining to remove the template is unavoidable [21-23], typically requiring the use of harmful solution or high temperature, which often leads to the cracking and aggregation of inorganic shells.

Current studies of hollow microspheres mainly focus on the homogeneous materials, such as polymeric hollow microspheres or inorganic hollow microspheres, and only little attention has been paid on the hollow microspheres with polymeric-inorganic composite shells, which enable materials to integrate the excellent properties of hollow structure with polymer as well as inorganic materials. Yang et al. [24] prepared hollow spheres with doubleshelled and sandwiched composite structure by preprocessing the commercial polymeric hollow spheres and adsorbing the desired precursor. Liu et al. [25] reported the hollow inorganic-organic hybrid microspheres with the exterior silica shell and the interior poly (methacrylic acid) (PMAA) functionalized shell prepared based on the capillary force and the competitive hydrogen bond interaction. Teo et al. [26] investigated the influence of the monomer, cross-linker and initiator type on the synthesis of hollow polymer-graphene oxide particles via Pickering miniemulsion polymerization.

In this work, we present an approach to prepare polymeric- Laponite composite microspheres with hollow structure inside by armoring Laponite directly on the surface of hollow polymer latex particles without any sacrifice of template. Hollow polymer latex particles with controllable morphology were prepared from synthesizing multistage hydrophilic core/hydrophobic shell latex particles and performing alkali post-treatment based on the osmotic swelling principle. Subsequently, composite polymeric- Laponite shells were formed by alternate adsorption of PEI and Laponite. The morphology, zeta potential and compression resistance of the hollow microspheres were studied. The results and approach represented an attractive route towards the preparation of such composite hollow microspheres.

According to osmotic swelling principle, the preparation of hollow polymer latex particles began with the core/shell structure latex particles which were synthesized via seeded emulsion polymerization using carboxyl-containing core latex as seeds. During the whole polymerization process, the feeding speed of monomers was strictly controlled in order to decrease the local electrolyte concentration and increase the instantaneous monomer conversion. In addition, a polar intermediate layer was inserted to balance the polar difference between the hydrophilic core and hydrophobic shell, avoiding the mutual penetration of polymer chains and ensuring the preparation of designed core/ shell structure [27]. The morphology of latex particles obtained at each stage was shown in Fig. 1, and microspheres with one, two and three layers could be clearly identified, corresponding to core, core/ interlayer and core/interlayer/shell, with average diameter (Dp) of 126 nm, 178 nm and 251 nm, respectively.

Figure 1

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Figure 1. TEM micrographs of microspheres prepared at each stage: (a) core, (b) core/interlayer, and (c) core/interlayer/shell.

When the core/interlayer/shell latex particles suffered alkali treatment, the carboxyl groups inside were ionized and water molecules permeated into the interior of latex particles due to the osmotic pressure, causing the volume expansion of latex particles. Our previous research work found that the alkali dosage had the most significant impact on the morphology of alkali-treated particles, followed by alkali post-treatment temperature, and the least effective for alkali post-treatment time [28]. By controlling the mole ratio of NaOH used in the post-treatment to MAA used in the polymerization (MR_alkali/acid) during the alkali post-treatment process, the alkali-treated microspheres presented different size and morphology, as shown in Fig. 2. It should be attributed to the different ionization degree of polymer chains at different MR_alkali/acid. When MR_alkali/acid was 1.0, the ionized polymer chains generated by the neutralization were insufficient, only several small pores and little hollow structure with thick exterior covering were found in the alkali-treated particles. Along with MR_alkali/acid climbed to 1.15 and 1.3, the increasing of ionized polymer chains promoted the osmotic swelling, leading to effective volume expansion and formation of uniform hollow structure with the shell thickness less than 60 nm and hollow ratio (Hr) higher than 45% (Fig. 2c). However, superfluous NaOH solution, such as MR_alkali/acid reached 1.75, would bring deformed structure, which might result from the migration of some ionized polymer chains towards the water phase and the collapse of the thinning shell affecting together.

Figure 2

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Figure 2. TEM micrographs of alkali-treated particles with different MRalkali/acid: (a) 1.0, (b) 1.15, (c) 1.3, and (d) 1.75.

Layer by layer self-assembly technique allows the fabrication of monolayer and multilayer buildup on charged substrates by the sequential adsorption of oppositely charged materials. Both the resultant polymeric hollow microspheres and Laponite, a kind of synthetic hectorite-like clay, are surface negative charge, so PEI was used to modify the polymeric hollow latex particles shown in Fig. 2c, and the zeta potential turned from -45.1 mV to +44.9 mV after the adsorption of the cationic polyelectrolyte PEI. Directly mixing PEI modified polymeric hollow latex particles and Laponite in distilled water resulted in the irreversible precipitation even if the sample was drastically ultrasonic dispersed. It could be explained that charged microspheres tended to adsorb opposite charges around themselves to form large clusters if the electrostatic attraction between each microsphere was strong enough, and were easily precipitated out from the continuous phase along with the increasing of the volume. Stable emulsion system could be formed by using 0.1 mol/L NaCl solution instead of distilled water, because adding NaCl could squeeze the thickness of electric double layer of the colloidal particles and weaken the electrostatic attraction, which finally eliminated the risk of flocculation formation.

The morphology of polymeric hollow microspheres coated with different layers of Laponite was shown in Fig. 3. With the increasing of coating layers, the coating around the microspheres became more uniform and homogenous, and the thickness of clay increased, although no clear interface of each Laponite layer could be distinguished because of the extremely thin lamellar structure of Laponite. When three layers of Laponite were built up, the stacked clay around the surface of the hollow microspheres could be clearly identified, as shown in Fig. 3c. In order to further confirming the immobilization principle of Laponite on the polymeric hollow microspheres was based on the electrostatic adsorption, the surface charge of each step was characterized and results were listed in Table 1. After the adsorption of PEI and Laponite in turn, the zeta potential alternatively changed from negative to positive. It was noted that from the third to the fifth layer, the charge densities of particles modified with PEI were 45.7 mV, 44.6 mV and 43.2 mV respectively, almost kept in the same value, and the similar tendency was also observed in the Laponite adsorption layer, which demonstrated even more layers could be generated by repeating the cycles of sequential adsorption PEI and Laponite.

Figure 3

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Figure 3. TEM micrographs of the polymeric hollow microspheres coated with different layers of Laponite: (a) 1, (b) 2, (c) 3, and (d) 5.

Table 1

Table 1 Zeta potential (ζ) of the microspheres absorbed PEI and Laponite sequentially.

Table 1 Zeta potential (ζ) of the microspheres absorbed PEI and Laponite sequentially.

The pressure resistance of hollow microspheres was measured by mercury intrusion method. The mercury was intruded into the sample gradually with the increase of pressure and typical threesegment mercury intrusion curves were exhibited in Fig. 4. The first rising stage represented the compression of incompact solid power, and the second flat stage meant the intrusion of mercury into the interspaces between each microsphere. If the pressure was higher than the critical point that the hollow structure could be tolerated, the hollow structures would begin to break and the curve entered into its final stage. Subsequently, the cumulate volume increased with the increase of the pressure until all of the hollow spaces were filled with mercury. The result demonstrated that the broken pressure of polymeric hollow microspheres was about 1000 psi, and the polymeric hollow microspheres armored 1 layer Laponite also collapsed at the pressure of about 1000 psi, while the polymeric hollow microspheres armored 5 layers could keep intact until the pressure was higher than 1200 psi. For the microspheres with 1 inorganic layer, the Laponite platelets covered on the latex surface without overlapping each other to large extent, and the broken pressure almost kept at the same position of the naked polymeric hollow microspheres. Compared to the particles with no or less inorganic material, 5 layers of Laponite in exterior covering had resulted in the complex and overlap state, which made the Laponite layers form an integrative inorganic shell to resist the pressure of mercury. Therefore, armoring the polymeric hollow microspheres with multilayer of Laponite could increase the pressure resistance of the hollow structure.

Figure 4

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Figure 4. Cumulate volume versus pressure curves obtained by mercury porosimetry: (a) polymeric hollow microspheres (), (b) 1 layer Laponite armored hollow microspheres (), and (c) 5 layers Laponite armored hollow microspheres (Δ).

In this work, hollow microspheres containing polymeric- Laponite composite shells were prepared. The hollow polymeric microspheres could be obtained by performing alkali posttreatment on the multilayer core/shell particles at 100 ℃ for 3 h with MR_alkali/acid in the range of 1.15-1.3. After the polymeric hollow microspheres were modified by PEI, Laponite had been successfully coated on the surface of the hollow microspheres, and more Laponite layers could be generated by repeating the cycles of sequential adsorption PEI and Laponite. With the increasing of the Laponite layers, not only the thickness of inorganic layer, but also the pressure resistance of the hollow microspheres increased obviously.

Methyl methacrylate (MMA), butyl acrylate (BA), methacrylic acid (MAA) and styrene (St) (All analytical reagent, First Chemical Reagent Factory, Tianjin, China) were purified by distillation under reduced pressure. Ethylene glycol dimethacrylate (EGDMA) (98.0%, Alfa Aesar, Tianjin, China), polyethyleneimine (PEI) (99.0%, Acros, Beijing, China), sodium dodecyl sulfate (SDS), sodium hydroxide (NaOH), and Sodium chloride (NaOH) (All analytical reagent, Beijing Chemical Works, Beijing, China) were used as received. Ammonium persulfate (APS) (Analytical reagent, Aijian Modern Reagent Factory, Shanghai, China) was purified by recrystallization twice. Laponite was provided by Akzo Nobel (Shanghai) Co., and distilled water was used throughout.

The preparation process of polymeric hollow microspheres was illustrated in Scheme 1. First, 90 g H₂O, 0.10 g APS, 0.03 g SDS and the mixture of 0.64 g MMA, 0.03 g MAA, and 0.57 g BA were charged into the reactor, and polymerized at 80 ℃ for 40 min; Then, the mixture of 15.86 g MMA, 10.02 g MAA, 14.13 g BA, and 0.25 g EGDMA, as well as 10 g APS aqueous solution (3 wt%) were simultaneously dripped into the reactor at 80 ℃ within 4.5 h, then maintained at 90 ℃ for 30 min to obtain core latex. In the interlayer preparation, 10 g resultant core latex was diluted with 25 g H₂O, then the seeded emulsion copolymerization of 7.33 g MAA, 2.03 g St and 0.84 g MMA was carried out at 80 ℃ by constant feeding the monomer mixture and 10 g APS aqueous solution (1 wt%) simultaneously into the diluted core emulsion within 40 min. Next, 11.84 g St and 10 g APS aqueous solution (1 wt%) were also simultaneously dropwise added into the system at 90 ℃ within 2 h to form the outer shell. Finally, alkali post-treatment was performed at 100 ℃ with different dosage of NaOH aqueous solution (5 wt%) for 3 h.

Scheme1

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Scheme1. The schematic of the preparation process for polymeric hollow microspheres.

As-prepared emulsion containing polymeric hollow microspheres (1 g) was diluted with 20 g H₂O, and then slowly dripped with 60 g PEI aqueous solution (0.1 wt%) under continuous agitation. The modification procedure was maintained for 6 h and excessive polyelectrolyte was removed by three cycles of centrifugating-washing-redispersing with water at 9000 r/min. Next, 5 g Laponite aqueous solution (1 wt%) was added into the resultant PEI modified polymer latex which was redispersed with 0.1 mol/L NaCl aqueous solution, and adsorption was allowed to last for at least 6 h. Excessive inorganic materials were removed by centrifugation-washing-redispersing with water at 9000 r/min.

The zeta potential (ζ) of the microspheres was measured on Zetasizer 3000HS (Malvern, UK) with a fixed scattering angle of 90° at 25 ℃. The size and morphology of the prepared microspheres were characterized by TEM (JEOL, JEM-2010, Japan) at 70 kV. The samples were first diluted with water to solid content about 1 wt% and stained with 4-5 droplets of phosphotungstic acid (2 wt%) for 3 h, then mounted on carbon-coated copper grids and dried overnight. The number average diameter (Dp) and hollow ratio (Hr) of the microspheres were obtained by measuring at least 50 microspheres and calculated as below:

where n_i was the number of the microspheres with a diameter of D_i, and D_h as well as DAP denoted the diameters of the hollow structure inside and the alkali-treated microspheres, respectively.

The compression resistance of the hollow microspheres was measured on a mercury intrusion porosimeter (Autopore IV 9510, China). The samples were dried to white solid in an oven at 30 ℃, then gently grinded into powder and subjected to the mercury intrusion analysis.

This work was supported by Heilongjiang Provincial Natural Science Foundation for Youth, China (No. QC₂₀14C052), Fund of Key Laboratory of Advanced materials of Ministry of Education (No. 2016AML06), and the training project for innovation and entrepreneurship of the Harbin University of Science and Technology, China (2016).