Synthesis of polystyrene microspheres in the presence of zinc oxide nanoparticles

Plasticheskie Massy, No. 9, 2011, pp. 60 64 Synthesis of polystyrene microspheres in the presence of zinc oxide nanoparticles Yu.M. Shiryakina, 1 N.S. Serkhacheva, 1 N.I. Prokopov, 1 I.A. Gritskova, 1 and S.M. Levachev 2 M.V. Lomonosov State University of Fine Chemical Technologies, Moscow, Russia Selected from International Polymer Science and Technology, 39, No. 2, 2012, reference PM 11/09/60; transl. serial no. 16498 Translated by P. Curtis Summary Zinc oxide nanoparticles stable in styrene were obtained, and the factors determining the possibility of immobilising them in polymer microspheres were studied. INTRODUCTION Heterophase polymerisation is one of the most effective methods for introducing (immobilising) solid particles into submicron particles [1 5]. Thus, clays, limestone, aluminium oxide, silicon dioxide, carbon black, and magnetic materials have been immobilised into polymer matrices [6]. Inorganic particles have been used as nuclei for the formation of polymer particles. In recent literature [7, 8] it has been concluded that it is most expedient to immobilise inorganic solid particles in emulsions consisting of monomer microdroplets. The aim of the present work was to produce polymer microspheres with zinc oxide nanoparticles on their surface by heterophase polymerisation. This work is timely in view of the need to expand methods for synthesising polymeric materials containing inorganic nanoparticles of different morphology. EXPERIMENTAL Initial results Zinc acetate dihydrate, Acros, >98.0%; potassium hydroxide, chemically pure, GOST 24363-80, company MSD ; propanol-2, Khimmed LLC, TU 6-09-07-1718- 91; oleic acid, GOST TU 9145-002-51043152-2005, TTs Vympel LLC; acetone, Reakhim LLC, TU 6-09- 3513-86; styrene, Aldrich, 99% (purified from stabiliser with a 5% aqueous solution of sodium hydroxide, washed with water until neutral, dried over calcined calcium chloride, and twice distilled in vacuum; use was made of 41 C (2.1 kpa) fraction, d 20 = 0.906 g/ cm 3, n 20 = 1.5450); water, bidistillate; cetyl alcohol (CA), Aldrich, 99%; sodium dodecyl sulphate (SDS), Sigma, >98.0%; potassium persulphate (PP), Sigma Aldrich, 99.9%. Synthesis of zinc oxide nanoparticles Zinc oxide nanoparticles were obtained by hydrolysis by reaction between zinc acetate dihydrate and potassium hydroxide according to the procedure described in Dazhi et al. [9]. Hydrophobisation of the surface of zinc oxide nanoparticles A quantity of 10 ml of a solution of oleic acid in styrene was added to washed zinc oxide nanoparticles, and the mixture was subjected to ultrasound treatment (a Cole- Parmer Instruments (USA) ultrasonic processor was used). The characteristics of the ultrasound treatment were: amplitude 25% of the maximum possible (70 khz), total treatment time 30 s, pulse duration 10 s, time between pulses 2 s. 2012 Smithers Rapra Technology T/15

Synthesis of polymer microspheres with zinc oxide fillers The heterophase polymerisation of styrene was conducted in an HWS-Labortechnik glass reactor of 100 ml volume, equipped with a heat exchange jacket and an anchorimpeller mixer of PETP with a PE-8310 motor (Ekros, Russia). The aqueous phase was first prepared: water was heated to 70 C, emulsifier (sodium dodecyl sulphate) and a cosurfactant (cetyl alcohol) were dissolved in it, an initiator (potassium persulphate) was added, then a styrene dispersion containing zinc oxide nanoparticles was poured in, and ultrasound treatment was carried out. The ultrasound treatment was conducted with the following parameters: amplitude 20%, total treatment time 12.5 min, pulse duration 10 s, time between pulses 2 s. To maintain the necessary polymerisation temperature, water was pumped through the heat exchange jacket of the reactor by means of a DC 30 ultrathermostat (HAAKE, Germany), and the temperature of the water was maintained at a level of 70 ± 0.5 C. Highly dispersed emulsion prepared by the above method was charged into the reactor, mixing was begun (280 r/min), and the polymerisation start time was noted. The process was conducted for 5 h. Methods of investigation Observations were conducted on an LEO 912 electron scanning microscope (Karl Zeiss, Germany) by the standard procedure. For the investigations, specimens of dispersions were diluted to weak opalescence. A Zetasizer NanoZS laser particle analyser (Malvern, UK) was used to assess the particle size distribution of the polymer suspension. Measurement was conducted in an automatic regime by the standard procedure recommended by the manufacturer. The solids content of the suspension was determined by the gravimetric method. The interphase tension at the phase boundary was measured by the Wilhelmi method on a K100MK2 tensiometer (Krüss, Germany). RESULTS AND DISCUSSION Synthesis of zinc oxide nanoparticles Zinc oxide nanoparticles were synthesised by chemical condensation, i.e. alkaline hydrolysis of zinc salt in isopropyl alcohol. The following reaction was conducted between zinc acetate dihydrate and potassium hydroxide in isopropanol [9]: Zn(CH 3 COO) 2 2H 2 O+2KOH ZnO +2CH 3 COOK+ 3H 2 O Production of a dispersion of zinc oxide nanoparticles in styrene As zinc oxide nanoparticles have a hydrophilic surface, to transfer zinc oxide suspension from isopropanol to a hydrophobic medium (styrene), the surface of the particles have to be hydrophobised. From the literature [10] it is known that, in most cases, to obtain stable dispersions of inorganic nanoparticles in different media, use is made of fatty acids and their salts. The most effective stabiliser [11] is oleic acid, which possesses the ability to be chemisorbed on the surface of zinc oxide nanoparticles [12], and here its hydrocarbon radicals form a shell on the surface of the nanoparticles that is compatible with an aromatic hydrocarbon such as styrene. By dynamic light scattering it was established that the average diameter of the hydrophobised zinc oxide nanoparticles in styrene hardly varies and amounts to 6 11 nm. Thus, a stable dispersion of zinc oxide nanoparticles in styrene was obtained, containing zinc oxide nanoparticles in a quantity of 6.6 wt%, which it was proposed to use for immobilisation in polymer microspheres during the polymerisation of styrene. Production of highly dispersed monomer emulsions It was proposed that the heterophase polymerisation of styrene with the inclusion of zinc oxide nanoparticles in polymer microspheres be conducted in a highly dispersed styrene emulsion in which the entire volume of monomer was dispersed in the form of microdroplets with a unimodal size distribution. For the investigations, a method was chosen for producing highly dispersed emulsions by emulsification of the monomer with a mixture of ionogenic emulsifiers and long-chain fatty alcohols during intense mixing of the system by the procedure proposed by Hansen [13]. In this case, an aqueous solution of emulsifier and fatty acid is premixed at a temperature above the melting point of the alcohol, and then the monomer is added. The ratio of fatty alcohol and ionogenic emulsifier in such systems is varied in the range from 1:1 to 4:1. The change in the order of introduction of the ingredients, for example the introduction of alcohol into monomer with the subsequent addition of aqueous solution of emulsifier, leads to the formation of an unstable, coarsely dispersed emulsion. An important role in emulsion preparation is played by the length of the hydrocarbon radical in the alcohol T/16 International Polymer Science and Technology, Vol. 39, No. 12, 2012

molecule. According to data in Grimm et al. [14], a stable, highly dispersed styrene emulsion is obtained in the presence of cetyl alcohol (CA), in which the length of the alkyl radical amounts to 16 carbon atoms. Sodium dodecyl sulphate (SDS) was selected as the ionogenic emulsifier. Given that the literature contains no data on the colloid-chemical properties of these surfactants at high temperature, the investigation was begun with a study of the influence of the SDS/CA molar ratio on interphase tension at the styrene/water phase boundary at a temperature of 70 C. The data obtained showed that the interphase tension does not change up to an SDS/CA molar ratio of 1:2, and then increases considerably, and at a ratio of 1:3 it reaches a value of 15 mn/m. The given surfactants should create on the surface of the monomer microdroplets, and then also on the polymer monomer particles (PMPs), a strong interphase layer capable of ensuring their stability, the localisation of nanoparticles in the surface layer of the monomer microdroplets, and their subsequent immobilisation in the surface layer of the PMPs formed from the microdroplets. The high strength of the interphase adsorption layers is due to the fact that the selected SDS/CA molar ratio on the water CA SDS phase diagram lies in a region corresponding to the formation of mesomorphic structures. The formation of a mesomorphic film in interphase adsorption layers of particles guarantees them high strength and resistance to coalescence. Immobilisation of solid zinc oxide particles in the surface layers of particles enhances these properties. SDS added to the system forms bilayers in the interphase layers of the zinc oxide nanoparticles, thereby changing the hydrophilic lipophilic balance of the surface and increasing the surface-active properties of the particles, which enables them to be localised in adsorption layers of the PMPs, which means redistribution of the zinc oxide nanoparticles within the microdroplets to their surface. The formation of polymer in the surface layers of the PMPs on initiation of styrene polymerisation promotes immobilisation of the nanoparticles on the surface, and they, along with acetyl alcohol, SDS, and polymer, form strong adsorption layers on the surface of the microdroplets, thereby determining the ability of the PMPs not to change dimensions in the course of polymerisation by coagulation with particles similar to them. The above is confirmed by data given in Tables 1 and 2. The surface-active properties of hydrophobised zinc oxide nanoparticles in xylene emulsion in water are characterised by an interphase tension of 25.93 mn/m. The addition of cetyl alcohol leads to a further reduction in interphase tension, and it becomes equal to 16.18 mn/m. The addition of sodium dodecyl sulphate leads to a reduction in interphase tension to 3.58 mn/m. A general characteristic of a system obtained with the selected CA:SDS ratio of 1:2 in the presence of hydrophobised zinc oxide nanoparticles is a low interphase tension, which amounts to 11.19 mn/m. As stated above, it was proposed that the heterophase polymerisation of styrene in the presence of zinc oxide nanoparticles be carried out in a highly dispersed emulsion with the formation of polymer microspheres from monomer microdroplets. It was suggested that such an emulsion be obtained by creating conditions for intense mass transfer of SDS and CA through the phase boundary in order to reduce the interphase tension and microemulsion formation when the system is treated with ultrasound. Table 1. Values of interphase tension with different compositions of the measured system No. Composition of measured system Amount of oleic acid for hydrophobisation of zinc oxide nanoparticles (µl) Interphase tension (mn/m) 1 o-xylene water No nanoparticles 32.54 2 (o-xylene + hydrophobised ZnO) water 100 25.93 3 (o-xylene + hydrophobised ZnO + CA) water 100 16.18 4 o-xylene (SDS + water) 100 4.43 5 (o-xylene + hydrophobised ZnO) (SDS + water) 100 3.58 Table 2. Values of the interphase tension as a function of the content of hydrophobised zinc oxide nanoparticles in a styrene water system with a CA:SDS mass ratio of 2:1 No. Composition of measured system Content of hydrophobised zinc oxide nanoparticles in styrene (wt%) Interphase tension (mn/m) 1 Styrene water No nanoparticles 28.07 2 Styrene (CA + SDS) + water (2:1) 0 7.20 3 Styrene (CA + SDS) + water (2:1) 3.3 8.64 4 Styrene (CA + SDS) + water (2:1) 6.6 11.19 2012 Smithers Rapra Technology T/17

Under these conditions, the entire volume of monomer containing zinc oxide nanoparticles should be dispersed, with the formation of monomer microdroplets with zinc oxide nanoparticles uniformly distributed within them. Synthesis of polystyrene suspensions in the presence of zinc oxide nanoparticles The process of heterophase polymerisation of styrene in the presence of zinc oxide nanoparticles is shown in Figure 1. At the first stage, a surfactant mixture is formed from sodium dodecyl sulphate and the cetyl alcohol that has been introduced into the aqueous phase at 70 C. At the second stage, a monomer emulsion containing zinc oxide nanoparticles is formed, in which practically 100% of the monomer phase is dispersed in the form of microdroplets, and interphase adsorption layers of mesomorphic structure are formed on their surface. Then, a water-soluble initiator (potassium persulphate) is introduced and the system is subjected to ultrasound treatment with the aim of producing a stable, highly dispersed emulsion in which the monomer microdroplets become the main source of PMPs. At the third stage, the system is transferred into a reactor in which polymerisation proceeds for 5 h at a temperature of 70 C. An investigation was made of the influence of different parameters on the polymerisation rate, the stability of the system at all polymerisation stages, and the size of the polymer particles and PMPs: the volume ratio of monomer phase to water, the nature and concentration of initiator, the concentration of emulsifier (SAS), and the duration of ultrasound treatment. Generalised results are presented in Table 3. The conducted experiments on a model system (without zinc oxide) made it possible to propose the following optimal conditions for heterophase polymerisation of styrene in the presence of zinc oxide nanoparticles. Analysis of the obtained transmission microscopy data showed that the nanoparticles were immobilised in polymer microspheres (Figure 2). The results obtained make it possible to propose the following scheme of formation of polymer microspheres with zinc oxide nanoparticles immobilised in them. The formation of such particles occurs in several stages. The first stage consists in the production of a stable dispersion of zinc oxide nanoparticles in hydrocarbon, the surface of which are characterised by a certain hydrophilic lipophilic balance providing them with surface-active properties. The second stage guarantees stability of the monomer microdroplets containing zinc oxide nanoparticles and an improvement in their surfaceactive properties for localisation of the zinc oxide particles in the surface layer of the polymer particles. For this, cetyl alcohol and sodium dodecyl sulphate, capable of Figure 1. Scheme of heterophase polymerisation of styrene in the presence of zinc oxide nanoparticles Table 3. Determination of the optimum conditions for polymerisation in highly dispersed styrene emulsions in the absence of zinc oxide nanoparticles No. Measured parameter a Particle size distribution Average diameter (nm) Monomer phase:aqueous phase volume ratio 1 1:9 Unimodal 77 2 1:6 Unimodal 86 3 1:3 Unimodal 139 Nature of initiator 4 PP Unimodal 86 5 DAA Unimodal 133 Concentration of initiator (potassium persulphate) 6 PP (2 wt%) Unimodal 86 7 PP (3 wt%) Bimodal 83, 230 Concentration of emulsifier (SDS) 8 SDS (2 wt%) Unimodal 79 9 SDS (3 wt%) Unimodal 64 10 SDS (5 wt%) Unimodal 63 Duration of ultrasound treatment 11 Absent Bimodal 68, 257 12 Ultrasound (6 min) Unimodal 80 13 Ultrasound (12.5 min) Unimodal 75 a All weight percentages are in terms of styrene T/18 International Polymer Science and Technology, Vol. 39, No. 12, 2012

Table 4. Formulation for producing polymer microspheres in the presence of zinc oxide nanoparticles No. Components Amount (parts) 1 Dispersion of zinc oxide in styrene 100 (with different zinc oxide content) 2 Potassium persulphate 2 3 Sodium dodecyl sulphate 3 4 Cetyl alcohol 6 5 Distilled water 648 Figure 2. Transmission electron micrograph of polymer microspheres with immobilised zinc oxide nanoparticles forming mesomorphic films in the interphase layer a structural mechanical factor of stabilisation are added to the system. Here, the interphase tension decreases considerably. At the third stage, polymerisation is carried out with initiation at the phase boundary by potassium persulphate, and the polymer formed precipitates in the surface layer, as water is a precipitating agent of the polymer. The participation in the formation of the interphase adsorption layer of polymer, sodium dodecyl sulphate, cetyl alcohol, and oligomeric surface-active radicals promotes immobilisation of the zinc oxide particles in this layer (Figure 3). It is this that determines the morphology of the polymer microspheres. The arrangement of the nanoparticles mainly on the surface of the polymer microspheres (raspberry-like structure) was confirmed by scanning microscopy (Figure 4). The zinc oxide content in the polymer microspheres was determined by thermogravimetric analysis. An analysis was made of two specimens polymer microspheres obtained in the absence of zinc oxide nanoparticles and polymer microspheres obtained in their presence with a zinc oxide concentration of 6.6 wt% of the monomer. Thermogravimetric analysis makes it possible to record the change in weight of the specimen as a function of temperature. The zinc oxide content in the polymer microspheres was 6.43 wt%, and as a result it can be assumed that practically all PMPs contain zinc oxide nanoparticles. CONCLUSIONS Figure 3. Schematic of the formation of the interphase adsorption layer Thus, it was shown that, by giving zinc oxide nanoparticles surface-active properties, combined with the use of a certain mixture of surfactant, cosurfactant, and polymer in the formation of interphase adsorption layers of polymer microspheres, it is possible to produce polystyrene microspheres with zinc oxide nanoparticles immobilised on their surface. REFERENCES Figure 4. Scanning electron micrograph of polystyrene microspheres containing zinc oxide nanoparticles 1. Pomogailo A.D. et al., Nanoparticles of Metals in Polymers. Khimiya, Moscow, 672 pp. (2000). 2. Tomczak N. et al., Designer polymer-quantum dot architectures. Prog. Polym. Sci., 34:393 430 (2009). 3. Hu J. et al., Organic inorganic nanocomposites synthesized via miniemulsion polymerization. Polym. Chem., 2:760 772 (2011). 4. Sheng W. et al., In-situ encapsulation of quantum dots into polymer microspheres. Langmuir, 22:3782 3790 (2006). 2012 Smithers Rapra Technology T/19

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