Poly(methyl methacrylate)/polystyrene composite latex particles with a novel core/shell morphology

Abstract

The development of colloidal chemistry and nanotechnology make it possible to precisely control the architecture of the latex particles of submicron and micron size [1–4]. Seeded polymerization is a useful technique for the design and preparation of latex particles with desirable morphology. Recently, our group has found various internal and surface morphologies in the synthesis of composite latex particles through seeded soap-free emulsion or seeded dispersion polymerization [5–9]. The particle morphologies are tunable by exquisitely controlling the thermodynamic and kinetic factors during reaction. We report here the synthesis of poly(methyl methacrylate)/polystyrene (PMMA/PS) composite latex particles which have a novel core/shell morphology. The particles were synthesized via seeded soap-free emulsion polymerization of styrene in the presence of PMMA particles by using a water-soluble nonionic azo-initiator, 2,20-azobis[2-methylN-(2-hydroxyethyl)propionamide] (VA086). Though the seeded emulsion or soap-free emulsion polymerization of styrene with PMMA latex has been widely investigated, the use of this kind of initiator was the first time to our knowledge. The polymerization kinetics and the morphology of the resulting PMMA/PS particles was studied and compared with the case using the ionic potassium persulfate (KPS) initiator. All polymerizations were performed in a 300 mL roundbottom separated flask equipped with a nitrogen inlet, a sampling syringe and a mechanical agitator. PMMA seed latex was prepared via a standard soap-free emulsion polymerization at 70 C for 5 h using KPS as initiator. The seed latex was dialyzed against deionized water for 3 days before use. A typical seeded soap-free emulsion polymerization was as follows. Appropriate amount of PMMA seed latex, deionized water and initiator VA086 or KPS were charged into the flask and bubbled with nitrogen gas for 30 min. Then, the oxygen-free styrene monomer was added, followed by the initiation of the polymerization by immersing the flask into the water bath thermostated at 70 C. The seeded polymerization was carried out for 4 h with continuous stirring. During the polymerization, fractions of 1.5 mL were withdrawn from the reaction mixture at various time intervals. The typical recipes for the preparation of PMMA seed and PMMA/PS composite latex particles were given in Table 1. The latex particles sampled during the polymerization were analyzed using a particle size analyzer (Microtrac S3500). It was confirmed that no appreciable agglomeration and new particle formation occurred during polymerization whether VA086 or KPS was used. The measurement of these samples by Fourier transform infrared spectroscopy (FT-IR, JASCO FT/IR-8000) showed that some new absorption peaks such as those near 700 and 3026 cm appeared as compared with the IR spectrum of PMMA seed particles. These peaks could be reasonably assigned to the characteristic absorptions of aromatic C–H bending and aromatic C–H stretching vibrations. The above results suggested that PS component was successfully introduced into the PMMA seed particles. The PS content (relative to PMMA) was quantified by the absorbance ratio of the peak at 700 cm (for PS) to that at 1375 cm (CH3 bending, for PMMA), with a calibration curve obtained from mixtures of PMMA and PS latex particles. From the PS content, the monomer conversion was calculated. T. Wang S. Shi (&) F. Yang Department of Materials Science and Engineering, Shenyang Institute of Chemical Technology, Shenyang 110142, China e-mail: shan.shi@hotmail.com