Communications to the Editor Microwave Preparation of Narrowly Distributed Surfactant-Free Stable Polystyrene Nanospheres
نویسندگان
چکیده
Recently, much attention has been paid to the preparation of polymeric nanoparticles in the size range of 10-100 nm. The microemulsion polymerization of polystyrene is a typical example.1-10 It has been shown that the particle size can be quantitatively controlled by the initial fleet ratio, namely the macroscopic surfactant-to-monomer weight ratio.1,8-10 A normal microemulsion contains at least water, monomer (oil), and surfactant. Often, a cosurfactant and/or salt are added to stabilize the microemulsion, so that the number of components could be four or more. In practice, a complete removal of the added surfactant and other components from the resulted polymeric particles is rather difficult, if not impossible. In this communication, we report, for the first time to our knowledge, a novel method of using microwave radiation to prepare narrowly distributed surfactant-free stable polystyrene nanospheres. It should be stated that using microwave radiation to initiate a reaction is not new in chemistry,11 but it has several distinct advantages in the preparation of the surfactant-free stable polymeric nanoparticles. In this study, a microwave oven (Whirlpool-VIP20) with a double emission system, operating at 2450 MHz with a maximum output power of 900 W, was used. A flask equipped with a glass stirrer, a reflux condenser, and feeding heads was assembled inside the oven. A mixture of proper amounts of freshly distilled styrene monomer and water inside the flask was stirred for 10 min at a speed of 3 × 102 rpm under N2 before a given amount of initiator, potassium persulfate (KPS), was added to start the polymerization. The total volume of the reaction mixture is 250 mL. Typically, under the microwave radiation, the reaction temperature of ∼70 °C was reached within just 2 min, and the reaction was carried out at ∼70 °C under N2 for 1 h, with a reduced microwave radiation of only 80 W. In this way, more than 98% of styrene monomer were polymerized within ∼40 min. It should be stated that no surfactant was added, but the resultant polystyrene nanoparticles are stable with a constant particle size distribution over months. Moreover, such formed polystyrene nanoparticles are narrowly distributed (Figure 4). Figure 1 shows a typical hydrodynamic radius distribution f(Rh) of the nanoparticles. In comparison, using a conventional heating method to polymerize the same reaction mixture under an identical condition not only took a much longer time (>10 h) to reach the same extent of conversion but also resulted in a much broadly distributed particles, as shown in Figure 1. Since no cross-linking agent was added, individual polystyrene chains inside the nanoparticles are soluble in toluene to form a polymer solution. Both the polystyrene nanoparticles in water and individual polystyrene chains in toluene were characterized by using a modified commercial laser light scattering (LLS) spectrometer (ALV/SP-125) equipped with a multi-τ digital time correlator (ALV-5000) and a solid-state laser (ADLAS DPY 425 II, output power ∼400 mW at λ0 ) 532 nm). In static LLS, the weight average molecules (Mw) and the z-average radius of gyration (〈Rg〉) were determined. In dynamic LLS, the measured intensityintensity time correlation function was analyzed by both the cumulants and CONTIN (a Laplace inversion program) methods equipped with the correlator to give the average line width 〈Γ〉 and line-width distribution G(Γ). Γ was further reduced to the translational diffusion coefficient (D) and the hydrodynamic radius (Rh). The specific refractive index increments (dn/dC) of the polystyrene nanoparticles in water and individual polystyrene chains in toluene are 0.256 ( 0.002 and 0.110 ( 0.001 mL/g, respectively.12,13 The details of LLS instrumentation and theory can be found elsewhere.14,15 The LLS results are summarized in Table 1. The ratios of 〈Rg〉/〈Rh〉 are close to the theoretical value of 0.774 for a uniform hard sphere, indicating that these polystyrene nanoparticles are spherical and uniform. The small values of (1 + μ2/D) clearly indicate that the resulted nanospheres are narrowly distributed, where μ2 ) ∫0G(D)(D 〈D〉) dD with D, G(D), and 〈D〉 being the translational diffusion coefficient, the translational diffusion coefficient distribution, and the average translational diffusion coefficient, respectively. Figure 2 shows that the plot of the average hydrodynamic volume (〈Vh〉) of the nanospheres vs the initial monomer concentration (Cmonomer) is a straight line for a given initiator concentration of 3.02 × 10-4 g/mL, * To whom correspondence should be addressed. † Anhui Normal University. ‡ The Chinese University of Hong Kong. Figure 1. Comparison of the typical hydrodynamic radius distributions (f(Rh)) of the polystyrene nanospheres prepared by microwave radiation (O) and by the conventional heating method (0), where the styrene monomer and initiator concentrations are 1.13 × 10-2 and 3.02 × 10-4 g/mL, respectively. 6388 Macromolecules 1997, 30, 6388-6390
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