Materials and Reagents
Silica nanoparticles with average particle size of 10 nm were MN1P obtained from Zhoushan Mingri Nano-materials Co. Ltd., Zhejiang, China. They were dried in vacuum at 110 °C for 48 h before use.
γ-Aminopropyltriethoxysilane (APTES) (Gaizhou Chemical Industrial Co. Ltd., Liaoning, China) was used as received. Bromoacetylbromide was analytical reagent grade and purchased from Acros Organics (Phillipsburg, New Jersey, USA). Cu(I)Br (Tianjin Chemical Co., Tianjin, China) was analytical reagent grade and purified by stirring in glacial acetic acid, filtered, washed with ethanol, and dried. 2,2′-bipyridine (bpy) (A.R., 97.0%) provided by Tianjin Chemical Co., China, was recrystallized twice from acetone. Hexamethylene diisocyanate (HDI) was used as received from Aldrich. Styrene (St, analytical reagent, Tianjin Chemicals Co. Ltd., China) was dried over CaH2and distilled under reduced pressure. Triethylamine (TEA) and tetrahydrofuran (THF) were dried by CaH2overnight, and then distilled under reduced pressure before use.
Toluene, dimethylformamide (DMF), tetrahydrofuran (THF), ethanol, hydrofluoric acid, and other solvents used were all of analytical reagent grade and obtained from Tianjin Chemical Co., Tianjin, China, and were used without further purification. Distilled water was used throughout.
Polystyrene Grafted Silica Nanoparticles (PS-SNs)
The preparation procedure of the crosslinked polymeric nanocapsules (CPNs) is shown schematically as Scheme 1. The bromo-acetyl modified silica nanoparticles (BrA-SNs) used as the macroinitiators in the surface-initiated atom transfer radical polymerization (SI-ATRP) of styrene were prepared with the same procedures as reported previously .
The SI-ATRP of styrene (St) from the BrA-SN macroinitiators was accomplished by the following procedure (Scheme 1): BrA-SN 0.5 g, the monomer (St) 15 mL, 215 mg (1.5 mmol) of CuBr, and 470 mg (3 mmol) of bpy were added into a dry round-bottom flask. The mixture was irradiated with ultrasonic vibrations for 30 min, bubbling with nitrogen (N2). The reaction proceeded at 90 °C for 10 h with magnetic stirring. N2was bubbled throughout the polymerization period. The products, polystyrene grafted silica nanoparticles (PS-SNs), were separated by centrifugation and subjected to intense washing by toluene. Ultrasonication was used in combination with above solvents to remove the impurities, and then dried in vacuum at 40 °C.
Crosslinked Polystyrene Nanocapsules
The dispersion of polystyrene grafted silica nanoparticles (PS-SNs) in dimethylformamide (0.02 g/mL) was irradiated at a distance of about 5 cm for 6 h with a 300 W mercury UV lamp having a maximum emission wavelength at 365 nm. The crosslinked polystyrene grafted silica nanoparticles (CP-SNs) were collected by centrifugation and washed thoroughly with THF. Then the CP-SNs obtained were resuspended in DMF (10 mL) and 24% aqueous HF solution (10 mL) was added. The mixture was stirred at room temperature for 10 h. The resulting products, crosslinked polystyrene nanocapsules (CPNs), were collected by centrifugation, washed thoroughly with THF, and dried under vacuum.
Analysis and Characterization
Elemental analysis (EA) of C, N, and H was performed on Elementar vario EL instrument (Elementar Analysensysteme GmbH, Munich, German). Bruker IFS 66 v/s infrared spectrometer (Bruker, Karlsruhe, Germany) was used for the Fourier transform infrared (FT-IR) spectroscopy analysis in the range of 400–4000 cm−1with the resolution of 4 cm−1. The KBr pellet technique was adopted to prepare the sample for recording the IR spectra. Thermogravimetric analysis (TGA) was performed with a Perkin-Elmer TGA-7 system (Norwalk, CT, USA) at a scan rate of 10 °C min to 800 °C in N2atmosphere. The morphologies of the polymer grafted silica nanoparticles and the polymeric nanocapsules were characterized with a JEM-1200 EX/S transmission electron microscope (TEM) (JEOL, Tokyo, Japan). The samples were dispersed in toluene (PS-SNs) and dimethylformamide (CPNs) in an ultrasonic bath for 5 min, and then deposited on a copper grid covered with a perforated carbon film.