CONTROL OF INTERFACIAL PROPERTIES OF SILICA NANOPARTICLES WITH GRAFTED MIXED POLYMER SHELL

. Amphiphilic properties can be easily imparted by creating organic shell by chemical grafting of polymers, which exhibit amphiphilic behavior. Such an organic shell can be used to regulate interparticle interactions and interactions between the particles and their environment, which allows for control of physical properties of colloidal dispersions 3,4 . Here we report on the first experimental attempts to apply a grafted mixed polymer layer on silica nanoparticles to prepare responsive colloids, which demonstrated drastic transformation/switching of their properties upon external stimuli. Experimental Materials. Poly(styrene-b-2-vinylpyridine-b-ethylene oxide) P(S-b2VP-b-EO) was purchased from Polymer Source Inc., Canada (Sample P1109S2VPEO), PS(Mn=14,100 g/mol)-b-2VP (Mn=12,300 g/mol)-b-EO (Mn=35,000 g/mol), PI=1.08. Silica nanoparticles (SiO2-F-0.2; d=0.193 µm) were purchased from Microparticles GmbH, Berlin, Germany. Nitromethane, toluene, ethanol, and dichloromethane (Sigma) were used as received. Highlypolished silicon wafers (Semiconductor Processing, Union Miniere USA) were cleaned in an ultrasonic bath three times with ethanol, then treated with cleaning solution (NH4OH and H2O2) for 1h at 60 0 C and carefully rinsed with Millipore water (18 MΩ.cm). 11-bromoundodeciltrimethoxisilane (BTMS) (Gelest, Inc., USA) was used as received. Instrumentation. The amount of the grafted copolymer was estimated from elemental analysis using a “2400 Series II” CHNS/O Analyzer (Perkin Elmer, USA). AFM studies were performed using a Dimension 3100 (Digital Instruments, Inc., Santa Barbara, CA) microscope. The tapping and phase contrast modes were used to map the film morphology at ambient conditions. Silicon tips with a radius of 20 ± 5 nm, a spring constant of 1.5-6.3 N/m and frequency of 63-100 KHz were used. The size of the particles and Zeta-potential were measured by a dynamic light scattering (DLS) instrument (Brookhaven Instruments Corp., USA) with a detection angle of 90° and ZetaPlus Particle Sizing extension software. X-ray Photoelectron Spectroscopy (XPS). XPS experiments were performed with an AXISULTRA spectrometer (Kratos AXIS 165,U.K.) in survey regime with Al as a reference at angles of incidence of 90 and 60 deg . The analyzer’s pass energy was set to 80 eV to obtain element composition of the layers. Grafting procedure. Silica nanoparticles were annealed at 110 o C in a vacuum oven overnight and were dispersed in toluene with concentration of 2% by ultrasonication. BTMS was added to the particle dispersion under vigorous stirring. The concentration of the particles in toluene was 2% and the concentration of BTMS in the toluene particle dispersion was 1%. The mixture was stirred at ambient temperature under nitrogen atmosphere overnight. The nanoparticles were isolated by centrifugation, redispersed in toluene, and centrifuged again. This washing process was repeated three times. Finally, the particles were redispersed in nitromethane. The nitromethane suspension of BTMS-functionalized particles was added in a flask containing the P(S-b-2VP-b-EO) solution in nitromethane. The concentration of particles and block copolymer in the mixture was 2%. The mixture was stirred at 60 o C for 68-72h. The particles with the grafted polymer layer were isolated by centrifugation, redispersed in dichloromethane and centrifuged again. The washing process with dichloromethane was repeated three times, followed by washing one time with ethanol, and final redispersion in water (pH2).