Xinya Zhang

Stabilization of polymer colloid dispersions with pH-sensitive poly-acrylic acid brushes

Polyelectrolyte brushes are widely used for surface modification of nano-and colloidal particles because of their ability to dramatically change their conformation, hydrophobicity, polarity, charge, etc., as a response to smooth variations in environmental conditions. In this work, we have studied experimentally the stability behavior of polymer colloids with grafted poly-acrylic acid (PAA) surface brushes. We have measured the Fuchs stability ratio (W) as a function of electrolyte concentrations at different pH. It is observed that at pHu2009<u20093, the W values with 1xa0% and 2xa0% PAA brushes do not differ significantly from those without PAA, indicating that in their protonated state, the carboxylic groups do not contribute significantly to the colloidal stability. At the intermediate pHu2009~u20095, the PAA brushes are partially deprotonated, and their contribution to the colloidal stability is substantial and increases as the length of the PAA brushes increases. Under alkaline conditions (pHu2009>u20098), since most of the carboxylic groups are ionized, the colloidal stability is much higher than that at pHu2009~u20095. However, the W values are basically the same with 1xa0% and 2xa0% PAA, implying that the contribution of the ionized AA in the two cases is practically the same. This experimental evidence indicates that only the ionized AA groups in the outer region of long brushes contribute to colloidal stability, thus supporting the hypothesis of local electroneutrality in the inner region of long brushes (LEA).

Synthesis and characterization of core/shell titanium dioxide nanoparticle/polyacrylate nanocomposite colloidal microspheres

Core/shell titanium dioxide (TiO2) nanoparticle/poly(methyl methacrylate-butyl acrylate-methacrylic acid) [P(MMA-BA-MAA)] nanocomposite colloidal microspheres have been successfully synthesized via in situ emulsion polymerization. TiO2 nanoparticles were firstly modified by silane coupling agent, vinyl triethoxysilane (A-151), to increase the dispersibility of TiO2 nanoparticles into the polyacrylate matrix. The A-151-modified TiO2 nanoparticles were characterized by Fourier transform infrared spectra (FTIR), thermogravimetric analysis (TGA), dynamic light scattering (DLS), and transmission electron microscopy (TEM) techniques. The synthesized nanocomposite colloidal microspheres were characterized by TEM, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), and differential scanning calorimetry analysis (DSC). The results showed that A-151 coupling molecules was chemically bonded on the TiO2 nanoparticles surface, and the amount of coated A-151 was 3.0 wt%. According to TEM micrographs and DLS results, the dispersibility of modified TiO2 nanoparticles was obviously improved. TEM analysis revealed that an obvious core/shell structure morphology was observed with the core TiO2 particles surrounded by a uniform 10∼15-nm thick polymer shell. SEM–EDS result showed that TiO2 nanoparticles were homogenously monodispersed in the polymer matrix. DSC measurements indicated a glass transition temperature (Tg) enhancement of P(MMA-BA-MAA).

Behavior of adipic dihydrazide and silica in the preparation of acrylate redispersible polymer powders

The modified acrylate redispersible polymer powders were obtained in a free-flowing form by spray drying the structured latex. The latex with configuration of soft-core and hard-shell was produced by emulsion polymerization, where methyl methacrylate (MMA) and butyl acrylate (BA) were used as comonomers, methacrylic acid (MAA) as functional monomer. The latex was modified by adipic dihydrazide (ADH) to lower the minimum film-forming temperature (MFFT), and it was found that the MFFT of the latex decreased as the content of ADH increased. The obtained latex was transformed into redispersible polymer powders by spray drying, where the silica-sol was used as anticaking agent. Transmission electron microscopy (TEM) was used to detect the distribution of silica sol in the latex before spray drying, and a raspberry morphology was found. Meanwhile, zeta potential showed that the polymer/silica system was rather stable. After spray drying, the morphology of the polymer powders and the distribution of silica were characterized by SEM and EDS. It indicated that the polymer powders had a standard spherical shape with silica-rich surface. The polymer powders could be redispersed in water with the properties similar to the “mother” latex; SEM showed that the redispersion formed a dense and smooth film at 25xa0°C.

Self-assembly of anisotropic red blood cell (RBC)-like colloidal particles

Non-spherical colloidal particles, as basic building blocks, exhibit special capability in constructing novel materials. In this work, red blood cell (RBC)-like, anisotropic particles were synthesized and the self-assembly of the RBC-like particles was then carried out at the air-water interface. Subsequently, multilayer 3D structured colloidal crystals were also fabricated. The as-prepared colloidal crystal film displays beautiful Bragg diffraction, which can be used to construct a photonic crystal. After that, the self-assembly of binary colloidal particles was explored to design well-patterned binary colloidal crystals. This facile self-assembly approach to prepare colloidal crystals may extend to other anisotropic building blocks, providing guidance for the fabrication of more complex and flexible materials.

Optimization of microencapsulation of silane coupling agent by spray drying using response surface methodology

ABSTRACT A microcapsule containing a silane coupling agent was prepared by spray drying. The optimal encapsulating condition was determined by employing a response surface methodology with a central composite design. The effect of emulsion’s solid content and oil concentration, inlet air temperature, and atomizer speed on properties of microcapsule were investigated. The resulting microcapsule containing silane coupling agent and possessing an unbroken surface and a good shape under optimized conditions exhibited an encapsulation efficiency of 82.7%, average size of 21.3u2009µm, and moisture content of 0.93%. The experimental results agree well with the model prediction, with a relative error below 5.0%. GRAPHICAL ABSTRACT