Prepartion Of Rasp Berry-like Polymer/Silica Composite Particles And Their Application In Wettability Gradient Surface

Raspberry-like polymer/SiO₂composite particles are one type of particulate materials withmicro-nano-binary structure. As it combines the property advantages of both polymer andsilica components, and further possesses some synergy properties of composite materials, thistype of composite particles plays important roles in the fields of chemical mechanicalpolishing, superhydrophobic coatings and so on. However there still exist several technicalproblems in the preparation of raspberry-like polymer/SiO₂composite particles, such as themorphological defects and low silica composite efficiency, which consequently hinder theapplications of composite particles in high-precision conditions.Wettability gradient surface generally refers to surface with gradually varied surface energyalong one dimension, which has numerous potential applications in fields of microfluidics,biomedical research and enhanced heat transfer. But the current methods of wettabilitygradient surface preparation have several defects, such as complicated preparation process,limited tunable range of contact angles, and being difficult to precisely control the contactangle of certain position. In this study, raspberry-like polymer/SiO₂composite particles wouldbe prepared via Pickering emulsion polymerization, sol-gel template processing and hydrogel-shell template particle method. The resultant polymer/SiO₂composite particles would beapplied in fields of wettability gradient surface, to build up surfaces with wettability rangingfrom hydrophobic to superhydrophilic, which is of great significance in the applicationextension of raspberry-like polymer/SiO₂composite particles as well as the propertyimprovement of wettability gradient surfaces. The main research contents are listed as follows:1. Raspberry-like composite particles were prepared via cationic soap-free emulsionpolymerization with silica sol （colloidal SiO₂particles） as stabilizer and2-（methacryloyl）ethyltrimethylammonium chloride （MTC） as cationic auxiliary monomer. The SiO₂particleswere assembled onto the oil/water interface by electrostatic attraction, and influences of theamounts of MTC, AIBA and SiO₂were studied in this research. The conversion of monomerswent up accompanied with the increasing of AIBA amount, suggesting that the involvementof SiO₂hardly influenced the efficiency of initiator. The SiO₂coverage and SiO₂content ofthe composite particles appeared to be enhanced with the increasing of SiO₂amount. Byadding more cationic monomer MTC, more SiO₂particles were attracted onto the surface of composite particles by electrostatic attraction and the particle morphology turned to be moreraspberry-like. The optimal formulation of synthesis was AIBA amount0.25g, SiO₂amount28g and MTC amount1.2g.2. The preparation of composite microspheres was through the ethanol sol–gel templateprocessing of TEOS on carboxyl functionalized polystyrene （PSt） template particles. Thefactors influencing the raspberry-like morphology and the deposition efficiency of SiO₂ontotemplates were discussed, including the surface charge density, TEOS amount, water amountand the reaction temperature. As there exist electrostatic repulsion effect between anionicsiloxane oligomers and carboxyl groups on templates, the SiO₂content of the compositeparticles decreased with the increasing of surface charge density so that the raspberry-likemorphology weakened. By increasing TEOS amount, morphology of composite particlesturned to be more raspberry-like. When more water was added or the reaction temperatureincreased, the reaction rate of―water condensation‖would simultaneously speed up andrapidly consume silanol groups, making it more difficult to form raspberry-like microspheres.The optimal formulation of preparation was given as: AA amount2.5g, TEOS amount1.0g,H₂O amount2.5g, and sol-gel temperature50℃.3. The suspension of composite microspheres was prepared according to above optimalformulation of the sol–gel template method. The resultant suspension containing compositemicrospheres and free SiO₂particles was spin coated on glass slides and calcinated in thermalgradient field. Then surface with gradient wettability could be generated, which ranged fromhydrophobic （CA>110o） to superhydrophilic （CA<5o）. Combined with the TGA&DTG dataof composite microspheres, the contact angle variation could be divided into three segmentsaccording to the calcination temperature. The1st temperature segment was below thedecomposition temperature of PSt （275°C）. In this segment, the calcinated surfacesdemonstrated hydrophobic property and the apparent contact angle of the wetting surface wasover110o（CA>110o）. The2nd temperature segment was between225and about400°C, inwhich PSt decomposed. In this part, the apparent contact angles of surfaces changed rapidly,from hydrophobic to hydrophilic. The3rd temperature segment was the part over400°C, inwhich the calcinated surface remained superhydrophilic. Wenzel and Cassie/Cassie–Baxter models were tried in turn to analyze the wetting transitions.In the1st temperature segment, with the temperature increased, the polymer began to meltand the roughness decreased. The Cassie–Baxter model was applicable in this situation. In the2nd temperature segment, with the decomposition of polymer components, the texture ofsurface collapsed to form relatively flat surface and the roughness further decreased.Meanwhile the chemical composition of surface changed from PSt to SiO₂, so the apparentcontact angle of surface fell quickly. In the3rd temperature segment, the texture of surfacecompletely became pore-type texture, and the superhydrophilic phenomenon appeared（CA<5o）. The Cassie–Baxter model was also applicable in this situation. Though the surfacewas superhydrophilic, the water drop still could not completely spread on it due to theexistence of SiO₂particles.4. Hydrogel-shell sulfonated polystyrene （PSS） particles were used as template toprepare raspberry-like polymer/SiO₂composite particles. At first the shells of polystyrenetemplate particles were sulfonated into hydrogel, then the hydrogel shell was used as templateto absorb aniline and SiO₂particles via electrostatic attraction. With the polymerization ofaniline, SiO₂particles could be fixed on the surface of template particle and raspberry-likepolymer/SiO₂composite particles were obtained.Factors such as sulfonation time, sulfonation temperature and sulfuric acid concentration werestudied. It was found that with the increasing of sulfonation time, the sulfonation extent ofPSS templates and SiO₂content of the composite particles were firstly increased and thendecreased. When the sulfonation temperature rose, sulfonation extent of PSS templatesimproved and the SiO₂content increased consequently. The SiO₂content as well as theraspberry–like morphology of composite particles was found to decline due to the decreasingof the concentration of the sulfuric acid. The optimal parameters of preparation weresulfonation time4h, sulfonation temperature60℃and concentration of sulfuric acid98%.Composite particles with different micro-nano-binary-structures were prepared by assemblingdifferent sizes of polymer templates and SiO₂particles together, through the hydrogeltemplate method. Surfaces with different binary roughness were prepared correspondinglyand the relationship of wettability with binary structure was studied. Base on polymertemplates of the same size, the contact angles were found to increase with the increasing ofSiO₂particle size. Meanwhile the contact angles appeared no significant changes with the increasing of polymer template size, when the size of SiO₂particles was the same. Abovephenomenons were explained by Cassie equations.