Preparation, assembly and surface wettability of hierarchical structured composite microspheres

Abstract

The development of polymer microspheres has great influence on our economy and life. It has become an increasingly important subject to achieve desirable physical properties for coatings, light-sensing materials, bio-medicine, and so on. Furthermore, the morphology and size of particles are key factors to determine potential applications of polymer microspheres. So that, great interest has been focused on the morphology control of polymer microspheres, and the microspheres with different novel morphologies have been prepared. Amongst, the hierachical microspheres have attracted more and more attention because of their wide range of applications, e.g. surface enhanced Raman scattering (SERS), heterogeneous catalysts, and most importantly, fabrication of superhydrophobic materials. As we know, the raspberry hybrid particle possesses well-defined hierarchical structure, which consists of a microsized core decorated with nanoscaled particles on the surface. Thus, nowadays, raspberry hybrid particles have been intensively employed by more and more researchers to prepare superhydrophobic films with a dual-size hierarchical structured surface. This study aims at the preparation of raspberry, core-shell and triangle particles with controllable morphologies by different emulsion system (seed emulsion, miniemulsion) based on the interface physical chemistry theory, and the assembly of these particles to fabricate particulate surface with particular wettability. The main content is as follows: 1. raspberry-like SiO2/PS particles were prepared by introducing of MPS-modified SiO2 particles into radiation miniemulsion polymerization of St. MPS modification with a lower cover density of 0.90 molecules/nm2 on the surface of SiO2 is a crucial step to form a raspberry-like structure. The WSt/o-SiO2 (weight ratio of St to o-SiO2) and surfactant concentration also influence the morphology and wettability of the resultant SiO2/PS hybrid particles. The optimum WSt/o-SiO2 and SDS concentration for well-defined raspberry SiO2/PS particles is 50 and 6.94 mM, respectively. When the raspberry SiO2/PS particles were deposited on a glass substrate from ethanol dispersion, a film with dual-size roughness on the surface can be obtained. The surface of the film exhibites superhydrophobic property with a water contact angle of 151o and a large contact angle hysteresis (~116°). Compared with the traditional superhydrophobic surface, the as-prepared layer shows strong adhesion to water. The mechanism of the superhydrophobilicity and high adhesion to water produced on the raspberry SiO2/PS particulate film was considered to be the Gecko’s state because of the van der Waals force between water and PS nanospheres on the film surface and the large density of the PS nanoparticles contacted with water (~1.55×107/mm2). This kind of superhydrophobic particulate film can be used as a “mechanical hand” for transportation of small water droplets without lost, thus it may have potential applications in industrial field. 2. Submicron-sized polystyrene/polyacrylonitrile (PS/PAN) hybrid particles were prepared by γ-ray-induced seeded polymerization, in which the mono-disperse poly (acrylic acid) (PAA)-functionalized PS particles were used as seed particles and acrylonitrile (AN) as the second monomer. The morphology of the PS/PAN hybrid particles could be easily controlled by the pH value of seeded emulsion system. In particular, with the increase of pH value from 4 to 9.6, the morphology of the corresponding PS/PAN hybrid particles gradually changed from core-shell to well-defined raspberry-like structure. The result of X-ray photoelectron spectroscopy (XPS) spectrum revealed the existence of PAN on the surface of the resultant hybrid particles. In addition, the particulate films were constructed by assembling these raspberry-like particles on glass substrates. Without any treatment, superhydrophilic surfaces can be obtained and the contact angle of water on the dual-sized structured surface is highly influenced by the micro/nano surface structure of raspberry-like particles. 3. It is well know that silica must be modified with silane coupling agent, such as MPS, to improve the affinity of SiO2 particles to organic components before they can be used as carrier to load polymer. However, recently, we have successfully prepared raspberry silica/polyacrylonitrile (SiO2/PAN) hybrid particles by γ-ray radiation induced seeded polymerization using unmodified SiO2 particles (590 nm) as seeds. The surface morphologies of the obtained SiO2/PAN can be easily adjusted by the weight ratio of AN to SiO2 seeds. Most importantly, superhydrophilic surfaces can be obtained by assembling these raspberry-like SiO2/PAN particles on glass substrates, and the contact angle of water on the dual-sized structured surface is greatly influenced by the micro/nano surface structure of raspberry-like particles. 4. PS/silica hybrid microspheres with core-shell and raspberry-like structure were prepared respectively with different sol-gel reaction catalysts (NH3·H2O, diethanolamine ， triethylolamine and triethylamine) in the presence of PS microspheres. The PS microspheres were prepared by dispersion polymerization using poly(vinylpyrrolidone) (PVP) as the stabilizer. Since the seed PS microspheres contain PVP molecules on the surface, which was facilitated to give perfect location of silica on PS microspheres. After calcination of the core/shell structured particles at 800 °C, hollow silica microspheres were successfully prepared. The formation of hollow silica particles strongly indicates that PVP-stabilized PS microspheres prepared by dispersion polymerization can be successfully employed to the fabrication of a well-defined silica layer due to the existence of PVP molecules on the surface. The wettability of the PS and PS/SiO2 microspheres was also characterized by measuring the water contact angle of the corresponding particulate film. Furthermore, a novel strategy has been designed for the preparation of triangle polystyrene/SiO2 (PS/SiO2) hybrid particles using radiation-induced seeded emulsion polymerization from above PS/SiO2 seed particles. The triangle particles can be fabricated in relatively high yield when the weight ratio of monomer/seed particles (WSt/PS) is 5. Moreover, the size of the protruded PS bulbs of the triangle particles obviously increases with the weight ratio of monomer/seed particles from 1.25 to 5. This novel method is applicable to the preparation of anisotropic hybrid particles from polymer/inorganic core-shell particles. 5. Polystyrene (PS) microspheres with hydrophilic β-cyclodextrin (β-CD) shell were fabricated one-step via γ-ray radiation induced emulsion polymerization in a simple ternary system of styrene/β-CD/water (St/β-CD/water) at room temperature. The solid inclusion complex of St and β-CD (St/β-CD IC) particles formed at the St droplets-water interface act in-situ as the surfactant not only to stabilize the emulsion but also make contribution to the formation of a hydrophilic β-CD shell on the prepared PS microspheres, which was observed by TEM and XPS investigation. The average size of the PS particles is dependent on the weight ratio of β-CD to St (Wβ-CD/St), which increases from 186 nm to 294 nm as Wβ-CD/St rises from 5% to 12.5%. The water contact angle (CA) of the obtained PS latex film was measured to study the hydrophilic property of PS microspheres. The results show that all the CA of the obtained PS latex film are lower than 90o, and reduce with the content of β-CD even to 36o. Thus, this work provides a new and one-pot strategy to surface hydrophilic modification on hydrophobic polymer particles with cyclodextrins through radiation emulsion polymerization.

Date of Award	16 Jul 2012
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Hon Wah Michael LAM (Supervisor)