One-pot synthesis of organo-functionalized monodisperse silica particles in W/O microemulsion and the effect of functional groups on addition into polystyrene

Abstract

The direct synthesis from TEOS and organosilanes is carried out in W/O microemulsion in order to prepare monodisperse functionalized silica nanoparticles. The well monodisperse particles are obtained successfully from a wide range of concentration of 3-aminopropyltriethoxysilane (APTES), 3-mercaptopropyltrimethoxysilane (MPTMS), phenyltrimethoxysilane (PTMS), vinyltriethoxysilane (VTES) and 3-cyanoethyltriethoxysilane (CETES) in the solution of TEOS and polyoxyethylene nonylphenol ether (NP-5). The particle size can be controlled without widening the distributions. On the other hand, the direct synthesis with these organosilanes and TEOS by conventional Stöber method using no surfactant provides colloidal solutions with DLS plots with wide or bimodal dispersions. NP-5 is removed by washing the colloids with DEGDEE and the decline of IR absorptions due to the benzene ring stretching is observed. The resultant colloidal powder is occluded into polystyrene by the polymerization of styrene monomer in the presence of these functionalized silica nanoparticles. The heat resistance of modified polystyrene is investigated by TG–DTA in dry air. The position of DTA peak due to combustion of the polymer is shifted to a higher temperature with the addition of silica nanoparticles. The considerably large shift is found in silica nanoparticles prepared with VTES and PTMS, whose difference is higher than that by the addition of simple silica nanoparticle with no functional groups.

Introduction

The preparation of monodisperse inorganic particles in 10–100 nm range has attracted a considerable attention in recent years due to their potential importance in technological applications in the fields of optical devises, magnetic particles, support materials in chromatography, catalysts and polymer materials. The use of water-in-oil (W/O) microemulsions has been applied to the sol–gel synthesis of silica in order to obtain narrow size distributions [1], [2], [3], [4], [5], [6], [7]. The authors of these studies have demonstrated that various surfactants such as bis(2-ethylhexyl)sulfosuccinate [2], [3], nonylphenol ethers [4], polyethylene glycol alkyl ether [5], [6], triblock copolymer Altox [5], [6] and PEG [8] can be used, while the particle distribution range depends not only on the kind of surfactant but also on the concentrations of alcohol [2], [5], water [3] and surfactant [4], [6].

Monodispersion is especially important for the additives in the size of 10 nm range into polymers where the modification of mechanical and permeability properties is concerned. The agglomeration of silica nanoparticles has been often pointed out as a major problem in the incorporation into polymers [9]. The formation of chemical bond with a specific functional group in the polymer matrix is one of the possible solutions for uniform dispersion of the nanoparticles and the functionalization of the surface of silica particles is required for such bond formation. Several methods have been proposed for surface functionalization of monodisperse silica nanoparticles. Grafting [10], [11], [12], [13], [14], [15] has been used as widely as in conventional silica powders. Some of the functional groups grafted on the surface can be polymerized to form a polymer coat layer on the nanoparticles [11], [14], [15]. Although grafting, also known as post-synthetic modification, is a simple and versatile method, previous studies have often indicated self-condensations of the organosilane and a non-uniform distribution of organic groups on silica [16], [17]. Direct synthesis, also known as co-condensation or one-pot synthesis, is an alternative method, which is also versatile and theoretically results in a uniform dispersion of organic functions [16]. The uniform organic layer on the surface will be an advantage of nanoparticles for their various applications such as for dispersion in the polymer matrix.

Recently, co-condensation has been applied to the introduction of 3-aminopropyl [18], [19], [20] and phenyl [20] groups in Stöber synthesis. The data in these studies clearly shows that the shape and size of the particles as well as the rate of formation changes depending on the concentration of organosilane. The presence of basic silane will influence on the condensation rate and, consequently, geometric properties of the product. This effect can be controlled modestly in the condensation reaction of silica precursor and silanes in W/O microemulsions. In this study, one-pot syntheses of monodisperse silica nanoparticles are carried in the presence of polyoxyethylene(5) nonylphenyl ether surfactant and organosilanes as follows: 3-aminopropyltriethoxysilane (APTES), 3-mercaptopropyltrimethoxysilane (MPTMS), phenyltrimethoxysilane (PTMS), vinyltriethoxysilane (VTES) and 3-cyanoethyltriethoxysilane (CETES). We explore the effect of organosilane on the particles size distributions and the removal efficiencies of the surfactant. We further investigate the improvement of thermal properties of polystyrene polymer where the functionalized silica monodisperse particles are dispersed.