Modification of Nanosize Silica Particle Surfaces to Improve Dispersion in a Polymer Matrix

Abstract

Composite materials composed of inorganic particles and polymers have been developed with the purpose of improving the physical properties, such as strength, heat resistance, and elastic modulus. There is no repulsive force between particle surfaces in an organic medium, hence, the interaction between particles is always attractive. Because of this, it is difficult to uniformly disperse inorganic particles in polymers. To achieve good dispersion in a polymer, it is necessary to modify the surface of inorganic particles. In this paper, we used chemical surface modification to change the surface design of inorganic particles. To uniformly disperse particles in a polymer we used organic compounds with a chemical structure similar to the polymer to chemically modify surfaces and change the molecular level particle surface design. INTRODUCTION In the fabrication of organic/inorganic composite, improvements in processing, cost reductions, and improvements in physical property can be expected with increasing particle loading in a polymer matrix. '' In order to increase the leading efficiency, the interfacial energy between the particles and the polymer matrix should be considered as well as the shapesize, and surface of the particles. Recently, with the development of nanotechnology, particle size has become to be controlled on a nano scale. Relative to larger particles, nanosize particles are known to have different electrical, optical, magnetical, and chemical properties. However, nanosize particles aggregate sassily owing to their specific surface area. Therefore, it is difficult to uniformly disperse nanoparticles into a matrix using only mechanical mixing. This is due to the interracial interaction between the particle surface and the matrix during the mixing process. One way to improve the wettability of particles by the matrix is through chemical surface modification using a coupling reagent. 3 It has been reported a polymer coating on the particles can prevent aggregation. In order to decrease the difference in interfacial energy between the particles and the matrix, a novel molecular level surface treatment was proposed. It is expected that this surface modification will prevent particle aggregation. In this study, polyimide was used as the polymer matrix and its monomers were used as reagents to modify particle surfaces. Surface characterization of the treated particles was performed using simultaneous thermogravimetric and differential thermal analysis (TG-DTA), and Fourier Transform Infrared spectrometry (FTIR). Additionally, an improvement in the dispersability in N-methyl-2-pyrrolidone (NMP), which is a solvent for the polyimide precursor, was confirmed. 39 Characterization, Design, and Processing of Nanosize Powders and Nanostructured Materials Edited by Kevin G. Ewsuk and Yury Gogotsi Copyright © 2006 The American Ceramics Society Modification of Nanosize Silica Particle Surfaces to Improve Dispersion in a Polymer Matrix EXPERIMENTAL SECTION Materials Fine silica powder (OX50, specific surface area 50 m/g, particle size 40 nm) and Aminopropyl triethoxysilane (APTS) were purchased from Nippon Aerosil Co., and Shin-Etsu Chemical Co., Ltd., respectively. N-methyl-2-pyrrolidone (NMP) was obtained from Aldrich. Hexane, Pyromellitic dianhydride (PMDA), and 4, 4'-Diaminodiphenylether (DDE) were purchased from Wako Pure Chemical Industries Ltd., Osaka, Japan. Modification of silica with APTS Chemical surface modification was accomplished through is a reaction of APTS with the hydroxyl groups of the silica substrate. This reaction was carried out using the autoclave method' at 235 D and under 30 atm for 1 h. The degree of surface modification was controlled using a modifier content of 3 -OH/nm surface density of the hydroxyl groups determined by the Grignard reagent method. Before the measurement, the sample was degassed at 200 G for 4 h. Synthesis of surface treated samples The surface treatment was performed as follows: Modified silica with APTS, NH2 modified silica, was suspended in NMP, and then PMDA was introduced into the suspension. The one functional anhydride group of PMDA reacts with the surface amino group. Then, DDE was dissolved into the suspension to react with the other groups of PMDA. After repeating these two reactions, a surface treated sample (i.e., coated particle) was obtained. Determination of the surface density of the modifier The surface density of the modifier was estimated from the number of modifiers and the specific surface area. The number of aminopropyl groups was determined using TG/DTA (Thermo Plus TG8120, Rigaku Co., Japan) as shown in Figure 1. * The measurement was carried out under oxygen at a flowing at 250 ml/min. The surface aminopropyl group density, dA, was calculated using Eq. (1), C / A [ W ] ^ ( A W C H A W O H ) ^ X 1 0 1 8 (1) MWSN1 where AWCH and AWOH are the ratio of weight loss for the modified and unmodified samples measured by thermogravimetry, Mw is the molar weight of the aminopropyl group, Sm is the BET specific surface area measured from the nitrogen adsorption isotherm and N is Avogadro's number. 40 • Nanosize Powders and Nanostructured Materials Modification of Nanosize Silica Particle Surfaces to Improve Dispersion in a Polymer Matrix Figure 1. Estimation of the number of aminopropyl groups introduced onto the silica surface from simultaneous thermogravimetric analysis (TG) and differential thermal analysis (DTA); (a) DTA curve for modified silica, (b) TGA curve for modified silica, (c) TGA curve for unmodified silica.