Synthesis and silicon gradient distribution of emulsifier-free TRIS-containing acrylate copolymer

Abstract

In this study, a series of emulsifier-free 3-[tris(trimethylsilyloxy)silyl] propyl methacrylate and methyl methacrylate copolymer emulsion was prepared via the semi-continuous technique. The polymerization conditions in the presence of a reactive emulsifier were discussed in detail, and the suitable polymerization condition was provided. Proton nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and transmission electron microscopy proved that emulsifier-free silicon-containing acrylate copolymers were obtained, and silicon content reached 60%. The gradient distribution of silicon in the latex film was characterized by contact angle (CA) analysis, X-ray photoelectron spectroscopy (XPS), and confocal Raman spectroscopy (CRS). CA and XPS results demonstrated the enrichment of silicon at the film–air (F–A) interface in the film that was prepared using an emulsifier-free silicon-containing acrylate copolymer. Furthermore, CRS provided considerable insights into the distribution behavior of silicon in the structure of the film from the F–A interface to the film–glass interface. CRS not only showed that silicon preferentially located at the F–A interface, but also that its distribution behavior was formed in the direction across the film thickness in the sigmoidal model.

Introduction

Silicon polymers exhibit desirable properties, such as low surface energy, water repellency, stain resistance, flexibility at low temperature, and thermal stability at high temperature. Their application, however, is limited by their poor mechanical properties and adhesive performance. By contrast, acrylate polymers present good mechanical properties and adhesive performance, but poor water resistance and thermal stability. Therefore, silicon-containing acrylate polymers are of considerable scientific interest and widely applied in coating and adhesives, by virtue of their extraordinary compound properties, in which the advantages of both components are combined and their individual limitations are surmounted [1], [2], [3], [4], [5], [6], [7]. Various methods for the preparation of silicon-containing acrylate polymers have been applied; examples include chemical copolymerization of unsaturated acrylate and silicon monomer containing one pair of carbon atoms linked by a double bond [1], [2], [3], [4], [5], [6], [7], physical preparation of silicon/polymer bicomponent blends [8], and fabrication of interpenetrating polymer networks via silicon crosslinking [8].

The surface properties of polymers can be enhanced by increasing the amount of silicon components in the polymer. However, high silicon content in silicon-containing acrylate emulsion may enhance the instability of latex, resulting in silicon content of no more than 50% in the copolymer [6], [7]. 3-[Tris(trimethylsilyloxy)silyl] propyl methacrylate (TRIS) is a type of monomer that has high silicon content. Its Si(OSi(CH₃)₃)₃ group prevents it from easily hydrolyzing to form Si–OH that may lead to the restriction of the amount of silicon [6], [7]. To date, researchers have focused on TRIS [5], [9], [10], [11], [12], [13], but few studies on the emulsion polymerization of TRIS have been conducted. Compared with traditional emulsion polymerization, latex particles without emulsifiers have remarkable advantages: they have a clear and well-designed surface, homogeneous size, and excellent stabilities, and do not pose negative effects on the environment [14]. Therefore, emulsifier-free polymerization, which is expected to effectively eliminate the negative effects of surfactants on the bulk and surface properties of films, has emerged as a promising method for synthesizing silicon-containing acrylate polymers.

Silicon-containing groups have the tendency to migrate toward interface and preferentially locate at the interface to minimize the interfacial energy due to its low surface free energy; nevertheless, no paper has reported on the concentration profile of silicon along the overall cross-section of films. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR) and X-ray photoelectron spectroscopy (XPS) effectively identify the presence of components at the interface. However, these do not provide information on overall distribution [15], [16], [17]. Herein, the use of confocal Raman spectroscopy (CRS) proved to be a powerful method for the in-depth profiling of thin films, coatings, membranes, and composites [18], [19], [20]. In the present paper, a series of silicon-containing acrylate latex was successfully synthesized via emulsifier-free semi-continuous polymerization of TRIS and methyl methacrylate (MMA), with sodium 3-allyloxy-2-hydroxy-propane sulfonate (COPS-1) as the reactive emulsifier. The polymerization conditions are discussed in detail and the best polymerization condition is provided. The characterization of the emulsion was studied using various methods, such as proton nuclear magnetic resonance (¹H NMR) spectroscopy, FT-IR spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The distribution of silicon on the film along the overall cross-section was also investigated using contact angle (CA) analysis, XPS, and CRS.