Preparation and characterization of fluorinated acrylate copolymer latexes by miniemulsion polymerization under microwave irradiation

Abstract

Fluoroacrylate copolymer miniemulsion was prepared by miniemulsion polymerization under microwave irradiation. The composition of the copolymer was determined by FTIR, DSC, ¹H NMR and ¹⁹F NMR. The morphology, size, and size distribution of the latex particles as well as changes in the size during polymerization were characterized by TEM and photon correlation spectroscopy (PCS). The effects of kinetic parameters on the polymerization were evaluated. The particle size of latex underwent almost no change during microwave irradiation polymerization. The diameters of latex particles prepared by microwave irradiation were smaller and more monodispersed than those prepared by conventional heating and the latex had good centrifugal stability. Polymerization under microwave irradiation had a higher reaction rate and higher conversion than traditional heating. By using 10 wt% fluoromonomer, the surface energy of the latex film could be reduced from 27.24 mJ/m² (latex film of fluorine-free) to 17.59 mJ/m² and the decomposition temperature increased by 25 °C.

Introduction

Fluorinated polymers are known to have many useful and desirable features such as unique surface/optical properties, high thermal stability, superb chemical resistance, excellent mechanical properties at extreme temperatures owing to the low polarizability and the strong electronegativity of fluorine atom [1], [2]. Acrylic polymers with fluorine-containing groups or perfluoroalkyl groups, in particular, can provide the material with low surface energies and the acrylic groups ensure that the polymers can adhere well to various substrates [3], [4]. Therefore, fluorinated polyacrylate latexes have been used progressively in a wide range of applications such as biomaterials, microelectronics, antifogging, and antifouling applications [5], [6], [7], especially as surface coatings for textile, paper and leather, because their presence can introduce a number of unique physical and chemical surface properties [1], [8]. Many fluorinated or perfluorinated (meth)acrylate monomers have been studied with regard to the synthesis of copolymers with conventional acrylate in aqueous emulsions by radical polymerization methods [1], [5], [6], [7], [8], [9], [10]. Most of them are the copolymers of fluorinated acrylate with long perfluoroalkyl groups (>7 fluorocarbons). However, poly (fluoroalkyl acrylates) with long perfluoroalkyl groups have been found to degrade to form perfluorooctanoic acid (PFOA), which can resist degradation and bioaccumulate in the human body [11], [12]. But the properties of fluorinated polyacrylate latexes which contain long fluoroalkyl groups are better than those containing short-chain fluoroalkyl groups [13]. Hence, the perfluoroalkyl acrylate with short fluorinated side chain (<7 fluorocarbons) and have relatively more number of fluorine atoms becomes alternative fluorinated materials to avoid the adverse reaction associated with PFOA.

In a classical emulsion polymerization technique the monomer relies on transport from the droplets to the growing particles. However, because the solubility of fluorinated monomers in water is very low [7], [14], [15], [16], [17], therefore, FA-containing copolymer latexes are usually synthesized by the addition of a large quantity of an organic solvent to the continuous phase [7], [14] or in supercritical carbon dioxide [18] or by use of some special emulsifier such as cationic surfactant [17], polymerizable emulsifier [19]. Miniemulsion polymerization is an in situ method to prepare fluorinated acrylate copolymers because of the unique features. In miniemulsion polymerization, both the particle nucleation and subsequent propagation reaction occur primarily in submicrometer monomer droplets of 50–500 nm. Polymerization in miniemulsion does not depend on the monomer transport through the water phase. The predominant initiation mechanism is droplet nucleation in which each small stabilized droplet in the minimeulsion can be regarded as a nanoreactor [15], [20], [21], [22]. This enables polymerization of the miniemulsions, and the problem associated with polymerization of extremely hydrophobic fluoromonomers can be simplified to the successful preparation stable miniemulsions while solubility questions are avoided. Fluorinated monomers produced by miniemulsion polymerization have been reported [7], [15], [23], [24].

Microwave radiation as a fast and efficient means to heat the reaction media has significant advantages over conventional thermal methods. Studies have shown that in comparison with reactions under conventional heating, reactions under microwave irradiation have the advantages of higher reaction rates and higher monomer conversion in a shorter period of time [25], [26], [27]. Recently, many researchers have focused on polymerization with microwave irradiation [28], [29], [30]. Hoogenboom and Schubert have reviewed the applications of microwave irradiation to polymer chemistry [31]. Concerning the mechanism of miniemulsion polymerization, the major interest in miniemulsion polymerization is droplet nucleation to avoid micellar nucleation because polymerization inside a droplet starts as soon as a radical enters. Therefore, miniemulsion polymerization seems to be particularly suitable for the combination utilizing a very fast microwave heating. In this way, microwave heating may provide ultra-fast processing assuming that the nucleation step does become rate-limiting.

So far, miniemulsion polymerization under microwave irradiation has not been investigated extensively. Holtze [32], [33] and Zhu [34] have indeed conducted some research on the use of microwave irradiation in miniemulsion polymerization. Holtze [32], [33] indicated that ultra-fast conversion and molecular weight could be obtained through temperature programming in microwave induced miniemulsion polymerization. In Zhu's work [34], well-defined and stable polystyrene latexes were obtained by nitroxide-mediated radical miniemulsion polymerization under microwave irradiation.

In this work, we first applied microwave irradiation to the miniemulsion copolymerization of fluoroacrylate. Dodecafluoroheptyl methacrylate (DFHMA) (<7 fluorocarbons and has 11 fluorine atoms) was adopted as a fluorine-containing monomer. The particle morphology and size change during miniemulsion polymerization were determined. The effects of operation parameters such as temperature, concentration of initiator and emulsifier on monomer conversion were investigated systematically. The surface properties and thermal stability of the latex films were also characterized by contact angle, atomic force microscopy (AFM) and thermo-gravimetric analyses.