Poly(glycidyl methacrylate)-grafted clay nanofiller for highly transparent and mechanically robust epoxy composites

Highlights

• Exfoliated poly(glycidyl methacrylate)/clay nanocomposites were prepared by free radical photopolymerization.

• The bentonite clay was intercalated with mercaptosilane prior to photopolymerization.

• The clay-silane/polymer nanocomposite served as hybrid nanofiller for epoxy matrix.

• Poly(glycidyl methacrylate) permits to disperse clay in- and to couple it to the epoxy matrix through the aminated hardener.

• Thermal and mechanical properties of hybrid filler-reinforced epoxy are superior to those of epoxy–pristine clay composites.

Abstract

Poly(glycidyl methacrylate) (PGMA)/clay nanocomposites were prepared by free radical in situ photopolymerization using intercalated chain transfer agent (3-mercaptopropyl)trimethoxysilane (MPS) in the layers of bentonite (B) clay. The natural bentonite was treated with MPS (at 1–5% v/v) which acts as both intercalant and chain transfer agent. Glycidyl methacrylate (GMA) was photopolymerized in situ in the presence of the silanized bentonite (B-MPS) using 2,2-azobisisobutyronitrile (AIBN). The intercalation ability of MPS and exfoliated nanocomposite structure were evidenced by both X-ray diffraction spectroscopy (XRD) and transmission electron microscopy (TEM). Surface analysis and morphologies of the resultant nanocomposites were also studied. The mass loading of PGMA reached 81 wt.% while XPS spectra, particularly the high resolution C1s region, resemble those of pure PGMA. The primary B-MPS/PGMA nanocomposite was then used in the preparation of nanocomposite coatings by mixing it with di-glycidyl ether of bisphenol A (DGEBA) epoxy matrix and 4,4-diaminodiphenyl sulfone (DDS) hardener in order to obtain ternary Epoxy/B-MPS/PGMA. The latter was cured and then was compressed into 5 mm thick sheets; it was found to be transparent with mixed intercalated/exfoliated structure. The thermal and mechanical performances of Epoxy/B-MPS/PGMA are by far superior to those of the reference epoxy–pristine bentonite nanocomposite.

Introduction

Polymer–clay nanocomposites have been widely investigated in recent years due to enhancement in mechanical, thermal, gas barrier, catalytic and fire retardant properties that nanoclay imparts to polymer matrices [1], [2], [3], [4], [5]. The improvement in properties combined with the low cost of manufacturing of polymer–clay nanocomposites has led to their use in engineering applications for aerospace, automobiles, food packaging, and electronics among others [6]. One well known approach to maximize performances of nanocomposite materials is to obtain well dispersed single clay sheets in the polymer matrix [7], [8]. In addition, it is important to strengthen molecular interactions between the clay nanofiller and polymer matrix in order to obtain robust nanocomposites [9].

Thermoplastic-based polymer–clay nanocomposites can be manufactured through three main strategies according to the starting materials and processing techniques: solution exfoliation, melt intercalation and in situ intercalative polymerization [10]. Disadvantages of the first two methods are the co-intercalation of solvents in the case of the solution reaction and slow polymer transport into the interlayer space [11]. In contrast, in situ polymerization holds great promises as demonstrated by several investigations [7]. The approach rests on the modification of clay by intercalating into its interlayer space functional compounds such as quaternary ammonium [12], [13], [14], iodonium [15], phosphonium [16], diazonium salts [17], silane coupling agents [18], [19] or quaternary ammonium-bearing functional monomer [20]. It follows that intercalation of monomers triggers the growth of polymer chains in the clay galleries followed by exfoliation and the nanocomposite.

In situ polymerization can be initiated either by thermal polymerization and photopolymerization using a variety of initiating mechanisms [21], [22], [23], [24]. Moreover, several conceptually different approaches, namely copper (I) catalyzed azide/alkyne cycloaddition (CuAAC) “click” reaction [25], [26] and multiple hydrogen bonding interactions [27] were shown to be attractive processes for the clay/polymer nanocomposite preparation. Particularly, the photochemical route for clay/polymer nanocomposites offers distinct advantages over thermal methods in the sense it is a green process, does not necessarily require drastic monomer purification procedures, can be adapted to many systems, can be confined at surfaces and especially permits to provide patterned polymer coatings [28], [29]. Among the photochemical routes for clay–polymer nanocomposites, the use of chain transfer agents strategy is well-documented and provides polymers by free radical polymerization [30], [31], [32], [33]. It was demonstrated that the end groups borne by the polymer chains depend on the nature of the transfer agent [33], [34]. In the present work, the chain transfer agent is intercalated within the layered silicate through silylation reaction by using commercially available mercaptosilane. The designed strategy appeared to be more simple and economical than the conventional ammonium ion based intercalation methodology.

The aim of this manuscript is to describe the in situ preparation of polymer/clay nanocomposites using (3-mercaptopropyl)trimethoxysilane acting both as intercalation and chain transfer agent. Glycidyl methacrylate (GMA) was deliberately selected as the monomer in the subsequent polymerization process in order to provide not only intercalated and exfoliated polymer chains but also compatibility with the reinforcement material epoxy resin through the glycidyl groups [19]. Moreover, PGMA chains can readily be crosslinked by amines such as 4,4′-diamminodiphenyl sulfone (DDS) [35]. The thermal and mechanical properties of epoxy reinforced by clay–PGMA nanocomposite filler were conducted and compared with the reinforcement by unmodified clay in order to verify the role of the interface chemistry of the PGMA chains grafted to the bentonite lamellae.