Room temperature ionic liquids for epoxy nanocomposite synthesis: Direct dispersion and cure

Abstract

Nanocomposites of silica, single-wall carbon nanotubes, and graphite nanoplatelets in an epoxy matrix were prepared using a novel single-step room temperature ionic liquid (RTIL) preparation strategy. Silica nanocomposites showed improved fracture toughness and modulus, carbon nanotube composites showed electrical network percolation at 8.6 × 10⁻⁵ volume fraction, and graphite nanoplatelets composites showed electrical percolation at 1.7 × 10⁻² volume fraction. These results were compared with literature results for nanocomposites made with volatile solvents, demonstrating the viability of the RTIL nanocomposite preparation strategy.

Introduction

Nanocomposites have attracted significant research attention in recent years. They promise to open new areas of applications for thermosets and thermoplastics with potentially facile processing and enhanced mechanical and electrical properties [1]. Epoxies are an important class of thermosetting polymers that can gain functionality by modification with nanoparticles. Epoxies are widely used in structural composite applications and as adhesives, coatings, and as encapsulants for electronics in a wide array of industries. They are considered high performance thermosetting polymer matrices and generally possess high modulus, low creep, good adhesion to many substrate materials, and chemical and corrosion resistance. However they are brittle in nature, with a poor resistance to crack propagation. Inorganic nanoparticles have been shown to enhance the inherent properties of epoxy systems. Specifically, silica nanosphere composites [2] show improved modulus and fracture toughness, SWNT composites [3], [4] show enhanced electrical and thermal conductivity at low loadings, and GNP composites [5], [6], [7], [8] provide increases in modulus and electrical conductivity with a decrease in vapor permeation relative to an unmodified epoxy matrix.

Effective nanoscale dispersion of individual inorganic particles is important in order for a nanocomposite to obtain property improvements. Because of this, significant nanocomposite research focuses on processing methods, including solvent selection and physical dispersion methods. A typical nanocomposite preparation scheme involves the following steps: (i) chemical pre-treatment of the nanoparticle, (ii) dispersion into a volatile solvent [9] by physical processing [10], (iii) mixture with the polymer matrix, (iv) removal of the volatile solvent, and (v) cure of the final composite. Silica, SWNT, and GNP nanocomposites are made in this way using a variety of modifying agents, solvent chemistries, and mechanical dispersion methods.

For the preparation of silica nanocomposites, nanoparticles are covalently modified with an organosilane coupling agent, and subsequently dispersed in a solvent such as acetone [11], [12], isopropanol [13], methyl ethyl ketone (MEK) [14], [15], or methyl isobutyl ketone (MIBK) [16], [17], using sonication [11], [12], or elevated temperature mixing [11], [13], [18]. A commercially available approach uses silica nanoparticles suspended directly in epoxy via organosilane surface modification, with both dispersion and solvent removal accomplished by proprietary processes [19], [20], [21], [22], [23], [24].

In the preparation of SWNT composites, SWNTs can be covalently modified to improve dispersion. However, the improved dispersability is offset by reduced aspect ratios [25] and carbon defects that decrease the electrical performance [26] of the modified nanotubes. Non-covalent surfactants have shown some promise for improving dispersion without affecting SWNT structure, but surfactant removal poses additional processing challenges. Unmodified nanotubes can also be directly dispersed using volatile solvents such as ethanol [25], [27], dimethyl formamide [25], [28], acetone [25], [29], [30], [31], chloroform/toluene [32], and water [33]. While some studies report SWNTs dispersed directly into the polymer matrix solely via mechanical methods [34], [35], [36], [37], [38], [39], the resulting SWNT composites generally do not possess the levels of nanoscale dispersion obtained by surface modification or solvent-aided methods.

In the preparation of graphite-based nanocomposites there are two primary methods [40], [41] for chemical exfoliation of graphite sheets: the reduction of graphite oxide (rGO), which produces single-layer, chemically altered, graphene sheets; and acid intercalation, which produces few-layer stacks of still-pristine expanded graphite (EG). These chemically modified nanosheets are then dispersed via a combination of miscible solvent and physical processing methods. For the preparation of epoxy nanocomposites, expanded graphite has been dispersed in cyclohexane [42], acetone [43], [44], [45], [46], other solvent, [47] and directly into the resin [48].

An alternative to both covalent modification and volatile solvent dispersion is the use of room temperature ionic liquids (RTILs) [49]. Already used as green solvents for organic synthesis [50], RTILs are organic ion pairs with negligible vapor pressure, relatively low viscosity, high ionic conductivity, and tunable physicochemical properties [51]. Additionally, some RTILs have shown promise as a solvent for non-covalently modified graphite and carbon nanotube systems. Researchers have used an imidazolium-based ionic liquid to electrochemically exfoliate graphene directly from graphite [52], [53], [54], and to create an ionic-liquid/SWNT gel with well-distributed SWNTs [55], [56]. Theoretical and spectroscopic studies have suggested cation-pi [57] stacking and electrostatic shielding between graphitic particles by the RTIL solvent [58] as reasons for the favorable interactions.

In addition to its properties as a solvent, a particular class of ionic liquids that includes 1-ethyl-3-methyl imidazolium dicyanamide (EMIM-DCN) has been shown by our group [59] and others [60] to act as a latent curing agent for epoxies, demonstrating excellent liquid miscibility, long-term room temperature stability, and a highly cross-linked structure with good thermomechanical properties of the cast material.

We present an in situ nanocomposite preparation method that takes advantages of both the favorable nanoparticles interactions and the thermoset cure capability of EMIM-DCN so that the entire nanocomposite synthesis process can be accomplished by a single mixing step requiring no energy-intensive evaporation or other separation process. The advantages are shown schematically in Fig. 1 in which the new processing method is compared to a typical volatile-solvent based scheme. The elimination of the solvent removal step not only simplifies processing, but also removes a processing variable that could provide non-uniformity in the final product [25]. Additionally, minimizing volatile waste makes the process environmentally friendlier, both during production and over the lifetime of the epoxy composite.

Thus, we demonstrate the use of this streamlined method enabled by EMIM-DCN for the synthesis of diglycidyl ether of bisphenol A (DGEBA) nanocomposites that have excellent properties prepared using silica nanoparticles, SWNTs, and graphite nanoplatelets.