Observation of two α-relaxation peaks in a nanoclay-filled epoxy compound

Abstract

For over 10 years, the effect of nanosilicate layers on glass transition temperature of epoxy resins has been studied by various research groups. Unfortunately, there appears to be no consensus in the literature regarding the role of nanoclay addition on a-relaxation peak (Tg-peak) in montmorillonite (MMT)-filled epoxy compounds [1–8]. Several studies reported an increase in the overall glass transition temperature (Tg) of MMT-filled epoxies compared to the neat epoxy [1–3], while other studies claimed suppression of the Tg in these nanocomposites [4–6]. There are even some results indicating that the addition of nanoclay has no effect on a-relaxation peak of epoxy resins [7, 8]. Table 1 contains a description of the materials investigated in these studies. The following sections contain discussions of glass transition behavior in several types of nanoclay-filled epoxies. Messersmith and Giannelis [1] reported broadening of the tan d peak in the glass transition region and a slight increase in the Tg of mica-filled epoxy compound with the interlayer distances larger than 100 A. The glass transition temperature was characterized using a dynamic mechanical analyzer (DMA). The increase in Tg was attributed to the strong interactions between silicate platelets and epoxy network that restrict molecular motion of polymer segments near the silicate layer surface. Regarding the observations of Tg depression, several hypotheses have been proposed. Chen et al. [4] attributed this phenomenon to the presence of interphase region between the silicate layers where the surface modifiers of clay act as plasticizer in epoxy resin. They predicted the effective size of interphase region in the order of 50 A [4]. It was reported that the glass transition temperature decreased as the volume fraction of the interphase region inside the galleries increased by increasing the interlayer distance between silicate platelets [4]. However, Jackson and McKenna [9] found that confining organic liquids (1,2-diphenylbenzene and benzyl alcohol) in both surface treated and untreated nano-size pores resulted in similar Tg suppression. Becker et al. [5] believed that the presence of unreacted epoxy monomers leads to Tg depression in clay nanocomposites. Specifically, it was claimed that the drop in the Tg can be ascribed to the lower crosslink density of epoxy network between silicate layers, i.e. in the galleries [5]. The purpose of this letter is to report an observation on the effect of nanoclay fillers on the a-relaxation in an epoxy resin. This observation, which is a small portion of a more detailed study on MMT-filled epoxies, provides useful insight on glass transition processes in nanoclay-filled epoxy composites.