Effects of sonication energy on the dispersion of carbon nanotubes in a vinyl ester matrix and associated thermo-mechanical properties

In the present paper, we report a systematic examination of sonication energy and the subsequent dispersion condition of nanoparticles in a vinyl ester resin. The nanoparticles were multi-walled carbon nanotubes (MWCNTs) functionalized with carboxylic acid groups. Two nanoparticle concentrations of 0.25 and 0.5 wt% with a variety of sonication amplitudes and duration were considered. Elastic moduli were determined using a 3-point flexural method and a nanoindentation technique. Glass transition temperatures (Tg) were also evaluated by differential scanning calorimetry. The dispersion quality of MWCNTs was investigated by high-resolution scanning electron microscopy (HR-SEM). Results indicated that with both concentrations, there is a gradual increase in elastic modulus and Tg up to a certain sonication energy level beyond which both properties decreased. However, this threshold energy level varied with MWCNT concentration. For example, the threshold energy level was 60 kJ in case of 0.25 wt% while it was only 15 kJ with 0.5 wt% concentration. It was also observed that regardless of the level of threshold energy, enhancement in properties remained more or less the same. This suggests that there is interplay between particle concentration and sonication energy that dictates the dispersion condition and hence controls the nanocomposite properties. It is further evidenced by SEM studies that nanotubes undergo significant structural changes such as length reduction even at lower energies that eventually limits the threshold level.