Investigation of the low-temperature mechanical behavior of elastomers and their carbon nanotube composites using microindentation

The micromechanical properties of epoxy resin elastomers and their carbon nanotube composites were studied using a microhardness tester equipped with low-temperature chamber. X-ray diffraction analysis indicated that all specimens were free of any crystalline components and were amorphous with only short-range order domains. The Vickers microhardness of all samples has been estimated in the temperature range 230–300 K. The measurements demonstrated that at room temperature these materials are elastomers (notably, they are in high-elastic state) and on cooling in the range of 250–270 K the glass transition takes place. Analysis of the temperature dependence of microhardness suggested that the thermomechanical and relaxation properties of the materials studied are consistent with a rheological model of a standard linear solid where the relaxation time (or viscosity) depends exponentially on the temperature in accordance with the Arrhenius equation for the rate of thermally activated process. Empirical estimates for the nonrelaxed and relaxed Young’s moduli and also for the activation energy (U = 0.75 eV) and the period of attempts (τ₀ = 10^–12 s) of the molecular process which determines the relaxation properties and the glasstransition of the materials have been obtained. The addition of carbon nanotubes into elastomeric epoxy resin had no effect on its micromechanical characteristics as measured by the microhardness tester. It is shown that the conventional microindentation method is an efficient tool of investigating the thermomechanical properties of elastomers nearby and below the glass transition temperature.