Viscoelastic properties and long-term stability of polystyrene-carbon nanotube nanocomposites. Effect of the nature of the carbon nanotubes and modification by ionic liquid

Dynamic mechanical analysis (DMA) measurements have been used for determining the viscoelastic behaviour of polystyrene (PS) nanocomposites containing a 1 wt.% of single- or multi-walled carbon nanotubes, both pristine (SWCNT and MWCNT) and modified by previous treatment with a room-temperature ionic liquid (SWCNTm and MWCNTm). Two sets of multiple frequency tests have been carried out. The Arrhenius relationship has been used to estimate the apparent activation energies for the glass transition. Master curves have been generated using the time-temperature superposition principle. The nanocomposites show an increase of the activation energy up to an 8% with respect to PS, in the order PS + MWCNT > PS + MWCNTm > PS + SWCNTm > PS + SWCNT > PS. The nanophases also induce a displacement of the onset of the storage modulus to longer times following the order PS + SWCNTm > PS + SWCNT > PS + MWCNT > PS + MWCNTm > PS, at 40 °C. At higher temperatures, the storage modulus onset is very similar for all materials. In the case of PS + SWCNTm, the period of time for the onset of the storage modulus is multiplied by a factor of 28 with respect to PS. The surface modification of the SWCNT by the ionic liquid could improve the interfacial compatibility. The results described here show that a very low proportion of carbon nanotubes can effectively improve the long-term stability of PS under service temperature conditions.