Viscoelastic properties and determination of free volume fraction of multi-walled carbon nanotube/epoxy composite using dynamic mechanical thermal analysis

Various weight fractions of multi-walled carbon nanotubes (MWNTs) were added to the epoxy as reinforcement to fabricate MWNT/epoxy nanocomposites by using ultrasonication and cast molding method. Dilatometeric measurements and Dynamic mechanical thermal analysis (DMTA) were used to determine the William–Landel–Ferry (WLF) constants and the effect of MWNT on viscoelastic behavior of nanocomposites in the glass transition region respectively. The results showed that addition of nanotubes to epoxy had significant effect on the viscoelastic properties. However, use of 0.5 wt.% increased the viscoelastic properties more significantly. The WLF constants C1 and C2 were affected by MWNT amount. They seem to be interrelated as C2/C1 = 3.21–6.27 K. It explains a relationship between the free volume fraction (fg) at glass transition temperature (Tg) and the thermal expansion coefficient (CTE). The relationship found between the experimental values of C1, C2 and Δα, clearly disagrees with the classical free volume interpretation of the WLF relationship.

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► The optimum percentage for improvement of the viscoelastic properties was 0.5 wt.%.
► The WLF constants C1 and C2 were affected by MWNT amount. The value C2/C1 was between 3.21 and 6.27 K.
► The free volume fraction derived from WLF equation was increased by the addition of MWNT.