Thermal properties and glass transition in PMMA + SWCNT composites

The effective thermal and dynamic properties of composites containing polymethyl-metha-acralyte (PMMA) dispersed with single-wall carbon nanotubes (SWCNTs) were studied by modulation/ac calorimetry (ACC) and modulation-differential-scanning (MDSC) calorimetric techniques. The specific heat and effective thermal conductivity were determined by ACC from 300 to 400 K as a function of SWCNT content. A large enhancement of the effective thermal conductivity is observed as the mass fraction (φm) of SWCNTs increases from 0.014 to 0.083. These results are in good agreement with a simple geometric model at low SWCNT content but are better described by more sophisticated models above φm ∼ 0.039. The dynamics of the glass transition were studied by MDSC as a function of temperature scan rate. The hysteresis of the reversible specific heat between heating and cooling scans decreases with decreasing scan rate for pure PMMA but is essentially zero in the composites, indicating that the SWCNT may be quenching slow glass dynamics. In all samples, the effective glass transition temperature, Tg, increases with increasing scan rate (though less so for higher φm) but the MDSC determined Tg does not fall inline with that determined by the ACC method. This discrepancy is attributed to the effect of prolonged heat treatment of the composite sample used in the ACC experiments.