Do Carbon Nanotubes Improve the Thermomechanical Properties of Benzoxazine Thermosets?

Fillers are widely
used to improve the thermomechanical response
of polymer matrices, yet often in an unpredictable manner because
the relationships between the mechanical properties of the composite
material and the primary (chemical) structure of its molecular components
have remained elusive so far. Here, we report on a combined theoretical
and experimental study of the structural and thermomechanical properties
of carbon nanotube (CNT)–reinforced polybenzoxazine resins,
as prepared from two monomers that only differ by the presence of
two ethyl side groups. Remarkably, while addition of CNT is found
to have no impact on the glass-transition temperature (Tg) of the ethyl-decorated resin, the corresponding ethyl-free
composite features a surge by ∼47 °C (50 °C) in Tg, from molecular dynamics simulations (dynamic
mechanical analysis measurements), as compared to the neat resin.
Through a detailed theoretical analysis, we propose a microscopic
picture for the differences in the thermomechanical properties of
the resins, which sheds light on the relative importance of network
topology, cross-link and hydrogen-bond density, chain mobility, and
free volume.