Steric guided anomalous thermal expansion in a dimorphic organic system

The thermal expansion behaviour of two polymorphs of 4,4'-methylenebis(2,6-dimethylaniline) (MBDA) has been studied. The monoclinic form exhibits a very large uniaxial negative thermal expansion (alpha(l) = -174(8) MK-1) and 1.6 times higher alpha(v) than the triclinic form, which exhibits a normal positive thermal expansion. This anomalous thermal expansion behaviour in the monoclinic form is guided by the steric effects of the strongly vibrating methyl groups in the solid.