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 (αl = −174(8) MK−1) and 1.6 times higher αv 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.
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Bhattacharya et al. (2014) studied this question.
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