Influence of conformational change of chain unit on the intrinsic negative thermal expansion of polymers

Negative thermal expansion (NTE) behavior has roused wide interest for the control of thermomechanical properties of functional materials. Although NTE behaviors have been found in kinds of compounds, it remains challenging for polymers to achieve intrinsic NTE property. In this work, we systematically studied the conformational change of dibenzocyclooctadiene (DBCOD) derivatives between chair (C) and twist-boat (TB) forms based on density-functional theory (DFT) calculations, and found clear evidence of the relationship between the structure of DBCOD units and the thermal contraction behavior of the related polymers. In order to obtain the polymer with NTE property, two conditions should be met for the thermal contracting DBCOD related units as follows: (i) the TB conformation can turn into C conformation as the temperature increases, and (ii) the volume of C conformation is smaller than that of TB conformation. This rule should offer a guidance to exploration of the new intrinsic NTE polymers in the future.