Design and Synthesis of Epoxy Vitrimers with a Low or Negative Coefficient of Thermal Expansion

Mitigating thermally induced stress in epoxy polymers is significantly challenging. An effective strategy of preparing a dual-functional epoxy vitrimer with a low coefficient of thermal expansion (CTE) and rapid stress relaxation was developed to mitigate stress. The epoxy vitrimer was synthesized using AFG90, a dicarboxylic acid, and a dibenzocyclooctadiene (DBCOD)-based motif (DBCOD-NH₂). The samples exhibited low or even negative thermal expansion below their T_g, owing to the conformational transition of the DBCOD unit, and rapid stress relaxation above the T_g, owing to the carboxylate transesterification of the dynamic ester bonds. The two functions operated independently over different temperature ranges. One of the typical samples exhibited a CTE of -21.8 ppm/K (40-70 °C) and a stress relaxation time of 268 s (240 °C). Structure-property correlation revealed that increasing the DBCOD-NH₂/dicarboxylic acid ratio or decreasing the dicarboxylic acid chain length could decrease the thermal expansion and slow the stress relaxation owing to the variation in the content of DBCOD or dynamic bonds and the increased chain rigidity and cross-link density. Stress relaxation could be further enhanced using the synergistic effect of dual dynamic bonds while maintaining the low thermal expansion. This strategy is promising for mitigating thermally induced stress in epoxy matrices.