Natural glycyrrhizic acid: improving stress relaxation rate and glass transition temperature simultaneously in epoxy vitrimers

Abstract

Epoxy vitrimers based on transesterification reactions (TERs) are a type of recyclable thermosets which have been developed sucessfully in recent years. However, the good thermal performance and the fast network rearrangements are somehow contradictory and difficult to achieve simultaneously in TER-based epoxy vitrimers. This contradiction has been resolved in this work by use of a green and simple, yet effective strategy, where natural glycyrrhizic acid (GL) with sebacic acid (SA) were used as curing agents to prepare epoxy vitrimers based on TERs with different monomer compositions (V1 to V5). Thanks to the unique structure of GL, the glass transition temperatures (T_g) of the vitrimers rose from 31 to 89 °C, and their stress relaxation time shortened from 390 to 130 s at 180 °C with the increase of GL content. In particular, the vitrimer V4 exhibited good thermal stability (a decomposition temperature T_d5 of 263 °C), fast stress relaxation (a relaxation time of 130 s at 180 °C), low malleability temperature (a T_mall of 116 °C) and usable T_g (61 °C). Moreover, this vitrimer could be welded, reprocessed, repaired, recycled and chemically degraded at a relatively mild temperature (130 °C). This work provides a green strategy for vitrimer design, which might benefit the preparation of recyclable thermosets with a highly comprehensive performance in the future.