Linear viscoelasticity of covalent adaptable network (CAN) polymers comprising β-amino esters

Abstract

We present an experimental investigation of the linear viscoelasticity for a series of covalent adaptable network (CAN) polymers of β-amino esters possessing tertiary amines at the β position of ester linkages. CAN polymers were synthesized by aza-Michael addition from di-acrylate monomers with or without β-hydroxyl groups using a tri-amine cross-linker. The prepared CAN polymers exhibited dissociative-type bond exchanges by aza-Michael reaction. The additional inclusion of β-hydroxyl group endowed them with associative-type bond exchanges by transesterification. Time–temperature superposition (TTS) was used to construct pseudo-master curves of storage, loss, and stress-relaxation moduli over wide timescales. The results showed that without transesterification, the slow kinetics of aza-Michael reaction considerably retarded terminal relaxation. The introduction of transesterification accelerated terminal relaxation rates but did not modify the overall broadness of terminal relaxation modes. Horizontal shift factors displayed Williams–Landel–Ferry (WLF) dependence below 120 °C but Arrhenius dependence above 120 °C. The former was due to slow segmental dynamics, whereas the latter reflected the characteristic of exchange reaction kinetics. In addition, we also compared and discussed two definitions of topology freezing transition temperature, an important concept for associative-type CANs. Conclusively, the topology freezing transition temperature obtained from the transition of shift factors (WLF → Arrhenius) was a more practical definition for the potential processing and applications of CAN polymers.