Time Scale-Dependent Structure–Property Relationships in Dynamic Imine-Benzoxazine Networks

TL;DR

This paper studies how the structure of dynamic polymer networks affects their viscoelastic properties at different time scales.

Contribution

The study reveals how structural variations in iBOX vitrimers influence activation energy across time scales.

Findings

iBOX vitrimers exhibit distinct short- and long-time viscoelastic behaviors governed by different activation energies.

Polymer architecture significantly affects the dynamics of bond exchange processes in dynamic networks.

Time–temperature superposition reveals nonuniform changes in activation energy due to structural variations.

Abstract

Dynamic
polymer networks, or vitrimers, incorporate labile
cross-links
that enable topological rearrangement of the network via associative
bond exchange, a process accelerated by the application of strain
and heat. The linear viscoelasticity of vitrimers is governed by the
behavior of the network at both short and long time scales, influenced
by both local segmental motions and the kinetics of the dynamic exchange
mechanism itself. Herein, we investigate the linear viscoelasticity
of imine-containing benzoxazine (iBOX) networks, the majority of which
possess glass transition temperatures (T
g) exceeding 100 °C and resemble nondynamic thermosets used in
structural applications. Nine iBOX monomers were synthesized by the
condensation reaction of an aldehyde-functionalized benzoxazine precursor
with a series of difunctional amines. Subsequent cationic ring-opening
polymerization produced iBOX vitrimer networks with varying poly­(propylene
oxide) (PPO), poly­(ethylene oxide) (PEO), and poly­(ethylene) (PE)
backbone structures, each represented by three different molecular
weights between cross-links. Time–temperature superposition
(TTS) was applied to small-amplitude oscillatory shear (SAOS) and
stress relaxation data to elucidate the influence of polymer architecture
on viscoelastic response. The occurrence of multiple relaxation processes
with distinct temperature dependences necessitated separate shift
factors for short- and long-time dynamics, each exhibiting an Arrhenius
temperature dependence and yielding different estimates of apparent
activation energy (E
a). Notably, the structural
variations lead to nonuniform changes in E
a across time scales, which exemplifies the complexity of tuning exchange
kinetics in dynamic polymer networks.

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