Towards improved dual network, disulfide based, self-healing thermosets for fibre reinforced composites

Abstract

In order to address the need for a mechanically strong, self-healing thermosetting polymer for fiber reinforced composites, a novel dual network epoxy containing reversible disulfide bonds has been developed recently. This hybrid epoxy had the ability to heal multiple times when incorporated into fiber reinforced composites along with easy processing routes. However, one of the problems faced by this system was the instability of thiols, which were used to prevent phase separation of the components and promote healing by increasing the S-S bond content. These thiols, however, undergo side reactions which reduce the mechanical properties of the final network along with reduced long-term healing ability.
The goal of this thesis was to study the effect of eliminating thiol from this dual-network epoxy on its mechanical and healing properties and to explore the effect of CuCl2 as a catalyst in case there is a reduction in the healing efficiency. The hybrid epoxy without thiol has been synthesized and its microscopic and macroscopic reversibility have been studied using stress relaxation and mode I opening of Double Cantilever Beam (DCB) specimens respectively.
Eliminating thiol lead to an increase in Young’s modulus and strength of the hybrid epoxy. The same epoxy without thiol shows higher stress relaxation than the epoxy with thiol at 80°C. Given that in a crosslinked network, the ability to relax stress depends on the reversibility of the disulfide bonds, this indicates that the disulfide bond reversibility in the epoxy without thiol does not decrease in the absence of thiol in spite of having a stronger network at room temperature. Macroscopically, however, the epoxy with thiol had higher healing efficiency compared to the epoxy without thiol which showed almost no healing.
To address the loss in healing efficiency, the effect of CuCl2 as a catalyst to promote disulfide exchange reactions was studied. Stress relaxation experiments indicated greater relaxation in the presence of CuCl2 along with a small increase in the stiffness of the hybrid epoxy. CuCl2 can thus be used as an alternative for thiol to promote disulfide exchange reactions at higher temperatures.
Given that there is a dearth of literature comparing thiol-disulfide reactions with disulfide exchange reactions, the effect of thiols in this systems was unclear before. From these experiments, it can be observed that in the absence of thiols, disulfide bonds undergo stress relaxation most likely using a thermally triggered radical mediated exchange mechanism. However, the polymer architecture and the amount of disulfide content plays a key role in the macroscopic healing and this could be tuned to increase both healing and mechanical properties simultaneously.