(322d) Investigation of Crack-Healing Characteristics in Polymer Networks of Dgeba Epoxy Cured with a Cycloaliphatic Diamine
Crack healing in polymer thermoset systems have been carried out using the reversible chemistry of Diels-Alder reactions, catalyst/monomer inclusions that cause autonomous healing or polymer composites comprised of a thermoset and a linear polymer. There have been almost no systematic studies so far into the healing characteristics of unmodified thermosetting polymers and a fundamental understanding into elucidating possible mechanisms and requirements for the design of healing materials, which is the topic of this presentation.
In this work, the crack healing behavior of polymer networks formed by reacting a difunctional epoxy, diglycidyl ether of bisphenol A (EPON®828) and a tetrafunctional amine, 4,4' methylenebiscyclohexanamine (AMICURE®PACM) both at and off stoichiometry are tested using a modified compact tension geometry. After complete cure as determined by Infra-Red analysis and total fracture, these thermosets exhibit the ability to regain their load bearing capacity to various degrees at temperatures above the glass transition temperature. The effects of temperature, pressure, stoichiometric ratio and reheal cycles are studied here with an effort to understand the basic phenomenon of healing in polymer networks. Load recovery of approximately 60 percent is observed at the stoichiometric ratio and this and other values of the healing efficiency are found to depend on the conditions employed for healing. Visual inspection of specimen crack interfaces indicates that the crack has disappeared. Scanning electron micrographs of fractured and healed sections of the specimen suggests that there may be a diffusible sol-phase that lends adhesive strength at the crack interface and the higher load recovery of samples that are significantly off-stoichiometry tend to support the observation. On the other hand, secondary chemical reactions at elevated temperatures cannot be ruled out at this point although in the absence of excess epoxide groups, there does not seem to be evidence based on infra red analysis. Our results thus show that an unmodified thermosetting system can exhibit healing behavior similar to other designed healable thermosets. The conclusions drawn in this work will also throw light upon the possibility of designing polymer networks with embedded mobile, diffusible and reactive sol-species.