Recyclable dual-curing thiol-isocyanate-epoxy vitrimers with sequential relaxation profiles

Two series of poly(thiourethane)s-poly(thiol-epoxy) hybrids were prepared from dual-curing, stoichiometric thiol:isocyanate:epoxy mixtures. 1-methylimidazole (MI) was used as initiator for the thiol-isocyanate and thiol-epoxy curing reactions. A salt of tetraphenylborate derived from highly basic amine 1,5,7 triazabicyclo [4,4,0] dec-5-ene (TBD) was used as catalyst for the activation of bond exchange reactions. The curing kinetics and the glass transition temperature (Tg) of the samples were studied by differential scanning calorimetry (DSC). Thermal stability was characterized by thermogravimetric analysis (TGA). Stress relaxation dynamics related to the bond exchange reactions were studied by dynamic mechanical analysis (DMA). Recyclability of the fully cured samples according to the DMA results was carried out by hot-pressing at elevated temperatures. The results show that the curing process takes place in a controlled and sequential way: the thiol-isocyanate reaction takes place first, at moderate temperatures, followed by the thiol-epoxy reaction, which is completed at higher tem-peratures. The analysis of stress relaxation evidences a complex two-step relaxation behavior depending on the contribution of isocyanate and epoxy groups to the mixture. Isocyanate-rich materials show a simple relaxation process corresponding to the trans-thiocarbamoylation exchange reactions. Epoxy-rich materials show, in contrast, a two-step relaxation processes evidencing that trans-thiocarbamoylation alone may not be sufficient to relax the stress completely, due to an apparently permanent network structure. However, complete stress relaxation is possible for epoxy-rich materials through additional bond exchange reactions such as trans-esterification or dynamic thiol-Michael, but at a slower rate. Depending on the composition and the temperature, full recycling of the material can be achieved at moderate temperatures in a timescale of minutes to hours.

Automated summary

Two series of poly(thiourethane)s-poly(thiol-epoxy) hybrids were synthesized from dual-curing, stoichiometric thiol:isocyanate:epoxy mixtures. The curing process occurs in a controlled and sequential manner, with the thiol-isocyanate reaction occurring first followed by the thiol-epoxy reaction. The stress relaxation behavior is dependent on the contribution of isocyanate and epoxy groups, with isocyanate-rich materials showing simple relaxation and epoxy-rich materials exhibiting a two-step relaxation process. Complete stress relaxation can be achieved through additional bond exchange reactions, allowing for full recycling of the material at moderate temperatures.