Reprocessable vinylogous urethane cross-linked polyethylene via reactive extrusion†
Vitrimers are an emerging class of permanently cross-linked materials, where exchange reactions of cross-links provide malleability. The most promising application of these materials is to impart reprocessability to thermoset polymers, such as cross-linked polyethylene (PE). Here, a facile and easy to scale-up method for the preparation of cross-linked PE vitrimers is presented by combining reactive extrusion and injection molding using exchange reactions of vinylogous urethanes. Very low retention times (<15 min) inside the extruder were required and the obtained elastomeric vitrimers have increased melt strength and a cross-link density dependent plateau modulus. Control over the amount of cross-links per chain in the final polymer was possible by varying the co-monomer feed in the preparation of the precursor polymers, which allowed the tuning of the final material properties such as melt viscosity and rubber plateau modulus. For polymers with an amount of >1.9 cross-links per chain, stress-relaxation is largely controlled by the exchange reactions of vinylogous urethanes. It was found that the activation energy depended on the cross-link density. Reprocessability of the vitrimers is demonstrated by four cycles of molding and tensile strength testing revealing minor degradation of the material.