Fluorinated Vitrimer Elastomers with a Dual Temperature Response.

Abstract

Vitrimers are an emerging new class of permanently cross-linked polymeric materials that show a liquid behavior upon heating wherein the macroscopic deformation is controlled by the rate of internal chemical bond exchange reactions. Thus, quite uniquely among polymeric materials, flow rates and material viscosities can be enhanced or controlled by the addition of catalysts and additives. We now report a catalyst-free vitrimer system, prepared from mixing two simple components, wherein two competing bond exchange mechanisms coexist, each showing a strikingly different temperature dependence, related to the large difference in activation energy for the different exchange pathways (60 vs 130-170 kJ/mol). The low barrier process is predominant at lower temperatures, but is outcompeted by the high barrier process that becomes dominant at higher temperatures because of its much more pronounced temperature dependence. The result is an interesting and highly unusual dual viscosity profile for this new class of vitrimer materials: a very gradual decrease in viscosity at lower temperatures, intercepted by a much sharper drop in viscosity at higher temperatures. The highly counterintuitive effect where a higher barrier pathway is dominant over a much lower barrier process can be rationalized by the exchange mechanisms that involve different reactive species, but lead to the overall same exchange. We observed this unusual but highly promising behavior first for fluorinated vitrimer elastomers, aimed at high performance materials, but the effect was also shown to hold in related nonfluorinated elastomers. A new way to control and design the rheological behavior of vitrimers toward finely tuned and precisely controlled processing applications has thus been provided.